Cytokine manipulation of primitive human hematopoietic cell self-renewal

Ex vivo expansion of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are multipotent progenitor cells that can differentiate into various mature blood cells and immune cells, thus reconstituting hematopoiesis. By taking advantage of the tremendous potential of HSCs, varied hereditary and hematologic diseases are promised to be alleviated or cured. To solve the contradiction between the growing demand for HSCs in disease treatment and the low population of HSCs in both cord blood and bone marrow, ex vivo HSC expansion along with multiple protocols has been investigated for harvesting adequate HSCs over the past two decades. This review surveys the state-of-the-art techniques for ex vivo HSC self-renewal and provides a concise summary of the effects of diverse intrinsic and extrinsic factors on the expansion of HSCs. The remaining challenges and emerging opportunities in the field of HSC expansion are also presented.

Human Hematopoietic Stem Cells Co-cultured in 3D with Stromal Support to Optimize Lentiviral Vector-mediated Gene Transduction

HSC transplantation (HSCT) has emerged as a promising treatment option for hematological and immunological disorders. Unfortunately, many viral vectors are inefficient at transduction, limiting the number of cells available for gene therapy in cord blood HSC transplantation. Combining ex vivo expansion and genetic manipulation of cord blood cells is a potential gene therapy approach. We present a 3D co-culture method using a demineralized bone matrix scaffold to optimize lentiviral vector-mediated gene transduction. pLenti-III-miR-GFP-has-miR-124 was transduced into cord blood HSCs. Transduced CD34 + cells co-cultured on the stromal layer for 72 h under cytokine-free conditions. We performed flow cytometry, colony assays, real-time polymerase chain reaction, and SEM morphological analysis. Seventy-two hours after transduction, when pLentiIII-miR-GFP-has-miR-124 and control vector-transduced expanded cord blood HSCs were compared to non-transduced expanded cord blood HSCs, the findings revealed 15 ± 3.04 and 55 ± 3.05-fold increases in miR-124 mRNA expression, respectively. Compared to a control culture on the same day, the expansion of CD34+, CD38-HSCs in 3D culture increased 544 ± 31.09 fold. This result demonstrated that the 3D-culture system could emerge as a novel approach to overcoming the current limitations of cord blood HSC transduction. In the future, this research could be applied in a therapeutic setting.

Clever Experimental Designs: Shortcuts for Better iPSC Differentiation

For practical use of pluripotent stem cells (PSCs) for disease modelling, drug screening, and regenerative medicine, the cell differentiation process needs to be properly refined to generate end products with consistent and high quality. To construct and optimize a robust cell-induction process, a myriad of cell culture conditions should be considered. In contrast to inefficient brute-force screening, statistical design of experiments (DOE) approaches, such as factorial design, orthogonal array design, response surface methodology (RSM), definitive screening design (DSD), and mixture design, enable efficient and strategic screening of conditions in smaller experimental runs through multifactorial screening and/or quantitative modeling. Although DOE has become routinely utilized in the bioengineering and pharmaceutical fields, the imminent need of more detailed cell-lineage specification, complex organoid construction, and a stable supply of qualified cell-derived material requires expedition of DOE utilization in stem cell bioprocessing. This review summarizes DOE-based cell culture optimizations of PSCs, mesenchymal stem cells (MSCs), hematopoietic stem cells (HSCs), and Chinese hamster ovary (CHO) cells, which guide effective research and development of PSC-derived materials for academic and industrial applications.

Secreted factors from mouse embryonic fibroblasts maintain repopulating function of single cultured hematopoietic stem cells

Hematopoietic stem cell self-renewal, proliferation, and differentiation are independently regulated by intrinsic as well as extrinsic mechanisms. We previously demonstrated that murine proliferation of hematopoietic stem cells is supported in serum-free medium supplemented with two growth factors, stem cell factor and interleukin 11. The survival of hematopoietic stem cells is additionally improved by supplementing this medium with two more growth factors, neural growth factor and collagen 1 (four growth factors) or serum-free medium conditioned by the hematopoietic stem cell-supportive stromal UG26-1B6 cells1. Here, we describe a robust and versatile alternative source of conditioned medium from mouse embryonic fibroblasts. We found that this conditioned medium supports survival and phenotypical identity of hematopoietic stem cells, as well as cell cycle entry in single cell cultures of CD34- CD48- CD150+ Lineage- SCA1+ KIT+ cells supplemented with two growth factors. Strikingly, in comparison with cultures in serum-free medium with four growth factors, conditioned medium from mouse embryonic fibroblasts increases the numbers of proliferating clones and the number of Lineage- SCA1+ KIT+ cells, both with two and four growth factors. In addition, conditioned medium from mouse embryonic fibroblasts supports self-renewal in culture of cells with short- and long-term hematopoiesis-repopulating ability in vivo. These findings identify conditioned medium from mouse embryonic fibroblasts as a robust alternative serumfree source of factors to maintain self-renewal of in vivo-repopulating hematopoetic stem cells in culture.

Tailored Cytokine Optimization for ex vivo Culture Platforms Targeting the Expansion of Human Hematopoietic Stem/Progenitor Cells

Umbilical cord blood (UCB) has been established as an alternative source for hematopoietic stem/progenitor cells (HSPC) for cell and gene therapies. Limited cell yields of UCB units have been tackled with the development of cytokine-based ex vivo expansion platforms. To improve the effectiveness of these platforms, namely targeting clinical approval, in this study, we optimized the cytokine cocktails in two clinically relevant expansion platforms for HSPC, a liquid suspension culture system (CS_HSPC) and a co-culture system with bone marrow derived mesenchymal stromal cells (BM MSC) (CS_HSPC/MSC). Using a methodology based on experimental design, three different cytokines [stem cell factor (SCF), fms-like tyrosine kinase 3 ligand (Flt-3L), and thrombopoietin (TPO)] were studied in both systems during a 7-day culture under serum-free conditions. Proliferation and colony-forming unit assays, as well as immunophenotypic analysis were performed. Five experimental outputs [fold increase (FI) of total nucleated cells (FI TNC), FI of CD34+ cells, FI of erythroid burst-forming unit (BFU-E), FI of colony-forming unit granulocyte-monocyte (CFU-GM), and FI of multilineage colony-forming unit (CFU-Mix)] were followed as target outputs of the optimization model. The novel optimized cocktails determined herein comprised concentrations of 64, 61, and 80 ng/mL (CS_HSPC) and 90, 82, and 77 ng/mL (CS_HSPC/MSC) for SCF, Flt-3L, and TPO, respectively. After cytokine optimization, CS_HSPC and CS_HSPC/MSC were directly compared as platforms. CS_HSPC/MSC outperformed the feeder-free system in 6 of 8 tested experimental measures, displaying superior capability toward increasing the number of hematopoietic cells while maintaining the expression of HSPC markers (i.e., CD34+ and CD34+CD90+) and multilineage differentiation potential. A tailored approach toward optimization has made it possible to individually maximize cytokine contribution in both studied platforms. Consequently, cocktail optimization has successfully led to an increase in the expansion platform performance, while allowing a rational side-by-side comparison among different platforms and enhancing our knowledge on the impact of cytokine supplementation on the HSPC expansion process.

Three-Dimensional Co-culture of Human Hematopoietic Stem/Progenitor Cells and Mesenchymal Stem/Stromal Cells in a Biomimetic Hematopoietic Niche Microenvironment

The development of cellular therapies to treat hematological malignancies has motivated researchers to investigate ex vivo culture systems capable of expanding the number of hematopoietic stem/progenitor cells (HSPC) before transplantation. The strategies exploited to achieve relevant cell numbers have relied on culture systems that lack biomimetic niche cues thought to be essential to promote HSPC maintenance and proliferation. Although stromal cells adhered to 2-D surfaces can be used to support the expansion of HSPC ex vivo, culture systems aiming to incorporate cell-cell interactions in a more intricate 3-D environment can better contribute to recapitulate the bone marrow (BM) hematopoietic niche in vitro.Herein, we describe the development of a 3-D co-culture system of human umbilical cord blood (UCB)-derived CD34⁺ cells and BM mesenchymal stem/stromal cell (MSC) spheroids in a microwell-based platform that allows to attain large numbers of spheroids with uniform sizes. Further comparison with a traditional 2-D co-culture system exploiting the supportive features of feeder layers of MSC is provided, while functional in vitro assays to assess the features of HSPC expanded in the 2-D vs. 3-D MSC co-culture systems are suggested.

A Human Hematopoietic Niche Model Supporting Hematopoietic Stem and Progenitor Cells In Vitro

Niches in the bone marrow regulate hematopoietic stem and progenitor cell (HSPC) fate and behavior through cell-cell interactions and soluble factor secretion. The niche-HSPC crosstalk is a very complex process not completely elucidated yet. To aid further investigation of this crosstalk, a functional in vitro 3D model that closely represents the main supportive compartments of the bone marrow is developed. Different combinations of human stromal cells and hydrogels are tested for their potential to maintain CD34⁺ HSPCs. Cell viability, clonogenic hematopoietic potential, and surface marker expression are assessed over time. Optimal HSPC support is obtained in presence of adipogenic and osteogenic cells, together with progenitor derived endothelial cells. When cultured in a bioactive hydrogel, the supportive cells self-assemble into a hypoxic stromal network, stimulating CD34⁺ CD38⁺ cell formation, while maintaining the pool of CD34⁺ 38^- HSPCs. HSPC clusters colocalize with the stromal networks, in close proximity to sinusoidal clusters of CD31⁺ endothelial cells. Importantly, the primary in vitro niche model supports HSPCs with no cytokine addition. Overall, the engineered primary 3D bone marrow environment provides an easy and reliable model to further investigate interactions between HSPCs and their endosteal and perivascular niches, in the context of normal hematopoiesis or blood-related diseases.

Distinct Bone Marrow Sources of Pleiotrophin Control Hematopoietic Stem Cell Maintenance and Regeneration

Bone marrow (BM) perivascular stromal cells and vascular endothelial cells (ECs) are essential for hematopoietic stem cell (HSC) maintenance, but the roles of distinct niche compartments during HSC regeneration are less understood. Here we show that Leptin receptor-expressing (LepR+) BM stromal cells and ECs dichotomously regulate HSC maintenance and regeneration via secretion of pleiotrophin (PTN). BM stromal cells are the key source of PTN during steady-state hematopoiesis because its deletion from stromal cells, but not hematopoietic cells, osteoblasts, or ECs, depletes the HSC pool. Following myelosuppressive irradiation, PTN expression is increased in bone marrow endothelial cells (BMECs), and PTN⁺ ECs are more frequent in the niche. Moreover, deleting Ptn from ECs impairs HSC regeneration whereas Ptn deletion from BM stromal cells does not. These findings reveal dichotomous and complementary regulation of HSC maintenance and regeneration by BM stromal cells and ECs.

Dissociation of Survival, Proliferation, and State Control in Human Hematopoietic Stem Cells

The role of growth factors (GFs) in controlling the biology of human hematopoietic stem cells (HSCs) remains limited by a lack of information concerning the individual and combined effects of GFs directly on the survival, Mitogenesis, and regenerative activity of highly purified human HSCs. We show that the initial input HSC activity of such a purified starting population of human cord blood cells can be fully maintained over a 21-day period in serum-free medium containing five GFs alone. HSC survival was partially supported by any one of these GFs, but none were essential, and different combinations of GFs variably stimulated HSC proliferation. However, serial transplantability was not detectably compromised by many conditions that reduced human HSC proliferation and/or survival. These results demonstrate the dissociated control of these three human HSC bio-responses, and set the stage for future improvements in strategies to modify and expand human HSCs ex vivo.

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Growth factors alone can maintain serially transplantable human cord blood HSCs

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Growth factors tunably and combinatorially control HSC survival and proliferation

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SCF is a critical factor for stimulating human HSC proliferation

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HSC regenerative activity is regulated independent of HSC survival or proliferation

Hematopoietic Stem Cells in Neural-crest Derived Bone Marrow

Hematopoietic stem cells (HSCs) in the endosteum of mesoderm-derived appendicular bones have been extensively studied. Neural crest-derived bones differ from appendicular bones in developmental origin, mode of bone formation and pathological bone resorption. Whether neural crest-derived bones harbor HSCs is elusive. Here, we discovered HSC-like cells in postnatal murine mandible, and benchmarked them with donor-matched, mesoderm-derived femur/tibia HSCs, including clonogenic assay and long-term culture. Mandibular CD34 negative, LSK cells proliferated similarly to appendicular HSCs, and differentiated into all hematopoietic lineages. Mandibular HSCs showed a consistent deficiency in lymphoid differentiation, including significantly fewer CD229 + fractions, PreProB, ProB, PreB and B220 + slgM cells. Remarkably, mandibular HSCs reconstituted irradiated hematopoietic bone marrow in vivo, just as appendicular HSCs. Genomic profiling of osteoblasts from mandibular and femur/tibia bone marrow revealed deficiencies in several HSC niche regulators among mandibular osteoblasts including Cxcl12. Neural crest derived bone harbors HSCs that function similarly to appendicular HSCs but are deficient in the lymphoid lineage. Thus, lymphoid deficiency of mandibular HSCs may be accounted by putative niche regulating genes. HSCs in craniofacial bones have functional implications in homeostasis, osteoclastogenesis, immune functions, tumor metastasis and infections such as osteonecrosis of the jaw.

Distinct signaling programs control human hematopoietic stem cell survival and proliferation

Several growth factors (GFs) that together promote quiescent human hematopoietic stem cell (HSC) expansion ex vivo have been identified; however, the molecular mechanisms by which these GFs regulate the survival, proliferation. and differentiation of human HSCs remain poorly understood. We now describe experiments in which we used mass cytometry to simultaneously measure multiple surface markers, transcription factors, active signaling intermediates, viability, and cell-cycle indicators in single CD34⁺ cord blood cells before and up to 2 hours after their stimulation with stem cell factor, Fms-like tyrosine kinase 3 ligand, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor (5 GFs) either alone or combined. Cells with a CD34⁺CD38^-CD45RA^-CD90⁺CD49f⁺ (CD49f⁺) phenotype (∼10% HSCs with >6-month repopulating activity in immunodeficient mice) displayed rapid increases in activated STAT1/3/5, extracellular signal-regulated kinase 1/2, AKT, CREB, and S6 by 1 or more of these GFs, and β-catenin only when the 5 GFs were combined. Certain minority subsets within the CD49f⁺ compartment were poorly GF-responsive and, among the more GF-responsive subsets of CD49f⁺ cells, different signaling intermediates correlated with the levels of the myeloid- and lymphoid-associated transcription factors measured. Phenotypically similar, but CD90^-CD49f^- cells (MPPs) contained lower baseline levels of multiple signaling intermediates than the CD90⁺CD49f⁺ cells, but showed similar response amplitudes to the same GFs. Importantly, we found activation or inhibition of AKT and β-catenin directly altered immediate CD49f⁺ cell survival and proliferation. These findings identify rapid signaling events that 5 GFs elicit directly in the most primitive human hematopoietic cell types to promote their survival and proliferation.

HIF1α-induced PDGFRβ signaling promotes developmental HSC production via IL-6 activation

Hematopoietic stem cells (HSCs) have the ability to both self-renew and differentiate each of the mature blood cell lineages and thereby reconstitute the entire blood system. Therefore, HSCs are therapeutically valuable for treatment of hematological malignances and bone marrow failure. We showed recently that transient glucose elevation elicited dose-dependent effects on HSCs through elevated metabolic activity and subsequent reactive oxygen species-mediated induction of Hypoxia-Inducible Factor 1α (Hif1α). Platelet-Derived Growth Factor B (pdgfb), a Hif1α-target, and its receptor, pdgfrb, were significantly upregulated in response to metabolic stimulation. Although the function of PDGF signaling is well established in vascular development, its role in hematopoiesis is less understood. Exposure to either a pan-PDGF inhibitor or a PDGFRβ-selective antagonist in the context of Hif1α stimulation blocked elevations in hematopoietic stem and progenitor cell (HSPC) formation as determined by runx1;cmyb whole-mount in situ hybridization (WISH) and HSPC-reporter flow cytometry analysis. Similar results were observed for morpholino (MO) knockdown of pdgfrb or dominant-negative pdgfrb expression, indicating that PDGFRβ signaling is a key downstream mediator of Hif1α-mediated induction of HSPCs. Notably, overexpression of Pdgfb ligand enhanced HSPC numbers in the aorta-gonado-mesonephros (AGM) at 36 hours postfertilization (hpf) and in the caudal hematopoietic tissue at 48 hpf. A survey of known PDGF-B/PDGFRβ regulatory targets by expression analysis revealed a significant increase in inflammatory intermediates, including Interleukin 6 (IL-6) and its receptor (IL-6R). MO-mediated knockdown of il6 or chemical inhibition of IL-6R antagonized the effect of Pdgfb overexpression. Furthermore, epistatic analysis of IL-6/IL-6R function confirmed activity downstream of Hif1α. Together, these findings define a Hif1α-regulated signaling axis mediated through PBFGB/PDGFRβ and IL-6/IL-6R that acts to control embryonic HSPC production.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

Early production of human neutrophils and platelets posttransplant is severely compromised by growth factor exposure

The critical human cells that produce neutrophils and platelets within 3 weeks in recipients of hematopoietic transplants are thought to produce these mature blood cells with the same kinetics in sublethally irradiated immunodeficient mice. Quantification of their numbers indicates their relative underrepresentation in cord blood (CB), likely explaining the clinical inadequacy of single CB units in rescuing hematopoiesis in myelosuppressed adult patients. We here describe that exposure of CD34(+) CB cells ex vivo to growth factors that markedly expand their numbers and colony-forming cell content also rapidly (within 24 hours) produce a significant and sustained net loss of their original short-term repopulating activity. This loss of short-term in vivo repopulating activity affects early platelet production faster than early neutrophil output, consistent with their origin from distinct input populations. Moreover, this growth factor-mediated loss is not abrogated by published strategies to increase progenitor homing despite evidence that the effect on rapid neutrophil production is paralleled in time and amount by a loss of the homing of their committed clonogenic precursors to the bone marrow. These results highlight the inability of in vitro or phenotype assessments to reliably predict clinical engraftment kinetics of cultured CB cells.

An Overview on Human Umbilical Cord Blood Stem Cell-Based Alternative In Vitro Models for Developmental Neurotoxicity Assessment

The developing brain is found highly vulnerable towards the exposure of different environmental chemicals/drugs, even at concentrations, those are generally considered safe in mature brain. The brain development is a very complex phenomenon which involves several processes running in parallel such as cell proliferation, migration, differentiation, maturation and synaptogenesis. If any step of these cellular processes hampered due to exposure of any xenobiotic/drug, there is almost no chance of recovery which could finally result in a life-long disability. Therefore, the developmental neurotoxicity (DNT) assessment of newly discovered drugs/molecules is a very serious concern among the neurologists. Animal-based DNT models have their own limitations such as ethical concerns and lower sensitivity with less predictive values in humans. Furthermore, non-availability of human foetal brain tissues/cells makes job more difficult to understand about mechanisms involve in DNT in human beings. Although, the use of cell culture have been proven as a powerful tool for DNT assessment, but many in vitro models are currently utilizing genetically unstable cell lines. The interpretation of data generated using such terminally differentiated cells is hard to extrapolate with in vivo situations. However, human umbilical cord blood stem cells (hUCBSCs) have been proposed as an excellent tool for alternative DNT testing because neuronal development from undifferentiated state could exactly mimic the original pattern of neuronal development in foetus when hUCBSCs differentiated into neuronal cells. Additionally, less ethical concern, easy availability and high plasticity make them an attractive source for establishing in vitro model of DNT assessment. In this review, we are focusing towards recent advancements on hUCBSCs-based in vitro model to understand DNTs.

Molecular integration of HoxB4 and STAT3 for self-renewal of hematopoietic stem cells: a model of molecular convergence for stemness

The upregulation of HoxB4 promotes self-renewal of hematopoietic stem cells (HSCs) without overriding the normal stem cell pool size. A similar enhancement of HSC self-renewal occurs when signal transducer and activator of transcription 3 (STAT3) is activated in HSCs. In this study, to gain insight into the functional organization of individual transcription factors (TFs) that have similar effects on HSCs, we investigated the molecular interplay between HoxB4 and STAT3 in the regulation of HSC self-renewal. We found that while STAT3-C or HoxB4 similarly enhanced the in vitro self-renewal and in vivo repopulating activities of HSCs, simultaneous transduction of both TFs did not have additive effects, indicating their functional redundancy in HSCs. In addition, activation of STAT3 did not cause changes in the expression levels of HoxB4. In contrast, the inhibition of STAT3 activity in HoxB4-overexpressing hematopoietic cells significantly abrogated the enhancing effects of HoxB4, and the upregulation of HoxB4 caused a ligand-independent Tyr-phosphorylation of STAT3. Microarray analysis revealed a significant overlap of the transcriptomes regulated by STAT3 and HoxB4 in undifferentiated hematopoietic cells. Moreover, a gene set enrichment analysis showed significant overlap in the candidate TFs that can recapitulate the transcriptional changes induced by HoxB4 or STAT3. Interestingly, among these common TFs were the pluripotency-related genes Oct-4 and Nanog. These results indicate that tissue-specific TFs regulating HSC self-renewal are functionally organized to play an equivalent role in transcription and provide insights into the functional convergence of multiple entries of TFs toward a conserved transcription program for the stem cell state.

Myeloid-Derived Suppressor Cells are Generated during Retroviral Transduction of Murine Bone Marrow

Previous work by our group showed that transferring bone marrow cells transduced with an autoantigen into nonmyeloablated mice with experimental autoimmune encephalomyelitis induced immune tolerance and improved symptoms of the disease. Because this effect occurred in the absence of molecular chimerism, we hypothesized that the cells responsible did not have repopulating ability and that they were not mediating central but peripheral tolerance mechanisms. In the present study, we analyzed the immunophenotype of the cells that are generated in the transduction cultures and we evaluated the immunosuppressive activity of the main cell subpopulations produced. We show that both granulocytic (CD11b+ Gr-1hi) and monocytic (CD11b+Gr-1lo) myeloid-derived suppressor cells (G- and M-MDSCs, respectively) are generated during standard 4-day γ-retroviral transduction cultures (representing about 25% and 40% of the total cell output, respectively) and that the effectively transduced cells largely consist of these two cell types. A third cell population representing about 15% of the transduced cells did not express CD45 or hematopoietic lineage markers and expressed mesenchymal stromal cell markers. Transduced total bone marrow cells and sorted M-MDSCs expressed arginase and inducible nitric oxide synthase activities, produced reactive oxygen species, and inhibited antigen-induced T-cell proliferation in vitro. Transgene-expressing MDSCs could be exploited therapeutically to induce tolerance in autoimmune diseases and in gene therapy protocols.

Blastic leukaemias (AML): a biologist's view

Acute myeloblastic leukaemia is characterised by the extreme clonal proliferation of haematopoietic precursor cells with abnormal or arrested differentiation. Chemotherapy of acute leukaemia is channelled towards the reduction and eradication of leukaemic cells. However, relapse is generally assumed to occur in residual host cells, which are refractory to or elude therapy. The cancer stem cell hypothesis has gained considerable importance in recent years and could interpret this behaviour. This persuasive theory states that cells within a tumour are organised in a hierarchy similar to that of normal tissues and are maintained by a small subset of cells responsible for tumour dormancy. These cells, defined as 'tumour initiating cells' (TICs), possess several properties of normal tissue stem cells. Recently, the TICs associated with AML have been shown to comprise distinct, hierarchically arranged classes similar to those observed for haematopoietic stem cells. We know now that the growth and survival of blasts in AML are driven by the same growth factors that stimulate normal cells. Furthermore, direct evidence of the role of membrane stem cell factor and its receptor c-Kit in cell-cell interactions and cell survival in primary AML blasts have been provided, defining the importance of juxtacrine stimulation. Inhibition of c-Kit signalling induces combinations of cell death: autophagy (compensatory mechanism towards survival) and apoptosis. While recent work confirmed that c-Kit inhibitors reduce cancer cell proliferation, it also demonstrated that future inappropriate prescriptions could cause normal tissue deterioration. The purpose of this paper was to review some of the salient features of leukaemic blasts in support of the proposal that research into neoplasia be increased. Rather than presenting the details of various studies, I have attempted to indicate general areas in which work has been done or is in progress. It is hoped that this survey of the subject will demonstrate a variety of opportunities for additional research in human neoplasia.

Developmental changes in hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) comprise a rare population of cells that can regenerate and maintain lifelong blood cell production. This functionality is achieved through their ability to undergo many divisions without activating a poised, but latent, capacity for differentiation into multiple blood cell types. Throughout life, HSCs undergo sequential changes in several key properties. These affect mechanisms that regulate the self-renewal, turnover and differentiation of HSCs as well as the properties of the committed progenitors and terminally differentiated cells derived from them. Recent findings point to the Lin28b-let-7 pathway as a master regulator of many of these changes with important implications for the clinical use of HSCs for marrow rescue and gene therapy, as well as furthering our understanding of the different pathogenesis of childhood and adult-onset leukemia.

Establishment of a human allergy model using human IL-3/GM-CSF-transgenic NOG mice

The development of animal models that mimic human allergic responses is crucial to study the pathophysiology of disease and to generate new therapeutic methodologies. Humanized mice reconstituted with human immune systems are essential to study human immune reactions in vivo and are expected to be useful for studying human allergies. However, application of this technology to the study of human allergies has been limited, largely because of the poor development of human myeloid cells, especially granulocytes and mast cells, which are responsible for mediating allergic diseases, in conventional humanized mice. In this study, we developed a novel transgenic (Tg) strain, NOD/Shi-scid-IL2rγ(null) (NOG), bearing human IL-3 and GM-CSF genes (NOG IL-3/GM-Tg). In this strain, a large number of human myeloid cells of various lineages developed after transplantation of human CD34⁺ hematopoietic stem cells. Notably, mature basophils and mast cells expressing FcεRI were markedly increased. These humanized NOG IL-3/GM-Tg mice developed passive cutaneous anaphylaxis reactions when administered anti-4-hydroxy-3-nitrophenylacetyl IgE Abs and 4-hydroxy-3-nitrophenylacetyl. More importantly, a combination of serum from Japanese cedar pollinosis patients and cedar pollen extract also elicited strong passive cutaneous anaphylaxis responses in mice. Thus, to our knowledge, our NOG IL-3/GM-Tg mice are the first humanized mouse model to enable the study of human allergic responses in vivo and are excellent tools for preclinical studies of allergic diseases.

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.

Replication of bone marrow differentiation niche: comparative evaluation of different three-dimensional matrices

The comparative evaluation of different 3D matrices-Matrigel, Puramatrix, and inverted colloidal crystal (ICC) scaffolds-provides a perspective for studying the pathology and potential cures for many blood and bone marrow diseases, and further proves the significance of 3D cultures with direct cell-cell contacts for in vitro mimicry of the human stem cell niche.

Biochemical measurements on single erythroid progenitor cells shed light on the combinatorial regulation of red blood cell production

Adult bone marrow (BM) erythrocyte colony-forming units (CFU-Es) are important cellular targets for the treatment of anemia and also for the manufacture of red blood cells (RBCs) ex vivo. We obtained quantitative biochemical measurements from single and small numbers of CFU-Es by isolating and analyzing c-Kit(+)CD71(high)Ter119(-) cells from adult mouse BM and this allowed us to identify two mechanisms that can be manipulated to increase RBC production. As expected, maximum RBC output was obtained when CFU-Es were stimulated with a combination of Stem Cell Factor (SCF) and Erythropoietin (EPO) mainly because SCF supports a transient CFU-E expansion and EPO promotes the survival and terminal differentiation of erythroid progenitors. However, we found that one of the main factors limiting the output in RBCs was that EPO induces a downregulation of c-Kit expression which limits the transient expansion of CFU-Es. In the presence of SCF, the EPO-mediated downregulation of c-Kit on CFU-Es is delayed but still significant. Moreover, treatment of CFU-Es with 1-Naphthyl PP1 could partially inhibit the downregulation of c-Kit induced by EPO, suggesting that this process is dependent on a Src family kinase, v-Src and/or c-Fyn. We also found that CFU-E survival and proliferation was dependent on the level of time-integrated extracellular-regulated kinase (ERK) activation in these cells, all of which could be significantly increased when SCF and EPO were combined with mouse fetal liver-derived factors. Taken together, these results suggest two novel molecular strategies to increase RBC production and regeneration.

Gastrointestinal stromal tumor and its targeted therapeutics

Over the past 60 years, investigators of basic science, pathology, and clinical medicine have studied gastrointestinal stromal tumor (GIST) and made minor advances in patient care. Recent discoveries have led to an understanding of the biological role of KIT and platelet-derived growth factor receptor-α in GIST and the development of the tyrosine kinase inhibitor imatinib mesylate (Gleevec, formerly STI-571), one of the most exciting examples of targeted therapy to date. The success of targeted therapy in GIST has lead to new developments in our understanding of the medical and surgical management of the disease. Intense study of GIST may lead to new paradigms in the management of cancer.

Progress and prospects for stem cell engineering

Stem cells offer tremendous biomedical potential owing to their abilities to self-renew and differentiate into cell types of multiple adult tissues. Researchers and engineers have increasingly developed novel discovery technologies, theoretical approaches, and cell culture systems to investigate microenvironmental cues and cellular signaling events that control stem cell fate. Many of these technologies facilitate high-throughput investigation of microenvironmental signals and the intracellular signaling networks and machinery processing those signals into cell fate decisions. As our aggregate empirical knowledge of stem cell regulation grows, theoretical modeling with systems and computational biology methods has and will continue to be important for developing our ability to analyze and extract important conceptual features of stem cell regulation from complex data. Based on this body of knowledge, stem cell engineers will continue to develop technologies that predictably control stem cell fate with the ultimate goal of being able to accurately and economically scale up these systems for clinical-grade production of stem cell therapeutics.

Chronic myelogenous leukemia stem and progenitor cells demonstrate chromosomal instability related to repeated breakage-fusion-bridge cycles mediated by increased nonhomologous end joining

Chromosomal aberrations are an important consequence of genotoxic exposure and contribute to pathogenesis and progression of several malignancies. We investigated the susceptibility to chromosomal aberrations in chronic myelogenous leukemia (CML) progenitors after exposure to ionizing radiation. In normal progenitors, ionizing radiation induced both stable and unstable chromosomal lesions, but only stable aberrations persisted after multiple divisions. In contrast, radiation of chronic phase CML progenitors resulted in enhanced generation of unstable lesions that persisted after multiple divisions. CML progenitors demonstrated active cell cycle checkpoints and increased nonhomologous end joining DNA repair, suggesting that persistence of unstable aberrations was the result of continued generation of these lesions. CML progenitors demonstrated enhanced susceptibility to repeated cycles of chromosome damage, repair, and damage through a breakage-fusion-bridge mechanism. Perpetuation of breakage-fusion-bridge cycles in CML progenitors was mediated by classic nonhomologous end joining repair. These studies reveal a previously unrecognized mechanism of chromosomal instability in leukemia progenitors because of continued generation of unstable chromosomal lesions through repeated cycles of breakage and repair of such lesions.

Survival of cord blood haematopoietic stem cells in a hyaluronan hydrogel for ex vivo biomimicry

Haematopoietic stem cells (HSCs) and haematopoietic progenitor cells (HPCs) grow in a specified niche in close association with the microenvironment, the so-called 'haematopoietic niche'. Scaffolds have been introduced to overcome the liquid culture limitations, mimicking the presence of the extracellular matrix (ECM). In the present study the hyaluronic acid scaffold, already developed in the laboratory, has been used for the first time to maintain long-term cultures of CD34⁺ haematopoietic cells obtained from human cord blood. One parameter investigated was the impact on ex vivo survival of CD34⁺ cord blood cells (CBCs) on the hyaluronic acid surface, immobilized with peptides containing the RGD motif. This peptide was conjugated by coating the hyaluronan hydrogel and cultured in serum-free liquid phase complemented with stem cell factor (SCF), a commonly indispensable cytokine for haematopoiesis. Our work demonstrated that these hyaluronan hydrogels were superior to traditional liquid cultures by maintaining and expanding the HPCs without the need for additional cytokines, and a colonization of 280-fold increment in the hydrogel compared with liquid culture after 28 days of ex vivo expansion.

Robust, quantitative tools for modelling ex-vivo expansion of haematopoietic stem cells and progenitors

Despite substantial research activity on bioreactor design and experiments, there are very few reports of modelling tools that can be used to generate predictive models describing how bioreactor parameters affect performance. New developments in mathematics, such as sparse Bayesian feature selection methods and nonlinear model-free modelling regression methods, offer considerable promise for modelling diverse types of data. The utility of these mathematical tools in stem cell biology are demonstrated by analysis of a large set of bioreactor data derived from the literature. In spite of the diversity of the data sources, and the inherent difficulty in representing bioreactor variables, these modelling methods were able to develop robust, quantitative, predictive models. These models relate bioreactor operational parameters to the degree of expansion of haematopoietic stem cells or their progenitors, and also identify the bioreactor variables that are most likely to affect performance across many experiments. These methods show substantial promise in assisting the design and optimisation of stem cell bioreactors.

Targeting of GSK3β promotes imatinib-mediated apoptosis in quiescent CD34+ chronic myeloid leukemia progenitors, preserving normal stem cells

The targeting of BCR-ABL, a hybrid oncogenic tyrosine (Y) kinase, does not eradicate chronic myeloid leukemia (CML)-initiating cells. Activation of β-catenin was linked to CML leukemogenesis and drug resistance through its BCR-ABL-dependent Y phosphorylation and impaired binding to GSK3β (glycogen synthase kinase 3β). Herein, we show that GSK3β is constitutively Y(216) phospho-activated and predominantly relocated to the cytoplasm in primary CML stem/progenitor cells compared with its balanced active/inactive levels and cytosolic/nuclear distribution in normal cells. Under cytokine support, persistent GSK3β activity and its altered subcellular localization were correlated with BCR-ABL-dependent and -independent activation of MAPK and p60-SRC/GSK3β complex formation. Specifically, GSK3β activity and nuclear import were increased by imatinib mesylate (IM), a selective ABL inhibitor, but prevented by dasatinib that targets both BCR-ABL- and cytokine-dependent MAPK/p60-SRC activity. SB216763, a specific GSK3 inhibitor, promoted an almost complete suppression of primary CML stem/progenitor cells when combined with IM, but not dasatinib, while sparing bcr-abl-negative cells. Our data indicate that GSK3 inhibition acts to prime a pro-differentiative/apoptotic transcription program in the nucleus of IM-treated CML cells by affecting the β-catenin, cyclinD1, C-EBPα, ATF5, mTOR, and p27 levels. In conclusion, our data gain new insight in CML biology, indicating that GSK3 inhibitors may be of therapeutic value in selectively targeting leukemia-initiating cells in combination with IM but not dasatinib.

Sprouting angiogenesis under a chemical gradient regulated by interactions with an endothelial monolayer in a microfluidic platform

Microfluidic cell culture assays are versatile tools for studying cell migration, particularly angiogenesis. Such assays can deliver precisely controlled linear gradients of chemical stimuli to cultured cells in a microfluidic channel, offering excellent optical resolution and in situ monitoring of cellular morphogenesis in response to a gradient. Microfluidic cell culture assays provide a chemical gradient subject to molecular diffusion, although cellular metabolism can perturb it. The actual gradient perturbed by cells has not been precisely described in the context of regulated cellular morphogenesis. We modeled the chemical gradient in a microfluidic channel by simulating the analyte(VEGF) distribution during cellular interactions. The results were experimentally verified by monitoring sprouting angiogenic response from a monolayer of human umbilical vein endothelial cells (hUVECs) into a type 1 collagen scaffold. The simulation provided a basis for understanding a real distribution of the analyte interrupted by cells in microfluidic device. The new protocol enables one to quantify the morphogenesis of hUVECs under a flat, less-steep, or steep gradient.

Towards predictive models of stem cell fate

Quantitative approaches are essential for the advancement of strategies to manipulate stem cells or their derivatives for therapeutic applications. Predictive models of stem cell systems would provide the means to pose and validate non-intuitive hypotheses and could thus serve as an important tool for discerning underlying regulatory mechanisms governing stem cell fate decisions. In this paper we review the development of computational models that attempt to describe mammalian adult and embryonic stem (ES) cell responses. Early stochastic models that relied exclusively on statistical distributions to describe the in vitro or in vivo output of stem cells are being revised to incorporate the contributions of exogenous and endogenous parameters on specific stem cell fate processes. Recent models utilize cell specific data (for example, cell-surface receptor distributions, transcription factor half-lives, cell-cycle status, etc.) to provide mechanistic descriptions that are consistent with biologically observed phenomena. Ultimately, the goal of these computational models is to, a priori, predict stem cell output given an initial set of conditions. Our efforts to develop a predictive model of ES cell fate are discussed. The quantitative studies presented in this review represent an important step in developing bioengineering approaches to characterize and predict stem cell behavior. Ongoing efforts to incorporate genetic and signaling network data into computational models should accelerate our understanding of fundamental principles governing stem cell fate decisions.

What is the future for cord blood stem cells?

Stem and progenitor cells are present in cord blood at a high frequency making these cells a major target population for experimental and clinical studies. Over the past decade there has been considerable developments in cord blood research and transplantation but despite the rapid progress many problems remain. The initial hope that cord blood would be an alternative source of haemopoietic cells for transplantation has been tempered by the fact that there are insufficient cells in most cord blood collections to engraft an adult of average weight. In attempts to increase the cell number, a plethora of techniques for ex-vivo expansion have been developed.These techniques have also proved useful for gene therapy. As cord blood cells possess unique properties this allows them to be utilised as suitable vehicles for gene therapy and long-term engraftment of transduced cells has been achieved. Current work examining the nature of the stem cells present in this haematological source indicates that cord blood contains not only haemopoietic stem cells but also primitive non-haemopoietic cells with high proliferative and developmental potential. As attention focuses on stem cell biology and the controversies surrounding the potential use of embryonic stem cells in treatment of disease, the properties of stem cells from other sources including cord blood are being re-appraised. The purpose of this article is to review some of the current areas of work and highlight biological problems associated with the use of cord blood cells.

In vitro 3D collective sprouting angiogenesis under orchestrated ANG-1 and VEGF gradients

Sprouting angiogenesis requires a coordinated guidance from a variety of angiogenic factors. Here, we have developed a unique hydrogel incorporating microfluidic platform which mimics the physiological microenvironment in 3D under a precisely orchestrated gradient of soluble angiogenic factors, VEGF and ANG-1. The system enables the quantified investigation in chemotactic response of endothelial cells during the collective angiogenic sprouting process. While the presence of a VEGF gradient alone was sufficient in inducing a greater number of tip cells, addition of ANG-1 to the VEGF gradient enhanced the number of tip cells that are attached to collectively migrated stalk cells. The chemotactic response of tip cells attracted by the VEGF gradient and the stabilizing role of ANG-1 were morphologically investigated, elucidating the 3D co-operative migration of tip and stalk cells as well as their structures. We found that ANG-1 enhanced the connection of the stalk cells with the tip cells, and then the direct connection regulated the morphogenesis and/or life cycle of stalk cells.

Optimal conditions for lentiviral transduction of engrafting human CD34+ cells

Cytokines are required for γ-retroviral transduction of human CD34+ cells. However, cytokines may reduce engraftment of CD34+ cells and may not be necessary for their lentiviral transduction. We sought to optimize transduction and engraftment of human CD34+ cells using lentiviral vectors. Single 24 h transduction of human CD34+ cells with human immunodeficiency virus type 1 (HIV1)-based lentiviral vectors in media containing stem cell factor (SCF), FMS-like tyrosine kinase 3 (FLT3) ligand, thrombopoietin (each 100 ng ml⁻¹) and 10% fetal bovine serum was compared with various cytokine conditions during ex vivo culture and assayed using humanized xenograft mice for 6 months after transplantation. Serum-free media improved transduction efficiency of human CD34+ cells. Interleukin-3 (20 ng ml⁻¹) had little effect on transduction efficiency or engraftment. Threefold higher cytokine mixture (each 300 ng ml⁻¹) reduced engraftment of CD34+ cells. SCF alone (100 ng ml⁻¹) proved insufficient for maintaining engraftment ability and reduced transduction efficiency. Short-term prestimulation had little effect on transduction efficiency or engraftment, yet 24 h prestimulation showed higher transduction efficiency, higher gene expression levels and lower engraftment. In summary, 24 h prestimulation followed by single 24-h lentiviral transduction in serum-free media with SCF, FLT3 ligand and thrombopoietin yields high transduction efficiency to engrafting human CD34+ cells, and is applicable in human clinical gene therapy trials.

Establishment and regulation of the HSC niche: Roles of osteoblastic and vascular compartments

Hematopoietic stem cells (HSC) are multi-potent cells that function to generate a lifelong supply of all blood cell types. During mammalian embryogenesis, sites of hematopoiesis change over the course of gestation: from extraembryonic yolk sac and placenta, to embryonic aorta-gonad-mesonephros region, fetal liver, and finally fetal bond marrow where HSC reside postnatally. These tissues provide microenviroments for de novo HSC formation, as well as HSC maturation and expansion. Within adult bone marrow, HSC self-renewal and differentiation are thought to be regulated by two major cellular components within their so-called niche: osteoblasts and vascular endothelial cells. This review focuses on HSC generation within, and migration to, different tissues during development, and also provides a summary of major regulatory factors provided by osteoblasts and vascular endothelial cells within the adult bone marrow niche.

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

BACKGROUND AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Toward modeling the bone marrow niche using scaffold-based 3D culture systems

In the bone marrow, specialized microenvironments, called niches, regulate hematopoietic stem cell (HSC) maintenance and function through a complex crosstalk between different cell types. Although in vivo studies have been instrumental to elucidate some of the mechanisms by which niches exert their function, the establishment of an in vitro model that recapitulates the fundamental interactions of the niche components in a controlled setting would be of great benefit. We have previously shown that freshly harvested bone marrow- or adipose tissue-derived cells can be cultured under perfusion within porous scaffolds, allowing the formation of an organized 3D stromal tissue, composed by mesenchymal and endothelial progenitors and able to support hematopoiesis. Here we describe 3D scaffold-based perfusion systems as potential models to reconstruct ex vivo the bone marrow stem cell niche. We discuss how several culture parameters, including scaffold properties, cellular makeup and molecular signals, can be varied and controlled to investigate the role of specific cues in affecting HSC fate. We then provide a perspective of how the system could be exploited to improve stem cell-based therapies and how the model can be extended toward the engineering of other specialized stromal niches.

Insights into the stem cells of chronic myeloid leukemia

Chronic myeloid leukemia (CML) has long served as a paradigm for generating new insights into the cellular origin, pathogenesis and improved approaches to treating many types of human cancer. Early studies of the cellular phenotypes and genotypes represented in leukemic populations obtained from CML patients established the concept of an evolving clonal disorder originating in and initially sustained by a rare, multipotent, self-maintaining hematopoietic stem cell (HSC). More recent investigations continue to support this model, while also revealing new insights into the cellular and molecular mechanisms that explain how knowledge of CML stem cells and their early differentiating progeny can predict the differing and variable features of chronic phase and blast crisis. In particular, these emphasize the need for new agents that effectively and specifically target CML stem cells to produce non-toxic, but curative therapies that do not require lifelong treatments.

Role of erythropoietin receptor signaling in parvovirus B19 replication in human erythroid progenitor cells

Parvovirus B19 (B19V) infection is highly restricted to human erythroid progenitor cells. Although previous studies have led to the theory that the basis of this tropism is receptor expression, this has been questioned by more recent observation. In the study reported here, we have investigated the basis of this tropism, and a potential role of erythropoietin (Epo) signaling, in erythroid progenitor cells (EPCs) expanded ex vivo from CD34(+) hematopoietic cells in the absence of Epo (CD36(+)/Epo(-) EPCs). We show, first, that CD36(+)/Epo(-) EPCs do not support B19V replication, in spite of B19V entry, but Epo exposure either prior to infection or after virus entry enabled active B19V replication. Second, when Janus kinase 2 (Jak2) phosphorylation was inhibited using the inhibitor AG490, phosphorylation of the Epo receptor (EpoR) was also inhibited, and B19V replication in ex vivo-expanded erythroid progenitor cells exposed to Epo (CD36(+)/Epo(+) EPCs) was abolished. Third, expression of constitutively active EpoR in CD36(+)/Epo(-) EPCs led to efficient B19V replication. Finally, B19V replication in CD36(+)/Epo(+) EPCs required Epo, and the replication response was dose dependent. Our findings demonstrate that EpoR signaling is absolutely required for B19V replication in ex vivo-expanded erythroid progenitor cells after initial virus entry and at least partly accounts for the remarkable tropism of B19V infection for human erythroid progenitors.

Adventures in time and space: Nonlinearity and complexity of cytokine effects on stem cell fate decisions

Cytokines are central factors in the control of stem cell fate decisions and, as such, they are invaluable to those interested in the manipulation of stem and progenitor cells for clinical or research purposes. In their in vivo niches or in optimized cultures, stem cells are exposed to multiple cytokines, matrix proteins and other cell types that provide individual and combinatorial signals that influence their self-renewal, proliferation and differentiation. Although the individual effects of cytokines are well-characterized in terms of increases or decreases in stem cell expansion or in the production of specific cell lineages, their interactions are often overlooked. Factorial design experiments in association with multiple linear regression is a powerful multivariate approach to derive response-surface models and to obtain a quantitative understanding of cytokine dose and interactions effects. On the other hand, cytokine interactions detected in stem cell processes can be difficult to interpret due to the fact that the cell populations examined are often heterogeneous, that cytokines can exhibit pleiotropy and redundancy and that they can also be endogenously produced. This perspective piece presents a list of possible biological mechanisms that can give rise to positive and negative two-way factor interactions in the context of in vivo and in vitro stem cell-based processes. These interpretations are based on insights provided by recent studies examining intra- and extra-cellular signaling pathways in adult and embryonic stem cells. Cytokine interactions have been classified according to four main types of molecular and cellular mechanisms: (i) interactions due to co-signaling; (ii) interactions due to sequential actions; (iii) interactions due to high-dose saturation and inhibition; and (iv) interactions due to intercellular signaling networks. For each mechanism, possible patterns of regression coefficients corresponding to the cytokine main effects, quadratic effects and two-way interactions effects are provided. Finally, directions for future mechanistic studies are presented.

Mobilization of CD34+ progenitor cells in association with decreased proliferation in the bone marrow of macaques after administration of the Fms-like tyrosine kinase 3 ligand

Fms-like tyrosine kinase 3 ligand (FLT3-L) is critical for the differentiation and self-renewal of CD34+ progenitor cells in primates and has been used therapeutically to mobilize progenitor and dendritic cells in vivo. However, little is known regarding the expansion of progenitor cells outside of peripheral blood, particularly in bone marrow (BM), where progenitor cells primarily reside. Evaluation of FLT3-L-mediated cell mobilization during lentivirus infections, where the numbers of CD34+ progenitor cells are reduced, is limited. We enumerated frequencies and absolute numbers of CD34+ progenitor cells in blood and BM of naive and SIV- or SHIV-infected macaques during and after the administration of FLT3-L. Flow cytometric analyses revealed that, while CD34+ cells increased in the circulation, no expansion was observed in BM. Furthermore, in the BM intracellular Ki67, a marker of cell proliferation, was downregulated in CD34+ progenitor cells but was upregulated significantly in the bulk cell population. Although the exact mechanism(s) remains unclear, these data suggest that CD34+ cell mobilization in blood was the result of cellular emigration from BM and not the proliferation of CD34+ cells already in the periphery. It is possible that the decreased progenitor cell proliferation observed in BM is evidence of a negative regulatory mechanism preventing hyperproliferation and development of neoplastic cells.

Presentation counts: microenvironmental regulation of stem cells by biophysical and material cues

Stem cells reside in adult and embryonic tissues in a broad spectrum of developmental stages and lineages, and they are thus naturally exposed to diverse microenvironments or niches that modulate their hallmark behaviors of self-renewal and differentiation into one or more mature lineages. Within each such microenvironment, stem cells sense and process multiple biochemical and biophysical cues, which can exert redundant, competing, or orthogonal influences to collectively regulate cell fate and function. The proper presentation of these myriad regulatory signals is required for tissue development and homeostasis, and their improper appearance can potentially lead to disease. Whereas these complex regulatory cues can be challenging to dissect using traditional cell culture paradigms, recently developed engineered material systems offer advantages for investigating biochemical and biophysical cues, both static and dynamic, in a controlled, modular, and quantitative fashion. Advances in the development and use of such systems have helped elucidate novel regulatory mechanisms controlling stem cell behavior, particularly the importance of solid-phase mechanical and immobilized biochemical microenvironmental signals, with implications for basic stem cell biology, disease, and therapeutics.

Controlled growth factor delivery for tissue engineering

Growth factors play a crucial role in information transfer between cells and their microenvironment in tissue engineering and regeneration. They initiate their action by binding to specific receptors on the surface of target cells and the chemical identity, concentration, duration, and context of these growth factors contain information that dictates cell fate. Hence, the importance of exogenous delivery of these molecules in tissue engineering is unsurprising, considering their importance for tissue regeneration. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and their potential toxicity at high systemic levels, suggest that conventional routes of administration are unlikely to be effective. In this review, we provide an overview of the design criteria for growth factor delivery vehicles with respect to the growth factor itself and the microenvironment for delivery. We discuss various methodologies that could be adopted to achieve this localized delivery, and strategies using polymers as delivery vehicles in particular.

Cell-cell interaction networks regulate blood stem and progenitor cell fate

Communication networks between cells and tissues are necessary for homeostasis in multicellular organisms. Intercellular (between cell) communication networks are particularly relevant in stem cell biology, as stem cell fate decisions (self-renewal, proliferation, lineage specification) are tightly regulated based on physiological demand. We have developed a novel mathematical model of blood stem cell development incorporating cell-level kinetic parameters as functions of secreted molecule-mediated intercellular networks. By relation to quantitative cellular assays, our model is capable of predictively simulating many disparate features of both normal and malignant hematopoiesis, relating internal parameters and microenvironmental variables to measurable cell fate outcomes. Through integrated in silico and experimental analyses, we show that blood stem and progenitor cell fate is regulated by cell–cell feedback, and can be controlled non-cell autonomously by dynamically perturbing intercellular signalling. We extend this concept by demonstrating that variability in the secretion rates of the intercellular regulators is sufficient to explain heterogeneity in culture outputs, and that loss of responsiveness to cell–cell feedback signalling is both necessary and sufficient to induce leukemic transformation in silico.

Stem cell bioprocessing: fundamentals and principles

In recent years, the potential of stem cell research for tissue engineering-based therapies and regenerative medicine clinical applications has become well established. In 2006, Chung pioneered the first entire organ transplant using adult stem cells and a scaffold for clinical evaluation. With this a new milestone was achieved, with seven patients with myelomeningocele receiving stem cell-derived bladder transplants resulting in substantial improvements in their quality of life. While a bladder is a relatively simple organ, the breakthrough highlights the incredible benefits that can be gained from the cross-disciplinary nature of tissue engineering and regenerative medicine (TERM) that encompasses stem cell research and stem cell bioprocessing. Unquestionably, the development of bioprocess technologies for the transfer of the current laboratory-based practice of stem cell tissue culture to the clinic as therapeutics necessitates the application of engineering principles and practices to achieve control, reproducibility, automation, validation and safety of the process and the product. The successful translation will require contributions from fundamental research (from developmental biology to the 'omics' technologies and advances in immunology) and from existing industrial practice (biologics), especially on automation, quality assurance and regulation. The timely development, integration and execution of various components will be critical-failures of the past (such as in the commercialization of skin equivalents) on marketing, pricing, production and advertising should not be repeated. This review aims to address the principles required for successful stem cell bioprocessing so that they can be applied deftly to clinical applications.