Apart from posing a public health threat, poxviruses have gathered research interest for their use as oncolytic vectors. Viral oncolysis takes advantage of the virus’ robust replicative and lethal nature to kill cancer cells. Poxviruses are an ideal choice for viral oncolytic agents. Replication and host cell lysis occurs relatively quickly; the first viral particles are secreted within 8 hours and infected cells are destroyed 2 to 3 days after infection. Poxvirus cell-tropism is broad and can enter multiple cell types, and the EEV form spreads quickly through the bloodstream to distant tumors. Poxvirus genomes do not integrate into the host cell chromosome, and thus there is no concern of inducing potentially dangerous gene disruptions. Of particular importance for genetic engineering, vaccinia can accommodate multiple large transgenes to enhance oncolytic function . Finally, in case of an adverse response, many antiviral agents are available to treat poxvirus infections.
Poxviruses naturally selectively replicate within cancer cells, but engineering can enhance that specificity. The activities of cancer cells (blocking apoptosis, deregulating the cell cycle, and evading the immune system) are also characteristic of viruses and provide an optimal condition for poxvirus replication. Engineered viruses missing genes regulating these functions will replicate in cancer cells but not normal cells. In addition, deletion of these genes reduces the virulence of the virus towards normal cells. For example, vaccinia replication relies on the activation of the EFGR – Ras signaling pathway; a vaccinia strain engineered with a defective EFGR-Ras signaling pathway will selectively replicate in most cancer cells, where EFGR-Ras pathway is activated, but not in normal cells. Once established within a tumor cell, poxviruses induce cell-death in multiple ways. First, poxviruses kill cancer cells directly by infection, leading to cell lysis and death (both apoptosis and necrosis). Poxvirus infection also elicits immune responses, which induces an adaptive immune response to the tumor. Finally, vaccinia infection of tumor-associated endothelial cells results in vascular collapse (Kim, 2009).
There are many transgenes that can be inserted into engineered oncolytic vaccinia. These transgenes must be carefully selected, because a transgene secreted during cell death could elicit bystander effects in surrounding non-infected cells. In addition, it is necessary to ensure that the gene products do not have direct antiviral effects and do not clear the viral vector before it destroys the tumor. Transgene products include cytokines and other factors that enhance immunogenicity, and agents that improve viral spread within the tumor by disrupting the extracellular matrix.
Transgenes can also enhance the “stealth” of poxviruses to evade premature removal by the host immune system by coating the virus with cationic liposomes or polymers, or by increasing production of the EEV form, which is capable of evading complement and antibodies due to its additional envelope. Apart from oncolytic properties, transgenes can also be used for imaging purposes. For example, expression of genes such as luciferase or those encoding fluorescent proteins can be used to determine tumor sites and size. Several promising poxvirus oncolytic vectors are currently undergoing clinical trials. JX-594, the first strain of vaccinia to be used clinically transgenically expresses granulocyte macrophage colony stimulating factor (GM-CSF) to enhance tumor cell death via immune-stimulation. However, several hurdles remain to oncolytic viral application, including balancing benefits and drawbacks of the host immune response and possible transmission through viral shedding to the environment.
I was drawn to this article because of Khang’s recent presentation on viral vector therapy. I like this research because, unlike other research on smallpox, it seems like something that we can actually use. Vaccine and antiviral research seems to me a bit unproductive; why not redirect that energy and resources to research on a pathogen that actually still poses a real threat? Additionally, I’m not sure how much of the bioterror threat is real, and it seems that preservation of smallpox strains in the name of research poses a big bioterror threat itself (who is to say that the US or Russia are not engaging in bioweapons research?). It also gives me a greater appreciation for bioengineering for their smart ability to manipulate nature to our needs. For example, I never realized how similar cancer cells and viral particles are in function. Oncolytic therapy seems to me like a smart way to take advantage of smallpox virulence and apply it in a therapeutic way, shedding some good on this historical scourge of humanity.
Kirn & Thorne (2009) Targeted and armed oncolytic poxvirus: a novel multi-mechanistic therapeutic class for cancer. Nature Reviews Cancer, 9:64-71.