MMP9 and neoplasm: Soluble factors (e.g., VEGF, TGFβ, TNFα, CXCL12) secreted from the primary tumor can activate lung cells to express S100A8/S100A9 and MMP-9 (74), and circulating CD11b+ myeloid cells are recruited to the lung in a VEGFR1-dependent manner (75).