IDO2 and melanoma: IFN-γ released by NK cells induces indoleamine 2,3-dioxygenase (IDO) expression and prostaglandin E2 (PGE2) production by melanoma cells that inhibit NK cell function by down-regulating the expression of NKp30- and NKG2D-activating receptors further contributing to melanoma escape (78, 79).