KLRK1 and neoplasm: Indoleamine 2,3-dioxygenase (IDO) promotes the production of the immunosuppressive tryptophan catabolite, L-kynurenine, which interferes with the IL-2-induced upregulation of NKp46 and NKG2D expression, thereby reducing the ability of NK cells to recognize and kill tumor cells (50–52).