Escaping the Norwalk Virus
This past May, American soldiers encamped in northern Iraq fell victim to an invisible enemy: a norovirus, common cause of the so-called stomach flu. Within weeks, as many as 2,500 soldiers, mostly from the 101st Airborne Division, developed symptoms—diarrhea, vomiting, nausea, headaches, and weakness—that ranged from mild to devastating. The virus probably spread on the spigots of jury-rigged faucets. But many soldiers who used those same faucets remained healthy, despite regular exposure to the highly contagious norovirus.
Such widely variable responses, typical of norovirus epidemics, have long puzzled scientists. Outbreaks tend to occur in close quarters, such as cruise ships, nursing homes, summer camps, or military encampments. As with the outbreak in northern Iraq, a sizable proportion of exposed individuals usually remains unaffected. Now, scientists have discovered possible clues to this viral resistance. For many individuals, the secret to protection appears to lie in a gene known as FUT2.
In a study conducted at the NCRR-supported General Clinical Research Center (GCRC) at the University of North Carolina at Chapel Hill, Dr. Christine L. Moe and her colleagues found that more than half of 77 volunteers exposed to Norwalk virus, a type of norovirus, were resistant to infection. About half of these protected individuals had FUT2 mutations that blocked production of H type-1, a carbohydrate and blood group antigen found on many cell surfaces. All volunteers who had two copies of the nonfunctional FUT2 gene remained healthy, even after receiving high doses of the virus. The findings suggest that H type-1 is the cellular receptor that binds Norwalk virus and allows it to enter cells. The study also jibes with earlier clinical studies that identified possible links between blood group antigens and norovirus susceptibility.
Photo: In clinical trials, Dr. Christine Moe (left) and her colleagues observed that some volunteers had a quick immune response when exposed to Norwalk virus and remained uninfected, perhaps because of past exposure to a similar virus. This immunoprotection may hold promise for vaccine development. (Photo by Jack Kearse, Emory Health Sciences Photography)
The North Carolina research also confirmed what had long been suspected based on anecdotal evidence: that the virus is infectious at incredibly low levels. “For those who were susceptible, the amount of virus necessary to develop infection was down to the limits of detection,” says Dr. Moe, formerly at the University of North Carolina and now associate professor of international health at Emory University in Atlanta.
Knowledge of the mechanisms that reduce vulnerability to viruses could provide important clues for treating and preventing communicable diseases like norovirus infections, which cause an estimated 23 million cases of gastrointestinal illness in the United States each year. In developing countries, nearly 100 percent of children have contracted the viruses and developed antibodies to them by age five. Most often, noroviruses are spread through contaminated water and food.
Dr. Moe has been investigating noroviruses and other infectious agents for more than a decade. For the past five years, her clinical studies often have depended on the specialized research staff and infrastructure of the GCRC, which offers an ideal environment for analyzing the clinical aspects of viral exposure while ensuring the safety of volunteers. “It is incredibly important to be able to do these studies under safe and controlled conditions, like those available at the GCRCs,” Dr. Moe says.
In preparing for the norovirus study, the GCRC staff recruited and carefully screened all volunteers to ensure that their exposure to the virus would not injure them, their families, or community members. When the study began, each of the 77 volunteers, men and women aged 18 to 50, was given a dose of virus mixed with water. Volunteers spent the next five days in the GCRC, receiving round-the-clock care. GCRC staff collected saliva, blood, and stool samples that were used for genetic screening and for tracking immune response and viral load. Volunteers returned at days 8, 14, and 21 for follow-up evaluations.
As expected, the researchers observed that a substantial number of volunteers—56 percent—did not become infected. To determine possible causes, Dr. Moe’s team collaborated with scientists at the Institute of Biology in Nantes, France. In earlier studies, the French scientists had evaluated norovirus infection in rabbit cells and shown that blood group antigens similar to H type-1 are required for viral docking, and possibly entry, to the cells. Previous clinical studies also have identified a link between red blood cell antigens and Norwalk virus infection.
The French scientists screened the volunteers’ DNA for the FUT2 gene. Normal versions of the gene produce the enzyme a-(1,2)fucosyltransferase, needed to generate the H type-1 molecule. The researchers found that all individuals with deactivated versions of FUT2 remained healthy even after high-dose exposure to Norwalk virus. “H type-1 is the receptor that provides a door into the cell,” comments lead author Lisa Lindesmith, a laboratory research specialist at the University of North Carolina’s School of Public Health. “Without the receptor, the virus cannot gain access to the cell, regardless of how much virus gets into the gastrointestinal tract.” The malfunctioning FUT2 gene appears to provide genetic resistance to Norwalk virus, Lindesmith notes. “Because of this study, FUT2 has become one of only a few human genes with a known link to viral resistance,” she says.
But genes are only part of the story. Dr. Moe and her colleagues also discovered that some volunteers with normal, nonprotective versions of the FUT2 gene remained healthy despite exposure to high doses of the virus. Studies led by Dr. Ralph Baric, professor of epidemiology at the University of North Carolina’s School of Public Health, showed that these men and women mount an impressive mucosal immune response that peaks between the second and third days, perhaps because they had been exposed to a similar virus in the past. In contrast, volunteers who became sick did not produce mucosal antibodies to the virus until a week or more after viral challenge.
Such rapid development of immunoprotection is good news, says Dr. Moe, because it suggests that vaccination may one day protect against the virus. Since Norwalk virus accounts for only about five percent of norovirus outbreaks each year, finding signs of acquired immunity suggests either that people can develop long-term immunity to Norwalk virus, or that infection with a related norovirus confers protective immunity for more than one viral strain.
Many questions remain. For instance, not all of the norovirus variants enter a cell through the same molecular door. The H type-1 molecule, generated via the FUT2 gene, has been implicated only in Norwalk virus infection, Dr. Moe notes. “We need to discover which receptors serve as cellular doorways for other noroviruses,” she says. “We also must identify the factors that allow individuals to acquire and maintain immunity. Which viral strains must they be exposed to, and at what frequency, to gain and maintain protection?”
Dr. Moe, in collaboration with Lindesmith and Dr. Baric, plans to conduct additional norovirus studies at the Emory GCRC to further test the hypothesis that acquired immunity protects some people from infection. It is possible that a quick antibody response could signify reactivation of an earlier, so-called memory immune response. Additional evidence of acquired immunity would come if volunteers who got sick after a single viral challenge were later able to remain uninfected after a second challenge. If such studies ultimately lend support to the effectiveness of acquired immunity, Dr. Moe says, she and her colleagues are prepared to start designing a vaccine.
—Bernice Wuethrich
This research is supported by the Division of Clinical Research of the National Center for Research Resources and by the National Institute of Allergy and Infectious Diseases, the National Institute of General Medical Sciences, and the U.S. Environmental Protection Agency.
For more information, visit the NCRR Division of Clinical Research page.
Additional Reading
Lindesmith, L., Moe, C., Marionneau, S., et al., Human susceptibility and resistance to Norwalk virus infection. Nature Medicine 9:548-553, 2003.
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Originally Posted by ann32
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