Antigenic drift originating from changes to the lateral surface of the neuraminidase head of influenza A virus.

Influenza viruses possess two surface glycoproteins, haemagglutinin and neuraminidase (NA). Although haemagglutinin plays a major role as a protective antigen, immunity to NA also contributes to protection. The NA protein consists of a stalk and a head portion, the latter of which possesses enzymatic NA (or sialidase) activity. Like haemagglutinin, NA is under immune pressure, which leads to amino acid alterations and antigenic drift. Amino acid changes accumulate around the enzymatic active site, which is located at the top of the NA head. However, amino acid alterations also accumulate at the lateral surface of the NA head. The reason for this accumulation remains unknown. Here, we isolated seven anti-NA monoclonal antibodies (mAbs) from individuals infected with A(H1N1)pdm09 virus. We found that amino acid mutations on the lateral surface of the NA head abolished the binding of all of these mAbs. All seven mAbs activated Fcγ receptor (FcγR)-mediated signalling pathways in effector cells and five mAbs possessed NA inhibition activity, but the other two did not; however, all seven protected mice from lethal challenge infection through their NA inhibition activity and/or FcγR-mediated antiviral activity. Serological analysis of individuals infected with A(H1N1)pdm09 virus revealed that some possessed or acquired the anti-NA-lateral-surface antibodies following infection. We also found antigenic drift on the lateral surface of the NA head of isolates from 2009 and 2015. Our results demonstrate that anti-lateral-surface mAbs without NA inhibition activity can provide protection by activating FcγR-mediated antiviral activity and can drive antigenic drift at the lateral surface of the NA head. These findings have implications for NA antigenic characterization in that they demonstrate that traditional NA inhibition assays are inadequate to fully characterize NA antigenicity.