Antigenic characteristics and genome composition of a naturally occurring recombinant influenza virus isolated from a pig in Japan.

We performed antigenic analysis of the haemagglutinin and neuraminidase subunits of a recombinant virus (A/swine/Kanagawa/2/78) isolated from a pig in Japan in 1978, using a series of monoclonal antibodies to H1 (Hsw1) haemagglutinin and N2 neuraminidases of H2N2 and H3N2 viruses. Results obtained in haemagglutination inhibition tests with five monoclonal antibodies to the haemagglutinin of A/NJ/8/76 (H1N1) revealed that the haemagglutinin of three H1N1 and the recombinant viruses were indistinguishable from that of A/NJ/8/76. The neuraminidase of A/swine/Kanagawa/2/78 was found to be antigenically similar to A/Kumamoto/22/76 (H3N2, A/Victoria/3/75-like strain). The oligonucleotide maps of the entire RNAs of H1N1, H1N2 and H3N2 viruses showed that A/swine/Kanagawa/2/78 (H1N2) virus was more similar to swine (H1N1) virus than to A/Kumamoto/22/76 (H3N2) virus. Radioactive cDNA was prepared by reverse transcription of the recombinant virus RNA using a dodecadeoxyribonucleotide primer and used in DNA-RNA hybridization experiments. The results obtained in molecular hybridization based on blotting procedures showed that all cDNA segments except gene 6 hybridized efficiently with RNAs of swine (H1N1) influenza virus. The sixth cDNA segment was homologous to the corresponding RNA segment of H3N2 virus. The genetic relatedness of A/swine/Kanagawa/2/78 (H1N2) with either A/swine/Kanagawa/4/78 (H1N1) or A/Kumamoto/22/76 (H3N2) was clearly established by hybridization between the cDNA segment probes and viral RNA. It was concluded that the neuraminidase gene of A/swine/Kanagawa/2/78 (H1N2) was derived from a human H3N2 virus, while the seven other genes were from a swine H1N1 virus.