Comparison of the effects of peptide pretreatment of Env-expressing versus CCR5-expressing cells. (left) Target cells (HeLa.T4 infected with vTF7-3 and transfected with the CCR5 plasmid) or (right) effector cells (HeLa cells coinfected with vCB-21R-LacZ and vCB-43) were preincubated in the absence or presence of the PL2 peptide (25 μg/mL). After incubation for 1 hour at 37°C, the cells were mixed with their fusion partners without or with washing to remove excess peptide. Fusion assay was performed as described in “Materials and methods.” Error bars indicate SD of mean values obtained from duplicate fusion assays.