Mechanism of factor VIII inactivation by human antibodies. IV. Antibody binding prevents factor VIII proteolysis by thrombin.

Factor VIII activation by thrombin is the result of a proteolytic cleavage of the procoagulant component. These studies examine the effect of human antibody on this activation step in a solid phase immunoadsorbent assay system. Radiolabeled factor VIII antibody: factor VIII protein immune complexes were bound to agarose beads by mouse monoclonal antifactor VIII R:Ag antibody. The incubation of these bound labeled immune complexes with high ionic strength buffers (1 M NaCl, 0.24 M CaCl2), or with acidic buffers (0.01 M glycine-0.1 M NaCl, pH 3.0 or 3.5), or with trypsin (1, 5, and 20 mg per ml) dissociated 14 to 62 percent of the bound radiolabel. Thrombin at a concentration of 0.05 U per ml, however, only dissociated 2.9 percent of the label, an amount not significantly different than borate buffered saline control. It is concluded that inactivation of factor VIII is the result of human antibody inhibition of thrombin-induced proteolysis of factor VIII procoagulant protein.