IL-9 and T_h9 cells: Progress and challenges

T_h9 cells are a new subset of helper T cells, and the signature cytokine for T_h9 cells is IL-9. Both T_h9 cells and T_h9 products are implicated in multiple disease settings. Thus, a clear understanding of how T_h9 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_h9 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_h9 cells.