FOXP3 and neoplasm: To further define whether DPP-4i is involved in the infiltration of immunosuppressive cells in metastatic sites (7–13), we further investigated the effect of DPP-4i on the infiltration of Treg cells and MDSCs in metastasis sites of 4T1-bearing BALB/c mice and observed a significant increase in tumor-infiltrating Foxp3+ cells in metastasis sites (Figure 6C), indicating that immunosuppressive Treg cells may be responsible for the increased tumor-infiltrating CD4+ T cells after DPP-4i treatment.