Anti-FVIII IgG2 antibodies isolated from hemophilia dog #3 interact with functional sites on the therapeutic cFVIII protein. (A) Screening a peptide library consisting of 321 peptides spanning the canine BDD FVIII with an ELISA-based assay revealed 12 peptides that interacted with the purified anti-FVIII IgG2 antibodies isolated from dog #3 (column 1). The canine and corresponding human amino acids along with the known functional sites are indicated. (B) After subtracting the background signal (negative control) for each of the peptides, the signal intensity for each peptide was compared with the signal intensity obtained from recombinant cFVIII. For overlapping peptides, the peptide with the strongest signal intensity is displayed on the horizontal axis. APC, activated protein C; FIX, factor IX; FXa, factor Xa.