Immunocytochemical localization of the major surfactant apoproteins in type II cells, Clara cells, and alveolar macrophages of rat lung.

The adsorptive properties of phospholipids of pulmonary surfactant are markedly influenced by the presence of three related proteins (26-38 KD, reduced) found in purified surfactant. Whether these proteins are pre-assembled with lipids before secretion is uncertain but would be expected for a lipoprotein secretion. We performed indirect immunocytochemistry on frozen thin sections of rat lung to identify cells and intracellular organelles that contain these proteins. The three proteins, purified from lavaged surfactant, were used to generate antisera in rabbits. Immunoblotting of rat surfactant showed that the IgG reacted with the three proteins and a 55-60 KD band which may be a polymer of the lower MW species. Specific gold labeling occurred over alveolar type II cells, bronchiolar Clara cells, alveolar macrophages, and tubular myelin. In type II cells labeling occurred in synthetic organelles and lamellar bodies, which contain surfactant lipids. Lamellar body labeling was increased fivefold by pre-treating tissue sections with a detergent. Multivesicular bodies and some small apical vesicles in type II cells were also labeled. Secondary lysosomes of alveolar macrophages were immunoreactive. Labeling in Clara cells exceeded that of type II cells, with prominent labeling in secretory granules, Golgi apparatus, and endoplasmic reticulum. These observations clarify the organelles and pathways utilized in the elaboration of surfactant. After synthesis, the proteins move, probably via multivesicular bodies, to lamellar bodies. Both lipids and proteins are present in tubular myelin. Immunologically identical or closely similar proteins are synthesized by Clara cells and secreted from granules which appear not to contain lipid. The role of these proteins in bronchiolar function is unknown.