CCL2 and breast cancer: Various cytokines secreted by tumor cells including breast cancer (BC) cells have been demonstrated to modulate M2 polarization and infiltration in TME, such as colony-stimulating factor 1 (CSF1) [6], interleukin10 (IL-10) [7], chemokine (C-C motif) ligand 2 (CCL2, also known as MCP-1) [8], transforming growth factor beta (TGF-β) [9], and etc. It’s also well recognized that the acquired drug resistance of existing anti-cancer therapy may be mediated by their effects on TAMs [10].