Perforin mediated apoptosis of cerebral microvascular endothelial cells during experimental cerebral malaria

Single cell RNA sequencing reveals endothelial cell killing and resolution pathways in experimental malaria-associated acute respiratory distress syndrome

Plasmodium parasites cause malaria, a global health disease that is responsible for more than 200 million clinical cases and 600 000 deaths each year. Most deaths are caused by various complications, including malaria-associated acute respiratory distress syndrome (MA-ARDS). Despite the very rapid and efficient killing of parasites with antimalarial drugs, 15% of patients with complicated malaria succumb. This stresses the importance of investigating resolution mechanisms that are involved in the recovery from these complications once the parasite is killed. To study the resolution of MA-ARDS, P. berghei NK65-infected C57BL/6 mice were treated with antimalarial drugs after onset of symptoms, resulting in 80% survival. Micro-computed tomography revealed alterations of the lungs upon infection, with an increase in total and non-aerated lung volume due to edema. Whole body plethysmography confirmed a drastically altered lung ventilation, which was restored during resolution. Single-cell RNA sequencing indicated an increased inflammatory state in the lungs upon infection, which was accompanied by a drastic decrease in endothelial cells, consistent with CD8+ T cell-mediated killing. During resolution, anti-inflammatory pathways were upregulated and proliferation of endothelial cells was observed. MultiNicheNet interactome analysis identified important changes in the ligand-receptor interactions during disease resolution that warrant further exploration in order to develop new therapeutic strategies. In conclusion, our study provides insights in pro-resolving pathways that limit inflammation and promote endothelial cell proliferation in experimental MA-ARDS. This information may be useful for the design of adjunctive treatments to enhance resolution after Plasmodium parasite killing by antimalarial drugs.

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.

Coenzyme Q10 exhibits anti-inflammatory and immune-modulatory thereby decelerating the occurrence of experimental cerebral malaria

Cerebral Malaria (CM) is associated with the complex neurological syndrome, whose pathology is mediated by severe inflammatory processes following infection with Plasmodium falciparum. Coenzyme-Q10 (Co-Q10) is a potent anti-inflammatory, anti-oxidant, and anti-apoptotic agent with numerous clinical applications. The aim of this study was to elucidate the role of oral administration of Co-Q10 on the initiation or regulation of inflammatory immune response during experimental cerebral malaria (ECM). For this purpose, the pre-clinical effect of Co-Q10 was evaluated in C57BL/6 J mice infected with Plasmodium berghei ANKA (PbA). Treatment with Co-Q10 resulted in the reduction of infiltrating parasite load, greatly improved the survival rate of PbA-infected mice that occurred independent of parasitaemia and prevented PbA-induced disruption of the blood-brain barrier (BBB) integrity. Exposure to Co-Q10 resulted in the reduction of infiltration of effector CD8 + T cells in the brain and secretion of cytolytic Granzyme B molecules. Notably, Co-Q10-treated mice had reduced levels of CD8 +T cell chemokines CXCR3, CCR2, and CCR5 in the brain following PbA-infection. Brain tissue analysis showed a reduction in the levels of inflammatory mediators TNF- α, CCL3, and RANTES in Co-Q10 administered mice. In addition, Co-Q10 modulated the differentiation and maturation of both splenic and brain dendritic cells and cross-presentation (CD8α+DCs) during ECM. Remarkably, Co-Q10 was very effective in decreasing levels of CD86, MHC-II, and CD40 in macrophages associated with ECM pathology. Exposure to Co-Q10 resulted in increased expression levels of Arginase-1 and Ym1/chitinase 3-like 3, which is linked to ECM protection. Furthermore, Co-Q10 supplementation prevented PbA-induced depletion of Arginase and CD206 mannose receptor levels. Co-Q10 abrogated PbA-driven elevation in pro-inflammatory cytokines IL-1β, IL-18, and IL-6 levels. In conclusion, the oral supplementation with Co-Q10 decelerates the occurrence of ECM by preventing lethal inflammatory immune responses and dampening genes associated with inflammation and immune-pathology during ECM, and offers an inimitable opening for developing an anti-inflammatory agent against cerebral malaria.

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.

Dynamics and immunomodulation of cognitive deficits and behavioral changes in non-severe experimental malaria

Data recently reported by our group indicate that stimulation with a pool of immunogens capable of eliciting type 2 immune responses can restore the cognitive and behavioral dysfunctions recorded after a single episode of non-severe rodent malaria caused by Plasmodium berghei ANKA. Here we explored the hypothesis that isolated immunization with one of the type 2 immune response-inducing immunogens, the human diphtheria-tetanus (dT) vaccine, may revert damages associated with malaria. To investigate this possibility, we studied the dynamics of cognitive deficits and anxiety-like phenotype following non-severe experimental malaria and evaluated the effects of immunization with both dT and of a pool of type 2 immune stimuli in reversing these impairments. Locomotor activity and long-term memory deficits were assessed through the open field test (OFT) and novel object recognition task (NORT), while the anxiety-like phenotype was assessed by OFT and light/dark task (LDT). Our results indicate that poor performance in cognitive-behavioral tests can be detected as early as the 12^th day after the end of antimalarial treatment with chloroquine and may persist for up to 155 days post infection. The single immunization strategy with the human dT vaccine showed promise in reversal of long-term memory deficits in NORT, and anxiety-like behavior in OFT and LDT.

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.

Malaria Related Neurocognitive Deficits and Behavioral Alterations

Typical of tropical and subtropical regions, malaria is caused by protozoa of the genus Plasmodium and is, still today, despite all efforts and advances in controlling the disease, a major issue of public health. Its clinical course can present either as the classic episodes of fever, sweating, chills and headache or as nonspecific symptoms of acute febrile syndromes and may evolve to severe forms. Survivors of cerebral malaria, the most severe and lethal complication of the disease, might develop neurological, cognitive and behavioral sequelae. This overview discusses the neurocognitive deficits and behavioral alterations resulting from human naturally acquired infections and murine experimental models of malaria. We highlighted recent reports of cognitive and behavioral sequelae of non-severe malaria, the most prevalent clinical form of the disease worldwide. These sequelae have gained more attention in recent years and therapies for them are required and demand advances in the understanding of neuropathogenesis. Recent studies using experimental murine models point to immunomodulation as a potential approach to prevent or revert neurocognitive sequelae of malaria.

Hemozoin: a Complex Molecule with Complex Activities

Purpose of Review

Malaria is a disease caused by parasites that reside in host red blood cells and use hemoglobin as a nutrient source. Heme released by hemoglobin catabolism is modified by the parasite to produce hemozoin (HZ), which has toxic effects on the host. Experimentation aiming to elucidate how HZ contributes to malaria pathogenesis has utilized different preparations of this molecule, complicating interpretation and comparison of findings. We examine natural synthesis and isolation of HZ and highlight studies that have used multiple preparations, including synthetic forms, in a comparative fashion.

Recent Findings

Recent work utilizing sophisticated imaging and detection techniques reveals important molecular characteristics of HZ synthesis and biochemistry. Other recent studies further refine understanding of contributions of HZ to malaria pathogenesis yet highlight the continuing need to characterize HZ preparations and contextualize experimental conditions in the in vivo infection milieu.

Summary

This review highlights the necessity of collectively determining what is physiologically relevant HZ. Characterization of isolated natural HZ and use of multiple preparations in each study are recommended with application of in vivo studies whenever possible. Adoption of such practices is expected to improve reproducibility of results and elucidate the myriad of ways that HZ participates in malaria pathogenesis.

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.

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.

Immune system challenge improves recognition memory and reverses malaria-induced cognitive impairment in mice

The immune system plays a role in the maintenance of healthy neurocognitive function. Different patterns of immune response triggered by distinct stimuli may affect nervous functions through regulatory or deregulatory signals, depending on the properties of the exogenous immunogens. Here, we investigate the effect of immune stimulation on cognitive-behavioural parameters in healthy mice and its impact on cognitive sequelae resulting from non-severe experimental malaria. We show that immune modulation induced by a specific combination of immune stimuli that induce a type 2 immune response can enhance long-term recognition memory in healthy adult mice subjected to novel object recognition task (NORT) and reverse a lack of recognition ability in NORT and anxiety-like behaviour in a light/dark task that result from a single episode of mild Plasmodium berghei ANKA malaria. Our findings suggest a potential use of immunogens for boosting and recovering recognition memory that may be impaired by chronic and infectious diseases and by the effects of ageing.

Monocyte Locomotion Inhibitory Factor confers neuroprotection and prevents the development of murine cerebral malaria

Cerebral malaria (CM) is a neurological complication derived from the Plasmodium falciparum infection in humans. The mechanisms involved in the disease progression are still not fully understood, but both the sequestration of infected red blood cells (iRBC) and leukocytes and an exacerbated host inflammatory immune response are significant factors. In this study, we investigated the effect of Monocyte Locomotion Inhibitory Factor (MLIF), an anti-inflammatory peptide, in a well-characterized murine model of CM. Our data showed that the administration of MLIF increased the survival and avoided the neurological signs of CM in Plasmodium berghei ANKA (PbA) infected C57BL/6 mice. MLIF administration down-regulated systemic inflammatory mediators such as IFN-γ, TNF-α, IL-6, CXCL2, and CCL2, as well as the in situ expression of TNF-α in the brain. In the same way, MLIF reduced the expression of CD31, CD36, CD54, and CD106 in the cerebral endothelium of infected animals and prevented the sequestration of iRBC and leucocytes in the brain microvasculature. Furthermore, MLIF inhibited the activation of astrocytes and microglia and preserved the integrity of the blood-brain barrier (BBB). In conclusion, our results demonstrated that the administration of MLIF increased survival and conferred neuroprotection by decreasing neuroinflammation in murine CM.

Attenuated T Cell Responses Are Associated With the Blockade of Cerebral Malaria Development by YOP1-Deficient Plasmodium berghei ANKA

Reticulon and the REEP family of proteins stabilize the high curvature of endoplasmic reticulum tubules. The REEP5 homolog in Plasmodium, Plasmodium berghei YOP1 (PbYOP1), plays an important role in the erythrocytic cycle of the P. berghei ANKA and the pathogenesis of experimental cerebral malaria (ECM), but the mechanisms are largely unknown. Here, we show that protection from ECM in Pbyop1Δ-infected mice is associated with reduced intracerebral Th1 accumulation, decreased expression of pro-inflammatory cytokines and chemokines, and attenuated pathologies in the brainstem, though the total number of CD4⁺ and CD8⁺ T cells sequestered in the brain are not reduced. Expression of adhesive molecules on brain endothelial cells, including ICAM-1, VCAM-1, and CD36, are decreased, particularly in the brainstem, where fatal pathology is always induced during ECM. Subsequently, CD8⁺ T cell-mediated cell apoptosis in the brain is compromised. These findings suggest that Pbyop1Δ parasites can be a useful tool for mechanistic investigation of cerebral malaria pathogenesis.

Perillyl alcohol reduces parasite sequestration and cerebrovascular dysfunction during experimental cerebral malaria

Cerebral malaria (CM) is a severe immunovasculopathy which presents high mortality rate (15-20%), despite the availability of artemisinin-based therapy. More effective immunomodulatory and/or antiparasitic therapies are urgently needed. Experimental Cerebral Malaria (ECM) in mice is used to elucidate aspects involved in this pathology since manifests many of the neurological features of CM. In the present study, we evaluated the potential mechanisms involved in the protection afforded by perillyl alcohol (POH) in mouse strains susceptible to CM caused by Plasmodium berghei ANKA (PbA) infection through intranasal preventive treatment. Additionally, to evaluate the interaction of POH with the cerebral endothelium using an in vitro model of human brain endothelial cells (HBEC). Pharmacokinetic approaches demonstrated constant and prolonged levels of POH in the plasma and brain after a single intranasal dose. Treatment with POH effectively prevented vascular dysfunction. Furthermore, treatment with POH reduced the endothelial cell permeability and PbA s in the brain and spleen. Finally, POH treatment decreased the accumulation of macrophages and T and B cells in the spleen and downregulated the expression of endothelial adhesion molecules (ICAM-1, VCAM-1, and CD36) in the brain. POH is a potent monoterpene that prevents cerebrovascular dysfunction in vivo and in vitro, decreases parasite sequestration, and modulates different processes related to the activation, permeability, and integrity of the blood brain barrier (BBB), thereby preventing cerebral oedema and inflammatory infiltrates.

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 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

Necroptosis and Caspase-2-Mediated Apoptosis of Astrocytes and Neurons, but Not Microglia, of Rat Hippocampus and Parenchyma Caused by Angiostrongylus cantonensis Infection

Infection with the roundworm Angiostrongylus cantonensis is the main cause of eosinophilic meningitis worldwide. The underlying molecular basis of the various pathological outcomes in permissive and non-permissive hosts infected with A. cantonensis remains poorly defined. In the present study, the histology of neurological disorders in the central nervous system (CNS) of infected rats was assessed by using hematoxylin and eosin staining. Quantitative reverse transcription polymerase chain reaction (RT-qPCR), western blot and immunofluorescence (IF) were used in evolutions of the transcription and translation levels of the apoptosis-, necroptosis-, autophagy-, and pyroptosis-related genes. The distribution of apoptotic and necroptotic cells in the rat hippocampus and parenchyma was further detected using flow cytometry, and the features of the ultrastructure of the cells were examined by transmission electron microscopy (TEM). The inflammatory response upon CNS infection with A. cantonensis evolved, as characterized by the accumulation of a small number of inflammatory cells under the thickened meninges, which peaked at 21 days post-infection (dpi) and returned to normal by 35 dpi. The transcription levels and translation of caspase-2, caspase-8, RIP1 and RIP3 increased significantly at 21 and 28 dpi but decreased sharply at 35 dpi compared to those in the normal control group. However, the changes in the expression of caspase-1, caspase-3, caspase-11, Beclin-1 and LC3B were not obvious, suggesting that apoptosis and necroptosis but not autophagy or pyroptosis occurred in the brains of infected animals at 21 and 28 dpi. The results of RT-qPCR, western blot analysis, IF, flow cytometry and TEM further illustrated that necroptosis and caspase-2-mediated apoptosis occurred in astrocytes and neurons but not in microglia in the parenchyma and hippocampus of infected animals. This study provides the first evidence that neuronal and astrocytic necroptosis and caspase-2-mediated apoptosis are induced by A. cantonensis infection in the parenchymal and hippocampal regions of rats at 21 and 28 dpi but these processes are negligible at 35 dpi. These findings enhance our understanding of the pathogenesis of A. cantonensis infection and provide new insights into therapeutic approaches targeting the occurrence of cell death in astrocytes and neurons in infected patients.

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.

The Liver-Stage Plasmodium Infection Is a Critical Checkpoint for Development of Experimental Cerebral Malaria

Cerebral malaria is a life-threatening complication of malaria in humans, and the underlying pathogenic mechanisms are widely analyzed in a murine model of experimental cerebral malaria (ECM). Here, we show abrogation of ECM by hemocoel sporozoite-induced infection of a transgenic Plasmodium berghei line that overexpresses profilin, whereas these parasites remain fully virulent in transfusion-mediated blood infection. We, thus, demonstrate the importance of the clinically silent liver-stage infection for modulating the onset of ECM. Even though both parasites triggered comparable splenic immune cell expansion and accumulation of antigen-experienced CD8⁺ T cells in the brain, infection with transgenic sporozoites did not lead to cerebral vascular damages and suppressed the recruitment of overall lymphocyte populations. Strikingly, infection with the transgenic strain led to maintenance of CD115⁺Ly6C⁺ monocytes, which disappear in infected animals prone to ECM. An early induction of IL-10, IL-12p70, IL-6, and TNF at the time when parasites emerge from the liver might lead to a diminished induction of hepatic immunity. Collectively, our study reveals the essential role of early host interactions in the liver that may dampen the subsequent pro-inflammatory immune responses and influence the occurrence of ECM, highlighting a novel checkpoint in this fatal pathology.

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.

Experimental severe malaria is resolved by targeting newly-identified monocyte subsets using immune-modifying particles combined with artesunate

Current treatment of severe malaria and associated cerebral malaria (CM) and respiratory distress syndromes are directed primarily at the parasite. Targeting the parasite has only partial efficacy in advanced infection, as neurological damage and respiratory distress are due to accumulation of host blood cells in the brain microvasculature and lung interstitium. Here, computational analysis identifies Ly6C^lo monocytes as a major component of the immune infiltrate in both organs in a preclinical mouse model. Specifically targeting Ly6C^lo monocyte precursors, identified by adoptive transfer, with immune-modifying particles (IMP) prevents experimental CM (ECM) in 50% of Plasmodium berghei ANKA-infected mice in early treatment protocols. Furthermore, treatment at onset of clinical ECM with 2 doses of a novel combination of IMP and anti-malarial drug artesunate results in 88% survival. This combination confers protection against ECM and mortality in late stage severe experimental malaria and provides a viable advance on current treatment regimens.

Sex-Associated Differential mRNA Expression of Cytokines and Its Regulation by Sex Steroids in Different Brain Regions in a Plasmodium berghei ANKA Model of Cerebral Malaria

Cerebral malaria (CM) is the major complication associated with death in malaria patients, and its pathogenesis is associated with excessive proinflammatory cytokine production. Notably, the severity and mortality of natural infections with Plasmodium are higher in males than females, suggesting that sexual hormones influence both the pathogenesis of and immune response in CM. However, no studies on inflammation mediators in the brains of both sexes have been reported. In this work, the mRNA expression levels of the proinflammatory cytokines IL-1β, IFN-γ, TNF-α, and IL-2 were measured in the preoptic area, hypothalamus, hippocampus, olfactory bulb, frontal cortex, and lateral cortex regions of gonadectomized female and male CBA/Ca mice infected with P. berghei ANKA (a recognized experimental CM model). Our findings demonstrate that both infection with P. berghei ANKA and gonadectomy trigger a cerebral sex dimorphic mRNA expression pattern of the cytokines IL-1β, TNF-α, IFN-γ, and IL-2. This dimorphic cytokine pattern was different in each brain region analysed. In most cases, infected males exhibited higher mRNA expression levels than females, suggesting that sexual hormones differentially regulate the mRNA expression of proinflammatory cytokines in the brain and the potential use of gonadal steroids or their derivates in the immunomodulation of cerebral malaria.

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.

Long-term effect of uncomplicated Plasmodium berghei ANKA malaria on memory and anxiety-like behaviour in C57BL/6 mice

Background

Cerebral malaria, the main complication of Plasmodium falciparum infection in humans, is associated with persistent neurocognitive sequels both in human disease and the murine experimental model. In recent years, cognitive deficits related to uncomplicated (non-cerebral) malaria have also been reported in chronically exposed residents of endemic areas, but not in some murine experimental models of non-cerebral malaria. This study aimed at evaluating the influence of uncomplicated malaria on different behavioural paradigms associated with memory and anxiety-like parameters in a murine model that has the ability to develop cerebral malaria.

Methods

Plasmodium berghei ANKA-infected and non-infected C57BL/6 mice were used. Development of cerebral malaria was prevented by chloroquine treatment starting on the fourth day of infection. The control group (non-infected mice) were treated with PBS. The effect of uncomplicated malaria infection on locomotor habituation, short and long-term memory and anxious-like behaviour was evaluated 64 days after parasite clearance in assays including open field, object recognition, Y-maze and light/dark tasks.

Results

Plasmodium berghei ANKA-infected mice showed significant long-lasting disturbances reflected by a long-term memory-related behaviour on open field and object recognition tasks, accompanied by an anxious-like phenotype availed on open field and light-dark tasks.

Conclusions

Long-term neurocognitive sequels may follow an uncomplicated malaria episode in an experimental model prone to develop cerebral malaria, even if the infection is treated before the appearance of clinical signs of cerebral impairment.

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.

The Effects of Intestinal Nematode L4 Stage on Mouse Experimental Autoimmune Encephalomyelitis

Helminths use various immunomodulatory and anti-inflammatory strategies to evade immune attack by the host. During pathological conditions, these strategies alter the course of disease by reducing immune-mediated pathology. The study examines the therapeutic effect of the nematode L4 stage based on an in vivo model of multiple sclerosis, monophasic encephalomyelitis (EAE), induced by sensitization with MOG_35-55 peptide in C57BL/6 female mice infected with the intestinal nematode Heligmosomoides polygyrus. The EAE remission was correlated with altered leukocyte number identified in the central nervous system (CNS), and temporary permeability of the blood-brain barrier at the histotrophic phase of infection. At 6 days post-infection, when the L4 stage had almost completely attenuated the clinical severity and pathological signs of EAE, CD25⁺ cell numbers expanded significantly, with parallel growth of CD8⁺ and CD4⁺, both CD25⁺Foxp3⁺ and CD25⁺Foxp3^- subsets and alternatively activated macrophages. The phenotypic changes in distinct subsets of cerebrospinal fluid cells were correlated with an inhibited proliferative response of encephalitogenic T cells and elevated levels of nerve growth factor and TGF-β. These results enhance our understanding of mechanisms involved in the inhibition of immune responses in the CNS during nematode infection.

S-Nitrosoglutathione Reductase Deficiency Confers Improved Survival and Neurological Outcome in Experimental Cerebral Malaria

Artesunate remains the mainstay of treatment for cerebral malaria, but it is less effective in later stages of disease when the host inflammatory response and blood-brain barrier integrity dictate clinical outcomes. Nitric oxide (NO) is an important regulator of inflammation and microvascular integrity, and impaired NO bioactivity is associated with fatal outcomes in malaria. Endogenous NO bioactivity in mammals is largely mediated by S-nitrosothiols (SNOs). Based on these observations, we hypothesized that animals deficient in the SNO-metabolizing enzyme, S-nitrosoglutathione reductase (GSNOR), which exhibit enhanced S-nitrosylation, would have improved outcomes in a preclinical model of cerebral malaria. GSNOR knockout (KO) mice infected with Plasmodium berghei ANKA had significantly delayed mortality compared to WT animals (P < 0.0001), despite higher parasite burdens (P < 0.01), and displayed markedly enhanced survival versus the wild type (WT) when treated with the antimalarial drug artesunate (77% versus 38%; P < 0.001). Improved survival was associated with higher levels of protein-bound NO, decreased levels of CD4⁺ and CD8⁺ T cells in the brain, improved blood-brain barrier integrity, and improved coma scores, as well as higher levels of gamma interferon. GSNOR KO animals receiving WT bone marrow had significantly reduced survival following P. berghei ANKA infection compared to those receiving KO bone barrow (P < 0.001). Reciprocal transplants established that survival benefits of GSNOR deletion were attributable primarily to the T cell compartment. These data indicate a role for GSNOR in the host response to malaria infection and suggest that strategies to disrupt its activity will improve clinical outcomes by enhancing microvascular integrity and modulating T cell tissue tropism.

Perforin Expression by CD8 T Cells Is Sufficient To Cause Fatal Brain Edema during Experimental Cerebral Malaria

Human cerebral malaria (HCM) is a serious complication of Plasmodium falciparum infection. The most severe outcomes for patients include coma, permanent neurological deficits, and death. Recently, a large-scale magnetic resonance imaging (MRI) study in humans identified brain swelling as the most prominent predictor of fatal HCM. Therefore, in this study, we sought to define the mechanism controlling brain edema through the use of the murine experimental cerebral malaria (ECM) model. Specifically, we investigated the ability of CD8 T cells to initiate brain edema during ECM. We determined that areas of blood-brain barrier (BBB) permeability colocalized with a reduction of the cerebral endothelial cell tight-junction proteins claudin-5 and occludin. Furthermore, through small-animal MRI, we analyzed edema and vascular leakage. Using gadolinium-enhanced T1-weighted MRI, we determined that vascular permeability is not homogeneous but rather confined to specific regions of the brain. Our findings show that BBB permeability was localized within the brainstem, olfactory bulb, and lateral ventricle. Concurrently with the initiation of vascular permeability, T2-weighted MRI revealed edema and brain swelling. Importantly, ablation of the cytolytic effector molecule perforin fully protected against vascular permeability and edema. Furthermore, perforin production specifically by CD8 T cells was required to cause fatal edema during ECM. We propose that CD8 T cells initiate BBB breakdown through perforin-mediated disruption of tight junctions. In turn, leakage from the vasculature into the parenchyma causes brain swelling and edema. This results in a breakdown of homeostatic maintenance that likely contributes to ECM pathology.

Targeting Angiotensin II Type-1 Receptor (AT1R) Inhibits the Harmful Phenotype of Plasmodium-Specific CD8+ T Cells during Blood-Stage Malaria

CD8⁺ T-cell response is critical in the pathogenesis of cerebral malaria during blood-stage. Our group and other have been shown that angiotensin II (Ang II) and its receptor AT₁ (AT₁R), a key effector axis of renin-angiotensin system (RAS), have immune regulatory effects on T cells. Previously, we showed that inhibition of AT₁R signaling protects mice against the lethal disease induced by Plasmodium berghei ANKA infection However, most of the Ang II/AT₁R actions were characterized by using only pharmacological approaches, the effects of which may not always be due to a specific receptor blockade. In addition, the mechanisms of action of the AT₁R in inducing the pathogenic activity of Plasmodium-specific CD8⁺ T cells during blood-stage were not determined. Here, we examined how angiotensin II/AT₁R axis promotes the harmful response of Plasmodium-specific CD8⁺ T-cell during blood-stage by using genetic and pharmacological approaches. We evaluated the response of wild-type (WT) and AT₁R^−/−Plasmodium-specific CD8⁺ T cells in mice infected with a transgenic PbA lineage expressing ovalbumin; and in parallel infected mice receiving WT Plasmodium-specific CD8⁺ T cells were treated with losartan (AT₁R antagonist) or captopril (ACE inhibitor). Both, AT₁R^−/− OT-I cells and WT OT-I cells from losartan- or captopril-treated mice showed lower expansion, reduced IL-2 production and IL-2Rα expression, lower activation (lower expression of CD69, CD44 and CD160) and lower exhaustion profiles. AT₁R^−/− OT-I cells also exhibit lower expression of the integrin LFA-1 and the chemokine receptors CCR5 and CXCR3, known to play a key role in the development of cerebral malaria. Moreover, AT₁R^−/− OT-I cells produce lower amounts of IFN-γ and TNF-α and show lower degranulation upon restimulation. In conclusion, our results show the pivotal mechanisms of AT₁R-induced harmful phenotype of Plasmodium-specific CD8⁺ T cells during blood-stage malaria.

CD8+ T Cells Induce Fatal Brainstem Pathology during Cerebral Malaria via Luminal Antigen-Specific Engagement of Brain Vasculature

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that results in thousands of deaths each year, mostly in African children. The in vivo mechanisms underlying this fatal condition are not entirely understood. Using the animal model of experimental cerebral malaria (ECM), we sought mechanistic insights into the pathogenesis of CM. Fatal disease was associated with alterations in tight junction proteins, vascular breakdown in the meninges / parenchyma, edema, and ultimately neuronal cell death in the brainstem, which is consistent with cerebral herniation as a cause of death. At the peak of ECM, we revealed using intravital two-photon microscopy that myelomonocytic cells and parasite-specific CD8⁺ T cells associated primarily with the luminal surface of CNS blood vessels. Myelomonocytic cells participated in the removal of parasitized red blood cells (pRBCs) from cerebral blood vessels, but were not required for the disease. Interestingly, the majority of disease-inducing parasite-specific CD8⁺ T cells interacted with the lumen of brain vascular endothelial cells (ECs), where they were observed surveying, dividing, and arresting in a cognate peptide-MHC I dependent manner. These activities were critically dependent on IFN-γ, which was responsible for activating cerebrovascular ECs to upregulate adhesion and antigen-presenting molecules. Importantly, parasite-specific CD8⁺ T cell interactions with cerebral vessels were impaired in chimeric mice rendered unable to present EC antigens on MHC I, and these mice were in turn resistant to fatal brainstem pathology. Moreover, anti-adhesion molecule (LFA-1 / VLA-4) therapy prevented fatal disease by rapidly displacing luminal CD8⁺ T cells from cerebrovascular ECs without affecting extravascular T cells. These in vivo data demonstrate that parasite-specific CD8⁺ T cell-induced fatal vascular breakdown and subsequent neuronal death during ECM is associated with luminal, antigen-dependent interactions with cerebrovasculature.

Cerebral malaria (CM) is a severe and potentially fatal complication of malaria in humans that results in swelling and bleeding within the brain. The mechanisms that cause this fatal disease in humans are not completely understood. We studied an animal model known as experimental cerebral malaria to learn more about the factors that drive this disease process. Using a technique referred to as intravital microscopy, we captured movies of immune cells operating in the living brain as the disease developed. At the peak of disease, we observed evidence of immune cells interacting with and aggregating along blood vessels throughout the brain. These interactions were directly associated vascular leakage. This caused the brain to swell, which gave rise to an unsustainable pressure that ultimately killed neurons responsible for heart and lung function. The fatal swelling was induced by immune cells (referred to as T cells) interacting with bits of parasite presented by blood vessels in the brain. Removal of this parasite presentation protected the mice from fatal disease. We also evaluated a straightforward therapy that involved intravenous administration of antibodies that interfered with T cell sticking to blood vessels. Our movies revealed that this therapeutic approach rapidly displaced T cells from the blood vessels in the brain and prevented fatal disease.

A chimeric protein-based malaria vaccine candidate induces robust T cell responses against Plasmodium vivax MSP119

The most widespread Plasmodium species, Plasmodium vivax, poses a significant public health threat. An effective vaccine is needed to reduce global malaria burden. Of the erythrocytic stage vaccine candidates, the 19 kDa fragment of the P. vivax Merozoite Surface Protein 1 (PvMSP1₁₉) is one of the most promising. Our group has previously defined several promiscuous T helper epitopes within the PvMSP1 protein, with features that allow them to bind multiple MHC class II alleles. We describe here a P. vivax recombinant modular chimera based on MSP1 (PvRMC-MSP1) that includes defined T cell epitopes genetically fused to PvMSP1₁₉. This vaccine candidate preserved structural elements of the native PvMSP1₁₉ and elicited cytophilic antibody responses, and CD4+ and CD8+ T cells capable of recognizing PvMSP1₁₉. Although CD8+ T cells that recognize blood stage antigens have been reported to control blood infection, CD8+ T cell responses induced by P. falciparum or P. vivax vaccine candidates based on MSP1₁₉ have not been reported. To our knowledge, this is the first time a protein based subunit vaccine has been able to induce CD8+ T cell against PvMSP1₁₉. The PvRMC-MSP1 protein was also recognized by naturally acquired antibodies from individuals living in malaria endemic areas with an antibody profile associated with protection from infection. These features make PvRMC-MSP1 a promising vaccine candidate.

Molecular basis of human cerebral malaria development

Cerebral malaria is still a deleterious health problem in tropical countries. The wide spread of malarial drug resistance and the lack of an effective vaccine are obstacles for disease management and prevention. Parasite and human genetic factors play important roles in malaria susceptibility and disease severity. The malaria parasite exerted a potent selective signature on the human genome, which is apparent in the genetic polymorphism landscape of genes related to pathogenesis. Currently, much genomic data and a novel body of knowledge, including the identification of microRNAs, are being increasingly accumulated for the development of laboratory testing cassettes for cerebral malaria prevention. Therefore, understanding of the underlying complex molecular basis of cerebral malaria is important for the design of strategy for cerebral malaria treatment and control.

Behavioural and neurological symptoms accompanied by cellular neuroinflammation in IL-10-deficient mice infected with Plasmodium chabaudi

Background

Cerebral malaria is one of the most severe complications of Plasmodium falciparum infection and occurs mostly in young African children. This syndrome results from a combination of high levels of parasitaemia and inflammation. Although parasite sequestration in the brain is a feature of the human syndrome, sequestering strains do not uniformly cause severe malaria, suggesting interplay with other factors. Host genetic factors such as mutations in the promoters of the cytokines IL-10 and TNF are also clearly linked to severe disease. Plasmodium chabaudi, a rodent malaria parasite, leads to mild illness in wildtype animals. However, IL-10^−/− mice respond to parasite with increased levels of pro-inflammatory cytokines IFN-γ and TNF, leading to lethal disease in the absence of sequestration in the brain. These mice also exhibit cerebral symptoms including gross cerebral oedema and haemorrhage, allowing study of these critical features of disease without the influence of sequestration.

Methods

The neurological consequences of P. chabaudi infection were investigated by performing a general behavioural screen (SHIRPA). The immune cell populations found in the brain during infection were also analysed using flow cytometry and confocal microscopy.

Results

IL-10^−/− mice suffer significant declines in behavioural and physical capacities during infection compared to wildtype. In addition, grip strength and pain sensitivity were affected, suggestive of neurological involvement. Several immune cell populations were identified in the perfused brain on day 7 post-infection, suggesting that they are tightly adherent to the vascular endothelium, or potentially located within the brain parenchyma. There was an increase in both inflammatory monocyte and resident macrophage (CD11b^hi, CD45⁺, MHCII⁺, Ly6C^+/−) numbers in IL-10^−/− compared to wildtype animals. In addition, the activation state of all monocytes and microglia (CD11b^int, CD45⁻, MHC-II⁺) were increased. T cells making IFN-γ were also identified in the brain, but were localized within the vasculature, and not the parenchyma.

Conclusions

These studies demonstrate exacerbated neuroinflammation concurrent with development of behavioural symptoms in P. chabaudi infection of IL-10^−/− animals.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1477-1) contains supplementary material, which is available to authorized users.

Cannabinoid Receptor 2 Modulates Susceptibility to Experimental Cerebral Malaria through a CCL17-dependent Mechanism

Cerebral malaria is a severe and often fatal complication of Plasmodium falciparum infection. It is characterized by parasite sequestration, a breakdown of the blood-brain barrier, and a strong inflammation in the brain. We investigated the role of the cannabinoid receptor 2 (CB2), an important modulator of neuroinflammatory responses, in experimental cerebral malaria (ECM). Strikingly, mice with a deletion of the CB2-encoding gene (Cnr2(-/-)) inoculated with Plasmodium berghei ANKA erythrocytes exhibited enhanced survival and a diminished blood-brain barrier disruption. Therapeutic application of a specific CB2 antagonist also conferred increased ECM resistance in wild type mice. Hematopoietic derived immune cells were responsible for the enhanced protection in bone marrow (BM) chimeric Cnr2(-/-) mice. Mixed BM chimeras further revealed that CB2-expressing cells contributed to ECM development. A heterogeneous CD11b(+) cell population, containing macrophages and neutrophils, expanded in the Cnr2(-/-) spleen after infection and expressed macrophage mannose receptors, arginase-1 activity, and IL-10. Also in the Cnr2(-/-) brain, CD11b(+) cells that expressed selected anti-inflammatory markers accumulated, and expression of inflammatory mediators IFN-γ and TNF-α was reduced. Finally, the M2 macrophage chemokine CCL17 was identified as an essential factor for enhanced survival in the absence of CB2, because CCL17 × Cnr2 double-deficient mice were fully susceptible to ECM. Thus, targeting CB2 may be promising for the development of alternative treatment regimes of ECM.

Synergistic induction of CXCL10 by interferon-gamma and lymphotoxin-alpha in astrocytes: Possible role in cerebral malaria

Cerebral malaria (CM) has a high mortality rate and incidence of neurological sequelae in survivors. Hypoxia and cytokine expression in the brain are two mechanisms thought to contribute to the pathogenesis of CM. The cytokines interferon (IFN)-γ and lymphotoxin (LT)-α and the chemokine CXCL10 are essential for the development of CM in a mouse model. Furthermore, serum IFN-γ protein levels are higher in human CM than in controls, and CXCL10 is elevated in both serum and cerebrospinal fluid in Ghanaian paediatric CM cases. Astrocytes actively participate in CNS pathologies, becoming activated in response to various stimuli including cytokines. Astrocyte activation also occurs in murine and human CM. We here determined the responsiveness of mouse and human astrocytes to IFN-γ and LT-α, with the aim of further elucidating the role of astrocytes in CM pathogenesis. Initially we confirmed that Ifn-γ and Cxcl10 are expressed in the brain in murine CM, and that the increased Cxcl10 expression is IFN-γ-dependant. IFN-γ induced CXCL10 production in human and murine astrocytes in vitro. The degree of induction was increased synergistically in the presence of LT-α. IFN-γ induced the expression of receptors for LT-α, while LT-α increased the expression of the receptor for IFN-γ, in the astrocytes. This cross-induction may explain the synergistic effect of the two cytokines on CXCL10 production. Expression of these receptors also was upregulated in the brain in murine CM. The results suggest that astrocytes contribute to CM pathogenesis by producing CXCL10 in response to IFN-γ and LT-α.

The immunological balance between host and parasite in malaria

Coevolution of humans and malaria parasites has generated an intricate balance between the immune system of the host and virulence factors of the parasite, equilibrating maximal parasite transmission with limited host damage. Focusing on the blood stage of the disease, we discuss how the balance between anti-parasite immunity versus immunomodulatory and evasion mechanisms of the parasite may result in parasite clearance or chronic infection without major symptoms, whereas imbalances characterized by excessive parasite growth, exaggerated immune reactions or a combination of both cause severe pathology and death, which is detrimental for both parasite and host. A thorough understanding of the immunological balance of malaria and its relation to other physiological balances in the body is of crucial importance for developing effective interventions to reduce malaria-related morbidity and to diminish fatal outcomes due to severe complications. Therefore, we discuss in this review the detailed mechanisms of anti-malarial immunity, parasite virulence factors including immune evasion mechanisms and pathogenesis. Furthermore, we propose a comprehensive classification of malaria complications according to the different types of imbalances.

Mechanisms of murine cerebral malaria: Multimodal imaging of altered cerebral metabolism and protein oxidation at hemorrhage sites

Using a multimodal biospectroscopic approach, we settle several long-standing controversies over the molecular mechanisms that lead to brain damage in cerebral malaria, which is a major health concern in developing countries because of high levels of mortality and permanent brain damage. Our results provide the first conclusive evidence that important components of the pathology of cerebral malaria include peroxidative stress and protein oxidation within cerebellar gray matter, which are colocalized with elevated nonheme iron at the site of microhemorrhage. Such information could not be obtained previously from routine imaging methods, such as electron microscopy, fluorescence, and optical microscopy in combination with immunocytochemistry, or from bulk assays, where the level of spatial information is restricted to the minimum size of tissue that can be dissected. We describe the novel combination of chemical probe-free, multimodal imaging to quantify molecular markers of disturbed energy metabolism and peroxidative stress, which were used to provide new insights into understanding the pathogenesis of cerebral malaria. In addition to these mechanistic insights, the approach described acts as a template for the future use of multimodal biospectroscopy for understanding the molecular processes involved in a range of clinically important acute and chronic (neurodegenerative) brain diseases to improve treatment strategies.

Perivascular Arrest of CD8+ T Cells Is a Signature of Experimental Cerebral Malaria

There is significant evidence that brain-infiltrating CD8⁺ T cells play a central role in the development of experimental cerebral malaria (ECM) during Plasmodium berghei ANKA infection of C57BL/6 mice. However, the mechanisms through which they mediate their pathogenic activity during malaria infection remain poorly understood. Utilizing intravital two-photon microscopy combined with detailed ex vivo flow cytometric analysis, we show that brain-infiltrating T cells accumulate within the perivascular spaces of brains of mice infected with both ECM-inducing (P. berghei ANKA) and non-inducing (P. berghei NK65) infections. However, perivascular T cells displayed an arrested behavior specifically during P. berghei ANKA infection, despite the brain-accumulating CD8⁺ T cells exhibiting comparable activation phenotypes during both infections. We observed T cells forming long-term cognate interactions with CX₃CR1-bearing antigen presenting cells within the brains during P. berghei ANKA infection, but abrogation of this interaction by targeted depletion of the APC cells failed to prevent ECM development. Pathogenic CD8⁺ T cells were found to colocalize with rare apoptotic cells expressing CD31, a marker of endothelial cells, within the brain during ECM. However, cellular apoptosis was a rare event and did not result in loss of cerebral vasculature or correspond with the extensive disruption to its integrity observed during ECM. In summary, our data show that the arrest of T cells in the perivascular compartments of the brain is a unique signature of ECM-inducing malaria infection and implies an important role for this event in the development of the ECM-syndrome.

Cerebral malaria is the most severe complication of Plasmodium falciparum infection. Utilizing the murine experimental model of cerebral malaria (ECM), it has been found that CD8⁺ T cells are a key immune cell type responsible for development of cerebral pathology during malaria infection. To identify how CD8⁺ T cells cause cerebral pathology during malaria infection, in this study we have performed detailed in vivo analysis (two photon imaging) of CD8⁺ T cells within the brains of mice infected with strains of malaria parasites that cause or do not cause ECM. We found that CD8⁺ T cells appear to accumulate in similar numbers and in comparable locations within the brains of mice infected with parasites that do or do not cause ECM. Importantly, however, brain accumulating CD8⁺ T cells displayed significantly different movement characteristics during the different infections. CD8⁺ T cells interacted with myeloid cells within the brain during infection with parasites causing ECM, but this association was not required for development of cerebral complications. Furthermore, our results suggest that CD8⁺ T cells do not cause ECM through the widespread killing of brain microvessel cells. The results in this study significantly improve our understanding of the ways through which CD8⁺ T cells can mediate cerebral pathology during malaria infection.

The application of anti-Toso antibody enhances CD8(+) T cell responses in experimental malaria vaccination and disease

Toso is a molecule highly expressed on B cells. It influences their survival and was identified as an IgM binding molecule. B cells and natural antibodies play a role in vaccination-induced CD8(+) T cell responses. We investigated the impact of an anti-Toso antibody on vaccination efficiency in a malaria vaccination model. In this model, CD8(+) T cells exert antiparasitic functions on infected hepatocytes in the liver stage of the disease. In vaccinated anti-Toso treated mice, more antigen-specific CD8(+) T cells were induced than in control mice and after infection with Plasmodium berghei ANKA (PbA) sporozoites, the liver parasite burden was lower. In B cell deficient mice, the anti-Toso antibody did not stimulate the CD8(+) T cell response, indicating that B cells were mediating this effect. Furthermore, we analyzed the influence of anti-Toso treatment on non-vaccinated mice in the PbA infection model, in which CD8(+) T cells cause brain pathology. Anti-Toso treatment increased cerebral pathology and the accumulation of CD8(+) T cells in the brain. Thus, anti-Toso treatment enhanced the CD8(+) T cell response against PbA in a vaccination and in an infection model. Our findings indicate that Toso may be a novel target to boost vaccine-induced CD8(+) T cell responses.

An effective, low-cost method for achieving and maintaining hypoxia during cell culture studies

Here we report a simple new method for exposing cells to normoxic and hypoxic conditions using vacuum bags, normally employed for food storage, to establish and maintain low oxygen levels in vitro. Vacuum bags were gassed with a mixture containing specified levels of oxygen, then sealed, creating a hypoxic microenvironment for cells cultured in flasks placed therein. Oxygen levels in the gas mixture and culture medium in flasks inside the sealed bags equilibrated after two hours of incubation. The vacuum bags maintained low oxygen levels (either <2% or 5%) in medium for at least 4 days. Human fetal astrocytes grew normally in flasks for at least 4 days in a 5% oxygen/ 5% CO2/ 90% nitrogen atmosphere, but viability decreased at <2% oxygen. Vacuum bags can accommodate varying oxygen levels that would otherwise require systems with separate chambers or modules, but are less useful when repeated experimental manipulations of individual cultures are required.

Activated Brain Endothelial Cells Cross-Present Malaria Antigen

In the murine model of cerebral malaria caused by P. berghei ANKA (PbA), parasite-specific CD8+ T cells directly induce pathology and have long been hypothesized to kill brain endothelial cells that have internalized PbA antigen. We previously reported that brain microvessel fragments from infected mice cross-present PbA epitopes, using reporter cells transduced with epitope-specific T cell receptors. Here, we confirm that endothelial cells are the population responsible for cross-presentation in vivo, not pericytes or microglia. PbA antigen cross-presentation by primary brain endothelial cells in vitro confers susceptibility to killing by CD8+ T cells from infected mice. IFNγ stimulation is required for brain endothelial cross-presentation in vivo and in vitro, which occurs by a proteasome- and TAP-dependent mechanism. Parasite strains that do not induce cerebral malaria were phagocytosed and cross-presented less efficiently than PbA in vitro. The main source of antigen appears to be free merozoites, which were avidly phagocytosed. A human brain endothelial cell line also phagocytosed P. falciparum merozoites. Besides being the first demonstration of cross-presentation by brain endothelial cells, our results suggest that interfering with merozoite phagocytosis or antigen processing may be effective strategies for cerebral malaria intervention.

Iron prevents the development of experimental cerebral malaria by attenuating CXCR3-mediated T cell chemotaxis

Cerebral malaria is a severe neurological complication of Plasmodium falciparum infection. Previous studies have suggested that iron overload can suppress the generation of a cytotoxic immune response; however, the effect of iron on experimental cerebral malaria (ECM) is yet unknown. Here we determined that the incidence of ECM was markedly reduced in mice treated with iron dextran. Protection was concomitant with a significant decrease in the sequestration of CD4+ and CD8+ T cells within the brain. CD4+ T cells demonstrated markedly decreased CXCR3 expression and had reduced IFNγ-responsiveness, as indicated by mitigated expression of IFNγR2 and T-bet. Additional analysis of the splenic cell populations indicated that parenteral iron supplementation was also associated with a decrease in NK cells and increase in regulatory T cells. Altogether, these results suggest that iron is able to inhibit ECM pathology by attenuating the capacity of T cells to migrate to the brain.

IRGM3 contributes to immunopathology and is required for differentiation of antigen-specific effector CD8+ T cells in experimental cerebral malaria

Gamma interferon (IFN-γ) drives antiparasite responses and immunopathology during infection with Plasmodium species. Immunity-related GTPases (IRGs) are a class of IFN-γ-dependent proteins that are essential for cell autonomous immunity to numerous intracellular pathogens. However, it is currently unknown whether IRGs modulate responses during malaria. We have used the Plasmodium berghei ANKA (PbA) model in which mice develop experimental cerebral malaria (ECM) to study the roles of IRGM1 and IRGM3 in immunopathology. Induction of mRNA for Irgm1 and Irgm3 was found in the brains and spleens of infected mice at times of peak IFN-γ production. Irgm3-/- but not Irgm1-/- mice were completely protected from the development of ECM, and this protection was associated with the decreased induction of inflammatory cytokines, as well as decreased recruitment and activation of CD8+ T cells within the brain. Although antigen-specific proliferation of transferred CD8+ T cells was not diminished compared to that of wild-type recipients following PbA infection, T cells transferred into Irgm3-/- recipients showed a striking impairment of effector differentiation. Decreased induction of several inflammatory cytokines and chemokines (interleukin-6, CCL2, CCL3, and CCL4), as well as enhanced mRNA expression of type-I IFNs, was found in the spleens of Irgm3-/- mice at day 4 postinfection. Together, these data suggest that protection from ECM pathology in Irgm3-/- mice occurs due to impaired generation of CD8+ effector function. This defect is nonintrinsic to CD8+ T cells. Instead, diminished T cell responses most likely result from defective initiation of inflammatory responses in myeloid cells.

Experimental cerebral malaria pathogenesis--hemodynamics at the blood brain barrier

Cerebral malaria claims the lives of over 600,000 African children every year. To better understand the pathogenesis of this devastating disease, we compared the cellular dynamics in the cortical microvasculature between two infection models, Plasmodium berghei ANKA (PbA) infected CBA/CaJ mice, which develop experimental cerebral malaria (ECM), and P. yoelii 17XL (PyXL) infected mice, which succumb to malarial hyperparasitemia without neurological impairment. Using a combination of intravital imaging and flow cytometry, we show that significantly more CD8(+) T cells, neutrophils, and macrophages are recruited to postcapillary venules during ECM compared to hyperparasitemia. ECM correlated with ICAM-1 upregulation on macrophages, while vascular endothelia upregulated ICAM-1 during ECM and hyperparasitemia. The arrest of large numbers of leukocytes in postcapillary and larger venules caused microrheological alterations that significantly restricted the venous blood flow. Treatment with FTY720, which inhibits vascular leakage, neurological signs, and death from ECM, prevented the recruitment of a subpopulation of CD45(hi) CD8(+) T cells, ICAM-1(+) macrophages, and neutrophils to postcapillary venules. FTY720 had no effect on the ECM-associated expression of the pattern recognition receptor CD14 in postcapillary venules suggesting that endothelial activation is insufficient to cause vascular pathology. Expression of the endothelial tight junction proteins claudin-5, occludin, and ZO-1 in the cerebral cortex and cerebellum of PbA-infected mice with ECM was unaltered compared to FTY720-treated PbA-infected mice or PyXL-infected mice with hyperparasitemia. Thus, blood brain barrier opening does not involve endothelial injury and is likely reversible, consistent with the rapid recovery of many patients with CM. We conclude that the ECM-associated recruitment of large numbers of activated leukocytes, in particular CD8(+) T cells and ICAM(+) macrophages, causes a severe restriction in the venous blood efflux from the brain, which exacerbates the vasogenic edema and increases the intracranial pressure. Thus, death from ECM could potentially occur as a consequence of intracranial hypertension.

Plasmodium and mononuclear phagocytes

Plasmodium, the causative agent of malaria, initially multiplies inside liver cells and then in successive cycles inside erythrocytes, causing the symptoms of the disease. In this review, we discuss interactions between the extracellular and intracellular forms of the Plasmodium parasite and innate immune cells in the mammalian host, with a special emphasis on mononuclear phagocytes. We overview here what is known about the innate immune cells that interact with parasites, mechanisms used by the parasite to evade them, and the protective or detrimental contribution of these interactions on parasite progression through its life cycle and pathology in the host.

Cerebral malaria: gamma-interferon redux

There are two theories that seek to explain the pathogenesis of cerebral malaria, the mechanical obstruction hypothesis and the immunopathology hypothesis. Evidence consistent with both ideas has accumulated from studies of the human disease and experimental models. Thus, some combination of these concepts seems necessary to explain the very complex pattern of changes seen in cerebral malaria. The interactions between malaria parasites, erythrocytes, the cerebral microvascular endothelium, brain parenchymal cells, platelets and microparticles need to be considered. One factor that seems able to knit together much of this complexity is the cytokine interferon-gamma (IFN-γ). In this review we consider findings from the clinical disease, in vitro models and the murine counterpart of human cerebral malaria in order to evaluate the roles played by IFN-γ in the pathogenesis of this often fatal and debilitating condition.