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Influenza A virus strains adapt to achieve successful entry into host species. Entry is mediated by the viral membrane protein hemagglutinin (HA), which triggers membrane fusion and genome release under acidic conditions in the endosome. In addition to changes in the receptor binding domain, the acid stability of HA has been linked to the successful transmission of virus between avian and human hosts. However, to fully understand the connection between changes in HA and host tropism, additional factors relevant to HA structure-function and membrane fusion are also likely to be important. Using single-particle-tracking (SPT) techniques, individual membrane fusion events can be observed under specific conditions, which provide detailed information regarding HA pH sensitivity and acid stability and the rate and extent of membrane fusion. This provides a comparative way to characterize and distinguish influenza virus fusion properties among virus strains. We used SPT to quantify the fusion properties of three H3 influenza strains: A/Aichi/68/H3N2 (X:31), A/Udorn/72/H3N2 (Udorn), and A/Brisbane/07/H3N2 (Brisbane). The rate of fusion for the most clinically relevant strain, Brisbane, is generally insensitive to decreasing pH, while the fusion of the egg-adapted strains Udorn and X:31 is strongly dependent on pH (and is faster) as the pH decreases. All strains exhibit similar acid stability (the length of time that they remain fusogenic in an acidic environment) at higher pHs, but the egg-adapted strains become less acid stable at lower pHs. Thus, it appears that the laboratory-adapted H3 strains tested may have evolved to compensate for the faster HA deactivation at low pH, with a commensurate increase in the rate of fusion and number of proteins facilitating fusion, relative to the Brisbane strain.

Variations in pH Sensitivity, Acid Stability, and Fusogenicity of Three Influenza Virus H3 Subtypes