Umbilical cord blood cells capable of engrafting in primary, secondary, and tertiary xenogeneic hosts are preserved after ex vivo culture in a noncontact system

Ex vivo HSC expansion challenges the paradigm of unidirectional human hematopoiesis

Understanding mechanisms that determine the behavior of human hematopoietic stem cells (HSCs) is essential for developing novel strategies to expand ex vivo the number of fully functional HSCs. In this review, we focus on the complex interplay between intrinsic mechanisms regulated by transcriptional and mitochondrial networks and extrinsic signals imposed by the bone marrow microenvironment, which in concert regulate the balance between HSC self‐renewal and differentiation. Such integrated signaling mechanisms that dictate the fate of HSCs in vivo must be recapitulated ex vivo to achieve successful expansion of clinically relevant HSCs. We also highlight some of the most recent ex vivo HSC expansion strategies that have currently entered clinical development. Finally, based on the evidence reviewed here and lessons learned from ex vivo HSC expansion, we raise some critical questions regarding HSC fate and the cellular plasticity of hematopoietic cells that challenge the unidirectional model of human hematopoiesis.

Current approaches in biomaterial-based hematopoietic stem cell niches

Hematopoietic stem cells (HSCs) are multipotent progenitor cells that can differentiate and replenish blood and immune cells. While there is a growing demand for autologous and allogeneic HSC transplantation owing to the increasing incidence of hereditary and hematologic diseases, the low population of HSCs in cord-blood and bone marrow (the main source of HSCs) hinders their medical applicability. Several cytokine and growth factor-based methods have been developed to expand the HSCs in vitro; however, the expansion rate is low, or the expanded cells fail to survive upon engraftment. This is at least in part because the overly simplistic polystyrene culture substrates fail to fully replicate the microenvironments or niches where these stem cells live. Bone marrow niches are multi-dimensional, complex systems that involve both biochemical (cells, growth factors, and cytokines) and physiochemical (stiffness, O₂ concentration, and extracellular matrix presentation) factors that regulate the quiescence, proliferation, activation, and differentiation of the HSCs. Although several studies have been conducted on in vitro HSC expansion via 2D and 3D biomaterial-based platforms, additional work is required to engineer an effective biomaterial platform that mimics bone marrow niches. In this study, the factors that regulate the HSC in vivo were explained and their applications in the engineering of a bone marrow biomaterial-based platform were discussed. In addition, current approaches, challenges, and the future direction of a biomaterial-based culture and expansion of the HSC were examined.

STATEMENT OF SIGNIFICANCE

Hematopoietic stem cells (HSC) are multipotent cells that can differentiate and replace the blood and immune cells of the body. However, in vivo, there is a low population of these cells, and thus their use in biotherapeutic and medical applications is limited (i.e., bone marrow transplantation). In this review, the biochemical factors (growth factors, cytokines, co-existing cells, ECM, gas concentrations, and differential gene expression) that may regulate the over-all fate of HSC, in vivo, were summarized and discussed. Moreover, different conventional and recent biomaterial platforms were reviewed, and their potential in generating a biomaterial-based, BM niche-mimicking platform for the efficient growth and expansion of clinically relevant HSCs in-vitro, was discussed.

Human mesenchymal stem cells promote CD34+ hematopoietic stem cell proliferation with preserved red blood cell differentiation capacity

Studies showed that co-transplantation of mesenchymal stem cells (MSCs) and cord blood-derived CD34⁺ hematopoietic stem cells (HSCs) offered greater therapeutic effects but little is known regarding the effects of human Wharton's jelly derived MSCs on HSC expansion and red blood cell (RBC) generation in vitro. This study aimed to investigate the effects of MSCs on HSC expansion and differentiation. HSCs were co-cultured with MSCs or with 10% MSCs-derived conditioned medium, with HSCs cultured under standard medium served as a control. Cell expansion rates, number of mononuclear cell post-expansion and number of enucleated cells post-differentiation were evaluated. HSCs showed superior proliferation in the presence of MSC with mean expansion rate of 3.5 × 10⁸  ± 1.8 × 10⁷ after day 7 compared to the conditioned medium and the control group (8.9 × 10⁷  ± 1.1 × 10⁸ and 7.0 × 10⁷  ± 3.3 × 10⁶ respectively, P < 0.001). Although no significant differences in RBC differentiation were observed between groups at passage IV, the number of enucleated cell was greater compared to earlier passages, indicating successful RBC differentiation. Cord blood-derived CD34⁺ HSCs can be greatly expanded by co-culturing with MSCs without affecting the RBC differentiation capability, suggesting the importance of direct MSC-HSCs contact in HSC expansion and RBC differentiation.

Reprogramming Mouse Embryonic Fibroblasts with Transcription Factors to Induce a Hemogenic Program

This protocol details the induction of a hemogenic program in mouse embryonic fibroblasts (MEFs) via overexpression of transcription factors (TFs). We first designed a reporter screen using MEFs from human CD34-tTA/TetO-H2BGFP (34/H2BGFP) double transgenic mice. CD34⁺ cells from these mice label H2B histones with GFP, and cease labeling upon addition of doxycycline (DOX). MEFS were transduced with candidate TFs and then observed for the emergence of GFP⁺ cells that would indicate the acquisition of a hematopoietic or endothelial cell fate. Starting with 18 candidate TFs, and through a process of combinatorial elimination, we obtained a minimal set of factors that would induce the highest percentage of GFP⁺ cells. We found that Gata2, Gfi1b, and cFos were necessary and the addition of Etv6 provided the optimal induction. A series of gene expression analyses done at different time points during the reprogramming process revealed that these cells appeared to go through a precursor cell that underwent an endothelial to hematopoietic transition (EHT). As such, this reprogramming process mimics developmental hematopoiesis "in a dish," allowing study of hematopoiesis in vitro and a platform to identify the mechanisms that underlie this specification. This methodology also provides a framework for translation of this work to the human system in the hopes of generating an alternative source of patient-specific hematopoietic stem cells (HSCs) for a number of applications in the treatment and study of hematologic diseases.

Identification of factors promoting ex vivo maintenance of mouse hematopoietic stem cells by long-term single-cell quantification

AFT024-induced HSC maintenance correlates with early survival/proliferation whereas early death is a major reason for HSC loss in culture. Dermatopontin is required for ex vivo HSC maintenance, and also improves HSC clonogenicity in stroma-based and stroma-free cultures.

Effect of systemic heparan sulfate haploinsufficiency on steady state hematopoiesis and engraftment of hematopoietic stem cells

Heparan sulfate (HS) proteoglycans on stromal and hematopoietic stem/progenitor cells (HSPC) help form the stem cell niche, co-localize molecules that direct stem cell fate, and modulate HSPC homing and retention. Inhibition of HS function mobilizes marrow HSPC. In vitro, HSPC maintenance is influenced by stromal HS structure and concentration. Because inhibition of HS activity or synthesis may be developed for HSPC transplantation, it is important to examine if systemic HS deficiency influences hematopoiesis in vivo. In a transgenic mouse model of HS haploinsufficiency, we examined endogenous hematopoiesis and engraftment of allogeneic bone marrow. Endogenous hematopoiesis was normal except gender-specific alterations in peripheral blood monocyte and platelet counts. Donor engraftment was achieved in all mice following myeloablative irradiation, but HS deficiency in the stromal microenvironment, on HSPC, or both (the 3 test conditions), was associated with a trend towards lower donor engraftment percentage in the bone marrow. Following non-myeloablative irradiation, competitive engraftment was achieved in 22% of mice in the test conditions, vs 50% of control animals (P = 0.03). HS deficiency did not re-direct donor engraftment from bone marrow to spleen or liver. Normal HS levels in the stromal microenvironment and HSPC are required for HSPC engraftment following non-myeloablative conditioning.

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

BACKGROUND AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Concise review: umbilical cord blood transplantation: past, present, and future

Allogeneic hematopoietic stem cell transplantation is an important treatment option for fit patients with poor-risk hematological malignancies; nevertheless, the lack of available fully matched donors limits the extent of its use. Umbilical cord blood has emerged as an effective alternate source of hematopoietic stem cell support. Transplantation with cord blood allows for faster availability of frozen sample and avoids invasive procedures for donors. In addition, this procedure has demonstrated reduced relapse rates and similar overall survival when compared with unrelated allogeneic hematopoietic stem cell transplantation. The limited dose of CD34-positive stem cells available with single-unit cord transplantation has been addressed by the development of double-unit cord transplantation. In combination with improved conditioning regimens, double-unit cord transplantation has allowed for the treatment of larger children, as well as adult patients with hematological malignancies. Current excitement in the field revolves around the development of safer techniques to improve homing, engraftment, and immune reconstitution after cord blood transplantation. Here the authors review the past, present, and future of cord transplantation.

Concise review: ex vivo expansion of cord blood-derived hematopoietic stem and progenitor cells: basic principles, experimental approaches, and impact in regenerative medicine

Hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) play key roles in the production of mature blood cells and in the biology and clinical outcomes of hematopoietic transplants. The numbers of these cells, however, are extremely low, particularly in umbilical cord blood (UCB); thus, ex vivo expansion of human UCB-derived HSCs and HPCs has become a priority in the biomedical field. Expansion of progenitor cells can be achieved by culturing such cells in the presence of different combinations of recombinant stimulatory cytokines; in contrast, expansion of actual HSCs has proved to be more difficult because, in addition to needing recombinant cytokines, HSCs seem to deeply depend on the presence of stromal cells and/or elements that promote the activation of particular self-renewal signaling pathways. Hence, there is still controversy regarding the optimal culture conditions that should be used to achieve this. To date, UCB transplants using ex vivo-expanded cells have already been performed for the treatment of different hematological disorders, and although results are still far from being optimal, the advances are encouraging. Recent studies suggest that HSCs may also give rise to nonhematopoietic cells, such as neural, cardiac, mesenchymal, and muscle cells. Such plasticity and the possibility of producing nonhematopoietic cells at the clinical scale could bring new alternatives for the treatment of neural, metabolic, orthopedic, cardiac, and neoplastic disorders. Once standardized, ex vivo expansion of human HSCs/HPCs will surely have a positive impact in regenerative medicine.

Cis and trans acting factors involved in human cytomegalovirus experimental and natural latent infection of CD14 (+) monocytes and CD34 (+) cells

The parameters involved in human cytomegalovirus (HCMV) latent infection in CD14 (+) and CD34 (+) cells remain poorly identified. Using next generation sequencing we deduced the transcriptome of HCMV latently infected CD14 (+) and CD34 (+) cells in experimental as well as natural latency settings. The gene expression profile from natural infection in HCMV seropositive donors closely matched experimental latency models, and included two long non-coding RNAs (lncRNAs), RNA4.9 and RNA2.7 as well as the mRNAs encoding replication factors UL84 and UL44. Chromatin immunoprecipitation assays on experimentally infected CD14 (+) monocytes followed by next generation sequencing (ChIP-Seq) were employed to demonstrate both UL84 and UL44 proteins interacted with the latent viral genome and overlapped at 5 of the 8 loci identified. RNA4.9 interacts with components of the polycomb repression complex (PRC) as well as with the MIE promoter region where the enrichment of the repressive H3K27me3 mark suggests that this lncRNA represses transcription. Formaldehyde Assisted Isolation of Regulatory Elements (FAIRE), which identifies nucleosome-depleted viral DNA, was used to confirm that latent mRNAs were associated with actively transcribed, FAIRE analysis also showed that the terminal repeat (TR) region of the latent viral genome is depleted of nucleosomes suggesting that this region may contain an element mediating viral genome maintenance. ChIP assays show that the viral TR region interacts with factors associated with the pre replication complex and a plasmid subclone containing the HCMV TR element persisted in latently infected CD14 (+) monocytes, strongly suggesting that the TR region mediates viral chromosome maintenance.

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus where infection is usually subclinical. HCMV initial infection is followed by the establishment of latency in CD34 (+) myeloid cells and CD14 (+) monocytes. Primary infection or reactivation from latency can be associated with significant morbidity and mortality can occur in immune compromised patients. Latency is marked by the persistence of the viral genome, lack of production of infectious virus and the expression of only a few previously recognized latency associated transcripts. Despite the significant interest in HCMV latent infection, little is known regarding the mechanism involved in establishment or maintenance of the viral chromosome. We have now identified the transacting factors present in latently infected CD14 (+) monocytes and CD34 (+) progenitor cells as well as identification of a region of the HCMV genome, the terminal repeat locus that mediates viral DNA maintenance. This is a major step toward understanding the mechanism of HCMV latent infection.

Stem cell bioengineering strategies to widen the therapeutic applications of haematopoietic stem/progenitor cells from umbilical cord blood

Umbilical cord blood (UCB) transplantation has observed a significant increase in recent years, due to the unique features of UCB haematopoietic stem/progenitor cells (HSCs) for the treatment of blood-related disorders. However, the low cell numbers available per UCB unit significantly impairs the widespread use of this source for transplantation of adult patients, resulting in graft failure, delayed engraftment and delayed immune reconstitution. In order to overcome this issue, distinct approaches are now being considered in clinical trials, such as double-UCB transplantation, intrabone injection or ex vivo expansion. In this article the authors review the current state of the art, future trends and challenges on the ex vivo expansion of UCB HSCs, focusing on culture parameters affecting the yield and quality of the expanded HSC grafts: novel HSC selection schemes prior to cell culture, cytokine/growth factor cocktails, the impact of biochemical factors (e.g. O2 ) or the addition of supportive cells, e.g. mesenchymal stem/stromal cell (MSC)-based feeder layers) were addressed. Importantly, a critical challenge in cellular therapy is still the scalability, reproducibility and control of the expansion process, in order to meet the clinical requirements for therapeutic applications. Efficient design of bioreactor systems and operation modes are now the focus of many bioengineers, integrating the increasing 'know-how' on HSC biology and physiology, while complying with the GMP standards for the production of cellular products, i.e. through the use of commercially available, highly controlled, disposable technologies.

Ex vivo expansion of umbilical cord blood: where are we?

Since the first successful clinical use of umbilical cord blood (UCB) in 1988, UCB grafts have been used for over 20,000 patients with both malignant and non-malignant diseases. UCB has several practical advantages over other transplantable graft sources. For example, the ease of procurement, the absence of donor risks, the reduced risk of transmissible infections, and the availability for immediate use make UCB an appealing graft choice. However, UCB grafts suffer from a few limitations related to the limited cell dose available for transplantation in each UCB unit and to defects in UCB stem cell homing. These limitations lead to increased post-transplant complications. In this review, we focus on the issue of limited cell dose in UCB units and discuss the possible approaches to overcome this limitation. We also summarize the various cellular pathways that have been explored to expand UCB units.

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.

Mesenchymal stem cells in ex vivo cord blood expansion

Umbilical cord blood (CB) is becoming an important source of haematopoietic support for transplant patients lacking human leukocyte antigen matched donors. The ethnic diversity, relative ease of collection, ready availability as cryopreserved units from CB banks, reduced incidence and severity of graft versus host disease and tolerance of higher degrees of HLA disparity between donor and recipient, are positive attributes when compared to bone marrow or cytokine-mobilized peripheral blood. However, CB transplantation is associated with significantly delayed neutrophil and platelet engraftment and an elevated risk of graft failure. These hurdles are thought to be due, at least in part, to low total nucleated cell and CD34(+) cell doses transplanted. Here, current strategies directed at improving TNC and CD34(+) cell doses at transplant are discussed, with particular attention paid to the use of a mesenchymal stem cell (MSC)/CB mononuclear cell ex vivo co-culture expansion system.

Natural killer cell-triggered vascular transformation: maternal care before birth?

Natural killer (NK) cells are found in lymphoid and non-lymphoid organs. In addition to important roles in immune surveillance, some NK cells contribute to angiogenesis and circulatory regulation. The uterus of early pregnancy is a non-lymphoid organ enriched in NK cells that are specifically recruited to placental attachment sites. In species with invasive hemochorial placentation, these uterine natural killer (uNK) cells, via secretion of cytokines, chemokines, mucins, enzymes and angiogenic growth factors, contribute to the physiological change of mesometrial endometrium into the unique stromal environment called decidua basalis. In humans, uNK cells have the phenotype CD56(bright)CD16(dim) and they appear in great abundance in the late secretory phase of the menstrual cycle and early pregnancy. Gene expression studies indicate that CD56(bright)CD16(dim) uterine and circulating cells are functionally distinct. In humans but not mice or other species with post-implantation decidualization, uNK cells may contribute to blastocyst implantation and are of interest as therapeutic targets in female infertility. Histological and genetic studies in mice first identified triggering of the process of gestation spiral arterial modification as a major uNK cell function, achieved via interferon (IFN)-γ secretion. During spiral arterial modification, branches from the uterine artery that traverse the endometrium/decidua transiently lose their muscular coat and ability to vasoconstrict. The expression of vascular markers changes from arterial to venous as these vessels dilate and become low-resistance, high-volume channels. Full understanding of the vascular interactions of human uNK cells is difficult to obtain because endometrial time-course studies are not possible in pregnant women. Here we briefly review key information concerning uNK cell functions from studies in rodents, summarize highlights concerning human uNK cells and describe our preliminary studies on development of a humanized, pregnant mouse model for in vivo investigations of human uNK cell functions.

Ex vivo expansion of cord blood

A marked increase in the utilization of umbilical cord blood (UCB) transplantation has been observed in recent years; however, the use of UCB as a hematopoietic stem cell (HSC) source is limited primarily by the number of progenitor cells contained in the graft. Graft failure, delayed engraftment and profound delay in immune reconstitution lead to significant morbidity and mortality in adults. The lack of cells available for post transplant therapies, such as donor lymphocyte infusions, has also been considered to be a disadvantage of UCB. To improve outcomes and extend applicability of UCB transplantation, one potential solution is ex vivo expansion of UCB. Investigators have used several methods, including liquid suspension culture with various cytokines and expansion factors, co-culture with stromal elements and continuous perfusion systems. Techniques combining ex vivo expanded and unmanipulated UCB are being explored to optimize the initial engraftment kinetics as well as the long-term durability. The optimal expansion conditions are still not known; however, recent studies suggest that expanded UCB is safe. It is hoped that by ex vivo expansion of UCB, a resulting decrease in the morbidity and mortality of UCB transplantation will be observed, and that the availability of additional cells may allow adoptive immunotherapy or gene transfer therapies in the UCB setting.

Unrelated-Donor Cord Blood Transplantation for Adult Hematological Malignancies

Umbilical cord blood (UCB) is potentially an attractive new source of hematopoietic stem cells. The extensive and ready availability of UCB offers important logistical advantages compared with traditional unrelated bone marrow. However, the initial concern about engraftment due to the low number of progenitor cells contained in a UCB unit, has limited the use of UCB, mainly for adult patients. In this review, we summarize the most important data available regarding the main aspects of unrelated-donor cord blood transplantation (UD-CBT) in adult patients. We will review the characteristics of UCB and their practical implications, the outcomes of and clinical results after UD-CBT used for hematological malignancies, and the current advances in UD-CBT designed to optimize this procedure. Current data confirm that UD-CBT is a reasonable alternative for those patients lacking a matched bone-marrow donor.

Protocols for hematopoietic stem cell expansion from umbilical cord blood

The reconstitution of adult stem cells may be a promising source for the regeneration of damaged tissues and for the reconstitution of organ dysfunction. However, there are two major limitations to the use of such cells: they are rare, and only a few types exist that can easily be isolated without harming the patient. The best studied and most widely used stem cells are of the hematopoietic lineage. Pioneering work on hematopoietic stem cell (HSC) transplantation was done in the early 1970s by ED. Thomas and colleagues. Since then HSCs have been used in allogenic and autologous transplantation settings to reconstitute blood formation after high-dose chemotherapy for various blood disorders. The cells can be easily harvested from donors, but the cell number is limited, especially when the HSCs originate from umbilical cord blood (UCB). It would be desirable to set up an ex vivo strategy to expand HSCs in order to overcome the cell dose limit, whereby the expansion would favor cell proliferation over cell differentiation. This review provides an overview of the various existing HSC expansion strategies-focusing particularly on stem cells derived from UCB-of the parameters that might affect the outcome, and of the difficulties that may occur when trying to expand such cells.

Forward RNAi screens in primary human hematopoietic stem/progenitor cells

The mechanisms regulating key fate decisions such as self-renewal and differentiation in hematopoietic stem and progenitor cells (HSPC) remain poorly understood. We report here a screening strategy developed to assess modulators of human hematopoiesis using a lentiviral short hairpin RNA (shRNA) library transduced into cord blood-derived stem/progenitor cells. To screen for modifiers of self-renewal/differentiation, we used the limited persistence of HSPCs under ex vivo culture conditions as a baseline for functional selection of shRNAs conferring enhanced maintenance or expansion of the stem/progenitor potential. This approach enables complex, pooled screens in large numbers of cells. Functional selection identified novel specific gene targets (exostoses 1) or shRNA constructs capable of altering human hematopoietic progenitor differentiation or stem cell expansion, respectively, thereby demonstrating the potential of this forward screening approach in primary human stem cell populations.

Development and characterization of a novel CD34 monoclonal antibody that identifies sheep hematopoietic stem/progenitor cells

OBJECTIVE

We and many others have long used sheep as a predictive model system in which to explore stem cell transplantation. Unfortunately, while numerous markers are available to identify and isolate human hematopoietic stem cells (HSC), no reagents exist that allow HSC/progenitors from sheep to be identified or purified, greatly impeding the application of this well-established large animal model to the study of autologous or allogeneic HSC transplantation. The current studies were undertaken to create a monoclonal antibody to sheep CD34 that would enable isolation and study of sheep HSC/progenitors.

MATERIALS AND METHODS

A partial cDNA to the extracellular domain of the sheep CD34 antigen was polymerase chain reaction cloned, characterized, and used to genetically immunize mice and create hybridomas.

RESULTS

The resultant monoclonal antibody to sheep CD34 allows flow cytometric detection of sheep HSC/progenitors present within bone marrow, cord blood, and mobilized peripheral blood. Moreover, this antibody can be used to enrich for HSC/progenitors with enhanced in vitro colony-forming potential, and also identifies endothelial cells in situ within paraffin-embedded tissue sections, similarly to antibodies to human CD34.

CONCLUSIONS

The availability of this monoclonal antibody recognizing the stem cell antigen CD34 in sheep will greatly facilitate the study of autologous and allogeneic HSC transplantation using this clinically relevant large animal model.

Hematopoietic progenitor cells and interleukin-stimulated endothelium: expansion and differentiation of myeloid precursors

BACKGROUND

Cytokine-stimulated endothelial cells (EC) propagate hematopoietic progenitor cell (HPC) expansion. However, the effects on the functional capacities of cultured progenitors have not been evaluated. HPC were assessed by flow cytometry, colony and cobblestone assays and long-term cultures (LTC) after culturing in the supernatant of EC stimulated by IL-1beta, IL-3 or IL-6.

RESULTS

EC incubation with IL-6 did not improve cell expansion in comparison to non-stimulated EC supernatant, while the HPCs' phenotype and functional capacities were retained. In contrast, IL-1beta and IL-3 stimulation resulted in a 10- and 100-fold increase in cell numbers with more than 90% of these cells being CD33(+). Plating efficiencies and LTC initiating cells were greatest in IL-6 supernatants, whereas the highest numbers of burst-forming units were observed using IL-3. IL-1beta supernatants diminished the number of 5-week cobblestone-areas, whereas the number of 2-week cobblestone areas remained equal to freshly isolated HPC. Fewer 2-week cobblestones and greater amounts of 5-week cobblestones were observed with IL-6 and IL-3. Expanded progenitors from all interleukin conditions were further matured into functional granulocytes.

CONCLUSION

IL-1beta and IL-3 stimulated endothelium induces proliferation and differentiation of myeloid precursors, while IL-6 treatment induced a benefit of HPC survival.

Umbilical cord blood transplantation: basic biology and clinical challenges to immune reconstitution

Allogeneic stem cell transplantation has continued to evolve as a common procedure for the treatment of hematological malignancies and bone marrow failure. Donor bone marrow and mobilized peripheral stem cells are routinely employed for the reconstitution of immune function in leukemia and lymphoma patients following radiation and/or chemotherapy. Unfortunately, only 30% of patients have an HLA-identical sibling donor and the identification of matched unrelated donors, particularly for minorities, can present an exceptional challenge. The transplantation of umbilical cord blood (UCB) represents the most recent strategy to expand the potential donor pool while maintaining an acceptable level of treatment-related complications. First utilized in children, UCB transplantation permits a higher degree of HLA disparity while demonstrating a reduction in the incidence and severity of graft-versus-host disease (GvHD) compared to previous transplantation modalities. Despite the apparent decrease in GvHD, relapse rates remain comparable to transplantation with bone marrow or mobilized peripheral blood suggesting a strong graft-versus-leukemia/lymphoma (GvL) effect. However, several issues complicate the use of UCB transplantation and its extension to the treatment of adults. Many infections that afflict transplant patients are particularly frequent and more severe in the context of UCB transplantation. UCB T-cells are naive and therefore display less proliferation and IFN-gamma production in response to cognate antigen and also appear to demonstrate defects in signal transduction mechanisms. In addition, UCB contains T regulatory cells (Treg) with more potent suppressor function than adult Treg. Furthermore, adult patients often require more total cells and CD34+ progenitors for transplantation than a single UCB unit can provide. Thus, strategies to expand selected subpopulations from UCB and the use of multi-unit transplantation are areas of active research. This review will provide a condensed summary of the clinical history of UCB transplantation and emphasize the advantages and disadvantages of this approach to hematological malignancies in comparison to other methods of hematopoietic stem cell transplantation. Subsequently, it will mainly focus on the current challenges to immune reconstitution presented by UCB transplantation, recent research into their cellular and molecular mechanisms, and experimental approaches to overcome them.

Human hematopoietic stem cells can survive in vitro for several months

We previously reported that long-lasting in vitro hematopoiesis could be achieved using the cells differentiated from primate embryonic stem (ES) cells. Thus, we speculated that hematopoietic stem cells differentiated from ES cells could sustain long-lasting in vitro hematopoiesis. To test this hypothesis, we investigated whether human hematopoietic stem cells could similarly sustain long-lasting in vitro hematopoiesis in the same culture system. Although the results varied between experiments, presumably due to differences in the quality of each hematopoietic stem cell sample, long-lasting in vitro hematopoiesis was observed to last up to nine months. Furthermore, an in vivo analysis in which cultured cells were transplanted into immunodeficient mice indicated that even after several months of culture, hematopoietic stem cells were still present in the cultured cells. To the best of our knowledge, this is the first report to show that human hematopoietic stem cells can survive in vitro for several months.

Humanized SCID mouse models for biomedical research

There is a growing need for effective animal models to carry out experimental studies on human hematopoietic and immune systems without putting individuals at risk. Progress in development of small animal models for the in vivo investigation of human hematopoiesis and immunity has seen three major breakthroughs over the last three decades. First, CB 17-Prkdc(scid) (abbreviated CB 17-scid) mice were discovered in 1983, and engraftment of these mice with human fetal tissues (SCID-Hu model) and peripheral blood mononuclear cells (Hu-PBL-SCID model) was reported in 1988. Second, NOD-scid mice were developed and their enhanced ability to engraft with human hematolymphoid tissues as compared with CB17-scid mice was reported in 1995. NOD-scid mice have been the "gold standard" for studies of human hematolymphoid engraftment in small animal models over the last 10 years. Third, immunodeficient mice bearing a targeted mutation in the IL-2 receptor common gamma chain (IL2rgamma(null)) were developed independently by four groups between 2002 and 2005, and a major increase in the engraftment and function of human hematolymphoid cells as compared with NOD-scid mice has been reported. These new strains of immunodeficient IL2rgamma(null) mice are now being used for studies in human hematopoiesis, innate and adaptive immunity, autoimmunity, infectious diseases, cancer biology, and regenerative medicine. In this chapter, we discuss the current state of development of these strains of mice, the remaining deficiencies, and how approaches used to increase the engraftment and function of human hematolymphoid cells in CB 17-scid mice and in previous models based on NOD-scid mice may enhance human hematolymphoid engraftment and function in NOD-scid IL2rgamma(null) mice.

Cell polarity and asymmetric cell division within human hematopoietic stem and progenitor cells

Like other somatic stem cells, hematopoietic stem cells (HSC) contain the capacity to self-renew and to give rise to committed progenitor cells that are able to replenish all hematopoietic cell types. To keep a constant level of HSC, the decision whether their progeny maintain the stem cell fate or become committed to differentiation needs to be highly controlled. In this context it became evident that HSC niches fulfill important functions in keeping the level of HSC more or less constant. Before discovering such niches, it was widely assumed that HSC divide asymmetrically to give birth to a daughter cell maintaining the stem cell fate and to another one which is committed to differentiation. Here, I summarize some of the experimental data being compatible with the model of asymmetric cell division and review some of our latest findings, which demonstrate the occurrence of asymmetric cell divisions within the HSC and hematopoietic progenitor cell compartment. Since cell polarity is an essential prerequisite for asymmetrically dividing as well as for migrating cells, I will also discuss some aspects of cell polarity of primitive hematopoietic cells.

Human mesenchymal stem cells improve ex vivo expansion of adult human CD34+ peripheral blood progenitor cells and decrease their allostimulatory capacity

OBJECTIVE

Bone marrow mesenchymal stem cells (MSC) participate in the bone marrow microenvironment by providing growth factors and matrix proteins, which play a role in the regulation of hematopoiesis, through cell-to-cell interactions. Recently, MSC have been demonstrated to improve expansion of cord blood heamtopoietic stem cells (HSC).

METHODS

In this report, we evaluated the impact of MSC on ex vivo expansion of adult mobilized peripheral blood stem cells (PBSC). Moreover, the effect of MSC on the expanded PBSC allostimulatory capacity was also investigated, due to the well-known immunomodulatory properties of MSC. In addition, the requirement for cell-cell contact in this MSC coculture system was investigated using a transwell system.

RESULTS

Our results show that MSC greatly improved the expansion rate of adult PBSC cells relative to the absolute number of 1) clonogenic cells, 2) long-term cultured cells, or 3) CD34(+) cells. Whereas high levels of IL-6 on its own was sufficient to significantly improve PBSC expansion, direct contact between MSC and PBSC was required to achieve maximal expansion. Finally, MSC decreased the allostimulatory capacity of expanded PBSC.

CONCLUSION

Our data show that MSC efficiently improve expansion of adult PBSC, together with decreasing their allostimulatory capacity. Therefore, this study should provide a clinically relevant method for optimizing PBSC ex-vivo expansion, in particular when poor grafts are obtained.

Human umbilical vein endothelial cells increase ex vivo expansion of human CD34(+) PBPC through IL-6 secretion

BACKGROUND

Ex vivo expansion of hematopoietic stem cells (HSC) can help reduce cytopenia following transplantation, especially in NHL patients whose BM is deficient because of extensive chemotherapy. We have previously reported that human umbilical vein endothelial cells (HUVEC) can contribute to improved PBPC expansion when used in co-culture with CD34(+) cells.

METHODS

We evaluated the roles of direct HUVEC CD34(+) contact and HUVEC-produced soluble factors. We cultured CD34(+) PBPC harvested from NHL patients in four different conditions: (1) liquid culture without HUVEC; (2) co-culture in contact with HUVEC; (3) co-culture with HUVEC but without direct contact; (4) liquid culture with HUVEC-conditioned medium (CM). Thrombopoietin (Tpo), Flk2Flt3 ligand (FL) and c-kit ligand (KL) with or without rhIL-6 were added to these four culture conditions.

RESULTS AND DISCUSSION

Our results showed that HUVEC co-culture or addition of HUVEC-CM to Tpo, FL and KL (TFK) improved CD34(+) PBPC expansion compared with liquid culture, as determined by total viable nucleated cells (TNC), colony-forming cell assay (CFC) and week-6 cobblestone area-forming cells (Wk-6 CAFC) expansions. Non-contact culture led to similar PBPC expansion as contact co-culture; moreover, HUVEC-CM improved PBPC expansion. However, when rhIL-6 was added to HUVEC-CM with TFK, no significant difference was observed. Finally, high quantities of IL-6 were detected in HUVEC-CM and addition of anti-IL-6 Ab inhibited the positive effect of HUVEC on PBPC expansion. Our results thus suggest that HUVEC may improve PBPC expansion, at least through IL-6 secretion.

A novel direct competitive repopulation assay for human hematopoietic stem cells using NOD/SCID mice

BACKGROUND

The major problem in cord blood (CB) transplantation for adult patients is shortage of stem cell number. To overcome this disadvantage, several studies on ex vivo expansion have been performed. However, such efforts are always troubled by the lack of a reliable and simple assay system for stem cells. Our aim was to establish an in vivo assay system to compare the directly repopulating ability of two populations of human hematopoietic stem cells using a xenogeneic transplant system.

METHODS

Thirty CB samples from infants of each sex were pooled and enriched for CD34(+) progenitor cells. Enriched CD34(+) cells were transplanted into irradiated NOD/SCID mice at different male to female ratios, and human hematopoietic cells recovered 7 weeks after transplantation were analyzed by a quantitative DNA sex test using competitive PCR for the amelogenin gene. Using this assay system, ex vivo cultured and non-cultured CB cells were compared for repopulating ability.

RESULTS

The sex ratio of human CB cells transplanted was found to be maintained for 7 weeks in matured and progenitor cells. The competitive repopulation assay of cultured and non-cultured CB cells showed a marked defect in the repopulating ability of cultured cells, although the LTCIC count was maintained during cultivation.

DISCUSSION

Our assay system is a simple and reliable quantitative method that permits direct comparison of two stem cell compartments. The assay system will be useful for the assessment of the functional abilities of various human hematopoietic stem cells.

Bone morphogenetic protein 4 contributes to the maintenance of primitive cord blood hematopoietic progenitors in an ex vivo stroma-noncontact co-culture system

Establishment of conditions supporting hematopoietic stem cell (HSC) maintenance and expansion ex vivo is critical for wider clinical application of cord blood (CB) transplantation. AFT024 is a murine fetal liver cell line that expands primitive hematopoietic cells via a process that is not understood. Here we show that bone morphogenic protein 4 (BMP4) is produced by AFT024 and contributes significantly to the maintenance of co-cultured CB-derived primitive cells. Significant amounts of BMP4 mRNA are produced by the supportive AFT024 stromal cell line, and secreted BMP4 protein accumulates in AFT024 conditioned medium. Blockade of BMP4 activity in this coculture model using neutralizing BMP4 monoclonal antibody reduced expansion of primitive CB cells on the basis of phenotypic (CD34(+)CD38(-)) and functional criteria [long-term culture initiating cells (LTC-IC)] and significantly reduced the capacity of the cultured CB stem cells to support repopulation in the nonobese diabetic-severe combined immunodeficiency (NOD-SCID) xenograft model. Therefore, BMP4 is a key growth factor for maintenance of HSC and contributes to the unique properties of the AFT024 stromal noncontact culture.

Ex vivo expansion of umbilical cord blood stem cells for transplantation: growing knowledge from the hematopoietic niche

Umbilical cord blood transplantation (UCBT) in adults is limited by the small number of primitive hematopoietic stem cells (HSC) in each graft, resulting in delayed engraftment post transplant, and both short- and long-term infectious complications. Initial efforts to expand UCB progenitors ex vivo have resulted in expansion of mature rather than immature HSC, confounded by the inability to accurately and reliably measure long-term reconstituting cells. Ex vivo expansion of UCB HSC has failed to improve engraftment because of resulting defects that promote apoptosis, disrupt marrow homing and initiate cell cycling. Here we discuss the future of ex vivo expansion, which we suggest will include the isolation of immature hematopoietic progenitors on the basis of function rather than surface phenotype and will employ both cytokines and stroma to maintain and expand the stem cell niche. We suggest that ex vivo expansion could be enhanced by manipulating newly discovered signaling pathways (Notch, Wnt, bone morphogenetic protein 4 and Tie2/angiopoietin-1) and intracellular mediators (phosphatase and tensin homolog and glycogen synthase kinase-3) in a manner that promotes HSC expansion with less differentiation. Improved methods for ex vivo expansion will make UCBT available to more patients, decrease engraftment times and allow more rapid immune reconstitution post transplant.

Cord blood banking for potential future transplantation

In recent years, umbilical cord blood, which contains a rich source of hematopoietic stem and progenitor cells, has been used successfully as an alternative allogeneic donor source to treat a variety of pediatric genetic, hematologic, immunologic, and oncologic disorders. Because there is diminished risk of graft-versus-host disease after transplantation of cord stem cells using matched related donors, the use of less-than-completely matched HLA cord blood stem cells may incur less risk of graft-versus-host disease than mismatched cells from either a related or unrelated "walking" donor, although this remains to be proven. Gene-therapy research involving modification of autologous cord blood stem cells for the treatment of childhood genetic disorders, although experimental at the present time, may prove to be of value. These scientific advances have resulted in the establishment of not-for-profit and for-profit cord blood-banking programs for allogeneic and autologous cord blood transplantation. Many issues confront institutions that wish to establish or participate in such programs. Parents often seek information from their physicians about this new biotechnology option. This document is intended to provide information to guide physicians in responding to parents' questions about cord blood donation and banking and the types and quality of cord blood banks. Provided also are recommendations about appropriate ethical and operational standards, including informed consent policies, financial disclosures, and conflict-of-interest policies for physicians, institutions, and organizations that operate or have a relationship with cord blood-banking programs.

All-trans retinoic acid induces proliferation of an irradiated stem cell supporting stromal cell line AFT024

OBJECTIVE

We have previously shown that all-trans retinoic acid (ATRA) enhanced the maintenance of early human hematopoietic progenitor cells (HPCs) in the presence of an irradiated stromal cell line AFT024. In this study, we examined the effects of ATRA on the stromal cell component with particular reference to cellular proliferation and gene expression.

METHODS

Irradiated AFT024 cells were cultured in Dulbecco's Modified Eagle's Medium supplemented with fetal bovine serum and were incubated with ATRA at 1 mumol/L up to 21 days. The cells were examined in terms of immunostaining for proliferative cell nuclear antigen (PCNA) and BrdU incorporation, apoptosis assay, cell cycle analysis, and gene expression using semiquantitative reverse-transcriptase polymerase chain reaction.

RESULTS

In the control experiments, AFT024 cells lost their confluence in culture after 15-Gy irradiation and were arrested in G2/M phase on days 7 and 21. ATRA restored the cellular confluence with an increase in proliferation on day 21 (BrdU incorporation: 20.6-fold; PCNA staining: 51.7-fold) with reversal of cell cycle arrest (S phase: 2.7-fold increase; G2/M phase: 2.0-fold decrease). There was no effect on apoptosis as shown by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling staining. ATRA significantly upregulated the expression of cell cycle genes for checkpoint transition, including cyclin A2, B2, and aurora kinase B, as well as genes associated with a putative role in HPC maintenance, including osteopontin, HoxA5, enhancer of zeste homolog 2, and peroxisome proliferator-activated receptor gamma.

CONCLUSION

We concluded that ATRA induced cellular proliferation of irradiated AFT024 cells and expression of a number of genes whose relevance to HPC homeostasis would have to be further examined.

A Stro-1(+) human universal stromal feeder layer to expand/maintain human bone marrow hematopoietic stem/progenitor cells in a serum-free culture system

OBJECTIVE

To compare the ability of allogeneic versus autologous purified human Stro-1(+) mesenchymal stem cell (MSC) populations from different human donors to support the ex vivo expansion and maintenance of human hematopoietic stem/progenitor cells (HSCs). Furthermore, we compared the results obtained with MSC as a feeder layer to traditional allogeneic stromal layers grown in long-term bone marrow culture media (LT-ST).

METHODS

Adult human bone marrow CD34(+)-enriched cells were cultured in serum-free medium for 2 to 3 weeks over the respective MSC-irradiated feeder layers or over traditional allogeneic LT- ST stromal layers in the presence of stem cell factor, basic fibroblast growth factor, leukemia inhibitory factor, and Flt-3 and analyzed every 2 to 4 days for expansion, phenotype, and clonogenic ability.

RESULTS

There was a progressive expansion of total numbers of cells in all the experimental groups; however, allogeneic MSCs were more efficient at expanding CD34(+)CD38(-) cells and showed a higher clonogenic potential than both allogeneic LT-ST and autologous MSCs. The differentiative potential of cells cultured on both MSC and LT-ST was primarily shifted toward myeloid lineage; however, only MSCs were able to maintain/expand a CD7(+) population with lymphocytic potential. Importantly, transplantation into preimmune fetal sheep demonstrated that the HSCs cultured over MSCs retained their engraftment capability.

CONCLUSION

These results indicate that purified Stro-1(+) MSCs may be used as a universal and reproducible stromal feeder layer to efficiently expand and maintain human bone marrow HSCs ex vivo.

Growth kinetics of progenitor cell-enriched hematopoietic cell populations in long-term liquid cultures under continuous removal of mature cells

BACKGROUND

During long-term culture of primitive hematopoietic cells large numbers of mature cells are generated that, on the one hand, consume nutrients and cytokines present in the medium and, on the other hand, may produce or elicit the production of soluble factors that limit the growth of primitive cells. Thus it is possible that under standard culture conditions hematopoietic stem and progenitor cells are unable to display their true proliferation and expansion potentials.

METHODS

Hematopoietic cell populations, enriched for CD34+ cells, were obtained from both umbilical cord blood (UCB) and mobilized peripheral blood (MPB), and cultured in cytokine-supplemented liquid culture, under continuous removal of mature cells by means of weekly re-selection of primitive, lineage-negative (Lin-) cells. Proliferation and expansion capacities of such cells were determined weekly for a 42-day culture period.

RESULTS

As expected, based on our previous studies in standard liquid cultures, throughout the culture period there was a continuous decrease in the proportion of progenitor cells; however, after every re-selection on days 7, 14 and 21, there was a significant enrichment for both CD34+ cells and colony-forming cells (CFC). As a result of such an enrichment, the cumulative increase in the numbers of total cells and CFC in cultures with two, three or four selections was significantly higher than the increments observed in standard cultures, in which only a single selection was performed on day 0. Cultures of UCB cells showed consistently higher levels of both total cells and CFC than cultures of MPB cells.

DISCUSSION

Taken together, these results indicate that continuous removal of mature cells from liquid cultures of primitive progenitors results in higher increments in the levels of both total cells and CFC.

Evaluation of different protocols for gene transfer into non-obese diabetes/severe combined immunodeficiency disease mouse repopulating cells

PURPOSE

Although gene transfer with retroviral vectors has shown distinct clinical success in defined settings, efficient genetic manipulation of hematopoietic progenitor cells remains a challenge. To address this issue we have evaluated different transduction protocols and retroviral constructs in the non-obese diabetes (NOD)/severe combined immunodeficiency disease (SCID) xenograft model.

METHODS

An extended transduction protocol requiring 144 h of in vitro manipulation was compared to a more conventional protocol requiring 96 h only.

RESULT

While pretransplantation analysis of cells transduced with a retroviral vector, expressing the enhanced green fluorescent protein (EGFP) marker gene, demonstrated significantly higher overall transduction rates for the extended protocol (33.6 +/- 2.3 vs. 22.1 +/- 3.8%), EGFP expression in CD34+ cells before transplantation (4.0 +/- 0.9 vs. 11.6 +/- 2.5%), engraftment of human cells in NOD/SCID bone marrow 4 weeks after transplantation (4.5 +/- 1.7 vs. 36.5 +/- 9.4%) and EGFP expression in these cells (0 +/- 0 vs. 11.3 +/- 2.8%) were significantly impaired. When the 96 h protocol was used in combination with the spleen focus forming virus (SFFV)/murine embryonic stem cell (MESV) hybrid vector SFbeta11-EGFP, high transduction rates for CD45+ (41.0 +/- 5.3%) and CD34+ (38.5 +/- 3.7%) cells prior to transplantation, as well as efficient human cell engraftment in NOD/SCID mice 4 weeks after transplantation (32.4 +/- 3.5%), was detected. Transgene expression was observed in B-lymphoid (15.9 +/- 2.0%), myeloid (36.5 +/- 3.5%) and CD34+ cells (10.1 +/- 1.5%).

CONCLUSION

Our data show that CD34+ cells maintained in cytokines for multiple days may differentiate and loose their capacity to contribute to the haematological reconstitution of NOD/SCID mice. In addition, the SFFV/MESV hybrid vector SFbeta11-EGFP allows efficient transduction of and gene expression in haematopoietic progenitor cells.

Proteomics of primary mesenchymal stem cells

Tissue and functional regeneration takes place in the body at various stages throughout life. However, bone, cartilage, tendons, blood vessels and cardiac muscle have a limited capacity for self repair and, after injury or disease, the regenerative ability of these adult tissues is often insufficient and leads to nonfunctional scar tissue. In this context, mesenchymal stem cells, which are adult multipotential progenitors of mesoderm cells (osteoblasts, chondrocytes, adipocytes and stroma cells), represent a major hope for tissue-engineered replacement and regenerative medicine. Furthermore, the autologous use of these cells prevents immunological responses against new tissues and the risks of disease transmission from donors, which are both common problems of organ transplantation. While the existence of mesenchymal stem cells is undisputed, many questions remain regarding their self-renewal and capacity to differentiate, their homogenous nature as a cell population throughout the body and their true potential in regenerative medicine. In this article, the proteomics studies carried out to characterize mesenchymal stem cells and to help understand their physiology are reviewed.

Superior ex vivo cord blood expansion following co-culture with bone marrow-derived mesenchymal stem cells

One factor limiting the therapeutic efficacy of cord blood (CB) hematopoietic progenitor cell (HPC) transplantation is the low cell dose of the graft. This is associated with an increased incidence of delayed or failed engraftment. Cell dose can be increased and the efficacy of CB transplantation potentially improved, by ex vivo CB expansion before transplantation. Two ex vivo CB expansion techniques were compared: (1) CD133+ selection followed by ex vivo liquid culture and (2) co-culture of unmanipulated CB with bone-marrow-derived mesenchymal stem cells (MSCs). Ex vivo culture was performed in medium supplemented with granulocyte colony-stimulating factor, stem cell factor and either thrombopoietin or megakaryocyte growth and differentiation factor. Expansion was followed by measuring total nucleated cell (TNC), CD133+ and CD34+ cell, colony-forming unit and cobblestone area-forming cell output. When compared to liquid culture, CB-MSC co-culture (i) required less cell manipulation resulting in less initial HPC loss and (ii) markedly improved TNC and HPC output. CB-MSC co-culture therefore holds promise for improving engraftment kinetics in CB transplant recipients.

Stromal cells selectively reduce the growth advantage of human committed CD34+ hematopoietic cells ectopically expressing HOXB4

Key players in self-renewal of hemopoietic stem cells are homeobox (HOX) transcription factors. In murine cells, overexpression of HOXB4 results in expansion of hematopoietic stem- and committed progenitor cells in vitro without obvious hematopoietic alterations. In vivo, HOXB4 induced HSC expansion continued until stem cell regeneration reached pretransplantation levels. HOXB4 is thus an attractive candidate for amplification of stem cells provided that human HOXB4 overexpressing cells can also be restricted to normal growth in vivo. The stromal microenvironment provides the regulatory mechanisms controlling the balance of stem cell self-renewal and differentiation. Here, we compared the response of HOXB4- and GFP-control vector transduced human CD34(+) cells to stroma encoded signals in vitro. In serum-sustained cocultures MS-5 stroma contact reduced the output of late CD34- HOXB4(+) cells in relation to GFP-controls 9-fold whereas the expansion of early CD34(+)HOXB4(+) cells remained unchanged as compared to liquid cultures. In presence of insulin HOXB4 overexpressing cells do not react to stroma encoded growth-restricting signals. Our results show that ectopic expression of HOXB4 in combination with MS-5 stroma exerts different effects in early and late human cord blood CD34(+) cells resulting in an enhanced proliferation of early CD34(+) cells in absence or presence of MS-5 stroma and an impaired output of late committed CD34(+) cells on MS-5 stroma.