IDO1 and neoplasm: Suppressive cytokines, metabolites L-kynurenine, prostaglandine E2, nitric oxide (NO) produced by immunosuppressive enzymes indolamine-2,3-dioxygenase (IDO), cyclooxygenase 2 (COX2), inducible NO synthase (iNOS), respectively, and vascular endothelial growth factor (VEGF), produced by suppressive and regulatory immune cells and tumor cells, further downregulate the expression of activating NK cell receptors (Della Chiesa et al., 2006; Harizi, 2013; Konjević et al., 2017a; Park et al., 2018; Table 1).