Figure 2.

Polykaryon formation by cells expressing HA or both the HA and NA. 293T cells were transfected with plasmids expressing the WSN HA (a–c), WSN HA and WSN NA (d–f) or WSN HA and HK NA (g–i). At 24-h posttransfection, the cells were treated with 0.5 μg/ml trypsin (a, d, and g), 1% FCS (b, e, and h), or 5 μg/ml human plasminogen (c, f, and i) for 30 min. After exposure to PBS (pH 5.0), the cells were then incubated for 6 h, fixed with 3% formaldehyde, and stained with Giemsa solution.