Aerosolized endotoxin is immediately bound by pulmonary surfactant proteinD in vivo

Collectins are carbohydrate binding proteins that are implicated in innate host defense. The lung collectins, surfactant proteins A and D (SP-A and SP -D), bind a variety of pathogens in vitro and influence phagocytosis by alv eolar macrophages. In this report we show that SP-D binds endotoxin (lipopo lysaccharide, LPS) in vivo in a rat model of acute respiratory distress syn drome (ARDS). Intratracheal aerosolization of LPS in rats resulted in the t ypical features of human ARDS. Total amounts of SP-D, as well as the carboh ydrate binding properties of SP-D were measured in lung lavage as a functio n of time. The amount of SP-D did not change during 24 h. Interestingly, SP -D in lung lavage isolated from rats during the first 2 h after LPS treatme nt, was not able to bind to carbohydrate. Further analysis revealed that th e carbohydrate binding sites of SP-D were occupied by LPS, suggesting that SP-D is an LPS scavenging molecule in vivo. Electron microscopic analysis i ndicated that, 1 h after LPS aerosolization, aggregates of SP-D with LPS we re found in lysosomal structures in alveolar macrophages. We conclude that the lung collectin SP-D binds inhaled endotoxin in vivo, which may help to protect the lung from endotoxin-induced disease. (C) 1999 Elsevier Science B.V. All rights reserved.