Mechanism of activation of coagulation factor XI by factor XIIa studied with monoclonal antibodies.

The interaction of Factor XIIa with Factor XI was investigated using two monoclonal antibodies, one (3Cl) directed against the heavy chain of Factor XIa and the other (5F4) against its light chain. 3C1 either as intact IgG or as Fab' fragment, enhanced the rate of Factor XIa generation in the fluid phase but inhibited it in the presence of kaolin and high molecular weight (HMW) kininogen. In contrast, the Fab' fragments of 5F4 inhibited only the fluid phase activation and had no effect on the surface-mediated activation. 3C1 was found to block the binding of Factor XI to HMW kininogen, whereas 5F4 did not. We conclude: a domain on the heavy chain region of Factor XI is essential for binding to HMW kininogen and for optimal surface-mediated activation by Factor XIIa; and binding of 3C1 to Factor XI changes its conformation rendering it a more favorable substrate for Factor XIIa in the fluid phase.