ICOS and neoplasm: On the one hand, ICOS exerts its anti-tumor effect through the enhancement of CD4+ and CD8+ effector T cells (11, 12); on the other hand, ICOS significantly activates and upregulates regulatory T cells (Tregs) (13), which are a subpopulation of T cells that mainly function as an immunosuppressor and consequently facilitate the immune escape in tumors (14).