Quantitation of a murine lung endothelial cell protein, P112, with a double monoclonal antibody assay

Two monoclonal antibodies (MoAb) have been isolated from two different F344 rats immunized with normal BALB/c mouse lung homogenates. The MoAbs 273-34A and 411-201B demonstrate similar staining properties when used in immunohistochemistry and have been shown to bind to endothelial cells in the lung using immunogold electron microscopy. Radiolabeled MoAb 273-34A injected intravenously localizes rapidly to lung. Ratios of MoAb to control and values for percentage of recovered dose/gram are 50-fold higher in lung than in other organs. Both MoAbs recognize the same 112 kilodalton protein (P112) on the surface of lung cells in culture but do not compete for binding with each other. Both antibodies can be used to purify P112 from normal mouse lung homogenate using immunoaffinity chromatography. These MoAb have been used in a two-site, quantitative assay for P112. Data show that normal lung contains high amounts of P112 (500 to 700 ng/mg P), whereas other organs have low (spleen, uterus) or undetectable (liver) levels of P112. We conclude that endothelial cells in different organs are not identical and that P112 is preferentially expressed in lung endothelial cells. Quantitation of P112 may be useful to assess the role of endothelial cells in toxic injury, lung diseases, and growth and vascularization of lung tumors.