ARG1 and neoplasm: TAMs can enhance tumor evasion of the immune surveillance system in two ways: (1) by directly inhibiting anti-tumoral cytotoxic CD8+ T cell responses via PD-L1/PD-L2 expression [23]; and (2) by secreting immunosuppressive cytokines and proteases such as arginase-1, IL-10, TGF-β and prostaglandins, which prevent T cell activation [17,24,25].