Science Editor

Late last year, and on into 2003, fleets of vacation cruise ships saw legions of vacationers slink or stagger to their cabins. They were not seasick but virus-smitten. What laid them low was the flu-like "winter vomiting disease."

Throwing up was only a fraction of their misery, compounded by diarrhea and severe gastrointestinal upset. Victims wondered feebly what terrorist group was responsible for the mysterious floating pandemic.

"Certainly the cruise ships were more affected than in other years," observed viral immunologist Herbert Virgin. "Then troops in Afghanistan," he added, "as well as hospitals, nursing homes, the elderly and disabled, wherever people lived in close, cramped quarters. "CDC - the Centers for Disease Control and Prevention," Virgin continued, "estimates at more than 20 million cases a year of the Norwalk-like virus infection, with 50,000 hospital admissions and 300 deaths per year - in the U.S. alone."

Virgin, a professor of pathology and molecular microbiology at Washington University in St. Louis, is senior author of an article in the current Science, dated March 7, 2003. Its title: "STAT-1-dependent innate immunity to a Norwalk-like virus."

"Our principal finding," Virgin told BioWorld Today, "is using a newly discovered mouse virus that is closely related to an important class of Norwalk-like human viruses. We also determined that for these viruses, STAT-1-dependent innate immunity is critically important to infection."

More than a year ago, Virgin and his co-authors discovered the hitherto-unknown virus in a strain of immune-deficient mice they were rearing for other research. "When five of six mice died in one cage," he recounted, "we filtered tissue from the dead mice and injected it into healthy animals, some of which had normal immunity while others were immune-deficient.

"We found a group of immunocompromised mice," Virgin went on, "lacking both the RAG gene and STAT-1 gene, that died spontaneously. We then tried to passage the tissues from those mice to additional ones, and discovered a passable interferon-sensitive filterable agent, which was responsible for the death of that initial cageful of RAG-STAT-deficient animals."

Fast-Acting Innate Immunity, Mightier Adaptive

"The original mice had been engineered to lack the two proteins expressed by those two genes," Virgin explained. "They were RAG [for recombination activating gene] and STAT-1 [for signal transducer and activator of transcription 1]. Animals that lack the RAG," he went on, "do not develop T cells, B cells and antibodies. Therefore, they cannot recognize specific proteins, or antigens, found on viruses, bacteria and parasites. That is, these animals lack adaptive immunity, which begins fighting viruses within five to seven days of infection.

"Mice without STAT-1 lack the other half of the immune system - innate immunity. This kicks in immediately to fight infection, in part by the effects of antiviral interferon. Mice deficient in both RAG and STAT-1 have no adaptive and very limited innate immunity."

"The first one defines the Norwalk viruses, with the innate immune system a potential target for antivirals or other possible therapies for this important class of viruses. The second is that the Norwalk-like viruses - which cause more than 90 percent of epidemic nonbacterial gastroenteritis in the world - are very important pathogens. There hasn't been a good model for studying their pathogenesis and immunity. This finding provides us the opportunity to do that.

"In addition, I think it's important that we found this virus using immunocompromised mice. The fact that we were successful at identifying a new agent in such animals raises the possibility that there are yet undiscovered additional pathogens in these rodents, which may be an important source in future model systems for analyzing human diseases.

"We noted that some of the caged mice died on their own, without our inoculating them with anything. That led to our experiments to identify the new murine virus. Once we had it, we knew that mice lacking both RAG and STAT-l, when infected, died. So we took the individual RAG-1 or RAG-2 and STAT-1 mutations, and asked: Since the combination created an animal that was highly susceptible to infection, did either gene individually result in that deadly immune deficiency?'

"The data we obtained from this experiment was that the new [Murine norovirus] doesn't kill RAG-deficient animals, or induce lethal infection in a RAG-deficient mouse. So one doesn't need T and B cells to prevent lethal infection with this virus."

Interesting Parallel Between Mouse, Human Disease

"Mice lacking STAT-1, whether or not they have any immunity, were highly susceptible to lethal infection. So the combination of those two things is the fact that the virus does not cause lethal infection in RAG-deficient animals, and therefore adaptive immunity is not required for resistance. But if the virus does cause lethal infection in a STAT-1-deficient animal - and therefore STAT-1 plays an important role in resistance - allowed us to draw the conclusion that STAT-1-dependent innate immunity is fundamentally vital for control of this virus.

"That has an interesting parallel in human disease, which has a very short incubation period of about 24 hours. After 24 hours people have severe nausea, vomiting and diarrhea. But generally the infection is over within 48 hours of the symptoms initiating. The symptomatic disease itself is very short-lived. That time period in humans, which is basically three days, maybe four at the outside, is considered to be too brief a period for the induction of innate immunity.

"In this way, the mouse is telling us something that is directly related to the human disease. In that the human disease appears to be cleared too rapidly for the generation of typical adaptive immunity - B-cell immunity. So maybe the humans, like mice, depend on the innate response in a critical way for resistance to this set of infections.

"In other ongoing work," Virgin observed, "we will have a new virus model, which presents us with a number of opportunities to answer critical questions in the field. But the Norwalk virus itself is limited to experiments in humans. And that is serial passage of the virus through filtrates of emesis or diarrheal stool. That's the only source of the virus. Of course it doesn't grow in small animals. So now we have a small animal model where we can begin to address some important obvious questions."