MMP9 and neoplasm: TANs recruited to the tumor site, mediated by C-X-C motif chemokine receptor 2 (CXCR2) and its ligands (i.e., CXCL1, CXCL2, CXCL3, CXCL5, CXCL7 and CXCL8), contribute to tumor progression by secreting matrix metalloproteinase-9 (MMP-9), vascular endothelial growth factor (VEGF), arginase, hepatocyte growth factor and numerous chemokines, such as CCL2, CCL5 and CXCL4, which can exert paracrine effects on the TME [17,18].