Differential cellular recognition of antigens during acute Plasmodium falciparum and Plasmodium vivax malaria

Immunosuppression in Malaria: Do Plasmodium falciparum Parasites Hijack the Host?

Malaria reflects not only a state of immune activation, but also a state of general immune defect or immunosuppression, of complex etiology that can last longer than the actual episode. Inhabitants of malaria-endemic regions with lifelong exposure to the parasite show an exhausted or immune regulatory profile compared to non- or minimally exposed subjects. Several studies and experiments to identify and characterize the cause of this malaria-related immunosuppression have shown that malaria suppresses humoral and cellular responses to both homologous (Plasmodium) and heterologous antigens (e.g., vaccines). However, neither the underlying mechanisms nor the relative involvement of different types of immune cells in immunosuppression during malaria is well understood. Moreover, the implication of the parasite during the different stages of the modulation of immunity has not been addressed in detail. There is growing evidence of a role of immune regulators and cellular components in malaria that may lead to immunosuppression that needs further research. In this review, we summarize the current evidence on how malaria parasites may directly and indirectly induce immunosuppression and investigate the potential role of specific cell types, effector molecules and other immunoregulatory factors.

A Synthetic Nanoparticle Based Vaccine Approach Targeting MSP4/5 Is Immunogenic and Induces Moderate Protection Against Murine Blood-Stage Malaria

Malaria remains a significant health problem in many tropical and sub-tropical regions. The development of vaccines against the clinically active blood-stage of infection needs to consider variability and polymorphism in target antigens, and an adjuvant system able to induce broad spectrum immunity comprising both antibodies and helper T cells. Moreover, recent studies have shown some conventional pro-inflammatory adjuvants can also promote expansion of immunosuppressive regulatory T cells (Treg) and myeloid derived suppressor cells (MDSC), both of which could negatively impact malaria disease progression. Herein, we explore the ability of a model nanoparticle delivery system (polystyrene nanoparticles; PSNPs), previously proven to not induce conventional inflammation, Treg or MDSC, to induce immunity to MSP4/5 from Plasmodium yoelii, a member of the MSP4 and MSP5 family of proteins which are highly conserved across diverse malaria species including P. falciparum. The results show PSNPs-MSP4/5 conjugates are highly immunogenic, inducing immune responses comprising both T helper 1 (Th1) and Th2 cellular immunity, and a spectrum of antibody subclasses including IgG1, IgG2a, and IgG2b. Benchmarked against Alum and Complete Freund's Adjuvant (CFA), the immune responses that were induced were of comparable or higher magnitude, for both T cell frequencies by ELISpot and antibody responses in terms of ELISA end titer. Importantly, immunization with PSNPs-MSP4/5 induced partial protection against malaria blood-stage infection (50–80%) shown to be mechanistically dependent on interferon gamma (IFN-γ) production. These results expand the scope of adjuvants considered for malaria blood-stage vaccine development to those that do not use conventional adjuvant pathways and emphasizes the critical role of cellular immunity and specifically IFN-γ producing cells in providing moderate protection against blood-stage malaria comparable to Freunds adjuvant.

Naturally Acquired Binding-Inhibitory Antibodies to Plasmodium vivax Duffy Binding Protein in Pregnant Women Are Associated with Higher Birth Weight in a Multicenter Study

A vaccine to eliminate malaria would need a multi-stage and multi-species composition to achieve robust protection, but the lack of knowledge about antigen targets and mechanisms of protection precludes the development of fully efficacious malaria vaccines, especially for Plasmodium vivax (Pv). Pregnant women constitute a risk population who would greatly benefit from a vaccine preventing the adverse events of Plasmodium infection during gestation. We hypothesized that functional immune responses against putative targets of naturally acquired immunity to malaria and vaccine candidates will be associated with protection against malaria infection and/or poor outcomes during pregnancy. We measured (i) IgG responses to a large panel of Pv and Plasmodium falciparum (Pf) antigens, (ii) the capacity of anti-Pv ligand Duffy binding protein (PvDBP) antibodies to inhibit binding to Duffy antigen, and (iii) cellular immune responses to two Pv antigens, in a subset of 1,056 pregnant women from Brazil, Colombia, Guatemala, India, and Papua New Guinea (PNG). There were significant intraspecies and interspecies correlations for most antibody responses (e.g., PfMSP1₁₉ versus PfAMA1, Spearman’s rho = 0.81). Women from PNG and Colombia had the highest levels of IgG overall. Submicroscopic infections seemed sufficient to boost antibody responses in Guatemala but not antigen-specific cellular responses in PNG. Brazil had the highest percentage of Duffy binding inhibition (p-values versus Colombia: 0.040; Guatemala: 0.047; India: 0.003, and PNG: 0.153) despite having low anti-PvDBP IgG levels. Almost all antibodies had a positive association with present infection, and coinfection with the other species increased this association. Anti-PvDBP, anti-PfMSP1, and anti-PfAMA1 IgG levels at recruitment were positively associated with infection at delivery (p-values: 0.010, 0.003, and 0.023, respectively), suggesting that they are markers of malaria exposure. Peripheral blood mononuclear cells from Pv-infected women presented fewer CD8⁺IFN-γ⁺ T cells and secreted more G-CSF and IL-4 independently of the stimulus used in vitro. Functional anti-PvDBP levels at recruitment had a positive association with birth weight (difference per doubling antibody levels: 45 g, p-value: 0.046). Thus, naturally acquired binding-inhibitory antibodies to PvDBP might confer protection against poor outcomes of Pv malaria in pregnancy.

Characterizing Antibody Responses to Plasmodium vivax and Plasmodium falciparum Antigens in India Using Genome-Scale Protein Microarrays

Understanding naturally acquired immune responses to Plasmodium in India is key to improving malaria surveillance and diagnostic tools. Here we describe serological profiling of immune responses at three sites in India by probing protein microarrays consisting of 515 Plasmodium vivax and 500 Plasmodium falciparum proteins with 353 plasma samples. A total of 236 malaria-positive (symptomatic and asymptomatic) plasma samples and 117 malaria-negative samples were collected at three field sites in Raurkela, Nadiad, and Chennai. Indian samples showed significant seroreactivity to 265 P. vivax and 373 P. falciparum antigens, but overall seroreactivity to P. vivax antigens was lower compared to P. falciparum antigens. We identified the most immunogenic antigens of both Plasmodium species that were recognized at all three sites in India, as well as P. falciparum antigens that were associated with asymptomatic malaria. This is the first genome-scale analysis of serological responses to the two major species of malaria parasite in India. The range of immune responses characterized in different endemic settings argues for targeted surveillance approaches tailored to the diverse epidemiology of malaria across the world.

Although malaria deaths have fallen by 60% worldwide since 2000, the disease remains a significant public health problem. India has the highest burden of malaria in the South-East Asia Region, where Plasmodium vivax and Plasmodium falciparum are its main causes. While the two major malaria parasite species co-occur in India, their proportion varies across the country. Antibodies in an individual indicate current or past Plasmodium infection, and can be used to identify suitable vaccine candidates, as well as develop novel tools for malaria surveillance. We present the results of a pilot study undertaking the first large-scale characterization of antibody responses to ~1000 Plasmodium antigens at three field sites in India using high-throughput protein microarray technology. Individuals from the eco-epidemiologically diverse sites showed reactivity to 265 P. vivax and 373 P. falciparum antigens, regardless of infection status. Further comparison of individuals with symptomatic and asymptomatic malaria revealed the most immunogenic Plasmodium antigens, as well as antigens that were recognized with greater intensity in individuals that were asymptomatic at the point of sample collection. These results are a valuable addition to existing data from other malaria endemic regions, and will help to expand our understanding of host immunity against the disease.

Insights into the naturally acquired immune response toPlasmodium vivaxmalaria

Plasmodium vivaxis the most geographically widespread of the malaria parasites causing human disease, yet it is comparatively understudied compared withPlasmodium falciparum.In this article we review what is known about naturally acquired immunity toP. vivax, and importantly, how this differs to that acquired againstP. falciparum.Immunity to clinicalP. vivaxinfection is acquired more quickly than toP. falciparum, and evidence suggests humans in endemic areas also have a greater capacity to mount a successful immunological memory response to this pathogen. Both of these factors give promise to the idea of a successfulP. vivaxvaccine. We review what is known about both the cellular and humoral immune response, including the role of cytokines, antibodies, immunoregulation, immune memory and immune dysfunction. Furthermore, we discuss where the future lies in terms of advancing our understanding of naturally acquired immunity toP. vivax, through the use of well-designed longitudinal epidemiological studies and modern tools available to immunologists.

Immunity to malaria in Plasmodium vivax infection: a study in central China

Background

P. vivax infection is characterised by relapsing fever, indicating reinfection by previously hidden parasites in the host. Relapsed infection can lead to the activation of the memory T cell pool, which may lead to protective immunity. This study aims to characterise immune responses in acute P. vivax-infected patients living in an area of central China characterised by only P. vivax infection.

Methodology/Principal Findings

We conducted a cross-sectional immune-phenotypic analysis of adults using the following inclusion criteria: acute P. vivax infection (N = 37), a history of P. vivax infection (N = 17), and no known history of P. vivax infection (N = 21). We also conducted a 2-week longitudinal analysis following acute P. vivax infection, in which PBMC proliferation was measured in response to P. vivax and P. falciparum blood stage lysates. Using flow cytometry, we showed elevated memory T cells in the blood during acute P. vivax infection. The levels of γδ T cells were two-fold higher than those measured in naive controls. This result suggested that in the two populations, memory and γδ T cells promptly responded to P. vivax parasites. Interestingly, P. falciparum antigens stimulated T cells obtained from P. vivax-infected patients during a day 14-convalescence, whereas lymphocytes from the naïve control group responded to a lower degree of convalescence.

Conclusions/Significance

Cell-mediated immunity during the convalescent period of the P. vivax-infected hosts was comprised of T cells that were specifically able to recognise P. falciparum antigens. Although the magnitude of the response was only half that measured after stimulation with P. vivax antigens, the matter of cross-antigenic stimulation is of great interest.