Graphical abstract
The study describes NA-specific human antibodies that target the underside of the NA globular head domain, inhibit viral propagation of a wide range of human H3N2, swine-origin variant H3N2, and H2N2 viruses, and confer both pre- and post-exposure protection against lethal H3N2 infection in mice. Cryo-EM structures of two such antibodies in complex with NA reveal non-overlapping epitopes covering the underside of the NA head. These sites are highly conserved among N2 NAs yet inaccessible unless the NA head tilts or dissociates.

Seeking a universal flu vaccine, scientists have identified two antibodies that block a wide range of influenza viruses by targeting a region of the viral structure that was discovered 40 years ago but has yet to be successfully exploited.   

Flu vaccines typically target hemagglutinin, the most abundant and accessible protein on the surface of the virus. But the head of the hemagglutinin molecule mutates often. That means vaccines that target these regions must be reformulated each year based on a best guess of which influenza strain will emerge as dominant for that season. 

The new research, a very early step toward a vaccine, focuses on neuraminidase, a less-studied essential protein on the surface of influenza viruses and one that is less prone to mutation. This makes it a potential target for vaccines that might provide universal protection from the flu, but only a handful of neuraminidase antibodies have been identified.  

The two newly identified antibodies inhibit viral proliferation in multiple strains of influenza, including lethal H3N2 infection in mice, either when given as a preventative or as a treatment. These findings may help guide future development of a flu vaccine that doesn't need to be modified annually. Such a vaccine might be more effective because it's not trying to hit a moving target, the scientists reported in the journal Immunity. The research team was led by Masaru Kanekiyo of the National Institute of Allergy and Infectious Diseases with collaborators including Yaroslav Tsybovsky, Ph.D., of the Frederick National Laboratory. 

The research group obtained monoclonal antibodies from two volunteer individuals who were recovering from confirmed cases of influenza A H3N2. From these, the scientists isolated six monoclonal antibodies for further experiments. 

Cryo-electron microscopy showed that when exposed to the flu virus, two of the monoclonal antibodies attached to the so-called dark side of the neuraminidase (NA) structure. This underside area was among four antigenic regions of the protein first described in 1984 by collaborators from St. Jude Children's Research Hospital and Australian National University. While the topside regions of the protein structure became the target of successful vaccine development, the underside regions have been largely overlooked. 

"We found that all NA dark side-directed mAbs (monoclonal antibodies) inhibited the growth of all viruses tested, including non-circulating H3N2v and H2N2 viruses," the group reported. 

In further testing, the scientists gave three of the monoclonal antibodies to laboratory mice 24 hours before exposing them to an H3N2 influenza virus. Each antibody was given in three different doses to a group of 10, or in one case nine, mice before virus exposure. Each of the three antibodies was effective.   

"Overall, NA dark side-directed mAbs provided potent protection from mortality and morbidity caused by H3N2 virus infection in mice when prophylactically administered," the scientists observed. 

Many laboratories are working toward a vaccine design that will provide broad protection against multiple versions of the flu virus. This would make it possible for people to get long-lasting protection from multiple strains of the virus with just one shot. But many additional experiments will be needed before designing a vaccine candidate suitable for testing in humans. 

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