Extinct horsepox virus recreated in lab in bid to build a better smallpox vaccine

David Evans and Ryan Noyce have created a new synthetic virus they hope could lead to the development of a more effective vaccine against smallpox. Photo: Melissa Fabrizio
David Evans and Ryan Noyce have created a new synthetic virus they hope could lead to the development of a more effective vaccine against smallpox. Photo: Melissa Fabrizio

Researchers who stitched together the infectious horsepox virus, which is believed to be extinct in the wild, say it could lead to the development of a more effective vaccine against smallpox.

A potentially better vaccine in humans would be welcomed, given the toxicity of current smallpox vaccines, but the recreation of an extinct infectious virus is likely to be seen as controversial.

The researchers from the University of Alberta in Canada say their discovery demonstrates how techniques based on the use of synthetic DNA can be used to advance public health measures.

Their findings are reported today in the open-access peer-reviewed journal PLOS ONE.

Virologist David Evans and his research associate, Ryan Noyce, synthetically reconstructed the horsepox virus using the genome sequence published in 2006. They used DNA fragments made entirely by chemical methods.

The team went on to show that the synthetic horsepox virus could provide vaccine protection in a mouse model of poxvirus infection, which may be indicative of vaccine protection in humans.

“This application of synthetic DNA technology has the potential to revolutionize how we manufacture complex biologicals, including recombinant viruses,” says Evans, a professor of microbiology and member of the Li Ka-Shing Institute of Virology.

“These methods advance the capacity to produce next-generation vaccines and offer special promise as a tool for constructing the complicated synthetic viruses that will likely be needed to treat cancer.”

It is the largest virus assembled to date using chemically synthesized DNA.

Horsepox is not a hazard to humans. It is closely related to vaccinia virus, the virus that was used as a vaccine to eradicate human smallpox 40 years ago.

While there have been no cases of naturally occurring smallpox since 1977, it remains a concern to public health agencies.

Tonix Pharmaceuticals is developing the synthetic version of horsepox as a potential vaccine to prevent smallpox (variola virus) infection in humans. The company’s president and chief executive, Seth Lederman, is co-investigator on the research and co-inventor on the designated TNX-801 patent.

He says his company’s goal is to develop a vaccine that has a better safety profile than the current vaccines for broader usage and to provide greater protection to the public.

Current smallpox vaccines are used to protect first-responders and military service members but are rarely used except in special circumstances.

Because of the toxicity of most modern smallpox vaccines, Canada and the United States have long discontinued immunizing whole populations, as was the policy before smallpox eradication.

A transmission electron micrograph of a smallpox virus particle. Photo: Dr Graham Beards at en.wikipedia CC BY-SA 4.0 via Wikimedia Commons

The University of Alberta’s Evans hopes the research will add to informed discussions about the potential applications of synthetic biology for the benefit of society.

His research team had previously used more traditional recombinant DNA technologies to engineer a vaccinia virus with the aim of improving the treatment for bladder cancer.

The virus is an oncolytic virus, which means it was modified to selectively kill rapidly dividing cancer cells while remaining safe for surrounding healthy cells. In pre-clinical models these viruses can infect and kill cancer cells, while promoting the development of an immune response that is needed to prevent the cancer from returning.

However, future generations of oncolytic viruses will require a greater degree of modification than is possible using older technologies. Synthetic biology offers a powerful tool for manufacturing these more complicated biological therapeutics.

“We are invested as a research laboratory in taking that same technology and applying it to other poxviruses,” Evans said.

The work of assembling TNX-801 was funded by a research contract from Tonix Pharmaceuticals, but was made possible by a long history of grant funding from the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council.

The complete synthesis of horsepox virus by the team was first announced in March 2017.

“Evolutionary evidence is building that the vaccines which largely eradicated smallpox in the US and Europe in the 19th Century were closely related to horsepox,” explains Lederman, from Tonix.

Horsepox virus is believed to have become extinct through some natural process. No known horsepox outbreaks have been reported since 1976, at which time the US Department of Agriculture obtained the viral sample used for the sequence published in 2006 in the Journal of Virology – crucial recreating the virus.

Dr Edward Jenner
Dr Edward Jenner

In 1798, English physician and scientist Dr Edward Jenner speculated that a vaccine derived from cowpox could protect humans against smallpox.

Jenner had a strong suspicion that his vaccine, which he isolated from what he called cowpox in cows, began as a pox disease in horses. As a result of Jenner’s vaccine, smallpox was eradicated.

No cases of naturally occurring smallpox have been reported since 1977. A recent analysis of a 1902 US vaccine showed 99.7% similarity to horsepox in the core genome.

Modern vaccines were developed after smallpox had been largely eradicated from the US and Europe and before it was known that the 19th century vaccines were closely related to horsepox. Smallpox-preventing vaccines are maintained in the US Strategic National Stockpile.

Evans, Noyce and Lederman said in their PLOS ONE paper that their studies were provoked by their interest in the history of Jenner’s vaccine.

“Contemporary accounts provide support for Jenner’s speculation that the vaccine probably originated as an equine disease called ‘grease’.

Most notably, J.G. Loy, in 1801, showed that one of two forms of grease (or horsepox) provided protection against smallpox without requiring passage through cows.

“Modern sequencing supports this hypothesis with the one known horsepox virus strain most closely resembling both old French and early American smallpox vaccines, and sharing a common origin with all known smallpox virus.

“Most viruses could be assembled nowadays using reverse genetics,” they say, “and these methods have been combined with gene synthesis technologies to assemble poliovirus and other extinct pathogens like the 1918 influenza strain.

“Given that the sequence of variola [smallpox] virus has been known since 1993, our studies show that it is clearly accessible to current synthetic biology technology, with important implications for public health and biosecurity.

“Our hope is that this work will promote new and informed public health discussions relating to synthetic biology, stimulate new evaluation of horsepox-based vaccines, and advance the capacity to rapidly produce next-generation vaccines and poxvirus-based therapeutics.”

Noyce RS, Lederman S, Evans DH (2018) Construction of an infectious horsepox virus vaccine from chemically synthesized DNA fragments. PLoS ONE 13(1): e0188453. https://doi.org/10.1371/journal.pone.0188453

The study, published under a Creative Commons License, can be read here

Earlier report 


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