Leishmania major is a tropical protozoan parasite transmitted to mammalian hosts by its sand fly vector. The parasite can persist in its host, but how the pathogen is able to establish this latency remains unclear. In December 5 Nature, Yasmine Belkaid and colleagues at the National Institutes of Health, Bethesda, USA, show that dermal L. major persistence after healing is controlled by an endogenous population of CD4+ CD25+ regulatory T cells (Nature, 420:502-507, December 5, 2002).

Belkaid et al. inoculated promastigotes of L. major into the ear dermis of C57BL/6 mice and examined the resulting chronically infected lesions. They observed that in these areas CD4+ CD25+ T cells accumulated in the dermis, where they suppressed (by both interleukin-10-dependent and -independent mechanisms) the ability of CD4+ CD25+ effector T cells to eliminate the parasite from the site. In addition, they showed...

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