Tolerance induction in hemophilia A animal models: battling inhibitors with antigen-specific immunotherapies.

Hemophilia A is an X-linked recessive bleeding disorder due to either a lack of or greatly reduced activity in the blood coagulation protein factor VIII (FVIII), due to mutations in the F8 gene. This poses significant challenges for FVIII replacement therapy since hemophilic patients are not immunologically tolerant to the protein. Thus, a proportion of patients who receive plasma-derived or recombinant FVIII replacement therapy develop anti FVIII neutralizing antibodies, known as "inhibitors." These patients require long-term regimens of high dose FVIII administration, which has varying success rates and prohibitive costs. Therefore, therapeutics for tolerance induction in such patients with inhibitors are desired. In this review, we address the current progress of immunotherapies for inducing FVIII specific tolerance in animal models of hemophilia A. Specifically we discuss the beneficial effects of B-cell depletion on immune tolerance induction (ITI), B-cell mediated gene therapy, antigen-coupled lymphocyte therapy, and regulatory T-cell epitopes (Tregitopes).