The p110delta isoform of phosphatidylinositol 3-kinase controls susceptibility to Leishmania major by regulating expansion and tissue homing of regulatory T cells.

Resistance to Leishmania major and most intracellular pathogens is usually associated with a strong T cell-mediated immunity, particularly a CD4(+) Th1 response. Mice with an inactivating knock-in mutation in the p110delta isoform of PI3K (referred to as p110delta(D910A)) show severely impaired T cell responses. Because a strong T cell response is thought to mediate resistance to intracellular pathogens, we examined the outcome of L. major infection in p110delta(D910A) mice. Paradoxically, p110delta(D910A) mice on "resistant" and "susceptible" genetic backgrounds showed more robust resistance manifested as significantly reduced lesion size and accelerated parasite clearance. This enhanced resistance was associated with dramatically diminished immune responses, including impaired cell proliferation and effector cytokine (IFN-gamma and TNF) production. Interestingly, the ability of macrophages and dendritic cells from p110delta(D910A) mice to produce NO and destroy Leishmania parasites was similar to those of wild-type mice. We show that the enhanced resistance of p110delta(D910A) mice was due to impaired expansion and effector functions of regulatory T cells (Tregs). Adoptive transfer studies demonstrated that p110delta(D910A) mice lost their increased resistance when given enriched Tregs from wild-type mice. We suggest on the basis of these and further observations that the lack of this enzyme prominently affects Treg expansion and homing to infection sites, and that in the absence of Tregs, weak Th1 responses are capable of containing parasites and prevent pathology. We also suggest that temporary pharmacological inhibition of this enzyme may be a very effective form of treatment against cutaneous leishmaniasis.