Role of anti-idiotypic antibodies in immune tolerance induction

Replacement therapy using factor VIII (FVIII) elicits FVIII‐specific antibodies (abs) in about 25% of the patients. A majority of such abs are directed towards specific FVIII regions in which major epitopes have been identified (C‐terminal end of the C2 domain, the N‐terminal end of the A2 domain and C1 domain in cases of mild/moderate haemophilia A). We derived five human monoclonal abs (mabs) that react with high affinity to the FVIII C1, C2 or A2 domains respectively and are representative of most of the specific inhibitors observed in haemophilia A patients. We generated mouse anti‐idiotypic mabs (anti‐Ids) against the paratope of each of the inhibitors. We demonstrated that a combination of these anti‐Ids (anti‐anti‐A2, ‐C1, ‐C2) had the ability to neutralize the inhibitory properties of human polyclonal abs in plasma. In 16 of the 18 plasmas tested, the inhibiting FVIII activity was neutralized up to 100% by the anti‐Ids mixture with restoration of full FVIII activity. These data allow us to conclude that polyclonal high‐affinity FVIII inhibitors could be neutralized with an anti‐Ids mixture and that only a limited number of anti‐Ids were required for inhibitor neutralization in 90% of the patients. We also demonstrated that anti‐Id Abs bound to anti‐FVIII human B cell line produced the corresponding anti‐FVIII Ab and that this binding was followed by surface capping of complexes. Data obtained in vitro at monoclonal and polyclonal level, confirmed by in vivo assays, and the preliminary results obtained at BCR level, indicate that anti‐id mixture made of only a limited number of anti‐Ids could be useful in the restoration of haemostasis in haemophilia patients with inhibitor.