FIG 4.

Influenza virus fusion threshold measured by syncytium formation assay. Vero cells were infected with H7 influenza viruses for 16 h before treatment with 5 μg/ml of TPCK-trypsin for 15 min, and then the cells were incubated with fusion buffer with pH values ranging from 4.8 to 7.4 at 37°C for 5 min, followed by incubation with cell culture medium containing 10% FBS for three additional hours. NP-positive cells were identified by immunofluorescence microscopy with anti-NP antibody after fixation. The syncytia among NP-positive cells at the highest pH are shown on the left, and the syncytia among NP-positive cells upon fusion induction at a pH 0.1 unit higher are shown on the right.