Characterization of the effects and potential mechanisms leading to increased megakaryocytic differentiation under mild hyperthermia

Generation and manipulation of human iPSC-derived platelets

The discovery of iPSCs has led to the ex vivo production of differentiated cells for regenerative medicine. In the case of transfusion products, the derivation of platelets from iPSCs is expected to complement our current blood-donor supplied transfusion system through donor-independent production with complete pathogen-free assurance. This derivation can also overcome alloimmune platelet transfusion refractoriness by resulting in autologous, HLA-homologous or HLA-deficient products. Several developments were necessary to produce a massive number of platelets required for a single transfusion. First, expandable megakaryocytes were established from iPSCs through transgene expression. Second, a turbulent-type bioreactor with improved platelet yield and quality was developed. Third, novel drugs that enabled efficient feeder cell-free conditions were developed. Fourth, the platelet-containing suspension was purified and resuspended in an appropriate buffer. Finally, the platelet product needed to be assured for competency and safety including non-tumorigenicity through in vitro and in vivo preclinical tests. Based on these advancements, a clinical trial has started. The generation of human iPSC-derived platelets could evolve transfusion medicine to the next stage and assure a ubiquitous, safe supply of platelet products. Further, considering the feasibility of gene manipulations in iPSCs, other platelet products may bring forth novel therapeutic measures.

On the Quest for In Vitro Platelet Production by Re-Tailoring the Concepts of Megakaryocyte Differentiation

The demand of platelet transfusions is steadily growing worldwide, inter-donor variation, donor dependency, or storability/viability being the main contributing factors to the current global, donor-dependent platelet concentrate shortage concern. In vitro platelet production has been proposed as a plausible alternative to cover, at least partially, the increasing demand. However, in practice, such a logical production strategy does not lack complexity, and hence, efforts are focused internationally on developing large scale industrial methods and technologies to provide efficient, viable, and functional platelet production. This would allow obtaining not only sufficient numbers of platelets but also functional ones fit for all clinical purposes and civil scenarios. In this review, we cover the evolution around the in vitro culture and differentiation of megakaryocytes into platelets, the progress made thus far to bring the culture concept from basic research towards good manufacturing practices certified production, and subsequent clinical trial studies. However, little is known about how these in vitro products should be stored or whether any safety measure should be implemented (e.g., pathogen reduction technology), as well as their quality assessment (how to isolate platelets from the rest of the culture cells, debris, microvesicles, or what their molecular and functional profile is). Importantly, we highlight how the scientific community has overcome the old dogmas and how the new perspectives influence the future of platelet-based therapy for transfusion purposes.

In vitro platelet production for transfusion purposes: Where are we now?

Over the last decade there has been a worldwide increase in the demand of platelet concentrates (PCs) for transfusion. This is, to a great extent, due to a growing and aging population with the concomitant increase in the incidence of onco-hematological diseases, which require frequent platelet (PLT) transfusions. Currently, PLTs are sourced uniquely from donations, and their storage time is limited only to a few days. The necessity to store PCs at room temperature (to minimize loss of PLT functional integrity), poses a major risk for bacterial contamination. While the implementation of pathogen reduction treatments (PRTs) and new-generation PLT additive solutions have allowed the extension of the shelf life and a safer PLT transfusion product, the concern of PCs shortage still pressures the scientific community to find alternative solutions with the aim of meeting the PLT transfusion increasing demand. In this concise report, we will focus on the efforts made to produce, in in vitro culture, high yields of viable and functional PLTs for transfusion purposes in a cost-effective manner, meeting not only current Good Manufacturing Practices (cGMPs), but also transfusion safety standards.

Platelet production from induced pluripotent stem cells

Ex vivo production of human platelets has been pursued as an alternative measure to resolve limitations in the supply and safety of current platelet transfusion products. To this end, induced pluripotent stem cells (iPSCs) are considered an ideal global source, as they are not only pluripotent and self-renewing, but are also available from basically any person, have relatively few ethical issues, and are easy to manipulate. From human iPSCs, megakaryocyte (MK) lines with robust proliferation capacity have been established by the introduction of specified sets of genes. These expandable MKs are also cryopreservable and thus would be suitable as master cells for good manufacturing practice (GMP)-grade production of platelets, assuring availability on demand and safety against blood-borne infections. Meanwhile, developments in bioreactors that physically mimic the in vivo environment and discovery of substances that promote thrombopoiesis have yielded competent platelets with improved efficiency. The derivation of platelets from iPSCs could further resolve transfusion-related alloimmune complications through the manufacturing of autologous products and human leukocyte antigen (HLA)-compatible platelets from stocked homologous HLA-type iPSC libraries or by manipulation of HLAs and human platelet antigens (HPAs). Considering these key advances in the field, HLA-deleted platelets could become a universal product that is manufactured at industrial level to safely fulfill almost all demands. In this review, we provide an overview of the ex vivo production of iPSC-derived platelets toward clinical applications, a production that would revolutionize the blood transfusion system and lead the field of iPSC-based regenerative medicine.

Platelet bioreactor-on-a-chip

Platelet transfusions total >2.17 million apheresis-equivalent units per year in the United States and are derived entirely from human donors, despite clinically significant immunogenicity, associated risk of sepsis, and inventory shortages due to high demand and 5-day shelf life. To take advantage of known physiological drivers of thrombopoiesis, we have developed a microfluidic human platelet bioreactor that recapitulates bone marrow stiffness, extracellular matrix composition,micro-channel size, hemodynamic vascular shear stress, and endothelial cell contacts, and it supports high-resolution live-cell microscopy and quantification of platelet production. Physiological shear stresses triggered proplatelet initiation, reproduced ex vivo bone marrow proplatelet production, and generated functional platelets. Modeling human bone marrow composition and hemodynamics in vitro obviates risks associated with platelet procurement and storage to help meet growing transfusion needs.

Regulation of the genetic code in megakaryocytes and platelets

Platelets are generated from nucleated precursors referred to as megakaryocytes. The formation of platelets is one of the most elegant and unique developmental processes in eukaryotes. Because they enter the circulation without nuclei, platelets are often considered simple, non-complex cells that have limited functions beyond halting blood flow. However, emerging evidence over the past decade demonstrates that platelets are more sophisticated than previously considered. Platelets carry a rich repertoire of messenger RNAs (mRNAs), microRNAs (miRNAs), and proteins that contribute to primary (adhesion, aggregation, secretion) and alternative (immune regulation, RNA transfer, translation) functions. It is also becoming increasingly clear that the 'genetic code' of platelets changes with race, genetic disorders, or disease. Changes in the 'genetic code' can occur at multiple points including megakaryocyte development, platelet formation, or in circulating platelets. This review focuses on regulation of the 'genetic code' in megakaryocytes and platelets and its potential contribution to health and disease.

Manipulating megakaryocytes to manufacture platelets ex vivo

Historically, platelet transfusion has proven a reliable way to treat patients suffering from thrombocytopenia or similar ailments. An undersupply of donors, however, has demanded alternative platelet sources. Scientists have therefore sought to recapitulate the biological events that convert hematopoietic stem cells into platelets in the laboratory. Such platelets have shown good function and potential for treatment. Yet the number manufactured ex vivo falls well short of clinical application. Part of the reason is the remarkable gaps in our understanding of the molecular mechanisms driving platelet formation. Using several stem cell sources, scientists have progressively clarified the chemical signaling and physical microenvironment that optimize ex vivo platelets and reconstituted them in synthetic environments. Key advances in cell reprogramming and the ability to propagate self-renewal have extended the lifetime of megakaryocytes to increase the pool of platelet progenitors.

Scalable generation of universal platelets from human induced pluripotent stem cells

Human induced pluripotent stem cells (iPSCs) provide a potentially replenishable source for the production of transfusable platelets. Here, we describe a method to generate megakaryocytes (MKs) and functional platelets from iPSCs in a scalable manner under serum/feeder-free conditions. The method also permits the cryopreservation of MK progenitors, enabling a rapid “surge” capacity when large numbers of platelets are needed. Ultrastructural/morphological analyses show no major differences between iPSC platelets and human blood platelets. iPSC platelets form aggregates, lamellipodia, and filopodia after activation and circulate in macrophage-depleted animals and incorporate into developing mouse thrombi in a manner identical to human platelets. By knocking out the β2-microglobulin gene, we have generated platelets that are negative for the major histocompatibility antigens. The scalable generation of HLA-ABC-negative platelets from a renewable cell source represents an important step toward generating universal platelets for transfusion as well as a potential strategy for the management of platelet refractoriness.

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Large-scale production of platelets from human iPSCs under defined conditions

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iPSC platelets are functional both in vivo and in vitro

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Knockout of β2-microglobulin gene in iPSCs generates universal platelets

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.

Challenges and promises for the development of donor-independent platelet transfusions

Platelet transfusions are often a life-saving intervention, and the use of platelet transfusions has been increasing. Donor-derived platelet availability can be challenging. Compounding this concern are additional limitations of donor-derived platelets, including variability in product unit quality and quantity, limited shelf life and the risks of product bacterial contamination, other transfusion-transmitted infections, and immunologic reactions. Because of these issues, there has been an effort to develop strategies to generate platelets from exogenously generated precursor cells. If successful, such platelets have the potential to be a safer, more consistent platelet product, while reducing the necessity for human donations. Moreover, ex vivo-generated autologous platelets or precursors may be beneficial for patients who are refractory to allogeneic platelets. For patients with inherited platelet disorders, ex vivo-generated platelets offer the promise of a treatment via the generation of autologous gene-corrected platelets. Theoretically, ex vivo-generated platelets also offer targeted delivery of ectopic proteins to sites of vascular injury. This review summarizes the current, state-of-the-art methodologies in delivering a clinically relevant ex vivo-derived platelet product, and it discusses significant challenges that must be overcome for this approach to become a clinical reality.

Ex vivo differentiation of cord blood stem cells into megakaryocytes and platelets

Megakaryocytes (MK) are hematopoietic cells present in the bone marrow that are responsible for the production and release of platelets in the circulation. Given their very low frequency (<1%), human MK often need to be derived in culture to study their development or to generate sufficient material for biological studies. This chapter describes a simplified 14-day culture protocol that efficiently leads to the production of MK and platelets from cord blood enriched progenitor cells. A serum-free medium is suggested for the growth of the CB cells together with an optimized cytokine cocktail developed specifically for MK differentiation, expansion, and maturation. Methodologies for flow cytometry analysis, MK and platelets estimation, and MK progenitor assay are also presented.

Mild hyperthermia promotes and accelerates development and maturation of erythroid cells

Hyperthermia treatment has at times been associated with increased platelet levels in humans. The heat shock protein HSP70, which can be induced by hyperthermia in megakaryocytes and erythrocytes, was recently shown to protect GATA-1 from degradation and to be required for erythroid differentiation. Based on these findings, we hypothesize that mild hyperthermia (MH), such as fever (39°C), could impact the differentiation of hematopoietic progenitors into erythrocytes and their subsequent maturation. Cell growth and erythroid differentiation increased dramatically in cord blood CD34(+) cell cultures incubated under MH. Erythroid maturation was also strongly promoted, which resulted in an increased proportion of hemoglobinized and enucleated erythroids. The rise in erythroid development was traced to a strong synergistic activity between MH and erythropoietin (EPO). The molecular basis for this potent synergy appears to originate from the capacity of MH to increase the basal activation of several signaling molecules downstream of the EPO receptor and the transcriptional activity of GATA-1. Moreover, the potent impact of MH on erythroid development was found be dependent on increased intracellular levels of reactive oxygen species. Thus, fever-like temperatures can promote the differentiation of progenitors along the erythroid lineage and accelerate their maturation through normal regulatory circuitry.

Insulin-like growth factor binding protein-2 and neurotrophin 3 synergize together to promote the expansion of hematopoietic cells ex vivo

Co-culture of Umbilical Cord Blood (UCB) CD34+ cells with irradiated Mesenchymal Stem Cells (MSCs) without contact increase the expansion of Hematopoietic Progenitor Cells (HPC). Neurotrophin-3 (NT-3) and insulin-like growth factor binding protein-2 (IGFBP-2) are two factors whose expressions were significantly elevated in conditioned media derived from irradiated MSCs. To determine whether these factors are partly responsible for the growth promoting potential of MSCs, we investigated their impact on the growth and differentiation of UCB-CD34+ cells. Addition of either factor alone had little impact on cell growth, however both factors synergized together to increase the expansion of total nucleated cells, erythroids, megakaryocytes (Mk) and CD34+ cells. However, in contrast to MSCs they failed to significantly improve the expansion of hematopoietic progenitors. Consistent with the impact of these factors on hematopoietic cells, both synergized to activate ERK1/2 and AKT in primary human UCB cells. In conclusion, the study demonstrates for the first time that a neurotrophin factor can synergize with IGFBP-2 to promote hematopoietic cell expansion.

Single-cell level analysis of megakaryocyte growth and development

Several fundamental questions regarding cell growth and development can be answered by recording and analyzing the history of cells and their progeny. Herein, long-term and large-field live cell imaging was used to study the process of megakaryopoiesis at the single cell level (n = 9300) from human CD34+ cord blood (CB) in the presence of thrombopoietin (TPO) or the cytokine cocktail BS1 with or without nicotinamide (NIC). Comparative analyses revealed that the cocktail BS1 increased the mitotic and proplatelet rate of diploid and polyploid cells, respectively. Conversely, only NIC treatment increased the endomitotic rate of megakaryocytes (MKs) leading to the formation of CB-MKs with ploidy level frequently observed with BM-MKs. However, NIC failed to enhance platelet production. Rather, a 7- and 31-fold reduction in proplatelet formation was observed in tetraploid and octaploid CB-MKs, respectively, and ex vivo platelet production output was reduced by half due to a reduction in MK output in NIC cultures. Unexpectedly, a significant fraction of di- and polyploid CB-MKs were seen to undergo complete proplatelet regression. Though rare (< 0.6%), proplatelet reversal led to the formation of regular round cells that could at times resume normal development. The cell tracking data was then used to investigate the impact of "developmental fate" and ploidy on cell cycling time, and to identify potential developmental patterns. These analyses revealed that cell fate and ploidy level have major impacts on the cell cycling time of the cells, and that four recurrent cell lineage patterns could be identified for CD34+ cells undergoing MK differentiation.

Megakaryocyte and platelet production from human cord blood stem cells

The cloning of thrombopoietin together with advances in the culture of hematopoietic stem cells have paved the way for the study of megakaryopoiesis, ongoing clinical trials and, in the future, for the potential therapeutic use of ex vivo produced blood substitutes, such as platelets. This chapter describes a 14-day culture protocol for the production of human megakaryocytes (MKs) and platelets, and assays that can be used to characterize the functional properties of the platelets produced ex vivo. CD34(+) cells isolated from cord blood cells are grown in a serum-free medium supplemented with newly developed cytokine cocktails optimized for MK differentiation, expansion, and maturation. Detailed methodologies for flow cytometry analysis of MKs and platelets, for the purification of platelets and functional assays, are presented together with supporting figures. The chapter also provides a brief review on megakaryocytic differentiation and ex vivo MK cultures.

Effects of extracellular matrix proteins on the growth of haematopoietic progenitor cells

Umbilical cord blood (UCB) transplantation and haematological recovery are currently limited by the amount of haematopoietic progenitor cells (HPCs) present in each unit. HPCs and haematopoietic stem cells (HSCs) normally interact with cells and extracellular matrix (ECM) proteins present within the endosteal and vascular niches. Hence, we investigated whether coating of culture surfaces with ECM proteins normally present in the marrow microenvironment could benefit the ex vivo expansion of HPCs. Towards this, collagen types I and IV (COL I and IV), laminin (LN) and fibronectin (FN) were tested individually or as component of two ECM-mix complexes. Individually, ECM proteins had both common and unique properties on the growth and differentiation of UCB CD34+ cells; some ECM proteins favoured the differentiation of some lineages over that of others (e.g. FN for erythroids), some the expansion of HPCs (e.g. LN and megakaryocyte (MK) progenitor) while others had less effects. Next, two ECM-mix complexes were tested; the first one contained all four ECM proteins (4ECMp), while the second 'basement membrane-like structure' was without COL I (3ECMp). Removal of COL I led to strong reductions in cell growth and HPCs expansion. Interestingly, the 4ECMp-mix complex reproducibly increased CD34+ (1.3-fold) and CD41+ (1.2-fold) cell expansions at day 6 (P < 0.05) versus control, and induced greater myeloid progenitor expansion (P < 0.05) than 3ECMp. In conclusion, these results suggest that optimization of BM ECM protein complexes could provide a better environment for the ex vivo expansion of haematopoietic progenitors than individual ECM protein.

Irradiated Mesenchymal Stem Cells improve the ex vivo expansion of Hematopoietic Progenitors by partly mimicking the bone marrow endosteal environment

Mesenchymal Stem Cells (MSCs) regulate the growth and differentiation of Hematopoietic Progenitor cells (HPCs) through the release of soluble factors or through their differentiation into osteoblasts. We recently demonstrated that expansion of megakaryocyte (MK) progenitors ex vivo had reached a plateau when CD34(+) cells were grown with two optimized cytokine cocktails developed for the growth of MK. Hence, we sought to determine whether co-culture of CD34(+) cells with Bone Marrow (BM) MSCs could further increase the expansion of myeloid and MK progenitors. First, we tested the impact of cell-cell contact and pre-irradiation treatment of the MSCs to identify the condition that best supports HPC expansion. This screen revealed that HPC expansions were generally greater in the non-contact conditions, and that pre-irradiation of the MSCs appeared to be of added benefits. Improved expansion of both myeloid and MK progenitors in co-culture with irradiated MSCs without contact was subsequently confirmed. Next, cytokine array profiling was carried out to investigate why irradiation promoted progenitor expansion. This revealed that the levels of as many as 33 factors were potentially altered. ELISA confirmed the significant up regulation of NT-3 and IGFBP-2. Since, these factors are known to be released by and important for osteogenic and endothelial cells, we investigated and confirmed that irradiation of MSCs induced their rapid differentiation into osteogenic-like cells, but not into endothelial-like cells. Supporting this finding, expansions of myeloid and MK progenitors were increased when CD34(+) cells were co-culture with MSCs-derived osteoblasts. Altogether, these results indicate that the improved expansion of HPCs obtained with irradiated MSCs is due in part to their differentiation into osteoblast-like cells, thereby recreating an endosteal-like environment that provides improved support for HPCs expansion.

Individual and synergistic cytokine effects controlling the expansion of cord blood CD34(+) cells and megakaryocyte progenitors in culture

BACKGROUND AIMS

Expansion of hematopoietic progenitors ex vivo is currently investigated as a means of reducing cytopenia following stem cell transplantation. The principal objective of this study was to develop a new cytokine cocktail that would maximize the expansion of megakaryocyte (Mk) progenitors that could be used to reduce periods of thrombocytopenia.

METHODS

We measured the individual and synergistic effects of six cytokines [stem cell factor (SCF), FLT-3 ligand (FL), interleukin (IL)-3, IL-6, IL-9 and IL-11] commonly used to expand cord blood (CB) CD34(+) cells on the expansion of CB Mk progenitors and major myeloid populations by factorial design.

RESULTS

These results revealed an elaborate array of cytokine individual effects complemented by a large number of synergistic and antagonistic interaction effects. Notably, strong interactions with SCF were observed with most cytokines and its concentration level was the most influential factor for the expansion and differentiation kinetics of CB CD34(+) cells. A response surface methodology was then applied to optimize the concentrations of the selected cytokines. The newly developed cocktail composed of SCF, thrombopoietin (TPO) and FL increased the expansion of Mk progenitors and maintained efficient expansion of clonogenic progenitors and CD34(+) cells. CB cells expanded with the new cocktail were shown to provide good short- and long-term human platelet recovery and lymphomyeloid reconstitution in NOD/SCID mice.

CONCLUSIONS

Collectively, these results define a complex cytokine network that regulates the growth and differentiation of immature and committed hematopoietic cells in culture, and confirm that cytokine interactions have major influences on the fate of hematopoietic cells.

Glycoprotein Ibα receptor instability is associated with loss of quality in platelets produced in culture

The development of culture processes for hematopoietic progenitors could lead to the development of a complementary source of platelets for therapeutic purposes. However, functional characterization of culture-derived platelets remains limited, which raises some uncertainties about the quality of platelets produced in vitro. The aim of this study was to define the proportion of functional platelets produced in cord blood CD34+ cell cultures. Toward this, the morphological and functional properties of culture-derived platelet-like particles (PLPs) were critically compared to that of blood platelets. Flow cytometry combined with transmission electron microscopy analyses revealed that PLPs formed a more heterogeneous population of platelets at a different stage of maturation than blood platelets. The majority of PLPs harbored the fibrinogen receptor αIIbβ3, but a significant proportion failed to maintain glycoprotein (GP)Ibα surface expression, a component of the vWF receptor essential for platelet functions. Importantly, GPIbα extracellular expression correlated closely with platelet function, as the GPIIb+ GPIbα+ PLP subfraction responded normally to agonist stimulation as evidenced by α-granule release, adhesion, spreading, and aggregation. In contrast, the GPIIb+ GPIbα⁻ subfraction was unresponsive in most functional assays and appeared to be metabolically inactive. The present study confirms that functional platelets can be generated in cord blood CD34+ cell cultures, though these are highly susceptible to ectodomain shedding of receptors associated with loss of function. Optimization of culture conditions to prevent these deleterious effects and to homogenize PLPs is necessary to improve the quality and yields of culture-derived platelets before they can be recognized as a suitable complementary source for therapeutic purposes.

In vitro megakaryocyte production and platelet biogenesis: state of the art

The exciting and extraordinary capabilities of stem cells to proliferate and differentiate into numerous cell types not only offers promises for changing how diseases are treated but may also impact how transfusion medicine may be practiced in the future. The possibility of growing platelets in the laboratory to some day supplement and/or replace standard platelet products has clear advantages for blood centers and patients. Because of the high utilization of platelets by patients undergoing chemotherapy or receiving stem cell transplants, platelet transfusions have steadily increased over the past decades. This trend is likely to continue as the number of adult and pediatric patients receiving stem cell transplants is also continuously rising. As a result of increased demand, coupled with the short shelf-life of platelet concentrates, providing platelets to patients can stretch the resources of most blood centers and drive donor recruitment efforts, and on occasion, platelet shortages can compromise the care of thrombocytopenic patients.

Ex vivo megakaryocyte expansion and platelet production from human cord blood stem cells

The identification and cloning of thrombopoietin was certainly a defining moment for the study of megakaryopoiesis and thrombopoiesis ex vivo. This and other progresses made in the development of culture processes for hematopoietic stem cells have paved the way for ongoing clinical trials and, in the future, for the potential therapeutic use of ex vivo produced blood substitutes such as platelets. This chapter describes a 14-day culture protocol for the production of megakaryocytes (MK) and platelets from human cord blood stem cells. The CD34+ cells are grown in a serum-free medium supplemented with a newly developed cytokine cocktail optimizing MK differentiation, expansion, and maturation. A detailed methodology for flow cytometry analysis of the cells and platelets is also presented together with supporting figures. A brief review on megakaryocytic differentiation and ex vivo MK cultures is first presented.