Fusion can be rescued by the addition of exogenous PDI. Sup T-1 cells initially treated with 2.5 mM DTNB or PBS (control) were next incubated with 0 (control), 20 or 40 μg/mL PDI in 2 mM GSSG/20 mM GSH buffer (90 minutes, 22°C), then washed and cocultured with untreated Env-expressing cells (1 hour at 37°C). Bar no. 4 is the untreated control; bar no. 5 represents an additional control not exposed to DTNB but treated with 40 μg/mL PDI. The extent of fusion was measured following coincubation of Sup T-1 and Env⁺ cells as the ratio of bound, dye-redistributed Sup T-1 cells to the total number of bound Sup T-1. Each bar represents mean fusion ± SE for 200 cells counted.