ENDOGENOUS PROTEASE-DEPENDENT REPLICATION OF HUMAN INFLUENZA-VIRUSES IN 2 MDCK CELL-LINES

Multi-cycle replication and plaque formation of influenza A and B viru ses and cleavage activation of their hemagglutinin (HA) by an endogeno us protease(s) were examined in two MDCK cell lines, MDCK(-) and MDCK( +). No exogenous trypsin was required for multi-cycle replication and plaque formation of all the influenza A viruses tested in the MDCK(+) cell, while those of the viruses in the MDCK(-) cell were completely t rypsin-dependent. In both cell lines, on the other hand, influenza B v iruses grew well in the absence of trypsin. The capability of multiple replication and plaque formation of the influenza viruses correlated with cleavage of the HA precursor (HA(0)) to HA(1) and HA(2), indicati ng that both cell lines express an HA activating endoprotease(s); that of the MDCK(+) cell activates the HA of influenza A and B viruses, an d that of the MDCK(-) cell does only the HA of influenza B virus. Furt hermore, the protease of the MDCK(+) cell was strongly suggested to be present on the cell surface and a serine protease. The MDCK(+) cell w ould be useful fur isolation of influenza viruses from clinical specim ens and for screening of protease inhibitors for anti-influenza virus drugs.