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¹ Institute of Stem Cell Biology and Regenerative Medicine Stanford University School of Medicine Stanford, CA 94305
² Immune Disease Institute, Harvard Stem Cell Institute Department of Pathology, Harvard Medical School Boston, MA 02115
³ Immunology Unit Institution for Experimental Medical Science Lund University, BMC I13, 221 84 Lund, Sweden

CORRESPONDENCE Deepta Bhattacharya: deeptab{at}wustl.edu

Hematopoietic stem cells (HSCs) are thought to reside in discrete niches through stable adhesion, yet previous studies have suggested that host HSCs can be replaced by transplanted donor HSCs, even in the absence of cytoreductive conditioning. To explain this apparent paradox, we calculated, through cell surface phenotyping and transplantation of unfractionated blood, that 1–5% of the total pool of HSCs enters into the circulation each day. Bromodeoxyuridine (BrdU) feeding experiments demonstrated that HSCs in the peripheral blood incorporate BrdU at the same rate as do HSCs in the bone marrow, suggesting that egress from the bone marrow to the blood can occur without cell division and can leave behind vacant HSC niches. Consistent with this, repetitive daily transplantations of small numbers of HSCs administered as new niches became available over the course of 7 d led to significantly higher levels of engraftment than did large, single-bolus transplantations of the same total number of HSCs. These data provide insight as to how HSC replacement can occur despite the residence of endogenous HSCs in niches, and suggest therapeutic interventions that capitalize upon physiological HSC egress.

D. Bhattacharya’s present address is Dept. of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO 63110.

Abbreviations used: EPO, erythropoietin; G-CSF, granulocyte colony stimulating factor; HSC, hematopoietic stem cell; KLS, c-kit⁺ lineage- Sca-1⁺; M-CSF, macrophage colony stimulating factor; MP, myeloid progenitor; MPP, multipotent progenitor; SCF, stem cell factor; TPO, thrombopoietin.

© 2009 Bhattacharya et al.
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This article has been cited by other articles:

• Bhattacharya, D., Czechowicz, A., Ooi, A.G. L., Rossi, D. J., Bryder, D., Weissman, I. L. (2009). Niche recycling through division-independent egress of hematopoietic stem cells. JCB 187: i6-i6 [Full Text]  

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