Reoxygenation of cells acclimated to 6% O₂ leads to shedding of CXCR4 and release of microparticles. (A) Protein concentration in the supernatant of 2 × 10⁷ OCI-AML3 cells under 6% and after reexposure with 21% shows a significant increase. The protein fraction also contains CXCR4 as shown by Western blotting. (B) Dot plot of flow cytometry analysis of OCI-AML3 cells. R1 indicates gate for cells. A total of 10 000 cells were counted. R2 indicates gate for microparticles. (C) Number of events in R2 are significantly (P < .05) increased upon reoxygenation in OCI-AML3 and U937 cell lines (left) and patient samples (n = 4, right). (D) OCI-AML3 cells show a highly significant correlation between increase in microparticles and decrease of CXCR4 expression over a period of 30 minutes. Graphs show mean values and SD.