Abstract
Hepatic adeno-associated viral (AAV) gene transfer has the potential to cure the X-linked bleeding disorder hemophilia A. However, declining therapeutic coagulation factor VIII (FVIII) expression has plagued clinical trials. To assess the mechanistic underpinnings of this loss of FVIII expression, we developed a hemophilia A mouse model that shares key features observed in clinical trials. Following liver-directed AAV8 gene transfer in the presence of rapamycin, initial FVIII protein expression declines over time in the absence of antibody formation. Surprisingly, loss of FVIII protein production occurs despite persistence of transgene and mRNA, suggesting a translational shutdown rather than a loss of transduced hepatocytes. Some of the animals develop ER stress, which may be linked to hepatic inflammatory cytokine expression. FVIII protein expression is preserved by interleukin-15/interleukin-15 receptor blockade, which suppresses CD8+ T and natural killer cell responses. Interestingly, mice with initial FVIII levels >100% of normal had diminishing expression while still under immune suppression. Taken together, our findings of interanimal variability of the response, and the ability of the immune system to shut down transgene expression without utilizing cytolytic or antibody-mediated mechanisms, illustrate the challenges associated with FVIII gene transfer. Our protocols based upon cytokine blockade should help to maintain efficient FVIII expression.