XCL2 and neoplasm: Both M-MDSCs and PMN-MDSCs are recruited to tumour sites by tumour-derived cytokines: CCL2 and C-C motif chemokine ligand 5 CCL5 for M-MDSCs, C-X-C motif chemokine ligand 1 (CXCL1), C-X-C motif chemokine ligand 2 (CXCL2), C-X-C motif chemokine ligand 5 (CXCL5), C-X-C motif chemokine ligand 6 (CXCL6), C-X-C motif chemokine ligand 8 (CXCL8), CXCL12, CCL2, C-C motif chemokine ligand 3 (CCL3), and C-C motif chemokine ligand 15 (CCL15) for PMN-MDSCs [58].