IL17A and neoplasm: ICOS has dual effects in tumor advancement: On the one hand, ICOS can promote the differentiation of effector T helper (Th) cells and enhances the secretion of cytokines, such as human interleukin (IL)-10, IL-17, and interferon gamma (IFN-γ), to enhance the anti-tumor T cell response; on the other hand, ICOS promotes regulatory T cells (Tregs), which support tumor development 7, 8.