ARG1 and neoplasm: In mouse tumor models, it was demonstrated that tumor-infiltrating M-MDSCs could produce CCR5 ligands to chemoattract Tregs with high CCR5 expression into tumor tissues.121 Furthermore, MDSCs can induce Tregs proliferation through either a direct cell-cell interaction or secretion of soluble factors like IL-10 and TGF-β.122 The expression of ARG1, IDO, and CD40 by MDSCs have also been reported to participate in Tregs induction.123 Macrophage is another accomplice of MDSCs.