Interaction between a 1998 human influenza virus N2 neuraminidase and monoclonal antibody Mem5

Influenza virus constantly escapes antibody inhibition by introducing mutations that disrupt protein–protein interactions. Based on the structure of the complex between neuraminidase (NA) of influenza A/Memphis/31/98 (H3N2) and the Fab of a monoclonal antibody (Mem5) that binds and inhibits the Memphis/98 NA, we investigated the contribution made by individual amino acids of NA to the stability of the complex. We made mutations D147A, D147N, H150A, H197A, D198A, D198N, E199A, E199Q, K221R, A246K, D251N, and D251A. Binding of each mutant to NA was quantitated by NA inhibition assays and ELISA. Most of the mutant NAs were inhibited by Mem5 to the same extent as wild-type, but with lower affinity. The exceptions were E199A, E199Q, and K221R, in which binding was abrogated. The ELISA results confirmed a correlation between NA inhibition and binding. The Mem5 epitope is dominated by a few high-energy interactions as was found in the epitope on an avian subtype N9 NA that binds antibody NC41 and different to the more diffuse energy distribution in the NC10 epitope on N9 NA. Energetic dominance of a particular interaction, which is associated with potential for antibody escape mutations, may be associated with the absence of water molecules in the vicinity. Critical contacts in a dominant antigenic site are likely to mutate, allowing some predictions of antigenic drift.