The role of CCR5 in directing the mobilization and biological function of CD11b+Gr1+Ly6Clow polymorphonuclear myeloid cells in cancer

Bone marrow (BM) cells of the hematopoietic system, also known as BM-derived leukocytes (BMD), are mobilized from the BM to the blood and then colonize tumor sites. These cells then become key players in either promoting or regulating the development and progression of tumors. Among the cells that suppress anti-tumor immunity are regulatory T cells (Tregs), tumor-associated macrophages (TAMS) and myeloid-derived suppressor cells (MDSC). MDSC comprise CD11b+Gr1+Ly6Clow polymorphonuclear myeloid cells (PMN-MDSC), and CD11b+Gr1+Ly6Chigh monocytic myeloid cells (Mo-MDSC). Several studies including ours have identified the CCR2–CCL2 axis as the key driver of the mobilization of monocytic cells from the BM to the blood and later their colonization at the tumor site. The current review focuses on the mechanisms by which PMN-MDSC are mobilized from the BM to the blood and later to the tumor site, and their clinical implications.