Studies on the significance of antigenic variation in influenza C epidemiology

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1. Antigenic and genetic analyses of three influenza C strains, isolated in the Kinki district in 1982-1983, showed that they were very similar to one another but dissimilar to any of the strains isolated in Japan before 1982. A comparison of the HE gene-sequences of these strains with those of the previously published ones revealed that the Kinki isolates had a high degree of homology with the virus isolated in 1980 in the United States (Mississippi/1/80). The isolates from Kinki and Mississippi could be distinguished from the previous isolates in serological tests with polyclonal sera. These observations suggest that an influenza C variant closely related to Mississippi/1/80 has spread rapidly in Kinki in 1982-1983 presumably because this imported virus was largely different in antigenicity from the previously prevalent ones.2. Antigenic and genetic analyses of eight influenia C strains isolated in Japan in 1985-1989 were also performed. One of the two 1985 isolates was similar to Mississippi/1/80 whereas the other was dissimilar to any of the previously isolated strains. Six strains isolated after 1986, however, all resembled the virus isolated in 1981 in the Aichi prefecture (Aichi/1/81), raising the possibility that in 1986-1989 the Aichi/l/81-like variant may have possessed an epidemiological advantage over any of the other coexisting strains.3. The technique for isolation of influenza C virus has been confined to the amniotic inoculation of embryonated hen's eggs that is too laborious for routine use. Therefore we attempted to develop a tissue culture method for isolation of the virus and found that a human melanoma cell line (HMV- II) is useful for this purpose.