Analysis Of T Cell Responses In Human 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 conditions favoring the persistence of low numbers of parasites in the skin following healing have also been studied. The chronic site is characterized by a high number of both CD8+ and CD4+ T lymphocytes in the dermis that are able to produce IFN g in response to the antigen. In vivo treatment with either anti-IFNg, anti-CD4+, or anti-CD8+ antibodies resulted in a dramatic increase in parasite numbers and reactivation of dermal lesions. Thus IFNg pressure is necessary to maintain a low parasite burden but not sufficient to clear it. The persistence of the parasite depends upon the production of IL-10 in the site, as evidenced by a) the presence of IL-10 staining cells in the ear and draining lymph node, b) treatment with anti-IL-10R antibodies resulted in complete clearance of parasites from the skin, and c) the complete clearance of L. major from the skin in IL-10 KO and IL-4/IL-10 KO mice. 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 leishmaniasis , this protection lasted longer in the DNA vaccinated mice. Furthermore, only the DNA vaccine reduced the parasitic burden in the skin during the acute and chronic stages to the low levels achieved in healed mice, and only the DNA vaccine eliminated the capacity of challenged mice to serve as infection reservoirs for vector sand flies. 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. Can the same level of immunity be induced and sustained using live attenuated L. major? 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. We have also initiated studies using Affymetrix Gene Chip analysis to compare parasite-induced gene expression patterns in human DCs. Hierarchical cluster analysis has revealed that while the majority of genes regulated by exposure to Leishmania are common between the species, a number of clusters are L. major or L. donovani specific.