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Tomato, Tobacco Plants Produce SARS Vaccine

Stefan Lovgren
for National Geographic News
June 14, 2005
 
Scientists have genetically engineered tomato and tobacco plants to produce a vaccine against the virus that causes severe acute respiratory syndrome, or SARS, the disease that killed nearly 800 people in 2003.

Meanwhile, another team of researchers has developed a vaccine that protects monkeys against the deadly Ebola and Marburg viruses, which have plagued central Africa. An ongoing Marburg outbreak in Angola has killed at least 357 people.

While both vaccines are far from ready for human use, the studies raise the possibility of producing economical vaccines for diseases for which there is no known cure.

"There was a need to prepare quickly a vaccine that is inexpensive and safe," said Hilary Koprowski, who is the director of the Center for Neurovirology at Jefferson Medical College in Philadelphia, Pennsylvania.

Koprowski, who produced the first oral polio vaccine and developed the rabies vaccine, is the lead researcher on the SARS vaccine study, which is published today in the Proceedings of the National Academy of Sciences, a science journal.

The Ebola and Marburg research was published last week in the academic journal Nature Medicine.

Modifying Tobacco

One reason it is difficult to develop vaccines for diseases like SARS and Ebola is because some viruses are believed to mutate constantly. If a virus changes quickly, a vaccine might be suitable for a while but not forever.

The SARS, Ebola, and Marburg viruses all have RNA genomes, notes Doris Bucher, an associate professor in the Department of Microbiology and Immunology at New York Medical College in Valhalla.

RNA is chemical similar in structure to DNA that plays an important role in the chemical activities of cells.

"RNA viruses are subject to a great deal of antigenic variation"—changes that evade the defense mechanisms of host species—"due to the high mutability of the RNA genome as a result of errors in transcription [reproduction] of the genome," Bucher said.

"Two viruses with RNA genomes, influenza virus and HIV, are especially notorious for their antigenic variation … [which] requires the re-formulation of the influenza vaccine on a near annual basis," she added. Bucher notes that the development of an HIV vaccine has been similarly challenged.

Genetic sequencing of the SARS virus reveals a pathogen that began its life in an animal, then mutated before it picked up the power to infect people.

Health officials have a strong need for vaccines that are both economical and easily administered. Plants have long been seen as an attractive source for vaccines, since plants can be grown inexpensively on a large scale.

The easiest plant to genetically modify is tobacco. For the SARS study, Koprowski and his colleagues engineered low-nicotine plants to produce a so-called spike protein of the SARS virus. The protein is used to trigger an immune response in the human body.

Once the researchers showed that tobacco could produce the SARS virus spike protein, they applied the same technique to tomatoes. Mice that were fed genetically modified tomatoes developed antibodies to the virus that causes SARS.

However, the scientists did not report whether they injected the mice with the SARS virus.

"It does not show that the antibody offers any protection," said Michael Lai, a virologist at the University of Southern California in Los Angeles.

Don't expect to see "vaccine vegetables" anytime soon. Researchers say it would be impossible to standardize the amount of vaccine in a vegetable. In fact no vaccine produced in plants has ever been licensed for human use.

However, the scientists envision that the SARS vaccine could be ground into a powder, formed into capsules, and consumed as a dietary supplement.

Battling Ebola

Another team of scientists, meanwhile, is claiming a breakthrough in the battle against Ebola and Marburg, two related viruses that have struck central Africa several times in recent years.

Ebola and Marburg are hemorrhagic fevers, meaning they cause major internal and external bleeding. They kill 50 to 90 percent of their victims within a few days. There is no cure for either virus.

Scientists based the new vaccines on a virus called vesicular stomatitis virus (VSV). The virus was "rewired" to carry a gene that encoded a protein from either the Ebola or Marburg virus.

When monkeys were inoculated with the vaccines, the primates' immune systems mounted a response against either the Ebola or Marburg virus.

With their immune systems accordingly "primed," the monkeys were able to fend off authentic doses of Ebola or Marburg virus that they were later exposed to.

"This is the first vaccine system that has been shown to protect monkeys against both Ebola and Marburg viruses," said Tom Geisbert of the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland.

Geisbert, who was one of the lead researchers of the study, says he expects the vaccine to work in people because monkeys are even more sensitive to the viruses than humans are.

"The fact that we could completely protect monkeys against such a high-challenge dose given by a very lethal route is very encouraging," Geisbert said. "[It] bodes well for future use in humans."

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