Update on herpesvirus amplicon vectors

Herpesviruses come in a vast variety; more than 120 different viruses are known and still more are being identified. All herpesviruses have a large genome of linear, double-stranded DNA (120-250 kb) and share the ability to replicate as infectious defective particles, the basis of amplicon vectors. The potential of herpesviruses to infect a broad range of host cells and to either induce or avert immune reactions makes them important entities to exploit in the context of development of both new vaccines and vectors for gene therapy. The development of HSV-1-based vectors has been stimulated by the desire to deliver therapeutic genes into cells of the nervous system, but many other tissues, including muscle, liver, and dendritic, are targets as well. The most attractive feature of herpesvirus-based amplicon vector is their large transgene capacity, which allows the insertion of multiple expression cassettes as well as entire genomic loci. Moreover, a variety of genomic elements, including transcriptional regulatory sequences for cell-type-specific or induced expression, and genetic elements from other viruses can be incorporated in a modular fashion to control the fate of transgenes. Further, the virion envelope can be readily modified for targeting infection to specific cell types, and functional proteins can be carried into cells during infection through fusion to tegument proteins. Since their development more than 20 years ago, HSV-1 amplicon vectors have been extensively modified to increase: (i) their safety profile, (ii) the efficiency of infection, and (iii) the stability of transgene expression. Some of these modifications involved preparation of vector stocks, such as the development of helper virus-free packaging systems, the inclusion of foreign polypeptides in the virus particle to target specific cells, and optimization of vector purification. Other modifications have involved the amplicon vector itself, for example, the inclusion of genetic elements from other viruses to create hybrid and even tribrid vectors, which opens the possibility to change the biological properties of the vector DNA in a very specific, customized manner. Future developments to optimize vector production will be crucial to allow the transition of amplicon vector from experimental to clinical gene therapy protocols.