MMP9 and neoplasm: PMN-MDSCs can condition tumor cells at the primary site to facilitate metastasis, possibly through pathways that regulate the production of hepatocyte growth factor and TGF-β to induce tumor epithelial-mesenchymal transition14, the production of matrix metalloproteinase 9 to facilitate tumor invasion15, 16, direct immunosuppressive activity that promotes metastasis13, and by tethering tumor cells to the vascular endothelium to promote lung metastasis17.