ARG1 and neoplasm: This tumor microenvironment is characterized by containing cells with an immunosuppressive phenotype including myeloid-derived suppressor cells (MDSCs), M2-polarized macrophages, and regulatory T cells (Tregs), and by the upregulation of several immune inhibitory molecules such as programmed death ligand 1 (PD-L1) and -2 (PD-L2), TGF-β, indoleamine 2,3-dioxygenase (IDO), and arginase 1 (ARG1) (66–69).