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

Characteristics of splenic PD-1+ γδT cells in Plasmodium yoelii nigeriensis infection

Although the functions of programmed death-1 (PD-1) on αβ T cells have been extensively reported, a role for PD-1 in regulating γδT cell function is only beginning to emerge. Here, we investigated the phenotypic and functional characteristics of PD-1-expressing γδT cells, and the molecular mechanism was also explored in the Plasmodium yoelii nigeriensis (P. yoelii NSM)-infected mice. Flow cytometry and single-cell RNA sequencing (scRNA-seq) were performed. An inverse agonist of RORα, SR3335, was used to investigate the role of RORα in regulating PD-1+ γδT cells. The results indicated that γδT cells continuously upregulated PD-1 expression during the infection period. Higher levels of CD94, IL-10, CX3CR1, and CD107a; and lower levels of CD25, CD69, and CD127 were found in PD-1+ γδT cells from infected mice than in PD-1- γδT cells. Furthermore, GO enrichment analysis revealed that the marker genes in PD-1+ γδT cells were involved in autophagy and processes utilizing autophagic mechanisms. ScRNA-seq results showed that RORα was increased significantly in PD-1+ γδT cells. GSEA identified that RORα was mainly involved in the regulation of I-kappaB kinase/NF-κB signaling and the positive regulation of cytokine production. Consistent with this, PD-1-expressing γδT cells upregulated RORα following Plasmodium yoelii infection. Additionally, in vitro studies revealed that higher levels of p-p65 were found in PD-1+ γδT cells after treatment with a RORα selective synthetic inhibitor. Collectively, these data suggest that RORα-mediated attenuation of NF-κB signaling may be fundamental for PD-1-expressing γδT cells to modulate host immune responses in the spleen of Plasmodium yoelii nigeriensis-infected C57BL/6 mice, and it requires further investigation.

Fasciola hepatica infection modifies IgG1 specific immune response to foot-and-mouth disease virus induced by vaccination

Fasciola hepatica, a worldwide distributed helminth, has a robust immunoregulatory effect in the host, increasing the susceptibility to secondary infections. Foot and mouth disease (FMD) is a highly contagious acute vesicular viral disease effectively controlled by vaccination in endemic regions. Despite the evidence of immunoregulatory effects, the impact of fasciolosis on the immune response induced by FMD vaccination in cattle has never been assessed. Our objective was to evaluate whether the infection by F. hepatica in cattle influences the long-term immunity elicited by the currently used commercial FMD-inactivated vaccines. Aberdeen Angus steers negative for F. hepatica were vaccinated twice against FMD virus (FMDV) during the first 6 months of age using a commercial oil vaccine formulated with A24/Cruzeiro and O1/Campos strains. When maternal antibodies against F. hepatica were weaned (18--20 months of age) animals were divided into groups of 12 and infected or mock-infected with 500 metacercariae/animal. Individual serum samples were collected at 0-, 28-, 59-, 87- and 157-days post-infection (dpi). Indirect ELISAs were used to detect A24/Cruzeiro specific bovine IgG and IgG subtypes. The total IgG antibody levels and avidity against FMDV did not show significant differences between all the groups. The commercial vaccine induced higher IgG2 than IgG1 titers in vaccinated animals. Anti-FMDV IgG1 levels significantly decreased in the infected group at 28 dpi. In addition, the avidity of IgG1 FMDV-specific antibodies at day 28 in the infected group was reduced compared to the control. These results show that F. hepatica infection modified anamnestic responses against FMDV, reducing serum IgG1 titers and avidity. To our knowledge, this is the first report of immune-regulation of F. hepatica altering the immune response of FMD vaccines, one of the most globally used animal vaccines.

IgM+ and IgM- memory B cells represent heterogeneous populations capable of producing class-switched antibodies and germinal center B cells upon rechallenge with P. yoelii

Memory B cells (MBCs) are essential for maintaining long-term humoral immunity to infectious organisms, including Plasmodium. MBCs are a heterogeneous population whose function can be dictated by isotype or expression of particular surface proteins. Here, aided by antigen-specific B-cell tetramers, MBC populations were evaluated to discern their phenotype and function in response to infection with a nonlethal strain of P. yoelii. Infection of mice with P. yoelii 17X resulted in 2 predominant MBC populations: somatically hypermutated isotype-switched (IgM- ) and IgM+ MBCs that coexpressed CD73 and CD80 that produced antigen-specific antibodies in response to secondary infection. Rechallenge experiments indicated that IgG-producing cells dominated the recall response over the induction of IgM-secreting cells, with both populations expanding with similar timing during the secondary response. Furthermore, using ZsGreen1 expression as a surrogate for activation-induced cytidine deaminase expression alongside CD73 and CD80 coexpression, ZsGreen1+ CD73+ CD80+ IgM+ , and IgM- MBCs gave rise to plasmablasts that secreted Ag-specific Abs after adoptive transfer and infection with P. yoelii. Moreover, ZsGreen1+ CD73+ CD80+ IgM+ and IgM- MBCs could differentiate into B cells with a germinal center phenotype after adoptive transfer. A third population of B cells (ZsGreen1- CD73- CD80- IgM- ) that is apparent after infection responded poorly to reactivation in vitro and in vivo, indicating that these cells do not represent a canonical population of MBCs. Together these data indicated that MBC function is not defined by immunoglobulin isotype, nor does coexpression of key surface markers limit the potential fate of MBCs after recall.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Immunization with CSP and a RIG-I Agonist is Effective in Inducing a Functional and Protective Humoral Response Against Plasmodium

Malaria is a major public health concern, as a highly effective human vaccine remains elusive. The efficacy of a subunit vaccine targeting the most abundant protein of the sporozoite surface, the circumsporozoite protein (CSP) has been hindered by difficulties in generating an effective humoral response in both quantity and quality. Using the rodent Plasmodium yoelii model we report here that immunization with CSP adjuvanted with 5’ppp-dsRNA, a RIG-I agonist, confers early and long-lasting sterile protection in mice against stringent sporozoite and mosquito bite challenges. The immunization induced high levels of antibodies, which were functional in targeting and killing the sporozoites and were sustained over time through the accumulation of long-lived plasma cells in the bone marrow. Moreover, 5’ppp-dsRNA-adjuvanted immunization with the CSP of P. falciparum was also significantly protective against challenges using a transgenic PfCSP-expressing P. yoelii parasite. Conversely, using the TLR3 agonist poly(A:U) as adjuvant resulted in a formulation that despite inducing high antibody levels was unable to generate equally functional antibodies and was, consequently, less protective. In conclusion, we demonstrate that using 5’ppp-dsRNA as an adjuvant to vaccines targeting CSP induces effective anti-Plasmodium humoral immunity.

The Dynamic Change of Immune Responses Between Acute and Recurrence Stages of Rodent Malaria Infection

Malaria infections are persistent as frequent recrudescence of the disease may occur following the acute infection stage, but the different immune responses that control the acute and recrudescence stages are still largely unknown. Using single-cell RNA sequencing (scRNA-seq), we showed that the number of Th1 and plasma cells in the spleen was significantly reduced during the recurrence stage compared to the acute stage of Plasmodium chabaudi chabaudi AS (P. chabaudi) infection. Additionally, the ability of both CD4+ T cell responses and B cells to control P. chabaudi recurrence was significantly reduced compared to their roles in the control of acute infection. In contrast, the number of innate immune cells, including red pulp macrophages (RPMs), gamma delta (γδ) T cells, and Dendritic cells (DCs) were significantly increased during the recurrence stage and showed to be critical for P. chabaudi infection recurrence control. Thus, our data strongly suggest the complementary role of innate immune responses in controlling malaria recrudescence when adaptive immune responses are suppressed. These findings shed new light on the development of immune interventions against malaria.

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.

Development of B Cell Memory in Malaria

A single exposure to many viral and bacterial pathogens typically induces life-long immunity, however, the development of the protective immunity to Plasmodium parasites is strikingly less efficient and achieves only partial protection, with adults residing in endemic areas often experiencing asymptomatic infections. Although naturally acquired immunity to malaria requires both cell-mediated and humoral immune responses, antibodies govern the control of malarial disease caused by the blood-stage form of the parasites. A large body of epidemiological evidence described that antibodies to Plasmodium antigens are inefficiently generated and rapidly lost without continued parasite exposure, suggesting that malaria is accompanied by defects in the development of immunological B cell memory. This topic has been of focus of recent studies of malaria infection in humans and mice. This review examines the main findings to date on the processes that modulate the acquisition of memory B cell responses to malaria, and highlights the importance of closing outstanding gaps of knowledge in the field for the rational design of next generation therapeutics against malaria.

Immune checkpoint blockade in infectious diseases

The upregulation of immune checkpoint molecules, such as programmed cell death protein 1 (PD1) and cytotoxic T lymphocyte antigen 4 (CTLA4), on immune cells occurs during acute infections, such as malaria, as well as during chronic persistent viral infections, including HIV and hepatitis B virus. These pathways are important for preventing immune-driven pathology but can also limit immune-mediated clearance of the infection. The recent success of immune checkpoint blockade in cancer therapy suggests that targeting these pathways would also be effective for preventing and treating a range of infectious diseases. Here, we review our current understanding of immune checkpoint pathways in the pathogenesis of infectious diseases and discuss the potential for therapeutically targeting these pathways in this setting.

Blood Stage Malaria Disrupts Humoral Immunity to the Pre-erythrocytic Stage Circumsporozoite Protein

Many current malaria vaccines target the pre-erythrocytic stage of infection in the liver. However, in malaria-endemic regions, increased blood stage exposure is associated with decreased vaccine efficacy, thereby challenging current vaccine efforts. We hypothesized that pre-erythrocytic humoral immunity is directly disrupted by blood stage infection. To investigate this possibility, we used Plasmodium-antigen tetramers to analyze B cells after infection with either late liver stage arresting parasites or wild-type parasites that progress to the blood stage. Our data demonstrate that immunoglobulin G (IgG) antibodies against the pre-erythrocytic antigen, circumsporozoite protein (CSP), are generated only in response to the attenuated, but not the wild-type, infection. Further analyses revealed that blood stage malaria inhibits CSP-specific germinal center B cell differentiation and modulates chemokine expression. This results in aberrant memory formation and the loss of a rapid secondary B cell response. These data highlight how immunization with attenuated parasites may drive optimal immunity to malaria.

Isolation and Characterization of Vaccine Candidate Genes Including CSP and MSP1 in Plasmodium yoelii

Malaria is an infectious disease affecting humans, which is transmitted by the bite of Anopheles mosquitoes harboring sporozoites of parasitic protozoans belonging to the genus Plasmodium. Despite past achievements to control the protozoan disease, malaria still remains a significant health threat up to now. In this study, we cloned and characterized the full-unit Plasmodium yoelii genes encoding merozoite surface protein 1 (MSP1), circumsporozoite protein (CSP), and Duffy-binding protein (DBP), each of which can be applied for investigations to obtain potent protective vaccines in the rodent malaria model, due to their specific expression patterns during the parasite life cycle. Recombinant fragments corresponding to the middle and C-terminal regions of PyMSP1 and PyCSP, respectively, displayed strong reactivity against P. yoelii-infected mice sera. Specific native antigens invoking strong humoral immune response during the primary and secondary infections of P. yoelii were also abundantly detected in experimental ICR mice. The low or negligible parasitemia observed in the secondary infected mice was likely to result from the neutralizing action of the protective antibodies. Identification of these antigenic proteins might provide the necessary information and means to characterize additional vaccine candidate antigens, selected solely on their ability to produce the protective antibodies.

Differing rates of antibody acquisition to merozoite antigens in malaria: implications for immunity and surveillance

Antibodies play a key role in acquired human immunity to Plasmodium falciparum (Pf) malaria and target merozoites to reduce or prevent blood-stage replication and the development of disease. Merozoites present a complex array of antigens to the immune system, and currently, there is only a partial understanding of the targets of protective antibodies and how responses to different antigens are acquired and boosted. We hypothesized that there would be differences in the rate of acquisition of antibodies to different antigens and how well they are boosted by infection, which impacts the acquisition of immunity. We examined responses to a range of merozoite antigens in 2 different cohorts of children and adults with different age structures and levels of malaria exposure. Overall, antibodies were associated with age, exposure, and active infection, and the repertoire of responses increased with age and active infection. However, rates of antibody acquisition varied between antigens and different regions within an antigen following exposure to malaria, supporting our hypothesis. Antigen-specific responses could be broadly classified into early response types in which antibodies were acquired early in childhood exposure and late response types that appear to require substantially more exposure for the development of substantial levels. We identified antigen-specific responses that were effectively boosted after recent infection, whereas other responses were not. These findings advance our understanding of the acquisition of human immunity to malaria and are relevant to the development of malaria vaccines targeting merozoite antigens and the selection of antigens for use in malaria surveillance.

Malaria Parasites: The Great Escape

Parasites of the genus Plasmodium have a complex life cycle. They alternate between their final mosquito host and their intermediate hosts. The parasite can be either extra- or intracellular, depending on the stage of development. By modifying their shape, motility, and metabolic requirements, the parasite adapts to the different environments in their different hosts. The parasite has evolved to escape the multiple immune mechanisms in the host that try to block parasite development at the different stages of their development. In this article, we describe the mechanisms reported thus far that allow the Plasmodium parasite to evade innate and adaptive immune responses.

Chemically Attenuated Blood-Stage Plasmodium yoelii Parasites Induce Long-Lived and Strain-Transcending Protection

The development of a vaccine is essential for the elimination of malaria. However, despite many years of effort, a successful vaccine has not been achieved. Most subunit vaccine candidates tested in clinical trials have provided limited efficacy, and thus attenuated whole-parasite vaccines are now receiving close scrutiny. Here, we test chemically attenuated Plasmodium yoelii 17X and demonstrate significant protection following homologous and heterologous blood-stage challenge. Protection against blood-stage infection persisted for at least 9 months. Activation of both CD4⁺ and CD8⁺ T cells was shown after vaccination; however, in vivo studies demonstrated a pivotal role for both CD4⁺ T cells and B cells since the absence of either cell type led to loss of vaccine-induced protection. In spite of significant activation of circulating CD8⁺ T cells, liver-stage immunity was not evident. Neither did vaccine-induced CD8⁺ T cells contribute to blood-stage protection; rather, these cells contributed to pathogenesis, since all vaccinated mice depleted of both CD4⁺ and CD8⁺ T cells survived a challenge infection. This study provides critical insight into whole-parasite vaccine-induced immunity and strong support for testing whole-parasite vaccines in humans.

Protection against malaria in mice is induced by blood stage-arresting histamine-releasing factor (HRF)-deficient parasites

Although most vaccines against blood stage malaria in development today use subunit preparations, live attenuated parasites confer significantly broader and more lasting protection. In recent years, Plasmodium genetically attenuated parasites (GAPs) have been generated in rodent models that cause self-resolving blood stage infections and induce strong protection. All such GAPs generated so far bear mutations in housekeeping genes important for parasite development in red blood cells. In this study, using a Plasmodium berghei model compatible with tracking anti-blood stage immune responses over time, we report a novel blood stage GAP that lacks a secreted factor related to histamine-releasing factor (HRF). Lack of HRF causes an IL-6 increase, which boosts T and B cell responses to resolve infection and leave a cross-stage, cross-species, and lasting immunity. Mutant-induced protection involves a combination of antiparasite IgG2c antibodies and FcγR(+) CD11b(+) cell phagocytes, especially neutrophils, which are sufficient to confer protection. This immune-boosting GAP highlights an important role of opsonized parasite-mediated phagocytosis, which may be central to protection induced by all self-resolving blood stage GAP infections.

Mice lacking Programmed cell death-1 show a role for CD8(+) T cells in long-term immunity against blood-stage malaria

Even after years of experiencing malaria, caused by infection with Plasmodium species, individuals still have incomplete immunity and develop low-density parasitemia on re-infection. Previous studies using the P. chabaudi (Pch) mouse model to understand the reason for chronic malaria, found that mice with a deletion of programmed cell death-1 (PD-1KO) generate sterile immunity unlike wild type (WT) mice. Here we investigated if the mechanism underlying this defect during acute immunity also impacts on long-term immunity. We infected WT and PD-1KO mice with Pch-malaria and measured protection as well as immune responses against re-infections, 15 or 20 weeks after the original infection had cleared. WT mice showed approximately 1% parasitemia compared to sterile immunity in PD-1KO mice on re-infection. An examination of the mechanisms of immunity behind this long-term protection in PD-1KO mice showed a key role for parasite-specific CD8⁺ T cells even when CD4⁺ T cells and B cells responded to re-infection. These studies indicate that long-term CD8⁺ T cell-meditated protection requires consideration for future malaria vaccine design, as part of a multi-cell type response.

Novel flow cytometry technique for detection of Plasmodium falciparum specific B-cells in humans: increased levels of specific B-cells in ongoing infection

BACKGROUND

Malaria caused by Plasmodium falciparum is still a major health threat in endemic areas especially for children below 5 years of age. While it is recognized that antibody immunity plays an important role in controlling the disease, knowledge of the mechanisms of sustenance and natural boosting of immunity is very limited. Before, it has not been possible to investigate malaria specific B-cells directly in flow cytometry, making it difficult to know how much of a B cell response is due to malaria, or how much is due to other immunological stimulators.

METHODS

This study developed a technique using quantum dots and schizont extract made from ghosts of infected erythrocytes, to be able to investigate P. falciparum specific B-cells, something that has never been done before.

RESULTS

Major differences in P. falciparum specific B-cells were found between samples from immune (22.3 %) and non-immune (1.7 %) individuals. Samples from parasite positive individuals had the highest proportions of specific B-cells (27.9 %).

CONCLUSION

The study showed increased levels of P. falciparum-specific B-cells in immune individuals, with the highest levels in active malaria infections, using a new technique that opens up new possibilities to study how these cells are sustained in vivo after natural infections. It will also be useful in vaccine studies.

Transient Loss of Protection Afforded by a Live Attenuated Non-typhoidal Salmonella Vaccine in Mice Co-infected with Malaria

In immunocompetent individuals, non-typhoidal Salmonella serovars (NTS) are associated with gastroenteritis, however, there is currently an epidemic of NTS bloodstream infections in sub-Saharan Africa. Plasmodium falciparum malaria is an important risk factor for invasive NTS bloodstream in African children. Here we investigated whether a live, attenuated Salmonella vaccine could be protective in mice, in the setting of concurrent malaria. Surprisingly, mice acutely infected with the nonlethal malaria parasite Plasmodium yoelii 17XNL exhibited a profound loss of protective immunity to NTS, but vaccine-mediated protection was restored after resolution of malaria. Absence of protective immunity during acute malaria correlated with maintenance of antibodies to NTS, but a marked reduction in effector capability of Salmonella-specific CD4 and CD8 T cells. Further, increased expression of the inhibitory molecule PD1 was identified on memory CD4 T cells induced by vaccination. Blockade of IL-10 restored protection against S. Typhimurium, without restoring CD4 T cell effector function. Simultaneous blockade of CTLA-4, LAG3, and PDL1 restored IFN-γ production by vaccine-induced memory CD4 T cells but was not sufficient to restore protection. Together, these data demonstrate that malaria parasite infection induces a temporary loss of an established adaptive immune response via multiple mechanisms, and suggest that in the setting of acute malaria, protection against NTS mediated by live vaccines may be interrupted.

In children, malaria is a predisposing factor for invasive bacterial infections with non-typhoidal Salmonella (NTS) serovars, a frequent cause of morbidity and mortality in sub-Saharan Africa. Since development of vaccines against NTS has been proposed as a strategy to protect African children against disseminated NTS infection, we interrogated the effect of malaria on vaccine-induced memory responses to NTS. Our results from a mouse infection model show that infection with malaria parasites temporarily suspends protective immunity conferred by a live, attenuated vaccine and suppresses adaptive immune responses to NTS that are mediated by T cells. These results suggest that in the setting of acute malaria, live attenuated NTS vaccines may lose their effectiveness.

Parasite Specific Antibody Increase Induced by an Episode of Acute P. falciparum Uncomplicated Malaria

Introduction

There is no approved vaccine for malaria, and precisely how human antibody responses to malaria parasite components and potential vaccine molecules are developed and maintained remains poorly defined. In this study, antibody anamnestic or memory response elicited by a single episode of P. falciparum infection was investigated.

Methods

This study involved 362 malaria patients aged between 6 months to 60 years, of whom 19% were early-diagnosed people living with HIV/AIDS (PLWHA). On the day malaria was diagnosed and 42 days later, blood specimens were collected. Parasite density, CD4⁺ cells, and antibodies specific to synthetic peptides representing antigenic regions of the P. falciparum proteins GLURP, MSP3 and HRPII were measured.

Results

On the day of malaria diagnosis, Immunoglobulin (IgG) antibodies against GLURP, MSP3 and HRP II peptides were present in the blood of 75%, 41% and 60% of patients, respectively. 42 days later, the majority of patients had boosted their serum IgG antibody more than 1.2 fold. The increase in level of IgG antibody against the peptides was not affected by parasite density at diagnosis. The median CD4⁺ cell counts of PLWHAs and HIV negative individuals were not statistically different, and median post-infection increases in anti-peptide IgG were similar in both groups of patients.

Conclusion

In the majority (70%) of individuals, an infection of P. falciparum elicits at least 20% increase in level of anti-parasite IgG. This boost in anti-P. falciparum IgG is not affected by parasite density on the day of malaria diagnosis, or by HIV status.

Antibody and B cell responses to Plasmodium sporozoites

Antibodies are capable of blocking infection of the liver by Plasmodium sporozoites. Accordingly the induction of anti-sporozoite antibodies is a major aim of various vaccine approaches to malaria. In recent years our knowledge of the specificity and quantities of antibodies required for protection has been greatly expanded by clinical trials of various whole sporozoite and subunit vaccines. Moreover, the development of humanized mouse models and transgenic parasites have also aided our ability to assess the specificity of antibodies and their ability to block infection. Nonetheless, considerable gaps remain in our knowledge – in particular in understanding what antigens are recognized by infection blocking antibodies and in knowing how we can induce robust, long-lived antibody responses. Maintaining high levels of circulating antibodies is likely to be of primary importance, as antibodies must block infection in the short time it takes for sporozoites to reach the liver from the skin. It is clear that a better understanding of the development of protective B cell-mediated immunity will aid the development and refinement of malaria vaccines.

Plasmodium falciparum malaria and invasive bacterial co-infection in young African children: the dysfunctional spleen hypothesis

Children with recent or acute malaria episodes are at increased risk of invasive bacterial infections (IBI). However, the exact nature of the malaria-IBI association is still unclear. Young children have an age-related spleen immunologic immaturity, mainly due to the still ongoing development of the marginal zone (MZ) B cell subset. By mounting a rapid antibody response against encapsulated bacteria, these cells are critical for the defence against highly pathogenic microorganisms that do not elicit classical T cell-dependent responses. There is increasing evidence that the anatomy of the spleen becomes disorganized during malaria infection, with complete dissolution of the MZ and apoptosis of MZ B cells. Correspondingly, a reduction in the frequency of the peripheral equivalent of the MZ B cells has been found in malaria endemic areas. A remarkable similarity exists in IBI susceptibility between African children with malaria and hyposplenic or splenectomized patients. However, studies specifically assessing the immune function of the spleen in controlling bacterial infections in young children with malaria are scarce.

Here, it is hypothesized that Plasmodium falciparum malaria infection constitutes a detrimental factor in the still immature spleen function of young children, resulting in a factually hyposplenic state during malaria episodes, putting children with malaria at a high risk to develop life-threatening bacterial infections. Studies to confirm or reject this hypothesis are greatly needed, as well as the development of affordable and feasible tools to assess the immune spleen function against encapsulated bacteria in children with malaria.

Memory B-cell and antibody responses induced by Plasmodium falciparum sporozoite immunization

Background

Immunization of healthy volunteers during receipt of chemoprophylaxis with Plasmodium falciparum sporozoites (CPS-immunization) induces sterile protection from malaria. Antibody responses have long been known to contribute to naturally acquired immunity against malaria, but their association with sterile protection after whole sporozoite immunization is not well established. We therefore studied the induction and kinetics of malaria parasite antigen-specific antibodies and memory B-cells (MBCs) during CPS-immunization and their correlation with protection from challenge infection.

Methods

We assessed humoral reactivity to 9 antigens representing different stages of the life cycle of P. falciparum by performing standardized MBC enzyme-linked immunospot and enzyme-linked immunosorbent assays on peripheral blood mononuclear cells and plasma samples from 38 Dutch volunteers enrolled in 2 randomized controlled clinical trials.

Results

MBCs and antibodies recognizing pre-erythrocytic and cross-stage antigens were gradually acquired during CPS-immunization. The magnitude of these humoral responses did not correlate with protection but directly reflected parasite exposure in CPS-immunization and challenge.

Conclusions

Humoral responses to the malarial antigens circumsporozoite protein, liver-stage antigen-1, apical membrane antigen-1, and merozoite surface protein-1 do not to predict protection from challenge infection but can be used as sensitive marker of recent parasite exposure.

Clinical Trials Registration

NCT01236612 and NCT01218893.

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.

Analysis of human B-cell responses following ChAd63-MVA MSP1 and AMA1 immunization and controlled malaria infection

Acquisition of non-sterilizing natural immunity to Plasmodium falciparum malaria has been shown in low transmission areas following multiple exposures. However, conflicting data from endemic areas suggest that the parasite may interfere with the induction of effective B-cell responses. To date, the impact of blood-stage parasite exposure on antigen-specific B cells has not been reported following controlled human malaria infection (CHMI). Here we analysed human B-cell responses in a series of Phase I/IIa clinical trials, which include CHMI, using candidate virus-vectored vaccines encoding two blood-stage antigens: merozoite surface protein 1 (MSP1) and apical membrane antigen 1 (AMA1). Previously vaccinated volunteers show boosting of pre-existing antigen-specific memory B-cell (mBC) responses following CHMI. In contrast, unvaccinated malaria-naive control volunteers developed an mBC response against MSP1 but not AMA1. Serum IgG correlated with the mBC response after booster vaccination but this relationship was less well maintained following CHMI. A significant reduction in peripheral MSP1-specific mBC was observed at the point of diagnosis of blood-stage infection. This was coincident with a reduction in peripheral blood B-cell subsets expressing CXCR3 and elevated serum levels of interferon-γ and CXCL9, suggesting migration away from the periphery. These CHMI data confirm that mBC and antibody responses can be induced and boosted by blood-stage parasite exposure, in support of epidemiological studies on low-level parasite exposure.

BAFF and BAFF receptor levels correlate with B cell subset activation and redistribution in controlled human malaria infection

Characteristic features of Plasmodium falciparum malaria are polyclonal B cell activation and an altered composition of the blood B cell compartment, including expansion of CD21(-)CD27(-) atypical memory B cells. BAFF is a key cytokine in B cell homeostasis, but its potential contribution to the modulation of the blood B cell pool during malaria remains elusive. In the controlled human malaria model (CHMI) in malaria-naive Dutch volunteers, we therefore examined the dynamics of BAFF induction and B cell subset activation and composition, to investigate whether these changes are linked to malaria-induced immune activation and, in particular, induction of BAFF. Alterations in B cell composition after CHMI closely resembled those observed in endemic areas. We further found distinct kinetics of proliferation for individual B cell subsets across all developmental stages. Proliferation peaked either immediately after blood-stage infection or at convalescence, and for most subsets was directly associated with the peak parasitemia. Concomitantly, plasma BAFF levels during CHMI were increased and correlated with membrane-expressed BAFF on monocytes and dendritic cells, as well as blood-stage parasitemia and parasite-induced IFN-γ. Correlating with increased plasma BAFF and IFN-γ levels, IgD(-)CD38(low)CD21(-)CD27(-) atypical B cells showed the strongest proliferative response of all memory B cell subsets. This provides unique evidence for a link between malaria-induced immune activation and temporary expansion of this B cell subset. Finally, baseline BAFF-R levels before CHMI were predictive of subsequent changes in proportions of individual B cell subsets. These findings suggest an important role of BAFF in facilitating B cell subset proliferation and redistribution as a consequence of malaria-induced immune activation.

Recovery of an antiviral antibody response following attrition caused by unrelated infection

The homeostatic mechanisms that regulate the maintenance of immunological memory to the multiple pathogen encounters over time are unknown. We found that a single malaria episode caused significant dysregulation of pre-established Influenza A virus-specific long-lived plasma cells (LLPCs) resulting in the loss of Influenza A virus-specific Abs and increased susceptibility to Influenza A virus re-infection. This loss of LLPCs involved an FcγRIIB-dependent mechanism, leading to their apoptosis. However, given enough time following malaria, the LLPC pool and humoral immunity to Influenza A virus were eventually restored. Supporting a role for continuous conversion of Influenza A virus-specific B into LLPCs in the restoration of Influenza A virus immunity, B cell depletion experiments also demonstrated a similar requirement for the long-term maintenance of serum Influenza A virus-specific Abs in an intact LLPC compartment. These findings show that, in addition to their established role in the anamnestic response to reinfection, the B cell pool continues to be a major contributor to the maintenance of long-term humoral immunity following primary Influenza A virus infection, and to the recovery from attrition following heterologous infection. These data have implications for understanding the longevity of protective efficacy of vaccinations in countries where continuous infections are endemic.

Antibody responses to infectious pathogens are critical in host survival, recovery and protection from reinfection; they also correlate with the success of vaccination. It is currently thought that antibody serum titers are maintained at protective levels over long periods of time by specialized long-lived antibody-secreting plasma cells residing in the bone marrow. Indeed, antibodies against the original virus can still be found in survivors of the 1918 Spanish Flu, more than 90 years ago. However, it is also becoming clear that subsequent infection with heterologous pathogens may cause attrition of previously established immunological memory, in order to accommodate new lymphocyte specificities in the finite space of the host. This phenomenon is seemingly at odds with long-term maintenance of immunological memory. We also show that a single episode of malaria, caused by infection by Plasmodium chabaudi, leads to the loss of preexisting plasma cells, serum antibodies and protective immunity against Influenza A virus. However, Influenza A virus-specific immunity does eventually recover in these animals with the replenishment of plasma cells by B cells over the course of several weeks. Thus, the reported mechanism reconciles attrition of immunological memory by heterologous infection and long-term stability, and places B cells, instead of their descendant plasma cells, at the center of humoral memory.

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.

An essential role for C5aR signaling in the optimal induction of a malaria-specific CD4+ T cell response by a whole-killed blood-stage vaccine

The protective immunity induced by the whole-killed parasite vaccine against malarial blood-stage infection is dependent on the CD4(+) T cell response. However, the mechanism underlying this robust CD4(+) T cell response elicited by the whole-killed parasite vaccine is still largely unknown. In this study, we observe that immunization with Plasmodium yoelii-parasitized RBC lysate activates complement C5 and generates C5a. However, the protective efficacy against P. yoelii 17XL challenge is considerably reduced, and the malaria-specific CD4(+) T cell activation and memory T cell differentiation are largely suppressed in the C5aR-deficient (C5aR(-/-)) mice. An adoptive transfer assay demonstrates that the reduced protection of C5aR(-/-) mice is closely associated with the severely impaired CD4(+) T cell response. This is further confirmed by the fact that administration of C5aR antagonist significantly reduces the protective efficacy of the immunized B cell-deficient mice. Further study indicates that the defective CD4(+) T cell response in C5aR(-/-) mice is unlikely involved in the expansion of CD4(+)CD25(+)Foxp3(+) T cells, but strongly linked to a defect in dendritic cell (DC) maturation and the ability to allostimulate CD4(+) T cells. These results demonstrate that C5aR signaling is essential for the optimal induction of the malaria-specific CD4(+) T cell response by the whole-killed parasite vaccine through modulation of DCs function, which provides us with new clues to design an effective blood-stage subunit vaccine and helps us to understand the mechanism by which the T cell response is regulated by the complement system.

Hemoglobinopathies: slicing the Gordian knot of Plasmodium falciparum malaria pathogenesis

Plasmodium falciparum malaria kills over 500,000 children every year and has been a scourge of humans for millennia. Owing to the co-evolution of humans and P. falciparum parasites, the human genome is imprinted with polymorphisms that not only confer innate resistance to falciparum malaria, but also cause hemoglobinopathies. These genetic traits—including hemoglobin S (HbS), hemoglobin C (HbC), and α-thalassemia—are the most common monogenic human disorders and can confer remarkable degrees of protection from severe, life-threatening falciparum malaria in African children: the risk is reduced 70% by homozygous HbC and 90% by heterozygous HbS (sickle-cell trait). Importantly, this protection is principally present for severe disease and largely absent for P. falciparum infection, suggesting that these hemoglobinopathies specifically neutralize the parasite's in vivo mechanisms of pathogenesis. These hemoglobin variants thus represent a “natural experiment” to identify the cellular and molecular mechanisms by which P. falciparum produces clinical morbidity, which remain partially obscured due to the complexity of interactions between this parasite and its human host. Multiple lines of evidence support a restriction of parasite growth by various hemoglobinopathies, and recent data suggest this phenomenon may result from host microRNA interference with parasite metabolism. Multiple hemoglobinopathies mitigate the pathogenic potential of parasites by interfering with the export of P. falciparum erythrocyte membrane protein 1 (PfEMP1) to the surface of the host red blood cell. Few studies have investigated their effects upon the activation of the innate and adaptive immune systems, although recent murine studies suggest a role for heme oxygenase-1 in protection. Ultimately, the identification of mechanisms of protection and pathogenesis can inform future therapeutics and preventive measures. Hemoglobinopathies slice the “Gordian knot” of host and parasite interactions to confer malaria protection, and offer a translational model to identify the most critical mechanisms of P. falciparum pathogenesis.

The enigma of memory B cells in malaria

Malaria is a major public health problem particularly in the tropics. It is caused by protozoan parasites belonging to the genus Plasmodium and is transmitted by Anopheles mosquitoes. Currently, strategies to control malaria include vector control measures, chemoprophylaxis, and efficient diagnosis and treatment. The availability of a highly efficacious malaria vaccine would greatly facilitate malaria control and possibly eradicate malaria. Efforts to design such malaria vaccines are underway but are greatly hampered by the poor understanding of how immune memory to malaria is generated and maintained. In this issue of the European Journal of Immunology, Wykes and colleagues [Eur. J. Immunol. 2012. 42: 3291-3301] demonstrate that experimental malaria infection lowers the expression of B-cell-activating factor in DCs, thereby compromising the ability of these DCs to stimulate memory B cells and sustain the survival of Ab-secreting cells. These findings provide potential clues in the quest for better understanding of immunity to malaria as discussed in this Commentary.

Long-term antibody memory induced by synthetic peptide vaccination is protective against Streptococcus pyogenes infection and is independent of memory T cell help

Streptococcus pyogenes (group A Streptococcus [GAS]) is a leading human pathogen associated with a diverse array of mucosal and systemic infections. Vaccination with J8, a conserved region synthetic peptide derived from the M-protein of GAS and containing only 12 aa from GAS, when conjugated to diphtheria toxoid, has been shown to protect mice against a lethal GAS challenge. Protection has been previously shown to be Ab-mediated. J8 does not contain a dominant GAS-specific T cell epitope. The current study examined long-term Ab memory and dissected the role of B and T cells. Our results demonstrated that vaccination generates specific memory B cells (MBC) and long-lasting Ab responses. The MBC response can be activated following boost with Ag or limiting numbers of whole bacteria. We further show that these memory responses protect against systemic infection with GAS. T cell help is required for activation of MBC but can be provided by naive T cells responding directly to GAS at the time of infection. Thus, individuals whose T cells do not recognize the short synthetic peptide in the vaccine will be able to generate a protective and rapid memory Ab response at the time of infection. These studies significantly strengthen previous findings, which showed that protection by the J8-diphtheria toxoid vaccine is Ab-mediated and suggest that in vaccine design for other organisms the source of T cell help for Ab responses need not be limited to sequences from the organism itself.

Malaria infection alters the expression of B-cell activating factor resulting in diminished memory antibody responses and survival

Malaria is a major cause of morbidity worldwide with reports of over 200-500 million infected individuals and nearly 1 million deaths each year. Antibodies have been shown to play a critical role in controlling the blood stage of this disease; however, in malaria-endemic areas antibody immunity is slow to develop despite years of exposure to Plasmodium spp. the causative parasite. Using rodent Plasmodium yoelii YM, we provide evidence that malarial infections result in a decrease in the proportion of DCs that express the B-cell survival factor, BAFF, resulting in a decreased ability of these DCs to support memory B-cell differentiation into antibody secreting cells (ASCs) and/or the survival of ASCs. Further, compared with infected WT mice, ASC numbers were significantly increased in malaria-infected transgenic mice that either overexpressed BAFF or mice with BAFF-independent B-cell survival (B-cell-restricted TRAF3 deletion). Remarkably, BAFF-overexpressing mice were protected from lethal malaria infections, indicating the significance of the role BAFF plays in determining the outcome of malaria infections. These findings describe a previously unappreciated mechanism by which Plasmodium spp. can depress the generation of protective antibody responses.

Plasmodium riboprotein PfP0 induces a deviant humoral immune response in Balb/c mice

Passive immunization with antibodies to recombinant Plasmodium falciparum P0 riboprotein (rPfP0, 61–316 amino acids) provides protection against malaria. Carboxy-terminal 16 amino acids of the protein (PfP0C0) are conserved and show 69% identity to human and mouse P0. Antibodies to this domain are found in 10–15% of systemic lupus erythematosus patients. We probed the nature of humoral response to PfP0C0 by repeatedly immunizing mice with rPfP0. We failed to raise stable anti-PfP0C0 hybridomas from any of the 21 mice. The average serum anti-PfP0C0 titer remained low (5.1 ± 1.3 × 10⁴). Pathological changes were observed in the mice after seven boosts. Adsorption with dinitrophenyl hapten revealed that the anti-PfP0C0 response was largely polyreactive. This polyreactivity was distributed across all isotypes. Similar polyreactive responses to PfP0 and PfP0C0 were observed in sera from malaria patients. Our data suggests that PfP0 induces a deviant humoral response, and this may contribute to immune evasion mechanisms of the parasite.

Kinetics of humoral and memory B cell response induced by the Plasmodium falciparum 19-kilodalton merozoite surface protein 1 in mice

The 19-kDa carboxyl-terminal fragment of the merozoite surface protein-1 (MSP-1(19)) has been shown to regulate antibody (Ab)-mediated protective immunity to blood-stage malaria infection. But the serological memory to this antigen tends to be short-lived, and little is known of the mechanisms that regulate the formation of B cell memory to MSP-1(19) antigen. We studied the formation of B cell memory response after immunization with the recombinant 19-kDa Plasmodium falciparum merozoite surface protein 1 (PfMSP-1(19)). Immunization with PfMSP-1(19) resulted in delayed increase in germinal center (GC) B cell numbers. This poor GC reaction correlated with short-lived PfMSP-1(19)-specific antibodies in serum and the short life of PfMSP-1(19)-specific plasma cells and memory B cells (MBCs) in spleen and bone marrow. PfMSP-1(19)-specific MBCs were capable of producing antigen (Ag)-specific Ab-secreting cell (ASC) responses that were short-lived following challenge immunization of the immune mice with antigen or transgenic Plasmodium berghei parasite expressing PfMSP-1(19) in place of native P. berghei MSP-1(19) at 8 weeks after the last immunization or following adoptive transfer into naive hosts. However, no protection was achieved in PfMSP-1(19) immune mice or recipient mice with PfMSP-1(19)-specific MBCs following challenge with transgenic P. berghei. Our findings suggest that PfMSP-1(19)-specific IgG production by short-lived plasma cells combined with the poor ability of the PfMSP-1(19)-induced MBCs to maintain the anamnestic IgG responses failed to contribute to protection against 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.

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.

Our impasse in developing a malaria vaccine

Malaria presents a challenge to world health that to date has been beyond the abilities of researchers to conquer. This critique presents some of the strategies employed by the parasite to overcome immunity and the immunological challenges that we face to develop vaccines. A conclusion is that a vaccine must identify novel antigens or epitopes that are not normally immunogenic and which are therefore not under immune pressure and most likely to be conserved between different strains. Such antigens are most likely to be targets of cellular immunity. The case for a whole parasite blood stage vaccine is presented based on these premises.

Effect of schistosoma infection on malaria immune response: A systematic review

EXECUTIVE SUMMARY

Background Worldwide an estimated 225 million cases and about 800, 000 deaths due to malaria were documented in 2009. Malaria vaccines have been developed as a malaria control strategy. Immune response to these vaccines might be affected by the blood fluke schistosoma which is often co-endemic with malaria in Sub-Saharan Africa where most of phase II and Phase III malaria vaccine trials were conducted.Objectives To systematically search, appraise and synthesize the best available evidence on the effect of schistosoma infection on the immune response to malaria antigens and provide direction to future malaria vaccination trials.Types of participants The review considered studies with above 5 year old individuals as participants.Phenomenon of interest The phenomenon of interest was the presence of schistosoma infectionTypes of outcomes Blood serum levels of Th1 and Th2 specific to Merozoite Surface Proteins 1, 2, and 3 of malaria were considered as primary outcomes. While blood serum levels of IgG1, IgG2, IgG3, IFN-γ, IL-10 and TGF-β directed against Merozoite Surface Proteins were considered as secondary outcomes.Types of studies Studies with any quantitative study designs were considered for inclusion.Search Strategy Any quantitative English language articles published between 1994 and April 2011 were sought using a comprehensive search strategy.Assessment of methodological quality It was done using Joanna Briggs Institutes' Meta Analysis of Statistical Assessment and Review Instrument critical appraisal tools.Data extraction Data extraction was carried out using the Joanna Briggs Institute Meta Analysis of Statistical Assessment and Review Instrument data extraction tool.Data synthesis Meta- analysis was conducted using random effects model with an inverse variance method with RevMan5 software. Heterogeneity between the studies was assessed using ξ test at a p-value of <0.1. Summary statistics were expressed as Standardized Mean Difference with 95% confidence intervals at a p-value of <0.05.Results From 3985 titles identified during the initial search, 3 cohort studies were included in the review following detailed examination and critical appraisal. Mean blood serum level of IgG1 directed against MSP of malaria was increased in S.hematobium positive individuals than in schistosoma negative individuals (0.478 and 0.181). This effect was small with no statistical significance, SMD (95% CI), 0.15 (-2.00, 2.31), p=0.89.Similarly a small and statistically not significant increase in mean blood serum levels of IgG3 and IFN-γ directed against MSP of malaria were found in schistosoma positive individuals than in schistosoma negative individuals with a SMD (95% CI) of 0.31 (-0.66, 1.29), p=0.52 and 0.16 (-0.27, 0.59), p=0.46, respectively.

CONCLUSIONS

Implications for Practice Concurrent schistosoma infection increases humoral immune response measures to malaria. This could confound an increase in humoral immune response measures after the administration of malaria vaccine. In addition, it might increase the incidence of malaria complication such as cerebral malaria by increasing IFN-γ levels.Implications for Research Further longitudinal studies aimed at determining the difference in long term response (cellular immunity) to malaria vaccine in individuals with and without schistosoma infection need to be undertaken.

Distinct kinetics of memory B-cell and plasma-cell responses in peripheral blood following a blood-stage Plasmodium chabaudi infection in mice

B cell and plasma cell responses take place in lymphoid organs, but because of the inaccessibility of these organs, analyses of human responses are largely performed using peripheral blood mononuclear cells (PBMC). To determine whether PBMC are a useful source of memory B cells and plasma cells in malaria, and whether they reflect Plasmodium-specific B cell responses in spleen or bone marrow, we have investigated these components of the humoral response in PBMC using a model of Plasmodium chabaudi blood-stage infections in C57BL/6 mice. We detected memory B cells, defined as isotype-switched IgD⁻ IgM⁻ CD19⁺ B cells, and low numbers of Plasmodium chabaudi Merozoite Surface Protein-1 (MSP1)-specific memory B cells, in PBMC at all time points sampled for up to 90 days following primary or secondary infection. By contrast, we only detected CD138⁺ plasma cells and MSP1-specific antibody-secreting cells within a narrow time frame following primary (days 10 to 25) or secondary (day 10) infection. CD138⁺ plasma cells in PBMC at these times expressed CD19, B220 and MHC class II, suggesting that they were not dislodged bone-marrow long-lived plasma cells, but newly differentiated migratory plasmablasts migrating to the bone marrow; thus reflective of an ongoing or developing immune response. Our data indicates that PBMC can be a useful source for malaria-specific memory B cells and plasma cells, but extrapolation of the results to human malaria infections suggests that timing of sampling, particularly for plasma cells, may be critical. Studies should therefore include multiple sampling points, and at times of infection/immunisation when the B-cell phenotypes of interest are likely to be found in peripheral blood.

Plasmodium berghei circumvents immune responses induced by merozoite surface protein 1- and apical membrane antigen 1-based vaccines

Background

Two current leading malaria blood-stage vaccine candidate antigens for Plasmodium falciparum, the C-terminal region of merozoite surface protein 1 (MSP1₁₉) and apical membrane antigen 1 (AMA1), have been prioritized because of outstanding protective efficacies achieved in a rodent malaria Plasmodium yoelii model. However, P. falciparum vaccines based on these antigens have had disappointing outcomes in clinical trials. Discrepancies in the vaccine efficacies observed between the P. yoelii model and human clinical trials still remain problematic.

Methodology and Results

In this study, we assessed the protective efficacies of a series of MSP1₁₉- and AMA1-based vaccines using the P. berghei rodent malarial parasite and its transgenic models. Immunization of mice with a baculoviral-based vaccine (BBV) expressing P. falciparum MSP1₁₉ induced high titers of PfMSP1₁₉-specific antibodies that strongly reacted with P. falciparum blood-stage parasites. However, no protection was achieved following lethal challenge with transgenic P. berghei expressing PfMSP1₁₉ in place of native PbMSP1₁₉. Similarly, neither P. berghei MSP1₁₉- nor AMA1-BBV was effective against P. berghei. In contrast, immunization with P. yoelii MSP1₁₉- and AMA1-BBVs provided 100% and 40% protection, respectively, against P. yoelii lethal challenge. Mice that naturally acquired sterile immunity against P. berghei became cross-resistant to P. yoelii, but not vice versa.

Conclusion

This is the first study to address blood-stage vaccine efficacies using both P. berghei and P. yoelii models at the same time. P. berghei completely circumvents immune responses induced by MSP1₁₉- and AMA1-based vaccines, suggesting that P. berghei possesses additional molecules and/or mechanisms that circumvent the host's immune responses to MSP1₁₉ and AMA1, which are lacking in P. yoelii. Although it is not known whether P. falciparum shares these escape mechanisms with P. berghei, P. berghei and its transgenic models may have potential as useful tools for identifying and evaluating new blood-stage vaccine candidate antigens for P. falciparum.

Anaplasma marginale infection with persistent high-load bacteremia induces a dysfunctional memory CD4+ T lymphocyte response but sustained high IgG titers

Control of blood-borne infections is dependent on antigen-specific effector and memory T cells and high-affinity IgG responses. In chronic infections characterized by a high antigen load, it has been shown that antigen-specific T and B cells are vulnerable to downregulation and apoptosis. Anaplasma marginale is a persistent infection of cattle characterized by acute and chronic high-load bacteremia. We previously showed that CD4(+) T cells primed by immunization with an A. marginale outer membrane protein were rapidly deleted following infection. Furthermore, peripheral blood T cell responses to bacteria were not observed after acute infection was controlled, suggesting dysfunctional T cell priming to other A. marginale antigens. The current study more closely investigated the kinetics of A. marginale-specific CD4(+) T cell responses primed during infection. Frequent sampling of peripheral blood and spleens revealed that antigen-specific CD4(+) T cell responses were first detected at 5 to 7 weeks, but the responses were sporadic and transient thereafter. A similar pattern was observed in animals sampled weekly for nearly 1 year. Paradoxically, by 2 weeks of infection, cattle had developed high titers of A. marginale-specific IgG, which remained high throughout persistent infection. This dysfunctional CD4(+) T cell response to infection is consistent with continual downregulation or deletion of newly primed effector T cells, similar to what was observed for immunization-induced T cells following A. marginale infection. The failure to establish a strong memory T cell response during A. marginale infection likely contributes to bacterial persistence.

Protective immune responses elicited by immunization with a chimeric blood-stage malaria vaccine persist but are not boosted by Plasmodium yoelii challenge infection

An efficacious malaria vaccine remains elusive despite concerted efforts. Using the Plasmodium yoelii murine model, we previously reported that immunization with the C-terminal 19 kDa domain of merozoite surface protein 1 (MSP1(19)) fused to full-length MSP8 protected against lethal P. yoelii 17XL, well beyond that achieved by single or combined immunizations with the component antigens. Here, we continue the evaluation of the chimeric PyMSP1/8 vaccine. We show that immunization with rPyMSP1/8 vaccine elicited an MSP8-restricted T cell response that was sufficient to provide help for both PyMSP1(19) and PyMSP8-specific B cells to produce high and sustained levels of protective antibodies. The enhanced efficacy of immunization with rPyMSP1/8, in comparison to a combined formulation of rPyMSP1(42) and rPyMSP8, was not due to improved conformation of protective B cell epitopes in the chimeric molecule. Unexpectedly, rPyMSP1/8 vaccine-induced antibody responses were not boosted by exposure to P. yoelii 17XL infected RBCs. However, rPyMSP1/8 immunized and infected mice mounted robust responses to a diverse set of blood-stage antigens. The data support the further development of an MSP1/8 chimeric vaccine but also suggest that vaccines that prime for responses to a diverse set of parasite proteins will be required to maximize vaccine efficacy.

The Plasmodium falciparum-specific human memory B cell compartment expands gradually with repeated malaria infections

Immunity to Plasmodium falciparum (Pf) malaria is only acquired after years of repeated infections and wanes rapidly without ongoing parasite exposure. Antibodies are central to malaria immunity, yet little is known about the B-cell biology that underlies the inefficient acquisition of Pf-specific humoral immunity. This year-long prospective study in Mali of 185 individuals aged 2 to 25 years shows that Pf-specific memory B-cells and antibodies are acquired gradually in a stepwise fashion over years of repeated Pf exposure. Both Pf-specific memory B cells and antibody titers increased after acute malaria and then, after six months of decreased Pf exposure, contracted to a point slightly higher than pre-infection levels. This inefficient, stepwise expansion of both the Pf-specific memory B-cell and long-lived antibody compartments depends on Pf exposure rather than age, based on the comparator response to tetanus vaccination that was efficient and stable. These observations lend new insights into the cellular basis of the delayed acquisition of malaria immunity.

Plasmodium falciparum (Pf) is a mosquito-borne parasite that causes over 500 million cases of malaria annually, one million of which result in death, primarily among African children. The development of an effective malaria vaccine would be a critical step toward the control and eventual elimination of this disease. To date, most licensed vaccines are for pathogens that induce long-lived protective antibodies after a single infection. In contrast, immunity to malaria is only acquired after repeated infections. Antibodies play a key role in protection from malaria, yet several studies indicate that antibodies against some Pf proteins are generated inefficiently and lost rapidly. The cells that are responsible for the maintenance of antibodies over the human lifespan are memory B-cells and long-lived plasma cells. To determine how these cells are generated and maintained in response to Pf infection, we conducted a year-long study in an area of Mali that experiences a six-month malaria season. We found memory B-cells and long-lived antibodies specific for the parasite were generated in a gradual, step-wise fashion over years despite intense Pf exposure. This contrasts sharply with the efficient response to tetanus vaccination in the same population. This study lends new insights into the delayed acquisition of malaria immunity. Future studies of the cellular and molecular basis of these observations could open the door to strategies for the development of a highly effective malaria vaccine.