Niche Recycling through Division-Independent Egress of Hematopoietic Stem Cells.

Abstract 79

Hematopoietic stem cells (HSCs) are thought to reside in discrete niches through stable adhesion. However, previous studies through unfractionated bone marrow transplantation experiments, have suggested that host HSCs can be replaced by transplanted donor HSCs, even in the absence of cytoreductive conditioning. The need for ablating host HSCs prior to transplantation to achieve high levels of donor HSC engraftment has been a hotly debated issue over the years, with a number of groups claiming efficient HSC replacement in the absence of prior cytoreductive conditioning of the host, while experimental and clinical studies from our group and others found little evidence for extensive HSC replacement in unconditioned recipients. We specifically examined the intrinsic behavior and replacement properties of HSCs rather than that of unfractionated bone marrow, which contains a number of different cell types that have been reported to influence engraftment and replacement, such as host-reactive T cells and stromal cells. In order to study the physiologic properties of peripheral blood HSCs, we isolated KLS CD27+ IL-7Ra- CD150+ CD34- cells from peripheral blood and were the first to our knowledge to identify these cells as peripheral blood HSCs. We calculated then, through cell surface phenotyping and transplantation of unfractionated blood, that up to 1-5% of the total pool of HSCs enter into the circulation each day. Bromodeoxyuridine (BrdU) feeding of 3, 6, 9 and 12 days demonstrated that HSCs in the bone marrow incorporate BrdU at the same rate as do HSCs in the peripheral blood, suggesting that egress from the bone marrow to the blood can occur without cell division and can leave behind vacant HSC niches. Indeed, transplantation of over 10,000 purified HSCs, representing approximately 50% of the total number of HSCs in a normal animal, into unconditioned wild type mice led to the occupancy of 2-5% of the total number of appropriate niches by the donor HSCs, a similar percentage as was estimated for the fraction of HSCs that egress daily. Additionally, repetitive daily transplantations of small numbers of HSCs administered as new niches became available over the course of 7 days led to significantly higher levels of engraftment than did large single bolus transplantations of the same total number of HSCs. These data demonstrate that niche saturation following transplantation is transient and provide insight as to how HSC replacement can occur despite the residence of endogenous HSCs in niches. We, for the first time, have specifically assessed the number of available HSC niches in normal wild type animals and the rate of their emptying under steady-state conditions. Moreover, our study provides a model that is consistent with host HSC replacement following donor HSC transplantation in unconditioned recipients, yet is also consistent with data suggesting the existence of a physically discrete niche which effectively retains and regulates HSCs. Importantly these data suggest therapeutic interventions that capitalize upon physiological HSC egress, and allow for increased levels of HSC engraftment with non-myeloablative conditioning.