Infection-Induced Resistance to Experimental Cerebral Malaria Is Dependent Upon Secreted Antibody-Mediated Inhibition of Pathogenic CD8+ T Cell Responses

Cerebral malaria (CM) is one of the most severe complications of Plasmodium falciparum infection. There is evidence that repeated parasite exposure promotes resistance against CM. However, the immunological basis of this infection-induced resistance remains poorly understood. Here, utilizing the Plasmodium berghei ANKA (PbA) model of experimental cerebral malaria (ECM), we show that three rounds of infection and drug-cure protects against the development of ECM during a subsequent fourth (4X) infection. Exposure-induced resistance was associated with specific suppression of CD8+ T cell activation and CTL-related pathways, which corresponded with the development of heterogeneous atypical B cell populations as well as the gradual infection-induced generation and maintenance of high levels of anti-parasite IgG. Mechanistically, transfer of high-titer anti-parasite IgG did not protect 1X infected mice against ECM and depletion of atypical and regulatory B cells during 4X infection failed to abrogate infection-induced resistance to ECM. However, IgMi mice that were unable to produce secreted antibody, or undergo class switching, during the repeated rounds of infection failed to develop resistance against ECM. The failure of infection-induced protection in IgMi mice was associated with impaired development of atypical B cell populations and the inability to suppress pathogenic CD8+ T cell responses. Our results, therefore, suggest the importance of anti-parasite antibody responses, gradually acquired, and maintained through repeated Plasmodium infections, for modulating the B cell compartment and eventually suppressing memory CD8+ T cell reactivation to establish infection-induced resistance to ECM.

PDL1 Fusion Protein Protects Against Experimental Cerebral Malaria via Repressing Over-Reactive CD8+ T Cell Responses

Cerebral malaria (CM), mainly caused by Plasmodium falciparum (P. f.), is one of the most lethal complications of severe malaria. As immunopathology mediated by brain-infiltrating CD8⁺ T cells is the major pathogenesis of CM, there is no safe and efficient treatment clinically focused on CD8⁺ T cells. New methods are needed to protect the host from injury. As evidence has shown that programmed death-1 (PD-1) is one of the most efficient immunomodulatory molecules, we constructed two soluble fusion proteins, PDL1-IgG1Fc and PDL2-IgG1Fc, to enhance PD-1/PDL signaling pathways in innate and adaptive immune cells, including macrophages and CD8⁺ T cells. Firstly, we confirmed that PD-1 signal pathway deficiency led to higher levels of CD8⁺ T cell proliferation and shorter survival time in PD-1-deficient (Pdcd1^−/−) mice than WT mice. Secondly, PDL1-IgG1Fc-treated mice exhibited a more prolonged survival time than control groups. Moreover, PDL1-IgG1Fc was observed to ameliorate blood-brain barrier (BBB) disruption by limiting the over-reactive CD8⁺ T cell cytotoxicity during experimental cerebral malaria (ECM). Further studies found thatPDL1-IgG1Fc-treated macrophages showed significant suppression in macrophage M1 polarization and their antigen presentation capability to CD8⁺ T cells. In conclusion, our results demonstrated that the administration of PDL1-IgG1Fc in the early stage before ECM onset has an obvious effect on the maintenance of immune microenvironment homeostasis in the brain and is deemed a promising candidate for protection against CM in the future.

Soluble markers of neutrophil, T-cell and monocyte activation are associated with disease severity and parasitemia in falciparum malaria

Background

The immune response during P. falciparum infection is a two-edged sword, involving dysregulation of the inflammatory responses with several types of immune cells participating. Here we examined T-cell, monocyte/macrophage and neutrophil activation during P. falciparum infection by using soluble activation markers for these leukocyte subsets.

Methods

In a prospective cross-sectional study clinical data and blood samples were collected from adults in Mozambique with P. falciparum infection, with (n = 70) and without (n = 61) co-infection with HIV-1, as well as HIV-infected patients with similar symptoms but without malaria (n = 58) and healthy controls (n = 52). Soluble (s)CD25, sCD14, sCD163 and myeloperoxidase (MPO) as markers for T-cell, monocyte/macrophage and neutrophil activation, respectively as well as CX3CL1, granzyme B and TIM-3 as markers of T-cell subsets and T-cell exhaustion, were analyzed.

Results

All patient groups had raised levels of activation markers compared with healthy controls. Levels of sCD25 and MPO increased gradually from patient with HIV only to patient with malaria only, with the highest levels in the HIV/malaria group. In the malaria group as a whole, MPO, sCD14 and in particular sCD25 were correlated with disease severity. sCD163, sCD25 and in particular MPO correlated with the degree of parasitemia as assessed by qPCR. Patients with falciparum malaria also had signs of T-cell subset activation (i.e. increased granzyme B and CX3CL1) and T-cell exhaustion as assessed by high levels of TIM-3 particularly in patients co-infected with HIV.

Conclusion

Our data support a marked immune activation in falciparum malaria involving all major leukocyte subsets with particular enhanced activation of neutrophils and T-cells in patients co-infected with HIV. Our findings also support a link between immune activation and immune exhaustion during falciparum malaria, particularly in relation to T-cell responses in patients co-infected with HIV.

Electronic supplementary material

The online version of this article (10.1186/s12879-018-3593-8) contains supplementary material, which is available to authorized users.

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.

Cerebral Malaria in Mouse and Man

Cerebral malaria (CM) is an acute encephalopathy caused by the malaria parasite Plasmodium falciparum, which develops in a small minority of infected patients and is responsible for the majority of deaths in African children. Despite decades of research on CM, the pathogenic mechanisms are still relatively poorly defined. Nevertheless, many studies in recent years, using a combination of animal models, in vitro cell culture work, and human patients, provide significant insight into the pathologic mechanisms leading to CM. In this review, we summarize recent findings from mouse models and human studies on the pathogenesis of CM, understanding of which may enable development of novel therapeutic approaches.

A Plasmodium Cross-Stage Antigen Contributes to the Development of Experimental Cerebral Malaria

Cerebral malaria is a complex neurological syndrome caused by an infection with Plasmodium falciparum parasites and is exclusively attributed to a series of host–parasite interactions at the pathological blood-stage of infection. In contrast, the preceding intra-hepatic phase of replication is generally considered clinically silent and thereby excluded from playing any role in the development of neurological symptoms. In this study, however, we present an antigen PbmaLS_05 that is presented to the host immune system by both pre-erythrocytic and intra-erythrocytic stages and contributes to the development of cerebral malaria in mice. Although deletion of the endogenous PbmaLS_05 prevented the development of experimental cerebral malaria (ECM) in susceptible mice after both sporozoite and infected red blood cell (iRBC) infections, we observed significant differences in contribution of the host immune response between both modes of inoculation. Moreover, PbmaLS_05-specific CD8⁺ T cells contributed to the development of ECM after sporozoite but not iRBC-infection, suggesting that pre-erythrocytic antigens like PbmaLS_05 can also contribute to the development of cerebral symptoms. Our data thus highlight the importance of the natural route of infection in the study of ECM, with potential implications for vaccine and therapeutic strategies against malaria.

The Glycosylphosphatidylinositol Transamidase Complex Subunit PbGPI16 of Plasmodium berghei Is Important for Inducing Experimental Cerebral Malaria

In animal models of experimental cerebral malaria (ECM), the glycosylphosphatidylinositols (GPIs) and GPI anchors are the major factors that induce nuclear factor kappa B (NF-κB) activation and proinflammatory responses, which contribute to malaria pathogenesis. GPIs and GPI anchors are transported to the cell surface via a process called GPI transamidation, which involves the GPI transamidase (GPI-T) complex. In this study, we showed that GPI16, one of the GPI-T subunits, is highly conserved among Plasmodium species. Genetic knockout of pbgpi16 (Δpbgpi16) in the rodent malaria parasite Plasmodium berghei strain ANKA led to a significant reduction of the amounts of GPIs in the membranes of merozoites, as well as surface display of several GPI-anchored merozoite surface proteins. Compared with the wild-type parasites, Δpbgpi16 parasites in C57BL/6 mice caused much less NF-κB activation and elicited a substantially attenuated T helper type 1 response. As a result, Δpbgpi16 mutant-infected mice displayed much less severe brain pathology, and considerably fewer Δpbgpi16 mutant-infected mice died from ECM. This study corroborated the GPI toxin as a significant inducer of ECM and further suggested that vaccines against parasite GPIs may be a promising strategy to limit the severity of malaria.

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.

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.

Interferon regulatory factor 1 is essential for pathogenic CD8+ T cell migration and retention in the brain during experimental cerebral malaria

Host immune response has a key role in controlling the progression of malaria infection. In the well-established murine model of experimental cerebral malaria (ECM) with Plasmodium berghei ANKA infection, proinflammatory Th1 and CD8+ T cell response are essential for disease development. Interferon regulatory factor 1 (IRF1) is a transcription factor that promotes Th1 responses, and its absence was previously shown to protect from ECM death. Yet the exact mechanism of protection remains unknown. Here we demonstrated that IRF1-deficient mice (IRF1 knockout) were protected from ECM death despite displaying early neurological signs. Resistance to ECM death was a result of reduced parasite sequestration and pathogenic CD8+ T cells in the brain. Further analysis revealed that IRF1 deficiency suppress interferon-γ production and delayed CD8+ T cell proliferation. CXCR3 expression was found to be decreased in pathogenic CD8+ T cells, which limited their migration to the brain. In addition, reduced expression of adhesion molecules by brain endothelial cells hampered leucocyte retention in the brain. Taken together, these factors limited sequestration of pathogenic CD8+ T cells and consequently its ability to induce extensive damage to the blood-brain barrier.

A T Cell Receptor Locus Harbors a Malaria-Specific Immune Response Gene

Immune response (Ir) genes, originally proposed by Baruj Benacerraf to explain differential antigen-specific responses in animal models, have become synonymous with the major histocompatibility complex (MHC). We discovered a non-MHC-linked Ir gene in a T cell receptor (TCR) locus that was required for CD8⁺ T cell responses to the Plasmodium berghei GAP50_40-48 epitope in mice expressing the MHC class I allele H-2D^b. GAP50_40-48-specific CD8⁺ T cell responses emerged from a very large pool of naive Vβ8.1⁺ precursors, which dictated susceptibility to cerebral malaria and conferred protection against recombinant Listeria monocytogenes infection. Structural analysis of a prototypical Vβ8.1⁺ TCR-H-2D^b-GAP50_40-48 ternary complex revealed that germline-encoded complementarity-determining region 1β residues present exclusively in the Vβ8.1 segment mediated essential interactions with the GAP50_40-48 peptide. Collectively, these findings demonstrated that Vβ8.1 functioned as an Ir gene that was indispensable for immune reactivity against the malaria GAP50_40-48 epitope.

A single rapamycin dose protects against late-stage experimental cerebral malaria via modulation of host immunity, endothelial activation and parasite sequestration

Background

Maladaptive immune responses during cerebral malaria (CM) result in high mortality despite opportune anti-malarial chemotherapy. Rapamycin, an FDA-approved immunomodulator, protects against experimental cerebral malaria (ECM) in mice through effects on the host. However, the potential for reduced adaptive immunity with chronic use, combined with an incomplete understanding of mechanisms underlying protection, limit translational potential as an adjunctive therapy in CM.

Results

The results presented herein demonstrate that a single dose of rapamycin, provided as late as day 4 or 5 post-infection, protected mice from ECM neuropathology and death through modulation of distinct host responses to infection. Rapamycin prevented parasite cytoadherence in peripheral organs, including white adipose tissue, via reduction of CD36 expression. Rapamycin also altered the splenic immune response by reducing the number of activated T cells with migratory phenotype, while increasing local cytotoxic T cell activation. Finally, rapamycin reduced brain endothelial ICAM-1 expression concomitant with reduced brain pathology. Together, these changes potentially contributed to increased parasite elimination while reducing CD8 T cell migration to the brain.

Conclusions

Rapamycin exerts pleotropic effects on host immunity, vascular activation and parasite sequestration that rescue mice from ECM, and thus support the potential clinical use of rapamycin as an adjunctive therapy in CM.

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.

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

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

Characterising the effect of antimalarial drugs on the maturation and clearance of murine blood-stage Plasmodium parasites in vivo

The artemisinins are the first-line therapy for severe and uncomplicated malaria, since they cause rapid declines in parasitemia after treatment. Despite this, in vivo mechanisms underlying this rapid decline remain poorly characterised. The overall decline in parasitemia is the net effect of drug inhibition of parasites and host clearance, which competes against any ongoing parasite proliferation. Separating these mechanisms in vivo was not possible through measurements of total parasitemia alone. Therefore, we employed an adoptive transfer approach in which C57BL/6J mice were transfused with Plasmodium berghei ANKA strain-infected, fluorescent red blood cells, and subsequently drug-treated. This approach allowed us to distinguish between the initial drug-treated generation of parasites (Gen₀), and their progeny (Gen₁). Artesunate efficiently impaired maturation of Gen₀ parasites, such that a sufficiently high dose completely arrested maturation after 6h of in vivo exposure. In addition, artesunate-affected parasites were cleared from circulation with a half-life of 6.7h. In vivo cell depletion studies using clodronate liposomes revealed an important role for host phagocytes in the removal of artesunate-affected parasites, particularly ring and trophozoite stages. Finally, we found that a second antimalarial drug, mefloquine, was less effective than artesunate at suppressing parasite maturation and driving host-mediated parasite clearance. Thus, we propose that in vivo artesunate treatment causes rapid decline in parasitemia by arresting parasite maturation and encouraging phagocyte-mediated clearance of parasitised RBCs.

High pathogen burden in childhood promotes the development of unconventional innate-like CD8+ T cells

Cellular and humoral constituents of the immune system differ significantly between children and adults, yet very little is known about the impact of early-life pathogen exposure on this immunologic transition. We examined CD4+ and CD8+ T cell subsets defined by CCR7 and CD45RA expression in two longitudinal pediatric cohorts experiencing divergent levels of pathogen burden. Using multiparameter flow cytometry, along with serological, cytokine, and transcriptomic data, we show that cumulative pathogen burden promotes the development of atypical CD8dim T cells with an innate-like profile (Granzyme Bhi, IFNγlow, TNFαlow, PLFZhi, ID2hi, IKZF2hi) in contrast to age-matched children residing in a low pathogen-exposure area who display a more conventional CD8bright profile (IFNγ+, TNFα+, CCL4+). Furthermore, these unconventional T cells had stunted proliferation, distinct transcriptional programs, and impaired T cell receptor signaling and were enriched in hallmark TNFα, NF-κB, and IL-6 gene signaling pathways, reminiscent of NK cells and type-1 innate lymphoid cells. Our findings suggest that these unconventional CD8dim T cells arise in a very particular immunological context and may provide a deeper understanding of the heterogeneity in human immune responses.

Cytokines and Chemokines in Cerebral Malaria Pathogenesis

Cerebral malaria is among the major causes of malaria-associated mortality and effective adjunctive therapeutic strategies are currently lacking. Central pathophysiological processes involved in the development of cerebral malaria include an imbalance of pro- and anti-inflammatory responses to Plasmodium infection, endothelial cell activation, and loss of blood-brain barrier integrity. However, the sequence of events, which initiates these pathophysiological processes as well as the contribution of their complex interplay to the development of cerebral malaria remain incompletely understood. Several cytokines and chemokines have repeatedly been associated with cerebral malaria severity. Increased levels of these inflammatory mediators could account for the sequestration of leukocytes in the cerebral microvasculature present during cerebral malaria, thereby contributing to an amplification of local inflammation and promoting cerebral malaria pathogenesis. Herein, we highlight the current knowledge on the contribution of cytokines and chemokines to the pathogenesis of cerebral malaria with particular emphasis on their roles in endothelial activation and leukocyte recruitment, as well as their implication in the progression to blood-brain barrier permeability and neuroinflammation, in both human cerebral malaria and in the murine experimental cerebral malaria model. A better molecular understanding of these processes could provide the basis for evidence-based development of adjunct therapies and the definition of diagnostic markers of disease progression.

Small molecule-based inhibition of MEK1/2 proteins dampens inflammatory responses to malaria, reduces parasite load, and mitigates pathogenic outcomes

Malaria infections cause several systemic and severe single- or multi-organ pathologies, killing hundreds of thousands of people annually. Considering the existing widespread resistance of malaria parasites to anti-parasitic drugs and their high propensity to develop drug resistance, alternative strategies are required to manage malaria infections. Because malaria is a host immune response-driven disease, one approach is based on gaining a detailed understanding of the molecular and cellular processes that modulate malaria-induced innate and adaptive immune responses. Here, using a mouse cerebral malaria model and small-molecule inhibitors, we demonstrate that inhibiting MEK1/2, the upstream kinases of ERK1/2 signaling, alters multifactorial components of the innate and adaptive immune responses, controls parasitemia, and blocks pathogenesis. Specifically, MEK1/2 inhibitor treatment up-regulated B1 cell expansion, IgM production, phagocytic receptor expression, and phagocytic activity, enhancing parasite clearance by macrophages and neutrophils. Further, the MEK1/2 inhibitor treatment down-regulated pathogenic pro-inflammatory and helper T cell 1 (Th1) responses and up-regulated beneficial anti-inflammatory cytokine responses and Th2 responses. These inhibitor effects resulted in reduced granzyme B expression by T cells, chemokine and intracellular cell adhesion molecule 1 (ICAM-1) expression in the brain, and chemokine receptor expression by both myeloid and T cells. These bimodal effects of the MEK1/2 inhibitor treatment on immune responses contributed to decreased parasite biomass, organ inflammation, and immune cell recruitment, preventing tissue damage and death. In summary, we have identified several previously unrecognized immune regulatory processes through which a MEK1/2 inhibitor approach controls malaria parasitemia and mitigates pathogenic effects on host organs.

Host-mediated impairment of parasite maturation during blood-stage Plasmodium infection

Severe malaria and associated high parasite burdens occur more frequently in humans lacking robust adaptive immunity to Plasmodium falciparum Nevertheless, the host may partly control blood-stage parasite numbers while adaptive immunity is gradually established. Parasite control has typically been attributed to enhanced removal of parasites by the host, although in vivo quantification of this phenomenon remains challenging. We used a unique in vivo approach to determine the fate of a single cohort of semisynchronous, Plasmodium berghei ANKA- or Plasmodium yoelii 17XNL-parasitized red blood cells (pRBCs) after transfusion into naive or acutely infected mice. As previously shown, acutely infected mice, with ongoing splenic and systemic inflammatory responses, controlled parasite population growth more effectively than naive controls. Surprisingly, however, this was not associated with accelerated removal of pRBCs from circulation. Instead, transfused pRBCs remained in circulation longer in acutely infected mice. Flow cytometric assessment and mathematical modeling of intraerythrocytic parasite development revealed an unexpected and substantial slowing of parasite maturation in acutely infected mice, extending the life cycle from 24 h to 40 h. Importantly, impaired parasite maturation was the major contributor to control of parasite growth in acutely infected mice. Moreover, by performing the same experiments in rag1^-/- mice, which lack T and B cells and mount weak inflammatory responses, we revealed that impaired parasite maturation is largely dependent upon the host response to infection. Thus, impairment of parasite maturation represents a host-mediated, immune system-dependent mechanism for limiting parasite population growth during the early stages of an acute blood-stage Plasmodium infection.

Host Resistance to Plasmodium-Induced Acute Immune Pathology Is Regulated by Interleukin-10 Receptor Signaling

The resolution of malaria infection is dependent on a balance between proinflammatory and regulatory immune responses. While early effector T cell responses are required for limiting parasitemia, these responses need to be switched off by regulatory mechanisms in a timely manner to avoid immune-mediated tissue damage. Interleukin-10 receptor (IL-10R) signaling is considered to be a vital component of regulatory responses, although its role in host resistance to severe immune pathology during acute malaria infections is not fully understood. In this study, we have determined the contribution of IL-10R signaling to the regulation of immune responses during Plasmodium berghei ANKA-induced experimental cerebral malaria (ECM). We show that antibody-mediated blockade of the IL-10R during P. berghei ANKA infection in ECM-resistant BALB/c mice leads to amplified T cell activation, higher serum gamma interferon (IFN-γ) concentrations, enhanced intravascular accumulation of both parasitized red blood cells and CD8⁺ T cells to the brain, and an increased incidence of ECM. Importantly, the pathogenic effects of IL-10R blockade during P. berghei ANKA infection were reversible by depletion of T cells and neutralization of IFN-γ. Our findings underscore the importance of IL-10R signaling in preventing T-cell- and cytokine-mediated pathology during potentially lethal malaria infections.

IL-33 receptor ST2 regulates the cognitive impairments associated with experimental cerebral malaria

Cerebral malaria (CM) is associated with a high mortality rate and long-term neurocognitive impairment in survivors. The murine model of experimental cerebral malaria (ECM) induced by Plasmodium berghei ANKA (PbA)-infection reproduces several of these features. We reported recently increased levels of IL-33 protein in brain undergoing ECM and the involvement of IL-33/ST2 pathway in ECM development. Here we show that PbA-infection induced early short term and spatial memory defects, prior to blood brain barrier (BBB) disruption, in wild-type mice, while ST2-deficient mice did not develop cognitive defects. PbA-induced neuroinflammation was reduced in ST2-deficient mice with low Ifng, Tnfa, Il1b, Il6, CXCL9, CXCL10 and Cd8a expression, associated with an absence of neurogenesis defects in hippocampus. PbA-infection triggered a dramatic increase of IL-33 expression by oligodendrocytes, through ST2 pathway. In vitro, IL-33/ST2 pathway induced microglia expression of IL-1β which in turn stimulated IL-33 expression by oligodendrocytes. These results highlight the IL-33/ST2 pathway ability to orchestrate microglia and oligodendrocytes responses at an early stage of PbA-infection, with an amplification loop between IL-1β and IL-33, responsible for an exacerbated neuroinflammation context and associated neurological and cognitive defects.

The cerebral complication of malaria caused by Plasmodium falciparum infection, is associated with long-term neurological sequelae in survivors. The mechanisms involved in neurocognitive impairments during cerebral malaria development are still unknown. We reported recently the essential role of IL-33/ST2 pathway in experimental cerebral malaria (ECM) development. In this study we investigated the capacity of IL-33, highly expressed in oligodendrocytes, to promote ECM-associated neurological and cognitive damages. We found that IL-33/ST2 pathway through glial cells is involved in neurocognitive impairments, associated with exacerbated neuroinflammation, and altered neurogenesis. Interestingly, the implication of glial cells with a high level of IL-33 production in neurocognitive disorders, occurs at an early stage of ECM development, prior to blood brain barrier permeabilization. We propose the link between microglial IL-1β and oligodendrocytes IL-33 production in neurological symptoms associated with ECM.

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.

A quantitative brain map of experimental cerebral malaria pathology

The murine model of experimental cerebral malaria (ECM) has been utilised extensively in recent years to study the pathogenesis of human cerebral malaria (HCM). However, it has been proposed that the aetiologies of ECM and HCM are distinct, and, consequently, no useful mechanistic insights into the pathogenesis of HCM can be obtained from studying the ECM model. Therefore, in order to determine the similarities and differences in the pathology of ECM and HCM, we have performed the first spatial and quantitative histopathological assessment of the ECM syndrome. We demonstrate that the accumulation of parasitised red blood cells (pRBCs) in brain capillaries is a specific feature of ECM that is not observed during mild murine malaria infections. Critically, we show that individual pRBCs appear to occlude murine brain capillaries during ECM. As pRBC-mediated congestion of brain microvessels is a hallmark of HCM, this suggests that the impact of parasite accumulation on cerebral blood flow may ultimately be similar in mice and humans during ECM and HCM, respectively. Additionally, we demonstrate that cerebrovascular CD8+ T-cells appear to co-localise with accumulated pRBCs, an event that corresponds with development of widespread vascular leakage. As in HCM, we show that vascular leakage is not dependent on extensive vascular destruction. Instead, we show that vascular leakage is associated with alterations in transcellular and paracellular transport mechanisms. Finally, as in HCM, we observed axonal injury and demyelination in ECM adjacent to diverse vasculopathies. Collectively, our data therefore shows that, despite very different presentation, and apparently distinct mechanisms, of parasite accumulation, there appear to be a number of comparable features of cerebral pathology in mice and in humans during ECM and HCM, respectively. Thus, when used appropriately, the ECM model may be useful for studying specific pathological features of HCM.

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 cell cytotoxicity mediates pathology in the skin by inflammasome activation and IL-1β production

Deregulated CD8+ T cell cytotoxicity plays a central role in enhancing disease severity in several conditions. However, we have little understanding of the mechanisms by which immunopathology develops as a consequence of cytotoxicity. Using murine models of inflammation induced by the protozoan parasite leishmania, and data obtained from patients with cutaneous leishmaniasis, we uncovered a previously unrecognized role for NLRP3 inflammasome activation and IL-1β release as a detrimental consequence of CD8+ T cell-mediated cytotoxicity, ultimately resulting in chronic inflammation. Critically, pharmacological blockade of NLRP3 or IL-1β significantly ameliorated the CD8+ T cell-driven immunopathology in leishmania-infected mice. Confirming the relevance of these findings to human leishmaniasis, blockade of the NLRP3 inflammasome in skin biopsies from leishmania-infected patients prevented IL-1β release. Thus, these studies link CD8+ T cell cytotoxicity with inflammasome activation and reveal novel avenues of treatment for cutaneous leishmaniasis, as well as other of diseases where CD8+ T cell-mediated cytotoxicity induces pathology.

Leishmaniasis is a neglected tropical disease endemic in 98 countries and approximately 1 million new cases occur each year. Disease caused by Leishmania braziliensis, the main causative agent of leishmaniasis in South America, leads to skin ulcers that are difficult to heal with drugs that target the parasites. This is because disease severity seen in patients infected with L. braziliensis is largely due to the immune response that develops, rather than the number of parasites in the skin. CD8+ T cells induce cell death in the lesions of L. braziliensis-infected mice, as well as in the lesions from L. braziliensis-infected patients, which promotes disease. However, the mechanism mediating CD8+ T cell dependent pathology is unknown. Here, using studies in mice and experiments with L. braziliensis patients’ samples we show that increased disease severity is due to inflammasome activation, and furthermore that therapies that block either inflammasome activation or IL-1β ameliorate disease in mouse models of severe leishmaniasis. Based on these studies we propose a novel strategy of therapy for L. braziliensis infection and other diseases in which cytotoxicity plays a central role in promoting disease severity.

The Deubiquitinating Enzyme Cylindromatosis Dampens CD8+ T Cell Responses and Is a Critical Factor for Experimental Cerebral Malaria and Blood-Brain Barrier Damage

Cerebral malaria is a severe complication of human malaria and may lead to death of Plasmodium falciparum-infected individuals. Cerebral malaria is associated with sequestration of parasitized red blood cells within the cerebral microvasculature resulting in damage of the blood-brain barrier and brain pathology. Although CD8⁺ T cells have been implicated in the development of murine experimental cerebral malaria (ECM), several other studies have shown that CD8⁺ T cells confer protection against blood-stage infections. Since the role of host deubiquitinating enzymes (DUBs) in malaria is yet unknown, we investigated how the DUB cylindromatosis (CYLD), an important inhibitor of several cellular signaling pathways, influences the outcome of ECM. Upon infection with Plasmodium berghei ANKA (PbA) sporozoites or PbA-infected red blood cells, at least 90% of Cyld^-/- mice survived the infection, whereas all congenic C57BL/6 mice displayed signatures of ECM, impaired parasite control, and disruption of the blood-brain barrier integrity. Cyld deficiency prevented brain pathology, including hemorrhagic lesions, enhanced activation of astrocytes and microglia, infiltration of CD8⁺ T cells, and apoptosis of endothelial cells. Furthermore, PbA-specific CD8⁺ T cell responses were augmented in the blood of Cyld^-/- mice with increased production of interferon-γ and granzyme B and elevated activation of protein kinase C-θ and nuclear factor "kappa light-chain enhancer" of activated B cells. Importantly, accumulation of CD8⁺ T cells in the brain of Cyld^-/- mice was significantly reduced compared to C57BL/6 mice. Bone marrow chimera experiments showed that the absence of ECM signatures in infected Cyld^-/- mice could be attributed to hematopoietic and radioresistant parenchymal cells, most likely endothelial cells that did not undergo apoptosis. Together, we were able to show that host deubiqutinating enzymes play an important role in ECM and that CYLD promotes ECM supporting it as a potential therapeutic target for adjunct therapy to prevent cerebral complications of severe 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.

Tacrolimus prevents murine cerebral malaria

Tacrolimus and mycophenolate mofetil are immunosuppressants frequently used in human organ transplantation. Tacrolimus is also reported to inhibit Plasmodium falciparum growth in vitro. Here, we report that tacrolimus prevented the death from cerebral malaria of Plasmodium berghei ANKA-infected C57BL/6J mice, but not their death from malaria due to the high parasitaemia and severe anaemia. The mycophenolate mofetil-treated mice showed higher mortality from cerebral malaria and succumbed to malaria earlier than tacrolimus-treated littermates. Tacrolimus attenuated the infiltration of mononuclear cells including pathogenic CD8⁺ T cells into the brain. It appears to prevent murine cerebral malaria through the inhibition of cerebral infiltration of CD8⁺ T cells.

Establishment of a murine model of cerebral malaria in KunMing mice infected with Plasmodium berghei ANKA

Malaria remains one of the most devastating diseases. Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection resulting in high mortality and morbidity worldwide. Analysis of precise mechanisms of CM in humans is difficult for ethical reasons and animal models of CM have been employed to study malaria pathogenesis. Here, we describe a new experimental cerebral malaria (ECM) model with Plasmodium berghei ANKA infection in KunMing (KM) mice. KM mice developed ECM after blood-stage or sporozoites infection, and the development of ECM in KM mice has a dose-dependent relationship with sporozoites inoculums. Histopathological findings revealed important features associated with ECM, including accumulation of mononuclear cells and red blood cells in brain microvascular, and brain parenchymal haemorrhages. Blood-brain barrier (BBB) examination showed that BBB disruption was present in infected KM mice when displaying clinical signs of CM. In vivo bioluminescent imaging experiment indicated that parasitized red blood cells accumulated in most vital organs including heart, lung, spleen, kidney, liver and brain. The levels of inflammatory cytokines interferon-gamma, tumour necrosis factor-alpha, interleukin (IL)-17, IL-12, IL-6 and IL-10 were all remarkably increased in KM mice infected with P. berghei ANKA. This study indicates that P. berghei ANKA infection in KM mice can be used as ECM model to extend further research on genetic, pharmacological and vaccine studies of CM.

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.

Suppressor of cytokine signaling 2 modulates the immune response profile and development of experimental cerebral malaria

Plasmodium falciparum infection results in severe malaria in humans, affecting various organs, including the liver, spleen and brain, and resulting in high morbidity and mortality. The Plasmodium berghei ANKA (PbA) infection in mice closely recapitulates many aspects of human cerebral malaria (CM); thus, this model has been used to investigate the pathogenesis of CM. Suppressor of cytokine signaling 2 (SOCS2), an intracellular protein induced by cytokines and hormones, modulates the immune response, neural development, neurogenesis and neurotrophic pathways. However, the role of SOCS2 during CM remains unknown. SOCS2 knockout (SOCS2(-/-)) mice infected with PbA show an initial resistance to infection with reduced parasitemia and production of TNF, TGF-β, IL-12 and IL-17 in the brain. Interestingly, in the late phase of infection, SOCS2(-/-) mice display increased parasitemia and reduced Treg cell infiltration, associated with enhanced levels of Th1 and Th17 cells and related cytokines IL-17, IL-6, and TGF-β in the brain. A significant reduction in protective neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), was also observed. Moreover, the molecular alterations in the brain of infected SOCS2(-/-) mice were associated with anxiety-related behaviors and cognition impairment. Mechanistically, these results revealed enhanced nitric oxide (NO) production in PbA-infected SOCS2(-/-) mice, and the inhibition of NO synthesis through l-NAME led to a marked decrease in survival, the disruption of parasitemia control and more pronounced anxiety-like behavior. Treatment with l-NAME also shifted the levels of Th1, Th7 and Treg cells in the brains of infected SOCS2(-/-) mice to the background levels observed in infected WT, with remarkable exception of increased CD8(+)IFN(+) T cells and inflammatory monocytes. These results indicate that SOCS2 plays a dual role during PbA infection, being detrimental in the control of the parasite replication but crucial in the regulation of the immune response and production of neurotrophic factors. Here, we provided strong evidence of a critical relationship between SOCS2 and NO in the orchestration of the immune response and development of CM during PbA infection.

Expression of complement and toll-like receptor pathway genes is associated with malaria severity in Mali: a pilot case control study

Background

The host response to infection by Plasmodium falciparum, the parasite most often responsible for severe malaria, ranges from asymptomatic parasitaemia to death. The clinical trajectory of malaria is influenced by host genetics and parasite load, but the factors determining why some infections produce uncomplicated malaria and some proceed to severe disease remain incompletely understood.

Methods

To identify molecular markers of severe falciparum malaria, human gene expression patterns were compared between children aged 6 months to 5 years with severe and uncomplicated malaria who were enrolled in a case–control study in Bandiagara, Mali. Microarrays were used to obtain expression data on severe cases and uncomplicated controls at the time of acute disease presentation (five uncomplicated and five severe), 1 week after presentation (three uncomplicated and three severe) and treatment initiation, and in the subsequent dry season (late convalescence, four uncomplicated and four severe). This is a pilot study for the first use of microarray technology in Mali.

Results

Complement and toll-like receptor (TLR) pathways were differentially expressed, with severe cases showing higher expression of the C1q, TLR2, TLR4, TLR8, and CR1 genes. Other genes previously associated with malaria pathogenesis, GZMB, FOS and HSPA6, were also higher among severe cases. TLR2, TLR4, TLR8, CR1, GZMB, FOS, and HSPA6 genes were expressed at lower levels in severe cases at late convalescence.

Conclusions

Overexpression of genes previously associated with uncomplicated malaria was associated with severe disease. Low baseline expression of these genes may represent candidate markers for severe malaria. Despite the small sample size, results of this pilot study offer promising targets for follow-up analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1189-6) contains supplementary material, which is available to authorized users.

Blimp-1-Dependent IL-10 Production by Tr1 Cells Regulates TNF-Mediated Tissue Pathology

Tumor necrosis factor (TNF) is critical for controlling many intracellular infections, but can also contribute to inflammation. It can promote the destruction of important cell populations and trigger dramatic tissue remodeling following establishment of chronic disease. Therefore, a better understanding of TNF regulation is needed to allow pathogen control without causing or exacerbating disease. IL-10 is an important regulatory cytokine with broad activities, including the suppression of inflammation. IL-10 is produced by different immune cells; however, its regulation and function appears to be cell-specific and context-dependent. Recently, IL-10 produced by Th1 (Tr1) cells was shown to protect host tissues from inflammation induced following infection. Here, we identify a novel pathway of TNF regulation by IL-10 from Tr1 cells during parasitic infection. We report elevated Blimp-1 mRNA levels in CD4⁺ T cells from visceral leishmaniasis (VL) patients, and demonstrate IL-12 was essential for Blimp-1 expression and Tr1 cell development in experimental VL. Critically, we show Blimp-1-dependent IL-10 production by Tr1 cells prevents tissue damage caused by IFNγ-dependent TNF production. Therefore, we identify Blimp-1-dependent IL-10 produced by Tr1 cells as a key regulator of TNF-mediated pathology and identify Tr1 cells as potential therapeutic tools to control inflammation.

Many parasitic diseases are associated with the generation of potent inflammatory responses. These are often needed to control infection, but can also cause tissue damage if not appropriately regulated. IL-10 has emerged as an important immune regulator that protects tissues by dampening inflammation. Recently, some T cells that initially produce inflammatory cytokines have been found to start producing IL-10 as a mechanism of auto-regulation. We identified an important transcriptional regulator called B lymphocyte-induced maturation protein 1 (Blimp-1), which promotes IL-10 production by IFNγ-producing CD4⁺ T (Tr1) cells during malaria and visceral leishmaniasis, two important diseases caused by protozoan parasites. We found that Tr1 cell-derived IL-10 suppressed anti-parasitic immunity, but played a critical role in preventing tissue damage caused by the potent pro-inflammatory cytokine TNF. Specifically, IL-10 protected macrophages from TNF-mediated destruction, and this enabled lymphocytes to continue to migrate to regions in the spleen where T and B cell responses are generated. These findings allow us to better understand how parasites persist in a host, but also identify new opportunities to control inflammation to prevent disease.

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.

Mycobacterium tuberculosis Coinfection Has No Impact on Plasmodium berghei ANKA-Induced Experimental Cerebral Malaria in C57BL/6 Mice

Cerebral malaria (CM) is the most severe complication of human infection with Plasmodium falciparum. The mechanisms predisposing to CM are still not fully understood. Proinflammatory immune responses are required for the control of blood-stage malaria infection but are also implicated in the pathogenesis of CM. A fine balance between pro- and anti-inflammatory immune responses is required for parasite clearance without the induction of host pathology. The most accepted experimental model to study human CM is Plasmodium berghei ANKA (PbANKA) infection in C57BL/6 mice that leads to the development of a complex neurological syndrome which shares many characteristics with the human disease. We applied this model to study the outcome of PbANKA infection in mice previously infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. Tuberculosis is coendemic with malaria in large regions in the tropics, and mycobacteria have been reported to confer some degree of unspecific protection against rodent Plasmodium parasites in experimental coinfection models. We found that concomitant M. tuberculosis infection did not change the clinical course of PbANKA-induced experimental cerebral malaria (ECM) in C57BL/6 mice. The immunological environments in spleen and brain did not differ between singly infected and coinfected animals; instead, the overall cytokine and T cell responses in coinfected mice were comparable to those in animals solely infected with PbANKA. Our data suggest that M. tuberculosis coinfection is not able to change the outcome of PbANKA-induced disease, most likely because the inflammatory response induced by the parasite rapidly dominates in mice previously infected with M. tuberculosis.

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.

Measuring antigen presentation in mouse brain endothelial cells ex vivo and in vitro

We have recently demonstrated that brain endothelial cells cross-present parasite antigen during mouse experimental cerebral malaria (ECM). Here we describe a 2-d protocol to detect cross-presentation by isolating the brain microvessels and incubating them with a reporter cell line that expresses lacZ upon detection of the relevant peptide-major histocompatibility complex. After X-gal staining, a typical positive result consists of hundreds of blue spots, compared with fewer than 20 spots from a naive brain. The assay is generalizable to other disease contexts by using reporter cells that express appropriate specific T cell receptors. Also described is the protocol for culturing endothelial cells from brain microvessels isolated from naive mice. After 7-10 d, an in vitro cross-presentation assay can be performed by adding interferon-γ, antigen (e.g., Plasmodium berghei-infected red blood cells) and reporter cells in sequence over 3 d. This is useful for comparing different antigen forms or for probing the effects of various interventions.

Pathogenesis of cerebral malaria: new diagnostic tools, biomarkers, and therapeutic approaches

Cerebral malaria is a severe neuropathological complication of Plasmodium falciparum infection. It results in high mortality and post-recovery neuro-cognitive disorders in children, even after appropriate treatment with effective anti-parasitic drugs. While the complete landscape of the pathogenesis of cerebral malaria still remains to be elucidated, numerous innovative approaches have been developed in recent years in order to improve the early detection of this neurological syndrome and, subsequently, the clinical care of affected patients. In this review, we briefly summarize the current understanding of cerebral malaria pathogenesis, compile the array of new biomarkers and tools available for diagnosis and research, and describe the emerging therapeutic approaches to tackle this pathology effectively.

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.

Predominant role of interferon-γ in the host protective effect of CD8(+) T cells against Neospora caninum infection

It is well established that CD8⁺ T cells play an important role in protective immunity against protozoan infections. However, their role in the course of Neospora caninum infection has not been fully elucidated. Here we report that CD8-deficient mice infected with N. caninum presented higher parasitic loads in the brain and lungs and lower spleen and brain immunity-related GTPases than their wild-type counterparts. Moreover, adoptive transfer of splenic CD8⁺ T cells sorted from N. caninum-primed immunosufficient C57BL/10 ScSn mice prolonged the survival of infected IL-12-unresponsive C57BL/10 ScCr recipients. In both C57BL/6 and C57BL/10 ScSn mice CD8⁺ T cells are activated and produce interferon-γ (IFN-γ) upon challenged with N. caninum. The host protective role of IFN-γ produced by CD8⁺ T cells was confirmed in N. caninum-infected RAG2-deficient mice reconstituted with CD8⁺ T cells obtained from either IFN-γ-deficient or wild-type donors. Mice receiving IFN-γ-expressing CD8⁺ T cells presented lower parasitic burdens than counterparts having IFN-γ-deficient CD8⁺ T cells. Moreover, we observed that N. caninum-infected perforin-deficient mice presented parasitic burdens similar to those of infected wild-type controls. Altogether these results demonstrate that production of IFN-γ is a predominant protective mechanism conferred by CD8⁺ T cells in the course of neosporosis.

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.

Inhibiting the Mammalian target of rapamycin blocks the development of experimental cerebral malaria

Malaria is an infectious disease caused by parasites of several Plasmodium spp. Cerebral malaria (CM) is a common form of severe malaria resulting in nearly 700,000 deaths each year in Africa alone. At present, there is no adjunctive therapy for CM. Although the mechanisms underlying the pathogenesis of CM are incompletely understood, it is likely that both intrinsic features of the parasite and the human host's immune response contribute to disease. The kinase mammalian target of rapamycin (mTOR) is a central regulator of immune responses, and drugs that inhibit the mTOR pathway have been shown to be antiparasitic. In a mouse model of CM, experimental CM (ECM), we show that the mTOR inhibitor rapamycin protects against ECM when administered within the first 4 days of infection. Treatment with rapamycin increased survival, blocked breakdown of the blood-brain barrier and brain hemorrhaging, decreased the influx of both CD4(+) and CD8(+) T cells into the brain and the accumulation of parasitized red blood cells in the brain. Rapamycin induced marked transcriptional changes in the brains of infected mice, and analysis of transcription profiles predicted that rapamycin blocked leukocyte trafficking to and proliferation in the brain. Remarkably, animals were protected against ECM even though rapamycin treatment significantly increased the inflammatory response induced by infection in both the brain and spleen. These results open a new avenue for the development of highly selective adjunctive therapies for CM by targeting pathways that regulate host and parasite metabolism.

IMPORTANCE

Malaria is a highly prevalent infectious disease caused by parasites of several Plasmodium spp. Malaria is usually uncomplicated and resolves with time; however, in about 1% of cases, almost exclusively among young children, malaria becomes severe and life threatening, resulting in nearly 700,000 deaths each year in Africa alone. Among the most severe complications of Plasmodium falciparum infection is cerebral malaria with a fatality rate of 15 to 20%, despite treatment with antimalarial drugs. Cerebral malaria takes a second toll on African children, leaving survivors at high risk of debilitating neurological defects. At present, we have no effective adjunctive therapies for cerebral malaria, and developing such therapies would have a large impact on saving young lives in Africa. Here we report results that open a new avenue for the development of highly selective adjunctive therapies for cerebral malaria by targeting pathways that regulate host and parasite metabolism.

Phenylhydrazine administration accelerates the development of experimental cerebral malaria

Phenylhydrazine (PHZ) treatment is generally used to enhance parasitemia in infected mice models. Transient reticulocytosis is commonly observed in iron-deficient anemic hosts after treatment with iron supplementation, and is also associated with short-term hemolysis caused by PHZ treatment. In this study, we investigated the relationship between reticulocytosis and cerebral malaria (CM) in a murine model induced by PHZ administration before Plasmodium berghei ANKA (PbA) infection. Mortality and parasitemia were checked daily. Pro-inflammatory cytokines and IL-10 were quantified by ELISA. The expression of CXCL9, CXCL10, CCL5, and CXCR3 mRNAs was determined by real-time PCR. Brain sequestration of CD4(+) and CD8(+) T cells and populations of splenic Th1 CD4(+) T cells, dendritic cells (DCs), CD11b(+) Gr1(+) cells, and regulatory T cells (Tregs) were assessed by FACS. PHZ administration dramatically increased parasitemia from day 3 to day 5 post infection (p.i.) compared with the untreated control infected mice group; also, CM developed at day 5 p.i., compared with day 7 p.i. in untreated control infected mice, as well as significantly decreased blood-brain barrier function (P < 0.001). PHZ administration during PbA infection significantly increased the expression of CXCL9 (P <0.05) and VCAM-1 (P <0.001) in the brain, increased the expression of CXCL10, CCL5 and CXCR3, and significantly increased the recruitment of CD4(+) and CD8(+) T cells (P <0.001 and P <0.01, respectively) as well as CD11b(+) Gr1(+) cells to the brain. In addition, PHZ administration significantly increased the numbers of IL-12-secreting DCs at days 3 and 5 p.i. compared to those of untreated control infected mice (P <0.001 and P <0.01, respectively). Consequently, the activation of CD4(+) T cells, especially the expansion of the Th1 subset (P <0.05), was significantly and dramatically enhanced and was accompanied by marked increases in the production of protein and/or mRNA of the Th1-type pro-inflammatory mediators, IFN-γ and TNF-α (P <0.01 for both for protein; P <0.05 for TNF-α mRNA). Our results suggest that, compared to healthy individuals, people suffering from reticulocytosis may be more susceptible to severe malaria infection in malaria endemic areas. This has implications for the most appropriate selection of treatment, which may also cause reticulocytosis in patients living in such areas.

Reduced erythrocyte susceptibility and increased host clearance of young parasites slows Plasmodium growth in a murine model of severe malaria

The best correlate of malaria severity in human Plasmodium falciparum (Pf) infection is the total parasite load. Pf-infected humans could control parasite loads by two mechanisms, either decreasing parasite multiplication, or increasing parasite clearance. However, few studies have directly measured these two mechanisms in vivo. Here, we have directly quantified host clearance of parasites during Plasmodium infection in mice. We transferred labelled red blood cells (RBCs) from Plasmodium infected donors into uninfected and infected recipients, and tracked the fate of donor parasites by frequent blood sampling. We then applied age-based mathematical models to characterise parasite clearance in the recipient mice. Our analyses revealed an increased clearance of parasites in infected animals, particularly parasites of a younger developmental stage. However, the major decrease in parasite multiplication in infected mice was not mediated by increased clearance alone, but was accompanied by a significant reduction in the susceptibility of RBCs to parasitisation.

Disruption of Parasite hmgb2 Gene Attenuates Plasmodium berghei ANKA Pathogenicity

Eukaryotic high-mobility-group-box (HMGB) proteins are nuclear factors involved in chromatin remodeling and transcription regulation. When released into the extracellular milieu, HMGB1 acts as a proinflammatory cytokine that plays a central role in the pathogenesis of several immune-mediated inflammatory diseases. We found that the Plasmodium genome encodes two genuine HMGB factors, Plasmodium HMGB1 and HMGB2, that encompass, like their human counterparts, a proinflammatory domain. Given that these proteins are released from parasitized red blood cells, we then hypothesized that Plasmodium HMGB might contribute to the pathogenesis of experimental cerebral malaria (ECM), a lethal neuroinflammatory syndrome that develops in C57BL/6 (susceptible) mice infected with Plasmodium berghei ANKA and that in many aspects resembles human cerebral malaria elicited by P. falciparum infection. The pathogenesis of experimental cerebral malaria was suppressed in C57BL/6 mice infected with P. berghei ANKA lacking the hmgb2 gene (Δhmgb2 ANKA), an effect associated with a reduction of histological brain lesions and with lower expression levels of several proinflammatory genes. The incidence of ECM in pbhmgb2-deficient mice was restored by the administration of recombinant PbHMGB2. Protection from experimental cerebral malaria in Δhmgb2 ANKA-infected mice was associated with reduced sequestration in the brain of CD4(+) and CD8(+) T cells, including CD8(+) granzyme B(+) and CD8(+) IFN-γ(+) cells, and, to some extent, neutrophils. This was consistent with a reduced parasite sequestration in the brain, lungs, and spleen, though to a lesser extent than in wild-type P. berghei ANKA-infected mice. In summary, Plasmodium HMGB2 acts as an alarmin that contributes to the pathogenesis of cerebral malaria.

Critical role of IL-33 receptor ST2 in experimental cerebral malaria development

Cerebral malaria, a severe complication of Plasmodium falciparum infection, can be modeled in murine Plasmodium berghei ANKA (PbA) infection. PbA-induced experimental cerebral malaria (ECM) is CD8(+) T-cell mediated, and influenced by TH 1/TH 2 balance. Here, we show that IL-33 expression is increased in brain undergoing ECM and we address the role of the IL-33/ST2 pathway in ECM development. ST2-deficient mice were resistant to PbA-induced neuropathology. They survived >20 days with no ECM neurological sign and a preserved cerebral microcirculation, while WT mice succumbed within 10 days with ECM, brain vascular leakage, distinct microvascular pathology obstruction, and hemorrhages. Parasitemia and brain parasite load were similar in ST2-deficient and WT mice. Protection was accompanied by reduced brain sequestration of activated CD4(+) T cells and perforin(+) CD8(+) T cells. While IFN-γ and T-cell-attracting chemokines CXCL9 and CXCL10 were not affected in the absence of functional ST2 pathway, the local expression of ICAM-1, CXCR3, and LT-α, crucial for ECM development, was strongly reduced, and this may explain the diminished pathogenic T-cell recruitment and resistance to ECM. Therefore, IL-33 is induced in PbA sporozoite infection, and the pathogenic T-cell responses with local microvascular pathology are dependent on IL-33/ST2 signaling, identifying IL-33 as a new actor in ECM development.