TGF-beta 1 inhibition of IFN-gamma-induced signaling and Th1 gene expression in CD4+ T cells is Smad3 independent but MAP kinase dependent.

In addition to classic Smad signaling pathways, the pleiotropic immunoregulatory cytokine TGF-beta1 can activate MAP kinases, but a role for TGF-beta1-MAP kinase pathways in T cells has not been defined heretofore. We have shown previously that TGF-beta1 inhibits Th1 development by inhibiting IFN-gamma's induction of T-bet and other Th1 differentiation genes, and that TGF-beta1 inhibits receptor-proximal IFN-gamma-Jak-Stat signaling responses. We now show that these effects of TGF-beta1 are independent of the canonical TGF-beta1 signaling module Smad3, but involve a specific MAP kinase pathway. In primary T cells, TGF-beta1 activated the MEK/ERK and p38 MAP kinase pathways, but not the JNK pathway. Inhibition of the MEK/ERK pathway completely eliminated the inhibitory effects of TGF-beta1 on IFN-gamma responses in T cells, whereas inhibition of the p38 pathway had no effect. Thus, TGF-beta1's inhibition of IFN-gamma signaling in T cells is mediated through a highly specific Smad3 independent, MEK/ERK-dependent signaling pathway.