Influenza neuraminidase-inhibiting antibodies are induced in the presence of zanamivir.

Seasonal influenza epidemics cause illness and death each year, and the emergence of antigenically novel influenza A viruses are a continual pandemic threat. Disease and death can be averted by vaccination. The potency of killed virus vaccines is based on hemagglutinin (HA) content. However, antibodies that inhibit enzyme activity of the neuraminidase (NA) also reduce virus replication and protect against disease. Monoclonal NA-inhibiting (NI) antibodies recognize conformational epitopes, and it is anticipated that native tertiary structure is required for their induction. NA assembles as a tetramer and only this form has enzyme activity. Since small inhibitors of NA do not significantly alter conformation, we sought to determine whether neuraminidase-inhibiting (NI) antibodies would be induced by inhibitor-inactivated NA. We therefore evaluated responses of mice immunized with purified NA that was either inactivated by addition of zanamivir or denatured by heat treatment. NI antibodies were induced following immunization with NA from A/Wisconsin/67/2005 (H3N2) in which enzyme activity was inhibited by the former method but not the latter. Protection of mice against challenge with virus containing an antigenically matched NA correlated with the detection of NI antibodies in serum. Similar results were obtained when mice were immunized with whole H1N1 virus in which NA activity had been inhibited by the same two modalities. This demonstrates that native conformation of NA is necessary for induction of NI antibodies; enzyme activity provides a useful marker of intact structure, but absence of activity in NA that is correctly folded does not result in loss of immunogenicity. Thus any assay to assess potency of influenza vaccines with respect to NA content should consider the proportion of NA that is in a structurally native state.