IDO1 and cancer: Tumors can evade immune surveillance due to ADO-AR signals, which decrease the anti-cancer function of immune cells like CD8+ T cells, DCs, NK cells, and M1 macrophages while boosting the immunosuppressive function of cells like MDSCs and Tregs, increasing the expression of IL-10, TGF-β, arginase-2, and IDO-1, lead to Th2 or M2 differentiation (196).