Molecular study of influenza C virus evolution

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We sequenced the hemagglutinin-esterase (HE) genes of 20 influenza C strains isolated in Japan between 1964 and 1988 and performed a combined evolutionary analysis with the sequences of 10 strains isolated in other countries. The followings are the summaries of the results obtained.1. The Japanese isolates analyzed could be divided into four distinct lineages (I-IV). At present, however, three lineages (I,II,IV) seemes to be cocirculating in our country because no virus belonging to lineage III (represented by Aomori/74) has been isolated at all after 1977.2. The 10 strains isolated outside Japan were all found to belong to one of the three lineages currently circulating in our country, suggesting that influenza C virus is capable of spreading worldwide.3. By comparing the HE gene sequence among the 1971-1988 isolates that belong to lineage I, the evolutionary rate of HE gene was estimated to be 0.509x10^<-3> substitutions/site/year, indicating that the HE gene evolves much more slowly than the HA gene of influenza A virus.4. We reported previously that in the HE protein most of neutralizing epitopes are clustered in the region spanning from residue 180 to 214. It became clear in the present study that this domain is considerably variable among the strains belonging to different lineages but is nearly identical among the strains belonging to the same lineage, raising the possibility that antibody pressure may have playd a role in producing new lineage but that once such a lineage was established, the virus was no longer able to undergo antigenic changes even in the presence of antibody pressure, presumably because of the hunctional constraints.