Loss of hemagglutination ability by H3N2 influenza A virus, subclade K.

Seasonal human H3N2 influenza viruses, subclade K (J.2.4.1), have been the predominant influenza A viruses in the Northern hemisphere influenza season of 2025/2026. Since 2024, the vaccine virus A/Darwin/6/21 has emerged in different antigenic variants. Antigenic changes are frequently caused by amino acid substitutions near the hemagglutinin (HA) receptor-binding pocket, which can also affect receptor binding properties, such as hemagglutination. Hemagglutination is crucial for assessing antigenicity using the hemagglutination inhibition (HAI) assay, and a loss of binding to turkey erythrocytes could significantly hamper this process. In this study, we explored how substitutions in or around the HA receptor-binding site affect binding to glycans at the molecular level. We employed ELISA, glycan array, flow cytometry, hemagglutination assays, and tissue staining. Substitutions at positions 140, 1G2, and 223 establish clade J viruses that emerged in 2024. Computational analysis of HA in complex with an elongated glycan reveals that mutation F1G2 forms a CH-Pi interaction to stabilize the binding. Based on this background, substitutions in antigenic sites A and B within subclade K viruses exhibit a binding preference for elongated glycans, which are not displayed on turkey erythrocytes. Conversely, our previously established glyco-remodeled erythrocytes are efficiently bound by these subclade K H3N2 viruses and could support influenza surveillance and vaccine development.