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. This evidence concerns the gene IDO1 and neoplasm.