Influenza neuraminidase as adjuvanted protein or mRNA vaccine protects mice against influenza A and B virus challenge

For almost a century, influenza vaccines have been produced using embryonated chicken eggs and their immunogenicity has traditionally been assessed by measuring hemagglutination inhibition antibodies titers in serum, a recognized correlate of protection against influenza. The current split influenza vaccines elicit a limited immune response against neuraminidase (NA), which is the second major surface glycoprotein next to the hemagglutinin. Antibodies against NA are associated with protection against influenza in humans. However, producing NA as a stable, soluble tetrameric antigen remains challenging because of its instability. Here, we compared the immunogenicity and protective potential of two NA-based approaches: i) a soluble recombinant tetrameric NA stabilized with tetrabrachion and ii) mRNA-LNP formulations encoding full-length NA, in naïve mice. Both NA immunogen formats induced high NA inhibition titers against viruses with homologous NA. Consistent with this, both formats protected mice against challenge with antigenically homologous H1N1s, HxN2s, or IBV virus strains. However, only immunization with NA-encoding mRNA-LNP protected against challenge with a heterologous HxN2 virus, whereas immunization with recombinant NA was associated with weight loss following heterologous HxN2 virus challenge. Although serum transfer confirmed the protection induced by mRNA immunization, sera from mice immunized with recombinant NA did not reproduce the early onset of morbidity. On the contrary, it conferred partial protection. Finally, neuraminidase inhibition assays against a panel of antigenically distinct N2 proteins showed similar breadth of inhibition, suggesting that the enhanced protection conferred by mRNA is attributable to its increased immunogenicity.