Natural regulatory T cells in malaria: host or parasite allies?

Cerebral malaria pathogenesis: Dissecting the role of CD4+ and CD8+ T-cells as major effectors in disease pathology

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum (P. falciparum) infection, with complex pathogenesis involving multiple factors, including the host's immunological response. T lymphocytes, specifically CD4+ T helper cells and CD8+ cytotoxic T cells, are crucial in controlling parasite growth and activating cells for parasite clearance via cytokine secretion. Contrary to this, reports also suggest the pathogenic nature of T lymphocytes as they are often involved in disease progression and severity. CD8+ cytotoxic T cells migrate to the host's brain vasculature, disrupting the blood-brain barrier and causing neurological manifestations. CD4+ T helper cells on the other hand play a variety of functions as they differentiate into different subtypes which may function as pro-inflammatory or anti-inflammatory. The excessive pro-inflammatory response in CM can lead to multi-organ failure, necessitating a check mechanism to maintain immune homeostasis. This is achieved by regulatory T cells and their characteristic cytokines, which counterbalance the pro-inflammatory immune response. Maintaining a critical balance between pro and anti-inflammatory responses is crucial for determining disease outcomes in CM. A slight change in this balance may contribute to a disease severity owing to an extreme inflammatory response or unrestricted parasite growth, a potential target for designing immunotherapeutic treatment approaches. The review briefly discusses the pathogenesis of CM and various mechanisms responsible for the disruption of the blood-brain barrier. It also highlights the role of different T cell subsets during infection and emphasizes the importance of balance between pro and anti-inflammatory T cells that ultimately decides the outcome of the disease.

Dysfunction of CD169+ macrophages and blockage of erythrocyte maturation as a mechanism of anemia in Plasmodium yoelii infection

Plasmodium parasites cause malaria with disease outcomes ranging from mild illness to deadly complications such as severe malarial anemia (SMA), pulmonary edema, acute renal failure, and cerebral malaria. In young children, SMA often requires blood transfusion and is a major cause of hospitalization. Malaria parasite infection leads to the destruction of infected and noninfected erythrocytes as well as dyserythropoiesis; however, the mechanism of dyserythropoiesis accompanied by splenomegaly is not completely understood. Using Plasmodium yoelii yoelii 17XNL as a model, we show that both a defect in erythroblastic island (EBI) macrophages in supporting red blood cell (RBC) maturation and the destruction of reticulocytes/RBCs by the parasites contribute to SMA and splenomegaly. After malaria parasite infection, the destruction of both infected and noninfected RBCs stimulates extramedullary erythropoiesis in mice. The continuous decline of RBCs stimulates active erythropoiesis and drives the expansion of EBIs in the spleen, contributing to splenomegaly. Phagocytosis of malaria parasites by macrophages in the bone marrow and spleen may alter their functional properties and abilities to support erythropoiesis, including reduced expression of the adherence molecule CD169 and inability to support erythroblast differentiation, particularly RBC maturation in vitro and in vivo. Therefore, macrophage dysfunction is a key mechanism contributing to SMA. Mitigating and/or alleviating the inhibition of RBC maturation may provide a treatment strategy for SMA.

Preclinical Study of Plasmodium Immunotherapy Combined with Radiotherapy for Solid Tumors

Immune checkpoint blockade therapy (ICB) is ineffective against cold tumors and, although it is effective against some hot tumors, drug resistance can occur. We have developed a Plasmodium immunotherapy (PI) that can overcome these shortcomings. However, the specific killing effect of PI on tumor cells is relatively weak. Radiotherapy (RT) is known to have strong specific lethality to tumor cells. Therefore, we hypothesized that PI combined with RT could produce synergistic antitumor effects. We tested our hypothesis using orthotopic and subcutaneous models of mouse glioma (GL261, a cold tumor) and a subcutaneous model of mouse non-small cell lung cancer (NSCLC, LLC, a hot tumor). Our results showed that, compared with each monotherapy, the combination therapy more significantly inhibited tumor growth and extended the life span of tumor-bearing mice. More importantly, the combination therapy could cure approximately 70 percent of glioma. By analyzing the immune profile of the tumor tissues, we found that the combination therapy was more effective in upregulating the perforin-expressing effector CD8⁺ T cells and downregulating the myeloid-derived suppressor cells (MDSCs), and was thus more effective in the treatment of cancer. The clinical transformation of PI combined with RT in the treatment of solid tumors, especially glioma, is worthy of expectation.

Pathophysiology of Cerebral Malaria: Implications of MSCs as A Regenerative Medicinal Tool

The severe form of malaria, i.e., cerebral malaria caused by Plasmodium falciparum, is a complex neurological syndrome. Surviving persons have a risk of behavioral difficulties, cognitive disorders, and epilepsy. Cerebral malaria is associated with multiple organ dysfunctions. The adhesion and accumulation of infected RBCs, platelets, and leucocytes (macrophages, CD4⁺ and CD8⁺ T cells, and monocytes) in the brain microvessels play an essential role in disease progression. Micro-vascular hindrance by coagulation and endothelial dysfunction contributes to neurological damage and the severity of the disease. Recent studies in human cerebral malaria and the murine model of cerebral malaria indicate that different pathogens as well as host-derived factors are involved in brain microvessel adhesion and coagulation that induces changes in vascular permeability and impairment of the blood-brain barrier. Efforts to alleviate blood-brain barrier dysfunction and de-sequestering of RBCs could serve as adjunct therapies. In this review, we briefly summarize the current understanding of the pathogenesis of cerebral malaria, the role of some factors (NK cells, platelet, ANG-2/ANG-1 ratio, and PfEMP1) in disease progression and various functions of Mesenchymal stem cells. This review also highlighted the implications of MSCs as a regenerative medicine.

IL-6 dependent expansion of inflammatory MDSCs (CD11b+ Gr-1+) promote Th-17 mediated immune response during experimental cerebral malaria

Myeloid derived suppressor cells (MDSCs) are a group of heterogeneous cell populations that can suppress T cell responses. Various aspects of MDSCs in regulating immune responses in several cancer and infectious diseases have been reported till date. But the role and regulation of MDSCs have not been systematically studied in the context of malaria. This study depicts the phenotypic and functional characteristics of splenic MDSCs and how they regulate Th-17 mediated immune response during Experimental Cerebral Malaria (ECM). Flow cytometric analysis reveals that MDSCs in the spleen and bone marrow expand at 8 dpi during ECM. Among subtypes of MDSCs, PMN-MDSCs show significant expansion in the spleen but M-MDSCs remain unaltered. Functional analysis of sorted MDSCs from spleens of Plasmodium berghei ANKA (PbA) infected mice shows suppressive nature of these cells and high production of Nitric oxide (NO). Besides, MDSCs were also found to express various inflammatory markers during ECM suggesting the M1 type phenotype of these cells. In-vivo depletion of MDSCs by the use of Anti Gr-1 increases mice survival but doesn't significantly alter the parasitemia. Previously, it has been reported that Treg/Th-17 balance in the spleen is skewed towards Th-17 during ECM. Depletion of MDSCs was found to regulate Th-17 percentages to homeostatic levels and subvert various inflammatory changes in the spleen. Among different factors, IL-6 was found to play an important role in the expansion of MDSCs and expression of inflammatory markers on MDSCs in a STAT3-dependent manner. These findings provide a unique insight into the role of IL-6 in the expansion of the MDSC population which causes inflammatory changes and increased Th-17 responses during ECM.

Possible Interactions between Malaria, Helminthiases and the Gut Microbiota: A Short Review

Malaria, caused by the Plasmodium species, is an infectious disease responsible for more than 600 thousand deaths and more than 200 million morbidity cases annually. With above 90% of those deaths and cases, sub-Saharan Africa is affected disproportionately. Malaria clinical manifestations range from asymptomatic to simple, mild, and severe disease. External factors such as the gut microbiota and helminthiases have been shown to affect malaria clinical manifestations. However, little is known about whether the gut microbiota has the potential to influence malaria clinical manifestations in humans. Similarly, many previous studies have shown divergent results on the effects of helminths on malaria clinical manifestations. To date, a few studies, mainly murine, have shown the gut microbiota’s capacity to modulate malaria’s prospective risk of infection, transmission, and severity. This short review seeks to summarize recent literature about possible interactions between malaria, helminthiases, and the gut microbiota. The knowledge from this exercise will inform innovation possibilities for future tools, technologies, approaches, and policies around the prevention and management of malaria in endemic countries.

A comprehensive study of epitopes and immune reactivity among Plasmodium species

Background

Malaria is a life-threatening disease caused by protozoan parasite of genus Plasmodium. Various antigenic proteins of Plasmodium are considered as the major targets for the development of an effective vaccine. The aim of the current study was a comprehensive analysis of the experimentally validated epitopes of Plasmodium obtained from various immunoassays.

Methods

Plasmodium species epitopes were prefetched from Immune Epitope Database (IEDB). Species specific classification of available epitopes was done for both human and murine malaria parasites. Further, these T cell and B cell epitopes along with MHC I/II binders of different Plasmodium species were examined to find out their capability to induce IFN-γ and IL-10 using IFNepitope and IL-10 Pred, respectively.

Results

The species-specific classification of 6874 unique epitopes resulted in the selection of predominant human and murine Plasmodium species. Further, the attempt was made to analyse the immune reactivity of these epitopes for their ability to induce cytokines namely IFN-γ and IL-10. Total, 2775 epitopes were predicted to possess IFN-γ inducing ability, whereas 1275 epitopes were found to be involved in the induction of IL-10.

Conclusions

This study facilitates the assessment of Plasmodium epitopes and associated proteins as a potential approach to design and develop an epitope-based vaccine. Moreover, the results highlight the epitope-based immunization in malaria to induce a protective immune response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02480-7.

CD4+ICOS+Foxp3+: a sub-population of regulatory T cells contribute to malaria pathogenesis

BACKGROUND

Regulatory T cells are known to play a key role to counter balance the protective immune response and immune mediated pathology. However, the role of naturally occurring regulatory cells CD4⁺CD25⁺Foxp3⁺ in malaria infection during the disease pathogenesis is controversial. Beside this, ICOS molecule has been shown to be involved in the development and function of regulatory T cell enhance IL-10 production. Therefore, possible involvement of the ICOS dependent regulatory CD4⁺ICOS⁺Foxp3⁺ T cells in resistance/susceptibility during malaria parasite is explored in this study.

METHODS

5 × 10⁵ red blood cells infected with non-lethal and lethal parasites were inoculated in female Balb/c mice by intra-peritoneal injection. Infected or uninfected mice were sacrificed at early (3rd day post infection) and later stage (10th day post infection) of infection. Harvested cells were analysed by using flow cytometer and serum cytokine by Bioplex assay.

RESULTS

Thin blood films show that percentages of parasitaemia increases with disease progression in infections with the lethal malaria parasite and mice eventually die by day 14th post-infection. Whereas in case of non-lethal malaria parasite, parasitaemia goes down by 7th day post infection and gets cleared within 13th day. The number of CD4⁺ ICOS⁺ T cells increases in lethal infection with disease progression. Surprisingly, in non-lethal parasite, ICOS expression decreases after day 7th post infection as parasitaemia goes down. The frequency of CD4⁺ICOS⁺FoxP3⁺ Tregs was significantly higher in lethal parasitic infection as compared to the non-lethal parasite. The level of IL-12 cytokine was remarkably higher in non-lethal infection compared to the lethal infection. In contrast, the level of IL-10 cytokines was higher in lethal parasite infection compared to the non-lethal parasite.

CONCLUSION

Taken together, these data suggest that lethal parasite induce immunosuppressive environment, protecting from host immune responses and help the parasite to survive whereas non-lethal parasite leads to low frequencies of Treg cells seldom impede immune response that allow the parasite to get self-resolved.

Comparison of the impact of allelic polymorphisms in PfAMA1 on the induction of T Cell responses in high and low malaria endemic communities in Ghana

BACKGROUND

Malaria eradication requires a combined effort involving all available control tools, and these efforts would be complemented by an effective vaccine. The antigen targets of immune responses may show polymorphisms that can undermine their recognition by immune effectors and hence render vaccines based on antigens from a single parasite variant ineffective against other variants. This study compared the influence of allelic polymorphisms in Plasmodium falciparum apical membrane antigen 1 (PfAMA1) peptide sequences from three strains of P. falciparum (3D7, 7G8 and FVO) on their function as immunodominant targets of T cell responses in high and low malaria transmission communities in Ghana.

METHODS

Peripheral blood mononuclear cells (PBMCs) from 10 subjects from a high transmission area (Obom) and 10 subjects from a low transmission area (Legon) were tested against 15 predicted CD8 + T cell minimal epitopes within the PfAMA1 antigen of multiple parasite strains using IFN-γ ELISpot assay. The peptides were also tested in similar assays against CD8 + enriched PBMC fractions from the same subjects in an effort to characterize the responding T cell subsets.

RESULTS

In assays using unfractionated PBMCs, two subjects from the high transmission area, Obom, responded positively to four (26.7%) of the 15 tested peptides. None of the Legon subject PBMCs yielded positive peptide responses using unfractionated PBMCs. In assays with CD8 + enriched PBMCs, three subjects from Obom made positive recall responses to six (40%) of the 15 tested peptides, while only one subject from Legon made a positive recall response to a single peptide. Overall, 5 of the 20 study subjects who had positive peptide-specific IFN-γ recall responses were from the high transmission area, Obom. Furthermore, while subjects from Obom responded to peptides in PfAMA1 from multiple parasite strains, one subject from Legon responded to a peptide from 3D7 strain only.

CONCLUSIONS

The current data demonstrate the possibility of a real effect of PfAMA1 polymorphisms on the induction of T cell responses in malaria exposed subjects, and this effect may be more pronounced in communities with higher parasite exposure.

Expansion of Functional Myeloid-Derived Suppressor Cells in Controlled Human Malaria Infection

Malaria can cause life-threatening complications which are often associated with inflammatory reactions. More subtle, but also contributing to the burden of disease are chronic, often subclinical infections, which result in conditions like anemia and immunologic hyporesponsiveness. Although very frequent, such infections are difficult to study in endemic regions because of interaction with concurrent infections and immune responses. In particular, knowledge about mechanisms of malaria-induced immunosuppression is scarce. We measured circulating immune cells by cytometry in healthy, malaria-naïve, adult volunteers undergoing controlled human malaria infection (CHMI) with a focus on potentially immunosuppressive cells. Infectious Plasmodium falciparum (Pf) sporozoites (SPZ) (PfSPZ Challenge) were inoculated during two independent studies to assess malaria vaccine efficacy. Volunteers were followed daily until parasites were detected in the circulation by RT-qPCR. This allowed us to analyze immune responses during pre-patency and at very low parasite densities in malaria-naïve healthy adults. We observed a consistent increase in circulating polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) in volunteers who developed P. falciparum blood stage parasitemia. The increase was independent of preceding vaccination with a pre-erythrocytic malaria vaccine. PMN-MDSC were functional, they suppressed CD4⁺ and CD8⁺ T cell proliferation as shown by ex-vivo co-cultivation with stimulated T cells. PMN-MDSC reduced T cell proliferation upon stimulation by about 50%. Interestingly, high circulating PMN-MDSC numbers were associated with lymphocytopenia. The number of circulating regulatory T cells (T_reg) and monocytic MDSC (M-MDSC) showed no significant parasitemia-dependent variation. These results highlight PMN-MDSC in the peripheral circulation as an early indicator of infection during malaria. They suppress CD4⁺ and CD8⁺ T cell proliferation in vitro. Their contribution to immunosuppression in vivo in subclinical and uncomplicated malaria will be the subject of further research. Pre-emptive antimalarial pre-treatment of vaccinees to reverse malaria-associated PMN-MDSC immunosuppression could improve vaccine response in exposed individuals.

Balancing in a black box: Potential immunomodulatory roles for TGF-β signaling during blood-stage malaria

Malarial disease caused by Plasmodium parasites challenges the mammalian immune system with a delicate balancing act. Robust inflammatory responses are required to control parasite replication within red blood cells, which if unchecked, can lead to severe anemia and fatality. However, the same inflammatory response that controls parasite replication is also associated with immunopathology and severe disease, as is exemplified by cerebral malaria. A robust literature has identified critical roles for innate, cellular, and humoral immune responses orchestrated by IFN-γ and T_H1 type responses in controlling blood stage malarial disease. In contrast, TGF-β and IL-10 have been identified as important anti–inflammatory immunomodulators that help to limit inflammation and pathology during malaria. TGF-β is a pleiotropic cytokine, with the ability to exert a wide variety of context-dependent immunomodulatory roles.

The specific mechanisms that allow TGF-β to protect against malarial pathology remain essentially unexplored and offer a promising avenue to dissect the most critical elements of immunomodulation in avoiding severe malaria. Here we discuss potential immunomodulatory roles for TGF-β during malaria in light of recent advances in our understanding of the role of Tregs during blood-stage malaria.

Mesenchymal Stem Cells: A Novel Therapeutic Approach to Enhance Protective Immunomodulation and Erythropoietic Recovery in Malaria

Mesenchymal stem cells (MSCs) are self-renewing, multi-potent heterogeneous stem cells that display strong tissue protective and restorative properties by differentiating into cells of the mesodermal lineages. In addition to multi-lineage differentiation capacity, MSCs play important roles in regulating immune responses, inflammation, and tissue regeneration. MSCs play a role in the outcome of the pathogenesis of several infectious diseases. A unique subset of MSCs accumulates in secondary lymphoid organs during malaria disease progression. These MSCs counteract the capacity of malaria parasites to subvert activating co-stimulatory molecules and to regulate expression of negative co-stimulatory molecules on T lymphocytes. Consequently, MSCs have the capacity to restore the functions of CD34⁺ haematopoietic cells and CD4⁺ and CD8⁺ T cells during malaria infection. These observations suggest that cell-based therapeutics for intervention in malaria may be useful in achieving sterile clearance and preventing disease reactivation. In addition, MSCs provide host protection against malaria by reprogramming erythropoiesis through accelerated formation of colony-forming-units-erythroid (CFU-E) cells in the bone marrow. These findings suggest that MSCs are positive regulators of erythropoiesis, making them attractive targets for treatment of malarial anemia. MSC-based therapies, unlike anti-malarial drugs, display therapeutic effects by targeting a large variety of cellular processes rather than a single pathway. In the present review we focus on these recent research findings and discuss clinical applications of MSC-based therapies for malaria.

Accelerator or Brake: Immune Regulators in Malaria

Malaria is a life-threatening infectious disease, affecting over 250 million individuals worldwide each year, eradicating malaria has been one of the greatest challenges to public health for a century. Growing resistance to anti-parasitic therapies and lack of effective vaccines are major contributing factors in controlling this disease. However, the incomplete understanding of parasite interactions with host anti-malaria immunity hinders vaccine development efforts to date. Recent studies have been unveiling the complexity of immune responses and regulators against Plasmodium infection. Here, we summarize our current understanding of host immune responses against Plasmodium-derived components infection and mainly focus on the various regulatory mechanisms mediated by recent identified immune regulators orchestrating anti-malaria immunity.

T cell-mediated immunity to malaria

Immunity to malaria has been linked to the availability and function of helper CD4⁺ T cells, cytotoxic CD8⁺ T cells and γδ T cells that can respond to both the asymptomatic liver stage and the symptomatic blood stage of Plasmodium sp. infection. These T cell responses are also thought to be modulated by regulatory T cells. However, the precise mechanisms governing the development and function of Plasmodium-specific T cells and their capacity to form tissue-resident and long-lived memory populations are less well understood. The field has arrived at a point where the push for vaccines that exploit T cell-mediated immunity to malaria has made it imperative to define and reconcile the mechanisms that regulate the development and functions of Plasmodium-specific T cells. Here, we review our current understanding of the mechanisms by which T cell subsets orchestrate host resistance to Plasmodium infection on the basis of observational and mechanistic studies in humans, non-human primates and rodent models. We also examine the potential of new experimental strategies and human infection systems to inform a new generation of approaches to harness T cell responses against malaria.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

IL35 modulation altered survival, cytokine environment and histopathological consequences during malaria infection in mice

BACKGROUND

The immune modulating potential of IL-35 in multiple human disorders has been reported. Consequent upon the recognition of inflammatory cytokine activation and its preponderance for mediating pathology during malaria infection, the study aimed to characterize the expression and functional contribution(s) of IL-35 in Plasmodium berghei (strain ANKA) infected mice.

METHODS

Plasmodium berghei infection in male ICR mice was used as the rodent model of choice. The time course of IL-35 expression in the systemic circulation and tissues of P. berghei infected mice as well as their healthy control counterparts was assessed by enzyme linked immunosorbent assay and immunohistochemistry respectively. The effect of modulating IL-35 by recombinant IL-35 protein or neutralizing anti-Epstein-Barr virus-induced gene 3 antibody on the cytokine environment during P. berghei infection was assessed by flow cytometry. Furthermore, the influence of modulating IL-35 on histopathological hallmarks of malaria and disease progression was evaluated.

RESULTS

Interleukin-35 was significantly up regulated in serum and tissues of P. berghei infected mice and correlated with parasitaemia. Neutralization of IL-35 significantly enhanced the release of IFN-γ, decreased the expression of IL-6 and decreased parasitaemia patency. Neutralization of IL-35 was also associated with a tendency towards increased survival as well as the absence of pathological features associated with malaria infection unlike recombinant IL-35 protein administration which sustained a normal course of infection and unfavourable malaria associated histological outcomes in P. berghei infected mice.

CONCLUSION

These results indicate the involvement of IL-35 in P. berghei induced malaria infection. IL-35 neutralization strategies may represent viable therapeutic modalities beneficial for the resolution of malaria infection.

Unexpected encounter of the parasitic kind

Both parasitology and stem cell research are important disciplines in their own right. Parasites are a real threat to human health causing a broad spectrum of diseases and significant annual rates morbidity and mortality globally. Stem cell research, on the other hand, focuses on the potential for regenerative medicine for a range of diseases including cancer and regenerative therapies. Though these two topics might appear distant, there are some “unexpected encounters”. In this review, we summarise the various links between parasites and stem cells. First, we discuss how parasites’ own stem cells represent interesting models of regeneration that can be translated to human stem cell regeneration. Second, we explore the interactions between parasites and host stem cells during the course of infection. Third, we investigate from a clinical perspective, how stem cell regeneration can be exploited to help circumvent the damage induced by parasitic infection and its potential to serve as treatment options for parasitic diseases in the future. Finally, we discuss the importance of screening for pathogens during organ transplantation by presenting some clinical cases of parasitic infection following stem cell therapy.

Immune effector mechanisms in malaria: An update focusing on human immunity

The past decade has witnessed dramatic decreases in malaria-associated mortality and morbidity around the world. This progress has largely been due to intensified malaria control measures, implementation of rapid diagnostics and establishing a network to anticipate and mitigate antimalarial drug resistance. However, the ultimate tool for malaria prevention is the development and implementation of an effective vaccine. To date, malaria vaccine efforts have focused on determining which of the thousands of antigens expressed by Plasmodium falciparum are instrumental targets of protective immunity. The antigenic variation and antigenic polymorphisms arising in parasite genes under immune selection present a daunting challenge for target antigen selection and prioritization, and is a given caveat when interpreting immune recall responses or results from monovalent vaccine trials. Other immune evasion strategies executed by the parasite highlight the myriad of ways in which it can become a recurrent infection. This review provides an update on immune effector mechanisms in malaria and focuses on our improved ability to interrogate the complexity of human immune system, accelerated by recent methodological advances. Appreciating how the human immune landscape influences the effectiveness and longevity of antimalarial immunity will help explain which conditions are necessary for immune effector mechanisms to prevail.

Induction of Plasmodium-Specific Immune Responses Using Liposome-Based Vaccines

In the development of vaccines, the ability to initiate both innate and subsequent adaptive immune responses need to be considered. Live attenuated vaccines achieve this naturally, while inactivated and sub-unit vaccines generally require additional help provided through delivery systems and/or adjuvants. Liposomes present an attractive adjuvant/delivery system for antigens. Here, we review the key aspects of immunity against Plasmodium parasites, liposome design considerations and their current application in the development of a malaria vaccine.

Characterization of T cell activation and regulation in children with asymptomatic Plasmodium falciparum infection

Background

Asymptomatic Plasmodium infections are characterized by the absence of clinical disease and the ability to restrict parasite replication. Increasing levels of regulatory T cells (Tregs) in Plasmodium falciparum infections have been associated with the risk of developing clinical disease, suggesting that individuals with asymptomatic infections may have reduced Treg frequency. However, the relationship between Tregs, cellular activation and parasite control in asymptomatic malaria remains unclear.

Methods

In a cross-sectional study, the levels of Tregs and other T cell activation phenotypes were compared using flow cytometry in symptomatic, asymptomatic and uninfected children before and after stimulation with infected red blood cell lysates (iRBCs). In addition, the association between these T cell phenotypes and parasitaemia were investigated.

Results

In children with asymptomatic infections, levels of Tregs and activated T cells were comparable to those in healthy controls but significantly lower than those in symptomatic children. After iRBC stimulation, levels of Tregs remained lower for asymptomatic versus symptomatic children. In contrast, levels of activated T cells were higher for asymptomatic children. Strikingly, the pre-stimulation levels of two T cell activation phenotypes (CD8+CD69+ and CD8+CD25+CD69+) and the post-stimulation levels of two regulatory phenotypes (CD4+CD25+Foxp3+ and CD8+CD25+Foxp3+) were significantly positively correlated with and explained 68% of the individual variation in parasitaemia. A machine-learning model based on levels of these four phenotypes accurately distinguished between asymptomatic and symptomatic children (sensitivity = 86%, specificity = 94%), suggesting that these phenotypes govern the observed variation in disease status.

Conclusion

Compared to symptomatic P. falciparum infections, in children asymptomatic infections are characterized by lower levels of Tregs and activated T cells, which are associated with lower parasitaemia. The results indicate that T cell regulatory and activation phenotypes govern both parasitaemia and disease status in paediatric malaria in the studied sub-Saharan African population.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2410-6) contains supplementary material, which is available to authorized users.

Avian malaria and bird humoral immune response

Background

Plasmodium parasites are known to impose fitness costs on their vertebrate hosts. Some of these costs are due to the activation of the immune response, which may divert resources away from self-maintenance. Plasmodium parasites may also immuno-deplete their hosts. Thus, infected individuals may be less able to mount an immune response to a new pathogen than uninfected ones. However, this has been poorly investigated.

Methods

The effect of Plasmodium infection on bird humoral immune response when encountering a novel antigen was tested. A laboratory experiment was conducted on canaries (Serinus canaria) experimentally infected with Plasmodium relictum (lineage SGS1) under controlled conditions. Birds were immune challenged with an intra-pectoral injection of a novel non-pathogenic antigen (keyhole limpet haemocyanin, KLH). One week later they were challenged again. The immune responses to the primary and to the secondary contacts were quantified as anti-KLH antibody production via enzyme-linked immunosorbent assay (ELISA).

Results

There was no significant difference in antibody production between uninfected and Plasmodium infected birds at both primary and secondary contact. However, Plasmodium parasite intensity in the blood increased after the primary contact with the antigen.

Conclusions

There was no effect of Plasmodium infection on the magnitude of the humoral immune response. However, there was a cost of mounting an immune response in infected individuals as parasitaemia increased after the immune challenge, suggesting a trade-off between current control of chronic Plasmodium infection and investment against a new immune challenge.

Regulatory T cells impede acute and long-term immunity to blood-stage malaria through CTLA-4

Malaria, caused by the protozoan Plasmodium, is a devastating mosquito-borne disease with the potential to affect nearly half the world's population. Despite mounting substantial T and B cell responses, humans fail to efficiently control blood-stage malaria or develop sterilizing immunity to reinfections. Although forkhead box P3 (FOXP3)⁺CD4⁺ regulatory T (T_reg) cells form a part of these responses, their influence remains disputed and their mode of action is unknown. Here we show that T_reg cells expand in both humans and mice in blood-stage malaria and interfere with conventional T helper cell responses and follicular T helper (T_FH)-B cell interactions in germinal centers. Mechanistically, T_reg cells function in a critical temporal window to impede protective immunity through cytotoxic-T-lymphocyte-associated protein-4 (CTLA-4). Targeting T_reg cells or CTLA-4 in this precise window accelerated parasite clearance and generated species-transcending immunity to blood-stage malaria in mice. Our study uncovers a critical mechanism of immunosuppression associated with blood-stage malaria that delays parasite clearance and prevents development of potent adaptive immunity to reinfection. These data also reveal a temporally discrete and potentially therapeutically amenable functional role for T_reg cells and CTLA-4 in limiting antimalarial immunity.

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

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

Antecedent Nippostrongylus infection alters the lung immune response to Plasmodium berghei

In endemic regions, it is not uncommon for patients to be co-infected with soil-transmitted helminths and malaria. Although both malaria and many helminth species use the lungs as a site of development, little attention has been paid to the impact that pulmonary immunity induced by one parasite has on the lung response to the other. To model the consequences of a prior hookworm exposure on the development of immunity to malaria in the lungs, mice were infected with Nippostrongylus brasiliensis and 2 weeks later challenged with Plasmodium berghei. We found that a pre-existing hookworm-induced type 2 immune environment had a measurable but modest impact on the nature of the malaria-driven type 1 cytokine response in the lungs that was associated with a transient effect on parasite development and no significant changes in morbidity and mortality after malaria infection. However, prior hookworm infection did have a lasting effect on lung macrophages, where the malaria-induced M1-like response was blunted by previous M2 polarization. These results demonstrate that, although helminth parasites confer robust changes to the immunological status of the pulmonary microenvironment, lung immunity is plastic and capable of rapidly adapting to consecutive heterologous infections.

TOLLIP gene variant is associated with Plasmodium vivax malaria in the Brazilian Amazon

Background

Toll-interacting protein is a negative regulator in the TLR signaling cascade, particularly by impeding the TLR2 and, TLR4 pathway. Recently, TOLLIP was shown to regulate human TLR signaling pathways. Two common TOLLIP polymorphisms (rs5743899 and rs3750920) were reported to be influencing IL-6, TNF and IL-10 expression. In this study, TOLLIP variants were investigated to their relation to Plasmodium vivax malaria in the Brazilian Amazon.

Methods

This cohort study was performed in the municipalities of Careiro and, Manaus, in Western Brazilian Amazon. A total of 319 patients with P. vivax malaria and, 263 healthy controls with no previous history of malaria were included in the study. Genomic DNA was extracted from blood collected on filter paper, using the QIAamp^® DNA Mini Kit, according to the manufacturer’s suggested protocol. The rs5743899 and rs3750920 polymorphisms of the TOLLIP gene were typed by PCR–RFLP.

Results

Homozygous individuals for the rs3750920 T allele gene had twice the risk of developing malaria when compared to individuals homozygous for the C allele (OR 2.0 [95% CI 1.23–3.07]; p = 0.004). In the dominant model, carriers the C allele indicates protection to malaria, carriers of the C allele were compared to individuals with the T allele, and the difference is highly significant (OR 0.52 [95% CI 0.37–0.76]; p = 0.0006). The linkage disequilibrium between the two polymorphisms was weak (r² = 0.037; D′ = 0.27).

Conclusions

These findings suggest that genes involved in the TLRs-pathway may be involved in malaria susceptibility. The association of the TOLLIP rs3750920 T allele with susceptibility to malaria further provides evidence that genetic variations in immune response genes may predispose individuals to malaria.

On the cytokine/chemokine network during Plasmodium vivax malaria: new insights to understand the disease

Background

The clinical outcome of malaria depends on the delicate balance between pro-inflammatory and immunomodulatory cytokine responses triggered during infection. Despite the numerous reports on characterization of plasma levels of cytokines/chemokines, there is no consensus on the profile of these mediators during blood stage malaria. The identification of acute phase biomarkers might contribute to a better understanding of the disease, allowing the use of more effective therapeutic approaches to prevent the progression towards severe disease. In the present study, the plasma levels of cytokines and chemokines and their association with parasitaemia and number of previous malaria episodes were evaluated in Plasmodium vivax-infected patients during acute and convalescence phase, as well as in healthy donors.

Methods

Samples of plasma were obtained from peripheral blood samples from four different groups: P. vivax-infected, P. vivax-treated, endemic control and malaria-naïve control. The cytokine (IL-6, IL-10, IL-17, IL-27, TGF-β, IFN-γ and TNF) and chemokine (MCP-1/CCL2, IP-10/CXCL10 and RANTES/CCL5) plasma levels were measured by CBA or ELISA. The network analysis was performed using Spearman correlation coefficient.

Results

Plasmodium vivax infection induced a pro-inflammatory response driven by IL-6 and IL-17 associated with an immunomodulatory profile mediated by IL-10 and TGF-β. In addition, a reduction was observed of IFN-γ plasma levels in P. vivax group. A lower level of IL-27 was observed in endemic control group in comparison to malaria-naïve control group. No significant results were found for IL-12p40 and TNF. It was also observed that P. vivax infection promoted higher levels of MCP-1/CCL2 and IP-10/CXCL10 and lower levels of RANTES/CCL5. The plasma level of IL-10 was elevated in patients with high parasitaemia and with more than five previous malaria episodes. Furthermore, association profile between cytokine and chemokine levels were observed by correlation network analysis indicating signature patterns associated with different parasitaemia levels.

Conclusions

The P. vivax infection triggers a balanced immune response mediated by IL-6 and MCP-1/CCL2, which is modulated by IL-10. In addition, the results indicated that IL-10 plasma levels are influenced by parasitaemia and number of previous malaria episodes.

Lymph node - an organ for T-cell activation and pathogen defense

The immune system is a multicentered organ that is characterized by intimate interactions between its cellular components to efficiently ward off invading pathogens. A key constituent of this organ system is the distinct migratory activity of its cellular elements. The lymph node represents a pivotal meeting point of immune cells where adaptive immunity is induced and regulated. Additionally, besides barrier tissues, the lymph node is a critical organ where invading pathogens need to be eliminated in order to prevent systemic distribution of virulent microbes. Here, we explain how the lymph node is structurally and functionally organized to fulfill these two critical functions - pathogen defense and orchestration of adaptive immunity. We will discuss spatio-temporal aspects of cellular immune responses focusing on CD8 T cells and review how and where these cells are activated in the context of viral infections, as well as how viral antigen expression kinetics and different antigen presentation pathways are involved. Finally, we will describe how such responses are regulated and 'helped', and discuss how this relates to intranodal positioning and cellular migration of the various cellular components that are involved in these processes.

Plasmodium infection and oxidative status in breeding great tits, Parus major

Background

Plasmodium parasites may affect the oxidative status of their hosts, defined as the balance of pro-oxidant compounds and antioxidant defences in an organism. An increased energy requirement, the activation of immune functions or the parasite itself may lead to a higher production of pro-oxidants and/or an antioxidant depletion resulting in a higher oxidative stress and associated damage in infected individuals. Relatively little is known about the mechanisms underlying oxidative processes at play during host-Plasmodium interaction in the wild.

Methods

The effect of Plasmodium infection on host oxidative status was investigated in wild populations of breeding great tits, Parus major, naturally infected by Plasmodium spp. When chicks were 14 days old, the parents were blood-sampled to measure four complementary oxidative status markers: pro-oxidant production as mitochondrial superoxide production in red blood cells (RBC), antioxidant defences as plasma antioxidant capacity and oxidative damage as reactive oxygen metabolites in the plasma and RBC membrane resistance to oxidative attack.

Results

Plasmodium-infected individuals produced more pro-oxidants compared to uninfected ones and pro-oxidant production positively correlated to infection intensity. There was also a conditional effect of reproductive effort on oxidative damage depending on Plasmodium infection status. There was no direct effect of infection on oxidative damage and no effect on antioxidant defences.

Conclusions

The results suggest that Plasmodium parasites may impose a cost in terms of increased oxidative stress possibly mediated via a higher energy requirement in infected hosts. This further suggests that Plasmodium parasites may modify host life history traits via an induction of oxidative stress. This study highlights that measuring several complementary oxidative status markers may enable to capture oxidative processes at play during host-Plasmodium interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1579-9) contains supplementary material, which is available to authorized users.

A Novel Model of Asymptomatic Plasmodium Parasitemia That Recapitulates Elements of the Human Immune Response to Chronic Infection

In humans, immunity to Plasmodium sp. generally takes the form of protection from symptomatic malaria (i.e., 'clinical immunity') rather than infection ('sterilizing immunity'). In contrast, mice infected with Plasmodium develop sterilizing immunity, hindering progress in understanding the mechanistic basis of clinical immunity. Here we present a novel model in which mice persistently infected with P. chabaudi exhibit limited clinical symptoms despite sustaining patent parasite burdens for many months. Characterization of immune responses in persistently infected mice revealed development of CD4⁺ T cell exhaustion, increased production of IL-10, and expansion of B cells with an atypical surface phenotype. Additionally, persistently infected mice displayed a dramatic increase in circulating nonclassical monocytes, a phenomenon that we also observed in humans with both chronic Plasmodium exposure and asymptomatic infection. Following pharmacological clearance of infection, previously persistently infected mice could not control a secondary challenge, indicating that persistent infection disrupts the sterilizing immunity that typically develops in mouse models of acute infection. This study establishes an animal model of asymptomatic, persistent Plasmodium infection that recapitulates several central aspects of the immune response in chronically exposed humans. As such, it provides a novel tool for dissection of immune responses that may prevent development of sterilizing immunity and limit pathology during infection.

Immune response pattern in recurrent Plasmodium vivax malaria

Background

Plasmodium vivax is the causative agent of human malaria of large geographic distribution, with 35 million cases annually. In Brazil, it is the most prevalent species, being responsible by around 70 % of the malaria cases.

Methods

A cross-sectional study was performed in Manaus (Amazonas, Brazil), including 36 adult patients with primary malaria, 19 with recurrent malaria, and 20 endemic controls. The ex vivo phenotypic features of circulating leukocyte subsets (CD4⁺ T-cells, CD8⁺ T-cells, NK, NKT, B, B1 and Treg cells) as well as the plasmatic cytokine profile (IL-2, IL-4, IL-6, IL-10, TNF and IFN-γ) were assessed, aiming at establishing patterns of immune response characteristic of primary malaria vs recurrent malaria as compared to endemic controls.

Results

The proportion of subjects with high levels of WBC was reduced in malaria patients as compared to the endemic control. Monocytes were diminished particularly in patients with primary malaria. The proportion of subjects with high levels of all lymphocyte subsets was decreased in all malaria groups, regardless their clinical status. Decreased proportion of subjects with high levels of CD4⁺ and CD8⁺ T-cells was found especially in the group of patients with recurrent malaria. Data analysis indicated significant increase in the proportion of the subjects with high plasmatic cytokine levels in both malaria groups, characterizing a typical cytokine storm. Recurrent malaria patients displayed the highest plasmatic IL-10 levels, that correlated directly with the CD4⁺/CD8⁺ T-cells ratio and the number of malaria episodes.

Conclusion

The findings confirm that the infection by the P. vivax causes a decrease in peripheral blood lymphocyte subsets, which is intensified in the cases of “recurrent malaria”. The unbalanced CD4⁺/CD8⁺ T-cells ratio, as well as increased IL-10 levels were correlated with the number of recurrent malaria episodes. These results suggest that the gradual remodelling of the immune response is dependent on the repeated exposure to the parasite, which involves a strict control of the immune response mediated by the CD4⁺/CD8⁺ T-cell unbalance and exacerbated IL-10 secretion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1501-5) contains supplementary material, which is available to authorized users.

Regulatory issues in immunity to liver and blood-stage malaria

T cells play a major role in control of both blood and liver stage of plasmodium infection. While immunization with certain attenuated whole-parasite vaccines that are attenuated at the liver stage of the infection induces protective T cell responses, even multiple exposures to natural infection in endemic areas do not lead to stable T cell memory or humoral immunity and sterilizing protection. One of the key differences between vaccination and natural exposure is the absence of blood stage during vaccination. Here we will discuss possible immunoregulatory strategies employed by blood stage of malaria leading to generation of severely compromised T cell and humoral immune responses and subsequent lack of sterilizing immunity.

Surface expression of inhibitory (CTLA-4) and stimulatory (OX40) receptors by CD4+ regulatory T cell subsets circulating in human malaria

Several CD4⁺ T cell subtypes contribute to immune homeostasis in malaria, but the markers that define the main suppressive T cell subsets induced by this infection remain largely unknown. Here we provide a detailed phenotypic characterization of immunoregulatory CD4⁺ T cell populations in uncomplicated human malaria. We found an increased proportion of CD4⁺ T cells expressing CTLA-4, OX40, GITR, TNFRII, and CD69 in acute-phase single-species infections with Plasmodium vivax, Plasmodium falciparum, or both. Such an increase was not proportional to parasite density in P. vivax infections, and did not persist after parasite clearance. Significantly, less than 10% of CD4⁺ T cells expressing these regulatory molecules had the classical T regulatory (Treg) phenotype (CD4⁺CD25⁺CD127^-FoxP3⁺). Two major Treg cell subpopulations, which together accounted for 19-23% of all Treg cells circulating in malaria patients, expressed surface receptors with opposing regulatory functions, either CTLA-4 or OX40. OX40⁺ Treg cells outnumbered their CTLA-4⁺ counterparts (1.8:1) during acute P. vivax infection, while a more balanced ratio (1.3:1) was observed following parasite clearance These data reveal new players in the complex CD4⁺ Treg cell network that maintains immune homeostasis in malaria and suggest potential targets for therapeutic interventions to improve parasite-specific effector immune responses.

Cytokine modulation of human blood viscosity from vivax malaria patients

Malaria is a major infectious disease in several countries and is caused by protozoa of the genus Plasmodium. In vivax malaria patients, inflammatory processes occur, as well as changes in cytokines and blood flow. The present study analyzed the cytokine modulation of blood viscosity from patients infected with Plasmodium vivax (P. vivax). Blood samples were collected from 42 non-infected individuals (control group) and 37 individuals infected with P. vivax. The IL-2, IL-4, IL-6, IL-10, TNFα, TGF-β and IL-17 cytokine concentrations in the serum were assessed, and the blood rheological properties were determined. The analysis of blood viscosity for shear rates revealed that the blood viscosity of the infected patients was significantly greater than that of the non-infected individuals. The viscosity of the blood was greater in the infected individuals than in the non-infected subjects. The serum from individuals with P. vivax infections exhibited higher IFN-γ and IL-17 concentrations and lower TGF-β levels. Incubation of the blood from infected individuals with IL-17 or IL-17 associated with IFN-γ reduced the viscosity to rates equivalent to the blood from non-infected individuals. Independently of cytokine modulation, no correlation was found between the parasitemia and blood viscosity of the infected patients. These data suggest that the alterations of blood viscosity are relevant as an auxiliary tool for the clinical diagnosis of disease. In malaria, erythrocytes are more sensitive to osmotic shock, and the reduction of viscosity by IL-17 may be related to a possible immunomodulator agent during infection.

The role of regulatory T cells during Plasmodium chabaudi chabaudi AS infection in BALB/c mice

An inappropriate immune response to parasite infection is one of the primary drivers of malaria pathogenesis. Regulatory T cells (Tregs), an important subset of CD4(+) T cells, can maintain self-tolerance and prevent autoimmune diseases. However, there is little consensus about their role in malaria pathogenesis. In this study, we transiently depleted Tregs (CD25(+) T cells) using an anti-CD25 mAb (7D4 clone) at different time points following Plasmodium chabaudi chabaudi AS infection in BALB/c mice and investigated the effect of depletion of Tregs in this model. In control mice, Tregs proliferated significantly and their suppressive function was enhanced after infection. IL-10 was increased drastically during infection. Depletion of Tregs at various time points can lead to divergent outcomes. When Tregs were depleted prior to or during the early phase of infection, most mice survived and had a robust Th1 immune response. In contrast, when Tregs were depleted close to peak parasitemia, all mice died as a result of inflammation. Taken together, these data suggest that in P. c. chabaudi AS-infected BALB/c mice, Tregs inhibit the Th1 response and macrophage activation, leading to increased parasite load; however, they also control inflammation-mediated pathology by secreting high levels of IL-10.

Depletion of Phagocytic Cells during Nonlethal Plasmodium yoelii Infection Causes Severe Malaria Characterized by Acute Renal Failure in Mice

In the current study, we examined the effects of depletion of phagocytes on the progression of Plasmodium yoelii 17XNL infection in mice. Strikingly, the depletion of phagocytic cells, including macrophages, with clodronate in the acute phase of infection significantly reduced peripheral parasitemia but increased mortality. Moribund mice displayed severe pathological damage, including coagulative necrosis in liver and thrombi in the glomeruli, fibrin deposition, and tubular necrosis in kidney. The severity of infection was coincident with the increased sequestration of parasitized erythrocytes, the systematic upregulation of inflammation and coagulation, and the disruption of endothelial integrity in the liver and kidney. Aspirin was administered to the mice to minimize the risk of excessive activation of the coagulation response and fibrin deposition in the renal tissue. Interestingly, treatment with aspirin reduced the parasite burden and pathological lesions in the renal tissue and improved survival of phagocyte-depleted mice. Our data imply that the depletion of phagocytic cells, including macrophages, in the acute phase of infection increases the severity of malarial infection, typified by multiorgan failure and high mortality.

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

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

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.

Polymorphism in liver-stage malaria vaccine candidate proteins: immune evasion and implications for vaccine design

The pre-erythrocytic stage of infection by malaria parasites represents a key target for vaccines that aim to eradicate malaria. Two important broad immune evasion strategies that can interfere with vaccine efficacy include the induction of dendritic cell (DC) dysfunction and regulatory T cells (Tregs) by blood-stage malaria parasites, leading to inefficient priming of T cells targeting liver-stage infections. The parasite also uses 'surgical strike' strategies, whereby polymorphism in pre-erythrocytic antigens can interfere with host immunity. Specifically, we review how even single amino acid changes in T cell epitopes can lead to loss of binding to major histocompatibility complex (MHC), lack of cross-reactivity, or antagonism and immune interference, where simultaneous or sequential stimulation with related variants of the same T cell epitope can cause T cell anergy or the conversion of effector to immunosuppressive T cell phenotypes.

Suppression of CD4+ Effector Responses by Naturally Occurring CD4+ CD25+ Foxp3+ Regulatory T Cells Contributes to Experimental Cerebral Malaria

The role of naturally occurring CD4(+) CD25(+) Foxp3(+) regulatory T cells (nTreg) in the pathogenesis of cerebral malaria (CM), which involves both pathogenic T cell responses and parasite sequestration in the brain, is still unclear. To assess the contribution and dynamics of nTreg during the neuropathogenesis, we unbalanced the ratio between nTreg and naive CD4(+) T cells in an attenuated model of Plasmodium berghei ANKA-induced experimental CM (ECM) by using a selective cell enrichment strategy. We found that nTreg adoptive transfer accelerated the onset and increased the severity of CM in syngeneic C57BL/6 (B6) P. berghei ANKA-infected mice without affecting the level of parasitemia. In contrast, naive CD4(+) T cell enrichment prevented CM and promoted parasite clearance. Furthermore, early during the infection nTreg expanded in the spleen but did not efficiently migrate to the site of neuroinflammation, suggesting that nTreg exert their pathogenic action early in the spleen by suppressing the protective naive CD4(+) T cell response to P. berghei ANKA infection in vivo in both CM-susceptible (B6) and CM-resistant (B6-CD4(-/-)) mice. However, their sole transfer was not sufficient to restore CM susceptibility in two CM-resistant congenic strains tested. Altogether, these results demonstrate that nTreg are activated and functional during P. berghei ANKA infection and that they contribute to the pathogenesis of CM. They further suggest that nTreg may represent an early target for the modulation of the immune response to malaria.

Regulatory T-Cells at the Interface between Human Host and Pathogens in Infectious Diseases and Vaccination

Regulatory T-cells (Tregs) act at the interface of host and pathogen interactions in human infectious diseases. Tregs are induced by a wide range of pathogens, but distinct effects of Tregs have been demonstrated for different pathogens and in different stages of infection. Moreover, Tregs that are induced by a specific pathogen may non-specifically suppress immunity against other microbes and parasites. Thus, Treg effects need to be assessed not only in homologous but also in heterologous infections and vaccinations. Though Tregs protect the human host against excessive inflammation, they probably also increase the risk of pathogen persistence and chronic disease, and the possibility of disease reactivation later in life. Mycobacterium leprae and Mycobacterium tuberculosis, causing leprosy and tuberculosis, respectively, are among the most ancient microbes known to mankind, and are master manipulators of the immune system toward tolerance and pathogen persistence. The majority of mycobacterial infections occur in settings co-endemic for viral, parasitic, and (other) bacterial coinfections. In this paper, we discuss recent insights in the activation and activity of Tregs in human infectious diseases, with emphasis on early, late, and non-specific effects in disease, coinfections, and vaccination. We highlight mycobacterial infections as important models of modulation of host responses and vaccine-induced immunity by Tregs.

Proinflammatory responses and higher IL-10 production by T cells correlate with protection against malaria during pregnancy and delivery outcomes

Pregnancy triggers immunological changes aimed to tolerate the fetus. However, it has not been properly addressed whether similar changes occur in tropical areas with high infection pressure and whether these changes render women more susceptible to infectious diseases. We compared the frequencies of T cell subsets, including regulatory T cells, in pregnant and nonpregnant women from Papua New Guinea, a high malaria transmission area, and from Spain, a malaria-free country. We also assessed the relationship among these cellular subsets, malaria infection, and delivery outcomes. CD4(+)FOXP3(+)CD127(low) T cells (Tregs) were decreased in pregnant women in both countries but were not associated with malaria infection or poor delivery outcomes. An expansion of IFN-γ-producing cells and intracytoplasmic IFN-γ levels was found in pregnant compared with nonpregnant women only in Papua New Guinea. Increased CD4(+)IL-10(+)IFN-γ(+) frequencies and Treg-IFN-γ production were found in women with current Plasmodium falciparum infection. Higher CD4(+)IL-10(-)IFN-γ(+) T cells frequencies and production of proinflammatory cytokines (including TNF and IL-2) at recruitment (first antenatal visit) had a protective association with birth weight and future (delivery) P. falciparum infection, respectively. Higher intracellular IL-10 levels in T cells had a protective association with future P. falciparum infection and hemoglobin levels at delivery. The protective associations were found also with nonmalaria-specific T cell responses. Treg frequencies positively correlated with plasma eotaxin concentrations, but this subset did not express eotaxin receptor CCR3. Thus, an activated immune system during pregnancy might contribute to protection against malaria during pregnancy and poor delivery outcomes.

Vitamin D status is associated with mortality, morbidity, and growth failure among a prospective cohort of HIV-infected and HIV-exposed Tanzanian infants

BACKGROUND

Vitamin D is a potent immunomodulator, but its impact on morbidity and mortality among infants remains unclear.

OBJECTIVE

The objective of the study was to prospectively assess the association of vitamin D status with mortality, morbidity, and growth during the first 2 y of life.

METHODS

A prospective cohort of 253 HIV-infected and 948 HIV-exposed Tanzanian infants enrolled in a randomized trial of multivitamins (not including vitamin D) was studied. Serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured at 5-7 wk of age and infants were followed at monthly clinic visits until 24 mo. Physicians performed a clinical exam every 3 mo or when an illness was noted.

RESULTS

Serum 25(OH)D concentrations were (means ± SDs) 18.6 ± 10.3 ng/mL and 18.1 ± 9.2 ng/mL for HIV-infected and HIV-exposed infants, respectively. Unexpectedly, serum 25(OH)D concentrations ≥30 ng/mL were significantly associated with higher mortality as compared to the 20-29.9 ng/mL reference for HIV-infected (HR: 2.47; 95% CI: 1.13, 5.44; P = 0.02) and HIV-exposed (HR: 4.00; 95% CI: 1.67, 9.58; P < 0.01) infants after multivariate adjustment. We found no statistically significant association between 25(OH)D concentrations <10 ng/mL and mortality for HIV-infected (HR: 1.43; 95% CI: 0.74, 2.78; P = 0.29) and HIV-exposed (HR: 1.56; 95% CI: 0.60, 4.03; P = 0.36) infants. Among HIV-exposed infants, 25(OH)D concentrations ≥30 ng/mL were significantly associated with clinical [incidence ratio rate (IRR): 1.34; 95% CI: 1.06,1.70; P = 0.02] and confirmed (IRR: 1.71; 95% CI: 1.71; 1.15, 2.54; P < 0.01) malaria diagnoses, whereas concentrations of <10 ng/mL were associated with oral candidiasis (IRR: 1.47; 95% CI: 1.00-2.15; P = 0.046) and wasting (HR: 1.71; 95% CI: 1.20, 2.43; P < 0.01).

CONCLUSION

The observational design of this study does not allow for causal interpretation; however, the results indicate a strong need for additional studies of vitamin D among HIV-infected and -exposed children, particularly in malaria-endemic settings. The parent trial was registered at clinicaltrials.gov as NCT00197730.

Plasmodium falciparum induces Foxp3hi CD4 T cells independent of surface PfEMP1 expression via small soluble parasite components

Elevated levels of regulatory T cells following Plasmodium infection are a well-reported phenomenon that can influence both protective and pathological anti-parasite responses, and might additionally impact on vaccine responses in acutely malaria infected individuals. The mechanisms underlying their induction or expansion by the parasite, however, are incompletely understood. In a previous study, Plasmodium falciparum infected red blood cells (iRBCs) were shown to induce effector-cytokine producing Foxp3int CD4+ T cells, as well as regulatory Foxp3hi CD4+ T cells in vitro. The aim of the present study was to determine the contribution of parasite components to the induction of Foxp3 expression in human CD4+ T cells. Using the surface PfEMP1-deficient parasite line 1G8, we demonstrate that induction of Foxp3hi and Foxp3int CD4+ T cells is independent of PfEMP1 expression on iRBCs. We further demonstrate that integrity of iRBCs is no requirement for the induction of Foxp3 expression. Finally, transwell experiments showed that induction of Foxp3 expression, and specifically the generation of Foxp3hi as opposed to Foxp3int CD4 T cells, can be mediated by soluble parasite components smaller than 20 nm and thus likely distinct from the malaria pigment hemozoin. These results suggest that the induction of Foxp3hi T cells by P. falciparum is largely independent of two key immune modulatory parasite components, and warrant future studies into the nature of the Foxp3hi inducing parasite components to potentially allow their exclusion from vaccine formulations.

Exposure-dependent control of malaria-induced inflammation in children

In malaria-naïve individuals, Plasmodium falciparum infection results in high levels of parasite-infected red blood cells (iRBCs) that trigger systemic inflammation and fever. Conversely, individuals in endemic areas who are repeatedly infected are often asymptomatic and have low levels of iRBCs, even young children. We hypothesized that febrile malaria alters the immune system such that P. falciparum re-exposure results in reduced production of pro-inflammatory cytokines/chemokines and enhanced anti-parasite effector responses compared to responses induced before malaria. To test this hypothesis we used a systems biology approach to analyze PBMCs sampled from healthy children before the six-month malaria season and the same children seven days after treatment of their first febrile malaria episode of the ensuing season. PBMCs were stimulated with iRBC in vitro and various immune parameters were measured. Before the malaria season, children's immune cells responded to iRBCs by producing pro-inflammatory mediators such as IL-1β, IL-6 and IL-8. Following malaria there was a marked shift in the response to iRBCs with the same children's immune cells producing lower levels of pro-inflammatory cytokines and higher levels of anti-inflammatory cytokines (IL-10, TGF-β). In addition, molecules involved in phagocytosis and activation of adaptive immunity were upregulated after malaria as compared to before. This shift was accompanied by an increase in P. falciparum-specific CD4⁺Foxp3⁻ T cells that co-produce IL-10, IFN-γ and TNF; however, after the subsequent six-month dry season, a period of markedly reduced malaria transmission, P. falciparum–inducible IL-10 production remained partially upregulated only in children with persistent asymptomatic infections. These findings suggest that in the face of P. falciparum re-exposure, children acquire exposure-dependent P. falciparum–specific immunoregulatory responses that dampen pathogenic inflammation while enhancing anti-parasite effector mechanisms. These data provide mechanistic insight into the observation that P. falciparum–infected children in endemic areas are often afebrile and tend to control parasite replication.

Malaria remains a major cause of disease and death worldwide. When mosquitoes infect people with malaria parasites for the first time, the parasite rapidly multiplies in the blood and the body responds by producing molecules that cause inflammation and fever, and sometimes the infection progresses to life-threatening disease. However, in regions where people are repeatedly infected with malaria parasites, most infections do not cause fever and parasites often do not multiply uncontrollably. For example, in Mali where this study was conducted, children are infected with malaria parasites ≥100 times/year but only get malaria fever ∼2 times/year and often manage to control parasite numbers in the blood. To understand these observations we collected immune cells from the blood of healthy children before the malaria season and 7 days after malaria fever. We simulated malaria infection at these time points by exposing the immune cells to malaria parasites in a test-tube. We found that re-exposing immune cells to parasites after malaria fever results in reduced expression of molecules that cause fever and enhanced expression of molecules involved in parasite killing. These findings help explain how the immune system prevents fever and controls malaria parasite growth in children who are repeatedly infected with malaria parasites.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Alterations in cytokines and haematological parameters during the acute and convalescent phases of Plasmodium falciparum and Plasmodium vivax infections

Haematological and cytokine alterations in malaria are a broad and controversial subject in the literature. However, few studies have simultaneously evaluated various cytokines in a single patient group during the acute and convalescent phases of infection. The aim of this study was to sequentially characterise alterations in haematological patters and circulating plasma cytokine and chemokine levels in patients infected with Plasmodium vivax or Plasmodium falciparum from a Brazilian endemic area during the acute and convalescent phases of infection. During the acute phase, thrombocytopaenia, eosinopaenia, lymphopaenia and an increased number of band cells were observed in the majority of the patients. During the convalescent phase, the haematologic parameters returned to normal. During the acute phase, P. vivax and P. falciparum patients had significantly higher interleukin (IL)-6, IL-8, IL-17, interferon-γ, tumour necrosis factor (TNF)-α, macrophage inflammatory protein-1β and granulocyte-colony stimulating factor levels than controls and maintained high levels during the convalescent phase. IL-10 was detected at high concentrations during the acute phase, but returned to normal levels during the convalescent phase. Plasma IL-10 concentration was positively correlated with parasitaemia in P. vivax and P. falciparum-infected patients. The same was true for the TNF-α concentration in P. falciparum-infected patients. Finally, the haematological and cytokine profiles were similar between uncomplicated P. falciparum and P. vivax infections.

Relationship of regulatory T cells to Plasmodium falciparum malaria symptomatology in a hypoendemic region

BACKGROUND

Previous data have suggested that regulatory T cells (Tregs) balance protective immune responses with immune mediated pathology in malaria. This study aimed to determine to test the hypothesis that Treg proportions or absolute levels are associated with parasitaemia and malaria symptoms.

METHODS

Treg cells were quantified by flow cytometry as CD4+ CD25+, Foxp3+, CD127(low) T cells. Three patient groups were assessed: patients with symptomatic Plasmodium falciparum malaria (S), subjects with asymptomatic P. falciparum parasitaemia (AS) and uninfected control individuals (C).

RESULTS

S, AS and C groups had similar absolute numbers and percentage of Tregs (3.9%, 3.5% and 3.5% respectively). Levels of parasitaemia were not associated with Treg percentage (p = 0.47).

CONCLUSION

Neither relative nor absolute regulatory T cell numbers were found to be associated with malaria-related symptomatology in this study. Immune mechanisms other than Tregs are likely to be responsible for the state of asymptomatic P. falciparum parasitaemia in the Peruvian Amazon; but further study to explore these mechanisms is needed.