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. In this model, the evolution of small, healing dermal lesions occurs in three distinct phases that have not been appreciated before: 1) an initial ?silent? phase, lasting 4-5 wks, favoring the establishment of the peak load of parasites in the dermis in the absence of lesion formation; 2) an acute phase, lasting 5-10 wks, corresponding to the development and resolution of a lesion that is associated with an acute infiltration of neutrophils, macrophages, and eosinophils into the dermis, and is coincident with the onset of immunity and the killing of parasites in the site; and 3) a chronic phase, lasting for the life of the animal, during which a low number of parasites persist in the skin in the absence of overt pathology. 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 ?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 low dose, intradermal infection model has been extended to L. tropica, which is responsible for anthroponotic cutaneous leishmaniasis. While the growth of the parasite is contained, there is no dermal pathology, parasite clearance does not occur, and a transmissible level of parasitemia persists in the skin for the life of the animal. This equilibrium was again found to be established by counter-balanced levels of IFNg and IL-10. The chronic infections due to L. tropica were cleared from the skin in IL-10 deficient mice, and in mice treated with anti-IL-10R antibodies. 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. 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, and these intrinsic differences in parasite interactions with DC may account, at least in part, for the evolution of healing and non-healing forms of leishmanial disease.
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