Isolation of a novel type of interfering influenza B virus defective in the function of M gene.

A novel type of interfering influenza B virus which is defective in the function of M gene has been reported. Clone 301, a B type virus clone obtained by successive back-crosses of A/Aichi/2/68 (H 3 N 2) with B/Yamagata/1/73, grew normally in MDCK cells when inoculated at a low multiplicity, but was easily converted to a hemagglutinating but non-infectious form by one cycle of high multiplicity infection. Within MDCK cells infected with infectious clone 301 at a high multiplicity, synthesis of M protein was greatly reduced. The virus particle produced by a high multiplicity infection was devoid of RNA segment 7 (M gene), contained less amount of M protein compared with the standard virus, and interfered with the replication of wild type B/Yamagata, again accompanied by a selective suppression of M protein synthesis within the co-infected cells.