The Role of Apoptosis in Immune Hyporesponsiveness Following AAV8 Liver Gene Transfer

Gene therapy provides a significant opportunity to treat a variety of inherited and acquired diseases. However, adverse immune responses toward the adeno-associated virus (AAV) antigens may limit its success. The mechanisms responsible for immunity or tolerance toward AAV-encoded transgene products remain poorly defined. Studies in mice demonstrate that AAV2/8 gene transfer to liver is associated with immunological hyporesponsiveness toward both AAV vector and antigenic transgene product. To evaluate the role of activation-induced cell death (AICD) and cytokine withdrawal (intrinsic cell death) in the deletion of mature T lymphocytes, we compared immunological responses in hepatic AAV2/8 transfer in murine recipients lacking the Fas receptor, and recipients overexpressing Bcl-xL, to WT murine counterparts. Prolonged transgene expression was dependent on both Fas signaling and Bcl-xL regulated apoptosis in T cells. Abrogation of intrinsic cell death enhanced Th1 responses, whereas AICD functioned to limit neutralizing antibody production toward AAV2/8. In addition, immune hyporesponsiveness and stable transgene expression was dependent on upregulation of FasL expression on transduced hepatocytes and a corresponding apoptosis of infiltrating Fas (+) cells. These data provide evidence that both AICD and apoptosis due to cytokine withdrawal of lymphocytes are essential for immune hyporesponsiveness toward hepatic AAV2/8-encoded transgene product in the setting of liver gene transfer.