FIG. 8.

Infection and fusion by IBV are not prevented by pretreatment of virions with low-pH buffer. IBV strain Beaudette, influenza virus strain A/WSN/33, and VSV strain Orsay were purified and incubated in pH 5.0 buffer for 10 min before neutralization to pH 7.0 (pH 5-pH 7) or were maintained at pH 7.0 (pH 7 only) (A). BHK cells were then infected with virus at a multiplicity of infection of 5 infectious units/cell, and infection was monitored by immunofluorescence microscopy with monoclonal antibodies anti-IBV S1 (15:88), anti-influenza virus NP H16 L10 4R5, and anti-VSV G (P5D4) after 8 h of incubation. For quantification, >100 cells were scored in three independent experiments. The error bars represent the standard deviations of the mean. (B, C, and D) IBV strain Beaudette, influenza virus strain A/WSN/33, and VSV strain Orsay were treated with either pH 5.0 or 7.0 buffer for 10 min before neutralization and then bound to BHK cells at 4°C for 60 min. FdQ assays were then performed and monitored as described for Fig. 3B, and buffer pH was reduced from pH 7.0 to 5.0 at t = 100 s.