Preferential ex vivo expansion of megakaryocytes from human cord blood CD34+-enriched cells in the presence of thrombopoietin and limiting amounts of stem cell factor and Flt-3 ligand

Multiphoton In Vivo Microscopy of Embryonic Thrombopoiesis Reveals the Generation of Platelets through Budding

Platelets are generated by specialized cells called megakaryocytes (MKs). However, MK's origin and platelet release mode have remained incompletely understood. Here, we established direct visualization of embryonic thrombopoiesis in vivo by combining multiphoton intravital microscopy (MP-IVM) with a fluorescence switch reporter mouse model under control of the platelet factor 4 promoter (Pf4CreRosa26mTmG). Using this microscopy tool, we discovered that fetal liver MKs provide higher thrombopoietic activity than yolk sac MKs. Mechanistically, fetal platelets were released from MKs either by membrane buds or the formation of proplatelets, with the former constituting the key process. In E14.5 c-Myb-deficient embryos that lack definitive hematopoiesis, MK and platelet numbers were similar to wild-type embryos, indicating the independence of embryonic thrombopoiesis from definitive hematopoiesis at this stage of development. In summary, our novel MP-IVM protocol allows the characterization of thrombopoiesis with high spatio-temporal resolution in the mouse embryo and has identified membrane budding as the main mechanism of fetal platelet production.

Apoptosis in megakaryocytes: Safeguard and threat for thrombopoiesis

Platelets, generated from precursor megakaryocytes (MKs), are central mediators of hemostasis and thrombosis. The process of thrombopoiesis is extremely complex, regulated by multiple factors, and related to many cellular events including apoptosis. However, the role of apoptosis in thrombopoiesis has been controversial for many years. Some researchers believe that apoptosis is an ally of thrombopoiesis and platelets production is apoptosis-dependent, while others have suggested that apoptosis is dispensable for thrombopoiesis, and is even inhibited during this process. In this review, we will focus on this conflict, discuss the relationship between megakaryocytopoiesis, thrombopoiesis and apoptosis. In addition, we also consider why such a vast number of studies draw opposite conclusions of the role of apoptosis in thrombopoiesis, and try to figure out the truth behind the mystery. This review provides more comprehensive insights into the relationship between megakaryocytopoiesis, thrombopoiesis, and apoptosis and finds some clues for the possible pathological mechanisms of platelet disorders caused by abnormal apoptosis.

Ricolinostat promotes the generation of megakaryocyte progenitors from human hematopoietic stem and progenitor cells

Background

Ex vivo production of induced megakaryocytes (MKs) and platelets from stem cells is an alternative approach for supplying transfusible platelets. However, it is difficult to generate large numbers of MKs and platelets from hematopoietic stem cells and progenitor cells (HSPCs).

Methods

To optimize the differentiation efficiency of megakaryocytic cells from HSPCs, we first employed a platelet factor 4 (PF4)-promoter reporter and high-throughput screening strategy to screen for small molecules. We also investigated the effects and possible mechanisms of candidate small molecules on megakaryocytic differentiation of human HSPCs.

Results

The small molecule Ricolinostat remarkably promoted the expression of PF4-promoter reporter in the megakaryocytic cell line. Notably, Ricolinostat significantly enhanced the cell fate commitment of MK progenitors (MkPs) from cord blood HSPCs and promoted the proliferation of MkPs based on cell surface marker detection, colony-forming unit-MK assay, and quantitative real-time PCR analyses. MkPs generated from Ricolinostat-induced HSPCs differentiated into mature MKs and platelets. Mechanistically, we found that Ricolinostat enhanced MkP fate mainly by inhibiting the secretion of IL-8 and decreasing the expression of the IL-8 receptor CXCR2.

Conclusion

The addition of Ricolinostat to the culture medium promoted MkP differentiation from HSPCs and enhanced the proliferation of MkPs mainly by suppressing the IL-8/CXCR2 pathway. Our results can help the development of manufacturing protocols for the efficient generation of MKs and platelets from stem cells in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02722-5.

Isolation of megakaryocytes using magnetic cell separation and adverse effects induced by diclofenac toxicity in an experiment

This study investigates the response of bone marrow (particularly megakaryocytes) in mice under the influence of diclofenac sodium for 10 days using intraperitoneal injection at various doses. A fundamentally new immunomagnetic separation method was applied during the experiment, which helped obtain pure lines of bone marrow cells, particularly megakaryocytes (MC), without admixtures of other cells or their particles. The resulting cells completely retain their structure and can be used in further research. The study determined that different doses of diclofenac sodium have different effects on different groups of diabetes mellitus cells CD34-megakaryocytes. The use of 1.0 mg/ml sharply negatively affects the state of early populations of megakaryocytes (decrease by 80%, p=0.05), a dose of 0.025 mg/ml had the least effect on this population of cells (22.8%, p=0.05). The greatest number of average forms of diabetes mellitus 34 was observed when using a dose of 0.95 mg/ml (22.8%, p=0.05), with a gradual decrease in the dose, the indicator of this group of cells decreased. A dose of 0.03 mg/ml did not affect the quantitative state of megakaryocytes, and a dose of 0.025 mg/ml caused a slight decrease (16.6%, p=0.05). Indicators of mature cells of megakaryocytes CD 34- decreased in all studied groups, however, their maximum value reached a maximum decrease by 0.25 mg/ml (55.2%, p=0.05), the dose of diclofenac sodium 0.03 mg/ml, lower (18.4%, p=0.05). Diclofenac sodium in different doses has different effects on the degree of differentiation of CD 34-. Its introduction positively affects the state of intermediate forms of megakaryocytes, except for minimal doses, while the effect on early and mature forms in all cases turned out to be negative.

Generation of HLA Universal Megakaryocytes and Platelets by Genetic Engineering

Patelet transfusion refractoriness remains a relevant hurdle in the treatment of severe alloimmunized thrombocytopenic patients. Antibodies specific for the human leukocyte antigens (HLA) class I are considered the major immunological cause for PLT transfusion refractoriness. Due to the insufficient availability of HLA-matched PLTs, the development of new technologies is highly desirable to provide an adequate management of thrombocytopenia in immunized patients. Blood pharming is a promising strategy not only to generate an alternative to donor blood products, but it may offer the possibility to optimize the therapeutic effect of the produced blood cells by genetic modification. Recently, enormous technical advances in the field of in vitro production of megakaryocytes (MKs) and PLTs have been achieved by combining progresses made at different levels including identification of suitable cell sources, cell pharming technologies, bioreactors and application of genetic engineering tools. In particular, use of RNA interference, TALEN and CRISPR/Cas9 nucleases or nickases has allowed for the generation of HLA universal PLTs with the potential to survive under refractoriness conditions. Genetically engineered HLA-silenced MKs and PLTs were shown to be functional and to have the capability to survive cell- and antibody-mediated cytotoxicity using in vitro and in vivo models. This review is focused on the methods to generate in vitro genetically engineered MKs and PLTs with the capacity to evade allogeneic immune responses.

Biotechnologisch hergestellte Megakaryozyten und Thrombozyten

Angesichts der ständig steigenden Nachfrage nach Thrombozyten zielen neue Zell-Pharming-Strategien auf die Generierung von Megakaryozyten und Thrombozyten in vitro ab. Dieser Übersichtsartikel analysiert den aktuellen Stand der Methoden zur biotechnologischen Herstellung von Megakaryozyten und Thrombozyten und zeigt die Erarbeitung von Strategien, die darauf abzielen, diese Methoden in die Klinik zu bringen.

Endocytosed factor V is trafficked to CD42b+ proplatelet extensions during differentiation of human umbilical cord blood-derived megakaryocytes

Plasma- and platelet-derived factor Va are essential for thrombin generation catalyzed by the prothrombinase complex; however, several observations demonstrate that the platelet-derived cofactor, which is formed following megakaryocyte endocytosis and modification of the plasma procofactor, factor V, is more hemostatically relevant. Factor V endocytosis, as a function of megakaryocyte differentiation and proplatelet formation, was assessed by flow cytometry and microscopy in CD34⁺ hematopoietic progenitor cells isolated from human umbilical cord blood and cultured for 12 days in the presence of cytokines to induce ex vivo differentiation into megakaryocytes. Expression of an early marker of megakaryocyte differentiation, CD41, endocytosis of factor V, and the percentage of CD41⁺ cells that endocytosed factor V increased from days 6 to 12 of differentiation. In contrast, statistically significant decreases in expression of the stem cell marker, CD34, and in the percentage of CD34⁺ cells that endocytosed factor V were observed. A statistically significant increase in the expression of CD42b, a late marker of megakaryocyte differentiation, was also observed over time, such that by Day 12, all CD42b⁺ cells endocytosed factor V and expressed CD41. This endocytosed factor V was trafficked to proplatelet extensions and was localized in a punctate pattern in the cytoplasm consistent with its storage in α-granules. In conclusion, loss of CD34 and expression of CD42b define cells capable of factor V endocytosis and trafficking to proplatelet extensions during differentiation of megakaryocytes ex vivo from progenitor cells isolated from umbilical cord blood.

Megakaryocyte Differentiation and Platelet Formation from Human Cord Blood-derived CD34+ Cells

Platelet production occurs principally in the bone marrow in a process known as thrombopoiesis. During thrombopoiesis, hematopoietic progenitor cells differentiate to form platelet precursors called megakaryocytes, which terminally differentiate to release platelets from long cytoplasmic processes termed proplatelets. Megakaryocytes are rare cells confined to the bone marrow and are therefore difficult to harvest in sufficient numbers for laboratory use. Efficient production of human megakaryocytes can be achieved in vitro by culturing CD34⁺ cells under suitable conditions. The protocol detailed here describes isolation of CD34⁺ cells by magnetic cell sorting from umbilical cord blood samples. The necessary steps to produce highly pure, mature megakaryocytes under serum-free conditions are described. Details of phenotypic analysis of megakaryocyte differentiation and determination of proplatelet formation and platelet production are also provided. Effectors that influence megakaryocyte differentiation and/or proplatelet formation, such as anti-platelet antibodies or thrombopoietin mimetics, can be added to cultured cells to examine biological function.

Safety and Efficacy of Megakaryocytes Induced from Hematopoietic Stem Cells in Murine and Nonhuman Primate Models

Because of a lack of platelet supply and a U.S. Food and Drug Administration‐approved platelet growth factor, megakaryocytes have emerged as an effective substitute for alleviating thrombocytopenia. Here, we report the development of an efficient two‐stage culture system that is free of stroma, animal components, and genetic manipulations for the production of functional megakaryocytes from hematopoietic stem cells. Safety and functional studies were performed in murine and nonhuman primate models. One human cryopreserved cord blood CD34⁺ cell could be induced ex vivo to produce up to 1.0 × 10⁴ megakaryocytes that included CD41a⁺ and CD42b⁺ cells at 82.4% ± 6.1% and 73.3% ± 8.5% (mean ± SD), respectively, yielding approximately 650‐fold higher cell numbers than reported previously. Induced human megakaryocytic cells were capable of engrafting and producing functional platelets in the murine xenotransplantation model. In the nonhuman primate model, transplantation of primate megakaryocytic progenitors increased platelet count nadir and enhanced hemostatic function with no adverse effects. In addition, primate platelets were released in vivo as early as 3 hours after transplantation with autologous or allogeneic mature megakaryocytes and lasted for more than 48 hours. These results strongly suggest that large‐scale induction of functional megakaryocytic cells is applicable for treating thrombocytopenic blood diseases in the clinic. Stem Cells Translational Medicine2017;6:897–909

Development of autologous blood cell therapies

Allogeneic hematopoietic stem cell transplantation and blood cell transfusions are performed commonly in patients with a variety of blood disorders. Unfortunately, these donor-derived cell therapies are constrained due to limited supplies, infectious risk factors, a lack of appropriately matched donors, and the risk of immunologic complications from such products. The use of autologous cell therapies has been proposed to overcome these shortcomings. One can derive such therapies directly from hematopoietic stem and progenitor cells of individuals, which can then be manipulated ex vivo to produce the desired modifications or differentiated to produce a particular target population. Alternatively, pluripotent stem cells, which have a theoretically unlimited self-renewal capacity and an ability to differentiate into any desired cell type, can be used as an autologous starting source for such manipulation and differentiation approaches. Such cell products can also be used as a delivery vehicle for therapeutics. In this review, we highlight recent advances and discuss ongoing challenges for the in vitro generation of autologous hematopoietic cells that can be used for cell therapy.

Producing megakaryocytes from a human peripheral blood source

BACKGROUND

Cultured megakaryocytes could prove useful in the study of human diseases, but it is difficult to produce sufficient numbers for study. We describe and evaluate the use of an expansion process to develop mature megakaryocytes from peripheral blood-derived human hematopoietic stem and progenitor cells (HSPCs).

STUDY DESIGN AND METHODS

HSPCs (CD34+) were isolated from peripheral blood by positive selection and expanded using an optimal CD34+ expansion supplement. We evaluated megakaryocyte growth, maturation, and morphology in response to thrombopoietin (TPO) stimulation using flow cytometry and electron microscopy. TPO demonstrated a dose-dependent stimulatory effect on both megakaryocyte number and maturation.

RESULTS

From 90 to 120 mL of unmanipulated peripheral blood, we isolated a mean of 1.5 × 10(5) HSPCs (1.5 × 10(3) cells/mL of whole blood). HSPCs expanded nine-fold after a 4-day culture using an expansion supplement. Expanded cells were cultured for an additional 8 days with TPO (20 ng/mL), which resulted in a 2.9-fold increase in megakaryocytic cells where 83% of live cells expressed CD41a+, a marker of megakaryocyte commitment, and 50% expressed CD42b+, a marker for megakaryocyte maturation. The expanded HSPCs responded to TPO stimulation to yield more than 1.0 × 10(6) megakaryocytes. This cell number was sufficient for morphologic studies that demonstrated these expanded HSPCs produced mature polyploid megakaryocytes capable of forming proplatelet extensions.

CONCLUSIONS

Peripheral blood HSPCs can be expanded and differentiated into functional, mature megakaryocytes, a finding that supports the use of this process to study inherent platelet (PLT) production disorders as well as study factors that impair normal PLT production.

Developing a co-culture system for effective megakaryo/thrombopoiesis from umbilical cord blood hematopoietic stem/progenitor cells

BACKGROUND AIMS

Platelet transfusion can be a life-saving procedure in different medical settings. Thus, there is an increasing demand for platelets, of which shelf-life is only 5 days. The efficient ex vivo biomanufacturing of platelets would allow overcoming the shortages of donated platelets.

METHODS

We exploited a two-stage culture protocol aiming to study the effect of different parameters on the megakaryo/thrombopoiesis ex vivo. In the expansion stage, human umbilical cord blood (UCB)-derived CD34(+)-enriched cells were expanded in co-culture with human bone marrow mesenchymal stromal cells (BM-MSCs). The megakaryocytic commitment and platelet generation were studied, considering the impact of exogenous addition of thrombopoietin (TPO) in the expansion stage and a cytokine cocktail (Cyt) including TPO and interleukin-3 in the differentiation stage, with the use of different culture medium formulations, and in the presence/absence of BM-MSCs (direct versus non-direct cell-cell contact).

RESULTS

Our results suggest that an early megakaryocytic commitment, driven by TPO addition during the expansion stage, further enhanced megakaryopoiesis. Importantly, the results suggest that co-culture with BM-MSCs under serum-free conditions combined with Cyt addition, in the differentiation stage, significantly improved the efficiency yield of megakaryo/thrombopoiesis as well as increasing %CD41, %CD42b and polyploid content; in particular, direct contact of expanded cells with BM-MSCs, in the differentiation stage, enhanced the efficiency yield of megakaryo/thrombopoiesis, despite inhibiting their maturation.

CONCLUSIONS

The present study established an in vitro model for the hematopoietic niche that combines different biological factors, namely, the presence of stromal/accessory cells and biochemical cues, which mimics the BM niche and enhances an efficient megakaryo/thrombopoiesis process ex vivo.

Separation of in-vitro-derived megakaryocytes and platelets using spinning-membrane filtration

In-vitro-derived platelets (PLTs) could potentially overcome problems associated with donated PLTs, including contamination and alloimmunization. Although several groups have produced functional PLTs from stem cells in vitro, the challenge of developing this technology to yield transfusable PLT units has yet to be addressed. The asynchronous nature of in vitro PLT generation makes a single harvest point infeasible for collecting PLTs as soon as they are formed. The current standard of performing manual centrifugations to separate PLTs from nucleated cells at multiple points during culture is labor-intensive, imprecise, and difficult to standardize in accordance with current Good Manufacturing Practices (cGMP). In an effort to develop a more effective method, we adapted a commercially-available, spinning-membrane filtration device to separate in-vitro-derived PLTs from nucleated cells and recover immature megakaryocytes (MKs), the precursor cells to PLTs, for continued culture. Processing a mixture of in-vitro-derived MKs and PLTs on the adapted device yielded a pure PLT population and did not induce PLT pre-activation. MKs recovered from the separation process were unaffected with respect to viability and ploidy, and were able to generate PLTs after reseeding in culture. Being able to efficiently harvest in-vitro-derived PLTs brings this technology one step closer to clinical relevance.

Bone marrow stromal cells transduced with a thrombopoietin, interleukin-6, and interleukin-11 syncretic gene induce cord mononuclear cells to generate platelets in vitro

BACKGROUND

The induction of hematopoietic stem cells to produce mass numbers of platelets (PLTs) in vitro is an effective method to address a lack of PLTs and PLT transfusion resistance in the clinic. However, the design of a low-cost and sustainable culture system is currently problematic.

STUDY DESIGN AND METHODS

Here, the thrombopoietin, interleukin (IL)-6, and IL-11 genes, three regulatory factors important for thrombopoiesis, were used to construct self-splicing fusion genes linked by foot and mouth disease (F2A) and Theiler's murine encephalitis (T2A) viruses. Bone marrow stromal cells (BMSCs) transduced with the fusion gene acted as nourishing cells and induced cord blood mononuclear cells (MNCs) to generate PLTs in vitro. We counted these cells; determined the percentage of cells expressing specific cell surface markers (CD41); and measured their ability to aggregate via flow cytometry, immunohistochemical staining, and aggregation remote analyzer.

RESULTS

BMSCs transduced with the fusion gene successfully induced cord blood MNCs to generate PLT-sized fragments in the absence of exogenous cytokines. The output was higher than that of the control groups, and the PLT-sized fragments were similar to endogenous PLTs in terms of shape, CD41 expression, and aggregation function.

CONCLUSION

These results suggest that our method could be used to develop a low-cost sustainable cultivation system that generates PLTs in vitro by enhancing the autocrine production of related cytokines through the nourishment provided by cells transduced with a syncretic gene.

Manufacturing blood ex vivo: a futuristic approach to deal with the supply and safety concerns

Blood transfusions are routinely done in every medical regimen and a worldwide established collection, processing/storage centers provide their services for the same. There have been extreme global demands for both raising the current collections and supply of safe/adequate blood due to increasingly demanding population. With, various risks remain associated with the donor derived blood, and a number of post collection blood screening and processing methods put extreme constraints on supply system especially in the underdeveloped countries. A logistic approach to manufacture erythrocytes ex-vivo by using modern tissue culture techniques have surfaced in the past few years. There are several reports showing the possibilities of RBCs (and even platelets/neutrophils) expansion under tightly regulated conditions. In fact, ex vivo synthesis of the few units of clinical grade RBCs from a single dose of starting material such as umbilical cord blood (CB) has been well established. Similarly, many different sources are also being explored for the same purpose, such as embryonic stem cells, induced pluripotent stem cells. However, the major concerns remain elusive before the manufacture and clinical use of different blood components may be used to successfully replace the present system of donor derived blood transfusion. The most important factor shall include the large scale of RBCs production from each donated unit within a limited time period and cost of their production, both of these issues need to be handled carefully since many of the recipients among developing countries are unable to pay even for the freely available donor derived blood. Anyways, keeping these issues in mind, present article shall be focused on the possibilities of blood production and their use in the near future.

Three-stage ex vivo expansion of high-ploidy megakaryocytic cells: toward large-scale platelet production

Hematopoietic stem and progenitor cells (HSPCs) have been cultured using a wide variety of cytokines to promote differentiation into megakaryocytic cells (Mks), the precursors to platelets. Greater Mk DNA content, or ploidy, has been correlated with increased platelet release. Gradients of pH, pO2, and signaling factors regulate megakaryopoiesis in the bone marrow niche. In this study, we demonstrate that a 3-phase culture process with increasing pH and pO2 and different cytokine cocktails greatly increases megakaryocyte production. CD34(+) HSPCs were first cultured at 5% O2 and pH 7.2 with a cytokine cocktail previously shown to promote Mk progenitor production. At day 5, cells were shifted to 20% O2 and pH 7.4 and maintained in 1 of 17 cytokine cocktails identified using a 2(4) factorial design of experiments method to evaluate the effects of interleukin (IL)-3, IL-6, IL-9, and high- or low-dose stem cell factor (SCF), in conjunction with thrombopoietin (Tpo) and IL-11, on expansion of mature Mks from progenitors. The combination of Tpo, high-dose SCF, IL-3, IL-9, and IL-11 best promoted Mk expansion. IL-3 greatly increased total cell fold expansion, but this was partially offset by lower Mk purity. IL-9 promoted CD41 and CD42b expression. High-dose (100 ng/mL) SCF increased Mk production and ploidy. Different commercial media and IL-3 sources substantially impacted differentiation, and X-VIVO 10 serum-free media best supported mature Mk expansion. Shifting from pH 7.4 to pH 7.6 at day 7 increased Mk production by 30%. Treatment with nicotinamide at day 7 or day 8 more than doubled the fraction of high-ploidy (>4N) Mks. Ultimately, the 3-phase culture system gave rise to 44.5±8.1 Mks and 8.5±3.1 high-ploidy Mks per input HSPC. Further optimization was required to improve platelet production. Using Iscove's modified Dulbecco's medium (IMDM)+20% BSA, insulin and transferin (BIT) 9500 Serum Substitute greatly improved the frequency and quality of Mk proplatelet extensions without affecting Mk expansion, commitment, or polyploidization in the 3-phase process. Mks cultured in IMDM+20% BIT 9500 gave rise to platelets with functional activity similar to that of fresh platelets from normal donors, as evidenced by basal tubulin distribution and the expression of surface markers and spreading in response to platelet agonists.

Designer cytokine hyper interleukin 11 (H11) is a megakaryopoietic factor

Interleukin-11 (IL-11) displays megakaryopoietic activity. We constructed super-cytokine Hyper- IL11 (H11) by linking soluble IL-11 receptor α (sIL-11Rα) with IL-11, which directly targets β-receptor (gp130) signal transducing subunit. The effects of H11 on hematopoiesis with a focus on megakaryopoiesis were studied. The expansion, differentiation and type of colony formation of cord blood progenitor Lin-CD34+ cells were analyzed. H11 was more effective than recombinant human IL-11 (rhIL-11) in enhancement of the Lin-CD34+ cells expansion and differentiation into megakaryocytes (Mk). It induced higher expression of CD41a and CD61 antigens, resulting in a substantially larger population of CD34-CD41a(high)CD61(high) cells. H11 treatment led to increased number of small and mainly medium megakaryocyte colony formation (Mk-CFU). Moreover, it induced the formation of a small number of large colonies, which were not observed following rhIL-11 treatment. Significantly higher number of H11 derived Mk colonies released platelets-like particles (PLP). Furthermore, H11 was considerably more potent than rhIL-11 in promoting differentiation of Lin-CD43+ cells toward erythrocytes. Our results indicate that H11 is more effective than rhIL-11 in enhancing expansion of early progenitors and directing them to megakaryocyte and erythroid cells and in inducing maturation of Mk. Thus, H11 may prove beneficial for thrombocytopenia treatment and/or an ex vivo expansion of megakaryocytes.

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.

Human endometrial stromal stem cells differentiate into megakaryocytes with the ability to produce functional platelets

Human endometrium is a high dynamic tissue that contains endometrial stromal stem cells (hESSCs). The hESSCs have been differentiated into a number of cell lineages. However, differentiation of hESSCs into megakaryocytes (MKs) has not yet been investigated. The aim of this study was to investigate the feasibility of MK generation from hESSCs and subsequent production of functional platelets (PLTs). In our study, hESSCs were cultured from endometrial stromal cells as confirmed by positive stromal cell specific markers (CD90 and CD29) and negative hematopoietic stem cell markers (CD45 and CD34) expression. Then, hESSCs were differentiated in a medium supplemented with thrombopoietin (TPO) for 18 days. The MK differentiation was analyzed by flow cytometry and confocal microscopy. The differentiation medium was collected for PLT production analysis by flow cytometry, transmission electron microscopy and functional measurements. Our results show: 1) MKs were successfully generated from hESSCs as identified by expression of specific markers (CD41a: 1±0.09% and 39±3.0%; CD42b: 1.2±0.06% and 28±2.0%, control vs. differentiation) accompanied with reduction of pluripotent transcription factors (Oct4 and Sox2) expression; 2) The level of PLTs in the differentiation medium was 16±1 number/µl as determined by size (2–4 µm) and CD41a expression (CD41a: 1±0.4% and 90±2.0%, control vs. differentiation); 3) Generated PLTs were functional as evidenced by the up-regulation of CD62p expression and fibrinogen binding following thrombin stimulation; 4) Released PLTs showed similar ultra-structure characteristics (alpha granules, vacuoles and dense tubular system) as PLTs from peripheral blood determined by electron microscopic analysis. Data demonstrate the feasibility of generating MKs from hESSCs, and that the generated MKs release functional PLTs. Therefore, hESSCs could be a potential new stem cell source for in vitro MK/PLT production.

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.

A journey to produce platelets in vitro

Allogeneic platelet transfusions protect patients from bleeding episodes and also make aggressive medical procedures such as those involving marrow transplants requiring chemotherapy and/or radiotherapy possible. These patients are dependent upon an unfailing supply of platelets that can sometimes be in short supply due to high demands coupled with an extremely short expiration date for platelet products of only 5 days. One approach that is under investigation to overcome platelet shortages is to harness the extraordinary capabilities of stem cells to proliferate and differentiate into various cell types and to use this ability to specifically produce clinical scale quantities of functional platelets in bioreactors. To accomplish such an enormous and complex task requires an appreciation of the regulatory mechanisms that occur during the development of megakaryocytes (MKs) and the subsequent biogenesis of functional platelets from mature MKs. This means understanding the complex network of intracellular and extracellular regulatory mechanisms that act at each phase of a developmental process that ushers stem cells along the MK lineage to produce billions of platelets per day in a healthy individual.

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.

Bone marrow niche-inspired, multiphase expansion of megakaryocytic progenitors with high polyploidization potential

BACKGROUND AIMS

Megakaryopoiesis encompasses hematopoietic stem and progenitor cell (HSPC) commitment to the megakaryocytic cell (Mk) lineage, expansion of Mk progenitors and mature Mks, polyploidization and platelet release. pH and pO2 increase from the endosteum to sinuses, and different cytokines are important for various stages of differentiation. We hypothesized that mimicking the changing conditions during Mk differentiation in the bone marrow would facilitate expansion of progenitors that could generate many high-ploidy Mks.

METHODS

CD34+ HSPCs were cultured at pH 7.2 and 5% O2 with stem cell factor (SCF), thrombopoietin (Tpo) and all combinations of Interleukin (IL)-3, IL-6, IL-11 and Flt-3 ligand to promote Mk progenitor expansion. Cells cultured with selected cytokines were shifted to pH 7.4 and 20% O2 to generate mature Mks, and treated with nicotinamide (NIC) to enhance polyploidization.

RESULTS

Using Tpo + SCF + IL-3 + IL-11, we obtained 3.5 CD34+ CD41+ Mk progenitors per input HSPC, while increasing purity from 1% to 17%. Cytokine cocktails with IL-3 yielded more progenitors and mature Mks, although the purities were lower. Mk production was much greater at higher pH and pO2. Although fewer progenitors were present, shifting to 20% O2 /pH 7.4 at day 5 (versus days 7 or 9) yielded the greatest mature Mk production, 14 per input HSPC. NIC more than doubled the percentage of high-ploidy Mks to 40%.

CONCLUSIONS

We obtained extensive Mk progenitor expansion, while ensuring that the progenitors could produce high-ploidy Mks. We anticipate that subsequent optimization of cytokines for mature Mk production and delayed NIC addition will greatly increase high-ploidy Mk production.

Platelet formation

Thrombocytopenia is the underlying cause of a number of major clinical conditions and genetic disorders worldwide. While therapeutic agents that bind and stimulate the thrombopoietin receptor are currently available, the development of drugs that directly stimulate megakaryocytes to generate platelets has lagged behind. To improve the management of thrombocytopenia, we will need to define the cell biological pathways that drive the production of platelets from megakaryocytes. This review integrates the latest research of platelet biogenesis and focuses on the molecular pathways that power and regulate proplatelet production.

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 human bone marrow analog: clinical potential

Bone marrow is the primary site of hematopoiesis in adult humans. Bone marrow can be cultured in vitro but few simple culture systems fully support hematopoiesis beyond a few months. Human bone marrow analogs are long-term in vitro cultures of marrow stromal and hematopoietic stem cells that can be used to produce cells and products normally harvested from human donors. Bone marrow analog systems should exhibit confluence of the stromal cell populations, persistence of hematopoietic progenitor cells, presence of active regions of hematopoiesis and capacity to produce mature cell types for extended periods of time. Although we are still years away from realizing clinical application of products formed by artificial bone marrow analogs, the process of transitioning this research tool from bench to bedside should be fairly straightforward. The most obvious application of artificial marrow would be for production of autologous hematopoietic CD34(+) stem cells as a stem cell therapy for individuals experiencing bone marrow failure due to disease or injury. Another logical application is for 'blood farming', a process for large-scale in vitro production of red blood cells, white blood cells or platelets, for transfusion or treatment. Other possibilities include production of nonhematopoietic stem cells such as osteogenic stromal cells, osteoblasts and rare pluripotent stem cells. Bone marrow analogs also have great potential as ex vivo human test systems and could play a critical role in drug discovery, drug development and toxicity testing in the future.

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.

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

The physical culture parameters have important influences on the proliferation and differentiation fate of hematopoietic stem cells. Recently, we have demonstrated that CD34+ cord blood (CB) cells undergo accelerated and increased megakaryocyte (Mk) differentiation when incubated under mild hyperthermic conditions (i.e., 39 degrees C). In this study, we investigated in detail the impacts of mild hyperthermia on Mk differentiation and maturation, and explored potential mechanisms responsible for these phenomena. Our results demonstrate that the qualitative and quantitative effects on Mk differentiation at 39 degrees C appear rapidly within 7 days, and that early transient culture at 39 degrees C led to even greater Mk yields (p<0.03). Surprisingly, cell viability was only found to be significantly reduced in the early stages of culture, suggesting that CB cells are able with time to acclimatize themselves to 39 degrees C. Although mild hyperthermia accelerated differentiation and maturation of CB-derived Mks, it failed to promote their polyploidization further but rather led to a small reduction in the proportion of polyploid Mks (p=0.01). Conversely, gene arrays analysis demonstrated that Mks derived at 39 degrees C have a normal gene expression program consistent with an advanced maturation state. Finally, two independent mechanisms that could account for the accelerated Mk differentiation were investigated. Our results suggest that the accelerated and increased Mk differentiation induced by mild hyperthermia is not mediated by cell-secreted factors but could perhaps be mediated by the increased expression of Mk transcription factors.

Effects of a 2-step culture with cytokine combinations on megakaryocytopoiesis and thrombopoiesis from carbon-ion beam-irradiated human hematopoietic stem/progenitor cells

To evaluate whether the continuous treatment of two cytokine combinations is effective in megakaryocytopoiesis and thrombopoiesis in hematopoietic stem/progenitor cells exposed to heavy ion beams, the effects of a 2-step culture by a combination of recombinant human interleukin-3 (IL-3) + stem cell factor (SCF) + thrombopoietin (TPO), which just slightly protected against carbon-ion beam-induced damages, and a combination of IL-3 + TPO, which selectively stimulated the differentiation of the hematopoietic stem/progenitor cells to megakaryocytes and platelets, were examined. CD34(+)-hematopoietic stem/progenitor cells isolated from the human placental and umbilical cord blood were exposed to carbon-ion beams (LET = 50 keV/microm) at 2 Gy. These cells were cultured under three cytokine conditions. The number of megakaryocytes, platelets and hematopoietic progenitors were assessed using a flow cytometer and a clonogenic assay at 14 and 21 days after irradiation, respectively. However, the efficacy of each 2-step culture was equal or lower than that of using the IL-3 + SCF + TPO combination alone and the 2-step culture could not induce megakaryocytes and platelets from hematopoietic stem/progenitor cells exposed to high LET-radiation such as carbon-ion beams. Therefore, additional cytokines and/or hematopoietic promoting compounds might be required to overcome damage to hematopoietic cells by high LET radiation.

Increased production of megakaryocytes near purity from cord blood CD34+ cells using a short two-phase culture system

Expansion of hematopoietic progenitor cells (HPC) ex vivo remains an important focus in fundamental and clinical research. The aim of this study was to determine whether the implementation of such expansion phase in a two-phase culture strategy prior to the induction of megakaryocyte (Mk) differentiation would increase the yield of Mks produced in cultures. Toward this end, we first characterized the functional properties of five cytokine cocktails to be tested in the expansion phase on the growth and differentiation kinetics of CD34+-enriched cells, and on their capacity to expand clonogenic progenitors in cultures. Three of these cocktails were chosen based on their reported ability to induce HPC expansion ex vivo, while the other two represented new cytokine combinations. These analyses revealed that none of the cocktails tested could prevent the differentiation of CD34+ cells and the rapid expansion of lineage-positive cells. Hence, we sought to determine the optimum length of time for the expansion phase that would lead to the best final Mk yields. Despite greater expansion of CD34+ cells and overall cell growth with a longer expansion phase, the optimal length for the expansion phase that provided greater Mk yield at near maximal purity was found to be 5 days. Under such settings, two functionally divergent cocktails were found to significantly increase the final yield of Mks. Surprisingly, these cocktails were either deprived of thrombopoietin or of stem cell factor, two cytokines known to favor megakaryopoiesis and HPC expansion, respectively. Based on these results, a short resource-efficient two-phase culture protocol for the production of Mks near purity (>95%) from human CD34+ CB cells has been established.

Long-term platelet production assessed in NOD/SCID mice injected with cord blood CD34+ cells, thrombopoietin-amplified in clinical grade serum-free culture

OBJECTIVE

Delayed platelet recovery post-cord blood (CB) transplantation might be due to CB characteristics: low maturity of stem cell compartment, poor production of CD34+/CD41+ cells when induced to differentiate along the megakaryocytic (MK) lineage, retention of a low ploidy in the expanded MKs. Ex vivo expansion of CB hematopoietic progenitor cells for reconstitution of different human hematopoietic lineages has already been developed in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. However, optimal conditions for MK-progenitor engraftment to reduce hemorrhaging risk still to be developed. This study assesses the hypothesis that CB-CD34+ amplification with thrombopoietin (TPO) can be applied to a portion of a CB transplant unit to stimulate recovery along MK differentiation program.

MATERIALS AND METHODS

Human CB-CD34+ cells were amplified in a serum-free, clinical grade medium with 100 ng/mL TPO alone and in addition to other cytokines (Kit ligand, interleukin-6, and Flt-3 ligand). Seven-day cultured cells were transplanted into irradiated NOD/SCID mice and engraftment, megakaryocytopoiesis, and platelet production were assessed.

RESULTS

Platelet release was successful and continuously present for at least 8 weeks in NOD/SCID mice transplanted with CB cells stimulated by TPO. Thrombocytopoiesis was more effective with transplanted TPO-amplified cells than with the cytokine cocktails.

CONCLUSION

Platelet number obtained is within the minimum level considered sufficient for hemostasis. Furthermore, amplified cells maintain their self-renewal capacity and multilineage potential differentiation. Thus, transplantation of TPO-expanded CB cells has the potential favoring both platelet recovery and human engraftment.

Comparison of promoter activities for efficient expression into human B cells and haematopoietic progenitors with adenovirus Ad5/F35

Adenoviral gene transfer into human B lymphocytes and haematopoietic progenitors would allow the characterization of their function on cellular growth, differentiation and survival. Efficient gene expression is however strongly dependent on the promoter used. In this study, we investigated the relative strength of various promoters by following and measuring the expression of the reporter gene EYFP in human peripheral B lymphocytes, cord blood CD34(+) cells and the megakaryocytic cell line M-07e. The murine PGK promoter provided the best level of transgene expression in CD34(+) cells among the four promoters tested, followed closely by the CMV promoter, and to a lesser extend by a CMV promoter with a beta-globin/IgG chimeric intron, whereas the human CD40 promoter provided the lowest levels of expression. In contrast, the strongest promoters in B lymphocytes were the two CMV promoters. Surprisingly, even the best promoters were unable to induce transgene expression in more than 75-80% of the primary B and CD34(+) cells, even though 100% of the cells were infected. Finally and in contrast to retroviruses, only a minority of B lymphocytes and CD34(+) cells were able to induce the transcription of IRES-containing bicistronic expression cassettes from adenovirus.

Telomere length of cord blood-derived CD34(+) progenitors predicts erythroid proliferative potential

Excessive telomere shortening has been demonstrated in inherited and acquired blood disorders, including aplastic anemia and myelodysplastic syndromes. It is possible that replicative exhaustion, owing to critical telomere shortening in hematopoietic progenitor cells (HPCs), contributes to the development of cytopenias in these disorders. However to date, a direct link between the telomere length (TL) of human HPCs and their proliferative potential has not been demonstrated. In the present investigation, the TL and level of telomerase enzyme activity (TA) detected in cord blood (CB)-derived HPCs was found to predict erythroid expansion (P<0.01 and P=0.01 respectively). These results were corroborated by a correlation between proliferation of erythroid cells and telomere loss (P=0.01). In contrast, no correlations were found between initial TL, telomere loss or TA and the expansion of other myeloid lineage-committed cells. There was also no correlation between TL or TA and the number of clonogenic progenitors, including primitive progenitors derived from long-term culture. Our investigations revealed upregulation of telomerase to tumor cell levels in CD34- cells undergoing erythroid differentiation. Together, these results provide new insight into the regulation of TL and TA during myeloid cell expansion and demonstrate that TL is an important determinant of CB-derived erythroid cell proliferation.

Efficient in vitro megakaryocyte maturation using cytokine cocktails optimized by statistical experimental design

OBJECTIVE

A multi-step statistical strategy was applied to quantify individual and interactive effects of cytokines on megakaryopoiesis and to determine the concentration of the selected cytokines that optimize ex vivo megakaryocyte (MK) expansion, maturation, and platelet production in stromal- and serum-free conditions.

MATERIALS AND METHODS

Immature MK were first generated from human CD34(+)-enriched cord blood cells cultured for 7 days in conditions favoring MK commitment. Then, the effect of different combinations of cytokines at various concentrations on MK differentiation and platelet production was tested on the day-7 MK.

RESULTS

A large-scale screening of 13 cytokines in the presence of thrombopoietin (TPO) using Placket-Burman designs (PBD) was initially performed to identify stimulators of MK maturation. Afterwards, a statistical analysis of the two-level factorial designs revealed that in the presence of TPO, MK maturation was significantly stimulated by stem cell factor (SCF), interleukin (IL)-6, and IL-9, whereas Flt-3 ligand (FL) had a positive effect only on the expansion of MK progenitors. In contrast, erythropoietin (EPO) and IL-8 were inhibitors of MK maturation. A response surface methodology was then used to optimize the concentrations of the selected cytokines (TPO, SCF, IL-6, and IL-9) and defined a new cytokine cocktail that maximized MK expansion and maturation. Importantly, the increased MK output was accompanied by a very high MK purity ( approximately 90%). Another optimum was also found at a higher SCF concentration, which further improved MK expansion and maturation, but reduced MK purity.

CONCLUSION

These statistical methods provide an efficient tool to analyze complex systems of cytokines and to develop promising ex vivo MK culture systems for clinical applications.

Increased megakaryopoiesis in cultures of CD34-enriched cord blood cells maintained at 39 degrees C

Based on previous evidence suggesting positive effects of fever on in vivo hematopoiesis, we tested the effect of hyperthermia on megakaryopoiesis (MK) in ex vivo cultures of CD34-enriched cord blood (CB) cells. The cells were cultured at 37 degrees C or 39 degrees C for 14 days in cytokine conditions optimized for megakaryocyte development and analyzed periodically. Compared to 37 degrees C, the cultures maintained at 39 degrees C produced significantly more (up to 10-fold) total cells, myeloid and MK progenitors, and total MKs, and showed accelerated and enhanced MK maturation with increased yields of proplatelets and platelets. This observation could facilitate clinical applications requiring ex vivo expansion of hematopoietic cells.