Visceral leishmaniasis: immunology and prospects for a vaccine

Human visceral leishmaniasis (HVL) is the most severe clinical form of a spectrum of neglected tropical diseases caused by protozoan parasites of the genus Leishmania. Caused mainly by L. donovani and L. infantum/chagasi, HVL accounts for more than 50 000 deaths every year. Drug therapy is available but costly, and resistance against several drug classes has evolved. Here, we review our current understanding of the immunology of HVL and approaches to and the status of vaccine development against this disease.

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.

Pulmonary dendritic cells and alveolar macrophages are regulated by gammadelta T cells during the resolution of S. pneumoniae-induced inflammation

γδ T cells commonly associate with mucosal and epithelial sites, fulfilling a variety of immunoregulatory functions. While lung γδ T cells have well-characterized pro-inflammatory activity, their potential role in the resolution of lung inflammation has yet to be explored in any detail. Indeed, given the importance of minimizing inflammation, the cellular mechanisms driving the resolution of lung inflammation are poorly understood. Using a murine model of acute Streptococcus pneumoniae-mediated lung inflammation, we now show that resolution of inflammation following bacterial clearance is associated with a > 30-fold increase in γδ T-cell number. Although inflammation eventually resolves in TCRδ^−/− mice, elevated numbers of alveolar macrophages and pulmonary dendritic cells, and the appearance of well-formed granulomas in lungs of TCRδ^−/− mice, together indicated a role for γδ T cells in regulating mononuclear phagocyte number. Ex vivo, both alveolar macrophages and pulmonary dendritic cells were susceptible to lung γδ T cell-mediated cytotoxicity, the first demonstration of such activity against a dendritic cell population. These findings support a model whereby expansion of γδ T cells helps restore mononuclear phagocyte numbers to homeostatic levels, protecting the lung from the consequences of inappropriate inflammation. Copyright © 2007 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Immunization with H1, HASPB1 and MML Leishmania proteins in a vaccine trial against experimental canine leishmaniasis

The protective capabilities of three Leishmania recombinant proteins – histone 1 (H1) and hydrophilic acylated surface protein B1 (HASPB1) immunized singly, or together as a protein cocktail vaccine with Montanide™, and the polyprotein MML immunized with MPL^®-SE adjuvant – were assessed in beagle dogs. Clinical examination of the dogs was carried out periodically under blinded conditions and the condition of the dogs defined as asymptomatic or symptomatic. At the end of the trial, we were able to confirm that following infection with L. infantum promastigotes, five out of eight dogs immunized with H1 Montanide™, and four out of eight dogs immunized with either the combination of HASPB1 with Montanide™ or the combination of H1 + HASPB1 with Montanide™, remained free of clinical signs, compared with two out of seven dogs immunized with the polyprotein MML and adjuvant MPL^®-SE, and two out of eight dogs in the control group. The results demonstrate that HASPB1 and H1 antigens in combination with Montanide™ were able to induce partial protection against canine leishmaniasis, even under extreme experimental challenge conditions.

IgG subclass responses in a longitudinal study of canine visceral leishmaniasis

Visceral leishmaniasis (VL), caused by Leishmania infantum, is an important disease of domestic dogs. Here, we present data on the IgG subclass antibody response to crude L. infantum antigen in a cohort of naturally infected Brazilian dogs. Specific IgG1-IgG4 responses could be detected in 98, 58, 70 and 82%, respectively of 57 dogs that were seropositive for specific IgG. Levels of all IgG subclasses were strongly inter-correlated. Levels of all IgG subclasses increased at the time of seroconversion, before reaching a plateau after several months. Levels of all IgG subclasses were higher in sick dogs than healthy dogs, and levels of all except IgG2 were higher in parasite-positive (by PCR) than parasite-negative dogs. However, levels of IgG2 relative to IgG1 were lower in sick or parasite-positive dogs compared to healthy or parasite-negative infected dogs. In contrast to previous studies, the results suggest that canine VL is associated with upregulation of specific antibody of all IgG subclasses, particularly IgG1, IgG3 and IgG4.

Tissue cytokine responses in canine visceral leishmaniasis

To elucidate the local tissue cytokine response of dogs infected with Leishmania chagasi, cytokine mRNA levels were measured in bone marrow aspirates from 27 naturally infected dogs from Brazil and were compared with those from 5 uninfected control animals. Interferon-gamma mRNA accumulation was enhanced in infected dogs and was positively correlated with humoral (IgG1) but not with lymphoproliferative responses to Leishmania antigen in infected dogs. Increased accumulation of mRNA for interleukin (IL)-4, IL-10, and IL-18 was not observed in infected dogs, and mRNA for these cytokines did not correlate with antibody or proliferative responses. However, infected dogs with detectable IL-4 mRNA had significantly more severe symptoms. IL-13 mRNA was not detectable in either control or infected dogs. These data suggest that clinical symptoms are not due to a deficiency in interferon-gamma production. However, in contrast to its role in human visceral leishmaniasis, IL-10 may not play a key immunosuppressive role in dogs.

CD95 is required for the early control of parasite burden in the liver of Leishmania donovani-infected mice

In this study we show an increased incidence of T cell apoptosis in the liver and spleen of mice infected with Leishmania donovani. T cells from L. donovani-infected mice were found to be increasingly susceptible to CD95-mediated apoptosis in vitro, compared to controls. To test if suboptimal T cell function resulting from CD95-mediated apoptosis contributes to sustained parasite burden in L. donovani parasitized mice, B6.gld mice (lacking functional CD95 ligand) were infected with L. donovani. Surprisingly, at four different time points no difference in levels of T cell apoptosis in the spleen and liver was found between these mice and controls following intravenous delivery of L. donovani amastigotes, indicating that the CD95 / CD95L interaction is not essential for T cell apoptosis in the L. donovani-infected liver and spleen. However, B6.gld mice were increasingly susceptible to L. donovani infection, associated with less efficient granuloma formation in the liver and uncontrolled parasite growth in the spleen. Late in infection (day 56 post-infection), B6.gld mice had higher numbers of IFN-gamma-producing CD4(+) T cells in the liver and spleen, indicating a role for CD95 signaling in the homeostasis of this subset of cytokine-producing T cells in L. donovani-parasitized mice. Adoptive transfer of CD4(+) and CD8(+) T cells into recombinase activating gene 1 knockout (RAG-1(- / -)) recipients, revealed that CD95L expressed on CD4(+) T cells contributes to early control of L. donovani infection in the liver via mechanisms that are independent of granuloma formation and induction of apoptosis. These results indicate important roles for CD95 and CD95L that are unrelated to regulation of apoptosis in the early control of L. donovani infection.

Interleukin-13 in Iranian patients with visceral leishmaniasis: relationship to other Th2 and Th1 cytokines

The role of interleukin (IL)-13, a Th2 cytokine sharing many of the features of IL-4, has not previously been examined in patients with visceral leishmaniasis (VL). We examined sera from Iranian patients with VL caused by Leishmania infantum. Serum IL-13 was detected in 50% (22/44) of patients with active primary disease. In comparison, IL-10 was detected in 79.5% (35/44), interferon gamma (IFN gamma) in 38.5% (17/44), and IL-4 in only 5% (2/44) of these patients. With few exceptions all 3 cytokines were undetectable after clinical recovery following antimony therapy. Five of 7 patients (71%) who failed antimony therapy and had relapsing disease had similar levels of IL-10 to patients with active primary disease. However, with only 1 exception, IL-13, IFN gamma and IL-4 were not detected in such patients. These data suggest that relapsing disease may result from defective cellular immunity, unrelated to immunosuppression mediated by IL-10.

Immunization with a recombinant stage-regulated surface protein from Leishmania donovani induces protection against visceral leishmaniasis

Vaccination against visceral leishmaniasis has received limited attention compared with cutaneous leishmaniasis, although the need for an effective vaccine against visceral leishmaniasis is pressing. In this study, we demonstrate for the first time that a recombinant stage-specific hydrophilic surface protein of Leishmania donovani, recombinant hydrophilic acylated surface protein B1 (HASPB1), is able to confer protection against experimental challenge. Protection induced by rHASPB1 does not require adjuvant and, unlike soluble Leishmania Ag + IL-12, extends to the control of parasite burden in the spleen, an organ in which parasites usually persist and are refractory to a broad range of immunological and chemotherapeutic interventions. Both immunohistochemistry (for IL-12p40) and enzyme-linked immunospot assay (for IL-12p70) indicate that immunization with rHASPB1 results in IL-12 production by dendritic cells, although an analysis of Ab isotype responses to rHASPB1 suggests that this response is not sufficient in magnitude to induce a polarized Th1 response. Although both vaccinated and control-infected mice have equivalent frequencies of rHASPB1-specific CD4(+) T cells producing IFN-gamma, vaccine-induced protection correlates with the presence of rHASPB1-specific, IFN-gamma-producing CD8(+) T cells. Thus, we have identified a novel vaccine candidate Ag for visceral leishmaniasis, which appears to operate via a mechanism similar to that previously associated with DNA vaccination.

Enhanced hematopoietic activity accompanies parasite expansion in the spleen and bone marrow of mice infected with Leishmania donovani

In this study, we have analyzed hematopoietic activity in the spleen, bone marrow, and blood of BALB/c and scid mice infected with Leishmania donovani. Our analysis demonstrates that infection induces a rapid but transient mobilization of progenitor cells into the circulation, associated with elevated levels of granulocyte/macrophage colony-stimulating factor (GM-CSF) and MIP-1alpha. From 14 to 28 days postinfection, when parasite expansion begins in the spleen and bone marrow, both the frequency and cell cycle activity of hematopoietic progenitors, particulary CFU-granulocyte, monocyte, are dramatically increased in these organs. This is associated with increased accumulation of mRNA for GM-CSF, M-CSF, and G-CSF, but not interleukin-3. Our data also illustrate that hematopoietic activity, as assessed by changes in the frequency of progenitor cell populations and their levels of cell cycle activity, can be regulated in both a T-cell-independent and T-cell-dependent, as well as in an organ-specific, manner. Collectively, these data add to our knowledge of the long-term changes which occur in organs in which L. donovani is able to persist.

B cell-deficient mice are highly resistant to Leishmania donovani infection, but develop neutrophil-mediated tissue pathology

Resolution of Leishmania infection is T cell-dependent, and B lymphocytes have been considered to play a minimal role in host defense. In this study, the contribution of B lymphocytes to the response against Leishmania donovani was investigated using genetically modified IgM transmembrane domain (muMT) mutant mice, which lack mature B lymphocytes. When compared with wild-type mice, muMT mice cleared parasites more rapidly from the liver, and infection failed to establish in the spleen. The rapid clearance of parasites in muMT mice was associated with accelerated and more extensive hepatic granuloma formation compared with wild-type mice. However, the liver of infected muMT mice also showed signs of destructive pathology, associated with the presence of increased numbers of neutrophils. The role of neutrophils in controlling parasite growth in the viscera was determined by depletion with the mAb RB6-8C5. This treatment led to a dramatic enhancement of parasite growth in both the liver and spleen of muMT and wild-type mice. As assessed by transfer of both normal and chronic-infection serum, Ig protects microMT mice from destructive hepatic pathology, but minimally alters their resistance compared with wild-type mice. However, adoptive transfer of CD4+ and CD8+ T cells into recombinase activating gene 1 (RAG1-/-) recipients, suggested that T cell function was not altered by maturation in a B cell-deficient environment. Taken together, these data suggest an inhibitory role for B lymphocytes in resistance to L. donovani unrelated to the presence or absence of Ig. However, Ig protects muMT mice from the exaggerated pathology that occurs during infection.

Leishmania donovani infection of bone marrow stromal macrophages selectively enhances myelopoiesis, by a mechanism involving GM-CSF and TNF-alpha

Alterations in hematopoiesis are common in experimental infectious disease. However, few studies have addressed the mechanisms underlying changes in hematopoietic function or assessed the direct impact of infectious agents on the cells that regulate these processes. In experimental visceral leishmaniasis, caused by infection with the protozoan parasite Leishmania donovani, parasites persist in the spleen and bone marrow, and their expansion in these sites is associated with increases in local hematopoietic activity. The results of this study show that L donovani targets bone marrow stromal macrophages in vivo and can infect and multiply in stromal cell lines of macrophage, but not other lineages in vitro. Infection of stromal macrophages increases their capacity to support myelopoiesis in vitro, an effect mediated mainly through the induction of granulocyte macrophage-colony stimulating factor and tumor necrosis factor-alpha. These data are the first to directly demonstrate that intracellular parasitism of a stromal cell population may modify its capacity to regulate hematopoiesis during infectious disease. (Blood. 2000;95:1642-1651)

Organ-specific immune responses associated with infectious disease

The immune response to infection can vary markedly in different organs of the same animal. In some organs, the infection can resolve with subsequent immunity to re-infection, whereas in other organs, pathogens can persist. Here, Christian Engwerda and Paul Kaye highlight the importance of defining organ-specific immune mechanisms for developing strategies that deal effectively with infectious diseases and their associated pathologies.

Leishmania donovani infection initiates T cell-independent chemokine responses, which are subsequently amplified in a T cell-dependent manner

Control of Leishmania donovani infection in immunocompetent mice is associated with hepatic inflammation and granuloma formation, both of which are absent in severe combined immunodeficient (scid) mice. In both BALB/c and scid mice, L. donovani infection induced a rapid hepatic accumulation of mRNA encoding macrophage inflammatory protein-1alpha (MIP-(1alpha), monocyte chemoattractant protein-1 (MCP-1) and interferon-gamma inducible protein-10 (gammaIP-10). This response was not preceded by increased IL-4 production in either strain, unlike that reported in other infectious disease models. Interestingly, only gammaIP-10 mRNA was maintained at elevated levels throughout the first 7 days of infection, by mechanisms involving CD4+ and CD8+ T cells, and CD4+CD8+ cells not activated in scid mice. By in vivo depletion and reconstitution of scid mice it was demonstrated that T cells regulate the expression of all three chemokines studied, while they themselves only produce gammaIP-10 in appreciable quantities.

Blockade of CTLA-4 enhances host resistance to the intracellular pathogen, Leishmania donovani

CTLA-4 has recently been shown to act as a negative regulator of T cell activation. Here we provide evidence that blockade of CTLA-4 can result in enhanced host resistance to an intracellular pathogen. The administration of anti-CTLA-4 mAb 4F10 to BALB/c mice, 1 day following infection with Leishmania donovani, enhanced the frequency of IFN-gamma and IL-4 producing cells in both spleen and liver, and dramatically accelerated the development of a hepatic granulomatous response. The expression of mRNA for the CXC chemokine gammaIP-10 was also elevated above that seen in control Ab treated mice, and was directly correlated with the frequency of IFN-gamma producing cells. In contrast, macrophage inflammatory protein-1alpha (MIP-1alpha) and monocyte chemotactic protein-1 (MCP-1) mRNA levels were unaffected by anti-CTLA-4 treatment, suggesting that CTLA-4 blockade may exert selective effects on chemokine expression. These changes in tissue response and cytokine/chemokine production were accompanied by a 50 to 75% reduction of parasite load in the spleen and liver of anti-CTLA-4-treated animals compared to controls. Furthermore, administration of anti-CTLA-4 mAb 15 days after L. donovani infection, when parasite burden is increasing in both organs, also resulted in enhanced resistance. Thus, these studies indicate a potent immunomodulatory and potentially therapeutic role for interventions targeted at CTLA-4.

Neutralization of IL-12 demonstrates the existence of discrete organ-specific phases in the control of Leishmania donovani

IL-12 plays a key role in stimulating both innate and antigen-specific immune responses against a number of intracellular pathogens. A neutralizing anti-IL-12 monoclonal antibody (mAb) was used to define and compare the role of endogenous IL-12 in the liver and spleen of mice infected with Leishmania donovani. IL-12 neutralization both early and late in infection caused delayed resolution of parasite load, a transient decrease in IFN-gamma, IL-4, TNF-alpha and inducible nitric oxide synthase (NOS-2) production, and suppressed tissue granuloma formation in the liver of genetically susceptible BALB/c mice. In contrast to the liver of BALB/c mice, neutralization of IL-12 had no effect on parasite burden in the spleen over the first 28 days of infection. However, IL-12 appeared to be critical for the development of mechanisms which subsequently contain the growth of persistent parasites in this organ in that neutralization of IL-12 dramatically enhanced parasite growth after day 28 of infection. Following IL-12 neutralization, the later unchecked growth of parasites in the spleen was coincident with an extensive breakdown of the tissue microarchitecture. Immunohistochemical studies revealed that IL-12 was largely produced by uninfected cells in L. donovani-infected BALB/c mice. In contrast, the course of infection in the liver and spleen of genetically resistant CBA/n mice was unaffected by the administration of anti-IL-12 mAb. These results suggest that the liver and spleen in susceptible BALB/c mice have different temporal requirements for IL-12 in controlling L. donovani infection, whereas IL-12 plays little role in either organ in resistant CBA/n mice. In addition, IL-12 appears to be involved in the generation of both Th1 and Th2 responses during L. donovani infection in BALB/c mice.

Dendritic cells, but not macrophages, produce IL-12 immediately following Leishmania donovani infection

Infection with Leishmania, an obligate intracellular parasite of mononuclear phagocytes, stimulates the production of IFN-gamma from NK cells, via a pathway which is dependent upon IL-12 and IL-2. IL-12 is also essential for the development of host protective T cell responses to this parasite. However, previous in vitro studies have indicated that macrophages fail to make IL-12 following infection with Leishmania, and that subsequent to infection, macrophages become refractory to normal IL-12-inducing stimuli. We have used an in situ approach to attempt to resolve this apparent paradox, and by immunostaining for IL-12 p40 protein, we now demonstrate for the first time, that dendritic cells (DC) are the critical source of early IL-12 production following Leishmania infection. IL-12 production by DC is transient, peaking at 1 day post infection and returning to the levels seen in uninfected mice by day 3. Although resident tissue macrophages fail to produce IL-12 after Leishmania infection, these cells are not totally refractory to cytokine inducing stimuli, as TNF-alpha production is induced by day 3 post infection. Not only do these data satisfactorily explain the differences between in vivo and in vitro data by identifying the cellular source of IL-12, but they also suggest a novel model for NK cell activation; namely that in response to pathogens which fail to trigger IL-12 production by macrophages, DC-T cell clusters provide the microenvironment for initial NK cell activation.

B7-2 blockade enhances T cell responses to Leishmania donovani

Infection with Leishmania donovani has been reported to induce a dominant Th1-type response in all strains of mice examined, providing a model for examining the regulation of Th1 responses in the relative absence of Th2 cross-regulation. Here we demonstrate that blockade of the costimulatory molecule B7-2, but not B7-1, has significant effects on disease progression, measured as day 28 parasite burden in the liver. The effects of B7-2 blockade were associated with increased IFN-gamma production, as determined 1) following restimulation with specific Ag, 2) by enumeration of IFN-gamma-secreting cells using ELISPOT assays, and 3) by analysis of IFN-gamma mRNA by reverse transcription-PCR. Surprisingly, IL-4-producing cells were also readily detected in infected mice by ELISPOT analysis. The frequency of these IL-4-producing cells and of IL-4 mRNA levels was also enhanced in the liver of infected mice treated with anti-B7-2 mAb. Administration of anti-B7-2 from the day of infection or delaying its administration until day 3 after infection had similar effects. Parasite-specific IgG1 and IgG2a responses were either unaffected or marginally increased following anti-B7-2 administration, contrary to the inhibitory effect of this treatment on responses to the T-dependent Ag DNP-BSA. These data support a model of T cell activation whereby B7-2/CD28 interactions play a relatively redundant role in initial T cell activation, but in which interference with later B7-2/CTLA4 interaction potentiates established cytokine responses.

B7-2 blockade enhances T cell responses to Leishmania donovani

Infection with Leishmania donovani has been reported to induce a dominant Th1-type response in all strains of mice examined, providing a model for examining the regulation of Th1 responses in the relative absence of Th2 cross-regulation. Here we demonstrate that blockade of the costimulatory molecule B7-2, but not B7-1, has significant effects on disease progression, measured as day 28 parasite burden in the liver. The effects of B7-2 blockade were associated with increased IFN-gamma production, as determined 1) following restimulation with specific Ag, 2) by enumeration of IFN-gamma-secreting cells using ELISPOT assays, and 3) by analysis of IFN-gamma mRNA by reverse transcription-PCR. Surprisingly, IL-4-producing cells were also readily detected in infected mice by ELISPOT analysis. The frequency of these IL-4-producing cells and of IL-4 mRNA levels was also enhanced in the liver of infected mice treated with anti-B7-2 mAb. Administration of anti-B7-2 from the day of infection or delaying its administration until day 3 after infection had similar effects. Parasite-specific IgG1 and IgG2a responses were either unaffected or marginally increased following anti-B7-2 administration, contrary to the inhibitory effect of this treatment on responses to the T-dependent Ag DNP-BSA. These data support a model of T cell activation whereby B7-2/CD28 interactions play a relatively redundant role in initial T cell activation, but in which interference with later B7-2/CTLA4 interaction potentiates established cytokine responses.

Destruction of follicular dendritic cells during chronic visceral leishmaniasis

Follicular dendritic cells (FDCs) play a pivotal role in the germinal center (GC) response and in the development and regulation of B lymphocytes. Pathologic changes in GCs and a loss of FDCs have previously been noted in various viral infections, notably HIV-1. However, such changes have not been formally described in a chronic parasitic infection. In BALB/c mice infected with Leishmania donovani, parasites persist in the spleen for long periods, with associated splenomegaly. To examine the fate of FDC during the course of this chronic infection, we used 1) immunohistology, with FDC-specific mAbs; and 2) passive immunization with immune complexes, followed by light and electron microscopy. This study demonstrates that destruction of FDCs and a concomitant loss of GCs are associated with chronic visceral leishmaniasis. These pathologic effects are notable from 4 wk postinfection. At 8 wk postinfection and beyond, FDC are almost undetectable by both immunohistology and functional immune complex trapping. The loss of FDCs is associated with the infiltration of heavily parasitized macrophages into the GC, and reduction in parasite burden by chemotherapy is able to retard the process of FDC destruction. These data directly demonstrate for the first time the loss of FDCs during a chronic parasite infection and suggest a mechanism underlying the aberrant regulation of B cell function in murine visceral leishmaniasis.

Destruction of follicular dendritic cells during chronic visceral leishmaniasis

Follicular dendritic cells (FDCs) play a pivotal role in the germinal center (GC) response and in the development and regulation of B lymphocytes. Pathologic changes in GCs and a loss of FDCs have previously been noted in various viral infections, notably HIV-1. However, such changes have not been formally described in a chronic parasitic infection. In BALB/c mice infected with Leishmania donovani, parasites persist in the spleen for long periods, with associated splenomegaly. To examine the fate of FDC during the course of this chronic infection, we used 1) immunohistology, with FDC-specific mAbs; and 2) passive immunization with immune complexes, followed by light and electron microscopy. This study demonstrates that destruction of FDCs and a concomitant loss of GCs are associated with chronic visceral leishmaniasis. These pathologic effects are notable from 4 wk postinfection. At 8 wk postinfection and beyond, FDC are almost undetectable by both immunohistology and functional immune complex trapping. The loss of FDCs is associated with the infiltration of heavily parasitized macrophages into the GC, and reduction in parasite burden by chemotherapy is able to retard the process of FDC destruction. These data directly demonstrate for the first time the loss of FDCs during a chronic parasite infection and suggest a mechanism underlying the aberrant regulation of B cell function in murine visceral leishmaniasis.

Epitope cleavage by Leishmania endopeptidase(s) limits the efficiency of the exogenous pathway of major histocompatibility complex class I-associated antigen presentation

The activation of CD8+ T cell responses is commonplace during infection with a number of nonviral pathogens. Consequently, there has been much interest in the pathways of presentation of such exogenous antigens for major histocompatibility complex class I-restricted recognition. We had previously shown that Leishmania promastigotes transfected with the ovalbumin (OVA) gene could efficiently target OVA to the parasitophorous vacuole (PV), with subsequent recognition by class II-restricted T cells. We now report the results of studies aimed at evaluating the PV as a route of entry into the exogenous class I pathway. Bone marrow-derived macrophages can present soluble OVA (albeit at high concentrations) to the OVA(257-264)-specific T cell hybridoma 13.13. In contrast, infection with OVA-transfected Leishmania promastigotes failed to result in the stimulation of this hybridoma. This appeared unrelated to variables such as antigen concentration, parasite survival, and macrophage activation status. These results prompted an analysis of the effects of promastigotes on class I peptide binding using RMA-S cells and OVA(257-264). Our data indicate that the major surface protease of Leishmania, gp63, inhibits this interaction by virtue of its endopeptidase activity against the OVA(257-264) peptide. The data suggest that this activity, if maintained within the PV, would result in loss of the OVA(257-264) epitope. Although we can therefore draw no conclusions from these studies regarding the efficiency of the PV as a site of entry of antigen into the exogenous class I pathway, we have identified a further means by which parasites may manipulate the immune repertoire of their host.

An in vivo analysis of cytokine production during Leishmania donovani infection in scid mice

Leishmania donovani, the causative agent of visceral leishmaniasis, fails to induce NK cell activation in scid mice. In order to further our understanding of the host response to L. donovani, we analysed cytokine mRNA accumulation and TNF alpha protein synthesis in the liver of scid and BALB/c mice infected with this parasite. scid mice infected with L. donovani exhibited very little proinflammatory response, as measured by cytokine mRNA accumulation and TNF alpha protein expression, supporting the notion of a relatively "quiet" interaction between L. donovani and macrophages in these animals. In contrast, immunocompetent BALB/c mice were found to generate an early IFN gamma response, coincident with a rise in IL-2 mRNA levels and elaboration of a tissue response involving TNF alpha production by infected Kupffer cells. These results extend our understanding of the response of BALB/c and scid mice to L. donovani infection and highlight the value of cytokine analysis at both the tissue and cellular levels.

C-reactive protein binds to a novel ligand on Leishmania donovani and increases uptake into human macrophages

C-reactive protein (CRP) is a major acute phase protein of man, with serum concentrations increasing dramatically following stimulation of hepatocytes by inflammatory cytokines. However, the role of CRP in inflammation and resistance to infection is still poorly understood. Here, the specificity of CRP binding to the surface of Leishmania donovani, an obligate intracellular parasite of mononuclear phagocytes, is described. CRP is shown to bind to promastigotes at the infectious metacyclic stage of development, at concentrations found in normal human serum. The presence of CRP on the surface of promastigotes substantially increases uptake into human monocyte-derived macrophages. Unusually, CRP does not bind via its characteristic ligand, phosphorylcholine. We show that CRP binds to the lipophosphoglycan (LPG) component of the promastigote cell surface, a molecule implicated in both uptake and survival of these parasites within the macrophage, and also to the major secreted protein of promastigotes, secreted acid phosphatase. Using mAb to LPG with known ligand specificities, we define a novel ligand for CRP as the repeating phosphorylated disaccharide units that form the backbone of LPG.

C-reactive protein binds to a novel ligand on Leishmania donovani and increases uptake into human macrophages

C-reactive protein (CRP) is a major acute phase protein of man, with serum concentrations increasing dramatically following stimulation of hepatocytes by inflammatory cytokines. However, the role of CRP in inflammation and resistance to infection is still poorly understood. Here, the specificity of CRP binding to the surface of Leishmania donovani, an obligate intracellular parasite of mononuclear phagocytes, is described. CRP is shown to bind to promastigotes at the infectious metacyclic stage of development, at concentrations found in normal human serum. The presence of CRP on the surface of promastigotes substantially increases uptake into human monocyte-derived macrophages. Unusually, CRP does not bind via its characteristic ligand, phosphorylcholine. We show that CRP binds to the lipophosphoglycan (LPG) component of the promastigote cell surface, a molecule implicated in both uptake and survival of these parasites within the macrophage, and also to the major secreted protein of promastigotes, secreted acid phosphatase. Using mAb to LPG with known ligand specificities, we define a novel ligand for CRP as the repeating phosphorylated disaccharide units that form the backbone of LPG.

Strategies for immune intervention in visceral leishmaniasis

Therapeutic intervention remains a major tool for control of visceral leishmaniasis (VL). Studies in murine models of VL have demonstrated the efficacy of various cytokines, either alone or in conjunction with antimony chemotherapy. Here, some basic aspects of the models used to study VL are summarized and the recent data in this field are reviewed. Finally, alternative strategies to harnessing the protective potential of the host immune response, based on augmenting communication between antigen-presenting cells and T cells, are discussed.

Costimulation and the regulation of antimicrobial immunity

The regulated expression of costimulatory molecules is a major factor limiting T-cell responses to self-antigens. However, the development of effective antimicrobial immunity requires that these molecules be induced on a variety of tissues, but most notably on macrophages. Here, Paul Kaye discusses the regulation of costimulatory molecules on macrophages and suggests that microbial interference in this process has important implications for immune regulation.

Deficient expression of co-stimulatory molecules on Leishmania-infected macrophages

Co-stimulatory signals are necessary for the full activation of T cells for growth and effector function. As co-stimulatory molecules are normally regulated in their expression, it has been suggested that microorganisms enhance their expression on host antigen-presenting cells (APC), thus allowing efficient generation of anti-microbial immunity. We here describe experiments which demonstrate that infection of macrophages, both in vitro and in vivo, by the protozoan parasite Leishmania donovani fails to trigger expression of co-stimulatory molecules B7-1 and heat-stable antigen on these APC. Furthermore, infection with this parasite inhibits the macrophage response to normal regulatory signals, such as bacterial lipopolysaccharide. These changes in the cell surface are mirrored in functional studies of co-stimulation in vitro. Together, these data suggest a further facet of parasite interference in host immunity, but also indicate a potential new target for immunotherapy.

Leishmania donovani-infected macrophages: characterization of the parasitophorous vacuole and potential role of this organelle in antigen presentation

Leishmania donovani amastigotes, the etiological agents of visceral leishmaniasis, are obligate intracellular parasites residing in membrane-bound compartments of macrophages called parasitophorous vacuoles (PV). The study of these organelles is of paramount importance to understanding how these parasites resist the microbicidal mechanisms of macrophages and how they escape the immune response of their hosts. Confocal microscopy of mouse bone marrow-derived macrophages infected with L. donovani amastigotes and stained for various prelysosomal/lysosomal markers and for major histocompatibility complex (MHC) molecules was used to define PV with respect to the endocytic compartments of the host cells and to address the issue of their potential role in antigen processing and presentation. Forty-eight hours after infection, many PV contained cathepsins B, D, H and L and they were all surrounded by a membrane enriched for the lysosomal glycoprotein lgp120/lamp 1 but apparently devoid of the cation-independent mannose 6-phosphate receptor, a membrane protein generally absent from the lysosomes. These data suggested that PV acquire within 48 hours the characteristics of a lysosomal compartment. However, both macrosialin and the GTP-binding protein rab7p (specific markers of the prelysosomal compartment) were found to be highly expressed in/on PV membrane. Thus, at this stage, PV appear to exhibit both lysosomal and prelysosomal features. Infected macrophages activated with IFN-gamma before or after infection showed PV strongly stained for MHC class II molecules but not for MHC class I molecules. This suggests that, if infected macrophages can act as antigen-presenting cells for class I-restricted CD8+ T lymphocytes, Leishmania antigens must exit the PV. MHC class II molecules reached the PV progressively, indicating that they were not plasma membrane-bound molecules trapped during internalization of the parasites. The redistribution of class II observed in infected cells did not alter their quantitative expression on the plasma membrane at least during the first 48 hours following the phagocytosis of the parasites. The invariant chains, which are transiently associated with class II molecules during their intracellular transport and which mask their peptide-binding sites, did not reach PV or were rapidly degraded in these sites, suggesting that PV-associated class II are able to bind peptides. This last assumption is strengthened by the fact that class II located in PV could bind conformational antibodies that preferentially recognize class II with tightly associated peptides.(ABSTRACT TRUNCATED AT 400 WORDS)

Resistance induced by drug abbreviated Schistosoma mansoni infections: treatment with the drug Ro11-3128 leads to enhanced antigen presentation

Treatment of mice with the benzodiazepine derivative Ro11-3128 1-2 days post-infection with Schistosoma mansoni leads to arrest of virtually all schistosomula at the skin stage, and results in the development of protective immunity to challenge infection. A characteristic feature of Ro11-3128 treatment in vitro is the formation of exudates and membranous blebs at the schistosomular surface; other drugs tested, such as Ro15-5458 and oxamniquine which are also effective against the skin stages but relatively ineffective in inducing protection, do not induce this reaction. Here, we have examined whether such in vitro treatment causes enhanced presentation of schistosomular antigens by host antigen-presenting cells (APC) using an in vitro assay with activated peritoneal adherent cells as APC and T cells from S. mansoni antigen-sensitized mice. We have shown that viable mechanically transformed schistosomula (MS) can be processed and presented with similar kinetics to soluble antigen. However, in vitro drug treatment leads to enhanced presentation of MS. Experiments in which membranous blebs and antigen released by Ro11-3128-treated parasites during in vitro culture were separated from the remaining intact schistosomula, demonstrated significant stimulatory activity in the soluble and particulate-released antigen fractions. Filtration, antigen transfer experiments and SDS-PAGE analysis of the released material further suggested that most of the activity resided in the particulate fraction. Thus, quantitative and qualitative changes to antigen presentation by Ro11-3128 treatment early after infection may underlie the immunoprotective efficacy of Ro11-3128-abbreviated infections.

Antigens targeted to the Leishmania phagolysosome are processed for CD4+ T cell recognition

Processing of antigen for recognition by class II-restricted CD4+ T cells occurs within acidic compartments of the antigen-presenting cell. The exact nature of this compartment has yet to be precisely defined, however, but may vary depending upon the cell type studied and the antigen used. The acidic compartments of macrophages are also responsible for the degradation of ingested micro-organisms and play host to others which are adapted to an intracellular existence. To determine whether the phagolysosome (PL) formed in activated macrophages after ingestion of Leishmania parasites is also a site for entry of antigen into the class II presentation pathway, we have used the approach of genetic transformation. Hence, Leishmania were transfected with the genes for the protein antigens ovalbumin (OVA) and beta-galactosidase (beta-gal) and after infection were able to deliver these antigens specifically into the PL. Delivery of antigen to this site resulted in the ability of infected macrophages to present these antigens to antigen-specific CD4+ T cells. After taking into account the absolute levels of antigen uptake by macrophages, a 4-h processing period for OVA delivered by this or a soluble route led to equivalent levels of T cell activation. Unlike macrophages pulsed with soluble OVA, those with PL-targeted OVA still retained the ability to stimulate T cells after a 24-h processing period. This enhanced lifespan of antigen in macrophages corresponded to the kinetics of degradation of the parasite, suggesting slow release of antigen into the processing pathway. beta-gal presentation from the PL was tenfold less efficient under the same conditions. In addition to providing the first information on antigen processing in a protozoan PL, these studies highlight the usefulness of genetically transformed parasites for these types of studies.

Leishmania donovani infection in scid mice: lack of tissue response and in vivo macrophage activation correlates with failure to trigger natural killer cell-derived gamma interferon production in vitro

Infection of immunocompetent mice with Leishmania donovani is characterized by the development of a tissue granulomatous response, in vivo macrophage activation, and a predominantly Th1-type CD4+ T-cell response. To determine whether a recently described T-cell-independent pathway of gamma interferon (IFN-gamma) production involving the collaboration of macrophages and natural killer (NK) cells contributed to this pattern of events, we have investigated the responses of scid mice to L. donovani infection. The multiplication of parasites in the livers of scid mice progressed at a rate equivalent to that seen in BALB/c mice over the first 14 days of infection, but by day 28 scid mice had a fivefold-higher parasite burden. This infection was not, however, accompanied by any demonstrable histological response in the liver or by elevated major histocompatibility complex class II expression on splenic macrophages. In vitro, L. donovani was unable to trigger IFN-gamma production from scid spleen cell cultures under conditions which allowed efficient triggering by bacterial stimuli. Although L. donovani also failed to stimulate the release of tumor necrosis factor, an important macrophage-derived cofactor for IFN-gamma secretion by NK cells, exogenous recombinant tumor necrosis factor alpha could not restore the IFN-gamma response. Even with the potent synergistic effect of exogenous interleukin-2, L. donovani was unable to stimulate this pathway to the same extent as Listeria monocytogenes. Indeed, L. donovani inhibited the response to L. monocytogenes in a dose-dependent fashion. Experiments involving the transfer of supernatants and the use of neutralizing monoclonal antibodies have failed to find evidence that interleukin-10 is involved in this inhibition. These data suggest that NK cell-derived IFN-gamma is unlikely to participate in the early regulation of visceral leishmaniasis in the mouse.

Recombinant interleukin-1 alpha augments granuloma formation and cytokine production but not parasite clearance in mice infected with Leishmania donovani

In vivo administration of various doses of recombinant interleukin-1 alpha to B10.D2/n mice chronically infected with Leishmania donovani resulted in enhanced formation of granulomas and in vitro production of gamma interferon. By direct microscopical enumeration, reduction in gross parasite burden in the viscera was not observed, however. These data highlight an important discordance between granuloma formation per se and parasite elimination and suggest that interleukin-1 deficiency alone cannot account for the chronicity of this disease.

Altered course of visceral leishmaniasis in mice expressing transgenic I-E molecules

Previous studies had shown that the outcome of infection with Leishmania donovani was exquisitely sensitive to the influence of the major histocompatibility complex. In this study, we have examined the course of infection in non-obese diabetic (NOD) and NOD-E-3 mice, the latter expressing an I-E molecule as a result of transgenic introduction of the wild-type Ed alpha gene. Introduction of this transgene significantly altered the course of infection allowing for enhanced parasite multiplication in the viscera from day 14 to day 28. This was associated with both a delayed and reduced tissue granulomatous response in NOD-E-3 mice. In vitro, spleen cells from these mice produced equivalent levels of interferon (IFN)-gamma during the early phase of infection but this originated from populations having a different balance of T cells subsets. In NOD mice CD8+ T cells contribute substantially to the total levels of IFN-gamma produced, but in transgenic mice the contribution from this subset is significantly decreased. This is reflected in a reduction in the proportion of Leishmania-specific CD8+ T cells, which could only partially be accounted for by deletion of V beta 5- and V beta 3-expressing CD8+ T cells in NOD-E-3 mice. This study highlights the impact of the introduction of a class II gene product on disease outcome and unexpectedly on the functional potential of CD8+ T cells.

Antigen presentation by dendritic cells provides optimal stimulation for the production of interleukin (IL) 2, IL 4 and interferon-gamma by allogeneic T cells

Previous studies have shown that dendritic cells are the most potent inducers of T cell proliferation in vitro and that this is reflected in the release of interleukin (IL) 2 into culture supernatants during dendritic cell-T cell interaction. However, the role of the dendritic cells, and, indeed, of the antigen-presenting step, has not yet been explored with respect to other T cell-derived cytokines, in either a qualitative or relative fashion. In this study, therefore, we have examined the comparative role of different antigen-presenting cells (APC) as inducers of T cell cytokine release in allogeneic responses. We have confirmed that dendritic cells are the most effective inducers for IL2 and have shown that this is true not only in primary alloresponses, but also in alloresponder T cells maintained for extended periods and then rechallenged. Dendritic cells were also the most potent inducers of IL3 and interferon-gamma (IFN-gamma) in primary cultures. No IL4 was demonstrable irrespective of the type of presenting cells used, and both tissue macrophages and dendritic cells can induce synthesis of IL6. Likewise, in secondary alloresponses both dendritic cells and to a lesser extent tissue macrophages induce release of IL3, no IL4 is detectable, and activated macrophages and B cells raise IFN-gamma levels in the supernatants albeit to a lower concentration than that seen when dendritic cells are used as stimulators. The results were similar in the tertiary alloresponse except that (a) IL4 was now detectable in the supernatants but only where dendritic cells had been used as APC, and (b) both resting and activated macrophages induced IL2 and IFN-gamma. By the eighth cycle of allostimulation there is negligible IL2. Dendritic cells, tissue macrophages and activated B cells constitute a hierarchy of APC for IL3, IFN-gamma and IL4. These findings therefore demonstrate the role of dendritic cells as potent in vitro inducers of IL3, IL4 and IFN-gamma synthesis as well as of IL2.

Presentation of Leishmania donovani promastigotes occurs via a brefeldin A-sensitive pathway

For the presentation of Leishmania promastigotes to polyclonal CD4+ T cells, a processing period within activated macrophages of 3-4 h is required. Presentation can be inhibited by both chloroquine and brefeldin A (BFA), the latter implicating a requirement for newly synthesized MHC class II molecules. This inhibition is both reversible and specific, in that BFA did not inhibit mixed lymphocyte reaction stimulation by these infected macrophages. Immunogold labeling demonstrated that class II was associated with the parasite-containing phagolysosome. The level of class II was not significantly altered in BFA-treated cells in the time period studied, suggesting that antigen may exist the phagolysosome and interact with class II in another cellular compartment.

Differential production of Th1- and Th2-derived cytokines does not determine the genetically controlled or vaccine-induced rate of cure in murine visceral leishmaniasis

Recent studies with models of cutaneous leishmaniasis have provoked much interest in the role of CD4+ T cell subsets in determining the outcome of infectious disease. In Leishmania major infections, cure vs progressive disease correlates with the expansion of Th1-like or Th2-like CD4+ populations, respectively. We have investigated whether similar responses are associated with the differential patterns of infection seen in models of visceral leishmaniasis, caused by L. donovani. Splenic lymphocytes from infected Lsh congenic C57BL/10 (Lshs;H-2b) and B10.L-Lshr (Lshr;H-2b) mice and MHC congenic non-curing B10.D2/n (Lshs;H-2d) mice were examined for the production of cytokines representative of these CD4+ populations (IL-2, IL-3, IL-4, IL-5, and IFN-gamma). In all three strains examined, there was no evidence for the production of Th2-restricted cytokines. In addition, levels of serum IgE were depressed during the early phase of infection, indicative of in vivo IFN-gamma production. In the non-curing B10.D2/n strain, late phase of infection was associated with the decreased ability to produce cytokines in response to Ag and not with the production of IL-4 or IL-5 in response to Ag or mitogen. Serum IgE levels were also not raised above levels seen in uninfected controls. C57BL/10 mice were vaccinated with SDS-PAGE fractionated amastigote Ag bound to nitrocellulose and cytokine levels determined at various times after infection. The protocol used for vaccination was able to induce significant modulation of the course of infection in this strain and it was clear that IFN-gamma production in vitro provided an excellent correlate of rate of cure. Occasional individuals produced low levels of IL-5 in culture in response to parasite Ag, but this did not correlate with disease progression. Together, these data suggest that over-expansion of Th2-type cells and production of their specific cytokines (IL-4 and IL-5) is not a contributing factor to the variable long term course of L. donovani infection in these strains of mice.

Differential production of Th1- and Th2-derived cytokines does not determine the genetically controlled or vaccine-induced rate of cure in murine visceral leishmaniasis

Recent studies with models of cutaneous leishmaniasis have provoked much interest in the role of CD4+ T cell subsets in determining the outcome of infectious disease. In Leishmania major infections, cure vs progressive disease correlates with the expansion of Th1-like or Th2-like CD4+ populations, respectively. We have investigated whether similar responses are associated with the differential patterns of infection seen in models of visceral leishmaniasis, caused by L. donovani. Splenic lymphocytes from infected Lsh congenic C57BL/10 (Lshs;H-2b) and B10.L-Lshr (Lshr;H-2b) mice and MHC congenic non-curing B10.D2/n (Lshs;H-2d) mice were examined for the production of cytokines representative of these CD4+ populations (IL-2, IL-3, IL-4, IL-5, and IFN-gamma). In all three strains examined, there was no evidence for the production of Th2-restricted cytokines. In addition, levels of serum IgE were depressed during the early phase of infection, indicative of in vivo IFN-gamma production. In the non-curing B10.D2/n strain, late phase of infection was associated with the decreased ability to produce cytokines in response to Ag and not with the production of IL-4 or IL-5 in response to Ag or mitogen. Serum IgE levels were also not raised above levels seen in uninfected controls. C57BL/10 mice were vaccinated with SDS-PAGE fractionated amastigote Ag bound to nitrocellulose and cytokine levels determined at various times after infection. The protocol used for vaccination was able to induce significant modulation of the course of infection in this strain and it was clear that IFN-gamma production in vitro provided an excellent correlate of rate of cure. Occasional individuals produced low levels of IL-5 in culture in response to parasite Ag, but this did not correlate with disease progression. Together, these data suggest that over-expansion of Th2-type cells and production of their specific cytokines (IL-4 and IL-5) is not a contributing factor to the variable long term course of L. donovani infection in these strains of mice.

Antigen processing and presentation: modelling with Leishmania

Most foreign antigens are "processed" by APC into a form in which they can bind MHC molecules and be recognised by the TCR. Functional and immuno-gold labelling studies have been used to determine the putative sites of MHC-Ag interaction following uptake of Leishmania donovani into a phagolysosome. During infection with this parasite in vivo, upregulation of class II occurs. Based on studies using scid mice, NK cells appear to contribute little to this process. During chronic infection, however, APC function is diminished in spite of high class II levels. We propose that a lack of coordinate costimulator expression may lead to subsequent loss of T cell function late in infection, by a mechanism involving the induction of T cell unresponsiveness.

A modified colorimetric assay of macrophage activation for intracellular cytotoxicity against Leishmania parasites

An in vitro method is described which colorimetrically assesses the activation of macrophages for intracellular cytotoxicity against the obligate intracellular parasite Leishmania donovani. The assay system uses a highly purified macrophage population derived from 10-day murine bone marrow cultures. These were infected in vitro as a suspension culture with viable L. donovani amastigotes and then exposed to activating agents. After 48 h the intracellular parasites were released by SDS lysis of the macrophages. Surviving Leishmania organisms were quantitated by their conversion of the chromophore MTT. The sensitivity of this method was comparable with the established method of [3H]dThd incorporation. This assay system has been used to show that there is a dual signal requirement (recombinant interferon-gamma and bacterial endotoxin (LPS] for effective activation of macrophages for leishmanicidal activity.

Acquisition of cell-mediated immunity to Leishmania. II. LSH gene regulation of accessory cell function

The macrophage natural resistance gene. Lsh, regulates the ability of a selective population of tissue macrophages to control intracellular multiplication of Leishmania donovani by a T-cell independent mechanism. We show here, using mice congenic for Lsh, that this gene also contributes to the acquisition of T-cell-mediated immunity. Whereas both resistant and susceptible mice generate equivalent primary T-cell responses to infection, resistant mice show a rapid increase in accessory cell activity, allowing for greater subsequent T-cell expansion. This change in accessory cell function correlates with increased class II antigen expression relative to susceptible mice, both in vivo during early infection and in vitro in response to induction by interferon-gamma (IFN-gamma). Differences in vitro were independent of, but differentially affected by, amastigote infection.

Inflammatory cells in murine visceral leishmaniasis express a dendritic cell marker

Immunohistological studies of the murine liver following Leishmania donovani infection have been performed. We describe here the identification of cells expressing a recently defined dendritic cell marker, as detected by monoclonal antibody NLDC 145. Such cells are numerous in the developing inflammatory foci but are not detected individually at any other site in the infected liver. This finding suggests that mature tissue DC are not recruited during infection and that expression of this antigen is under precise microenvironmental control.

Antigen presentation and the response to parasitic infection

Antigen presentation to T-cells is central to the induction and maintenance of the cell-mediated immune response. Many approaches have been used to define a complex accessory cell compartment, which performs the variety of functional roles encompassed by this term. Resulting from this, antigen presentation has become the cornerstone behind many theories of disease susceptibility and is an important consideration for vaccine design. In this article, Paul Kaye reviews aspects of antigen presentation by accessory cells with particular emphasis on how parasites and their antigens interact with this heterogeneous group of cells.