Bone marrow endothelial cells (ECs) are essential for reconstitution of hematopoiesis,
but their role in self-renewal of long-term hematopoietic stem cells (LT-HSCs) is
unknown. We have developed angiogenic models to demonstrate that EC-derived angiocrine
growth factors support in vitro self-renewal and in vivo repopulation of authentic
LT-HSCs. In serum/cytokine-free cocultures, ECs, through direct cellular contact,
stimulated incremental expansion of repopulating CD34(-)Flt3(-)cKit(+)Lineage(-)Sca1(+)
LT-HSCs, which retained their self-renewal ability, as determined by single-cell and
serial transplantation assays. Angiocrine expression of Notch ligands by ECs promoted
proliferation and prevented exhaustion of LT-HSCs derived from wild-type, but not
Notch1/Notch2-deficient, mice. In transgenic notch-reporter (TNR.Gfp) mice, regenerating
TNR.Gfp(+) LT-HSCs were detected in cellular contact with sinusoidal ECs. Interference
with angiocrine, but not perfusion, function of SECs impaired repopulation of TNR.Gfp(+)
LT-HSCs. ECs establish an instructive vascular niche for clinical-scale expansion
of LT-HSCs and a cellular platform to identify stem cell-active trophogens.
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