ANALYSIS OF THE HOST-SPECIFIC HEMAGGLUTINATION OF INFLUENZA A(H1N1) VIRUSES ISOLATED IN THE 1995 6 SEASON/

Two phenotypes of human influenza A(H1N1) virus are currently circulat ing in Japan. One (group 1) agglutinates both chicken and goose red bl ood cells (CRBC and GRBC), the other (group 2) agglutinates GRBC but n ot CRBC. In the 1995/6 season, group 2 viruses accounted for 70% of th e H1N1 viruses isolated in MDCK cells. The 1995/6 viruses were located on two branches of the genetic tree. One branch contained both group 1 and group 2 viruses and the other branch contained only group 2 viru ses. Group 2 viruses had aspartic acid at residue 225 in the haemagglu tinin (HA) protein, the key amino acid residue for group 2 phenotype. The HA protein of group 1 viruses had a change from aspartic acid to a sparagine at residue 225 and the expressed HA protein of these viruses adsorbed CRBC. Serial passage of group 2 viruses in MDCK cells or emb ryonated chicken eggs caused these viruses to gain the ability to aggl utinate CRBC. MDCK-adapted viruses had the same amino acid sequences o f HA polypeptide as the original ones, but egg-adapted viruses had cha nged amino acid sequences. The expressed HA protein from one egg-adapt ed virus that originally belonged to group 2 adsorbed CRBC.