In order to increase production and lower the costs of making anthrax vaccines, a research team grew a protein from anthrax bacteria in tobacco leaves.
Researchers at the University of Central Florida are betting that transplanting anthrax genes into tobacco plants can cheaply produce enormous amounts of a vaccine against the deadly bacteria.
Dr. Henry Daniell, a professor of molecular biology and microbiology at the University of Central Florida, identified three problems with how anthrax vaccine is currently produced: cost, capacity, and contamination. The new production solves all three, he says.
Traditional vaccine production methods rely on fermenting machines to grow large quantities of anthrax bacteria. These are very expensive, Daniell said. The protein protective antigen (PA) is purified from fermented anthrax cell cultures.
PA enables anthrax to inject its toxin into cells once the spores have entered the body. The purified PA is used, along with an adjuvant, to vaccinate people against anthrax. Exposing the immune system to the protein allows recipients to develop antibodies against PA, which is harmless.
The size and number of fermenters limits the production of PA, Daniell said. Worse, the purification fails to remove trace amounts of anthrax toxin, Daniell said, which probably cause side effects and reactions such as pain, swelling, inflammation, and flu-like symptoms.
He and his colleagues inserted a clone of the gene for PA into tobacco plants. They purified the protein from the leaves and injected it into mice, along with the alhydrogel adjuvant.
“What we have done is improve the method of production,” Daniell said.
The mice were tested for the presence of active antibodies against PA and then sent to the National Institute of Health. The NIH challenged their immunity with an exposure to anthrax toxin, which all of the mice survived. The mice vaccinated with plant-derived PA had the same ability to survive this exposure as mice injected with PA harvested from anthrax bacteria.
Daniell claims his method would yield an average of 150 milligrams of PA per plant. So one acre of tobacco could vaccinate 360 million people, he said.
Why this large capacity? Daniell inserts the PA gene into DNA in the plant’s chloroplasts, rather than the nucleus. Chloroplasts, the cellular organelles where photosynthesis takes place, have their own DNA.
Because there are hundreds of chloroplasts in every cell, inserting a gene into this DNA can produce a great deal of protein, explained Kent Bradford, director of the Seed Biotechnology Center at University of California Davis. In addition, Bradford said, the chloroplast DNA does not leave the plant – as nuclear DNA does via pollen and seeds – so there is no worry about contaminating other plants in the wild.
“Tobacco is probably a good target for this because there are not many relatives around,” he added. This offers an extra layer of protection against contamination.
He said using chloroplast DNA is a good strategy for producing pharmaceuticals in plants. The technique has one major drawback: protein must be harvested immediately after the leaves are picked, since it cannot remain viable in dead leaves. When genes are inserted into nuclear DNA, dried seeds can store the protein for long periods.
The next step, according to Daniell, is to demonstrate that PA derived from plants is not harmful to humans and is equivalent to the traditional vaccine. The end product should be no different, he said. Only the production method has changed.
Daniell and his colleagues are also working on inserting PA into carrot cells, which he believes could be dried and administered orally in pill form, and on producing other drugs, including insulin, in plants.
“I believe it’s going to significantly change the way medicine is delivered,” he said.