STUDIES ON THE MOLECULAR-BASIS FOR LOSS OF THE ABILITY OF RECENT INFLUENZA-A (H1N1) VIRUS-STRAINS TO AGGLUTINATE CHICKEN ERYTHROCYTES

Recent strains of influenza A but not B viruses have lost the ability to agglutinate chicken red blood cells (CRBC), The H1N1 viruses isolat ed in Japan during the 1991/92 season could be divided into two groups . Group 1 viruses (A/Aichi/4/92 and A/Aichi/7/92) agglutinated goose r ed blood cells (GRBC) and CRBC, while group 2 viruses (A/Aichi/24/92 a nd A/Aichi/26/92) did not agglutinate CRBC. There were no amino acid d ifferences between them in the haemagglutinin (HA) polypeptide. Reasso rtment experiments between a group 1 virus(A/Aichi/4/92) or a group 2 virus (A/Aichi/24/92) and the A/WSN/33 influenza A (H1N1) virus strain suggested that the HA gene products of the viruses of both groups had lost the capacity to agglutinate CRBC, The HA proteins expressed on C os cells by transfecting the cDNAs of the virus HA gene of A/Aichi/4/9 2 and A/Aichi/24/92 agglutinated GRBC but not CRBC. These experiments indicated that the HA proteins of H1N1 viruses of both groups isolated in 1992 had lost the ability to agglutinate CRBC even though the grou p 1 virions showed haemagglutinating capacity with CRBC. By using the cDNAs of the HA gene of seven natural isolates obtained from 1977 to 1 992, it was found that the expressed HA proteins of influenza A (H1N1) viruses isolated since 1988 had lost the ability to agglutinate CRBC, Experiments with chimeric and point-mutated HA cDNAs of A/Aichi/24/92 showed that an amino acid change at residue 225, which occurred after 1986, and a cluster of amino acid changes at residues 193, 196 and 19 7, which occurred before 1986, were responsible for loss of the abilit y to agglutinate CRBC, Egg-adapted virus derived from A/Aichi/24/92 ha d one amino acid change at residue 225 compared to the parental virus.