Adenoviral vector systems for gene therapy

Adenoviral (Ad) vectors are widely used in gene therapies, recombinant viral vaccines, and basic science studies. The vectors can deliver therapeutic genes into cells to recover the lost function of some genes, to enhance the ability of host immune systems, or to increase the sensitivity of cancer cells to chemotherapeutic drugs. Several adenoviral vector systems have been developed: first-generation vectors are those with deletion of the E1a and E1b genes, second-generation vectors with deletions of the E1 and another viral gene, and gutless vectors removed all coding sequences for viral proteins. The firstand second-generation vectors are relatively easy to construct and produce. The gutless Ad vectors are substantially superior to any other adenoviral systems for achieving high-level and longterm expression in vivo. The recent developed oncolytic adenoviruses can selectively replicate in cancer cells but not in normal cells, allowing the spread of the viruses throughout the tumors. This review focuses on the advantages and limitations of adenovirus vectors and discusses potential strategies for further improvement of vectors for gene therapy.