MyD88 is essential to sustain mTOR activation necessary to promote T helper 17 cell proliferation by linking IL-1 and IL-23 signaling.

Myeloid differentiation primary response protein 88 (MyD88) is classically known as an adaptor, linking TLR and IL-1R to downstream signaling pathways in the innate immune system. In addition to its role in innate immune cells, MyD88 has been shown to play an important role in T cells. How MyD88 regulates helper T-cell differentiation remains largely unknown, however. Here we demonstrate that MyD88 is an important regulator of IL-17-producing CD4(+) T helper cells (Th17) cell proliferation. MyD88-deficient CD4(+) T cells showed a defect in Th17 cell differentiation, but not in Th1 cell or Th2 cell differentiation. The impaired IL-17 production from MyD88-deficient CD4(+) T cells is not a result of defective RAR-related orphan receptor γt (RORγt) expression. Instead, MyD88 is essential for sustaining the mammalian target of rapamycin (mTOR) activation necessary to promote Th17 cell proliferation by linking IL-1 and IL-23 signaling. MyD88-deficient CD4(+) T cells showed impaired mTOR activation and, consequently, reduced Th17 cell proliferation. Importantly, the absence of MyD88 in T cells ameliorated disease in the experimental autoimmune encephalomyelitis model. Taken together, our results demonstrate that MyD88 has a dual function in Th17 cells by delivering IL-1 signaling during the early differentiation stage and integrating IL-23 signaling to the mTOR complex to expand committed Th17 cells.