Strain-specific rate of shutdown of CMV enhancer activity in murine liver confirmed by use of persistent [E1 −, E2b −] adenoviral vectors

Abstract

The systemic delivery of [E1 −] adenoviral (Ad) vectors encoding a transgene results in efficient viral uptake and abundant transgene expression in the liver. However, [E1 −]Ad vector persistence is transient due to cytotoxic T lymphocyte (CTL)-mediated loss of the Ad-infected cells. Our laboratory has previously demonstrated that additional modifications to the [E1 −]Ad vector genome, by deletion of the Ad E2b genes, significantly decreased virus-genome-derived gene expression and simultaneously improved the long-term performance of the resultant [E1 −, E2b −]Ad vector. In this study, we confirmed that [E1 −, E2b −]Ad vector genomes could persist equally well in C57Bl/6 or Balb/c mouse hepatocytes. Despite vector genome persistence, we observed a strain-dependent variability in the duration of CMV enhancer/promoter-driven transgene expression in the liver. While Balb/c mice rapidly shut down [E1 −, E2b −]Ad-derived transgene expression, C57Bl/6 mice allowed for prolonged transgene expression. This occurred even when both strains were crossed into a severe combined immune-deficient background, demonstrating that host adaptive immune responses are not responsible for the phenomenon. Furthermore, differential methylation of the CMV enhancer/promoter was also not demonstrated in either strain of mouse, eliminating this mechanism as causative. Thus, alternative mechanisms for this phenomenon are discussed.