Antigenic structure of the haemagglutinin of human influenza A/H2N2 virus

The antigenic structure of influenza A/H2N2 virus haemagglutinin (HA) was a nalysed using 19 monoclonal antibodies (MAbs) against the HA of A/Kayano/57 . The antibodies were classified into three groups: group I had both haemag glutination inhibition and neutralization activities, group II had neutrali zation activity but no haemagglutination inhibition activity and group III had neither activity. Analysis of escape mutants selected by each of the gr oup I and II antibodies identified six distinct antigenic sites: four (I-A to I-D) were recognized by group I MAbs and two (II-A and II-B) were recogn ized by group II MAbs. Sequence analysis of the HA genes of the escape muta nts demonstrated that sites I-A, I-B and I-C form a contiguous antigenic ar ea that contains the regions corresponding to antigenic sites A, B and D on the H3 molecule and that sites I-D and II-B are the equivalents of sites E and C, respectively, suggesting that the antigenic structure of the H2 mol ecule is largely similar to that of the H3 molecule. However, the H2 molecu le differed from the H3 molecule in having a highly conserved antigenic sit e (II-A) in the stem domain. It was also found that most of the escape muta nts selected by antibodies to sites I-A, I-B and I-C acquired a new glycosy lation site at position 160, 187 or 131, respectively, which indicates that A/H2N2 viruses have the potential to gain at least one additional oligosac charide on the tip of the HA, although this has never occurred during 11 ye ars of its circulation in humans.