Cerebral malaria in mice: interleukin-2 treatment induces accumulation of gammadelta T cells in the brain and alters resistant mice to susceptible-like phenotype

γδ T Cells in Brain Homeostasis and Diseases

γδ T cells are a distinct subset of T cells expressing γδ T cell receptor (TCR) rather than αβTCR. Since their discovery, the critical roles of γδ T cells in multiple physiological systems and diseases have been investigated. γδ T cells are preferentially located at mucosal surfaces, such as the gut, although a small subset of γδ T cells can circulate the blood. Additionally, a subset of γδ T cells reside in the meninges in the central nervous system. Recent findings suggest γδ T cells in the meninges have critical roles in brain function and homeostasis. In addition, several lines of evidence have shown γδ T cells can infiltrate the brain parenchyma and regulate inflammatory responses in multiple diseases, including neurodegenerative diseases. Although the importance of γδ T cells in the brain is well established, their roles are still incompletely understood due to the complexity of their biology. Because γδ T cells rapidly respond to changes in brain status and regulate disease progression, understanding the role of γδ T cells in the brain will provide critical information that is essential for interpreting neuroimmune modulation. In this review, we summarize the complex role of γδ T cells in the brain and discuss future directions for research.

γδ T Cells Kill Plasmodium falciparum in a Granzyme- and Granulysin-Dependent Mechanism during the Late Blood Stage

Plasmodium spp., the causative agent of malaria, have a complex life cycle. The exponential growth of the parasites during the blood stage is responsible for almost all malaria-associated morbidity and mortality. Therefore, tight immune control of the intraerythrocytic replication of the parasite is essential to prevent clinical malaria. Despite evidence that the particular lymphocyte subset of γδ T cells contributes to protective immunity during the blood stage in naive hosts, their precise inhibitory mechanisms remain unclear. Using human PBMCs, we confirmed in this study that γδ T cells specifically and massively expanded upon activation with Plasmodium falciparum culture supernatant. We also demonstrate that these activated cells gain cytolytic potential by upregulating cytotoxic effector proteins and IFN-γ. The killer cells bound to infected RBCs and killed intracellular P. falciparum via the transfer of the granzymes, which was mediated by granulysin in a stage-specific manner. Several vital plasmodial proteins were efficiently destroyed by granzyme B, suggesting proteolytic degradation of these proteins as essential in the lymphocyte-mediated death pathway. Overall, these data establish a granzyme- and granulysin-mediated innate immune mechanism exerted by γδ T cells to kill late-stage blood-residing P. falciparum.

Malawian children with uncomplicated and cerebral malaria have decreased activated Vγ9Vδ2 γδ T cells which increase in convalescence

Malaria is responsible for almost half a million deaths annually. The role of Vγ9Vδ2 γδ T cells in malaria is still unclear. Studies have reported an association between this cell subset and malaria symptoms and severity. Profiles of Vγ9Vδ2 γδ T cells in bigger cohorts with different levels of clinical severity have not been described. Proportion, numbers, and activation status of Vγ9Vδ2 γδ T cells were measured by flow cytometry in 59 healthy controls (HCs), 58 children with uncomplicated malaria (UM) and 67 with cerebral malaria (CM,) during acute malaria and in convalescence 28 days later. Vγ9Vδ2 γδ T cell were lower in children presenting with UM and CM than in HCs. Cell counts did not vary with malaria severity (CM median counts 40 x 10³ cells/μL, IQR [23–103]; UM median counts 30 x 10³ cells/μL [10–90], P = 0.224). Vγ9Vδ2 γδ T cell counts increased during convalescence for UM (70 [40–60] x 10³ cells/μL and CM (90 [60–140] x 10³ cells/μL), to levels similar to those in HCs (70 [50–140] x 10³ cells/μL), p = 0.70 and p = 0.40 respectively. Expression of the activation markers CD69 and HLA-DR on Vγ9Vδ2 γδ T cells was higher in malaria cases than in controls (HCs vs UM or CM, p < 0.0001) but was similar between UM and CM. HLA-DR expression remained elevated at 28 days, suggesting sustained activation of Vγ9Vδ2 γδ T cells during recovery. Vγ9Vδ2 γδ T cell proportions and cells counts were suppressed in acute disease and normalized in convalescence, a phenomenon previously hypothesized to be due to transient migration of the cells to secondary lymphoid tissue. The presence of highly activated Vγ9Vδ2 γδ T cells suggests that this T cell subset plays a specific role in response to malaria infection.

Resistance and Susceptibility to Malarial Infection: A Host Defense Strategy against Malaria

BACKGROUND

In an effort to understand what limits the virulence of malaria parasites in relation to the host genetic and immunogenic background, we investigated the possibility that the parasite and host genotype crossover interactions constrain virulence.

METHODS

Two groups of mice from different genotypes were used (C57BL/6 (B6) and DBA/2 mice). The mice were infected with a virulent parasite line Plasmodium yoelii 17XL (P. yoelii 17XL). Parasitemia, hematocrit value and lymphocytes yielded by livers and spleens were evaluated. Fluorescence Activated Cell Sorting (FACS) analysis illustrated phenotypic characterization of lymphocytes.

RESULTS

Infection with P. yoelii 17XL did not result in the death of DBA/2 mice. In contrast, B6 mice developed significantly high parasitemia and succumbed to death. Using (FACS) analysis, DBA/2 mice were found to experience a marked expansion of interleukin (IL)-2Rβ(+) CD3(int) cells and γδ T cells in the liver, especially in the recovery phase. The expansion of unconventional T cells (i.e. B220(+) T cells) was also marked in DBA/2 mice.

CONCLUSION

The outcome of murine malaria infections depends on the dynamic interplay between the immune-mediator and the genotype of the host.

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.

A new hypothesis on the manifestation of cerebral malaria: the secret is in the liver

Despite the abundance of information on cerebral malaria (CM), the pathogenesis of this disease is not completely understood. At present, two nonexclusive dominant hypotheses exist to explain how the neurological syndrome manifests: the sequestration (or mechanical) hypothesis and the inflammatory hypothesis. The sequestration hypothesis states that sequestration of Plasmodium falciparum-parasitized red blood cells (pRBCs) to brain capillary endothelia causes obstruction of capillary blood flow followed by brain tissue anoxia and coma. The inflammatory hypothesis postulates that P. falciparum infection releases toxic molecules in the circulation, inducing an imbalanced systemic inflammatory response that leads to coagulopathy, brain endothelial cell dysfunction, accumulation of leukocytes in the brain microcirculation, blood brain barrier (BBB) leakage, cerebral vasoconstriction, edema, and coma. However, both hypotheses, even when considered together, are not sufficient to fully explain the pathogenesis of CM. Here, we propose that the development of acute liver failure (ALF) together with BBB breakdown are the necessary and sufficient conditions for the genesis of CM. ALF is characterized by coagulopathy and hepatic encephalopathy (HE) in a patient without pre-existing liver disease. Signs of hepatic dysfunction have been shown to occur in 2.5-40% of CM patients. In addition, recent studies with murine models demonstrated that mice presenting experimental cerebral malaria (ECM) had hepatic damage and brain metabolic changes characteristic of HE. However, the occurrence of CM in patients with mild or without apparent hepatocellular liver damage and the presence of liver damage in non-CM murine models indicate that the development of ALF during malaria infection is not the single factor responsible for neuropathology. To solve this problem, we also propose that BBB breakdown contributes to the pathogenesis of CM and synergizes with hepatic failure to cause neurological signs and symptoms. BBB dysfunction would thus occur in CM by a mechanism similar to the one occurring in sepsis and is in agreement with the inflammatory hypothesis. Nevertheless, differently from in the inflammatory hypothesis, BBB leakage would facilitate the penetration of ammonia and other toxins into the brain parenchyma, but would not be sufficient to cause CM when occurring alone. We believe our hypothesis better explains the pathogenesis of CM, does not have problems to deal with the exception data not explained by the previous hypotheses, and reveals new targets for adjunctive therapy.

Plasmodium berghei ANKA infection increases Foxp3, IL-10 and IL-2 in CXCL-10 deficient C57BL/6 mice

Background

Cerebral malaria (CM) is a major cause of malaria mortality. Sequestration of infected red blood cells and leukocytes in brain vessels coupled with the production of pro-inflammatory factors contribute to CM. CXCL-10 a chemokine that is chemotactic to T cells has been linked to fatal CM. Mice deficient for CXCL-10 gene are resistant to murine CM, while antibody ablation of CXCL-10 enhanced the production of regulatory T cells (CD4+Cd25+Foxp3+) and IL-10 which regulate the immune system. Interleukin-2 (IL-2), a pro-inflammatory cytokine implicated in malaria pathogenesis has also been shown to be a key regulator of Foxp3. However the role of Foxp3 in resistant murine CM is not well understood.

Methods

The hypothesis that resistance of CXCL-10-/- mice to murine CM may be due to enhanced expression of Foxp3 in concert with IL-10 and IL-2 was tested. CXCL-10-/- and WT C57BL/6 mice were infected with Plasmodium berghei ANKA and evaluated for CM symptoms. Brain, peripheral blood mononuclear cells (PBMCs) and plasma were harvested from infected and uninfected mice at days 2, 4 and 8. Regulatory T cells (CD4+CD25+) and non-T regs (CD4+CD25-) were isolated from PBMCs and cultured with P. berghei antigens in vitro with dendritic cells as antigen presenting cells. Regulatory T cell transcription and specific factor Foxp3, was evaluated in mouse brain and PBMCs by realtime-PCR and Western blots while IL-10, and IL-2 were evaluated in plasma and cultured supernatants by ELISA.

Results

Wild type mice exhibited severe murine CM symptoms compared with CXCL-10-/- mice. Foxp3 mRNA and protein in brain and PBMC's of CXCL-10-/- mice was significantly up-regulated (p < 0.05) by day 4 post-infection (p.i) compared with WT. Plasma levels of IL-10 and IL-2 in infected CXCL-10-/- were higher than in WT mice (p < 0.05) at days 2 and 4 p.i. Ex-vivo CD4+CD25+ T cells from CXCL-10-/- re-stimulated with P. berghei antigens produced more IL-10 than WT CD4+CD25+ T cells.

Conclusion

The results indicate that in the absence of CXCL-10, the resulting up-regulation of Foxp3, IL-10 and IL-2 may be involved in attenuating fatal murine CM.

Interferon-γ--central mediator of protective immune responses against the pre-erythrocytic and blood stage of malaria

Immune responses against Plasmodium parasites, the causative organisms of malaria, are traditionally dichotomized into pre-erythrocytic and blood-stage components. Whereas the central role of cellular responses in pre-erythrocytic immunity is well established, protection against blood-stage parasites has generally been ascribed to humoral responses. A number of recent studies, however, have highlighted the existence of cellular immunity against blood-stage parasites, in particular, the prominence of IFN-γ production. Here, we have undertaken to chart the contribution of this prototypical cellular cytokine to immunity against pre-erythrocytic and blood-stage parasites. We summarize the various antiparasitic effector functions that IFN-γ serves to induce, review an array of data about its protective effects, and scrutinize evidence for any deleterious, immunopathological outcome in malaria patients. We discuss the activation and contribution of different cellular sources of IFN-γ production during malaria infection and its regulation in relation to exposure. We conclude that IFN-γ forms a central mediator of protective immune responses against pre-erythrocytic and blood-stage malaria parasites and identify a number of implications for rational malaria vaccine development.

CD4(+) T cell response in early erythrocytic stage malaria: Plasmodium berghei infection in BALB/c and C57BL/6 mice

Plasmodium berghei ANKA causes lethal malaria in mice. It is well established that C57BL/6 mice die early with fulminant symptoms including convulsion, whereas BALB/c mice survive this phase and die later of anemia and prostration. Early death in C57BL/6 mice has been considered to result from the adverse effects of inflammatory cytokines. To elucidate the CD4(+) T cell responses in early death due to severe malaria, the kinetics of CD4(+) T cells were compared by analyzing cell surface markers and the production of cytokines and transcription factors. The results revealed that cytokine production by CD4(+) T cells was induced as early as 5 days after infection and the maintenance of higher levels of IL-4 and IL-10 may be associated with the protection of BALB/c mice from early death. These results suggest that parasite control in the early phase of infection may be important for the development of an effective vaccine.

Scanning electron microscopy of the neuropathology of murine cerebral malaria

Background

The mechanisms leading to death and functional impairments due to cerebral malaria (CM) are yet not fully understood. Most of the knowledge about the pathomechanisms of CM originates from studies in animal models. Though extensive histopathological studies of the murine brain during CM are existing, alterations have not been visualized by scanning electron microscopy (SEM) so far. The present study investigates the neuropathological features of murine CM by applying SEM.

Methods

C57BL/6J mice were infected with Plasmodium berghei ANKA blood stages. When typical symptoms of CM developed perfused brains were processed for SEM or light microscopy, respectively.

Results

Ultrastructural hallmarks were disruption of vessel walls, parenchymal haemorrhage, leukocyte sequestration to the endothelium, and diapedesis of macrophages and lymphocytes into the Virchow-Robin space. Villous appearance of observed lymphocytes were indicative of activated state. Cerebral oedema was evidenced by enlargement of perivascular spaces.

Conclusion

The results of the present study corroborate the current understanding of CM pathophysiology, further support the prominent role of the local immune system in the neuropathology of CM and might expose new perspectives for further interventional studies.

Reasons why DBA/2 mice are resistant to malarial infection: expansion of CD3int B220+ gammadelta T cells with double-negative CD4- CD8- phenotype in the liver

DBA/2 (H-2(d)) mice are known to be more resistant than C57BL/6 (B6, H-2(b)) mice to the non-lethal 17XNL strain of Plasmodium yoelii. This is a very strange phenomenon because the functions of conventional T cells, especially CD8(+) T cells, are known to be somewhat lower in DBA/2 mice than in other strains of mice. We examined herein how immune responses differed between DBA/2 mice and B6 mice during malarial infection. DBA/2 mice and (DBA/2 x B6)F(1) (BDF(1), H-2(b/d)) mice were found to have milder parasitaemia and to recover more quickly from malarial infection than B6 mice. These DBA/2 and BDF(1) mice were also found to experience a marked expansion of interleukin (IL)-2Rbeta(+) CD3(int) cells and gammadelta T cells in the liver, especially in the recovery phase. The expansion of unconventional T cells (i.e. B220(+) T cells) was also marked in DBA/2 and BDF(1) mice. The majority of B220(+) T cells were gammadelta T cells and these T cells were double-negative CD4(-) CD8(-). More importantly, the production of immunoglobulin M (IgM)-type anti-DNA autoantibody was also higher in DBA/2 and BDF(1) mice than in B6 mice. In conjunction with data on cytokine production, these results indicate that primitive T and B cells, namely autoreactive extrathymic T cells and autoantibody-producing B cells, may be much more activated in DBA/2 mice and therefore resistant to the non-lethal 17XNL strain of P. yoelii.

Cell-mediated effector molecules and complicated malaria

In this review I attempt to advance hypotheses that might help contribute toward understanding the molecular pathogenesis of cerebral malaria (CM) and other complications based on a now widely accepted argument that the illness and pathology occasioned by Plasmodiumfalciparum infection might not necessarily be due to the direct effects of the parasite's 'toxins' and/or exoantigens or even its sequestration and consequent attendant effects in vital organs but rather to the parasite's mediated production of microbicidal molecules by the host. Tumor necrosis factor (TNF)-alpha is implicated in the pathogenesis of complicated malaria. There is a positive correlation between high levels of TNF-alpha and severity of malaria. The role of nitric oxide in the pathophysiology of complicated malaria is not clearly understood. Mononuclear phagocytes by virtue of their capacity to secrete toxic intermediates like reactive oxygen intermediates can inhibit the growth of both murine and human plasmodia. The role of interleukin-10 (IL-10) in malaria is also not well characterized to date. IL-10 is a powerful immunosuppressor factor. It acts as a natural dampener of immunoproliferative and inflammatory responses. Although transforming growth factor-beta has a crucial role in inflammation and repair, its role in complicated malaria is not too clearly understood. Furthermore, the anatomical source of these microbicidal molecules is not precisely known. The role of immune complexes (IC) in the pathophysiology of complicated malaria has hitherto not been tested. I argue here that IC play a critical role in influencing the outcome of malarial disease; IC-mediated stimulation of leukocytes to produce high levels of both TNF-alpha and NO and the fact that leukocytes are probably the principal anatomical source of these microbicidal and other pro-inflammatory mediators in complicated malaria provide a much more plausible explanation for the pathogenesis of CM and other complications. I also review the arguments that help contribute to rationalize hypoglycemia and hyperlactatemia in malarial disease and to some extent severe anemia. I am therefore tempted to conclude that CM and other complications are probably immune-mediated diseases or, at least, they present an inflammatory pathogenesis.

Murine γδ T cells in infections: beneficial or deleterious?

Although the importance of gammadelta T cells in pathogen-induced immune responses is becoming increasingly apparent, it is not clear that their involvement is always of benefit to the host. Here we review evidence for the protective and damaging roles of gammadelta T cells in infection and discuss how these disparate findings might be resolved by considering the nature and properties of the pathogen, the sites of infection and conditions under which gammadelta T cell responses are initiated, and the involvement of different subsets of gammadelta T cells.

Malaria Blood Stage Parasites Activate Human Plasmacytoid Dendritic Cells and Murine Dendritic Cells through a Toll-Like Receptor 9-Dependent Pathway

A common feature of severe Plasmodium falciparum infection is the increased systemic release of proinflammatory cytokines that contributes to the pathogenesis of malaria. Using human blood, we found that blood stage schizonts or soluble schizont extracts activated plasmacytoid dendritic cells (PDCs) to up-regulate CD86 expression and produce IFN-alpha. IFN-alpha production was also detected in malaria-infected patients, but the levels of circulating PDCs were markedly reduced, possibly because of schizont-stimulated up-regulation of CCR7, which is critical for PDC migration. The schizont-stimulated PDCs elicited a poor T cell response, but promoted gamma delta T cell proliferation and IFN-gamma production. The schizont immune stimulatory effects could be reproduced using murine DCs and required the Toll-like receptor 9 (TLR9)-MyD88 signaling pathway. Although the only known TLR9 ligand is CpG motifs in pathogen DNA, the activity of the soluble schizont extract was far greater than that of schizont DNA, and it was heat labile and precipitable with ammonium sulfate, unlike the activity of bacterial DNA. These results demonstrate that schizont extracts contain a novel and previously unknown ligand for TLR9 and suggest that the stimulatory effects of this ligand on PDCs may play a key role in immunoregulation and immunopathogenesis of human falciparum malaria.

Differential requirements for proliferation of CD4+ and γδ+ T cells to spirochetal antigens

Alphabeta+ and gammadelta+ T cells have different mechanisms of epitope recognition and are stimulated by antigens of different chemical nature. An immunization model with antigens from the spirochete Brachyspira hyodysenteriae was used to examine the requirements for proliferation of circulating porcine CD4+ and gammadelta+ T cells in mixed lymphocyte cultures. CD4+ T cells only responded to stimulation with B. hyodysenteriae antigens, whereas gammadelta+ T cells proliferated when cultures were stimulated with either spirochetal antigens or interleukin-2 (IL-2). T cells that had proliferated expressed high levels of IL-2-receptor-alpha (IL-2Ralpha). Furthermore, neutralization of IL-2 at the beginning of the culture period was more efficient in blocking gammadelta+ than CD4+ T cell proliferation. Immunization induced interferon-gamma (IFN-gamma) production by CD4+ T cells, whereas only a small fraction of the antigen-stimulated gammadelta+ T cells produced this cytokine. Our results indicate that, under the same environmental conditions, CD4+ T cell functions are more tightly regulated when compared to gammadelta+ T cells. We conclude that these differences are due, in part, to the enhanced gammadelta+ T cell responsiveness to IL-2.

Cerebral malaria: the contribution of studies in animal models to our understanding of immunopathogenesis

Cerebral malaria is a serious and often fatal complication of Plasmodium falciparum infections. The precise mechanisms involved in the onset of neuropathology remain unknown, but parasite sequestration in the brain, metabolic disturbances and host immune responses are all thought to be involved. This review outlines the current state of knowledge of cerebral disease in humans, and discusses the contribution of studies of animal models to elucidation of the underlying mechanisms.

The influence of gammadelta T cells on the CD4+ T cell and antibody response during a primary Plasmodium chabaudi chabaudi infection in mice

A primary infection with Plasmodium chabaudi chabaudi (AS) is characterized by an expansion of gammadelta cells after the acute phase of infection in mice. This is particularly marked during chronic infections in B cell-deficient mice. Infections in gammadelta T cell-deficient mice suggest that, although these cells play some role in the control of parasitaemia and can produce interferon-gamma, they do not appear to be involved in the development of hypoglycaemia, loss of weight and temperature during a P. c. chabaudi infection. However, gammadelta T cells do influence the nature of the CD4+ T cell response during infection since, in their absence, Th2-like responses, such as interleukin (IL)-4 production and help for malaria-specific antibody responses, are more pronounced. This alteration in CD4+ T cells is reflected in a more rapid and greater immunoglobulin (Ig)G1 and IgG3 antibody response to the parasite. The large gammadelta T cell expansion normally observed in infected B cell-deficient mice did not take place in the absence of IL-2, and double-knockout mice lacking both B cells and functional IL-2 were highly susceptible to lethal infection with P. c. chabaudi. The majority of the single IL-2 knockout mice, in contrast, were able to control and clear a primary infection, suggesting that for the CD4+ T cell and antibody response, IL-2 could be replaced by other cytokines.