Human CD34⁺ Cells from Different Sources Disclose a Specific Stemness Signature

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.HSCs are maintained in their niche by binding to cellular determinants through adhesion molecules and diverse strategies are currently used to promote their 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, other compounds have been recently tested.

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 blood. The use of this drug is currently approved by FDA and EMA in combination with G-CSF, in patients affected by lymphoma or multiple myeloma whose cells mobilize poorly with G-CSF alone. Clinical trials demonstrated that Plerixafor alone safely and rapidly mobilizes HSCs also in healthy donors, beta-thalassemia patients and pediatric patients affected by malignancies. Previous characterization studies on non-human primates and human samples of Plerixafor mobilized cells in comparison to cells mobilized by G-CSF alone or in combination with Plerixafor showed a different expression profile, cell composition and engrafting potential in a xenotransplant model. From these studies remains unsolved whether Plerixafor, G-CSF, or their combination mobilizes different primitive HSC populations, defined both by multimarker immunophenotype and in vivo functional analysis.

In the present study we investigated by controlled comparative analysis the functional and molecular hallmarks of human HSCs collected from BM, G-CSF and/or Plerixafor mobilized peripheral blood. We show that Plerixafor alone mobilizes preferentially long-term hematopoietic stem cells (LT-HSCs), defined as CD34⁺CD38^/lowCD90⁺CD45RA^-CD49f⁺ cells and primitive populations of HSCs. These cells possess higher ability to home to hematopoietic niches and engraft in NOD/SCID/IL2rγ^null (NSG) mice, resulting in enriched scid-repopulating cell frequency, in comparison to other sources. The higher content of CXCR4⁺ and CD49f⁺ cells correlates with this feature. Furthermore, global gene expression profiling highlights the superior in vivo reconstitution activity of Plerixafor mobilized cells. The "stemness" signature of cells dislodged from their niche by the drug is attenuated by the combined use with G-CSF, which emphasizes the gene expression profile induced by G-CSF treatment.

These data indicate that a qualitative advantage accounts for the superior performance of Plerixafor mobilized cells. These findings provide the rationale for using a suboptimal dose of more primitive HSCs when target cell number for transplantation is limited, or when G-CSF mobilization is too risky like in sickle cell anemia patients.

Moreover, CD34⁺ cells mobilized by Plerixafor alone or with the combination of G-CSF are efficiently transduced by a lentiviral vector encoding for human ß-globin gene (GLOBE LV) and are able to engraft and differentiate in vivo, supporting their use for gene therapy applications.