CD4-independent internalization of cell-surface CXCR4 after interaction with gp120. Cell surface expression of CXCR4 on the human Th2 clone CB-828 (A) was analyzed by flow cytometry after staining with an isotype-matched control MoAb (a) and the FITC-conjugated anti-CXCR4 12G5 MoAb (b). The Th2 cells (clone CB-828) were incubated with either gp120 wt (10 μg/mL; B), a mutant gp120 ▵HX1 that associates with CXCR4 but not CD4 (10 μg/mL; C), SDF1 (10 μg/mL; D), or medium alone (E) for 1 hour at 37°C. After fixation and permeabilization, cells were stained with an FITC-conjugated anti-CXCR4 12G5 MoAb and analyzed by confocal microscopy as described in Materials and Methods.