An immune system gene that flunked its first tryout as an antiviral factor has triumphed in its second, proving that it can help fight the flu, herpes and the Sindbis virus.
Picking the right opponents for interferon stimulated gene 15 (ISG15) to square off against proved to be key to uncovering its potential. Earlier tests found it provided no help in the battle against a pair of other viruses, but scientists at Washington University School of Medicine in St. Louis selected new viral opponents and found the gene significantly enhanced mouse survival after infection.
Among the viruses that ISG15 helps fight, influenza type A and type B cause the flu. Type A has been linked to serious flu pandemics, while type B is generally milder. When active, herpes simplex virus type 1 causes relatively mild symptoms including blisters and cold sores; in patients with weakened immune systems, though, it can cause more severe disease. Sindbis virus is prevalent in Africa and Asia, where it is spread by mosquitoes and typically causes brief colds, fevers and muscle aches.
The results are published this week in the online edition of Proceedings of the National Academy of Sciences.
"We're very curious about what ISG15 is doing and what it can teach us about fighting viruses," says first author Deborah J. Lenschow, M.D., Ph.D., assistant professor of medicine. "Our labs have identified over 200 proteins that ISG15 targets and binds to."
That binding labels ISG15's target proteins, but researchers don't yet know the effect of the labeling. It could change the enzymatic activity of the target protein the ISG15 protein attaches to, or it could make the target protein more likely to end up in a particular compartment within the cell, potentially increasing the chances that it will be degraded.
Scientists originally identified ISG15 in the 1980s as one of several genes that respond to interferons, a family of immune system proteins that inhibit viral replication. It came to researchers' attention again in the 1990s, when experiments with new gene chip technology showed that ISG15 became more active in cells after infection with pathogens or exposure to interferon.
This suggested the gene might play a role in defense against viral invaders. However, when scientists tested whether ISG15 could help fight off infections of two viruses commonly used in research, vesicular stomatitis and lymphocytic choriomeningitis, it failed.
Recent research in the laboratory of senior author Skip Virgin, M.D., Ph.D., the Edward Mallinckrodt Professor and head of Pathology and Immunology, renewed suspicion that ISG15 could be an antiviral factor.
"We showed that when the gene was transplanted into Sindbis virus and overexpressed, it helped mice deficient in the receptor for interferon to resist infection by the virus," Lenschow explains. "This led us to think that ISG15's first tryout as an antiviral just hadn't tested it against the right opponents."
Researchers retested ISG15's abilities by exposing mice lacking the ISG15 gene to different viruses. Infections with influenza type A and B, herpes simplex and Sindbis virus caused greater mortality in mice lacking the ISG15 gene than in controls. For example, influenza type A killed 70 percent of mice lacking the gene but only 23 percent of mice that had the gene.
ISG15's selective usefulness against viruses may be the result of a tightly specialized approach to fighting infection or of adaptations that allow some viruses to avoid ISG15's effects.
The viruses beaten back in the new study have relatively little in common. Such links could help researchers better understand how ISG15 works, but so far scientists know of only one commonality: infection by any of the four viruses causes the body to make more interferon and to turn on the genes regulated by interferon.
In addition to binding target proteins, prior research has suggested that ISG15 also may be acting like a cytokine, a type of signaling protein mainly recognized for its role in recruiting immune system cells to the sites of inflammatory reactions.
"As we work to learn what happens to proteins tagged by ISG15, one issue we're going to be interested in is whether the protein is only active in antiviral activities or whether it contributes to a multitude of functions," Lenschow says.
Lenschow DJ, Lai C, Frias-Staheli N, Giannakopoulos NV, Lutz A, Wolff T, Osiak A, Levine B, Schmidt RE, García-Sastre A, Leib DA, Pekosz A, Knobeloch K-P, Horak I, Virgin HW IV. ISG 15 functions as a critical antiviral molecule against influenza, herpes and Sindbis viruses. Proceedings of the National Academy of Sciences, early online edition.
Funding from the National Institutes of Health supported this research.
Washington University School of Medicine's full-time and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked fourth in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.