A common bipotent progenitor generates the erythroid and megakaryocyte lineages in embryonic stem cell-derived primitive hematopoiesis. Blood. 2009;114:1506-1517. [PMID: 19478046 DOI: 10.1182/blood-2008-09-178863]

Modeling primitive and definitive erythropoiesis with induced pluripotent stem cells

ABSTRACT

During development, erythroid cells are produced through at least 2 distinct hematopoietic waves (primitive and definitive), generating erythroblasts with different functional characteristics. Human induced pluripotent stem cells (iPSCs) can be used as a model platform to study the development of red blood cells (RBCs) with many of the differentiation protocols after the primitive wave of hematopoiesis. Recent advances have established that definitive hematopoietic progenitors can be generated from iPSCs, creating a unique situation for comparing primitive and definitive erythrocytes derived from cell sources of identical genetic background. We generated iPSCs from healthy fetal liver (FL) cells and produced isogenic primitive or definitive RBCs which were compared directly to the FL-derived RBCs. Functional assays confirmed differences between the 2 programs, with primitive RBCs showing a reduced proliferation potential, larger cell size, lack of Duffy RBC antigen expression, and higher expression of embryonic globins. Transcriptome profiling by scRNA-seq demonstrated high similarity between FL- and iPSC-derived definitive RBCs along with very different gene expression and regulatory network patterns for primitive RBCs. In addition, iPSC lines harboring a known pathogenic mutation in the erythroid master regulator KLF1 demonstrated phenotypic changes specific to definitive RBCs. Our studies provide new insights into differences between primitive and definitive erythropoiesis and highlight the importance of ontology when using iPSCs to model genetic hematologic diseases. Beyond disease modeling, the similarity between FL- and iPSC-derived definitive RBCs expands potential applications of definitive RBCs for diagnostic and transfusion products.

Optimization of seeding density of OP9 cells to improve hematopoietic differentiation efficiency

BACKGROUND

OP9 mouse stromal cell line has been widely used to induce differentiation of human embryonic stem cells (hESCs) into hematopoietic stem/progenitor cells (HSPCs). However, the whole co-culture procedure usually needs 14-18 days, including preparing OP9 cells at least 4 days. Therefore, the inefficient differentiation system is not appreciated. We aimed to optimize the culture conditions to improve differentiation efficiency.

METHODS

In the experimental group, we set six different densities of OP9 cells and just cultured them for 24 h before co-culture, and in the control group, OP9 cells were cultured for 4 days to reach an overgrown state before co-culture. Then we compared the hematopoietic differentiation efficiency among them.

RESULTS

OP9 cells were randomly assigned into two groups. In the experimental group, six different plated numbers of OP9 cells were cultured for 1 day before co-culture with hESCs. In contrast, in the control group, OP9 cells were cultured for 4 days at a total number of 3.1 × 104 cells/cm2 in a 6-well plate to reach an overgrown state before co-culture. Hematopoietic differentiation was evaluated with CD34 immunostaining, and compared between these two groups. We could not influence the differentiation efficiency of OP9 cells with a total number of 10.4 × 104 cells/cm2 in a 6-well plate which was cultured just for 1 day, followed by co-culture with hESCs. It reached the same differentiation efficiency 5 days earlier than the control group.

CONCLUSION

The peak of CD34 + cells appeared 2 days earlier compared to the control group. A total number of 1.0 × 106 cells in a 6-well plate for OP9 cells was appropriate to have high differentiation efficiency.

Baffled-flow culture system enables the mass production of megakaryocytes from human embryonic stem cells by enhancing mitochondrial function

Human embryonic stem cells (hESCs) have become an ideal cell source for the ex vivo generation of megakaryocyte (MK) and platelet products for clinical applications. However, an ongoing challenge is to establish scalable culture systems to maximize the yield of stem cell-derived MKs that release platelets. We defined a specific dynamic 3D manufacturing system in a baffled-flow manner that could remarkably facilitate megakaryopoiesis and increase the yield of platelet-producing MKs from hESCs within a 12-day induction period. Additionally, an increased number of >16N ploidy MKs, proplatelets, and platelets were generated from induced cells harvested on Day 12 using the specific dynamic culture method. The specific dynamic culture method significantly enhanced endothelium-to-haematopoietic transition and early haematopoiesis. More importantly, MK fate was significantly facilitated in a specific dynamic manner during early haematopoiesis. Mechanistically, this dynamic culture significantly enhanced mitochondrial function via the oxidative phosphorylation pathway and caused differentiation skewing of hESCs toward megakaryopoiesis. This study can aid in the automatic and scalable production of MKs from stem cells using baffled-flow bioreactors and assist in the manufacturing of hESC-derived MK and platelet products.

Generating hematopoietic cells from human pluripotent stem cells: approaches, progress and challenges

Human pluripotent stem cells (hPSCs) have been suggested as a potential source for the production of blood cells for clinical application. In two decades, almost all types of blood cells can be successfully generated from hPSCs through various differentiated strategies. Meanwhile, with a deeper understanding of hematopoiesis, higher efficiency of generating progenitors and precursors of blood cells from hPSCs is achieved. However, how to generate large-scale mature functional cells from hPSCs for clinical use is still difficult. In this review, we summarized recent approaches that generated both hematopoietic stem cells and mature lineage cells from hPSCs, and remarked their efficiency and mechanisms in producing mature functional cells. We also discussed the major challenges in hPSC-derived products of blood cells and provided some potential solutions. Our review summarized efficient, simple, and defined methodologies for developing good manufacturing practice standards for hPSC-derived blood cells, which will facilitate the translation of these products into the clinic.

Generative pretraining from large-scale transcriptomes for single-cell deciphering

Exponential accumulation of single-cell transcriptomes poses great challenge for efficient assimilation. Here, we present an approach entitled generative pretraining from transcriptomes (tGPT) for learning feature representation of transcriptomes. tGPT is conceptually simple in that it autoregressive models the ranking of a gene in the context of its preceding neighbors. We developed tGPT with 22.3 million single-cell transcriptomes and used four single-cell datasets to evalutate its performance on single-cell analysis tasks. In addition, we examine its applications on bulk tissues. The single-cell clusters and cell lineage trajectories derived from tGPT are highly aligned with known cell labels and states. The feature patterns of tumor bulk tissues learned by tGPT are associated with a wide range of genomic alteration events, prognosis, and treatment outcome of immunotherapy. tGPT represents a new analytical paradigm for integrating and deciphering massive amounts of transcriptome data and it will facilitate the interpretation and clinical translation of single-cell transcriptomes.

In Vitro Human Haematopoietic Stem Cell Expansion and Differentiation

The haematopoietic system plays an essential role in our health and survival. It is comprised of a range of mature blood and immune cell types, including oxygen-carrying erythrocytes, platelet-producing megakaryocytes and infection-fighting myeloid and lymphoid cells. Self-renewing multipotent haematopoietic stem cells (HSCs) and a range of intermediate haematopoietic progenitor cell types differentiate into these mature cell types to continuously support haematopoietic system homeostasis throughout life. This process of haematopoiesis is tightly regulated in vivo and primarily takes place in the bone marrow. Over the years, a range of in vitro culture systems have been developed, either to expand haematopoietic stem and progenitor cells or to differentiate them into the various haematopoietic lineages, based on the use of recombinant cytokines, co-culture systems and/or small molecules. These approaches provide important tractable models to study human haematopoiesis in vitro. Additionally, haematopoietic cell culture systems are being developed and clinical tested as a source of cell products for transplantation and transfusion medicine. This review discusses the in vitro culture protocols for human HSC expansion and differentiation, and summarises the key factors involved in these biological processes.

FLI1 accelerates leukemogenesis through transcriptional regulation of pyruvate kinase-L/R and other glycolytic genes

In cancer cells, multiple oncogenes and tumor suppressors control glycolysis to sustain rapid proliferation. The ETS-related transcription factor Fli1 plays a critical role in the induction and progression of leukemia, yet, the underlying mechanism of this oncogenic event is still not fully understood. In this study, RNAseq analysis of FLI1-depleted human leukemic cells revealed transcriptional suppression of the PKLR gene and activation of multiple glycolytic genes, such as PKM1/2. Pharmacological inhibition of glycolysis by PKM2 inhibitor, Shikonin, significantly suppressed leukemic cell proliferation. FLI1 directly binds to the PKLR promoter, leading to the suppression of this inhibitor of glycolysis. In accordance, shRNA-mediated depletion of PKLR in leukemic HEL cells expressing high levels of FLI1 accelerated leukemia proliferation, pointing for the first time to its tumor suppressor function. PKLR knockdown also led to downregulation of the erythroid markers EPOR, HBA1, and HBA2 and suppression of erythroid differentiation. Interestingly, silencing of PKLR in HEL cells significantly increased FLI1 expression, which was associated with faster proliferation in culture. In FLI1-expressing leukemic cells, lower PKLR expression was associated with higher expression of PKM1 and PKM2, which promote aerobic glycolysis. Finally, injection of pyruvate, a known inhibitor of glycolysis, into leukemia mice significantly suppressed leukemogenesis. These results demonstrate that FLI1 promotes leukemia in part by inducing glycolysis, implicates PKLR in erythroid differentiation, and suggests that targeting glycolysis may be an attractive therapeutic strategy for cancers driven by FLI1 overexpression.

Direct chemical reprogramming of human cord blood erythroblasts to induced megakaryocytes that produce platelets

Reprogramming somatic cells into megakaryocytes (MKs) would provide a promising source of platelets. However, using a pharmacological approach to generate human MKs from somatic cells remains an unmet challenge. Here, we report that a combination of four small molecules (4M) successfully converted human cord blood erythroblasts (EBs) into induced MKs (iMKs). The iMKs could produce proplatelets and release functional platelets, functionally resembling natural MKs. Reprogramming trajectory analysis revealed an efficient cell fate conversion of EBs into iMKs by 4M via the intermediate state of bipotent precursors. 4M induced chromatin remodeling and drove the transition of transcription factor (TF) regulatory network from key erythroid TFs to essential TFs for megakaryopoiesis, including FLI1 and MEIS1. These results demonstrate that the chemical reprogramming of cord blood EBs into iMKs provides a simple and efficient approach to generate MKs and platelets for clinical applications.

Current insights into the role of Fli-1 in hematopoiesis and malignant transformation

Fli-1, a member of the ETS family of transcription factors, was discovered in 1991 through retroviral insertional mutagenesis as a driver of mouse erythroleukemias. In the past 30 years, nearly 2000 papers have defined its biology and impact on normal development and cancer. In the hematopoietic system, Fli-1 controls self-renewal of stem cells and their differentiation into diverse mature blood cells. Fli-1 also controls endothelial survival and vasculogenesis, and high and low levels of Fli-1 are implicated in the auto-immune diseases systemic lupus erythematosus and systemic sclerosis, respectively. In addition, aberrant Fli-1 expression is observed in, and is essential for, the growth of multiple hematological malignancies and solid cancers. Here, we review the historical context and latest research on Fli-1, focusing on its role in hematopoiesis, immune response, and malignant transformation. The importance of identifying Fli-1 modulators (both agonists and antagonists) and their potential clinical applications is discussed.

A universal approach for integrating super large-scale single-cell transcriptomes by exploring gene rankings

Advancement in single-cell RNA sequencing leads to exponential accumulation of single-cell expression data. However, there is still lack of tools that could integrate these unlimited accumulations of single-cell expression data. Here, we presented a universal approach iSEEEK for integrating super large-scale single-cell expression via exploring expression rankings of top-expressing genes. We developed iSEEEK with 11.9 million single cells. We demonstrated the efficiency of iSEEEK with canonical single-cell downstream tasks on five heterogenous datasets encompassing human and mouse samples. iSEEEK achieved good clustering performance benchmarked against well-annotated cell labels. In addition, iSEEEK could transfer its knowledge learned from large-scale expression data on new dataset that was not involved in its development. iSEEEK enables identification of gene-gene interaction networks that are characteristic of specific cell types. Our study presents a simple and yet effective method to integrate super large-scale single-cell transcriptomes and would facilitate translational single-cell research from bench to bedside.

SnRNA sequencing defines signaling by RBC-derived extracellular vesicles in the murine heart

In a unique model of fluorescent based mapping of EV recipient cells, RBC-EVs were found to signal to cardiac cells and regulate gene expression in a model of ischemic heart failure.

Extracellular vesicles (EVs) mediate intercellular signaling by transferring their cargo to recipient cells, but the functional consequences of signaling are not fully appreciated. RBC-derived EVs are abundant in circulation and have been implicated in regulating immune responses. Here, we use a transgenic mouse model for fluorescence-based mapping of RBC-EV recipient cells to assess the role of this intercellular signaling mechanism in heart disease. Using fluorescent-based mapping, we detected an increase in RBC-EV–targeted cardiomyocytes in a murine model of ischemic heart failure. Single cell nuclear RNA sequencing of the heart revealed a complex landscape of cardiac cells targeted by RBC-EVs, with enrichment of genes implicated in cell proliferation and stress signaling pathways compared with non-targeted cells. Correspondingly, cardiomyocytes targeted by RBC-EVs more frequently express cellular markers of DNA synthesis, suggesting the functional significance of EV-mediated signaling. In conclusion, our mouse model for mapping of EV-recipient cells reveals a complex cellular network of RBC-EV–mediated intercellular communication in ischemic heart failure and suggests a functional role for this mode of intercellular signaling.

Industrially Compatible Transfusable iPSC-Derived RBCs: Progress, Challenges and Prospective Solutions

Amidst the global shortfalls in blood supply, storage limitations of donor blood and the availability of potential blood substitutes for transfusion applications, society has pivoted towards in vitro generation of red blood cells (RBCs) as a means to solve these issues. Many conventional research studies over the past few decades have found success in differentiating hematopoietic stem and progenitor cells (HSPCs) from cord blood, adult bone marrow and peripheral blood sources. More recently, techniques that involve immortalization of erythroblast sources have also gained traction in tackling this problem. However, the RBCs generated from human induced pluripotent stem cells (hiPSCs) still remain as the most favorable solution due to many of its added advantages. In this review, we focus on the breakthroughs for high-density cultures of hiPSC-derived RBCs, and highlight the major challenges and prospective solutions throughout the whole process of erythropoiesis for hiPSC-derived RBCs. Furthermore, we elaborate on the recent advances and techniques used to achieve cost-effective, high-density cultures of GMP-compliant RBCs, and on their relevant novel applications after downstream processing and purification.

Complex Interactions in Regulation of Haematopoiesis-An Unexplored Iron Mine

Iron is one of the most abundant metals on earth and is vital for the growth and survival of life forms. It is crucial for the functioning of plants and animals as it is an integral component of the photosynthetic apparatus and innumerable proteins and enzymes. It plays a pivotal role in haematopoiesis and affects the development and differentiation of different haematopoietic lineages, apart from its obvious necessity in erythropoiesis. A large amount of iron stores in humans is diverted towards the latter process, as iron is an indispensable component of haemoglobin. This review summarises the important players of iron metabolism and homeostasis that have been discovered in recent years and highlights the overall significance of iron in haematopoiesis. Its role in maintenance of haematopoietic stem cells, influence on differentiation of varied haematopoietic lineages and consequences of iron deficiency/overloading on development and maturation of different groups of haematopoietic cells have been discussed.

Induced Pluripotent Stem Cells Enable Disease Modeling and Drug Screening in Calreticulin del52 and ins5 Myeloproliferative Neoplasms

Mutations in the calreticulin (CALR) gene are seen in about 30% of essential thrombocythemia and primary myelofibrosis patients. To address the contribution of the human CALR mutants to the pathogenesis of myeloproliferative neoplasms (MPNs) in an endogenous context, we modeled the CALRdel52 and CALRins5 mutants by induced pluripotent stem cell (iPSC) technology using CD34⁺ progenitors from 4 patients. We describe here the generation of several clones of iPSC carrying heterozygous CALRdel52 or CALRins5 mutations. We showed that CALRdel52 induces a stronger increase in progenitors than CALRins5 and that both CALRdel52 and CALRins5 mutants favor an expansion of the megakaryocytic lineage. Moreover, we found that both CALRdel52 and CALRins5 mutants rendered colony forming unit–megakaryocyte (CFU-MK) independent from thrombopoietin (TPO), and promoted a mild constitutive activation level of signal transducer and activator of transcription 3 in megakaryocytes. Unexpectedly, a mild increase in the sensitivity of colony forming unit-granulocyte (CFU-G) to granulocyte-colony stimulating factor was also observed in iPSC CALRdel52 and CALRins5 compared with control iPSC. Moreover, CALRdel52-induced megakaryocytic spontaneous growth is more dependent on Janus kinase 2/phosphoinositide 3-kinase/extracellular signal-regulated kinase than TPO-mediated growth and opens a therapeutic window for treatments in CALR-mutated MPN. The iPSC models described here represent an interesting platform for testing newly developed inhibitors. Altogether, this study shows that CALR-mutated iPSC recapitulate MPN phenotypes in vitro and may be used for drug screening.

Megakaryocyte progenitor cell function is enhanced upon aging despite the functional decline of aged hematopoietic stem cells

Age-related morbidity is associated with a decline in hematopoietic stem cell (HSC) function, but the mechanisms of HSC aging remain unclear. We performed heterochronic HSC transplants followed by quantitative analysis of cell reconstitution. Although young HSCs outperformed old HSCs in young recipients, young HSCs unexpectedly failed to outcompete the old HSCs of aged recipients. Interestingly, despite substantial enrichment of megakaryocyte progenitors (MkPs) in old mice in situ and reported platelet (Plt) priming with age, transplanted old HSCs were deficient in reconstitution of all lineages, including MkPs and Plts. We therefore performed functional analysis of young and old MkPs. Surprisingly, old MkPs displayed unmistakably greater regenerative capacity compared with young MkPs. Transcriptome analysis revealed putative molecular regulators of old MkP expansion. Collectively, these data demonstrated that aging affects HSCs and megakaryopoiesis in fundamentally different ways: whereas old HSCs functionally decline, MkPs gain expansion capacity upon aging.

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Reconstitution deficit by old HSCs was observed by chimerism and absolute cell numbers

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Young HSCs did not outcompete resident HSCs in aged recipient mice

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Old MkPs display remarkable capacity to engraft, expand, and reconstitute platelets

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Aging is associated with changes in MkP genome-wide expression signatures

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.

Current understanding of human megakaryocytic-erythroid progenitors and their fate determinants

PURPOSE OF REVIEW

This review focuses on our current understanding of fate decisions in bipotent megakaryocyte-erythroid progenitors (MEPs). Although extensive research has been carried out over decades, our understanding of how MEP commit to the erythroid versus megakaryocyte fate remains unclear.

RECENT FINDINGS

We discuss the isolation of primary human MEP, and focus on gene expression patterns, epigenetics, transcription factors and extrinsic factors that have been implicated in MEP fate determination. We conclude with an overview of the open debates in the field of MEP biology.

SUMMARY

Understanding MEP fate is important because defects in megakaryocyte and erythrocyte development lead to disease states such as anaemia, thrombocytopenia and leukaemia. MEP also represent a model system for studying fundamental principles underlying cell fate decisions of bipotent and pluripotent progenitors, such that discoveries in MEP are broadly applicable to stem/progenitor cell biology.

Lysophosphatidic acid receptors 2 and 3 regulate erythropoiesis at different hematopoietic stages

Hematopoiesis, the complex developmental process that forms blood components and replenishes the blood system, involves multiple intracellular and extracellular mechanisms. We previously demonstrated that lysophosphatidic acid (LPA), a lipid growth factor, has opposing regulatory effects on erythrocyte differentiation through activation of LPA receptors 2 and 3; yet the mechanisms underlying this process remain unclear. In this study, LPA₂ is observed that highly expressed in common myeloid progenitors (CMP) in murine myeloid cells, whereas the expression of LPA₃ displaces in megakaryocyte-erythroid progenitors (MEP) of later stage of myeloid differentiation. Therefore, we hypothesized that the switching expression of LPA₂ and LPA₃ determine the hematic homeostasis of mammalian megakaryocytic-erythroid lineage. In vitro colony-forming unit assays of murine progenitors reveal that LPA₂ agonist GRI reduces the erythroblast differentiation potential of CMP. In contrast, LPA₃ agonist OMPT increases the production of erythrocytes from megakaryocyte-erythrocyte progenitor cells (MEP). In addition, treatment with GRI reduces the erythroid, CMP, and MEP populations in mice, indicating that LPA₂ predominantly inhibits myeloid differentiation at an early stage. In contrast, activation of LPA₃ increases the production of terminally differentiated erythroid cells through activation of erythropoietic transcriptional factor. We also demonstrate that the LPA₃ signaling is essential for restoration of phenylhydrazine (PHZ)-induced acute hemolytic anemia in mice and correlates to erythropoiesis impairment of Hutchinson-Gilford progeria Symptom (HGPS) premature aging expressed K562 model. Our results reveal the distinct roles of LPA₂ and LPA₃ at different stages of hematopoiesis in vivo, providing potentiated therapeutic strategies of anemia treatment.

Nobiletin Promotes Megakaryocytic Differentiation through the MAPK/ERK-Dependent EGR1 Expression and Exerts Anti-Leukemic Effects in Human Chronic Myeloid Leukemia (CML) K562 Cells

Differentiation therapy is an alternative strategy used to induce the differentiation of blast cells toward mature cells and to inhibit tumor cell proliferation for cancer treatment. Nobiletin (NOB), a polymethoxyflavone phytochemical, is present abundantly in citrus peels and has been reported to possess anti-cancer activity. In this study, we investigated the anti-leukemic effects of NOB on cell differentiation and its underlying mechanisms in human chronic myeloid leukemia (CML) K562 cells. NOB (100 μM) treatment for 24 and 48 h significantly decreased viability of K562 cells to 54.4 ± 5.3% and 46.2 ± 9.9%, respectively. NOB (10–100 μM) significantly inhibited cell growth in K562 cells. Flow cytometry analysis and immunoblotting data showed that NOB (40 and 80 μM) could modulate the cell cycle regulators including p21, p27, and cyclin D2, and induce G1 phase arrest. NOB also increased the messenger RNA (mRNA) and protein expression of megakaryocytic differentiation markers, such as CD61, CD41, and CD42 as well as the formation of large cells with multi-lobulated nuclei in K562 cells. These results suggested that NOB facilitated K562 cells toward megakaryocytic differentiation. Furthermore, microarray analysis showed that expression of EGR1, a gene associated with promotion of megakaryocytic differentiation, was markedly elevated in NOB-treated K562 cells. The knockdown of EGR1 expression by small interference RNA (siRNA) could significantly attenuate NOB-mediated cell differentiation. We further elucidated that NOB induced EGR1 expression and CD61 expression through increases in MAPK/ERK phosphorylation in K562 cells. These results indicate that NOB promotes megakaryocytic differentiation through the MAPK/ERK pathway-dependent EGR1 expression in human CML cells. In addition, NOB when combined with imatinib could synergistically reduce the viability of K562 cells. Our findings suggest that NOB may serve as a beneficial anti-leukemic agent for differentiation therapy.

GWAS of mosaic loss of chromosome Y highlights genetic effects on blood cell differentiation

Mosaic loss of chromosome Y (mLOY) is frequently observed in the leukocytes of ageing men. However, the genetic architecture and biological mechanisms underlying mLOY are not fully understood. In a cohort of 95,380 Japanese men, we identify 50 independent genetic markers in 46 loci associated with mLOY at a genome-wide significant level, 35 of which are unreported. Lead markers overlap enhancer marks in hematopoietic stem cells (HSCs, P ≤ 1.0 × 10⁻⁶). mLOY genome-wide association study signals exhibit polygenic architecture and demonstrate strong heritability enrichment in regions surrounding genes specifically expressed in multipotent progenitor (MPP) cells and HSCs (P ≤ 3.5 × 10⁻⁶). ChIP-seq data demonstrate that binding sites of FLI1, a fate-determining factor promoting HSC differentiation into platelets rather than red blood cells (RBCs), show a strong heritability enrichment (P = 1.5 × 10⁻⁶). Consistent with these findings, platelet and RBC counts are positively and negatively associated with mLOY, respectively. Collectively, our observations improve our understanding of the mechanisms underlying mLOY.

Mosaic loss of chromosome Y (mLOY) is associated with age and smoking but also genetic factors play a role. Here, Terao et al. perform GWAS for mLOY in 95,380 Japanese men and identify 46 loci that overlap with hematopoietic stem cell enhancers and transcription factor binding sites critical for hematopoiesis.

Ex vivo culture of adult CD34+ stem cells using functional highly porous polymer scaffolds to establish biomimicry of the bone marrow niche

Haematopoiesis, the process of blood production, occurs from a tiny contingent of haematopoietic stem cells (HSC) in highly specialised three-dimensional niches located within the bone marrow. When haematopoiesis is replicated using in vitro two-dimensional culture, HSCs rapidly differentiate, limiting self-renewal. Emulsion-templated highly porous polyHIPE foam scaffolds were chosen to mimic the honeycomb architecture of human bone. The unmodified polyHIPE material supports haematopoietic stem and progenitor cell (HSPC) culture, with successful culture of erythroid progenitors and neutrophils within the scaffolds. Using erythroid culture methodology, the CD34⁺ population was maintained for 28 days with continual release of erythroid progenitors. These cells are shown to spontaneously repopulate the scaffolds, and the accumulated egress can be expanded and grown at large scale to reticulocytes. We next show that the polyHIPE scaffolds can be successfully functionalised using activated BM(PEG)₂ (1,8-bismaleimido-diethyleneglycol) and then a Jagged-1 peptide attached in an attempt to facilitate notch signalling. Although Jagged-1 peptide had no detectable effect, the BM(PEG)₂ alone significantly increased cell egress when compared to controls, without depleting the scaffold population. This work highlights polyHIPE as a novel functionalisable material for mimicking the bone marrow, and also that PEG can influence HSPC behaviour within scaffolds.

The GABA receptor GABRR1 is expressed on and functional in hematopoietic stem cells and megakaryocyte progenitors

GABRR1 is a rho subunit receptor of GABA, the major inhibitory neurotransmitter in the mammalian brain. While most investigations of its function focused on the nervous system, its regulatory role in hematopoiesis has not been reported. In this study, we found GABRR1 is mainly expressed on subsets of human and mouse hematopoietic stem cells (HSCs) and megakaryocyte progenitors (MkPs). GABRR1-negative (GR-) HSCs led to higher donor-derived hematopoietic chimerism than GABRR1-positive (GR+) HSCs. GR+ but not GR- HSCs and MkPs respond to GABA in patch clamp studies. Inhibition of GABRR1 via genetic knockout or antagonists inhibited MkP differentiation and reduced platelet numbers in blood. Overexpression of GABRR1 or treatment with agonists significantly promoted MkP generation and megakaryocyte colonies. Thus, this study identifies a link between the neural and hematopoietic systems and opens up the possibility of manipulating GABA signaling for platelet-required clinical applications.

UM171 expands distinct types of myeloid and NK progenitors from human pluripotent stem cells

Scaling up blood cell production from hPSCs is critical to advancing hPSC technologies for blood transfusion, immunotherapy, and transplantation. Here we explored the potential of the HSC agonist pyrimido-indole derivative UM171, to expand hematopoietic progenitors (HPs) derived from hPSCs in chemically defined conditions. We revealed that culture of hPSC-HPs in HSC expansion conditions (SFEM with added TPO, SCF, FLT3L, IL3 and IL6) in the presence of UM171 predominantly expanded HPs with a unique CD34⁺CD41a^loCD45⁺ phenotype that were enriched in granulocytic progenitors (G-CFCs). In contrast, in lymphoid cultures on OP9-DLL4, in the presence of SCF, FLT3L, and IL7, UM171 selectively expanded CD34⁺CD45⁺CD7⁺ lymphoid progenitors with NK cell potential, and increased NK cell output up to 10-fold. These studies should improve our understanding of the effect of UM171 on de novo generated HPs, and facilitate development of protocols for robust granulocyte and lymphoid cell production from hPSCs, for adoptive immunotherapies.

Single-cell trajectories reconstruction, exploration and mapping of omics data with STREAM

Single-cell transcriptomic assays have enabled the de novo reconstruction of lineage differentiation trajectories, along with the characterization of cellular heterogeneity and state transitions. Several methods have been developed for reconstructing developmental trajectories from single-cell transcriptomic data, but efforts on analyzing single-cell epigenomic data and on trajectory visualization remain limited. Here we present STREAM, an interactive pipeline capable of disentangling and visualizing complex branching trajectories from both single-cell transcriptomic and epigenomic data. We have tested STREAM on several synthetic and real datasets generated with different single-cell technologies. We further demonstrate its utility for understanding myoblast differentiation and disentangling known heterogeneity in hematopoiesis for different organisms. STREAM is an open-source software package.

Molecular pathogenesis of myelodysplastic syndromes with deletion 5q

The molecular pathogenesis of deletion 5q (del(5q)) myelodysplastic syndrome (MDS) has recently been realized as a result of major advances in our understanding of the mechanisms responsible for clinical phenotype. Identification of commonly deleted genes such as RPS14, miRNA-145, HSPA9, CD78, and CSNK1a1 have elucidated the precise biological changes responsible for the anemia, leukopenia, and thrombocytosis that characterizes del(5q) MDS and highlighted the importance of allelic haploinsufficiency in the hematological phenotype. Recent elegant investigations have also identified a critical role of innate immune signaling in del(5q) pathogenesis. TP53 and Wnt/β-catenin pathways have also been found to be involved in clonal expansion and progression of the disease as well as resistance and poor outcomes to available therapy. Understanding the molecular pathogenesis of the disease has provided a critical foundation in identifying the biological targets of lenalidomide in del(5q) MDS, which has led to the development of novel therapeutic agents in hematologic malignancies as well as potential alternative targets to exploit in patients who have failed lenalidomide treatment.

Arterial identity of hemogenic endothelium: a key to unlock definitive hematopoietic commitment in human pluripotent stem cell cultures

Human pluripotent stem cells (hPSCs) have been suggested as a potential source for the de novo production of blood cells for transfusion, immunotherapies, and transplantation. However, even with advanced hematopoietic differentiation methods, the primitive and myeloid-restricted waves of hematopoiesis dominate in hPSC differentiation cultures, whereas cell surface markers to distinguish these waves of hematopoiesis from lympho-myeloid hematopoiesis remain unknown. In the embryo, hematopoietic stem cells (HSCs) arise from hemogenic endothelium (HE) lining arteries, but not veins. This observation led to a long-standing hypothesis that arterial specification is an essential prerequisite to initiate the HSC program. It has also been established that lymphoid potential in the yolk sac and extraembryonic vasculature is mostly confined to arteries, whereas myeloid-restricted hematopoiesis is not specific to arterial vessels. Here, we review how the link between arterialization and the subsequent definitive multilineage hematopoietic program can be exploited to identify HE enriched in lymphoid progenitors and aid in in vitro approaches to enhance the production of lymphoid cells and potentially HSCs from hPSCs. We also discuss alternative models of hematopoietic specification at arterial sites and recent advances in our understanding of hematopoietic development and the production of engraftable hematopoietic cells from hPSCs.

Early Human Hemogenic Endothelium Generates Primitive and Definitive Hematopoiesis In Vitro

The differentiation of human embryonic stem cells (hESCs) to hematopoietic lineages initiates with the specification of hemogenic endothelium, a transient specialized endothelial precursor of all blood cells. This in vitro system provides an invaluable model to dissect the emergence of hematopoiesis in humans. However, the study of hematopoiesis specification is hampered by a lack of consensus in the timing of hemogenic endothelium analysis and the full hematopoietic potential of this population. Here, our data reveal a sharp decline in the hemogenic potential of endothelium populations isolated over the course of hESC differentiation. Furthermore, by tracking the dynamic expression of CD31 and CD235a at the onset of hematopoiesis, we identified three populations of hematopoietic progenitors, representing primitive and definitive subsets that all emerge from the earliest specified hemogenic endothelium. Our data establish that hemogenic endothelium populations endowed with primitive and definitive hematopoietic potential are specified simultaneously from the mesoderm in differentiating hESCs.

Review: In vitro generation of red blood cells for transfusion medicine: Progress, prospects and challenges

In vitro generation of red blood cells (RBCs) has the potential to circumvent the shortfalls in global demand for blood for transfusion applications. The conventional approach for RBC generation has been from differentiation of hematopoietic stem cells (HSCs) derived from cord blood, adult bone marrow or peripheral blood. More recently, RBCs have been generated from human induced pluripotent stem cells (hiPSCs) as well as from immortalized adult erythroid progenitors. In this review, we highlight the recent advances to RBC generation from these different approaches and discuss the challenges and new strategies that can potentially make large-scale in vitro generation of RBCs a feasible approach.

Inhibition of Retinoic Acid Production Expands a Megakaryocyte-Enriched Subpopulation with Islet Regenerative Function

Islet regeneration is stimulated after transplantation of human umbilical cord blood (UCB) hematopoietic progenitor cells with high aldehyde dehydrogenase (ALDH)-activity into NOD/SCID mice with streptozotocin (STZ)-induced β cell ablation. ALDH^hi progenitor cells represent a rare subset within UCB that will require expansion without the loss of islet regenerative functions for use in cell therapies. ALDH^hi cells efficiently expand (>70-fold) under serum-free conditions; however, high ALDH-activity is rapidly diminished during culture coinciding with emergence of a committed megakaryocyte phenotype CD41+/CD42+/CD38+. ALDH-activity is also the rate-limiting step in retinoic acid (RA) production, a potent driver of hematopoietic differentiation. We have previously shown that inhibition of RA production during 9-day cultures, using diethylaminobenzaldehyde (DEAB) treatment, enhanced the expansion of ALDH^hi cells (>20-fold) with vascular regenerative paracrine functions. Herein, we sought to determine if DEAB-treatment also expanded ALDH^hi cells that retain islet regenerative function following intrapancreatic transplantation into hyperglycemic mice. After DEAB-treatment, expanded ALDH^hi cell subset was enriched for CD34+/CD38- expression and demonstrated enhanced myeloid multipotency in vitro compared to the ALDH^lo cell subset. Unfortunately, DEAB-treated ALDH^hi cells did not support islet regeneration after transplantation. Conversely, expanded ALDH^lo cells from DEAB-treated conditions reduced hyperglycemia, and increased islet number and cell proliferation in STZ-induced hyperglycemic NOD/SCID mice. DEAB-treated ALDH^lo cells were largely committed to a CD41+/CD42+ megakaryocyte phenotype. Collectively, this study provides preliminary evidence that committed cells of the megakaryocyte-lineage support endogenous islet regeneration and/or function, and the retention of high ALDH-activity did not coincide with islet regenerative function after expansion under serum-free culture conditions.

Screening for genes that regulate the differentiation of human megakaryocytic lineage cells

Different combinations of transcription factors (TFs) function at each stage of hematopoiesis, leading to distinct expression patterns of lineage-specific genes. The identification of such regulators and their functions in hematopoiesis remain largely unresolved. In this study, we utilized screening approaches to study the transcriptional regulators of megakaryocyte progenitor (MkP) generation, a key step before platelet production. Promising candidate genes were generated from a microarray platform gene expression commons and individually manipulated in human hematopoietic stem and progenitor cells (HSPCs). Deletion of some of the candidate genes (the hit genes) by CRISPR/Cas9 led to decreased MkP generation during HSPC differentiation, while more MkPs were produced when some hit genes were overexpressed in HSPCs. We then demonstrated that overexpression of these genes can increase the frequency of mature megakaryocytic colonies by functional colony forming unit-megakaryocyte (CFU-Mk) assay and the release of platelets after in vitro maturation. Finally, we showed that the histone deacetylase inhibitors could also increase MkP differentiation, possibly by regulating some of the newly identified TFs. Therefore, identification of such regulators will advance the understanding of basic mechanisms of HSPC differentiation and conceivably enable the generation and maturation of megakaryocytes and platelets in vitro.

Large-scale production of megakaryocytes in microcarrier-supported stirred suspension bioreactors

Megakaryocytes (MKs) are the precursors of platelets (PLTs) and may be used for PLT production in vivo or in vitro, as well as a source for PLT-derived growth factors. Induced pluripotent stem cells represent an unlimited cell source for the in vitro production of MKs. This study aimed at developing an effective, xeno-free and scalable system to produce high numbers of MKs. In particular, microcarrier beads-assisted stirred bioreactors were evaluated as a means of improving MK yields. This method resulted in the production of 18.7 × 10⁷ MKs per 50 ml medium. Laminin-coated microcarriers increased MK production per iPSC by up to 10-fold. MKs obtained in this system showed typical features of mature MKs and were able to produce PLTs in vitro and in vivo. To increase safety, MKs produced in the bioreactors were irradiated; a procedure that did not affect their capability to form proPLTs and PTLs after transfusion. In vitro generated MKs represent a promising alternative to donor PLTs and open the possibility for the development of innovative MK-based cell therapies.

Cells with hematopoietic potential reside within mouse proepicardium

During embryonic development, hematopoietic cells are present in areas of blood-vessel differentiation. These hematopoietic cells emerge from a specific subpopulation of endothelial cells called the hemogenic endothelium. We have previously found that mouse proepicardium contained its own population of endothelial cells forming a network of vascular tubules. We hypothesize that this EC population contains cells of hematopoietic potential. Therefore, we investigated an in vitro hematopoietic potential of proepicardial cell populations. The CD31+/CD45-/CD71- cell population cultured for 10 days in MethocultTM gave numerous colonies of CFU-GEMM, CFU-GM, and CFU-E type. These colonies consisted of various cell types. Flk-1+/CD31-/CD45-/CD71-, and CD45+ and/or CD71+ cell populations produced CFU-GEMM and CFU-GM, or CFU-GM and CFU-E colonies, respectively. Immunohistochemical evaluations of smears prepared from colonies revealed the presence of cells of different hematopoietic lineages. These cells were characterized by labeling with various combinations of antibodies directed against CD31, CD41, CD71, c-kit, Mpl, Fli1, Gata-2, and Zeb1 markers. Furthermore, we found that proepicardium-specific marker WT1 co-localized with Runx1 and Zeb1 and that single endothelial cells bearing CD31 molecule expressed Runx1 in the proepicardial area of embryonic tissue sections. We have shown that cells of endothelial and/or hematopoietic phenotypes isolated from mouse proepicardium possess hematopoietic potential in vitro and in situ. These results are supported by RT-PCR analyses of proepicardial extract, which revealed the expression of mRNA for crucial regulatory factors for hemogenic endothelium specification, i.e., Runx1, Notch1, Gata2, and Sox17. Our data are in line with previous observation on hemangioblast derivation from the quail PE.

Concise Review: Bipotent Megakaryocytic-Erythroid Progenitors: Concepts and Controversies

Hematopoietic stem and progenitor cells maintain blood formation throughout our lifetime by undergoing long- and short-term self-renewal, respectively. As progenitor cells progress through the hematopoiesis process, their differentiation capabilities narrow, such that the precursors become committed to only one or two lineages. This Review focuses on recent advances in the identification and characterization of bipotent megakaryocytic-erythroid progenitors (MEP), the cells that can further produce two completely different functional outputs: platelets and red blood cells. The existence of MEP has sparked controversy as studies describing the requirement for this intermediate progenitor stage prior to commitment to the erythroid and megakaryocytic lineages have been potentially contradictory. Interpretation of these studies is complicated by the variety of species, cell sources, and analytical approaches used along with inherent challenges in the continuum of hematopoiesis, where hematopoietic progenitors do not stop at discrete steps on single paths as classically drawn in hematopoietic hierarchy models. With the goal of improving our understanding of human hematopoiesis, we discuss findings in both human and murine cells. Based on these data, MEP clearly represent a transitional stage of differentiation in at least one route to the generation of both megakaryocytes and erythroid cells. Stem Cells 2018;36:1138-1145.

Induced Pluripotent Stem Cell-Derived Red Blood Cells and Platelet Concentrates: From Bench to Bedside

Red blood cells and platelets are anucleate blood components indispensable for oxygen delivery and hemostasis, respectively. Derivation of these blood elements from induced pluripotent stem (iPS) cells has the potential to develop blood donor-independent and genetic manipulation-prone products to complement or replace current transfusion banking, also minimizing the risk of alloimmunization. While the production of erythrocytes from iPS cells has challenges to overcome, such as differentiation into adult-type phenotype that functions properly after transfusion, platelet products are qualitatively and quantitatively approaching a clinically-applicable level owing to advances in expandable megakaryocyte (MK) lines, platelet-producing bioreactors, and novel reagents. Guidelines that assure the quality of iPS cells-derived blood products for clinical application represent a novel challenge for regulatory agencies. Considering the minimal risk of tumorigenicity and the expected significant demand of such products, ex vivo production of iPS-derived blood components can pave the way for iPS translation into the clinic.

Human Bone Marrow Mesenchymal Stromal Cell-Derived Osteoblasts Promote the Expansion of Hematopoietic Progenitors Through Beta-Catenin and Notch Signaling Pathways

Coculture of hematopoietic stem cells (HSC) with primary stromal cells from HSC niches supports the maintenance and expansion of HSC and progenitors ex vivo. However, a major drawback is the availability of primary human samples for research and clinical applications. We investigated the use of in vitro derived osteoblasts as a new source of feeder cells and characterized the molecular pathways that mediate their growth-promoting activities. First, we compared the growth and differentiation modulating activities of mesenchymal stromal cells (MSC)-derived osteoblasts (M-OST) with those of their undifferentiated precursor on umbilical cord blood (UCB) progenitors. Feeder-free cultures were also included as baseline control. Cell growth and expansion of hematopoietic progenitors were significantly enhanced by both feeder cell types. However, progenitor cell growth was considerably greater with M-OST. Coculture also promoted the maintenance of immature CD34⁺ progenitor subsets and modulated in a positive fashion the expression of several homing-related cell surface receptors, in a feeder-specific fashion. Serial transplantation experiments revealed that M-OST coculture supported the maintenance of long-term lympho-myeloid reconstituting HSC that provided engraftment levels that were generally superior to those from MSC cocultures. Mechanistically, we found that coculture with M-OST was associated with enhanced beta-catenin (β-Cat) activity in UCB cells and that abrogation of β-Cat/T-cell factor activity blunted the growth-promoting activity of the M-OST coculture. Conversely, Notch inhibition reduced UCB cell expansion, but to a much lesser extent. In conclusion, this study demonstrates that M-OST are excellent feeder cells for HSC and progenitors, and it identifies key molecular pathways that are responsible for the growth-enhancing activities of osteoblasts on UCB progenitors.

Revisiting erythroleukemia

PURPOSE OF REVIEW

The 2016 WHO classification of hematopoietic and lymphoid neoplasms alters the diagnostic criteria for erythroleukemia, including eliminating the erythroid/myeloid type of acute erythroleukemia, which was a prior subcategory of acute myeloid leukemia, not otherwise specified. Only pure erythroid leukemia remains in the WHO classification. This review will summarize the literature that contributed to that classification change as well as recent literature on the significance of the change.

RECENT FINDINGS

There is now a large body of literature on the negative prognostic impact of erythroid predominance, defined as 50% or more bone marrow erythroid cells, in myelodysplastic syndromes (MDSs). Recent studies have found similarities between erythroleukemia, especially the erythroid/myeloid type, and the erythroid-rich MDS cases. On the basis of these data, the WHO now reclassifies cases of the prior erythroid/myeloid acute erythroleukemia group based on the total blast cell count. This change moves such cases into an MDS category, usually MDS with excess blasts. This approach, however, may cloud the significance of erythroid predominance in this group of patients.

SUMMARY

The report clarifies the current criteria for a diagnosis of erythroleukemia as well as the ongoing challenges in classifying this group of erythroid rich bone marrow disorders.

Germline variants in ETV6 underlie reduced platelet formation, platelet dysfunction and increased levels of circulating CD34+ progenitors

Variants in ETV6, which encodes a transcription repressor of the E26 transformation-specific family, have recently been reported to be responsible for inherited thrombocytopenia and hematologic malignancy. We sequenced the DNA from cases with unexplained dominant thrombocytopenia and identified six likely pathogenic variants in ETV6, of which five are novel. We observed low repressive activity of all tested ETV6 variants, and variants located in the E26 transformation-specific binding domain (encoding p.A377T, p.Y401N) led to reduced binding to corepressors. We also observed a large expansion of megakaryocyte colony-forming units derived from variant carriers and reduced proplatelet formation with abnormal cytoskeletal organization. The defect in proplatelet formation was also observed in control CD34+ cell-derived megakaryocytes transduced with lentiviral particles encoding mutant ETV6. Reduced expression levels of key regulators of the actin cytoskeleton CDC42 and RHOA were measured. Moreover, changes in the actin structures are typically accompanied by a rounder platelet shape with a highly heterogeneous size, decreased platelet arachidonic response, and spreading and retarded clot retraction in ETV6 deficient platelets. Elevated numbers of circulating CD34+ cells were found in p.P214L and p.Y401N carriers, and two patients from different families suffered from refractory anemia with excess blasts, while one patient from a third family was successfully treated for acute myeloid leukemia. Overall, our study provides novel insights into the role of ETV6 as a driver of cytoskeletal regulatory gene expression during platelet production, and the impact of variants resulting in platelets with altered size, shape and function and potentially also in changes in circulating progenitor levels.

Proper cytoskeletal architecture beneath the plasma membrane of red blood cells requires Ttll4

Mammalian red blood cells (RBCs) circulate through blood vessels, including capillaries, for tens of days under high mechanical stress. RBCs tolerate this mechanical stress while maintaining their shape because of their elastic membrane skeleton. This membrane skeleton consists of spectrin-actin lattices arranged as quasi-hexagonal units beneath the plasma membrane. In this study, we found that the organization of the RBC cytoskeleton requires tubulin tyrosine ligase-like 4 (Ttll4). RBCs from Ttll4-knockout mice showed larger average diameters in smear test. Based on the rate of hemolysis, Ttll4-knockout RBCs showed greater vulnerability to phenylhydrazine-induced oxidative stress than did wild-type RBCs. Ultrastructural analyses revealed the macromolecular aggregation of cytoskeletal components in RBCs of Ttll4-knockout mice. Immunoprecipitation using the anti-glutamylation antibody GT335 revealed nucleosome assembly protein 1 (NAP1) to be the sole target of TTLL4 in the RBCs, and NAP1 glutamylation was completely lost in Ttll4-knockout RBCs. In wild-type RBCs, the amount of glutamylated NAP1 in the membrane was nearly double that in the cytosol. Furthermore, the absence of TTLL4-dependent glutamylation of NAP1 weakened the binding of NAP1 to the RBC membrane. Taken together, these data demonstrate that Ttll4 is required for proper cytoskeletal organization in RBCs.

Hematopoietic stem cell-independent hematopoiesis: emergence of erythroid, megakaryocyte, and myeloid potential in the mammalian embryo

Steady-state production of all circulating blood cells in the adult ultimately depends on hematopoietic stem cells (HSCs), which first arise in small numbers beginning at embryonic day (E) 10.5 in large arterial vessels of the murine embryo. However, blood cell synthesis first begins in the yolk sac beginning at E7.25 and consists of two waves of hematopoietic progenitors. The first wave consists of primitive erythroid, megakaryocyte, and macrophage progenitors that rapidly give rise to maturing blood cells of all three lineages. This 'primitive' wave of progenitors is followed by a partially overlapping wave of 'erythro-myeloid progenitors', which contain definitive erythroid, megakaryocyte, macrophage, neutrophil, and mast cell progenitors that seed the fetal liver and jump-start hematopoiesis before the engraftment and expansion of HSCs. These two waves of progenitors that arise in the yolk sac are necessary and even sufficient to sustain the survival of the mouse embryo until birth in the absence of HSCs. They provide key signals to support HSC emergence. Finally, HSC-independent hematopoiesis also provides long-lived tissue-resident macrophage populations that function in multiple adult organs.

[Iron deficiency, thrombocytosis and thromboembolism]

Iron deficiency, the most common nutritional deficiency worldwide, is often associated with reactive thrombocytosis. Although secondary thrombocytosis is commonly considered to be harmless, there is accumulating evidence that elevated platelet counts, especially in the setting of iron deficiency, can lead to an increased thromboembolic risk in both arterial and venous systems. Here we present the mechanisms of iron deficiency-induced thrombocytosis and summarize its clinical consequences especially in patients with inflammatory bowel diseases, chronic kidney disease or cancer. We hypothesize that iron deficiency is an underestimated thromboembolic risk factor, and that iron replacement therapy can become an effective preventive strategy in a variety of clinical settings.

Polyurethane scaffolds seeded with CD34(+) cells maintain early stem cells whilst also facilitating prolonged egress of haematopoietic progenitors

We describe a 3D erythroid culture system that utilises a porous polyurethane (PU) scaffold to mimic the compartmentalisation found in the bone marrow. PU scaffolds seeded with peripheral blood CD34⁺ cells exhibit a remarkable reproducibility of egress, with an increased output when directly compared to human bone scaffolds over 28 days. Immunofluorescence demonstrated the persistence of CD34⁺ cells within the scaffolds for the entirety of the culture. To characterise scaffold outputs, we designed a flow cytometry panel that utilises surface marker expression observed in standard 2D erythroid and megakaryocyte cultures. This showed that the egress population is comprised of haematopoietic progenitor cells (CD36⁺GPA^−/low). Control cultures conducted in parallel but in the absence of a scaffold were also generally maintained for the longevity of the culture albeit with a higher level of cell death. The harvested scaffold egress can also be expanded and differentiated to the reticulocyte stage. In summary, PU scaffolds can behave as a subtractive compartmentalised culture system retaining and allowing maintenance of the seeded “CD34⁺ cell” population despite this population decreasing in amount as the culture progresses, whilst also facilitating egress of increasingly differentiated cells.

Cooperative Effect of Erythropoietin and TGF-β Inhibition on Erythroid Development in Human Pluripotent Stem Cells

Patient-specific human induced-pluripotent stem cells (hiPSCs) represent important cell sources to treat patients with acquired blood disorders. To realize the therapeutic potential of hiPSCs, it is crucial to understand signals that direct hiPSC differentiation to a hematopoietic lineage fate. Our previous study demonstrated that CD34(+)CD31(+) cells derived from human pluripotent stem cells (hPSCs) contain hemato-endothelial progenitors (HEPs) that give rise to hematopoietic cells and endothelial cells. Here, we established a serum-free and feeder-free system to induce the differentiation of hPSC-derived CD34(+)CD31(+) progenitor cells to erythroid cells. We show that extracellular matrix (ECM) proteins promote the differentiation of CD34(+)CD31(+) progenitor cells into CD235a(+) erythroid cells through CD41(+)CD235a(+) megakaryocyte-erythroid progenitors (MEP). Erythropoietin (EPO) is a predominant factor for CD34(+)CD31(+) progenitor differentiation to erythroid cells, whereas transforming growth factor beta (TGF-β) inhibits the development of CD34(+)CD31(+) progenitor cells. Apoptosis of progenitor cells is induced by TGF-β in early erythroid differentiation. Suppression of TGF-β signaling by SB431542 at early stage of CD34(+)CD31(+) progenitor differentiation induces the erythroid cell generation. Together, these findings suggest that TGF-β suppression and EPO stimulation promote erythropoiesis of CD34(+)CD31(+) progenitor cells derived from hPSCs.

Superior Red Blood Cell Generation from Human Pluripotent Stem Cells Through a Novel Microcarrier-Based Embryoid Body Platform

In vitro generation of red blood cells (RBCs) from human embryonic stem cells and human induced pluripotent stem cells appears to be a promising alternate approach to circumvent shortages in donor-derived blood supplies for clinical applications. Conventional methods for hematopoietic differentiation of human pluripotent stem cells (hPSC) rely on embryoid body (EB) formation and/or coculture with xenogeneic cell lines. However, most current methods for hPSC expansion and EB formation are not amenable for scale-up to levels required for large-scale RBC generation. Moreover, differentiation methods that rely on xenogenic cell lines would face obstacles for future clinical translation. In this study, we report the development of a serum-free and chemically defined microcarrier-based suspension culture platform for scalable hPSC expansion and EB formation. Improved survival and better quality EBs generated with the microcarrier-based method resulted in significantly improved mesoderm induction and, when combined with hematopoietic differentiation, resulted in at least a 6-fold improvement in hematopoietic precursor expansion, potentially culminating in a 80-fold improvement in the yield of RBC generation compared to a conventional EB-based differentiation method. In addition, we report efficient terminal maturation and generation of mature enucleated RBCs using a coculture system that comprised primary human mesenchymal stromal cells. The microcarrier-based platform could prove to be an appealing strategy for future scale-up of hPSC culture, EB generation, and large-scale generation of RBCs under defined and xeno-free conditions.

Large-scale production of megakaryocytes from human pluripotent stem cells by chemically defined forward programming

The production of megakaryocytes (MKs)--the precursors of blood platelets--from human pluripotent stem cells (hPSCs) offers exciting clinical opportunities for transfusion medicine. Here we describe an original approach for the large-scale generation of MKs in chemically defined conditions using a forward programming strategy relying on the concurrent exogenous expression of three transcription factors: GATA1, FLI1 and TAL1. The forward programmed MKs proliferate and differentiate in culture for several months with MK purity over 90% reaching up to 2 × 10(5) mature MKs per input hPSC. Functional platelets are generated throughout the culture allowing the prospective collection of several transfusion units from as few as 1 million starting hPSCs. The high cell purity and yield achieved by MK forward programming, combined with efficient cryopreservation and good manufacturing practice (GMP)-compatible culture, make this approach eminently suitable to both in vitro production of platelets for transfusion and basic research in MK and platelet biology.

Early hematopoiesis and macrophage development

The paradigm that all blood cells are derived from hematopoietic stem cells (HSCs) has been challenged by two findings. First, there are tissue-resident hematopoietic cells, including subsets of macrophages that are not replenished by adult HSCs, but instead are maintained by self-renewal of fetal-derived cells. Second, during embryogenesis, there is a conserved program of HSC-independent hematopoiesis that precedes HSC function and is required for embryonic survival. The presence of waves of HSC-independent hematopoiesis as well as fetal HSCs raises questions about the origin of fetal-derived adult tissue-resident macrophages. In the murine embryo, historical examination of embryonic macrophage and monocyte populations combined with recent reports utilizing genetic lineage-tracing approaches has led to a model of macrophage ontogeny that can be integrated with existing models of hematopoietic ontogeny. The first wave of hematopoiesis contains primitive erythroid, megakaryocyte and macrophage progenitors that arise in the yolk sac, and these macrophage progenitors are the source of early macrophages throughout the embryo, including the liver. A second wave of multipotential erythro-myeloid progenitors (EMPs) also arises in the yolk sac. EMPs colonize the fetal liver, initiating myelopoiesis and forming macrophages. Lineage tracing indicates that this second wave of macrophages are distributed in most fetal tissues, although not appreciably in the brain. Thus, fetal-derived adult tissue-resident macrophages, other than microglia, appear to predominately derive from EMPs. While HSCs emerge at midgestation and colonize the fetal liver, the relative contribution of fetal HSCs to tissue macrophages at later stages of development is unclear. The inclusion of macrophage potential in multiple waves of hematopoiesis is consistent with reports of their functional roles throughout development in innate immunity, phagocytosis, and tissue morphogenesis and remodeling. Understanding the influences of developmental origin, as well as local tissue-specific signals, will be necessary to fully decode the diverse functions and responses of tissue-resident macrophages.

Linkage between the mechanisms of thrombocytopenia and thrombopoiesis

Thrombocytopenia is defined as a status in which platelet numbers are reduced. Imbalance between the homeostatic regulation of platelet generation and destruction is 1 potential cause of thrombocytopenia. In adults, platelet generation is a 2-stage process entailing the differentiation of hematopoietic stem cells into mature megakaryocytes (MKs; known as megakaryopoiesis) and release of platelets from MKs (known as thrombopoiesis or platelet biogenesis). Until recently, information about the genetic defects responsible for congenital thrombocytopenia was only available for a few forms of the disease. However, investigations over the past 15 years have identified mutations in genes encoding >20 different proteins that are responsible for these disorders, which has advanced our understanding of megakaryopoiesis and thrombopoiesis. The underlying pathogenic mechanisms can be categorized as (1) defects in MK lineage commitment and differentiation, (2) defects in MK maturation, and (3) defect in platelet release. Using these developmental stage categories, we here update recently described mechanisms underlying megakaryopoiesis and thrombopoiesis and discuss the association between platelet generation systems and thrombocytopenia.