Isolation and preliminary characterization of a highly cytolytic influenza B virus variant with an aberrant NS gene.

By repeated backcrosses of influenza virus A/Aichi/2/68 (H3N2) with B/Yamagata/1/73 in MDCK cells, a virus clone with HA of B serotype (clone B/610B5B/201, or clone 201) was obtained, which formed sharp plaques in MDCK cells and induced a severe cell lysis early after infection. Its structural proteins were indistinguishable from those of B/Yamagata. Electrophoresis of the RNA of the clone also showed an identical pattern to that of B/Yamagata except RNA segment 8 (NS gene), which migrated faster than the corresponding segment of B/Yamagata in a 2.8% polyacrylamide gel. Within the clone 201-infected MDCK cells, only one species of nonstructural (NS) polypeptide was demonstrable, which had the same electrophoretic mobility as NS2 of B/Yamagata, and any band which might be taken as the counterpart of NS1 of B/Yamagata was not detectable on the gel. Peptide mapping revealed that NS of clone 201 was structurally different from either NS1 or NS2 of wild-type B/Yamagata. NS gene and its function of clone 201 was successfully transferred to B/Lee/40 by genetic reassortment.