Transfusion effect on hematopoietic microenvironment. Townes SS mice were transfused with 2 leuko-reduced RBC units to represent short-term transfusions and analyzed 3 days later. (A) Mean fluorescence intensity (MFI) of DCFDA signal measuring ROS among MSCs in transfused mice and SS control phosphate-buffered saline transfused mice (N = 3 mice). (B) Peripheral blood hematopoietic stem and progenitor cell population marked by Lineage-Sca1⁺Ckit⁺Flt3^– is represented as fold change in number per mL of the transfused group relative to control (N = 3-4 mice). (C) Bone marrow HSC frequency determined by flow cytometry (N = 3-4 mice). Gene expression for Cxcl12 (D) and Scf (E) among sorted MSCs in transfused and control groups as determined by using RT-PCR (N = 7-9 mice). Peripheral blood samples from patients with SCD on a chronic RBC exchange transfusion regimen were analyzed immediately before their transfusion and 4 hours after their transfusion . (F) Hemoglobin (HGB) levels in transfused patients before and after transfusion measured by ADVIA automated cell counter (N = 6). (G) Representative histogram of DCFDA levels in human SCD multipotent progenitor cells (Lin^–CD34⁺CD38^–CD45RA^–CD90^–CD49f^–). (H) Fold change of DCFDA levels in human SCD multipotent progenitor cells compared before and after transfusion (N = 6 patients). Data are presented as mean ± standard error of the mean. Data were analyzed with a 2-tailed, unpaired Student t test. (A-F), or a 1-sample Student t test. *P ≤ .05, ****P ≤ .0001.