946. CD8 T Cells Are Partially Mediate the Immune-Mediated Silencing of Adenoviral Vector-Encoded Transgene Expression in the Rat Brain

First generation adenoviral vectors (RAd) transfer genes into the brain with high efficiency, and provide high levels, long term expression, and therapeutic efficacy in models of brain tumors and neurodegeneration, in the absence of priming of an anti-adenoviral immune response. Systemic immunization against RAd, however, induces a cellular adaptive immune response which completely eliminates the expression of the transgene from the brain, and causes a long-term inflammatory response. To elucidate the mechanisms by which the immune response clears transgene expression from the brain, we examined, following the delivery of RAds into the striatum, and systemic immune priming, whether the immune mediated inhibition of transgene expression causes brain neurotoxicity, reduces the levels of transgene transcripts, reduces vector genome copy numbers, and is mediated by CD8+ T cells. Our experiments provided the following results: (1) at 7 and 14 days post-immunization there was an infiltration of lymphocytes and macrophages surrounding transduced cells; (2) macrophages containing remains of transduced cells could be identified indicating the existence of cytotoxicity of transduced cells; (3) there was neuronal loss in the substantia nigra, but not in the striatum; (4) transgene mRNA levels were reduced by more than 90%, but genome copy numbers were only reduced by 10-20%; (5) the depletion of CD8+ T cells using the monoclonal antibody OX8, reduced circulating CD8+ T cells by >90% and partially protected against the immunization-induced silencing of transgene expression. In summary, our data indicate that immune-mediated silencing of RAd-encoded transgene expression in the brain occurs through a combination of non-cytolytic, and, also cytotoxic mechanisms. The percentage contribution of each mechanism is currently being evaluated.