CXCL12 and neoplasm: C-X-C motif chemokine ligand 12 (CXCL12) is a cytokine which, if secreted in high concentration by senescent cells in the tumor microenvironment (TME), can create a “cytokine shield” that inhibits CD8+ T cell infiltration by downregulating C-X-C chemokine receptor type 4 (CXCR4) and impairing directional migration [134].