INDUCTION OF HUMAN FACTOR-VIII INHIBITORS IN RATS .2. FINE MAPPING OFRAT ANTI-HUMAN RFVIII ANTIBODIES

Inhibiting antibodies in patients with hemophilia A pose a significant therapeutic dilemma in the treatment of bleeding episodes, The geneti c factors which predispose hemophiliacs to inhibitors and the optimal method for inhibitor suppression remain obscure. Hence, an animal mode l of the human FVm inhibitor response is of potential value. Sprague-D awley rats immunized with human recombinant FVIII (rFVIII) subsequentl y developed abnormal coagulation parameters coincident with the develo pment of an immune response to the human protein. The epitopes for the resultant rat anti-rFVIII antibodies were mapped using a random fragm ent expression library constructed from the FVIII cDNA. Antigenic regi ons located within the Al, First and Second Acidic and B domains were mapped. Rat immunoglobulins reactive with the individual epitopes were immunoaffinity purified and assayed for inhibitory activity. Several of the epitopes mapped using the rat antibodies were similar to region s previously shown to be antigenic for human inhibitors. By contrast, no epitopes were mapped to the A2 domain with the techniques used. Thi s may be due to the possible presence of conformational epitopes in th is area which cannot undergo fragmentation and still retain antigenici ty or the presence of relatively low concentrations of antibodies to t his region. The rat model shares some similarity with both the auto- a nd alloimmune human response to FVIII and therefore may be a valuable model for studies on the induction and suppression of the inhibitor re sponse.