Failure of sgp120/CD4 to induce CXCR4 downmodulation in CD34⁺ progenitor cells. Purified CD34⁺cells or CD4⁺ cells from UCB were incubated without gp120, with sgp120 (5 μg/mL), or SDF-1 (1 μg/mL) for 2 hours at either +4°C (□) or 37°C (▪) in serum-free medium. Cells were then washed once, labeled with anti-CD4 PE in combination with anti-CXCR4 TC, and analyzed by flow cytometry. An aliquot of cells was incubated with PE- and TC-conjugated isotype control-matched antibodies. Columns represent the mean fluorescence intensity for CD4 expression (A) and CXCR4 expression (B) on CD4⁺ T cells (left) and CD34⁺ cells (right), respectively.