PHENOTYPIC MIXING WITH RECOMBINANT HEMAGGLUTININ OF HIGH CLEAVABILITYMEDIATES MULTICYCLE 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 su perinfected with the human influenza virus A/FM/1/47(H1N1) (FM1), phen otypically mixed progeny virus was observed. It contained clevaved FPV HA and uncleaved FM1 HA, was infectious without trypsin treatment and its infectivity was neutralizable by anti-FPV serum. When superinfect ion of H7 HA-expressing CV-1 cells was performed at a low multiplicity of infection, multicycle replication occurred. Control cells preinfec ted with an SV40-based recombinant not expressing FPV HA did not allow multi-cycle replication. Multi-cycle replication of FM1 virus was als o observed when cells were preinfected with a vector expressing a high ly cleavable mutant of influenza virus A/Port Chalmers/1/73(H3) HA car rying an insert of four arginine residues at the cleavage site. This w as not the case when cells expressing uncleaved wild-type H3 HA were u sed. The results show that by phenotypic mixing with recombinant HA of high cleavability, a human influenza virus can be obtained in infecti ous form from cells lacking a suitable protease to activitate this vir us.