R4⁺ cells functionally modulate CXCR4 expression.

(A) Cell surface CXCR4 staining. R4⁺ sorted cells (Ai) were cultured for 24 hours (Aii), and 48 hours (Aiii), with 5 cytokines, or for 48 hours with SCF plus IL-6 (Aiv) or in serum-free medium alone (Av). At each time point, cells were restained with antihuman CXCR4-PE for flow cytometry analysis. A representative FACS analysis of 3 independent experiments is shown. (B) Enhancement of CXCR4-dependent repopulation by cytokine stimulation. CXCR4 surface expression of cells from cultures in panels Ai-Aiii was analyzed before and after restaining with anti–CXCR4-PE. CXCR4 re-expression was calculated by dividing values obtained in restained samples by those of cells without restaining. (C) Surface CXCR4 re-expression. Cells cultured with 5 cytokines were also transplanted to determine their engraftment potential. R4⁺ cells were pretreated and coinjected with antihuman CXCR4 (10 μg/mouse) as indicated. Data present mean ± SE values of 3 independent experiments. *P < .03 compared to cells transplanted in day 0 without anti-CXCR4 mAb.