Cotransplantation of Ex Vivo Expanded Progenitors with Nonexpanded Cord Blood Cells Improves Platelet Recovery

Effect of Mesenchymal Stem Cell-derived Microvesicles on Megakaryocytic Differentiation of CD34+ Hematopoietic Stem Cells

Purpose: Mesenchymal stem cells (MSCs) release hematopoietic cytokines, growth factors, and Microvesicles (MVs) supporting the hematopoietic stem cells (HSCs). MVs released from various cells, playing a crucial role in biological functions of their parental cells. MSC-derived MVs contain microRNAs and proteins with key roles in the regulation of hematopoiesis. Umbilical cord blood (UCB) is a source for transplantation but the long-term recovery of platelets is a main problem. Therefore, we intend to show that MSC-MVs are able to improve the differentiation of UCB-derived CD34⁺ cells to megakaryocyte lineage.

Methods: In this descriptive study, MSCs were cultured in DMEM to collect the culture supernatant, which was ultracentrifuged for the isolation of MVs. HSCs were isolated from UCB using MACS method and cultured in IMDM supplemented with cytokines and MVs in three different conditions. Megakaryocyte differentiation was evaluated through the expression of specific markers and genes after 72 hours, and the data was analyzed by t test (P<0.05).

Results: The expression of specific megakaryocyte markers (CD41 and CD61) in the presence of different concentrations of MSC-MVs did not show any significant difference. Also, the expression of specific genes of megakaryocyte lineage was compared with control group. The expression of GATA2 and c-Mpl was significantly increased, GATA1 was not significantly decreased, and FLI1 was significantly decreased.

Conclusion: MSC-MVs could improve the expression of specific megakaryocyte genes; however, there was no significant expression of CD markers. Further studies, including the evaluation of late stages of megakaryocyte differentiation, are required to evaluate platelet production and shedding

Paracrine Factors Released by Osteoblasts Provide Strong Platelet Engraftment Properties

Ex vivo expansion of hematopoietic stem cell (HSCs) and progenitors may one day overcome the slow platelet engraftment kinetics associated with umbilical cord blood transplantation. Serum-free medium conditioned with osteoblasts (i.e., osteoblast-conditioned medium [OCM]) derived from mesenchymal stromal cells (MSC) was previously shown to increase cell growth and raise the levels of human platelets in mice transplanted with OCM-expanded progenitors. Herein, we characterized the cellular and molecular mechanisms responsible for these osteoblast-derived properties. Limiting dilution transplantation assays revealed that osteoblasts secrete soluble factors that synergize with exogenously added cytokines to promote the production of progenitors with short-term platelet engraftment activities, and to a lesser extent with long-term platelet engraftment activities. OCM also modulated the expression repertoire of cell-surface receptors implicated in the trafficking of HSC and progenitors to the bone marrow. Furthermore, OCM contains growth factors with prosurvival and proliferation activities that synergized with stem cell factor. Insulin-like growth factor (IGF)-2 was found to be present at higher levels in OCM than in control medium conditioned with MSC. Inhibition of the IGF-1 receptor, which conveys IGF-2' intracellular signaling, largely abolished the growth-promoting activity of OCM on immature CD34⁺ subsets and progenitors in OCM cultures. Finally, IGF-1R effects appear to be mediated in part by the coactivator β-catenin. In summary, these results provide new insights into the paracrine regulatory activities of osteoblasts on HSC, and how these can be used to modulate the engraftment properties of human HSC and progenitors expanded in culture. Stem Cells 2019;37:345-356.

Impact of osteoblast maturation on their paracrine growth enhancing activity on cord blood progenitors

BACKGROUND

Osteoblasts possess strong growth modulatory activity on haematopoietic stem cells and progenitors. We sought to characterise the growth and differentiation modulatory activities of human osteoblasts at distinct stages of maturation on cord blood (CB) progenitors in the context of osteoblast conditioned medium (OCM).

METHODS

OCM was produced from MSC-derived osteoblasts (M-OST) at distinct stages of maturation. The growth modulatory activities of the OCM were tested on CB CD34⁺ cells using different functional assays.

RESULTS

OCMs raised the growth of CB cells and expansion of CD34⁺ cells independently of the maturation status of M-OST. However, productions of immature CB cells including committed and multipotent progenitors were superior with OCM produced with immature osteoblasts. Osteogenic differentiation was accompanied by the upregulation of IGFBP-2, by several members of the Angpt-L family of growth factor, and by the Notch ligands Dll-1 and Dll-4. However, the growth activity of OCM and the in vivo engraftment properties of OCM-expanded CB cells were retained after IGFBP-2 neutralisation. Similarly, OCM-mediated expansion of CB myeloid progenitors was largely independent of Notch signalling.

CONCLUSIONS

These results demonstrate that immature osteoblasts possess greater regulatory activity over haematopoietic progenitors, and that this activity is not entirely dependent on Notch signalling.

Proliferation extent of CD34+ cells as a key parameter to maximize megakaryocytic differentiation of umbilical cord blood-derived hematopoietic stem/progenitor cells in a two-stage culture protocol

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A two-stage protocol established aiming at effective Mk differentiation of UCB CD34⁺-enriched cells.

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Proliferation extent of CD34⁺ cells during expansion identified as a key parameter to maximize Mk differentiation.

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Morphological analysis demonstrated the characteristic features of ex-vivo generated Mks and platelet-like particles.

Co-infusion of ex-vivo generated megakaryocytic progenitors with hematopoietic stem/progenitor cells (HSC/HPC) may contribute to a faster platelet recovery upon umbilical cord blood (UCB) transplantation. A two stage protocol containing cell expansion and megakaryocyte (Mk) differentiation was established using human UCB CD34⁺-enriched cells. The expansion stage used a pre-established protocol supported by a human bone marrow mesenchymal stem cells (MSC) feeder layer and the differentiation stage used TPO (100 ng/mL) and IL-3 (10 ng/mL). 18% of culture-derived Mks had higher DNA content (>4 N) and were able to produce platelet-like particles. The proliferation extent of CD34⁺ cells obtained in the expansion stage (FI-CD34⁺), rather than expansion duration, determined as a key parameter for efficient megakaryocytic differentiation. A maximum efficiency yield (EY) of 48 ± 7.7 Mks/input CD34⁺ cells was obtained for a FI-CD34⁺ of 17 ± 2.5, where a higher FI-CD34⁺ of 42 ± 13 resulted in a less efficient megakaryocytic differentiation (EY of 22 ± 6.7 and 19 ± 4.6 %CD41).

Medium conditioned with mesenchymal stromal cell-derived osteoblasts improves the expansion and engraftment properties of cord blood progenitors

Strategies to enhance the expansion of umbilical cord blood hematopoietic stem and progenitor cells (HSPCs) are crucial to enable their widespread application to adults and to overcome important limitations, such as delayed engraftment. Osteoblasts regulate HSPCs under steady-state and also under stress conditions, when HSPCs undergo numerous cycles of expansion. We hypothesized that osteoblasts could provide better stimulation for the expansion of multipotent HSPCs and subsequent hematopoietic recovery than mesenchymal stromal cells. Hence, we assessed the growth and engraftment modulatory activities of mesenchymal stromal cell-derived osteoblasts (M-OSTs) on hematopoietic progenitors. Mesenchymal stromal cells and M-OSTs favored the maintenance of CD34(+) cells. The expansion of cord blood CD34(+) cells and myeloid progenitors was highest in cultures supplemented with unfiltered M-OST-conditioned medium (M-OST CM). In addition, increased expression of cell surface receptors important for the homing of progenitors to the bone marrow, C-X-C chemokine receptor type 4 and lymphocyte function-associated antigen 1, was observed in CM-based cultures. Additionally, M-OST CM positively modulated the engraftment properties of expanded progenitors. Most notably, although human platelet levels remained steady in the first 2 weeks in mice transplanted with HSPCs expanded in standard medium, levels in mice transplanted with M-OST CM HSPCs rose continuously. Consistent with this, short-term human progenitor reconstitution was consistently greater in M-OST recipients. Finally, cytokine array-based profiling revealed increases in insulin-like growth factor binding protein 2, chemokines, and myeloid stimulating cytokines in M-OST CM. In conclusion, this study suggests that M-OSTs represent a new underappreciated source of feeder cells for the expansion of HSPCs with enhanced thrombopoietic activity.

Advances in megakaryocytopoiesis and thrombopoiesis: from bench to bedside

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, proliferation and terminal differentiation of megakaryocytic progenitors (MK-p) and maturation of megakaryocytes (MKs) to produce functional platelets. This complex process occurs in specialized niches in the bone marrow where MKs align adjacent to vascular endothelial cells, form proplatelet projections and release platelets into the circulation. Thrombopoietin (THPO, TPO) is the primary growth factor for the MK lineage and necessary at all stages of development. THPO is constitutively produced in the liver, and binds to MPL (c-Mpl) receptor on platelets and MKs. This activates a cascade of signalling molecules, which induce transcription factors to drive MK development and thrombopoiesis. Decreased turnover rate and platelet number result in increased levels of free THPO, which induces a concentration-dependent compensatory response of marrow-MKs to enhance platelet production. Newly developed thrombopoietic agents operating via MPL receptor facilitate platelet production in thrombocytopenic states, primarily immune thrombocytopenia. Other drugs are available for attenuating malignant thrombocytosis. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease states, and the innovative drugs and therapeutic modalities to stimulate or decrease thrombopoiesis.