The goal of these studies was to test whether adeno-associated virus (AAV) capsid-specific CD8⁺ T cells cause loss of hepatic AAV-mediated gene expression in experimental animals. Mice immunized with adenoviral vectors expressing AAV capsid or with AAV vectors developed CD8⁺ T cells in blood, lymphatic tissues, and liver to epitopes shared between AAV2 and AAV8, and serotype-specific neutralizing antibodies. At the height of the T cells' effector phase, mice were infused with a heterologous AAV vector expressing human factor IX under a hepatocyte-specific promoter. Despite the presence of lytic CD8⁺ T cells in the liver, hepatic Factor IX expression was sustained and comparable in AAV-preimmune and naïve animals. These results suggest that, in mice, pre-existing CD8⁺ T cells to AAV capsid do not affect the longevity of AAV-mediated hepatic gene transfer. These results are in contrast to the outcome of a recent gene therapy trial of hemophilia B patients who were treated by hepatic gene transfer of AAV2 vectors expressing Factor IX. The loss of Factor IX expression, accompanied by a rise in liver enzymes and detectable frequencies of circulating AAV capsid-specific T cells, suggested T-cell-mediated destruction of transduced hepatocytes following reactivation of AAV-specific T cells upon AAV transfer.