Induced CD4⁺ forkhead box protein-positive T cells inhibit mast cell function and established contact hypersensitivity through TGF-β1

Background: Induced CD4⁺ forkhead box protein 3-positve regulatory T (iTreg) cells are a promising source for cell-based therapies of established inflammatory and autoimmune diseases. However, their relationship to mast cell (MC) function and MC-driven diseases remains unknown. Objective: We sought to explore the roles of iTreg cells on MC function and the established MC-driven disease contact hypersensitivity (CHS). Methods: In vitro coculture studies were carried out to investigate the interaction between iTreg cells in murine or human MCs by using both direct cell-cell contact and transwell systems to separate cell-cell contact. In vivo mice iTreg cells were administered to mice with established CHS, and innate immunologic responses, such as MC infiltration and inflammatory cytokine expression at contact sites, were evaluated. Results: In vitro coculture under direct cell-cell contact resulted in indirect suppression of IgE-independent activation of MCs by murine or human iTreg cells. Mechanistically, iTreg cells suppressed proinflammatory cytokine levels by modulating nuclear factor κB p65 activation in MCs through T cell-derived TGF-β1. Injection of iTreg cells but not natural CD4 ⁺CD25⁺ regulatory T cells into animals with established CHS resulted in the suppression of infiltration and functions of MCs and also led to decreased production of inflammatory cytokines at allergic contact areas. iTreg cell-mediated immunosuppressive effects were abrogated when iTreg cells were pretreated with TGF-β1 small interfering RNA. Conclusions: Our study demonstrates that iTreg cells suppress MC function and attenuate established MC-driven CHS through TGF-β1-dependent mechanisms.