p172: An alveolar type II and Clara cell specific protein with late developmental expression and upregulation by hyperoxic lung injury

Identification of a novel antigen on the apical surface of rat alveolar epithelial type II and Clara cells

Here we describe a monoclonal antibody (MMC4) that recognizes a novel antigen on the apical surface of rat alveolar epithelial type II and Clara cells in the lung, proximal tubule epithelial cells in the kidney, and villus epithelial cells in the small intestine. Biochemical analysis showed that the MMC4 antigen was sensitive to heating and proteinase K digestion and that it is distributed in the detergent-rich phase after Triton X-114 phase separation. These data suggest that the MMC4 antigen is an integral membrane protein. Glycerol gradient sedimentation identified two forms of the MMC4 antigen: one with a sedimentation coefficient of 10.1 and one with a sedimentation coefficient of 1.66, suggesting that the antigen may be part of a multiprotein complex. During rat development (fetal day 16 to adult), the MMC4 antigen increased 12-fold in the lung and 200-fold in the kidney. In the intestine, the MMC4 antigen increased 150-fold by neonatal day 1 and then decreased to adult values. Our data demonstrate that the MMC4 antigen is unlike known type II cell- and Clara cell-associated proteins. The MMC4 monoclonal antibody will be useful as a marker of epithelial cell phenotype in development and injury studies.

HTI56, an integral membrane protein specific to human alveolar type I cells

The alveolar epithelium is composed of two morphologically distinct types of cells, Type I and Type II cells. The thin cytoplasmic extensions of Type I cells cover more than 95% of the internal surface area of the lungs. Type I cells provide the very short diffusion pathway essential for gas exchange. Because there were no biochemical markers specific for human Type I cells, we developed a strategy to produce a monoclonal antibody (MAb) specific for human Type I cells. Isolated human lung cells were used as immunogens; >5000 clones from seven fusions were screened to identify an MAb specific for a 56-kD protein of Type I cells, HTI56. By Western blotting, HTI56 is unique to the lung. By immunoelectron microscopy, it is localized to the Type I cell apical plasma membrane. The pI of HTI56 is 2.5-3.5. HTI56 is glycosylated and has the biochemical characteristics of an integral membrane protein. HTI56 is detectable by Week 20 of gestation and its expression increases in fetal lung explant culture. HTI56 should be useful as a marker for human Type I cells both morphologically and biochemically. It may also be useful in studies of disease and as a marker for lung injury.