IDO1 and cancer: When PD-1 binds to its ligand, PD-L1, expressed on cancer cells within the TME, it transmits regulatory signals to effector T cells, leading to T cell exhaustion, sending anti-apoptotic signals to tumor cells, and promoting tumor survival, thereby exacerbating immune suppression.565 Indoleamine 2,3-dioxygenase 1 (IDO-1) is a cytosolic enzyme overexpressed in tumor cells and DCs, enhancing immune suppression by increasing the kynurenine to tryptophan ratio, inhibiting the antitumor capacity of NK cells, and promoting Treg infiltration.566