Deletion of Plasmodium berghei-specific CD4+ T cells adoptively transferred into recipient mice after challenge with homologous parasite

Properties and Roles of γδT Cells in Plasmodium yoelii nigeriensis NSM Infected C57BL/6 Mice

Background

Many kinds of immune cells are involved in malaria infection. γδT cells represent a special type of immune cell between natural and adaptive immune cells that play critical roles in anti-parasite infection.

Methods

In this study, malaria infection model was constructed. Distribution of γδT cells in various immune organs and dynamic changes of γδT cells in the spleens of C57BL/6 mice after infection were detected by flow cytometry. And activation status of γδT cells was detected by flow cytometry. Then γδT cells in naive and infected mice were sorted and performed single-cell RNA sequencing (scRNA-seq). Finally, γδTCR KO mice model was constructed and the effect of γδT cell depletion on mouse T and B cell immunity against Plasmodium infection was explored.

Results

Here, splenic γδT cells were found to increase significantly on day 14 after Plasmodium yoelii nigeriensis NSM infection in C57BL/6 mice. Higher level of CD69, ICOS and PD-1, lower level of CD62L, and decreased IFN-γ producing after stimulation by PMA and ionomycin were found in γδT cells from infected mice, compared with naive mice. Moreover, 11 clusters were identified in γδT cells by scRNA-seq based t-SNE analysis. Cluster 4, 5, and 7 in γδT cells from infected mice were found the expression of numerous genes involved in immune response. In the same time, the GO enrichment analysis revealed that the marker genes in the infection group were involved in innate and adaptive immunity, pathway enrichment analysis identified the marker genes in the infected group shared many key signalling molecules with other cells or against pathogen infection. Furthermore, increased parasitaemia, decreased numbers of RBC and PLT, and increased numbers of WBC were found in the peripheral blood from γδTCR KO mice. Finally, lower IFN-γ and CD69 expressing CD4⁺ and CD8⁺ T cells, lower B cell percentage and numbers, and less CD69 expressing B cells were found in the spleen from γδTCR KO infected mice, and lower levels of IgG and IgM antibodies in the serum were also observed than WT mice.

Conclusions

Overall, this study demonstrates the diversity of γδT cells in the spleen of Plasmodium yoelii nigeriensis NSM infected C57BL/6 mice at both the protein and RNA levels, and suggests that the expansion of γδT cells in cluster 4, 5 and 7 could promote both cellular and humoral immune responses.

Whole-Killed Blood-Stage Vaccine: Is It Worthwhile to Further Develop It to Control Malaria?

Major challenges have been encountered regarding the development of highly efficient subunit malaria vaccines, and so whole-parasite vaccines have regained attention in recent years. The whole-killed blood-stage vaccine (WKV) is advantageous as it can be easily manufactured and efficiently induced protective immunity against a blood-stage challenge, as well as inducing cross-stage protection against both the liver and sexual-stages. However, it necessitates a high dose of parasitized red blood cell (pRBC) lysate for immunization, and this raises concerns regarding its safety and low immunogenicity. Knowledge of the major components of WKV that can induce or evade the host immune response, and the development of appropriate human-compatible adjuvants will greatly help to optimize the WKV. Therefore, we argue that the further development of the WKV is worthwhile to control and potentially eradicate malaria worldwide.

Unraveling Cell Death Pathways during Malaria Infection: What Do We Know So Far?

Malaria is a parasitic disease (caused by different Plasmodium species) that affects millions of people worldwide. The lack of effective malaria drugs and a vaccine contributes to this disease, continuing to cause major public health and socioeconomic problems, especially in low-income countries. Cell death is implicated in malaria immune responses by eliminating infected cells, but it can also provoke an intense inflammatory response and lead to severe malaria outcomes. The study of the pathophysiological role of cell death in malaria in mammalians is key to understanding the parasite-host interactions and design prophylactic and therapeutic strategies for malaria. In this work, we review malaria-triggered cell death pathways (apoptosis, autophagy, necrosis, pyroptosis, NETosis, and ferroptosis) and we discuss their potential role in the development of new approaches for human malaria therapies.

Blood-stage malaria parasites manipulate host innate immune responses through the induction of sFGL2

Malaria parasites suppress host immune responses to facilitate their survival, but the underlying mechanism remains elusive. Here, we found that blood-stage malaria parasites predominantly induced CD4⁺Foxp3⁺CD25⁺ regulatory T cells to release soluble fibrinogen-like protein 2 (sFGL2), which substantially enhanced the infection. This was attributed to the capacity of sFGL2 to inhibit macrophages from releasing monocyte chemoattractant protein-1 (MCP-1) and to sequentially reduce the recruitment of natural killer/natural killer T cells to the spleen and the production of interferon-γ. sFGL2 inhibited c-Jun N-terminal kinase phosphorylation in the Toll-like receptor 2 signaling pathway of macrophages dependent on FcγRIIB receptor to release MCP-1. Notably, sFGL2 were markedly elevated in the sera of patients with malaria, and recombinant FGL2 substantially suppressed Plasmodium falciparum from inducing macrophages to release MCP-1. Therefore, we highlight a previously unrecognized immune suppression strategy of malaria parasites and uncover the fundamental mechanism of sFGL2 to suppress host innate immune responses.

Recent advances on T-cell exhaustion in malaria infection

T-cell exhaustion reportedly leads to dysfunctional immune responses of antigen-specific T cells. Investigations have revealed that T cells expand into functionally defective phenotypes with poor recall/memory abilities to parasitic antigens. The exploitation of co-inhibitory pathways represent a highly viable area of translational research that has very well been utilized against certain cancerous conditions. Malaria, at times, evolve into a sustained chronic state where T cells express several co-inhibitory molecules (negative immune checkpoints) facilitating parasite escape and sub-optimal protective responses. Experimental evidence suggests that blockade of co-inhibitory molecules on T cells in malaria could result in the sustenance of protective responses together with dramatic parasite clearance. The role of several co-inhibitory molecules in malaria infection largely remain unclear, and here we discussed the potential applicability of co-inhibitory molecules in the management of malaria with a view to harness protective host responses against chronic disease and associated consequences.

Evaluating antidisease immunity to malaria and implications for vaccine design

Immunity to malaria could be categorized broadly as antiparasite or antidisease immunity. While most vaccine research efforts have focused on antiparasite immunity, the evidence from endemic populations suggest that antidisease immunity is an important component of natural immunity to malaria. The processes that mediate antidisease immunity have, however, attracted little to no attention, and most interests have been directed towards the antibody responses. This review evaluates the evidence for antidisease immunity in endemic areas and discusses the possible mechanisms responsible for it. Given the key role that inflammation plays in the pathogenesis of malaria, regulation of the inflammatory response appears to be a major mechanism for antidisease immunity in naturally exposed individuals.

Mechanism of splenic cell death and host mortality in a Plasmodium yoelii malaria model

Malaria is a fatal disease that displays a spectrum of symptoms and severity, which are determined by complex host-parasite interactions. It has been difficult to study the effects of parasite strains on disease severity in human infections, but the mechanisms leading to specific disease phenotypes can be investigated using strains of rodent malaria parasites that cause different disease symptoms in inbred mice. Using a unique mouse malaria model, here we investigated the mechanisms of splenic cell death and their relationship to control of parasitemia and host mortality. C57BL/6 mice infected with Plasmodium yoelii nigeriensis N67C display high levels of pro-inflammatory cytokines and chemokines (IL-6, IFN-γ, TNF-α, CXCL1, and CCL2) and extensive splenic damage with dramatic reduction of splenic cell populations. These disease phenotypes were rescued in RAG2^−/−, IFN-γ^−/−, or T cell depleted mice, suggesting IFN-γ and T cell mediated disease mechanisms. Additionally, apoptosis was one of the major pathways involved in splenic cell death, which coincides with the peaks of pro-inflammatory cytokines. Our results demonstrate the critical roles of T cells and IFN-γ in mediating splenic cell apoptosis, parasitemia control, and host lethality and thus may provide important insights for preventing/reducing morbidity associated with severe malaria in humans.

The effect of declining exposure on T cell-mediated immunity to Plasmodium falciparum - an epidemiological "natural experiment"

BACKGROUND

Naturally acquired immunity to malaria may be lost with lack of exposure. Recent heterogeneous reductions in transmission in parts of Africa mean that large populations of previously protected people may lose their immunity while remaining at risk of infection.

METHODS

Using two ethnically similar long-term cohorts of children with historically similar levels of exposure to Plasmodium falciparum who now experience very different levels of exposure, we assessed the effect of decreased parasite exposure on antimalarial immunity. Peripheral blood mononuclear cells (PBMCs) from children in each cohort were stimulated with P. falciparum and their P. falciparum-specific proliferative and cytokine responses were compared.

RESULTS

We demonstrate that, while P. falciparum-specific CD4⁺ T cells are maintained in the absence of exposure, the proliferative capacity of these cells is altered considerably. P. falciparum-specific CD4⁺ T cells isolated from children previously exposed, but now living in an area of minimal exposure ("historically exposed") proliferate significantly more upon stimulation than cells isolated from children continually exposed to the parasite. Similarly, PBMCs from historically exposed children expressed higher levels of pro-inflammatory cytokines and lower levels of anti-inflammatory cytokines after stimulation with P. falciparum. Notably, we found a significant positive association between duration since last febrile episode and P. falciparum-specific CD4⁺ T cell proliferation, with more recent febrile episodes associated with lower proliferation.

CONCLUSION

Considered in the context of existing knowledge, these data suggest a model explaining how immunity is lost in absence of continuing exposure to P. falciparum.

T-Cell Immunoglobulin- and Mucin-Domain-Containing Molecule 3 Signaling Blockade Improves Cell-Mediated Immunity Against Malaria

Cell-mediated immune responses play important roles in immune protection against Plasmodium infection. However, impaired immunity, such as lymphocyte exhaustion, is a common phenomenon in malaria. T-cell immunoglobulin- and mucin-domain-containing molecule 3 (Tim-3) is an important regulatory molecule in cell-mediated immunity and has been implicated in malaria. In this study, it was found that Tim-3 expression on key populations of lymphocytes was significantly increased in both Plasmodium falciparum-infected patients and Plasmodium berghei ANKA (PbANKA)-infected C57BL/6 mice. Upregulation of Tim-3 led to lymphocyte exhaustion, while blocking Tim-3 signaling with an anti-Tim-3 antibody restored lymphocyte activity in Plasmodium infections. Further, anti-Tim-3 treatment accelerated the parasite clearance and relieved the symptoms of cerebral malaria in PbANKA-infected mice. In conclusion, Tim-3 on immune cells negatively regulates cell-mediated immunity against Plasmodium infection, and blocking Tim-3 signaling enhances sterile immunity and may play a protective role during malarial parasite infections.

Examining cellular immune responses to inform development of a blood-stage malaria vaccine

Naturally acquired immunity to the blood-stage of the malaria parasite develops slowly in areas of high endemicity, but is not sterilizing. It manifests as a reduction in parasite density and clinical symptoms. Immunity as a result of blood-stage vaccination has not yet been achieved in humans, although there are many animal models where vaccination has been successful. The development of a blood-stage vaccine has been complicated by a number of factors including limited knowledge of human-parasite interactions and which antigens and immune responses are critical for protection. Opinion is divided as to whether this vaccine should aim to accelerate the acquisition of responses acquired following natural exposure, or whether it should induce a different response. Animal and experimental human models suggest that cell-mediated immune responses can control parasite growth, but these responses can also contribute to significant immunopathology if unregulated. They are largely ignored in most blood-stage malaria vaccine development strategies. Here, we discuss key observations relating to cell-mediated immune responses in the context of experimental human systems and field studies involving naturally exposed individuals and how this may inform the development of a blood-stage malaria vaccine.

The case for a rational genome-based vaccine against malaria

Historically, vaccines have been designed to mimic the immunity induced by natural exposure to the target pathogen, but this approach has not been effective for any parasitic pathogen of humans or complex pathogens that cause chronic disease in humans, such as Plasmodium. Despite intense efforts by many laboratories around the world on different aspects of Plasmodium spp. molecular and cell biology, epidemiology and immunology, progress towards the goal of an effective malaria vaccine has been disappointing. The premise of rational vaccine design is to induce the desired immune response against the key pathogen antigens or epitopes targeted by protective immune responses. We advocate that development of an optimally efficacious malaria vaccine will need to improve on nature, and that this can be accomplished by rational vaccine design facilitated by mining genomic, proteomic and transcriptomic datasets in the context of relevant biological function. In our opinion, modern genome-based rational vaccine design offers enormous potential above and beyond that of whole-organism vaccines approaches established over 200 years ago where immunity is likely suboptimal due to the many genetic and immunological host-parasite adaptations evolved to allow the Plasmodium parasite to coexist in the human host, and which are associated with logistic and regulatory hurdles for production and delivery.

Malaria drives T cells to exhaustion

Malaria is a significant global burden but after >30 years of effort there is no vaccine on the market. While the complex life cycle of the parasite presents several challenges, many years of research have also identified several mechanisms of immune evasion by Plasmodium spp. Recent research on malaria, has investigated the programmed cell death-1 (PD-1) pathway which mediates exhaustion of T cells, characterized by poor effector functions and recall responses and in some cases loss of the cells by apoptosis. Such studies have shown exhaustion of CD4(+) T cells and an unappreciated role for CD8(+) T cells in promoting sterile immunity against blood stage malaria. This is because PD-1 mediates up to a 95% reduction in numbers and functional capacity of parasite-specific CD8(+) T cells, thus masking their role in protection. The role of T cell exhaustion during malaria provides an explanation for the absence of sterile immunity following the clearance of acute disease which will be relevant to future malaria-vaccine design and suggests the need for novel therapeutic solutions. This review will thus examine the role of PD-1-mediated T cell exhaustion in preventing lasting immunity against malaria.

Changes in B Cell Populations and Merozoite Surface Protein-1-Specific Memory B Cell Responses after Prolonged Absence of Detectable P. falciparum Infection

Clinical immunity to malaria declines in the absence of repeated parasite exposure. However, little is known about how B cell populations and antigen-specific memory B cells change in the absence of P. falciparum infection. A successful indoor residual insecticide spraying campaign in a highland area of western Kenya, led to an absence of blood-stage P. falciparum infection between March 2007 and April 2008. We assessed memory B cell responses in 45 adults at the beginning (April 2008) and end (April 2009) of a subsequent 12-month period during which none of the adults had evidence of asymptomatic parasitemia or clinical disease. Antibodies and memory B cells to the 42-kDa portion of the merozoite surface protein-1 (MSP-1₄₂) were measured using ELISA and ELISPOT assays, respectively. B cell populations were characterized by flow cytometry. From 2008 to 2009, the prevalence of MSP-1₄₂-specific memory B cells (45% vs. 55%, respectively, P = 0.32) or antibodies (91% vs. 82%, respectively, P = 0.32) did not differ significantly, although specific individuals did change from positive to negative and vice versa, particularly for memory B cells, suggesting possible low-level undetected parasitemia may have occurred in some individuals. The magnitude of MSP-1₄₂-specific memory B cells and levels of antibodies to MSP-1₄₂ also did not differ from 2008 to 2009 (P>0.10 for both). However, from 2008 to 2009 the proportions of both class-switched atypical (CD19+IgD-CD27-CD21-IgM-) and class-switched activated (CD19+IgD-CD27+CD21-IgM-) memory B cells decreased (both P<0.001). In contrast, class-switched resting classical memory B cells (CD19+IgD-CD27+CD21+IgM-) increased (P<0.001). In this area of seasonal malaria transmission, a one- year absence of detectable P. falciparum infection was not associated with changes in the prevalence or level of MSP-1₄₂ specific memory B cells, but was associated with major changes in overall memory B cell subsets.

Induction of anti-Plasmodium immunity following subpatent infection with live erythrocytic stages and drug cure

An effective malaria vaccine remains an important priority for the millions of people living in malaria endemic regions. Subambitious goals for the development of a vaccine have been set, which aim to achieve a licensed first-generation P. falciparum malaria vaccine with more than 50% protective efficacy against severe disease and death, lasting for at least 1 year by 2015. These goals were set in the context of a subunit vaccine. However, a whole-parasite vaccine might be expected to induce substantially superior protection. Our group has been focusing on low dose blood-stage parasites as a valid vaccine approach, and we present here the relevant methodology for this.

Plasmodium berghei sporozoite challenge of vaccinated BALB/c mice leads to the induction of humoral immunity and improved function of CD8(+) memory T cells

Protection against malaria can be achieved by induction of a strong CD8(+) T-cell response against the Plasmodium circumsporozoite protein (CSP), but most subunit vaccines suffer from insufficient memory responses. In the present study, we analyzed the impact of postimmunization sporozoite challenge on the development of long-lasting immunity. BALB/c mice were immunized by a heterologous prime/boost regimen against Plasmodium berghei CSP that induces a strong CD8(+) T-cell response and sterile protection, which is short-lived. Here, we show that protective immunity is prolonged by a sporozoite challenge after immunization. Repeated challenges induced sporozoite-specific antibodies that showed protective capacity. The numbers of CSP-specific CD8(+) T cells were not substantially enhanced by sporozoite infections; however, CSP-specific memory CD8(+) T cells of challenged mice displayed a higher cytotoxic activity than memory T cells of immunized-only mice. CD4(+) T cells contributed to protection as well; but CD8(+) memory T cells were found to be the central mediator of sterile protection. Based on these data, we suggest that prolonged protective immunity observed after immunization and infection is composed of different antiparasitic mechanisms including CD8(+) effector-memory T cells with increased cytotoxic activity as well as CD4(+) memory T cells and neutralizing antibodies.

IFN-γ-producing CD4+ T cells promote experimental cerebral malaria by modulating CD8+ T cell accumulation within the brain

It is well established that IFN-γ is required for the development of experimental cerebral malaria (ECM) during Plasmodium berghei ANKA infection of C57BL/6 mice. However, the temporal and tissue-specific cellular sources of IFN-γ during P. berghei ANKA infection have not been investigated, and it is not known whether IFN-γ production by a single cell type in isolation can induce cerebral pathology. In this study, using IFN-γ reporter mice, we show that NK cells dominate the IFN-γ response during the early stages of infection in the brain, but not in the spleen, before being replaced by CD4(+) and CD8(+) T cells. Importantly, we demonstrate that IFN-γ-producing CD4(+) T cells, but not innate or CD8(+) T cells, can promote the development of ECM in normally resistant IFN-γ(-/-) mice infected with P. berghei ANKA. Adoptively transferred wild-type CD4(+) T cells accumulate within the spleen, lung, and brain of IFN-γ(-/-) mice and induce ECM through active IFN-γ secretion, which increases the accumulation of endogenous IFN-γ(-/-) CD8(+) T cells within the brain. Depletion of endogenous IFN-γ(-/-) CD8(+) T cells abrogates the ability of wild-type CD4(+) T cells to promote ECM. Finally, we show that IFN-γ production, specifically by CD4(+) T cells, is sufficient to induce expression of CXCL9 and CXCL10 within the brain, providing a mechanistic basis for the enhanced CD8(+) T cell accumulation. To our knowledge, these observations demonstrate, for the first time, the importance of and pathways by which IFN-γ-producing CD4(+) T cells promote the development of ECM during P. berghei ANKA infection.

The contribution of Plasmodium chabaudi to our understanding of malaria

Malaria kills close to a million people every year, mostly children under the age of five. In the drive towards the development of an effective vaccine and new chemotherapeutic targets for malaria, field-based studies on human malaria infection and laboratory-based studies using animal models of malaria offer complementary opportunities to further our understanding of the mechanisms behind malaria infection and pathology. We outline here the parallels between the Plasmodium chabaudi mouse model of malaria and human malaria. We will highlight the contribution of P. chabaudi to our understanding of malaria in particular, how the immune response in malaria infection is initiated and regulated, its role in pathology, and how immunological memory is maintained. We will also discuss areas where new tools have opened up potential areas of exploration using this invaluable model system.

A whole parasite vaccine to control the blood stages of Plasmodium: the case for lateral thinking

Now, 27 years following the cloning of malaria antigens with the promise of the rapid development of a malaria vaccine, we face significant obstacles that are belatedly being addressed. Poor immunogenicity of subunit vaccine antigens and significant antigenic diversity of target epitopes represent major hurdles for which there are no clear strategies for a way forward within the current paradigm. Thus, a different paradigm - a vaccine that uses the whole organism - is now being examined. Although most advances in this approach relate to a vaccine for the pre-erythrocytic stages (sporozoites, liver stages), this opinion paper will outline the possibilities of developing a whole parasite vaccine for the blood stage and address some of the challenges for this strategy, which are entirely different to the challenges for a subunit vaccine. It is the view of the author that both vaccine paradigms should be pursued, but that success will come more quickly using the paranormal approach of exposing individuals to ultra-low doses of whole attenuated or killed parasites.

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.

Effector memory Th1 CD4 T cells are maintained in a mouse model of chronic malaria

Protection against malaria often decays in the absence of infection, suggesting that protective immunological memory depends on stimulation. Here we have used CD4⁺ T cells from a transgenic mouse carrying a T cell receptor specific for a malaria protein, Merozoite Surface Protein-1, to investigate memory in a Plasmodium chabaudi infection. CD4⁺ memory T cells (CD44^hiIL-7Rα⁺) developed during the chronic infection, and were readily distinguishable from effector (CD62L^loIL-7Rα⁻) cells in acute infection. On the basis of cell surface phenotype, we classified memory CD4⁺ T cells into three subsets: central memory, and early and late effector memory cells, and found that early effector memory cells (CD62L^loCD27⁺) dominated the chronic infection. We demonstrate a linear pathway of differentiation from central memory to early and then late effector memory cells. In adoptive transfer, CD44^hi memory cells from chronically infected mice were more effective at delaying and reducing parasitemia and pathology than memory cells from drug-treated mice without chronic infection, and contained a greater proportion of effector cells producing IFN-γ and TNFα, which may have contributed to the enhanced protection. These findings may explain the observation that in humans with chronic malaria, activated effector memory cells are best maintained in conditions of repeated exposure.

Protective immunity against malaria develops only after several infections and can be lost on leaving an area in which malaria is transmitted. This suggests that the chronic infection may maintain the protective immune response. In this paper we have used a mouse model of a blood-stage malaria infection to examine the memory response of CD4⁺ T cells during chronic infection. These T cells are required for protective immunity, and also play a part in the inflammatory response that gives rise to malaria disease. Understanding what constitutes a protective CD4⁺ T cell may help us design more protective vaccines. We show that these memory CD4⁺ T cells persist in an activated state, produce the inflammatory cytokines TNFα and IFN-γ, and are more protective than “resting” memory CD4⁺ T cells obtained from mice in which the infection has been eliminated. This may explain why people are better protected against malaria disease when they are infected frequently.

Malaria vaccine design: immunological considerations

The concept of a malaria vaccine has sparked great interest for decades; however, the challenge is proving to be a difficult one. Immune dysregulation by Plasmodium and the ability of the parasite to mutate critical epitopes in surface antigens have proved to be strong defense weapons. This has led to reconsideration of polyvalent and whole parasite strategies and ways to enhance cellular immunity to malaria that may be more likely to target conserved antigens and an expanded repertoire of antigens. These and other concepts will be discussed in this review.

Targeting caspases in intracellular protozoan infections

Caspases are cysteine aspartases acting either as initiators (caspases 8, 9, and 10) or executioners (caspases 3, 6, and 7) to induce programmed cell death by apoptosis. Parasite infections by certain intracellular protozoans increase host cell life span by targeting caspase activation. Conversely, caspase activation, followed by apoptosis of lymphocytes and other cells, prevents effective immune responses to chronic parasite infection. Here we discuss how pharmacological inhibition of caspases might affect the immunity to protozoan infections, by either blocking or delaying apoptosis.

Immune-mediated mechanisms of parasite tissue sequestration during experimental cerebral malaria

Cerebral malaria is a severe complication of malaria. Sequestration of parasitized RBCs in brain microvasculature is associated with disease pathogenesis, but our understanding of this process is incomplete. In this study, we examined parasite tissue sequestration in an experimental model of cerebral malaria (ECM). We show that a rapid increase in parasite biomass is strongly associated with the induction of ECM, mediated by IFN-gamma and lymphotoxin alpha, whereas TNF and IL-10 limit this process. Crucially, we discovered that host CD4(+) and CD8(+) T cells promote parasite accumulation in vital organs, including the brain. Modulation of CD4(+) T cell responses by helminth coinfection amplified CD4(+) T cell-mediated parasite sequestration, whereas vaccination could generate CD4(+) T cells that reduced parasite biomass and prevented ECM. These findings provide novel insights into immune-mediated mechanisms of ECM pathogenesis and highlight the potential of T cells to both prevent and promote infectious diseases.

Blood-stage Plasmodium berghei infection leads to short-lived parasite-associated antigen presentation by dendritic cells

Despite extensive evidence that Plasmodium species are capable of stimulating the immune system, the association of malaria with a higher incidence of other infectious diseases and reduced responses to vaccination against unrelated pathogens suggests the existence of immune suppression. Recently, we provided evidence that blood-stage Plasmodium berghei infection leads to suppression of MHC class I-restricted immunity to third party (non-malarial) antigens as a consequence of systemic DC activation. This earlier study did not, however, determine whether reactivity was also impaired to MHC class II-restricted third party antigens or to Plasmodium antigens themselves. Here, we show that while P. berghei-expressed antigens were presented early in infection, there was a rapid decline in presentation within 4 days, paralleling impairment in MHC class I- and II-restricted presentation of third party antigens. This provides important evidence that P. berghei not only causes immunosuppression to subsequently encountered third party antigens, but also rapidly limits the capacity to generate effective parasite-specific immunity.

Baculovirus-based nasal drop vaccine confers complete protection against malaria by natural boosting of vaccine-induced antibodies in mice

Blood-stage malaria parasites ablate memory B cells generated by vaccination in mice, resulting in diminishing natural boosting of vaccine-induced antibody responses to infection. Here we show the development of a new vaccine comprising a baculovirus-based Plasmodium yoelii 19-kDa carboxyl terminus of merozoite surface protein 1 (PyMSP1(19)) capable of circumventing the tactics of parasites in a murine model. The baculovirus-based vaccine displayed PyMSP1(19) on the surface of the virus envelope in its native three-dimensional structure. Needle-free intranasal immunization of mice with the baculovirus-based vaccine induced strong systemic humoral immune responses with high titers of PyMSP1(19)-specific antibodies. Most importantly, this vaccine conferred complete protection by natural boosting of vaccine-induced PyMSP1(19)-specific antibody responses shortly after challenge. The protective mechanism is a mixed Th1/Th2-type immunity, which is associated with the Toll-like receptor 9 (TLR9)-dependent pathway. The present study offers a novel strategy for the development of malaria blood-stage vaccines capable of naturally boosting vaccine-induced antibody responses to infection.

Evaluation of the safety and immunogenicity of Plasmodium falciparum apical membrane antigen 1, merozoite surface protein 1 or RTS,S vaccines with adjuvant system AS02A administered alone or concurrently in rhesus monkeys

In an effort to broaden the immune response induced by the RTS,S/AS02(A),vaccine, we have evaluated the immunogenicity of the RTS,S antigen when combined with MSP1(42) and with AMA1, antigens derived from the asexual blood stage. The objectives of this study were (i) to determine whether MSP1(42) and AMA1 vaccines formulated with the AS02(A) Adjuvant System were safe and immunogenic in the rhesus monkey model; (ii) to investigate whether MSP1(42) or AMA1 induced immune interference to each other, or to RTS,S, when added singly or in combinations at a single injection site; (iii) in the event of immune interference, to determine if this could be reduced when antigens were administered at separate sites. We found that MSP1(42) and AMA1 were safe and immunogenic, eliciting antibodies, and Th1 and Th2 responses using IFN-gamma and IL-5 as markers. When malaria antigens were delivered together in one formulation, MSP1(42) and RTS,S reduced AMA1-specific antibody responses as measured by ELISA however, only MSP1(42) lowered parasite growth inhibitory activity of anti-AMA1 antibodies as measured by in vitro growth inhibition assay. Unlike RTS,S, MSP1(42) significantly reduced AMA1 IFN-gamma and IL-5 responses. MSP1(42) suppression of AMA1 IFN-gamma responses was not seen in animals receiving RTS,S+AMA1+MSP1(42) suggesting that RTS,S restored IFN-gamma responses. Conversely, AMA1 had no effect on MSP1(42) antibody and IFN-gamma and IL-5 responses. Neither AMA1 alone or combined with MSP1(42) affected RTS,S antibody or IFN-gamma and IL-5 responses. Immune interference by MSP1(42) on AMA1 antibody responses was also evident when AMA1, MSP1(42) and RTS,S were administered concurrently at separate sites. These results suggest that immune interference may be complex and should be considered for the design of multi-antigen, multi-stage vaccines against malaria.

What have we learnt from mouse models for the study of malaria?

Malaria is a serious cause of morbidity and mortality and yet a vaccine is not available. Studies have used animal models to understand the pathogenesis of disease and a large amount of data on parasite biology, immune regulation and disease processes have been gained from these studies. Moreover, these models have been used for pre-clinical testing of various drugs and vaccines. Here, we discuss the features of various mouse models used to study the immunobiology of malaria and test pre-clinical vaccines and conclude that animal models have a role in the study of malaria but the experimental conditions used for testing must reflect the environment of infected individuals.

Gradual decline in malaria-specific memory T cell responses leads to failure to maintain long-term protective immunity to Plasmodium chabaudi AS despite persistence of B cell memory and circulating antibody

The mechanisms responsible for the generation and maintenance of immunological memory to Plasmodium are poorly understood and the reasons why protective immunity in humans is so difficult to achieve and rapidly lost remain a matter for debate. A possible explanation for the difficulty in building up an efficient immune response against this parasite is the massive T cell apoptosis resulting from exposure to high-dose parasite Ag. To determine the immunological mechanisms required for long-term protection against P. chabaudi malaria and the consequences of high and low acute phase parasite loads for acquisition of protective immunity, we performed a detailed analysis of T and B cell compartments over a period of 200 days following untreated and drug-treated infections in female C57BL/6 mice. By comparing several immunological parameters with the capacity to control a secondary parasite challenge, we concluded that loss of full protective immunity is not determined by acute phase parasite load nor by serum levels of specific IgG2a and IgG1 Abs, but appears to be a consequence of the progressive decline in memory T cell response to parasites, which occurs similarly in untreated and drug-treated mice with time after infection. Furthermore, by analyzing adoptive transfer experiments, we confirmed the major role of CD4(+) T cells for guaranteeing long-term full protection against P. chabaudi malaria.

Blood stage malaria antigens induce different activation-induced cell death programs in splenic CD4+T cells

CD4(+) T cells respond to antigen immunization through a process of activation, clonal expansion to generate activated effector T cells followed by activation-induced clonal deletion of the responding T cells. While loss of responding T cells in post-activation death by apoptosis is a major factor regulating immune homeostasis, the precise pathways involved in downsizing of Plasmodium falciparum antigen-induced T cell expansions are not well characterized. We report in this study that splenic CD4(+) T cells from mice immunized with nonreplicating immunogens like OVA or recombinant blood stage P. falciparum antigens, PfMSP-3 and PfMSP-1(19) or crude parasite antigen (PfAg) undergo sequential T cell activation, proliferation followed by activation-induced cell death (AICD) in a dose- and time-dependent manner after Ag restimulation. While PfMSP-3 and OVA-induced AICD was mediated through a death receptor-dependent apoptotic program, PfMSP-1(19) and PfAg-induced AICD was via a mechanism dependent on the activation of mitochondria apoptosis signalling pathway through Bax activation. These results provide insights into the mechanism through which two blood stage merozoite antigens trigger different apoptotic programs of AICD in splenic CD4(+) T cells.

Interactions between dendritic cells and CD4+ T cells during Plasmodium infection

Background

During infection, dendritic cells (DCs) encounter pathogenic microorganisms that can modulate their function and shape the T cell responses generated. During the process of T cell activation, DCs establish strong, long-lasting interactions with naïve T cells.

Methods

Using a mouse malaria model, the interactions of DCs and naïve CD4⁺ T cells have been analysed.

Results

DCs, either incubated in vitro with infected erythrocytes or isolated from infected mice, are able to present exogenous antigens by MHC-II, but are not able to establish prolonged effective interactions with naïve CD4⁺ T cells and do not induce T cell activation. It was also found that effective T cell activation of naïve CD4⁺ T cells is impaired during late Plasmodium yoelii infection.

Conclusion

These data may provide a mechanism for the lack of effective adaptive immune responses induced by the Plasmodium parasite.

What can transgenic parasites tell us about the development of Plasmodium-specific immune responses?

Malaria infects 500 million people and kills an estimated 2.7 million annually, representing one of the most significant diseases in the world. However, efforts to develop effective vaccines have met with limited success. One reason is our lack of basic knowledge of how and where the immune system responds to parasite antigens. This is important as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our knowledge of the interactions of Plasmodia with the host immune system has largely been derived through in vitro study. This is a significant issue as the component parts of the immune system do not work in isolation and their interactions occur in distinct and specialized micro- and macro-anatomical locations that can only be assessed in the physiological context, in vivo. In this context, the availability of transgenic malaria parasites over the last 10 years has greatly enhanced our ability to understand and evaluate factors involved in host-parasite interactions in vivo. In this article, we review the current status of this area and speculate on what parasite transgenesis approaches will tell us about the development of Plasmodium-specific immune responses in the future.

T cell memory in malaria

The observation that individuals living in malaria endemic areas fail to develop sterilizing immunity to malaria infection has led to the assumption that malaria-specific immune responses are sub-optimal. Recently, T cell receptor (TCR) transgenic mice specific for the sporozoite and blood stages of the malaria parasite have been developed. Studies using these models have found that, unexpectedly, T cell memory in malaria is not noticeably defective. However, if T cell memory is 'normal' why are people not better protected? We suggest this is because protective immunity and T cell memory do not always correlate; moreover, T cells alone may simply not be able to provide the type of antibody-mediated sterilizing immunity induced by traditional vaccines.

Plasmodium strain determines dendritic cell function essential for survival from malaria

The severity of malaria can range from asymptomatic to lethal infections involving severe anaemia and cerebral disease. However, the molecular and cellular factors responsible for these differences in disease severity are poorly understood. Identifying the factors that mediate virulence will contribute to developing antiparasitic immune responses. Since immunity is initiated by dendritic cells (DCs), we compared their phenotype and function following infection with either a nonlethal or lethal strain of the rodent parasite, Plasmodium yoelii, to identify their contribution to disease severity. DCs from nonlethal infections were fully functional and capable of secreting cytokines and stimulating T cells. In contrast, DCs from lethal infections were not functional. We then transferred DCs from mice with nonlethal infections to mice given lethal infections and showed that these DCs mediated control of parasitemia and survival. IL-12 was necessary for survival. To our knowledge, our studies have shown for the first time that during a malaria infection, DC function is essential for survival. More importantly, the functions of these DCs are determined by the strain of parasite. Our studies may explain, in part, why natural malaria infections may have different outcomes.

Inhibition of dendritic cell maturation by malaria is dose dependent and does not require Plasmodium falciparum erythrocyte membrane protein 1

Red blood cells infected with Plasmodium falciparum (iRBCs) have been shown to modulate maturation of human monocyte-derived dendritic cells (DCs), interfering with their ability to activate T cells. Interaction between Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) and CD36 expressed by DCs is the proposed mechanism, but we show here that DC modulation does not require CD36 binding, PfEMP1, or contact between DCs and infected RBCs and depends on the iRBC dose. iRBCs expressing a PfEMP1 variant that binds chondroitin sulfate A (CSA) but not CD36 were phagocytosed, inhibited lipopolysaccharide (LPS)-induced phenotypic maturation and cytokine secretion, and abrogated the ability of DCs to stimulate allogeneic T-cell proliferation. CD36- and CSA-binding iRBCs showed comparable inhibition. P. falciparum lines rendered deficient in PfEMP1 expression by targeted gene knockout or knockdown also inhibited LPS-induced phenotypic maturation, and separation of DCs and iRBCs in transwells showed that inhibition was not contact dependent. Inhibition was observed at an iRBC:DC ratio of 100:1 but not at a ratio of 10:1. High doses of iRBCs were associated with apoptosis of DCs, which was not activation induced. Lower doses of iRBCs stimulated DC maturation sufficient to activate autologous T-cell proliferation. In conclusion, modulation of DC maturation by P. falciparum is dose dependent and does not require interaction between PfEMP1 and CD36. Inhibition and apoptosis of DCs by high-dose iRBCs may or may not be physiological. However, our observation that low-dose iRBCs initiate functional DC maturation warrants reevaluation and further investigation of DC interactions with blood-stage P. falciparum.

CD4+ CD25+ regulatory T cells suppress CD4+ T-cell function and inhibit the development of Plasmodium berghei-specific TH1 responses involved in cerebral malaria pathogenesis

The infection of mice with Plasmodium berghei ANKA constitutes the best available mouse model for human Plasmodium falciparum-mediated cerebral malaria, a devastating neurological syndrome that kills nearly 2.5 million people every year. Experimental data suggest that cerebral disease results from the sequestration of parasitized erythrocytes within brain blood vessels, which is exacerbated by host proinflammatory responses mediated by cytokines and effector cells including T lymphocytes. Here, T cell responses to P. berghei ANKA were analyzed in cerebral malaria-resistant and -susceptible mouse strains. CD4+ T-cell proliferation and interleukin-2 (IL-2) production in response to parasite-specific and polyclonal stimuli were strongly inhibited in cerebral malaria-resistant mice. In vitro and in vivo depletion of CD4+ CD25+ regulatory T (T(reg)) cells significantly reversed the inhibition of CD4+ T-cell proliferation and IL-2 production, indicating that this cell population contributes to the suppression of T-cell function during malaria. Moreover, in vivo depletion of T(reg) cells prevented the development of parasite-specific TH1 cells involved in the induction of cerebral malaria during a secondary parasitic challenge, demonstrating a regulatory role for this cell population in the control of pathogenic responses leading to fatal disease.

Immunogenicity and protective efficacy of Escherichia coli expressed Plasmodium falciparum merozoite surface protein-1(42) using human compatible adjuvants

The C-terminal 42-kDa fragment of the merozoite surface protein-1 of Plasmodium falciparum (PfMSP-1(42)) was expressed as a recombinant protein in Escherichia coli and purified to near homogeneity. We tested the immunogenicity of recombinant PfMSP-1(42) in three clinically acceptable adjuvants (Montanide ISA 720, alum and MF59) in mice and in rabbits. High antibody responses were obtained with two adjuvant formulations with IgGl being the predominant immunoglobulin isotype. Significant T-cell proliferation responses were also observed. Competitive enzyme linked immunosorbant assay (ELISA) showed the presence of both invasion and processing inhibitory antibodies in sera obtained from the immunized rabbits. Passive immunizations of mice with anti-PfMSP-1(42) IgG purified from the rabbit-sera were found to be protective against a parasite challenge with P. berghei/P. falciparum chimeric line (Pb-PfM19) that expresses Plasmodium falciparum MSP-1(19). These findings may be useful for the development of a malaria vaccine based on Plasmodium falciparum MSP-1(42).

Plasmodium yoelii can ablate vaccine-induced long-term protection in mice

Malaria is a serious cause of morbidity and mortality for people living in endemic areas, but unlike many other infections, individuals exposed to the parasite do not rapidly become resistant to subsequent infections. High titers of Ab against the 19-kDa C-terminal fragment of the merozoite surface protein-1 can mediate complete protection in model systems; however, previous studies had not determined whether this vaccine generated long-term protection. In this study, we report that functional memory cells generated by merozoite surface protein-1, per se, do not offer any protection. This is because the parasite induces deletion of vaccine-specific memory B cells as well as long-lived plasma cells including those specific for bystander immune responses. Our study demonstrates a novel mechanism by which Plasmodium ablates immunological memory of vaccines, which would leave the host immuno-compromised.

Early cytokine production is associated with protection from murine cerebral malaria

Cerebral malaria (CM) is an infrequent but serious complication of Plasmodium falciparum infection in humans. Animal and human studies suggest that the pathogenesis of CM is immune mediated, but the precise mechanisms leading to cerebral pathology are unclear. In mice, infection with Plasmodium berghei ANKA results in CM on day 6 postinoculation (p.i.), while infection with the closely related strain P. berghei K173 does not result in CM. Infection with P. berghei K173 was associated with increased plasma gamma interferon (IFN-gamma) at 24 h p.i. and with increased splenic and hepatic mRNAs for a range of cytokines (IFN-gamma, interleukin-10 [IL-10], and IL-12) as well as the immunoregulatory enzyme indoleamine 2,3-dioxygenase. In contrast, P. berghei ANKA infection was associated with an absence of cytokine production at 24 h p.i. but a surge of IFN-gamma production at 3 to 4 days p.i. When mice were coinfected with both ANKA and K173, they produced an early cytokine response, including a burst of IFN-gamma at 24 h p.i., in a manner similar to animals infected with P. berghei K173 alone. These coinfected mice failed to develop CM. In addition, in a low-dose P. berghei K173 infection model, protection from CM was associated with early production of IFN-gamma. Early IFN-gamma production was present in NK-cell-depleted, gammadelta-cell-depleted, and Jalpha281(-/-) (NKT-cell-deficient) mice but absent from beta2-microglobulin mice that had been infected with P. berghei K173. Taken together, the results suggest that the absence of a regulatory pathway involving IFN-gamma and CD8(+) T cells in P. berghei ANKA infection allows the development of cerebral immunopathology.

Malaria-specific transgenic CD4+ T cells protect immunodeficient mice from lethal infection and demonstrate requirement for a protective threshold of antibody production for parasite clearance

T cells are important in the immune response to malaria, both for their cytokines and their help for antibody production. To look at the relative importance of these roles, a T-cell receptor (TCR) transgenic mouse has been generated carrying a TCR specific for an epitope of the merozoite surface protein 1 (MSP-1) of the malaria parasite, Plasmodium chabaudi. In adoptive transfer experiments, malaria-specific CD4+ T cells expand and produce interferon γ (IFN-γ) early in infection, but the population contracts quickly despite prolonged persistence of the parasite. MSP-1-specific CD4+ cells can protect immunodeficient mice from lethal infection; however, the parasite is only completely cleared in the presence of B cells showing that T helper cells are critical. Levels of malaria-specific antibody and the speed of their production clearly correlate with the time of resolution of infection, indicating that a critical threshold of antibody production is required for parasite clearance. Furthermore, T cells specific for a shed portion of MSP-1 are able to provide help for antibody to the protective region, which remains bound to the infected erythrocyte, suggesting that MSP-1 has all of the components necessary for a good vaccine. (Blood. 2005;106:1676-1684)

DEVELOPMENT AND REGULATION OF CELL-MEDIATED IMMUNE RESPONSES TO THE BLOOD STAGES OF MALARIA: Implications for Vaccine Research

The immune response to the malaria parasite is complex and poorly understood. Although antibodies and T cells can control parasite growth in model systems, natural immunity to malaria in regions of high endemicity takes several years to develop. Variation and polymorphism of antibody target antigens are known to impede immune responses, but these factors alone cannot account for the slow acquisition of immunity. In human and animal model systems, cell-mediated responses can control parasite growth effectively, but such responses are regulated by parasite load via direct effects on dendritic cells and possibly on T and B cells as well. Furthermore, high parasite load is associated with pathology, and cell-mediated responses may also harm the host. Inflammatory cytokines have been implicated in the pathogenesis of cerebral malaria, anemia, weight loss, and respiratory distress in malaria. Immunity without pathology requires rapid parasite clearance, effective regulation of the inflammatory anti-parasite effects of cellular responses, and the eventual development of a repertoire of antibodies effective against multiple strains. Data suggest that this may be hastened by exposure to malaria antigens in low dose, leading to augmented cellular immunity and rapid parasite clearance.

Heterologous immunity in the absence of variant-specific antibodies after exposure to subpatent infection with blood-stage malaria

We examined immunity induced by subpatent blood-stage malaria (undetectable by microscopy) using the rodent malaria parasite, Plasmodium chabaudi chabaudi, postulating that limited infection may allow expansion of antigen-specific T cells that are normally deleted by apoptosis. After three infections drug cured at 48 h, mice were protected against high-dose challenge with homologous or heterologous parasites (different strain or variant). Immunity differed from that generated by three untreated, patent infections. Subpatently infected mice lacked immunoglobulin G (IgG) to variant surface antigens, despite producing similar titers of total malaria-specific IgG to those produced by patently infected mice, including antibodies specific for merozoite surface antigens conserved between heterologous strains. Antigen-specific proliferation of splenocytes harvested prechallenge was significantly higher in subpatently infected mice than in patently infected or naive mice. In subpatently infected mice, lymphoproliferation was similar in response to homologous and heterologous parasites, suggesting that antigenic targets of cell-mediated immunity were conserved. A Th1 cytokine response was evident during challenge. Apoptosis of CD4+ and CD8+ splenic lymphocytes occurred during patent but not subpatent infection, suggesting a reason for the relative prominence of cell-mediated immunity after subpatent infection. In conclusion, subpatent infection with blood stage malaria parasites induced protective immunity, which differed from that induced by patent infection and targeted conserved antigens. These findings suggest that alternative vaccine strategies based on delivery of multiple parasite antigens at low dose may induce effective immunity targeting conserved determinants.

Infection-induced apoptosis deletes bystander CD4+ T cells: a mechanism for suppression of autoimmunity during BCG infection

Infection with Mycobacterium bovis Bacille Calmette Guérin (BCG) induces high levels of apoptosis among activated CD4+ T cells. We have investigated the specificity of this pro-apoptotic response and its influence on CD4+ T cell mediated autoimmunity. Apoptosis induced by BCG-infection is unrelated to antigenic specificity, as demonstrated by the increased apoptosis of activated TCR transgenic CD4+ T cells of unrelated specificity. Moreover, infection-induced apoptosis promoted the deletion of CD4+ T cells activated either by peptide or anti-CD3/anti-CD28 stimulation. Infection-induced apoptosis required IFN-gamma production by the infected host, and expression of the IFN-gamma receptor on donor CD4+ T cells. We used an adoptive transfer model of experimental autoimmune encephalomyelitis (EAE) to assess the influence of infection-induced apoptosis on a CD4+ T cell-mediated response. A significantly higher level of apoptosis was seen among sorted encephalitogenic CD4+ T cells transferred to BCG-infected versus uninfected hosts. BCG-infected mice displayed a milder course of clinical disease than their uninfected counterparts and a decreased recovery of donor cells from the CNS. The data suggest that mycobacterial infection attenuates the severity of EAE, at least in part, by promoting the apoptotic elimination of autoreactive CD4+ T cells.

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).

The role of programmed cell death in Plasmodium-mosquito interactions

Many host-parasite interactions are regulated in part by the programmed cell death of host cells or the parasite. Here we review evidence suggesting that programmed cell death occurs during the early stages of the development of the malaria parasite in its vector. Zygotes and ookinetes of Plasmodium berghei have been shown to die by programmed cell death (apoptosis) in the midgut lumen of the vector Anopheles stephensi, or whilst developing in vitro. Several morphological markers, indicative of apoptosis, are described and evidence for the involvement of a biochemical pathway involving cysteine proteases discussed in relationship to other protozoan parasites. Malaria infection induces apoptosis in the cells of two mosquito tissues, the midgut and the follicular epithelium. Observations on cell death in both these tissues are reviewed including the role of caspases as effector molecules and the rescue of resorbing follicles resulting from inhibition of caspases. Putative signal molecules that might induce parasite and vector apoptosis are suggested including nitric oxide, reactive nitrogen intermediates, oxygen radicals and endocrine balances. Finally, we suggest that programmed cell death may play a critical role in regulation of infection by the parasite and the host, and contribute to the success or not of parasite establishment and host survival.

The immunological challenge to developing a vaccine to the blood stages of malaria parasites

Twenty-one years after malaria antigens were first cloned, a vaccine still appears to be a long way off. There have been periods of great excitement, and in model systems, subunit vaccine homologs can induce robust protection. However, significant challenges exist concerning antigenic variation and polymorphism, immunological non-responsiveness to individual vaccine antigens, parasite-induced apoptosis of immune effector and memory cells, and immune deviation as a result of maternal immunity and alterations of dendritic cell function. Novel approaches will be required. This review addresses some of the approaches that might present malaria antigens in a way designed to induce superior immune responses or that target novel conserved epitopes. Cell-mediated immunity, acting independently of antibody, may exert potent anti-parasite effects, and identification of multiple target antigens/epitopes could lead to the development of vaccines with profound efficacy.

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.