The relative quiescence of hematopoietic stem cells in nonhuman primates

Abstract

Quiescence has been thought to be required for the retention of the full biological potential of pluripotent hematopoietic stem cells (PHSCs). This hypothesis has been challenged recently by the observation that all murine PHSCs cycle continuously and constantly contribute to steadystate blood cell production. It was asked whether these observations could be extrapolated to describe hematopoiesis in higher mammals. In this series of experiments, the replicative history of PHSCs was examined in baboons by continuously administering bromodeoxyuridine (BrdU) for more than 85 weeks. The results indicate that under steady-state conditions, PHSCs remain largely quiescent but do cycle, albeit at a far lower rate than previously reported for rodent PHSCs. BrdU-labeled cycling PHSCs and progenitor cells were shown to have an extensive proliferative capacity and to contribute to blood cell production for prolonged periods of time. The proportion of PHSCs entering cell cycle could, however, be rapidly increased by the in vivo administration of granulocyte-colony stimulating factor. These data indicate that dur- IntroductionMammalian blood cell production is a tightly regulated process in which a hierarchy of cycling hematopoietic stem cell populations contributes to active hematopoiesis. [1][2][3][4][5] The formed elements of blood ultimately originate from rare pluripotent hematopoietic stem cells (PHSCs), which are capable of self-renewal and differentiation into cells belonging to each hematopoietic lineage. 6,7 PHSCs contribute to steady-state hematopoiesis and respond to a variety of stressful conditions without actually depleting their number. It is commonly believed that PHSCs remain quiescent during steady-state hematopoiesis and that stem cell quiescence plays a pivotal role in determining the self-renewal and proliferative potential of PHSCs. 8,9 In 1965, Kay 10 postulated that only a few PHSC clones actively contribute to blood cell production at any given time and that only after exhaustion of these active clones would additional PHSCs self-replicate and actively contribute to blood cell production. Experimental evidence supporting this clonal succession hypothesis was first gained by studies of the contribution of murine blood cell production by PHSCs that had been genetically modified by retroviral vectors. 11,12 The contribution of individual PHSCs to blood cell production was monitored by tracking retroviral integration sites in mature blood cells. Abkowitz and coworkers 13 also addressed this hypothesis by performing stem cell transplants in female cats who were heterozygotes for the X-linked gene glucose 6-phosphate dehydrogenase. They observed wide fluctuations in clonal contributions to hematopoiesis following initial engraftment but found that long-term reconstitution was due to a limited number of stem cell clones. 13 The validity of such reconstitution assays as an appropriate test of the clonal succession hypothesis has been questioned since the kinetics of hematopoietic reconstitution following st...

The CDK inhibitors: potential targets for therapeutic stem cell manipulations?

“…The rate of hematopoietic stem cell (HSC) cycling in small rodents was estimated at 30 days by 5-bromodeoxyuridine (BrdU) labeling 8 and up to 1 year in nonhuman primates. 9 Similar cycling kinetics have also been observed in other stem cell types such as mouse cornea epithelia 10 and hair follicles. 11 In the skin, four times as many dermal stem cells are in the S-G 2 /M phases of the cell cycle as compared to more primitive dermal cells.…”

A Nonhuman Primate Model for Urinary Bladder Regeneration Using Autologous Sources of Bone Marrow-Derived Mesenchymal Stem Cells

“…The BM was diluted 1:8 in Dulbecco's phosphate-buffered saline (DPBS; Invitrogen Corporation, San Diego, CA) and the mononuclear cell (MNC) fraction was isolated by centrifugation over a 60% Percoll (Pharmacia, Uppsala, Sweden) density gradient at 500g for 30 minutes at room temperature (RT) [19]. The MNC fraction was collected and cultured in MSC growth media (MSCGM, Lonza, Inc., Walkersville, MD) at a density of 4,000 cells/cm 2 and allowed to proliferate.…”

Cell cycle regulators and hematopoiesis

“…On the other hand, long-term in vivo BrDU labeling experiments indicate that in mice (Bradford et al, 1997;Pietrzyk et al, 1985) and to a lesser degree in primates (Mahmud et al, 2001) hematopoietic stem cells are constantly and slowly cycling. In mice continual BrDU administration demonstrated that Ho-lo/Rho-lo primitive HSC cycle with an average turnover time of 30 days.…”