Regulation of the Procoagulant Activity within the Bronchoalveolar Compartment of Normal Human Lung

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Abstract
The nature of the procoagulant activity of normal bronchalveolar fluid was examined both qualitatively and quantitatively. Unconcentrated, cell-free lavage freshly obtained from normal volunteers clotted normal plasma in a mean of 84 .+-. 20 s. The procoagulant activity was initiated by Factor VII-tissue factor complexes as judged by differential activity in various plasmas genetically deficient in single clotting factors, by neutralizaiton of the procoagulant activity with antibodies to either Factor VII or tissue factor, and by a Factor X activation assay. Preincubation of the lavage with calcium was required to demonstrate Factor VII activity in unconcentrated samples. The cell-free fluid contained about 8,500 thromboplastin units/mg protein, equivalent to a third of the thromboplastin standard and indicating high amounts of cofactor. Quantitation of Factor VII was estimated by functional analysis in coagulation and amidolytic assays with reference to dilutions of normal plasma of known Factor VII concentration. When lavage and diluted plasma were adjusted to yield equivalent amidolytic activities, the average ratio of the Factor VII-clotting activity of the alveolar fluid relative to plasma Factor VII was 19 .+-. 7, suggesting the presence of Factor VIIa in lavage. In contrast to previous reports with serum or activated plasma, immunoblots of concentrated lavage revealed only single-chain Factor VII, and 125I-Factor VII added to the fluid was not converted to 125I-Factor VIIa, suggesting a unique control mechanism in the lung compartment which differs from plasma. When equivalent Factor VII amidolytic activities in diluted plasma and cell-free lavage were compared, the rates of Factor Xa formation were very similar. However, the rates of thrombin formation were different, being accelerated 2- to 3-fold in the bronchoalveolar fluid in the presence of exogenous Factors II, V, and X and phospholipid. Additional studies showed that the enhanced clotting activity of alveolar Factor VII over that of plasma was due to an enzyme(s) in the epithelial lining layer which directly cleaved Factor V in a thrombin-like fashion and thereby enhanced prothrombinase complex activity. The data indicate that the normal bronchoalveolar surfaces are highly procoagulant, being lined with saturing amounts of active Factor VII-tissue factor compelxes and catalytic amounts of an enzyme(s) capable of activating Factor V. Normal alveolar macrophages may be the source of much of this procoagulant activity. In addition, thrombin formation is greatly enhanced in the presence of bronchoalveolar cells relative to endogenous alveolar lipoprotein, indicating that the cells provide a suitable surface for the coagulation process described in this report to occur within the alvolar compartment of normal lung.