DTNB does not modify the Env binding site on the CD4 or CXCR4. (A) Sup T-1 cells treated with 2.5 mM DTNB or PBS (control) were incubated with 0.5 μg gp120/1 × 10⁶ cells (30 minutes, 22°C) followed by anti-gp120 rabbit serum (15 minutes, 22°C) and PE-conjugated antirabbit secondary Ab or (B) with PE-labeled Leu3A (anti-CD4) MAb (15 minutes, 22°C). After staining, cells were fixed in 3.7% formaldehyde and analyzed with FACScan cytofluorometer. Data shown are representative of 3 experiments. (C) Sup T-1cells treated with 2.5 mM DTNB or PBS (control) were incubated with 12G5 (anti-CXCR4) MAb. After staining, cells were fixed in 3.7% formaldehyde and analyzed with FACScan cytofluorometer. (D) CXCR4 retains the ability to mobilize intracellular Ca²⁺ after DTNB treatment. Sup T-1 cells attached to poly-l-lysine–coated coverslips were loaded with 2.5 μM Fura-2/am and 2.5 mM DTNB or Hanks buffer (control). Fluorescence in cells was excited by alternating 340- and 380-nm light beams, and the emitted light intensity was measured at 520 nm. Data shown reflect a single-cell response in mobilizing intracellular Ca²⁺ induced with 500 ng/mL SDF-1α after following the baseline [Ca²⁺]_i for 60 seconds and are representative of 3 experiments.