Hypothesized roles of HGF and neutrophil peptidases in epithelial repair.
A: Airway epithelium is exposed to allergens, bacteria or other pathogens. B: HGF moves from bloodstream into airway via leaky and damaged endothelial and epithelial barriers. Some HGF is inactivated by neutrophil proteases, such as elastase (NE, red arrow), thereby generating HGF fragment NK4, which interferes with HGF activation of receptor c-Met. C: HGF blocks epithelial to mesenchymal transformation (blue arrow) and stimulates epithelial cell migration to repair damaged epithelium. D: Inflammation resolves and epithelial barrier is restored.