An Immune-Competent Murine Model to Study Elimination of AAV-Transduced Hepatocytes by Capsid-Specific CD8⁺ T Cells

Multiple independent adeno-associated virus (AAV) gene therapy clinical trials for hemophilia B, utilizing different AAV serotypes, have reported a vector dose-dependent loss of circulating factor IX (FIX) protein associated with capsid-specific CD8⁺ T cell (Cap-CD8) elimination of transduced hepatocytes. Hemophilia B patients who develop transient transaminitis and loss of FIX protein may be stabilized with the immune-suppressive (IS) drug prednisolone, but do not all recover lost FIX expression, whereas some patients fail to respond to IS. We developed the first animal model demonstrating Cap-CD8 infiltration and elimination of AAV-transduced hepatocytes of immune-deficient mice. Here, we extend this model to an immune-competent host where Cap-CD8 transfer to AAV2-F9-treated mice significantly reduced circulating and hepatocyte FIX expression. Further, we studied two high-expressing liver tropic AAV2 variants, AAV2-LiA and AAV2-LiC, obtained from a rationally designed capsid library. Unlike AAV2, Cap-CD8 did not initially reduce circulating FIX levels for either variant. However, FIX levels were significantly reduced in AAV2-LiC-F9-treated, but not AAV2-LiA-F9-treated, mice at the study endpoint. Going forward, the immune-competent model may provide an opportunity to induce immunological memory directed against a surrogate AAV capsid antigen and study recall responses following AAV gene transfer.