80. Pre-Existing Cytotoxic T-Lymphocyte (CTL) Responses to the AAV CAP Protein Do Not Interfere with AAV Transgene Expression in Mice

The use of adeno-associated viral (AAV) vectors as a delivery method for gene replacement strategies is currently being explored in several clinical indications. AAV vectors do not express any viral proteins suggesting that they represent a less immunogenic delivery system. However, recent data have suggested that administration of AAV vectors may actually result in the stimulation of T-cell responses that result in a loss of transgene expression and may limit the utility of these vectors. By using computer prediction algorithms, we identified several AAV peptides as putative MHC I binders. Immunization of mice with peptides predicted to have the highest binding affinities did not result in a detectable cytotoxic T lymphocyte (CTL) response. In addition, intravenous administration of increasing doses of AAV serotype 2 vector also failed to induce a measurable CTL response. In contrast, immunization with either a plasmid or an adenoviral vector encoding the full length AAV2 CAP protein, to mimic exposure to wild type AAV, generated a potent CTL response. A pre-existing CTL response is likely to be present in the human population and C57Bl/6 IgH6 knock-out (KO) mice were used to determine whether such CTLs could affect AAV transduction and subsequent transgene expression. When immunized with two doses of CAP plasmid, the IgH6 KO mice mounted a robust CTL response without the generation of AAV neutralizing antibodies. The mice were then injected with an AAV2 vector encoding the alpha-galactosidase (a-gal) protein. AAV transduction was unimpeded and the serum levels of a-gal protein as well as the longevity of transgene expression were comparable to those observed in mice pre-immunized with empty plasmid as a control. In contrast, wild-type C57Bl/6 mice pre-immunized with CAP plasmid, which developed both CTLs and neutralizing antibodies against AAV, did not show any evidence of a-gal expression after the injection of vector. Taken together, results of these experiments suggest that pre-existing AAV-specific CTLs may have little or no effect on AAV transduction or transgene expression and confirm that neutralizing antibodies significantly interfere with transduction by AAV vector.