Surfactant apoprotein A (SP-A) is synthesized in airway cells.

The pulmonary surfactant apoproteins A, B, and C (SP-A, SP-B, and SP-C, respectively) function in concert with surfactant phospholipids to reduce surface pressure in the alveolus. Surfactant apoproteins also regulate surfactant synthesis, secretion, adsorption, and recycling. SP-A and B have been localized by immunocytochemistry to alveolar epithelial (type II) cells, alveolar macrophages, and nonciliated bronchiolar epithelial (Clara) cells. In contrast, in situ hybridization to SP-A and B mRNA in human lung has shown SP-A and B transcripts in type II cells, but only SP-B message in Clara cells, implying that synthesis of SP-A occurs exclusively in type II cells. In this report, in situ hybridization to SP-A mRNA was performed on adult and developing rabbit lung and on human lung. SP-A transcripts were found in type II cells and bronchiolar epithelium of both species. The distribution of SP-A message-containing cells in the bronchiolar epithelium of rabbits and humans was similar to the distribution of Clara cells in these two species. These data indicate that SP-A is not only synthesized in type II cells but also in Clara cells.