Interleukin-10 from T Cells,but Not Macrophages and Granulocytes,Is Required for Chronic Disease in Leishmania mexicana Infection

Chronic cutaneous disease of mice caused by the protozoan parasite Leishmania mexicana requires interleukin-10 (IL-10) and FcγRIII (an activating IgG receptor). Macrophages readily secrete IL-10 in response to IgG-coated amastigotes, making macrophages a prime candidate as the critical source of IL-10. However, indirect evidence suggested that macrophage IL-10 is not essential for chronic disease. I now show directly that mice lacking IL-10 from macrophages and granulocytes still have chronic disease, like wild-type C57BL/6 mice. However, T cell-derived IL-10 is required for chronic disease. CD4-cre IL-10^flox/flox mice lack IL-10 from T cells (both CD4⁺ and CD8⁺) and heal their L. mexicana lesions, with parasite control. I had previously shown that depletion of CD25⁺ T cells had no effect on chronic disease, and thus, T cells other than CD25⁺ regulatory T (T_reg) cells should be the important source of IL-10. Given that conventional T cells do not express FcγRs, there is likely to be an indirect pathway by which FcγRIII on some other cell engaged by IgG1-amastigote immune complexes induces IL-10 from T cells. Further work is needed to delineate these pathways.