| Hematopoietic stem cells (HSC) are rare, self-perpetuating cells that give rise to all blood and immune cells. HSC migration into and out of sites of active hematopoiesis is a poorly understood but critical process that underlies clinical stem cell transplantation and that may be important for normal hematopoietic homeostasis. Although most HSC reside in bone marrow (BM), these cells can be “mobilized” from BM cavities to the blood and spleen by treatment with cytokines and other agents. Whereas HSC have been isolated prospectively from hematopoietic tissues such as BM and fetal liver, rigorously purified mobilized HSC have rarely been studied. And although HSC in untreated animals are found in low numbers in peripheral tissues such as blood and liver, the function of these extramedullary HSC has been unclear. Here is described the prospective isolation and characterization of mouse HSC mobilized to the periphery by treatment with cyclophosphamide (Cy) and G-CSF, and the kinetics of HSC mobilization. We show that mobilization by Cy/G-CSF is synchronized with the cell cycle, such that dividing HSC leave the BM and enter the bloodstream after M-phase but prior to the next S-phase. Although HSC mobilized to the blood appear to be quiescent due to their 2n DNA content, they are actually cycling cells whose residence time in the bloodstream is very short (seconds to a few minutes). We describe the unique pattern of responsiveness of HSC to chemotactic cyto kines (chemokines), and show that HSC mobilization, as well as engraftment of transplanted HSC, do not rely on the ubiquitous leukocyte adhesion molecule L-selectin. Finally, we demonstrate that in normal, untreated animals, large numbers of HSC and progenitor cells constitutively and rapidly migrate through the blood, and that these cells play a physiologic role in the functional re-engraftment of unconditioned BM. Taken together, these findings shed light on the physiology of induced stem cell mobilization, and the relevance of stem cell migration in normal hematopoietic homeostasis. |
