Human pulmonary alveolar type 2 cells contain an apical membrane glycoprotein common to malignant cells

A monoclonal antibody, Ca1, raised against a detergent extract of Hep 2 cells derived from a human laryngeal carcinoma, was shown in these studies to bind to the apical surface of normal alveolar type 2 cells but not type 1 cells in the human lung. In lung specimens from patients with alveolitis, the antibody also bound to hyperplastic type 2 cells and to transition cells which were in the process of becoming alveolar type 1 cells. Ca1 binds to the apical plasma membrane and to internal membranes of cytoplasmic vesicles thought to be involved in the packaging of pulmonary surfactant. A surfactant-enriched fraction of human lung lavage did not bind the Ca1 antibody suggesting that the antigen was not an integral component of secreted surfactant. In normal human lung parenchyma, Ca1 binds only to type 2 cells, however it also binds to the apical surface of Clara cells in areas of cellular hyperplasia. Solubilized homogenates of whole lung, of a cell membrane fraction and of Hep 2 cells, immunoprecipitated with Ca2, separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and probed with iodinated lectins, revealed that terminal glycosylation of the type 2 cell antigen differed from that of Hep 2 cells. Ca1 and a 330 kilodalton type 2 cell glycoprotein bind the lectin Maclura pomifera agglutinin. These two glycoproteins represent the first defined membrane markers of the apical surface of the human type 2 cell.