Chemotaxis and in vitro migration of CD34⁺ HSCs in response to extracellular nucleotides. The chemotactic effect of UTP on CD34⁺ HSCs was assessed in transwell migration assays, as described in “Materials and methods.” UTP alone (10 μM) slightly improved HSC migration but significantly enhanced CXCL12-induced chemotaxis (P < .05; A). The activity of UTP was exerted within 1 hour as shown in priming experiments (UTP/ATP-primed cells). UTP alone showed, per se, a slight chemotactic activity, but it significantly enhanced HSC migration rate when combined with CXCL12 in the lower chamber of transwell assays (P < .05; B). These results were confirmed by clonogenic assays (C-D). Conversely, ATP (1 nM) gave a borderline priming effect on CXCL12-induced chemotaxis (E,G) and no significant chemotactic activity (F,H). Results are expressed as mean ± SEM and were obtained from 6 independent experiments. *P values less than .05.