Toll-like receptor 9 mediates innate immune activation by the malaria pigment hemozoin

The role of Toll-like receptors in immune disorders

Toll-like receptors (TLRs) play an important role in innate immunity. Individual TLRs recognise microbial components that are conserved among pathogens. Such recognition initiates necessary inflammatory immune responses and induces subsequent activation of adaptive immunity. Studies in people with polymorphisms in genes encoding TLR signalling can elucidate the relationship between TLRs and human diseases, such as infectious diseases, atherosclerosis and immunodeficiency. Indeed, accumulating data in respect to TLR signalling suggest that TLRs are closely related with the pathogenesis of autoimmune diseases. This review looks at the role of TLRs in various immune disorders, and discusses the pathogenesis of diseases.

Malaria infection changes the ability of splenic dendritic cell populations to stimulate antigen-specific T cells

The capacity of splenic CD11c⁺ dendritic cell (DC) populations to present antigen (Ag) to T cells differs during malarial infection with Plasmodium chabaudi in mice. Both CD11c⁺CD8⁺ and CD8⁻ DCs presented malarial peptides on their surface during infection. However, although both DC subsets expressing malaria peptides could induce interferon-γ production by CD4 T cells, only CD8⁻ DCs isolated at the acute phase of infection stimulated Ag-specific T cell proliferation and interleukin (IL)-4 and -10 production from MSP1-specific T cell receptor for Ag transgenic T cells coincidental with our reported Th1 to Th2 switch at this stage in response to the pathogen. The timing of these distinct DC responses coincided with increased levels of apoptosis in the CD8⁺ population and an increase in the numbers of CD8⁻ DCs in the spleen. Our data suggest that the switch in CD4 T cell responses observed in P. chabaudi–infected mice may be the result of the presentation by different DC populations modified by the malaria infection.

Antitumor Activity of IFN-λ in Murine Tumor Models

IFN-lambda 1, -lambda 2 and -lambda 3 have been discovered as the latest members of the class II cytokine family and shown to possess antiviral activity. Murine B16 melanoma and Colon26 cancer cells were transduced with mouse IFN-lambda to determine whether IFN-lambda possesses antitumor activity. Overexpression of IFN-lambda induced cell surface MHC class I expression and Fas/CD95 Ag, induced significant caspase-3/7 activity, and increased p21(Waf1/Cip1) and dephosphorylated Rb (Ser(780)) in B16 cells in vitro. IFN-lambda expression in tumor cell lines markedly inhibited s.c. and metastatic tumor formation in vivo compared with mock transfections (p < 0.05). Moreover, IFN-lambda expression induced lymphocytic infiltrates, and an Ab-mediated immune cell depletion assay showed that NK cells were critical to IFN-lambda-mediated tumor growth inhibition. Hydrodynamic injection of IFN-lambda cDNA successfully targeted liver metastatic foci of Colon26 cells, and moderately decreased the mortality of mice with tumors. IFN-lambda overexpression in the liver increased NK/NKT cells and enhanced their tumor-killing activity, and suggested the activation of innate immune responses. Thus, IFN-lambda induced both tumor apoptosis and NK cell-mediated immunological tumor destruction through innate immune responses. These findings suggested that local delivery of IFN-lambda might prove a useful adjunctive strategy in the clinical treatment of human malignancies.

TLR9 regulates Th1 responses and cooperates with TLR2 in mediating optimal resistance to Mycobacterium tuberculosis

To investigate the role of Toll-like receptor (TLR)9 in the immune response to mycobacteria as well as its cooperation with TLR2, a receptor known to be triggered by several major mycobacterial ligands, we analyzed the resistance of TLR9^−/− as well as TLR2/9 double knockout mice to aerosol infection with Mycobacterium tuberculosis. Infected TLR9^−/− but not TLR2^−/− mice displayed defective mycobacteria-induced interleukin (IL)-12p40 and interferon (IFN)-γ responses in vivo, but in common with TLR2^−/− animals, the TLR9^−/− mice exhibited only minor reductions in acute resistance to low dose pathogen challenge. When compared with either of the single TLR-deficient animals, TLR2/9^−/− mice displayed markedly enhanced susceptibility to infection in association with combined defects in proinflammatory cytokine production in vitro, IFN-γ recall responses ex vivo, and altered pulmonary pathology. Cooperation between TLR9 and TLR2 was also evident at the level of the in vitro response to live M. tuberculosis, where dendritic cells and macrophages from TLR2/9^−/− mice exhibited a greater defect in IL-12 response than the equivalent cell populations from single TLR9-deficient animals. These findings reveal a previously unappreciated role for TLR9 in the host response to M. tuberculosis and illustrate TLR collaboration in host resistance to a major human pathogen.

Different T-bet expression patterns characterize particular reactive lymphoid tissue lesions

AIMS

To investigate T-bet expression profiles in various lymphoid tissue diseases caused by intracellular pathogens and to compare them in disorders without an infective aetiology. Murine and in vitro experiments have shown that the expression/induction of T-bet, the master regulator of Th1 differentiation, can be achieved by obligate intracellular pathogens and high interferon (IFN)-gamma levels.

METHODS

Lymph node biopsies were analysed immunohistochemically employing single and double labelling for T-bet and CD20, CD4, CD8 and CD30 detection.

RESULTS

In disorders associated with high IFN-gamma levels and intracellular pathogens (infectious mononucleosis, HIV-associated lymphadenopathy, cat-scratch disease, and toxoplasmic lymphadenitis), T-bet-expressing CD4 cells were accompanied by significant numbers of T-bet-positive CD8 and B cells. A similar profile was also found in histiocytic necrotizing (Kikuchi) lymphadenitis, a disease of unknown cause. In contrast, T-bet expression in disorders without an infective aetiology was observed in only a small portion of lymphocytes.

CONCLUSIONS

Increased T-bet expression does not only identify intracellular infections in lymphoid tissue associated with high IFN-gamma levels, but also implies that, under these conditions, it becomes induced in B cells, which apparently support the Th1 response. T-bet expression in Kikuchi lymphadenitis underscores the hypothesis that it is caused by an intracellular microorganism.

A critical role for the host mediator macrophage migration inhibitory factor in the pathogenesis of malarial anemia

The pathogenesis of malarial anemia is multifactorial, and the mechanisms responsible for its high mortality are poorly understood. Studies indicate that host mediators produced during malaria infection may suppress erythroid progenitor development (Miller, K.L., J.C. Schooley, K.L. Smith, B. Kullgren, L.J. Mahlmann, and P.H. Silverman. 1989. Exp. Hematol. 17:379–385; Yap, G.S., and M.M. Stevenson. 1991. Ann. NY Acad. Sci. 628:279–281). We describe an intrinsic role for macrophage migration inhibitory factor (MIF) in the development of the anemic complications and bone marrow suppression that are associated with malaria infection. At concentrations found in the circulation of malaria-infected patients, MIF suppressed erythropoietin-dependent erythroid colony formation. MIF synergized with tumor necrosis factor and γ interferon, which are known antagonists of hematopoiesis, even when these cytokines were present in subinhibitory concentrations. MIF inhibited erythroid differentiation and hemoglobin production, and it antagonized the pattern of mitogen-activated protein kinase phosphorylation that normally occurs during erythroid progenitor differentiation. Infection of MIF knockout mice with Plasmodium chabaudi resulted in less severe anemia, improved erythroid progenitor development, and increased survival compared with wild-type controls. We also found that human mononuclear cells carrying highly expressed MIF alleles produced more MIF when stimulated with the malarial product hemozoin compared with cells carrying low expression MIF alleles. These data suggest that polymorphisms at the MIF locus may influence the levels of MIF produced in the innate response to malaria infection and the likelihood of anemic complications.

Suppression of adaptive immunity to heterologous antigens during Plasmodium infection through hemozoin-induced failure of dendritic cell function

BACKGROUND

Dendritic cells (DCs) are central to the initiation and regulation of the adaptive immune response during infection. Modulation of DC function may therefore allow evasion of the immune system by pathogens. Significant depression of the host's systemic immune response to both concurrent infections and heterologous vaccines has been observed during malaria infection, but the mechanisms underlying this immune hyporesponsiveness are controversial.

RESULTS

Here, we demonstrate that the blood stages of malaria infection induce a failure of DC function in vitro and in vivo, causing suboptimal activation of T cells involved in heterologous immune responses. This effect on T-cell activation can be transferred to uninfected recipients by DCs isolated from infected mice. Significantly, T cells activated by these DCs subsequently lack effector function, as demonstrated by a failure to migrate to lymphoid-organ follicles, resulting in an absence of B-cell responses to heterologous antigens. Fractionation studies show that hemozoin, rather than infected erythrocyte (red blood cell) membranes, reproduces the effect of intact infected red blood cells on DCs. Furthermore, hemozoin-containing DCs could be identified in T-cell areas of the spleen in vivo.

CONCLUSION

Plasmodium infection inhibits the induction of adaptive immunity to heterologous antigens by modulating DC function, providing a potential explanation for epidemiological studies linking endemic malaria with secondary infections and reduced vaccine efficacy.

Malaria pigment paralyzes dendritic cells

The capacity of malarial infection to suppress the patient's immune responses both to the parasite and to other antigens has long puzzled researchers. A prime suspect, the parasite-produced pigment hemozoin, has now been clearly shown to mediate immunosuppression by inhibiting dendritic cell activity.

Frequencies of peripheral blood myeloid cells in healthy Kenyan children with alpha+ thalassemia and the sickle cell trait

The high frequencies of both alpha+ thalassemia and the sickle cell trait (hemoglobin AS [HbAS]) found in many tropical populations are thought to reflect selection pressure from Plasmodium falciparum malaria. For HbAS, but not for alpha+ thalassemia, protection appears to be mediated by the enhanced phagocytic clearance of ring-infected erythrocytes. We have investigated the genotype-specific distributions of peripheral blood leukocyte populations in two groups of children living on the coast of Kenya: a group of healthy P. falciparum parasite-negative children sampled at cross-sectional survey during a period of low malaria transmission, and a group of children attending the hospital with acute malaria. We report distinctive distributions of peripheral blood myeloid dendritic cells and monocytes in children with alpha+ thalassemia and HbAS during healthy periods and disease, and suggest ways in which these might relate to the mechanisms of protection afforded by these conditions.

Pathogen recognition and innate immunity

Microorganisms that invade a vertebrate host are initially recognized by the innate immune system through germline-encoded pattern-recognition receptors (PRRs). Several classes of PRRs, including Toll-like receptors and cytoplasmic receptors, recognize distinct microbial components and directly activate immune cells. Exposure of immune cells to the ligands of these receptors activates intracellular signaling cascades that rapidly induce the expression of a variety of overlapping and unique genes involved in the inflammatory and immune responses. New insights into innate immunity are changing the way we think about pathogenesis and the treatment of infectious diseases, allergy, and autoimmunity.

Priming of CD4+ T cells and development of CD4+ T cell memory; lessons for malaria

CD4 T cells play a central role in the immune response to malaria. They are required to help B cells produce the antibody that is essential for parasite clearance. They also produce cytokines that amplify the phagocytic and parasitocidal response of the innate immune system, as well as dampening this response later on to limit immunopathology. Therefore, understanding the mechanisms by which T helper cells are activated and the requirements for development of specific, and effective, T cell memory and immunity is essential in the quest for a malaria vaccine. In this paper on the CD4 session of the Immunology of Malaria Infections meeting, we summarize discussions of CD4 cell priming and memory in malaria and in vaccination and outline critical future lines of investigation. B. Stockinger and M.K. Jenkins proposed cutting edge experimental systems to study basic T cell biology in malaria. Critical parameters in T cell activation include the cell types involved, the route of infection and the timing and location and cell types involved in antigen presentation. A new generation of vaccines that induce CD4 T cell activation and memory are being developed with new adjuvants. Studies of T cell memory focus on differentiation and factors involved in maintenance of antigen specific T cells and control of the size of that population. To improve detection of T cell memory in the field, efforts will have to be made to distinguish antigen-specific responses from cytokine driven responses.

Molecular cloning and tissue expression of canine Toll-like receptor 2 (TLR2)

Toll-like receptor 2 (TLR2) is involved in an important role on inflammatory reactions against various pathogenic microorganisms. Although some investigations suggested a relation between human and murine TLR2 gene polymorphisms and infections or inflammatory reactions, canine TLR2 gene has not been investigated. In the present study, therefore, the canine full-length of cDNA of Toll-like receptor 2 gene was sequenced and the expression of canine TLR2 mRNA was investigated in various tissues. Canine TLR2 clone was 2552bp encoding 785 amino acids. The similarities of canine TLR2 with human, mouse, rat, cattle, horse and pig TLR2 were 87.6, 72.9, 73.2, 83.1, 85.1 and 80.7% at the nucleotide sequence level, and 84.6, 68.7, 68.5, 78.8, 82.9 and 79.0% at amino acid sequence level, respectively. By a semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis, mRNA of canine TLR2 was detected in blood mononuclear cells (BMCs), lymph node, lung, liver, spleen, bladder, pancreas, small intestine, large intestine and skin of the dogs.

MyD88-dependent pathways mediate resistance to Cryptosporidium parvum infection in mice

Cryptosporidium spp. cause diarrheal disease worldwide. Innate immune responses mediating resistance to this parasite are not completely understood. To determine whether MyD88-dependent pathways play a role in resistance to Cryptosporidium parvum, we compared the course of infection in MyD88(-/-) mice to that in their wild-type (WT) littermate controls. Three- to 4-week-old mice were infected with C. parvum, and infection was monitored by quantifying fecal oocyst shedding. Twelve days postinfection, the histology of the intestines was examined to quantify intestinal parasite burden and to determine if there were any pathological changes. Fecal oocyst shedding and intestinal parasite burden were significantly greater in MyD88(-/-) mice than in littermate controls. Nonetheless, both WT and MyD88(-/-) mice cleared the infection within 3 weeks. These results indicate that MyD88-dependent pathways are involved in mediating initial resistance to C. parvum. Since gamma interferon (IFN-gamma) is known to mediate resistance to C. parvum, we also studied infection in MyD88(-/-) mice and WT controls in which this cytokine was temporarily neutralized. Fecal oocyst shedding, as well as intestinal parasite burden, intestinal inflammation, and mortality, was significantly greater in MyD88(-/-) mice in which IFN-gamma was neutralized than in IFN-gamma-neutralized WT mice or in MyD88(-/-) mice in which this cytokine was active. These results suggest that MyD88 and IFN-gamma had an additive effect in conferring protection from C. parvum infection. While this study confirms the importance of IFN-gamma in conferring resistance to infection with C. parvum, it suggests that MyD88-mediated pathways also play a role in innate immunity to this parasite.

Yellow fever vaccine YF-17D activates multiple dendritic cell subsets via TLR2, 7, 8, and 9 to stimulate polyvalent immunity

The live attenuated yellow fever vaccine 17D (YF-17D) is one of the most effective vaccines available, with a 65-yr history of use in >400 million people globally. Despite this efficacy, there is presently no information about the immunological mechanisms by which YF-17D acts. Here, we present data that suggest that YF-17D activates multiple Toll-like receptors (TLRs) on dendritic cells (DCs) to elicit a broad spectrum of innate and adaptive immune responses. Specifically, YF-17D activates multiple DC subsets via TLRs 2, 7, 8, and 9 to elicit the proinflammatory cytokines interleukin (IL)-12p40, IL-6, and interferon-α. Interestingly, the resulting adaptive immune responses are characterized by a mixed T helper cell (Th)1/Th2 cytokine profile and antigen-specific CD8⁺ T cells. Furthermore, distinct TLRs appear to differentially control the Th1/Th2 balance; thus, whilst MyD88-deficient mice show a profound impairment of Th1 cytokines, TLR2-deficient mice show greatly enhanced Th1 and Tc1 responses to YF-17D. Together, these data enhance our understanding of the molecular mechanism of action of YF-17D, and highlight the potential of vaccination strategies that use combinations of different TLR ligands to stimulate polyvalent immune responses.

RNA interference reveals a role for TLR2 and TLR3 in the recognition ofLeishmania donovani promastigotes by interferon–γ-primed macrophages

Leishmania donovani promastigotes evade the induction of a proinflammatory response during their invasion of naive macrophages. However, their entry into IFN-gamma-primed macrophages is accompanied by the secretion of nitric oxide (NO) and proinflammatory cytokines. In the present study, we addressed the hypothesis that priming with IFN-gamma induces the expression of a receptor that enables mouse macrophages to recognize L. donovani promastigotes. We observed that in IFN-gamma-primed macrophages, L. donovani promastigotes stimulated Interleukin-1 receptor-associated kinase-1 (IRAK-1) activity. We next showed that Toll-like receptor (TLR)3 is barely detectable in naive macrophages but is expressed in IFN-gamma-treated macrophages. Silencing of TLR3, TLR2, IRAK-1 and myeloid differentiation factor 88 (MyD88) expression by RNA interference revealed that both TLR are involved in the secretion of NO and TNF-alpha induced by L. donovani promastigotes. Using L. donovani mutants, we showed that TLR2-mediated responses are dependent on Galbeta1,4Manalpha-PO(4)-containing phosphoglycans, whereas TLR3-mediated responses are independent of these glycoconjugates. Furthermore, our data indicate a participation of TLR2 and TLR3 in the phagocytosis of L. donovani promastigotes and a role for TLR3 in the leishmanicidal activity of the IFN-gamma-primed macrophages. Collectively, our data are consistent with a model where recognition of L. donovani promastigotes depends on the macrophage activation status and requires the expression of TLR3.

Stimulation of innate immune responses by malarial glycosylphosphatidylinositol via pattern recognition receptors

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.

Systemic activation of dendritic cells by Toll-like receptor ligands or malaria infection impairs cross-presentation and antiviral immunity

The mechanisms responsible for the immunosuppression associated with sepsis or some chronic blood infections remain poorly understood. Here we show that infection with a malaria parasite (Plasmodium berghei) or simple systemic exposure to bacterial or viral Toll-like receptor ligands inhibited cross-priming. Reduced cross-priming was a consequence of downregulation of cross-presentation by activated dendritic cells due to systemic activation that did not otherwise globally inhibit T cell proliferation. Although activated dendritic cells retained their capacity to present viral antigens via the endogenous major histocompatibility complex class I processing pathway, antiviral responses were greatly impaired in mice exposed to Toll-like receptor ligands. This is consistent with a key function for cross-presentation in antiviral immunity and helps explain the immunosuppressive effects of systemic infection. Moreover, inhibition of cross-presentation was overcome by injection of dendritic cells bearing antigen, which provides a new strategy for generating immunity during immunosuppressive blood infections.

Antigen presentation and dendritic cell biology in malaria

Dendritic cells (DCs) are important both in amplifying the innate immune response and in initiating adaptive immunity and shaping the type of T helper (Th) response. Although the role of DCs in immune responses to many intracellular pathogens has been delineated and research is underway to identify the mechanisms involved, relatively little is known concerning the role of DCs in immunity to malaria. In this review, we provide an overview and summary of previous and current studies aimed to investigate the role of DCs as antigen presenting cells (APCs). In addition, the role of DCs in inducing innate and adaptive immunity to blood-stage malaria is discussed and, where information is available, the mechanisms involved are presented. Data from studies in humans infected with Plasmodium falciparum, the major human parasite responsible for the high morbidity and mortality associated with malaria throughout many regions of the developing world, as well as data from experimental mouse models are presented. Overall, the data from these studies are conflicting. The possible reasons for these differences, including the use of different parasite species and parasite strains in the mouse studies, are discussed. Nevertheless, together the data have important implications for development of an effective malaria vaccine since the selection of appropriate Plasmodium antigens and/or adjuvants, targeting innate immune responses involving DCs, may provide optimal protection against malaria. It is hoped that this review promotes more investigation among malariologists and immunologists alike on DCs and malaria.

Regulating immunity to malaria

The optimal outcome of a malaria infection is that parasitized cells are killed and degraded without inducing significant pathology. Since much of the pathology of malaria infection can be immune-mediated, this implies that immune responses have to be carefully regulated. The mechanisms by which anti-malarial immune responses are believed to be regulated were discussed at the recent Malaria Immunology Workshop (Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland, USA; February 2005). Potential regulatory mechanisms include regulatory T cells, which have been shown to significantly modify cellular immune responses to various protozoan infections, including leishmania and malaria; neutralising antibodies to pro-inflammatory malarial toxins such as glycosylphosphatidylinositol and haemozoin; and self-regulating networks of effector molecules. Innate and adaptive immune responses are further moderated by the broader immunological environment, which is influenced by both the genetic background of the host and by co-infection with other pathogens. A detailed understanding of the interplay between these different immunoregulatory processes may facilitate the rationale design of vaccines and novel therapeutics.

Regulation of immune response by Plasmodium-infected red blood cells

During the asexual blood stage infection of the human malaria parasite, Plasmodium falciparum, parasite-derived proteins are inserted onto the surface of the host red blood cell membrane. These proteins are highly variable and were originally thought only to mediate antigenic variation, and sequestration of parasites from peripheral circulation, thus enabling immune evasion. Recent studies have revealed that PfEMP-1 and other molecules on the P. falciparum-infected red blood cell (PfRBC) activate and modulate the immune response. In this review, we discuss how PfRBCs interact with antigen-presenting cells (APCs) and other cells of the immune system, and how such interactions could modulate the host response to Plasmodium infections.

Toll-like receptor (TLR) polymorphisms in African children: Common TLR-4 variants predispose to severe malaria

Genetic host factors play a substantial role in susceptibility to and severity of malaria, which continues to cause at least one million deaths per year. Recently, members of the toll-like receptor (TLR) family have been shown to be involved in recognition of the etiologic organism Plasmodium falciparum: The glycosylphosphatidylinositol anchor induces signaling in host cells via TLR-2 and -4, whereas hemozoin-induced immune activation involves TLR-9. Binding of microbial ligands to the respective TLRs triggers the release of proinflammatory cytokines via the TLR/IL-1 receptor (TIR) domain and may contribute to the host response in malaria, including cytokine induction and fever. In a case-control study among 870 Ghanaian children, we examined the influence of TLR-2, -4, and -9 polymorphisms in susceptibility to severe malaria. TLR-2 variants common in Caucasians and Asians were completely absent. However, we found a rare previously undescribed mutation (Leu658Pro), which impairs signaling via TLR-2. We failed to detect any polymorphisms within the TLR-9 Toll/IL-1 receptor domain. Two frequent TLR-9 promoter polymorphisms did not show a clear association with malaria severity. In contrast, the TLR-4-Asp299Gly variant occurred at a high rate of 17.6% in healthy controls and was even more frequent in severe malaria patients (24.1%, P < 0.05). Likewise, TLR-4-Thr399Ile was seen in 2.4% of healthy children and in 6.2% of patients (P = 0.02). TLR-4-Asp299Gly and TLR-4-Thr399Ile conferred 1.5- and 2.6-fold increased risks of severe malaria, respectively. These findings suggest TLR4-mediated responses to malaria in vivo and TLR-4 polymorphisms to be associated with disease manifestation.

Leukocyte activation by malarial pigment

Malarial pigment, a unique hemozoin crystal composed of unit cells of heme dimers, is present in large amounts in circulating monocytes and neutrophils and can persist unchanged in macrophages for several months. In the present study, we investigated the effect of hemozoin not only on macrophages, but also on neutrophils. We used beta-hematin (BH), a chemically synthetic crystal structurally identical to hemozoin, for these studies. In vitro, BH up-regulated the expression of tumor necrosis factor-alpha in whole blood and in isolated peritoneal macrophages, indicating that hemozoin is able to stimulate monocytes. BH stimulated murine peritoneal neutrophils to express macrophage inflammatory protein-2 (MIP-2), a homologue of human interleukin-8 that is used as a marker of neutrophil activation. Injecting BH into the peritoneal cavity resulted in a dose-dependent migration of neutrophils and a high level of myeloperoxidase activity of peritoneal cells. Finally, BH directly induced neutrophil chemotaxis in vitro. Taken together, these results suggest that the malarial pigment hemozoin can activate leukocytes and may participate in the pathology of severe malaria.

Heterogeneous Human NK Cell Responses toPlasmodium falciparum-Infected Erythrocytes

Human NK cells can respond rapidly to Plasmodium falciparum-infected RBC (iRBC) to produce IFN-gamma. In this study, we have examined the heterogeneity of this response among malaria-naive blood donors. Cells from all donors become partially activated (up-regulating CD69, perforin, and granzyme) upon exposure to iRBC but cells from only a subset of donors become fully activated (additionally up-regulating CD25, IFN-gamma, and surface expression of lysosomal-associated membrane protein 1 (LAMP-1)). Although both CD56dim and CD56bright NK cell populations can express IFN-gamma in response to iRBC, CD25 and LAMP-1 are up-regulated only by CD56dim NK cells and CD69 is up-regulated to a greater extent in this subset; by contrast, perforin and granzyme A are preferentially up-regulated by CD56bright NK cells. NK cells expressing IFN-gamma in response to iRBC always coexpress CD69 and CD25 but rarely LAMP-1, suggesting that individual NK cells respond to iRBC either by IFN-gamma production or cytotoxicity. Furthermore, physical contact with iRBC can, in a proportion of donors, lead to NK cell cytoskeletal reorganization suggestive of functional interactions between the cells. These observations imply that individuals may vary in their ability to mount an innate immune response to malaria infection with obvious implications for disease resistance or susceptibility.

A Toll-like receptor-independent antiviral response induced by double-stranded B-form DNA

The innate immune system recognizes nucleic acids during infection or tissue damage; however, the mechanisms of intracellular recognition of DNA have not been fully elucidated. Here we show that intracellular administration of double-stranded B-form DNA (B-DNA) triggered antiviral responses including production of type I interferons and chemokines independently of Toll-like receptors or the helicase RIG-I. B-DNA activated transcription factor IRF3 and the promoter of the gene encoding interferon-beta through a signaling pathway that required the kinases TBK1 and IKKi, whereas there was substantial activation of transcription factor NF-kappaB independent of both TBK and IKKi. IPS-1, an adaptor molecule linking RIG-I and TBK1, was involved in B-DNA-induced activation of interferon-beta and NF-kappaB. B-DNA signaling by this pathway conferred resistance to viral infection in a way dependent on both TBK1 and IKKi. These results suggest that both TBK1 and IKKi are required for innate immune activation by B-DNA, which might be important in antiviral innate immunity and other DNA-associated immune disorders.

Immunological processes in malaria pathogenesis

Malaria is possibly the most serious infectious disease of humans, infecting 5-10% of the world's population, with 300-600 million clinical cases and more than 2 million deaths annually. Adaptive immune responses in the host limit the clinical impact of infection and provide partial, but incomplete, protection against pathogen replication; however, these complex immunological reactions can contribute to disease and fatalities. So, appropriate regulation of immune responses to malaria lies at the heart of the host-parasite balance and has consequences for global public health. This Review article addresses the innate and adaptive immune mechanisms elicited during malaria that either cause or prevent disease and fatalities, and it considers the implications for vaccine design.

Natural killer cell and macrophage cooperation in MyD88-dependent innate responses to Plasmodium falciparum

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.

The immunophysiological impact of bacterial CpG DNA on the gut

Both the endogenous commensal flora and a dysregulated mucosal immune response have been implicated as contributing to the pathogenesis of human intestinal disease. Unmethylated cytosine-guanine (CpG)-containing DNA, the ligand for Toll-like receptor 9 (TLR9), is a recently recognized microbial product with immunostimulatory and immunoregulatory effects. TLR9 is expressed by many cell types located in the intestine, including epithelial cells and classical immune cells. The physiological impact of the juxtaposition of these factors (bacterial DNA and responsive cells) in the gut therefore bears consideration. Here we discuss studies that examine the interaction between CpG DNA and the intestine, focusing on activation of epithelial cells, administration of CpG-containing oligonucleotides as therapy for experimental inflammatory enteropathies, and the role of CpG DNA in mediating the beneficial effects of bacterial probiotics.

Molecular mechanisms of autoimmunity triggered by microbial infection

Autoimmunity can be triggered by microbial infection. In this context, the discovery of Toll-like receptors (TLRs) provides new insights and research perspectives. TLRs induce innate and adaptive antimicrobial immune responses upon exposure to common pathogen-associated molecules, including lipopeptides, lipopolysaccharides, and nucleic acids. They also have the potential, however, to trigger autoimmune disease, as has been revealed by an increasing number of experimental reports. This review summarizes important facts about TLR biology, available data on their role in autoimmunity, and potential consequences for the management of patients with autoimmune disease.

Interactions between malaria parasites and the host immune system

Malaria remains one of the greatest impediments to development in many tropical regions of the world. Understanding host immune responses to malaria parasites is crucial for the effective design and implementation of new vaccines and drugs. Recent research has seen the identification of the first pattern recognition receptor (TLR9) on dendritic cells for a defined product of malaria infection (hemozoin). In addition, progress has been made in understanding the role of dendritic cell subsets in malaria, and how they promote specific components of the host immune response. Potentially important advances in vaccine design have also been made by inserting a Plasmodium sporozoite epitope into the yellow fever vaccine 17D, as well as using a whole, live-attenuated sporozoite vaccine.

Pathogen recognition with Toll-like receptors

The innate immune system is an evolutionarily conserved system of defense against microbial infections. The family of Toll-like receptors is a major class of receptors that sense molecular patterns associated with a broad range of pathogens including bacteria, viruses, fungi and protozoa. Following pathogen recognition, Toll-like receptors initiate intracellular signal transduction that results in the expression of genes involved in inflammation, antiviral responses and maturation of dendritic cells. Individual Toll-like receptors activate common and unique transcription factors through different signaling pathways to drive specific biological responses against microorganisms.

Negative regulation of toll-like receptor-mediated immune responses

Toll-like receptors (TLRs) are involved in host defence against invading pathogens, functioning as primary sensors of microbial products and activating signalling pathways that induce the expression of immune and pro-inflammatory genes. However, TLRs have also been implicated in several immune-mediated and inflammatory diseases. As the immune system needs to constantly strike a balance between activation and inhibition to avoid detrimental and inappropriate inflammatory responses, TLR signalling must be tightly regulated. Here, we discuss the various negative regulatory mechanisms that have evolved to attenuate TLR signalling to maintain this immunological balance.

The evolution of vertebrate Toll-like receptors

The complete sequences of Takifugu Toll-like receptor (TLR) loci and gene predictions from many draft genomes enable comprehensive molecular phylogenetic analysis. Strong selective pressure for recognition of and response to pathogen-associated molecular patterns has maintained a largely unchanging TLR recognition in all vertebrates. There are six major families of vertebrate TLRs. This repertoire is distinct from that of invertebrates. TLRs within a family recognize a general class of pathogen-associated molecular patterns. Most vertebrates have exactly one gene ortholog for each TLR family. The family including TLR1 has more species-specific adaptations than other families. A major family including TLR11 is represented in humans only by a pseudogene. Coincidental evolution plays a minor role in TLR evolution. The sequencing phase of this study produced finished genomic sequences for the 12 Takifugu rubripes TLRs. In addition, we have produced >70 gene models, including sequences from the opossum, chicken, frog, dog, sea urchin, and sea squirt.

Cutting Edge: The Acquisition of TLR Tolerance during Malaria Infection Impacts T Cell Activation

An effective immune response to infection requires control of pathogen growth while minimizing inflammation-associated pathology. During malaria infection, this balance is particularly important. Murine malaria is characterized by early production of proinflammatory cytokines, which declines in the face of continuing parasitemia. The mechanism by which this occurs remains poorly understood. In this study, we investigated the role of dendritic cells (DCs) in regulating pro- and anti-inflammatory cytokine responses. As malaria infection progresses, DCs become refractory to TLR-mediated IL-12 and TNF-alpha production, while increasing their ability to produce IL-10 and retaining the capacity for activation of naive T cells. IL-12-secreting DCs from early infection stimulate an IFN-gamma-dominated T cell response, whereas IL-10-secreting DCs from later stages induce an IL-10-dominated T cell response. We suggest that phenotypic changes in DCs during Plasmodium yoelii infection represent a mechanism of controlling host inflammation while maintaining effective adaptive immunity.

Innate immune recognition of nucleic acids: Beyond toll-like receptors

During infection or tissue damage, the innate immune system detects and responds to nucleic acids released from pathogens or damaged host cells. Accumulating evidence has showed that specific sequences, modifications or structures of nucleic acids influence their immunomodulatory activities. Resulting innate immune modulations are regulated by Toll-like receptor (TLR)-dependent or -independent signaling pathways. The first step in host defense against foreign or unwelcome self nucleic acids may play important roles in immune responses against infectious organisms, as well as in clearance of unnecessary tissues, which may be linked to autoimmune diseases and possibly to other immunological disorders. Elucidating mechanisms of innate immune activation by nucleic acids will help future development of more efficient or safer nucleic acid-based immunotherapies and gene therapies.

Glycosylphosphatidylinositols in malaria pathogenesis and immunity: potential for therapeutic inhibition and vaccination

Glycosylphosphatidylinositols (GPIs) are found in the outer cell membranes of all eukaryotes. GPIs anchor a diverse range of proteins to the surface of Plasmodium falciparum, but may also exist free of protein attachment. In vitro and in vivo studies have established GPIs as likely candidate toxins in malaria, consistent with the prevailing paradigm that attributes induction of inflammatory cytokines, fever and other pathology to parasite toxins released when schizonts rupture. Although evolutionarily conserved, sufficient structural differences appear to exist that impart upon plasmodial GPIs the ability to activate second messengers in mammalian cells and elicit immune responses. In populations exposed to P. falciparum, the antibody response to purified GPIs is characterised by a predominance of immunoglobulin (Ig)G over IgM and an increase in the prevalence, level and persistence of responses with increasing age. It remains unclear, however, if these antibodies or other cellular responses to GPIs mediate anti-toxic immunity in humans; anti-toxic immunity may comprise either reduction in the severity of disease or maintenance of the malaria-tolerant state (i.e. persistent asymptomatic parasitaemia). P. falciparum GPIs are potentially amenable to specific therapeutic inhibition and vaccination; more needs to be known about their dual roles in malaria pathogenesis and protection for these strategies to succeed.