Interleukin 9 (IL-9) belongs to the common cytokine receptor γ-chain-dependent family, together with IL-2, IL-4, IL-7, IL-15, and IL-21. It is secreted by stimulated T helper cells, and specifically by a subpopulation classified as Tʜ9 cells. The IL-9 receptor forms a complex that internalizes upon IL-9 binding, and it is expressed both as a membrane-bound and a soluble form. In combination with other immune-stimulatory cytokines, IL-9 acts as a mitogen of lymphoid cells and other immune cells, such as erythroid precursors, mast cells, Tʜ2 cells and eosinophils. Moreover, IL-9 regulates hematopoietic cell differentiation, increases immunoglobulin production, and induces the growth of lymphoma and other T cell-mediated tumors. Mouse and human IL-9 share only a partial homology. Whereas mouse IL-9 is effective on human cells, human IL-9 is inactive in murine cell culture.
