IDO1 and neoplasm: The secretion of nitric oxide (NO), PGE2, IL-6, IL-10, and kynurenine, generated from tryptophan by indoleamine 2,3-dioxygenase (IDO), induce a switch from the Th1 anti-tumor effector response to Th2 suppressive mode [16], expand immunosuppressive M2-polarized macrophages, tumor-associated macrophages (TAMs), and also Tregs populations [31,32].