Modulation of elastase binding to elastin by human alveolar macrophage-derived lipids

Human neutrophil elastase (HNE), an enzyme secreted by activated neutrophils, can bind to and degrade extracellular matrix including human lung elastin. This protease is believed to play an important role in several destructive processes including pulmonary emphysema. In this study, we hypothesized that an alveolar macrophage (AM) product or products may interact with neutrophil elastase (NE) and modulate its binding to elastin. Elastase binding to elastase was evaluated by a modified elastase functional assay using a synthetic substrate. Supernatants from cultured AM inhibited elastase binding to elastin at a dose-dependent manner without inhibiting functional elastase activity. The AM products had a heterogeneous molecular weight ranging from 440,000 to 54,000. The activity was heat-stable, but was lost after ultracentrifugation. After lipid fractionation, neither the aqueous nor the lipid fractions contained activity, suggesting that the factor may be a lipid complex. Culture supernatants from smokers' AM released significantly higher amounts of the factor than nonsmokers. In addition, high-molecular-weight elastase was present in bronchoalveolar lavage fluid (BALF) obtained from patients with pneumonia. Most of the in vivo high-molecular-weight elastase was lost after lipid extraction. In conclusion, macrophages release a factor or factors, probably lipid, which can interact with NE and inhibit its binding to human lung elastin without inhibiting elastase activity. This macrophage-derived factor may play a role in protecting the lung from NE by partitioning elastase into the airspace and thus protecting the interstitial connective tissue matrix from elastase degradation.