The immunology of susceptibility and resistance to Leishmania major in mice

The Leishmania infantum acidic ribosomal protein P0 administered as a DNA vaccine confers protective immunity to Leishmania major infection in BALB/c mice

In this study, we examined the immunogenic properties of the Leishmania infantum acidic ribosomal protein P0 (LiP0) in the BALB/c mouse model. The humoral and cellular responses induced by the administration of the LiP0 antigen, either as soluble recombinant LiP0 (rLiP0) or as a plasmid DNA formulation (pcDNA3-LiP0), were determined. Also, the immunological response associated with a prime-boost strategy, consisting of immunization with pcDNA3-LiP0 followed by a boost with rLiP0, was assayed. Immunization with rLiP0 induced a predominant Th2-like humoral response, but no anti-LiP0 antibodies were induced after immunization with pcDNA3-LiP0, whereas a strong humoral response consisting of a mixed immunoglobulin G2a (IgG2a)-IgG1 isotype profile was induced in mice immunized with the prime-boost regime. For all three immunization protocols, rLiP0-stimulated production of gamma interferon (IFN-gamma) in both splenocytes and lymph node cells from immunized mice was observed. However, it was only when mice were immunized with pcDNA3-LiP0 that noticeable protection against L. major infection was achieved, as determined by both lesion development and parasite burden. Immunization of mice with LiP0-DNA primes both CD4(+) and CD8(+) T cells, which, with the L. major challenge, were boosted to produce significant levels of IL-12-dependent, antigen-specific IFN-gamma. Taken together, these data indicate that genetic vaccination with LiP0 induces protective immunological effector mechanisms, yet the immunological response elicited by LiP0 is not sufficient to keep the infection from progressing.

Cross-talk between CD8+ and CD4+ T cells in experimental cutaneous leishmaniasis: CD8+ T cells are required for optimal IFN-gamma production by CD4+ T cells

Although the importance of CD8(+) T cells for vaccination and immunity to reinfection with Leishmania parasites is well established, their role in primary infections is disputed. In the present study we further characterized the role of CD8(+) T cells in primary L. major infections. We used two groups of L. major infected BALB/c mice: both groups were immunomanipulated to heal and in one group CD8(+) T cells were depleted throughout the course of infection. Our results show that the reversal of healing caused by the absence of CD8(+) T cells did not alter the proliferation of CD4(+) T cells, however, the frequency of CD4(+) T cells expressing IFN-gamma as well as the levels of this cytokine were clearly reduced. These lower levels of IFN-gamma correlated with a higher parasite load. Our results show that transient depletion of CD4(+) T cells allows the establishment of an equilibrium between CD4(+) and CD8(+) T cells and allows CD8(+) T cell activation and effector functions to develop. In addition, our results suggest that cross-talk between CD4(+) and CD8(+) T cells is crucial for the host defence against L. major.

Coxsackievirus B3-induced myocarditis: differences in the immune response of C57BL/6 and Balb/c mice

Coxsackievirus B3 (CVB3) infections are the most frequent causes of human myocarditis, often resulting in chronic stages characterized by fibrosis and loss of function. This disease is called dilated cardiomyopathy (DCM). Persistent virus in the myocardium may lead to chronic activation of fibroblasts, and subsequently, to fibrosis of the myocardium. Studies with immunodeficient mice have shown that certain defects of the immune system retard the rate at which virus is eliminated from the heart, thus leading to viral persistence. Therefore, we followed the immune response of two immunocompetent mouse strains (C57BL/6 and Balb/c) to CVB3 infection. These two strains have been reported to develop different immune responses to infections and we expected a similar reaction to viral infections as well. The two mouse strains recovered completely from CVB3 infection and expressed identical levels of cytokine mRNA in the heart. However, the virus in heart tissue decreased more slowly in Balb/c than in C57BL/6 mice. This was accompanied by a strong virus-specific IgG and weak IgM response in the C57BL/6 mice, in comparison to the Balb/c mice. We conclude, therefore, that viral-specific IgG is of importance for CVB3 elimination from infected hearts.

The role of CD4(+)CD25(+) regulatory T cells in Leishmania infection

There is growing evidence that regulatory T cells and, in particular, the endogenous CD4(+)CD25(+) T cells (T(reg)) are playing a fundamental role in the infectious process due to the protozoan parasite Leishmania. Endogenous CD4(+)CD25(+) T cells are a population of regulatory T cells, recently described for their capacity to control excessive or misdirected immune response. During human or murine Leishmaniasis, many features characteristic of T(reg )function, such as high levels of IL-10, transforming growth factor-beta (TGF-beta) or immunosuppression, have been extensively described. Recent reports formally involved T(reg) in the control of Leishmania major infection. Such control occurs by modulation of the effector immune response; in susceptible mouse strains, CD4(+)CD25(+) T cells suppress excessive T helper (Th)2 response, while in genetically resistant mouse strains, they control protective Th1 responses, allowing for parasite survival and maintenance of memory response. The mechanisms and consequences of such control in both susceptible or resistant mouse strains are discussed.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.

Interleukin 1alpha promotes Th1 differentiation and inhibits disease progression in Leishmania major-susceptible BALB/c mice

Protective immunity against pathogens such as Leishmania major is mediated by interleukin (IL)-12-dependent Th1-immunity. We have shown previously that skin-dendritic cells (DCs) from both resistant C57BL/6 and susceptible BALB/c mice release IL-12 when infected with L. major, and infected BALB/c DCs effectively vaccinate against leishmaniasis. To determine if cytokines other than IL-12 might influence disease outcome, we surveyed DCs from both strains for production of a variety of cytokines. Skin-DCs produced significantly less IL-1alpha in response to lipopolysaccharide/interferon gamma or L. major when expanded from BALB/c as compared with C57BL/6 mice. In addition, IL-1alpha mRNA accumulation in lymph nodes of L. major-infected BALB/c mice was approximately 3-fold lower than that in C57BL/6 mice. Local injections of IL-1alpha during the first 3 d after infection led to dramatic, persistent reductions in lesion sizes. In L. major-infected BALB/c mice, IL-1alpha administration resulted in increased Th1- and strikingly decreased Th2-cytokine production. IL-1alpha and IL-12 treatments were similarly effective, and IL-1alpha efficacy was strictly IL-12 dependent. These data indicate that transient local administration of IL-1alpha acts in conjunction with IL-12 to influence Th-development in cutaneous leishmaniasis and prevents disease progression in susceptible BALB/c mice, perhaps by enhancing DC-induced Th1-education. Differential production of IL-1 by C57BL/6 and BALB/c mice may provide a partial explanation for the disparate outcomes of infection in these mouse strains.

Production of eicosanoids and other oxylipins by pathogenic eukaryotic microbes

Oxylipins are oxygenated metabolites of fatty acids. Eicosanoids are a subset of oxylipins and include the prostaglandins and leukotrienes, which are potent regulators of host immune responses. Host cells are one source of eicosanoids and oxylipins during infection; however, another potential source of eicosanoids is the pathogen itself. A broad range of pathogenic fungi, protozoa, and helminths produce eicosanoids and other oxylipins by novel synthesis pathways. Why do these organisms produce oxylipins? Accumulating data suggest that phase change and differentiation in these organisms are controlled by oxylipins, including prostaglandins and lipoxygenase products. The precise role of pathogen-derived eicosanoids in pathogenesis remains to be determined, but the potential link between pathogen eicosanoids and the development of TH2 responses in the host is intriguing. Mammalian prostaglandins and leukotrienes have been studied extensively, and these molecules can modulate Th1 versus Th2 immune responses, chemokine production, phagocytosis, lymphocyte proliferation, and leukocyte chemotaxis. Thus, eicosanoids and oxylipins (host or microbe) may be mediators of a direct host-pathogen "cross-talk" that promotes chronic infection and hypersensitivity disease, common features of infection by eukaryotic pathogens.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.

The relative contribution of IL-4 receptor signaling and IL-10 to susceptibility to Leishmania major

The roles of IL-10 and IL-4 receptor signaling were evaluated in a murine model of Leishmania major infection. In previous studies the L. major substrain LV39 caused progressive, nonhealing lesions in BALB/c mice deficient for IL-4R alpha-chain (IL-4R alpha), while substrain IR173 was highly controlled. To explore whether IL-10 is responsible for inducing susceptibility to LV39, wild-type and IL-4R alpha(-/-) mice were treated with anti-IL-10R mAb, and in a genetic approach, the IL-4R alpha(-/-) mice were crossed with BALB/c IL-10(-/-) mice. In contrast to the lack of resistance conferred by IL-4R alpha gene deletion, partial resistance to LV39 was conferred by IL-10 gene deletion or treatment of BALB/c mice with anti-IL-10R mAb. Lesion sizes and LV39 parasite numbers were further and dramatically reduced in both anti-IL-10R-treated IL-4R alpha(-/-) mice and IL-4R alpha x IL-10 double knockouts. Anti-IL-10R mAb treatment further suppressed parasite growth in IL-4R alpha(-/-) mice infected with L. major IR173. Production of IFN-gamma was only increased relative to wild-type or littermate controls in IL-4R alpha(-/-) mice with complementary defects in IL-10. Comparisons of IFN-gamma-treated infected macrophages in vitro indicated that LV39 required 25- to 500-fold greater concentrations of IFN-gamma than IR173-infected macrophages to achieve a similar efficiency of parasite killing. These studies suggest that regardless of parasite substrain, IL-10 is as important as IL-4/IL-13 in promoting susceptibility to L. major and even more so for those substrains that are relatively resistant to IFN-gamma mediated killing.

Persistence of lesions in suppressor of cytokine signaling-1-deficient mice infected with Leishmania major

To investigate the role of the cytokine IFN-gamma and its negative regulator, the suppressor of cytokine signaling-1 (SOCS1) in the progression of cutaneous leishmaniasis, we infected mice lacking a single copy of the gene encoding SOCS1 (SOCS1(+/-)), mice lacking both copies of IFN-gamma (IFN-gamma(-/-)), or mice lacking copies of both SOCS1 and IFN-gamma (SOCS1(-/-) IFN-gamma(-/-)), with a moderate dose of 10(3) or 10(4) of the most virulent stage of parasites, metacyclic promastigotes. Surprisingly, SOCS1(+/-) mice developed larger lesions than wild-type mice, although the parasite load in the draining lymph node was not significantly altered. These mice also developed apparently normal Th1 responses, as indicated by elevated levels of IFN-gamma and low levels of IL-4 and IL-10. The persistence of lesions and the enlargement of draining lymph nodes despite a normal Th1 response and control of parasitemia indicate that there may be a dissociation of the inflammatory pathology and clearance of parasites in SOCS1(+/-) mice. We also investigated the role of the related suppressor of cytokine signaling, SOCS2, which has been implicated in the development of Th1 immunity. The progression of disease in SOCS2(-/-) mice did not differ from that in C57BL/6 control mice, suggesting that it is not involved in the host response to Leishmania major infection and supporting the specific role of SOCS1. These results suggest that SOCS1 plays an important role in the regulation of appropriate inflammatory responses during the resolution of L. major infection.

Protozomics: trypanosomatid parasite genetics comes of age

Trypanosomatid protozoans cause important diseases of humans and their domestic livestock. Various molecular genetic tools are now allowing rapid progress in understanding many of the unique aspects of the molecular and cell biology of these organisms. Diploidy and the lack or difficulty of sexual crossing has been a challenge for forward genetics, but powerful selections and functional complementation have helped to overcome it in Leishmania. RNA interference has been adapted for forward genetics in trypanosomes, in which it is also a powerful tool for reverse genetics. Interestingly, the efficacy of different genetic tools has steered research into different aspects of the biology of these parasites.