No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
African trypanosomosis: From immune escape and immunopathology to immune intervention
Abstract
African trypanosomes can cause prolonged chronic infections through a mechanism of antigen variation whereby they manipulate the humoral immune system of their hosts. However, besides antigenic variation these extracellular parasites exert other immunoregulatory activities mainly mediated by innate cells in particular macrophage-like (M) cells. In this review, the modulation of M cells through parasite factors and host cytokines as well as their role in parasite control and immunopathology will be examined. The concept of M cell polarization into distinct activation states (M1, M2) that may contribute to trypanosusceptibility or resistance will be discussed. Finally, the possibility to interfere with such activation states hereby providing new therapeutical modalities in the treatment of this infectious disease will be illustrated.
... So far, conventional immunological studies combined with bulk-RNA sequencing, have suggested that while antibody response are important for trypanosome parasitemia control (13), it is the balance of pro-and anti-inflammatory cytokines production, accompanied by activation and alteration of classical and alternatively activated macrophages populations, that determines the chronicity of AT infections (14)(15)(16)(17)(18). In general, the early stage of trypanosomosis is hallmarked by the induction of a Th1 T cell immune response and the presence of pro-inflammatory cytokines such as INFg, TNFa, and IL-1b (19-21). ...
... In line with this, our present study shows that maintaining high systemic IL-10 level coincides with prolonged survival time in chronically T. evansi infected mice. Previous AT studies have frequently correlated the diseaseinduced cytokine environment to alterations in macrophage polarization, i.e. the occurrence of classically activated socalled M1 and alternative activated so-call M2 macrophages (16,18). However, this bipolar paradigm, which was initially established using in vitro stimulated macrophages (67,68), has often been based on bulk RNA sequencing representing 'wholeorgan' cytokine expression levels. ...
... Important is that lactic acidosis itself has been linked to catabolic muscle degradation through several mechanisms, including (i) inhibition of the glycolytic flux in skeletal muscles, by blocking skeletal hexokinase and phosphofructokinase, leading to inhibition of the carbohydrate metabolism itself (95, 96), (ii) pain-induced reduced locomotor activity (97,98), resulting in reduced food intake, (iii) the breakdown of amino acids through the glutamine/glutamate/ a-ketoglutarate and aspartate/oxaloacetate shuttles that feeds proteins into the Krebs-cycle allowing NADH/ATP generation under starvation conditions, and (iv) the association with chronic kidney disease and kidney failure (99, 100). Combined, these factors aggravate each other as they drive a perpetuated cycle that can quickly deteriorate, and could explain the 'multi-organ failure' that is often quoted as the cause of death in livestock trypanosomosis (16,32). Interestingly, while a single publication on a human trypanosomosis case did report the occurrence of lactic acidosis (101), to our knowledge there are no studies documenting blood acidosis measurements in the context of AT. ...
Infection caused by extracellular single-celled trypanosomes triggers a lethal chronic wasting disease in livestock and game animals. Through screening of 10 Trypanosoma evansi field isolates, exhibiting different levels of virulence in mice, the current study identifies an experimental disease model in which infection can last well over 100 days, mimicking the major features of chronic animal trypanosomosis. In this model, despite the well-controlled parasitemia, infection is hallmarked by severe trypanosomosis-associated pathology. An in-depth scRNA-seq analysis of the latter revealed the complexity of the spleen macrophage activation status, highlighting the crucial role of tissue resident macrophages (TRMs) in regulating splenic extramedullary erythropoiesis. These new data show that in the field of experimental trypanosomosis, macrophage activation profiles have so far been oversimplified into a bi-polar paradigm (M1 vs M2). Interestingly, TRMs exert a double-sided effect on erythroid cells. On one hand, these cells express an erythrophagocytosis associated signature. On another hand, TRMs show high levels of Vcam1 expression, known to support their interaction with hematopoietic stem and progenitor cells (HSPCs). During chronic infection, the latter exhibit upregulated expression of Klf1, E2f8, and Gfi1b genes, involved in erythroid differentiation and extramedullary erythropoiesis. This process gives rise to differentiation of stem cells to BFU-e/CFU-e, Pro E, and Baso E subpopulations. However, infection truncates progressing differentiation at the orthochromatic erythrocytes level, as demonstrated by scRNAseq and flow cytometry. As such, these cells are unable to pass to the reticulocyte stage, resulting in reduced number of mature circulating RBCs and the occurrence of chronic anemia. The physiological consequence of these events is the prolonged poor delivery of oxygen to various tissues, triggering lactic acid acidosis and the catabolic breakdown of muscle tissue, reminiscent of the wasting syndrome that is characteristic for the lethal stage of animal trypanosomosis.
... The parasite-derived components sVSG and CpG DNA that are released trigger via specific receptors (SR-A, TLR9) myeloid cell activation (121-123, 127, 164). In turn, this triggers T cell activation and the release of IFN-γ (165), which primes macrophages to become fully activated/M1 polarized thereby releasing pro-inflammatory molecules (TNF/NO) needed for parasite control (166,167). This type 1 cytokine storm can also culminate in pathology development if maintained during later stages of infection (166)(167)(168)(169)(170)(171)(172)). ...
... In turn, this triggers T cell activation and the release of IFN-γ (165), which primes macrophages to become fully activated/M1 polarized thereby releasing pro-inflammatory molecules (TNF/NO) needed for parasite control (166,167). This type 1 cytokine storm can also culminate in pathology development if maintained during later stages of infection (166)(167)(168)(169)(170)(171)(172)). Yet, only animals able to produce tissue-protective IL-10 can exhibit an alleviated pathogenicity (167). ...
... This type 1 cytokine storm can also culminate in pathology development if maintained during later stages of infection (166)(167)(168)(169)(170)(171)(172)). Yet, only animals able to produce tissue-protective IL-10 can exhibit an alleviated pathogenicity (167). Importantly, the balance of these different activation/deactivation signals may determine the outcome of infection (173,174). ...
The diseases caused by African trypanosomes are of both medical and veterinary importance and have adversely influenced the economic development of sub-Saharan Africa. Moreover, so far not a single field applicable vaccine exists, and chemotherapy is the only strategy available to treat the disease. These strictly extracellular protozoan parasites are confronted with different arms of the host’s immune response (cellular as well as humoral) and via an elaborate and efficient (vector)-parasite-host interplay they have evolved efficient immune escape mechanisms to evade/manipulate the entire host immune response. This is of importance, since these parasites need to survive sufficiently long in their mammalian/vector host in order to complete their life cycle/transmission. Here, we will give an overview of the different mechanisms African trypanosomes (i.e. T. brucei as a model organism) employ, comprising both tsetse fly saliva as well as parasite-derived components, to modulate host innate immune responses thereby sculpturing an environment that allows survival and development within the mammalian host.
... In addition, IFN-γ is also important for production of optimal amounts and isotypes of parasite-specific IgG antibodies that are important for resistance via enhanced phagocytosis and complementmediated lysis [10][11][12]. However, uncontrolled production of IFN-γ and other proinflammatory cytokines (including tumor necrosis factor-α [TNF-α], IL-6, IL-1β and IL -12) has been incriminated as the major cause of death in the highly susceptible mice [13][14][15][16][17]. On the other hand, IL-10 plays a regulatory role in dampening the excessive proinflammatory cytokines produced during infection [13]. ...
... Enhanced production of disease exacerbating proinflammatory cytokines in infected Bam32 -/mice Previous studies have shown that susceptibility to T. congolense infection in mice is associated with the production of high levels of proinflammatory cytokines (including TNF-α, IL-6, IL-12, and IFN-γ) by spleen cells from infected mice leading to increased serum levels of the cytokines, systemic inflammatory response syndrome (SIRS) and death [14][15][16][17]. Because infected Bam32 -/mice had higher and uncontrolled late phase parasitemia and succumbed to the infection significantly earlier than their infected WT counterpart mice (Fig 1A), we hypothesized that the levels of these cytokines would be significantly higher than those of WT mice. ...
... The susceptibility to T. congolense infection in mice has been associated with several factors, including immunosuppression [48][49][50][51], systemic inflammatory response syndrome resulting from cytokine storm [14][15][16][17], impaired antibody response [28,52,53], induction of regulatory T cells [41,43], and hepatotoxicity particularly during the chronic phase of the disease [36]. We found that infected Bam32 -/mice were unable to control chronic (late stage) parasitemia and show shorter survival time than their wild type counterpart mice and this was associated with significantly increased splenomegaly and hepatomegaly. ...
Bam32, a 32 kDa adaptor molecule, plays important role in B cell receptor signalling, T cell receptor signalling and antibody affinity maturation in germinal centres. Since antibodies against trypanosome variant surface glycoproteins (VSG) are critically important for control of parasitemia, we hypothesized that Bam32 deficient (Bam32-/-) mice would be susceptible to T. congolense infection.
We found that T. congolense-infected Bam32-/- mice successfully control the first wave of parasitemia but then fail to control subsequent waves and ultimately succumb to their infection unlike wild type (WT) C57BL6 mice which are relatively resistant. Although infected Bam32-/- mice had significantly higher hepatomegaly and splenomegaly, their serum AST and ALT levels were not different, suggesting that increased liver pathology may not be responsible for the increased susceptibility of Bam32-/- mice to T. congolense. Using direct ex vivo flow cytometry and ELISA, we show that CD4+ T cells from infected Bam32-/- mice produced significantly increased amounts of disease-exacerbating proinflammatory cytokines (including IFN-γ, TNF-α and IL-6). However, the percentages of regulatory T cells and IL-10-producing CD4+ cells were similar in infected WT and Bam32-/- mice. While serum levels of parasite-specific IgM antibodies were normal, the levels of parasite-specific IgG, (particularly IgG1 and IgG2a) were significantly lower in Bam32-/- mice throughout infection. This was associated with impaired germinal centre response in Bam32-/- mice despite increased numbers of T follicular helper (Tfh) cells. Adoptive transfer studies indicate that intrinsic B cell defect was responsible for the enhanced susceptibility of Bam32-/- mice to T. congolense infection.
Collectively, our data show that Bam32 is important for optimal anti-trypanosome IgG antibody response and suppression of disease-promoting proinflammatory cytokines and its deficiency leads to inability to control T. congolense infection in mice.
... In terms of innate cell-mediated immune response against trypanosomes, macrophages play a central role as antigen presenting cells (APCs) and effector microbicidal cells. Trypanosomes modulate macrophages through parasite factors and host cytokines to control cell polarization into distinct activation states (M1, M2), which may further contribute to susceptibility or resistance to infection [176,177]. Trypanosome killing is assumed to occur via the induction of classically activated macrophages (M1-type macrophages) that produce high levels of inflammatory compounds such as tumour necrosis factor (TNF-), reactive oxygen intermediates, nitric oxide synthase 2-dependent reactive nitrogen intermediates, such as NO and associated molecules [177]. Interestingly, in murine models of T. evansi trypanosomosis, whereas infection causes the induction of interferon (IFN-), TNF-, and NO, none of these molecules was found to be crucial for parasitaemia control and survival of the infected animals [178]. ...
... Trypanosomes modulate macrophages through parasite factors and host cytokines to control cell polarization into distinct activation states (M1, M2), which may further contribute to susceptibility or resistance to infection [176,177]. Trypanosome killing is assumed to occur via the induction of classically activated macrophages (M1-type macrophages) that produce high levels of inflammatory compounds such as tumour necrosis factor (TNF-), reactive oxygen intermediates, nitric oxide synthase 2-dependent reactive nitrogen intermediates, such as NO and associated molecules [177]. Interestingly, in murine models of T. evansi trypanosomosis, whereas infection causes the induction of interferon (IFN-), TNF-, and NO, none of these molecules was found to be crucial for parasitaemia control and survival of the infected animals [178]. ...
... However, while existing in the case of T. evansi, it is not clear why IgM-mediated phagocytosis would be more efficient and protective than IgG-mediated phagocytosis of opsonised trypanosomes, which is classically reported [191]. Contrary to T. brucei and T. congolense, T. evansi exhibits distinct molecular and cellular dialogues and conflicts when interacting with a mammalian host, since despite an infection-associated induction of trypanocidal inflammatory molecules, only IgM antibodies were proved to significantly contribute to trypanosome control [177]. Moreover, to achieve immunosuppression of the host, even if demonstrated only with T. brucei, it has been proven that a nonrelated vaccine-induced protection was completely abolished during an ongoing trypanosome infection. ...
Trypanosoma evansi, the agent of "surra," is a salivarian trypanosome, originating from Africa. It is thought to derive from Trypanosoma brucei by deletion of the maxicircle kinetoplastic DNA (genetic material required for cyclical development in tsetse flies). It is mostly mechanically transmitted by tabanids and stomoxes, initially to camels, in sub-Saharan area. The disease spread from North Africa towards the Middle East, Turkey, India, up to 53° North in Russia, across all South-East Asia, down to Indonesia and the Philippines, and it was also introduced by the conquistadores into Latin America. It can affect a very large range of domestic and wild hosts including camelids, equines, cattle, buffaloes, sheep, goats, pigs, dogs and other carnivores, deer, gazelles, and elephants. It found a new large range of wild and domestic hosts in Latin America, including reservoirs (capybaras) and biological vectors (vampire bats). Surra is a major disease in camels, equines, and dogs, in which it can often be fatal in the absence of treatment, and exhibits nonspecific clinical signs (anaemia, loss of weight, abortion, and death), which are variable from one host and one place to another; however, its immunosuppressive effects interfering with intercurrent diseases or vaccination campaigns might be its most significant and questionable aspect.
... Resistance to trypanosomiasis was previously thought to be largely conferred by the adaptive immunity that include VSG-specific B-and T lymphocyte responses and has been the subject of several recent reviews [5][6][7]. However, evidence that the innate immunity acts as a key component of the host resistance against parasitic infections, including trypanosomiasis, by controlling their growth during acute infection, is now emerging [8][9][10][11][12][13][14][15][16] and is the subject of this review. ...
... During trypanosomiasis, a membrane-associated phospholipase C enzyme cleaves the membrane form VSG (mfVSG), releasing a soluble form VSG (sVSG) that retains only the glycosylinositolphosphate (GIP) while the dimyristoylglycerol compound (DMG) remains embedded in the parasite membrane [64]. Although the identity of the putative innate receptors on M cells that engage with mfVSG, sVSG and DMG is currently elusive, it seems that the induction and maintenance of an innate immune response following T. brucei infection are mediated by the interaction with the members of the TLR family that signal through myeloid differentiation primary response gene 88 (MyD88) ............................................................................................................................................................... [5,11]. Indeed, M cells from MyD88-deficient mice were shown to be non-responsive to sVSG and mfVSG, resulting in elevated levels of the first wave of parasitemia [11]. ...
... Development of M1 is a prerequisite to controlling trypanosome growth and therefore contributes to resistance to African trypanosome infections, mainly during the early stage of the disease [5,9,15,58,76]. Although the exact phenotype of the M1 population responsible for trypanotolerance is yet to be identified, engagement of the trypanosome galactose-residues of sVSG-GIP with MyD88 or trypanosome DNA with TLR9 could initiate signalling cascades that result in M1 activation and the production of inflammatory factors that amplify the innate response to infection and further stimulate adaptive immunity [11,73]. ...
During the course of African trypanosomiasis, an intact monocytic cell system appears to be crucial for the initiation and maintenance of antitrypanosome responses and could be critical for the survival of trypanosome-infected host. Monocytic cells in turn require support from other components of the innate immunity as well as adaptive immunity for effective and sustained control of trypanosome infections. In this review, the contribution of specific components of the innate immune system towards resistance to African trypanosomes is discussed in the context of host survival and the ideas presented are expected to stimulate more debate and research on host innate mechanisms of defence against African trypanosomiasis.
... Given the similarities between the anemic responses of cattle and C57Bl/6 mice upon trypanosome infection [8], the underlying mechanisms were mainly scrutinized in murine models [10,11]. The data collectively suggest that the pro-inflammatory type I immune response, involving TNF, IFN-γ and M1-type (classically activated) myeloid cells, contributes to pathogenicity in general and anemia in particular, and in combination with impaired B-cell functionality, results in reduced survival of the mice [10,12,13]. Thus, identification of gene-products regulating pro-inflammatory signals during the course of the disease might pave the way to develop novel intervention strategies. ...
... Alternatively, the virulence of T. b. brucei due to its tissue-invading capacity could be higher than that of T. congolense, which remains strictly in the blood vessels [51]. It is postulated that T. b. brucei-and T. congolenseinfected mice die from inflammation-mediated multiple organ failure [12], but the cause of death remains unclear and may differ between the two parasite species. In this respect, a refined mechanism for the death of T. congolense-infected WT mice could be envisaged based on the data reported herein. ...
Animal African trypanosomosis is a major threat to the economic development and human health in sub-Saharan Africa. Trypanosoma congolense infections represent the major constraint in livestock production, with anemia as the major pathogenic lethal feature. The mechanisms underlying anemia development are ill defined, which hampers the development of an effective therapy. Here, the contribution of the erythropoietic and erythrophagocytic potential as well as of hemodilution to the development of T. congolense-induced anemia were addressed in a mouse model of low virulence relevant for bovine trypanosomosis. We show that in infected mice, splenic extramedullary erythropoiesis could compensate for the chronic low-grade type I inflammation-induced phagocytosis of senescent red blood cells (RBCs) in spleen and liver myeloid cells, as well as for the impaired maturation of RBCs occurring in the bone marrow and spleen. Rather, anemia resulted from hemodilution. Our data also suggest that the heme catabolism subsequent to sustained erythrophagocytosis resulted in iron accumulation in tissue and hyperbilirubinemia. Moreover, hypoalbuminemia, potentially resulting from hemodilution and liver injury in infected mice, impaired the elimination of toxic circulating molecules like bilirubin. Hemodilutional thrombocytopenia also coincided with impaired coagulation. Combined, these effects could elicit multiple organ failure and uncontrolled bleeding thus reduce the survival of infected mice. MIF (macrophage migrating inhibitory factor), a potential pathogenic molecule in African trypanosomosis, was found herein to promote erythrophagocytosis, to block extramedullary erythropoiesis and RBC maturation, and to trigger hemodilution. Hence, these data prompt considering MIF as a potential target for treatment of natural bovine trypanosomosis.
... Overall, studies into the role of cells of the mononuclear phagocyte system (MPS) which include myeloid cells (i.e., macrophages, monocytes and granulocytes) during experimental murine trypanosome infections (focussing on T. brucei and T. congolense) have revealed that these cells make an important contribution to African trypanosomiasis development both during the early and later/chronic stages of infection, whereby they can play a protective or pathogenic role depending on their activation state (Noel et al. 2004) (see Fig. 16.1). In particular, trypanotolerance is associated with induction of Type-1 cytokine responses [interferon-γ (IFNγ)] during the early phase of infection, giving rise to classically activated macrophages (M1) producing tumor necrosis factor (TNF) and/or nitric oxide (NO) and hereby ensuring parasite control, followed by switching to Type-2 cytokine production, including IL-4, IL-13, IL-10, during later phases of infection, giving rise to alternatively activated macrophages (M2) (Kaushik et al. 1999a;Stijlemans et al. 2007a). On the other hand, a maintained Type-1 cytokine response or an early mixed Type-1/2 cytokine production confers susceptibility to trypanosome infections (Stijlemans et al. 2007a;Baetselier et al. 2001;Namangala et al. 2001a). ...
... In particular, trypanotolerance is associated with induction of Type-1 cytokine responses [interferon-γ (IFNγ)] during the early phase of infection, giving rise to classically activated macrophages (M1) producing tumor necrosis factor (TNF) and/or nitric oxide (NO) and hereby ensuring parasite control, followed by switching to Type-2 cytokine production, including IL-4, IL-13, IL-10, during later phases of infection, giving rise to alternatively activated macrophages (M2) (Kaushik et al. 1999a;Stijlemans et al. 2007a). On the other hand, a maintained Type-1 cytokine response or an early mixed Type-1/2 cytokine production confers susceptibility to trypanosome infections (Stijlemans et al. 2007a;Baetselier et al. 2001;Namangala et al. 2001a). The ability of more resistant mouse strains to alleviate excessive infection-elicited pathogenicity thereby relies on a control of the Type-1-mediated immune responses, allowing a transition from parasite control towards pathogenicity control. ...
African trypanosomiasis is a parasitic disease of medical and veterinary importance that has adversely influenced the economic development of sub-Saharan Africa. The causative agents, the trypanosomes, are hemoflagellated blood-borne unicellular protozoan parasites that are transmitted through the bite of their vector (i.e., the tsetse fly, glossina spp.) and cause fatal diseases in mammals, commonly called sleeping sickness in humans (HAT, Human African Trypanosomiasis) or Nagana in domestic livestock. Studies into the role of cells of the mononuclear phagocyte system (MPS), which include myeloid cells (i.e., macrophages, monocytes and granulocytes), during experimental murine trypanosome infections, have revealed that these cells make an important contribution to African trypanosomiasis development both during the early and later/chronic stages of infection, whereby they can play a protective or pathogenic role depending on their activation state. In this chapter, we will discuss (1) the parasite-host interactions with a focus on the role played by cells of the MPS and parasite-derived components triggering immune responses during the different stages/phases of experimental trypanosome infections and (2) the contribution of cells of the MPS to immunopathogenicity development with focus on liver injury and anemia. Finally, we will give (3) an overview of different strategies that can be employed to alleviate immunopathogenicity which might pave the way to develop new intervention strategies, as well as (4) discuss the potential link between murine models and HAT. © 2014 Springer Science+Business Media New York. All rights reserved.
... In particular, they manipulate cells of the myeloid phagocytes system (MYPS) which includes myeloid cells of the mononuclear phagocytes system (MPS, i.e., macrophages, monocytes, and immature DCs) as well as granulocytes (neutrophils) [14] and thereby affect the capacity of the host to (i) control parasite growth (referred to as resistance to infection) and (ii) to limit tissue pathogenicity caused by the immune response mounted for resistance to infection (referred to as (trypano)tolerance to infection). Trypanotolerance is associated with the sequential induction of IFN-and MyD88-dependent M1-type myeloid cells (i.e., classically activated myeloid cells) producing TNF and/or NO which reduce the fitness of the parasite and ensure parasite control, followed by a switch to IL-10 dependent M2-type myeloid cells (i.e., alternatively activated myeloid cells) ensuring pathogenicity control [15]. In contrast, trypanosusceptibility is associated with a persistence of M1-type myeloid cells and an inability to switch to M2-type myeloid cells, which culminates in pathogenicity. ...
... As far as parasites like African trypanosomes are concerned, their complex lifecycle alternating between the tsetse fly vector and the mammalian host adds an additional problem in the struggle for the supply of this metal. At the level of the mammalian host, the concept that polarization of cells of the MYPS into distinct M1-type or M2-type activation states contributes to trypanosusceptibility or tolerance, respectively, suggests that reprogramming of MYPS cells may provide new therapeutical modalities in the treatment of infection-associated pathogenicity development [15]. However, additional research is required to dissect the exact contribution of the different liver and spleen associated MYPS cell subsets (Ly6c+ and Ly6c− monocytes, resident and Ly6c+ monocyte-derived macrophages, granulocytes, and dendritic cells) in erythrophagocytosis or modulation of iron homeostasis and to Gal-3 or MIF production. ...
African trypanosomosis is a chronic debilitating disease affecting the health and economic well-being of developing countries. The immune response during African trypanosome infection consisting of a strong proinflammatory M1-type activation of the myeloid phagocyte system (MYPS) results in iron deprivation for these extracellular parasites. Yet, the persistence of M1-type MYPS activation causes the development of anemia (anemia of chronic disease, ACD) as a most prominent pathological parameter in the mammalian host, due to enhanced erythrophagocytosis and retention of iron within the MYPS thereby depriving iron for erythropoiesis. In this review we give an overview of how parasites acquire iron from the host and how iron modulation of the host MYPS affects trypanosomosis-associated anemia development. Finally, we also discuss different strategies at the level of both the host and the parasite that can/might be used to modulate iron availability during African trypanosome infections.
... This reaction controls parasite invasion and proliferation, but the exacerbated immune response can induce collateral tissue damage. 53 To alleviate parasiteelicited pathological changes the host can mount type 2 immune responses consisting of sequential production of interleukin 10 and interleukin 4 or interleukin 1 that can induce macrophage-2 cells with anti-infl ammatory 1 9 4 4 1 9 4 9 1 9 5 4 1 9 5 9 1 9 6 4 1 9 6 9 1 9 7 4 1 9 7 9 1 9 8 4 1 9 8 9 1 9 9 4 1 9 9 9 2 0 0 4 Year properties. 53 The mechanism of antigenic variation on the surface of the parasite allows it to persist and to elicit new parasitic waves. ...
... 53 To alleviate parasiteelicited pathological changes the host can mount type 2 immune responses consisting of sequential production of interleukin 10 and interleukin 4 or interleukin 1 that can induce macrophage-2 cells with anti-infl ammatory 1 9 4 4 1 9 4 9 1 9 5 4 1 9 5 9 1 9 6 4 1 9 6 9 1 9 7 4 1 9 7 9 1 9 8 4 1 9 8 9 1 9 9 4 1 9 9 9 2 0 0 4 Year properties. 53 The mechanism of antigenic variation on the surface of the parasite allows it to persist and to elicit new parasitic waves. ...
Human African trypanosomiasis (sleeping sickness) is caused by the unicellular parasite Trypanosoma brucei and transmitted by tsetse flies. It occurs exclusively in sub-Saharan Africa, usually in rural areas affected by civil conflicts and neglected health systems. Reported cases are fewer than 10,000/year, which classifies it as one of the most neglected tropical diseases. Because sleeping sickness is fatal if not treated, it has to be included in the differential diagnosis of every febrile traveler returning from a game park in East Africa. Elimination of the disease is considered feasible provided better tools for diagnosis and treatment can be made available.
... As such, natural selection has enabled African trypanosomes to develop very sophisticated mechanisms to evade immune killing to survive in the chronically infected host and hence allow transmission to the next host, via the tsetse vector. Well-documented evasion mechanisms include antigenic variation of the VSG (13)(14)(15) and the induction of alterations in the host's defence system (16)(17)(18)(19)(20)(21)(22). Antigenic variation remains one of the most spectacular adaptive mechanisms exhibited by African trypanosomes and is the central most important immune escape mechanism by these parasites (4,13,23). ...
... Instead, these parasites swim freely in the immunologically hostile host tissue fluids including blood, lymph and cerebro-spinal fluids. Although the alterations occurring during trypanosomosis could possibly be coincidental, there is cumulative evidence suggesting that trypanosomes target and modulate immune responses that are potentially detrimental to their survival (18)(19)(20)22,24,30,32,34,66,67,74). Continued contacts with the host's immune system could provide strong selective pressure for these parasites to evolve very sophisticated mechanisms to evade immune killing to survive in the chronically infected host. ...
Unlike other protozoan parasites, African trypanosomes never enter the host cell at any stage of their development. Instead, these parasites swim freely in the immunologically hostile host tissue fluids. During the course of infection, a complex interaction between the host immune responses and trypanosome survival strategies occurs. Continued contacts with the host's immune system occurring during the course of infection could have provided strong selection pressure for African trypanosomes to evolve very sophisticated mechanisms to evade immune killing to survive the hostile immunological environment in the infected host. This review discusses some of the documented immunological evasion mechanisms African trypanosomes employ for their survival and perpetuity.
... Compounds 16, 18, 21, 28, 31 and 33, have sub-μM potencies (773-350 nM) whereas 8, 12, 14, 26 and 30 display one digit μM EC 50 (1.2-7.7 μM). African trypanosomes are extracellular pathogens and macrophages play an important role as the host front line defense that contribute to control parasite infection (Stijlemans et al., 2007). Therefore, the cytotoxicity of the hit compounds was tested against murine (cell line J774; Table 1) and human macrophages (cell line THP-1; Table S1). ...
Earlier evidences showed that diglycosyl diselenides are active against the infective stage of African trypano-somes (top hits IC 50 0.5 and 1.5 μM) but poorly selective (selectivity index <10). Here we extended the study to 33 new seleno-glycoconjugates with the aim to improve potency and selectivity. Three selenoglycosides and three glycosyl selenenylsulfides displayed IC 50 against bloodstream Trypanosoma brucei in the sub-μM range (IC 50 0.35-0.77 μM) and four of them showed an improved selectivity (selectivity index >38-folds vs. murine and human macrohages). For the glycosyl selenylsulfides, the anti-trypanosomal activity was not significantly influenced by the nature of the moiety attached to the sulfur atom. Except for a quinoline-, and to a minor extent a nitro-derivative, the most selective hits induced a rapid (within 60 min) and marked perturbation of the LMWT-redox homeostasis. The formation of selenenylsulfide glycoconjugates with free thiols has been identified as a potential mechanism involved in this process.
... As such, natural selection has enabled African trypanosomes to develop very sophisticated mechanisms to evade immune killing to survive in the chronically infected host and hence allow transmission to the next host, via the tsetse vector. Well-documented evasion mechanisms include antigenic variation of the VSG [75] and the induction of alterations in the host's defense system [82,83,84,85,86,87] . Antigenic variation remains one of the most spectacular adaptive mechanisms exhibited by African trypanosomes and is the central most important immune escape mechanism by these parasites [88,78] . ...
The aim of this document is to review the current knowledge on Trypanosome parasites of animals with emphasis on the immunological response and points to the wealth of information available for trypanosomiasis, in contrast to the numerous gaps in our understanding of immune responses to trypanosomal infections. African trypanosomes are pathogens for humans and livestock. They are single-cell, extra-cellular parasites that cause persistent infections of the blood and induce profound immune-suppression. The specific immune response to the infecting parasites is complex and involves both the humoral and cellular branches of immune systems. In trypanosomosis, parasite growth is primarily controlled through T-cell dependent antibody responses to the variable surface glycoproteins and possibly to other molecules embedded on the surface of the parasites. Cellular immune responses also occur but at the level of immune-suppression directed against B cells. Additionally, a variety of immune-modulatory cytokines like TNF-α, IFN-γ, IL-10, IL-4, IL-6, IL-12 and etc are produced during the course of infection. The center of the immune-pathology is the T-cell-independent production of antibodies to the variant surface glycoprotein of trypanosomes, the anti-VSG antibody-mediated phagocytosis of trypanosomes by macrophages, and the subsequent profound dysregulation of the macrophage system. It has, however, been demonstrated that a T-cell subset is critical in protective immunity.
... However, high NO levels can be a disadvantage to the host, given the large spectra of cell disorders that are associated with NO activity. In sleeping sickness, the persistence of M1-MΦ is related to anaemia and systemic immune response [16][17][18][19]. ...
African trypanosomiasis or sleeping sickness is a zoonotic disease caused by Trypanosoma brucei, a protozoan parasite transmitted by Glossina spp. (tsetse fly). Parasite introduction into mammal hosts triggers a succession of events, involving both innate and adaptive immunity. Macrophages (MΦ) have a key role in innate defence since they are antigen-presenting cells and have a microbicidal function essential for trypanosome clearance. Adaptive immune defence is carried out by lymphocytes, especially by T cells that promote an integrated immune response. Like mammal cells, T. b. brucei parasites release extracellular vesicles (TbEVs), which carry macromolecules that can be transferred to host cells, transmitting biological information able to manipulate cell immune response. However, the exact role of TbEVs in host immune response remains poorly understood. Thus, the current study examined the effect elicited by TbEVs on MΦ and T lymphocytes. A combined approach of microscopy, nanoparticle tracking analysis, multiparametric flow cytometry, colourimetric assays and detailed statistical analyses were used to evaluate the influence of TbEVs in mouse mononuclear cells. It was shown that TbEVs can establish direct communication with cells of innate and adaptative immunity. TbEVs induce the differentiation of both M1- and M2-MΦ and elicit the expansion of MHCI+, MHCII+ and MHCI+MHCII+ MΦ subpopulations. In T lymphocytes, TbEVs drive the overexpression of cell-surface CD3 and the nuclear factor FoxP3, which lead to the differentiation of regulatory CD4+ and CD8+ T cells. Moreover, this study indicates that T. b. brucei and TbEVs seem to display opposite but complementary effects in the host, establishing a balance between parasite growth and controlled immune response, at least during the early phase of infection.
... Trypanosomosis prevalence was higher in poor body conditioned animal than medium and good. This might be due to high immunesuppression nature of trypanosomosis (Stijlemans et al., 2007;Namangala, 2011) which predisposed the animal to concurrent infections with resulting reluctance to feed and water intake which subsequently cause emaciation. It was also clinical manifestations of trypanosome infected animal (Ohaeri and Eluwa, 2011). ...
A cross-sectional study aimed to elucidate the prevalence of bovine trypanosomosis and its potential risk factors was conducted in tsetse suppression and non-suppression areas of South Omo Zone, Southern Ethiopia from November 2018- May 2019. A total of 1284 blood samples from local zebu cattle (642 each in dry and wet season) were examined by using buffy coat technique and thin blood smear method. The overall prevalence was 11.05% with 14.33% in dry and 7.78% in wet season. According to multiple logistic regression analysis of tsetse suppression areas, higher prevalence in female than male (OR = 0.48, 95% CI: 0.27, 0.83), in poor (OR = 3.25, 95% CI: 1.26, 11.09) and medium (OR = 2.07, 95% CI: 0.74, 7.37) than good body conditioned animals was recorded. Moreover, tethered animals (OR = 2.07, 95% CI: 1.06, 3.92) were more likely to be infected than communal grazers and also higher prevalence in dry season than wet season (OR = 0.52, 95% CI: 0.30, 0.87). Similarly, in tsetse non-suppression areas, higher prevalence in female than male (OR = 0.48, 95% CI: 0.27, 0.85) and in wet season (OR = 0.41, 95% CI: 0.23, 0.7) than dry season was recorded. Trypanosoma congolense and Trypanosoma vivax were found in cattle with the former more prevalent in both areas. Overall pooled mean packed cell volume (PCV) of parasitaemic animals (23.57 ± 3.13) was significantly lower than aparasitaemic animals (27.80 ± 4.95). Similarly, parasitaemic animals from tsetse suppression areas and tsetse non-suppression areas had significantly lower mean PCV than their aparasitaemic counterparts. Mean PCV of T. congolense (23.59 ± 3.22) infected animals was not different (P > 0.05) from T. vivax infected animals (23.26 ± 3.31). It was also indicated that the probability of anaemic animals to be parasitaemic was significantly higher (P < 0.05) than non-anaemic animals in both areas. In conclusion, the prevalence of trypanosomosis revealed its endemicity which bottlenecked the livestock production and productivity in the study area despite of tsetse suppression activities. Therefore, integrated parasite and vector control approach should be undertaken to curve the disease.
... As such, natural selection has enabled African trypanosomes to develop highly advanced mechanisms to evade immune killing to survive in the chronically infected host and thereby allow transmission to the next host [18]. Antigenic variation remains one of the most spectacular adaptive mechanisms exhibited by African trypanosomes and is the central most important immune escape mechanism of these parasites [20,21]. These conditions have all the attributes of concomitant or tolerance immunity, better known as premonition. ...
This study investigated the susceptibility of eight different animal species (rat, mice, guinea pigs, hamsters, rabbits, sheep, goats, and chicken) experimentally infected with Trypanosoma evansi isolated from camels. In all laboratory animals, the number of trypanosomes was standardized according to the weight of the animal, and daily examination of the blood was conducted to assess the presence of trypanosomes to determine the prepatent period and the peak of parasitemia. Results suggested that mice and rats were the most susceptible laboratory animals to infection, whereas hamsters and guinea pigs displayed a certain degree of tolerance to infection. Rabbits exhibited a chronic course of infection, but the level of parasitemia was very low. In sheep and goats, trypanosomes were detectable only by subpassage to rats and mice, whereas all the chickens remained uninfected even with increasing doses of infection.
... Although the role of adaptive immunity in protection against extracellular pathogens such as trypanosomes are well recognized [32,33], the independent contributions of the cell-mediated or humoral immune mechanisms are less well understood. Profiles of induced [34][35][36] have shown repeatedly that attempting to predict the outcome of an infection based on a single cytokine response is unlikely to be successful. For example, IFN-γ, an important cytokine of the Th1 subset, activates and stimulates the macrophages to produce IL-12 that not only helps in differentiation of Th1 cells, but also inhibits the expansion of Th2 type T-cell population. ...
... As such, natural selection has enabled African trypanosomes to develop very sophisticated mechanisms to evade immune killing to survive in the chronically infected host and hence allow transmission to the next host, via the tsetse vector. Well-documented evasion mechanisms include antigenic variation of the VSG [75] and the induction of alterations in the host's defense system [82,83,84,85,86,87] . Antigenic variation remains one of the most spectacular adaptive mechanisms exhibited by African trypanosomes and is the central most important immune escape mechanism by these parasites [88,78] . ...
The aim of this document is to review the current knowledge on Trypanosome parasites of animals with emphasis on the immunological response and points to the wealth of information available for trypanosomiasis, in contrast to the numerous gaps in our understanding of immune responses to trypanosomal infections. African trypanosomes are pathogens for humans and livestock. They are single-cell, extra-cellular parasites that cause persistent infections of the blood and induce profound immune-suppression. The specific immune response to the infecting parasites is complex and involves both the humoral and cellular branches of immune systems. In trypanosomosis, parasite growth is primarily controlled through T-cell dependent antibody responses to the variable surface glycoproteins and possibly to other molecules embedded on the surface of the parasites. Cellular immune responses also occur but at the level of immune-suppression directed against B cells. Additionally, a variety of immune-modulatory cytokines like TNF-α, IFN-γ, IL-10, IL-4, IL-6, IL-12 and etc are produced during the course of infection. The center of the immune-pathology is the T-cell-independent production of antibodies to the variant surface glycoprotein of trypanosomes, the anti-VSG antibody-mediated phagocytosis of trypanosomes by macrophages, and the subsequent profound dysregulation of the macrophage system. It has, however, been demonstrated that a T-cell subset is critical in protective immunity.
... The inflammatory changes observed during trypanosomosis are caused by tissue damage stimulated by the presence of the parasites in the body, leading to infiltration of the tissues by inflammatory cells and production of pro-inflammatory cytokines Maina et al., 2004). The host inflammatory responses produce free radicals and reactive oxygen species (ROS) to control parasite invasion and proliferation but the enhanced immune responses induce collateral tissue damage (Stijlemans et al., 2007). Free radicals destroy tissues by lipid peroxidation and are common in many pathological conditions. ...
Tissue inflammatory damage during trypanosomosis significantly affects the treatment and prognosis. The current study investigated the effects of water extracts of Solanum nigrum (SNE) on the liver pathology and survival of Swiss white mice infected with Trypanosoma brucei rhodesiense. Trypanosome infected mice treated with SNE had significantly (P<0.05) increased and dose dependent survival time and liver pathology. Mice treated with higher concentrations of SNE had minimal liver pathology with minimal infiltration by inflammatory cells compared with the dexamethasone treated and untreated mice which had massive infiltration suggesting that SNE could be superior to dexamethasone in reducing trypanosome mediated liver pathology. Therefore, SNE could be a better anti-inflammatory adjunct in the treatment of Human African trypanosomosis (HAT) and other inflammatory conditions such as hepatitis.
... Trypanosomatids are very proficient in evading the host's immune response, yet, these escape mechanisms are associated with a chronic inflammatory state that culminates into increased tissue damage and finally into host death [41]. IL-10 was shown to be essential to attenuate the inflammatory response and prevent early death of the host due to a hyper-inflammation syndrome [19]. ...
Bovine African Trypanosomosis is an infectious parasitic disease affecting livestock productivity and thereby impairing the economic development of Sub-Saharan Africa. The most important trypanosome species implicated is T. congolense, causing anemia as most important pathological feature. Using murine models, it was shown that due to the parasite’s efficient immune evasion mechanisms, including (i) antigenic variation of the variable surface glycoprotein (VSG) coat, (ii) induction of polyclonal B cell activation, (iii) loss of B cell memory and (iv) T cell mediated immunosuppression, disease prevention through vaccination has so far been impossible. In trypanotolerant models a strong, early pro-inflammatory immune response involving IFN-γ, TNF and NO, combined with a strong humoral anti-VSG response, ensures early parasitemia control. This potent protective inflammatory response is counterbalanced by the production of the anti-inflammatory cytokine IL-10, which in turn prevents early death of the host from uncontrolled hyper-inflammation-mediated immunopathologies. Though at this stage different hematopoietic cells, such as NK cells, T cells and B cells as well as myeloid cells (i.e. alternatively activated myeloid cells (M2) or Ly6c⁻ monocytes), were found to produce IL-10, the contribution of non-hematopoietic cells as potential IL-10 source during experimental T. congolense infection has not been addressed. Here, we report for the first time that during the chronic stage of T. congolense infection non-hematopoietic cells constitute an important source of IL-10. Our data shows that hepatocyte-derived IL-10 is mandatory for host survival and is crucial for the control of trypanosomosis-induced inflammation and associated immunopathologies such as anemia, hepatosplenomegaly and excessive tissue injury.
... The best studied immune evasion strategy employed by T. brucei is antigenic variation of the single variable surface glycoprotein (VSG) that covers the surface of the parasite (4). Macrophages act as one of the first lines of defense against T. brucei infection, with M1-type immune responses such as the production of pro-inflammatory mediators TNF-α and nitric oxide (NO) recognized as particularly important in parasitemia control [reviewed in (5)]. However, as strong immune responses pose a threat to the survival of trypanosomes and are potentially deleterious to the host, T. brucei acts to dampen the immune response in order to evade clearance by the immune system and promote host survival (3,6). ...
African trypanosomes, such as Trypanosoma brucei (T. brucei), are protozoan parasites of the mammalian vasculature and central nervous system that are best known for causing fatal human sleeping sickness. As exclusively extracellular parasites, trypanosomes are subject to constant challenge from host immune defenses but they have developed very effective strategies to evade and modulate these responses to maintain an infection while simultaneously prolonging host survival. Here we investigate host parasite interactions, especially within the CNS context, which are not well-understood. We demonstrate that T. brucei strongly upregulates the stress response protein, Heme Oxygenase 1 (HO-1), in primary murine glia and macrophages in vitro. Furthermore, using a novel AHADHin T. brucei cell line, we demonstrate that specific aromatic ketoacids secreted by bloodstream forms of T. brucei are potent drivers of HO-1 expression and are capable of inhibiting pro-IL1β induction in both glia and macrophages. Additionally, we found that these ketoacids significantly reduced IL-6 and TNFα production by glia, but not macrophages. Finally, we present data to support Nrf2 activation as the mechanism of action by which these ketoacids upregulate HO-1 expression and mediate their anti-inflammatory activity. This study therefore reports a novel immune evasion mechanism, whereby T. brucei secretes amino-acid derived metabolites for the purpose of suppressing both the host CNS and peripheral immune response, potentially via induction of the Nrf2/HO-1 pathway.
... One of the main strategies used by microorganisms to escape this macrophage activity involves altering the response profile of macrophages. 185,[235][236][237][238][239][240][241][242][243][244][245][246][247][248][249][250][251][252] The activation of a response profile mediated by M1 macrophages is commonly associated with a protective tissue environment and has been described for infections by pathogens such as Helicobacter pylori, M. tuberculosis, Mycobacterium leprae, Salmonella typhi, and Chlamydia. 253-258 M1 macrophages elicit an effective immune response against S. typhi and H. pylori, and in the response against H. pylori, the induction of iNOS associated with the M1 profile is closely related to the occurrence of gastric cancer. ...
Macrophages are a functionally heterogeneous group of cells with specialized functions depending not only on their subgroup but also on the function of the organ or tissue in which the cells are located. The concept of macrophage phenotypic heterogeneity has been investigated since the 1980s, and more recent studies have identified a diverse spectrum of phenotypic subpopulations. Several types of macrophages play a central role in the response to infectious agents and, along with other components of the immune system, determine the clinical outcome of major infectious diseases. Here, we review the functions of various macrophage phenotypic subpopulations, the concept of macrophage polarization, and the influence of these cells on the evolution of infections. In addition, we emphasize their role in the immune response in vivo and in situ, as well as the molecular effectors and signaling mechanisms used by these cells. Furthermore, we highlight the mechanisms of immune evasion triggered by infectious agents to counter the actions of macrophages and their consequences. Our aim here is to provide an overview of the role of macrophages in the pathogenesis of critical transmissible diseases and discuss how elucidation of this relationship could enhance our understanding of the host-pathogen association in organ-specific immune responses.
... For example, activities of immune regulation mediated by innate cells, in particular macrophage-like M cells have been reported. 106 The role of interleukin (IL)-10 in reducing IFN-γ-mediated pathology in trypanosome infections has also been elucidated in T. brucei, 107 suggesting a major role of this anti-inflammatory cytokine in the transition from parasite density control to infection pathology control. ...
... Other parasite peptidases that have also been shown to inhibit Th1 responses during infection include a cysteine protease B from Leishmania mexicana and cathepsin B from Leishmania chagasi, that cleave human TGF-β, a cytokine that regulates IL-10 and thereby affecting immunosuppression (Somanna et al. 2002;Buxbaum et al. 2003;Buxbaum, 2015). In classical trypanosome infections, initial pro-inflammatory type I immune responses mediated by IFN-γ, nitric oxide (NO) and TNF are necessary for controlling the first wave of parasitaemia (Namangala et al. 2001b;Stijlemans et al. 2007;Baral, 2010). Interleukin 1b is also a strong mediator of inflammatory immune responses during T. brucei infections (Nyakundi et al. 2002). ...
The protozoan parasite Trypanosoma evansi is responsible for causing Surra in a variety of mammalian hosts over a wide geographical area. In the absence of an effective vaccine and increasing resistance to current chemotherapeutic agents, peptidases from the S9 prolyl oligopeptidase family have been identified as potential drug and vaccine targets. In order to understand the function of these peptidases during infection, three null mutant clones for prolyl oligopeptidase ( Δpop ), prolyl oligopeptidase-like ( Δpop-like ) and oligopeptidase B ( Δopb ) were generated in T. evansi RoTat 1.2 parasites and used for infection of mice. Mice inoculated with T. evansi Δpop-like mutants were able to survive longer than other groups of mice inoculated with Δpop , Δopb mutants or wild-type parasites. The regression analysis of plasma from mice-infected over time using Δpop-like mutants showed stable levels of interleukin-10 (IL-10) (non-significant slope, P = 0·171) and declining IL-1b levels (negative slope, P = 0·04) when compared with the wild-type control that demonstrated increasing levels of IL-10 and IL-1b ( P < 0·01 for both). Further analysis using mouse spleen cells in an in vitro 24 h incubation assay revealed that the percentage of IL-10 producing CD3 positive cells display significantly lower values when incubated with Δpop-like parasites than the wild-type clone ( P = 0·002). These results suggest that prolyl oligopeptidase-like peptidase may play a role in immune responses during T. evansi infections by affecting interleukin concentrations in the host.
... In turn, these activated M1 develop upon exposure to parasite-derived molecules such as VSG and CpG a type-1 inflammatory immune response leading to the production of the potential trypanocidal molecules such as TNF and NO that in conjunction with Abs will contribute to parasite control (96,97). Yet, persistence of this type-1 immune response and hyperactivated M1 cells will culminate in trypanosusceptible animals into immunopathological features such as the systemic immune response syndrome and anemia (98). Trypanotolerant animals on the other hand are able to switch to a more type-2 immune response and the induction of alternatively activate macrophages (M2), whereby the antiinflammatory cytokine interleukin (IL)-10 was shown to play a pivotal "dampening" role (99,100). ...
African trypanosomosis is a debilitating disease of great medical and socioeconomical importance. It is caused by strictly extracellular protozoan parasites capable of infecting all vertebrate classes including human, livestock, and game animals. To survive within their mammalian host, trypanosomes have evolved efficient immune escape mechanisms and manipulate the entire host immune response, including the humoral response. This report provides an overview of how trypanosomes initially trigger and subsequently undermine the development of an effective host antibody response. Indeed, results available to date obtained in both natural and experimental infection models show that trypanosomes impair homeostatic B-cell lymphopoiesis, B-cell maturation and survival and B-cell memory development. Data on B-cell dysfunctioning in correlation with parasite virulence and trypanosome-mediated inflammation will be discussed, as well as the impact of trypanosomosis on heterologous vaccine efficacy and diagnosis. Therefore, new strategies aiming at enhancing vaccination efficacy could benefit from a combination of (i) early parasite diagnosis, (ii) anti-trypanosome (drugs) treatment, and (iii) anti-inflammatory treatment that collectively might allow B-cell recovery and improve vaccination.
... For example, activities of immune regulation mediated by innate cells, in particular macrophage-like M cells have been reported. 106 The role of interleukin (IL)-10 in reducing IFN-γ-mediated pathology in trypanosome infections has also been elucidated in T. brucei, 107 suggesting a major role of this anti-inflammatory cytokine in the transition from parasite density control to infection pathology control. ...
Human African trypanosomiasis (HAT) is in decline, thanks to sustained control efforts in recent decades. Yet, its complexity as a disease at the animal–human interface and its potential for resurgence represent a significant concern for the final elimination goal. Understanding the challenges underlying HAT control involves engaging deeply with the epidemiology, ecology, and evolution of trypanosomes and their hosts. Dissecting the importance of parasite-intrinsic biological factors, vector life-history contribution, and host immunological aspects, requires integrated efforts across disciplines. The prospects for control of HAT, reviewed here, comprise a spectrum of developments, from new tools for disease diagnosis and staging, tsetse control, and prevention of transmission, to more effective and non-toxic treatment options, including immune therapies. Although fundamentally pathogenic, trypanosomes can also be carried asymptomatically by their hosts. Trypanotolerance, recently recognized as an important factor in persistence of disease foci, especially for Trypanosoma brucei gambiense, remains an under-studied aspect of HAT epidemiology. With advancing technologies, a better cellular and molecular resolution of trypanosome infection processes is now possible. Integrating these data with quantitative models, and linking mechanistically different aspects of disease across biological scales, could bring key novel insights into HAT control strategies.
... This protein, highly immunogenic, is recognized by the host's immune system, evoking IgM and IgG humoral and type 2 cellular responses. These immune mechanisms, with the production of interleukin 10 and interleukin 4 or interleukin-1, have an anti-inflammatory effect and antibodies neutralize the majority of the parasites, leading to decreased parasitemia [186,187]. For this reason, host's immune system is capable of controlling parasitemia in a high degree mainly by neutralization of VSGs with antibodies [188]. ...
The Leishmania spp, Trypanosoma cruzi and Trypanosoma brucei spp are the causative agents of tropical infections, and over 20 million people worldwide suffer from these neglected diseases. During the last century, vaccine development has had an undeniable impact on public health and may offer some alternatives for the control of parasitic diseases. Immune protection against experimental infection with these parasites has been studied and many types of immunogens have been used. Use of new technologies has allowed the development of recombinant proteins and DNA-based vaccines against those protozoans, aiming to generate both humoral and cellular protective responses. A large amount of data have been obtained from preclinical model systems which gave us promising results. The main challenge at the present is to translate what has been succeeded in these models into efficient human vaccines. The objective of this review is to summarize the efforts of the science community about the development of recombinant vaccines against trypanosomatids.
... In contrast, higher levels of inflammatory interleukin 8 and tumor necrosis factor α were observed in the SERO group than in the SERO/HAT or HAT group [6]. Tumor necrosis factor α was previously shown to be induced ex vivo by T. brucei gambiense in human macrophages [34] and was described as having a role in controlling parasite growth through the induction of cytotoxic molecules [35]. Hence, our results are in line with sHLA-G-mediated inhibition of the host immune response associated with greater susceptibility to T. brucei gambiense infections. ...
Background:
Human African trypanosomiasis (HAT) caused by Trypanosoma brucei gambiense (Tbg) can be diagnosed in the early haemolymphatic stage (S1) or meningoencephalitic stage (S2). Importantly, individuals harbouring high and specific antibody responses to Tbg antigens but negative parasitology are also diagnosed in the field. Whereas some develop the disease in the months following their initial diagnosis (SERO/HAT), others remain parasitologically negative for long periods (SERO) and are apparently able to control infection. Human leukocyte antigen (HLA)-G, an immunosuppressive molecule, could play a critical role in this variability of progression between infection and disease.
Methods:
Soluble HLA-G (sHLA-G) was measured in plasma for SERO (n=65), SERO/HAT (n=14) and HAT patients (n=268) and in cerebrospinal fluid (CSF) for S1 (n=55), early S2 (S2E) (n=93) and late S2 (S2L) (n=110). Associations between these different statuses and the soluble level or genetic polymorphisms of HLA-G were explored.
Results:
Plasma sHLA-G levels were significantly higher in HAT (p=6 10(-7)) and SERO/HAT (p=0.007) than SERO patients. No difference was observed between the SERO/HAT and HAT groups. Within the HAT group, specific haplotypes (HG010102 and HG0103) displayed increased frequencies in S1 (p=0.013) and S2L (p=0.036), respectively.
Conclusions:
These results strongly suggest the involvement of HLA-G in HAT disease progression. Importantly, high plasma sHLA-G levels in SERO patients could be predictive of subsequent disease development and could represent a serological marker to help take the appropriate therapeutic decision. Further studies are necessary to assess the predictive nature of HLA-G and to estimate both sensitivity and specificity.
... As iron is an essential nutrient for the parasite, deprivation of iron could limit parasite growth. Indeed, iron starvation is a frequently employed host tactic to battle invading pathogens [54]. It is possible that this protective effect significantly outweighs the detrimental effect of anemia, and has therefore been selected throughout the evolution as a consequence of host-pathogen interactions. ...
African trypanosomes are the causative agents of Human African Trypanosomosis (HAT), otherwise termed ‘Sleeping Sickness’, and Animal African Trypanosomosis (AAT) or ‘Nagana’. These parasites infect humans and animals throughout the African continent, where they cause death and impair economic development. In this review we describe the events leading to the onset of inflammation in the mouse model for trypanosomosis, and we describe two important pathological features associated to the acute pro-inflammatory reaction: anemia and B cell destruction.
... However, in another study, a T-cellindependent anti-VSG IgM response was proposed as the first line of defense against proliferating parasites [54]. However, gaps still exist on how and whether antibodies play a significant role in parasite control [55]. Evidence is building up suggesting that cytokines might be key players in HAT pathogenesis [14,15,56]. ...
Human African trypanosomiasis due to Trypanosoma brucei rhodesiense is invariably fatal if untreated with up to 12.3 million people at a risk of developing the disease in Sub-Saharan Africa. The disease is characterized by a wide spectrum of clinical presentation coupled with differences in disease progression and severity. While the factors determining this varied response have not been clearly characterized, inflammatory cytokines have been partially implicated as key players. In this review, we consolidate available literature on the role of specific cytokines in the pathogenesis of T. b. rhodesiense sleeping sickness and further discuss their potential as stage biomarkers. Such information would guide upcoming research on the immunology of sleeping sickness and further assist in the selection and evaluation of cytokines as disease stage or diagnostic biomarkers.
... Although the role of adaptive immunity in protection against extracellular pathogens such as trypanosomes are well recognized [32,33], the independent contributions of the cell-mediated or humoral immune mechanisms are less well understood. Profiles of induced [34][35][36] have shown repeatedly that attempting to predict the outcome of an infection based on a single cytokine response is unlikely to be successful. For example, IFN-γ, an important cytokine of the Th1 subset, activates and stimulates the macrophages to produce IL-12 that not only helps in differentiation of Th1 cells, but also inhibits the expansion of Th2 type T-cell population. ...
Background:
Trypanosomosis or Surra, caused by the flagellated hemoprotozoan parasite Trypanosoma evansi, is a disease of economic importance through its wide prevalence in domestic livestock in tropical countries. In the absence of a protective vaccine, management of the disease relies on a few available chemotherapeutic agents. Although humoral immunity is the mainstay of resistance to T. evansi, the ability of the parasite to vary its immunodominant surface proteins to subvert the immune system has forced vaccine efforts to target a variety of invariant epitopes. Beta tubulin, an integral component of the trypanosome cytoskeleton, was therefore targeted using the recombinant form of the protein for immunization.
Methods:
The 1329 bp coding sequence of beta tubulin gene was PCR amplified and cloned in pQE-TriSystem expression vector. Recombinant beta tubulin was heterologously expressed in Escherichia coli as a 46 KDa fusion protein and used for immunization of mice. The Ig response was studied by ELISA, whereas the cytokine response was measured using a cytometric bead-based assay quantified by flow cytometry.
Result:
Immunization with recombinant beta (β)-tubulin protein induced a beta-tubulin specific humoral immune response of predominantly IgG2a isotype. Lethal challenge with T. evansi blood-form trypomastigotes post-immunization elicited a robust anamnestic response. An abundance of IFN-γ further confirmed the Th-1 bias of the protective response. We also observed extended survival and better control of the challenge infection in the immunized mice.
Conclusions:
A robust anamnestic response following challenge including a Th-1 serum cytokine profile coupled with increased survival is indicative of protective immunity in the immunized mice. These observations suggest that β-tubulin of T. evansi is a viable antigenic target for development of a vaccine against this important livestock pathogen.
... Such correlations have already been observed in several studies [19], [63], [23], [21] but with different magnitudes depending on the variable calculation, the cattle populations or the experimental conditions. It is obvious that parasite infection is the primary cause of anaemia, but the intensity of the latter is not a simple function of parasite burdens: it depends on interacting mechanisms, including host-inflammatory-mediated anaemia and parasite-induced anaemia [64], more or less buffered by the erythropoietic response [65] or the cell-membrane composition of RBC [66]. Thereby, some QTL were associated with both PCV and the parasitaemia indicators, whereas others were independently linked to one or the other [21]. ...
Background:
Animal African Trypanosomosis particularly affects cattle and dramatically impairs livestock development in sub-Saharan Africa. African Zebu (AFZ) or European taurine breeds usually die of the disease in the absence of treatment, whereas West African taurine breeds (AFT), considered trypanotolerant, are able to control the pathogenic effects of trypanosomosis. Up to now, only one AFT breed, the longhorn N'Dama (NDA), has been largely studied and is considered as the reference trypanotolerant breed. Shorthorn taurine trypanotolerance has never been properly assessed and compared to NDA and AFZ breeds.
Methodology/principal findings:
This study compared the trypanotolerant/susceptible phenotype of five West African local breeds that differ in their demographic history. Thirty-six individuals belonging to the longhorn taurine NDA breed, two shorthorn taurine Lagune (LAG) and Baoulé (BAO) breeds, the Zebu Fulani (ZFU) and the Borgou (BOR), an admixed breed between AFT and AFZ, were infected by Trypanosoma congolense IL1180. All the cattle were genetically characterized using dense SNP markers, and parameters linked to parasitaemia, anaemia and leukocytes were analysed using synthetic variables and mixed models. We showed that LAG, followed by NDA and BAO, displayed the best control of anaemia. ZFU showed the greatest anaemia and the BOR breed had an intermediate value, as expected from its admixed origin. Large differences in leukocyte counts were also observed, with higher leukocytosis for AFT. Nevertheless, no differences in parasitaemia were found, except a tendency to take longer to display detectable parasites in ZFU.
Conclusions:
We demonstrated that LAG and BAO are as trypanotolerant as NDA. This study highlights the value of shorthorn taurine breeds, which display strong local adaptation to trypanosomosis. Thanks to further analyses based on comparisons of the genome or transcriptome of the breeds, these results open up the way for better knowledge of host-pathogen interactions and, furthermore, for identifying key biological pathways.
... Increased IL10 and low TNFa levels were also observed in HAT patients confirming their association with disease susceptibility. Interestingly a line of evidences indicates that the production of TNFa is involved in the control of parasite growth but also in the development of pathogenesis in experimental trypanosomiasis [39]. The mechanisms by which TNFa interacts with trypanosomes (via direct versus indirect actions) are still controversial and differ in the various experimental models [40,41]. ...
In West Africa, Trypanosoma brucei gambiense, causing human African trypanosomiasis (HAT), is associated with a great diversity of infection outcomes. In addition to patients who can be diagnosed in the early hemolymphatic phase (stage 1) or meningoencephalitic phase (stage 2), a number of individuals can mount long-lasting specific serological responses while the results of microscopic investigations are negative (SERO TL+). Evidence is now increasing to indicate that these are asymptomatic subjects with low-grade parasitemia. The goal of our study was to investigate the type of immune response occurring in these "trypanotolerant" subjects. Cytokines levels were measured in healthy endemic controls (n = 40), stage 1 (n = 10), early stage 2 (n = 19), and late stage 2 patients (n = 23) and in a cohort of SERO TL+ individuals (n = 60) who were followed up for two years to assess the evolution of their parasitological and serological status. In contrast to HAT patients which T-cell responses appeared to be activated with increased levels of IL2, IL4, and IL10, SERO TL+ exhibited high levels of proinflammatory cytokines (IL6, IL8 and TNFα) and an almost absence of IL12p70. In SERO TL+, high levels of IL10 and low levels of TNFα were associated with an increased risk of developing HAT whereas high levels of IL8 predicted that serology would become negative. Further studies using high throughput technologies, hopefully will provide a more detailed view of the critical molecules or pathways underlying the trypanotolerant phenotype.
... IL-10 was shown to be essential, in both murine T. congolense and T. brucei infections, to limit the infectionassociated inflammation and to prolong the survival of the host by dampening the levels of the pathogenic type I immune effector molecules, including IFN-γ, TNF and NO [23,28,59,68,74,[76][77][78][79]. This was confirmed by the fact that IL-10-deficient mice exhibit severe immunopathological symptoms and have a drastically shortened survival since they can not control the strong type I immune response mounted during the early stage of infection [77]. ...
Bovine African trypanosomiasis causes severe economical problems on the African continent and one of the most prominent immunopathological parameters associated with this parasitic infection is anemia. In this report we review the current knowledge of the mechanisms underlying trypanosomiasis-associated anemia. In first instance, the central role of macrophages and particularly their activation state in determining the outcome of the disease (i.e. trypanosusceptibility versus trypanotolerance) will be discussed. In essence, while persistence of classically activated macrophages (M1) contributes to anemia development, switching towards alternatively activated macrophages (M2) alleviates pathology including anemia. Secondly, while parasite-derived glycolipids such as the glycosylphosphatidylinositol (GPI) induce M1, host-derived IL-10 blocks M1-mediated inflammation, promotes M2 development and prevents anemia development. In this context, strategies aimed at inducing the M1 to M2 switch, such as GPI-based treatment, adenoviral delivery of IL-10 and induction of IL-10 producing regulatory T cells will be discussed. Finally, the crucial role of iron-homeostasis in trypanosomiasis-associated anemia development will be documented to stress the analogy with anemia of chronic disease (ACD), hereby providing new insight that might contribute to the treatment of ACD.
... Within this period, the macrophage plays a crucial role in the production of an efficient immune response against African trypanosomes. This cell is responsible for the initial production of pro-inflammatory cytokines such as tumor necrosis factor (TNF) and nitric oxide (NO) (Stijlemans et al. 2007). Thus, BChE could be involved in activation of macrophages, as it has nicotinic receptors for acetylcholine. ...
The aim of this study is to evaluate the role of cholinesterases as an inflammatory marker in acute and chronic infection by Trypanosoma evansi in rabbits experimentally infected. Twelve adult female New Zealand rabbits were used and divided into two groups with 6 animals each: control group (rabbits 1-6) and infected group (rabbits 7-12). Infected group received intraperitoneally 0.5 mL of blood from a rat containing 108 parasites per animal. Blood samples used for cholinesterases evaluation were collected on days 0, 2, 7, 12, 27, 42, 57, 87, 102 and 118 days post-inoculation (PI). Increased activity (P<0.05) of butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) were observed in the blood on days 7 and 27, respectively and no differences were observed in cholinesterase activity in other periods. No significant difference in AChE activity (P>0.05) was observed in the encephalic structures. The increased activities of AChE and BChE probably have a pro-inflammatory purpose, attempting to reduce the concentration of acetylcholine, a neurotransmitter which has an anti-inflammatory property. Therefore, cholinesterase may be inflammatory markers in infection with T. evansi in rabbits.
Studies on the camel immune response to Trypanosoma (T.) evansi, the causative agent of Surra, are very limited. In the present study, flow cytometry was employed to investigate the modulatory effects of different T. evansi antigens on the in vitro differentiation of camel blood monocytes into macrophages. For this, in vitro, separated camel monocytes were differentiated into monocyte-derived macrophages (MDM) in the presence or absence (control) of formalin-fixed (inactivated) T. evansi whole parasite (T. evansi group) or the purified Ro Tat 1.2 antigen (Ro Tat 1.2 group). The analysis of the antimicrobial functions of MDM (phagocytosis and reactive oxygen species (ROS) production) revealed reduced phagocytosis activity of camel MDM generated in the presence of T. evansi antigens. In addition, a lack of ROS-response was observed in camel MDM generated in the presence of T. evansi antigens after stimulation with PMA. These results indicated a compromising effect of T. evansi on the innate defense mechanisms in camels. Phenotypic analysis revealed the upregulation of major histocompatibility complex (MHC) class II molecules together with the lower abundance of the scavenger receptor for haptoglobin–hemoglobin complexes (CD163) on MDM generated in the presence of whole T. evansi parasites, indicating a polarizing effect of T. evansi on the differentiation of camel monocytes into an M1 phenotype. However, the reduced antimicrobial functions of these cells argue against their pro-inflammatory nature. Although both MDM generated in the presence of whole T. evansi antigens or their purified Ro Tat 1.2 proteins indicated similar expression levels of CD14 and MHCII molecules, the different abundance of the cell surface molecules CD172a, CD163, CD45, and CD44 indicated different phenotypes of the two MDMs. The results of the present study revealed compromising effects of T. evansi antigens on camel macrophages differentiated in vitro from blood monocytes. Whether these effects contribute to the in vivo pathogenesis of T. evansi in camels remains to be determined in future studies.
African trypanosomes are vector-borne protozoa, which cause significant human and animal disease across sub-Saharan Africa, and animal disease across Asia and South America. In humans, infection is caused by variants of Trypanosoma brucei, and is characterised by varying rate of progression to neurological disease, caused by parasites exiting the vasculature and entering the brain. Animal disease is caused by multiple species of trypanosome, primarily T. congolense, T. vivax and T. brucei. These trypanosomes also infect multiple species of mammalian host, and this complexity of trypanosome and host diversity is reflected in the spectrum of severity of disease in animal trypanosomiasis, ranging from hyperacute infections associated with mortality to long term chronic infections, and is also a main reason why designing interventions for animal trypanosomiasis is so challenging. In this review, we will provide an overview of the current understanding of trypanosome determinants of infection progression and severity, covering laboratory models of disease, as well as human and livestock disease. We will also highlight gaps in knowledge and capabilities, which represent opportunities to both further our fundamental understanding of how trypanosomes cause disease, as well as facilitating the development of the novel interventions that are so badly needed to reduce the burden of disease caused by these important pathogens.
A tightly regulated innate immune response to trypanosome infections is critical to strike a balance between parasite control and inflammation-associated pathology. In this study, we make use of the recently established Trypanosoma carassii infection model in larval zebrafish to study the early response of macrophages and neutrophils to trypanosome infections in vivo. We consistently identified high- and low-infected individuals and were able to simultaneously characterize their differential innate response. Not only did macrophage and neutrophil number and distribution differ between the two groups, but also macrophage morphology and activation state. Exclusive to high-infected zebrafish, was the occurrence of foamy macrophages characterized by a strong pro-inflammatory profile and potentially associated with an exacerbated immune response as well as susceptibility to the infection. To our knowledge this is the first report of the occurrence of foamy macrophages during an extracellular trypanosome infection.
A tightly regulated innate immune response to trypanosome infections is critical to strike a balance between parasite control and inflammation-associated pathology. In the present study, we make use of the recently established Trypanosoma carassii infection model in larval zebrafish to study the early response of macrophages and neutrophils to trypanosome infections in vivo . We consistently identified high- and low-infected individuals and were able to simultaneously characterize their differential innate response. Not only did macrophage and neutrophil number and distribution differ between the two groups, but also macrophage morphology and activation state. Exclusive to high-infected zebrafish, was the appearance of macrophages rich in lipid droplets, confirmed to be foamy macrophages and characterized by a strong pro-inflammatory profile. Altogether, we provide an in vivo characterization of the differential response of macrophage and neutrophil to trypanosome infection and identify foamy macrophages as potentially associated with an exacerbated immune response and susceptibility to the infection. To our knowledge this is the first report of the occurrence of foamy macrophages during an extracellular trypanosome infection.
Les maladies à kinétoplastidés sont des parasitoses vectorielles dues à des protozoaires flagellés sanguicoles. Parmi celles-ci, la Trypanosomose Africaine due à un parasite du genre Trypanosoma touche à la fois les Hommes et les animaux. Chez l’Homme, cette maladie, plus connue sous le nom de maladie du sommeil, évolue classiquement en 2 stades. Le stade hémolymphatique où le parasite se multiplie dans le sang et la lymphe et le stade nerveux caractérisé par la présence du parasite au niveau cérébral. En l’absence d’une thérapeutique adapté, la mort est inéluctable. Actuellement les traitements proposés en médecine humaine comme vétérinaire sont anciens, non dénués de toxicité et sont à l’origine de cas de résistances de plus en plus marqués. La recherche de nouvelles molécules est donc primordiale pour pouvoir espérer maitriser cette pathologie. C’est dans ce contexte que nous avons étudié deux familles de molécules capables de reconnaitre des sites parasitaires : (i)Les nitroimidazolés qui vont interagir avec les nitroréductases pour générer des intermédiaires toxiques, et (ii) les dérivés phénanthroliniques ciblant les télomérases afin de perturber la synthèse d’ADN du trypanosome. Notre travail de thèse a permis d’évaluer le pouvoir trypanocide de différentes molécules de ces deux familles à la fois par des tests in vitro mais aussi sur un modèle murin infecté par une souche de Trypanosoma brucei brucei. La finalité de ce travail étant d’identifier de nouveaux candidats médicaments. Les résultats obtenus ont permis de mettre en évidence des composés d’intérêt qui ouvrent de nouvelles voies de recherche pour lutter contre cette parasitose, mais aussi plus largement contre tous les kinétoplastidés.
Human African trypanosomiasis (HAT), often called sleeping sickness in its second, central nervous system–involved stage, is a disease of Sub-Saharan Africa. It is caused by subspecies of the protozoan parasite Trypanosoma brucei, which is transmitted by tsetse flies. Two distinctive conditions, classically discriminated by diverging clinical progression, are caused by different trypanosome subspecies. In East and Southern Africa, Trypanosoma brucei rhodesiense causes a rapidly advancing form that within weeks passes from the hemolymphatic first stage to the neurological second stage. In West and Central Africa, Trypanosoma brucei gambiense takes many months to progress to its second-stage form. Other trypanosome subspecies infect animals but not humans, in whom they are killed by several trypanosome-lytic factors. Rare genetic polymorphisms in which one such factor, apolipoprotein L1 (APOL1), is absent can lead to human infection by other trypanosome species (e.g., Trypanosoma evansi). Genetic variants of APOL1, common in African Americans suffering chronic kidney disease, originate in West Africa, where their prevalence is high. These alleles might have been selected during human evolution because of their ability to lyse trypanosomes (e.g., Trypanosoma rhodesiense) that would otherwise bypass the lytic effects of other APOL1 variants. Sleeping sickness treatment is stratified depending on the causative subspecies and whether the disease is diagnosed in the first or the second stage. For a variety of reasons, current drugs are unsatisfactory in the treatment of HAT.
This chapter mainly focuses attention on trypanosome infections occurring in domestic animal species, with particular emphasis on those in the African continent. Specific trypanosome species cause three major disease syndromes within Africa namely nagana, surra and Dourine. Nagana, mainly affecting domestic ruminants, pigs and pets in sub-Saharan Africa, is caused by Trypanosoma congolense, Trypanosoma vivax, Trypanosoma brucei subspecies as well as Trypanosoma simiae, primarily transmitted by tsetse flies (Glossina species). Surra, caused by Trypanosoma evansi, mainly affects camels, horses, ruminants, pigs and dogs in North and East Africa. Beyond the African borders, surra occurs in the Middle East, Asia and Latin America. Unlike other trypanosome syndromes, Dourine is non-vector borne, but rather transmitted through coitus from stallions to mares and vice versa. It occurs worldwide wherever horses are reared in large numbers. Thus non-vector-borne trypanosomes have a wider geographical distribution beyond the African continent as they may also be spread through international trade.
Although some trypanosome species strictly cause disease to animals, others such as T. b. rhodesiense and T. b. gambiense cause disease to both animals and humans. In aggregate, trypanosome infections have serious socio-economic implications and significantly contribute to poverty and underdevelopment experienced in the affected regions where livestock production is the main livelihood of the local communities. As such, efforts towards effective control of the disease are justified.
The African trypanosomes (Trypanosoma brucei ssp.)Pathogenesis of sleeping sicknessVariant surface glycoprotein – the key to trypanosome-host interactionsThe humoral response to African trypanosomesT cell responses in African trypanosome infectionsInnate defence mechanisms: trypanosome lytic factorImmunopathology and VSGSummaryReferences for further reading
Trypanosoma brucei gambiense, a parasitic protozoan belonging to kinetoplastids, is the main etiological agent of human African trypanosomiasis (HAT), or sleeping sickness. One major characteristic of this disease is the dysregulation of the host immune system. The present study demonstrates that the secretome (excreted-secreted proteins) of T. b. gambiense impairs the lipopolysaccharide (LPS)-induced maturation of murine dendritic cells (DCs). The up-regulation of MHC II, CD40, CD80, and CD86 molecules, as well as the secretion of cytokines such as TNF-α, IL-10, and IL-6, which are normally released at high levels by LPS-stimulated DCs, are significantly reduced when these cells are cultured in the presence of the T. b. gambiense secretome. Moreover, the inhibition of DC maturation results in the loss of their allostimulatory capacity, leading to a dramatic decrease in Th1/Th2 cytokine production by co-cultured lymphocytes. These results provide new insights into a novel efficient immunosuppressive mechanism, directly involving the alteration of DC function, which might be used by T. b. gambiense to interfere with the host immune responses in HAT and promote the infectious process.
In this overview, we cover basic biology of select protozoan parasites of veterinary importance. These
include the Apicomplexans, Eimeria, Theileria, Cryptosporidium, and the Flagellates Giardia and
African trypanosomes. We have synthesised the available information into a relatively simple
information guide of potential interest to a wide reading audience. Where appropriate, references are
made to human disease and zoonoses, but the main focus is on the biology of these infections in
domestic animals. A more detailed and comprehensive treatise on different aspects of the biology of
these protozoan infections can be obtained from several scientific review articles which have been
listed and annotated in the bibliography section of this article.
Tolerogenicity of dendritic cells (DCs) has initially been attributed exclusively to immature/resting stages, while mature/activated DCs were considered strictly immunogenic. Later, all different subsets among the myeloid/conventional DCs and plasmacytoid DCs have been shown to bear tolerogenic potential, so that tolerogenicity could not be attributed to a specific subset. Immunosuppressive treatments of immature DC subsets could prevent re-programming into mature DCs or upregulated inhibitory surface markers or cytokines. Furthermore, the different T cell tolerance mechanisms anergy, deletion, immune deviation, and suppression require different quantities and qualities of costimulation by DCs. Since expansion of regulatory T cells (Tregs) has been shown to be promoted best by fully mature DCs the role of CD80/B7-1 and CD86/B7-2 as major costimulatory molecules for Treg biology is under debate. In this review, we discuss the role of these and other costimulatory molecules on myeloid DCs and their ligands CD28 and CD152/CTLA-4 on Tregs for peripheral conversion from naive CD4⁺ T cells into the major subsets of Foxp3⁺ Tregs and Foxp3⁻ IL-10⁺ regulatory type-1 T cells (Tr1) or Tr1-like cells and their role for peripheral maintenance in the steady state and after activation.
Trypanosomes are digenetic protozoans that infect domestic and wild animals, as well as humans. They cause important medical and veterinary diseases, making them a major public health concern. There are many species of trypanosomes that infect virtually all vertebrate taxa. They typically cycle between insect or leech vectors and vertebrate hosts, and they undergo biochemical and morphological changes in the process. Trypanosomes have received much attention in the last 4 decades because of the diseases they cause and their remarkable armamentarium of immune evasion mechanisms. The completed genome sequences of trypanosomes have revealed an extensive array of molecules that contribute to various immune evasion mechanisms. The different species interact uniquely with their vertebrate hosts with a wide range of evasion strategies and some of the most fascinating immune evasion mechanisms, including antigenic variation that was first described in the trypanosomes. This review focuses on the variety of strategies that these parasites have evolved to evade or modulate immunity of endothermic and ectothermic vertebrates.
DCs represent the major cell type leading to polarized T-helper (Th) cell responses in vivo. Here, we asked whether the instruction of murine Th2 responses by DCs matured with the proinflammatory cytokine TNF is qualitatively different from maturation by different types of TLR4/MyD88-dependent variant-specific surface glycoproteins (VSGs) of Trypanosoma brucei (T. brucei). The results obtained by analyzing DC surface markers, Notch ligand mRNA, cytokines, asthma, and experimental autoimmune encephalomyelitis (EAE) models as well as performing microarrays indicate that both types of stimuli induce similar inflammatory, semi-mature DC profiles. DCs matured by TNF or VSG treatment expressed a common inflammatory signature of 24 genes correlating with their Th2-polarization capacity. However, the same 24 genes and 4498 additional genes were expressed by DCs treated with LPS that went on to induce Th1 cells. These findings support the concept of a default pathway for Th2-cell induction in DCs matured under suboptimal or inflammatory conditions, independent of the surface receptors and signaling pathways involved. Our data also indicate that quantitative differences in DC maturation might direct Th2- vs Th1-cell responses, since suboptimally matured inflammatory DCs induce default Th2-cell maturation, whereas fully mature DCs induce Th1-cell maturation.
Trypanosomes are protozoan parasites of medical and veterinary importance. It is well established that different species, subspecies and strains of trypanosome can cause very different disease in the mammalian host, exemplified by the two human-infective subspecies of Trypanosoma brucei that cause either acute or chronic disease. We are beginning to understand how the host response shapes the course of the disease and how genetic variation in the host can be a factor in disease severity, particularly in the mouse model, but until recently the role of parasite genetic variation that determines differential disease outcome has been a neglected area. This review will discuss the recent advances in this field, covering both our current knowledge of the T. brucei genes involved and the approaches that are leading towards the identification of T. brucei virulence genes. Finally, the potential for using parasite genotype variation to examine the evolutionary context of virulence will be discussed.
In order to evaluate during experimental Trypanosoma brucei infections the potential role of tumor necrosis factor alpha (TNF-α) in the host-parasite interrelationship, C57BL/6 TNF-α knockout mice (TNF-α−/−) as well as C57BL/6 wild-type mice were infected with pleomorphic T. brucei AnTat 1.1 E parasites. In the TNF-α−/− mice, the peak levels of parasitemia were strongly increased compared to the peak levels recorded in wild-type mice. The increased parasite burden did not reflect differences in clearance efficacy or in production of T. brucei-specific immunoglobulin M (IgM) and IgG antibodies. Trypanosome-mediated immunopathological features, such as lymph node-associated immunosuppression and lipopolysaccharide hypersensitivity, were found to be greatly reduced in infected TNF-α−/− mice. These results demonstrate that, during trypanosome infections, TNF-α is a key mediator involved in both parasitemia control and infection-associated pathology.
The TNF-alpha-inducing capacity of different trypanosome components was analyzed in vitro, using as indicator cells a macrophage cell line (2C11/12) or peritoneal exudate cells from LPS-resistant C3H/HeJ mice and LPS-sensitive C3H/HeN mice. The variant-specific surface glycoprotein (VSG) was identified as the major TNF-alpha-inducing component present in trypanosome-soluble extracts. Both soluble (sVSG) and membrane-bound VSG (mfVSG) were shown to manifest similar TNF-alpha-inducing capacities, indicating that the dimyristoylglycerol (DMG) compound of the mfVSG anchor was not required for TNF-alpha triggering. Detailed analysis indicated that the glycosyl-inositol-phosphate (GIP) moiety was responsible for the TNF-alpha-inducing activity of VSG and that the presence of the GIP-associated galactose side chain was essential for optimal TNF-alpha production. Furthermore, the results showed that the responsiveness of macrophages toward the TNF-alpha-inducing activity of VSG was strictly dependent on the activation state of the macrophages, since resident macrophages required IFN-gamma preactivation to become responsive. Comparative analysis of the ability of both forms of VSG to activate macrophages revealed that mfVSG but not sVSG stimulates macrophages toward IL-1alpha secretion and acquisition of LPS responsiveness. The priming activity of mfVSG toward LPS responsiveness was also demonstrated in vivo and may be relevant during trypanosome infections, since Trypanosoma brucei-infected mice became gradually LPS-hypersensitive during the course of infection. Collectively, the VSG of trypanosomes encompasses two distinct macrophage-activating components: while the GIP moiety of sVSG mediates TNF-alpha induction, the DMG compound of the mfVSG anchor contributes to IL-1 alpha induction and LPS sensitization.
The mechanisms regulating resistance or susceptibility to African trypanosomes have been enigmatic. In this study, we assessed the production of several cytokines (IL-4, IFN-gamma, and TNF-alpha) in vivo and in vitro using genetically susceptible (BALB/c) or resistant (C57BL/6) mice infected with cloned Trypanosoma congolense and the role of these cytokines in pathogenesis of this infection. Plasma of infected BALB/c mice contained higher levels of IL-4 and IFN-gamma than the plasma of infected C57BL/6 mice. Conversely, plasma TNF-alpha levels were elevated significantly in the resistant mice relative to the susceptible ones. Splenic IFN-gamma mRNA appeared earlier and were maintained at higher levels in infected BALB/c than in C57BL/6 mice. Both spontaneous and Con A-induced secretions of IL-4 and IFN-gamma by splenocytes from infected BALB/c mice were significantly higher than those from their C57BL/6 counterparts. Con A-induced proliferation of splenocytes from infected BALB/c mice was progressively suppressed. Nitric oxide was not involved in this suppression, but the suppression was positively correlated with IFN-gamma secretion. Addition of neutralizing Abs to IFN-gamma to cultures of Con A-stimulated spleen cells from infected BALB/c mice effectively reversed this suppression. Furthermore, administration of anti-IFN-gamma Abs to BALB/c mice early during infection dramatically shifted the phenotype of these susceptible mice to a more resistant-like phenotype, as expressed by a low and undulating parasitemia and a >300% increase in survival period. These results strongly suggest that the enhanced induction and secretion of IFN-gamma during T. congolense infections contribute to the relative susceptibility of BALB/c mice to the disease.
Trypanosoma brucei gambiense a causative agent of sleeping sickness, induced a dose-dependent production of tumor necrosis factor (TNF)–α by human macrophages
in vitro. TNF-α was also induced in the Mono Mac 6 cell line, which indicates a direct effect of parasite components on macrophages.
Parasite-soluble factors were also potent inducers of TNF-α. The addition of anti–TNF-α to cocultures of macrophages and parasites
increased the number of trypanosomes and their life span, whereas irrelevant antibodies had no effect. TNF-α may have a direct
role (i.e., direct trypanolytic activity) and/or an indirect one, such as TNF-α–mediated induction of cytotoxic molecules.
A direct dose-dependent lytic effect of TNF-α on purified parasites was observed. This lytic effect was inhibited by anti–TNF-α.
These data suggest that, as in experimental trypanosomiasis, TNF-α is involved in parasite growth control in human African
trypanosomiasis
The activation of innate immune responses by genomic DNA from bacteria and several nonvertebrate organisms represents a novel
mechanism of pathogen recognition. We recently demonstrated the CpG-dependent mitogenic activity of DNA from the protozoan
parasiteBabesia bovis for bovine B lymphocytes (W. C. Brown, D. M. Estes, S. E. Chantler, K. A. Kegerreis, and C. E. Suarez, Infect. Immun. 66:5423–5432,
1998). However, activation of macrophages by DNA from protozoan parasites has not been demonstrated. The present study was
therefore conducted to determine whether DNA from the protozan parasites B. bovis, Trypanosoma cruzi, and T. brucei activates macrophages to secrete inflammatory mediators associated with protective immunity. DNA fromEscherichia coli and all three parasites stimulated B-lymphocyte proliferation and increased macrophage production of interleukin-12 (IL-12),
tumor necrosis factor alpha (TNF-α), and nitric oxide (NO). Regulation of IL-12 and NO production occurred at the level of
transcription. The amounts of IL-12, TNF-α, and NO induced by E. coli and protozoal DNA were strongly correlated (r
2 > 0.9) with the frequency of CG dinucleotides in the genome, and immunostimulation by DNA occurred in the order E. coli ≥ T. cruzi > T. brucei > B. bovis. Induction of inflammatory mediators by E. coli, T. brucei, and B. bovis DNA was dependent on the presence of unmethylated CpG dinucleotides. However, at high concentrations,E. coli and T. cruzi DNA-mediated macrophage activation was not inhibited following methylation. The recognition of protozoal DNA by B lymphocytes
and macrophages may provide an important innate defense mechanism to control parasite replication and promote persistent infection.
Resistance to Trypanosoma brucei brucei has been correlated with the ability of infected animals to produce interferon (IFN)–γ and tumor necrosis factor (TNF) in
an early phase of infection, followed by interleukin (IL)–4 and IL-10 in late and chronic stages of the disease. Contributions
of IFN-γ and IL-10 in the control of parasitemia and survival of mice infected with T. brucei brucei were investigated by using IFN-γ−/− and IL-10−/− mice. Results suggest that IFN-γ, mainly secreted by CD8+ T cells, is essential for parasite control via macrophage activation, which results in TNF and nitric oxide secretions. IL-10,
partially secreted by CD4+ T cells, seems to be important for the survival of infected mice. Its absence resulted in the sustained secretion of inflammatory
mediators, which indicated the role of IL-10 in maintaining the balance between pathogenic and protective immune responses
during African trypanosomosis
Alternatively activated macrophages (aaMphi) display molecular and biological characteristics that differ from those of classically activated macrophages (caMphi). Recently, we described an experimental model of murine trypanosomosis in which the early stage of infection of mice with a Trypanosoma brucei brucei variant is characterized by the development of caMphi, whereas in the late and chronic stages of infection, aaMphi develop. In the present study, we used suppression subtractive hybridization (SSH) to identify genes that are expressed differentially in aaMphi versus caMphi elicited during infection with this T. b. brucei variant. We show that FIZZ1 and Ym1 are induced strongly in in vivo- and in vitro-elicited aaMphi as compared with caMphi. Furthermore, we demonstrate that the in vivo induction of FIZZ1 and Ym1 in macrophages depends on IL-4 and that in vitro, IFN-gamma antagonizes the effect of IL-4 on the expression of FIZZ1 and Ym1. Collectively, these results open perspectives for new insights into the functional properties of aaMphi and establish FIZZ1 and Ym1 as markers for aaMphi.
African trypanosomes express a glycosylphosphatidyl inositol (GPI)-anchored variant-specific surface glycoprotein (VSG) as a protective coat. During infection, large amounts of VSG molecules are released into the circulation. Their interaction with various cells of the immune system underlies the severe infection-associated pathology. Recent results have shown that anti-GPI vaccination can prevent the occurrence of this pathology. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.
The contribution of cytokines and chemokines to resistance and susceptibility to African trypanosomiasis remains controversial. In the present study, the levels of type I and type II cytokines and of the MCP-1 chemokine were compared during the early and late stages of Trypanosoma congolense infection in susceptible BALB/c and resistant C57BL/6 mice. Moreover, the status of macrophage activation was compared in these animals by analyzing the inducible nitric oxide synthase-arginase balance, tumor necrosis factor secretion, and expression of the FIZZ1 and YM genes. Data show that changing from a predominant type I cytokine environment in the early stage of infection to a predominant type II cytokine environment and an enhanced MCP-1 secretion in the late stage of infection correlates with resistance to T. congolense. Concomitantly, macrophage activation evolves from a classical to a predominant alternative phenotype. We further confirmed that the simultaneous occurrence of type I/type II cytokines in the early stage of infection in susceptible BALB/c mice, reflected by the presence of macrophages exhibiting a mixed classical/alternative activation phenotype, is associated with uncontrolled parasite growth and early death. Interleukin-4 (IL-4) and IL-13 signaling did not influence the susceptibility of BALB/c mice to T. congolense infection and interestingly were not the main trigger to alternative macrophage activation. In T. congolense-resistant C57BL/6 mice, our results corroborated the induction of FIZZ1 and YM gene expressions with the alternative pathway of macrophage activation. In susceptible BALB/c mice, however, YM but not FIZZ1 induction reflected the emergence of alternatively activated macrophages. Hence, the FIZZ1 and YM genes may be useful markers to discriminate between distinct populations of alternatively activated macrophages.
Immunohistochemical double-label technique was used to detect trypanosomal antigen in macrophages. Immunoglobulin (Ig)M as well as IgG2a monoclonal antibodies (mAb) specific for the variant surface glycoprotein (VSG) mediated phagocytosis of Trypanosoma congolense variant antigenic type (VAT) TC13 by macrophages [bone marrow-derived macrophage cell line from BALB/c (BALB.BM)] in vitro. Administration of these IgM or IgG2a antibodies to BALB/c mice 30 min after injection of 3 x 10(8) T. congolense mediated phagocytosis of trypanosomes by Kupffer cells of the liver within 1 h. Plasma levels of the monokines interleukin (IL)-1beta, IL-10, and IL-12p40 were significantly increased 6-48 h after phagocytosis. In BALB/c mice infected with 10(3) T. congolense, a small degree of phagocytosis of trypanosomes by Kupffer cells, mediated by actively synthesized antibodies, was detected as early as 5 days after infection. Phagocytosis of trypanosomes was dramatically enhanced on day 6. Concomitantly, the Kupffer cells trippled in size. In BALB/c mice infected for 6 days, treatment with IgM or IgG2a mAb specific for T. congolense VSG led to clearance of VAT TC13 parasitemia but did not prevent death at the second parasitemia of a different VAT. We conclude that IgM as well as IgG antibody mediate phagocytosis of trypanosomes by Kupffer cells.
The initial host response toward the extracellular parasite Trypanosoma brucei is characterized by the early release of inflammatory mediators associated with a type 1 immune response. In this study, we show that this inflammatory response is dependent on activation of the innate immune system mediated by the adaptor molecule MyD88. In the present study, MyD88-deficient macrophages are nonresponsive toward both soluble variant-specific surface glycoprotein (VSG), as well as membrane-bound VSG purified from T. brucei. Infection of MyD88-deficient mice with either clonal or nonclonal stocks of T. brucei resulted in elevated levels of parasitemia. This was accompanied by reduced plasma IFN-gamma and TNF levels during the initial stage of infection, followed by moderately lower VSG-specific IgG2a Ab titers during the chronic stages of infection. Analysis of several TLR-deficient mice revealed a partial requirement for TLR9 in the production of IFN-gamma and VSG-specific IgG2a Ab levels during T. brucei infections. These results implicate the mammalian TLR family and MyD88 signaling in the innate immune recognition of T. brucei.
In highly susceptible BALB/c mice infected with Trypanosoma congolense, the total number of Kupffer cells in the liver remains constant; however, their mean size increases fivefold towards the
terminal stage. About 25% of Kupffer cells undergo apoptosis. We suggest that development of an impairment of the macrophage
system might be a major mechanism for inefficient elimination of trypanosomes.
The glycosyl-phosphatidylinositol (GPI) protein-membrane anchors are ubiquitous among the eukaryotes. However, while mammalian cells typically express in the order of 100 thousand copies of GPI-anchor per cell, the parasitic protozoa, particularly the kinetoplastids, express up to 10–20 million copies of GPI-anchor and/or GPI-related glycolipids per cell. Thus GPI-family members dominate the cell surface molecular architecture of these organisms. In several cases, GPI-anchored proteins, such as the variant surface glycoprotein (VSG) of the African trypanosomes, or GPI-related glycolipids, such as the lipophosphoglycan (LPG) of the Leishmania, are known to be essential for parasite survival and infectivity. The highly elevated levels and specialised nature of GPI metabolism in the kinetoplastid parasites suggest that the GPI biosynthetic pathways might be good targets for the development of chemotherapeutic agents. This article introduces the range of GPI structures found in protozoan parasites, and their mammalian hosts, and discusses some aspects of GPI biosynthesis.
Macrophages collected from BCG-infected mice or exposed in vitro to interferon-γ plus lipopolysaccharide developed a cytostatic activity on Trypanosoma brucei gambiense and Trypanosoma brucei brucei. This trypanostatic activity of activated macrophages was inhibited by addition of N-monomethyl-l-arginine, an inhibitor of the l-arginine-nitric oxide (NO) metabolic pathway, indicating a role for NO as the effector molecule. Contrary to trypanosomes treated with N2 gas, trypanosomes treated with NO gas did not proliferate in vitro on normal macrophages. Compared to mice infected with control parasites, mice infected with NO-treated parasites had decreased parasitemias in the first days postinfection and had a prolonged survival. Addition of excess iron reversed the trypanostatic effect of both activated macrophages and NO gas. These data show that activated macrophages exert an antimicrobial effect on T.b. gambiense and T.b. brucei through the l-arginine-NO metabolic pathway. In trypanosomes, NO could trigger iron loss from critical targets involved in parasite division. The participation of this effector mechanism among the other immune elements involved in the control of African trypanosomes (antibodies, complement, phagocytic events) remains to be defined.
Intraperitoneal injection of Trypanosoma brucei AnTat 1.1 into mice of the C3H.He, BALB/c or C57BL/6 strains resulted in impaired immune responses from day 3 onwards, as measured by the reduction in DNA synthesis in spleen cell populations stimulated with concanavalin A (Con-A) in vitro. Adherent cells from the peritoneum (PC) or from the spleen of infected mice, consisting predominantly of macrophages, caused a 60-80% reduction of the Con-A response in spleen cells from syngeneic recipients 3-4 days after transfer in vivo. Adherent PC from irradiated or athymic mice were equally suppressive. Spleen cells from infected mice reduced the proliferative response of spleen cells from uninfected mice upon co-cultivation in vitro. This dominant suppressive effect was abolished after the selective removal of macrophages from the spleen cell population by treatment with L-leucine methylester. Moreover, the macrophage-depleted spleen cells from infected mice responded normally to Con-A provided they were supplemented with splenic adherent cells from naive mice as a source of accessory cells. Both the cell transfer and co-cultivation experiments suggest that infection with African trypanosomes changes the properties of macrophages to a state which allows them actively to suppress immune responses.
Parasitaemia of Trypanosoma brucei rhodesiense infection in rats peaked on day 5. Afterwards there was a reduction in parasite counts accompanied by anaemia. Prior to the peak, soluble trypanosome antigen was detected in blood of infected rats, but was not found afterwards until the end of the experiment when the parasite counts were again elevated. Onset of reduced parasitaemia with anaemia coincided with appearance of soluble immune complexes, immunoconglutinin and reduced titres of lytic complement. Complexes precipitated from plasma contained parasite antigen. Injection of rats with patent T. b. rhodesiense parasitaemia with antibody against the parasite combined with immunoconglutinin resulted in a prompt reduction in parasite and erythrocyte counts, but did not effect survival of the rats. Injection of the two antibodies did not affect erythrocyte counts of normal rats. The effect was not seen in infected rats injected with parasite antibody alone or with immunoconglutinin alone. From the data it is suggested that antibody against parasite reacted with soluble trypanosomal antigen, forming complexes which fixed complement and became bound to blood cells and trypanosomes and that these cells were immunoconglutinated and removed from the circulation by filter organs of the blood.
The exploitation of genetic resistance to disease is an important consideration in livestock development programs, where conventional disease control measures are not effective or are too costly. Such an approach may be directly applicable to African animal trypanosomiasis. In African trypanosomiasis, the control measures currently in use include diagnosis and treatment, chemoprophylaxis, and control or eradication of tsetse with insecticides. At present, there is no effective field vaccine available against African trypanosomiasis. The major constraints to the development of a vaccine include the existence of the different species of trypanosomes and of different serodemes within the same species, all with the capability of producing different repertoires of variable antigen types (VATS). This chapter discusses the available information on the aspects of trypanotolerance, with regard to cattle, sheep, goats, wildlife, and man. It describes experimental results derived from mouse models and the relevance of these models to trypanosomiasis of livestock. The genetic resistance to trypanosomiasis is not necessarily associated with low productivity. Trypanotolerance is not a stable characteristic, and evidence suggests that it can be supplemented or reduced by a number of factors affecting the host and its environment, which include age and sex; maternally-derived immunity, intensity of challenge, virulence, previous exposure, stress, and susceptibility to other diseases. Investigations of trypanotolerance also represent an exciting approach to the important interactions between host and parasite. Trypanotolerance exists as an innate characteristic and that it is probably inherited as a dominant trait. While the level of trypanotolerance can be reduced under certain circumstances, it can also be enhanced, for example, by previous exposure. Therefore, there is hope that it may be possible in the future to supplement the level of trypanotolerance both by genetic selection and by immunological or therapeutic procedures.
Light microscopic and scanning and transmission electron microscopic studies demonstrated that Trypanosoma brucei EATRO 110 produced several alterations in RBC structure including microspherocytes, schistocytosis, vacuolation, doughnut-cell formation, and keratocytosis. Mature RBC and reticulocytes were constantly observed to adhere firmly to trypanosomes in heart blood and in blood vessels of the testes, heart, liver, and kidney, as well as in the sinuses and pulp cords of the spleen. Adhesion of RBC to trypanosomes was also observed by light microscopy in thin blood films. Except for a few platelets that adhered to trypanosomes, other blood cells were not involved. Minute pores were sometimes observed on the RBC membrane at the point of adhesion to the trypanosome, but effects were not seen on the parasite. Erythrophagocytosis was marked in the spleen and to a lesser extent in the liver; mature RBC, as well as reticulocytes, were engulfed. Erythrophagocytosis was presumed to arise from the mechanical injury to RBC, the damage caused by the adhesion phenomenon and the hyperactivity of the enormously enlarged spleen.
Erythrocyte surface and free serum sialic acid concentrations were determined during experimental Trypanosoma vivax infection in cattle. All infected calves developed mild trypanosomiasis, with significant decreases in mean packed cell volume occurring 15, 16, 20, 22 and 24 days after infection. The anaemia was preceded by significant decreases in mean erythrocyte surface sialic acid concentrations on days 7, 13 and 14, with yet another significant decrease on day 31 after infection. These decreases in erythrocyte surface sialic acid concentrations coincided with the parasitaemic waves. Free serum sialic acid concentration, however, showed an increase, though non-significantly, on day 8, which coincided with both a decrease in erythrocyte surface sialic acid and the initial parasitaemic wave. It is postulated that the early anaemia observed in infected animals may be attributable to the activities of the circulating trypanosomes which produce neuraminidase which, in turn, cleaves off surface sialic acid, thus rendering the erythrocyte more prone to phagocytosis by the recticuloendothelial system.
Suppression of host T cell responses is one of the hallmarks of infection with the African trypanosomes. The cellular basis for immunosuppression includes the generation of suppressor macrophages that down-regulate T cell proliferative but not necessarily cytokine responses to both mitogen and trypanosome Ag. Since macrophages from infected animals display activation characteristics, we have asked whether products of activated cells, specifically nitric oxide (NO) and PG, may mediate the suppressor cell effects and immunosuppression observed. We demonstrate that cells isolated from B10.BR mice infected with Trypanosoma brucei rhodesiense exhibited transcriptional up-regulation of inducible NO synthase and released significant amounts of NO. The levels of NO released were elevated further after stimulation of cells with T cell mitogens or specific parasite Ag; antibody blocking experiments demonstrated that this up-regulation of NO synthesis was at least partially dependent upon IFN-gamma and TNF-alpha. The addition of inducible NO synthase substrate analogues such as NG-monomethyl-L-arginine to cell cultures inhibited NO release and also partially reversed the suppressor cell activity and immunosuppression displayed by such cultures. PG levels also were elevated in cell cultures from infected mice, but the PG inhibitor indomethacin had no effect on suppressor cells or suppression when added alone to the cultures. However, the concurrent inhibition of NO and PG synthesis by the addition of both NG-monomethyl-L-arginine and indomethacin completely blocked suppressor cell activity associated with infected macrophages and also resulted in further recovery of infected cells from immunosuppression, thus revealing an epistatic effect between these two mediators. We conclude that macrophage activation in trypanosomiasis induces the release of reactive nitrogen intermediates and PG, which down-regulate proliferative responses by T cells during infection.
Tumor necrosis factor (TNF), but not lymphotoxin (LT), is directly trypanolytic for salivarian trypanosomes. This activity was not blocked by soluble 55-kilodalton and 75-kilodalton TNF receptors, but was potently inhibited by N,N'-diacetylchitobiose, an oligosaccharide that binds TNF. Comparative sequence analysis of TNF and LT localized the trypanocidal region, and synthetic peptides were trypanolytic. TNF molecules in which the trypanocidal region was mutated or deleted retained tumoricidal activity. Thus, trypanosome-TNF interactions occur via a TNF domain, probably with lectin-like affinity, which is functionally and spatially distinct from the mammalian TNF receptor binding sites.
Lymph node cells (LNC) from mice infected with Trypanosoma brucei contain macrophage-like cells that inhibit interleukin-2 receptor (IL-2R) expression (M. Sileghem, A. Darji, R. Hamers, M. Van De Winkel, and P. De Baetselier, Eur. J. Immunol. 19:829-835, 1989). Evidence that gamma interferon (IFN-gamma) is actively involved in (i) the inhibition of IL-2R expression and (ii) the generation of suppressive cells during infections with T. brucei is presented. First, despite an impaired T-cell mitogenic response, LNC from infected mice are hyperresponsive for IFN-gamma production. Second, addition of neutralizing anti-IFN-gamma antibodies to cocultures of normal LNC and suppressive LNC populations reduces the level of suppression and restores the level of IL-2R expression. Third, administration of anti-IFN-gamma to T. brucei-infected animals increases the blastogenic response and reduces the suppressive activity of LNC.
It has been reported that some breeds of cattle such as the N'Dama mount a more effective antibody response to the variable surface glycoprotein coat of trypanosomes and that this may contribute to their ability to control the infection. Thus we have investigated antibody responses to surface exposed epitopes of the variable surface glycoprotein in Trypanosoma congolense-infected N'Dama (trypanotolerant) and Boran (susceptible) cattle. Similar titres and isotypes were found in both N'Damas and Borans indicating that trypanotolerance is not associated with superior antibody-mediated destruction of trypanosomes. However, significant differences in antibody responses to cryptic VSG epitopes and non-trypanosome antigens were identified. Trypanosusceptible Boran cattle had low IgG1 responses to cryptic epitopes but high IgM responses to non-trypanosome antigens such as beta-galactosidase. In contrast the N'Dama cattle had significantly higher IgG1 responses to cryptic VSG epitopes and negligible responses to beta-galactosidase. These results indicate differences in the induction of anti-trypanosome immune responses between trypanotolerant and susceptible cattle infected with T. congolense.
This study examines B-cell immunoglobulin (Ig) class-switching events in the context of parasite antigen-specific Th-cell responses in experimental African trypanosomiasis. Inbred mice were infected with Trypanosoma brucei rhodesiense, and the coordinate stimulation of Th-cell cytokine responses and B-cell responses to the trypanosome variant surface glycoprotein (VSG) was measured. The cytokines produced by T cells in response to VSG, at both the transcript and protein levels, were gamma interferon and interleukin-2 (IL-2) but not IL-4 or IL-5. Isotype profiles of antibodies specific for VSG showed that IgG1, IgG2a, and IgG3 switch responses predominated; no VSG-specific IgE responses were detected. To determine whether cryptic IL-4 responses played a role in promoting the unexpected IgG1 switch response, IL-4 knockout mice were infected; the cytokine responses and Ig isotype profiles of IL-4 knockout mice were identical to those of the wild-type control mice except for dramatically reduced IgG1 levels in response to VSG. Thus, these results revealed an IL-4-dependent component of the VSG-driven B-cell Cmu-to-Cgamma1 switch. We speculate that an IL-4 response is mediated primarily by cells other than T lymphocytes since IL-4-secreting but parasite antigen-unresponsive, "background" cells were detected in all infected mice and since infected nude mice also displayed a detectable IgG1 switch response. Overall, our results suggest that B-cell clonal stimulation, maturation, and Ig class switching in African trypanosomiasis may be partially regulated by unusual mechanisms that do not include antigen-specific Th1 or Th2 cells.
During murine Trypanosoma brucei infection, macrophages contribute significantly to the inhibition of T cell responses. Although nitric oxide (NO) was shown to play a central role in macrophage-mediated splenic suppression, macrophage-mediated lymph node suppression occurred in an interferon-gamma (IFN-gamma)-dependent manner. In this study, using NO inhibitor NG-monomethyl-L-arginine and anti-IFN-gamma antibodies, the relative contribution of NO and IFN-gamma to the active inhibition of ex vivo concanavalin A-induced T cell proliferation taking place in the spleen and the lymph nodes of T. brucei-infected mice was investigated. NO contributes to the suppressive activity of spleen and lymph node cells only during early-stage infection. The existence of NO-independent suppressive pathway was further evidenced in IFN-gamma(-/-)-infected mice. Spleen cells from such animals do not produce NO but exert significant suppressive activity during the whole course of infection. In contrast in the lymph nodes, no suppressive activity is recorded at any moment of infection. Moreover, addition of exogenous IFN-gamma to cultures containing lymph node cells from IFN-gamma(-/-)-infected mice does not impair proliferation despite NO production in such cultures. Thus during late-stage infection, an IFN-gamma-independent suppressive mechanism is elicited in the spleen, whereas in the lymph nodes, IFN-gamma is required yet not sufficient to inhibit T cell proliferation.
Trypanosomosis is one of the major constraints on animal production in areas of Africa which have the greatest potential for significant increases in domestic livestock populations and livestock productivity. While the eradication of trypanosomosis from the entire continent is an unrealistic goal, considerable effort has been invested in the control of this disease through the use of trypanocidal drugs, management of the vector and exploitation of the genetic resistance exhibited by indigenous breeds. There is little hope that a conventional, anti-infection vaccine will be produced in the near future. Drug resistance is developing faster than generally thought. The control of the tsetse fly has been attempted over many decades. The decreasing efficacy of available trypanocidal drugs and the difficulties of sustaining tsetse control increase the imperative need to enhance trypanotolerance through selective breeding, either within breeds or through cross-breeding. Trypanotolerance has been defined as the relative capacity of an animal to control the development of the parasites and to limit their pathological effects, the most prominent of which is anaemia. A major constraint on selection for trypanotolerance in cattle, for both within-breed and cross-breeding programmes, has been the absence of practical reliable markers of resistance or susceptibility. Distinct humoral immune response to trypanosome infection is the major feature of bovine trypanotolerance. The role that these responses play in the control of infection or disease is being addressed by ongoing research, but remains a matter of speculation at present. Results in recent years have shown that packed cell volume (PCV) in particular and parasitaemia, the two principal indicators of trypanotolerance, are strongly correlated to animal performance. However, although direct effects of trypanosome infections on PCV and growth are obvious, more sensitive diagnostic methods for reflecting parasite control are required so that individual animals can be categorised reliably for their parasite control capability. One key finding is the major contribution made by each of the indicators evaluated to the overall trypanotolerance variance. Preliminary genetic parameters for PCV provide evidence that trypanotolerance is not only a breed characteristic but is also a heritable trait within the N'Dama population; this brings new opportunities for improved productivity through selection for trypanotolerance. More reliable estimation of genetic parameters of the indicators may well show that these parameters must be handled simultaneously for optimal progress. This would require diagnostics for assessing parasite control capability that identify trypanosome species more accurately, especially in mixed infections. A major advantage of trypanotolerant livestock, particularly N'Dama cattle, is the resistance or adaptation of this breed to many of the important pathogenes which prevail in the sub-humid and humid tropics. Research on practical indicators of resistance to these conditions will be required to establish relevant integrated strategies based on disease-resistant livestock. Selective breeding will require the integration of the traits that farmers hold important for their production systems.
We infected highly susceptible BALB/c and relatively resistant C57BL/6 mice with cloned Trypanosoma congolense and followed the effects of these infections on the circulating parasite numbers, mouse mortality and cytokine expression. C57BL/6 mice controlled their parasitaemia and survived for up to 163 +/- 12 days, while BALB/c mice could not control their parasitaemia and succumbed to the infection within 8.4 +/- 0.5 days. Susceptible BALB/c mice had dramatically higher plasma levels of IL-10 than the resistant C57BL/6 mice from day 7 forward. This was preceded by an earlier and higher level induction of splenic IL-10 messenger RNA (mRNA) expression in the infected BALB/c mice. There was a strong negative correlation between the splenocyte proliferative responses to Concanavalin-A (Con-A) and their production of IL-10 in these infected BALB/c mice. Co-treatment of the Con-A-stimulated spleen cell cultures with monoclonal anti-IL-10 antibodies, but not isotype-matched control antibodies, could completely reverse this suppression of the splenocyte proliferative response. Finally, in three experiments, anti-IL-10 antibody treatment in vivo reduced the peak circulating parasitaemia of infected BALB/c mice by 43% and increased their median survival periods by 38% relative to isotype-matched control antibody-treated mice.
The role of variant surface glycoprotein (VSG)-specific Th cell responses in determining resistance to the African trypanosomes was examined by comparing Th cell responses in relatively resistant and susceptible mice as well as in cytokine gene knockout mice infected with Trypanosoma brucei rhodesiense. Resistant B10.BR and C57BL/6 mice expressed Th1 cell cytokine responses to VSG stimulation during infection, while susceptible C3H mice produced weak or no Th1 cell cytokine responses. Neither resistant B10.BR and C57BL/6 mice nor susceptible C3H mice made detectable Th2 cell cytokine responses to parasite Ag. To more closely examine the potential role of IFN-gamma and other cytokines in host resistance, we determined the resistance phenotypes and Th cell responses of IFN-gamma and IL-4 knockout mice. Infected C57BL/6-IFN-gamma knockout mice were as susceptible as C57BL/6-scid mice and made an IL-2, but not an IL-4, cytokine response to VSG, while C57BL/6-IL-4 knockout mice were as resistant as the wild-type strain and exhibited both IL-2 and IFN-gamma cytokine responses. Passive transfer of spleen cells from wild-type mice to IFN-gamma knockout mice resulted in enhanced survival. Both wild-type and IFN-gamma knockout mice controlled parasitemia with VSG-specific Ab responses, although parasitemias were higher in the IFN-gamma knockout mice. Overall, this study demonstrates for the first time that relative resistance to African trypanosomes is associated with a strong Th1 cell response to parasite Ags, that IFN-gamma, but not IL-4, is linked to host resistance, and that susceptible animals do not make compensatory Th2 cell responses in the absence of Th1 cell cytokine responses.
Resistance to African trypanosomes is dependent on B cell and Th1 cell responses to the variant surface glycoprotein (VSG). While B cell responses to VSG control levels of parasitemia, the cytokine responses of Th1 cells to VSG appear to be linked to the control of parasites in extravascular tissues. We have recently shown that IFN-gamma knockout (IFN-gamma KO) mice are highly susceptible to infection and have reduced levels of macrophage activation compared to the wild-type C57BL/6 (WT) parent strain, even though parasitemias were controlled by VSG-specific antibody responses in both strains. In the present work, we examine the role of IFN-gamma in the induction of nitric oxide (NO) production and host resistance and in the development of suppressor macrophage activity in mice infected with Trypanosoma brucei rhodesiense. In contrast to WT mice, susceptible IFN-gamma KO mice did not produce NO during infection and did not develop suppressor macrophage activity, suggesting that NO might be linked to resistance but that suppressor cell activity was not associated with resistance or susceptibility to trypanosome infection. To further examine the consequence of inducible NO production in infection, we monitored survival, parasitemia, and Th cell cytokine production in iNOS KO mice. While survival times and parasitemia of iNOS KO mice did not differ significantly from WT mice, VSG-specific Th1 cells from iNOS KO mice produced higher levels of IFN-gamma and IL-2 than cells from WT mice. Together, these results show for the first time that inducible NO production is not the central defect associated with susceptibility of IFN-gamma KO mice to African trypanosomes, that IFNgamma-induced factors other than iNOS may be important for resistance to the trypanosomes, and that suppressor macrophage activity is not linked to either the resistance or the susceptibility phenotypes.
BALB/c and C57B1/6 mice differ in resistance to Trypanosoma congolense infections. Evidence suggests that macrophages play a central role in the resistance to trypanosomiasis. Nitric oxide (NO) produced by macrophages in response to various stimuli or pathogens is one of the important arms of nonspecific immunity. We investigated the production of NO by the peritoneal macrophages and bone marrow-derived macrophages (BMDM) from trypanosome-resistant C57B1/6 and -susceptible BALB/c mice following stimulation with T. congolense whole cell extract (WCE) or following phagocytosis of T. congolense mediated by anti-variant surface glycoprotein (VSG) antibodies of IgM or IgG2a isotype. C57B1/6 peritoneal macrophages as well as BMDM produced significantly more NO than similar BALB/c macrophages in response to T. congolense lysate and IFN-gamma. In both BALB/c and C57B1/6 BMDM cultures, phagocytosis of T. congolense mediated by anti-VSG antibodies of IgG2a isotype in the presence of IFNgamma induced two- to ninefold more NO than phagocytosis mediated by IgM antibodies and C57B1/6 BMDM produced significantly higher amounts of NO than BALB/c BMDM under these conditions. NO produced by BMDM was found to exert trypanostatic effect on T. congolense in vitro, but was not found to influence the in vivo infectivity of these treated parasites under the conditions used in this study.
Mechanisms regulating resistance to African trypanosomes were addressed by comparing the immune responses of mice infected
with attenuated Trypanosoma brucei brucei lacking the phospholipase C gene (PLC−/−) and those of mice infected with wild-type (WT) parasites. Inhibition of concanavalin A (ConA)-induced T cell proliferation
occurred in spleen and lymph nodes of PLC−/−- and WT-infected mice. Although suppressive cells were elicited in spleen and lymph nodes of WT-infected animals, such cells
were not detected in lymph nodes of PLC−/−-infected mice. PLC−/−-infected mice had more interleukin-4 and -10 in their blood than did WT-infected mice. Correspondingly, PLC−/−-infected mice had higher IgGl antibody levels against variant surface glycoprotein than did WT-infected mice. These data
indicate that attenuation of T. b. brucei correlates with the absence of cells suppressing ConA-induced T cell proliferation in the lymph nodes, with increased production
of Th2 cytokines and a stronger IgGl antibody response to trypanosome antigens.
We have put emphasis on recent findings in experimental Trypanosoma congolense infections in highly susceptible BALB/c and relatively resistant C57Bl/6 mice. Based on various analyses, it has been shown that a major difference in resistance to T. congolense infections is expressed early in infection at the macrophage level. A novel plastic-adherent Thy1.2(+) suppressor lymphocyte, which in absolute synergy with a Thy 1.2(-) cell exerts its suppression via interleukin-10 and interferon-gamma opens up an exciting new field of research.
The type I/type II cytokine balance may influence the development of different subsets of suppressive macrophages, i.e., classically activated macrophages (caMphi, type I) versus alternatively activated macrophages (aaMphi, type II). Recently, we showed that although mice infected with phospholipase C-deficient (PLC-/-) Trypanosoma brucei brucei exhibit a clear shift from type I to the type II cytokine production, wild type (WT)-infected mice remain locked in a type I cytokine response. In the present study, phenotype and accessory cell function of macrophages elicited during WT and PLC-/- T. b. brucei infection were compared. Results indicate that caMphi develop in a type I cytokine environment in the early phase of WT and PLC-/- trypanosome infection, correlating with inhibition of T cell activation triggered by a mitogen, a superantigen, or an antigen. In the late stage of infection, only PLC(-/-)-infected mice resisting the infection develop type II cytokine-associated aaMphi correlating with impaired antigen- but not mitogen- or superantigen-induced T cell activation.
The membrane-associated form of the variable surface glycoprotein (mfVSG) from African trypanosomes is a potent macrophage activator capable of inducing production of tumor necrosis factor alpha (TNFalpha) in both bovine and murine models. Using a bovine model, we have re-investigated the hypothesis that the diacylglycerol moiety of the glycosylphosphatodylinositol (GPI) anchor is involved in macrophage activation and might be the actual parasite toxin. The anchor of the variable surface glycoprotein (VSG) was labeled with (3)H-myristic acid and VSG purified in its membrane-associated form. The dimyristylglycerol moiety of the anchor was released by phospholipase C cleavage. Integrity of the anchor and efficiency of cleavage was verified by autoradiography and methanol:hexane extraction. For analysis of biological function, bovine monocytes were used which had been incubated with bovine interferon gamma (primed) or with culture medium (unprimed). The VSG purified in its membrane-associated form was found to stimulate both primed and unprimed cells to secrete TNFalpha. The same preparation from which the dimyristylglycerol moiety had been cleaved was no longer able to stimulate unprimed cells but could still stimulate primed cells. Our data indicate that the presence of the dimyristylglycerol is not an absolute requirement for induction of TNFalpha production but can substitute for the interferon gamma priming. Therefore, we favor the hypothesis that stimulation of macrophages to secrete TNFalpha by the mfVSG is mediated by an as yet unknown trigger moiety and is facilitated by the dimyristylglycerol anchor.
The type I/type II cytokine balance may influence the development of different subsets of suppressive macrophages, i.e., classically activated macrophages (caM phi, type I) versus alternatively activated macrophages (aaM phi, type II). Recently, we showed that although mice infected with phospholipase C-deficient (PLC-/(-)) Trypanosoma brucei brucei exhibit a clear shift from type I to the type II cytokine production, wild type (WT)-infected mice remain locked in a type I cytokine response, In the present study, phenotype and accessory cell function of macrophages elicited during WT and PLC-/(-) T. b. brucei infection were compared, Results indicate that caM phi develop in a type T cytokine environment in the early phase of WT and PLC-/(-) trypanosome infection, correlating with inhibition of T cell activation triggered by a mitogen, a superantigen, or an antigen. In the late stage of infection, only PLC-/(-)-infected mice resisting the infection develop type TT cytokine-associated aaM phi correlating with impaired antigen- but not mitogen- or superantigen-induced T cell activation.
The TNF-alpha gene on mouse chromosome MMU17 is among the candidates for the trypanosomosis resistance QTL Tir1. Tir1 has the largest effect of those loci so far detected which influence degree of resistance to murine trypanosomosis caused by Trypanosoma congolense infection. We therefore studied the survival to 180 days after challenge with T. congolense of mice that were homozygous and hemizygous with respect to a disruption of the TNF-alpha gene on a > 99% C57BL/6 (resistant) background. We also examined the responses of TNF-alpha hemizygous mice produced by crossing the deletion line with mice of the C57BL/6J strain, and with mice of the susceptible A/J strain. Mice lacking a functional TNF-alpha gene were shown to be highly susceptible to challenge with T. congolense with a median survival time of 37 days. This was comparable to 71 days for control wild-type mice, and 61 and 111 days for mice of the susceptible A/J and resistant C57BL/6J strains, respectively. In mice of the deletion line, the C57BL/6 TNF-alpha allele tended to be dominant to the TNF knockout in terms of resistance. We conclude that TNF-alpha plays an important role in resistance to the effects of T. congolense infection in mice.
The involvement of intestinal damage in experimental African trypanosomiasis was investigated in rats infected with Trypanosoma brucei brucei by measuring the urinary excretion of the previously administered non-metabolizable sugar probes, D-mannitol and lactulose, and the flux of FITC-dextran across isolated, everted gut segments. There was increased urinary recovery and flux of the sugar probes across the intestine which were significant (P < 0.05) and maximum at day 21 of the infection, but subsequently reduced, in the terminal stages of infection (day 33 p.i.). In the case of the everted sac studies the reductions were to less than 25% control values (P < 0.001). Levels of circulating endotoxin were increased approximately 3-fold at day 21 p.i., 4-fold at day 33 p.i., compared to controls. At day 21 there was a significant correlation (r = 0.63, P < 0.01) between the log endotoxin levels and the increased sugar excretion expressed as the lactulose/mannitol ratios. Histological studies showed damage to the villi, wall thinning and marked cellular infiltrations, which were very prominent in the proximal jejunum and duodenum. These results demonstrate that during trypanosome infections in rats, increased intestinal leakage and increased circulating endotoxins are significant pathological features.
West African N'Dama cattle have developed a genetic capacity to survive, reproduce and remain productive under trypanosomosis risk. The cellular and molecular bases of this so-called trypanotolerance are not known, but the trait is manifested by the N'Dama's greater capacity to control parasitaemia and anaemia development during an infection. In order to examine the role of the haematopoietic system in trypanotolerance, we have exploited the tendency for the placentas of bovine twin embryos to fuse. Placental fusion in cattle results in bone marrow chimaerism in twins. By comparison with the N'Dama, cattle of the East African Boran breed are relatively susceptible. We evaluated the role of the haemopoietic system in trypanotolerance by comparing the performance of five Chimaeric Boran/N'Dama twin calves with that of singletons of the two breeds. Chimaeric Boran/N'Dama pairs of twins were produced in recipient Boran cows by embryo transfer, and the majority of haemopoietic cells in all twinned individuals were of Boran origin. Thus, N'Dama chimaeras differed from N'Dama singletons in that the bulk of their haemopoietic system was derived from their susceptible Boran twins, while Boran chimaeras differed little from Boran control animals. All cattle became parasitaemic and developed anaemia. The N'Dama chimaeras did not manage their anaemia and white blood cell counts effectively. However, they were able to limit parasitaemia development. These results suggest that trypanotolerance is the result of two mechanisms, one that improves parasite control and is independent of the genetic origin of the haemopoietic tissue, and another that is influenced by haemopoietic tissue genotype and which improves control over anaemia. The capacity to maintain growth during infection was similarly dependent on the genetic origin of the haemopoietic tissue.
Macrophage heterogeneity used to be a research topic on which the careers of many postdoctoral fellows were misspent. The lack of definitive markers and dubious biochemical assays prevented the unequivocal identification of specific cell subsets. There has now been significant progress in establishing the heterogeneity of activated macrophages. There are at least three distinct populations of macrophages, and each cell type appears to have different biological roles. The interplay among these populations of cells may help to shape not only the magnitude but also the character of the immune response. The manipulation of these cells may lead to new approaches to treat or prevent disease.
In this study, we demonstrate that Kupffer cells in the livers of highly susceptible BALB/c mice infected with Trypanosoma congolense were loaded with trypanosomal antigen and appeared highly activated. This was associated with an enlarged capillary bed in the livers and decreased blood pressure of these mice towards the terminal stage. Blocking of murine IL-10 receptor (IL-10R)in vivo shortened the survival time of highly susceptible T. congolense-infected BALB/c mice. Anti-IL-10R treatment decreased the survival of relatively resistant T. congolense-infected C57BL/6 mice dramatically. Blocking of the IL-10R also significantly shortened the survival time of mice infected with T. brucei. The acute death of trypanosome-infected mice treated with anti-IL-10R antibodies in vivo was associated with focal liver necrosis, with significantly increased plasma levels of IL-6, IL-10, IL-12p40 and IFN-gamma and enhanced synthesis of IL-6, IL-12p40 and IFN-gamma by spleen cell cultures. Anti-IL-10R-induced death of T. congolense-infected C57BL/6 mice could be prevented by administration of a neutralizing antibody specific forIFN-gamma. We conclude that phagocytosis of a critical number of trypanosomes by Kupffer cells leads to a systemic inflammatory response syndrome and, depending on the degree of Kupffer cell activation, is followed by death that is mediated by IFN-gamma. The role of trypanosome-pulsed macrophages, T cells and genetic influences is discussed in a synopsis.
Biological functions of selenium are exerted by selenoproteins that contain selenocysteine in their primary structure. Selenocysteine is synthesized and inserted into proteins cotranslationally by a complex process. Families of selenoproteins include the glutathione peroxidases, the iodothyronine deiodinases and the thioredoxin reductases. These are redox enzymes that take advantage of the chemical properties of selenium to catalyze, respectively, removal of hydroperoxides by glutathione, deiodination of thyroid hormones and support of cellular processes requiring reduction of disulfides. Approximately 10 additional selenoproteins have been identified. One of them, selenoprotein P, is an extracellular protein that contains most of the selenium in plasma. It associates with endothelial cells, probably through its heparin-binding properties. Selenoprotein P has been postulated to protect against oxidative injury and to transport selenium from the liver to peripheral tissues. Selenium-dependent protection against diquat-induced liver necrosis and lipid peroxidation in the rat correlates with the presence of selenoprotein P. Recent results support a transport function. When (75)SeO(3)(2-) was administered intravenously to rats, liver tissue took up (75)Se within minutes, associated with a rapid decline in plasma (75)Se. Brain tissue did not begin accumulating (75)Se until (75)Se-labeled selenoprotein P had begun appearing in the plasma after 30 min. These results suggest that tissues like liver can take up small-molecule forms of selenium whereas presence of the element in selenoprotein P facilitates uptake by tissues like brain. Thus, there is evidence for both antioxidant and selenium transport functions of selenoprotein P.
Depending on the cytokine environment, macrophages can differentiate into distinct subsets that perform specific immunological roles. In this regard, the functions of macrophages activated by interferon gamma, referred to as classically activated macrophages, have been extensively documented, particularly during immune responses to infection. Recently, it was recognized that macrophages exposed to cytokines generated by T helper cell type 2 (Th2) cells exert an alternative activation program. However, the nature and functions of alternatively activated macrophages are ill defined. Evidence for the presence of alternatively activated macrophages and their possible influence in the outcome of several parasite diseases are discussed here.
Plasticity and functional polarization are hallmarks of the mononuclear phagocyte system. Here we review emerging key properties of different forms of macrophage activation and polarization (M1, M2a, M2b, M2c), which represent extremes of a continuum. In particular, recent evidence suggests that differential modulation of the chemokine system integrates polarized macrophages in pathways of resistance to, or promotion of, microbial pathogens and tumors, or immunoregulation, tissue repair and remodeling.
Development of anaemia in inflammatory diseases is cytokine-mediated. Specifically, the levels of tumour necrosis factor-alpha (TNF-alpha), produced by activated macrophages, are correlated with severity of disease and anaemia in infections and chronic disease. In African trypanosomiasis, anaemia develops very early in infection around the time when parasites become detectable in the blood. Since the anaemia persists after the first waves of parasitaemia when low numbers of trypanosomes are circulating in the blood, it is generally assumed that anaemia is not directly induced by a parasite factor, but might be cytokine-mediated, as in other cases of anaemia accompanying inflammation. To clarify the role of TNF-alpha in the development of anaemia, blood parameters of wild type (TNF-alpha+/+), TNF-alpha-null (TNF-alpha-/-) and TNF-alpha-hemizygous (TNF-alpha-/+) trypanotolerant mice were compared during infections with the cattle parasite Trypanosoma congolense. No differences in PCV, erythrocyte numbers or haemoglobin were observed between TNF-alpha-deficient and wild type mice, suggesting that the decrease in erythrocytes was not mediated by TNF-alpha. Erythropoetin (EPO) levels increased during infection and no significant differences in EPO levels were observed between the three mouse strains. In contrast, during an infection with the human pathogen Trypanosoma brucei rhodesiense, the number of red blood cells in TNF-alpha-deficient mice remained significantly higher than in the wild type mice. These data suggest that more than one mechanism promotes the development of anaemia associated with trypanosomiasis.
In experimental murine trypanosomiasis, resistance is often scored as the capacity to control peak parasitemia levels, which results in prolonged survival. Infection-induced pathology has not systematically been used as a resistance criterion. Because this parameter could be the most relevant for comparative analysis of natural and experimental infections, as well as for understanding of pathology-associated immune alterations, we analyzed Trypanosoma brucei infections in 4 different established conventional mouse models, as well as in tumor necrosis factor (TNF)-deficient and TNF-receptor-deficient mice. Results indicate the following: (1) there is no correlation between peak parasitemia control or survival and the induction of infection-associated anemia, loss of body weight, liver pathology, reduced locomotor activity, and general morbidity; (2) serum levels of TNF, interferon- gamma, and interleukin-10, which are known to affect survival, do not correlate with induction of pathology; and (3) infection-induced occurrence of lipopolysaccharide hypersensitivity does not correlate with survival. However, one parameter that was found to correlate with the inhibition of trypanosomiasis-associated pathology in all models was the shedding of soluble p75 TNF-receptor during peak parasitemia stages. These results are important for future cytokine and trypanosomiasis pathology studies, because the interplay between TNF and the soluble receptors it sheds has not been considered in either human clinical sleeping sickness studies or in veterinary trypanosomiasis research.
African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.