Surfactant protein-D regulates the postnatal maturation of pulmonary surfactant lipid pool sizes.

Surfactant protein (SP)-D plays an important role in host defense and pulmonary surfactant homeostasis. In SP-D-deficient (Sftpd−/−) mice, the abnormal large surfactant forms seen at the ultrastructural level are taken up inefficiently by type II cells, resulting in an over threefold increase in the surfactant pool size. The mechanisms by which SP-D influences surfactant ultrastructure are unknown. We hypothesized that SP-D binds to surfactant immediately after being secreted and influences surfactant ultrastructure conversion. In newborn and adult sheep lungs, immunogold-labeled SP-D was associated with both lamellated membranous lipid structures of newly secreted surfactant and with small aggregate surfactant but not with tubular myelin. Since SP-D preferentially binds to phosphatidylinositol (PI) in vitro, the postnatal changes in PI were assessed. PI content in the bronchoalveolar lavage fluid increased after birth and peaked at 2–5 days of age, a time of rapid conversion of surfactant forms that is a...