Blood-stage Plasmodium infection induces CD8+ T lymphocytes to parasite-expressed antigens, largely regulated by CD8alpha+ dendritic cells

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.

Immunity to Cryptosporidium: insights into principles of enteric responses to infection

Cryptosporidium parasites replicate within intestinal epithelial cells and are an important cause of diarrhoeal disease in young children and in patients with primary and acquired defects in T cell function. This Review of immune-mediated control of Cryptosporidium highlights advances in understanding how intestinal epithelial cells detect this infection, the induction of innate resistance and the processes required for activation of T cell responses that promote parasite control. The development of a genetic tool set to modify Cryptosporidium combined with tractable mouse models provide new opportunities to understand the principles that govern the interface between intestinal epithelial cells and the immune system that mediate resistance to enteric pathogens.

Discrete class I molecules on brain endothelium differentially regulate neuropathology in experimental cerebral malaria

Cerebral malaria is the deadliest complication that can arise from Plasmodium infection. CD8 T-cell engagement of brain vasculature is a putative mechanism of neuropathology in cerebral malaria. To define contributions of brain endothelial cell major histocompatibility complex (MHC) class I antigen-presentation to CD8 T cells in establishing cerebral malaria pathology, we developed novel H-2Kb LoxP and H-2Db LoxP mice crossed with Cdh5-Cre mice to achieve targeted deletion of discrete class I molecules, specifically from brain endothelium. This strategy allowed us to avoid off-target effects on iron homeostasis and class I-like molecules, which are known to perturb Plasmodium infection. This is the first endothelial-specific ablation of individual class-I molecules enabling us to interrogate these molecular interactions. In these studies, we interrogated human and mouse transcriptomics data to compare antigen presentation capacity during cerebral malaria. Using the Plasmodium berghei ANKA model of experimental cerebral malaria (ECM), we observed that H-2Kb and H-2Db class I molecules regulate distinct patterns of disease onset, CD8 T-cell infiltration, targeted cell death and regional blood-brain barrier disruption. Strikingly, ablation of either molecule from brain endothelial cells resulted in reduced CD8 T-cell activation, attenuated T-cell interaction with brain vasculature, lessened targeted cell death, preserved blood-brain barrier integrity and prevention of ECM and the death of the animal. We were able to show that these events were brain-specific through the use of parabiosis and created the novel technique of dual small animal MRI to simultaneously scan conjoined parabionts during infection. These data demonstrate that interactions of CD8 T cells with discrete MHC class I molecules on brain endothelium differentially regulate development of ECM neuropathology. Therefore, targeting MHC class I interactions therapeutically may hold potential for treatment of cases of severe malaria.

Pathogenetic mechanisms and treatment targets in cerebral malaria

Malaria, the most prevalent mosquito-borne infectious disease worldwide, has accompanied humanity for millennia and remains an important public health issue despite advances in its prevention and treatment. Most infections are asymptomatic, but a small percentage of individuals with a heavy parasite burden develop severe malaria, a group of clinical syndromes attributable to organ dysfunction. Cerebral malaria is an infrequent but life-threatening complication of severe malaria that presents as an acute cerebrovascular encephalopathy characterized by unarousable coma. Despite effective antiparasite drug treatment, 20% of patients with cerebral malaria die from this disease, and many survivors of cerebral malaria have neurocognitive impairment. Thus, an important unmet clinical need is to rapidly identify people with malaria who are at risk of developing cerebral malaria and to develop preventive, adjunctive and neuroprotective treatments for cerebral malaria. This Review describes important advances in the understanding of cerebral malaria over the past two decades and discusses how these mechanistic insights could be translated into new therapies.

Plasmodium Infection-Cure Cycles Increase the Capacity of Phagocytosis in Conventional Dendritic Cells

Malaria stands as one of the most pervasive human infectious diseases globally and represents a prominent cause of mortality. Immunity against clinical malaria disease is achieved through multiple infection and treatment cycles, culminating in a substantial reduction in parasite burden. To investigate this phenomenon, we established a murine model involving repeated infection-cure cycles, whereby mice were infected with the lethal rodent malarial parasite Plasmodium berghei ANKA and subsequently treated with the anti-malarial drug pyrimethamine. Our earlier study revealed a significant decrease in the capacity of conventional dendritic cells (cDCs) to produce cytokines upon stimulation in infection-cured mice. In the present study, we aimed to further elucidate the modulation of cDC functionality during repeated infection-cure cycles by examining their phagocytic capacity. Administering fluorescent beads to mice resulted in no significant difference in the total number of bead-positive cells within the spleens of both uninfected and 3-cure (three cycles of infection-cure) mice. However, the proportion of the CD11c+F4/80- population within bead-positive cells was notably higher in 3-cure mice compared to uninfected mice. Subsequent in vitro analysis of bead phagocytosis by purified CD11c+cDCs revealed that the cDC2 subset from 3-cure mice exhibited significantly enhanced phagocytic capacity in comparison to their uninfected counterparts. These findings underscore the substantial impact of repeated infection-cure cycles on various facets of cDC function, potentially influencing the trajectory of immune responses against subsequent malaria infections.

Systemic host inflammation induces stage-specific transcriptomic modification and slower maturation in malaria parasites

Maturation rates of malaria parasites within red blood cells (RBCs) can be influenced by host nutrient status and circadian rhythm; whether host inflammatory responses can also influence maturation remains less clear. Here, we observed that systemic host inflammation induced in mice by an innate immune stimulus, lipopolysaccharide (LPS), or by ongoing acute Plasmodium infection, slowed the progression of a single cohort of parasites from one generation of RBC to the next. Importantly, plasma from LPS-conditioned or acutely infected mice directly inhibited parasite maturation during in vitro culture, which was not rescued by supplementation, suggesting the emergence of inhibitory factors in plasma. Metabolomic assessments confirmed substantial alterations to the plasma of LPS-conditioned and acutely infected mice, and identified a small number of candidate inhibitory metabolites. Finally, we confirmed rapid parasite responses to systemic host inflammation in vivo using parasite scRNA-seq, noting broad impairment in transcriptional activity and translational capacity specifically in trophozoites but not rings or schizonts. Thus, we provide evidence that systemic host inflammation rapidly triggered transcriptional alterations in circulating blood-stage Plasmodium trophozoites and predict candidate inhibitory metabolites in the plasma that may impair parasite maturation in vivo. IMPORTANCE Malaria parasites cyclically invade, multiply, and burst out of red blood cells. We found that a strong inflammatory response can cause changes to the composition of host plasma, which directly slows down parasite maturation. Thus, our work highlights a new mechanism that limits malaria parasite growth in the bloodstream.

The deubiquitinating enzyme OTUD7b protects dendritic cells from TNF-induced apoptosis by stabilizing the E3 ligase TRAF2

The cytokine tumor necrosis factor (TNF) critically regulates the intertwined cell death and pro-inflammatory signaling pathways of dendritic cells (DCs) via ubiquitin modification of central effector molecules, but the intrinsic molecular switches deciding on either pathway are incompletely defined. Here, we uncover that the ovarian tumor deubiquitinating enzyme 7b (OTUD7b) prevents TNF-induced apoptosis of DCs in infection, resulting in efficient priming of pathogen-specific CD8⁺ T cells. Mechanistically, OTUD7b stabilizes the E3 ligase TNF-receptor-associated factor 2 (TRAF2) in human and murine DCs by counteracting its K48-ubiquitination and proteasomal degradation. TRAF2 in turn facilitates K63-linked polyubiquitination of RIPK1, which mediates activation of NF-κB and MAP kinases, IL-12 production, and expression of anti-apoptotic cFLIP and Bcl-xL. We show that mice with DC-specific OTUD7b-deficiency displayed DC apoptosis and a failure to induce CD8⁺ T cell-mediated brain pathology, experimental cerebral malaria, in a murine malaria infection model. Together, our data identify the deubiquitinating enzyme OTUD7b as a central molecular switch deciding on survival of human and murine DCs and provides a rationale to manipulate DC responses by targeting their ubiquitin network downstream of the TNF receptor pathway.

Lymphotoxin-α Orchestrate Hypoxia and Immune factors to Induce Experimental Cerebral Malaria: Inhibition Mitigates Pathogenesis, Neurodegeneration, and Increase Survival

Knockdown studies have shown lymphotoxin-α (Lt-α) as a critical molecule for Experimental cerebral malaria (ECM) pathogenesis. We investigated the role of lymphotoxin-α in regulating active caspase-3 and calpain1. T cell infiltration into the brains, and subsequent neuronal cell death are the essential features of Plasmodium berghei ANKA(PbA)-induced ECM. Our results showed increased Lt-α levels during ECM. Treatment of naïve mice with serum from ECM mice and exogenous Lt-α was lethal. We inhibited Lt-α in vivo during PbA infection by injecting the mice with anti-Lt-α antibody. Inhibition of Lt-α mitigated neuronal cell death and increased mice's survival until 30-day post-infection (p.i.) compared to only 15 days survival of PbA control mice.

Host immunity to Plasmodium infection: Contribution of Plasmodium berghei to our understanding of T cell-related immune response to blood-stage malaria

Malaria is a life-threatening disease caused by infection with Plasmodium parasites. The goal of developing an effective malaria vaccine is yet to be reached despite decades of massive research efforts. CD4⁺ helper T cells, CD8⁺ cytotoxic T cells, and γδ T cells are associated with immune responses to both liver-stage and blood-stage Plasmodium infection. The immune responses of T cell-lineages to Plasmodium infection are associated with both protection and immunopathology. Studies with mouse model of malaria contribute to our understanding of host immune response. In this paper, we focus primarily on mouse malaria model with blood-stage Plasmodium berghei infection and review our knowledge of T cell immune responses against Plasmodium infection. Moreover, we also discuss findings of experimental human studies. Uncovering the precise mechanisms of T cell-mediated immunity to Plasmodium infection can be accomplished through further investigations using mouse models of malaria with rodent Plasmodium parasites. Those findings would be invaluable to advance the efforts for development of an effective malaria vaccine.

Increased Expression of Multiple Co-Inhibitory Molecules on Malaria-Induced CD8+ T Cells Are Associated With Increased Function Instead of Exhaustion

Activated cytotoxic CD8+ T cells can selectively kill target cells in an antigen-specific manner. However, their prolonged activation often has detrimental effects on tissue homeostasis and function. Indeed, overwhelming cytotoxic activity of CD8+ T cells can drive immunopathology, and therefore, the extent and duration of CD8+ T cell effector function needs to be tightly regulated. One way to regulate CD8+ T cell function is their suppression through engagement of co-inhibitory molecules to their cognate ligands (e.g., LAG-3, PD-1, TIM-3, TIGIT and CTLA-4). During chronic antigen exposure, the expression of co-inhibitory molecules is associated with a loss of T cell function, termed T cell exhaustion and blockade of co-inhibitory pathways often restores T cell function. We addressed the effect of co-inhibitory molecule expression on CD8+ T cell function during acute antigen exposure using experimental malaria. To this end, we infected OT-I mice with a transgenic P. berghei ANKA strain that expresses ovalbumin (PbTG), which enables the characterization of antigen-specific CD8+ T cell responses. We then compared antigen-specific CD8+ T cell populations expressing different levels of the co-inhibitory molecules. High expression of LAG-3 correlated with high expression of PD-1, TIGIT, TIM-3 and CTLA-4. Contrary to what has been described during chronic antigen exposure, antigen-specific CD8+ T cells with the highest expression of LAG-3 appeared to be fully functional during acute malaria. We evaluated this by measuring IFN-γ, Granzyme B and Perforin production and confirmed the results by employing a newly developed T cell cytotoxicity assay. We found that LAG-3high CD8+ T cells are more cytotoxic than LAG-3low or activated but LAG-3neg CD8+ T cells. In conclusion, our data imply that expression of co-inhibitory molecules in acute malaria is not necessarily associated with functional exhaustion but may be associated with an overwhelming T cell activation. Taken together, our findings shed new light on the induction of co-inhibitory molecules during acute T cell activation with ramifications for immunomodulatory therapies targeting these molecules in acute infectious diseases.

A reverse vaccinology approach on transmembrane carbonic anhydrases from Plasmodium species as vaccine candidates for malaria prevention

BACKGROUND

Malaria is a significant parasitic infection, and human infection is mediated by mosquito (Anopheles) biting and subsequent transmission of protozoa (Plasmodium) to the blood. Carbonic anhydrases (CAs) are known to be highly expressed in the midgut and ectoperitrophic space of Anopheles gambiae. Transmembrane CAs (tmCAs) in Plasmodium may be potential vaccine candidates for the control and prevention of malaria.

METHODS

In this study, two groups of transmembrane CAs, including α-CAs and one group of η-CAs were analysed by immunoinformatics and computational biology methods, such as predictions on transmembrane localization of CAs from Plasmodium spp., affinity and stability of different HLA classes, antigenicity of tmCA peptides, epitope and proteasomal cleavage of Plasmodium tmCAs, accessibility of Plasmodium tmCAs MHC-ligands, allergenicity of Plasmodium tmCAs, disulfide-bond of Plasmodium tmCAs, B cell epitopes of Plasmodium tmCAs, and Cell type-specific expression of Plasmodium CAs.

RESULTS

Two groups of α-CAs and one group of η-CAs in Plasmodium spp. were identified to contain tmCA sequences, having high affinity towards MHCs, high stability, and strong antigenicity. All putative tmCAs were predicted to contain sequences for proteasomal cleavage in antigen presenting cells (APCs).

CONCLUSIONS

The predicted results revealed that tmCAs from Plasmodium spp. can be potential targets for vaccination against malaria.

Similarly efficacious anti-malarial drugs SJ733 and pyronaridine differ in their ability to remove circulating parasites in mice

Background

Artemisinin-based combination therapy (ACT) has been a mainstay for malaria prevention and treatment. However, emergence of drug resistance has incentivised development of new drugs. Defining the kinetics with which circulating parasitized red blood cells (pRBC) are lost after drug treatment, referred to as the “parasite clearance curve”, has been critical for assessing drug efficacy; yet underlying mechanisms remain partly unresolved. The clearance curve may be shaped both by the rate at which drugs kill parasites, and the rate at which drug-affected parasites are removed from circulation.

Methods

In this context, two anti-malarials, SJ733, and an ACT partner drug, pyronaridine were compared against sodium artesunate in mice infected with Plasmodium berghei (strain ANKA). To measure each compound’s capacity for pRBC removal in vivo, flow cytometric monitoring of a single cohort of fluorescently-labelled pRBC was employed, and combined with ex vivo parasite culture to assess parasite maturation and replication.

Results

These three compounds were found to be similarly efficacious in controlling established infection by reducing overall parasitaemia. While sodium artesunate acted relatively consistently across the life-stages, single-dose SJ733 elicited a biphasic effect, triggering rapid, partly phagocyte-dependent removal of trophozoites and schizonts, followed by arrest of residual ring-stages. In contrast, pyronaridine abrogated maturation of younger parasites, with less pronounced effects on mature parasites, while modestly increasing pRBC removal.

Conclusions

Anti-malarials SJ733 and pyronaridine, though similarly efficacious in reducing overall parasitaemia in mice, differed markedly in their capacity to arrest replication and remove pRBC from circulation. Thus, similar parasite clearance curves can result for anti-malarials with distinct capacities to inhibit, kill and clear parasites.

Spatiotemporal Adaptations of Macrophage and Dendritic Cell Development and Function

Macrophages and conventional dendritic cells (cDCs) are distributed throughout the body, maintaining tissue homeostasis and tolerance to self and orchestrating innate and adaptive immunity against infection and cancer. As they complement each other, it is important to understand how they cooperate and the mechanisms that integrate their functions. Both are exposed to commensal microbes, pathogens, and other environmental challenges that differ widely among anatomical locations and over time. To adjust to these varying conditions, macrophages and cDCs acquire spatiotemporal adaptations (STAs) at different stages of their life cycle that determine how they respond to infection. The STAs acquired in response to previous infections can result in increased responsiveness to infection, termed training, or in reduced responses, termed paralysis, which in extreme cases can cause immunosuppression. Understanding the developmental stage and location where macrophages and cDCs acquire their STAs, and the molecular and cellular players involved in their induction, may afford opportunities to harness their beneficial outcomes and avoid or reverse their deleterious effects. Here we review our current understanding of macrophage and cDC development, life cycle, function, and STA acquisition before, during, and after infection. We propose a unified framework to explain how these two cell types adjust their activities to changing conditions over space and time to coordinate their immunosurveillance functions. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

T cells expressing multiple co-inhibitory molecules in acute malaria are not exhausted but exert a suppressive function in mice

Overwhelming activation of T cells in acute malaria is associated with severe outcomes. Thus, counter-regulation by anti-inflammatory mechanisms is indispensable for an optimal resolution of disease. Using Plasmodium berghei ANKA (PbA) infection of C57BL/6 mice, we performed a comprehensive analysis of co-inhibitory molecules expressed on CD4⁺ and CD8⁺ T cells using an unbiased cluster analysis approach. We identified similar T cell clusters co-expressing several co-inhibitory molecules like programmed cell death protein 1 (PD-1) and lymphocyte activation gene 3 (LAG-3) in the CD4⁺ and the CD8⁺ T cell compartment. Interestingly, despite expressing co-inhibitory molecules, which are associated with T cell exhaustion in chronic settings, these T cells were more functional compared to activated T cells that were negative for co-inhibitory molecules. However, T cells expressing high levels of PD-1 and LAG-3 also conferred suppressive capacity and thus resembled type I regulatory T cells. To our knowledge, this is the first description of malaria-induced CD8⁺ T cells with suppressive capacity. Importantly, we found an induction of T cells with a similar co-inhibitory rich phenotype in Plasmodium falciparum-infected patients. In conclusion, we demonstrate that malaria-induced T cells expressing co-inhibitory molecules are not exhausted, but acquire additional suppressive capacity, which might represent an immune regulatory pathway to prevent further activation of T cells during acute malaria.

Eosinophils Suppress the Migration of T Cells Into the Brain of Plasmodium berghei-Infected Ifnar1-/- Mice and Protect Them From Experimental Cerebral Malaria

Cerebral malaria is a potentially lethal disease, which is caused by excessive inflammatory responses to Plasmodium parasites. Here we use a newly developed transgenic Plasmodium berghei ANKA (PbA_Ama1OVA) parasite that can be used to study parasite-specific T cell responses. Our present study demonstrates that Ifnar1^-/- mice, which lack type I interferon receptor-dependent signaling, are protected from experimental cerebral malaria (ECM) when infected with this novel parasite. Although CD8⁺ T cell responses generated in the spleen are essential for the development of ECM, we measured comparable parasite-specific cytotoxic T cell responses in ECM-protected Ifnar1^-/- mice and wild type mice suffering from ECM. Importantly, CD8⁺ T cells were increased in the spleens of ECM-protected Ifnar1^-/- mice and the blood-brain-barrier remained intact. This was associated with elevated splenic levels of CCL5, a T cell and eosinophil chemotactic chemokine, which was mainly produced by eosinophils, and an increase in eosinophil numbers. Depletion of eosinophils enhanced CD8⁺ T cell infiltration into the brain and increased ECM induction in PbA_Ama1OVA-infected Ifnar1^-/- mice. However, eosinophil-depletion did not reduce the CD8⁺ T cell population in the spleen or reduce splenic CCL5 concentrations. Our study demonstrates that eosinophils impact CD8⁺ T cell migration and proliferation during PbA_Ama1OVA-infection in Ifnar1^-/- mice and thereby are contributing to the protection from ECM.

Memory CD8+ T cells exhibit tissue imprinting and non-stable exposure-dependent reactivation characteristics following blood-stage Plasmodium berghei ANKA infections

Experimental cerebral malaria (ECM) is a severe complication of Plasmodium berghei ANKA (PbA) infection in mice, characterized by CD8⁺ T‐cell accumulation within the brain. Whilst the dynamics of CD8⁺ T‐cell activation and migration during extant primary PbA infection have been extensively researched, the fate of the parasite‐specific CD8⁺ T cells upon resolution of ECM is not understood. In this study, we show that memory OT‐I cells persist systemically within the spleen, lung and brain following recovery from ECM after primary PbA‐OVA infection. Whereas memory OT‐I cells within the spleen and lung exhibited canonical central memory (Tcm) and effector memory (Tem) phenotypes, respectively, memory OT‐I cells within the brain post‐PbA‐OVA infection displayed an enriched CD69⁺CD103⁻ profile and expressed low levels of T‐bet. OT‐I cells within the brain were excluded from short‐term intravascular antibody labelling but were targeted effectively by longer‐term systemically administered antibodies. Thus, the memory OT‐I cells were extravascular within the brain post‐ECM but were potentially not resident memory cells. Importantly, whilst memory OT‐I cells exhibited strong reactivation during secondary PbA‐OVA infection, preventing activation of new primary effector T cells, they had dampened reactivation during a fourth PbA‐OVA infection. Overall, our results demonstrate that memory CD8⁺ T cells are systemically distributed but exhibit a unique phenotype within the brain post‐ECM, and that their reactivation characteristics are shaped by infection history. Our results raise important questions regarding the role of distinct memory CD8⁺ T‐cell populations within the brain and other tissues during repeat Plasmodium infections.

CD49d marks Th1 and Tfh-like antigen-specific CD4+ T cells during Plasmodium chabaudi infection

Upon activation, specific CD4+ T cells up-regulate the expression of CD11a and CD49d, surrogate markers of pathogen-specific CD4+ T cells. However, using T-cell receptor transgenic mice specific for a Plasmodium antigen, termed PbT-II, we found that activated CD4+ T cells develop not only to CD11ahiCD49dhi cells, but also to CD11ahiCD49dlo cells during acute Plasmodium infection. CD49dhi PbT-II cells, localized in the red pulp of spleens, expressed transcription factor T-bet and produced IFN-γ, indicating that they were type 1 helper T (Th1)-type cells. In contrast, CD49dlo PbT-II cells resided in the white pulp/marginal zones and were a heterogeneous population, with approximately half of them expressing CXCR5 and a third expressing Bcl-6, a master regulator of follicular helper T (Tfh) cells. In adoptive transfer experiments, both CD49dhi and CD49dlo PbT-II cells differentiated into CD49dhi Th1-type cells after stimulation with antigen-pulsed dendritic cells, while CD49dhi and CD49dlo phenotypes were generally maintained in mice infected with Plasmodium chabaudi. These results suggest that CD49d is expressed on Th1-type Plasmodium-specific CD4+ T cells, which are localized in the red pulp of the spleen, and can be used as a marker of antigen-specific Th1 CD4+ T cells, rather than that of all pathogen-specific CD4+ T cells.

Perivascular macrophages create an intravascular niche for CD8+ T cell localisation prior to the onset of fatal experimental cerebral malaria

Objectives

The immunologic events that build up to the fatal neurological stage of experimental cerebral malaria (ECM) are incompletely understood. Here, we dissect immune cell behaviour occurring in the central nervous system (CNS) when Plasmodium berghei ANKA (PbA)‐infected mice show only minor clinical signs.

Methods

A 2‐photon intravital microscopy (2P‐IVM) brain imaging model was used to study the spatiotemporal context of early immunological events in situ during ECM.

Results

Early in the disease course, antigen‐specific CD8⁺ T cells came in contact and arrested on the endothelium of post‐capillary venules. CD8⁺ T cells typically adhered adjacent to, or were in the near vicinity of, perivascular macrophages (PVMs) that line post‐capillary venules. Closer examination revealed that CD8⁺ T cells crawled along the inner vessel wall towards PVMs that lay on the abluminal side of large post‐capillary venules. ‘Activity hotspots’ in large post‐capillary venules were characterised by T‐cell localisation, activated morphology and clustering of PVM, increased abutting of post‐capillary venules by PVM and augmented monocyte accumulation. In the later stages of infection, when mice exhibited neurological signs, intravascular CD8⁺ T cells increased in number and changed their behaviour, actively crawling along the endothelium and displaying frequent, short‐term interactions with the inner vessel wall at hotspots.

Conclusion

Our study suggests an active interaction between PVM and CD8⁺ T cells occurs across the blood–brain barrier (BBB) in early ECM, which may be the initiating event in the inflammatory cascade leading to BBB alteration and neuropathology.

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.

Plasmodium berghei Hsp90 contains a natural immunogenic I-Ab-restricted antigen common to rodent and human Plasmodium species

Thorough understanding of the role of CD4 T cells in immunity can be greatly assisted by the study of responses to defined specificities. This requires knowledge of Plasmodium-derived immunogenic epitopes, of which only a few have been identified, especially for the mouse C57BL/6 background. We recently developed a TCR transgenic mouse line, termed PbT-II, that produces CD4⁺ T cells specific for an MHC class II (I-A^b)-restricted Plasmodium epitope and is responsive to both sporozoites and blood-stage P. berghei. Here, we identify a peptide within the P. berghei heat shock protein 90 as the cognate epitope recognised by PbT-II cells. We show that C57BL/6 mice infected with P. berghei blood-stage induce an endogenous CD4 T cell response specific for this epitope, indicating cells of similar specificity to PbT-II cells are present in the naïve repertoire. Adoptive transfer of in vitro activated T_H1-, or particularly T_H2-polarised PbT-II cells improved control of P. berghei parasitemia in C57BL/6 mice and drastically reduced the onset of experimental cerebral malaria. Our results identify a versatile, potentially protective MHC-II restricted epitope useful for exploration of CD4 T cell-mediated immunity and vaccination strategies against malaria.

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Identification of a novel MHC-II-restricted epitope in P. berghei Hsp90 that is the cognate antigen of PbT-II CD4+ T cells.

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This epitope is conserved among mouse malaria parasites and in Plasmodium falciparum, which causes human malaria.

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Exposure to liver or blood stage P. berghei infection expands a population of endogenous Hsp90-specific CD4+ T cells.

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Dendritic cell-targeted vaccination generates memory PbT-II cells and endogenous Hsp90-specific CD4+ T cells.

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TH1- and TH2-polarised PbT-II cells reduce P. berghei parasitaemia and mitigate development of experimental cerebral malaria.

Functional Human CD141+ Dendritic Cells in Human Immune System Mice

BACKGROUND

For the purpose of studying functional human dendritic cells (DCs) in a humanized mouse model that mimics the human immune system (HIS), a model referred to as HIS mice was established.

METHODS

Human immune system mice were made by engrafting NOD/SCID/IL2Rgammanull (NSG) mice with human hematopoietic stem cells (HSCs) following the transduction of genes encoding human cytokines and human leukocyte antigen (HLA)-A2.1 by adeno-associated virus serotype 9 (AAV9) vectors.

RESULTS

Our results indicate that human DC subsets, such as CD141+CD11c+ and CD1c+CD11c+ myeloid DCs, distribute throughout several organs in HIS mice including blood, bone marrow, spleen, and draining lymph nodes. The CD141+CD11c+ and CD1c+CD11c+ human DCs isolated from HIS mice immunized with adenoviruses expressing malaria/human immunodeficiency virus (HIV) epitopes were able to induce the proliferation of malaria/HIV epitopes-specific human CD8+ T cells in vitro. Upregulation of CD1c was also observed in human CD141+ DCs 1 day after immunization with the adenovirus-based vaccines.

CONCLUSIONS

Establishment of such a humanized mouse model that mounts functional human DCs enables preclinical assessment of the immunogenicity of human vaccines in vivo.

The C-type Lectin Receptor CLEC12A Recognizes Plasmodial Hemozoin and Contributes to Cerebral Malaria Development

Malaria represents a major cause of death from infectious disease. Hemozoin is a Plasmodium-derived product that contributes to progression of cerebral malaria. However, there is a gap of knowledge regarding how hemozoin is recognized by innate immunity. Myeloid C-type lectin receptors (CLRs) encompass a family of carbohydrate-binding receptors that act as pattern recognition receptors in innate immunity. In the present study, we identify the CLR CLEC12A as a receptor for hemozoin. Dendritic cell-T cell co-culture assays indicate that the CLEC12A/hemozoin interaction enhances CD8⁺ T cell cross-priming. Using the Plasmodium berghei Antwerpen-Kasapa (ANKA) mouse model of experimental cerebral malaria (ECM), we find that CLEC12A deficiency protects mice from ECM, illustrated by reduced ECM incidence and ameliorated clinical symptoms. In conclusion, we identify CLEC12A as an innate sensor of plasmodial hemozoin.

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CLEC12A recognizes plasmodial hemozoin

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The CLEC12A/hemozoin interaction enhances CD8⁺ T cell cross-priming in vitro

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CLEC12A^−/− mice are protected from experimental cerebral malaria

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.

Protection of Batf3-deficient mice from experimental cerebral malaria correlates with impaired cytotoxic T-cell responses and immune regulation

Excessive inflammatory immune responses during infections with Plasmodium parasites are responsible for severe complications such as cerebral malaria (CM) that can be studied experimentally in mice. Dendritic cells (DCs) activate cytotoxic CD8+ T-cells and initiate immune responses against the parasites. Batf3-/- mice lack a DC subset, which efficiently induces strong CD8 T-cell responses by cross-presentation of exogenous antigens. Here we show that Batf3-/- mice infected with Plasmodium berghei ANKA (PbA) were protected from experimental CM (ECM), characterized by a stable blood-brain barrier (BBB) and significantly less infiltrated peripheral immune cells in the brain. Importantly, the absence of ECM in Batf3-/- mice correlated with attenuated responses of cytotoxic T-cells, as their parasite-specific lytic activity as well as the production of interferon gamma and granzyme B were significantly decreased. Remarkably, spleens of ECM-protected Batf3-/- mice had elevated levels of regulatory immune cells and interleukin 10. Thus, protection from ECM in PbA-infected Batf3-/- mice was associated with the absence of strong CD8+ T-cell activity and induction of immunoregulatory mediators and cells.

Cytotoxic T Cell-Derived Granzyme B Is Increased in Severe Plasmodium Falciparum Malaria

In Plasmodium falciparum malaria, CD8⁺ T cells play a double-edged role. Liver-stage specific CD8⁺ T cells can confer protection, as has been shown in several vaccine studies. Blood-stage specific CD8⁺ T cells, on the other hand, contribute to the development of cerebral malaria in murine models of malaria. The role of CD8⁺ T cells in humans during the blood-stage of P. falciparum remains unclear. As part of a cross-sectional malaria study in Ghana, granzyme B levels and CD8⁺ T cells phenotypes were compared in the peripheral blood of children with complicated malaria, uncomplicated malaria, afebrile but asymptomatically infected children and non-infected children. Granzyme B levels in the plasma were significantly higher in children with febrile malaria than in afebrile children. CD8⁺ T cells were the main T cell subset expressing granzyme B. The proportion of granzyme B⁺ CD8⁺ T cells was significantly higher in children with complicated malaria than in uncomplicated malaria, whereas the activation marker CD38 on CD8⁺ T cells showed similar expression levels. This suggests a pathogenic role of cytotoxic CD8⁺ T cells in the development of malaria complications in humans.

Origin and development of classical dendritic cells

Classical dendritic cells (cDCs) are mononuclear phagocytes of hematopoietic origin specialized in the induction and regulation of adaptive immunity. Initially defined by their unique T cell activation potential, it became quickly apparent that cDCs would be difficult to distinguish from other phagocyte lineages, by solely relying on marker-based approaches. Today, cDCs definition increasingly embed their unique ontogenetic features. A growing consensus defines cDCs on multiple criteria including: (1) dependency on the fms-like tyrosine kinase 3 ligand hematopoietic growth factor, (2) development from the common DC bone marrow progenitor, (3) constitutive expression of the transcription factor ZBTB46 and (4) the ability to induce, after adequate stimulation, the activation of naïve T lymphocytes. cDCs are a heterogeneous cell population that contains two main subsets, named type 1 and type 2 cDCs, arising from divergent ontogenetic pathways and populating multiple lymphoid and non-lymphoid tissues. Here, we present recent knowledge on the cellular and molecular pathways controlling the specification and commitment of cDC subsets from murine and human hematopoietic stem cells.

Comparison of whole blood and spleen transcriptional signatures over the course of an experimental malaria infection

Although the spleen is broadly accepted as the major lymphoid organ involved in generating immune responses to the erythrocytic stages of the malaria parasite, Plasmodium, human splenic tissue is not readily available in most cases. As a result, most studies of malaria in humans rely on peripheral blood to assess cellular immune responses to malaria. The suitability of peripheral blood as a proxy for splenic immune responses is however unknown. Here, we have simultaneously analysed the transcriptomes of whole blood and spleen over 12 days of erythrocytic stage Plasmodium chabaudi infection in C57BL/6 mice. Using both unsupervised and directed approaches, we compared gene expression between blood and spleen over the course of infection. Taking advantage of publicly available datasets, we used machine learning approaches to infer cell proportions and cell-specific gene expression signatures from our whole tissue transcriptome data. Our findings demonstrate that spleen and blood are quite dissimilar, sharing only a small amount of transcriptional information between them, with transcriptional differences in both cellular composition and transcriptional activity. These results suggest that while blood transcriptome data may be useful in defining surrogate markers of protection and pathology, they should not be used to predict specific immune responses occurring in lymphoid organs.

Expression of CD300lf by microglia contributes to resistance to cerebral malaria by impeding the neuroinflammation

Genetic mapping and genome-wide studies provide evidence for the association of several genetic polymorphisms with malaria, a complex pathological disease with multiple severity degrees. We have previously described Berr1and Berr2 as candidate genes identified in the WLA/Pas inbreed mouse strain predisposing to resistance to cerebral malaria (CM) induced by P. berghei ANKA. We report in this study the phenotypic and functional characteristics of a congenic strain we have derived for Berr2^WLA allele on the C57BL/6JR (B6) background. B6.WLA-Berr2 was found highly resistant to CM compared to C57BL/6JR susceptible mice. The mechanisms associated with CM resistance were analyzed by combining genotype, transcriptomic and immune response studies. We found that B6.WLA-Berr2 mice showed a reduced parasite sequestration and blood-brain barrier disruption with low CXCR3⁺ T cell infiltration in the brain along with altered glial cell response upon P. berghei ANKA infection compared to B6. In addition, we have identified the CD300f, belonging to a family of Ig-like encoding genes, as a potential candidate associated with CM resistance. Microglia cells isolated from the brain of infected B6.WLA-Berr2 mice significantly expressed higher level of CD300f compared to CM^S mice and were associated with inhibition of inflammatory response.

Immunisation of cattle against Babesia bovis combining a multi-epitope modified vaccinia Ankara virus and a recombinant protein induce strong Th1 cell responses but fails to trigger neutralising antibodies required for protection

Protection against the intraerythrocytic protozoan parasite Babesia bovis depends on both strong innate and adaptive immune response, this latter involving the presentation of parasite antigens to CD4⁺ T-lymphocytes by professional antigen-presenting cells. Secretion of Th1 cytokines by CD4⁺ T cell is also very important for isotype switching to IgG₂, the best opsonising antibody isotype in cattle, to target extracellular parasites and parasite antigens displayed at the erythrocyte surface. In the field of vaccinology, heterologous prime-boost schemes combining protein-adjuvant formulations with a modified vaccinia Ankara vector expressing the same antigen have demonstrated the induction of both humoral and cellular immune responses. It has been previously demonstrated that MVA-infected dendritic cells can present antigens in the context of MHC II and activate CD4⁺ T cell. These results support the use of the MVA viral vector for a pathogen like Babesia bovis, which only resides within erythrocytes. In this study, 13-15-months-old Holstein-Friesian steers were immunised with a subunit vaccine as a prime and a modified vaccinia Ankara vector as a boost, both expressing a chimeric multi-antigen (rMABbo - rMVA). This antigen includes the immunodominant B and T cell epitopes of three B. bovis proteins: merozoite surface antigen - 2c (MSA - 2c), rhoptry associated protein 1 (RAP - 1) and heat shock protein 20 (HSP20). Responses were compared with the Babesia bovis live attenuated vaccine used in Argentina (R1A). Eleven weeks after the first immunisation, all bovines were challenged by the inoculation of a virulent B. bovis strain. All groups were monitored daily for hyperthermia and reduction of packed cell volume. Both the rMABbo - rMVA and R1A vaccinated animals developed high titters of total IgG antibodies and an antigen-specific Th1 cellular response before and after challenge. However, all rMABbo - rMVA steers showed clinical signs of disease upon challenge. Only the R1A live vaccine group developed an immune response associated with in vitro neutralising antibodies at a level that significantly inhibited the parasite invasion. The lack of protection observed with this recombinant formulation indicates the need to perform further basic and clinical studies in the bovine model in order to achieve the desired effectiveness. This is the first report in which a novel vaccine candidate against Babesia bovis was constructed based on a recombinant and rationally designed viral vector and evaluated in the biological model of the disease.

Investigating immune responses to parasites using transgenesis

Parasites comprise diverse and complex organisms, which substantially impact human and animal health. Most parasites have complex life-cycles, and by virtue of co-evolution have developed multifaceted, often life-cycle stage-specific relationships with the immune system of their hosts. The complexity in the biology of many parasites often limits our knowledge of parasite-specific immune responses, to in vitro studies only. The relatively recent development of methods to stably manipulate the genetic make-up of many parasites has allowed a better understanding of host-parasite interactions, particularly in vivo. In this regard, the use of transgenic parasites can facilitate the study of immunomodulatory mechanisms under in vivo conditions. Therefore, in this review, we specifically highlighted the current developments in the use of transgenic parasites to unravel the host's immune response to different life-cycle stages of some key parasite species such as Leishmania, Schistosoma, Toxoplasma, Plasmodium and Trypanosome and to some degree, the use of transgenic nematode parasites is also briefly discussed.

The Ins and Outs of Cerebral Malaria Pathogenesis: Immunopathology, Extracellular Vesicles, Immunometabolism, and Trained Immunity

Complications from malaria parasite infections still cost the lives of close to half a million people every year. The most severe is cerebral malaria (CM). Employing murine models of CM, autopsy results, in vitro experiments, neuroimaging and microscopic techniques, decades of research activity have investigated the development of CM immunopathology in the hope of identifying steps that could be therapeutically targeted. Yet important questions remain. This review summarizes recent findings, primarily mechanistic insights on the essential cellular and molecular players involved gained within the murine experimental cerebral malaria model. It also highlights recent developments in (a) cell-cell communication events mediated through extracellular vesicles (EVs), (b) mounting evidence for innate immune memory, leading to “trained“ increased or tolerised responses, and (c) modulation of immune cell function through metabolism, that could shed light on why some patients develop this life-threatening condition whilst many do not.

Oral administration of Coenzyme Q10 protects mice against oxidative stress and neuro-inflammation during experimental cerebral malaria

In animal model of experimental cerebral malaria (ECM), the genesis of neuropathology is associated with oxidative stress and inflammatory mediators. There is limited progress in the development of new approaches to the treatment of cerebral malaria. Here, we tested whether oral supplementation of Coenzyme Q₁₀ (CoQ₁₀) would offer protection against oxidative stress and brain associated inflammation following Plasmodium berghei ANKA (PbA) infection in C57BL/6 J mouse model. For this purpose, one group of C57BL/6 mice was used as control; second group of mice were orally supplemented with 200 mg/kg CoQ₁₀ and then infected with PbA and the third group was PbA infected alone. Clinical, biochemical, immunoblot and immunological features of ECM was monitored. We observed that oral administration of CoQ₁₀ for 1 month and after PbA infection was able to improve survival, significantly reduced oedema, TNF-α and MIP-1β gene expression in brain samples in PbA infected mice. The result also shows the ability of CoQ₁₀ to reduce cholesterol and triglycerides lipids, levels of matrix metalloproteinases-9, angiopoietin-2 and angiopoietin-1 in the brain. In addition, CoQ₁₀ was very effective in decreasing NF-κB phosphorylation. Furthermore, CoQ₁₀ supplementation abrogated Malondialdehyde, and 8-OHDG and restored cellular glutathione. These results constitute the first demonstration that oral supplementation of CoQ₁₀ can protect mice against PbA induced oxidative stress and neuro-inflammation usually observed in ECM. Thus, the need to study CoQ₁₀ as a candidate of antioxidant and immunomodulatory molecule in ECM and testing it in clinical studies either alone or in combination with antimalaria regimens to provide insight into a potential translatable therapy.

Repeated clinical malaria episodes are associated with modification of the immune system in children

BACKGROUND

There are over 200 million reported cases of malaria each year, and most children living in endemic areas will experience multiple episodes of clinical disease before puberty. We set out to understand how frequent clinical malaria, which elicits a strong inflammatory response, affects the immune system and whether these modifications are observable in the absence of detectable parasitaemia.

METHODS

We used a multi-dimensional approach comprising whole blood transcriptomic, cellular and plasma cytokine analyses on a cohort of children living with endemic malaria, but uninfected at sampling, who had been under active surveillance for malaria for 8 years. Children were categorised into two groups depending on the cumulative number of episodes experienced: high (≥ 8) or low (< 5).

RESULTS

We observe that multiple episodes of malaria are associated with modification of the immune system. Children who had experienced a large number of episodes demonstrated upregulation of interferon-inducible genes, a clear increase in circulating levels of the immunoregulatory cytokine IL-10 and enhanced activation of neutrophils, B cells and CD8⁺ T cells.

CONCLUSION

Transcriptomic analysis together with cytokine and immune cell profiling of peripheral blood can robustly detect immune differences between children with different numbers of prior malaria episodes. Multiple episodes of malaria are associated with modification of the immune system in children. Such immune modifications may have implications for the initiation of subsequent immune responses and the induction of vaccine-mediated protection.

Different doses of vitamin C supplementation enhances the Th1 immune response to early Plasmodium yoelii 17XL infection in BALB/c mice

Vitamin C (ascorbate) is maintained at high levels in most immune cells and can affect many aspects of the immune response. Here, we evaluated the effect of vitamin C supplementation on the immune response to Plasmodium yoelii 17XL (P. yoelii 17XL) infection in BALB/c mice. Two orally administered doses (25 mg/kg/day and 250 mg/kg/day) of vitamin C significantly reduced levels of parasitemia during the early stages of P. yoelii 17XL infection. The numbers of activated Th1 cells and macrophages in the groups receiving vitamin C supplementation were both higher than those in the untreated group. Meanwhile, vitamin C administration reduced the levels of tumor necrosis factor α secreted by splenocytes. Vitamin C also regulated the protective anti-malarial immune response by increasing the number of plasmacytoid dendritic cells, as well as the expression of dendritic cell maturation markers, such as major histocompatibility complex class II and cluster of differentiation 86. In conclusion, the doses of vitamin C (25 mg/kg/day, 250 mg/kg/day) during the early stages of malaria infection may better enhance host protective immunity, but have no dose dependence.

Dendritic Cell Responses and Function in Malaria

Malaria remains a serious threat to global health. Sustained malaria control and, eventually, eradication will only be achieved with a broadly effective malaria vaccine. Yet a fundamental lack of knowledge about how antimalarial immunity is acquired has hindered vaccine development efforts to date. Understanding how malaria-causing parasites modulate the host immune system, specifically dendritic cells (DCs), key initiators of adaptive and vaccine antigen-based immune responses, is vital for effective vaccine design. This review comprehensively summarizes how exposure to Plasmodium spp. impacts human DC function in vivo and in vitro. We have highlighted the heterogeneity of the data observed in these studies, compared and critiqued the models used to generate our current understanding of DC function in malaria, and examined the mechanisms by which Plasmodium spp. mediate these effects. This review highlights potential research directions which could lead to improved efficacy of existing vaccines, and outlines novel targets for next-generation vaccine strategies to target malaria.

T-lymphocytes response persists following Plasmodium berghei strain Anka infection resolution and may contribute to later experimental cerebral malaria outcomes

Several studies have proposed cerebral malaria (CM) as a CD4+ and CD8+ T lymphocyte-mediated disease. However, there are no data regarding the recruitment and/or persistence of these cells in the CNS following the phase of infection resolution. Glutamate-mediate excitotoxicity has also been implicated in CM. Blockade of glutamate NMDA receptors by its noncompetitive antagonist MK801 modulates cytokine and neurotrophic factors expression preventing cognitive and depressive-like behavior in experimental CM. Herein, we aim to investigate the role of T lymphocytes in later outcomes in CM, and whether the protective role of MK801 is associated with T lymphocytes response.

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.

Controlled Infection Immunization Using Delayed Death Drug Treatment Elicits Protective Immune Responses to Blood-Stage Malaria Parasites

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop.

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop. Evidence suggests that apoptosis of immune lymphocytes due to uncontrolled parasite growth contributes to the slow acquisition of immunity. To hasten and augment the development of natural immunity, we studied controlled infection immunization (CII) using low-dose exposure to different parasite species (Plasmodium chabaudi, P. yoelii, or P. falciparum) in two rodent systems (BALB/c and C57BL/6 mice) and in human volunteers, with drug therapy commencing at the time of initiation of infection. CIIs with infected erythrocytes and in conjunction with doxycycline or azithromycin, which are delayed death drugs targeting the parasite’s apicoplast, allowed extended exposure to parasites at low levels. In turn, this induced strong protection against homologous challenge in all immunized mice. We show that P. chabaudi/P. yoelii infection initiated at the commencement of doxycycline therapy leads to cellular or antibody-mediated protective immune responses in mice, with a broad Th1 cytokine response providing the best correlate of protection against homologous and heterologous species of Plasmodium. P. falciparum CII with doxycycline was additionally tested in a pilot clinical study (n = 4) and was found to be well tolerated and immunogenic, with immunological studies primarily detecting increased cell-associated immune responses. Furthermore, we report that a single dose of the longer-acting drug, azithromycin, given to mice (n = 5) as a single subcutaneous treatment at the initiation of infection controlled P. yoelii infection and protected all mice against subsequent challenge.

MRI demonstrates glutamine antagonist-mediated reversal of cerebral malaria pathology in mice

The deadliest complication of Plasmodium falciparum infection is cerebral malaria (CM), with a case fatality rate of 15 to 25% in African children despite effective antimalarial chemotherapy. No adjunctive treatments are yet available for this devastating disease. We previously reported that the glutamine antagonist 6-diazo-5-oxo-l-norleucine (DON) rescued mice from experimental CM (ECM) when administered late in the infection, a time by which mice had already suffered blood-brain barrier (BBB) dysfunction, brain swelling, and hemorrhaging. Herein, we used longitudinal MR imaging to visualize brain pathology in ECM and the impact of a new DON prodrug, JHU-083, on disease progression in mice. We demonstrate in vivo the reversal of disease markers in symptomatic, infected mice following treatment, including the resolution of edema and BBB disruption, findings usually associated with a fatal outcome in children and adults with CM. Our results support the premise that JHU-083 is a potential adjunctive treatment that could rescue children and adults from fatal CM.

HVEM and CD160: Regulators of Immunopathology During Malaria Blood-Stage

CD8⁺ T cells are key players during infection with the malaria parasite Plasmodium berghei ANKA (PbA). While they cannot provide protection against blood-stage parasites, they can cause immunopathology, thus leading to the severe manifestation of cerebral malaria. Hence, the tight control of CD8⁺ T cell function is key in order to prevent fatal outcomes. One major mechanism to control CD8⁺ T cell activation, proliferation and effector function is the integration of co-inhibitory and co-stimulatory signals. In this study, we show that one such pathway, the HVEM-CD160 axis, significantly impacts CD8⁺ T cell regulation and thereby the incidence of cerebral malaria. Here, we show that the co-stimulatory molecule HVEM is indeed required to maintain CD8⁺ T effector populations during infection. Additionally, by generating a CD160^-/- mouse line, we observe that the HVEM ligand CD160 counterbalances stimulatory signals in highly activated and cytotoxic CD8⁺ T effector cells, thereby restricting immunopathology. Importantly, CD160 is also induced on cytotoxic CD8⁺ T cells during acute Plasmodium falciparum malaria in humans. In conclusion, CD160 is specifically expressed on highly activated CD8⁺ T effector cells that are harmful during the blood-stage of 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.

CXCL10 stabilizes T cell-brain endothelial cell adhesion leading to the induction of cerebral malaria

Malaria remains one of the world's most significant human infectious diseases and cerebral malaria (CM) is its most deadly complication. CM pathogenesis remains incompletely understood, hindering the development of therapeutics to prevent this lethal complication. Elevated levels of the chemokine CXCL10 are a biomarker for CM, and CXCL10 and its receptor CXCR3 are required for experimental CM (ECM) in mice, but their role has remained unclear. Using multiphoton intravital microscopy, CXCR3 receptor- and ligand-deficient mice and bone marrow chimeric mice, we demonstrate a key role for endothelial cell-produced CXCL10 in inducing the firm adhesion of T cells and preventing their cell detachment from the brain vasculature. Using a CXCL9 and CXCL10 dual-CXCR3-ligand reporter mouse, we found that CXCL10 was strongly induced in the brain endothelium as early as 4 days after infection, while CXCL9 and CXCL10 expression was found in inflammatory monocytes and monocyte-derived DCs within the blood vasculature on day 8. The induction of both CXCL9 and CXCL10 was completely dependent on IFN-γ receptor signaling. These data demonstrate that IFN-γ-induced, endothelium-derived CXCL10 plays a critical role in mediating the T cell-endothelial cell adhesive events that initiate the inflammatory cascade that injures the endothelium and induces the development of ECM.

Plasmodium parasite as an effective hepatocellular carcinoma antigen glypican-3 delivery vector

We have previously demonstrated that malaria parasite infection has an anti-tumor effect in a mouse model. This research aimed to investigate the possibility of using Plasmodium parasite as a novel vaccine vector for hepatocellular carcinoma (HCC) immunotherapy. We constructed a Plasmodium yoelii 17XNL strain (P.y) expressing murine glypican-3 (GPC3) protein (P.y-GPC3), and examined its therapeutic potency in a murine Hepa1-6-induced hepatoma model that highly expressed GPC3 protein. The prerequisites for invoking a CD8+ T cell response were assessed after P.y-based immunization, which included obviously increased concentrations of T helper cell type 1 (Th1)-associated cytokines, such as IL-2, IFN-γ and TNF-α, in serum and preferential expansion of the CD8α+ dendritic cell (DC) subset with higher expression of CD80 and CD86 molecules. Compared with uninfected and wild-type P.y-infected mice, a significant GPC3-specific cytotoxic T lymphocyte (CTL) response was detected in P.y-GPC3 vaccinated mice. Furthermore, P.y-GPC3-based vaccination dramatically inhibited Hepa1-6-induced tumor growth in the implanted HCC and prolonged the survival of tumor-bearing mice. We concluded that a Plasmodium-based vector is highly efficient in inducing tumor antigen-specific T cell-mediated immunity and protection against tumor cells. More broadly, this strategy supported our hypothesis that Plasmodium parasites, as novel therapeutic antigen vectors, may be applicable to tumor immunotherapy for patients with HCC.

Interleukin-15 Complex Treatment Protects Mice from Cerebral Malaria by Inducing Interleukin-10-Producing Natural Killer Cells

Cerebral malaria is a deadly complication of Plasmodium infection and involves blood brain barrier (BBB) disruption following infiltration of white blood cells. During experimental cerebral malaria (ECM), mice inoculated with Plasmodium berghei ANKA-infected red blood cells develop a fatal CM-like disease caused by CD8⁺ T cell-mediated pathology. We found that treatment with interleukin-15 complex (IL-15C) prevented ECM, whereas IL-2C treatment had no effect. IL-15C-expanded natural killer (NK) cells were necessary and sufficient for protection against ECM. IL-15C treatment also decreased CD8⁺ T cell activation in the brain and prevented BBB breakdown without influencing parasite load. IL-15C induced NK cells to express IL-10, which was required for IL-15C-mediated protection against ECM. Finally, we show that ALT-803, a modified human IL-15C, mediates similar induction of IL-10 in NK cells and protection against ECM. These data identify a regulatory role for cytokine-stimulated NK cells in the prevention of a pathogenic immune response.

Doxycycline inhibits experimental cerebral malaria by reducing inflammatory immune reactions and tissue-degrading mediators

Malaria ranks among the most important infectious diseases worldwide and affects mostly people living in tropical countries. Mechanisms involved in disease progression are still not fully understood and specific treatments that might interfere with cerebral malaria (CM) are limited. Here we show that administration of doxycycline (DOX) prevented experimental CM (ECM) in Plasmodium berghei ANKA (PbA)-infected C57BL/6 wildtype (WT) mice in an IL-10-independent manner. DOX-treated mice showed an intact blood-brain barrier (BBB) and attenuated brain inflammation. Importantly, if WT mice were infected with a 20-fold increased parasite load, they could be still protected from ECM if they received DOX from day 4-6 post infection, despite similar parasitemia compared to control-infected mice that did not receive DOX and developed ECM. Infiltration of T cells and cytotoxic responses were reduced in brains of DOX-treated mice. Analysis of brain tissue by RNA-array revealed reduced expression of chemokines and tumour necrosis factor (TNF) in brains of DOX-treated mice. Furthermore, DOX-administration resulted in brains of the mice in reduced expression of matrix metalloproteinase 2 (MMP2) and granzyme B, which are both factors associated with ECM pathology. Systemic interferon gamma production was reduced and activated peripheral T cells accumulated in the spleen in DOX-treated mice. Our results suggest that DOX targeted inflammatory processes in the central nervous system (CNS) and prevented ECM by impaired brain access of effector T cells in addition to its anti-parasitic effect, thereby expanding the understanding of molecular events that underlie DOX-mediated therapeutic interventions.

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Development of a Novel CD4+ TCR Transgenic Line That Reveals a Dominant Role for CD8+ Dendritic Cells and CD40 Signaling in the Generation of Helper and CTL Responses to Blood-Stage Malaria

We describe an MHC class II (I-A^b)-restricted TCR transgenic mouse line that produces CD4⁺ T cells specific for Plasmodium species. This line, termed PbT-II, was derived from a CD4⁺ T cell hybridoma generated to blood-stage Plasmodium berghei ANKA (PbA). PbT-II cells responded to all Plasmodium species and stages tested so far, including rodent (PbA, P. berghei NK65, Plasmodium chabaudi AS, and Plasmodium yoelii 17XNL) and human (Plasmodium falciparum) blood-stage parasites as well as irradiated PbA sporozoites. PbT-II cells can provide help for generation of Ab to P. chabaudi infection and can control this otherwise lethal infection in CD40L-deficient mice. PbT-II cells can also provide help for development of CD8⁺ T cell-mediated experimental cerebral malaria (ECM) during PbA infection. Using PbT-II CD4⁺ T cells and the previously described PbT-I CD8⁺ T cells, we determined the dendritic cell (DC) subsets responsible for immunity to PbA blood-stage infection. CD8⁺ DC (a subset of XCR1⁺ DC) were the major APC responsible for activation of both T cell subsets, although other DC also contributed to CD4⁺ T cell responses. Depletion of CD8⁺ DC at the beginning of infection prevented ECM development and impaired both Th1 and follicular Th cell responses; in contrast, late depletion did not affect ECM. This study describes a novel and versatile tool for examining CD4⁺ T cell immunity during malaria and provides evidence that CD4⁺ T cell help, acting via CD40L signaling, can promote immunity or pathology to blood-stage malaria largely through Ag presentation by CD8⁺ DC.

Protein Tyrosine Phosphatase Inhibition Prevents Experimental Cerebral Malaria by Precluding CXCR3 Expression on T Cells

Cerebral malaria induced by Plasmodium berghei ANKA infection is dependent on the sequestration of cytotoxic T cells within the brain and augmentation of the inflammatory response. Herein, we demonstrate that inhibition of protein tyrosine phosphatase (PTP) activity significantly attenuates T cell sequestration within the brain and prevents the development of neuropathology. Mechanistically, the initial upregulation of CXCR3 on splenic T cells upon T cell receptor stimulation was critically decreased through the reduction of T cell-intrinsic PTP activity. Furthermore, PTP inhibition markedly increased IL-10 production by splenic CD4⁺ T cells by enhancing the frequency of LAG3⁺CD49b⁺ type 1 regulatory cells. Overall, these findings demonstrate that modulation of PTP activity could possibly be utilized in the treatment of cerebral malaria and other CXCR3-mediated diseases.