The Molecular Basis for Antigenic Drift of Human A/H2N2 Influenza Viruses

Antigenic map of human A/H2N2 influenza viruses as measured in HI assays with ferret postinfection antisera. Circles indicate the position of viruses, squares represent two ferret antisera raised against each of the viruses A/Japan/305/57, A/Singapore/1/57, A/Netherlands/K1/63, A/England/1/66, A/Tokyo/3/67, and A/Netherlands/B1/68. The underlying grid depicts the scale of antigenic difference between the viruses, with each square representing one antigenic unit or a 2-fold difference in HI titer. Years of isolation of the A/H2N2 virus isolates are indicated, ranging from 1957 (red) to 1968 (blue).

Summary of substitutions responsible for antigenic differences between NL/M1/57 and NL/B1/68. Antigenic maps showing the antigenic change caused by individual amino acid substitutions introduced into NL/M1/57 (A) or NL/B1/68 (B) and combinations of mutations introduced into NL/M1/57 (C) or NL/B1/68 (D). Viruses are shown as circles of different colors, with a diamond indicating the mutant virus with the largest antigenic distance to the corresponding wild-type strain. Sera are indicated as open squares. The underlying map of wild-type viruses from Fig. 2 is shown in gray, and its positioning is kept constant. The arrows indicate the antigenic distance of a double mutant that spans a long distance between the earliest and latest isolates of A/H2N2. Structure of an HA trimer (E) with individual monomers in shades of gray, the RBS in yellow, and mutations near the RBS with a measurable effect on antigenicity in orange. The two mutations with the biggest combined effect in panel C are colored in red (T128D and N139K).