Transient Expression of Genes Transferred In Vivo into Heart Using First-Generation Adenoviral Vectors: Role of the Immune Response

Gene therapy for heart diseases requires availability of an efficient vector for gene transfer into myocardium. Recombinant adenovirus expressing the Escherichia coli β-galactosidase (β-Gal) gene was shown to infect rat cardiocytes efficiently in vivo. However, a time course of gene expression showed that transgene expression was maximal during the first week following injection, then declined and disappeared by day 21. An immunosuppressive treatment prolonged β-Gal expression for at least 21 days. On the contrary, a preimmunization of the animals by two intraperitoneal injections of the vector led to a decreased transgene expression 48 hr after intramyocardial injection and to a barely detectable expression at the sixth day. Appearance of adenovirus neutralizing antibodies in preimmunized animals could have contributed to such a refractoriness to further adenoviral infection. Finally, a neonatal intrathymic injection of the vector was able to induce long-term LacZ expression for more than 2 months after heart injection, although neutralizing as well as anti-β-Gal antibodies were detected in sera of the animals. These results indicate that an immune response against first-generation replication-defective adenoviral vectors is a major cause of transient transgene expression, a cellular response being most probably responsible for ablation of transgene expression in immunocompetent animals. First-generation recombinant adenovirus have been shown to infect adult rat cardiocytes efficiently in vivo. However, transgene expression is transient. Here, we have used direct intramyocardial injections of a first-generation recombinant adenovirus expressing β-galactosidase (β-Gal) to determine whether an immune response could be involved in the decreased expression of the transgene with time. Antibodies directed against β-Gal and anti-adenovirus neutralizing antibodies were detected after a single intramyocardial injection. A prolonged transgene expression was observed when rats were treated with cyclosporine or tolerized to antigens encoded by the recombinant adenovirus by a neonatal intrathymic injection. In contrast, such expression was barely detectable 6 days after injection when rats were preinjected intraperitoneally with the adenovirus vector. Our results suggest that an immune response is induced by direct intramyocardial injection of such vectors, and that the cellular response is mostly responsible for transient efficiency of adenovirus-mediated gene transfer.