The development of anti‐factor VIII (FVIII) antibodies (inhibitors) is a critical concern when considering gene therapy as a potential treatment modality for hemophilia A. We used a hemophilia A mouse model bred on different genetic backgrounds to explore genetically controlled differences in the immune response to FVIII gene therapy.

C57BL/6 FVIII knockout (C57‐FVIIIKO) mice were bred with normal BALB/c (BAL) mice, to generate a recombinant congenic BAL‐FVIIIKO model of hemophilia A. Early generation adenoviral (Ad) vectors containing the canine FVIII B‐domain‐deleted transgene under the control of either the CMV promoter or a tissue‐restricted (TR) promoter were administered to C57‐FVIIIKO, C57xBAL(F1)‐FVIIIKO crosses, and BAL‐FVIIIKO mice. FVIII expression, inhibitor development, inflammation, and vector‐mediated toxicity were assessed.

In response to administration of Ad‐CMV‐cFVIII, C57‐FVIIIKO mice attain 3‐fold higher levels of FVIII expression than BAL‐FVIIIKO. All strains injected with Ad‐CMV‐FVIII displayed FVIII expression lasting only 2 weeks, with associated inhibitor development. C57‐FVIII‐KO mice that received Ad‐TR‐FVIII expressed FVIII for 12 months post‐injection, whereas FVIII expression was limited to 1 week in C57xBAL(F1)‐FVIIIKO and BAL‐FVIIIKO mice. This loss of expression was associated with anti‐FVIII inhibitor development. BAL‐FVIIIKO mice showed increased hepatotoxicity with alanine aminotransferase levels reaching 4‐fold higher levels than C57‐FVIIIKO mice. However, C57‐FVIIIKO mice initiate a more rapid and effective cell‐mediated clearance of virally transduced cells than BAL‐FVIIIKO, as evidenced by real‐time PCR analysis of transduced tissues. Overall, strain‐dependent differences in the immune response to FVIII gene delivery were only noted in the adaptive response, and not in the innate response.

Our results indicate that the genetic background of the murine model of hemophilia A influences FVIII expression levels, the development of anti‐FVIII inhibitors, clearance of transduced cells, and the severity of vector‐mediated hepatotoxicity. Copyright © 2004 John Wiley & Sons, Ltd.