Inhibition of the human neutrophil respiratory burst by native and synthetic surfactant

Production of oxygen radicals by phagocytic cells and loss of surfactant function have each been implicated in the pathogenesis of acute lung injury. Therapeutic administration of exogenous surfactant to injured lungs in which neutrophils are the dominant cell type has been proposed. To understand the role of surfactant in modulating pulmonary inflammation and the impact of surfactant supplementation on diseased lungs, we studied the effect of native porcine and synthetic surfactant preparations on human neutrophil respiratory burst oxidase activity in vitro. We found that surfactant inhibited neutrophil superoxide production induced by either receptor-mediated [formylmethionylleucylphenylalanine (fMLP)] or non-receptor-mediated [phorbol myristate acetate (PMA)] agonists with an IC50 of similar to 0.015 mg phospholipid/ml for porcine surfactant or similar to 0.050 mg phospholipid/ml for synthetic surfactant. Surfactant had no effect on detection of superoxide generation in a noncellular system using xanthine and xanthine oxidase and only minimally inhibited superoxide generation by neutrophils that had been fully stimulated by prior exposure to PMA. There was no effect of surfactant on neutrophil calcium mobilization in response to fMLP, on lactoferrin release in response to PMA, or on membrane protein kinase C activity in response to PMA. Suspensions of dipalmitylphosphatidylcholine alone had no effect on neutrophil superoxide production. Taken together, these findings indicate that certain components of lung surfactant may effect relatively late steps in the activation of the respiratory burst or may alter subsequent steps involved in the assembly of the respiratory burst oxidase.