CXCR4 expression and down-modulation in response to CXCL12. Purified CLL cells from ZAP-70⁺ CLL samples (n = 7) or ZAP-70⁻ CLL samples (n = 8) were incubated with or without CXCL12 (80nM) for 30 minutes. The CLL cells were either collected immediately and stained to assess CXCR4 down-modulation after incubation with its ligand (A) or washed and recultured for another 30, 60, or 240 minutes and then collected and stained to study CXCR4 reexpression overtime in the absence of the ligand (B). Cells were analyzed by flow cytometry, and the data shown depict CXCR4 expression as mean fluorescence intensity (MFI). (A) CXCR4 expression level is compared in the presence or absence of CXCL12 for 30 minutes. (B) CXCR4 reexpression is expressed as a percentage of control, which is the level of CXCR4 remaining on the surface after 30 minutes of CXCL12 stimulation, and corresponds to time 0.