CD4-independent internalization of CXCR4 and gp120 in early endosomes. CD4⁻/CXCR4⁺ CHO-K1 cells were incubated in medium alone (A), in medium with 10 μg/mL of SDF1 (B), or in medium with 10 μg/mL of gp120 wt (C and D) in the presence of 125 μg/mL of Texas Red-conjugated transferrin for 1 hour at 37°C. After fixation and permeabilization, cells were stained with the FITC-conjugated anti-CXCR4 12G5 MoAb (A, B, and C) or the anti-gp120 110-4 MoAb and an FITC-conjugated antimouse IgG (D). The intracellular localizations of CXCR4 and transferrin (A, B, and C) or gp120 and transferrin (D) were analyzed by confocal microscopy. Yellow spots are indicative of the colocalization of transferrin (red) with either CXCR4 (green) or gp120 (green) in early endosomes.