Delayed myeloid engraftment following cord blood transplantation (CBT) is thought to result from inadequate numbers of progenitor cells in the graft and is associated with increased early transplant related morbidity and mortality. Novel culture strategies that increase the number of cord blood (CB) progenitors capable of rapid myeloid engraftment following CBT would allow more widespread use of this stem cell source for transplantation. Here we report development of a clinically relevant Notchmediated ex vivo expansion system for human CD34 + CB progenitors that results in a >100 fold increase in the absolute number of stem/progenitor cells, including those capable of enhanced repopulation in the marrow of immunodeficient NOD/SCID mice. Furthermore, when Notchmediated ex vivo expanded CB progenitors were infused in a clinical setting, rapid recovery of myeloid cells was achieved, demonstrating the first observation of rapid engraftment derived from ex vivo expanded stem/progenitor cells in humans.

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 co-cultures, 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 and interfering 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.

Administration of the antibody-targeted chemotherapy agent Mylotarg to patients with CD33-positive AML in first relapse induces complete remissions with what appears to be a favorable safety profile.