Tranilast inhibits the function of cancer-associated fibroblasts responsible for the induction of immune suppressor cell types.

Cancer-associated fibroblasts (CAFs) are the dominant stromal component in the tumour microenvironment (TME), playing critical roles in generation of pro-tumourigenic TME; however, their contribution to suppression of antitumour immune responses has not been fully understood. To elucidate the interaction between CAFs and immune suppressor cells, we examined whether inhibition of CAFs function would impair the induction of immune suppressor cell types in vitro. In this study, we applied an anti-allergic and antifibrotic agent tranilast, which is used clinically, and evaluated a potential of tranilast to serve as a CAFs inhibitor. CAFs that had been isolated from E.G7 or LLC1 tumour-bearing mice were cultured in the presence of tranilast, and thereafter, CAFs functions on the secretion of some soluble factors as well as the induction of immune suppressor cells were evaluated. As a result, tranilast inhibited the proliferation of CAFs and reduced the levels of stromal cell-derived factor-1, prostaglandin E2 and transforming growth factor-β1 from CAFs in a dose-dependent manner. On the other hand, tranilast exerted no inhibitory effects on immune cells at doses under 100 μm. The induction of regulatory T cells and myeloid-derived suppressor cells from their progenitor cells was suppressed in the medium that CAFs had been cultured in the presence of tranilast; however, these findings were not observed when those progenitor cells were cultured in the medium containing tranilast alone. These data demonstrate that tranilast inhibits CAFs function, which is responsible for the induction of immune suppressor cells, and possesses a potential to serve as a specific CAFs inhibitor.