Effect of Amino Acid Substitutions in the Small Subunit of the Avian H5N2 Influenza Virus Hemagglutinin on Selection of the Mutants, Resistant to Neutralizing Monoclonal Antibodies

Mutations in the hemagglutinin (HA) protein molecule of influenza A viruses associated with the virus’s resistance to physical and chemical factors can play important roles in the selection of influenza variants during circulation in nature. In this study, we examined escape mutants of the A/mallard/Pennsylvania/10218/84 (H5N2) influenza virus, which were selected with a monoclonal antibody specific to an epitope in the large HA subunit (HA1). We obtained escape mutant m4F11(4), which carries a single amino acid substitution S145P1 in the HA1 subunit and two other mutants, m4G10(10) and m4G10(6), which accumulated additional mutations in the small subunit (HA2), i.e., L124F2 and L124F2+N79D2, respectively. We demonstrate that amino acid substitutions in HA2 of m4G10(10) and m4G10(6) viruses can compensate for the negative effect of the S145P1 mutation, which manifested in a significant increase in the capacity for viral replication at the early stages of infection in chicken embryos, as well as in growth in the HA thermostability compared with the m4F11(4) escape mutant. We propose that these variations in phenotypic properties, which provide advantages during viral replication, play a role in the positive selection factor and are retained in the viral population.