IDO2 and neoplasm: l-kynurenine, a tryptophan-derived catabolite resulting from the activity of indoleamine 2,3-dioxygenase (IDO), an enzyme that is often expressed in tumor cells or tumor-associated cells (TAMs, dendritic cells, etc.)(81, 82), interferes with NK cells by regulating the surface expression of activating receptors and thus their antitumor function (83).