Cell tropism of influenza virus mediated by hemagglutinin activation at the stage of virus entry.

When influenza virus A/WSN/33 (H1N1) was grown in MDBK or CV-1 cells in serum-free medium, progeny virus released from these cells contained only uncleaved hemagglutinin (HA). This virus was unable to infect CV-1 cells unless subjected to cleavage activation by trypsin, but it was infectious for MDBK cells without such treatment. Temperature shift experiments demonstrated that HA of radiolabeled input virus was cleaved within 30 min after inoculation of MDBK cells. As indicated by its resistance to trypsin cleavage, the virus was already internalized at this stage. Cleavage was not observed after inoculation of CV-1 cells. The serine-protease inhibitor leupeptin blocked virus growth when added to MDBK cells during the initial phase of the replication cycle. The inhibitor did not show this effect, when the input virus contained already cleaved HA. These results demonstrate that activation of HA of the WSN strain can occur in MDBK cells, but not in CV-1 cells, at the stage of virus entry, presumably by an endosomal protease.