Distinct Mechanisms Underlying the Ex Vivo Expansion of Human Cord Blood Stem Cells with Different Strategies Currently Used for Allogeneic Transplantation

• Published in: Blood 2019-11, Vol.134 (Supplement_1), p.4470-4470
• Main Authors: Papa, Luena, Djedaini, Mansour, Schaniel, Christoph, Hoffman, Ronald
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2019-127040

Umbilical cord blood (UCB) is an alternative source of hematopoietic stem cells (HSCs) for patients that require allogeneic stem cell transplantation. The use of UCB in adults is restricted due to the limited number of HSCs within a single unit. This limitation can be overcome by ex-vivo expansion. Several strategies have successfully expanded the numbers of clinically functional HSCs. Such strategies, including the aryl hydrocarbon receptor antagonist, SR1, pyrimidoindole derivative, UM171, nicotinamide, NAM and more recently, valproic acid (VPA), have entered clinical development. HSC grafts expanded by SR1 and UM171 have been reported to establish rapid and sustained hematological reconstitution in allogeneic transplant recipients [1-3]. Our group has started a clinical trial using VPA-expanded allogeneic UCB grafts (NCT03885947). Initial data have shown multi-lineage donor cell reconstitution with especially rapid T cell and platelet engraftment in patients with treatment refractory acute leukemias. To gain insight into the mechanisms underlying these clinical HSC expansion procedures, we recreated the different approaches to compare the various expanded grafts. Although there is successful clinical data with the NAM approach, we were unable to recreate these conditions in our laboratory and therefore, we could not compare these grafts to those expanded with SR1, UM171 and VPA [4-7]. These comparative studies indicated that each strategy increased to varying degrees both the percentage and absolute numbers of CD34+CD133+CD90+EPCR+ as opposed to cultures treated with their respective cytokine cocktail alone. Although SR1 treatment resulted in the generation of greater numbers of total nucleated cells from purified 1x106 UCB-CD34+ cells (16x107 in SR1 vs 8x107 in UM171 and 7x107 in VPA), the numbers of CD34+CD133+CD90+EPCR+ cells (4x105 in SR1 vs, 7x105 in UM171 and 8.7x105 in VPA) as well as of other CD34+ subpopulations with different degrees of stem cell activity were lower than those observed in VPA and UM171-expanded cultures. In order to understand the underlying mechanism by which these agents act, we isolated CD34+CD90-EPCR- cells and treated with VPA, SR1 or UM171. VPA treatment for 24 hrs resulted in acquisition of CD90 and EPCR expression (80% and 25% of cells acquired CD90 and EPCR, respectively). UM171 treatment led to a modest increase in cells expressing EPCR and CD90. By contrast, SR1 treatment did not induce a phenotypic switch. Comparis