Inhibition of direct binding of [I¹²⁵]-labeled gp120 to CCR5. NIH-3T3 cells expressing recombinant CCR5 were prepared by infection with vHC-1. [I¹²⁵]-labeled gp120 molecules derived from the indicated HIV-1 strains were preincubated with 100 nM sCD4 then treated with (A) 50 μg/mL of PNt or PL2 or (B) increasing concentrations of PL2 before mixing with CCR5-expressing cells. As a background control for specificity of gp120 binding to CCR5, samples of CCR5-expressing cells were preincubated with 500 nM cold RANTES before mixing with the sCD4-activated [I¹²⁵]-labeled gp120 (A). Error bars indicate SD of mean values obtained from duplicate experiments.