Inhibition of host cell signal transduction by Leishmania: observations relevant to the selective impairment of IL-12 responses

IFN-γ inhibitory molecules derived from Eimeria maxima inhibit IL-12 secretion by modulating MAPK pathways in chicken macrophages

IFN-γ plays a crucial role in resisting intracellular parasitic protozoa, such as Eimeria species. In our previous study, we identified 4 molecules derived from Eimeria maxima (E. maxima) that significantly inhibited IFN-γ production. However, the mechanism underlying this inhibitory effect remains unknown. In this study, we first investigated the effects of these 4 IFN-γ inhibitory molecules on the expression levels of chicken Toll-like receptors (chTLRs), IL-12, IL-10, TGF-β, and TNF-α in chicken macrophage HD11 and bone marrow-derived dendritic cells (BMDCs). The results demonstrated that these 4 inhibitory molecules significantly downregulated the mRNA levels of chTLR-2, chTLR-4, chTLR-21, and both mRNA and protein levels of IL-12. Subsequently, to clarify the effects of these 4 inhibitory molecules on the IL-12 secretion-related signaling pathways in chicken macrophages, qRT-PCR and Western blot were used to detect the changes of key molecules involved in the signaling pathways of IL-12 secretion (NF-κB, ERK1/2, p38, JNK, STAT3) following coincubation with these inhibitory molecules. Finally, RNAi was employed to verify the function of key molecules in the signaling pathway. The results revealed a significant upregulation in the expression of ERK1/2 phosphorylated protein induced by the 4 inhibitory molecules. Knockdown of the ERK1/2 gene significantly reduced the inhibitory effect of the 4 E. maxima inhibitory molecules on IL-12. These findings indicate that the 4 inhibitory molecules can inhibit the secretion of IL-12 by upregulating the expression of ERK1/2 phosphorylated protein, which is a key molecule in the ERK-MAPK pathway. Our study may contribute to elucidating the mechanisms underlying immune evasion during E. maxima infections, thereby providing new insights for the control of chicken coccidiosis.

Inhibitory effect of Eimeria maxima IFN-γ inhibitory molecules on the immune function of T cell subsets in chickens

It has been reported that infection of chicken coccidian could inhibit the production of Th1 cytokine IFN-γ, thereby evading clearance by the host immune system. The present study aimed to have a further investigation into the effects of Eimeria maxima IFN-γ inhibitory molecules (EmHPSP-2 and EmHPSP-3) on the immune function of chicken peripheral blood mononuclear cells (PBMC) and various T cell subsets. First, separated PBMC or sorted T cell subsets were used for incubation with recombinant proteins of EmHPSP-2 (rEmHPSP-2) and EmHPSP-3 (rEmHPSP-3). Subsequently, the effects of rEmHPSP-2 and rEmHPSP-3 on proliferative capacity, nitric oxide (NO) release and mRNA levels of cytokines of the above cells were detected. The sorting purity of CD8+, CD4+ CD25-, CD4+, and CD4+ CD25+ T cells was 93.01, 88.88, 87.04, and 81.26%, respectively. The NO release of PBMC was significantly inhibited by rEmHPSP-2 and rEmHPSP-3. The proliferation of PBMC and CD4+ T cells was significantly inhibited by rEmHPSP-2 and rEmHPSP-3, whereas CD8+, CD4+ CD25-, and CD4+ CD25+ T cells was significantly promoted by the 2 proteins. The 2 proteins significantly downregulated interferon-gamma (IFN-γ) mRNA level, upregulated the transcriptional levels of interleukin-10 (IL-10) and transforming growth factor-beta1 (TGF-β1) in PBMC. IFN-γ and IL-2 transcriptional levels were markedly inhibited in CD8+ T cells. IFN-γ transcriptional level was significantly inhibited, but IL-4 was promoted by rEmHPSP-2 and rEmHPSP-3 in CD4+ CD25- T cells. Meanwhile, the inhibitory effects of rEmHPSP-2 and rEmHPSP-3 on the transcriptional levels of IFN-γ and IL-2 were more obvious in CD4+ T cells containing CD25+ cells compared with the CD25+ cells depletion group. It was found that IL-10, TGF-β1, and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) mRNA levels were significantly upregulated upon stimulation of chicken CD4+ CD25+ T cells by proteins. This study is not only of great significance to clarify the immune evasion mechanism of chicken coccidia, but also provides candidate antigen molecules for development of a novel vaccine against chicken coccidiosis.

Mechanisms of immune evasion in leishmaniasis

Diseases caused by Leishmania present a worldwide problem, and current therapeutic approaches are unable to achieve a sterile cure. Leishmania is able to persist in host cells by evading or exploiting host immune mechanisms. A thorough understanding of these mechanisms could lead to better strategies for effective management of Leishmania infections. Current research has focused on parasite modification of host cell signaling pathways, entry into phagocytic cells, and modulation of cytokine and chemokine profiles that alter immune cell activation and trafficking to sites of infection. Immuno-therapeutic approaches that target these mechanisms of immune evasion by Leishmania offer promising areas for preclinical and clinical research.

Visceral leishmaniasis: Revisiting current treatments and approaches for future discoveries

The current treatments for visceral leishmaniasis are old and toxic with limited routes of administration. The emergence of drug-resistant Leishmania threatens the efficacy of the existing reservoir of antileishmanials, leading to an urgent need to develop new treatments. It is particularly important to review and understand how the current treatments act against Leishmania in order to identify valid drug targets or essential pathways for next-generation antileishmanials. It is equally important to adapt newly emerging biotechnologies to facilitate the current research on the development of novel antileishmanials in an efficient fashion. This review covers the basic background of the current visceral leishmaniasis treatments with an emphasis on the modes of action. It briefly discusses the role of the immune system in aiding the chemotherapy of leishmaniasis, describes potential new antileishmanial drug targets and pathways, and introduces recent progress on the utilization of high-throughput phenotypic screening assays to identify novel antileishmanial compounds.

Acetyl shikonin induces IL-12, nitric oxide and ROS to kill intracellular parasite Leishmania donovani in infected hosts

Acetyl shikonin (AS), a naphthoquinone isolated from Arnebia nobilis, was tested against visceral leishmaniasis in this study, revealing that AS provides a chemo-immunotherapeutic strategy against visceral leishmaniasis.

Targeted extracellular signal-regulated kinase activation mediated by Leishmania amazonensis requires MP1 scaffold

Leishmania amazonensis infection promotes alteration of host cellular signaling and intracellular parasite survival, but specific mechanisms are poorly understood. We previously demonstrated that L. amazonensis infection of dendritic cells (DC) activated extracellular signal-regulated kinase (ERK), an MAP-kinase kinase kinase, leading to altered DC maturation and non-healing cutaneous leishmaniasis. Studies using growth factors and cell lines have shown that targeted, robust, intracellular phosphorylation of ERK1/2 from phagolysosomes required recruitment and association with scaffolding proteins, including p14/MP1 and MORG1, on the surface of late endosomes. Based on the intracellular localization of L. amazonensis within a parasitophorous vacuole with late endosome characteristics, we speculated that scaffolding proteins would be important for intracellular parasite-mediated ERK signaling. Our findings demonstrate that MP1, MORG1, and ERK all co-localized on the surface of parasite-containing LAMP2-positive phagolysosomes. Infection of MEK1 mutant fibroblasts unable to bind MP1 demonstrated dramatically reduced ERK1/2 phosphorylation following L. amazonensis infection but not following positive control EGF treatment. This novel mechanism for localization of intracellular L. amazonensis-mediated ERK1/2 phosphorylation required the endosomal scaffold protein MP1 and localized to L. amazonensis parasitophorous vacuoles. Understanding how L. amazonensis parasites hijack host cell scaffold proteins to modulate signaling cascades provides targets for antiprotozoal drug development.

Indoleamine 2,3-dioxygenase (IDO) induced by Leishmania infection of human dendritic cells

Dendritic cells (DC) play a pivotal role in regulating immunity, establishing immunologically privileged tissue microenvironments and maintaining homoeostasis. It is becoming increasingly clear that one key mechanism that mediates many DC functions is production of the immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO). For pathogens that cause chronic infection, exploitation of host DCs is a solution to establish and persist within a host. Leishmania parasites cause a range of clinical manifestations, all involving chronic infection, and are proficient at avoiding immune responses. We demonstrate here that infection of human myeloid-derived DC with L. major and L. donovani induces IDO expression using a mechanism that involves autocrine or paracrine stimulation with a DC-secreted factor. Leishmania-induced IDO suppresses allogeneic and tetanus toxoid-specific lymphocyte proliferation, an inhibition that is reversed with the IDO inhibitor, 1-methyl tryptophan (1-MT). Furthermore, IDO expression by human DC does not require live Leishmania infection, as parasite lysates also up-regulate IDO mRNA production. Our data suggest that one mechanism Leishmania parasites utilize to circumvent immune clearance may be to promote the induction of IDO among host DC within the infection microenvironment.

Immunological perspectives of leishmaniasis

Leishmania parasites have been widely used in experimental models to understand generation, maintenance and failure of immune responses underlying resistance and susceptibility to infection. The clinical outcomes of Leishmania infection depend on the infecting species and the immune status of the host. Noticeably most people exposed Leishmania never develop overt disease. Understanding the immunological events that result in failure or successful control of the parasites is fundamental to both design and evaluation of vaccines and therapies against the leishmaniases. Recent studies visualizing immune response to Leishmania major in the skin have given new insights into the different immune cells acting as hosts the parasite during different stage of infection. Control of Leishmania infection and disease progression has been associated with generation of T-helper (Th) 1 and Th2 responses respectively. Though still valid in several aspects, the Th1/Th2 paradigm is an oversimplification in need of revision. Th2 polarization has never explained severity of human leishmanial disease and a number of other T-cell subsets, including regulatory T- and Th17- cells, have important roles in susceptibility and resistance of both experimental and human leishmanial disease. This review gives an updated overview of immunological response considered to be of importance in protection, susceptibility, disease progression and cure of leishmaniasis, with a special emphasis on human diseases.

Cytokines and their STATs in cutaneous and visceral leishmaniasis

Cytokines play a critical role in shaping the host immune response to Leishmania infection and directing the development of protective and non-protective immunities during infection. Cytokines exert their biological activities through the activation and translocation of transcription factors into the nucleus whether they drive the expression of specific cytokine-responsive genes. Signal transducer and activator of transcription (STATs) are transcription factors which play a critical role in mediating signaling downstream of cytokine receptors and are important for shaping the host immune response during Leishmania infection. Here we discuss the signature cytokines and their associated STATs involved in the host immune response during cutaneous and visceral leishmaniasis.

Leishmania major phosphoglycans influence the host early immune response by modulating dendritic cell functions

The precise role of Leishmania glycoconjugate molecules including phosphoglycans (PGs) and lipophosphoglycan (LPG) on host cellular responses is still poorly defined. Here, we investigated the interaction of Leishmania major LPG2 null mutant (lpg2(-)), which lacks both PGs and LPG, with dendritic cells (DCs) and the subsequent early immune response in infected mice. Surprisingly, the absence of phosphoglycans did not influence expression pattern of major histocompatibility complex class II (MHC II), CD40, CD80, and CD86 on DCs in vitro and in vivo. However, lpg2(-) L. major induced significantly higher production of interleukin-12p40 (IL-12p40) by infected bone marrow-derived DCs (BMDCs) than wild-type (WT) parasites in vitro. Furthermore, the production of IL-12p40 by draining lymph node cells from lpg2(-) mutant-infected mice was higher than those from WT L. major-infected mice. In model antigen presentation experiments, DCs from lpg2(-) mutant-infected mice induced more gamma interferon (IFN-gamma) and IL-2 production by Leishmania-specific T cells than those from WT-infected mice. Lymphocytes isolated from mice infected for 3 days with lpg2(-) parasites produce similar levels of IFN-gamma, but significantly less IL-4 and IL-10 than WT controls. Decreased IL-4 production was also seen in another general PG-deficient mutant lacking the Golgi UDP-galactose transporters (lpg5A(-) lpg5B(-)), but not with the lpg1(-) mutant lacking only LPG, thereby implicating PGs generally in the reduction of IL-4 production. Thus, Leishmania PGs influence host early immune response by modulating DC functions in a way that inhibits antigen presentation and promotes early IL-4 response, and their absence may impact the balance between Th1 and Th2 responses.

Macrophage and Leishmania: An Unacceptable Coexistence

Research over the past year has revealed several significant and interesting advances in the biology of macrophage, key cells responsible in body's host defense against invading pathogens and in immune responses. Perturbation of macrophage surface with different bacterial pathogens leads to activate general signal transduction pathways of macrophages, including activation of NADPH oxidase, nitric oxide synthase, and so on. However, in this review, the results of macrophage interactions only with Leishmania parasites, which harbors the host macrophages, are discussed. It appears that interference in transduction of regulatory signals during leishmanial invasion lead to an inadequate leishmanicidal response. In this connection, information concerning regulation of MHC molecules and other current events related to macrophage function after invasion by the parasites are also discussed.

Leishmania braziliensis infection induces dendritic cell activation, ISG15 transcription, and the generation of protective immune responses

Leishmania (Viannia) braziliensis is the causative agent of cutaneous and mucosal leishmaniasis in South America, and the latter is a severe and disfiguring form of the disease. Our understanding of how L. braziliensis parasites interact with dendritic cells (DCs) is limited, partially due to the difficulty in generating axenic amastigotes. In this study, we successfully generated axenic amastigotes of L. braziliensis and used them to test the hypothesis that L. braziliensis infection efficiently triggers innate responses in DCs and the subsequent adaptive immune responses for parasite clearance. This study has revealed unique immunological features of L. braziliensis infection. Firstly, axenic amastigotes showed higher infectivity and the potential to stimulate C57BL/6 (B6) bone marrow-derived dendritic cells to produce IL-12p40 when compared with their promastigote counterparts. Both parasite-carrying and bystander DCs displayed an activated (CD11c(high)CD45RB(-)CD83(+)CD40(+)CD80(+)) phenotype. Secondly, L. braziliensis infection triggered transcription and phosphorylation of STAT molecules and IFN-stimulated gene 15 (ISG15). Finally, the self-healing of the infection in mice was correlated to the expansion of IFN-gamma- and IL-17-producing CD4(+) cells, suggesting the existence of active mechanisms to regulate local inflammation. Collectively, this study supports the view that innate responses at the DC level determine parasite-specific T cell responses and disease outcomes.

Leishmania major: effects of proteophosphoglycan on reactive oxygen species, IL-12, IFN-gamma and IL-10 production in healthy individuals

Protozoan parasites of the genus Leishmania secrete a range of proteophosphoglycans (PPG) known to be important for successful colonization of Leishmania in the sandfly and for virulence in the mammalian host. PPGs are a large family of extensively glycosylated proteins with some unusual and unique features. In this study we purified PPG from culture supernatant of Leishmania major metacyclic promastigotes. In discontinuous SDS-PAGE, PPG could not enter the resolving gel but after mild acid hydrolysis several bands resolved. Agarose gel electrophoresis and immunoblot analysis using monoclonal antibody (WIC 79.3) indicated that the PPG preparation consisted of heterogeneous molecules. Compositional analysis showed that the PPG preparation contained 67% glycan, 28% protein and 5% phosphate. Additionally, the effect of PPG on reactive oxygen species (ROS) production and induction of IL-10, IL-12 and IFN-gamma secretion by human peripheral blood mononuclear cells (PBMCs) isolated from healthy individuals was investigated. The water-soluble secreted form of PPG at a concentration of 1 microg glycan/ml seems to be a potent inducer of ROS and IL-10 and to a lesser extent of IFN-gamma and IL-12. Cytokines and ROS production was decreased in a dose-dependent manner as the concentration of PPG was increased to 100 microg glycan/ml.

Transcriptional inhibition of interleukin-12 promoter activity in Leishmania spp.-infected macrophages

To establish and persist within a host, Leishmania spp. parasites delay the onset of cell-mediated immunity by suppressing interleukin-12 (IL-12) production from host macrophages. Although it is established that Leishmania spp.-infected macrophages have impaired IL-12 production, the mechanisms that account for this suppression remain to be completely elucidated. Using a luciferase reporter assay assessing IL-12 transcription, we report here that Leishmania major, Leishmania donovani, and Leishmania chagasi inhibit IL-12 transcription in response to interferon-gamma, lipopolysaccharide, and CD40 ligand and that Leishmania spp. lipophosphoglycan, phosphoglycans, and major surface protein are not necessary for inhibition. In addition, all the Leishmania spp. strains and life-cycle stages tested inhibited IL-12 promoter activity. Our data further reveal that autocrine-acting host factors play no role in the inhibitory response and that phagocytosis signaling is necessary for inhibition of IL-12.

The amastigote forms of Leishmania are experts at exploiting host cell processes to establish infection and persist

Leishmania are dimorphic protozoan parasites that live as flagellated forms in the gut of their sandfly vector and as aflagellated forms in their mammalian hosts. Although both parasite forms can infect macrophages and dendritic cells, they elicit distinct responses from mammalian cells. Amastigotes are the parasites forms that persist in the infected host; they infect cells recruited to lesions and disseminate the infection to secondary sites. In this review I discuss studies that have investigated the mechanisms that Leishmania amastigotes employ to harness the host cell's response to infection. It should be acknowledged that our understanding of the mechanisms deployed by Leishmania amastigotes to modulate the host cell's response to infection is still rudimentary. Nonetheless, the results show that amastigote interactions with mammalian cells promote the production of anti-inflammatory cytokines such as IL-10 and TGF-beta while suppressing the production of IL-12, superoxide and nitric oxide. An underlying issue that is considered is how these parasites that reside in sequestered vacuolar compartments target host cell processes in the cytosol or the nucleus; does this occur through the release of parasite molecules from parasitophorous vacuoles or by engaging and sustaining signalling pathways throughout the course of infection?

Cutaneous Leishmania infection: progress in pathogenesis research and experimental therapy

Studies in murine experimental Leishmania major infection have helped to understand the requirements for efficient development of T helper (Th)1/cytotoxic T (Tc)1-mediated protection against the parasite. As such they have revealed that Fc gamma receptor (Fc gamma R)I and Fc gamma RIII-mediated uptake of L. major amastigotes by dendritic cells (DC) is an important prerequisite for Th1 development. In addition, DC-derived cytokines contribute to adequate T-cell education. DC-based vaccines may thus provide an important tool for both the development of a prophylactic vaccine against leishmaniasis and - together with leishmanicidal drugs - for eliciting immune-deviating functions towards protective immunity in non-healing leishmaniasis. This review highlights recent advances in the understanding of the role of DC for the induction of Th1/Tc1-predominant immunity against L. major and how this knowledge may translate into clinical approaches.

Multiplex analysis of circulating cytokines in the sera of patients with different clinical forms of visceral leishmaniasis

BACKGROUND

The clinical spectrum of visceral leishmaniasis (VL), a chronic intracellular parasitic disease, ranges from a subclinical, asymptomatic infection to severe clinical disease (kala-azar). In experimental leishmaniasis, mice that have a Th1 response to infection tend to have limited disease while a Th2 response is associated with disease progression. Humans with VL most often have mixed rather than polarized responses. However, most clinical studies have used methods that require a relatively large sample volume, thus limiting their scope. Measuring multiple cytokine levels in blood samples using a multiplexed microsphere assay (MMA) may be useful to further evaluate the Th1/Th2 paradigm in humans.

METHODS

Bangladeshi individuals (n=120) living in an area endemic for VL were categorized into one of the five clinical categories. Sera from these individuals were measured for levels of IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, IFN-gamma, and TNF-alpha by multiplexed microsphere cytokine immunoassay.

RESULTS

Circulating IL-8, IL-10, and IL-12 differed significantly among the clinical groups. Persons with kala-azar demonstrated the highest median levels of IL-8 and IL-10 but lower median levels of IL-12.

CONCLUSIONS

The MMA for cytokines is an extremely time-and sample-efficient method for characterizing circulating cytokine levels in visceral leishmaniasis patients.

Apoptotic cells at the crossroads of tolerance and immunity

Clearance of apoptotic cells by phagocytes can result in either anti-inflammatory and immunosuppressive effects or prostimulatory consequences through presentation of cell-associated antigens to T cells. The differences in outcome are due to the conditions under which apoptosis is induced, the type of phagocytic cell, the nature of the receptors involved in apoptotic cell capture, and the milieu in which phagocytosis of apoptotic cells takes place. Preferential ligation of specific receptors on professional antigen-presenting cells (dendritic cells) has been proposed to induce potentially tolerogenic signals. On the other hand, dendritic cells can efficiently process and present antigens from pathogen-infected apoptotic cells to T cells. In this review, we discuss how apoptotic cells manipulate immunity through interactions with dendritic cells.

The immunopathology of experimental visceral leishmaniasis

Experimental murine infection with the parasites that cause human visceral leishmaniasis (VL) results in the establishment of infection in the liver, spleen, and bone marrow. In most strains of mice, parasites are eventually cleared from the liver, and hepatic resistance to infection results from a coordinated host response involving a broad range of effector and regulatory pathways targeted within defined tissue structures called granulomas. In contrast, parasites persist in the spleen and bone marrow by mechanisms that are less well understood. Parasite persistence is accompanied by the failure of granuloma formation and by a variety of pathologic changes, including splenomegaly, disruption of lymphoid tissue microarchitecture, and enhanced hematopoietic activity. Here, we review the salient features of these distinct tissue responses and highlight the varied roles that cytokines of the tumor necrosis factor family play in immunity to this infection. In addition, we also discuss recent studies aimed at understanding how splenomegaly affects the survival and function of memory cells specific for heterologous antigens, an issue of considerable importance for our understanding of the disease-associated increase in secondary infections characteristic of human VL.

The role(s) of lipophosphoglycan (LPG) in the establishment of Leishmania major infections in mammalian hosts

The abundant cell surface glycolipid lipophosphoglycan (LPG) was implicated in many steps of the Leishmania infectious cycle by biochemical tests. The presence of other abundant surface or secreted glycoconjugates sharing LPG domains, however, has led to uncertainty about the relative contribution of LPG in vivo. Here we used an Leishmania major lpg1- mutant, which lacks LPG alone and shows attenuated virulence, to dissect the role of LPG in the establishment of macrophage infections in vivo. lpg1- was highly susceptible to human complement, had lost the ability to inhibit phagolysosomal fusion transiently, and was oxidant sensitive. Studies of mouse mutants defective in relevant defense mechanisms confirmed the role of LPG in oxidant resistance but called into question the importance of transient inhibition of phagolysosomal fusion for Leishmania macrophage survival. Moreover, the limited lytic activity of mouse complement appears to be an ineffective pathogen defense mechanism in vitro and in vivo, unlike human hosts. In contrast, lpg1- parasites bound C3b and resisted low pH and proteases normally, entered macrophages efficiently and silently, and continued to inhibit host-signaling pathways. These studies illustrate the value of mechanistic approaches focusing on both parasite and host defense pathways in dissecting the specific biological roles of complex virulence factors such as LPG.

Leishmania donovani-induced expression of suppressor of cytokine signaling 3 in human macrophages: a novel mechanism for intracellular parasite suppression of activation

Leishmania donovani protozoan parasites, the causative agent of visceral leishmaniasis, establish an infection partly by interfering with cytokine signaling in the host macrophages. Therefore, we investigated the expression of the suppressor of cytokine signaling (SOCS) genes in human macrophages infected with L. donovani. The expression of SOCS3 mRNA was induced transiently after exposure to live or heat-killed parasites, but not purified lipophosphoglycan, while that of other SOCS genes remained unchanged. SOCS3 gene expression was not dependent on phagocytosis or on cytokines released by L. donovani-infected macrophages, such as interleukin-1beta or tumor necrosis factor alpha. In addition, Leishmania used a different signaling pathway(s) than bacterial lipopolysaccharide to induce SOCS3 mRNA, as indicated by the kinetics of induction and sensitivity to polymyxin B inhibition. Finally, phosphorylation of the STAT1 transcription factor was significantly reduced in L. donovani-infected macrophages and required de novo transcription. The induction of SOCS3 provides a potent inhibitory mechanism by which intracellular microorganisms may suppress macrophage activation and interfere with the host immune response.

IL-12Rbeta2-deficient mice of a genetically resistant background are susceptible to Leishmania major infection and develop a parasite-specific Th2 immune response

The cytokine IL-12 plays a critical role in inducing the production of IFN-gamma from T and NK cells and in the polarization of T cells towards the Th1 phenotype. IL-12 is comprised of two subunits (IL-12p40 and IL-12p35) that together form the biologically active p70 molecule, and IL-12 functions via binding to a heterodimeric receptor (IL-12Rbeta1 and IL-12Rbeta2). Previous studies utilizing mice deficient for either the IL-12 cytokine or the IL-12-induced signaling molecule STAT4 have established a critical role for IL-12 during infection with Leishmania major. However, these studies warrant careful re-interpretation in light of the recent discovery of the IL-12-related cytokine, IL-23, which utilizes the IL-12p40 chain in combination with an IL-12p35-related molecule, called p19, and a receptor comprised of the IL-12Rbeta1 chain plus a unique chain referred to as IL-23R. We analyzed the course of L. major infection in mice deficient for the IL-12-specific IL-12Rbeta2 subunit in order to assess the role of IL-12 signaling without disruption of the IL-23 pathway. After infection with L. major, IL-12Rbeta2KO mice of a resistant background (C57Bl/6) developed large cutaneous lesions similar to those developed by susceptible BALB/c mice. Draining lymph node cells from L. major-infected IL-12Rbeta2KO mice released the Th2 cytokines IL-4 and IL-5 after in vitro stimulation with Leishmania lysate but were completely devoid of IFN-gamma, consistent with a default towards a strong parasite-specific Th2 response. L. major-infected IL-12Rbeta2KO mice were also devoid of parasite-specific IgG2a antibodies, and interestingly, their footpad lesions ulcerated earlier than those of susceptible BALB/c mice.

Immunomodulatory effects of Maxadilan and Phlebotomus papatasi sand fly salivary gland lysates on human primary in vitro immune responses

Leishmaniasis is a parasitic disease transmitted by the bite of Leishmania-infected sand flies. Here we show for the first time the ability of Maxadilan (Max), a vasodilatory peptide isolated from the sand fly Lutzomyia longipalpis, and salivary gland lysate (SGL) from Phlebotomus papatasi to decrease the secretion of Type 1 cytokines and to enhance the production of the Type 2 cytokine interleukin (IL)-6 by human peripheral blood mononuclear cells (PBMC) and monocytes. We found Max decreased the secretion of interferon (IFN)-gamma and IL-12p40 by PBMC and TNF-alpha by monocytes. SGL reduced the production of IFN-gamma by PBMC. In contrast, production of the Type 2 cytokine IL-6 was increased in Max or SGL-exposed cells. Finally, we determined that Max interacts with human cells through at least the pituitary adenylate cyclase activating polypeptide receptor. These results show that sand fly salivary gland components have an immunomodulatory effect on human cells, and this has important implications for the development of vaccines against leishmaniasis for humans.

Leishmania major activates IL-1 alpha expression in macrophages through a MyD88-dependent pathway

Leishmania species present unusual challenges to the immune system with their capacity to downregulate inflammatory responses as well as their ability to live within macrophages. Although toll-like receptor (TLR) pathways have been implicated in the recognition of several classes of pro-inflammatory microbes, it is not known if pathogens with anti-inflammatory properties activate the host response through this family of proteins. In this study, Leishmania major stimulation of cytokine promoter-luciferase reporter constructs was examined in transfected macrophages to detect early signs of cellular activation. L. major selectively activated the promoter region of IL-1 alpha, but not IL-6, IL-8, IL-10, or an NF-kappa B reporter. IL-1 alpha mRNA expression was also stimulated by L. major, although at lower levels than lipopolysacharide-stimulated macrophages. No IL-1 alpha protein was detectable in stimulated cell lysates or culture supernatants. Transfection of macrophages with a dominant-negative version of myeloid differentiation factor 88 (MyD88), an adaptor protein which interacts with TLRs, inhibited activation of the IL-1 alpha promoter. Furthermore, stimulation of IL-1 alpha RNA expression by L. major was inhibited in peritoneal macrophages from MyD88-/- as compared to MyD88+/+ mice. These observations indicate that L. major stimulates IL-1 alpha promoter activity and mRNA expression in macrophages through MyD88-dependent pathways. However, additional anti-inflammatory pathways must also be activated which downregulate transcription and ultimately inhibit translation of the IL-1 alpha protein. Examination of promoter activation is a powerful tool for understanding the early events in macrophage activation for anti-inflammatory pathogens such as Leishmania that have mechanisms to downregulate transcription and translation.

Evasion of innate immunity by parasitic protozoa

Parasitic protozoa are a major cause of global infectious disease. These eukaryotic pathogens have evolved with the vertebrate immune system and typically produce long-lasting chronic infections. A critical step in their host interaction is the evasion of innate immune defenses. The ability to avoid attack by humoral effector mechanisms, such as complement lysis, is of particular importance to extracellular parasites, whereas intracellular protozoa must resist killing by lysosomal enzymes and toxic metabolites. They do so by remodeling the phagosomal compartments in which they reside and by interfering with signaling pathways that lead to cellular activation. In addition, there is growing evidence that protozoan pathogens modify the antigen-presenting and immunoregulatory functions of dendritic cells, a process that facilitates their evasion of both innate and adaptive immunity.

Mechanism of entry determines the ability of Toxoplasma gondii to inhibit macrophage proinflammatory cytokine production

Macrophages (Mphi) infected with tachyzoites of the opportunistic protozoan Toxoplasma gondii are blocked in production of the proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12) in response to lipopolysaccharide (LPS) triggering, and this is associated with parasite-induced inhibition of NFkappaB translocation. Here, we demonstrate a requirement for active invasion in the ability of the parasite to mediate suppression. Neither soluble tachyzoite antigen nor secreted products were suppressive, and heat-inactivated, antibody-coated tachyzoites, which efficiently entered the cell through receptor-mediated uptake, failed to inhibit LPS responses. Cytochalasin D, a drug blocking tachyzoite invasion of, but not adherence to, Mphi, severely curtailed Toxoplasma-induced suppression. In addition, parasite-induced nonresponsiveness, as measured by TNF-alpha production, was reversed by treating infected cells with the toxoplasmastatic drugs pyrimethamine and 6-thioxanthine prior to LPS stimulation. A divergence in IL-12 and TNF-alpha responses was found during extended incubation of tachyzoites and Mphi in that 24 h of incubation of infected Mphi resulted in IL-12, but not TNF-alpha, secretion, and production of the latter cytokine remained suppressed when these cells were subjected to LPS triggering. Our results demonstrate that active invasion and survival of the parasite within the parasitophorous vacuole are required to induce and maintain Mphi cytokine-specific nonresponsiveness to LPS. They also show that the effects of Toxoplasma on IL-12 and TNF-alpha production are nonidentical, with the parasite exerting a longer-lasting suppression of the latter.

Opsonization modulates Rac-1 activation during cell entry by Leishmania amazonensis

Lesions caused by Leishmania amazonensis normally heal, but relapses occur due to parasite persistence in host tissues. It has been proposed that infection of fibroblasts plays an important role in this process by providing the parasites with a safe haven in which to replicate. However, most previous studies have focused on the entry of Leishmania into macrophages, a process mediated by serum opsonins. To gain insight into a possible role of nonopsonic entry in the intracellular persistence of amastigotes, we examined the invasion of Chinese hamster ovary (CHO) cells. Amastigotes entered CHO cells by a cytochalasin D, genistein, wortmannin, and 2,3-butanedione monoxime-sensitive pathway and replicated within phagolysosomes. However, unlike most phagocytic processes described to date, amastigote internalization in CHO cells involved activation of the GTPases Rho and Cdc42 but not Rac-1. When uptake was mediated by fibronectin or when amastigotes were opsonized with immunoglobulin G and internalized by Fc receptor-expressing CHO cells, Rac-1 activation was restored and found to be required for parasite internalization. Given the essential role of Rac in assembly of the respiratory burst oxidase, invasion through this nonopsonic, Rac-1-independent pathway may play a central role in the intracellular survival of Leishmania in immune hosts.

Phagocytosis of microbes: complexity in action

The phagocytic response of innate immune cells such as macrophages is defined by the activation of complex signaling networks that are stimulated by microbial contact. Many individual proteins have been demonstrated to participate in phagocytosis, and the application of high-throughput tools has indicated that many more remain to be described. In this review, we examine this complexity and describe how during recognition, multiple receptors are simultaneously engaged to mediate internalization, activate microbial killing, and induce the production of inflammatory cytokines and chemokines. Many signaling molecules perform multiple functions during phagocytosis, and these molecules are likely to be key regulators of the process. Indeed, pathogenic microorganisms target many of these molecules in their attempts to evade destruction.

Leishmania amazonensis-dendritic cell interactions in vitro and the priming of parasite-specific CD4(+) T cells in vivo

The progressive disease following Leishmania amazonensis infection in mice requires functional CD4(+) T cells, which are primed to a disease-promoting phenotype during the infection. To understand how these pathogenic T cells are generated and the role of dendritic cells (DCs) in this process, we use DCs of susceptible BALB/c and resistant C3H/HeJ mice to examine parasite-DC interactions in vitro as well as the effector phenotype of T cells primed by parasite-exposed DCs in vivo. Our results demonstrate that amastigotes and metacyclics efficiently enter and activate DCs of both genetic backgrounds. Infection with amastigotes fails to induce CD40-dependent IL-12 production, but rather potentiates IL-4 production in BALB/c DCs. Upon transfer into syngeneic recipients, amastigote-exposed BALB/c DCs prime parasite-specific Th cells to produce significantly higher levels of IL-4 and IL-10 than their C3H/HeJ counterparts. Transfer studies with IL-4(-/-) DCs indicate that this enhanced Th2 priming seen in BALB/c mice is partially due to the IL-4 production by amastigote-carrying DCs. These results suggest that L. amazonensis amastigotes may condition DCs of a susceptible host to a state that favors activation of pathogenic CD4(+) T cells, and thereby provide a new perspective on the pathogenesis of cutaneous leishmaniasis and protozoan parasite-host interactions in general.

An IFN-gamma-inducible transcription factor, IFN consensus sequence binding protein (ICSBP), stimulates IL-12 p40 expression in macrophages

IL-12 is a cytokine that links innate and adaptive immunity. Its subunit p40 is induced in macrophages following IFN-gamma/LPS stimulation. Here we studied the role for IFN consensus sequence binding protein (ICSBP), an IFN-gamma/LPS-inducible transcription factor of the IFN regulatory factor (IRF) family in IL-12 p40 transcription. Macrophage-like cells established from ICSBP-/- mice did not induce IL-12 p40 transcripts, nor stimulated IL-12 p40 promoter activity after IFN-gamma/LPS stimulation, although induction of other inducible genes was normal in these cells. Transfection of ICSBP led to a marked induction of both human and mouse IL-12 p40 promoter activities in ICSBP+/+ and ICSBP-/- cells, even in the absence of IFN-gamma/LPS stimulation. Whereas IRF-1 alone was without effect, synergistic enhancement of promoter activity was observed following cotransfection of ICSBP and IRF-1. Deletion analysis of the human promoter indicated that the Ets site, known to be important for activation by IFN-gamma/LPS, also plays a role in the ICSBP activation of IL-12 p40. A DNA affinity binding assay revealed that endogenous ICSBP is recruited to the Ets site through protein-protein interaction. Last, transfection of ISCBP alone led to induction of the endogenous IL-12 p40 mRNA in the absence of IFN-gamma and LPS. Taken together, our results show that ICSBP induced by IFN-gamma/LPS, acts as a principal activator of IL-12p40 transcription in macrophages.