CXCL12 and cancer: Treg cells recruitment in a range of human cancer types is mediated by several soluble factors, such as C-C motif chemokine ligand 22 (CCL22), C-C motif chemokine ligand 28 (CCL28), and C-X-C motif chemokine ligand 12 (CXCL12), produced by tumour cells, cancer associated fibroblasts and immunosuppressive cells [35].