Similar repopulating capacity of mitotically active and resting umbilical cord blood CD34(+) cells in NOD/SCID mice.

It was hypothesized that during mammalian development, the extensive need for hematopoietic cells requires equal contribution to blood cell production from both quiescent and cycling hematopoietic stem cells (HSCs) while maintaining the stem cell pool. To investigate this hypothesis, the engraftment potential of umbilical cord blood (UCB) CD34(+) cells residing in either G(0) (G(0)CD34(+) cells) or G(1) (G(1)CD34(+) cells) phases of the cell cycle was assessed in nonobese diabetic/severe combined immune-deficient (NOD/SCID) mice. Whereas the level of chimerism in mice transplanted with UCB G(0)CD34(+) cells was 69.9% +/- 24.0%, mice receiving equal numbers of G(1)CD34(+) cells harbored 46.7% +/- 21.3% human cells 8 weeks posttransplantation. Both groups of cells sustained multilineage differentiation and the production of CD34(+) cells in recipient animals. The relationship between the number of transplanted G(0)CD34(+) or G(1)CD34(+) cells and the level of chimerism was analyzed by a general linear models procedure. Although the initial level of chimerism following transplantation of G(0)CD34(+) cells was higher than that sustained by G(1)CD34(+) cells, the increment in the degree of chimerism obtained with each additional 10(3) cells of either phenotype was identical, suggesting that the reconstitution potential of these 2 types of cells was similar. Of interest is that human cells recovered from primary recipients of both G(0)CD34(+) and G(1)CD34(+) cells engrafted in secondary NOD/SCID recipients, albeit at a substantially lower level, confirming the primitive nature of UCB CD34(+) cells residing in G(1).