IDO1 and neoplasm: ​ Suppressive immune cells, such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), in the tumor microenvironment can secrete large amounts of PEG2, TGF-β, IL-10, nitric acid, and indoleamine 2, 3-dioxygenase (IDO) to inhibit T-cell proliferation.