CXCL12 and cancer: Another suppressive subset of cells is the heterogeneous population of MDSC that are expanded in cancer and have the capacity to suppress the immune response; MDSC are generated in the bone marrow in response to cancer-derived factors and are recruited to the tumor site by CCL2, CXCL12, and CXCL5.40 The MDSCs suppress the activation of T effector and natural killer cells and induce expansion of Tregs.