TH9 cells are required for tissue mast cell accumulation during allergic inflammation

Background: IL-9 is important for the growth and survival of mast cells. IL-9 is produced by T cells, natural killer T cells, mast cells, eosinophils, and innate lymphoid cells, although the cells required for mast cell accumulation during allergic inflammation remain undefined. Objective: We sought to elucidate the role of T(H)9 cells in promoting mast cell accumulation in models of allergic lung inflammation. Methods: Adoptive transfer of ovalbumin-specific T(H)2 and T(H)9 cells was used to assess the ability of each subset to mediate mast cell accumulation in tissues. Mast cell accumulation was assessed in wild-type mice and mice with PU.1-deficient T cells subjected to acute and chronic models of allergic inflammation. Results: Adoptive transfer experiments demonstrated that recipients of T(H)9 cells had significantly higher mast cell accumulation and expression of mast cell proteases compared with control or T(H)2 recipients. Mast cell accumulation was dependent on IL-9, but not IL-13, a cytokine required for many aspects of allergic inflammation. In models of acute and chronic allergic inflammation, decreased IL-9 levels in mice with PU.1-deficient T cells corresponded to diminished tissue mast cell numbers and expression of mast cell proteases. Mice with PU.1-deficient T cells have defects in IL-9 production from CD4(+) T cells, but not natural killer T cells or innate lymphoid cells, suggesting a TH cell-dependent phenotype. Rag1(-/-) mice subjected to a chronic model of allergic inflammation displayed reduced mast cell infiltration comparable with accumulation in mice with PU.1-deficient T cells, emphasizing the importance of IL-9 produced by T cells in mast cell recruitment. Conclusion: T(H)9 cells are a major source of IL-9 in models of allergic inflammation and play an important role in mast cell accumulation and activation.