Introduction of Vaccinia Virus as Oncolytic Virus

Despite great advances in technology, the formidable challenge of treating cancer still has no significant improvement in survival rates, even with the most common cancer. Oncolytic viral therapies have illustrated an encouraging promise for the treatment of different cancers, with the potential advantages of stronger therapeutic efficacy compared to conventional therapies because of less toxicity and higher tumor selectivity. They're able to preferentially reproduce in cancer cells, eventually destroying tumor tissues basically through cell lysis, whilst leaving non-cancerous tissues undamaged. Currently, many wild-types and genetically engineered vaccinia virus (VV) strains have been assessed in both preclinical and clinical trials with exciting results. Deeper understanding in molecular biology enables the generation of genetically engineered oncolytic viruses (OVs) for safer and more efficient therapeutics, including arming oncolytic VVs with other drug candidates, conventional therapies, and/or other virus strains.

Vaccinia Virus

VV is a large enveloped, double-stranded DNA (dsDNA) virus of ~200 kb that can infect many types of mammalian cell lines and some invertebrate cell lines. Its replication cycle is restricted to the cytoplasm and classified into an early phase and a late phase. The late phase is featured by the synthesis of structural proteins and enzymes, packaged into progeny virions. Expression of foreign genes from recombinant VV makes use of active promoters during the late phase to maximize yields of the desired protein. Several characteristics make the VV system an attractive platform for the research of viral assembly and structure. The large size of the VV genome allows the insertion of multiple foreign genes under the control of various promoters. Expression takes place in the cytoplasm, removing splicing of transcripts owing to the existence of cryptic splice sites. Synthesis of the desired protein initiates in mammalian cells, ensuring proper posttranslational modifications and the presence of host factors that may be needed for folding and assembly.

Fig.1 Vaccinia virus, a representative poxvirus: virion structure (A) and genome organization with an expanded view of the Hin dIII D restriction enzyme fragment (B). (Harrison, 2004)

Vaccinia Viruses as Oncolytic Virotherapy

Replication-competent OV therapies have displayed great value preclinically and in clinical trials for the treatment of various cancers. Oncolytic VV strains are of particular interest due to a number of advantages. They have a broad host range, and their highlights as an OV are their large genome size with an ability to accommodate multiple foreign genes, as well as their wide tropism, fast replication, and ease of recombination for producing viral mutants. Here, there are more reports regarding replication-competent oncolytic VVs, with a special focus on its potential for cancer treatment, transgene delivery, and imaging capacity.

Fig.2 Construction of recombinant vaccinia virus expression vector. (Strauss, 2008)

• Oncolytic VV-mediated antitumor mechanisms

VV's oncolytic potential depends on the strain the vectors are based on. Copenhagen, Lister, and Wyeth strains have revealed oncolytic potency, while some strains such as Ankara and NYVAC (vP866) can't replicate in mammalian cells and thus have no oncolytic potential at all. The antitumor effect mediated by oncolytic VVs is mainly affected by three different mechanisms of action. The first is direct infection of tumor cells and then replication resulting in cell lysis. This principle appears to have properties of both necrosis and apoptosis. The second mechanism involves immune-based cell death. VV infection gives rise to cell destruction and release of cellular danger signals and viral danger signals, together with tumor-associated antigens. Third, oncolytic VVs have been shown to elicit vascular collapse within tumors in preclinical and clinical settings.

• Key-benefits of oncolytic VV therapy

VV's robust oncolytic effect is originated from its high infectivity, rapid replication cycle, efficient cell-to-cell spread and productive cell lysis. The entire life cycle of VVs occurs in the cytosol and is achieved within 24 hours producing as many as 10,000 new virions. There are several advantages of using VVs as an agent for OV therapy. Recombinant VVs' large 192-kb genome permits a large amount of foreign DNA to be incorporated without meaningfully declining the replication efficiency of the virus, promoting genetic engineering for safer attenuated viruses and transgene delivery systems. Cytoplasmic replication of the virus reduces the chance of the recombination of viral DNA into normal cells, and its DNA-based genome also makes it more stable than RNA-based viruses. It has been demonstrated to escape the immune system and infect a wide array of cells, enabling more efficacious systemic delivery.

References

1. Harrison, S.C.; et al. Perspective: Discovery of antivirals against smallpox. Proc Natl Acad Sci USA. 2004, 101(31): 11178-11192.
2. Strauss, J.H.; et al. CHAPTER 11 - Gene therapy. Viruses and Human Disease (Second Edition). 2008, 423-447.

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