One of H1N1’s mysteries explained

Laura Sanders follows the twists and turns of the pandemic virus

Scientists now have solid evidence that may explain one of the oddities of last year’s pandemic H1N1 virus. Two new studies, one published in Science and the other in Science Translational Medicine, point out how the latest pandemic flu strain is eerily similar to viruses present before 1957, explaining why the current virus spares the elderly.

The results are interesting, but not jaw-dropping, which is why you’re reading about them here in the Deleted Scenes blog and not in the Science News’ news section. There have been strong hints that influenzas circulating before 1957 — the 1918 pandemic flu strain and its offspring — were similar to the current H1N1 pandemic.

The new studies seem to nicely confirm a hunch discussed in our previous coverage of H1N1: Older folks’ immune systems have been around the influenza block, so to speak, and encountered flu strains similar to the current pandemic. Since their bodies had been exposed to those viruses, the reasoning went, their immune systems remembered how to fight them off. In contrast, young people’s naïve immune systems were caught unawares by this recent H1N1. 

Although the 1918 influenza and the current pandemic swept the globe over 90 years apart, they possess key similarities, the new studies show. In the Science Translational Medicine paper, which appeared March 24, a team of researchers from the National Institutes of Health in Bethesda, Md., and the Centers for Disease Control and Prevention in Atlanta show that antibodies in mice that had been vaccinated against the 1918 pandemic could also dispatch the 2009 pandemic virus, and mice vaccinated against the current pandemic flu were impervious to the 1918 virus. This coprotection appears to be why elderly people aren’t as easily infected with the current pandemic flu.

The new studies go further and pinpoint similarities between a key protein of the H1N1 virus and one of the 1918 pandemic strain’s. The protein in question is hemagglutinin, which decorates the outside of the viral particle — making it a big target for antibodies and vaccines. (The version of hemagglutinin also gives viruses part of their names — both the current pandemic and the 1918 pandemic carry H1 subtypes of the protein.)

Unlike most seasonal flus, parts of the hemagglutinin proteins in both strains lack the ability to get decorated with a type of sugar molecule called a glycan, the team found. This lack of glycosylation allows the virus to infect a target cell.

The other recent study, appearing March 25 in Science, resolved very detailed structures of the hemagglutinin proteins from the current pandemic and the 1918 virus. Scientists from The Scripps Research Institute in La Jolla, Calif., Vanderbilt University in Nashville and Mount Sinai School of Medicine in New York compared the shape of the two proteins and concluded that hemagglutinin from the 1918 virus is “a remarkably close relative” of the current virus. That team also detected that both viruses lack glycosylation sites.

As time goes on, the H1N1 virus is fading. CDC reports that right now, its activity is relatively low. But studies that help illuminate H1N1’s quirks, like these, will give scientists a better idea about how to handle the virus, which they say is likely resurface next flu season.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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