Opposing actions of IL-2 and IL-21 on Th9 differentiation correlate with their differential regulation of BCL6 expression.

Interleukin 9 (IL-9) is a γc-family cytokine that is highly produced by T-helper 9 (Th9) cells and regulates a range of immune responses, including allergic inflammation. Here we show that IL-2-JAK3-STAT5 signaling is required for Th9 differentiation, with critical STAT5 binding sites in the Il9 (the gene encoding IL-9) promoter. IL-2 also inhibited B cell lymphoma 6 (BCL6) expression, and overexpression of BCL6 impaired Th9 differentiation. In contrast, IL-21 induced BCL6 and diminished IL-9 expression in wild-type but not Bcl6(-/-) cells, and Th9 differentiation was increased in Il21(-/-) and Il21r(-/-) T cells. Interestingly, BCL6 bound in proximity to many STAT5 and STAT6 binding sites, including at the Il9 promoter. Moreover, there was increased BCL6 and decreased STAT binding at this site in cells treated with blocking antibodies to IL-2 and the IL-2 receptor, suggesting a possible BCL6-STAT5 binding competition that influences IL-9 production. BCL6 binding was also increased when cells were Th9-differentiated in the presence of IL-21. Thus, our data reveal not only direct IL-2 effects via STAT5 at the Il9 gene, but also opposing actions of IL-2 and IL-21 on BCL6 expression, with increased BCL6 expression inhibiting IL-9 production. These data suggest a model in which increasing BCL6 expression decreases efficient Th9 differentiation, indicating possible distinctive approaches for controlling this process.