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Activated Factor B (Bb) of the alternative pathway of complement (APC) induces human monocytes and murine macrophages to spread on a glass substrate (Sundsmo and Gątze (1980), Cell. Immunol., 52:1). In studies designed to investigate the effects of Bb on plasminogen activator secretion by human monocytes, it became apparent that Bb possesses innate plasminogen activator (PA) activity. PA activity of Factor Bb was determined by release of radiolabeled fibrin peptides (modified fibrin plate assay) over 4 hrs. at 37°C using purified human plasminogen, 125I-labeled human fibrinogen, and 20% fresh normal human serum as a source of thrombin. Native Factor B did not express significant PA activity, however, purified Bb, or Bb in complex with cobra venom factor released, 52±8% and 79±4%, respectively, of the 125I- fibrin that is released by urokinase. Intermediate complement complexes on rabbit erythrocytes (Er) were tested for their PA activity: ErC3b, Bb released 76±7% of 125I-fibrin, and control Er, C3, and B released &lt;15%. Factor D was without activity. Cleavage of plasminogen by Factor B was investigated by incubating cellular complement intermediates (ErC3b, Bb; EsC3b, Bb, NF) 30 min./37°C with 125iI-labeled plasminogen; and, conducting SDS-PAGE (reducing conditions) to separate native plasminogen from the heavy and light chains of plasmin. It was found that Bb cleaves predominantly the Lys-form of plasminogen to generate fragments with apparent molecular weights of 72,000 and 30,000. As a control, 125I-labeled plasminogen was incubated with Er, C3, or D, and &lt;10% cleavage was observed; EsC3b or native Factor B cleaved &lt;5%. Affinity-purified goat anti-B Ig inhibited Factor Bb- dependent plasminogen cleavage by 99%. These results suggest that activated Factor B, the central serine esterase of the APC, can serve as a plasminogen activator and raise the possibility that Factor B may play a role in initiation of fibrinolysis as well as in complement-dependent humoral and cellular mechanisms of immunity.

Complement-Dependent Activation Of Plasminogen By Activated Factor B (Bb) Of The Alternative Pathway Of Complement