Duration of POL5551-induced expansion. (A) Schematic presentation of washout studies. C57BL/6 mice (CD45.2⁺) were treated with continuous infusion of the CXCR4 antagonist POL5551 (100 mg/kg per day for 2 weeks) and either analyzed directly (day 0 group) or 1, 3, 7, or 14 days after pump removal. Control mice were sham-operated. PB and BM hematopoiesis was analyzed. CFU-C/LSK concentration in (B) blood and (C) BM is shown. (D-E) BM LSK SLAM (D) numbers and (E) frequency are shown. Corresponding cell cycle analysis was performed on BM LSK and LSK SLAM cells and (F) shows G₀/G₁/G_2/S/M phase distribution. (G-H) At the end of the treatment described in (A), BM (test BM) was mixed at a 1:1 ratio with CD45.1/2⁺ competitor BM (5 × 10⁵ BM cells each per recipient) and transplanted into lethally irradiated CD45.1⁺ recipients. (G) The CD45.2⁺ percentage within the graft-derived (ie, CD45.2⁺ or CD45.1/2⁺) CD45⁺CD3^– population was analyzed at the indicated time points after transplantation (Tx). (H) At 20 weeks after transplantation, the frequency of repopulating units in test BM suspensions was calculated on the basis of the contribution within the non–T-cell fraction (4-12 mice per treatment [test BM donor] group). The data from control and POL5551-treated BM recipients (Day 0) have also been included in calculations presented in Figure 3C (n = 3 competitor BM donor mice; data represent the mean ± SEM of 5-10 mice per transplant recipient group). ***P < .001; **P < .01; *P < .05.