Analysis Of T Cell Responses In Leishmaniasis

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Abstract

We previously established a model of cutaneous leishmaniasis due to L. major infection combining two main features of natural transmission; inoculation of a low number of metacyclic promastigotes into the mouse ear dermis. Adaptive immunity in this model confirmed a role for Th1 cells, and in addition revealed a requirement for CD8+ T cells, based on the results obtained in beta 2 microglobulin KO mice, CD8 KO mice, and CD8 depleted mice, which in each case failed to control infection in the skin. The induction of CD8+ T cells during infection is being studied using transgenic parasites expressing ovalbumin, for which TCR transgenic T cells specific for a dominant Class I restricted eptiope are available. Infected dendritic cells efficiently process the epitope for class I presentation to the transgenic T cells in vivo and in vitro in a TAP independent manner, suggesting that cross-priming of leishmania-specific CD8+ T cells does not necessarily involve cytosolic processing and ER translocation of exogenous antigens. The conditions favoring the persistence of low numbers of parasites in the skin following healing have also been studied. The persistence of the parasite depends upon the production of IL-10 in the site, as evidenced by the complete clearance of L. major from the skin in IL-10 KO and IL-4/IL-10 KO mice and in wild type mice treated during the chronic phase with anti-IL-10R antibodies. The IL-10 was shown to be produced by CD4+CD25+ immunoregulatory T cells, a lineage of cells whose primary fucntion was previously thought to be confined to the regulation of peripheral tolerance. Mice that achieved sterile cure as a consequence of anti-IL-10 receptor antibody treatment were no longer immune to reinfection. The natural challenge model was used to compare the potency and durability of vaccination with a cocktail of plasmid DNAs encoding the antigens LACK, M15, and MAPS, with that of heat killed promastigotes plus recombinant IL-12 (rIL-12). While both vaccines conferred complete protection against dermal disease , this protection lasted longer in the DNA vaccinated mice, and was dependent, at least in part, on CD8+ T cells. Inclusion of oligodeoxynucleotide CpG immunostimulatory sequences as adjuvant significantly enhanced the durability of killed or recombinant vaccines. again in a CD8 dependent manner. Since individuals with healed lesions have life long immunity to reinfection, vaccination using virulent L. major promastigotes, termed leishmanization, remains the gold standard in terms of the potency and durability of acquired immunity that can be achieved, but remains problematic because of the severity of the vaccination-lesions. In preliminary studies, inclusion of oligodeoxynucleotide CpG immunostimulatory sequences, with or without addition of killed promastigotes to the live inoculum, has achieved virtually complete attenuation of dermal pathology without compromising the ability of the primary infection to establish long lived immunity to reinfection. The clinical forms of leishmaniasis in humans range from self-healing cutaneous lesions to often fatal visceral disease. These diverse clinical outcomes are attributed primarily to differences in the Leishmania species initiating the infections. Animal models using a species associated with self-limiting cutaneous disease, L. major, have revealed that protective immunity requires CD40/CD40L-dependent, IL-12-driven Th1 responses. We have found that in contrast to L. major, Leishmania species responsible for visceral disease (L dononvani), as well as species associated with persistent, cutaneous lesions and occasional systemic disease (L. tropica), do not prime human dendritic cells for CD40L induced IL-12p70 production. All Leishmania species upregulated surface expression of CD40, CD86 and HLA-DR, and appreciable CD40L-induced IL-12p40 secretion was observed in uninfected as well as infected DCs, regardless of species. RT-PCR analysis confirmed that the production of heterodimeric IL-12 was limited by transcriptional regulation of the IL-12p35 subunit, which was especially dependent on both a microbial priming signal and CD40 engagement for it high level induction. The intrinsic differences in the ability of Leishmania species to prime dendritic cells for CD40L dependent IL-12p70 production may account, at least in part, for the evolution of healing and non-healing forms of leishmanial disease. The inter-species differences in microbial priming of DC may be related to expression of polymorphic phosphoglycan-containing molecules, since L. major mutants spefically deficient in L. major restricted polygalactose epitopes no longer prime DC and produce non-helaing lesions in normally resistant mice.