“Influenza viruses weren’t known to exist in 1918, so there were no isolates, and no way to understand what happened,” says Taubenberger, chief of the Viral Pathogenesis and Evolution Section of the National Institute of Allergy and Infectious Diseases’ Laboratory of Infectious Diseases.
Using archived autopsy tissue material from people who died from the pandemic, as well as samples from frozen bodies of pandemic victims interred in permafrost in Northern Alaska, Taubenberger made headlines a decade ago when he and his colleagues successfully extracted influenza RNA-positive tissue and sequenced the viral genome.
Work from his lab and other groups has shown that the 1918 H1 hemagglutinin was a key virulence factor in the 1918 flu. In work his lab published in 2012, they were able to show unexpectedly that a low pathogenicity avian influenza virus H1 hemagglutinin apparently shared a similar virulence property in mice. The lab’s newest work, published this week in mBio, finds that certain other subtypes of avian flu viruses also have the potential to cause more severe disease in humans and should be monitored carefully to prevent spread of disease.
In experiments conducted in an enhanced BSL3 laboratory, a set of avian flu viruses was constructed which were identical except for the hemagglutinin gene. Those viruses expressing the low pathogenicity avian H1, H6, H7, H10 or H15 hemagglutinins (genes that encode the major surface protein for the virus) led to fatal infections in mice and caused more cell damage in normal human lung cells grown in culture than did avian influenza viruses expressing other hemagglutinin subtypes.
“Viruses with these avian hemagglutinins have some type of inherent virulence motif to them, in that they induce a marked inflammatory response in mammals including human cells in culture,” Taubenberger said. “From a public health and epidemiology standpoint, it’s useful to know that avian viruses of these subtypes might lead to more severe infections in humans and is something to look out for.” They speculate that these subtypes of avian influenza may share a virulence property with the 1918 pandemic H1 hemagglutinin.
In 2013-2014 there have been close to 400 cases of avian influenza H7N9 infections in people in China, many severe. More than 100 people have died. Small numbers of severe human infections with H10N8 and H6N1 avian flu subtypes also have occurred, he said.
The investigators developed a series of viruses mimicking 13 subtypes of contemporary low pathogenicity avian influenza A viruses. Each avian influenza virus tested was genetically identical to each other except that they expressed different hemagglutinin subtypes. After growing the viruses in culture, the researchers inoculated them into mice and evaluated their pathogenicity in mice. This approach allowed a direct comparison of the role of different hemagglutinins in virulence.
The viruses expressing the H1, H6, H7, H10 and H15 subtypes all caused rapid weight loss and fatal pneumonia infections within a week. By contrast, the H2, H3, H5, H9, H11, H13, H14 and H16-expressing viruses caused only mild weight loss but no significant disease.
The research team performed a similar test using hemagglutinins from two 2013 H7N9 flu viruses from outbreaks in Anhui and Shanghai, China, with similar results in mice. They also took a subset of these viruses and put them in culture with normal human lung cells that line the airways. The cells had differentiated into a thick layer of epithelium with ciliated and goblet cells as seen along the respiratory tract. The disease-causing viruses like H1 and H7 caused mature cells to rapidly die over a couple of days, leaving just a thin lining of basal cells behind.
These results suggest that hemagglutinins may not initially require immune cells to trigger cell damage but instead may cause programmed cell death or other molecular processes that could ultimately lead to enhanced inflammation, severe disease or fatalities, Taubenberger said. In future studies it will be important to tease out differences in the hemagglutinins’ structural features and investigate the molecular processes involved as the viruses infect mammalian cells, he said.
He won’t be out of work any time soon. “There’s an unimaginably large variety of influenza viruses in hundreds of other animal species that occasionally cause infection in humans,” he said.
Meanwhile, until more is understood about how flu viruses cross from animals to humans and spread, more research is needed into producing a more broadly protective “universal” flu vaccine that may ultimately offer the best protection against future pandemics, he said.