CCL2 and neoplasm: MDSCs are generated in the bone marrow and their recruitment to the tumor site is dependent on diverse set of chemokines, such as CCL2, CCL3 and CCL4, for the recruitment of M-MDSCs via the C-C chemokine receptor 2 (CCR2) [44] and the ligands of the CXC-chemokine receptor 2 (CXCR2) for PMN-MDSC [45].