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Query: UMLS:C0024530 (malaria)
 44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
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PMID:Malaria blood stage parasites activate human plasmacytoid dendritic cells and murine dendritic cells through a Toll-like receptor 9-dependent pathway. 1506 72

The glycosylphosphatidylinositol (GPI) anchors of Plasmodium falciparum have been proposed to be the major factors that contribute to malaria pathogenesis through their ability to induce proinflammatory responses. In this study we identified the receptors for P. falciparum GPI-induced cell signaling that leads to proinflammatory responses and studied the GPI structure-activity relationship. The data show that GPI signaling is mediated mainly through recognition by TLR2 and to a lesser extent by TLR4. The activity of sn-2-lyso-GPIs is comparable with that of the intact GPIs, whereas the activity of Man(3)-GPIs is about 80% that of the intact GPIs. The GPIs with three (intact GPIs and Man(3)-GPIs) and two fatty acids (sn-2-lyso-GPIs) appear to differ considerably in the requirement of the auxiliary receptor, TLR1 or TLR6, for recognition by TLR2. The former are preferentially recognized by TLR2/TLR1, whereas the latter are favored by TLR2/TLR6. However, the signaling pathways initiated by all three GPI types are similar, involving the MyD88-dependent activation of extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 and NF-kappaB-signaling pathways. The signaling molecules of these pathways differentially contribute to the production of various cytokines and nitric oxide (Zhu, J., Krishnegowda, G., and Gowda, D. C. (2004) J. Biol. Chem. 280, 8617-8627). Our data also show that GPIs are degraded by the macrophage surface phospholipases predominantly into inactive species, indicating that the host can regulate GPI activity at least in part by this mechanism. These results imply that macrophage surface phospholipases play important roles in the GPI-induced innate immune responses and malaria pathogenesis.
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PMID:Induction of proinflammatory responses in macrophages by the glycosylphosphatidylinositols of Plasmodium falciparum: cell signaling receptors, glycosylphosphatidylinositol (GPI) structural requirement, and regulation of GPI activity. 1562 12

IFN-gamma secretion by natural killer (NK) cells is pivotal to several tumor and viral immune responses, during which NK and dendritic cells cooperation is required. We show here that macrophages are mandatory for NK cell IFN-gamma secretion in response to erythrocytes infected with Plasmodium falciparum (Pf), a causative agent of human malaria. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine CXCL8, without triggering their granule-mediated cytolytic programs. Despite their reported role in Pf recognition, Toll-like receptor (TLR) 2, TLR9, and TLR11 are individually dispensable for NK cell activation induced by Pf-infected erythrocytes. However, IL-18R expression on NK cells, IL-18 production by macrophages, and MyD88 on both cell types are essential components of this previously undescribed pathway of NK cell activation in response to a parasite infection.
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PMID:Natural killer cell and macrophage cooperation in MyD88-dependent innate responses to Plasmodium falciparum. 1620 71

The glycosylphosphatidylinositol (GPI) anchor of Plasmodium falciparum is thought to function as a critical toxin that contributes to severe malarial pathogenesis by eliciting the production of proinflammatory responses by the innate immune system of mammalian hosts. Analysis of the fine structure of P. falciparum GPI suggests a requirement for the presence of both core glycan and lipid moieties in the recognition and signalling of parasite glycolipids by host immune cells. It has been demonstrated that GPI anchors of various parasitic protozoa can mediate cellular immune responses via members of the Toll-like family of pattern recognition receptors (TLRs). Recent studies indicate that GPI anchors of P. falciparum and other protozoa are preferentially recognized by TLR-2, involving the MyD88-dependent activation of specific signalling pathways that mediate the production of proinflammatory cytokines and nitric oxide from host macrophages in vitro. However, the contribution of malaria GPI toxin to severe disease syndromes and the role of specific TLRs or other pattern recognition receptors in innate immunity in vivo is only just beginning to be characterized. A better understanding of the molecular mechanisms underlying severe malarial pathogenesis may yet lead to substantial new insights with important implications for the development of novel therapeutics for malaria treatment.
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PMID:Stimulation of innate immune responses by malarial glycosylphosphatidylinositol via pattern recognition receptors. 1628 92

Malaria pigment hemozoin was reported to activate the innate immunity by Toll-like receptor (TLR)-9 engagement. However, the role of TLR activation for the development of cerebral malaria (CM), a lethal complication of malaria infection in humans, is unknown. Using Plasmodium berghei ANKA (PbA) infection in mice as a model of CM, we report here that TLR9-deficient mice are not protected from CM. To exclude the role of other members of the TLR family in PbA recognition, we infected mice deficient for single TLR1, -2, -3, -4, -6, -7, or -9 and their adapter proteins MyD88, TIRAP, and TRIF. In contrast to lymphotoxin alpha-deficient mice, which are resistant to CM, all TLR-deficient mice were as sensitive to fatal CM development as wild-type control mice and developed typical microvascular damage with vascular leak and hemorrhage in the brain and lung, together with comparable parasitemia, thrombocytopenia, neutrophilia, and lymphopenia. In conclusion, the present data do not exclude the possibility that malarial molecular motifs may activate the innate immune system. However, TLR-dependent activation of innate immunity is unlikely to contribute significantly to the proinflammatory response to PbA infection and the development of fatal CM.
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PMID:Murine cerebral malaria development is independent of toll-like receptor signaling. 1745 69

We investigated the role of different TLRs and MyD88 in host resistance to infection and malaria pathogenesis. TLR2(-/-), TLR4(-/-), TLR6(-/-), TLR9(-/-) or CD14(-/-) mice showed no change in phenotypes (parasitemia, body weight and temperature) when infected with Plasmodium chabaudi chabaudi (AS). MyD88(-/-) mice displayed comparable ability to wild type animals in controlling and clearing parasitemia. Importantly, MyD88(-/-) mice exhibited impaired production of TNF-alpha and IFN-gamma as well as attenuated symptoms, as indicated by changes in body weight and temperature during parasitemia. Consistently, CD11b(+) monocytes and CD11c(+) dendritic cells from infected MyD88(-/-) mice were shown impaired for production of pro-inflammatory cytokines, and in initiating CD4(+) T cell responses. Importantly, the inhibition of T cell activation with anti-CD134L, mostly inhibited IFN-gamma, partially inhibited TNF-alpha production, and protected the animals from malaria symptoms. Our findings suggest that MyD88 and possibly its associated TLRs expressed by dendritic cells play an important role in pro-inflammatory responses, T cell activation, and pathogenesis of malaria, but are not critical for the immunological control of the erythrocytic stage of P. chabaudi.
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PMID:MyD88-dependent activation of dendritic cells and CD4(+) T lymphocytes mediates symptoms, but is not required for the immunological control of parasites during rodent malaria. 1753 66

Proinflammatory responses to malaria have crucial roles in controlling parasite growth and disease pathogenesis. The glycosylphosphatidylinositol (GPI) of Plasmodium falciparum is thought to be an important factor in the induction of proinflammatory responses. The GPI induces host cellular responses mainly through Toll-like receptor (TLR)2/MyD88-mediated signaling. Knowledge of the parasite-host factors involved in activating and regulating innate immune responses and of the associated signaling mechanisms is likely to provide insights into the modulation of parasite-specific adaptive immunity and offer targets for the development of novel therapeutics or a vaccine for malaria. This article focuses on the malaria GPI-mediated cell-signaling mechanisms.
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PMID:TLR-mediated cell signaling by malaria GPIs. 1798 Jun 63

Infection with Plasmodium berghei ANKA is a well-established model of human cerebral malaria (CM). We show herein that Toll-like receptor (TLR) signaling influences the development of lethal CM in P. berghei ANKA-infected mice. Modulation of outcome was dependent on genetic background, such that deletion of myeloid differentiation factor (MyD) 88 on the susceptible C57BL/6 background resulted in resistance to CM, whereas deletion of MyD88 on the resistant BALB/c background led to increased mortality. Our data show that MyD88 influenced the production of T helper-polarizing cytokines, including interferon (IFN)- gamma, interleukin (IL)-4, and IL-17, as well as the total number of Foxp3(+) regulatory T (T(reg)) cells in a manner dependent on host genetic background. In addition, mRNA levels of IFN- gamma, CXCL10, and CXCL9 were strongly up-regulated in the brains of susceptible wild-type but not MyD88(-/-) infected mice. These results suggest that TLR signaling and host genetic background influences the pathogenesis of CM via modulation of cytokine production and T(reg) cell numbers.
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PMID:Toll-like receptor modulation of murine cerebral malaria is dependent on the genetic background of the host. 1800 36

Plasmodium sporozoites traverse several host cells before infecting hepatocytes. In the process, the plasma membranes of the cells are ruptured, resulting in the release of cytosolic factors into the microenvironment. This released endogenous material is highly stimulatory/immunogenic and can serve as a danger signal initiating distinct responses in various cells. Thus, our study aimed at characterizing the effect of cell material leakage during Plasmodium infection on cultured mouse primary hepatocytes and HepG2 cells. We observed that wounded cell-derived cytosolic factors activate NF-kappaB, a main regulator of host inflammatory responses, in cells bordering wounded cells, which are potential host cells for final parasite infection. This activation of NF-kappaB occurred shortly after infection and led to a reduction of infection load in a time-dependent manner in vitro and in vivo, an effect that could be reverted by addition of the specific NF-kappaB inhibitor BAY11-7082. Furthermore, no NF-kappaB activation was observed when Spect(-/-) parasites, which are devoid of hepatocyte traversing properties, were used. We provide further evidence that NF-kappaB activation causes the induction of inducible NO synthase expression in hepatocytes, and this is, in turn, responsible for a decrease in Plasmodium-infected hepatocytes. Furthermore, primary hepatocytes from MyD88(-/-) mice showed no NF-kappaB activation and inducible NO synthase expression upon infection, suggesting a role of the Toll/IL-1 receptor family members in sensing cytosolic factors. Indeed, lack of MyD88 significantly increased infection in vitro and in vivo. Thus, host cell wounding due to parasite migration induces inflammation which limits the extent of parasite infection.
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PMID:Sporozoite-mediated hepatocyte wounding limits Plasmodium parasite development via MyD88-mediated NF-kappa B activation and inducible NO synthase expression. 1832 8

In this study, 2-Cys Plasmodium berghei ANKA (PbA) peroxiredoxin (Prx) was identified as an antigenic protein recognized by an anti-PbA IgE antibody using two-dimensional polyacrylamide gel electrophoresis and proteomic analysis. Innate immune responses to PbAPrx were examined using cells from mice deficient in Toll-like receptors (TLR) or related molecules, and it was demonstrated that responses were severely impaired in TLR4(-/-), MyD88(-/-) and MD-2(-/-) mice, but not in Toll/IL-1 receptor domain-containing adaptor inducing IFN-gamma (TRIF)(-/-), TLR2(-/-) or radioprotective 105 (RP105)(-/-) mice. An association between PbAPrx and TLR4 was observed following immunoprecipitation and immunoblotting, suggesting that PbAPrx was associated with TLR4/MD-2. Interactions between Prx and TLR4/MD-2 were also examined by flow cytometry using TLR4/MD-2- or TLR2-expressing cells. NFkappaB/GFP activity was observed in TLR4/MD-2- but not in TLR2-expressing cells following stimulation with Prx. However, this effect was not observed after treatment with proteinase K, suggesting that PbAPrx is a protein ligand for TLR4 and that the PbAPrx activity observed in this study is not due to contamination with LPS. These findings indicate that malarial Prx induces IgE-mediated protection through FcepsilonRI on mast cells and innate immunity through TLR4 with MyD88 and MD-2, suggesting a novel function for malarial Prx in innate and acquired immune responses in malaria.
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PMID:Mast cell-mediated immune responses through IgE antibody and Toll-like receptor 4 by malarial peroxiredoxin. 1839 34


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