The production of fibrinolysis inhibitors as a parameter of the activation state in murine macrophages

Macrophages from peritoneal exudates which had been induced by various irritants were cultured, and the supernatants were tested for inhibitors of fibrinolysis using the plasmin‐dependent lysis of 125I‐labeled fibrinogen as assay. Washout macrophages and casein or proteose peptone‐elicited macrophages were found to release fibrinolysis inhibitors in contrast to lipopolysaccharide or thioglycollate‐induced macrophages. The molecular weight of inhibitors was determined at 60 000, 45 000 and 15 000 by Sephadex chromatography. Whereas the inhibitors at 15 000 and 45 000 could be detected in all experiments, the inhibitor at mol. wt. 60 000 was not present in all preparations. The isoelectric point of inhibitor I (mol. wt. 15 000) and inhibitor II (mol. wt. 45 000) was determined at 4.15. The proteolytic and esterolytic activity of trypsin and chymotrypsin were both inhibited by each of the two inhibitors. On the other hand, only the proteolytic activity of plasmin could be inhibited. Evidence for the active synthesis of inhibitors by macrophages came from several experiments. Macrophages in serum‐free cultures continued to release inhibitors for at least 48 h; normal mouse serum did not contain inhibitors of the same molecular size, and [3H]leucine was incorporated into the inhibitors which were specifically detected by the absorption of plasmin inhibitor complexes to lysyl‐Sepharose. Since inhibitors and plasminogen activator could not be detected in the same macrophage culture supernatants, it appears that the production of inhibitors and plasminogen activator by the same macrophage population is mutually exclusive.