Despite increased CD4+Foxp3+ cells within the infection site, BALB/c IL-4 receptor-deficient mice reveal CD4+Foxp3-negative T cells as a source of IL-10 in Leishmania major susceptibility.

BALB/c IL-4Ralpha(-/-) mice, despite the absence of IL-4/IL-13 signaling and potent Th2 responses, remain highly susceptible to Leishmania major substain LV39 due exclusively to residual levels of IL-10. To address the contribution of CD4(+)CD25(+) T regulatory (Treg) cells to IL-10-mediated susceptibility, we depleted CD4(+)CD25(+) cells in vivo and reconstituted IL-4Ralpha x RAG2 recipients with purified CD4(+)CD25(-) T cells. Although anti-CD25 mAb treatment significantly decreased parasite numbers in IL-4Ralpha(-/-) mice, treatment with anti-IL-10R mAb virtually eliminated L. major parasites in both footpad and dermal infection sites. In addition, IL-4Ralpha x RAG2 mice reconstituted with CD4(+) cells depleted of CD25(+) Treg cells remained highly susceptible to infection. Analysis of L. major-infected BALB/c and IL-4Ralpha(-/-) inflammatory sites revealed that the majority of IL-10 was secreted by the CD4(+)Foxp3(-) population, with a fraction of IL-10 coming from CD4(+)Foxp3(+) Treg cells. All T cell IFN-gamma production was also derived from the CD4(+)Foxp3(-) population. Nevertheless, the IL-4Ralpha(-/-)-infected ear dermis, but not draining lymph nodes, consistently displayed 1.5- to 2-fold greater percentages of CD4(+)CD25(+) and CD4(+)Foxp3(+) Treg cells compared with the BALB/c-infected dermis. Thus, CD4(+)Foxp3(-) T cells are a major source of IL-10 that disrupts IFN-gamma activity in L. major-susceptible BALB/c mice. However, the increase in CD4(+)Foxp3(+) T cells within the IL-4Ralpha(-/-) dermis implies a possible IL-10-independent role for Treg cells within the infection site, and may indicate a novel immune escape mechanism used by L. major parasites in the absence of IL-4/IL-13 signaling.