TISSUE FACTOR PROCOAGULANT EXPRESSION BY RAT ALVEOLAR EPITHELIAL-CELLS

Fibrin deposition in the alveolar space is characteristic of inflammatory lung injury. The formation of fibrin in the alveolus results from the coagulation of extravasated plasma. The cellular elements that promote intra-alveolar clotting have not been completely defined. We have investigated the capacity of alveolar epithelial cells (AEC) to promote coagulation through the expression of procoagulant activity (PCA) in tissue culture. Using a single-stage coagulation assay, rat AEC monolayers were found to contain 20,750 +/- 4,035 procoagulant units (PCU)/10(6) cells; 10- to 20-fold greater activity than that found in concomitantly isolated alveolar macrophages. The epithelial-derived procoagulant was shown to be tissue factor by a series of assays using clotting factor-deficient human plasmas. Freshly isolated AEC also possessed PCA (2,500 +/- 1,000 PCU/10(6) cells) and expressed a 2.1-kb mRNA that hybridized with a cDNA for murine tissue factor. Using a kinetic turbidometric assay of clot acceleration, PCA was found on the surface of unstimulated epithelial monolayers and could be increased to 170% of control by incubation with phorbol myristate acetate (PMA). This response to PMA was accompanied by a parallel increase in the relative abundance of tissue factor mRNA. AEC shed particulate PCA into the culture media that displayed a specific activity similar to that recovered from alveolar lining fluid. Therefore, by expressing both cell surface and particulate PCA, the alveolar epithelium likely contributes significantly to the modulation of intra-alveolar coagulation.