Innate immune recognition controls adaptive immune responses through multiple mechanisms. The MyD88 signaling adaptor operates in many cell types downstream of Toll-like receptors (TLRs) and interleukin-1 (IL-1) receptor family members. Cell-type-specific functions of MyD88 signaling remain poorly characterized. Here, we have shown that the T cell-specific ablation of MyD88 in mice impairs not only T helper 17 (Th17) cell responses, but also Th1 cell responses. MyD88 relayed signals of TLR-induced IL-1, which became dispensable for Th1 cell responses in the absence of T regulatory (Treg) cells. Treg cell-specific ablation of MyD88 had no effect, suggesting that IL-1 acts on naive CD4(+) T cells instead of Treg cells themselves. Together, these findings demonstrate that IL-1 renders naive CD4(+) T cells refractory to Treg cell-mediated suppression in order to allow their differentiation into Th1 cells. In addition, IL-1 was also important for the generation of functional CD4(+) memory T cells.
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