IDO1 and neoplasm: These functions include arginine and cysteine deprivation, which impede T cell proliferation, production of nitric oxide (NO) and reactive oxygen species (ROS), which oxidize T cell receptors (TCRs) and cytokines leading to cell apoptosis, extensive production of immunosuppressive cytokines (primary IL-10 and TGF-β), enzymes (such as IDO) and angiogenic factors aiding both tumor dissemination and the formation of Treg cells, and a high expression of PD-1/PD-L1, receptors and ligands that are involved in direct killing of anti-tumoral T cells [46].