CCL2 and neoplasm: To date, various signaling pathways, including PGE2 [93], CCL2 [94,95], CSF-1 [96], TGF-β [97], CXCL5, CXCL12 [98], IL-1β [99,100], and IL-6 [101], have been identified to be involved in the infiltration and activation of MDSCs in tumor microenvironment [102,103,104,105,106].