The development of inhibitory antibodies against coagulation factor VIII (FVIII) is currently the most serious complication for hemophilia A patients that undergo FVIII replacement therapy. In addition, non-hemophilia A patients can spontaneously develop inhibitory auto-antibodies to FVIII, which results in acquired haemophilia A. The control of the allo- or autoimmune response to FVIII apparently includes the elicitation of anti-idiotypic antibodies. In this study the capacity of anti-idiotypic single-chain variable antibody fragments (scFvs) for neutralization of inhibitory anti-FVIII antibodies (FVIII inhibitors) was evaluated in vitro and in vivo.

Anti-idiotypic scFvs were selected from phage-displayed libraries against murine monoclonal FVIII-specific inhibitors. As the majority of inhibitory antibodies is directed against the A2 or C2 domain of FVIII, strongly inhibitory A2- and C2-specific antibodies served as targets. Selected scFvs were expressed as scFv-Fc fusion proteins.

Analysis of the scFv-Fcs by ELISA confirmed the specific binding to the cognate targets and binding studies via surface plasmon resonance revealed high affinities within the nanomolar range. Further characterization showed that binding of inhibitors to immobilized FVIII was blocked by specific scFv-Fcs in vitro. The ability of scFv-Fcs to neutralize their corresponding inhibitors was analyzed in a functional clotting assay. By adding scFv-Fcs to plasma spiked with inhibitors, FVIII activity was restored to 80% in a concentration dependent manner.

FVIII knockout mice served as model organism for testing the capacity of scFv-Fcs to restore coagulation in vivo. Subsequent injection of FVIII following the injection of the inhibitors resulted in a largely reduced FVIII activity. However, FVIII activity was recovered in a concentration dependent manner by adding cognate anti-idiotypes. The scFv-Fcs were either preincubated with the corresponding inhibitor or added to the FVIII mixture without preincubation. The latter represents an adaption to a therapeutic setting.

In conclusion, phage display selected anti-idiotypic scFvs are able to bind and effectively neutralize their target inhibitors in vitro and in vivo. Based on these promising results the potential of anti-idiotypic scFvs for the development of specific cell based immunotherapies for hemophilia A patients with inhibitors is currently under investigation.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.