IL-9 and Th9 cells: progress and challenges.

T(h)9 cells are a new subset of helper T cells, and the signature cytokine for T(h)9 cells is IL-9. Both T(h)9 cells and T(h)9 products are implicated in multiple disease settings. Thus, a clear understanding of how T(h)9 cells are induced and controlled is an important and clinically relevant issue. There are different molecular pathways identified thus far in the induction of T(h)9 cells, and activation of such diverse pathways requires integration of signals from TGF-β and IL-4 cytokine receptors as well as costimulatory molecules. These signals converge on the induction of multiple transcription factors that collectively drive the development of T(h)9 cells.