CCL2 and cancer: Macrophages recruited to TME are induced to undergo polarization towards immunosuppressive M2 phenotypes by a variety of signaling molecules produced by cancer cells, including lactic acid produced by tumor metabolism, multiple miRNAs and lncRNAs, CSF-1, CCL2, CCL3, and CCL14, and bone morphogenetic protein 4 (BMP4) (8).