DNA-Based Methods to Prepare Helper Virus-Free Herpes Amplicon Vectors and Versatile Design of Amplicon Vector Plasmids

The herpes simplex virus (HSV) amplicon vector is a versatile plasmid-based gene delivery vehicle with a large transgene capacity (up to 150 kb) and the ability to infect a broad range of cell types. The vector system was originally developed by Frenkel and her colleagues in 1980. Ever since, a great deal of effort by various investigators has been directed at minimizing the toxicity associated with the inevitable contamination by helper virus. In 1996, Fraefel and his colleagues successfully devised a cosmid-based packaging system that was free of contamination by helper virus (socalled helper virus-free packaging), which utilized as helper a set of 5 overlapping cosmid clones that covered the entire HSV genome, which lacked the DNA packaging/cleavage signals. With the helper virus-free system, broader applications of the vector became possible. Cloning of the entire HSV genome in bacteria artificial chromosome (BAC) plasmids enabled stable maintenance and propagation of the helper HSV genome in bacteria. It also allowed for the development of BAC-based helper virus-free packaging systems. In this article, we review various versions of DNA-based methods to prepare HSV amplicon vectors free of helper virus contamination. We also examine recent advances in vector design, including methods of vector construction, hybrid amplicon vectors, and the infectious BAC system. Future directions in improving packaging systems and vector designs are discussed.