ARG1 and neoplasm: MDSCs exert their immunosuppressive activities by inhibiting the production of immunosuppressive mediators, such as prostaglandin E2 (PGE2), arginase-1 (ARG1), and inducible nitric oxide synthase (iNOS), thus decreasing the intratumoral CD8+ T cell: Treg ratio, suppressing the cell cycle of T cells and restricting their recruitment into tumor sites [48].