Analysis of variables associated with promotion of resistance and its abrogation in T cell-reconstituted nude mice infected with Leishmania major.

Upon intradermal challenge with the protozoan parasite Leishmania major, some mouse strains develop chronic cutaneous lesions, whereas other mouse strains show a resolving pattern of disease. The importance of T cell-dependent immunity in resistance to cutaneous leishmaniasis is substantiated by the susceptibility to infection of athymic nude mice of both resistant and susceptible strains. Small numbers of T lymphocytes from uninfected euthymic mice promote resistance in nude mice but T cells from chronically infected mice can impair this protective effect. In the present study we used an adoptive transfer system in which nude mice were reconstituted with T cells from normal or chronically infected mice in order to further investigate protection against disease or disease promotion. The results supported the following conclusions: (a) the host-protective activity of T cells from uninfected mice is highly effective even in long-term chronically infected nude mice, (b) T cell-mediated exacerbation of cutaneous disease does not involve enhancement of lesion development and is thus unlikely to be based on an accelerated proliferation of parasites in the lesion, (c) disease-promoting cells are not only found in genetically susceptible mice but can also be induced in genetically resistant mice, and (d) lymphoid organs of genetically susceptible mice chronically infected with L. major contain resistance-promoting cells in addition to disease-promoting cells. The data, together with those of others, continue to support the notion that recruitment with expansion and/or activation of different T cell subsets underlies genetically based resistance and susceptibility of mice to L. major.