Description: Interleukin-9 (IL-9) is a 14 kDa cytokine originally named P40 and identified by its proliferative effects on T cell populations. The receptor, which is a heterodimer of the γ chain portion of the IL-2 receptor and the IL-9R chain, activates Jak/STAT signaling pathways upon binding its ligand. Since the discovery of IL-9, numerous other functions have been observed. It induces Th17 and Treg differentiation in CD4+ T cells, IgE production in B cells, and the differentiation and proliferation of mast cells. IL-9 expression was initially observed in Th2 cells, but has since been found in Th17, eosinophil, and mast cells. Th9 cells, a newly discovered subset of CD4+ T cells, are characterized by the secretion of large amounts of IL-9 and IL-10. These cells are derived from Th2 cells reprogrammed by the presence of TGFβ, making IL-9 the probable mechanism by which TGFβ drives Th17 and Treg differentiation.
Mouse and human IL-9 share 69% sequence homology at the nucleotide level and 57% at the protein level. Although mouse IL-9 exhibits cross-species activity, human IL-9 is species-specific.
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