The development of T helper (T-H)17 and regulatory T (T-reg) cells is reciprocally regulated by cytokines. Transforming growth factor (TGF)-beta alone induces FoxP3(+) T-reg cells, but together with IL-6 or IL-21 induces T(H)17 cells. Here we demonstrate that IL-9 is a key molecule that affects differentiation of T(H)17 cells and T-reg function. IL-9 predominantly produced by T(H)17 cells, synergizes with TGF-beta 1 to differentiate naive CD4(+) T cells into T(H)17 cells, while IL-9 secretion by T(H)17 cells is regulated by IL-23. Interestingly, IL-9 enhances the suppressive functions of FoxP3(+) CD4(+) T-reg cells in vitro, and absence of IL-9 signaling weakens the suppressive activity of nT(regs) in vivo, leading to an increase in effector cells and worsening of experimental autoimmune encephalomyelitis. The mechanism of IL-9 effects on TH17 and T-regs is through activation of STAT3 and STAT5 signaling. Our findings highlight a role of IL-9 as a regulator of pathogenic versus protective mechanisms of immune responses.
Elyaman, W., Bradshaw, E. M., Uyttenhove, C., Dardalhon, V., Awasthi, A., Imitola, J., Bettelli, E., Oukka, M., Van Snick, J., Renauld, J.-C., Kuchroo, V. K., & Khoury, S. J. (2009). IL-9 induces differentiation of T(H)17 cells and enhances function of FoxP3(+) natural regulatory T cells. National Academy of Sciences. Proceedings, 106(31), 12885-12890. https://doi.org/10.1073/pnas.0812530106 (Original work published 2009)