Tobacco plants have been enlisted as bioreactors for making what scientists hope will become the first vaccine for the norovirus, the cause of many food-poisoning outbreaks and the dreaded winter vomiting disease.
Using a production system that combines viruses, bacteria and plants to produce large quantities of vaccine rapidly, researchers at Arizona State University and biotech company Vaxx say they have a fast, cheap and reliable way to fight fast-changing pathogens like the norovirus.
With an estimated 23 to 74 million yearly cases in the U.S. alone, the norovirus is thought to be the second most common cause of infectious disease after the flu. Furthermore, like the flu, it has the ability to produce new strains every year or two, which has made it difficult to develop a vaccine.
Going through a lengthy and costly vaccine development process for a nonlethal virus that changes quickly presents a high financial risk for companies, says Charles Arntzen, one of the lead researchers on the project, who presented the work at this year’s fall meeting of the American Chemical Society in Washington, D.C.
“With the norovirus, a government mandate to vaccinate is unlikely, because it is inconvenient rather than a major death threat for people, meaning there would be no guaranteed market,” he says.
The technology used by Arntzen and his colleagues exploits the fact that plants are cheap to grow and incur little regulation compared with animal-based vaccine development systems, since plant-borne pathogens are not dangerous to humans. The new approach also addresses a number of shortcomings that have held back plant-based vaccine development in the past. In particular, the length of time it takes to make a transgenic plant that expresses enough vaccine proteins and the difficulty of processing plants in such a way that a stable, precisely dosed vaccine can be obtained.
To overcome the first challenge, the researchers developed a three-step system that allows faster production of foreign proteins in plants. First, genes that encode the proteins are engineered into an inactivated version of a plant virus, in this case the tobacco mosaic virus (TMV). Since TMV uses RNA instead of DNA as its genetic material–but genetic manipulation in the lab is done on DNA–the second step is to trick bacteria that can infect tobacco into producing a DNA version of the virus along with the vaccine genes. In step three, RNA copies of the viral genes are produced inside the infected plant cells and hijack their biochemical machinery to make copies of the desired protein. “Within 5 days [to] 2 weeks, the tobacco plant dies, but as it does so it is producing massive amounts of our protein, up to one gram per kilo of biomass,” says Arntzen.
Mario Pezotti at Verona University in Italy says that Arntzen’s work “demonstrates that you can easily speed up the production systems for any vaccine and especially for viruses that change quickly.”
The second main challenge for making plant-based vaccines has been to develop a final product that meets pharmaceutical standards, says Carole Cramer, executive director of the Arkansas Biosciences Institute at Arkansas State University in Jonesboro. The aim of earlier research was to produce edible vaccines that would allow cheap and safe delivery in developing countries where facilities for storing purified protein jabs that need to be cooled are limited.
Although transgenic potatoes, tomatoes or rice can produce functioning vaccine molecules, including those that are effective against the norovirus, “not having completely controlled doses has been a challenge, especially in Western medicine,” says Cane. “Any viable product will have to be completely purified and precisely deliverable–for example, as a nose spray,” she says.
Tobacco’s high biomass production in combination with affordable extraction technologies adapted from the food industry makes Arntzen confident that the norovirus vaccine can be produced at very low costs. He anticipates that clinical trials will start in early 2010.
“The current concern about swine flu demonstrates that the world needs vaccines at high speed and low costs, and this is where plant-based products like this norovirus vaccine have high potential,” says Pezotti.
Yuri Gleba of Nomad Bioscience in Munich, Germany, who first developed the three-step plant infection system used for the norovirus project, has tested it on more than 50 pharmaceutically- relevant proteins. Gleba believes the approach could become an effective tool anywhere where fast-response production of proteins is required.
“When it comes to bioterrorism, for example, such a plant-based system could be used to cheaply grow compounds in anticipation of a problem, [saving] only the most expensive final processing steps for the case of an emergency.”
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