In silico prediction of FVIII epitopes recognised by natural autoantibodies in polyvalent immunoglobulin concentrates

Inhibitory antibodies directed against blood coagulation factor VIII (FVIII) impair FVIII replacement therapy, constituting a serious complication in haemophilic and autoimmune patients. Identifying B-cell FVIII epitopes and mapping them on the molecule remain important challenges. Using a combination of different algorithms, more than 30 hypothetical linear epitopes were predicted on the FVIII molecule surface. We selected several major predicted sequences, spanning all FVIII domains, for specific antibody induction in rabbits. All peptides tested successfully induced production of specific anti-FVIII rabbit antibodies, supporting the relevance of our approach. To investigate the presence of FVIII-reactive antibodies in the healthy donor population, a pooled fraction rich in all IgG subclasses was purified on peptide-Sepharose columns. Substantial amounts of Ig, specific for each FVIII peptide, were purified with yields ranging from 8 to 223 ng/mg immunoglobulins. Our results confirm the diversity of FVIII epitopes recognised by natural human anti-FVIII autoantibodies. All IgG subclasses were found in the affinity-isolated anti-peptide material, with overrepresentation of IgG2 and IgG4. Evidence was also found for new FVIII epitopes. Five human anti-peptide preparations displayed FVIII-neutralising activity, ranging from 1.3 to 5.3 BU/mg. Although the presence of naturally occurring anti-FVIII antibodies in healthy donors has been previously described, our methodology has allowed, for the first time, a fine mapping of several inhibitory and non-inhibitory epitopes. Our observations support the hypothesis that FVIII inhibitors in haemophilia A and autoimmune disease may originate from the proliferation of natural FVIII-specific B-cell clones.