Transduction of B cells by lentiviral vectors encoding FVIII domains that harbour the most common immuno-epitopes has the potential

to diminish antibodies to the protein in HA mice with inhibitors [38]. We initiated an experimental study in HA dogs with inhibitors to canine FVIII (cFVIII) prior to liver-directed gene transfer by AAV-cFVIII. Inhibitor-prone HA dogs from the UNC-Chapel Hill colony or Queen’s University are excellent models for studying tolerance induction, as both colonies have the same mutation found in the majority of the human HA population (inversion of FVIII intron 22) [52,53]. Notably, the most common mutation (∼40% of cases) in severe HA patients is the FVIII intron 22 inversion, which together with rare mutations (large gene deletion and nonsense mutations) phosphatase inhibitor library presents a higher risk for inhibitor formation when compared with patients with FVIII missense mutations. The overall purpose of this strategy is to determine whether an alternative approach, i.e. endogenous liver expression of cFVIII, will lead to the eradication of inhibitors. Because AAV vectors SCH772984 price provide long-term FVIII expression, this strategy has the potential to not only eradicate inhibitor by induction of FVIII-specific immune tolerance but also provide increased therapeutic FVIII levels to ameliorate the disease phenotype. To date,

four HA dogs have been injected with AAV8 expressing B-domain-deleted canine FVIII (BDD-cFVIII) via the peripheral vein. Long-term follow-up of these animals demonstrated that eradication of the inhibitor to cFVIII is feasible in most animals. After inhibitor eradication, repeated challenges with intravenous injections of recombinant BDD-cFVIII did not induce antibody formation to cFVIII and pharmacokinetic studies following

such administration demonstrated normal recovery and half-life of BDD-cFVIIII. In these dogs, the disease phenotype was improved by the persistence of therapeutic levels of cFVIII and a significant reduction in the numbers of spontaneous bleeds. In one animal, AAV-cFVIII induced a strong transient anamnestic response SPTBN5 to the endogenous expressed cFVIII that after 18 months is now <1 BU. Factors such as exposure to xeno-antigens, the nature of the antibody response, or age may influence the outcome of the tolerance induction protocol. Collectively, these data demonstrate the potential of AAV-FVIII gene delivery not only to treat genetic deficiencies such as haemophilia, but also to induce tolerance to the transgene in the setting of pre-existing inhibitory antibodies. Studies on the immune response to FVIII and FIX in haemophilic mice will continue to provide new information relating to immunogenic and tolerance mechanisms. In addition, novel therapeutic approaches will probably require initial evaluation in these animal models.