Researchers showed that the mouse norovirus MNV-1 could be grown inside cells from mice with defective immune systems. Their findings make it much easier to learn about the mouse virus and may help other researchers seeking to duplicate the accomplishment with human forms of the virus.
In a study published this week in the online journal Public Library of Science-Biology, scientists who developed the new technique report it may already have led them to a good target for vaccine development.
"By looking at the mouse virus we'd grown in the lab, we were able to identify a part of the capsid, the virus' protein shell, that is essential to its ability to cause disease," says senior author Skip Virgin, M.D., Ph.D., professor of pathology and immunology and of molecular microbiology. "If this part of the capsid has an equivalent in human noroviruses, altering or disabling it may give us a way to produce forms of the viruses that are weak enough to serve as vaccines."
According to the U.S. Centers for Disease Control and Prevention, noroviruses are involved in about half of all food poisoning cases and annually cause about 23 million cases of acute gastroenteritis in the United States.
Norovirus disease is characterized by frequent vomiting and diarrhea over the course of one to two days. The most infamous norovirus, the Norwalk virus, was first identified after a 1968 outbreak at a school in Norwalk, Ohio. The Norwalk virus also caused a series of repeated outbreaks on cruise ships in 2002 and in military personnel in Afghanistan.
Although such infections rarely lead to serious or life-threatening illness in the United States and other Western countries, they spread rapidly, are difficult to prevent from spreading and can create considerable discomfort. Dehydration from the diarrhea and vomiting induced by the virus sometimes leads to hospitalization in the elderly, the young or those with weakened immune systems. In the developing world, these viruses are a major cause of human illness.
All previous attempts to culture human noroviruses in tissues in the laboratory have been unsuccessful.
"As a group, noroviruses have defied characterization for decades because there just hasn't been a way to get the virus to grow outside of a human host," Virgin says.
In 2003, Christianne Wobus, Ph.D., and Stephanie Karst, Ph.D., two postdoctoral fellows in Virgin's lab, identified MNV-1, the first known mouse norovirus. Virgin's group showed that the mice's ability to fight MNV-1 relied heavily on the innate immune system, the branch of the immune system that attacks invaders soon after they enter the body.
In the new paper, Virgin's group reveals that MNV-1 likes to infect cells of the innate immune system. In tests in mice, the researchers found the virus thrived in macrophages, immune system cells that normally engulf and destroy pathogens, and in dendritic cells, sentry-like cells that pick up and display proteins from pathogens.
"We think there may be dendritic cells just beneath the lining of the human gut that are providing the gateway the virus needs to cause disease," Virgin says.
To grow the virus in the lab, researchers took dendritic cells and macrophages from mice with defective innate immune systems and exposed them to the virus.
"The virus grew beautifully," Virgin says. "It's a very facile and robust system."
Comparisons of MNV-1 and human noroviruses have revealed many similarities in gene sequence, structure and overall arrangement of the genome. But Virgin acknowledges that differences between mouse and human physiology may significantly alter MNV-1's interactions with its host. For example, mice do not appear to be able to vomit. Additionally, researchers aren't sure yet whether MNV-1 can make mice with normal immune systems sick.
"The bottom line is that this mouse model provides us with a very useful way to examine certain similar aspects of the noroviruses," Virgin says. "Among other things, we'll be using it to look at how the capsid protein enables infection, viral replication processes and the receptors on host cells that enable the virus to infect specific cell types."
Wobus CE, Karst SM, Thackray L, Chang K-O, Sosnovtsev SV, Beliot G, Krug A, Mackenzie JM, Green KY, Virgin HW IV. Replication of a norovirus in cell culture reveals a tropism for dendritic cells and macrophages. Public Library of Science Biology, Nov. 30, 2004.
Funding from the National Institute of Allergy and Infectious Diseases.
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