CCL2 and breast cancer: The absence of CCL2 in the host reduced the tumor weight by more than 4-fold, the number of lung metastases, and the proportion of MDSCs (CD45+/CD11b+/Gr1+) in both tumors and metastases, suggesting that the expression of CCL2 in the host could promote the proliferation and metastasis of BC via the recruitment of MDSCs [104].