Expression and affinity of homing-related molecules on steady-state adult and neonate human PB CD34+ cells and their SRC activity.

Although the vast majority of hematopoietic progenitor cells (HPCs) reside within the bone marrow (BM), a small number of HPCs also continuously circulate in the peripheral blood (PB). The examination of the fate of blood-borne HPCs in parabiotic mice, which are surgically conjoined and share a common circulation, recently revealed that steady-state PB HPCs play a physiological role in, at least, the functional re-engraftment of unconditioned BM. To assess the possibility that human HPCs have a similar function, in this study we examined the expression level and affinity of the homing-related molecules, as well as the SCID mouse reconstituting cell (SRC) activity of human PB CD34+ cells, and compared adults with neonates. There was no remarkable difference between adults and neonates in the expression of E- and/or P-selectin ligands by PB CD34+ cells or in these cells' affinity to VCAM-1. In contrast, the expression level of CXCR4 on PB CD34+ cells was much lower in adults than in neonates. Adult cells also showed a much lower SRC activity than neonates. These results suggest that human PB HPCs may contribute to steady-state hematopoiesis in the BM of neonates to some extent, but not so much in adults.