Hemophilia A (HA) is an X-linked bleeding disorder caused by a deficiency in factor VIII (FVIII) due to mutations in the F8 gene. The disease affects 1:5,000 male born worldwide. Replacement therapy with FVIII protein is effective in preventing/controlling bleeding but ~30% of patients develop inhibitors to FVIII (neutralizing alloantibodies) that renders FVIII ineffective; thus, increasing morbidity and mortality. Immune tolerance induction (ITI) is the only successful strategy for eradication of inhibitors/restore immune tolerance to FVIII. ITI regimens are based on FVIII protein injections for months/years and is efficacious in ~60% of cases, but the high cost (~1million/year) prevents its use outside the developed word. Thus, new approaches for inhibitor eradication are urgently needed. Ongoing AAV liver gene therapy clinical trials for HA without inhibitors resulted in therapeutic FVIII. These studies are using vectors manufactured in either HEK-293 or Sf9 cells that differ in their basic biology and clinical outcomes. Our central hypothesis is that AAV-FVIII liver gene therapy is an ideal ITI regimen based on our proof-of-concept report in inhibitor HA dogs (Finn et al Blood, 2010) and novel preliminary data. We will use 2 novel distinct high-responding inhibitor HA dog models with the canine F8 gene mutations associated with the most challenging inhibitor patient group that likely would benefit from AAV ITI. The rationale of this proposal is that a single injection of AAV liver gene therapy provides (A) efficient eradication of high titer inhibitors, (B) restoration and maintenance of immune tolerance to FVIII in high responding HA dogs and (C) continuous FVIII expression that improves the bleeding phenotype after inhibitor eradication. The specific aims are Aim 1: Determine the efficacy of AAV gene therapy in inducing immune tolerance in high-responding HA dog models.We will advance our efforts testing AAV gene therapy in dog models across several distinct breeds to define the factors associated with kinetics of inhibitor eradication (transgene levels/duration). Aim 2: Determine the potential of ITI by rAAV-Sf9-derived AAV-cFVIII gene therapy in high-responding HA dog models. Recent unexpected decline of FVIII expression after 3 years post- AAV-Sf9- FVIII in HA patients raised concerns of durability. To date, the underlying mechanism is unknown. We hypothesized that this could be a combination of Sf9-derived vector and/or FVIII. We will determine in dogs if AAV-Sf9 impacts FVIII expression levels over time and its ability to eradicate inhibitors and to induce tolerance to FVIII. Aim 3: Define the mechanism(s) underlying AAV-mediated ITI. In these novel dog models, we will characterize specific B cells and especially T regulatory cells pool and function following AAV-ITI and to determine the underlying mechanism of immune tolerance in both cell line-derived vector systems. Successful completion of this proposal would support AAV liver gene...