Interleukin-9 (IL-9), also known as P40 and MEA (mast cell growth-enhancing activity), is a 30-40 kDa glycosylated member of a cytokine family that includes Interleukins-2, -4, -7, -15, and -21. These proteins utilize heteromeric receptors containing the Common gamma chain ( gamma c) in addition to ligand-specific subunits. IL-9 interacts selectively with IL-9 R which then associates with gamma c to form the functional receptor complex. IL-9 contributes to allergic inflammation, autoimmunity-induced inflammation, parasite clearance from the GI tract, and Treg-mediated immune suppression (1, 2). It enhances the expansion and recruitment of mast cells and eosinophils as well as the production of IgE and Th2 cytokines (3?6). It is required for anaphylactic responses to ingested allergens but not to systemic allergens (7). IL-9 plays multiple roles in the development and function of subsets within the CD4+ T cell lineage (8). It is expressed by activated Th9, Th17, Treg, and Th2 cells (3, 9?12). IL-9 acts as an autocrine growth and activation factor for Th17, Treg, and mast cells (3, 11, 13). It also can inhibit immune responses by enhancing the suppressive properties of Treg and by recruiting immune-suppressive mast cells to sites of inflammation (11, 12). Mature human IL-9 shares 57% amino acid sequence identity with mouse and rat IL-9 (14, 15).
