Intrinsic Molecular Features of Human Hematopoietic Stem Cells from Different Sources Define Their Specific Functional Properties

Over the past decades outcomes of clinical hematopoietic stem cell transplants have established a clear relationship between the sources of hematopoietic stem cells (HSCs) infused and their differential homing and engraftment properties. For a long time, bone marrow (BM) harvest has been the preferred source of hematopoietic stem and progenitor cells (HSPCs) for hematopoietic reconstitution following myeloablative conditioning regimen. At present, mobilized peripheral blood (PB) is commonly used for hematopoietic cells transplantation in both adults and children, particularly in the autologous setting, and it has progressively replaced BM as the source of HSCs. So far, the intrinsic molecular features of human primitive HSCs from different sources have not been investigated in comparative studies to unravel their variable reconstitution potential.

Diverse strategies are currently used to disengage HSCs from the niche, promoting egress from BM to PB. Traditionally the growth factor granulocyte-colony stimulating factor (G-CSF) represents the gold standard agent to mobilize HSPCs for transplantation. Nevertheless, many other compounds have been tested to this regard. One of the most successful mobilizing agents is Plerixafor (AMD3100, Mozobil™), a bicyclam molecule that selectively and reversibly antagonizes the binding of stromal cell derived factor-1 (SDF-1), located on the surface of BM stromal cells and osteoclasts, to chemokine CXC-receptor-4 (CXCR4), located on the surface of HSPCs, with the subsequent mobilization in the PB. This drug, which was shown in preclinical combination studies with G-CSF to enhance mobilization compared to G-CSF alone, is currently approved by FDA and EMA "in combination with G-CSF to enhance the mobilization of HSCs into the peripheral blood for collection and autologous transplantation of patients affected by lymphoma or multiple myeloma whose cells mobilize poorly"

We investigated functional and molecular hallmarks of human HSCs from different sources, i.e. BM and PB following mobilization by G-CSF and/or Plerixafor. We show that Plerixafor alone mobilizes preferentially long-term hematopoietic stem cells (LT-HSCs), defined as CD34⁺ CD38^/low CD90⁺ CD45RA^- CD49f⁺ cells and primitive populations of HSCs. These cells are able to provide stable long-term hematopoietic engraftment in NOD/SCID/IL2rγ^null (NSG) mice, resulting in enriched scid-repopulating cell frequency, in comparison to other sources. The quiescence status of these cells correlates with the enriched scid-repopulating cell frequency. Noteworthy, the combined use of G-CSF and Plerixafor mobilizes a CD34⁺ population enriched in immature cells and with a lower engraftment capacity respect to cells mobilized by Plerixafor alone.

Since the signaling provided by the interaction of SDF-1 with CXCR4, plays an essential role in maintaining HSC quiescence and regulating homing, we analyzed the CXCR4 expression. Interestingly, this analysis reveals that the proportion of CXCR4⁺ primitive cells was lower when using G-CSF combined to Plerixafor in respect to Plerixafor alone. These data indicate that the combination of the two mobilizing agents induce a higher amount of circulating CD34⁺ cells but containing a lower proportion of cells capable of homing to BM in NSG mice. . As a result, at a defined dose of transplanted CD34+ cells, less SRCs are observed when G-CSF is added to Plerixafor. Indeed, it is expected to observe also a rapid rescue of hematopoiesis in myeloablated subjects conferred by high amount of short-term progenitors.

Insights into the transcriptional program reveal the molecular machinery underlying stemness features of cells derived from different sources, defining their specific functional properties. Noteworthy, CD34⁺ cells exposed to Plerixafor but still resident in the BM acquire an intermediate signature between steady-state and circulating cells, suggesting an effect of this agent on HSC function.

From preliminary data, genes of Prostaglandin signaling are up-regulated in HSCs mobilized by Plerixafor, suggesting a role of this pathway.

These data uncover unique HSCs properties shaped by their origin and illuminate the choice of different transplantation strategies accordingly to the clinical need.