Figure 2. Proposed model of crosstalk between producers and immune targets of IL-9.

IL-9 can exert various effects on both adaptive and innate immune cells. These effects include stimulating the differentiation and proliferation of Th17, promoting suppressive functions of Treg cells, enhancing cytotoxicity of cytotoxic T lymphocytes (Tc, CTL), inducing activation and accumulation of MCs, activating and maintaining ILCs, activating DCs, regulating memory B cell development and antibody recall response. After being stimulated by IL-9, IL-9 responders secrete cytokines which, in turn, exert their biological effects on IL-9 producing cells. There are also significant autocrine effects of IL-9 in Th17, NKT, MC, ILC and memory B cells. Indirect effects of IL-9 on Th1 and Th2 are not included in this illustration.