Reciprocal TH17 and regulatory T cell differentiation mediated by retinoic acid

The cytokine transforming growth factor-beta (TGF-beta) converts naive T cells into regulatory T (Treg) cells that prevent autoimmunity. However, in the presence of interleukin-6 (IL-6), TGF-beta has also been found to promote the differentiation of naive T lymphocytes into proinflammatory IL-17 cytokine-producing T helper 17 (TH17) cells, which promote autoimmunity and inflammation. This raises the question of how TGF-beta can generate such distinct outcomes. We identified the vitamin A metabolite retinoic acid as a key regulator of TGF-beta-dependent immune responses, capable of inhibiting the IL-6-driven induction of proinflammatory TH17 cells and promoting anti-inflammatory Treg cell differentiation. These findings indicate that a common metabolite can regulate the balance between pro- and anti-inflammatory immunity.