Glioma-derived cytokines are the primary stimuli that induce the recruitment and proliferation of MDSCs, which can be MDSCs recruiters (including CCL2, CXCL8, SDF-1, and CCL2) and/or MDSCs expanders (including IL-6, PGE2, IL-10, VEGF, and GM-CSF) (134, 137). Here, CCL2 is linked to glioma.