The quantification of multipotent HSPCs is commonly performed according to the lineage-relationships proposed by the classical model of human hematopoiesis. One promising strategy employs a feeder-based co-culture system to mimic the bone marrow (BM) stem cell niche for the expansion of multipotent HSPCs for experimental, pre-clinical as well as clinical approaches 13, 14, 15, 16, reviewed in 17, 18. Since the HSPC yield of a single UCB unit in many cases is insufficient to transplant adults, a variety of ex vivo cultures conditions supporting the expansion of multipotent HSPCs has been reported within the last years 8, 9, 10, 11, 12. To overcome these limitations, many researchers aim to use umbilical cord blood (UCB)-derived HSPCs for allogeneic stem cell transplantation (alloSCT), which have been shown to require a lower cell dose per kg body weight, and HLA-mismatches are better tolerated by the host 6, 7. However, matched donors can be difficult to identify for some patients and transplantation of partly-matched, unrelated donors can result in life-threatening side-effects such as Graft-vs-Host disease (GVHD) 2, 3, 4, 5. The common treatment for many of these diseases involves the transplantation of allogeneic hematopoietic stem cells (HSCs) from HLA-matched unrelated or related donors 1. Loss, exhaustion, or acquisition of DNA damage in these HSPCs can lead to imbalanced lineage-output, anemia or hematological malignancies. Self-renewal and differentiation of multipotent hematopoietic stem and progenitor cells (HSPCs) need to be highly controlled in order to warrant a life-long supply of mature blood cells. Instead, enhanced expansion and a consistent bias towards lympho-myeloid committed LMPPs were observed. In our conditions, neither classical co-cultures of HSPCs with primary ECs or MSCs, even in combination, nor the xenograft environment in immunocompromised mice efficiently support the expansion of multipotent HSPCs. Consistent with the in vitro findings, a bias towards lympho-myeloid lineage potentials was observed. Similarly, following transplantation into immunocompromised mice the percentage of multipotent HSPCs within the engrafted HSPC population was significantly decreased compared to the original graft. Instead, mainly lympho-myeloid primed progenitors (LMPPs) were expanded. Even though a massive expansion of total CD34 + HSPCs was observed, none of the tested culture conditions supported the expansion or maintenance of multipotent HSPCs. With the aim to exploit niche cell properties for experimental and potential clinical applications, we analyzed the potential of primary ECs alone and in combination with MSCs to support the ex vivo expansion/maintenance of human hematopoietic stem and progenitor cells (HSPCs). hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). Both cell types appear to be required to support the maintenance and expansion of multipotent hematopoietic cells, i.e. Endothelial and mesenchymal stromal cells (ECs/MSCs) are crucial components of hematopoietic bone marrow stem cell niches.
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