Effect of a tyrosine sulfated CCR5 peptide [25, 56, 57] on expression on HIV-1 BaL of binding sites for mAb NC-1. Purified HIV-1 BaL (quantity indicated in the legend for Fig. 2) was mixed with the tyrosine sulfated S peptide and a control non-sulfated peptide, respectively, from CCR5 for 5 min at 37°C, followed by 30 min at 4°C. Control virus was incubated under the same conditions in the absence of the peptide. Serial dilutions (indicated on the abscissa) of the treated and untreated virus were added to mAb NC-1 and control normal mouse IgG coated wells, respectively, and the bound virus was quantitated by ELISA for p24. The quantity of virus bound to control wells was ≤ 2.3% of that bound to mAb NC-1 coated wells. All experiments were done at least in triplicate.