Surfactant protein D (SP-D) counteracts the inhibitory effect of surfactant protein A (SP-A) on phospholipid secretion by alveolar type II cells. Interaction of native SP-D with SP-A.

The surfactant proteins SP-A and SP-D were obtained from rats given intratracheal instillation of silica. SP-D was isolated from the 33,000 g supernatant of rat bronchoalveolar lavage fluids, and we examined whether SP-D affects surfactant secretion by alveolar type II cells. Native SP-D affected neither basal secretion nor stimulated secretion by type II cells. However, native SP-D counteracted the inhibitory effect of SP-A on surfactant secretion in a concentration-dependent manner; however, SP-D failed to counteract the inhibitory effect of concanavalin A. The activity of SP-D was unaffected by inclusion of excess methyl alpha-mannoside. Excess native SP-D competed with 125I-SP-A for high-affinity binding to type II cells. Heat treatment of SP-D and antibody against SP-D both decreased SP-D activity. Butanol extraction of native SP-D was most effective at destroying SP-D activity and attenuated the ability of the protein to compete with labelled SP-A for binding to type II cells. The butanol-soluble fraction of SP-D possessed the ability to alter the inhibitory effect of SP-A to the same extent as native SP-D. Direct binding of 125I-SP-A on nitrocellulose sheets demonstrated that SP-A could bind native SP-D, but not butanol-extracted SP-D. We conclude that native SP-D alters SP-A activity in type II cells through interaction with it via SP-D-associated lipids.