Prolonged transgene expression mediated by a helper-dependent adenoviral vector (hdAd) in the central nervous system

Conventional adenoviral vectors such as E1-deleted first-generation adenovirus (fgAd) elicit striking host immune response, resulting in limited expression of the transgene. A recently described helper-dependent, or gutless, adenoviral vector (hdAd) can promote stable transgene expression in peripheral organs, including the liver. We therefore investigated the safety and durability of hdAd-mediated gene transfer to the central nervous system (CNS) of rats compared with gene delivery by fgAd. Equal amounts of either fgAd or hdAd carrying the beta geo transgene were stereotactically injected into the right hippocampus of adult rats. Transgene expression was assessed by histochemical staining, transgene stability by PCR analysis, and immune infiltration of T lymphocytes and macrophages by immunocytochemical methods. Strong transgene expression from either vector was detected in brain tissue examined on day 6 postinoculation. Thereafter, fgAd-mediated gene expression rapidly decreased, becoming undetectable by day 66, while expression from the hdAd vector persisted throughout the test period. PCR confirmed the presence of hdAd-associated DNA at 66 days postinoculation. The hdAd injection elicited apparently lower numbers of brain-infiltrating macrophages and T cells than did administration of fgAd. These results indicate improved transgene expression and reduced immunogenicity with use of hdAd to deliver genes to the CNS.