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-cell responses to the DBLα-tag, a short semi-conserved region of the Plasmodium falciparum membrane erythrocyte protein 1

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a variant surface antigen expressed on mature forms of infected erythrocytes. It is considered an important target of naturally acquired immunity. Despite its extreme sequence heterogeneity, variants of PfEMP1 can be stratified into distinct groups. Group A PfEMP1 have been independently associated with low host immunity and severe disease in several studies and are now of potential interest as vaccine candidates. Although antigen-specific antibodies are considered the main effector mechanism in immunity to malaria, the induction of efficient and long-lasting antibody responses requires CD4+ T-cell help. To date, very little is known about CD4+ T-cell responses to PfEMP1 expressed on clinical isolates. The DBLα-tag is a small region from the DBLα-domain of PfEMP1 that can be amplified with universal primers and is accessible in clinical parasite isolates. We identified the dominant expressed PfEMP1 in 41 individual clinical parasite isolates and expressed the corresponding DBLα-tag as recombinant antigen. Individual DBLα-tags were then used to activate CD4+ T-cells from acute and convalescent blood samples in children who were infected with the respective clinical parasite isolate. Here we show that CD4+ T-cell responses to the homologous DBLα-tag were induced in almost all children during acute malaria and maintained in some for 4 months. Children infected with parasites that dominantly expressed group A-like PfEMP1 were more likely to maintain antigen-specific IFNγ-producing CD4+ T-cells than children infected with parasites dominantly expressing other PfEMP1. These results suggest that group A-like PfEMP1 may induce long-lasting effector memory T-cells that might be able to provide rapid help to variant-specific B cells. Furthermore, a number of children induced CD4+ T-cell responses to heterologous DBLα-tags, suggesting that CD4+ T-cells may recognise shared epitopes between several DBLα-tags.

Chimeric parasites as tools to study Plasmodium immunology and assess malaria vaccines

The study of pathogen immunity relies upon being able to track antigen specific immune responses and assess their protective capacity. To study immunity to Plasmodium antigens, chimeric rodent or human malaria parasites that express proteins from other Plasmodium species or unrelated species have been developed. Different types of chimeric parasites have been used to address a range of specific questions. Parasites expressing model T cell epitopes have been used to monitor cellular immune responses to the preerythrocytic and blood stages of malaria. Other parasites have been used to assess the functional significance of immune responses targeting particular proteins. Finally, a number of rodent malaria parasites that express vaccine-candidate antigens from P. falciparum and P. vivax have been used in functional assays of vaccine-induced antibody responses. Here, I review the experimental contributions that have been made using these parasites, and discuss the potential of these approaches to continue advancing our understanding of malaria immunology and vaccine research.

Development and use of TCR transgenic mice for malaria immunology research

T-cell receptor transgenic mice are powerful tools to study T cell responses to malaria parasites. They allow for a population of antigen specific T cells to be monitored during developing responses to immunization or parasite infection; this makes them particularly useful to study fundamental aspects of T cell activation, differentiation, and migration in different tissue compartments. Moreover, the use of these cells allows for a thorough analysis of the mechanisms of antiparasite activity by T cells.

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.

Regulating the adaptive immune response to blood-stage malaria: role of dendritic cells and CD4⁺Foxp3⁺ regulatory T cells

Although a clearer understanding of the underlying mechanisms involved in protection and immunopathology during blood-stage malaria has emerged, the mechanisms involved in regulating the adaptive immune response especially those required to maintain a balance between beneficial and deleterious responses remain unclear. Recent evidence suggests the importance of CD11c⁺ dendritic cells (DC) and CD4⁺Foxp3⁺ regulatory T cells in regulating immune responses during infection and autoimmune disease, but information concerning the contribution of these cells to regulating immunity to malaria is limited. Here, we review recent findings from our laboratory and others in experimental models of malaria in mice and in Plasmodium-infected humans on the roles of DC and natural regulatory T cells in regulating adaptive immunity to blood-stage malaria.

Homeostatic proliferation and IL-7R alpha expression do not correlate with enhanced T cell proliferation and protection in chronic mouse malaria

While chronic infection has been shown to enhance protection from disease caused by several pathogens, the mechanisms are not known. The gamma-c family of cytokines IL-7, IL-2, and IL-15 are implicated in homeostatic proliferation, which is thought to maintain T cell memory. However in chronic infection, prolonged antigen exposure itself may contribute to lymphocyte survival. We have previously observed that chronic malaria infection enhances protection to re-infection, as well as enhancing B cell responses. Here, we show that chronic Plasmodium chabaudi malaria infection in mice enhances the expansion of CD4⁺ T cells in a second infection, and that this correlates with increased expression of the IL-2/15 Receptor beta (CD122) on memory T cells, as well as increasing IL-2 producers on re-infection. IL-2 has been recently linked to improved secondary proliferation, while the role of IL-7 in maintenance of CD4⁺ memory cells has been demonstrated in homeostatic proliferation, but its role in protective memory populations in infectious disease protective has not been fully investigated. Increased IL-7Rα (CD127) expression correlated, as previously reported with increased turnover of CD4 memory cells, however, this was not linked to protection or enhanced response to rechallenge, These data support the idea that antigen or IL-2 production resulting from chronic stimulation may play a role in an enhanced secondary T cell response.

Virus infection stages and distinct Th1 or Th17/Th22 T-cell responses in malaria/SHIV coinfection correlate with different outcomes of disease

BACKGROUND

Malaria and AIDS represent 2 leading causes of death from infectious diseases worldwide, and their high geographic overlap means coinfection is prevalent. It remains unknown whether distinct immune responses during coinfection with malaria and human immunodeficiency virus (HIV) affect clinical outcomes.

METHODS

We tested this hypothesis by employing macaque models of coinfection with malaria and simian-human immunodeficiency virus (SHIV).

RESULTS

Plasmodium fragile malaria coinfection of acutely SHIV-infected macaques induced hyperimmune activation and remarkable expansion of CD4+ and CD8+ T effector cells de novo producing interferon γ or tumor necrosis factor α. Malaria-driven cellular hyperactivation/expansion and high-level Th1-cytokines enhanced SHIV disease characterized by increasing CD4+ T-cell depletion, profound lymphoid depletion or destruction, and even necrosis in lymph nodes and spleens. Importantly, malaria/SHIV-mediated depletion, destruction, and necrosis in lymphoid tissues led to bursting parasite replication and fatal virus-associated malaria. Surprisingly, chronically SHIV-infected macaques without AIDS employed different defense mechanisms during malaria coinfection, and mounted unique ∼200-fold expansion of interleukin 17+/interleukin 22+ T effectors with profound Th1 suppression. Such remarkable expansion of Th17/Th22 cells and inhibition of Th1 response coincided with development of immunity against fatal virus-associated malaria without accelerating SHIV disease.

CONCLUSIONS

These novel findings suggest that virus infection status and selected Th1 or Th17/Th22 responses after malaria/AIDS-virus coinfection correlate with distinct outcomes of virus infection and malaria.

Experimental asexual blood stage malaria immunity

Immunity to asexual blood stages of malaria is complex, involving both humoral and cell-mediated immune mechanisms. The availability of murine models of malaria has greatly facilitated the analysis of immune mechanisms involved in resistance to the asexual blood stages. This unit details the materials and methods required for inducing protective immunity toward experimental blood stage malaria parasites by vaccination, repeated infection, and drug cure, as well as adoptive transfer of antigen-specific T cells.

Acute Disruption of Bone Marrow B Lymphopoiesis and Apoptosis of Transitional and Marginal Zone B Cells in the Spleen following a Blood-Stage Plasmodium chabaudi Infection in Mice

B cells and antibodies are essential for the protective immune response against a blood-stage Plasmodium infection. Although extensive research has focused on memory as well as plasma B-cell responses during infection, little is known about how malaria affects B-cell development and splenic maturation into marginal zone B (MZB) and follicular B (FoB) cells. In this study, we show that acute Plasmodium chabaudi AS infection in C57Bl/6 mice causes severe disruption of B lymphopoiesis in the bone marrow, affecting in particular pro-, pre-, and immature B cells as well as the expression of the bone marrow B-cell retention chemokine CXCL12. In addition, elevated apoptosis of transitional T2 and marginal zone (MZ) B cells was observed during and subsequent to the control of the first wave of parasitemia. In contrast, Folllicular (Fo) B cells levels were retained in the spleen throughout the infection, suggesting that these are essential for parasite clearance and proper infection control.

Harnessing immune responses against Plasmodium for rational vaccine design

In recent years, groundbreaking advances have been made in understanding the biology of and immune mechanisms against the Plasmodium spp. parasite, the causative agent of malaria. Novel features of the Plasmodium life cycle have been unravelled and immune mechanisms, which take place during both infection and immunization, have been dissected. We have undoubtedly enhanced our knowledge, but the question now is how to use this information to manipulate immune responses against Plasmodium and to develop an efficacious malaria vaccine. In this review, we discuss the latest developments in the field and speculate on how immune responses against Plasmodium could be harnessed for rational vaccine design and application.

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.

Antigen presentation in immunity to murine malaria

Understanding the initiation of cellular immune responses during blood-stage malaria infection is essential for the development of an effective vaccine that improves upon the naturally acquired immune response and induces rapid and long-lasting protection against disease. Recent studies have identified the dendritic cell (DC) subtypes responsible for priming Plasmodium-specific T cells that mediate protection and/or pathology during blood-stage infection. Significant progress has also been made towards understanding DC recognition of Plasmodium parasites through engagement of TLR signalling pathways, as well as the potential for non-TLR ligands to mediate Plasmodium-induced suppression of DC antigen presentation.

New candidate vaccines against blood-stage Plasmodium falciparum malaria: prime-boost immunization regimens incorporating human and simian adenoviral vectors and poxviral vectors expressing an optimized antigen based on merozoite surface protein 1

Although merozoite surface protein 1 (MSP-1) is a leading candidate vaccine antigen for blood-stage malaria, its efficacy in clinical trials has been limited in part by antigenic polymorphism and potentially by the inability of protein-in-adjuvant vaccines to induce strong cellular immunity. Here we report the design of novel vectored Plasmodium falciparum vaccines capable of overcoming such limitations. We optimized an antigenic insert comprising the four conserved blocks of MSP-1 fused to tandemly arranged sequences that represent both allelic forms of the dimorphic 42-kDa C-terminal region. Inserts were expressed by adenoviral and poxviral vectors and employed in heterologous prime-boost regimens. Simian adenoviral vectors were used in an effort to circumvent preexisting immunity to human adenoviruses. In preclinical studies these vaccines induced potent cellular immune responses and high-titer antibodies directed against MSP-1. The antibodies induced were found to have growth-inhibitory activity against dimorphic allelic families of P. falciparum. These vectored vaccines should allow assessment in humans of the safety and efficacy of inducing strong cellular as well as cross-strain humoral immunity to P. falciparum MSP-1.

In vivo imaging of infection immunology--4I's!

As predicted by the red queen hypothesis, microbial pathogens are probably the major reason for the evolution of the immune system (Paterson et al., Nature 464:275-278, 2010). In general, at the population level, i.e., for most of us, most of the time, the immune response to infection is highly effective. However, there remain significant challenges with particularly intransigent organisms or those that are crossing species barriers. Thus, in some cases, efforts to develop new and effective vaccines and drugs have met with limited success. To paraphrase Rudyard Kipling, "I keep six honest serving men--they taught me all I know; their names are what, and why, and when and how and where and who". Addressing these key tenets will be key to understanding the interaction between infection and the immune system. This is particularly important, as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our understanding of the interactions of pathogens with the host immune system has largely been derived through in vitro or static in vivo 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, dynamically in vivo. To this end, the increasing availability of genetically manipulable pathogens and high resolution, real-time in vivo imaging over the preceding 5 years has greatly enhanced our ability to understand and evaluate factors involved in host-pathogen interactions in vivo. This article will review the current status of this area, highlight why progress has been faster with some pathogens and tissues (e.g., protozoa and accessible site such as skin), and speculate on what recent developments in biology and imaging will tell us about pathogen-specific immune responses in the future. This will be done by following the chronological development of the infection process from invasion, to recognition, and dissemination.

Long-lived antibody and B Cell memory responses to the human malaria parasites, Plasmodium falciparum and Plasmodium vivax

Antibodies constitute a critical component of the naturally acquired immunity that develops following frequent exposure to malaria. However, specific antibody titres have been reported to decline rapidly in the absence of reinfection, supporting the widely perceived notion that malaria infections fail to induce durable immunological memory responses. Currently, direct evidence for the presence or absence of immune memory to malaria is limited. In this study, we analysed the longevity of both antibody and B cell memory responses to malaria antigens among individuals who were living in an area of extremely low malaria transmission in northern Thailand, and who were known either to be malaria naïve or to have had a documented clinical attack of P. falciparum and/or P. vivax in the past 6 years. We found that exposure to malaria results in the generation of relatively avid antigen-specific antibodies and the establishment of populations of antigen-specific memory B cells in a significant proportion of malaria-exposed individuals. Both antibody and memory B cell responses to malaria antigens were stably maintained over time in the absence of reinfection. In a number of cases where antigen-specific antibodies were not detected in plasma, stable frequencies of antigen-specific memory B cells were nonetheless observed, suggesting that circulating memory B cells may be maintained independently of long-lived plasma cells. We conclude that infrequent malaria infections are capable of inducing long-lived antibody and memory B cell responses.

Memory T-cell subsets in parasitic infections

Parasitic infections remain a major health problem throughout the world and unlike many viral or bacterial diseases, there are no vaccines to help control parasitic diseases. While several important advances have been made that will contribute to the development of parasite vaccines, such as cloning of dominant parasite antigens and a better understanding of the effector T-cell subsets needed for immunity, fundamental questions remain about how to induce long-term immunologic memory in vaccines. Here we examine a few of the experimental models that have been used to elucidate the nature of the memory T cells that are generated during parasitic infections. Although significant hurdles remain in the development of parasite vaccines, studies with both protozoa and gastrointestinal nematodes suggest that long-term immunity induced by vaccination is a realistic goal for control of parasitic infections.

Functional memory B cells and long-lived plasma cells are generated after a single Plasmodium chabaudi infection in mice

Antibodies have long been shown to play a critical role in naturally acquired immunity to malaria, but it has been suggested that Plasmodium-specific antibodies in humans may not be long lived. The cellular mechanisms underlying B cell and antibody responses are difficult to study in human infections; therefore, we have investigated the kinetics, duration and characteristics of the Plasmodium-specific memory B cell response in an infection of P. chabaudi in mice. Memory B cells and plasma cells specific for the C-terminal region of Merozoite Surface Protein 1 were detectable for more than eight months following primary infection. Furthermore, a classical memory response comprised predominantly of the T-cell dependent isotypes IgG2c, IgG2b and IgG1 was elicited upon rechallenge with the homologous parasite, confirming the generation of functional memory B cells. Using cyclophosphamide treatment to discriminate between long-lived and short-lived plasma cells, we demonstrated long-lived cells secreting Plasmodium-specific IgG in both bone marrow and in spleens of infected mice. The presence of these long-lived cells was independent of the presence of chronic infection, as removal of parasites with anti-malarial drugs had no impact on their numbers. Thus, in this model of malaria, both functional Plasmodium-specific memory B cells and long-lived plasma cells can be generated, suggesting that defects in generating these cell populations may not be the reason for generating short-lived antibody responses.

Malaria causes considerable human suffering resulting from associated high mortality, morbidity and reduced economic productivity in endemic areas. Current control methods are thwarted by a multiplicity of problems including rapidly developing resistance for anti-malarial drugs and insecticide-treated nets, and huge costs and hence poor coverage with bed nets in poor countries. Understanding the basis of the inefficiency of immunity to malaria in childhood will greatly aid the search for effective vaccines, which together with drugs and vector control, will be essential in the drive to eliminate malaria. Because of the strong evidence associating anti-malarial antibodies with anti-parasitic and anti-disease effects, vaccines inducing protective long-lasting antibody responses are attractive. However, it has been suggested that antibody responses to some Plasmodium antigens may be not long-lived. It would be important to determine whether long-lived plasma cells and memory B cells are generated after a malaria infection; however, these studies are difficult to perform in humans. Therefore we investigated the kinetics, duration and characteristics of the two cell types responsible for long-term antibody production in a mouse model of malaria. We show here that malaria-specific memory B cells and plasma cells are still detectable more than eight months after infection, and that both long-lived malaria-specific antibody-secreting cells and functional malaria-specific memory B cells can be made after a single infection.

Classical CD11c+ dendritic cells, not plasmacytoid dendritic cells, induce T cell responses to Plasmodium chabaudi malaria

Dendritic cells play an important role in the development of immune responses in malaria, but the contribution of plasmacytoid dendritic cells (pDC) to CD4 T cell activation and immunopathology is unknown. We have investigated pDC in a Plasmodium chabaudi infection in mice. During infection, pDC increased in number and transiently up-regulated expression of Major Histocompatibility Complex class II and co-stimulatory molecules. However, in contrast to classical CD11c(high) DC, pDC could not phagocytose parasites or process parasite proteins, to activate CD4 T cells. Activation of naïve pDC, but not CD11c(high) DC, by infected red blood cells induced IFN alpha in vitro, which was dependent on the Toll-like receptor, TLR9. However, inactivation of TLR9 in knock-out mice had no effect on a P. chabaudi infection suggesting that TLR9 was not crucial for parasite elimination or pathology. Neither pDC nor IFN alpha beta were essential for parasite clearance as mice depleted of pDC or IFN alpha beta Receptor-knock-out mice could control infection. However, these mice lost significantly more weight than untreated or wild-type mice. We conclude that classical DC are the major antigen-presenting cells for CD4 T cells in this infection, but that pDC and IFN alpha beta may play minor roles in controlling the magnitude of acute stage pathology.

Migrating monocytes recruited to the spleen play an important role in control of blood stage malaria

Host responses controlling blood-stage malaria include both innate and acquired immune effector mechanisms. During Plasmodium chabaudi infection in mice, a population of CD11b(high)Ly6C(+) monocytes are generated in bone marrow, most of which depend on the chemokine receptor CCR2 for migration from bone marrow to the spleen. In the absence of this receptor mice harbor higher parasitemias. Most importantly, splenic CD11b(high)Ly6C(+) cells from P chabaudi-infected wild-type mice significantly reduce acute-stage parasitemia in CCR2(-/-) mice. The CD11b(high)Ly6C(+) cells in this malaria infection display effector functions such as production of inducible nitric oxide synthase and reactive oxygen intermediates, and phagocytose P chabaudi parasites in vitro, and in a proportion of the cells, in vivo in the spleen, suggesting possible mechanisms of parasite killing. In contrast to monocyte-derived dendritic cells, CD11b(high)Ly6C(+) cells isolated from malaria-infected mice express low levels of major histocompatibility complex II and have limited ability to present the P chabaudi antigen, merozoite surface protein-1, to specific T-cell receptor transgenic CD4 T cells and fail to activate these T cells. We propose that these monocytes, which are rapidly produced in the bone marrow as part of the early defense mechanism against invading pathogens, are important for controlling blood-stage malaria parasites.

Viral vectors in malaria vaccine development

Traditional vaccine technologies have resulted in an impressive array of efficacious vaccines against a variety of infectious agents. However, several potentially deadly pathogens, including retroviruses and parasites, have proven less amenable to the application of traditional vaccine platforms, indicating the need for new approaches. Viral vectors represent an attractive way to deliver and present vaccine antigens that may offer advantages over traditional platforms. Due to their ability to induce strong cell-mediated immunity (CMI) in addition to antibodies, viral vectors may be suitable for infectious agents, such as malaria parasites, where potent CMI is required for protection. Poxvirus-vectored malaria vaccines have been the most extensively studied in the clinic, achieving significant reductions in liver-stage parasite burden. More recently, adenovirus-vectored malaria vaccines have entered clinical testing. The most promising approach - heterologous prime-boost regimens, in which different viral vectors are sequentially paired with each other or with DNA or recombinant protein vaccines - is now being explored, and could provide high-grade protection, if findings in animal models are translatable to humans. Significant barriers remain, however, such as pre-existing immunity to the vector particle and an unexplained safety signal observed in one trial suggesting an increased risk of HIV acquisition in volunteers with pre-existing immunity to the vector.

Temporal stability of naturally acquired immunity to Merozoite Surface Protein-1 in Kenyan adults

Background

Naturally acquired immunity to blood-stage Plasmodium falciparum infection develops with age and after repeated infections. In order to identify immune surrogates that can inform vaccine trials conducted in malaria endemic populations and to better understand the basis of naturally acquired immunity it is important to appreciate the temporal stability of cellular and humoral immune responses to malaria antigens.

Methods

Blood samples from 16 adults living in a malaria holoendemic region of western Kenya were obtained at six time points over the course of 9 months. T cell immunity to the 42 kDa C-terminal fragment of Merozoite Surface Protein-1 (MSP-1₄₂) was determined by IFN-γ ELISPOT. Antibodies to the 42 kDa and 19 kDa C-terminal fragments of MSP-1 were determined by serology and by functional assays that measure MSP-1₁₉ invasion inhibition antibodies (IIA) to the E-TSR (3D7) allele and growth inhibitory activity (GIA). The haplotype of MSP-1₁₉ alleles circulating in the population was determined by PCR. The kappa test of agreement was used to determine stability of immunity over the specified time intervals of 3 weeks, 6 weeks, 6 months, and 9 months.

Results

MSP-1 IgG antibodies determined by serology were most consistent over time, followed by MSP-1 specific T cell IFN-γ responses and GIA. MSP-1₁₉ IIA showed the least stability over time. However, the level of MSP-1₁₉ specific IIA correlated with relatively higher rainfall and higher prevalence of P. falciparum infection with the MSP-1₁₉ E-TSR haplotype.

Conclusion

Variation in the stability of cellular and humoral immune responses to P. falciparum blood stage antigens needs to be considered when interpreting the significance of these measurements as immune endpoints in residents of malaria endemic regions.

IP-10-mediated T cell homing promotes cerebral inflammation over splenic immunity to malaria infection

Plasmodium falciparum malaria causes 660 million clinical cases with over 2 million deaths each year. Acquired host immunity limits the clinical impact of malaria infection and provides protection against parasite replication. Experimental evidence indicates that cell-mediated immune responses also result in detrimental inflammation and contribute to severe disease induction. In both humans and mice, the spleen is a crucial organ involved in blood stage malaria clearance, while organ-specific disease appears to be associated with sequestration of parasitized erythrocytes in vascular beds and subsequent recruitment of inflammatory leukocytes. Using a rodent model of cerebral malaria, we have previously found that the majority of T lymphocytes in intravascular infiltrates of cerebral malaria-affected mice express the chemokine receptor CXCR3. Here we investigated the effect of IP-10 blockade in the development of experimental cerebral malaria and the induction of splenic anti-parasite immunity. We found that specific neutralization of IP-10 over the course of infection and genetic deletion of this chemokine in knockout mice reduces cerebral intravascular inflammation and is sufficient to protect P. berghei ANKA-infected mice from fatality. Furthermore, our results demonstrate that lack of IP-10 during infection significantly reduces peripheral parasitemia. The increased resistance to infection observed in the absence of IP-10-mediated cell trafficking was associated with retention and subsequent expansion of parasite-specific T cells in spleens of infected animals, which appears to be advantageous for the control of parasite burden. Thus, our results demonstrate that modulating homing of cellular immune responses to malaria is critical for reaching a balance between protective immunity and immunopathogenesis.

About 2.5 million people die of severe Plasmodium falciparum malaria every year. Experimental evidence from human studies and animal models indicates that severe disease syndromes arise in many organs through the sequestration of parasitized erythrocytes in vascular beds and the resulting recruitment of inflammatory leukocytes. Thus in this infection, cell-mediated immune responses appear to play a dual role by mediating protection against the parasite and also contributing to pathogenesis. Using a rodent model of cerebral malaria, we have previously found that during infection, inflammatory leukocytes are recruited to the brain via the CXCR3 trafficking pathway. Here we investigated whether blockade of the CXCR3 ligand, IP-10, alleviates brain intravascular inflammation and has an impact on the development of parasite-specific cellular immune responses involved in the control of parasitemia. We found that mice lacking IP-10 or receiving anti-IP-10 neutralizing antibodies had reduced cerebral intravascular inflammation and were protected against fatality. Inhibition of IP-10-mediated trafficking also resulted in retention of parasite-specific T cells in the spleen, facilitating control of parasite burden. Thus, IP-10-dependent trafficking critically controls the balance between pathogenic organ-specific inflammation and spleen-mediated protective immunity to malaria.

Immunity to malaria: more questions than answers

Malaria is one of the main health problems facing developing countries today. At present, preventative and treatment strategies are continuously hampered by the issues of the ever-emerging parasite resistance to newly introduced drugs, considerable costs and logistical problems. The main hope for changing this situation would be the development of effective malaria vaccines. An important part of this process is understanding the mechanisms of naturally acquired immunity to malaria. This review will highlight key aspects of immunity to malaria, about which surprisingly little is known and which will prove critical in the search for effective malaria vaccines.

Alterations of splenic architecture in malaria are induced independently of Toll-like receptors 2, 4, and 9 or MyD88 and may affect antibody affinity

Splenic microarchitecture is substantially altered during acute malaria infections, which may affect the development and regulation of immune responses. Here we investigated whether engagement of host Toll-like receptor 2 (TLR2), TLR4, TLR9, and the adaptor protein MyD88 is required for induction of the changes and whether antibody responses are modified when immunization takes place during the period of splenic disruption. The alterations in splenic microarchitecture were maximal shortly after the peak of parasitemia and were not dependent on engagement of TLR2, TLR4, or TLR9, and they were only minimally affected by the absence of the MyD88 adaptor molecule. Although germinal centers were formed in infected mice, they did not contain the usual light and dark zones. Immunization of mice with chicken gamma globulin 2 weeks prior to acute Plasmodium chabaudi infection did not affect the quantity or avidity of the immunoglobulin G antibody response to this antigen. However, immunization at the same time as the primary P. chabaudi infection resulted in a clear transient reduction in antibody avidity in the month following immunization. These data suggest that the alterations in splenic structure, particularly the germinal centers, may affect the quality of an antibody response during a malaria infection and could impact the development of immunity to malaria or to other infections or immunizations given during a malaria infection.

The severity of malarial anaemia in Plasmodium chabaudi infections of BALB/c mice is determined independently of the number of circulating parasites

BACKGROUND

Severe malarial anaemia is a major complication of malaria infection and is multi-factorial resulting from loss of circulating red blood cells (RBCs) from parasite replication, as well as immune-mediated mechanisms. An understanding of the causes of severe malarial anaemia is necessary to develop and implement new therapeutic strategies to tackle this syndrome of malaria infection.

METHODS

Using analysis of variance, this work investigated whether parasite-destruction of RBCs always accounts for the severity of malarial anaemia during infections of the rodent malaria model Plasmodium chabaudi in mice of a BALB/c background. Differences in anaemia between two different clones of P. chabaudi were also examined.

RESULTS

Circulating parasite numbers were not correlated with the severity of anaemia in either BALB/c mice or under more severe conditions of anaemia in BALB/c RAG2 deficient mice (lacking T and B cells). Mice infected with P. chabaudi clone CB suffered more severe anaemia than mice infected with clone AS, but this was not correlated with the number of parasites in the circulation. Instead, the peak percentage of parasitized RBCs was higher in CB-infected animals than in AS-infected animals, and was correlated with the severity of anaemia, suggesting that the availability of uninfected RBCs was impaired in CB-infected animals.

CONCLUSION

This work shows that parasite numbers are a more relevant measure of parasite levels in P. chabaudi infection than % parasitaemia, a measure that does not take anaemia into account. The lack of correlation between parasite numbers and the drop in circulating RBCs in this experimental model of malaria support a role for the host response in the impairment or destruction of uninfected RBC in P. chabaudi infections, and thus development of acute anaemia in this malaria model.

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.

The immunological challenges of malaria vaccine development

Malaria remains an important public health problem throughout the tropical world causing immense human suffering and impeding economic development. Despite extensive research for > 100 years, options for preventing malaria remain limited to vector control and chemoprophylaxis. The complexity of the organism and its life cycle have, thus far, thwarted vaccine development and exacerbated the perennial problems of drug resistance. Nevertheless, development of a vaccine against malaria that reduces morbidity and mortality, and ideally also reduces transmission, has long been seen as an essential component of a sustainable malaria control strategy. In this article the authors review the biological challenges of malaria vaccine development, summarise some of the recent advances and offer some immunological insights which might facilitate further research.

Malaria protection in beta 2-microglobulin-deficient mice lacking major histocompatibility complex class I antigens: essential role of innate immunity, including gammadelta T cells

It is still controversial whether malaria protection is mediated by conventional immunity associated with T and B cells or by innate immunity associated with extrathymic T cells and autoantibody-producing B cells. Given this situation, it is important to examine the mechanism of malaria protection in beta(2)-microglobulin-deficient (beta(2)m(-/-)) mice. These mice lack major histocompatibility complex class I and CD1d antigens, which results in the absence of CD8(+) T cells and natural killer T (NKT) cells. When C57BL/6 and beta(2)m(-/-) mice were injected with parasitized (Plasmodium yoelii 17XNL) erythrocytes, both survived from the infection and showed a similar level of parasitaemia. The major expanding T cells were NK1.1(-) alphabeta T-cell receptor(int) cells in both mice. The difference was a compensatory expansion of NK and gammadelta T cells in beta(2)m(-/-) mice, and an elimination experiment showed that these lymphocytes were critical for protection in these mice. These results suggest that malaria protection might be events of the innate immunity associated with multiple subsets with autoreactivity. CD8(+) T and NKT cells may be partially related to this protection.

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 yoelii: Influence of antimalarial treatment on acquisition of immunity in BALB/c and DBA/2 mice

The effect of antimalarial drugs on immune responses to the malaria infection is evaluated in vivo using two experimental self-cured rodent models. BALB/c and DBA/2 mice were infected by Plasmodium yoelii 17XNL and 17XL strains, respectively, and then treated with different doses of antimalarial drugs: chloroquine (228mg/kg or 114mg/kg of the body weight) or artesunate (78mg/kg or 39mg/kg). The effect of antimalarial drugs on host immune responses was evaluated by parasitemia, splenocyte IFN-gamma production level, and parasite-specific IgG level in the serum, however, no significant differences were observed between drug-treated and untreated groups. Moreover, most of the infected mice of all groups showed the ability to resist homologous reinfection (challenged on day 60 post-infection), only a few mice experienced transient, low parasitemia. The rechallenged mice were accompanied by high level of parasite-specific IgG. Therefore, this research implicated that, for BALB/c and DBA/2 mice, chloroquine or artesunate treatment of blood-stage P. yoelii infections does not compromise acquired immunity to malaria in either primary infection or upon rechallenge.

Malaria infection changes the ability of splenic dendritic cell populations to stimulate antigen-specific T cells

The capacity of splenic CD11c⁺ dendritic cell (DC) populations to present antigen (Ag) to T cells differs during malarial infection with Plasmodium chabaudi in mice. Both CD11c⁺CD8⁺ and CD8⁻ DCs presented malarial peptides on their surface during infection. However, although both DC subsets expressing malaria peptides could induce interferon-γ production by CD4 T cells, only CD8⁻ DCs isolated at the acute phase of infection stimulated Ag-specific T cell proliferation and interleukin (IL)-4 and -10 production from MSP1-specific T cell receptor for Ag transgenic T cells coincidental with our reported Th1 to Th2 switch at this stage in response to the pathogen. The timing of these distinct DC responses coincided with increased levels of apoptosis in the CD8⁺ population and an increase in the numbers of CD8⁻ DCs in the spleen. Our data suggest that the switch in CD4 T cell responses observed in P. chabaudi–infected mice may be the result of the presentation by different DC populations modified by the malaria infection.

Immunogenicity of a recombinant protein containing the Plasmodium vivax vaccine candidate MSP1(19) and two human CD4+ T-cell epitopes administered to non-human primates (Callithrix jacchus jacchus)

One of the most promising vaccine candidates against the erythrocytic forms of malaria is the 19 kDa C-terminal region of the merozoite surface protein 1 (MSP1(19)). As part of our studies aimed at the development of a Plasmodium vivax malaria vaccine, we characterized the immunogenic properties of a new bacterial recombinant protein containing the P. vivax MSP1(19) and two helper T-cell epitopes, the synthetic universal pan allelic DR epitope (PADRE) and a new internal MSP1 P. vivax epitope (DYDVVYLKPLAGMYK). We found that the recognition of His6MSP1(19)-DYDVVYLKPLAGMYK-PADRE was as good as the recognition of His6MSP1(19) indicating that the presence of the T-cell epitopes PADRE and DYDVVYLKPLAGMYK did not modify the MSP1(19) epitopes recognized by human IgG. The recombinant protein His6MSP1(19)-DYDVVYLKPLAGMYK-PADRE proved to be highly immunogenic in marmosets (Callithrix jacchus jacchus) when administered in incomplete Freund's adjuvant. However, when administered in other adjuvant formulations such as Quil A, CpG ODN 2006 or MPL/TDM, antibody titers to MSP1(19) were significantly lower. Among these three adjuvants, Quil A proved to be the most efficient one generating antibody titers significantly higher than the others. These results indicated that under the circumstances evaluated, adjuvants were key for the immunogenicity of the recombinant protein His6MSP1(19)-DYDVVYLKPLAGMYK-PADRE.