Differential preference of egg-adaptive mutations in human H3N2 vaccine strains.

(A) Frequencies of mutations among different egg-passaged seasonal H3N2 vaccine strains are shown as sequence logos. Amino acid variants from residues 183 to 196 are shown (H3 numbering). The number of egg-passaged strains included in this analysis is indicated in the parenthesis. The relative size of each amino acid letter represents its frequency in the sequences. Grey letters represent the amino acid variants that are observed in the corresponding unpassaged parental strains. (B) Key egg-adaptive mutations in this study are shown in orange (PDB 6BKT) [17]. Sialylated glycan receptor is in yellow sticks representation. Strains in parentheses are associated with the corresponding egg-adaptive mutations. Only three strains of interest (Sing16, Switz17, and Kansas17) are included here. (C) Replication fitness of different mutants of human H3N2 vaccine strains was examined in a virus rescue experiment in mammalian cells (293T/hMDCK). Viral titers were measured by TCID50. (D) The viral titer of each human H3N2 vaccine strain with either V186/L194 or G186/P194 was measured during egg-passaging. (C-D) The means of three independent experiments are shown with SD indicated by the error bars. The dashed line represents the lower detection limit. Amino acid variant representing an egg-adaptive mutation is underlined. (E) Frequencies of mutations in the receptor-binding subdomain (residues 117–265) [21] that emerged during serial passaging in eggs. The strain of inoculum for each passaging experiment is indicated above each plot, with those representing egg-adaptive mutations underlined. Frequencies are shown as means of three biological replicates. Only those mutations that reached a minimum average frequency of 10% after the fifth passage are plotted.</p

H3N2 genes that were used in this study.

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