COMMON INHIBITORY EFFECTS OF HUMAN ANTI-C2 DOMAIN INHIBITOR ALLOANTIBODIES ON FACTOR-VIII BINDING TO VON-WILLEBRAND-FACTOR

Factor VIII (FVIII) inhibitor alloantibodies obtained from seven sever e haemophilia A patients were examined for their binding regions and t heir effects on FVIII binding to non Willebrand factor (vWF). Immunobl otting analysis with a panel of recombinant fragments demonstrated tha t the binding regions of antibodies in cases 1-5 were contained in the C2 domain of the light chain. Antibodies from cases 1 and 2, which re cognized an epitope within residues 2248-2312, completely inhibited FV III/vWF binding in an ELISA (IC50: 5.0 and 9.0 mu g/ml, respectively). Antibodies from case 3 recognizing 2170-2312 and case 5 recognizing 2 170-2327 also inhibited FVIII/vWF binding (IC50: 110 and 400 mu g/ml, respectively). Case 4 antibodies recognizing 2218-2307 showed barely d etectable inhibition and cases 6 and 7 antibodies recognizing the 44 k D heavy chain, did not inhibit, Our results demonstrate that all anti- C2 alloantibodies with epitopes that extend to the residue 2312 inhibi t vWF binding and that an overlap of the inhibitor epitope with residu es 2308-2312 is critical for maximal inhibition of vWF binding. Preven tion of FVIII/vWF binding appears to be a common property of anti-C2 d omain inhibitor alloantibodies.