Surfactant Protein-D Regulates Surfactant Phospholipid Homeostasis in Vivo

Surfactant protein D (SP-D) is a 43-kDa member of the collectin family of collagenous lectin domain-containing proteins that is expressed in epithelial cells of the lung. The SP-D gene was targeted by homologous recombination in embryonic stem cells that were used to produce SP-D (±) and SP-D (−/−) mice. Both SP-D (−/−) and SP-D (±) mice survived normally in the perinatal and postnatal periods. Whereas no abnormalities were observed in SP-D (±) mice, alveolar and tissue phosphatidylcholine pool sizes were markedly increased in SP-D (−/−) mice. Increased numbers of large foamy alveolar macrophages and enlarged alveoli were also observed in SP-D (−/−) mice. Phospholipid composition was unaltered in SP-D (−/−) mice, but surfactant morphology was abnormal, consisting of dense phospholipid membranous arrays with decreased tubular myelin. The pulmonary lipoidosis in the SP-D (−/−) mice was not associated with accumulation of surfactant proteins B or C, or their mRNAs, distinguishing the disorder from alveolar proteinosis syndromes. Surfactant protein A mRNA was reduced and, SP-A protein appeared to be reduced in SP-D (−/−) compared with wild type mice. Targeting of the mouse SP-D gene caused accumulation of surfactant lipid and altered phospholipid structures, demonstrating a previously unsuspected role for SP-D in surfactant lipid homeostasis in vivo.