NOS2 and infection: Trauma, infection, cancer and abnormal inflammatory conditions promote MDSC proliferation, and MDSCs are recruited to the inflammatory microenvironment, exerting immunosuppressive activities by secreting Arg-1, inducible nitric oxide synthase (iNOS) and indolamine dioxygenase (IDO), which inhibit TCR ζ-chain synthesis and promote T-cell apoptosis (18).