[show abstract][hide abstract] ABSTRACT: How is it that individuals exposed to intense malaria transmission can tolerate the presence of malaria parasites in their blood at levels that would produce fever in others? In light of evidence discounting a role for nitric oxide or antibodies to plasmodial glycosylphosphatidylinositols in maintaining this tolerant state, refractoriness to toxin-induced Toll-like receptor-mediated signalling has emerged as a likely explanation that links malarial and bacterial endotoxin tolerance. Understanding the mechanisms underlying tolerance and the potential for cross-tolerization has significant implications for understanding the potential for antitoxic vaccine strategies, as well as interactions between different malaria species and between malaria and other human parasites.
Trends in Parasitology 09/2006; 22(8):371-7. · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cerebral malaria is associated with decreased production of nitric oxide and decreased levels of its precursor, l-arginine. Abnormal amino acid metabolism may thus be an important factor in malaria pathogenesis. We sought to determine if other amino acid abnormalities are associated with disease severity in falciparum malaria. Subjects were enrolled in Dar es Salaam, Tanzania (children) (n = 126), and Papua, Indonesia (adults) (n = 156), in two separate studies. Plasma samples were collected from subjects with WHO-defined cerebral malaria (children), all forms of severe malaria (adults), and uncomplicated malaria (children and adults). Healthy children and adults without fever or illness served as controls. Plasma amino acids were measured using reverse-phase high-performance liquid chromatography with fluorescence detection. Several plasma amino acids were significantly lower in the clinical malaria groups than in healthy controls. Despite the differences, phenylalanine was the only amino acid with mean levels outside the normal range (40 to 84 microM) and was markedly elevated in children with cerebral malaria (median [95% confidence interval], 163 [134 to 193] microM; P < 0.0001) and adults with all forms of severe malaria (median [95% confidence interval], 129 [111 to 155] microM; P < 0.0001). In adults who survived severe malaria, phenylalanine levels returned to normal, with clinical improvement (P = 0.0002). Maintenance of plasma phenylalanine homeostasis is disrupted in severe malaria, leading to significant hyperphenylalaninemia. This is likely a result of an acquired abnormality in the function of the liver enzyme phenylalanine hydroxylase. Determination of the mechanism of this abnormality may contribute to the understanding of neurological complications in malaria.
Infection and Immunity 06/2006; 74(6):3355-9. · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: The cytoadherence-linked asexual gene 9 (clag 9) of Plasmodium falciparum has been implicated in the cytoadherence of infected erythrocytes. To determine the immunogenicity of the clag 9 gene product (CLAG 9 protein) in humans, we measured antibody responses to 11 synthetic CLAG 9 peptides in a group of 177 asymptomatic children and adults subject to intense malaria exposure in Madang, Papua New Guinea. The CLAG 9 peptides were immunogenic in adults and children. Antibody responses to peptides 4 and 10 were high across all age groups and detectable in a majority of children less than five years of age. While CLAG 9 peptides are immunogenic in humans, longitudinal studies will be required to determine the longevity of antibody responses to CLAG 9 and their role in protection from disease.
The American journal of tropical medicine and hygiene 07/2005; 72(6):708-13. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: Glycosylphosphatidylinositols (GPIs) are found in the outer cell membranes of all eukaryotes. GPIs anchor a diverse range of proteins to the surface of Plasmodium falciparum, but may also exist free of protein attachment. In vitro and in vivo studies have established GPIs as likely candidate toxins in malaria, consistent with the prevailing paradigm that attributes induction of inflammatory cytokines, fever and other pathology to parasite toxins released when schizonts rupture. Although evolutionarily conserved, sufficient structural differences appear to exist that impart upon plasmodial GPIs the ability to activate second messengers in mammalian cells and elicit immune responses. In populations exposed to P. falciparum, the antibody response to purified GPIs is characterised by a predominance of immunoglobulin (Ig)G over IgM and an increase in the prevalence, level and persistence of responses with increasing age. It remains unclear, however, if these antibodies or other cellular responses to GPIs mediate anti-toxic immunity in humans; anti-toxic immunity may comprise either reduction in the severity of disease or maintenance of the malaria-tolerant state (i.e. persistent asymptomatic parasitaemia). P. falciparum GPIs are potentially amenable to specific therapeutic inhibition and vaccination; more needs to be known about their dual roles in malaria pathogenesis and protection for these strategies to succeed.
Current topics in microbiology and immunology 02/2005; 297:145-85. · 4.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: Individuals in areas of intense malaria transmission exhibit resistance (or tolerance) to levels of parasitemia in their blood that would normally be associated with febrile illness in malaria-naive subjects. The resulting level of parasitemia associated with illness (the pyrogenic threshold) is highest in childhood and lowest in adulthood. Clinical parallels between malarial and bacterial endotoxin tolerance have led to the supposition that both share common physiological processes, with nitric oxide (NO) proposed as a candidate mediator. The hypotheses that NO mediates tolerance and blood stage parasite killing in vivo were tested by determining its relationship to age and parasitemia cross-sectionally and longitudinally in a population of 195 children and adults from Papua New Guinea encountering intense malaria exposure. Despite pharmacological clearance of asymptomatic parasitemia, NO production and mononuclear cell NO synthase (NOS) activity were remarkably stable within individuals over time, were not influenced by parasitemia, and varied little with age. These results contrast with previous smaller cross-sectional studies. Baseline NO production and NOS activity did not protect against recurrent parasitemia, consistent with previous data suggesting that NO does not have antiparasitic effects against blood stage infection in vivo. The NO indices studied were markedly higher in specimens from study subjects than in samples from Australian controls, and NOS activity was significantly associated with plasma immunoglobulin E levels, consistent with induction of NO by chronic exposure to other infections and/or host genetic factors. These results suggest that NO is unlikely to mediate killing of blood stage parasites in this setting and is unlikely to be the primary mediator in the acquisition or maintenance of malarial tolerance.
Infection and Immunity 01/2005; 72(12):6932-8. · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: Polymorphisms in the inducible nitric oxide synthase gene (NOS2) promoter have been associated with clinical outcome from malaria. These include a CCTTT repeat (CCTTTn) 2.5 kilobases upstream from the NOS2 transcription start site, and two single nucleotide substitutions: G-->C at position -954 (G-954C), and C-->T at position -1173 (C-1173T). Although hypothesized to influence NO production in vivo, the functional relevance of (CCTTT)n and G-954C is uncertain because disease association studies have yielded inconsistent results. This study found no association between CCTTT repeat number and levels of plasma NO metabolites or peripheral blood mononuclear cell NOS activity in a cohort of asymptomatic malaria-exposed coastal Papua New Guineans 1-60 years old. This suggests that (CCTTT)n does not independently influence NOS2 transcription in vivo. Neither the G-954C nor the C-1173T polymorphisms were identified in this population, indicating the variability and complexity of selection for NOS2 promoter polymorphisms in different malaria-endemic populations.
The American journal of tropical medicine and hygiene 01/2004; 69(6):569-73. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: Interleukin-12 (IL-12) has been inversely associated with disease severity in human and murine malaria, and a polymorphism in the IL-12 p40 subunit gene (IL12B) has been associated with susceptibility to human cerebral malaria and reduced nitric oxide (NO) production. To better define the relationships between IL-12, NO, malaria parasitemia, and IL12B polymorphisms during malarial tolerance, plasma IL-12 levels and peripheral blood mononuclear cell NO synthase (NOS) activity were measured in asymptomatic Papua New Guineans exposed to intense malaria transmission. The IL-12 level was strongly inversely correlated with the density of Plasmodium falciparum parasitemia (rho = -0.45; P < 0.001) and was predicted to decrease by 19% (95% confidence interval [CI], 10 to 27%) for each twofold increase in P. falciparum parasitemia. This is consistent with a suppressive effect of parasitemia on IL-12 production, an effect previously shown in vitro and in rodent models of disease. The IL-12 level was inversely correlated with NOS activity (r = -0.22; P = 0.007), with each twofold increase in NOS activity being predictive of a 25% (95% CI, 7 to 38%) decrease in plasma IL-12 levels. This probably reflects additional down-regulation of IL-12 by the high basal NO production and monocyte NOS expression found in the malaria-tolerant state. Neither the IL-12 level nor NOS activity was associated with either of two IL12B polymorphisms, reflecting the diversity of genetic control over immune responses in different populations.
Infection and Immunity 11/2003; 71(11):6354-7. · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: Individuals living in regions of intense malaria transmission exhibit natural immunity that allows them to be without fever and other symptoms for most of the time despite frequent parasitization. Although this tolerance of parasitemia appears to be more effective in children than in adults (as evidenced by lower parasitemia fever thresholds with age), adults do exhibit a degree of tolerance but the mechanism(s) underlying this are unclear. Asymptomatic malaria-exposed children have higher levels of nitric oxide (NO) than children with severe disease, and NO has been proposed as a mediator of malarial tolerance. However, the ability of highly malaria-exposed asymptomatic adults to generate high-level basal NO is unknown, as is the relationship between NO and malaria tolerance in adults. The relationship between NO and malaria parasitemia was therefore determined in asymptomatic adults from Papua, Indonesia. Adults with Plasmodium falciparum parasitemia had markedly increased basal systemic NO production relative to aparasitemic Papuan controls, who in turn produced more NO than healthy controls from a region without malaria. Immunoglobulin E levels were universally elevated in malaria-exposed Papuan subjects, suggesting that the prevalence of intestinal parasitosis may be high and that nonmalarial infection may also contribute to high basal NO production. Basal peripheral blood mononuclear cell (PBMC) NO synthase activity was elevated in Papuans but poorly correlated with systemic NO production, suggesting that NO production in this setting arises not only from PBMCs but also from other tissue and cellular sources. NO production was associated with and may contribute to malaria tolerance in Papuan adults.
Infection and Immunity 08/2003; 71(7):3682-9. · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: The induction of neutralizing immunity to Plasmodium falciparum toxins by vaccination has been proposed as a preventive strategy to limit the severity of malaria. For this approach to be successful, generation of a sustained immune response would be necessary. This study shows that immunoglobulin G (IgG)-subclass responses elicited by the proposed P. falciparum toxin glycosylphosphatidylinositol (GPI) in Papua New Guinean subjects 5-60 years old predominantly involve IgG(3), with a lesser contribution from IgG(1) and an absence of IgG(2) and IgG(4). IgG(3) levels declined sharply within 6 weeks of pharmacological clearance of parasitemia in all subjects, whereas a significant decrease in IgG(1) levels was seen only in subjects < or =19 years old. Because the natural antibody response to P. falciparum GPIs is skewed toward the short-lived IgG(3) subclass, a vaccination strategy with GPI analogues would likely require augmentation by costimulatory molecules, to induce a more persistent anti-GPI response.
The Journal of Infectious Diseases 04/2003; 187(5):862-5. · 5.85 Impact Factor
[show abstract][hide abstract] ABSTRACT: Interleukin-12 (IL-12) is an important regulatory cytokine in infection and immunity. Administration of IL-12 may reduce complications of severe malaria in rodents. Polymorphisms in IL12B, the gene encoding the IL-12 p40 subunit, influence the secretion of IL-12 and susceptibility to Type 1 diabetes. We therefore investigated whether IL12B polymorphisms may affect the outcome of severe malaria. Homozygosity for a polymorphism in the IL12B promoter was associated with increased mortality in Tanzanian children having cerebral malaria but not in Kenyan children with severe malaria. Furthermore, homozygotes for the IL12B promotor polymorphism had decreased production of nitric oxide, which is in part regulated by IL-12 activity. These studies suggest that IL12B polymorphisms, via regulation of IL-12 production, may influence the outcome of malaria infection in at least one African population.
Genes and Immunity 12/2002; 3(7):414-8. · 3.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Individuals living in regions of intense malaria transmission exhibit natural immunity that facilitates persistence of parasitemia at controlled densities for much of the time without symptoms. This aspect of immunity has been referred to as malarial "tolerance" and is thought to partly involve inhibition of the chain of events initiated by a parasite toxin(s) that may otherwise result in cytokine release and symptoms such as fever. Antibodies to the candidate Plasmodium falciparum glycosylphosphatidylinositol (GPI) toxin have been viewed as likely mediators of such tolerance. In this study, the relationship between antibodies to P. falciparum GPIs, age, and parasitemia was determined in asymptomatic children and adults living in Madang, Papua New Guinea. The prevalence and intensity of antibody responses increased with age and were lowest in children 1 to 4 years old with the highest-density parasitemias. In children of this age group who were tolerant of parasitemia during the study, only 8.3% had detectable immunoglobulin G (IgG) and none had IgM antibodies to GPI. This suggests that anti-GPI antibodies are unlikely to be the sole mediator of malarial tolerance, especially in children younger than 5 years. Following antimalarial treatment, clearance of parasitemia led to a fall in anti-GPI IgG response in children and adolescents within 6 weeks. As anti-GPI antibodies potentially play a role in protecting against disease progression, our results caution against the treatment of asymptomatic parasitemia and suggest that generation of a sustained antibody response in children poses a challenge to novel antitoxic vaccination strategies.
Infection and Immunity 10/2002; 70(9):5052-7. · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nitric oxide (NO) and the related species nitrosothiols, have multiple important physiological and pathological roles in health
and disease that have been extensively reviewed elsewhere (1-6). In brief, NO is involved in modulating or mediating host resistance to tumors and microbes, regulation of blood pressure
and vascular tone, neurotransmission, learning, neurotoxicity, and control of cellular growth, and differentiation.