High levels of susceptibility and T helper 2 response in MyD88-deficient mice infected with Leishmania major are interleukin-4 dependent.

Myeloid differentiation protein 88 (MyD88) is a general adaptor for the signaling cascade through receptors of the Toll/IL-1R family. When infected with Leishmania major parasites, MyD88-deficient mice displayed a dramatically enhanced parasite burden in their tissues similar to that found in susceptible BALB/c mice. In contrast, MyD88 knockout mice did not develop ulcerating lesions despite a lack of interleukin-12 (IL-12) production and a predominant T helper 2 cell response. Blockade of IL-4 produced early (day 1) after infection restored a protective T helper 1 response in MyD88 knockout mice.