Summary And Future Directions

This overview of HSV biology and gene transfer has focused on the use of highly defective HSV genomic vectors that are blocked very early in the virus lytic cycle. These vectors express few viral functions and are highly reduced in vector toxicity, even for primary neurons in culture that are readily killed by less-defective HSV vectors. Moreover, these vector backgrounds are suitable for expression of multiple transgenes or single large genes (e.g., dystrophin) in applications where expression of single- or multiple-gene products are required to achieve a therapeutic outcome (e.g., tumor cell killing, vaccination). Expression of these transgenes can be coordinated, even sequentially, using strategies similar to those employed by the virus to regulate its own genes. Expression can also be controlled by drug-sensitive transactivators, which may prove to be important for regulating the timing and duration of transgene expression. HSV vectors may be most suited for expression of genes in the nervous system where the virus has evolved to remain lifelong in a latent state. The highly defective viruses deleted for multiple IE genes are able to efficiently establish resistance in neurons and serve as a platform for long-term gene expression using the latency-active LAP2 promoter system. These mutants can not reactivate from latency and cannot spread to other nerves or tissues following infection of cells. Delivery of these vectors requires direct inoculation of tissue to achieve direct contact with neurons. Ideally, HSV vectors would be most effective if infection could be targeted to specific cell types using enveloped particles defective for their normal receptor recognition ligands, but modified to contain novel attachment and entry functions. This area of research is still very early in development, and it remains to be determined to what extent this will be feasible. Finally, it should be emphasized that current viral delivery systems may each become reduced to highly defective transfer vectors, retaining only those elements required for vector DNA maintenance and transgene expression. Fortunately, the natural biology of many persistent viruses, including HSV-1, indicates that long-term vector maintenance will be possible and we continue to learn from the highly evolved biology of persistent and latent viruses in order to mimic their strategies for gene transfer and therapy.

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