Generation and characterization of monoclonal antibodies to alveolar type II cell lamellar body membrane

Monoclonal antibodies against the limiting membrane of alveolar type II cell lamellar bodies were obtained after immunization of mice with a membrane fraction prepared from lamellar bodies isolated from rat lungs. The specificity of the antibodies was investigated with Western blot analysis, indirect immunofluorescence, and electron-microscopic immunogold studies of freshly isolated or cultured alveolar type II cells, alveolar macrophages, and rat lung tissue. One of the monoclonal antibodies identified, MAb 3C9, recognized a 180-kDa lamellar body membrane (lbm180) protein. Immunogold labeling of rat lung tissue with MAb 3C9 demonstrated that lbm180 protein is primarily localized at the lamellar body limiting membrane and is not found in the lamellar body contents. Most multivesicular bodies of type II cells were also labeled, as were some small cytoplasmic vesicles. Golgi complex labeling and plasma membrane labeling were weak. The appearance of lbm180 protein by immunofluorescence in fetal rat lung cryosections correlated with the biogenesis of lamellar bodies. The lbm180 protein decreased with time in type II cells cultured on plastic. The lbm180 protein is an integral membrane protein of lamellar bodies and was also found in the pancreas and the pancreatic beta HC9 cell line but not in the rat brain, liver, kidney, stomach, or intestine. The present study provides evidence that the lbm180 protein is a lung lamellar body and/or multivesicular body membrane protein and that its antibody, MAb 3C9, will be a valuable reagent in further investigations of the biogenesis and trafficking of type II cell organelles.