Phenotypic mixing with recombinant haemagglutinin of high cleavability mediates multi-cycle replication of human influenza virus in cell culture

When CV-1 cells expressing haemagglutinin (HA) of fowl plague virus A/FPV/34/Rostock(H7) (FPV) from an SV40-based recombinant vector were superinfected with the human influenza virus A/FM/1/47(H1N1) (FM1), phenotypically mixed progeny virus was observed. It contained cleaved FPV HA and uncleaved FM1 HA, was infectious without trypsin treatment and its infectivity was neutralizable by anti-FPV serum. When superinfection of H7 HA-expressing CV-1 cells was performed at a low multiplicity of infection, multi-cycle replication occurred. Control cells preinfected with an SV40-based recombinant not expressing FPV HA did not allow multi-cycle replication. Multi-cycle replication of FM1 virus was also observed when cells were preinfected with a vector expressing a highly cleavable mutant of influenza virus A/Port Chalmers/1/73(H3) HA carrying an insert of four arginine residues at the cleavage site. This was not the case when cells expressing uncleaved wild-type H3 HA were used. The results show that by phenotypic mixing with recombinant HA of high cleavability, a human influenza virus can be obtained in infectious form from cells lacking a suitable protease to activitate this virus.