NOS2 and neoplasm: They inhibit anti-tumor immunity through multiple mechanisms, including the depletion of L-arginine in the TME via inducible nitric oxide synthase (iNOS) and arginase-1 (Arg-1), and the exhaustion of tryptophan through indoleamine 2,3-dioxygenase (IDO), which directly suppresses T cell activation (12–15).