CD8(+) T cell activation by murine erythroblasts infected with malaria parasites

Identification and biophysical characterization of Plasmodium peptide binding by common African HLAs

Human Leukocyte Antigens (HLA) are immunoreceptors that present peptide antigens at the cell surface to T cells as a primary mechanism of immune surveillance. Malaria, a disease associated with the Plasmodium parasite, claims > 600,000 lives per year globally with most deaths occurring in Africa. Development of efficacious prophylactic vaccines or therapeutic treatments for malaria has been hindered by the lack of a basic understanding of the role of HLA-mediated peptide antigen presentation during Plasmodium infection. In particular, there is (i) little understanding of which peptide antigens are presented by HLAs in the context of malaria, and (ii) a lack of structural insights into Plasmodium peptide antigen presentation by HLAs, which underpins peptide/HLA stability, specificity, cross-presentation across HLA alleles, and recognition by T cell receptors. To begin to address these knowledge gaps, we identify and characterize candidate peptide antigens derived from Plasmodium falciparum with potential for presentation by common class I HLA alleles. We computationally screen nine proteins from the P. falciparum proteome to predict eight peptides with potential for cross-presentation by common alleles in African populations, HLA-A*02:01 and HLA-B*08:01. We then validate the predictions by producing recombinant HLAs in complex with the eight identified peptides by in vitro refolding. We evaluate the folding and thermal stability of the resulting sixteen peptide/HLA complexes by CD spectroscopy and nanoDSF. In silico modeling of peptide/HLA complexes informs a plausible structural basis for mechanisms for cross-presentation of P. falciparum peptides across HLA-A*02:01 and HLA-B*08:01 alleles. Finally, we expand our identified P. falciparum peptides to cover a broader range of HLA alleles in malaria endemic populations with experimental validation provided for HLA-C*07:01 and HLA-E*01:03. Together, our results are a step forward towards a deeper understanding of the potential for multi-allele cross-presentation of peptides in malaria. These results further inform future development of multivalent vaccine strategies targeting HLA profiles in malaria endemic populations.

Severe malaria enforces short-lived effector cell differentiation but does not prevent effective secondary responses by memory CD8 T cells

Parasitic infections are a major worldwide health burden, yet most studies of CD8 T cell differentiation focus on acute viral and bacterial infections. To understand effector and memory CD8 T cell responses during erythrocytic malaria infection in mice, we utilized transgenic OT-I T cells and compared CD8 T cell responses between infection with OVA-expressing strains of Listeria monocytogenes (Lm) and Plasmodium berghei ANKA (PbA). We find that CD8 T cells expand vigorously during both infections. However, in contrast to Lm infection, PbA infection induces T cells that are heavily biased toward an IL-7Ra-deficient and KLRG1+ short-lived effector cell (SLEC) phenotype at the expense of memory precursor effector cell (MPECs) formation. PbA-induced inflammation, including IFNγ, is partially responsible for this outcome. Following treatment with antimalarial drugs and T cell contraction, PbA-primed memory T cells are rarely found in the blood and peripheral tissues but do maintain a low presence in the spleen and bone marrow. Despite these poor numbers, PbA memory T cells robustly expand upon vaccination or viral infection, control pathogen burden, and form secondary memory pools. Thus, despite PbA enforced SLEC formation and limited memory, effective secondary responses can still proceed.

ATG5-regulated CCL2/MCP-1 production in myeloid cells selectively modulates anti-malarial CD4+ Th1 responses

Parasite-specific CD4+ Th1 cell responses are the predominant immune effector for controlling malaria infection; however, the underlying regulatory mechanisms remain largely unknown. This study demonstrated that ATG5 deficiency in myeloid cells can significantly inhibit the growth of rodent blood-stage malarial parasites by selectively enhancing parasite-specific CD4+ Th1 cell responses. This effect was independent of ATG5-mediated canonical and non-canonical autophagy. Mechanistically, ATG5 deficiency suppressed FAS-mediated apoptosis of LY6G- ITGAM/CD11b+ ADGRE1/F4/80- cells and subsequently increased CCL2/MCP-1 production in parasite-infected mice. LY6G- ITGAM+ ADGRE1- cell-derived CCL2 selectively interacted with CCR2 on CD4+ Th1 cells for their optimized responses through the JAK2-STAT4 pathway. The administration of recombinant CCL2 significantly promoted parasite-specific CD4+ Th1 responses and suppressed malaria infection. Conclusively, our study highlights the previously unrecognized role of ATG5 in modulating myeloid cells apoptosis and sequentially affecting CCL2 production, which selectively promotes CD4+ Th1 cell responses. Our findings provide new insights into the development of immune interventions and effective anti-malarial vaccines.Abbreviations: ATG5: autophagy related 5; CBA: cytometric bead array; CCL2/MCP-1: C-C motif chemokine ligand 2; IgG: immunoglobulin G; IL6: interleukin 6; IL10: interleukin 10; IL12: interleukin 12; MFI: mean fluorescence intensity; JAK2: Janus kinase 2; LAP: LC3-associated phagocytosis; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; pRBCs: parasitized red blood cells; RUBCN: RUN domain and cysteine-rich domain containing, Beclin 1-interacting protein; STAT4: signal transducer and activator of transcription 4; Th1: T helper 1 cell; Tfh: follicular helper cell; ULK1: unc-51 like kinase 1.

Plasmodium falciparum infection of human erythroblasts induces transcriptional changes associated with dyserythropoiesis

During development down the erythroid lineage, hematopoietic stem cells undergo dramatic changes to cellular morphology and function in response to a complex and tightly regulated program of gene expression. In malaria infection, Plasmodium spp parasites accumulate in the bone marrow parenchyma, and emerging evidence suggests erythroblastic islands are a protective site for parasite development into gametocytes. Although it has been observed that Plasmodium falciparum infection in late-stage erythroblasts can delay terminal erythroid differentiation and enucleation, the mechanism(s) underlying this phenomenon are unknown. Here, we apply RNA sequencing after fluorescence-activated cell sorting of infected erythroblasts to identify transcriptional responses to direct and indirect interaction with P falciparum. Four developmental stages of erythroid cells were analyzed: proerythroblast, basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast. We found extensive transcriptional changes in infected erythroblasts compared with that in uninfected cells in the same culture, including dysregulation of genes involved in erythroid proliferation and developmental processes. Although some indicators of cellular oxidative and proteotoxic stress were common across all stages of erythropoiesis, many responses were specific to cellular processes associated with developmental stage. Together, our results evidence multiple possible avenues by which parasite infection can induce dyserythropoiesis at specific points along the erythroid continuum, advancing our understanding of the molecular determinants of malaria anemia.

Plasmodium falciparum infection of human erythroblasts induces transcriptional changes associated with dyserythropoiesis

During development down the erythroid lineage, hematopoietic stem cells undergo dramatic changes to cellular morphology and function in response to a complex and tightly regulated program of gene expression. In malaria infection, Plasmodium spp . parasites accumulate in the bone marrow parenchyma, and emerging evidence suggests erythroblastic islands are a protective site for parasite development into gametocytes. While it has been observed that Plasmodium falciparum infection of late-stage erythroblasts can delay terminal erythroid differentiation and enucleation, the mechanism(s) underlying this phenomenon are unknown. Here, we apply RNA-seq after fluorescence-activated cell sorting (FACS) of infected erythroblasts to identify transcriptional responses to direct and indirect interaction with Plasmodium falciparum . Four developmental stages of erythroid cells were analyzed: proerythroblast, basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast. We found extensive transcriptional changes in infected erythroblasts compared to uninfected cells in the same culture, including dysregulation of genes involved in erythroid proliferation and developmental processes. Whereas some indicators of cellular oxidative and proteotoxic stress were common across all stages of erythropoiesis, many responses were specific to cellular processes associated with developmental stage. Together, our results evidence multiple possible avenues by which parasite infection can induce dyserythropoiesis at specific points along the erythroid continuum, advancing our understanding of the molecular determinants of malaria anemia.

Key Points

Erythroblasts at different stages of differentiation have distinct responses to infection by Plasmodium falciparum . P. falciparum infection of erythroblasts alters expression of genes related to oxidative and proteotoxic stress and erythroid development.

Escaping the enemy's bullets: an update on how malaria parasites evade host immune response

Malaria continues to cause untold hardship to inhabitants of malaria-endemic regions, causing significant morbidity and mortality that severely impact global health and the economy. Considering the complex life cycle of malaria parasites (MPs) and malaria biology, continued research efforts are ongoing to improve our understanding of the pathogenesis of the diseases. Female Anopheles mosquito injects MPs into its hosts during a blood meal, and MPs invade the host skin and the hepatocytes without causing any serious symptoms. Symptomatic infections occur only during the erythrocytic stage. In most cases, the host's innate immunity (for malaria-naïve individuals) and adaptive immunity (for pre-exposed individuals) mount severe attacks and destroy most MPs. It is increasingly understood that MPs have developed several mechanisms to escape from the host's immune destruction. This review presents recent knowledge on how the host's immune system destroys invading MPs as well as MPs survival or host immune evasion mechanisms. On the invasion of host cells, MPs release molecules that bind to cell surface receptors to reprogram the host in a way to lose the capacity to destroy them. MPs also hide from the host immune cells by inducing the clustering of both infected and uninfected erythrocytes (rosettes), as well as inducing endothelial activation. We hope this review will inspire more research to provide a complete understanding of malaria biology and promote interventions to eradicate the notorious disease.

Erythrocyte tropism of malarial parasites: The reticulocyte appeal

Erythrocytes are formed from the enucleation of erythroblasts in the bone marrow, and as erythrocytes develop from immature reticulocytes into mature normocytes, they undergo extensive cellular changes through their passage in the blood. During the blood stage of the malarial parasite life cycle, the parasite sense and invade susceptible erythrocytes. However, different parasite species display varying erythrocyte tropisms (i.e., preference for either reticulocytes or normocytes). In this review, we explore the erythrocyte tropism of malarial parasites, especially their predilection to invade reticulocytes, as shown from recent studies. We also discuss possible mechanisms mediating erythrocyte tropism and the implications of specific tropisms to disease pathophysiology. Understanding these allows better insight into the role of reticulocytes in malaria and provides opportunities for targeted interventions.

Live Vaccination with Blood-Stage Plasmodium yoelii 17XNL Prevents the Development of Experimental Cerebral Malaria

In our work, we aim to develop a malaria vaccine with cross-strain (-species) protection. C57BL/6 mice infected with the P. berghei ANKA strain (PbA) develop experimental cerebral malaria (ECM). In contrast, ECM development is inhibited in infected mice depleted of T cells. The clinical applications of immune-cell depletion are limited due to the benefits of host defense against infectious diseases. Therefore, in the present study we attempted to develop a new method for preventing ECM without immune cell depletion. We demonstrated that mice inoculated with a heterologous live-vaccine of P. yoelii 17XNL were able to prevent both ECM and lung pathology and survived longer than control mice when challenged with PbA. Live vaccination protected blood–organ barriers from PbA infection. Meanwhile, live vaccination conferred sterile protection against homologous challenge with the P. yoelii 17XL virulent strain for the long-term. Analysis of the immune response induced by live vaccination showed that cross-reactive antibodies against PbA antigens were generated. IL-10, which has an immunosuppressive effect, was strongly induced in mice challenged with PbA, unlike the pro-inflammatory cytokine IFNγ. These results suggest that the protective effect of heterologous live vaccination against ECM development results from IL-10-mediated host protection.

Rodent Malaria Erythrocyte Preference Assessment by an Ex Vivo Tropism Assay

Circulating red blood cells consist of young erythrocytes (early and late reticulocytes) and mature erythrocytes (normocytes). The human malaria parasites, Plasmodium falciparum and P. vivax, have a preference to invade reticulocytes during blood-stage infection. Rodent malaria parasites that also prefer reticulocytes could be useful tools to study human malaria reticulocyte invasion. However, previous tropism studies of rodent malaria are inconsistent from one another, making it difficult to compare cell preference of different parasite species and strains. In vivo measurements of cell tropism are also subjected to many confounding factors. Here we developed an ex vivo tropism assay for rodent malaria with highly purified fractions of murine reticulocytes and normocytes. We measured invasion into the different erythrocyte populations using flow cytometry and evaluated the tropism index of the parasite strains. We found that P. berghei ANKA displayed the strongest reticulocyte preference, followed by P. yoelii 17X1.1, whereas P. chabaudi AS and P. vinckei S67 showed mixed tropism. These preferences are intrinsic and were maintained at different reticulocyte and normocyte availabilities. Our study shed light on the true erythrocyte preference of the parasites and paves the way for future investigations on the receptor-ligand interactions mediating erythrocyte tropism.

Erythroid cells and malaria parasites: it's a match!

PURPOSE OF REVIEW

The current review outlines recent discoveries on the infection of erythroid cells by Plasmodium parasites, focusing on the molecular interactions governing the tropism of parasites for their host cell and the implications of this tropism for parasite biology and erythroid cell maturation.

RECENT FINDINGS

Although most studies about the interactions of Plasmodium parasites and their host cell focused on the deadliest human malaria parasite, Plasmodium falciparum, and the erythrocyte, there is increasing evidence that several Plasmodium species, including P. falciparum, also develop within erythroid precursors. These interactions likely modify the remodeling of the host cell by the parasite and affect the maturation of erythroblast and reticulocytes.

SUMMARY

A better understanding of the remodeling of immature erythroid cells by Plasmodium parasites will have important implications for the development of antimalarial drugs or vaccines. In addition, deciphering how Plasmodium parasites interfere with erythropoiesis will provide new insights on how these parasites contribute to anemia in malaria patients.

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

Contributions of IFN-γ and granulysin to the clearance of Plasmodium yoelii blood stage

P. vivax-infected Retics (iRetics) express human leukocyte antigen class I (HLA-I), are recognized by CD8⁺ T cells and killed by granulysin (GNLY) and granzymes. However, how Plasmodium infection induces MHC-I expression on Retics is unknown. In addition, whether GNLY helps control Plasmodium infection in vivo has not been studied. Here, we examine these questions using rodent infection with the P. yoelii 17XNL strain, which has tropism for Retics. Infection with P. yoelii caused extramedullary erythropoiesis, reticulocytosis and expansion of CD8⁺CD44⁺CD62L^- IFN-γ-producing T cells that form immune synapses with iRetics. We now provide evidence that MHC-I expression by iRetic is dependent on IFN-γ-induced transcription of IRF-1, MHC-I and β2-microglobulin (β2-m) in erythroblasts. Consistently, CTLs from infected wild type (WT) mice formed immune synapses with iRetics in an IFN-γ- and MHC-I-dependent manner. When challenged with P. yoelii 17XNL, WT mice cleared parasitemia and survived, while IFN-γ KO mice remained parasitemic and all died. β2-m KO mice that do not express MHC-I and have virtually no CD8⁺ T cells had prolonged parasitemia, and 80% survived. Because mice do not express GNLY, GNLY-transgenic mice can be used to assess the in vivo importance of GNLY. Parasite clearance was accelerated in GNLY-transgenic mice and depletion of CD8⁺ T cells ablated the GNLY-mediated resistance to P. yoelii. Altogether, our results indicate that in addition to previously described mechanisms, IFN-γ promotes host resistance to the Retic-tropic P. yoelii 17XNL strain by promoting MHC-I expression on iRetics that become targets for CD8⁺ cytotoxic T lymphocytes and GNLY.

CD8⁺ cytotoxic T lymphocytes (CTLs) are important for immune defense against intracellular pathogens, such as viruses, bacteria and parasites, and tumor surveillance. CTLs, which recognize peptide epitopes presented by MHC-I molecules expressed in nucleated cells, become activated and kill infected target cells by releasing the contents of cytotoxic granules into the immunological synapse. Since most Plasmodium spp. infect erythrocytes that are enucleated and do not express MHC-I, the role of CD8⁺ T cells in the blood-stage of malaria has been neglected. We recently showed that P. vivax-infected reticulocytes express MHC-I and are killed in a manner dependent on granulysin (GNLY), a cytotoxic granule effector protein. However, the protective role of CD8⁺ T cells is controversial and the role of GNLY in vivo remains to be demonstrated. Here, we show that CTLs and GNLY mediate mouse resistance to blood-stage infection with P. yoelii, a rodent malaria parasite that preferably infects reticulocytes.

Potential and Limitations of Cross-Protective Vaccine against Malaria by Blood-Stage Naturally Attenuated Parasite

Human malaria vaccine trials have revealed vaccine efficacy but improvement is still needed. In this study, we aimed to re-evaluate vaccination with blood-stage naturally attenuated parasites, as a whole-organism vaccine model against cross-strain and cross-species malaria, to establish a better vaccination strategy. C57BL/6 mice controlled blood-stage Plasmodium yoelii 17XNL (PyNL) within 1 month of infection, while mice with a variety of immunodeficiencies demonstrated different susceptibilities to PyNL, including succumbing to hyperparasitemia. However, after recovery, survivors had complete protection against a challenge with the lethal strain PyL. Unlike cross-strain protection, PyNL-recovered mice failed to induce sterile immunity against Plasmodium berghei ANKA, although prolonged survival was observed in some vaccinated mice. Splenomegaly is a typical characteristic of malaria; the splenic structure became reorganized to prioritize extra-medullary hematopoiesis and to eliminate parasites. We also found that the peritoneal lymph node was enlarged, containing activated/memory phenotype cells that did not confer protection against PyL challenge. Hemozoins remained in the spleen several months after PyNL infection. Generation of an attenuated human blood-stage parasite expressing proteins from multiple species of malaria would greatly improve anti-malaria vaccination.

Fluctuations of Spleen Cytokine and Blood Lactate, Importance of Cellular Immunity in Host Defense Against Blood Stage Malaria Plasmodium yoelii

Our previous studies of protective immunity and pathology against blood stage malaria parasites have shown that not only CD4⁺ T cells, but also CD8⁺ T cells and macrophages, are important for host defense against blood stage malaria infection. Furthermore, we found that Plasmodium yoelii 17XNL (PyNL) parasitizes erythroblasts, the red blood cell (RBC) precursor cells, which then express MHC class I molecules. In the present study, we analyzed spleen cytokine production. In CD8⁺ T cell-depleted mice, IL-10 production in early stage infection was increased over two-fold relative to infected control animals and IL-10⁺ CD3^- cells were increased, whereas IFN-γ production in the late stage of infection was decreased. At day 16 after PyNL infection, CD8⁺ T cells produced more IFN-γ than CD4⁺ T cells. We evaluated the involvement of the immunoproteasome in induction of immune CD8⁺ T cells, and the role of Fas in protection against PyNL both of which are downstream of IFN-γ. In cell transfer experiments, at least the single molecules LMP7, LMP2, and PA28 are not essential for CD8⁺ T cell induction. The Fas mutant LPR mouse was weaker in resistance to PyNL infection than WT mice, and 20% of the animals died. LPR-derived parasitized erythroid cells exhibited less externalization of phosphatidylserine (PS), and phagocytosis by macrophages was impaired. Furthermore, we tried to identify the cause of death in malaria infection. Blood lactate concentration was increased in the CD8⁺ T cell-depleted PyNL-infected group at day 19 (around peak parasitemia) to similar levels as day 7 after infection with a lethal strain of Py. When we injected mice with lactate at day 4 and 6 of PyNL infection, all mice died at day 8 despite demonstrating low parasitemia, suggesting that hyperlactatemia is one of the causes of death in CD8⁺ T cell-depleted PyNL-infected mice. We conclude that CD8⁺ T cells might control cytokine production to some extent and regulate hyperparasitemia and hyperlactatemia in protection against blood stage malaria parasites.

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.

The Contribution of Co-signaling Pathways to Anti-malarial T Cell Immunity

Plasmodium spp., the causative agent of malaria, caused 212 million infections in 2016 with 445,000 deaths, mostly in children. Adults acquire enough immunity to prevent clinical symptoms but never develop sterile immunity. The only vaccine for malaria, RTS,S, shows promising protection of a limited duration against clinical malaria in infants but no significant protection against severe disease. There is now abundant evidence that T cell functions are inhibited during malaria, which may explain why vaccine are not efficacious. Studies have now clearly shown that T cell immunity against malaria is subdued by multiple the immune regulatory receptors, in particular, by programmed cell-death-1 (PD-1). Given there is an urgent need for an efficacious malarial treatment, compounded with growing drug resistance, a better understanding of malarial immunity is essential. This review will examine molecular signals that affect T cell-mediated immunity against malaria.

Profiling MHC II immunopeptidome of blood-stage malaria reveals that cDC1 control the functionality of parasite-specific CD4 T cells

In malaria, CD4 Th1 and T follicular helper (T_FH) cells are important for controlling parasite growth, but Th1 cells also contribute to immunopathology. Moreover, various regulatory CD4 T‐cell subsets are critical to hamper pathology. Yet the antigen‐presenting cells controlling Th functionality, as well as the antigens recognized by CD4 T cells, are largely unknown. Here, we characterize the MHC II immunopeptidome presented by DC during blood‐stage malaria in mice. We establish the immunodominance hierarchy of 14 MHC II ligands derived from conserved parasite proteins. Immunodominance is shaped differently whether blood stage is preceded or not by liver stage, but the same ETRAMP‐specific dominant response develops in both contexts. In naïve mice and at the onset of cerebral malaria, CD8α⁺ dendritic cells (cDC1) are superior to other DC subsets for MHC II presentation of the ETRAMP epitope. Using in vivo depletion of cDC1, we show that cDC1 promote parasite‐specific Th1 cells and inhibit the development of IL‐10⁺CD4 T cells. This work profiles the P. berghei blood‐stage MHC II immunopeptidome, highlights the potency of cDC1 to present malaria antigens on MHC II, and reveals a major role for cDC1 in regulating malaria‐specific CD4 T‐cell responses.

Tissue-specific immunopathology during malaria infection

Systemic inflammation mediated by Plasmodium parasites is central to malaria disease and its complications. Plasmodium parasites reside in erythrocytes and can theoretically reach all host tissues via the circulation. However, actual interactions between parasitized erythrocytes and host tissues, along with the consequent damage and pathological changes, are limited locally to specific tissue sites. Such tissue specificity of the parasite can alter the outcome of malaria disease, determining whether acute or chronic complications occur. Here, we give an overview of the recent progress that has been made in understanding tissue-specific immunopathology during Plasmodium infection. As knowledge on tissue-specific host-parasite interactions accumulates, better treatment modalities and targets may emerge for intervention in malaria disease.

Plasmodium vivax but Not Plasmodium falciparum Blood-Stage Infection in Humans Is Associated with the Expansion of a CD8+ T Cell Population with Cytotoxic Potential

P. vivax and P. falciparum parasites display different tropism for host cells and induce very different clinical symptoms and pathology, suggesting that the immune responses required for protection may differ between these two species. However, no study has qualitatively compared the immune responses to P. falciparum or P. vivax in humans following primary exposure and infection. Here, we show that the two species differ in terms of the cellular immune responses elicited following primary infection. Specifically, P. vivax induced the expansion of a subset of CD8⁺ T cells expressing the activation marker CD38, whereas P. falciparum induced the expansion of CD38⁺ CD4⁺ T cells. The CD38⁺ CD8⁺ T cell population that expanded following P. vivax infection displayed greater cytotoxic potential compared to CD38^- CD8⁺ T cells, and compared to CD38⁺ CD8⁺ T cells circulating during P. falciparum infection. We hypothesize that P. vivax infection leads to a stronger CD38⁺ CD8⁺ T cell activation because of its preferred tropism for MHC-I-expressing reticulocytes that, unlike mature red blood cells, can present antigen directly to CD8⁺ T cells. This study provides the first line of evidence to suggest an effector role for CD8⁺ T cells in P. vivax blood-stage immunity. It is also the first report of species-specific differences in the subset of T cells that are expanded following primary Plasmodium infection, suggesting that malaria vaccine development may require optimization according to the target parasite.

The specific immune responses that contribute to protective immunity in humans following Plasmodium infection are yet to be fully characterized. The species P. vivax and P. falciparum account for most human infections, yet little is known about P. vivax specific immune responses and whether they are similar to or distinct from P. falciparum. Here, we establish that P. vivax and P. falciparum elicit distinct cellular immune responses following primary infection, with the expansion of a subset of CD38⁺ CD8⁺ T cells with a cytotoxic potential in P. vivax but not in P. falciparum infection. This study provides the first evidence for the activation of CD8⁺ T cells in P. vivax blood-stage infection and demonstrates the existence of species-dependent host immune responses to malaria. These findings have important implications for P. vivax vaccine development, and suggest that future malaria vaccine studies should be adapted according to the target Plasmodium spp.

Spleen-Dependent Immune Protection Elicited by CpG Adjuvanted Reticulocyte-Derived Exosomes from Malaria Infection Is Associated with Changes in T cell Subsets' Distribution

Reticulocyte-derived exosomes (rex) are 30–100 nm membrane vesicles of endocytic origin released during the maturation of reticulocytes to erythrocytes upon fusion of multivesicular bodies with the plasma membrane. Combination of CpG-ODN with rex obtained from BALB/c mice infected with the reticulocyte-prone non-lethal P. yoelii 17X malaria strain (rexPy), had been shown to induce survival and long lasting protection. Here, we show that splenectomized mice are not protected upon rexPy+CpG inmunizations and that protection is restored upon passive transfer of splenocytes obtained from animals immunized with rexPy+CpG. Notably, rexPy immunization of mice induced changes in PD1⁻ memory T cells with effector phenotype. Proteomics analysis of rexPy confirmed their reticulocyte origin and demonstrated the presence of parasite antigens. Our studies thus prove, for what we believe is the first time, that rex from reticulocyte-prone malarial infections are associated with splenic long-lasting memory responses. To try extrapolating these data to human infections, in vitro experiments with spleen cells of human transplantation donors were performed. Plasma-derived exosomes from vivax malaria patients (exPv) were actively uptaken by human splenocytes and stimulated spleen cells leading to changes in T cell subsets.

A whole-killed, blood-stage lysate vaccine protects against the malaria liver stage

Although the attenuated sporozoite is the most efficient vaccine to prevent infection with the malaria parasite, the limitation of a source of sterile sporozoites greatly hampers its application. In this study, we found that the whole-killed, blood-stage lysate vaccine could confer protection against the blood stage as well as the liver stage. Although the protective immunity induced by the whole-organism vaccine against the blood stage is dependent on parasite-specific CD4⁺ T-cell responses and antibodies, in mice immunized with the whole-killed, blood-stage lysate vaccine, CD8⁺ , but not CD4⁺ effector T-cell responses greatly contributed to protection against the liver stage. Thus, our data suggested that the whole-killed, blood-stage lysate vaccine could be an alternative promising strategy to prevent malaria infection and to reduce the morbidity and mortality of patients with malaria.

Persistence and immunogenicity of chemically attenuated blood stage Plasmodium falciparum in Aotus monkeys

Malaria is a disease caused by a protozoan of the Plasmodium genus and results in 0.5-0.7million deaths per year. Increasing drug resistance of the parasite and insecticide resistance of mosquitoes necessitate alternative control measures. Numerous vaccine candidates have been identified but none have been able to induce robust, long-lived protection when evaluated in malaria endemic regions. Rodent studies have demonstrated that chemically attenuated blood stage parasites can persist at sub-patent levels and induce homologous and heterologous protection against malaria. Parasite-specific cellular responses were detected, with protection dependent on CD4+ T cells. To investigate this vaccine approach for Plasmodium falciparum, we characterised the persistence and immunogenicity of chemically attenuated P. falciparum FVO strain parasites (CAPs) in non-splenectomised Aotus nancymaae monkeys following administration of a single dose. Control monkeys received either normal red blood cells or wild-type parasites followed by drug treatment. Chemical attenuation was performed using tafuramycin A, which irreversibly binds to DNA. CAPs were detected in the peripheral blood for up to 2days following inoculation as determined by thick blood smears, and for up to 8days as determined by quantitative PCR. Parasite-specific IgG was not detected in monkeys that received CAPs; however, in vitro parasite-specific T cell proliferation was observed. Following challenge, the CAP monkeys developed an infection; however, one CAP monkey and the infection and drug-cure monkeys showed partial or complete resistance. These experiments lay the groundwork for further assessment of CAPs as a potential vaccine against malaria.

A transient resistance to blood-stage malaria in interferon-γ-deficient mice through impaired production of the host cells preferred by malaria parasites

IFN-γ plays both pathological and protective roles during blood-stage malaria. One of its pathological roles is its contribution to anemia by suppressing erythropoiesis. Here, to evaluate the effects of IFN-γ-mediated alterations in erythropoiesis on the course of malaria infection, mice deficient in IFN-γ (GKO) were infected with two strains of the rodent malaria parasite Plasmodium yoelii, 17XL (PyL) and 17XNL (PyNL), whose host cell ranges differ. Regardless of genotype, all mice infected with PyL, which can invade any erythrocyte, developed high parasitemia and died quickly. Although PyNL caused a transient non-lethal infection in wild-type (WT) mice, some GKO mice were unable to control the infection and died. However, GKO mice were resistant to the early phase of infection, showing an impaired increase in parasitemia compared with WT mice. This resistance in the GKO mice was associated with having significantly fewer reticulocytes, which are the preferred host cells for PyNL parasites, than the WT mice. Compared with the amount of reticulocytes in GKO mice during the early stages of infection, there was a significant increase in the amount of these cells at later stages, which coincided with the inability of these mice to control the infection. We found that the growth of PyNL parasites correlated with the amount of reticulocytes. Thus, the reduced number of reticulocytes in mice lacking IFN-γ appears to be responsible for the limited parasite growth. Notably, these differences in GKO mice were at least partially reversed when the mice were injected with exogenous IFN-γ. Additionally, an artificial induction of hemolytic anemia and an increase in reticulocytes by phenylhydrazine treatment in GKO mice completely abolished the lower parasitemia and resistance during early phase infection. These results suggest that IFN-γ may contribute to the early growth of PyNL parasites by increasing the amount of reticulocytes, presumably by enhancing erythropoiesis.

Recent developments in malaria vaccinology

The development of a highly effective malaria vaccine remains a key goal to aid in the control and eventual eradication of this devastating parasitic disease. The field has made huge strides in recent years, with the first-generation vaccine RTS,S showing modest efficacy in a Phase III clinical trial. The updated 2030 Malaria Vaccine Technology Roadmap calls for a second generation vaccine to achieve 75% efficacy over two years for both Plasmodium falciparum and Plasmodium vivax, and for a vaccine that can prevent malaria transmission. Whole-parasite immunisation approaches and combinations of pre-erythrocytic subunit vaccines are now reporting high-level efficacy, whilst exciting new approaches to the development of blood-stage and transmission-blocking vaccine subunit components are entering clinical development. The development of a highly effective multi-component multi-stage subunit vaccine now appears to be a realistic ambition. This review will cover these recent developments in malaria vaccinology.

Cytotoxic activities of CD8⁺ T cells collaborate with macrophages to protect against blood-stage murine malaria

The protective immunity afforded by CD8⁺ T cells against blood-stage malaria remains controversial because no MHC class I molecules are displayed on parasite-infected human erythrocytes. We recently reported that rodent malaria parasites infect erythroblasts that express major histocompatibility complex (MHC) class I antigens, which are recognized by CD8⁺ T cells. In this study, we demonstrate that the cytotoxic activity of CD8⁺ T cells contributes to the protection of mice against blood-stage malaria in a Fas ligand (FasL)-dependent manner. Erythroblasts infected with malarial parasites express the death receptor Fas. CD8⁺ T cells induce the externalization of phosphatidylserine (PS) on the infected erythroblasts in a cell-to-cell contact-dependent manner. PS enhances the engulfment of the infected erythroid cells by phagocytes. As a PS receptor, T-cell immunoglobulin-domain and mucin-domain-containing molecule 4 (Tim-4) contributes to the phagocytosis of malaria-parasite-infected cells. Our findings provide insight into the molecular mechanisms underlying the protective immunity exerted by CD8⁺ T cells in collaboration with phagocytes.

DOI:http://dx.doi.org/10.7554/eLife.04232.001

The immune system consists of several different types of cell that work together to prevent infection and disease. For example, immune cells called cytotoxic CD8⁺ T cells kill tumor cells or other cells that are infected. To do so, the CD8⁺ T cells must recognize certain molecules on the surface of the tumor or infected cells and bind to them.

Malaria is an infectious disease caused by the Plasmodium parasite, which is transferred between individuals by mosquitoes. The parasite is able to evade the immune system—so much so that it is not well understood how the immune system tries to respond to stop the infection. This has made it difficult to develop a vaccine that protects against malaria.

During the latter stages of a malaria infection, the parasite infects the host's red blood cells. It was long believed that CD8⁺ T cells did not help to eliminate the red blood cells that had been infected by Plasmodium. However, recent work in mice suggested that CD8⁺ T cells do respond to infected erythroblasts—precursor cells that develop into red blood cells—and that CD8⁺ T cells help protect mice against blood-stage malaria.

Now, Imai et al. describe how the CD8⁺ T cells in mice help to kill erythroblasts infected with Plasmodium yoelli, a species of the parasite used to study malaria in mice. The infected cells display a protein called Fas on their surface. Imai et al. found that, during a malaria infection, the CD8⁺ T cells produce a protein that can interact with Fas. This interaction causes the infected cell to move a signaling molecule to its outside surface, which encourages another type of immune cell to engulf and destroy the infected cell. This knowledge of how CD8⁺ T cells fight Plasmodium parasites in the bloodstream could now help to develop new types of blood-stage vaccine for malaria.

DOI:http://dx.doi.org/10.7554/eLife.04232.002

PD-1 deficiency enhances humoral immunity of malaria infection treatment vaccine

Malaria infection treatment vaccine (ITV) is a promising strategy to induce homologous and heterologous protective immunity against the blood stage of the parasite. However, the underlying mechanism of protection remains largely unknown. Here, we found that a malaria-specific antibody (Ab) could mediate the protective immunity of ITV-immunized mice. Interestingly, PD-1 deficiency greatly elevated the levels of both malaria-specific total IgG and subclass IgG2a and enhanced the protective efficacy of ITV-immunized mice against the blood-stage challenge. A serum adoptive-transfer assay demonstrated that the increased Ab level contributed to the enhanced protective efficacy of the immunized PD-1-deficient mice. Further study showed that PD-1 deficiency could also promote the expansion of germinal center (GC) B cells and malaria parasite-specific TFH cells in the spleens of ITV-immunized mice. These results suggest that PD-1 deficiency improves the protective efficacy of ITV-immunized mice by promoting the generation of malaria parasite-specific Ab and the expansion of GC B cells. The results of this study provide new evidence to support the negative function of PD-1 on humoral immunity and will guide the design of a more effective malaria vaccine.

Malaria induces anemia through CD8+ T cell-dependent parasite clearance and erythrocyte removal in the spleen

Severe malarial anemia (SMA) in semi-immune individuals eliminates both infected and uninfected erythrocytes and is a frequent fatal complication. It is proportional not to circulating parasitemia but total parasite mass (sequestered) in the organs. Thus, immune responses that clear parasites in organs may trigger changes leading to anemia. Here, we use an outbred-rat model where increasing parasite removal in the spleen escalated uninfected-erythrocyte removal. Splenic parasite clearance was associated with activated CD8(+) T cells, immunodepletion of which prevented parasite clearance. CD8(+) T cell repletion and concomitant reduction of the parasite load was associated with exacerbated (40 to 60%) hemoglobin loss and changes in properties of uninfected erythrocytes. Together, these data suggest that CD8(+) T cell-dependent parasite clearance causes erythrocyte removal in the spleen and thus anemia. In children infected with the human malaria parasite Plasmodium falciparum, elevation of parasite biomass (not the number of circulating parasites) increased the odds ratio for SMA by 3.5-fold (95% confidence intervals [CI95%], 1.8- to 7.5-fold). CD8(+) T cell expansion/activation independently increased the odds ratio by 2.4-fold (CI95%, 1.0- to 5.7-fold). Concomitant increases in both conferred a 7-fold (CI95%, 1.9- to 27.4-fold)-greater risk for SMA. Together, these data suggest that CD8(+)-dependent parasite clearance may predispose individuals to uninfected-erythrocyte loss and SMA, thus informing severe disease diagnosis and strategies for vaccine development.

IMPORTANCE

Malaria is a major global health problem. Severe malaria anemia (SMA) is a complex disease associated with partial immunity. Rapid hemoglobin reductions of 20 to 50% are commonly observed and must be rescued by transfusion (which can carry a risk of HIV acquisition). The causes and risk factors of SMA remain poorly understood. Recent studies suggest that SMA is linked to parasite biomass sequestered in organs. This led us to investigate whether immune mechanisms that clear parasites in organs trigger anemia. In rats, erythropoiesis is largely restricted to the bone marrow, and critical aspects of the spleen expected to be important in anemia are similar to those in humans. Therefore, using a rat model, we show that severe anemia is caused through CD8(+) T cell-dependent parasite clearance and erythrocyte removal in the spleen. CD8 activation may also be a new risk factor for SMA in African children.

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.