High frequency of malaria-specific T cells in non-exposed humans

Plasmodium vivax Pv12 B-cell epitopes and HLA-DRβ1*-dependent T-cell epitopes in vitro antigenicity

Malaria is an infectious disease caused by parasites from the genus Plasmodium (P. falciparum and P. vivax are responsible for 90% of all clinical cases); it is widely distributed throughout the world’s tropical and subtropical regions. The P. vivax Pv12 protein is involved in invasion, is expressed on merozoite surface and has been recognised by antibodies from individuals exposed to the disease. In this study, B- and T-cell epitopes from Pv12 were predicted and characterised to advance in the design of a peptide-based vaccine against malaria.

For evaluating the humoral response of individuals exposed to natural P. vivax infection from two endemic areas in Colombia, BepiPred-1.0 software was used for selecting B-cell epitopes. B-cell epitope 39038 displayed the greatest recognition by naturally-acquired antibodies and induced an IgG2/IgG4 response. NetMHCIIpan-3.1 prediction software was used for selecting peptides having high affinity binding for HLA-DRβ1* allele lineages and this was confirmed by in-vitro binding assays. T-epitopes 39113 and 39117 triggered a memory T-cell response (Stimulation Index≥2) and significant cytokine production.

Combining in-silico, in-vitro and functional assays, two Pv12 protein regions (containing peptides 39038, 39040, 39113 and 39117) have thus been characterised as promising vaccine candidates against P. vivax malaria.

Modulation of immune responses by Plasmodium falciparum infection in asymptomatic children living in the endemic region of Mbita, western Kenya

Individuals living in malaria endemic areas become clinically immune after multiple re-infections over time and remain infected without apparent symptoms. However, it is unclear why a long period is required to gain clinical immunity to malaria, and how such immunity is maintained. Although malaria infection is reported to induce inhibition of immune responses, studies on asymptomatic individuals living in endemic regions of malaria are relatively scarce. We conducted a cross-sectional study of immune responses in asymptomatic school children aged 4-16years living in an area where Plasmodium falciparum and Schistosoma mansoni infections are co-endemic in Kenya. Peripheral blood mononuclear cells were subjected to flow cytometric analysis and cultured to determine proliferative responses and cytokine production. The proportions of cellular subsets in children positive for P. falciparum infection at the level of microscopy were comparable to the negative children, except for a reduction in central memory-phenotype CD8⁺ T cells and natural killer cells. In functional studies, the production of cytokines by peripheral blood mononuclear cells in response to P. falciparum crude antigens exhibited strong heterogeneity among children. In addition, production of IL-2 in response to anti-CD3 and anti-CD28 monoclonal antibodies was significantly reduced in P. falciparum-positive children as compared to -negative children, suggesting a state of unresponsiveness. These data suggest that the quality of T cell immune responses is heterogeneous among asymptomatic children living in the endemic region of P. falciparum, and that the responses are generally suppressed by active infection with Plasmodium parasites.

Atypical activation of dendritic cells by Plasmodium falciparum

Dendritic cells (DCs) are activated by pathogens to initiate and shape immune responses. We found that the activation of DCs by Plasmodium falciparum, the main causative agent of human malaria, induces a highly unusual phenotype by which DCs up-regulate costimulatory molecules and secretion of chemokines, but not of cytokines typical of inflammatory responses (IL-1β, IL-6, IL-10, TNF). Similar results were obtained with DCs obtained from malaria-naïve US donors and malaria-experienced donors from Mali. Contact-dependent cross-talk between the main DC subsets, plasmacytoid and myeloid DCs (mDCs) was necessary for increased chemokine and IFN-α secretion in response to the parasite. Despite the absence of inflammatory cytokine secretion, mDCs incubated with P. falciparum-infected erythrocytes activated antigen-specific naïve CD4⁺ T cells to proliferate and secrete Th1-like cytokines. This unexpected response of human mDCs to P. falciparum exhibited a transcriptional program distinct from a classical LPS response, pointing to unique P. falciparum-induced activation pathways that may explain the uncharacteristic immune response to malaria.

A Unique Subset of γδ T Cells Expands and Produces IL-10 in Patients with Naturally Acquired Immunity against Falciparum Malaria

Although expansions in γδ T cell populations are known to occur in the peripheral blood of patients infected with Plasmodium falciparum, the role of these cells in people with naturally acquired immunity against P. falciparum who live in malaria-endemic areas is poorly understood. We used a cross-sectional survey to investigate the role of peripheral blood γδ T cells in people living in Lao People's Democratic Republic, a malaria-endemic area. We found that the proportion of non-Vγ9 γδ T cells was higher in non-hospitalized uncomplicated falciparum malaria patients (UMPs) from this region. Notably, we found that the non-Vγ9 γδ T cells in the peripheral blood of UMPs and negative controls from this region had the potential to expand and produce IL-10 and interferon-γ when cultured in the presence of IL-2 and/or crude P. falciparum antigens for 10 days. Furthermore, these cells were associated with plasma interleukin 10 (IL-10), which was elevated in UMPs. This is the first report demonstrating that, in UMPs living in a malaria-endemic area, a γδ T cell subset, the non-Vγ9 γδT cells, expands and produces IL-10. These results contribute to understanding of the mechanisms of naturally acquired immunity against P. falciparum.

Safety and comparability of controlled human Plasmodium falciparum infection by mosquito bite in malaria-naïve subjects at a new facility for sporozoite challenge

Background

Controlled human malaria infection (CHMI) studies which recapitulate mosquito-borne infection are a critical tool to identify protective vaccine and drug candidates for advancement to field trials. In partnership with the Walter Reed Army Institute of Research, the CHMI model was established at the Seattle Biomedical Research Institute's Malaria Clinical Trials Center (MCTC). Activities and reagents at both centers were aligned to ensure comparability and continued safety of the model. To demonstrate successful implementation, CHMI was performed in six healthy malaria-naïve volunteers.

Methods

All volunteers received NF54 strain Plasmodium falciparum by the bite of five infected Anopheles stephensi mosquitoes under controlled conditions and were monitored for signs and symptoms of malaria and for parasitemia by peripheral blood smear. Subjects were treated upon diagnosis with chloroquine by directly observed therapy. Immunological (T cell and antibody) and molecular diagnostic (real-time quantitative reverse transcriptase polymerase chain reaction [qRT-PCR]) assessments were also performed.

Results

All six volunteers developed patent parasitemia and clinical malaria. No serious adverse events occurred during the study period or for six months post-infection. The mean prepatent period was 11.2 days (range 9–14 days), and geometric mean parasitemia upon diagnosis was 10.8 parasites/µL (range 2–69) by microscopy. qRT-PCR detected parasites an average of 3.7 days (range 2–4 days) earlier than blood smears. All volunteers developed antibodies to the blood-stage antigen merozoite surface protein 1 (MSP-1), which persisted up to six months. Humoral and cellular responses to pre-erythrocytic antigens circumsporozoite protein (CSP) and liver-stage antigen 1 (LSA-1) were limited.

Conclusion

The CHMI model was safe, well tolerated and characterized by consistent prepatent periods, pre-symptomatic diagnosis in 3/6 subjects and adverse event profiles as reported at established centers. The MCTC can now evaluate candidates in the increasingly diverse vaccine and drug pipeline using the CHMI model.

Trial Registration

ClinicalTrials.gov NCT01058226

Modulation of the immune and inflammatory responses by Plasmodium falciparum schizont extracts: role of myeloid dendritic cells in effector and regulatory functions of CD4+ lymphocytes

The optimal immune response to malaria infection comprises rapid induction of inflammatory responses promptly counteracted by regulatory mechanisms to prevent immunopathology. To evaluate the role of dendritic cells (DC) in the balance of parasite-induced inflammatory/anti-inflammatory mechanisms, we studied the activity of monocyte-derived dendritic cells (MDDC), previously exposed to soluble extracts of Plasmodium falciparum-infected red blood cells (PfSE), in the differentiation of CD4 cells isolated from donors never exposed to malaria infection. We show that MDDC exposed to PfSE are extremely efficient to induce a contemporary differentiation of TH1 effector cells and T regulatory (Treg) cells in CD4 T cells even when exposed to low concentrations of parasitic extracts. Treg cells induced by MDDC infected with PfSE (MDDC-PfSE) produce transforming growth factor beta (TGF-β) and interleukin 10 (IL-10) and are endowed with strong suppressive properties. They also show phenotypical and functional peculiarities, such as the contemporary expression of markers of Treg and TH1 differentiation and higher sensitivity to TLR4 ligands both inducing an increasing production of suppressive cytokines. On the whole, our data indicate that MDDC exposed to PfSE orchestrate a well-balanced immune response with timely differentiation of TH1 and Treg cells in CD4 cells from nonimmune donors and suggest that, during the infection, the role of MDCC could be particularly relevant in low-parasitemia conditions.

Longevity and composition of cellular immune responses following experimental Plasmodium falciparum malaria infection in humans

Cellular responses to Plasmodium falciparum parasites, in particular interferon-gamma (IFNγ) production, play an important role in anti-malarial immunity. However, clinical immunity to malaria develops slowly amongst naturally exposed populations, the dynamics of cellular responses in relation to exposure are difficult to study and data about the persistence of such responses are controversial. Here we assess the longevity and composition of cellular immune responses following experimental malaria infection in human volunteers. We conducted a longitudinal study of cellular immunological responses to sporozoites (PfSpz) and asexual blood-stage (PfRBC) malaria parasites in naïve human volunteers undergoing single (n = 5) or multiple (n = 10) experimental P. falciparum infections under highly controlled conditions. IFNγ and interleukin-2 (IL-2) responses following in vitro re-stimulation were measured by flow-cytometry prior to, during and more than one year post infection. We show that cellular responses to both PfSpz and PfRBC are induced and remain almost undiminished up to 14 months after even a single malaria episode. Remarkably, not only ‘adaptive’ but also ‘innate’ lymphocyte subsets contribute to the increased IFNγ response, including αβT cells, γδT cells and NK cells. Furthermore, results from depletion and autologous recombination experiments of lymphocyte subsets suggest that immunological memory for PfRBC is carried within both the αβT cells and γδT compartments. Indeed, the majority of cytokine producing T lymphocytes express an CD45RO⁺ CD62L^- effector memory (EM) phenotype both early and late post infection. Finally, we demonstrate that malaria infection induces and maintains polyfunctional (IFNγ⁺IL-2⁺) EM responses against both PfRBC and PfSpz, previously found to be associated with protection. These data demonstrate that cellular responses can be readily induced and are long-lived following infection with P. falciparum, with a persisting contribution by not only adaptive but also (semi-)innate lymphocyte subsets. The implications hereof are positive for malaria vaccine development, but focus attention on those factors potentially inhibiting such responses in the field.

A decade into the 21st century, malaria remains responsible for an intolerable global health burden and an effective vaccine is sorely needed. Compounding the many technical hurdles in developing such a vaccine, (naturally-acquired) immunity to malaria is generally perceived to be short-lived, although direct evidence from field studies is conflicting. To overcome this issue, we measured the development of immune responses against the malaria parasite Plasmodium falciparum in human volunteers undergoing experimental malaria infections for the first time, allowing a uniquely detailed analysis thereof. We found that cellular immune responses against two clinically-relevant life-stages of the parasite are not only rapidly acquired following even a single malaria infection, but also remain virtually undiminished over a year later – an unprecedented measurement. These findings refute conclusively the notion that an intrinsic defect exists in either the development or persistence of cellular immune responses against malaria. This realization, in conjunction with a growing recognition that such responses are indeed associated with clinical protection against malaria, markedly enhances the prospect of one day developing a successful vaccine. Simultaneously, however, these results re-focus attention on the question of why the development of long-lived immune responses is often inhibited under conditions of natural exposure.

Short-lived IFN-γ effector responses, but long-lived IL-10 memory responses, to malaria in an area of low malaria endemicity

Immunity to malaria is widely believed to wane in the absence of reinfection, but direct evidence for the presence or absence of durable immunological memory to malaria is limited. Here, we analysed malaria-specific CD4+ T cell responses of individuals living in an area of low malaria transmission in northern Thailand, who had had a documented clinical attack of P. falciparum and/or P. vivax in the past 6 years. CD4+ T cell effector memory (CD45RO+) IFN-γ (24 hours ex vivo restimulation) and cultured IL-10 (6 day secretion into culture supernatant) responses to malaria schizont antigens were detected only in malaria-exposed subjects and were more prominent in subjects with long-lived antibodies or memory B cells specific to malaria antigens. The number of IFN-γ-producing effector memory T cells declined significantly over the 12 months of the study, and with time since last documented malaria infection, with an estimated half life of the response of 3.3 (95% CI 1.9-10.3) years. In sharp contrast, IL-10 responses were sustained for many years after last known malaria infection with no significant decline over at least 6 years. The observations have clear implications for understanding the immunoepidemiology of naturally acquired malaria infections and for malaria vaccine development.

Interferon-γ--central mediator of protective immune responses against the pre-erythrocytic and blood stage of malaria

Immune responses against Plasmodium parasites, the causative organisms of malaria, are traditionally dichotomized into pre-erythrocytic and blood-stage components. Whereas the central role of cellular responses in pre-erythrocytic immunity is well established, protection against blood-stage parasites has generally been ascribed to humoral responses. A number of recent studies, however, have highlighted the existence of cellular immunity against blood-stage parasites, in particular, the prominence of IFN-γ production. Here, we have undertaken to chart the contribution of this prototypical cellular cytokine to immunity against pre-erythrocytic and blood-stage parasites. We summarize the various antiparasitic effector functions that IFN-γ serves to induce, review an array of data about its protective effects, and scrutinize evidence for any deleterious, immunopathological outcome in malaria patients. We discuss the activation and contribution of different cellular sources of IFN-γ production during malaria infection and its regulation in relation to exposure. We conclude that IFN-γ forms a central mediator of protective immune responses against pre-erythrocytic and blood-stage malaria parasites and identify a number of implications for rational malaria vaccine development.

Plasmodium falciparum erythrocyte membrane protein-1 specifically suppresses early production of host interferon-gamma

Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) is a variable antigen expressed by P. falciparum, the malarial parasite. PfEMP-1, present on the surface of infected host erythrocytes, mediates erythrocyte binding to vascular endothelium, enabling the parasite to avoid splenic clearance. In addition, PfEMP-1 is proposed to regulate host immune responses via interactions with the CD36 receptor on antigen-presenting cells. We investigated the immunoregulatory function of PfEMP-1 by comparing host cell responses to erythrocytes infected with either wild-type parasites or transgenic parasites lacking PfEMP-1. We showed that PfEMP-1 suppresses the production of the cytokine interferon-gamma by human peripheral blood mononuclear cells early after exposure to P. falciparum. Suppression of this rapid proinflammatory response was CD36 independent and specific to interferon-gamma production by gammadelta-T, NK, and alphabeta-T cells. These data demonstrate a parasite strategy for downregulating the proinflammatory interferon-gamma response and further establish transgenic parasites lacking PfEMP-1 as powerful tools for elucidating PfEMP-1 functions.

gammadelta-T cells expressing NK receptors predominate over NK cells and conventional T cells in the innate IFN-gamma response to Plasmodium falciparum malaria

Rapid production of interferon-gamma (IFN-gamma) in response to malaria by the innate immune system may determine resistance to infection, or inflammatory disease. However, conflicting reports exist regarding the identity of IFN-gamma-producing cells that rapidly respond to Plasmodium falciparum. To clarify this area, we undertook detailed phenotyping of IFN-gamma-producing cells across a panel of naive human donors following 24-h exposure to live schizont-infected red blood cells (iRBC). Here, we show that NK cells comprise only a small proportion of IFN-gamma-responding cells and that IFN-gamma production is unaffected by NK cell depletion. Instead, gammadelta-T cells represent the predominant source of innate IFN-gamma, with the majority of responding gammadelta-T cells expressing NK receptors. Malaria-responsive gammadelta-T cells more frequently expressed NKG2A compared to non-responding gammadelta-T cells, while non-responding gammadelta-T cells more frequently expressed CD158a/KIR2DL1. Unlike long-term gammadelta-T cell responses to iRBC, alphabeta-T cell help was not required for innate gammadelta-T cell responses. Diversity was observed among donors in total IFN-gamma output. This was positively associated with CD94 expression on IFN-gamma(+) NK-like gammadelta-T cells. Applied to longitudinal cohort studies in endemic regions, similar comparative phenotyping should allow assessment of the contribution of diverse cell populations and regulatory receptors to risk of infection and disease.

The role of leukocytes bearing Natural Killer Complex receptors and Killer Immunoglobulin-like Receptors in the immunology of malaria

The biology of Natural Killer (NK) cells and other NK Receptor (NKR)(+) leukocytes has largely been elucidated in viral or cancer systems, and involvement in other diseases or infectious states is less clearly defined. Recently, however, clear evidence has emerged for a role in malaria. NK cells and NKR(+) leukocytes significantly control susceptibility and resistance to both malaria infection and severe disease syndromes in murine models, in dependence upon receptors encoded within the Natural Killer Complex (NKC). Plasmodium falciparum can rapidly activate human NKR(+) gammadelta T cells and NK cells in vitro, and these responses are controlled partly by NKR loci encoded within the human syntenic NKC and Killer Immunoglobulin-like Receptor (KIR) genomic regions. Neither erythrocytes nor malaria parasites express HLA or MHC Class I-like homologues, or obvious stress-type ligands, suggesting the possibility of novel NKR recognition mechanisms. Parasite-derived ligands such as P. falciparum Erythrocyte Membrane Protein-1 (PfEMP-1) and glycosylphosphatidylinositol (GPI) regulate some of these diverse responses. Population-based immunogenetic analyses should allow the identification of NKC and KIR loci controlling innate and adaptive immune responses to malaria and associated with altered risk of infection and disease.

CD4 T cells from malaria-nonexposed individuals respond to the CD36-Binding Domain of Plasmodium falciparum erythrocyte membrane protein-1 via an MHC class II-TCR-independent pathway

We have studied the human CD4 T cell response to a functionally conserved domain of Plasmodium falciparum erythrocyte membrane protein-1, cysteine interdomain region-1alpha (CIDR-1alpha). Responses to CIDR-1alpha were striking in that both exposed and nonexposed donors responded. The IFN-gamma response to CIDR-1alpha in the nonexposed donors was partially independent of TCR engagement of MHC class II and peptide. Contrastingly, CD4 T cell and IFN-gamma responses in malaria-exposed donors were MHC class II restricted, suggesting that the CD4 T cell response to CIDR-1alpha in malaria semi-immune adults also has a TCR-mediated component, which may represent a memory response. Dendritic cells isolated from human peripheral blood were activated by CIDR-1alpha to produce IL-12, IL-10, and IL-18. IL-12 was detectable only between 6 and 12 h of culture, whereas the IL-10 continued to increase throughout the 24-h time course. These data strengthen previous observations that P. falciparum interacts directly with human dendritic cells, and suggests that the interaction between CIDR-1alpha and the host cell may be responsible for regulation of the CD4 T cell and cytokine responses to P. falciparum-infected erythrocytes reported previously.

Priming of CD4+ T cells and development of CD4+ T cell memory; lessons for malaria

CD4 T cells play a central role in the immune response to malaria. They are required to help B cells produce the antibody that is essential for parasite clearance. They also produce cytokines that amplify the phagocytic and parasitocidal response of the innate immune system, as well as dampening this response later on to limit immunopathology. Therefore, understanding the mechanisms by which T helper cells are activated and the requirements for development of specific, and effective, T cell memory and immunity is essential in the quest for a malaria vaccine. In this paper on the CD4 session of the Immunology of Malaria Infections meeting, we summarize discussions of CD4 cell priming and memory in malaria and in vaccination and outline critical future lines of investigation. B. Stockinger and M.K. Jenkins proposed cutting edge experimental systems to study basic T cell biology in malaria. Critical parameters in T cell activation include the cell types involved, the route of infection and the timing and location and cell types involved in antigen presentation. A new generation of vaccines that induce CD4 T cell activation and memory are being developed with new adjuvants. Studies of T cell memory focus on differentiation and factors involved in maintenance of antigen specific T cells and control of the size of that population. To improve detection of T cell memory in the field, efforts will have to be made to distinguish antigen-specific responses from cytokine driven responses.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

Uninfected erythrocytes inhibit Plasmodium falciparum–induced cellular immune responses in whole-blood assays

Whole-blood assays (WBAs) have been successfully used as a simple tool for immuno-epidemiological field studies evaluating cellular immune responses to mycobacterial and viral antigens. Rather unexpectedly, we found very poor cytokine responses to malaria antigens in WBAs in 2 immuno-epidemiological studies carried out in malaria endemic populations in Africa. We have therefore conducted a detailed comparison of cellular immune responses to live (intact) and lysed malaria-infected erythrocytes in WBAs and in peripheral blood mononuclear cell (PBMC) cultures. We observed profound inhibition of both proliferative and interferon-γ responses to malarial antigens in WBAs as compared with PBMC cultures. This inhibition was seen only for malaria antigens and could not be overcome by increasing either antigen concentration or responder cell numbers. Inhibition was mediated by intact erythrocytes and occurred early in the culture period, suggesting that failure of antigen uptake might underlie the lack of T-cell responses. In support of this hypothesis, we have shown that intact uninfected erythrocytes specifically inhibit phagocytosis of infected red blood cells by peripheral blood monocytes. We propose that specific biochemical interactions with uninfected erythrocytes inhibit the phagocytosis of malaria-infected erythrocytes and that this may impede T-cell recognition in vivo. (Blood. 2004; 103:3084-3092)

CTLA-4 positive T cells in contrast to procalcitonin plasma levels discriminate between severe and uncomplicated Plasmodium falciparum malaria in Ghanaian children

Procalcitonin (PCT) plasma levels and the fraction of CTLA-4-positive T cells are both elevated in acute Plasmodium falciparum malaria in human adults and the degree of elevation is positively correlated with other markers of disease severity, for example with parasitaemia. However, the clinical manifestations of malaria are strongly age-dependent and children from endemic areas carry the main disease burden. Therefore, we measured PCT plasma levels and CTLA-4 expression by T cells in four groups of children from the Ashanti Region in Ghana: asymptomatic children with or without parasitaemia, children with uncomplicated P. falciparum malaria and children with severe disease. PCT levels were highly elevated in both groups with acute malaria but they did not discriminate between uncomplicated and severe disease. In contrast, CTLA-4-expression by T cells was increased only in severe malaria. The fraction of CTLA-4 positive T cells in the blood of children with severe disease differed significantly from that in uncomplicated malaria, which was not elevated in spite of the high parasite loads observed in these children. This was unexpected, as in adults uncomplicated malaria is associated with a dramatic sixfold increase of the fraction of CTLA-4-positive T cells. The data from this study support the hypothesis that strong T cell activation as measured here by CTLA-4 expression is not just the by-product of a high parasite burden, but that it contributes to the pathogenesis of P. falciparum malaria.

The war between the malaria parasite and the immune system: immunity, immunoregulation and immunopathology

Throughout history malaria has proved to be a significant threat to human health. Between 300 and 500 million clinical cases occur each year worldwide, approximately 2 million of which are fatal, primarily in children. The vast majority of malaria-related deaths are due to infection with Plasmodium falciparum; P. vivax causes severe febrile illness but is rarely fatal. Following repeated exposure to infection, people living in malaria endemic areas gradually acquire mechanisms to limit the inflammatory response to the parasite that causes the acute febrile symptoms (clinical immunity) as well as mechanisms to kill parasites or inhibit parasite replication (antiparasite immunity). Children, who have yet to develop protective immune mechanisms are thus at greater risk of clinical malaria, severe disease and death than adults. However, two epidemiological observations indicate that this is, perhaps, an oversimplified model. Firstly, cerebral malaria - a common manifestation of severe malaria - typically occurs in children who have already acquired a significant degree of antimalarial immunity, as evidenced by lower mean parasite densities and resistance to severe anaemia. One potential explanation is that cerebral malaria is, in part, an immune-mediated disease in which immunological priming occurs during first infection, eventually leading to immunopathology on re-infection. Secondly, among travelers from nonendemic areas, severe malaria is more common - and death rates are higher - in adults than in children. If severe malaria is an immune-mediated disease, what might be priming the immune system of adults from nonendemic areas to cause immunopathology during their first malaria infection, and how do adults from endemic areas avoid severe immunopathology? In this review we consider the role of innate and adaptive immune responses in terms of (i) protection from clinical malaria (ii) their potential role in immunopathology and (iii) the subsequent development of clinical immunity. We conclude by proposing a model of antimalarial immunity which integrates both the immunological and epidemiological data collected to date.

Cytokine profile of Plasmodium falciparum-specific T cells in non-immune malaria patients

CD3+ T cells are important sources of both pro- and anti-inflammatory cytokines during Plasmodium falciparum malaria. We studied the frequency of interleukin-2 (IL-2), gamma interferon (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and IL-10 expressing CD3+ cells in 10 non-immune malaria patients with uncomplicated malaria and in one patient with cerebral malaria after P. falciparum-specific and non-specific mitogenic stimulation. Analysis by fluorescence-activated cell sorting was performed after drug-induced clearance of parasites to allow previously sequestered T cells to be detected in peripheral blood. CD3+ cells of patients responded to P. falciparum infected erythrocytes with significant increases in the percentage of IL-2, IFN-gamma, and TNF-alpha, but also IL-10, positive cells. CD3+ cells from malaria-naïve donors were also responsive to specific stimulation albeit to a much lesser extent. Mitogenic stimulation of PBMC revealed no significant differences between cells of patients and controls. CD3+ cells of the patient with cerebral malaria were hyporesponsive both to the infecting parasite isolate as well as to our laboratory-adapted P. falciparum isolate, whereas two patients with uncomplicated disease were more responsive to their infecting parasites than to the laboratory-adapted isolate. The results indicate that the increased responsiveness of in vivo primed compared to malaria-naïve CD3+ cells is Plasmodium-specific and biased towards production of IFN-gamma and TNF-alpha.

Innate immune response to malaria: rapid induction of IFN-gamma from human NK cells by live Plasmodium falciparum-infected erythrocytes

To determine the potential contribution of innate immune responses to the early proinflammatory cytokine response to Plasmodium falciparum malaria, we have examined the kinetics and cellular sources of IFN-gamma production in response to human PBMC activation by intact, infected RBC (iRBC) or freeze-thaw lysates of P. falciparum schizonts. Infected erythrocytes induce a more rapid and intense IFN-gamma response from malaria-naive PBMC than do P. falciparum schizont lysates correlating with rapid iRBC activation of the CD3(-)CD56(+) NK cell population to produce IFN-gamma. IFN-gamma(+) NK cells are detectable within 6 h of coculture with iRBC, their numbers peaking at 24 h in most donors. There is marked heterogeneity between donors in magnitude of the NK-IFN-gamma response that does not correlate with mitogen- or cytokine-induced NK activation or prior malaria exposure. The NK cell-mediated IFN-gamma response is highly IL-12 dependent and appears to be partially IL-18 dependent. Exogenous rIL-12 or rIL-18 did not augment NK cell IFN-gamma responses, indicating that production of IL-12 and IL-18 is not the limiting factor explaining differences in NK cell reactivity between donors or between live and dead parasites. These data indicate that NK cells may represent an important early source of IFN-gamma, a cytokine that has been implicated in induction of various antiparasitic effector mechanisms. The heterogeneity of this early IFN-gamma response between donors suggests a variation in their ability to mount a rapid proinflammatory cytokine response to malaria infection that may, in turn, influence their innate susceptibility to malaria infection, malaria-related morbidity, or death from malaria.

Cellular responses to Plasmodium falciparum erythrocyte membrane protein-1: use of relatively conserved synthetic peptide pools to determine CD4 T cell responses in malaria-exposed individuals in Benin, West Africa

BACKGROUND

Plasmodium falciparum erythrocyte membrane protein-1, a variant antigen of the malaria parasite, is potentially a target for the immune response. It would be important to determine whether there are CD4 T cells that recognise conserved regions. However, within the relatively conserved region, there is variation. It is not possible to test T cell responses from small field samples with all possible peptides.

METHODS

We have aligned sequences that are relatively conserved between several PfEMP1 molecules, and chosen a representative sequence similar to most of the PfEMP1 variants. Using these peptides as pools representing CIDRalpha, CIDRbeta and DBLbeta-delta domains, DBLalpha domain, and EXON 2 domain of PfEMP1, we measured the CD4 T cell responses of malaria-exposed donors from Benin, West Africa by a FACS based assay.

RESULTS

All the three peptide pools elicited a CD4 T cell response in a proportion of malaria-exposed and non-exposed donors. CD4 T cell proliferation occurs at a relatively higher magnitude to peptide pools from the DBLalpha and EXON 2 in the malaria-exposed donors living in Benin than in the UK malaria-unexposed donors.

CONCLUSIONS

These findings suggest that an immunological recall response to conserved peptides of a variant antigen can be measured. Further testing of individual peptides in a positive pool will allow us to determine those conserved sequences recognised by many individuals. These types of assays may provide information on conserved peptides of PfEMP1 which could be useful for stimulating T cells to provide help to P. falciparum specific B cells.

Central nervous system in cerebral malaria: 'Innocent bystander' or active participant in the induction of immunopathology?

Cerebral malaria (CM) is a major life-threatening complication of Plasmodium falciparum infection in humans, responsible for up to 2 million deaths annually. The mechanisms underlying the fatal cerebral complications are still not fully understood. Many theories exist on the aetiology of human CM. The sequestration hypo-thesis suggests that adherence of parasitized erythrocytes to the cerebral vasculature leads to obstruction of the microcirculation, anoxia or metabolic disturbances affecting brain function, resulting in coma. This mechanism alone seems insufficient to explain all the known features of CM. In this review we focus on another major school of thought, that CM is the result of an over-vigorous immune response originally evolved for the protection of the host. Evidence in support of this second hypothesis comes from studies in murine malaria models in which T cells, monocytes, adhesion molecules and cytokines, have been implicated in the development of the cerebral complications. Recent studies of human CM also indicate a role for the immune system in the neurological complications. However, it is likely that multiple mechanisms are involved in the induction of cerebral complications and both the presence of parasitized erythrocytes in the central nervous system (CNS) and immunopathological processes contribute to the pathogenesis of CM. Most studies examining immunopathological responses in CM have focused on reactions occurring primarily in the systemic circulation. However, these also do not fully account for the development of cerebral complications in CM. In this review we summarize results from human and mouse studies that demonstrate morphological and functional changes in the resident glial cells of the CNS. The degree of immune activation and degeneration of glial cells was shown to reflect the extent of neurological complications in murine cerebral malaria. From these results we highlight the need to consider the potentially important contribution within the CNS of glia and their secreted products, such as cytokines, in the development of human CM.

Cytokine responses to Plasmodium falciparum liver-stage antigen 1 vary in rainy and dry seasons in highland Kenya

Seasonal epidemics of malaria occur in highland areas of western Kenya where transmission intensity varies according to rainfall. This study describes the seasonal changes in cytokine responses to Plasmodium falciparum liver-stage antigen 1 (LSA-1) by children (< or =17 years old) and adults (> or =18 years old) living in such a highland area. Fourteen- to 24-mer peptides corresponding to the N- and C-terminal nonrepeat regions of LSA-1 stimulated production of interleukin-5 (IL-5), interleukin-10 (IL-10), gamma interferon (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha) by peripheral blood mononuclear cells (PBMC) from 17 to 73% of individuals in both age groups in both seasons. IL-10 and TNF-alpha responses were more frequent during the high-transmission, rainy season than during the low-transmission, dry season (73 and 67% versus 17 and 25% response rates, respectively). In contrast, there was no seasonal change in the proportion of LSA-1-driven IFN-gamma and IL-5 responses. Children produced less IFN-gamma than adults, but IL-5, IL-10, and TNF-alpha levels were similar for both age groups. Depletion of CD8(+) cells from PBMC decreased IFN-gamma but increased IL-10 production. Individuals with LSA-1-stimulated IL-10 responses in the dry season were less likely to become reinfected in the subsequent rainy season than those without IL-10 responses (25% versus 49%; P = 0.083). These data support the notion that maintenance of LSA-1-driven IL-10 and TNF-alpha responses requires repeated and sustained exposure to liver-stage P. falciparum. In contrast, IFN-gamma responses increase slowly with age but persist once acquired. CD8(+) T cells are the major source of IFN-gamma but may suppress production or secretion of IL-10.

Human and murine T-cell responses to allelic forms of a malaria circumsporozoite protein epitope support a polyvalent vaccine strategy

Mouse models and a recent vaccine trial have indicated the importance of T-cell immunity to the circumsporozoite protein (CSP) of malaria sporozoites. One of the major impediments for the development of a CSP-based vaccine is that human T-cell epitopes, identified on the CSP, span regions of significant point mutational polymorphism. Studies with human and mouse T-cell clones have indicated that this polymorphism affects T-cell cross-reactivity to Th2R and Th3R, the two most polymorphic and immunodominant epitopes. We extend this observation with polyclonal human T-cell lines, from 11 donors, raised to known variants of Th2R. These lines showed limited but variable cross-reactivity with the heterologous peptides. T cells from B10.A4(R) (I-Ak) mice immunized with each of 18 natural variants of Th2R indicated a similar, limited, cross-reactivity. I-Ak competition assays showed that a number of peptides were unable to bind because of a single polymorphic residue. In both the human and mouse assays, analysis of the sequences of immunogenic cross-reactive and non-cross-reactive peptides suggested that the individual polymorphic residues affect the three-dimensional conformation of the peptide within the major histocompatibility complex (MHC) groove in an, as yet, unpredictable way. These observations argue that design of an epitope able to generate broad cross-reactivity is, to date, not possible. However, despite the limited cross-reactivity of the individual human T-cell lines, most of the donors had T-cell repertoires capable of recognizing all or nearly all of the variants tested, which supports a strategy using a multivalent vaccine.

Gammadelta+ T cells preferentially respond to live rather than killed malaria parasites

We have compared the in vitro responses of peripheral blood T cells from malaria-unexposed donors to live Plasmodium falciparum schizonts, freeze-thawed schizont extracts (P. falciparum schizont extracts [PfSE]), and parasite culture supernatants. We show that the cells responding to PfSE and parasite culture supernatants are predominantly CD4+ TCR alphabeta+ while in the presence of live schizonts there is an additional activation of TCR gammadelta+ cells. Activation of TCR gammadelta+ cells in response to PfSE was seen only when irradiated autologous feeder cells or recombinant interleukin-2 (IL-2) was added to the cultures. Live schizonts but not PfSE induced significant IL-2 production in vitro in the first 5 days after stimulation, suggesting that induction of early IL-2 by live parasites may contribute to the marked activation of the TCR gammadelta+ population.

Molecular mimicry between cardiac myosin and Trypanosoma cruzi antigen B13: identification of a B13-driven human T cell clone that recognizes cardiac myosin

Previous reports from our group have demonstrated the association of molecular mimicry between cardiac myosin and the immunodominant Trypanosoma cruzi protein B13 with chronic Chagas' disease cardiomyopathy at both the antibody and heart-infiltrating T cell level. At the peripheral blood level, we observed no difference in primary proliferative responses to T. cruzi B13 protein between chronic Chagas' cardiopathy patients, asymptomatic chagasics and normal individuals. In the present study, we investigated whether T cells sensitized by T. cruzi B13 protein respond to cardiac myosin. T cell clones generated from a B13-stimulated T cell line obtained from peripheral blood of a B13-responsive normal donor were tested for proliferation against B13 protein and human cardiac myosin. The results showed that one clone responded to B13 protein alone and the clone FA46, displaying the highest stimulation index to B13 protein (SI = 25.7), also recognized cardiac myosin. These data show that B13 and cardiac myosin share epitopes at the T cell level and that sensitization of a T cell with B13 protein results in response to cardiac myosin. It can be hypothesized that this also occurs in vivo during T. cruzi infection which results in heart tissue damage in chronic Chagas' disease cardiomyopathy.

The gamma/delta T-cell response to Plasmodium falciparum malaria in a population in which malaria is endemic

Frequencies and absolute numbers of peripheral gamma/delta T cells have been reported to increase after episodes of Plasmodium falciparum malaria in adults with limited or no previous malaria exposure. In contrast, little is known about the gamma/delta T-cell response to malaria in children from areas where malaria is endemic, who bear the burden of malaria-related morbidity and mortality. We investigated the gamma/delta T-cell response in 19 Ghanaian children from an area of hyperendemic, seasonal malaria transmission. The children presented with cerebral malaria (n = 7), severe malarial anemia (n = 5), or uncomplicated malaria (n = 7) and were monitored from admission until 4 weeks later. We found no evidence of increased frequencies of gamma/delta T cells in any of the patient groups, whereas one adult expatriate studied in Ghana and three adults admitted to the hospital in Copenhagen, Denmark, all with uncomplicated, primary P. falciparum malaria, showed increased gamma/delta T-cell frequencies similar to those previously reported. All patients had lowered absolute numbers of peripheral gamma/delta T cells at admission, changing to increased numbers by days 7 to 14 and then returning to normal levels. The study raises questions regarding age and degree of previous exposure as determinants of malaria-induced gamma/delta T-cell responses.

Naive human alpha beta T cells respond to membrane-associated components of malaria-infected erythrocytes by proliferation and production of interferon-gamma

Crude extracts of Plasmodium falciparum schizont-infected erythrocytes (PfSE) induce polyclonal activation of peripheral blood T lymphocytes from naive (malaria unexposed) humans. We demonstrate that the active component of PfSE is membrane bound, soluble in sodium dodecyl sulphate (SDS) and partially heat stable, but distinct from the tumour necrosis factor (TNF)-inducing, exoantigen-like activity of schizont extracts. Malaria pigment induces little or no T-cell activation. The responding cells are predominately CD4+, CD45RO+, T-cell receptor (TCR) alpha beta+. Contrary to previous reports, expansion of the TCR gamma delta+ subset was observed in cells from only one of eight donors. Proliferating cells secrete interferon-gamma (IFN-gamma) and release large amounts of soluble interleukin-2R (sIL-2R) into the culture supernatant but produce no detectable interleukin-4 (IL-4), a phenotype typical of the T-helper (Th)1 subset of CD4+ T cells. We propose that these activated T cells may initiate the inflammatory response to malaria infection in non-immunes and may contribute to the pathology of the disease.

Identification of T-cell determinants in natural immune responses to the Plasmodium falciparum apical membrane antigen (AMA-1) in an adult population exposed to malaria

AMA-1 of Plasmodium falciparum is a promising candidate antigen in malaria vaccine development. In this study, we have mapped the immunodominant T-cell determinants in this antigen by using synthetic peptides. From the amphipathic scores, 17 putative T-cell determinants were identified. Nine of the 17 peptides complementary to the putative T-cell determinants induced proliferation of peripheral blood mononuclear cells (PBMC) from Kenyan residents who had lifelong exposure to malaria; none of these peptides induced proliferation of PBMC from donors who were not previously exposed to malaria. This indicates that AMA-1 peptides were stimulating T cells that were previously primed by prior exposure to P. falciparum. Many positive responders showed reactivity to more than one peptide, and some of the potent proliferative T epitopes were found to be localized in the highly conserved regions of AMA-1, suggesting that it may be possible to induce T-cell memory that can recognize different variant forms of the parasite. This information on the natural immune responses against the AMA-1 vaccine antigen in clinically immune adults will be helpful in the development of an AMA-1 antigen-based malaria vaccine and may also guide testing of AMA-1-based vaccine formulations.

Reactive arthritis-associated bacteria can stimulate lymphocyte proliferation in non-exposed individuals and newborns

In reactive arthritis (ReA) a specific T cell response to the triggering bacterial antigen is present in the synovial fluid, while in paired peripheral blood T cells the response is markedly reduced. The proliferative response to ReA-associated bacteria in the peripheral blood of ReA patients was compared with that seen in the blood of healthy adults, who denied exposure to these microbes, and in the umbilical cord blood of newborns, who have clearly not been exposed to bacterial antigen. Peripheral blood mononuclear cells (PBMC) from non-exposed adults and those from umbilical cord blood proliferated to ReA-associated bacteria, whilst little response was seen in ReA PBMC. The response was MHC class II-restricted, required processing of the bacterial antigen, was seen in both CD45RO+ and CD45RA+ subsets, and was not oligoclonal. These T cell responses are similar to those previously demonstrated in non-exposed individuals to malaria, leishmania and trypanosoma antigen, and may reflect the existence of 'natural' T cell immunity to ReA-associated bacteria. The lack of such responses in ReA peripheral blood may suggest that such 'natural' responses may restrict the dissemination or progression of infection.

Identification of immunogenic epitopes of the 170-kDa subunit adhesin of Entamoeba histolytica in patients with invasive amebiasis

Entamoeba histolytica causes amebic dysentery (AD) and liver abscess (ALA). Little is known about protective immunity to amebiasis, and studies in this area have been complicated by the paucity of defined ameba antigens. We examined the proliferative responses of peripheral blood mononuclear cells (PBMC) from patients with AD and ALA to a recombinant protein containing a portion of the 170 kDa adhesin of E. histolytica (170CR), and to two synthetic peptides (1 and 2) derived from the 170 kDa sequence that were predicted to contain T cell epitopes. A significant number of patients with AD and ALA had PBMC that proliferated to 170CR molecule, and several individuals with ALA and AD had T cells that recognized one or both peptides. Contrarily, individuals from a non-endemic region for amebiasis did not respond to 170CR protein, or to both peptides. In regard to antibody response, nine of fifteen patients with ALA showed antibodies to 170CR protein. These same patients had antibodies to peptide 2. We identified peptides from 170-kDa adhesin that may contain both T and B cell epitopes recognized by some patients with invasive amebiasis. These peptides may be valuable reagents in studies of the immune response to amebiasis.

Toxocara canis adult worm antigen induces proliferative response of healthy human peripheral blood mononuclear cells

The proliferative response of human peripheral blood mononuclear cells (PBMC) from healthy donors to Toxocara canis adult worm antigens (TcA) was examined. PBMC from all donors examined (n = 7) strongly responded to TcA in a dose-dependent fashion after six days of culture, irrespective of their serological reactivity. In contrast, cord blood mononuclear cells did not react to TcA. The proliferation of PBMC in response to TcA was completely inhibited by anti-HLA-DR antibody. Purified CD4+ T cells reconstituted with autologous irradiated antigen presenting cells (APC) vigorously proliferated in response to TcA, but this was abrogated by pretreatment of APC with paraformaldehyde. Significant IL-2, IL-3, IL-4, IL-5 and IFN-gamma mRNA expression was detected in PBMC stimulated with TcA, with expression peaking at 72 h after stimulation. IL-1 beta, IL-6, IL-10 and GM-CSF mRNA expression was also upregulated, peaking at 24 h after stimulation. Taken together, these results suggest that adult T. canis-derived antigens have the ability to activate human PBMC as conventional antigens, possibly due to their cross-reactivity, which may be involved in the host defence against helminth infection.

Life-spans of human T-cell responses to determinants from the circumsporozoite proteins of Plasmodium falciparum and Plasmodium vivax

The longevity of specific human memory T-cell responses is largely unknown. However, a knowledge of the duration of memory is important for understanding immunity to an organism and for planning vaccine intervention. To address this, we have examined T-cell memory to malaria by determining T-cell responses by subjects recently exposed to peptides spanning the circumsporozoite (CS) proteins of two species of malaria-causing organisms, Plasmodium falciparum and Plasmodium vivax. Responses to vivax CS peptides by exposed Thai subjects were more frequent than responses by nonexposed individuals, permitting identification of determinants seen by vivax-induced responses. At the population level, there appears to be life-long memory, as the time since individuals were exposed did not diminish responsiveness to these determinants. In contrast, falciparum-exposed subjects were largely indistinguishable from nonexposed controls in responsiveness to falciparum CS determinants. However, a single peptide (F16: DNEKLRKPKHKKLKQPGDGN) was recognized significantly more frequently by P. falciparum-exposed than nonexposed Thai subjects. T cells responsive to this peptide were CD450+ and produced gamma-interferon. In contrast to the response to the vivax determinants and the other falciparum determinants, responsiveness to F16 was undetectable or minimal 2 years after exposure. Our data provide the average life-spans of certain malaria-specific T cells and are consistent with, but do not prove, the hypothesis that antigenic persistence (in the form of P. vivax hypnozoites) correlates with persistence of human T-cell memory.

Natural amino acid polymorphisms of the circumsporozoite protein of Plasmodium falciparum abrogate specific human CD4+ T cell responsiveness

Sequence polymorphism has been reported for virtually all malaria antigens and, in the case of the circumsporozoite (CS) protein, this variation is in the form of point mutations concentrated primarily in several regions recognized by T cells. The factors responsible for the variation are unknown. We studied the T cell responses to all known variants in malaria-exposed Thais. Memory CD4+ T cells responded to variants of a polymorphic immunodominant region (denoted Th2R), and CD4+ T cell clones specific for one Thai Th2R variant were generated. There was minimal cross-reactivity to any of the naturally occurring variants, including the other Thai variant, and competition studies performed with the clones using analog peptides demonstrated that all the substitutions of the polymorphic residues modulate either the binding of the peptide to major histocompatibility complex (MHC) molecules or the recognition by the T cell receptor of the peptide-MHC complex. Our data suggest that CD4+ T cells may be able to select parasites expressing variant sequences and have implications for development of a CS-based vaccine.

Quantification of T cells reactive to Pf155/RESA peptides in Plasmodium falciparum-exposed individuals

This study was designed to estimate, among T lymphocytes circulating in the peripheral blood of donors exposed to malaria parasites, the frequencies of those responding to Plasmodium falciparum antigens, and more specifically to Pf155/RESA peptides. The values of peptide-reactive T-cell frequencies were dispersed and rather low, ranging from 1:24,000 to undetectable (superior to 1:300,000). Subjects presented with different past exposure to parasites: either they had been repetitively exposed to malaria parasite infestation or they were recovering from a recent treated malaria attack. However, they had similar frequencies of T cells reactive to Pf155/RESA peptides. With cells from a minority of patients, "saw-tooth curves" were obtained, suggesting the existence of suppressor mechanisms operating in vitro in our limiting dilution assay.

Polyclonal expansion of peripheral gamma delta T cells in human Plasmodium falciparum malaria

Plasmodium falciparum malaria in humans is associated with an increase in the percentage and absolute number of gamma delta T cells in the peripheral blood. This increase begins during the acute infection phase and persists for at least 4 weeks during convalescence. In the present study, 25 to 30% of the gamma delta T cells expressed HLA-DR antigens in vivo and in some patients they proliferated in response to further stimulation by purified human interleukin 2 in vitro. However, there was no in vitro proliferative response to various malarial antigens, including a 75-kDa heat shock protein and a 72-kDa glucose-regulated protein of P. falciparum during the acute infection phase. Cytofluorographic studies showed that although an increase of V delta 1- gamma delta T cells was largely responsible for the expansion of the total number of gamma delta T cells, there was also a proportional increase in V delta 1+ cells. These results were confirmed with anchored PCR and by DNA sequencing to characterize at the molecular level the set of T-cell receptor (TCR) delta mRNAs expressed in the peripheral blood of two patients with high levels of gamma delta T cells. In each case, most of the TCR delta mRNA transcripts corresponded to nonproductively rearranged delta genes (unrearranged J delta or near J delta spliced to C delta). In those sequences which did represent productively rearranged genes, most of the transcripts originated from a V delta 2/J delta 1 joining, as in normal individuals. A minority of transcripts originated from a V delta 1/J delta 1 rearrangement, and one originated from a V alpha 4/J delta 1 rearrangement. Polyclonal activation of gamma delta T cells was inferred from the extensive junctional diversity seen in the delta mRNAs analyzed. Expansion of a heterogeneous set of both V delta 1(-)- and V delta 1(+)-bearing T cells suggests that the elevated levels of gamma delta T cells seen during acute P. falciparum malaria arose from immune responses to multiple distinct parasite antigens or unidentified host factors.

Evidence for limited activation of distinct CD4+ T cell subsets in response to the Plasmodium falciparum circumsporozoite protein in Papua New Guinea

Both CD4+ and CD8+ T cells, as well as antibody, are known to be important in sporozoite immunity. Data from animal studies suggest that cytokines, in particular gamma-interferon and interleukin-6, are involved. The interplay of these various factors and their importance in vaccine development has, however, not yet been elucidated. In this study, we have studied cellular and humoral responses of individuals naturally exposed to malaria in a highly endemic region of Papua New Guinea to the circumsporozoite protein of Plasmodium falciparum, a prime vaccine candidate antigen. A paucity of any CD4+ lymphoproliferative response to this protein by Papua New Guineans was notable which parallels our recent observation of a paucity of CD8+ T cell response and contrasts markedly with the responses of other endemic populations. There was nevertheless a significant antibody response to the central conserved B cell epitope, (NANP)n, as well as to other critical epitopes. An inverse relationship between gamma-interferon production and interleukin-6 production and a positive correlation between gamma-interferon production and CS peptide-specific lymphoproliferation was observed. High levels of peptide-specific IL-6 production were associated with high levels of peptide-specific serum antibodies. Our data provide evidence for the limited activation of distinct CD4+ T cell subsets and for the existence of functionally distinct subpopulations of human CD4+ T cells with respect to cytokines known to be important in sporozoite immunity.

Phenotypic characterization of splenic T cells from mice infected with Plasmodium chabaudi chabaudi

T cells from spleens of mice infected with the erythrocytic stages of Plasmodium chabaudi chabaudi have been analysed with respect to their expression of surface molecules CD3, CD4 and CD8 and T-cell receptor (TCR) alpha beta and gamma delta. The majority of T cells from infected mice were alpha beta TCR+. However, there was an increase of approximately 8-10-fold in the proportion and total number of gamma delta T cells. Immunocytochemical analysis of sections of spleens taken from infected C57BL/6 mice during a primary infection showed that this increase took place particularly in the non-lymphoid areas. Within the alpha beta TCR+ T-cell population, both CD4+ T cells and CD8+ T cells were represented in proportions similar to those observed in normal uninfected mice. Stimulation of splenic T cells from infected mice with P. chabaudi-infected erythrocytes in vitro resulted in a blasted cell population composed predominantly of alpha beta TCR+ T cells with no preferential expansion of gamma delta TCR+ T cells. There was no evidence of superantigen-like stimulation of T cells bearing particular V beta chains of the TCR. The representation of the different V beta chains within the population was not significantly different from that seen in uninfected mice.

Immunity to blood stages of malaria

Those developmental stages of malaria parasites that infect erythrocytes are responsible for the severe morbidity and mortality associated with this disease. The nature and specificity of the slowly acquired immunity seen in endemic populations remain to be defined, but significant progress has been made recently in identifying specific blood-stage proteins, characterizing immune responses to them, and exploring the dynamics of non-specific host responses to infection.

'Original antigenic sin', T cell memory, and malaria sporozoite immunity: an hypothesis for immune evasion

Prior to any exposure to malaria, most adults have T cells specific for malaria parasites and various malaria proteins. The protein for which this has been shown more than any other is the circumsporozoite protein (CSP) of Plasmodium falciparum. These T cells can be present in high frequency and appear to have arisen through exposure to other (non-malaria) organisms. Although T cells are thought to provide protection against sporozoites, these T cells specific for cross-reactive organisms are clearly unable to protect against malaria, and may be preferentially expanded following exposure to malaria sporozoites. Thus, cross-reactive organisms have the potential to skew the repertoire of sporozoite-induced T cells and affect the induction of protective immunity. This is analogous to the concept of 'original antigenic sin' whereby prior exposure to one strain of influenza virus was shown to be able to divert the antibody response to a second challenging strain to focus on the shared (cross-reactive) epitopes.

Specific T-cell recognition of the merozoite proteins rhoptry-associated protein 1 and erythrocyte-binding antigen 1 of Plasmodium falciparum

The merozoite proteins merozoite surface protein 1 (MSP-1) and rhoptry-associated protein 1 (RAP-1) and synthetic peptides containing sequences of MSP-1, RAP-1, and erythrocyte-binding antigen 1, induced in vitro proliferative responses of lymphocytes collected from Ghanaian blood donors living in an area with a high rate of transmission of malaria. Lymphocytes from a large proportion of the Ghanaian blood donors proliferated in response to the RAP-1 peptide, unlike those of Danish control blood donors, indicating that this sequence contains a malaria-specific T-cell epitope broadly recognized by individuals living in an area with a high transmission rate of malaria. Most of the donor plasma samples tested contained immunoglobulin G (IgG) and IgM antibodies recognizing the merozoite proteins, while only a minority showed high IgG reactivity to the synthetic peptides.