Ex Vivo Expansion of Autologous Bone Marrow CD34+ Cells With Porcine Microvascular Endothelial Cells Results in a Graft Capable of Rescuing Lethally Irradiated Baboons

The challenges and optimization of cell-based therapy for cardiovascular disease

With the hope of achieving real cardiovascular repair, cell-based therapy raised as a promising strategy for the treatment of cardiovascular disease (CVD) in the past two decades. Various types of cells have been studied for their reparative potential for CVD in the ensuing years. Despite the exciting results from animal experiments, the outcome of clinical trials is unsatisfactory and the development of cell-based therapy for CVD has hit a plateau nowadays. Thus, it is important to summarize the obstacles we are facing in this field in order to explore possible solutions for optimizing cell-based therapy and achieving real clinical application.

Stem cell homing: From physiology to therapeutics

Stem cell homing is a multistep endogenous physiologic process that is also used by exogenously administered hematopoietic stem and progenitor cells (HSPCs). This multistep process involves cell migration and is essential for hematopoietic stem cell transplantation. The process can be manipulated to enhance ultimate engraftment potential, and understanding stem cell homing is also important to the understanding of stem cell mobilization. Homing is also of potential importance in the recruitment of marrow mesenchymal stem and stromal cells (MSCs) to sites of injury and regeneration. This process is less understood but assumes importance when these cells are used for repair purposes. In this review, the process of HSPC and MSC homing is examined, as are methods to enhance this process.

Targeting the bone marrow microenvironment in acute leukemia

Despite individual differences between certain leukemias, the overall survival rate in acute leukemia remains low at approximately 40%. Novel therapeutics, including targeted therapies like tyrosine kinase inhibitors, have been incorporated into treatment regimens, but most have failed at eradicating leukemic stem cells (LSCs). The causes of disease relapse, progression, and resistance to chemotherapy are as yet not entirely clear but thought to be linked to protection in the bone marrow microenvironment (BMM). In this review, we summarize current knowledge on the BMM in acute leukemias and examine the ongoing efforts to target the BMM, which include treatment strategies targeting (a) leukemia-BMM interactions, (b) leukemia-cell intrinsic pathways influenced by the BMM, and (c) direct BMM targeting strategies. It is likely that the future ploy against leukemia will involve these and other innovative strategies designed to eradicate the last remaining warrior - the LSC.

Blood on the tracks: hematopoietic stem cell-endothelial cell interactions in homing and engraftment

Cells of the hematopoietic system undergo rapid turnover. Each day, humans require the production of about one hundred billion new blood cells for proper function. Hematopoietic stem cells (HSCs) are rare cells that reside in specialized niches and are required throughout life to produce specific progenitor cells that will replenish all blood lineages. There is, however, an incomplete understanding of the molecular and physical properties that regulate HSC migration, homing, engraftment, and maintenance in the niche. Endothelial cells (ECs) are intimately associated with HSCs throughout the life of the stem cell, from the specialized endothelial cells that give rise to HSCs, to the perivascular niche endothelial cells that regulate HSC homeostasis. Recent studies have dissected the unique molecular and physical properties of the endothelial cells in the HSC vascular niche and their role in HSC biology, which may be manipulated to enhance hematopoietic stem cell transplantation therapies.

Regulation of the hematopoietic stem cell lifecycle by the endothelial niche

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) predominantly reside either in direct contact or in close proximity to the vascular endothelium throughout their lifespan. From the moment of HSC embryonic specification from hemogenic endothelium, endothelial cells (ECs) act as a critical cellular-hub that regulates a vast repertoire of biological processes crucial for HSC maintenance throughout its lifespan. In this review, we will discuss recent findings in endothelial niche-mediated regulation of HSC function during development, aging and regenerative conditions.

RECENT FINDINGS

Studies employing genetic vascular models have unequivocally confirmed that ECs provide the essential instructive cues for HSC emergence during embryonic development as well as adult HSC maintenance during homeostasis and regeneration. Aging of ECs may impair their ability to maintain HSC function contributing to the development of aging-associated hematopoietic deficiencies. These findings have opened up new avenues to explore the therapeutic application of ECs. ECs can be adapted to serve as an instructive platform to expand bona fide HSCs and also utilized as a cellular therapy to promote regeneration of the hematopoietic system following myelosuppressive and myeloablative injuries.

SUMMARY

ECs provide a fertile niche for maintenance of functional HSCs throughout their lifecycle. An improved understanding of the EC-HSC cross-talk will pave the way for development of EC-directed strategies for improving HSC function during aging.

Endothelial Cells Promote Expansion of Long-Term Engrafting Marrow Hematopoietic Stem and Progenitor Cells in Primates

Successful expansion of bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) would benefit many HSPC transplantation and gene therapy/editing applications. However, current expansion technologies have been limited by a loss of multipotency and self-renewal properties ex vivo. We hypothesized that an ex vivo vascular niche would provide prohematopoietic signals to expand HSPCs while maintaining multipotency and self-renewal. To test this hypothesis, BM autologous CD34⁺ cells were expanded in endothelial cell (EC) coculture and transplanted in nonhuman primates. CD34⁺ C38^- HSPCs cocultured with ECs expanded up to 17-fold, with a significant increase in hematopoietic colony-forming activity compared with cells cultured with cytokines alone (colony-forming unit-granulocyte-erythroid-macrophage-monocyte; p < .005). BM CD34⁺ cells that were transduced with green fluorescent protein lentivirus vector and expanded on ECs engrafted long term with multilineage polyclonal reconstitution. Gene marking was observed in granulocytes, lymphocytes, platelets, and erythrocytes. Whole transcriptome analysis indicated that EC coculture altered the expression profile of 75 genes in the BM CD34⁺ cells without impeding the long-term engraftment potential. These findings show that an ex vivo vascular niche is an effective platform for expansion of adult BM HSPCs. Stem Cells Translational Medicine 2017;6:864-876.

Angiocrine functions of organ-specific endothelial cells

Endothelial cells that line capillaries are not just passive conduits for delivering blood. Tissue-specific endothelium establishes specialized vascular niches that deploy sets of growth factors, known as angiocrine factors. These cues participate actively in the induction, specification, patterning and guidance of organ regeneration, as well as in the maintainance of homeostasis and metabolism. When upregulated following injury, they orchestrate self-renewal and differentiation of tissue-specific resident stem and progenitor cells into functional organs. Uncovering the mechanisms by which organotypic endothelium distributes physiological levels of angiocrine factors both spatially and temporally will lay the foundation for clinical trials that promote organ repair without scarring.

The hematopoietic system in the context of regenerative medicine

Hematopoietic stem cells (HSC) represent the prototype stem cell within the body. Since their discovery, HSC have been the focus of intensive research, and have proven invaluable clinically to restore hematopoiesis following inadvertent radiation exposure and following radio/chemotherapy to eliminate hematologic tumors. While they were originally discovered in the bone marrow, HSC can also be isolated from umbilical cord blood and can be "mobilized" peripheral blood, making them readily available in relatively large quantities. While their ability to repopulate the entire hematopoietic system would already guarantee HSC a valuable place in regenerative medicine, the finding that hematopoietic chimerism can induce immunological tolerance to solid organs and correct autoimmune diseases has dramatically broadened their clinical utility. The demonstration that these cells, through a variety of mechanisms, can also promote repair/regeneration of non-hematopoietic tissues as diverse as liver, heart, and brain has further increased their clinical value. The goal of this review is to provide the reader with a brief glimpse into the remarkable potential HSC possess, and to highlight their tremendous value as therapeutics in regenerative medicine.

Exogenous endothelial cells as accelerators of hematopoietic reconstitution

Despite the successes of recombinant hematopoietic-stimulatory factors at accelerating bone marrow reconstitution and shortening the neutropenic period post-transplantation, significant challenges remain such as cost, inability to reconstitute thrombocytic lineages, and lack of efficacy in conditions such as aplastic anemia. A possible means of accelerating hematopoietic reconstitution would be administration of cells capable of secreting hematopoietic growth factors. Advantages of this approach would include: a) ability to regulate secretion of cytokines based on biological need; b) long term, localized production of growth factors, alleviating need for systemic administration of factors that possess unintended adverse effects; and c) potential to actively repair the hematopoietic stem cell niche. Here we overview the field of hematopoietic growth factors, discuss previous experiences with mesenchymal stem cells (MSC) in accelerating hematopoiesis, and conclude by putting forth the rationale of utilizing exogenous endothelial cells as a novel cellular therapy for acceleration of hematopoietic recovery.

The vascular niche: home for normal and malignant hematopoietic stem cells

Hematopoietic stem cells (HSCs) are uniquely capable of self-renewal and provision of all of the mature elements of the blood and immune system throughout the lifetime of an individual. HSC self-renewal is regulated by both intrinsic mechanisms and extrinsic signals mediated via specialized microenvironments or 'niches' wherein HSCs reside. HSCs have been shown to reside in close association with bone marrow (BM) osteoblasts in the endosteal niche and also in proximity to BM sinusoidal vessels. An unresolved question surrounds whether the endosteal and vascular niches provide synchronous or redundant regulation of HSC fate or whether these niches provide wholly unique regulatory functions. Furthermore, while some aspects of the mechanisms through which osteoblasts regulate HSC fate have been defined, the mechanisms through which the vascular niche regulates HSC fate remain obscure. Here, we summarize the anatomic and functional basis supporting the concept of an HSC vascular niche as well as the precise function of endothelial cells, perivascular cells and stromal cells within the niche in regulating HSC fate. Lastly, we will highlight the role of the vascular niche in regulating leukemic stem cell fate in vivo.

Bone marrow CD34+ cells expanded on human brain endothelial cells reconstitute lethally irradiated baboons in a variable manner

Increased cell dose has a positive impact on the therapeutic outcome of bone marrow (BM) hematopoietic stem cell (HSC) transplant. However, methods to successfully expand BM HSCs have yet to be achieved. It has been shown previously that ex vivo expansion of BM cells using porcine microvascular endothelial cells can rescue a baboon from a lethal dose of radiation. However, in a prior study, baboons that received CD34+ cell doses less than 4 x 10(6) cells/kg body weight failed to achieve hematopoietic reconstitution. In our present study we used human brain endothelial cells (HUBECs) and cytokines to expand BM cells, and examined their ability to provide hematopoietic reconstitution in three lethally irradiated baboons following autologous transplant as a surrogate preclinical model. After ex vivo culture, the grafts represented a 1.8- to 2.1-fold expansion of CD34+ cells, a 3.7- to 13.2-fold increase of colony-forming cells, and a 1.9- to 3.2-fold increase of cobblestone area-forming cells, in comparison to the input cell numbers. Despite transplanting CD34+ cell grafts displaying a comparable degree of expansion, there was an obvious variability in the kinetics of hematopoietic reconstitution. The variation in hematopoietic reconstitution cannot be fully explained by the properties tested in expanded CD34+ cells, and warrant caution against taking into account such attributes as cell dose, expression of adhesion molecules, and migration as a measure of successful expansion of HSCs.

Effect of ex vivo culture of CD34+ bone marrow cells on immune reconstitution of XSCID dogs following allogeneic bone marrow transplantation

Successful genetic treatment of most primary immunodeficiencies or hematological disorders will require the transduction of pluripotent, self-renewing hematopoietic stem cells (HSC) rather than their progeny to achieve enduring production of genetically corrected cells and durable immune reconstitution. Current ex vivo transduction protocols require manipulation of HSC by culture in cytokines for various lengths of time depending upon the retroviral vector that may force HSC to enter pathways of proliferation, and possibly differentiation, which could limit their engraftment potential, pluripotentiality and long-term repopulating capacity. We have compared the ability of normal CD34(+) cells cultured in a standard cytokine cocktail for 18hours or 4.5 days to reconstitute XSCID dogs following bone marrow transplantation in the absence of any pretransplant conditioning with that of freshly isolated CD34(+) cells. CD34(+) cells cultured under standard gamma-retroviral transduction conditions (4.5 days) showed decreased engraftment potential and ability to sustain long-term thymopoiesis. In contrast, XSCID dogs transplanted with CD34(+) cells cultured for 18hours showed a robust T cell immune reconstitution similar to dogs transplanted with freshly isolated CD34(+) cells, however, the ability to sustain long-term thymopoiesis was impaired. These results emphasize the need to determine ex vivo culture conditions that maintain both the engraftment potential and "stem cell" potential of the cultured cells.

Endothelial cells mediate the regeneration of hematopoietic stem cells

Recent studies suggest that endothelial cells are a critical component of the normal hematopoietic microenvironment. Therefore, we sought to determine whether primary endothelial cells have the capacity to repair damaged hematopoietic stem cells. Highly purified populations of primary CD31(+) microvascular endothelial cells isolated from the brain or lung did not express the pan hematopoietic marker CD45, most hematopoietic lineage markers, or the progenitor marker c-kit and did not give rise to hematopoietic cells in vitro or in vivo. Remarkably, the transplantation of small numbers of these microvascular endothelial cells consistently restored hematopoiesis following bone marrow lethal doses of irradiation. Analysis of the peripheral blood of rescued recipients demonstrated that both short-term and long-term multilineage hematopoietic reconstitution was exclusively of host origin. Secondary transplantation studies revealed that microvascular endothelial cell-mediated hematopoietic regeneration also occurs at the level of the hematopoietic stem cell. These findings suggest a potential therapeutic role for microvascular endothelial cells in the self-renewal and repair of adult hematopoietic stem cells.

Differences amid bone marrow and cord blood hematopoietic stem/progenitor cell division kinetics

Human hematopoietic stem/progenitor cells (HSC) isolated based upon specific patterns of CD34 and CD38 expression, despite phenotypically identical, were found to be functionally heterogeneous, raising the possibility that reversible expression of these antigens may occur during cellular activation and/or proliferation. In these studies, we combined PKH67 tracking with CD34/CD38 immunostaining to compare cell division kinetics between human bone marrow (BM) and cord blood (CB)-derived HSC expanded in a serum-free/stromal-based system for 14 days (d), and correlated CD34 and CD38 expression with the cell divisional history. CB cells began dividing 24 h earlier than BM cells, and significantly higher numbers underwent mitosis during the time in culture. By d10, over 55% of the CB-cells reached the ninth generation, whereas BM-cells were mostly distributed between the fifth and seventh generation. By d14, all CB cells had undergone multiple cell divisions, while 0.7-3.8% of BM CD34(+) cells remained quiescent. Furthermore, the percentage of BM cells expressing CD34 decreased from 60.8 +/- 6.3% to 30.6 +/- 6.7% prior to initiating division, suggesting that downmodulation of this antigen occurred before commencement of proliferation. Moreover, with BM, all primitive CD34(+)CD38(-) cells present at the end of culture arose from proliferating CD34(+)CD38(+) cells that downregulated CD38 expression, while in CB, a CD34(+)CD38(-) population was maintained throughout culture. These studies show that BM and CB cells differ significantly in cell division kinetics and expression of CD34 and CD38, and that the inherent modulation of these antigens during ex vivo expansion may lead to erroneous quantification of the stem cell content of the expanded graft.

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.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

Chromatin-modifying agents permit human hematopoietic stem cells to undergo multiple cell divisions while retaining their repopulating potential

Human hematopoietic stem cells (HSCs) exposed to cytokines in vitro rapidly divide and lose their characteristic functional properties presumably due to the alteration of a genetic program that determines the properties of an HSC. We have attempted to reverse the silencing of this HSC genetic program by the sequential treatment of human cord blood CD34+ cells with the chromatin-modifying agents, 5-aza-2′-deoxycytidine (5azaD) and trichostatin A (TSA). We determined that all CD34+CD90+ cells treated with 5azaD/TSA and cytokines after 9 days of incubation divide, but to a lesser degree than cells exposed to only cytokines. When CD34+CD90+ cells that have undergone extensive number of cell divisions (5-10) in the presence of cytokines alone were transplanted into immunodeficient mice, donor cell chimerism was not detectable. By contrast, 5azaD/TSA-treated cells that have undergone similar numbers of cell divisions retained their marrow repopulating potential. The expression of several genes and their products previously implicated in HSC self-renewal were up-regulated in the cells treated with 5azaD/TSA as compared to cells exposed to cytokines alone. These data indicate that HSC treated with chromatin-modifying agents are capable of undergoing repeated cell divisions in vitro while retaining their marrow-repopulating potential.

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.

Molecular Profile and Partial Functional Analysis of Novel Endothelial Cell-Derived Growth Factors that Regulate Hematopoiesis

Recent progress has been made in the identification of the osteoblastic cellular niche for hematopoietic stem cells (HSCs) within the bone marrow (BM). Attempts to identify the soluble factors that regulate HSC self-renewal have been less successful. We have demonstrated that primary human brain endothelial cells (HUBECs) support the ex vivo amplification of primitive human BM and cord blood cells capable of repopulating non-obese diabetic/severe combined immunodeficient repopulating (SCID) mice (SCID repopulating cells [SRCs]). In this study, we sought to characterize the soluble hematopoietic activity produced by HUBECs and to identify the growth factors secreted by HUBECs that contribute to this HSC-supportive effect. Extended noncontact HUBEC cultures supported an eight-fold increase in SRCs when combined with thrombopoietin, stem cell factor, and Flt-3 ligand compared with input CD34(+) cells or cytokines alone. Gene expression analysis of HUBEC biological replicates identified 65 differentially expressed, nonredundant transcripts without annotated hematopoietic activity. Gene ontology studies of the HUBEC transcriptome revealed a high concentration of genes encoding extracellular proteins with cell-cell signaling function. Functional analyses demonstrated that adrenomedullin, a vasodilatory hormone, synergized with stem cell factor and Flt-3 ligand to induce the proliferation of primitive human CD34(+)CD38(-)lin(-) cells and promoted the expansion of CD34(+) progenitors in culture. These data demonstrate the potential of primary HUBECs as a reservoir for the discovery of novel secreted proteins that regulate human hematopoiesis.

Vascular Endothelial Cells Produce Soluble Factors That Mediate the Recovery of Human Hematopoietic Stem Cells after Radiation Injury

The risk of terrorism with nuclear or radiologic weapons is considered to be high over the coming decade. Ionizing radiation can cause a spectrum of hematologic toxicities, from mild myelosuppression to myeloablation and death. However, the potential regenerative capacity of human hematopoietic stem cells (HSCs) after radiation injury has not been well characterized. In this study, we sought to characterize the effects of ionizing radiation on human HSCs and to determine whether signals from vascular endothelial cells could promote the repair of irradiated HSCs. Exposure of human bone marrow CD34+ cells to 400 cGy caused a precipitous decline in hematopoietic progenitor cell content and primitive cells capable of repopulating nonobese diabetic/severe combined immunodeficient mice (SCID-repopulating cells), which was not retrievable via treatment with cytokines. Conversely, culture of 400 cGy-irradiated bone marrow CD34+ cells with endothelial cells under noncontact conditions supported the differential recovery of both viable progenitor cells and primitive SCID-repopulating cells. These data illustrate that vascular endothelial cells produce soluble factors that promote the repair and functional recovery of HSCs after radiation injury and suggest that novel factors with radiotherapeutic potential can be identified within this milieu.

Characterization of primitive marrow CD34+ cells that persist after a sublethal dose of total body irradiation

Knowledge of the molecular events that occur during hematopoietic stem/progenitor cell (HSPC) development is vital to our understanding of blood cell production. To study the functional groups of genes characteristic of HSPCs we isolated a subpopulation of CD34+ bone marrow (BM) cells from nonhuman primates that persisted in vivo after a sublethal dose of total body irradiation (TBI). CD34+ cells isolated during the phase of maximal hematopoietic suppression show a transcriptional profile characteristic of metabolically inactive cells, with strong coordinate downregulation of a large number of genes required for protein production and processing. Consistent with this profile, these CD34+ cells were not able to generate hematopoietic colonies. Transcriptional profiling of these CD34+ cells in conjunction with a pathway analysis method reveals several classes of functionally related genes that are upregulated in comparison to the CD34+ cells obtained prior to TBI. These families included genes known to be associated with self-renewal and maintenance of HSPCs (including bone morphogenetic proteins), resistance to apoptosis (Bcl-2) as well as genes characteristic of a variety of nonhematopoietic tissues (gamma-aminobutyric acid/glycine receptor, complement receptor C1qRp). In contrast, during the period of hematopoietic recovery, the CD34+ cells expressed higher level of genes encoding factors regulating maturation and differentiation of HSPCs. Our data indicate that the primitive BM CD34+ cell population that persists after radiation possesses a transcriptional profile suggestive of pluripotency.

Nonhuman primate embryonic stem cells as a preclinical model for hematopoietic and vascular repair

Stem cell-based regenerative medicine therapies have been touted recently as a novel therapeutic approach to treat and cure a wide range of diseases. Both adult and embryonic stem (ES) cells can serve as important sources of precursor cells to derive more mature cells potentially utilized for clinical applications. Nonhuman primates have proven useful as a preclinical model, as demonstrated in studies of hematopoietic cell transplantation, gene therapy, and other areas. The derivation of nonhuman primate ES cells now provides an optimal resource to characterize and test ES cell-based therapies prior to trials with human ES cells. This review describes work to define strategies and mechanisms to derive blood and endothelial cells from nonhuman primate ES cells isolated from various species. Preclinical testing that solely relies on studies of putative therapeutic cells derived from mouse ES cells transplanted into other mice, or analyses of human ES cell-derived cells transplanted into immunodeficient or immunosuppressed rodents may not be predictive of efficacy in subsequent human trials. However, future testing using nonhuman primate ES cell-derived therapeutic cells done as an allogeneic transplant may best predict success for subsequent studies using human ES cells. Therefore, additional research on nonhuman primate ES cells, in addition to work on mouse and human ES cells, is greatly needed to facilitate clinical translation of new stem cell treatments.

Mesenchymal stem cells rescue CD34+ cells from radiation-induced apoptosis and sustain hematopoietic reconstitution after coculture and cografting in lethally irradiated baboons: is autologous stem cell therapy in nuclear accident settings hype or reality?

Autologous stem cell therapy (ACT) has been proposed to prevent irradiated victims from bone marrow (BM) aplasia by grafting hematopoietic stem and progenitor cells (HSPCs) collected early after damage, provided that a functional graft of sufficient size could be produced ex vivo. To address this issue, we set up a baboon model of cell therapy in which autologous peripheral blood HSPCs collected before lethal total body irradiation were irradiated in vitro (2.5 Gy, D0 1 Gy) to mimic the cell damage, cultured in small numbers for a week in a serum-free medium in the presence of antiapoptotic cytokines and mesenchymal stem cells (MSCs) and then cografted. Our study shows that baboons cografted with expanded cells issued from 0.75 and 1 x 10(6)/kg irradiated CD34+ cells and MSCs (n=2) exhibited a stable long-term multilineage engraftment. Hematopoietic recovery became uncertain when reducing the CD34+ cell input (0.4 x 10(6)/kg CD34+ cells; n=3). However, platelet recovery was accelerated in all surviving cografted animals, when compared with baboons transplanted with unirradiated, unmanipulated CD34+ cells (0.5-1 x 10(6)/kg, n=4). Baboons grafted with MSCs alone (n=3) did not recover. In all cases, the nonhematopoietic toxicity remained huge. This baboon study suggests that ACT feasibility is limited.

A human stromal-based serum-free culture system supports the ex vivo expansion/maintenance of bone marrow and cord blood hematopoietic stem/progenitor cells

OBJECTIVE

We investigated the role of human stromal layers (hu-ST) on the ex vivo expansion/maintenance of human hematopoietic stem/progenitor cells (HSC) from adult bone marrow (BM) and umbilical cord blood (CB).

MATERIALS AND METHODS

BM and CB CD34(+)-enriched cells were cultured in serum-free medium supplemented with SCF, bFGF, LIF, and Flt-3, in the presence or absence of stroma, and analyzed for proliferation, phenotype, and clonogenic potential.

RESULTS

Significant expansion of BM and CB CD34(+) and CD34(+)CD38(-) cells were achieved in the presence of hu-ST. The differentiative potential of both BM and CB CD34(+)-enriched cells cocultured with hu-ST was primarily shifted toward the myeloid lineage, while maintaining/expanding a CD7(+) population. Clonogenic analysis of the expanded cells showed increases in progenitors of the myeloid lineage, including colony-forming unit-granulocyte, macrophage (CFU-GM) and colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte (CFU-Mix) for both BM (stroma and stroma-free conditions) and CB cells in the presence of stroma.

CONCLUSIONS

These results indicate that adult hu-ST in the presence of appropriate cytokines can be used to efficiently expand/maintain myeloid and lymphoid cell populations from human BM and CB HSC.

Effect of age on the frequency, cell cycle, and lineage maturation of rhesus monkey (Macaca mulatta) CD34+ and hematopoietic progenitor cells

The effects of maturation and aging on hematopoietic progenitor cells, blood and bone marrow from second- and third-trimester fetal, newborn, infant, adult, and aged rhesus monkeys (Macaca mulatta) were analyzed. CD34(+) cells were immunoselected and stained with propidium iodide for cell cycle analysis. Blood and bone marrow mononuclear cells were plated in methylcellulose, and erythroid and myeloid progenitors were grown and counted. A higher frequency of circulating CD34(+)CD38(-) and CD34(+)DR(-) cells was observed in second-trimester fetuses compared with the other age groups. The frequency of bone marrow CD34(+)CD38(-) and CD34(+)DR(-) cells declined in adult and aged animals when compared with the younger age groups. Cell-cycle analysis showed 4.5% second-trimester fetal bone marrow CD34(+) cells entering the G(2)/M phase, compared with 1.7% CD34(+) cells in aged animals. More than 95% of circulating CD34(+) cells remained quiescent for most age groups, except for second-trimester fetuses. Adult marrow myeloid progenitors were found in a lower quantity when compared with third-trimester fetuses, whereas erythroid progenitors were greatest in early-gestation fetuses and adults. The results of these studies suggest that 1) the greatest quantity of CD34(+)CD38(-) and CD34(+)DR(-) cells was found in fetal and infant bone marrow, 2) the frequency of cycling CD34(+) cells declines with maturation and aging, and 3) an age-dependent difference in lineage commitment occurs.

Growth factors mobilize CXCR4 low/negative primitive hematopoietic stem/progenitor cells from the bone marrow of nonhuman primates

Abstract The chemokine receptor CXCR4 is expressed by CD34 + hematopoietic stem/progenitor cells (HSC/HPC). Several investigators have suggested that expression of CXCR4 may be an important characteristic of HSC/HPC. We studied the dynamic expression of CXCR4 during growth factor-induced mobilization of HSC in a clinically relevant nonhuman primate model, Papio anubis (baboons). We evaluated whether CXCR4 expression in HSC/HPC varies during steady-state hematopoiesis as well as during growth factor-induced mobilization. Peripheral blood stem cells from 5 baboons were mobilized with growth factors. During mobilization, there was a consistent stepwise increase in the proportion of peripheral blood CD34 + cells that were CXCR4 -. The highest number of CD34 + CXCR4 - cells appeared in the peripheral blood at the same time as the maximum number of assayable colony-forming cells. The cloning efficiency of the CD34 + CXCR4 - population was 3-fold greater than that of CD34 + CXCR4 + cells, and the frequency of cobblestone area-forming cells was 6 times higher in the CD34 + CXCR4 - population in comparison to CD34 + CXCR4 + cells. Furthermore, the most quiescent CD34 + cells isolated on the basis of low Hoechst 33342 (Ho) and rhodamine 123 (Rho) staining (Ho Low /Rho Low ) were highly enriched in the CXCR4 Low/- cell population. Ex vivo incubation of mobilized peripheral blood CD34 + cells with growth factors for 40 hours resulted in increasing numbers of cells expressing CXCR4. Peripheral blood stem cell grafts containing CD34 + cells that consisted of predominantly CXCR4 - cells were able to rapidly engraft lethally irradiated baboons. Because the overwhelming number of CD34 + cells within the mobilized peripheral blood grafts were CXCR4 - and were capable of rescuing lethally irradiated baboons, it seems unlikely that the expression of CXCR4 in vitro is an absolute requirement for HSC homing and engraftment. In summary, our data suggest the dynamic nature of CXCR4 expression on CD34 + cells during growth factor-induced HSC/HPC mobilization. In addition, our data indicate that the lack of CXCR4 expression is possibly a characteristic of relatively more primitive HSC/HPC characterized by a higher proliferative capacity.

Hematopoietic stem cell repopulating ability can be maintained in vitro by some primary endothelial cells

OBJECTIVE

Murine hematopoietic stem cells (HSC) reside primarily in bone marrow but freely circulate throughout the systemic circulation with retention of transplantable hematopoietic repopulating ability. The mechanisms maintaining HSC potential during systemic circulation remain elusive. We hypothesized that vascular endothelial cells (EC) play an important role in maintaining circulating HSC repopulating ability.

METHODS

Using Tie2-green fluorescence protein transgenic mice, we have isolated primary EC populations derived from several nonhematopoietic organs and cocultured bone marrow Sca1+c-Kit+lin- cells for 7 days in the presence or absence of growth factors.

RESULTS

All cocultures promoted the growth of hematopoietic progenitor cells at day 7 of coculture in the presence of added growth factors. Compared to fresh sorted cells, brain and heart EC monolayers significantly increased, lung and liver EC monolayers maintained, and kidney EC monolayer markedly decreased the number of colony-forming unit-spleen day-8 colonies in the 7-day cocultures. HSC competitive repopulating unit activity was maintained during the heart and liver EC 7-day cocultures but was lost in the kidney EC coculture in vitro.

CONCLUSION

These results demonstrate that some but not all primary EC isolated from nonhematopoietic organs support HSC function ex vivo.

Quantitative analysis demonstrates expansion of SCID-repopulating cells and increased engraftment capacity in human cord blood following ex vivo culture with human brain endothelial cells

Initial clinical trials examining the transplantation of ex vivo expanded cord blood (CB) cells have failed to demonstrate an impact on hematopoietic recovery compared with historical unmanipulated CB controls. In this study, we tested whether coculture with primary human brain endothelial cells (HUBECs) could increase the engraftment capacity and repopulating cell frequency within CB CD34+ cells. Quantitative analysis demonstrated that HUBEC coculture for 7 days supported a 19-fold greater number of CD34+ cells and 3.4-fold and 2.6-fold greater severe combined immunodeficient (SCID)-repopulating cell (SRC) frequencies than fresh CB CD34+ cells and liquid suspension-cultured cells. Mice transplanted with day-14 HUBEC-cultured cells showed 4.2-fold higher levels of human engraftment than mice transplanted with day-7 HUBEC-cultured cells, indicating that SRC enrichment continued to occur through day 14. Noncontact HUBEC cultures also maintained SRCs at levels comparable with contact HUBEC cultures, demonstrating that HUBEC-secreted soluble factors critically supported SRC self-renewal. Seeding efficiency studies demonstrated that HUBEC-cultured CB CD34+ cells engrafted nonobese diabetic/SCID marrow at significantly higher levels than either fresh CB CD34+ cells or liquid suspension-cultured CD34+ cells. These studies indicate that the application of HUBEC coculture or HUBEC-conditioned media can potentially improve upon current strategies for the clinical expansion of CB stem cells.

Murine bone marrow cells cultured ex vivo in the presence of multiple cytokine combinations lose radioprotective and long-term engraftment potential

The desire to improve engraftment following transplantation of limited numbers of hematopoietic stem cells (HSC) has spurred the investigation of ex vivo stem cell expansion techniques. While surrogate outcomes, such as an increase in SCID-repopulating cells, suggest successful stem cell expansion in some studies, it is not clear that such assays predict outcomes using a more clinically relevant approach (e.g., myeloablation). We have addressed this by testing three cytokine combinations for their ability to increase the radioprotective and long-term marrow reconstitution capacity of hematopoietic cells cultured ex vivo. Low numbers of light-density (LD) mouse bone marrow (BM) cells or their expanded product were injected into lethally irradiated (9 Gy) congenic recipients. Survival rates and percent donor engraftment were compared at 2, 5, and 7 months post-transplant. The three cytokine combinations used were: (i) kit-ligand (L), thrombopoietin (Tpo), Flt-3 L; (ii) cytokines in (i) plus interleukin-11 (IL-11); (iii) cytokines in (ii) plus IL-3. At 7 months post-transplant, LD cell doses of 10(4), 2-2.5 x 10(4), and 0.5-1.0 x 10(5) gave predictable survivals of 20-30%, 40-70%, and 100%, respectively. Mean percent donor engraftments were 54.9% (SEM 36%), 55.7% (SEM 36%), and 76.3% (SEM 21%), respectively. When cells expanded for 3 or 5-7 days with the various cytokine combinations were transplanted into different groups of mice, survival rates and percent donor engraftment were almost uniformly poorer than results obtained with unmanipulated cells, and cells expanded for 5-7 days led to poorer outcomes than cells expanded for 3 days. Overall, ex vivo expansion of LD BM cells with the cytokine combinations chosen failed to improve transplant outcomes in this model.

Ex vivo culture rescues hematopoietic stem cells with long-term repopulating capacity following harvest from lethally irradiated mice

OBJECTIVE

High-dose ionizing radiation can cause lethal myeloablation in exposed individuals. We examined whether ex vivo culture could rescue hematopoietic stem cells with repopulating capacity following harvest from lethally irradiated animals.

METHODS

We exposed B6.SJL mice to 1050 cGy, harvested their irradiated bone marrow (BM), and examined whether ex vivo culture of the irradiated BM mononuclear cells (MNC) with porcine microvascular endothelial cells (PMVEC) or cytokines alone could rescue hematopoietic cells with in vitro colony-forming activity, in vivo radioprotective capacity, and long-term repopulating potential.

RESULTS

PMVEC coculture supported the recovery of fourfold and 80-fold greater numbers of total cells and colony-forming cells (CFC) compared to cyokines alone following 1050 cGy irradiation. All control mice irradiated with 1050 cGy died by day 30, as did mice transplanted with 1050 cGy-irradiated BM MNC. In contrast, transplantation of 1050 cGy-irradiated/PMVEC-cultured BM was fully radioprotective in 12 of 16 recipient mice (75%) exposed to 1050 cGy. Six of the 12 CD45.2+ mice (50%) transplanted with 1050 cGy-irradiated/PMVEC-cultured cells showed long-term (>6 months) multilineage repopulation derived from irradiated donor CD45.1+ cells. Surprisingly, transplantation of identical doses of 1050 cGy-irradiated/cytokine-cultured BM was also radioprotective in 50% of irradiated recipient mice and 50% of these mice demonstrated donor-derived repopulation.

CONCLUSIONS

Fully functional BM stem and progenitor cells can be rescued following harvest from lethally irradiated animals via ex vivo culture with PMVEC or cytokines alone. This method can serve as a model for the rapid ex vivo rescue and transplantation of autologous BM progenitors in the treatment of victims of radiation injury.

Modification of hematopoietic stem cell fate by 5aza 2'deoxycytidine and trichostatin A

Efforts to change the fate of human hematopoietic stem cells (HSCs) and progenitor cells (HPCs) in vitro have met with limited success. We hypothesized that previously utilized in vitro conditions might result in silencing of genes required for the maintenance of primitive HSCs/HPCs. DNA methylation and histone deacetylation are components of an epigenetic program that regulates gene expression. Using pharmacologic agents in vitro that might possibly interfere with DNA methylation and histone deacetylation, we attempted to maintain and expand cells with phenotypic and functional characteristics of primitive HSCs/HPCs. Human marrow CD34(+) cells were exposed to a cytokine cocktail favoring differentiation in combination with 5aza 2'deoxycytidine (5azaD) and trichostatin A (TSA), resulting in a significant expansion of a subset of CD34(+) cells that possessed phenotypic properties as well as the proliferative potential characteristic of primitive HSCs/HPCs. In addition, 5azaD- and TSA-pretreated cells but not the CD34(+) cells exposed to cytokines alone retained the ability to repopulate immunodeficient mice. Our findings demonstrate that 5azaD and TSA can be used to alter the fate of primitive HSCs/HPCs during in vitro culture.

Primary endothelial cells isolated from the yolk sac and para-aortic splanchnopleura support the expansion of adult marrow stem cells in vitro

The embryonic origin and development of hematopoietic and endothelial cells is highly interdependent. We hypothesized that primary endothelial cells from murine yolk sac and para-aortic splanchnopleura (P-Sp) may possess the capacity to expand hematopoietic stem cells (HSCs) and progenitor cells ex vivo. Using Tie2-GFP transgenic mice in combination with fluorochrome-conjugated monoclonal antibodies to vascular endothelial growth factor receptor-2 (Flk1) and CD41, we have successfully isolated pure populations of primary endothelial cells from 9.5-days after coitus (dpc) yolk sac and P-Sp. Adult murine bone marrow Sca-1+c-Kit+lin- cells were cocultured with yolk sac or P-Sp Tie2-GFP+Flk-1+CD41- endothelial cell monolayers for 7 days and the total number of nonadherent cells increased 47- and 295-fold, respectively, and hematopoietic progenitor counts increased 9.4- and 11.4-fold, respectively. Both the yolk sac and P-Sp endothelial cell cocultures facilitated long-term (> 6 months) HSC competitive repopulating ability (2.8- to 9.8-fold increases, respectively). These data suggest that 9.5-dpc yolk sac- and P-Sp-derived primary Tie2-GFP+Flk-1+CD41- endothelial cells possess the capacity to expand adult bone marrow hematopoietic progenitor cell and HSC repopulating ability ex vivo.

Hematopoietic stem cells: can old cells learn new tricks?

Since the establishment of cell lines derived from human embryonic stem (ES) cells, it has been speculated that out of such "raw material," we could some day produce all sorts of replacement parts for the human body. Human pluripotent stem cells can be isolated from embryonic, fetal, or adult tissues. Enormous self-renewal capacity and developmental potential are the characteristics of ES cells. Somatic stem cells, especially those derived from hematopoietic tissues, have also been reported to exhibit developmental potential heretofore not considered possible. The initial evidences for the plasticity potential of somatic stem cells were so encouraging that the opponents of ES cell research used them as arguments for restricting ES cell research. In the past months, however, critical issues have been raised challenging the validity and the interpretation of the initial data. Whereas hematopoietic stem-cell therapy has been a clinical reality for almost 40 years, there is still a long way to go in basic research before novel therapy strategies with stem cells as replacement for other organ systems can be established. Given the present status, we should keep all options open for research in ES cells and adult stem cells to appreciate the complexity of their differentiation pathways and the relative merits of various types of stem cells for regenerative medicine.

Clinical application of hematopoietic progenitor cell expansion: current status and future prospects

In the past decade, we have witnessed significant advances in ex vivo hematopoietic stem cell culture expansion, progressing to the point where clinical trials are being designed and conducted. Preclinical milestone investigations provided data to enable expansion of portions of hematopoietic grafts in a clinical setting, indicating safety and feasibility of this approach. Data derived from current clinical trials indicate successful reconstitution of hematopoiesis after myeloablative chemoradiotherapy using infusion of ex vivo-expanded perfusion cultures. Future avenues of exploration will focus upon refining preclinical and clinical studies in which cocktails of available cytokines, novel molecules and sophisticated expansion systems will explore expansion of blood, marrow and umbilical cord blood cells.

Mesenchymal stem cells home to injured tissues when co-infused with hematopoietic cells to treat a radiation-induced multi-organ failure syndrome

BACKGROUND

Recent studies have suggested that ex vivo expansion of autologous hematopoietic cells could be a therapy of choice for the treatment of bone marrow failure. We investigated the potential of a combined infusion of autologous ex vivo expanded hematopoietic cells with mesenchymal (MSCs) for the treatment of multi-organ failure syndrome following irradiation in a non-human primate model.

METHODS

Hematopoietic cells and MSCs were expanded from bone marrow aspirates. MSCs were transduced with the gene encoding for the green fluorescent protein (e-GFP), in order to track them following infusion. Twelve animals were studied. Nine animals received total-body irradiation at 8 Gy from a neutron/gamma source thus resulting in heterogeneous exposure; three animals were sham-irradiated. The animals were treated with expanded hematopoietic stem cells and MSCs, expanded hematopoietic stem cells alone, or MSCs alone. Unmanipulated bone marrow cell transplants were used as controls.

RESULTS

Depending on the neutron/gamma ratio, an acute radiation sickness of varying severity but of similar nature resulted. GFP-labeled cells were found in the injured muscle, skin, bone marrow and gut of the treated animals via PCR up to 82 days post-infusion.

CONCLUSIONS

This is the first evidence of expanded MSCs homing in numerous tissues following a severe multi-organ injury in primates. Localization of the transduced MSCs correlated to the severity and geometry of irradiation. A repair process was observed in various tissues. The plasticity potential of the MSCs and their contribution to the repair process in vivo remains to be studied.

Ex vivo culture with human brain endothelial cells increases the SCID-repopulating capacity of adult human bone marrow

Adult human bone marrow (ABM) is an important source of hematopoietic stem cells for transplantation in the treatment of malignant and nonmalignant diseases. However, in contrast to the recent progress that has been achieved with umbilical cord blood, methods to expand ABM stem cells for therapeutic applications have been disappointing. In this study, we describe a novel culture method that uses human brain endothelial cells (HUBECs) and that supports the quantitative expansion of the most primitive measurable cell within the adult bone marrow compartment, the nonobese diabetic/severe combined immunodeficient (NOD/SCID) repopulating cell (SRC). Coculture of human ABM CD34(+) cells with brain endothelial cells for 7 days supported a 5.4-fold increase in CD34(+) cells, induced more than 95% of the CD34(+)CD38(-) subset to enter cell division, and produced progeny that engrafted NOD/SCID mice at significantly higher rates than fresh ABM CD34(+) cells. Using a limiting dilution analysis, we found the frequency of SRCs within fresh ABM CD34(+) cells to be 1 in 9.9 x 10(5) cells. Following HUBEC culture, the estimated frequency of SRCs increased to 1 in 2.4 x 10(5) cells. All mice that received transplants of HUBEC-cultured cells showed B-lymphoid and myeloid differentiation, indicating that a primitive hematopoietic cell was preserved during culture. Noncontact HUBEC cultures also maintained SRCs at a level comparable to contact HUBEC cultures, suggesting that cell-to-cell contact was not required. These data demonstrate that human brain endothelial cells possess a unique hematopoietic activity that increases the repopulating capacity of adult human bone marrow.

Clinical and experimental uses of umbilical cord blood

Umbilical cord blood (UCB) has been used successfully as an alternative source of haemopoietic stem cells (HSC) in allogeneic stem-cell transplantation for the treatment of acquired and genetic diseases. Advantages of using UCB include: (i) no risk to the donor, (ii) no donor attrition, (iii) minimal risk of viral transmission and (iv) immediate availability. Early results have highlighted differences in engraftment rates and toxicity between UCB and other sources of HSC. These differences relate to the low cell dose in UCB and also to the intrinsic properties of UCB. In this article, the clinical outcome of UCB transplantation (UCBT) will be reviewed with a discussion of the biological characteristics of UCB that may account for some of the clinical outcomes. To overcome the limitations of low cell dose, novel approaches such as ex vivo expansion of HSC are being actively explored, and this will be summarized in the present study. Finally, the success of UCBT has led to the establishment of dedicated UCB banks worldwide and the regulatory issues surrounding this will be briefly discussed.

Ex vivo expansion of human UC blood primitive hematopoietic progenitors and transplantable stem cells using human primary BM stromal cells and human AB serum

BACKGROUND

In vitro maintenance and expansion of human hematopoietic stem cells is crucial for many clinical applications, and investigators have been using xenogeneic, especially murine, stromal cells for stem-cell expansion. In addition, many such culture systems utilize FCS-containing medium or serum-free medium that contains human- or animal-derived proteins. However, the possible transmission of infectious diseases has led to a debate about the safety of the delivery of grafts expanded in culture using cells and proteins of allogeneic or xenogeneic origin. Using primary human BM stromal cells, we have established an AB serum-based co-culture system to expand human primitive progenitors and transplantable stem cells.

METHODS

Cord blood CD34+ cells were cultured on a monolayer of human BM-derived primary stromal cells with thrombopoietin (TPO), stem-cell factor (SCF) and flt3/flk2 ligand (FL) in the presence of either FCS or AB serum. One to three weeks later, cells were examined for total cells, CD34+ cells, CD34+ CD38- cells, and clonogenic progenitors. SCID mouse reconstituting cell (SRC) activity was also studied.

RESULTS

Three weeks of culture with TPO, SCF, and FL supported more than a 250-fold expansion of CD34+ cells, CD34+ CD38- cells and CFU-C, regardless of the kind of serum used. SRC assay revealed that transplantable stem cells were moderately expanded as well.

DISCUSSION

This ex vivo expansion system should prove valuable in clinical settings in which stromal cells and serum are available from recipients or stem-cell donors.

Primate skeletal muscle contains cells capable of sustaining in vitro hematopoiesis

OBJECTIVE

Several investigators recently reported that adult murine skeletal muscle cells possess a remarkable capacity to differentiate into hematopoietic cells. We further examined this biologic process by studying the phenotype and in vitro functional behavior of primate skeletal muscle cells.

MATERIALS AND METHODS

Muscles from human abortuses as well as fetal and adult baboons were digested enzymatically and mononuclear cell fractions were isolated. Muscle tissue-derived mononuclear cells (mu-TDMNC) were phenotypically characterized. Both short-term and long-term hematopoietic progenitors were assayed from mu-TDMNC using standard techniques. Gene expression patterns characteristic of hematopoietic and endothelial cells were examined in primary and cultured muscle cells.

RESULTS

Primate muscle cells were shown to express the CD34 antigen. Such CD34(+) cells were shown to be CD45(-) and desmin(+), indicating they were not of hematopoietic origin. Fetal but not adult muscle cells contained assayable hematopoietic progenitors. In addition, muscles contained an additional class of progenitors that formed colonies composed of blast cells after prolonged incubation (3-4 weeks). A two-step culture system was established that permitted muscle cells to continue to proliferate when exposed to a hematopoietic environment for 8 months. During this prolonged period of time, the generation of CD34(+), CD56(+), CD11b(+), and CD31(+) as well as von Willebrand factor (vWF)(+) cells were observed.

CONCLUSIONS

Our studies indicate that although primate muscle cells contain a significant number of CD34(+) cells, they are likely not of hematopoietic origin. Important ontogenic differences in the hematopoietic potential of primate muscle cells were documented. When exposed to appropriate microenvironmental stimuli, mu-TDMNC displayed an extensive proliferative capacity and contained primitive progenitors with the capacity to generate cells in vitro with phenotypic and genetic properties of hematopoietic and endothelial cells for sustained periods of time. Whether this observation can be accounted for by true transdifferentiation of muscle cells or proliferation of reservoirs of hematopoietic and endothelial progenitor cells residing within skeletal muscle remains unresolved.

Bone marrow stromal cells prepared using AB serum and bFGF for hematopoietic stem cells expansion

BACKGROUND

An ex vivo culture system was previously established for stem cell expansion using human marrow stromal cells and serum-free medium. However, the stromal cells were prepared using long-term culture medium containing horse serum and FCS, which may transmit infectious diseases of xenogeneic origin. In this study, therefore, a method was established to prepare stromal cells using an AB serum-based medium. In the case that serum from a transplant recipient or PBPC donor is available, additional infectious diseases would not be transmitted.

STUDY DESIGN AND METHODS

Cord blood CD34+ cells were cultured with thrombopoietin, stem cell factor, and flt3/flk2 ligand on a monolayer of human marrow primary stromal cells prepared using long-term culture medium or AB serum-based medium. After 2 weeks, clonogenic progenitor activity and SCID mouse-reconstituting cell activity were assayed. mRNA expression of cytokines and Notch ligand by stromal cells was also examined.

RESULTS

There were no remarkable differences in expansion-supporting activity and mRNA expression between stromal cells established by the two methods.

CONCLUSION

An ex vivo expansion system completely based on AB serum has been established.

Cultivation of hematopoietic stem and progenitor cells: biochemical engineering aspects

The ex vivo expansion of hematopoietic cells is one of the most challenging fields in cell culture. This is a rapidly growing area of tissue engineering with many potential applications in bone marrow transplantation, transfusion medicine or gene therapy. Over the last few years much progress has been made in understanding hematopoietic differentiation, discovery of cytokines, isolation and identification of cellular subtypes and in the development of a variety of bioreactor concepts. All this has led to a number of (preliminary) clinical trials that gave a hint of the benefits that can be obtained from the use of expanded hematopoietic cells in therapy. Moreover, as we understand the complexity and the regulation of hematopoiesis, it becomes obvious that highly sophisticated cultivation techniques and bioreactor concepts are needed: a new challenge for bioprocess engineering in cell culture.

Reinjection of ex vivo-expanded primate bone marrow mononuclear cells strongly reduces radiation-induced aplasia

To assess the therapeutic efficacy of ex vivo-expanded hematopoietic cells in the treatment of radiation-induced pancytopenia, we have set up a non-human primate model. Two ex vivo expansion protocols for bone marrow mononuclear cells (BMMNC) were studied. The first consisted of a 7-day culture in the presence of stem cell factor (SCF), Flt3-ligand, thrombopoietin (TPO), interleukin-3 (IL-3), and IL-6, which induced preferentially the expansion of immature hematopoietic cells [3.1 +/- 1.4, 10.0 +/- 5.1, 2.2 +/- 1.9, and 1.0 +/- 0.3-fold expansion for mononuclear cells (MNC), colony-forming units-granulocyte-macrophage (CFU-GM), burst-forming units erythroid (BFU-E), and long-term culture initiating cells (LTC-IC) respectively]. The second was with the same cytokine combination supplemented with granulocyte colony-stimulating factor (G-CSF) with an increased duration of culture up to 14 days and induced mainly the production of mature hematopoietic cells (17.2 +/- 11.7-fold expansion for MNC and no detectable BFU-E and LTC-IC), although expansion of CFU-GM (13.7 +/- 18.8-fold) and CD34+ cells (5.2 +/- 1.4-fold) was also observed. Results showed the presence of mesenchymal stem cells and cells from the lymphoid and the megakaryocytic lineages in 7-day expanded BMMNC. To test the ability of ex vivo-expanded cells to sustain hematopoietic recovery after radiation-induced aplasia, non-human primates were irradiated at a supralethal dose of 8 Gy and received the product of either 7-day (24 h after irradiation) or 14-day (8 days after irradiation) expanded BMMNC. Results showed that the 7-day ex vivo-expanded BMMNC shortened the period and the severity of pancytopenia and improved hematopoietic recovery, while the 14 day ex vivo-expanded BMMNC mainly produced a transfusion-like effect during 8 days, followed by hematopoietic recovery. These results suggest that ex vivo expanded BMMNC during 7 days may be highly efficient in the treatment of radiation-induced aplasia.

Ex vivo expansion of stem and progenitor cells in co-culture of mobilized peripheral blood CD34+ cells on human endothelium transfected with adenovectors expressing thrombopoietin, c-kit ligand, and Flt-3 ligand

To optimize conditions for ex vivo expansion of adult hematopoietic stem cells, we evaluated the co-culture of G-CSF mobilized human peripheral blood (PB) CD34(+) cells with endothelial cells engineered to overexpress various hematopoietic growth factors. Immortalized human bone marrow endothelial cells (BMEC) transfected with an expression vector carrying cDNA encoding the human telomerase reverse transcriptase (hTERT) and human umbilical vein endothelial cells (HUVEC) were transfected with combinations of adenovectors expressing murine c-kit ligand (KL), human thrombopoietin (TPO), human Flt3 ligand (FL), and human granulocyte-macrophage colony-stimulating factor (GM-CSF). Ex vivo expansion of PB CD34(+) cells from normal donors and non-Hodgkin lymphoma (NHL) patients in endothelial co-culture was evaluated weekly for total cell production, progenitor (CFU-GM, BFU-E) cell production, and stem cell production as measured by Week-5 Cobblestone Area Forming Cell assay (Wk-5 CAFC). HUVEC transfected with adenovectors expressing TPO, KL, and FL provided the best co-culture system for expanding CD34(+) cells. Maximal total nuclear cell, CFU-GM, and Wk-5 CAFC production occurred between weeks 2 and 3 with 113-fold, 25-fold, and 2.2-5.5-fold expansions, respectively. We did not detect significant differences when GM-CSF was added to the co-culture system. Expansion was also obtained using recombinant human cytokines, but was not maintained beyond 3 weeks. We demonstrated that continuous generation of high levels of TPO, FL, and KL as well as other factors secreted by endothelium provided a clinically relevant co-culture method for ex vivo expansion of stem and progenitor cells from cryopreserved CD34(+) populations.

Cell cycle activation of peripheral blood stem and progenitor cells expanded ex vivo with SCF, FLT-3 ligand, TPO, and IL-3 results in accelerated granulocyte recovery in a baboon model of autologous transplantation but G0/G1 and S/G2/M graft cell content does not correlate with transplantability

Ex vivo expansion is a new strategy for hematopoietic stem and progenitor cell transplantation based on cytokine-induced amplification to produce grafts of controlled maturity. If the cell cycle position of CD34(+) cells has been reported to govern their engraftment potential, the respective role of stem and progenitor cells in short- and long-term hematopoietic recovery remains debated. Studies focused on long-term engraftment potential suggest impairment when using cultured grafts, but the capacity to sustain short-term recovery is still controverted. The aim of this study was: A) to evaluate the consequences of cell cycle activation on short and long-term engraftment capacity, and B) to determine if cell cycle status of grafts could predict hematopoietic recovery. We showed in a nonhuman primate model of autologous peripheral blood stem and progenitor cell transplantation that cell cycle activation of CD34(+) cells in the presence of stem cell factor + FLT3-ligand + thrombopoietin + interleukin 3 (six days of culture) which induced G1 and S/G2/M cell amplification (G0: 6.1% +/- 2.8%; G0/G1: 64.2% +/- 7.2%; S/G2/M: 30.4% +/- 7.3% respectively of expanded CD34(+) cells on average) resulted in the acceleration of short-term granulocyte recovery. By contrast, G0/G1 and S/G2/M cell content of expanded grafts did not correlate with short- or long-term engraftment.

A novel human packaging cell line with hematopoietic supportive capacity increases gene transfer into early hematopoietic progenitors

The hematopoietic stem/progenitor cell (HSPC) represents the ideal target for gene therapy of disorders of the hematopoietic system, but still faces problems related to ex vivo manipulation and gene transfer efficiency. We demonstrate that soluble factors from the human endothelial-like cell line ECV 304/T24 support the growth of human CD34(+) progenitor cells as primary human bone marrow stroma and increase the rate of gene transfer into progenitor cells up to 5-fold. ECV 304/T24 was used to generate split-function amphotropic packaging cell lines (named APEX) with the purpose of combining, in the same cells, hematopoietic support and gene transfer vehicle functions. The APEX cell lines were negative for the presence of replication-competent retroviruses and produced complement-resistant vector particles. When mobilized peripheral blood or umbilical cord blood CD34(+) cells were exposed once to APEX supernatants, the level of gene transfer was equivalent to that observed with GP + Am12, in spite of the lower titer of the APEX producers. More importantly, APEX supernatants gave rise reproducibly to a 2-fold increase in transduction of early progenitors (long-term culture-initiating cells), reaching on average 50% gene transfer. This novel packaging cell represents a significant advance in HSPC genetic modification technology, combining both a beneficial hematopoietic supportive effect and the gene transfer vector function in a human-based system.

A novel nuclear protein, 5qNCA (LOC51780) is a candidate for the myeloid leukemia tumor suppressor gene on chromosome 5 band q31

Interstitial deletion or loss of chromosome 5, del(5q) or -5, is a frequent finding in myeloid leukemias and myelodysplasias, suggesting the presence of a tumor suppressor gene within the deleted region. In our search for this gene, we identified a candidate, 5qNCA (LOC51780), which lies within a consistently-deleted segment of 5q31. 5qNCA expresses a 7.2-kb transcript with a 5286-bp open reading frame which is present at high levels in heart, skeletal muscle, kidney, placenta, and liver as well as CD34+ cells and AML cell lines. 5qNCA encodes a 191-kD nuclear protein which contains a highly-conserved C-terminus containing a zinc finger with the unique spacing Cys-X2-Cys-X7-His-X2-Cys-X2-Cys-X4-Cys-X2-Cys and a jmjC domain, which is often found in proteins that regulate chromatin remodeling. Expression of 5qNCA in a del(5q) cell line results in suppression of clonogenic growth. Preliminary sequence results in AML and MDS samples and cell lines has revealed a possible mutation in the KG-1 cell line resulting in a THR to ALA substitution that has not been found in over 100 normal alleles to date. We propose 5qNCA is a good candidate for the del(5q) tumor suppressor gene based on its predicted function and growth suppressive activities, and suggest that further mutational and functional study of this interesting gene is warranted.

Identification of a candidate human neurohematopoietic stem-cell population

It was recently reported that transplantation of clonally derived murine neurosphere cells into sublethally irradiated allogeneic hosts leads to a donor-derived hematopoietic reconstitution. The confirmation of the existence of a common neurohematopoietic stem cell in the human brain will have a significant effect on stem cell research and on clinical transplantation. Here, it is demonstrated that the human fetal brain contains separate but overlapping epidermal growth factor (EGF)-responsive and basic fibroblast growth factor (FGF-2)-responsive neural stem cells. The majority (> 85%) of cells within these EGF- and/or FGF-2-generated neurospheres express characteristic neural stem/progenitor cell markers including nestin, EGF receptor, and FGF-2 receptor. These neural stem cells can be continuously passaged in vitro, and demonstrate a constant 20-fold expansion in every passage for up to the fifth passage (the longest period that has been carried out in the authors' laboratory). These neural stem cells are multipotential for neurons, astrocytes, and oligodendrocytes. After transplantation into SCID-hu mice, all neural stem cells, regardless of passages, culture conditions, and donors, are able to establish long-term hematopoietic reconstitution in the presence of an intact human bone marrow microenvironment.