Viruses 2012, 4, 2766-2785; doi:10.3390/v4112766
Immune Responses and Lassa Virus Infection
Marion Russier 1, Delphine Pannetier 2 and Sylvain Baize 1,*
1 Unité de Biologie des Infections Virales Emergentes, Institut Pasteur, 21 avenue Tony Garnier,
69365 Lyon, France; E-Mail: email@example.com
2 Laboratoire P4 Jean Mérieux-Inserm, 21 avenue Tony Garnier, 69365 Lyon, France;
* Author to whom correspondence should be addressed; E-Mail: firstname.lastname@example.org;
Tel.: +33-4-3728-2440; Fax: +33-4-3728-2441.
Received: 30 September 2012; in revised form: 23 October 2012 / Accepted: 31 October 2012 /
Published: 5 November 2012
Abstract: Lassa fever is a hemorrhagic fever endemic to West Africa and caused by Lassa
virus, an Old World arenavirus. It may be fatal, but most patients recover from acute
disease and some experience asymptomatic infection. The immune mechanisms associated
with these different outcomes have not yet been fully elucidated, but considerable progress
has recently been made, through the use of in vitro human models and nonhuman primates,
the only relevant animal model that mimics the pathophysiology and immune responses
induced in patients. We discuss here the roles of the various components of the innate and
adaptive immune systems in Lassa virus infection and in the control of viral replication and
Keywords: Lassa virus; hemorrhagic fever; T-cell responses; antigen-presenting cells;
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Lassa fever (LF) is a viral hemorrhagic fever (VHF) caused by Lassa virus (LASV), an Old World
arenavirus . LASV is an enveloped virus with two single-stranded RNA segments. The large
segment encodes a small zinc-binding (Z) protein that plays a crucial role in the regulation of
transcription and replication [2–4] and in viral budding [5–7], and the RNA polymerase (L). The small
segment encodes the nucleoprotein (NP) and the two envelope glycoproteins (GP1 and GP2) mediating
cell entry after binding to α-dystroglycan [8,9]. The disease is endemic in West Africa, including
Nigeria, Liberia, Sierra Leone and Guinea, in particular, but LASV probably also circulates in
neighboring countries such as Mali, Ivory Coast, Ghana, or Burkina Faso [10,11]. The attack rate of
LF is difficult to be precisely quantified because of limited survey in endemic countries, similarity of
clinical signs with more common diseases and elevated incidence of asymptomatic LASV infection.
However, it is usually admitted that LASV may be responsible for about 300,000 cases and
5,000–6,000 deaths each year [12,13]. Humans become infected through contact with Mastomys
natalensis, a peridomestic rodent, which acts as the reservoir host . Large numbers of these rodents
live in the vicinity of, and even within, residences, and 80% of the rodent population is infected with
the virus. Contact between humans and infected reservoir animals is thus frequent in villages, and the
seroprevalence of humans living in endemic zones may be as high as 50%. The disease is then
transmitted between humans. Its severity ranges from asymptomatic infection to fatal HF, and
nosocomial outbreaks are frequently observed . Nonspecific signs, such as fever, headache,
arthralgia, myalgia and severe asthenia, are observed six to 12 days after infectious contact.
Pharyngitis, conjunctivitis, cough, abdominal pain, diarrhea and vomiting appear in the next few days.
Cervical and facial edema, hemorrhages, renal and liver failures and, in some of the more severe cases,
encephalopathy may occur. Death follows hypotensive, hypovolemic and hypoxic shock in severely
affected patients, whereas the symptoms disappear 10 to 15 days after disease onset in surviving
patients . LF is a major public health and economic problem in the regions in which it is endemic,
not only due to the limited health structures of these regions and the isolation of the affected
populations, but mostly due to the high morbidity and disabling aftereffects, such as deafness, which
occurs in one-third of all survivors and may persist throughout life . There is currently no licensed
vaccine against LF, and the only treatment available is based on ribavirin . However, this molecule
is not readily available in the countries in which LF is endemic, due to its high cost, and it must be
administered early if it is to be effective. This drug is therefore far from satisfactory as a treatment
The observation that most patients successfully control LASV infection and recover, and that many
cases of asymptomatic LASV infection do occur, suggests that LF can induce effective immunity.
By contrast, severe LASV infection seems to be associated with defective immune responses and even
immunosuppression [15,18]. The pathogenesis of LF and the immune responses occurring during the
disease have yet to be fully elucidated. This limited knowledge results principally from the remote
location of the areas in which LF is endemic and the high level of infectivity of LV, both of which
have hampered investigations of LF in humans. In addition, there is no rodent model other than strain
13 guinea pigs, which can be infected but do not reproduce the pathophysiology or immune responses
observed in humans . Thus, nonhuman primates (NHP) represent currently the most relevant
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model for LF, but investigations in these animals are limited by the need to manipulate them in BSL4
facilities. Despite these problems, substantial advances have recently been made through studies in
NHP and human in vitro models and the use of reverse genetic tools. We review these data here,
providing an overview of current knowledge concerning the immune responses associated with
2. Pathogenesis of Lassa Fever
Antigen-presenting cells (APC)—dendritic cells (DC) and macrophages (MP)—are probably the
first cells targeted by LASV [20,21]. The widespread distribution of these cells in the mucosal tissues
and skin, results in their early infection, allowing the first replicative cycles to occur. APC are
probably also responsible for spreading the virus and establishing systemic infection, due to their
mobility and their presence in many organs and tissues . Massive viral release then occurs in the
secondary lymphoid organs and liver, and hepatocytes, fibroblasts, endothelial cells and some
epithelial cells become targets for viral replication. However, changes in the endothelium and other
organs do not seem to be severe enough to account for terminal shock and death, which seem instead
to be linked to the host response. The most frequent microscopic alterations reported in patients and in
NHP are multifocal hepatocellular necrosis with weak inflammatory cell involvement, adrenal cortical
cell necrosis, substantial infiltration with mononuclear cells, mostly MP, in most organs, interstitial
pneumonitis, acute myocarditis and damage to reticuloendothelial tissues [23–27]. Lymphadenopathy
and splenomegaly are observed, and changes to lymphoid organs include the disruption of follicular
architecture and the depletion of cells from the bone marrow, spleen and lymph nodes.
Transient lymphopenia affecting CD4+ and CD8+ T cells, NK cells and B cells is observed early in
the disease, followed two weeks after disease onset by leukocytosis, principally involving neutrophils
[18,22,25,26,28]. Moderate and transient thrombocytopenia is also a feature of LF, accompanied by a
progressive depression of platelet function [18,25,28,29]. However, no important change in
coagulation occurs, and disseminated intravascular coagulation is never observed during LF.
Together, these changes are not severe enough to account for the hemorrhagic signs and plasma
leakage observed in LF, which seem instead to be mostly due to an increase in endothelial
permeability, probably induced by host factors. Plasma AST and ALT levels increase strongly in the
terminal stages of the disease in both humans and NHP, whereas they increase in a transient and
moderate manner in nonfatal cases of LF [17,22,25,26,30–33]. These events may reflect hepatic
abnormalities. However, the high AST/ALT ratio suggests that the source of AST may be an organ
other than the liver. Similarly, high concentrations of IL-6 have been detected in plasma during fatal
LASV infections in NHP [22,25]. IL-6 production may be associated with hepatic regeneration during
LF, a phenomenon described in NHP and humans [26,34], but it may also result from tissue damage in
other organs and muscles. IL-6 may be involved in neutrophilia [35,36], and the simultaneous
increases in AST levels and the number of circulating neutrophils in NHP suggest that tissue damage
may result, at least in part, from neutrophil infiltration.
Severe LF is not the result of a single organ failure. Instead, it is associated with multiple-organ
dysfunction, with death ultimately occurring in a context of hypoxic, hypovolemic and hypotensive
Viruses 2012, 4
shock. However, further investigations are required to elucidate more fully the pathogenic mechanisms
ultimately leading to catastrophic illness and fatal infection.
3. Antigen-Presenting Cells Play a Key Role in Lassa Fever
Antigen-presenting cells (DC and MP) play a crucial role in the induction and regulation of immune
responses. On the one hand, they are the key actors in innate immunity, mediating the induction of
inflammatory responses and the direct control of viral infections. On the other hand, the ability of these
cells to present antigens (Ag), enables DC and MP to initiate and to orchestrate adaptive humoral and
cellular immune responses [37,38]. DC and MP are the primary target for LASV replication. APC are
initially the site of early replication in the periphery, and, following the infection of most of these cells
and the relentless replication occurring in the lymphoid organs, these cells become the primary
reservoir for the systemic dissemination of LASV [22,25,29,31]. However, this dual role of APC is not
without consequences for the immune responses induced during LF.
3.1. LASV Infection of APC
The privileged tropism of LASV for DC and MP has been demonstrated both in vitro in human cells
and in vivo in NHP. Human DC and MP are highly permissive to LASV infection, leading to the
release of large numbers of viral particles, particularly in DC, with no effect on cell viability [1,26,39].
LASV has recently been reported to bind plasmacytoid DC (pDC), suggesting that these cells are
probably a viral target in vivo, as reported for lymphocytic choriomeningitis virus (LCMV) .
In NHP, most of the DC and MP are infected by day 7 in all lymph nodes, the splenic marginal zone
and, to a lesser extent, in the red pulp, thymus and liver (Kupffer cells) , and the viral load in the
lymphoid organs and liver remains high throughout the disease [22,25,30,31]. LASV infection induces
no change in the viability of APC, resulting, at least in vitro, in the sustained release of large numbers
of viral particles . Mopeia virus (MOPV) is an Old World arenavirus closely related to LASV.
Its amino acid sequences are about 75% identical to those of LASV and it shares the same rodent host,
but is nonpathogenic in humans and NHP, and can even induce cross-reactivity and immune protection
against subsequent LASV challenge [23,41–43]. This virus is thus studied in comparison with LASV,
as a model of nonfatal LF or asymptomatic LASV infection. Like LASV, MOPV can infect DC and
MP, resulting in the release of large numbers of viral particles with no apparent cytotoxicity [39,44].
These observations suggest that this privileged tropism of LASV for APC is not correlated with its
high level of pathogenicity, being instead a common feature of arenavirus infections.
3.2. Responses of APC to LASV Infection and Correlation with Pathogenicity
APC are not activated by LASV infection. Despite the massive release of viral particles,
LASV-infected DC do not produce inflammatory cytokines or express activation molecules at their
surface [20,21,45]. Moreover, LASV infection does not lead to DC maturation . This lack of DC
activation and maturation in response to LASV infection may be associated with the
immunosuppression observed in severe infection. Indeed, Ag presentation by immature DC is known
to result in tolerance and defective immunity [46,47], and proinflammatory cytokines are crucial for
Viruses 2012, 4
the induction of adaptive immunity . Furthermore, this lack of DC activation probably favors
LASV replication, as immature DC produce significantly more LASV particles than mature DC .
Similarly, no significant activation is observed after the infection of MP with LASV, other than the
production of small amounts of type I IFN [20,39,45]. This absence of MP activation may also favor
viral spread, as MP activation is known to increase the microbicidal activity of these cells . Data
obtained in vivo in patients and in NHP have confirmed the absence of massive inflammatory
responses during LF, by demonstrating a lack of substantial proinflammatory cytokine production
despite the massive infiltration of most tissues and organs with MP and neutrophils (Table 1)
[22,25,26,30,32,50]. By contrast, the MOPV infection of MP leads to cell activation, with an
upregulation of surface molecules, such as CD86, CD80 and CD54, and the production of considerable
amounts of IFNβ, α and λ [44,45]. However, MOPV infection does not result in the release of
substantial amounts of inflammatory cytokines by MP [39,44,45]. Similarly, but to a lesser extent,
MOPV infection induces a modest activation of DC, with the synthesis of mRNA encoding type I IFN
[44,45]. Such a correlation between a lack of pathogenicity and an ability to activate MP has also been
observed with variants of the New-World Pichinde arenavirus . These observations suggest that, in
arenaviruses, a lack of pathogenicity would be associated with APC activation. The type I IFN
response seems to be a key element in the difference in pathogenicity between LASV and MOPV.
Indeed, LASV is a poor inducer of type I IFN. Both Lassa and Mopeia viruses are equally sensitive to
the antiviral properties of type I IFN, but only MOPV-infected APC can produce these cytokines
[44,45,52]. The lack of type I IFN production by LASV-infected APC is probably a key element in the
pathogenesis and immunosuppression observed in severe disease, as these cytokines are involved not
only in the initial control of viral spread, but also in the induction of adaptive immunity .
Consistently, the early release of type I IFN has been observed in cynomolgus monkeys surviving
severe LASV infection, whereas the production of this cytokine was detected only at terminal stages in
monkeys that died . In these monkeys, the high levels of IFNα circulating in the bloodstream in
the last few days before death are probably unable to control the massive viral replication observed at
this point in the course of the disease and may, instead, contribute to pathogenesis. Indeed, type I IFN
are known to have both beneficial and detrimental effects. Type I IFN have been implicated in the
transient lymphopenia and structural changes to lymphoid organs [54,55] observed during the LCMV
infection of mice, and they play a crucial role in reducing platelet counts and in platelet dysfunction
. Additional experiments in NHP will be required to determine the sensitivity of LASV to type I
IFN in vivo, to clarify their role in the pathogenesis of LF and to determine whether and why the innate
responses induced differ as a function of outcome. Moreover, it would be important to evaluate the
involvement of IFN response in the outcome of LASV infection in its natural rodent host.
3.3. Inhibition of Innate Immunity by LASV NP
The arenavirus NP has recently been implicated in the defective production of type I IFN in
response to LASV infection. Indeed, the NP of most arenaviruses, with the exception of the Tacaribe
virus, inhibit IRF3 activation and nuclear translocation and type I IFN production [57,58].
These properties are dependent on the presence, in the C-terminal part of the protein, of a dsRNA-
specific 3’ to 5’ exonuclease related to the enzymes of the DEDDh family [59,60]. By digesting
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dsRNA, NP prevents its sensing by RIG-I and MDA-5 helicases, which have been shown to recognize
arenavirus RNA [61,62], and subsequent activation of the type I IFN response . The amino-acid
residues required for this activity have been identified, and mutations of the corresponding nucleotides
abolish the anti-IFN activity of NP . However, the difference in pathogenicity between LASV and
MOPV cannot be due to this property alone, as the MOPV NP also contains the DEDDh motif 
and probably retains some ability to inhibit IFN, albeit less strongly than the LASV NP. This is
suggested by the observation that recombinant LASV containing mutations in this region induce type I
IFN much more strongly than MOPV .
The arenavirus NP also seems to have another string to its bow, as it inhibits IRF3 activation, as
demonstrated by the recent description of its binding to the IκB kinase-related kinase IKKε, preventing
autocatalytic activity and IRF3 phosphorylation . The ability of NP to sequester IKKε in an
inactive form, preventing the induction of innate immunity, is probably important for the virulence of
LASV. It remains unclear whether the MOPV NP also has this ability. Finally, the arenavirus NP has
also recently been reported to prevent the nuclear translocation and transcriptional activity of
NFκB , consistent with the lack of APC activation and inflammatory cytokine production observed
after LASV infection. Thus, like many other viruses, arenaviruses have developed efficient strategies
for preventing the induction of innate immunity. Furthermore, as the NP of all tested arenaviruses are
able to inhibit type I IFN response and because the only exception is the NP of Tacaribe virus, which is
not a rodent-borne virus , it would be interesting to determine the consequences of this inhibition
in the interaction between LASV and its natural rodent host.
4. Humoral Responses During Lassa Fever
Despite the strong antibody (Ab) responses observed in most LASV-infected patients and in NHP,
no evidence of a direct role for humoral responses in the control of LF has been reported.
LASV-specific IgM and IgG are rapidly produced in large amounts in patients and NHP, regardless of
outcome, and are therefore not correlated with survival or with the disappearance of viremia
[23,25,28,33]. The IgG has broad specificity, with Ab directed against at least NP, GP1, GP2, and Z
[25,68]. However, LASV infection does not result in the induction of significant amounts of
neutralizing Ab (nAb), such antibodies being detected only after recovery and, even then, only in very
small amounts [25,69]. This observation contrasts with the strong induction of nAb described with
New World arenaviruses and the efficiency of passive transfer reported for these viruses . The lack
of nAb induction during LASV infection seems to be intrinsic to GP and independent of the
immunogenicity of the viral backbone . Passive transfer experiments in humans and NHP infected
with LASV have generated conflicting results and the efficacy of such treatment is likely to depend on
the early administration of Ab with a high neutralization index [72–74]. In addition, immunization
studies in NHP have shown that the induction of a humoral response is not sufficient to protect against
a lethal LASV challenge [75,76]. Thus, although Ab seem to be unable to mediate the control of
LASV replication directly, the humoral response may play an indirect role in protection.
Further studies are required to determine the role of Ab during the course of the disease.
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5. NK-Cell Responses During LASV Infection
NK cells were initially described as lymphocytes from the innate immune compartment, with
cytolytic activity against tumor cells. They are also located at the crossroads between the innate and
adaptive immune responses, because they secrete cytokines, such as IFN-γ, which plays an important
role in regulating T-cell differentiation and functions. NK cell activation is controlled by a balance
between inhibitory and activating signals from target cells . The dialog between NK cells and
accessory cells, such as APC, potentiates NK cell functions and the overall immune response. NK cells
are involved in various viral infections. Recent studies have shown that NK cells are activated by
LASV-infected MP but that they neither secrete IFN-γ nor kill the infected cells and control the
infection. Similar observations have been reported for LCMV infection and for the stimulation of NK
cells by MOPV-infected MP. NK cell responses during LASV infection in humans have been little
studied, whereas these responses have been extensively analyzed during LCMV infections in mice
[78,79]. Thus, studies of LCMV infection can provide tools to help us understand the NK
cell-mediated mechanisms involved in the immune responses triggered during LASV infection.
It has recently been shown that, following their stimulation with LASV-infected MP, NK cells
acquire an enhanced cytolytic potential in vitro, with increases in the expression of activating receptor
NKp30 and granzyme B and the killing of K562 cells lacking MHC-I . Moreover, the increase in
TRAIL mRNA synthesis in NK/MP cocultures has been shown to be correlated with the increase in
the cytolytic capacity of NK cells. NK cell-mediated cytotoxicity requires type I IFN during LCMV
infection . Similarly, it has been shown that type I IFN, which are produced by LASV-infected
MP, are responsible for NK cell activation and the modulation of NKp30 expression, even at low
levels . However, infected APC remain resistant to NK cell-mediated lysis. NK cells do not control
LCMV infection despite high levels of NK cell cytotoxicity , whereas cytolytic activity seems to
play an important role in controlling Pichinde virus infection in mice . LASV-infected APC
express constant HLA class I molecules, which bind to inhibitory KIRs, and the expression of the
activating NKG2D ligands, MIC-A/B, is also unaffected [20,80]. Negative signals, at least those
provided by MHC class I, probably account for the inhibition of NK cell cytotoxicity to
LASV-infected cells. This mechanism strongly resembles a game of viral “hide and seek,” rendering
the infected cells insensitive to NK cell-mediated lysis.
IFN-γ is a key cytokine in the initiation and regulation of adaptive immune responses. It is directly
involved in controlling the replication of many viruses. However, we and others have clearly
demonstrated that IFN-γ is not induced during LASV infection. It is not detected in LASV-infected
patients and NHP and its mRNA is produced, but not translated, in NK cells in vitro, following
stimulation with LASV-infected APCs [25,80]. Moreover, IFN-γ does not control LASV replication in
APCs and other cells [45,52]. No IFN-γ is induced after the in vitro MOPV infection of APCs either
and this seems to be a common feature of LCMV infection in immunocompetent mice [83,84].
Some studies have focused on the reasons for this absence of IFN-γ secretion during LASV infection.
IL-12, secreted by APC, is a well known inducer of T cell- and NK cell-mediated IFN-γ production.
Several studies have shown that IL-12 is induced neither in vitro, following the infection of human
APCs with LASV or MOPV, nor during the infection of mice with LCMV . These observations
suggest that the absence of NK cell-mediated IFN-γ production during LASV infection may be partly
Viruses 2012, 4
due to the lack of IL-12 secretion by LASV-infected APCs. Moreover, it has been shown that high
levels of type I IFN inhibit IL-12 secretion by accessory cells and, thus, IL-12-mediated IFN-γ
production by NK cells . This may occur despite the very low levels detected during
A transient depletion of circulating NK cells and of other lymphocyte populations has been
observed in LASV-infected NHP , suggesting that NK cells can be recruited to other tissues or
depleted. This transient lymphopenia in blood is also observed during the LCMV infection of
macaques , and can be accounted for by marginalization in the periphery and cell death.
For example, NK cells are recruited to the liver during LCMV infection in mice [87,88]. NK cells
express certain chemokine receptors, such as CXCR3, and they can migrate in response to chemotactic
signals. CXCR3 is the receptor of CXCL9 (Mig), CXCL10 (IP-10) and CXCL11 (I-TAC), which are
expressed in large numbers on LASV-infected NHP  and in vitro in humans (Pannetier et al., in
preparation) . CXCR3 is upregulated at the surface of NK cells, where it directly senses LASV via
PRRs, whereas it is downregulated when NK cells are stimulated by LASV-infected MP. We propose
a model according to which CXCR3 ligands secreted by LASV-infected APC in blood vessels induce
rapid desensitization and the internalization of CXCR3 at the cell surface of NK cells, resulting in the
relocalization of these cells to peripheral organs. NK cells may reach secondary lymphoid organs,
where they elicit immune responses, and the liver, where they mediate or regulate the hepatic
inflammation occurring during LF.
NK cells have been shown to proliferate moderately in response to LASV-infected MP.
The importance of NK cell proliferation during viral infections in vivo remains unknown. It has been
suggested that the increase in NK cell populations participates in the development of NK cell memory
. The role of NK cells in controlling most viral infections in humans remains a matter of debate, as
humans lacking functional NK cell responses do not seem to be particularly susceptibility to viral
infections [78,79]. NK cells do not appear to be crucial determinants during LASV infection, as in
LCMV infections. However, this requires confirmation in vivo, in infected patients or NHP. Infected
APCs express MHC class I molecules and, via Ag processing, are susceptible to cytotoxic T
cell-mediated lysis but not NK cell killing. It has recently been shown that NK cells can downregulate
T cell-mediated immunity in LCMV infections of mice [90,91]. There is no evidence for the NK cell-
mediated lysis of LASV-specific cytotoxic T cells, but such a mechanism may occur during LASV
infection, with NK cells being responsible for the immunopathogenesis occurring during LF.
6. T-Cell Responses and the Control of LASV Infection
Both in vitro studies in human models and NHP experiments have suggested that T cells play a
crucial role in the outcome of LF. Severe LASV infection seems to be associated with defective T-cell
responses, whereas the effective control of LF seems to be mediated by robust and efficient responses.
6.1. Defective T-Cell Immunity and Fatal Lassa Fever
Severe LF seems to be associated with defective T-cell responses. In NHP models, a general
depression of T-cell responses to several mitogens has been observed in animals with fatal infections
. In addition to the transient lymphopenia occurring during acute disease, lymphoid depletion has
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also been described in the spleen and lymph nodes of NHP and humans with severe LF, together with
the destruction of secondary lymphoid organ architecture [24,26]. Fatal LASV infection of
cynomolgus monkeys has been associated with a lack of T-cell activation in peripheral blood and a
lack of T cell-derived cytokines . Similar results were reported in a study evaluating a vaccine
candidate in NHP. In non immunized animals, no T-cell response was observed after a lethal challenge
with LASV . Similarly, LASV-infected human DC fail to activate CD4+ and CD8+ T cells in an in
vitro model of the induction of the primary LASV-specific T-cell response  and in a mixed
lymphocyte reaction assay . It remains unclear whether the lack of induction of a T-cell response
results from an active suppression of DC immunogenicity, the absence of DC activation/maturation
after LASV infection or changes in the structure of lymphoid organs. Indeed, LASV-infected DC that
have been matured with TNFα and IL-1β remain unable to induce efficient T-cell responses in vitro
. In addition, there may be immunopathogenic events linked to T-cell responses. Indeed, it was
recently suggested that T cells may be involved in deleterious innate inflammatory reactions and
pathogenesis in mice expressing humanized MHC class I . In this model, interactions between
infected monocytes/MP and T cells could lead to the overstimulation of MP and an exacerbation of
inflammatory responses, resulting in disruption of the splenic white and red pulp compartments, a loss
of the marginal zone MP layer, and severe hepatic and pulmonary damage. These data are consistent
with the well-known role of T cells in the pathogenesis of the closely related LCMV [95,96].
However, further investigations are required in more relevant animal models, such as NHP, to confirm
that these events are likely occur during severe LF in humans.
6.2. The Control of LASV Infection Is Associated with the Induction of T-Cell Responses
There is increasing evidence to suggest that T-cell responses play a crucial role in the control of
LASV infection. Indeed, strong memory CD4+ T-cell responses directed against LASV NP and GP
have been detected in LASV-seropositive healthy individuals living in zones in which LF is endemic,
suggesting that mild and/or asymptomatic infections are associated with the activation of CD4+ T cells
[97,98]. Moreover, nonfatal LASV infection in humans is associated with high serum concentrations
of IL-8 and CXCL-10, two chemokines involved in the attraction and activation of T cells [99,100],
whereas the concentrations of these mediators remain low in fatal cases . In cynomolgus monkeys,
the control of acute LF has been correlated with the circulation of activated CD4+ and CD8+ T cells six
to nine days after LASV infection . In these animals, a large increase in the total number of
circulating T lymphocytes has also been observed, from nine days after infection. In addition, survival
has been correlated with the ability of PBMC to proliferate in vitro in response to LASV Ag .
Further evidence has been provided by vaccine studies in NHP, in which protection against a lethal
LASV challenge has been shown to be associated with the induction of T-cell immunity [75,92]. It has
recently been shown that MOPV-infected DC induce strong and efficient specific T cells in an in vitro
human model of the induction of primary T-cell responses, whereas LASV-infected DC induce only
delayed and weak responses devoid of effective function . The CD4+ and CD8+ T cells stimulated
by MOPV-infected DC proliferate strongly, acquire activation and memory phenotypes and
differentiate into cytotoxic T cells able to control viral infection in DC. In this model, the different
T-cell responses probably result from differences in the activation of infected DC. Indeed, by contrast
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to the results obtained for LASV-infected DC, the coculture of MOPV-infected DC and T cells is
accompanied by the strong release of type I IFN, IL-12 and CXCL-10, probably favoring T-cell
activation [101–103]. Consistently, T cells cluster massively around MOPV-infected DC .
The more robust synthesis of type I IFN and IL-12 by MOPV-infected DC in the presence of T cells
 than by infected DC alone  suggests cross-talk between the two populations leading to
reciprocal activation and the differentiation/maturation of DC and T cells. MOPV is a nonpathogenic
virus closely related to LASV that can even immunize NHP against LF. This virus is used to model
non fatal LF. Thus, the T-cell responses induced by MOPV-infected DC may de facto reflect the
immune responses induced in patients surviving acute LF and/or in individuals experiencing
asymptomatic LASV infection. The main viral Ag recognized by activated T cells are probably NP and
GP, as suggested by studies in humans [97,98], vaccine assays in NHP [75,76,92,104], and the
prediction of potential epitopes [105–107]. However, the mechanisms leading to the induction of
innate immunity followed by early and effective T-cell responses in survivors or to defective immune
responses and fatal outcome during the course of LF remain unclear and should be investigated further.
There are several possible hypotheses, including differences in inoculum size , different routes of
infection [96,108], different cell populations targeted early in infection , different genetic
backgrounds (MHC) and preexisting homologous or heterologous immunity . These results
indicate that T cells are essential for the control of LF and that a vaccine able to induce T cells specific
for LASV GP, and possibly for NP, would probably be effective.
Table 1. Immunological features of Lassa fever in nonhuman primates as a function of outcome
High number of CD80+ circulating
Early and transient release of IFNα
Inflammatory cytokines: not detected
CXCL-10 and 11 mRNA
MCP-1 ?, eotaxin ?
Low number of CD80+ circulating
Late release of IFNα
Inflammatory cytokines: ND, except
for IL-6 (late)
CXCL-10 and 11 mRNA
Antibodies High levels of IgM/IgG
High levels of IgM/IgG
Transient depletion from the
T-cell responses T cell-derived cytokines: ND
Transient lymphopenia followed by
Early and robust activation of CD4+
and CD8+ T cells
In vitro proliferation of T cells in
response to LASV
T cell-derived cytokines: ND
Weak and late activation of CD4+ and
CD8+ T cells
No proliferation in response to LASV
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The acquisition of knowledge about the immune responses induced during severe LF or involved in
the control of acute infection has long been hampered by the need to handle LASV in BSL4 facilities,
the remote location of the zones in which LF is endemic and the lack of access to patients and a
relevant rodent model for studying the disease. Recently, both immunological investigation in NHP
models and in vitro studies in human immune cells have led to major advances in our understanding of
the complex interactions of LASV with the innate immune system and the responses involved in
controlling viral replication. These results have provided important clues to the pathogenesis of severe
disease and have highlighted the essential role of T cells in the control of LF (Figure 1), opening up
new possibilities for the treatment and prophylaxis of this disease. However, the complete sequence of
events leading to catastrophic illness and death and the mechanisms responsible for the control of acute
infection in patients and for the ability of individuals to eliminate LASV before symptoms appear
remain unclear and require further investigation.
Viruses 2012, 4
Figure 1. Comparison of the responses induced in vitro by LASV (a) and MOPV (b) in
human cells (adapted from references [20,21,39,44,45,80,93])
Viruses 2012, 4
M. Russier held a fellowship from the Délégation Générale pour l’Armement (G. Vergnaud, the
Conflict of Interest
The authors declare that they have no conflict of interest.
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