Abundant deposition of bronchoalveolar fibrin and fibronectin occurs during the exudative phase of the adult respiratory distress syndrome (ARDS), promoting hyaline-membrane formation and subsequent alveolar fibrosis. To explore the mechanisms that account for the persistence of bronchoalveolar fibrin and fibronectin, we compared the activity of urokinase, which is necessary for plasminogen activation and fibrin degradation, in cell-free bronchoalveolar-lavage fluid from 8 patients with ARDS, 9 patients with acute pulmonary diseases other than ARDS, and 10 normal subjects. The mean level of urokinase activity in the lavage fluid from the patients with ARDS was 0.003 IU per milliliter of fluid (range, 0 to 0.008), which was significantly lower (P = 0.001) than the level in the fluid from either the patients with pulmonary diseases other than ARDS (0.118 IU per milliliter [range, 0.032 to 0.295]) or the normal subjects (0.129 IU per milliliter [range, 0.045 to 0.198]). The lavage fluid from all the patients with ARDS also had antiplasmin activity, which would promote the persistence of fibrin. A true decrease in urokinase activity was confirmed by the failure of the lavage fluid from the patients with ARDS to convert [125I]plasminogen to plasmin. Despite the low urokinase activity, immunochemical assays revealed normal levels of urokinase antigen in the fluid from the patients with ARDS, suggesting the presence of urokinase inhibitors. Inhibitors were demonstrated directly by a fibrin gel-underlay assay that detects complexes of urokinase with inhibitors. Plasminogen-activator inhibitor type 1 was the principal inhibitor identified. We conclude that increased antifibrinolytic activity due to both urokinase inhibitors and antiplasmins in the bronchoalveolar compartment of patients with ARDS contributes to the formation and persistence of hyaline membranes, a key component of alveolar histopathology in ARDS.

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Human alveolar macrophages are known to synthesize urokinase (uPA) and a specific plasminogen activator inhibitor, PAI-2. In this study we have identified a uPA receptor expressed by these cells and defined the influence of PAI-2 on the interaction of uPA with its receptor. Alveolar macrophages from four normal volunteers were incubated with 55 kDa 125I-labeled uPA (0.24-8 nM) in the presence or absence of excess unlabeled uPA. Specific and saturable binding was demonstrable in all cases. Scatchard plots were linear; regression analysis revealed a mean Kd of 5.25 nM (range 3.2-6.7) and mean Bmax of 30.7 femtomoles/10(5) cells (range 21.5-34.5). The structure of the uPA receptor was defined by electroblotting membrane fractions of macrophages and sequentially exposing filters to uPA and uPA antibodies. Membranes from macrophages demonstrated binding of either uPA or a 15-kDa amino-terminal fragment of uPA to a 55- to 60-kDa glycosylated membrane protein. Binding of uPA to filters was blocked by a synthetic oligopeptide containing the known receptor binding region of native uPA. Preincubation of 125I-uPA with PAI-2 dramatically reduced the rate of association of uPA with macrophage uPA receptor. Conversely, receptor-bound uPA activity was less susceptible to inhibition by PAI-2 than soluble uPA activity. These data indicate that normal alveolar macrophages express uPA receptors. The receptor preferentially binds and protects free uPA over complexed enzyme, indicating that one function of the receptor is to allow the cells to express active uPA in an inhibitor-rich environment.