FIG 4.

Syncytium formation in HeLa cells expressing HA proteins after exposure to low pH. Cells were transfected with plasmids encoding HAs of pandemic viruses A/Brevig Mission/1/1918 (H1N1), A/Singapore/1/1957 (H2N2), A/Hong Kong/1/1968 (H3N2), A/Hamburg/5/2009 (H1N1), and HA of A/swine/Marseille/2260/1980 (H1N1). The cells were exposed to different pHs 16 h posttransfection, incubated to allow syncytium formation, and contrasted using Giemsa stain. The numbers of nuclei in syncytia were counted and are expressed as percentages of the total nuclei in the same microscopic field. The data represent mean values; error bars show 95% confidence intervals.