Naturally occurring human antibodies against two distinct functional domains in the heavy chain of FXI/FXIa

We have isolated and probed the mechanism of action of two naturally occurring antibodies (Baltimore and Winston-Salem) against factor XI (FXI), that developed in patients congenitally deficient in FXI after replacement therapy. Purification on immobilized protein A and neutralization with monospecific antibodies against IgG heavy and light chain subtypes indicated that both antibodies were of restricted heterogeneity. Both Winston-Salem (IgG3 kappa) and Baltimore (IgG1 kappa) completely inhibited FXI coagulant activity at titers of 200 and 8 Bethesda units, respectively. Immunoaffinity columns prepared from each antibody were able to bind the heavy but not the light chain of reduced and alkylated activated FXI (FXIa). The activation of purified FXI by activated bovine factor XII (FXIIa), a reaction independent of high molecular weight kininogen (HK), was not inhibited by either antibody. The active site on the FXIa light chain was unaffected by either patient's IgG, as measured by its amidolytic activity. In contrast, one antibody (Baltimore) or its Fab' blocked the surface- mediated proteolytic activation of FXI by human FXIIa in a concentration-dependent fashion by preventing its binding to HK, but had no effect on the rate of activation of FIX by FXIa. In contrast, the other antibody (Winston-Salem) or its Fab' inhibited the activation of FIX by FXIa in a concentration-dependent fashion but did not inhibit binding of FXI to HK. We conclude that each of these two naturally occurring antibodies is directed against a specific, separate, and distinct epitope located in the heavy chain of FXIa, one near or at the domain essential for the activation of FIX by FXIa and the other close to the domain required for binding to HK.