Highly Efficient Ex Vivo Expansion of Human Hematopoietic Stem Cells Using Delta1-Fc Chimeric Protein.

[Background and purposes] Ex vivo expansion of hematopoietic stem cells (HSCs) has been explored in the fields of stem cell biology, gene therapy and clinical transplantation. The use of Notch ligands or soluble IL-6 receptor combined with IL-6 has been a major technique that revealed several fold expansion of human cord blood SCID repopulating cells (SRCs). These studies, however, have been conducted in an independent manner, which hampered direct comparison and evaluation of the effect of combination of these methods. Our purpose of this study is to clarify these issues in a chemically defined serum-free medium that allows us to develop clinical usage of the culture condition. We also compared the efficiencies of SRC isolation with magnetic beads targeting CD34 and CD133.

[Methods] Human cord blood CD133-sorted cells were cultured on immobilized Delta1 supplemented with stem cell factor, thrombopoietin, flt-3 ligand, IL-3 and IL-6/soluble IL-6 receptor chimeric protein (FP6) for three weeks, and cultured cells were transplanted into NOD/SCID mice after limiting dilution to calculate the number of SRCs. To confirm whether full multipotency and self-renewal capacity of SRCs were maintained during the culture, cells were transplanted serially into NOD/SCID/γc^null (NOG) mice, and hematopoietic reconstitution was examined. To compare the efficiencies of CD34- and CD133-sorting, we divided each sample into two aliquots and separated CD34⁺ and CD133⁺ cells, and calculated the SRC numbers recovered by both separation methods.

[Results and discussion] The frequencies of SRCs in the culture-initiating CD133-sorted cells and cultured progeny were calculated as one out of 1,020 and one out of 175 (adjusted to culture-initiating cells), respectively, indicating 6-fold expansion of SRCs that was statistically significant. Delta1 significantly enhanced the expansion rate of SRCs, and addition of IL-3 to this condition further promoted the expansion. In the serial transplantation assays, we found human myeloid and lymphoid reconstitution both in the primary and secondary NOG recipients, verifying the SRC capacity in the cultured cells. Notably, the CD133-sorting was approximately 4.5 times more efficient in collecting SRCs than the CD34-sorting from the same number of mononuclear cells (MNCs) (308 and 254 SRCs by CD133-sorting vs. 67 and 59 SRCs by CD34-sorting from 10⁸ MNCs). Our study provides a promising method to expand HSCs and encourages future trials on clinical transplantation.