Efficient hematopoietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches

Human fetal liver stromal cell co-culture enhances the differentiation of pancreatic progenitor cells into islet-like cell clusters

Recent advance in directed differentiation of pancreatic stem cells offers potential to the development of replacement therapy for diabetic patients. However, the existing differentiation protocols are complex, time-consuming, and costly; thus there is a need for alternative protocols. Given the common developmental origins of liver and pancreas, we sought to develop a novel protocol, devoid of growth factors, by using liver stromal cells (LSCs) derived from human fetal liver. We examined the effects of the LSCs on the differentiation of pancreatic progenitor cells (PPCs) into islet-like cell clusters (ICCs). PPCs and LSCs isolated from 1st to 2nd trimester human fetal tissues underwent co-cultures; differentiation and functionality of ICCs were determined by examining expression of critical markers and secretion of insulin. Co-culture with 2nd but not 1st trimester LSCs enhanced ICC differentiation and functionality without the use of exogenous differentiation 'cocktails'. Differential expression profiles of growth factors from 1st versus 2nd trimester fetal liver were compared. Many morphogenic factors were expressed by LSCs, while insulin-like growth factor 1 (IGF1) was identified as one of the key molecules responsible for the ICC differentiation. This is the first report showing that an LSC-induced microenvironment can enhance ICC differentiation and functionality. Further modifications of the stroma microenvironment may offer an alternative, efficient and cost-effective approach to providing islets for transplantation.

Cellular reprogramming for understanding and treating human disease

In the last two decades we have witnessed a paradigm shift in our understanding of cells so radical that it has rewritten the rules of biology. The study of cellular reprogramming has gone from little more than a hypothesis, to applied bioengineering, with the creation of a variety of important cell types. By way of metaphor, we can compare the discovery of reprogramming with the archeological discovery of the Rosetta stone. This stone slab made possible the initial decipherment of Egyptian hieroglyphics because it allowed us to see this language in a way that was previously impossible. We propose that cellular reprogramming will have an equally profound impact on understanding and curing human disease, because it allows us to perceive and study molecular biological processes such as differentiation, epigenetics, and chromatin in ways that were likewise previously impossible. Stem cells could be called “cellular Rosetta stones” because they allow also us to perceive the connections between development, disease, cancer, aging, and regeneration in novel ways. Here we present a comprehensive historical review of stem cells and cellular reprogramming, and illustrate the developing synergy between many previously unconnected fields. We show how stem cells can be used to create in vitro models of human disease and provide examples of how reprogramming is being used to study and treat such diverse diseases as cancer, aging, and accelerated aging syndromes, infectious diseases such as AIDS, and epigenetic diseases such as polycystic ovary syndrome. While the technology of reprogramming is being developed and refined there have also been significant ongoing developments in other complementary technologies such as gene editing, progenitor cell production, and tissue engineering. These technologies are the foundations of what is becoming a fully-functional field of regenerative medicine and are converging to a point that will allow us to treat almost any disease.

In vitro human embryonic stem cell hematopoiesis mimics MYB-independent yolk sac hematopoiesis

Although hematopoietic precursor activity can be generated in vitro from human embryonic stem cells, there is no solid evidence for the appearance of multipotent, self-renewing and transplantable hematopoietic stem cells. This could be due to short half-life of hematopoietic stem cells in culture or, alternatively, human embryonic stem cell-initiated hematopoiesis may be hematopoietic stem cell-independent, similar to yolk sac hematopoiesis, generating multipotent progenitors with limited expansion capacity. Since a MYB was reported to be an excellent marker for hematopoietic stem cell-dependent hematopoiesis, we generated a MYB-eGFP reporter human embryonic stem cell line to study formation of hematopoietic progenitor cells in vitro. We found CD34(+) hemogenic endothelial cells rounding up and developing into CD43(+) hematopoietic cells without expression of MYB-eGFP. MYB-eGFP(+) cells appeared relatively late in embryoid body cultures as CD34(+)CD43(+)CD45(-/lo) cells. These MYB-eGFP(+) cells were CD33 positive, proliferated in IL-3 containing media and hematopoietic differentiation was restricted to the granulocytic lineage. In agreement with data obtained on murine Myb(-/-) embryonic stem cells, bright eGFP expression was observed in a subpopulation of cells, during directed myeloid differentiation, which again belonged to the granulocytic lineage. In contrast, CD14(+) macrophage cells were consistently eGFP(-) and were derived from eGFP-precursors only. In summary, no evidence was obtained for in vitro generation of MYB(+) hematopoietic stem cells during embryoid body cultures. The observed MYB expression appeared late in culture and was confined to the granulocytic lineage.

An AAVS1-targeted minigene platform for correction of iPSCs from all five types of chronic granulomatous disease

There are five genetic forms of chronic granulomatous disease (CGD), resulting from mutations in any of five subunits of phagocyte oxidase, an enzyme complex in neutrophils, monocytes, and macrophages that produces microbicidal reactive oxygen species. We generated induced pluripotent stem cells (iPSCs) from peripheral blood CD34(+) hematopoietic stem cells of patients with each of five CGD genotypes. We used zinc finger nuclease (ZFN) targeting the AAVS1 safe harbor site together with CGD genotype-specific minigene plasmids with flanking AAVS1 sequence to target correction of iPSC representing each form of CGD. We achieved targeted insertion with constitutive expression of desired oxidase subunit in 70-80% of selected iPSC clones. Neutrophils and macrophages differentiated from corrected CGD iPSCs demonstrated restored oxidase activity and antimicrobial function against CGD bacterial pathogens Staphylococcus aureus and Granulibacter bethesdensis. Using a standard platform that combines iPSC generation from peripheral blood CD34(+) cells and ZFN mediated AAVS1 safe harbor minigene targeting, we demonstrate efficient generation of genetically corrected iPSCs using an identical approach for all five genetic forms of CGD. This safe harbor minigene targeting platform is broadly applicable to a wide range of inherited single gene metabolic disorders.

Amyotrophic lateral sclerosis - cell based therapy and novel therapeutic development

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease, characterized by the predominant loss of motor neurons (MNs) in primary motor cortex, the brainstem, and the spinal cord, causing premature death in most cases. Minimal delay of pathological development by available medicine has prompted the search for novel therapeutic treatments to cure ALS. Cell-based therapy has been proposed as an ultimate source for regeneration of MNs. Recent completion of non-autologous fetal spinal stem cell transplant to ALS patients brought renewed hope for further human trials to cure the disease. Autologous somatic stem cell-based human trials are now in track to reveal the outcome of the ongoing trials. Furthermore, induced pluripotent stem cell (iPSC)-based ALS disease drug screen and autologous cell transplant options will broaden therapeutic options. In this review paper, we discuss recent accomplishments in cell transplant treatment for ALS and future options with iPSC technology.

Natural killer cells for cancer immunotherapy: pluripotent stem cells-derived NK cells as an immunotherapeutic perspective

Natural killer (NK) cells play an essential role in the fight against tumor development. Over the last years, the progress made in the NK-cell biology field and in deciphering how NK-cell function is regulated, is driving efforts to utilize NK-cell-based immunotherapy as a promising approach for the treatment of malignant diseases. Therapies involving NK cells may be accomplished by activating and expanding endogenous NK cells by means of cytokine treatment or by transferring exogenous cells by adoptive cell therapy and/or by hematopoietic stem cell transplantation. NK cells that are suitable for adoptive cell therapy can be derived from different sources, including ex vivo expansion of autologous NK cells, unstimulated or expanded allogeneic NK cells from peripheral blood, derived from CD34+ hematopoietic progenitors from peripheral blood and umbilical cord blood, and NK-cell lines. Besides, genetically modified NK cells expressing chimeric antigen receptors or cytokines genes may also have a relevant future as therapeutic tools. Recently, it has been described the derivation of large numbers of functional and mature NK cells from pluripotent stem cells, both embryonic stem cells and induced pluripotent stem cells, which adds another tool to the expanding NK-cell-based cancer immunotherapy arsenal.

HOXA9 promotes hematopoietic commitment of human embryonic stem cells

The molecular determinants regulating the specification of human embryonic stem cells (hESCs) into hematopoietic cells remain elusive. HOXA9 plays a relevant role in leukemogenesis and hematopoiesis. It is highly expressed in hematopoietic stem and progenitor cells (HSPCs) and is downregulated upon differentiation. Hoxa9-deficient mice display impaired hematopoietic development, and deregulation of HOXA9 expression is frequently associated with acute leukemia. Analysis of the genes differentially expressed in cord blood HSPCs vs hESC-derived HSPCs identified HOXA9 as the most downregulated gene in hESC-derived HSPCs, suggesting that expression levels of HOXA9 may be crucial for hematopoietic differentiation of hESC. Here we show that during hematopoietic differentiation of hESCs, HOXA9 expression parallels hematopoietic development, but is restricted to the hemogenic precursors (HEP) (CD31(+)CD34(+)CD45(-)), and diminishes as HEPs differentiate into blood cells (CD45(+)). Different gain-of-function and loss-of-function studies reveal that HOXA9 enhances hematopoietic differentiation of hESCs by specifically promoting the commitment of HEPs into primitive and total CD45(+) blood cells. Gene expression analysis suggests that nuclear factor-κB signaling could be collaborating with HOXA9 to increase hematopoietic commitment. However, HOXA9 on its own is not sufficient to confer in vivo long-term engraftment potential to hESC-hematopoietic derivatives, reinforcing the idea that additional molecular regulators are needed for the generation of definitive in vivo functional HSPCs from hESC.

Stem cell therapy. Use of differentiated pluripotent stem cells as replacement therapy for treating disease

Pluripotent stem cells (PSCs) directed to various cell fates holds promise as source material for treating numerous disorders. The availability of precisely differentiated PSC-derived cells will dramatically affect blood component and hematopoietic stem cell therapies and should facilitate treatment of diabetes, some forms of liver disease and neurologic disorders, retinal diseases, and possibly heart disease. Although an unlimited supply of specific cell types is needed, other barriers must be overcome. This review of the state of cell therapies highlights important challenges. Successful cell transplantation will require optimizing the best cell type and site for engraftment, overcoming limitations to cell migration and tissue integration, and occasionally needing to control immunologic reactivity, as well as a number of other challenges. Collaboration among scientists, clinicians, and industry is critical for generating new stem cell-based therapies.

An effective approach to prevent immune rejection of human ESC-derived allografts

Human embryonic stem cells (hESCs) hold great promise for cell therapy as a source of diverse differentiated cell types. One key bottleneck to realizing such potential is allogenic immune rejection of hESC-derived cells by recipients. Here, we optimized humanized mice (Hu-mice) reconstituted with a functional human immune system that mounts a vigorous rejection of hESCs and their derivatives. We established knockin hESCs that constitutively express CTLA4-Ig and PD-L1 before and after differentiation, denoted CP hESCs. We then demonstrated that allogenic CP hESC-derived teratomas, fibroblasts, and cardiomyocytes are immune protected in Hu-mice, while cells derived from parental hESCs are effectively rejected. Expression of both CTLA4-Ig, which disrupts T cell costimulatory pathways, and PD-L1, which activates T cell inhibitory pathway, is required to confer immune protection, as neither was sufficient on their own. These findings are instrumental for developing a strategy to protect hESC-derived cells from allogenic immune responses without requiring systemic immune suppression.

Utilizing regulatory T cells against rheumatoid arthritis

Regulatory T (Treg) cells are essential for normal immune surveillance systems, and their dysfunction leads to development of diseases, such as autoimmune disorders. CD4⁺CD25⁺ Treg cells are well-known suppressive cells, which express the transcription factor Foxp3, are indispensable for the maintenance of immune self-tolerance and homeostasis by suppressing aberrant or excessive immune response. Other Foxp3⁻ Treg cells include Tr1, Th3, CD8⁺CD28^−/−, and Qa1-restricted T cells; however, the contribution of these Treg cells to self-tolerance, immune homeostasis as well as preventing autoimmunity is not well defined. Here, we discuss the phenotypes and function of Foxp3⁺ Treg cells and the potential use of such Treg cells against rheumatoid arthritis (RA). Of note, even though most expanded populations of Foxp3⁺ Treg cells exhibit suppressive activity, tissue-associated or antigen-specific Treg cells appear superior in suppressing local autoimmune disorders such as RA. In addition, utilizing tissue-associated Foxp3⁺ Treg cells from stem cells may stable Foxp3 expression and avoid induction of a potentially detrimental systemic immunosuppression.

A systems biology approach for defining the molecular framework of the hematopoietic stem cell niche

Despite progress in identifying the cellular composition of hematopoietic stem/progenitor cell (HSPC) niches, little is known about the molecular requirements of HSPC support. To address this issue, we used a panel of six recognized HSPC-supportive stromal lines and less-supportive counterparts originating from embryonic and adult hematopoietic sites. Through comprehensive transcriptomic meta-analyses, we identified 481 mRNAs and 17 microRNAs organized in a modular network implicated in paracrine signaling. Further inclusion of 18 additional cell strains demonstrated that this mRNA subset was predictive of HSPC support. Our gene set contains most known HSPC regulators as well as a number of unexpected ones, such as Pax9 and Ccdc80, as validated by functional studies in zebrafish embryos. In sum, our approach has identified the core molecular network required for HSPC support. These cues, along with a searchable web resource, will inform ongoing efforts to instruct HSPC ex vivo amplification and formation from pluripotent precursors.

Early dynamic fate changes in haemogenic endothelium characterized at the single-cell level

Haematopoietic stem cells (HSCs) are the founding cells of the adult haematopoietic system, born during ontogeny from a specialized subset of endothelium, the haemogenic endothelium (HE) via an endothelial-to-haematopoietic transition (EHT). Although recently imaged in real time, the underlying mechanism of EHT is still poorly understood. We have generated a Runx1 +23 enhancer-reporter transgenic mouse (23GFP) for the prospective isolation of HE throughout embryonic development. Here we perform functional analysis of over 1,800 and transcriptional analysis of 268 single 23GFP(+) HE cells to explore the onset of EHT at the single-cell level. We show that initiation of the haematopoietic programme occurs in cells still embedded in the endothelial layer, and is accompanied by a previously unrecognized early loss of endothelial potential before HSCs emerge. Our data therefore provide important insights on the timeline of early haematopoietic commitment.

Reprogramming human endothelial cells to haematopoietic cells requires vascular induction

Generating engraftable human hematopoietic cells from autologous tissues promises new therapies for blood diseases. Directed differentiation of pluripotent stem cells yields hematopoietic cells that poorly engraft. Here, we devised a method to phenocopy the vascular-niche microenvironment of hemogenic cells, thereby enabling reprogramming of human endothelial cells (ECs) into engraftable hematopoietic cells without transition through a pluripotent intermediate. Highly purified non-hemogenic human umbilical vein-ECs (HUVECs) or adult dermal microvascular ECs (hDMECs) were transduced with transcription factors (TFs), FOSB, GFI1, RUNX1, and SPI1 (FGRS), and then propagated on serum-free instructive vascular niche monolayers to induce outgrowth of hematopoietic colonies containing cells with functional and immunophenotypic features of multipotent progenitor cells (MPP). These reprogrammed ECs- into human-MPPs (rEC-hMPPs) acquire colony-forming cell (CFC) potential and durably engraft in immune-deficient mice after primary and secondary transplantation, producing long-term rEC-hMPP-derived myeloid (granulocytic/monocytic, erythroid, megakaryocytic) and lymphoid (NK, B) progeny. Conditional expression of FGRS transgenes, combined with vascular-induction, activates endogenous FGRS genes endowing rEC-hMPPs with a transcriptional and functional profile similar to self-renewing MPPs. Our approach underscores the role of inductive cues from vascular-niche in orchestrating and sustaining hematopoietic specification and may prove useful for engineering autologous hematopoietic grafts to treat inherited and acquired blood disorders.

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.

Distinct stromal cell factor combinations can separately control hematopoietic stem cell survival, proliferation, and self-renewal

Hematopoietic stem cells (HSCs) are identified by their ability to sustain prolonged blood cell production in vivo, although recent evidence suggests that durable self-renewal (DSR) is shared by HSC subtypes with distinct self-perpetuating differentiation programs. Net expansions of DSR-HSCs occur in vivo, but molecularly defined conditions that support similar responses in vitro are lacking. We hypothesized that this might require a combination of factors that differentially promote HSC viability, proliferation, and self-renewal. We now demonstrate that HSC survival and maintenance of DSR potential are variably supported by different Steel factor (SF)-containing cocktails with similar HSC-mitogenic activities. In addition, stromal cells produce other factors, including nerve growth factor and collagen 1, that can antagonize the apoptosis of initially quiescent adult HSCs and, in combination with SF and interleukin-11, produce >15-fold net expansions of DSR-HSCs ex vivo within 7 days. These findings point to the molecular basis of HSC control and expansion.

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HSC viability, mitogenesis, and self-renewal are differentially controlled

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Stromal cells produce nonmitogenic factors that directly sustain HSC viability

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More adult bone marrow cells can produce HSCs than display HSC activity directly

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Nerve growth factor and collagen 1 promote serially transplantable HSCs

Multicolor staining of globin subtypes reveals impaired globin switching during erythropoiesis in human pluripotent stem cells

Adult hemoglobin composed of α- and β-globin reflects a change from expression of embryonic ε- and fetal γ-globin to adult β-globin in human erythroid cells, so-called globin switching. Human pluripotent stem cells (hPSCs) are a potential source for in vitro erythrocyte production, but they show prominent expression of γ-globin with little β-globin expression, which indicates incomplete globin switching. To examine the mechanism of this impaired globin switching, we optimized multicolor flow cytometry to simultaneously follow expression of different globin subtypes using different immunofluorescent probes. This enabled us to detect upregulation of β-globin and the corresponding silencing of γ-globin at the single-cell level during cord blood CD34(+) cell-derived erythropoiesis, examined as an endogenous control. Using this approach, we initially characterized the heterogeneous β-globin expression in erythroblasts from several hPSC clones and confirmed the predominant expression of γ-globin. These hPSC-derived erythroid cells also displayed reduced expression of BCL11A-L. However, doxycycline-induced overexpression of BCL11A-L in selected hPSCs promoted γ-globin silencing. These results strongly suggest that impaired γ-globin silencing is associated with downregulated BCL11A-L in hPSC-derived erythroblasts and that multicolor staining of globin subtypes is an effective approach to studying globin switching in vitro.

Broad T-cell receptor repertoire in T-lymphocytes derived from human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) have enormous potential for the treatment of inherited and acquired disorders. Recently, antigen-specific T lymphocytes derived from hiPSCs have been reported. However, T lymphocyte populations with broad T cell receptor (TCR) diversity have not been generated. We report that hiPSCs derived from skin biopsy are capable of producing T lymphocyte populations with a broad TCR repertoire. In vitro T cell differentiation follows a similar developmental program as observed in vivo, indicated by sequential expression of CD7, intracellular CD3 and surface CD3. The γδ TCR locus is rearranged first and is followed by rearrangement of the αβ locus. Both γδ and αβ T cells display a diverse TCR repertoire. Upon activation, the cells express CD25, CD69, cytokines (TNF-α, IFN-γ, IL-2) and cytolytic proteins (Perforin and Granzyme-B). These results suggest that most, if not all, mechanisms required to generate functional T cells with a broad TCR repertoire are intact in our in vitro differentiation protocol. These data provide a foundation for production of patient-specific T cells for the treatment of acquired or inherited immune disorders and for cancer immunotherapy.

Induced pluripotent stem cells in hematology: current and future applications

Reprogramming somatic cells into induced pluripotent stem (iPS) cells is nowadays approaching effectiveness and clinical grade. Potential uses of this technology include predictive toxicology, drug screening, pathogenetic studies and transplantation. Here, we review the basis of current iPS cell technology and potential applications in hematology, ranging from disease modeling of congenital and acquired hemopathies to hematopoietic stem and other blood cell transplantation.

Parallel assessment of globin lentiviral transfer in induced pluripotent stem cells and adult hematopoietic stem cells derived from the same transplanted β-thalassemia patient

A patient with β(E)/β(0) -thalassemia major was converted to transfusion-independence 4.5 years ago by lentiviral gene transfer in hematopoietic stem cells while showing a myeloid-biased cell clone. Induced pluripotent stem cells (iPSCs) are a potential alternative source of hematopoietic stem cells. If fetal to adult globin class, switching does not occur in vivo in iPSC-derived erythroid cells, β-globin gene transfer would be unnecessary. To investigate both vector integration skewing and the potential use of iPSCs for the treatment of thalassemia, we derived iPSCs from the thalassemia gene therapy patient and compared iPSC-derived hematopoietic cells to their natural isogenic somatic counterparts. In NSG immunodeficient mice, embryonic to fetal and a partial fetal to adult globin class switching were observed, indicating that the gene transfer is likely necessary for iPSC-based therapy of the β-hemoglobinopathies. Lentivector integration occurred in regions of low and high genotoxicity. Surprisingly, common integration sites (CIS) were identified across those iPSCs and cells retrieved from isogenic and nonisogenic gene therapy patients with β-thalassemia and adrenoleukodystrophy, respectively. This suggests that CIS observed in the absence of overt tumorigenesis result from nonrandom lentiviral integration rather than oncogenic in vivo selection. These findings bring the use of iPSCs closer to practicality and further clarify our interpretation of genome-wide lentivector integration.

Transcriptional profiling of the human fibrillin/LTBP gene family, key regulators of mesenchymal cell functions

The fibrillins and latent transforming growth factor binding proteins (LTBPs) form a superfamily of extracellular matrix (ECM) proteins characterized by the presence of a unique domain, the 8-cysteine transforming growth factor beta (TGFβ) binding domain. These proteins are involved in the structure of the extracellular matrix and controlling the bioavailability of TGFβ family members. Genes encoding these proteins show differential expression in mesenchymal cell types which synthesize the extracellular matrix. We have investigated the promoter regions of the seven gene family members using the FANTOM5 CAGE database for human. While the protein and nucleotide sequences show considerable sequence similarity, the promoter regions were quite diverse. Most genes had a single predominant transcription start site region but LTBP1 and LTBP4 had two regions initiating different transcripts. Most of the family members were expressed in a range of mesenchymal and other cell types, often associated with use of alternative promoters or transcription start sites within a promoter in different cell types. FBN3 was the lowest expressed gene, and was found only in embryonic and fetal tissues. The different promoters for one gene were more similar to each other in expression than to promoters of the other family members. Notably expression of all 22 LTBP2 promoters was tightly correlated and quite distinct from all other family members. We located candidate enhancer regions likely to be involved in expression of the genes. Each gene was associated with a unique subset of transcription factors across multiple promoters although several motifs including MAZ, SP1, GTF2I and KLF4 showed overrepresentation across the gene family. FBN1 and FBN2, which had similar expression patterns, were regulated by different transcription factors. This study highlights the role of alternative transcription start sites in regulating the tissue specificity of closely related genes and suggests that this important class of extracellular matrix proteins is subject to subtle regulatory variations that explain the differential roles of members of this gene family.

Disease modeling and cell based therapy with iPSC: future therapeutic option with fast and safe application

Induced pluripotent stem cell (iPSC) technology has shown us great hope to treat various human diseases which have been known as untreatable and further endows personalized medicine for future therapy without ethical issues and immunological rejection which embryonic stem cell (hES) treatment has faced. It has been agreed that iPSCs knowledge can be harnessed from disease modeling which mimics human pathological development rather than trials utilizing conventional rodent and cell lines. Now, we can routinely generate iPSC from patient specific cell sources, such as skin fibroblast, hair follicle cells, patient blood samples and even urine containing small amount of epithelial cells. iPSC has both similarity and dissimilarity to hES. iPSC is similar enough to regenerate tissue and even full organism as ES does, however what we want for therapeutic advantage is limited to regenerated tissue and lineage specific differentiation. Depending on the lineage and type of cells, both tissue memory containing (DNA rearrangement/epigenetics) and non-containing iPSC can be generated. This makes iPSC even better choice to perform disease modeling as well as cell based therapy. Tissue memory containing iPSC from mature leukocytes would be beneficial for curing cancer and infectious disease. In this review, the benefit of iPSC for translational approaches will be presented.

Functional assessment of hematopoietic niche cells derived from human embryonic stem cells

To evaluate hematopoietic niche cell populations isolated from human embryonic stem cells (hESCs), we tested the ability of hESC-derived stromal lines to support CD34(+) umbilical cord blood (UCB)- and hESC-derived CD34(+)45(+) cells in long-term culture initiating cell (LTC-IC) assays. Specifically, these hematopoietic populations were cocultured with hESC-derived mesenchymal stromal cells (hESC-MSCs) and hESC-derived endothelial cells (hESC-ECs), and then assessed for their LTC-IC potential in comparison to coculture with bone marrow (BM)-derived MSCs and the mouse stromal line M2-10B4. We found that the hESC-derived stromal lines supported LTC-ICs from UCB similar to M2-10B4 cells and better than BM-MSCs. However, none of the stromal populations supported LTC-IC from hESC-derived CD34(+)45(+) cells. Engraftment data using the output from LTC-IC assays showed long-term repopulation (12 weeks) of NSG mice to correlate with LTC-IC support on a given stromal layer. Therefore, hESC-derived stromal lines can be used to efficiently evaluate putative hematopoietic stem/progenitor cells derived from hESCs or other cell sources.

Pluripotent stem cells in regenerative medicine: challenges and recent progress

After years of incremental progress, several recent studies have succeeded in deriving disease-relevant cell types from human pluripotent stem cell (hPSC) sources. The prospect of an unlimited cell source, combined with promising preclinical data, indicates that hPSC technology may be on the verge of clinical translation. In this Review, we discuss recent progress in directed differentiation, some of the new technologies that have facilitated the success of hPSC therapies and the remaining hurdles on the road towards developing hPSC-based cell therapies.

The immunogenicity of cells derived from induced pluripotent stem cells

With their ability to undergo unlimited self-renewal in culture and to differentiate into all cell types in the body, human embryonic stem cells (hESCs) hold great potential for the treatment of currently incurable diseases. Two hESC-based cell therapies for spinal cord injury and macular degeneration have been advanced into human clinical trials. Despite this rapid progress, one key challenge of hESC-based cell therapy is the allogeneic immune rejection of hESC-derived cells by recipients. This problem could be mitigated by a recent breakthrough in the technology of induced pluripotent stem cells (iPSCs) by nuclear reprogramming of patient-specific somatic cells with defined factors, which could become a renewable source of autologous cells for cell therapy. However, recent studies revealing the abnormal epigenetics, genomic stability and immunogenicity of iPSCs have raised safety concerns over iPSC-based therapy. Recent findings related to the immunogenicity of iPSC derivatives will be summarized in this review.

Applications of human hematopoietic stem cells isolated and expanded from different tissues in regenerative medicine

Bone marrow transplantation is a well-established stem cell-based therapy for the management of malignant and nonmalignant hematological disorders. In addition to the bone marrow, therapeutic hematopoietic stem cells (HSCs) can also be obtained from umbilical cord blood and mobilized peripheral blood. Transplantation of HSCs isolated from these tissues can be carried out with or without prior enrichment of specific cell types. New methodologies have been developed for lineage-specific HSC expansion and their transplantation as a supplementary treatment to whole bone marrow transplantation. In this review we have described the current methodologies for isolating and processing HSCs from various tissues, and discussed strategies to generate sufficient and functional HSCs for clinical and preclinical applications by expansion ex vivo. The various disease conditions in which these cells could be used, and the methods for delivering the cells into patients, are also discussed.

Hematopoietic specification from human pluripotent stem cells: current advances and challenges toward de novo generation of hematopoietic stem cells

Significant advances in cellular reprogramming technologies and hematopoietic differentiation from human pluripotent stem cells (hPSCs) have already enabled the routine production of multiple lineages of blood cells in vitro and opened novel opportunities to study hematopoietic development, model genetic blood diseases, and manufacture immunologically matched cells for transfusion and cancer immunotherapy. However, the generation of hematopoietic cells with robust and sustained multilineage engraftment has not been achieved. Here, we highlight the recent advances in understanding the molecular and cellular pathways leading to blood development from hPSCs and discuss potential approaches that can be taken to facilitate the development of technologies for de novo production of hematopoietic stem cells.

A short history of hemogenic endothelium

Definitive hematopoietic cells are generated de novo during ontogeny from a specialized subset of endothelium, the so-called hemogenic endothelium. In this review we give a brief overview of the identification of hemogenic endothelium, explore its links with the HSC lineage, and summarize recent insights into the nature of hemogenic endothelium and the microenvironmental and intrinsic regulators contributing to its transition into blood. Ultimately, a better understanding of the processes controlling the transition of endothelium into blood will advance the generation and expansion of hematopoietic stem cells for therapeutic purposes.

Haematopoietic differentiation is inhibited when Notch activity is enhanced in FLK1(+) mesoderm progenitors

Notch signalling has been implicated during haematopoietic development in vivo and in the differentiation of haematopoietic cells from pluripotent cells in vitro. However interpretation of data from many of these studies has been complicated by the heterogeneous nature of cell populations under study and by the fact that the Notch pathway is active during embryogenesis prior to the development of the haematopoietic system. To define the role of Notch signalling in more precise cell populations during the early stages of haematopoietic development within the aorta-gonad-mesonephros (AGM) microenvironment we co-cultured differentiating ESCs on a stromal cell line derived from this region of the embryo. Our co-culture system had no effect on the production of FLK1(+) mesoderm progenitor cells but promoted their subsequent haematopoietic differentiation. We assessed the role of Notch signalling on haematopoietic differentiation of isolated FLK1(+) cells. Notch activity is dynamic and drops to basal levels as FLK1(+) cells commit to a haematopoietic fate. Further reduction of Notch activity by the inducible expression of dominant negative MAML had no functional consequences. In contrast, induction of Notch activity using an inducible NotchIC expression system had an inhibitory effect on haematopoietic differentiation. We used a Cre-mediated recombination strategy whereby NotchIC-expressing cells were marked with the hCD2 receptor and observed a reduction in the number of multi-lineage and myeloid colonies derived from NotchIC(+) compared to NotchIC(-) FLK1(+) cells isolated from the same culture. We believe that our culture system represents a good model for haematopoietic development within the AGM microenvironment and our data suggest that haematopoietic commitment of FLK1(+) cells in this setting occurs when Notch activity is below a specific threshold.

Reprogrammed cells for disease modeling and regenerative medicine

The conversion of somatic cells into pluripotent cells is transforming the way diseases are researched and treated. Induced pluripotent stem (iPS) cells' promise may soon be realized in the field of hematology, as hematopoietic stem cell transplants are already commonplace in clinics around the world. We provide a current comparison between induced pluripotent and embryonic stem cells, describe progress toward modeling hematological disorders using iPS cells, and illustrate the hurdles that must be overcome before iPS cell therapies will be available in clinics.

The transcriptional landscape of hematopoietic stem cell ontogeny

Transcriptome analysis of adult hematopoietic stem cells (HSCs) and their progeny has revealed mechanisms of blood differentiation and leukemogenesis, but a similar analysis of HSC development is lacking. Here, we acquired the transcriptomes of developing HSCs purified from >2,500 murine embryos and adult mice. We found that embryonic hematopoietic elements clustered into three distinct transcriptional states characteristic of the definitive yolk sac, HSCs undergoing specification, and definitive HSCs. We applied a network-biology-based analysis to reconstruct the gene regulatory networks of sequential stages of HSC development and functionally validated candidate transcriptional regulators of HSC ontogeny by morpholino-mediated knockdown in zebrafish embryos. Moreover, we found that HSCs from in vitro differentiated embryonic stem cells closely resemble definitive HSCs, yet lack a Notch-signaling signature, likely accounting for their defective lymphopoiesis. Our analysis and web resource will enhance efforts to identify regulators of HSC ontogeny and facilitate the engineering of hematopoietic specification.

Hematopoietic cell differentiation from embryonic and induced pluripotent stem cells

Pluripotent stem cells, both embryonic stem cells and induced pluripotent stem cells, are undifferentiated cells that can self-renew and potentially differentiate into all hematopoietic lineages, such as hematopoietic stem cells (HSCs), hematopoietic progenitor cells and mature hematopoietic cells in the presence of a suitable culture system. Establishment of pluripotent stem cells provides a comprehensive model to study early hematopoietic development and has emerged as a powerful research tool to explore regenerative medicine. Nowadays, HSC transplantation and hematopoietic cell transfusion have successfully cured some patients, especially in malignant hematological diseases. Owing to a shortage of donors and a limited number of the cells, hematopoietic cell induction from pluripotent stem cells has been regarded as an alternative source of HSCs and mature hematopoietic cells for intended therapeutic purposes. Pluripotent stem cells are therefore extensively utilized to facilitate better understanding in hematopoietic development by recapitulating embryonic development in vivo, in which efficient strategies can be easily designed and deployed for the generation of hematopoietic lineages in vitro. We hereby review the current progress of hematopoietic cell induction from embryonic stem/induced pluripotent stem cells.

Human iPS cell-derived hematopoietic progenitor cells induce T-cell anergy in in vitro-generated alloreactive CD8(+) T cells

Human induced pluripotent stem cells (iPSCs) have emerged as an alternative source of pluripotent stem cells that can be used for tissue regeneration in place of the controversial human embryonic stem cells. However, immunologic knowledge about iPSC derivatives remains enigmatic. Here, we characterized human iPS-derived CD34(+) hematopoietic progenitor cells (HPCs). These HPCs poorly express major histocompatibility complex (MHC) I antigens and are MHC-II negative. Interestingly, they moderately express nonclassical HLA-G and HLA-E molecules. Consequently, alloreactive HLA-A2-specific cytotoxic T cells failed to recognize HLA-A2-expressing HPCs but became anergic. Subsequent upregulation of MHC-I using interferon-γ stimulation and provision of CD28 cosignaling led to T-cell activation, confirming that poor delivery of signals 1 and 2 by the HPCs mediated T-cell anergy. These data indicate for the first time that HPCs induce T-cell anergy, a unique characteristic of iPSC-derived cells that confers immunologic advantage for allogenic transplantation. Although iPSCs are ideal for patient-tailored treatments with the anticipation that no immunosuppression will be required, in cases of gene defects, their derivatives could be used to treat diseases in nonhistocompatible recipients.

Generation of engraftable hematopoietic stem cells from induced pluripotent stem cells by way of teratoma formation

In vitro generation of hematopoietic stem cells (HSCs) from induced pluripotent stem cells (iPSCs) has the potential to provide novel therapeutic approaches for replacing bone marrow (BM) transplantation without rejection or graft versus host disease. Hitherto, however, it has proved difficult to generate truly functional HSCs transplantable to adult host mice. Here, we demonstrate a unique in vivo differentiation system yielding engraftable HSCs from mouse and human iPSCs in teratoma-bearing animals in combination with a maneuver to facilitate hematopoiesis. In mice, we found that iPSC-derived HSCs migrate from teratomas into the BM and their intravenous injection into irradiated recipients resulted in multilineage and long-term reconstitution of the hematolymphopoietic system in serial transfers. Using this in vivo generation system, we could demonstrate that X-linked severe combined immunodeficiency (X-SCID) mice can be treated by HSCs derived from gene-corrected clonal iPSCs. It should also be noted that neither leukemia nor tumors were observed in recipients after transplantation of iPSC-derived HSCs. Taken our findings together, our system presented in this report should provide a useful tool not only for the study of HSCs, but also for practical application of iPSCs in the treatment of hematologic and immunologic diseases.

Generation and establishment of murine adherent cell lines

We describe a method to derive cell lines and clones from cells of the murine midgestation aorta-gonads-mesonephros (AGM) microenvironment. We start from subdissected AGM regions in "explant" or "single cell suspension" type cultures from embryos transgenic for tsA58, a temperature-sensitive mutant of the SV40 T antigen gene. The number of cells in such cultures initially expand, but in most cases, this expansion phase is followed by a stable or even decline in cell number. After this so-called crisis phase, cell proliferation is noticeable in more than 90% of the cultures. Stromal cell clones can be isolated from these cultures, some of which have been cultured for more than 50 population doublings, and functionally characterized using various methods These stromal cell clones are valuable tools for the study of the regulation of hematopoietic stem and progenitor cells in the midgestation mouse embryo.

Pluripotent stem cells in research and treatment of hemoglobinopathies

Pluripotent stem cells (PSCs) hold great promise for research and treatment of hemoglobinopathies. In principle, patient-specific induced pluripotent stem cells could be derived from a blood sample, genetically corrected to repair the disease-causing mutation, differentiated into hematopoietic stem cells (HSCs), and returned to the patient to provide a cure through autologous gene and cell therapy. However, there are many challenges at each step of this complex treatment paradigm. Gene repair is currently inefficient in stem cells, but use of zinc finger nucleases and transcription activator-like effector nucleases appear to be a major advance. To date, no successful protocol exists for differentiating PSCs into definitive HSCs. PSCs can be directly differentiated into primitive red blood cells, but not yet in sufficient numbers to enable treating patients, and the cost of clinical scale differentiation is prohibitively expensive with current differentiation methods and efficiencies. Here we review the progress, promise, and remaining hurdles in realizing the potential of PSCs for cell therapy.

Generation of CD34+ cells from human embryonic stem cells using a clinically applicable methodology and engraftment in the fetal sheep model

Until now, ex vivo generation of CD34(+) hematopoietic stem cells (HSCs) from human embryonic stem cells (hESCs) mostly involved use of feeder cells of nonhuman origin. Although they provided invaluable models to study hematopoiesis, in vivo engraftment of hESC-derived HSCs remains a challenging task. In this study, we used a novel coculture system composed of human bone marrow-derived mesenchymal stromal/stem cells (MSCs) and peripheral blood CD14(+) monocyte-derived macrophages to generate CD34(+) cells from hESCs in vitro. Human ESC-derived CD34(+) cells generated using this method expressed surface makers associated with adult human HSCs and upregulated hematopoietic stem cell genes comparable to human bone marrow-derived CD34(+) cells. Finally, transplantation of purified hESC-derived CD34(+) cells into the preimmune fetal sheep, primed with transplantation of MSCs derived from the same hESC line, demonstrated multilineage hematopoietic activity with graft presence up to 16 weeks after transplantation. This in vivo demonstration of engraftment and robust multilineage hematopoietic activity by hESC-derived CD34(+) cells lends credence to the translational value and potential clinical utility of this novel differentiation and transplantation protocol.

Clinical-scale derivation of natural killer cells from human pluripotent stem cells for cancer therapy

Adoptive transfer of antitumor lymphocytes has gained intense interest in the field of cancer therapeutics over the past two decades. Human natural killer (NK) cells are a promising source of lymphocytes for anticancer immunotherapy. NK cells are part of the innate immune system and exhibit potent antitumor activity without need for human leukocyte antigen matching and without prior antigen exposure. Moreover, the derivation of NK cells from pluripotent stem cells could provide an unlimited source of lymphocytes for off-the-shelf therapy. To date, most studies on hematopoietic cell development from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have used incompletely defined conditions and been on a limited scale. Here, we have used a two-stage culture system to efficiently produce NK cells from hESCs and iPSCs in the absence of cell sorting and without need for xenogeneic stromal cells. This novel combination of embryoid body formation using defined conditions and membrane-bound interleukin 21-expressing artificial antigen-presenting cells allows production of mature and functional NK cells from several different hESC and iPSC lines. Although different hESC and iPSC lines had varying efficiencies in hematopoietic development, all cell lines tested could produce functional NK cells. These methods can be used to generate enough cytotoxic NK cells to treat a single patient from fewer than 250,000 input hESCs/iPSCs. Additionally, this strategy provides a genetically amenable platform to study normal NK cell development and education in vitro.

Clinical-scale derivation of natural killer cells from human pluripotent stem cells for cancer therapy

Adoptive transfer of antitumor lymphocytes has gained intense interest in the field of cancer therapeutics over the past two decades. Human natural killer (NK) cells are a promising source of lymphocytes for anticancer immunotherapy. NK cells are part of the innate immune system and exhibit potent antitumor activity without need for human leukocyte antigen matching and without prior antigen exposure. Moreover, the derivation of NK cells from pluripotent stem cells could provide an unlimited source of lymphocytes for off-the-shelf therapy. To date, most studies on hematopoietic cell development from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have used incompletely defined conditions and been on a limited scale. Here, we have used a two-stage culture system to efficiently produce NK cells from hESCs and iPSCs in the absence of cell sorting and without need for xenogeneic stromal cells. This novel combination of embryoid body formation using defined conditions and membrane-bound interleukin 21-expressing artificial antigen-presenting cells allows production of mature and functional NK cells from several different hESC and iPSC lines. Although different hESC and iPSC lines had varying efficiencies in hematopoietic development, all cell lines tested could produce functional NK cells. These methods can be used to generate enough cytotoxic NK cells to treat a single patient from fewer than 250,000 input hESCs/iPSCs. Additionally, this strategy provides a genetically amenable platform to study normal NK cell development and education in vitro.

Deciphering the hierarchy of angiohematopoietic progenitors from human pluripotent stem cells

Identification of sequential progenitors leading to blood formation from pluripotent stem cells (PSCs) will be essential for understanding the molecular mechanisms of hematopoietic lineage specification and for development of technologies for in vitro production of hematopoietic stem cells (HSCs). It is well established that during development, blood and endothelial cells in the extraembryonic and embryonic compartments are formed in parallel from precursors with angiogenic and hematopoietic potentials. However, the identity and hierarchy of these precursors in human PSC (hPSC) cultures remain obscure. Using developmental stage-specific mesodermal and endothelial markers and functional assays, we recently identified discrete populations of angiohematopoietic progenitors from hPSCs, including mesodermal precursors and hemogenic endothelial cells with primitive and definitive hematopoietic potentials. In addition, we discovered a novel population of multipotent hematopoietic progenitors with an erythroid phenotype, which retain angiogenic potential. Here we introduce our recent findings and discuss their implication for defining putative HSC precursor and factors required for activation of self-renewal potential in hematopoietic cells emerging from endothelium.

Hematopoietic and nature killer cell development from human pluripotent stem cells

Natural killer (NK) cells are key effectors of the innate immune system, protecting the host from a variety of infections, as well as malignant cells. Recent advances in the field of NK cell biology have led to a better understanding of how NK cells develop. This progress has directly translated to improved outcomes in patients receiving hematopoietic stem cell transplants to treat potentially lethal malignancies. However, key differences between mouse and human NK cell development and biology limits the use of rodents to attain a more in depth understanding of NK cell development. Therefore, a readily accessible and genetically tractable cell source to study human NK cell development is warranted. Our lab has pioneered the development of lymphocytes, specifically NK cells, from human embryonic stem cells (hESCs) and more recently induced pluripotent stem cells (iPSCs). This chapter describes a reliable method to generate NK cells from hESCs and iPSCs using murine stromal cell lines. Additionally, we include an updated approach using a spin-embryoid body (spin-EB) differentiation system that allows for human NK cell development completely defined in vitro conditions.

T lymphocyte potential marks the emergence of definitive hematopoietic progenitors in human pluripotent stem cell differentiation cultures

The efficient generation of hematopoietic stem cells from human pluripotent stem cells is dependent on the appropriate specification of the definitive hematopoietic program during differentiation. In this study, we used T lymphocyte potential to track the onset of definitive hematopoiesis from human embryonic and induced pluripotent stem cells differentiated with specific morphogens in serum- and stromal-free cultures. We show that this program develops from a progenitor population with characteristics of hemogenic endothelium, including the expression of CD34, VE-cadherin, GATA2, LMO2, and RUNX1. Along with T cells, these progenitors display the capacity to generate myeloid and erythroid cells. Manipulation of Activin/Nodal signaling during early stages of differentiation revealed that development of the definitive hematopoietic progenitor population is not dependent on this pathway, distinguishing it from primitive hematopoiesis. Collectively, these findings demonstrate that it is possible to generate T lymphoid progenitors from pluripotent stem cells and that this lineage develops from a population whose emergence marks the onset of human definitive hematopoiesis.

In vivo generation of transplantable human hematopoietic cells from induced pluripotent stem cells

Lineage-restricted cells can be reprogrammed to a pluripotent state known as induced pluripotent stem (iPS) cells through overexpression of 4 transcription factors. iPS cells are similar to human embryonic stem (hES) cells and have the same ability to generate all the cells of the human body, including blood cells. However, this process is extremely inefficient and to date has been unsuccessful at differentiating iPS into hematopoietic stem cells (HSCs). We hypothesized that iPS cells, injected into NOD.Cg-Prkdc(scid) Il2rg(tm1Wjl)/SzJ immunocompromised (NSG) mice could give rise to hematopoietic stem/progenitor cells (HSPCs) during teratoma formation. Here, we report a novel in vivo system in which human iPS cells differentiate within teratomas to derive functional myeloid and lymphoid cells. Similarly, HSPCs can be isolated from teratoma parenchyma and reconstitute a human immune system when transplanted into immunodeficient mice. Our data provide evidence that in vivo generation of patient customized cells is feasible, providing materials that could be useful for transplantation, human antibody generation, and drug screening applications.

Two-step differentiation of mast cells from induced pluripotent stem cells

Mast cells play important roles in the pathogenesis of allergic diseases. They are generally classified into 2 phenotypically distinct populations: connective tissue-type mast cells (CTMCs) and mucosal-type mast cells (MMCs). The number of mast cells that can be obtained from tissues is limited, making it difficult to study the function of mast cells. Here, we report the generation and characterization of CTMC-like mast cells derived from mouse induced pluripotent stem (iPS) cells. iPS cell-derived mast cells (iPSMCs) were generated by the OP9 coculture method or embryoid body formation method. The number of Safranin O-positive cells, expression levels of CD81 protein and histidine decarboxylase mRNA, and protease activities were elevated in the iPSMCs differentiated by both methods as compared with those in bone marrow-derived mast cells (BMMCs). Electron microscopic analysis revealed that iPSMCs contained more granules than BMMCs. Degranulation was induced in iPSMCs after stimulation with cationic secretagogues or vancomycin. In addition, iPSMCs had the ability to respond to stimulation with the IgE/antigen complex in vitro and in vivo. Moreover, when iPSMCs generated on OP9 cells were cocultured with Swiss 3T3 fibroblasts, protease activities as maturation index were more elevated, demonstrating that mature mast cells were differentiated from iPS cells. iPSMCs can be used as an in vitro model of CTMCs to investigate their functions.

Prospects and challenges of induced pluripotent stem cells as a source of hematopoietic stem cells

Many life-threatening hematological diseases are now treated by bone marrow transplantations, i.e., infusion of hematopoietic stem cells (HSCs). HSC transplantations are a valid option for the treatment of a variety of metabolic disorders, and even for solid tumors and some refractory severe autoimmune diseases. Unfortunately, the frequency and outcome of HSC transplantations are limited by a shortage of suitable donors. Induced pluripotent stem cells (iPSCs)--somatic cells that have acquired pluripotent stem cell characteristics by the ectopic expression of pluripotency-inducing factors--have been proposed as an alternative source of HSCs. Possible applications include cells of autologous, of autologous and genetically modified, or of allogeneic origin. Here, we provide a perspective on the distinct opportunities of iPSCs and discuss the challenges that lie ahead.

Identification of the hemogenic endothelial progenitor and its direct precursor in human pluripotent stem cell differentiation cultures

Hemogenic endothelium (HE) has been recognized as a source of hematopoietic stem cells (HSCs) in the embryo. Access to human HE progenitors (HEPs) is essential for enabling the investigation of the molecular determinants of HSC specification. Here, we show that HEPs capable of generating definitive hematopoietic cells can be obtained from human pluripotent stem cells (hPSCs) and identified precisely by a VE-cadherin(+)CD73(-)CD235a/CD43(-) phenotype. This phenotype discriminates true HEPs from VE-cadherin(+)CD73(+) non-HEPs and VE-cadherin(+)CD235a(+)CD41a(-) early hematopoietic cells with endothelial and FGF2-dependent hematopoietic colony-forming potential. We found that HEPs arise at the post-primitive-streak stage of differentiation directly from VE-cadherin-negative KDR(bright)APLNR(+)PDGFRα(low/-) hematovascular mesodermal precursors (HVMPs). In contrast, hemangioblasts, which are capable of forming endothelium and primitive blood cells, originate from more immature APLNR(+)PDGFRα(+) mesoderm. The demarcation of HEPs and HVMPs provides a platform for modeling blood development from endothelium with a goal of facilitating the generation of HSCs from hPSCs.

Embryonic development of hematopoietic stem cells: implications for clinical use

Hematopoietic stem cell (HSC) transplantation is an important treatment modality for hematological malignancies or to correct congenital immunodeficiency disorders. Several stem cell sources are currently applied clinically, with a recent increased application of umbilical cord blood. The low number of HSCs available, particularly in umbilical cord blood, is a limiting factor, and different lines of research are ongoing to circumvent this issue. In this review, we will describe the research strategies developed to expand adult HSCs in vitro and to generate new HSCs from pluripotent stem cell lines. We will also discuss the importance of studying the embryonic microenvironment since it allows both generation and extensive expansion of HSCs. Understanding the mechanisms that underlie HSC production, self-renewal and differentiation is necessary for the establishment of optimal in vitro HSC cultures, where a limitless and manipulatable resource of HSCs would be available for both clinical and fundamental research.

Prospects and challenges of reprogrammed cells in hematology and oncology

Induced pluripotent stem cells (iPSCs) have emerged as a promising basis for modeling pediatric genetic disorders, allowing the derivation, study, and genetic correction of disease and patient-specific cell lines in vitro. Similar to embryonic stem cells (ESCs), iPSCs are capable of unlimited in vitro expansion and derivation of many cell types, including hematopoietic stem cells (HSCs). These may not only allow large scale screenings to develop therapeutic compounds, but also help to overcome cross-species barriers of genetically engineered animal models, which do not adequately recapitulate the associated human phenotype. Here, we review the current state and emerging developments of iPSC research, which can be exploited as a tool in modeling pediatric hematopoietic disorders and could lead to new clinical applications in gene and cell therapies.