IL-21 and TGF-β are required for differentiation of human TH17 cells.

The recent discovery of CD4⁺ T cells characterized by secretion of interleukin (IL)-17 (T_H17 cells) and the naturally occurring regulatory FOXP3⁺ CD4 T cell (nT_reg) has had a major impact on our understanding of immune processes not readily explained by the T_H1/T_H2 paradigm. T_H17 and nT_reg cells have been implicated in the pathogenesis of human autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease and psoriasis. Our recent data and the work of others demonstrated that transforming growth factor-β (TGF-β) and IL-6 are responsible for the differentiation of naive mouse T cells into T_H17 cells, and it has been proposed that IL-23 may have a critical role in stabilization of the T_H17 phenotype. A second pathway has been discovered in which a combination of TGF-β and IL-21 is capable of inducing differentiation of mouse T_H17 cells in the absence of IL-6 (refs 6–8). However, TGF-β and IL-6 are not capable of differentiating human T_H17 cells and it has been suggested that TGF-β may in fact suppress the generation of human T_H17 cells. Instead, it has been recently shown that the cytokines IL-1β, IL-6 and IL-23 are capable of driving IL-17 secretion in short-term CD4⁺ T cell lines isolated from human peripheral blood, although the factors required for differentiation of naive human CD4 to T_H17 cells are still unknown. Here we confirm that whereas IL-1β and IL-6 induce IL-17A secretion from human central memory CD4⁺ T cells, TGF-β and IL-21 uniquely promote the differentiation of human naive CD4⁺ T cells into T_H17 cells accompanied by expression of the transcription factor RORC2. These data will allow the investigation of this new population of T_H17 cells in human inflammatory disease. [ABSTRACT FROM AUTHOR]