Impact of amino acid substitution T203I in hemagglutinin on growth characteristics in vitro and hemagglutinin thermostability of A/H3N2 influenza viruses

Background: Russian live attenuated influenza vaccines are a three-component preparations that contain vaccine strains based on current epidemic influenza A/H1N1, A/H3N2 and B strains. Recent influenza viruses A/H3N2 are most susceptible to drift antigenic changes, and therefore, this component of the live attenuated influenza vaccines must be constantly updated. Current vaccine strains of live attenuated influenza vaccines are obtained by the method of classical reassortment using selective factors in developing chicken embryos. During the process of preparation of live attenuated influenza vaccines strains, single reassortants can acquire various egg-adaptive amino acid substitutions in hemagglutinin and neuraminidase – the genes responsible for the antigenic correspondence of the vaccine strain to the epidemic parent. These amino acid substitutions can affect the biological properties of the vaccine strain, thereby reducing the effectiveness of this component of live attenuated influenza vaccines.

AIM: The aim of the study was to explore the effect of amino acid substitution T203I in hemagglutinin of A/H3N2 influenza viruses on growth characteristics and hemagglutinin thermostability.

MATERIALS AND METHODS: For the study, three pairs of A/H3N2 vaccine reassortants were prepared. Reassortants differed from each other by amino acid Thr or Ile at position 203 in the hemagglutinin. The growth properties of vaccine strains were assessed by titration in eggs at 26–40°C and in a MDCK cell culture at 33°C. The thermostability of the hemagglutinin of studied influenza viruses was assessed by determining their ability to agglutinate 1% erythrocytes after exposure to elevated temperatures in the range of 37–70°C.

RESULTS: The amino acid substitution T203I in hemagglutinin in reassortants obtained on the basis of current influenza A/H3N2 viruses acquired during the preparation of vaccine strains does not affect the temperature sensitivity of viruses. It was shown that viruses with an egg-adaptive substitution T203I in hemagglutinin have more pronounced cold-adapted phenotype and a higher reproductive activity in MDCK cell culture, compared to strains without this mutation. It was found that hemagglutinin of reassortants with 203 Ile is more thermostable than with 203 Thr.

CONCLUSIONS: Our data indicate that the amino acid substitution of T203I in hemagglutinin in current influenza A/H3N2 viruses does not have a negative effect on biological properties, but improves growth characteristics in eggs and MDCK cells, as well as the thermostability of viruses.