IDO1 and neoplasm: Tumor cells and CAFs produce immunosuppressive substances such as vascular endothelial growth factor (VEGF) [150], IL-6 [151], IL-10 [151], transforming growth factor-β (TGF-β) [152], soluble Fas ligand (Fas-L) [153], and indolamine-2,3-dioxygenase (IDO) [154].