MONDAY, March 15, 2004 (HealthDayNews) -- Laboratory mice can produce antibodies that will fend off an attack by the SARS virus, a new study says.
This means researchers developing vaccines that work by triggering antibodies, which is the simplest and most straightforward approach, are on the right track, scientists say.
"The mouse model is a first step, so it would be an easy model in which to test candidate vaccines," says Dr. Kanta Subbarao, a senior investigator in the Laboratory of Infectious Diseases at the National Institute of Allergy and Infectious Diseases. "The usual path for any vaccine development is to test candidate vaccines in one or two animal models before going on."
Subbarao is lead author of the study describing the new findings, which appear in the April 1 issue of the Journal of Virology.
The researchers first infected mice with the SARS virus and discovered that while the virus did not make the mice sick, it did infect the cells lining the airways and lungs and reproduced itself.
When these mice were given a second helping of SARS 28 days later, the mice produced antibodies so the virus was not able, this time around, to replicate itself.
Finally, the researchers took the antibodies produced by the mice and injected them into another group of mice that had not yet been infected with SARS. When these inoculated mice were exposed to SARS, the virus was unable to replicate itself.
In other words, the antibodies produced by the first group of mice were enough to ward off infection in the second group.
This may also mean that humans who had SARS once will be protected from getting it again.
"It seems more than likely that [previously infected] people will not get SARS," says Robert Garry, a professor of microbiology and immunology at Tulane University School of Medicine in New Orleans.
Adds Subbarao: "You can't extrapolate directly but... we did an experiment to see whether there was any hint that people might also be protected, and we got the hint. It doesn't necessarily mean people are protected, but it's a move in the right direction. We would be more discouraged if we couldn't protect the mouse."
Vaccines can work by stimulating the immune system to produce antibodies -- usually by injecting a dead virus, specialized cells or both -- to stop invading viruses.
"It is encouraging that a protective immune response was generated in the mice and that passive antibodies were protective. Antibodies aren't always protective, for example, in AIDS," says Garry.
"It is also easier to generate an antibody response by simply injecting protein than it is to produce a cellular response," he adds. "This doesn't mean that an effective SARS vaccine is right around the corner, but assuming these results translate somewhat to humans, the prospects seem a bit more likely."
Garry was part of the team to identify the structure of the SARS virus.
The new findings should also help in the development of drugs to treat people who have already been infected with the virus, experts say.
SARS -- severe acute respiratory syndrome -- first surfaced in southern China in late 2002; by early 2003, it had spread to several countries in Asia, as well as Canada. According to the latest figures from the World Health Organization, 8,437 people were infected and 813 people died between Nov. 1, 2002, and July 31, 2003.
Although SARS has not yet reappeared this year, experts aren't sure what the future will hold. Will the virus reemerge seasonally like the flu, or will it will be reintroduced to the human population in another, as yet unknown, way? Whatever the case, experts have been working furiously to develop ways to prevent and treat it.
For such research, they need animal models to assess any potential prevention or treatment strategies.
And that's what makes the new study significant, Garry says. "A small animal model can greatly facilitate the development of vaccines or drugs against any infectious disease," he says.
Mice represent a particularly attractive candidate for this type of work.
"A small animal model like mice are a lot less expensive than civet cats (believed to be a possible source of SARS) or chimpanzees. You can do a lot of experiments in a short period of time and at much reduced cost," Garry says.
Subbarao adds: "The laboratory mouse is a very useful model because it is small, easy to come by, and easy to do a large number of experiments. If your only model was a primate, you would be really limited in what you can do."
There are limitations to mice model, however, including the fact that the rodents didn't actually get sick from SARS, even though the virus replicated itself in their respiratory tract, Subbarao says. Nevertheless, "it's very useful for vaccine purposes. You can at least screen vaccine possibilities and take them into more expensive models," she says.
More information
Visit the National Institute of Allergy and Infectious Diseases or the U.S. Centers for Disease Control and Prevention for more on SARS.
