Summary
Inhibiting antibodies in patients with hemophilia A pose a significant therapeutic
dilemma in the treatment of bleeding episodes. The genetic factors which predispose
hemophiliacs to inhibitors and the optimal method for inhibitor suppression remain
obscure. Hence, an animal model of the human FVIII inhibitor response is of potential
value. Sprague-Dawley rats immunized with human recombinant FVIII (rFVIII) subsequently
developed abnormal coagulation parameters coincident with the development of an immune
response to the human protein. The epitopes for the resultant rat anti-rFVIII antibodies
were mapped using a random fragment expression library constructed from the FVIII
cDNA. Antigenic regions 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 regions previously shown to be antigenic for human
inhibitors. By contrast, no epitopes were mapped to the A2 domain with the techniques
used. This may be due to the possible presence of conformational epitopes in this
area which cannot undergo fragmentation and still retain antigenicity or the presence
of relatively low concentrations of antibodies to this region. The rat model shares
some similarity with both the auto- and alloimmune human response to FVIII and therefore
may be a valuable model for studies on the induction and suppression of the inhibitor
response.
