Distribution and subcellular localization of surfactant-associated glycoproteins in human lung

Human surfactant contains lung-specific, high molecular weight glycoproteins which are composed of disulfide-linked 34-kilodalton peptide subunits. We prepared antibodies to both isolated HMW glycoproteins and 34-kilodalton peptides and tested the antisera for specificity with immunochemical procedures. In the present study we have investigated the cellular distribution and subcellular localization of these glycoproteins in surgically excised human lung tissue with or without type II cell hyperplasia. An immunoperoxidase technique was used, and cytoplasmic staining reflecting labeling with antibodies to either high molecular weight glycoproteins or the 34-kilodalton peptide subunits was consistently observed in normal type II cells, Clara cells, and some alveolar macrophages and was more intense and diffuse in hyperplastic type II cells. The ultrastructural localization of surfactant-associated glycoproteins was investigated using the periodate-lysine-paraformaldehyde fixative and a peroxidase-labeled antibody technique. In both normal and proliferating type II cells the staining was localized in the rough endoplasmic reticulum, perinuclear cisternas, vesicles of the Golgi complex, vesicles and lamellar membranes of the multivesicular bodies, and some multivesicular body-lamellar body forms. In addition, staining was frequently found in peripheral portions of partially preserved lamellar bodies including those at the stage of secretion, as well as in association with of alveolar tubular myelin. Labeling was also observed in the rough endoplasmic reticulum of Clara cells. We conclude that antibodies against human surfactant-associated glycoproteins are markers for normal and regenerating type II cells, as well as for Clara cells which apparently retain limited ability to produce surfactant-associated glycoproteins independently of surfactant phospholipids. The results indicate that, in type II cells, synthesis and secretion of these glycoproteins involve the same cytoplasmic organelles that are responsible for synthesis, packaging, storage, and exocytosis of surfactant phospholipids. However, maturation of the lamellar bodies, known to be characterized by progressive accumulation of phospholipids, may not require parallel storage of surfactant-associated glycoproteins within the entire lamellar body compartment.