MMP9 and neoplasm: The expansion of m-MDSCs is induced by a combination of soluble factors produced by tumor and/or surrounding cells such as stromal cells, T-cells or macrophages including VEGF, GM-CSF, M-CSF, IL-4, IL-6, IL-10, prostaglandin E2 (PGE2), MMP9, C-X-C motif chemokine ligand 5 (CXCL5), and CXCL12/ stromal-derived factor 1 alpha (SDF1-alpha) (39).