Competitive homing assay in sublethally irradiated NOD/SCID mice coinjected with UTP- and control medium–incubated CD34⁺ HSCs. Sublethally irradiated NOD/SCID mice (2 different experiments with 6 animals for each group in each experiment) were injected with UTP- and control medium–treated CD34⁺ HSCs that had been previously stained with red and green PKH dyes (“Materials and methods”). Twenty-four hours after transplantation, BM and PB samples were evaluated by flow cytometry to assess the frequency of human CD34⁺ HSCs bearing red or green fluorescence. Panels B and C show the gate used to exclude platelets, dead cells, and debris and the gate used to select human CD45⁺CD34⁺ HSCs, respectively. (D) Negative control obtained from a mouse that did not undergo transplantation. (E) Representative evaluation of PKH fluorescence in control BM samples. (F) Representative evaluation of PKH fluorescence in UTP-treated BM samples. In PB, no significant difference in the number of circulating UTP- or control medium–primed CD34⁺ cells was detected. Conversely, in the BM, the percentage of human CD34⁺ cells was significantly increased by 1-hour preincubation with UTP compared with untreated cells (P < .001; G). Longer times of preincubation with the extracellular nucleotide (ie, 6 and 24 hours) did not give different results. These data indicate that short-term incubation with UTP favors the BM homing of CD34⁺ cells. Results are expressed as mean ± SEM. *P values less than .05.