Identification of the binding sites to monoclonal antibodies on A/USSR/90/77 (H1N1) hemagglutinin and their involvement in antigenic drift in H1N1 influenza viruses.

We have determined nucleotide sequences of the HA1 portion of the hemagglutinin (HA) gene of the parental A/USSR/90/70 (H1N1) virus and its eight variants selected in vitro with six monoclonal antibodies to study antigenic determinants. The HA1 gene of one of the variants (B-1-23) was cloned in bacteria and its nucleotide sequence was determined by the Maxam-Gilbert method. The nucleotide sequence of the variant was confirmed by the dideoxy chain termination method. The gene sequences of the other viruses were determined by the latter method. Three variants with reduced reactivity in HI test only with the selecting antibodies possessed one amino acid substitution. On the other hand, most other variants which had the changed reactivity to multiple antibodies in HI test possessed more than one substitution. Comparison of the amino acid sequences of the HA1 molecule, deduced from the nucleotide sequences, suggested that the monoclonal antibodies W18 and 264 reacted with epitopes located on the area involving amino acid residues 125C and 189-190, respectively, whereas, the antibodies 22 and 70 reacted with epitopes involving amino acid residues 129, 132, and 157. The epitope recognized by antibody 110 overlapped with that of W18, and the epitope recognized by antibody 385 was located on the area involving at least amino acid residues 129, 159, and 189, which overlapped with some of the above epitopes. The sequence analysis with B-1-23 variant selected with antibody 264 clearly showed that in A/USSR/77 viruses, a single substitution at amino acid residue 190 effectively changes the epitope and caused a significant antigenic variation detectable by postinfection ferret sera.