Activation of gammadelta T cells in malaria: interaction of cytokines and a schizont-associated Plasmodium falciparum antigen

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

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

Emerging role of γδ T cells in protozoan infection and their potential clinical application

γδ T cells are thymus derived heterogeneous and unconventional T- lymphocyte expressing TCR γ (V γ9) and TCRδ (Vδ2) chain and play an important role in connecting innate and adaptive armaments of immune response. These cells can recognize wide ranges of antigens even without involvement of major histocompatibility complex and exert their biological functions by cytotoxicity or activating various types of immune cells. In recent past, γδ T cells have emerged as an important player during protozoa infection and rapidly expand after exposure with them. They have also been widely studied in vaccine induced immune response against many bacterial and protozoan infections with improved clinical outcome. In this review, we will discuss the various roles of γδ T cells in immunity against malaria and leishmaniasis, the two important protozoan diseases causing significant mortality and morbidity throughout the world.

γδ T cells suppress Plasmodium falciparum blood-stage infection by direct killing and phagocytosis

Activated Vγ9Vδ2 (γδ2) T lymphocytes that sense parasite-produced phosphoantigens are expanded in Plasmodium falciparum-infected patients. Although previous studies suggested that γδ2 T cells help control erythrocytic malaria, whether γδ2 T cells recognize infected red blood cells (iRBCs) was uncertain. Here we show that iRBCs stained for the phosphoantigen sensor butyrophilin 3A1 (BTN3A1). γδ2 T cells formed immune synapses and lysed iRBCs in a contact, phosphoantigen, BTN3A1 and degranulation-dependent manner, killing intracellular parasites. Granulysin released into the synapse lysed iRBCs and delivered death-inducing granzymes to the parasite. All intra-erythrocytic parasites were susceptible, but schizonts were most sensitive. A second protective γδ2 T cell mechanism was identified. In the presence of patient serum, γδ2 T cells phagocytosed and degraded opsonized iRBCs in a CD16-dependent manner, decreasing parasite multiplication. Thus, γδ2 T cells have two ways to control blood-stage malaria-γδ T cell antigen receptor (TCR)-mediated degranulation and phagocytosis of antibody-coated iRBCs.

T cell-mediated immunity to malaria

Immunity to malaria has been linked to the availability and function of helper CD4⁺ T cells, cytotoxic CD8⁺ T cells and γδ T cells that can respond to both the asymptomatic liver stage and the symptomatic blood stage of Plasmodium sp. infection. These T cell responses are also thought to be modulated by regulatory T cells. However, the precise mechanisms governing the development and function of Plasmodium-specific T cells and their capacity to form tissue-resident and long-lived memory populations are less well understood. The field has arrived at a point where the push for vaccines that exploit T cell-mediated immunity to malaria has made it imperative to define and reconcile the mechanisms that regulate the development and functions of Plasmodium-specific T cells. Here, we review our current understanding of the mechanisms by which T cell subsets orchestrate host resistance to Plasmodium infection on the basis of observational and mechanistic studies in humans, non-human primates and rodent models. We also examine the potential of new experimental strategies and human infection systems to inform a new generation of approaches to harness T cell responses against malaria.

Metformin Promotes the Protection of Mice Infected With Plasmodium yoelii Independently of γδ T Cell Expansion

Adaptive immune responses are critical for protection against infection with Plasmodium parasites. The metabolic state dramatically changes in T cells during activation and the memory phase. Recent findings suggest that metformin, a medication for treating type-II diabetes, enhances T-cell immune responses by modulating lymphocyte metabolism. In this study, we investigated whether metformin could enhance anti-malaria immunity. Mice were infected with Plasmodium yoelii and administered metformin. Levels of parasitemia were reduced in treated mice compared with those in untreated mice, starting at ~2 weeks post-infection. The number of γδ T cells dramatically increased in the spleens of treated mice compared with that in untreated mice during the later phase of infection, while that of αβ T cells did not. The proportions of Vγ1⁺ and Vγ2⁺ γδ T cells increased, suggesting that activated cells were selectively expanded. However, these γδ T cells expressed inhibitory receptors and had severe defects in cytokine production, suggesting that they were in a state of exhaustion. Metformin was unable to rescue the cells from exhaustion at this stage. Depletion of γδ T cells with antibody treatment did not affect the reduction of parasitemia in metformin-treated mice, suggesting that the effect of metformin on the reduction of parasitemia was independent of γδ T cells.

Human Vγ9Vδ2 T Lymphocytes in the Immune Response to P. falciparum Infection

Malaria is an infectious disease caused by the protozoan parasite Plasmodium sp, the most lethal being Plasmodium falciparum. Clinical malaria is associated with the asexual replication cycle of Plasmodium parasites inside the red blood cells (RBCs) and a dysregulated immune response. Although the mechanisms of immune responses to blood—or liver-stage parasites have been extensively studied, this has not led to satisfactory leads for vaccine design. Among innate immune cells responding to infection are the non-conventional gamma-delta T-cells. The Vγ9Vδ2 T-cell subset, found only in primates, is activated in response to non-peptidic phosphoantigens produced by stressed mammalian cells or by microorganisms such as Mycobacteria, E.coli, and Plasmodium. The potential protective role of Vγ9Vδ2 T-cells against infections and cancer progression is of current research interest. Vγ9Vδ2 T-cells have been shown to play a role in the early control of P. falciparum parasitemia and to influence malaria adaptive immunity via cytokine release and antigen presentation. They are activated and expanded during a primary P. falciparum infection in response to malaria phosphoantigens and their activity is modulated upon subsequent infections. Here, we review the wide range of functions by which Vγ9Vδ2 T-cells could both contribute to and protect from malaria pathology, with a particular focus on their ability to induce both innate and adaptive responses. We discuss how the multifunctional roles of these T-cells could open new perspectives on gamma-delta T-cell-based interventions to prevent or cure malaria.

Exposure to Plasmodium vivax is associated with the increased expression of exhaustion markers on γδ T lymphocytes

Gamma delta (γδ) T cells exhibit potent anti-Plasmodium activity but are also implicated in the immunopathology of malaria. It is currently poorly understood how γδ T cells are affected in human suffering from Plasmodium vivax infection or in symptomless individuals living in an endemic region. We examined both the percentages and expression of markers associated with immune exhaustion in γδ T cells in individuals living in a P. vivax endemic region by flow cytometry. The percentage of γδ T cells in the blood was significantly higher both in acute P. vivax-positive patients and in individuals from an endemic region in comparison with control uninfected adults. The frequency of the expression of the exhaustion markers-Tim-3, Lag-3, CTLA-4 and PD-1 was higher in γδ and total T cells from P. vivax-infected patients than in those populations from control uninfected adults. Individuals from a P. vivax endemic region showed elevated percentages of Tim-3-, Lag-3- and CTLA-4-positive γδ T cells and an increased percentage of Tim-3-positive total T cells. The phenotypic exhaustion of these cells might be a protective mechanism preventing the immunopathology associated with activated T cells and may provide a rationale for targeted manipulation of this process in diseases such as malaria.

γδ T Cells in Antimalarial Immunity: New Insights Into Their Diverse Functions in Protection and Tolerance

Uniquely expressing diverse innate-like and adaptive-like functions, γδ T cells exist as specialized subsets, but are also able to adapt in response to environmental cues. These cells have long been known to rapidly proliferate following primary malaria infection in humans and mice, but exciting new work is shedding light into their diverse functions in protection and following repeated malaria infection. In this review, we examine the current knowledge of functional specialization of γδ T cells in malaria, and the mechanisms dictating recognition of malaria parasites and resulting proliferation. We discuss γδ T cell plasticity, including changing interactions with other immune cells during recurrent infection and potential for immunological memory in response to repeated stimulation. Building on recent insights from human and murine experimental studies and vaccine trials, we propose areas for future research, as well as applications for therapeutic development.

Vγ9Vδ2 T cells proliferate in response to phosphoantigens released from erythrocytes infected with asexual and gametocyte stage Plasmodium falciparum

Vγ9Vδ2 T cells, the dominant γδ T cell subset in human peripheral blood, are stimulated by phosphoantigens, of which (E)-4-Hydroxy-3-methyl-but-2-enyl pyrophosphate, is produced in the apicoplast of malaria parasites. Cell-free media from synchronised Plasmodium falciparum asexual ring, trophozoite, and schizont stage-cultures of high purity as well as media from ruptured schizont cultures, all stimulated Vγ9Vδ2 T cell proliferation, as did media from pure gametocyte cultures, whereas media from uninfected erythrocytes cultures did not. The media from ruptured schizont cultures and all the asexual and gametocyte stage cultures contained only background iron levels, suggesting that all erythrocyte haemoglobin is consumed as the parasites develop and supporting that the phosphoantigens were released from intact parasitized erythrocytes. The Vγ9Vδ2 T cell-stimulating agent was not affected by freezing, thawing or heating but was sensitive to phosphatase treatment, confirming its phosphoantigen identity. In summary, phosphoantigens are released from parasitised erythrocytes at all developmental blood stages.

Plasmodium falciparum PfEMP1 Modulates Monocyte/Macrophage Transcription Factor Activation and Cytokine and Chemokine Responses

Immunity to Plasmodium falciparum malaria is slow to develop, and it is often asserted that malaria suppresses host immunity, although this is poorly understood and the molecular basis for such activity remains unknown. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is a virulence factor that plays a key role in parasite-host interactions. We investigated the immunosuppressive effect of PfEMP1 on monocytes/macrophages, which are central to the antiparasitic innate response. RAW macrophages and human primary monocytes were stimulated with wild-type 3D7 or CS2 parasites or transgenic PfEMP1-null parasites. To study the immunomodulatory effect of PfEMP1, transcription factor activation and cytokine and chemokine responses were measured. The level of activation of NF-κB was significantly lower in macrophages stimulated with parasites that express PfEMP1 at the red blood cell surface membrane than in macrophages stimulated with PfEMP1-null parasites. Modulation of additional transcription factors, including CREB, also occurred, resulting in reduced immune gene expression and decreased tumor necrosis factor (TNF) and interleukin-10 (IL-10) release. Similarly, human monocytes released less IL-1β, IL-6, IL-10, monocyte chemoattractant protein 1 (MCP-1), macrophage inflammatory protein 1α (MIP-1α), MIP-1β, and TNF specifically in response to VAR2CSA PfEMP1-containing parasites than in response to PfEMP1-null parasites, suggesting that this immune regulation by PfEMP1 is important in naturally occurring infections. These results indicate that PfEMP1 is an immunomodulatory molecule that affects the activation of a range of transcription factors, dampening cytokine and chemokine responses. Therefore, these findings describe a potential molecular basis for immune suppression by P. falciparum.

A Unique Subset of γδ T Cells Expands and Produces IL-10 in Patients with Naturally Acquired Immunity against Falciparum Malaria

Although expansions in γδ T cell populations are known to occur in the peripheral blood of patients infected with Plasmodium falciparum, the role of these cells in people with naturally acquired immunity against P. falciparum who live in malaria-endemic areas is poorly understood. We used a cross-sectional survey to investigate the role of peripheral blood γδ T cells in people living in Lao People's Democratic Republic, a malaria-endemic area. We found that the proportion of non-Vγ9 γδ T cells was higher in non-hospitalized uncomplicated falciparum malaria patients (UMPs) from this region. Notably, we found that the non-Vγ9 γδ T cells in the peripheral blood of UMPs and negative controls from this region had the potential to expand and produce IL-10 and interferon-γ when cultured in the presence of IL-2 and/or crude P. falciparum antigens for 10 days. Furthermore, these cells were associated with plasma interleukin 10 (IL-10), which was elevated in UMPs. This is the first report demonstrating that, in UMPs living in a malaria-endemic area, a γδ T cell subset, the non-Vγ9 γδT cells, expands and produces IL-10. These results contribute to understanding of the mechanisms of naturally acquired immunity against P. falciparum.

Phosphoantigen Burst upon Plasmodium falciparum Schizont Rupture Can Distantly Activate Vγ9Vδ2 T Cells

Malaria induces potent activation and expansion of the Vγ9Vδ2 subpopulation of γδT cells, which inhibit the Plasmodium falciparum blood cycle through soluble cytotoxic mediators, abrogating merozoite invasion capacity. Intraerythrocytic stages efficiently trigger Vγ9Vδ2 T-cell activation and degranulation through poorly understood mechanisms. P. falciparum blood-stage extracts are known to contain phosphoantigens able to stimulate Vγ9Vδ2 T cells, but how these are presented by intact infected red blood cells (iRBCs) remains elusive. Here we show that, unlike activation by phosphoantigen-expressing cells, Vγ9Vδ2 T-cell activation by intact iRBCs is independent of butyrophilin expression by the iRBC, and contact with an intact iRBC is not required. Moreover, blood-stage culture supernatants proved to be as potent activators of Vγ9Vδ2 T cells as iRBCs. Bioactivity in the microenvironment is attributable to phosphoantigens, as it is dependent on the parasite DOXP pathway, on Vγ9Vδ2 TCR signaling, and on butyrophilin expression by Vγ9Vδ2 T cells. Kinetic studies showed that the phosphoantigens were released at the end of the intraerythrocytic cycle at the time of parasite egress. We document exquisite sensitivity of Vγ9Vδ2 T cells, which respond to a few thousand parasites. These data unravel a novel framework, whereby release of phosphoantigens into the extracellular milieu by sequestered parasites likely promotes activation of distant Vγ9Vδ2 T cells that in turn exert remote antiparasitic functions.

γδ T Cell-Mediated Immune Responses in Disease and Therapy

The role of γδ T cells in immunotherapy has gained specific importance in the recent years because of their prominent function involving directly or indirectly in the rehabilitation of the diseases. γδ T cells represent a minor population of T cells that express a distinct T cell receptor (TCR) composed of γδ chains instead of αβ chains. Unlike αβ T cells, γδ T cells display a restricted TCR repertoire and recognize mostly unknown non-peptide antigens. γδ T cells act as a link between innate and adaptive immunity, because they lack precise major histocompatibility complex (MHC) restriction and seize the ability to recognize ligands that are generated during affliction. Skin epidermal γδ T cells recognize antigen expressed by damaged or stressed keratinocytes and play an indispensable role in tissue homeostasis and repair through secretion of distinct growth factors. γδ T cell based immunotherapy strategies possess great prominence in the treatment because of the property of their MHC-independent cytotoxicity, copious amount of cytokine release, and a immediate response in infections. Understanding the role of γδ T cells in pathogenic infections, wound healing, autoimmune diseases, and cancer might provide knowledge for the successful treatment of these diseases using γδ T cell based immunotherapy. Enhancing the human Vγ9Vδ2 T cells functions by administration of aminobisphosphonates like zoledronate, pamidronate, and bromohydrin pyrophosphate along with cytokines and monoclonal antibodies shows a hopeful approach for treatment of tumors and infections. The current review summarizes the role of γδ T cells in various human diseases and immunotherapeutic approaches using γδ T cells.

The protective effect of CD40 ligand-CD40 signalling is limited during the early phase of Plasmodium infection

γδ T cells are essential for eliminating Plasmodium berghei XAT. Because administration of the agonistic anti-CD40 antibody can induce elimination of P. berghei XAT parasites in γδ T cell-deficient mice, we considered that γδ T cells might activate dendritic cells via CD40 signalling during infection. Here we report that administration of the anti-CD40 antibody to γδ T cell-deficient mice 3-10 days post-P. berghei XAT infection could eliminate the parasites. Our data suggest that dendritic cell activation via γδ T cells expressing CD40 ligand is critical during the early phase of infection.

Roles of IFN-γ and γδ T Cells in Protective Immunity Against Blood-Stage Malaria

Malaria is caused by infection with Plasmodium parasites. Various studies with knockout mice have indicated that IFN-γ plays essential roles in protective immunity against blood-stage Plasmodium infection. However, after Plasmodium infection, increased IFN-γ production by various types of cells is involved not only in protective immunity, but also in immunopathology. Recent reports have shown that IFN-γ acts as a pro-inflammatory cytokine to induce not only the activation of macrophages, but also the generation of uncommon myelolymphoid progenitor cells after Plasmodium infection. However, the effects of IFN-γ on hematopoietic stem cells and progenitor cells are unclear. Therefore, the regulation of hematopoiesis by IFN-γ during Plasmodium infection remains to be clarified. Although there are conflicting reports concerning the significance of γδ T cells in protective immunity against Plasmodium infection, γδ T cells may respond to infection and produce IFN-γ as innate immune cells in the early phase of blood-stage malaria. Our recent studies have shown that γδ T cells express CD40 ligand and produce IFN-γ after Plasmodium infection, resulting in the enhancement of dendritic cell activation as part of the immune response to eliminate Plasmodium parasites. These data suggest that the function of γδ T cells is similar to that of NK cells. Although several reports suggest that γδ T cells have the potential to act as memory cells for various infections, it remains to be determined whether memory γδ T cells are generated by Plasmodium infection and whether memory γδ T cells can contribute to the host defense against re-infection with Plasmodium. Here, we summarize and discuss the effects of IFN-γ and the various functions of γδ T cells in blood-stage Plasmodium infection.

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.

Experimental malaria infection triggers early expansion of natural killer cells

In order to gain a better understanding of gene expression during early malaria infection, we conducted microarray analysis of early blood responses in mice infected with erythrocytic-stage Plasmodium chabaudi. Immediately following infection, we observed coordinated and sequential waves of immune responses, with interferon-associated gene transcripts dominating by 16 h postinfection, followed by strong increases in natural killer (NK) cell-associated and major histocompatibility complex class I-related transcripts by 32 h postinfection. We showed by flow cytometry that the observed elevation in NK cell-associated transcripts was the result of a dramatic increase in the proportion of NK cells in the blood during infection. Subsequent microarray analysis of NK cells isolated from the peripheral blood of infected mice revealed a cell proliferation expression signature consistent with the observation that NK cells replicate in response to infection. Early proliferation of NK cells was directly observed in studies with adoptively transferred cells in infected mice. These data indicate that the early response to P. chabaudi infection of the blood is marked by a primary wave of interferon with a subsequent response by NK cells.

Plasmodium falciparum erythrocyte membrane protein-1 specifically suppresses early production of host interferon-gamma

Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) is a variable antigen expressed by P. falciparum, the malarial parasite. PfEMP-1, present on the surface of infected host erythrocytes, mediates erythrocyte binding to vascular endothelium, enabling the parasite to avoid splenic clearance. In addition, PfEMP-1 is proposed to regulate host immune responses via interactions with the CD36 receptor on antigen-presenting cells. We investigated the immunoregulatory function of PfEMP-1 by comparing host cell responses to erythrocytes infected with either wild-type parasites or transgenic parasites lacking PfEMP-1. We showed that PfEMP-1 suppresses the production of the cytokine interferon-gamma by human peripheral blood mononuclear cells early after exposure to P. falciparum. Suppression of this rapid proinflammatory response was CD36 independent and specific to interferon-gamma production by gammadelta-T, NK, and alphabeta-T cells. These data demonstrate a parasite strategy for downregulating the proinflammatory interferon-gamma response and further establish transgenic parasites lacking PfEMP-1 as powerful tools for elucidating PfEMP-1 functions.

Analysis of T-cell responses in malaria-exposed and non-exposed donors using Plasmodium falciparum asexual blood stages enriched by a simple centrifugation method

Several studies have reported on similar in vitro cellular responses to different malaria-antigen preparations in both malaria-primed and un-primed donors. Whether intact live parasites can exert a distinct type of response in either of the two groups is not well known. In this study, we developed a simple three-step centrifugation method for simultaneous enrichment of early and late blood stages from Plasmodium falciparum cultures. Such enriched P. falciparum fractions and other antigen preparations were used to stimulate lymphocytes from malaria-exposed and non-exposed individuals to examine the proliferative activity and expansion of CD3+, gammadelta+, CD4+, and CD8+ T cells. While lymphocytes from malaria non-exposed donors proliferated relatively higher than those from malaria-exposed donors in response to most antigens tested, the enriched fractions of live parasites exerted higher proliferative responses on cells from the latter donors. This suggests the existence of memory cells in the malaria-exposed donors, but not in the non-exposed ones. Flow cytometric analysis revealed a higher percentage expansion of CD4+ T cells in the responding cells of the exposed donors than the non-exposed ones. Taken together, this study reports on a simple method that simultaneously enriches for intact live early and late blood stages of P. falciparum parasites. Moreover, the study revealed higher expansion CD4+ T cells in the exposed individuals than the non-exposed in response to live malaria parasites and not to other parasite-antigen preparations.

Homing and memory patterns of human γδ T cells in physiopathological situations

Vgamma9Vdelta2 are a heterogeneous population of T cells and comprise distinct naive, memory and effector populations that can be distinguished on the basis of surface marker expression and effector functions. We review here these recently studied features of Vgamma9Vdelta2 T lymphocyte biology and the roles they play in infectious and autoimmune diseases.

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.

The paroxysm of Plasmodium vivax malaria

The paroxysms of Plasmodium vivax malaria are antiparasite responses that, although distressing to the human host, almost never impart serious acute pathology. Using plasma and blood cells from P. vivax patients, the cellular and noncellular mediators of these events have been studied ex vivo. The host response during a P. vivax paroxysm was found to involve T cells, monocytes and neutrophils, and the activity, among others, of the pyrogenic cytokines tumor necrosis factor alpha and interleukin 2 in addition to granulocyte macrophage-colony stimulating factor. However, interferon gamma activity, associated with serious acute pathogenesis in other studies on malaria, was absent. Induction of the cytokines active during a P. vivax paroxysm depends upon the presence of parasite products, which are released into the plasma before the paroxysm. Chemical identification of these natural parasite products will be important for our understanding of pathogenesis and protection in malaria.

Plasmodium falciparum and P. vivax gametocyte-specific exoantigens stimulate proliferation of TCR gammadelta+ lymphocytes

Immune modulation of Plasmodium vivax and P. falciparum gametocytes occurs over the course of erythrocytic infection. The response is linked to proliferative and inflammatory responses, which may be stimulated by stage-specific gametocyte proteins. Stage-specific exoantigens were purified from supernatants of P. falciparum and P. vivax gametocyte cultures, and either primary or secondary postinfection lymphocytes were stimulated for proliferation. Five of 25 exoantigens purified from P. falciparum gametocyte cultures and 6 of 28 exoantigens isolated from P. vivax were gametocyte stage specific. Metabolic labeling of soluble P. falciparum gametocyte proteins confirmed synthesis and secretion of 5 stage-specific exoantigens, with molecular masses of 118, 62, 52, 37, and 33 kDa. Purified gametocyte exoantigens within the range of 50 to 100 kDa stage-specifically stimulated proliferation of lymphocytes from postprimary P. falciparum infections, and from postprimary and secondary P. vivax infection patients with homologous purified exoantigens. T-cell receptor (TCR)gammadelta+, and CD3+ CD8+ and CD3+ CD4- CD8- T cells were specifically upregulated from P. falciparum primary- and P. vivax secondary-infection lymphocytes, respectively, using gametocyte stage-specific exoantigens. CD25+ was the major activation marker expressed by CD3+ and gammadelta T cells when stimulated with gametocyte exoantigens. None of the T cell markers was significantly upregulated using gametocyte stage-specific exoantigens with primary-infection P. vivax lymphocytes.

Postinjury malaria: a study of trauma victims in cambodia

BACKGROUND

The pattern of host defense against plasmodium is comparable to the immune response to bacterial infection. Posttraumatic immunosuppression may therefore cause relapses of malaria secondary to trauma and trauma surgery in asymptomatic carriers of the parasites in endemic areas. To our knowledge this has not been validated in epidemiologic studies.

METHODS

Postinjury malaria was registered retrospectively in 342 land mine and war victims from malaria-endemic areas in Cambodia. The incidence rate was analyzed in terms of age, gender, preinjury endemicity, evacuation times, anatomic injury severity, systolic blood pressure at admission, blood transfusion, and duration of the first surgical intervention as independent variables.

RESULTS

The rate of postinjury malaria in the study patients was 33.3% (95% CI, 28.3-38.3%). Injury Severity Score (ISS) and surgical operation time were risk factors (area under the curve in receiver operating characteristic plots were 0.73 and 0.79, respectively). The impact of the other risk factors was nonsignificant.

CONCLUSION

Despite difficulties in diagnosing postoperative malaria in endemic areas, the study demonstrates that the rate of postinjury malaria is high. The results legitimate controlled trials of immediate postinjury chemoprophylaxis to severely injured in endemic areas. The authors recommend staged surgical operations with brief primary interventions in victims with severe injuries.

Cellular basis of early cytokine response to Plasmodium falciparum

Uncertainty remains about the cellular origins of the earliest phase of the proinflammatory cytokine response to malaria. Here we show by fluorescence-activated cell sorter analysis that gammadelta T cells and CD14+ cells from nonimmune donors produce tumor necrosis factor and that gammadelta T cells also produce gamma interferon within 18 h of contact with mycoplasma-free Plasmodium falciparum-infected erythrocytes in vitro. This early cytokine response is more effectively induced by intact than by lysed parasitized erythrocytes. However, the IFN-gamma response to lysed parasites is considerably enhanced several days after peripheral blood mononuclear cells are primed with low numbers of intact parasitized erythrocytes, and in this case it derives from both alphabeta and gammadelta T cells. These data show that naïve gammadelta T cells can respond very rapidly to malaria infection but that malaria fever may involve a multistage process in which the priming of both gammadelta and alphabeta T-cell populations boosts the cytokine response to lysed parasite products released at schizont rupture.

V gamma 9V delta 2 T cells impair intracellular multiplication of Brucella suis in autologous monocytes through soluble factor release and contact-dependent cytotoxic effect

Human Vgamma9Vdelta2 T cells are considered to play an important role in brucellosis, as this population is dramatically increased in peripheral blood of patients during the acute phase of the infection. This T lymphocyte population has been largely demonstrated to be activated by small m.w. nonpeptidic molecules from natural or synthetic origin. We recently identified a nonpeptidic fraction of Brucella suis that specifically activates human Vgamma9Vdelta2 T cells. Using a two-separate-chambers system, we showed that Brucella fraction, as well as isopentenyl pyrophosphate-activated Vgamma9Vdelta2 T cells, impaired the multiplication of B. suis in differentiated THP-1 cells through TNF-alpha and IFN-gamma release. In the present study, using circulating Vgamma9Vdelta2 T cells and autologous monocytes infected with B. suis, we provide evidence that 1) intramonocytic multiplication of B. suis is impaired by supernatants of activated Vgamma9Vdelta2 T cells in part via TNF-alpha and IFN-gamma, this impairment occurring without host cell lysis; 2) unstimulated Vgamma9Vdelta2 T cells can impair intracellular bacterial multiplication after their activation by soluble factors released by infected monocytes; and 3) activated Vgamma9Vdelta2 T cells lyse Brucella-infected monocytes in a contact-dependent manner. Taken together, these results provide evidence that Vgamma9Vdelta2 T cells, in addition to being directly activated by soluble nonpeptidic molecules, can be stimulated to become highly cytotoxic in the specific presence of infected monocytes; moreover, they suggest how Vgamma9Vdelta2 T cells could be triggered and respond as antibacterial effector cells in the early stages of Brucella infection.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

Activation of human Vgamma9Vdelta2 T cells by a Brucella suis non-peptidic fraction impairs bacterial intracellular multiplication in monocytic infected cells

Human gamma delta T cells are considered to play an important role in the early response to infection with intracellular pathogens. Evidence has been presented that the percentage of gamma delta T cells with Vgamma9Vdelta2 phenotype is dramatically increased in the peripheral blood of patients with acute brucellosis. This specific gd T-cell subpopulation is known to be activated by small non-peptidic molecules that can either be produced by the pathogen itself or released from damaged cells after infection. In the present work we provide evidence that Vgamma9Vdelta2 T lymphocytes from peripheral blood mononuclear cells of healthy donors can be specifically activated by non-peptidic low-molecular-weight compound(s) from Brucella suis lysate. Moreover, we show that Vgamma9Vdelta2 T cells activated by this B. suis fraction produce tumour necrosis factor-alpha and interferon-gamma, which reduce bacterial multiplication inside infected cells.

Early cytokine induction by Plasmodium falciparum is not a classical endotoxin-like process

We have investigated the widely held view that malaria parasites induce pro-inflammatory cytokines primarily through an endotoxin-like stimulatory effect on macrophages. We report that the pattern of cytokine production by non-immune human peripheral blood mononuclear cells following stimulation by Plasmodium falciparum-infected erythrocytes (Pfe) in vitro differs considerably from that induced by bacterial endotoxin. The Pfe-induced TNF response at day 1 is associated with a much higher level of IFN-gamma production and a much lower level of IL-12 p40 and IL-10 expression than a comparable endotoxin-induced TNF response. Both CD3(+) and CD14(+) populations are required for this early TNF response to Pfe, whereas the endotoxin-induced response is unaffected by depletion of the CD3(+) population. Pfe fails to stimulate the monocyte-like cell line MonoMac6 to express pro-inflammatory cytokines. These findings suggest that the early inflammatory response to malaria is critically dependent on lymphocyte subpopulations that play a lesser role in the response to bacterial endotoxin.

γδ T Cells Contribute to Control of Chronic Parasitemia in Plasmodium chabaudi Infections in Mice

During a primary infection of mice with Plasmodium chabaudi, γδ T cells are stimulated and their expansion coincides with recovery from the acute phase of infection in normal mice or with chronic infections in B cell-deficient mice (μ-MT). To determine whether the large γδ T cell pool observed in female B cell-deficient mice is responsible for controlling the chronic infection, studies were done using double-knockout mice deficient in both B and γδ cells (μ-MT × δ−/−TCR) and in γδ T cell-depleted μ-MT mice. In both types of γδ T cell-deficient mice, the early parasitemia following the peak of infection was exacerbated, and the chronic parasitemia was maintained at significantly higher levels in the absence of γδ T cells. The majority of γδ T cells in C57BL/6 and μ-MT mice responding to infection belonged predominantly to a single family of γδ T cells with TCR composed of Vγ2Vδ4 chains and which produced IFN-γ rather than IL-4.

Gamma delta T-cell function in pathogenesis of cerebral malaria in mice infected with Plasmodium berghei ANKA

Mice depleted of gammadelta T cells by monoclonal antibody treatment and infected with Plasmodium berghei ANKA did not develop cerebral malaria (CM). In striking contrast, delta0/0 mice infected with P. berghei developed CM despite their gammadelta T-cell deficiency. gammadelta T cells appear to be essential for the pathogenesis of CM in mice having experienced normal ontogeny but not in mice genetically deprived of gammadelta T cells from the beginning of life.