IDO1 and cancer: In cancer, kynurenine/kynurenic acid and IDO1 levels regulate immunosuppression through suppression of antitumorigenic immune cells (DCs, NK, and effector T cells), expansion of protumorigenic immune cells (Tregs, M2 macrophages, and myeloid-derived suppressor cells), and upregulation of immune checkpoints (PD-1/PD-L1, CTLA4/CD80 or CD86) (61, 79, 80).