The immunopathology of experimental visceral leishmaniasis

Dynamic imaging of experimental Leishmania donovani-induced hepatic granulomas detects Kupffer cell-restricted antigen presentation to antigen-specific CD8 T cells

Kupffer cells (KCs) represent the major phagocytic population within the liver and provide an intracellular niche for the survival of a number of important human pathogens. Although KCs have been extensively studied in vitro, little is known of their in vivo response to infection and their capacity to directly interact with antigen-specific CD8⁺ T cells. Here, using a combination of approaches including whole mount and thin section confocal microscopy, adoptive cell transfer and intra-vital 2-photon microscopy, we demonstrate that KCs represent the only detectable population of mononuclear phagocytes within granulomas induced by Leishmania donovani infection that are capable of presenting parasite-derived peptide to effector CD8⁺ T cells. This restriction of antigen presentation to KCs within the Leishmania granuloma has important implications for the identification of new candidate vaccine antigens and for the design of novel immuno-therapeutic interventions.

Leishmania donovani is a protozoan parasite that causes severe disease in humans with associated pathology in the spleen and liver. In experimental models of L. donovani infection, the hepatic response to infection is characterised by the presence of a focal mononuclear cell-rich inflammatory response (a granuloma) surrounding cells infected with intracellular amastigotes. Granulomas provide focus to the ensuing immune response, helping to contain parasite dissemination and providing the major effector site responsible for parasites elimination from the liver. Although granulomas are believed to form around infected resident liver macrophages (Kupffer cells), the role of these cells in intra-granuloma antigen presentation is currently unknown. As CD8⁺ T cells have been shown to play an important role in hepatic resistance to L. donovani following natural infection, vaccination and during immunotherapy, we asked which cells within the granuloma microenvironment serve as targets for antigen recognition by effector CD8⁺ T cells. Here we provide evidence that the heavily infected mononuclear cell core of the granuloma is composed almost entirely of Kupffer cells, many having migrated from the surrounding sinusoids. Furthermore, by intra-vital 2-photon microscopy, we show that only Kupffer cells laden with intracellular amastigotes are able to form long-lasting antigen-specific interactions with CD8⁺ T cells within the granuloma microenvironment. These data have important implications for the understanding of how granulomas function to limit infection and may have important implications for the development of vaccines to Leishmania that are designed to induce CD8⁺ T cell responses.

Therapeutic glucocorticoid-induced TNF receptor-mediated amplification of CD4+ T cell responses enhances antiparasitic immunity

Chronic infectious diseases and cancers are often associated with suboptimal effector T cell responses. Enhancement of T cell costimulatory signals has been extensively studied for cancer immunotherapy but not so for the treatment of infectious disease. The few previous attempts at this strategy using infection models have lacked cellular specificity, with major immunoregulatory mechanisms or innate immune cells also being targeted. In this study, we examined the potential of promoting T cell responses via the glucocorticoid-induced TNF receptor (GITR) family-related protein in a murine model of visceral leishmaniasis. GITR stimulation during established infection markedly improved antiparasitic immunity. This required CD4(+) T cells, TNF, and IFN-gamma, but crucially, was independent of regulatory T (Treg) cells. GITR stimulation enhanced CD4(+) T cell expansion without modulating Treg cell function or protecting conventional CD4(+) T cells from Treg cell suppression. GITR stimulation substantially improved the efficacy of a first-line visceral leishmaniasis drug against both acute hepatic infection and chronic infection in the spleen, demonstrating its potential to improve clinical outcomes. This study identifies a novel strategy to therapeutically enhance CD4(+) T cell-mediated antiparasitic immunity and, importantly, achieves this goal without impairment of Treg cell function.

Dendritic cells matured by inflammation induce CD86-dependent priming of naive CD8+ T cells in the absence of their cognate peptide antigen

Dendritic cells (DC) licensed by the interaction between pathogen products and pattern recognition receptors can activate naive T cells to undergo Ag-dependent proliferation and cytokine production. In contrast, DC induced to mature by trans-acting inflammatory stimuli are believed to only be capable of supporting Ag-dependent proliferative responses. In this study, we show that uninfected DC matured as a consequence of Leishmania-induced inflammation induce CD8(+) T cells to proliferate in the absence of their cognate Ag. We separated splenic DC from Leishmania donovani-infected mice into those that contained parasites and had been activated to induce IL-12p40, from those that had undergone only partial maturation, measured by increased CD86 expression in the absence of IL-12p40 induction. We then showed that these partially matured DC could induce exogenous peptide-independent proliferation of OT-I and F5 CD8(+) TCR transgenic T cells, as well as polyclonal CD8(+) T cells. Proliferation of OT-I cells was significantly inhibited in vitro and in vivo by anti-CD86 mAb but not by anti-CD80 mAb and could also be inhibited by cyclosporine A. Proliferating OT-I cells did not produce IFN-gamma, even when re-exposed to mature DC. However, these primed OT-I cells subsequently produced effector cytokines, not just on exposure to their cognate peptide but, more importantly, to weak exogenous TCR agonists that otherwise failed to induce IFN-gamma. We further showed that OT-I cells undergoing locally driven proliferation to another pathogen, Streptococcus pneumoniae, rapidly seeded other lymphoid tissues, suggesting that CD8(+) T cells primed in this way may play a role in rapidly countering pathogen dissemination.

Immunoactivation and immunopathogeny during active visceral leishmaniasis

Visceral leishmaniasis is caused by protozoan parasites of the Leishmania donovani complex. During active disease in humans, high levels of IFN-γ and TNF-α detected in blood serum, and high expression of IFN-γ mRNA in samples of the lymphoid organs suggest that the immune system is highly activated. However, studies using peripheral blood mononuclear cells have found immunosuppression specific to Leishmania antigens; this poor immune response probably results from Leishmania antigen-engaged lymphocytes being trapped in the lymphoid organs. To allow the parasites to multiply, deactivating cytokines IL-10 and TGF-β may be acting on macrophages as well as anti-Leishmania antibodies that opsonize amastigotes and induce IL-10 production in macrophages. These high activation and deactivation processes are likely to occur mainly in the spleen and liver and can be confirmed through the examination of organ samples. However, an analysis of sequential data from studies of visceral leishmaniasis in hamsters suggests that factors outside of the immune system are responsible for the early inactivation of inducible nitric oxide synthase, which occurs before the expression of deactivating cytokines. In active visceral leishmaniasis, the immune system actively participates in non-lymphoid organ lesioning. While current views only consider immunocomplex deposition, macrophages, T cells, cytokines, and immunoglobulins by diverse mechanism also play important roles in the pathogenesis.

SIGNR1-negative red pulp macrophages protect against acute streptococcal sepsis after Leishmania donovani-induced loss of marginal zone macrophages

Marginal zone macrophages in the murine spleen play an important role in the capture of blood-borne pathogens and are viewed as an essential component of host defense against the development of pneumococcal sepsis. However, we and others have previously described the loss of marginal zone macrophages associated with the splenomegaly that follows a variety of viral and protozoal infections; this finding raises the question of whether these infected mice would become more susceptible to secondary pneumococcal infection. Contrary to expectations, we found that mice lacking marginal zone macrophages resulting from Leishmania donovani infection have increased resistance to Streptococcus pneumoniae type 3 and do not develop sepsis. Using biophotonic imaging, we observed that pneumococci are rapidly trapped in the spleens of L. donovani-infected mice. By selective depletion studies using clodronate liposomes, depleting monoclonal antibodies specific for Ly6C/G and Ly6G, and CD11c-DTR mice, we show that the enhanced early resistance in L. donovani-infected mice is entirely due to the activity of SIGNR1(-) red pulp macrophages. Our data demonstrate, therefore, that the normal requirement for SIGNR1(+) marginal zone macrophages to protect against a primary pneumococcal infection can, under conditions of splenomegaly, be readily compensated for by activated red pulp macrophages.

B7-H1 blockade increases survival of dysfunctional CD8(+) T cells and confers protection against Leishmania donovani infections

Experimental visceral leishmaniasis (VL) represents an exquisite model to study CD8⁺ T cell responses in a context of chronic inflammation and antigen persistence, since it is characterized by chronic infection in the spleen and CD8⁺ T cells are required for the development of protective immunity. However, antigen-specific CD8⁺ T cell responses in VL have so far not been studied, due to the absence of any defined Leishmania-specific CD8⁺ T cell epitopes. In this study, transgenic Leishmania donovani parasites expressing ovalbumin were used to characterize the development, function, and fate of Leishmania-specific CD8⁺ T cell responses. Here we show that L. donovani parasites evade CD8⁺ T cell responses by limiting their expansion and inducing functional exhaustion and cell death. Dysfunctional CD8⁺ T cells could be partially rescued by in vivo B7-H1 blockade, which increased CD8⁺ T cell survival but failed to restore cytokine production. Nevertheless, B7-H1 blockade significantly reduced the splenic parasite burden. These findings could be exploited for the design of new strategies for immunotherapeutic interventions against VL.

The protozoan parasite Leishmania donovani is the cause of visceral leishmaniasis, a chronic disease that currently affects 12 million people worldwide. We are interested in understanding the immune mechanisms that can control infection. Preliminary studies suggested that CD8⁺ T cells can kill parasites and limit disease; however, studying these important killer cells has been hindered, because we do not know what parasite molecules they recognize. To overcome this, we engineered parasites to express ovalbumin. Since many tools exist to track and measure immune cells targeted at ovalbumin, we can now track the specific CD8⁺ T cell responses that develop upon infection with Leishmania. We found that Leishmania initially induced CD8⁺ T cells to divide and produce molecules such as IFN-gamma that may help them to kill parasites. However, the CD8⁺ T cells rapidly lost their effector function and died off as infection progressed. More encouragingly, though, we were able to recover some CD8⁺ T cell function by blocking immune inhibitory molecules that are induced by parasite infection. The recovered T cells killed parasites and controlled infection. These results are important as they could be exploited for the design of new therapeutic vaccine strategies aimed at inducing protective CD8⁺ T cells.

Fast high yield of pure Leishmania (Leishmania) infantum axenic amastigotes and their infectivity to mouse macrophages

Leishmania (L.) infantum (syn. Leishmania chagasi) is a dimorphic protozoan parasite that lives in promastigote and amastigote form in its sandfly vector and mammalian hosts, respectively. Here, we describe an in vitro culture system for the generation of a pure population of L. infantum axenic amastigotes after only 4 days incubation in culture medium supplemented with fetal calf serum, human urine, L: -glutamine, and HEPES at 37 masculineC (pH 5.5). Ultrastrutural analysis and infection assays in two macrophage populations (Kupffer cells (KUP) and peritoneal macrophages (PM)) infected with axenic amastigotes demonstrated that they maintained morphological and biochemical (A2 expression) features and a similar infection pattern to tissue-derived L. infantum amastigotes. The susceptibility of the macrophage lines to axenic or tissue-derived amastigotes and promastigotes was investigated. We found a completely different susceptibility profile for KUP and PM. Liver macrophages, both KUP and immigrant macrophages, are intimately involved in the response to L. infantum infection; this difference in susceptibility is probably related to their capacity to eliminate these parasites. Our in vitro system was thus able to generate axenic amastigotes that resemble tissue-derived amastigotes both in morphology and infectivity pattern; this will help in further investigation of the biological characteristics of the host-parasite relationship as well as the process of pathogenesis.

Designing therapies against experimental visceral leishmaniasis by modulating the membrane fluidity of antigen-presenting cells

The membrane fluidity of antigen-presenting cells (APCs) has a significant bearing on T-cell-stimulating ability and is dependent on the cholesterol content of the membrane. The relationship, if any, between membrane fluidity and defective cell-mediated immunity in visceral leishmaniasis has been investigated. Systemic administration of cholesterol by liposome delivery (cholesterol liposomes) in Leishmania donovani-infected hamsters was found to cure the infection. Splenic macrophages as a prototype of APCs in infected hamsters had decreased membrane cholesterol and an inability to drive T cells, which was corrected by cholesterol liposome treatment. The effect was cholesterol specific because liposomes made up of the analogue 4-cholesten-3-one provided almost no protection. Infection led to increases in interleukin-10 (IL-10), transforming growth factor beta, and IL-4 signals and concomitant decreases in gamma interferon (IFN-gamma), tumor necrosis factor alpha, and inducible NO synthase signals, which reverted upon cholesterol liposome treatment. The antileishmanial T-cell repertoire, whose expansion appeared to be associated with protection, was presumably type Th1, as shown by enhanced IFN-gamma signals and the predominance of the immunoglobulin G2 isotype. The protected group produced significantly more reactive oxygen species and NO than the infected groups, which culminated in killing of L. donovani parasites. Therefore, cholesterol liposome treatment may be yet another simple strategy to enhance the cell-mediated immune response to L. donovani infection. To our knowledge, this is the first report on the therapeutic effect of cholesterol liposomes in any form of the disease.

Posttranscriptional regulation of II10 gene expression allows natural killer cells to express immunoregulatory function

Natural killer (NK) cells play a well-recognized role in early pathogen containment and in shaping acquired cell-mediated immunity. However, indirect evidence in humans and experimental models has suggested that NK cells also play negative regulatory roles during chronic disease. To formally test this hypothesis, we employed a well-defined experimental model of visceral leishmaniasis. Our data demonstrated that NKp46(+)CD49b(+)CD3(-) NK cells were recruited to the spleen and into hepatic granulomas, where they inhibited host protective immunity in an interleukin-10 (IL-10)-dependent manner. Although IL-10 mRNA could be detected in activated NK cells 24 hr after infection, the inhibitory function of NK cells was only acquired later during infection, coincident with increased IL-10 mRNA stability and an enhanced capacity to secrete IL-10 protein. Our data support a growing body of literature that implicates NK cells as negative regulators of cell-mediated immunity and suggest that NK cells, like CD4(+) T helper 1 cells, may acquire immunoregulatory functions as a consequence of extensive activation.

Immunoregulatory mechanisms and CD4-CD8- (double negative) T cell subpopulations in human cutaneous leishmaniasis: a balancing act between protection and pathology

Cellular immune responses directed against protozoan parasites are key for controlling pathogen replication and disease resolution. However, an uncontrolled, or improperly controlled, response can be deleterious to the host in terms of both allowing for the establishment of pathology, as well as less effective establishment of memory responses. Human cutaneous leishmaniasis is a disease caused by the infection with Leishmania spp. following a bite from the sandfly, the natural vector of this disease. Tens of millions worldwide are currently infected with Leishmania and no effective vaccines have been developed to date. In the face of the complexity presented by the interaction between a host (humans) with the parasite, Leishmania, and the fact that this parasite is inoculated by another complex, biologically active, vector, the sandfly, it is clearly important to study the immunoregulatory mechanisms that are induced in humans naturally infected by this parasite if we hope to develop effective vaccines and immunotherapeutic treatments in the future. Our laboratory has focused over the years on the study of the local and systemic T cell response during the first episode of cutaneous leishmaniasis suffered by individuals before they undergo antimony treatment. The goal of this review is to briefly outline our findings with hopes of putting our most recent studies concerning the dichotomy between alpha/beta TCR and gamma/delta TCR expressing, CD4-CD8- (double negative-DN) T cells in the context of a balanced immune response against Leishmania and to discuss the implications of these findings toward our understanding of human leishmaniasis.

Interferon-gamma (INF-gamma), IL4 expression levels and Leishmania DNA load as prognostic markers for monitoring response to treatment of leishmaniotic dogs with miltefosine and allopurinol

In this study, we searched for a connection between Leishmania load and cytokine expression levels in the tissues of Leishmaniainfantum naturally infected dogs and the efficacy of treatment with miltefosine and allopurinol. To this purpose, we exploited a real-time PCR system to detect Leishmania load and the expression levels of IFN-gamma and IL-4 mRNAs at the time of diagnosis and during the follow up post-treatment. In particular, we measured the amount of parasites in blood and lymph node samples, while the expression levels of IFN-gamma and IL-4 cytokines were assessed in the blood of the animals. We employed different targeted real-time PCR assays on 20 naturally infected dogs with clinical signs. Three healthy dogs living in a non-endemic area were selected as negative controls. The overall results obtained demonstrate that the simultaneous evaluation of parasites and cytokine levels in different kinds of tissue might represent a reliable tool to evaluate the immune response, the efficacy of the therapy and to predict the relapses during the treatment.

Postgenomic research on leishmaniasis: a critical self-appraisal

Scientific conferences, a major feature of academic life, rarely provide the opportunity for self-appraisal of a research field. During a 2007 meeting on leishmaniasis research in the postgenomic era, approximately 60 researchers participated in group discussions that aimed to provide a critical self-appraisal of the state of the field and to highlight major roadblocks that are likely to prevent the translation of new research into tools for leishmaniasis control. These discussions demonstrated a surprising concordance of views and highlighted several crucial areas for future development.

Transgenic Leishmania and the immune response to infection

Genetic manipulation of single-celled organisms such as the Leishmania parasite enables in depth analysis of the consequences of genotypic change on biological function. In probing the immune responses to infection, use of transgenic Leishmania has the potential to unravel both the contribution of the parasite to the infection process and the cellular interactions and mechanisms that characterize the innate and adaptive immune responses of the host. Here, we briefly review recent technical advances in parasite genetics and explore how these methods are being used to investigate parasite virulence factors, elucidate immune regulatory mechanisms and contribute to the development of novel therapeutics for the leishmaniases. Recent developments in imaging technology, such as bioluminescence and intravital imaging, combined with parasite transfection with fluorescent or enzyme-encoding marker genes, provides a rich opportunity for novel assessment of intimate, real-time host-parasite interactions at a previously unexplored level. Further advances in transgenic technology, such as the introduction of robust inducible gene cassettes for expression in intracellular parasite stages or the development of RNA interference methods for down-regulation of parasite gene expression in the host, will further advance our ability to probe host-parasite interactions and unravel disease-promoting mechanisms in the leishmaniases.

Expression of helminth genes in Leishmania: an experimental transfection system to test immunological function

Functional analysis of genes from parasitic helminths requires, at the present time, heterologous expression. We have adapted the well-characterized system of transfection in Leishmania protozoal parasites, as a means of analysing the effect of single filarial genes on the mammalian immune system. For example, testing the function of the Brugia malayi abundant larval transcript (ALT) gene-transfected Leishmania mexicana were found to be significantly more virulent in macrophages in vitro. The course of infection in vivo is also aggravated by expression of the ALT gene. Examples are also given of transgenes which reduced in vitro growth within macrophages, as well as others which exert no effect on the protozoal parasitism. Thus, Leishmania transfection provides a tractable system to analyse helminth gene function within the context of the host immune system.

Neutrophils contribute to development of a protective immune response during onset of infection with Leishmania donovani

Neutrophils are key components of the inflammatory response and as such contribute to the killing of microorganisms. In addition, recent evidence suggests their involvement in the development of the immune response. The role of neutrophils during the first weeks post-infection with Leishmania donovani was investigated in this study. When L. donovani-infected mice were selectively depleted of neutrophils with the NIMP-R14 monoclonal antibody, a significant increase in parasite numbers was observed in the spleen and bone marrow and to a lesser extent in the liver. Increased susceptibility was associated with enhanced splenomegally, a delay in the maturation of hepatic granulomas, and a decrease in inducible nitric oxide synthase expression within granulomas. In the spleen, neutrophil depletion was associated with a significant increase in interleukin 4 (IL-4) and IL-10 levels and reduced gamma interferon secretion by CD4(+) and CD8(+) T cells. Increased production of serum IL-4 and IL-10 and higher levels of Leishmania-specific immunoglobulin G1 (IgG1) versus IgG2a revealed the preferential induction of Th2 responses in neutrophil-depleted mice. Altogether, these data suggest a critical role for neutrophils in the early protective response against L. donovani, both as effector cells involved in the killing of the parasites and as significant players influencing the development of a protective Th1 immune response.

Temporal regulation of interleukin-12p70 (IL-12p70) and IL-12-related cytokines in splenic dendritic cell subsets during Leishmania donovani infection

Dendritic cells (DC) play an essential role in initiating and directing T-cell responses, in part by production of interleukin-12p70 (IL-12p70), IL-23, and IL-27. However, comparative studies on the capacity for cytokine production of DC subsets are rare. Here, we compare splenic CD8alpha+, CD4+, and double-negative (DN) DC, isolated 5 h to 28 days after Leishmania donovani infection, for (i) production of IL-12p70, (ii) accumulation of IL-12/23p40, IL-12p35, IL-23p19, and IL-27p28 mRNAs, and (iii) their capacity to direct CD4+ T-cell differentiation. At 5 h, conventional DC (cDC) accumulated mRNA for IL-12/23p40 (CD8alpha>CD4>DN), IL-23p19 (CD4>CD8alpha>DN), and IL-27p28 (CD8alpha>CD4>DN), in an infection dose-dependent manner. IL-12p70 was restricted to CD8alpha+ cDC, reflecting the subset-specific accumulation of IL-12p35 mRNA. In contrast, cDC from mice infected for 14 to 28 days accumulated little mRNA for IL-12p40 and IL-12p19, though IL-27p28 mRNA remained detectable (CD8alpha>DN>CD4). IL-12p70 secretion by CD8alpha+ cDC was also absent, reflecting deficient IL-12/23p40, rather than IL-12p35, mRNA accumulation. The capacity of CD8alpha+ cDC isolated early after infection to direct Th1 cell differentiation was mediated through IL-12/23p40, whereas this ability in CD4+ and DN cDC was independent of IL-12/23p40 and did not result from overexpression of Delta 4 Notch-like ligand. However, DN cDC produced gamma interferon (IFN-gamma) and also contained a rare population of CD11c(hi) DX5+ IFN-gamma-producing cells. Our data illustrate the extensive diversity in, and temporal regulation of, splenic cDC subsets during infection and suggest caution in interpreting data obtained with unfractionated or minimally purified DC.

Hepatic granulomas in canine visceral leishmaniasis and clinical status

The histopathological description of intralobular hepatic granulomas in animals with a defined clinical status (asymptomatic, oligosymptomatic and symptomatic animals) was reported. Seventy-one mongrel dogs naturally infected with Leishmania chagasi were obtained from two Brazilian endemic areas: João Pessoa, PB and Belo Horizonte, MG. The hepatic parasite load was determined and compared to granuloma formation. Liver fragments from all infected animals showed remarkable leishmaniotic granulomatous inflammatory reaction. Granulomas with variable size were constituted by macrophages (parasitized or not with amastigotes of L. chagasi), some epithelioid cells, small numbers of lymphocytes, plasma cells, and rare neutrophils. Asymptomatic dogs had higher numbers of granulomas than oligosymptomatic and symptomatic animals from both geographical regions. However, the average diametric size of granulomas was very heterogeneous in all groups, independently of the geographic region (P>0.05). Parasite tissue load did not show any difference among liver fragments of all animals, especially when considering the defined clinical status and/or their geographic origin.

Interleukin-10 and the pathogenesis of human visceral leishmaniasis

The mechanisms underlying the failure to control the growth and systemic spread of Leishmania parasites in human visceral leishmaniasis (VL) are not well understood. Although the absence of antigen-specific Th1 responses in the peripheral blood mononuclear cells from VL patients is thought to be causally related to disease progression, the finding that these patients also express elevated interferon-gamma mRNA in lesional tissue, as well as elevated serum levels of proinflammatory cytokines, suggests that their immunological defect cannot be explained simply by immune tolerance or Th2 polarization. As a possible homeostatic mechanism to control persistent infection-induced inflammation, elevated levels of the regulatory cytokine interleukin (IL)-10 have been reported repeatedly in clinical studies of VL. Here, we review the studies with relevance to immune responses in human VL and highlight the central role that IL-10 might have in the pathogenesis of VL and as a target for immune-based therapy.

Molecular dissection and expression of the LdK39 kinesin in the human pathogen, Leishmania donovani

In this study we show for the first time the intracellular distribution of a K39 kinesin homologue in Leishmania donovani, a medically important parasite of humans. Further, we demonstrated that this motor protein is expressed in both the insect and mammalian developmental forms (i.e. promastigote and amastigotes) of this organism. Moreover, in both of these parasite developmental stages, immunofluorescence indicated that the LdK39 kinesin accumulated at anterior and posterior cell poles and that it displayed a peripheral localization consistent with the cortical cytoskeleton. Using a molecular approach, we identified, cloned and characterized the first complete open reading frame for the gene (LdK39) encoding this large (> 358 kDa) motor protein in L. donovani. Based on these observations, we subsequently used a homologous episomal expression system to dissect and express the functional domains that constitute the native molecule. Cell fractionation experiments demonstrated that LdK39 was soluble and that it bound to detergent-extracted cytoskeletons of these parasites in an ATP-dependent manner. The cumulative results of these experiments are consistent with LdK39 functioning as an ATP-dependent kinesin which binds to and travels along the cortical cytoskeleton of this important human pathogen.

Histopathological and immunohistochemical study of type 3 complement receptors (CD11b/CD18) in livers and spleens of asymptomatic and symptomatic dogs naturally infected with Leishmania (Leishmania) chagasi

Leishmania promastigotes interact with macrophages through the association of multiple membrane surface receptors. Macrophage complement receptor CR3 (CD11b/CD18 or Mac-1) has been implicated in the interaction of both human and murine macrophages with serum-opsonized promastigotes. The aim of this study was to determine CR3 expression in the livers and spleens of dogs naturally infected with Leishmania (Leishmania) chagasi. CR3 expression in liver was higher in asymptomatic than in symptomatic animals. Moreover, the hepatic parasitism load determined by immunocytochemical analysis was lower in parallel with higher numbers of granulomas. In contrast, in spleens, CR3 expression was higher in symptomatic animals than in asymptomatic ones. However, the tissue parasite load was greater in spleens of symptomatic dogs. There was a strict correlation between the parasite load and cellular CR3 expression in the spleens of dogs naturally infected with L. chagasi. CR3 macrophage integrins could be essential receptors for Leishmania survival. Considering that the symptomatic animals showed higher parasite loads and higher CD11b/CD18 expression in their spleens, we can conclude that these splenic cells (monocyte-macrophages) might serve to perpetuate intracellular infection.

Balancing immunity and pathology in visceral leishmaniasis

Experimental visceral leishmaniasis (VL) caused by infection with Leishmania donovani results in the development of organ-specific immunity in the two main target tissues of infection, the spleen and the liver. The liver is the site of an acute resolving infection associated with the development of inflammatory granulomas around infected Kupffer cells, and resistance to reinfection. Paradoxically, the spleen is an initial site for the generation of cell-mediated immune responses, but ultimately becomes a site of parasite persistence with associated immunopathological changes. These include splenomegaly and a breakdown in tissue architecture that is postulated to contribute to the immunocompromized status of the host. The progressive development of splenic pathology is largely associated with high levels of TNF and interleukin (IL)-10. Follicular dendritic cell (DC) networks are lost, whereas TNF mediates the destruction of marginal zone macrophages and gp38(+) stromal cells, and IL-10 promotes impaired DC migration into T-cell areas with consequent ineffective T-cell priming. Splenic stromal cell function is also altered, promoting the selective development of IL-10-producing DC with immunoregulatory properties. Ultimately, a fine immunological balance determines responses that effectively promote parasite clearance in the liver and those that promote pathology in the spleen, and future investigation aims to separate these responses to offer further means of parasite control in chronically infected VL patients.

The Schistosoma mansoni hepatic egg granuloma provides a favorable microenvironment for sustained growth of Leishmania donovani

Parasitic co-infections are prevalent in many parts of the world. However, relatively little is known about how an underlying infection may impact on the host's ability to control a newly acquired parasite, especially if both infect the same organ. We have studied this using an experimental co-infection model in C57BL/6 mice involving Schistosoma mansoni and Leishmania donovani, two important human pathogens affecting the liver. We show that mice with established S. mansoni infections fail to control L. donovani growth in the liver and spleen. The failure occurs despite the development of a functional anti-L. donovani Th1 response that can mediate granuloma formation and effective clearance of amastigotes from foci of infection in the hepatic parenchyma. Instead, anti-leishmanial immunity fails within the S. mansoni egg granuloma, consistent with a lack of L. donovani granuloma assembly in this tissue microenvironment and consequent lack of NO production. Persisting amastigote replication in the S. mansoni egg granulomas may thus explain the increased L. donovani burden in the liver and spleen. These results may have implications for human S. mansoni and L. donovani co-infections and also demonstrate that granulomatous tissue responses to helminth organisms can form a discrete niche facilitating survival of intracellular pathogens.

Responses to Leishmania donovani in mice deficient in interleukin-12 (IL-12), IL-12/IL-23, or IL-18

Interleukin-12 (IL-12) orchestrates acquired resistance in intracellular Leishmania donovani infection in the liver, inducing gamma interferon and, in turn, macrophage activation and parasite killing. Nevertheless, testing in IL-18(-/-) mice compared to wild-type mice and in IL-12p40(-/-) compared to IL-12p35(-/-) mice also suggested both early-acting (IL-18) and late-acting (IL-23) antileishmanial effects independent of IL-12.

Endogenous interleukin-18 is involved in immunity to Leishmania donovani but its absence does not adversely influence the therapeutic activity of sodium stibogluconate

Immunity to Leishmania donovani is associated with an interleukin (IL)-12 driven T helper 1 (Th1) response. In addition, the ability to respond to chemotherapy with sodium stibogluconate (SSG) requires a fully competent immune response and both Th1 and Th2 responses have been shown to positively influence the outcome of drug treatment. In the present study, the influence of IL-18, which can modulate both interferon (IFN)-gamma and IL-4 production, on the outcome of primary L. donovani infection and SSG therapy following infection was assessed using BALB/c IL-18-deficient and wild type mice. IL-18 deficiency was associated with an increased susceptibility to L. donovani infection, evident by day 40 post infection, resulting in higher parasite burdens in the spleen, liver, and bone marrow compared with wild type control animals. Infected IL-18-deficient mice had significantly lower splenocyte concanavalin A (ConA) induced IFN-gamma production as well as lower serum IL-12 and IFN-gamma levels, indicating a reduced Th1 response. However, drug treatment was equally effective in both mouse strains and restored serum IL-12 and IFN-gamma levels, and IFN-gamma production by ConA stimulated splenocytes of IL-18-deficient mice, to levels equivalent to similarly treated wild type mice.

A standardized cytological and immunochemical method for the analysis of fine-needle spleen aspirates: Assessment of leukocyte population changes in canine visceral leishmaniosis

A method for the evaluation of splenic cellularity using samples collected by fine-needle aspirative biopsy was standardized in this work. The procedure includes erythrocyte lysing, preparation of cytospin films and staining by histochemical and immunocytochemical techniques. The cellular profiles of spleen preparations were compared with those observed in peripheral blood samples subjected to the same procedure. Two groups were compared, one consisting of 14 healthy uninfected and the other of 15 polysymptomatic Leishmania chagasi/infantum-infected dogs, from an endemic area for visceral leishmaniosis. Cell populations were identified by conventional hematoxilin-eosin and Wright' stainings, and by immunocytochemistry using monoclonal antibodies against canine CD45RA and CD45RB, phagocytes and a pan-leukocyte antigen. Larger neutrophil (P < 0.0001) and monocyte/macrophage (P = 0.0036) relative counts and lower lymphocyte relative counts (P < 0.0001) were found in the spleen, and not in the blood, of the animals with leishmaniosis than in those of the healthy animals. The proportions of CD45RB+ cells were higher, and of CD45RA+ cells were lower, both in the spleen and in the blood of animals with leishmaniosis than in those of healthy dogs (P < 0.05). Additionally, hematoxilin-eosin-stained cytospins of spleen aspirates from Leishmania-infected animals permitted the easy visualization of amastigote forms inside phagocytes, under light microscopy.

Immunohistological features of visceral leishmaniasis in BALB/c mice

It has been reported that the level of protection provided by vaccines against murine visceral leishmaniasis (VL) is low and that progress in research on VL may be due to the lack of appropriate models to study protective immunity. We have analysed the immunohistological features occurring in BALB/c mice after intravenous administration of 10(3), 10(5) and 10(6) parasites of Leishmania infantum. Our results show that in all cases parasite administration leads to the establishment of infection and to the development of quantifiable immunohistological features which are dependent on the inoculum size. This study demonstrates that differences in the parasite challenge result in changes in the evolution of some of the parameters associated with the degree of the infection in the BALB/c model: level of anti-Leishmania antibodies, up-regulation of spleen arginase activity, balance between IFN-gamma and IL-10, extent of lymphoid follicle depletion in the splenic white pulp and ineffective development of hepatic granulomas. Also, and depending on the initial infectious inoculum, the absence of parasites in the bone marrow and the number of mature and empty type granulomas were parameters associated with protection. We think that in this model a challenge of the order of 10(5) parasites should prove useful for vaccine studies against VL.

Genetic susceptibility to infectious disease: lessons from mouse models of leishmaniasis

Susceptibility to infectious disease is influenced by multiple host genes, most of which are low penetrance QTLs that are difficult to map in humans. Leishmaniasis is a well-studied infectious disease with a variety of symptoms and well-defined immunological features. Mouse models of this disease have revealed more than 20 QTLs as being susceptibility genes, studies of which have made important contributions to our understanding of the host response to infection. The functional effects of individual QTLs differ widely, indicating a networked regulation of these effects. Several of these QTLs probably also influence susceptibility to other infections, indicating that their characterization will contribute to our understanding of susceptibility to infectious disease in general.

Imprinting of BALB/c mice with low Leishmania infantum parasite dose markedly protects spleen against high-dose challenge

In this study, we investigated in the BALB/c model, the dose-dependent protective potential of previous infection with Leishmania infantum parasites, against a high-dose challenge and showed for the first time that low-dose imprinting conferred substantial spleen resistance. Mice were immunized for 1 month or 5 months by IV route with parasite inocula ranging from 10(4) to 10(7) and from 10(3) to 10(5), respectively, and challenged for 1 month with 3 x 10(7) parasites. Liver protection was directly proportional to the parasite dose used for infection and reached 90-95% whereas, only low doses (< or =10(5)) protected spleen. Maximal spleen resistance (80%) was reached in mice infected for 5 months with 10(5) parasites. In most cases, protection was accompanied in spleen, by restored in vitro responses to Leishmania antigens. Analysis of anti L. infantum isotype responses and in vitro antigen-induced cytokine production, indicated that the acquired protection was irrespective of a Th1/Th2 imbalance.

Chemokines in host-parasite interactions in leishmaniasis

Crucial to the defense against leishmaniasis is the ability of the host to mount a cell-mediated immune response capable of controlling and/or eliminating the parasite. Cell recruitment to the site of infection is essential to the development of the host cellular immune response. The process is controlled by chemokines, which are chemotactic cytokines produced by leukocytes and tissue cells.

Antagonizing deactivating cytokines to enhance host defense and chemotherapy in experimental visceral leishmaniasis

In experimental visceral leishmaniasis, inhibition of interleukin 10 (IL-10) signaling enhances Th1-cell-associated responses, promoting gamma interferon (IFN-gamma) secretion, granuloma assembly, macrophage activation with substantial liver parasite killing, and synergy with pentavalent antimony (Sb) chemotherapy. To determine if inhibiting other suppressive cytokines has similar therapeutic potential, Leishmania donovani-infected BALB/c mice were injected with anti-IL-4 monoclonal antibody or receptor fusion antagonists of IL-13 or transforming growth factor beta (TGF-beta). Targeting IL-13 or TGF-beta enabled inhibition of L. donovani replication but little parasite killing; anti-IL-4 had no effect. None of the three antagonists promoted IFN-gamma production, granuloma maturation, or Sb efficacy. Excess IL-13 and TGF-beta exacerbated liver infection; however, effects were transient. Among IL-10, IL-4, IL-13, and TGF-beta, cytokines capable of disabling Th1-cell mechanisms (including those which support chemotherapy), IL-10 appears to be the appropriate target for therapeutic inhibition in visceral L. donovani infection.