Purification of hematopoietic stem cells for further biological study

Guidelines for the use of flow cytometry and cell sorting in immunological studies (second edition)

These guidelines are a consensus work of a considerable number of members of the immunology and flow cytometry community. They provide the theory and key practical aspects of flow cytometry enabling immunologists to avoid the common errors that often undermine immunological data. Notably, there are comprehensive sections of all major immune cell types with helpful Tables detailing phenotypes in murine and human cells. The latest flow cytometry techniques and applications are also described, featuring examples of the data that can be generated and, importantly, how the data can be analysed. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid, all written and peer-reviewed by leading experts in the field, making this an essential research companion.

Multiplexed quantification of proteins and transcripts in single cells

We present a tool to measure gene and protein expression levels in single cells with DNA-labeled antibodies and droplet microfluidics. Using the RNA expression and protein sequencing assay (REAP-seq), we quantified proteins with 82 barcoded antibodies and >20,000 genes in a single workflow. We used REAP-seq to assess the costimulatory effects of a CD27 agonist on human CD8⁺ lymphocytes and to identify and characterize an unknown cell type.

Current Strategies and Challenges for Purification of Cardiomyocytes Derived from Human Pluripotent Stem Cells

Cardiomyocytes (CMs) derived from human pluripotent stem cells (hPSCs) are considered a most promising option for cell-based cardiac repair. Hence, various protocols have been developed for differentiating hPSCs into CMs. Despite remarkable improvement in the generation of hPSC-CMs, without purification, these protocols can only generate mixed cell populations including undifferentiated hPSCs or non-CMs, which may elicit adverse outcomes. Therefore, one of the major challenges for clinical use of hPSC-CMs is the development of efficient isolation techniques that allow enrichment of hPSC-CMs. In this review, we will discuss diverse strategies that have been developed to enrich hPSC-CMs. We will describe major characteristics of individual hPSC-CM purification methods including their scientific principles, advantages, limitations, and needed improvements. Development of a comprehensive system which can enrich hPSC-CMs will be ultimately useful for cell therapy for diseased hearts, human cardiac disease modeling, cardiac toxicity screening, and cardiac tissue engineering.

Core-shell silver nanoparticles for optical labeling of cells

Silver nanoparticles have been modified with self-assembled monolayers of hydroxyl-terminated long chain thiols and encapsulated with a silica shell. The resulting core-shell nanoparticles were used as optical labels for cell analysis using flow cytometry and microscopy. The excitation of plasmon resonances in nanoparticles results in strong depolarized scattering of visible light, permitting detection at the single nanoparticle level. The nanoparticles were modified with neutravidin via epoxide-azide coupling chemistry, to which biotinylated antibodies targeting cell surface receptors were bound. The nanoparticle labels exhibited long-term stability in solutions with high salt concentrations without aggregation or silver etching. Labeled cells exhibited two orders of magnitude enhancement of the scattering intensity compared with unlabeled cells.

Noninvasive sorting of stem cell aggregates based on intrinsic markers

Noninvasive biomarkers hold important potential for the characterization and purification of stem cells because the addition of exogenous labels, probes, or reporters, as well as the disruption of cell-cell and cell-extracellular matrix interactions, can unintentionally but dramatically alter stem cell state. We recently showed that intensity of the intrinsically fluorescent metabolite, nicotinamide adenine dinucleotide (NADH), fluctuates predictably with changes in stem cell viability and differentiation state. Here, we use multiphoton flow cytometry developed in our laboratory to rapidly and noninvasively characterize and purify populations of intact stem cell aggregates based on NADH intensity and assessed the differentiation capacity of sorted populations. We found removal of aggregates with NADH intensity indicative of cell death resulted in a remaining population of aggregates significantly more likely to produce beating cardiomyocytes (26% vs. 8%, P < 0.05). Similarly, we found isolation of stem cell aggregates with NADH intensity indicative of future cardiac differentiation gave rise to more aggregates with beating cardiomyocytes at later time points (50% vs. 28%, P < 0.05). Further, coupling NADH intensity with gating based on size, enhances the enrichment for EBs capable of giving rise to cardiomyocytes (59% vs. 27%, P < 0.05). Thus, we demonstrate that endogenous properties of cell aggregates, such as NADH and size, can serve as gating parameters for large particle sorting devices to purify populations of stem cells or their progeny in a noninvasive manner, leading the way for improved therapeutic applications.

Measurement of cell-penetrating peptide-mediated transduction of adult hematopoietic stem cells

The ability of cell-penetrating peptides (CPPs) to cross cell membranes and transport cargo into cells makes them an attractive tool for the molecular engineering of stem cells. Even though the exact mechanism of transduction remains unclear, their potential has been demonstrated for diverse applications, including hematopoietic stem cell expansion and the generation of islets cells from embryonic stem cells. Several parameters can affect the intracellular delivery of CPP-based constructs. Those include the type of cells targeted, the type of CPP used, and the properties of the cargo. For this reason, it is important to have a means to quantitatively assess the transduction efficiency of specific constructs in the cell type of interest in order to select the best vector for a specific application. In this chapter, we describe a method to measure the uptake of HIV transactivator of transcription (TAT) and the homeobox protein Antennapedia (Antp) constructs in primary hematopoietic progenitor cells and hematopoietic cell lines. This method is useful to compare, select, and optimize different strategies to deliver CPP-based constructs into a given cell type.

Human hematopoietic cell culture, transduction, and analyses

This unit provides methods for introducing genes into human hematopoietic progenitor cells. The Basic Protocol describes isolation of CD34(+) cells, transduction of these cells with a retroviral vector on fibronectin-coated plates, assaying the efficiency of transduction, and establishing long-term cultures. Support protocols describe methods for maintenance of vector-producing fibroblasts (VPF) and supernatant collection from these cells, screening medium components for the ability to support hematopoietic cell growth, and establishing colonies from long-term cultures. Other protocols provide PCR-based methods to analyze individual colonies for transduction, methods to analyze cells harvested from long-term cultures, and procedures for freezing and thawing of hematopoietic cells.

Blood progenitor cell separation from clinical leukapheresis product by magnetic nanoparticle binding and magnetophoresis

Positive selection of CD34+ blood progenitor cells from circulation has been reported to improve patient recovery in applications of autologous transplantation. Current magnetic separation methods rely on cell capture and release on solid supports rather than sorting from flowing suspensions, which limits the range of therapeutic applications and the process scale up. We tested CD34+ cell immunomagnetic labeling and isolation from fresh leukocyte fraction of peripheral blood (leukapheresis) using the continuous quadrupole magnetic flow sorter (QMS), consisting of a flow channel (SHOT, Greenville, IN) and a quadrupole magnet with a maximum field intensity (B(o)) of 1.42 T and a mean force field strength (S(m)) of 1.45 x 10(8) TA/m(2). Both the sample magnetophoretic mobility (m) and the inlet and outlet flow patterns highly affect the QMS performance. Seven commercial progenitor cell labeling reagent combinations were quantitatively evaluated by measuring magnetophoretic mobility of a high CD34 expression cell line, KG-1a, using the cell tracking velocimeter (CTV). The CD34 Progenitor Cell Isolation Kit (Miltenyi Biotec, Bergisch Gladbach, Germany) showed the strongest labeling of KG-1a cells and was selected for progenitor cell enrichment from 11 fresh and 11 cryopreserved clinical leukapheresis samples derived from different donors. The CD34+ cells were isolated with a purity of 60-96%, a recovery of 18-60%, an enrichment rate of 12-169, and a throughput of (1.7-9.3) x 10(4) cells/s. The results also showed a highly regular dependence of the QMS performance on the flow conditions that agreed with the theoretical predictions based on the CD34+ cell magnetophoretic mobility.

Negative selection of hematopoietic progenitor cells by continuous magnetophoresis

OBJECTIVE

To evaluate a negative selection technique for the hematopoietic progenitor cell enrichment from clinical leukapheresis product using continuous magnetophoresis.

METHODS

The leukapheresis product was labeled with a tetrameric antibody cocktail (TAC) and magnetic colloid against nonprogenitor leukocytes (StemSep enrichment cocktail kit, Stem Cell Technologies, Vancouver, Canada). The separation of hematopoietic progenitor cells was performed by flow-through magnetophoresis in an annular channel placed coaxially inside a quadrupole magnetic field, in a split-flow thin-cell fractionation configuration (referred to as quadrupole magnetic flow sorter, QMS). The TAC antibody cocktail and the magnetic colloid were titrated to determine minimum effective antibody and magnetic reagent concentrations by measuring cell magnetophoretic mobility (m) distribution using cell tracking velocimetry.

RESULTS

Leukapheresis products from eight donors having initial CD34+ cell purity between 0.37 and 9.7% were enriched to the final purity of 30 to 85% and yield of 49 to 84% with a maximum throughput of 6.7 x 10(4) cells/s. The progenitor cell enrichment was accompanied by a more than 3.5 log(10) T-lymphocyte depletion, a significant factor considering the intended application to allogeneic transplantation. Cell colony-forming unit assays showed that there was no deterioration of progenitor cell proliferation and differentiation following the QMS enrichment process.

CONCLUSIONS

The negative selection method of hematopoietic progenitor cells by continuous magnetophoresis is a promising approach to a process scale-up, important for clinical applications.

Flow cytometric screening of cell-penetrating peptides for their uptake into embryonic and adult stem cells

There is an increasing appreciation of the potential of cell-penetrating peptides (CPPs) as vectors to deliver peptides, proteins, and DNA into cells. However, the absolute and relative efficacy of various CPPs for applications targeting stem cells and primary cells is unclear. In this study, we have developed a two-step loading method and a flow cytometric assay to systematically compare the cellular uptake of five CPPs into embryonic stem cells, neurospheres (NSs), primary bone marrow hematopoietic progenitor (Sca-1(+)Lin(-)) cells, and hematopoietic cell lines (TF-1, K562, and FDCP Mix). The series of CPPs tested included three arginine-rich peptides; one was derived from HIV transactivator of transcription (TAT), one was derived from Antennapedia (Antp), and the third was a synthetic peptide known as protein transduction domain 4 (PTD4). Two hydrophobic peptides were also tested; one was derived from Kaposi fibroblast growth factor (K-FGF), and one was derived from PreS2 surface antigen of hepatitis B virus (PreS2-TLM). Our results indicate, for the first time, that arginine-rich CPPs can internalize into primary NSs and bone marrow Sca-1(+)Lin(-) cells. In addition, in all cell types examined, the uptake of arginine-rich CPPs is significantly greater than that of hydrophobic peptides.

Intermittent exposure of primitive quiescent chronic myeloid leukemia cells to granulocyte-colony stimulating factor in vitro promotes their elimination by imatinib mesylate

PURPOSE

Primitive quiescent chronic myeloid leukemia (CML) cells are biologically resistant to imatinib mesylate, an inhibitor of the p210(BCR-ABL) kinase. The present study was designed to investigate whether either continuous or intermittent exposure of these cells to granulocyte-colony stimulating factor (G-CSF) in vitro can overcome this limitation to the effectiveness of imatinib mesylate therapy.

EXPERIMENTAL DESIGN

CD34(+) leukemic cells were isolated from six newly diagnosed chronic phase CML patients and cultured for 12 days in serum-free medium with or without G-CSF and/or imatinib mesylate present either continuously or intermittently (three cycles of G-CSF for 0, 1, or 4 days +/- imatinib mesylate for 0, 3, or 4 days). Every 4 days, the number of residual undivided viable cells and the total number of viable cells present were measured.

RESULTS

Intermittent but not continuous exposure to G-CSF significantly accelerated the disappearance in vitro of initially quiescent CD34(+) CML cells. This resulted in 3- and 5-fold fewer of these cells remaining after 8 and 12 days, respectively, relative to continuous imatinib mesylate alone (P < 0.04). Cultures containing imatinib mesylate and intermittently added G-CSF also showed the greatest reduction in the total number of cells present after 12 days (5-fold more than imatinib mesylate alone).

CONCLUSION

Intermittent exposure to G-CSF can enhance the effect of imatinib mesylate on CML cells by specifically targeting the primitive quiescent leukemic elements. A protocol for treating chronic-phase CML patients with imatinib mesylate that incorporates intermittent G-CSF exposure may offer a novel strategy for obtaining improved responses in vivo.

Recent advances in hematopoietic stem cell biology

PURPOSE OF REVIEW

Exciting advances have been made in the field of hematopoietic stem cell biology during the past year. This review summarizes recent progress in the identification, culture, and in vivo tracking of hematopoietic stem cells.

RECENT FINDINGS

The roles of Wnt and Notch proteins in regulating stem cell renewal in the microenvironment, and how these molecules can be exploited in ex vivo stem cell culture, are reviewed. The importance of identification of stem cells using functional as well as phenotypic markers is discussed. The novel field of nanotechnology is then discussed in the context of stem cell tracking in vivo. This review concludes with a section on the unexpected potential of bone marrow-derived stem cells to contribute to the repair of damaged tissues. The contribution of cell fusion to explain the latter phenomenon is discussed.

SUMMARY

Because of exciting discoveries made recently in the field of stem cell biology, researchers now have improved tools to define novel populations of stem cells, examine them ex vivo using conditions that promote self-renewal, track them into recipients, and determine whether they can contribute to the repair of damaged tissues. These discoveries will significantly advance the field of stem cell transplantation.

Identification and isolation of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are defined by their ability to repopulate all of the hematopoietic lineages in vivo and sustain the production of these cells for the life span of the individual. In the absence of reliable direct markers for HSCs, their identification and enumeration depends on functional long-term, multilineage, in vivo repopulation assays. The extremely low frequency of HSCs in any tissue and the absence of a specific HSC phenotype have made their purification and characterization a highly challenging goal. HSCs and primitive hematopoietic cells can be distinguished from mature blood cells by their lack of lineage-specific markers and presence of certain other cell-surface antigens, such as CD133 (for human cells) and c-kit and Sca-1 (for murine cells). Functional analyses of purified subpopulations of primitive hematopoietic cells have led to the development of several procedures for isolating cell populations that are highly enriched in cells with in vivo stem cell activity. Simplified methods for obtaining these cells at high yield have been important to the practical exploitation of such advances. This article reviews recent progress in identifying human and mouse HSCs and current techniques for their purification.

Biology of human hematopoietic stem and progenitor cells present in circulation

Circulating hematopoietic stem and progenitor cells play important roles in the physiology and homeostasis of the hematopoietic system. The frequency of these cells varies throughout development, being more abundant during gestation. In the adult, the numbers of such cells are extremely low; however, they can be increased by intravenous administration of chemotherapy and/or recombinant cytokines to individuals. This mechanism--known as mobilization--involves the disruption of the interactions between primitive hematopoietic cells and microenvironment elements (stromal cells and extracellular matrix molecules), which are mediated by a group of molecules known as cell adhesion molecules. During the last two decades, circulating cells of newborns (those present in umbilical cord blood) and adults (mobilized peripheral blood) have gained relevance not only because of their biology, but also because of their clinical application. Indeed, at present the number of mobilized peripheral blood-derived hematopoietic cell transplants performed worldwide is clearly superior to the number of bone marrow transplants being done annually. On the other hand, the number of cord blood transplants has significantly increased during the last few years, and cord blood banking has expanded in a significant manner over the last decade. Circulating stem and progenitor cells are being manipulated ex vivo, both in cellular and molecular terms, and the clinical use of these manipulated cells is just beginning. Undoubtedly, hematopoietic cells present in circulation will play a key role in the development of both gene and cellular therapies for a variety of diseases.

Severe functional alterations in vitro in CD34(+) cell subpopulations from patients with chronic myeloid leukemia

Chronic myeloid leukemia (CML) arises from the malignant transformation of a hematopoietic stem cell (HSC) that gives rise to functionally defective progeny, including primitive and relatively mature progenitor cells (HPC). Both HSC and HPC are comprised within the population of CD34(+) cells, normally present in bone marrow (BM). In the present study, we have separated two different subpopulations of CD34(+) cells from CML marrow: Population I, enriched for CD34(+) Lin(-) cells; and Population II, enriched for CD34(+) CD36(-) CD38(-) CD45RA(-) Lin(-) cells, and assessed their progenitor cell content as well as their capacity to proliferate and expand in response to a combination of hematopoietic cytokines in serum- and stroma-free long-term liquid cultures. The absolute cell numbers recovered in Population I from normal and CML samples were similar; in contrast, we found that Population II from CML was amplified four-fold, as compared to normal. In spite of this latter observation, no significant differences were observed in terms of the absolute number of CFC when comparing Populations I and II from CML patients and normal subjects. Interestingly, the proliferation and expansion potentials of CML cells were clearly deficient as compared to their normal counterparts. Indeed, in cultures of Population I cells the maximum fold increase in total and progenitor cell numbers corresponded to 30 and 8%, respectively, of those observed in cultures of normal marrow-derived Population I cells. Such functional deficiencies were even more evident in Population II cells in which the maximum fold increase in total and progenitor cell numbers corresponded to 3 and 0.5%, respectively, of the levels found in cultures of Population II cells from normal marrow. The present study demonstrates that bone marrow-derived CD34(+) cells from CML patients possess functional abnormalities, clearly evident in the in vitro system used by us. Among the two CML subpopulations studied here, the more immature one (Population II; enriched for CD34(+) CD36(-) CD38(-) CD45RA(-) Lin(-) cells) was the one that showed the most severe abnormalities, as compared to its relatively more mature counterpart (Population I; enriched for CD34(+) Lin(-) cells).

The role of stem cells for treatment of cardiovascular disease

Cardiovascular disease is a global cause of mortality and morbidity. Current treatments fail to address the underlying scarring and cell loss, which are the causes of ischaemic heart failure. Cellular transplantation can overcome these problems and new impetus has been injected into this field following the isolation of human embryonic and adult stem cells. These cells have shown remarkable ability to produce cardiomyocytes and vascular cells in vitro and in vivo. Initial transplantation studies have demonstrated functional benefits and it is hoped further randomised clinical trials will concur with initial findings. Much basic science remains to be unearthed, such as the signals for homing, differentiation and engraftment of transplanted cells. Further matters of concern are the role of cell fusion and the mechanisms by which transplanted cells improve cardiac function. In spite of initial progress made in stem cell therapy there is still much to be done and we are some way off from achieving the goal of effective cellular regeneration.

[Current concepts in stem cell research]

In the last few years, advances in stem cell research have opened up new horizons in the treatment of human diseases and in regenerative medicine. It is not unusual to find news on stem cell research in newspapers and other media. This review describes some basic concepts in research needed to understand the medical literature on stem cells and to provide the information and bibliography necessary to be up to date in one of the subjects that has generated the greatest number of publications in the last few years.

A glance into somatic stem cell biology: basic principles, new concepts, and clinical relevance

Somatic stem cells are undifferentiated cells with a high capacity for self-renewal that can give rise to one or more specialized cell types with specific functions in the body. Profound characterization of these cells has been difficult due to the fact that their frequency in different tissues of the body is extremely low; furthermore, their identification is not based on their morphology but on immunophenotypic and functional assays. Nevertheless, significant advances in the study of these cells at both cellular and molecular levels have been achieved during the last decade. The majority of what we know concerning somatic stem cell biology has come from work on hematopoietic stem cells. More recently, however, there has been a great amount of information on neural and epithelial stem cells. The importance of stem cell research has gone beyond basic biology and is currently contributing to the development of new medical approaches for treatment of hematologic, neurologic, autoimmune, and metabolic disorders (cellular therapy).

Long-term ex vivo expansion of human fetal liver primitive haematopoietic progenitor cells in stroma-free cultures

Successful expansion of haematopoietic cells in ex vivo cultures will have important applications in transplantation, gene therapy, immunotherapy and potentially also in the production of non-haematopoietic cell types. Haematopoietic stem cells (HSC), with their capacity to both self-renew and differentiate into all blood lineages, represent the ideal target for expansion protocols. However, human HSC are rare, poorly characterized phenotypically and genotypically, and difficult to test functionally. Defining optimal culture parameters for ex vivo expansion has been a major challenge. We devised a simple and reproducible stroma-free liquid culture system enabling long-term expansion of putative haematopoietic progenitors contained within frozen human fetal liver (FL) crude cell suspensions. Starting from a small number of total nucleated cells, a massive haematopoietic cell expansion, reaching > 1013-fold the input cell number after approximately 300 d of culture, was consistently achieved. Cells with a primitive phenotype were present throughout the culture and also underwent a continuous expansion. Moreover, the capacity for multilineage lymphomyeloid differentiation, as well as the recloning capacity of primitive myeloid progenitors, was maintained in culture. With its better proliferative potential as compared with adult sources, FL represents a promising alternative source of HSC and the culture system described here should be useful for clinical applications.

Common and distinct features of cytokine effects on hematopoietic stem and progenitor cells revealed by dose-response surface analysis

Recent studies have identified thrombopoietin (TPO), flt-3 ligand (FL), Steel factor (SF), and interleukin-11 (IL-11) as cytokines able to stimulate amplification of the most primitive murine hematopoietic cells in vitro. However, dose-response and interaction parameters that predict how to optimize mixtures of these cytokines have not been previously defined. To obtain this information, Sca-1(+)lin(-) and c-kit(+)Sca-1(+)lin(-) adult mouse bone marrow cells were cultured for 10 and 14 days, respectively, in serum-free medium with varying concentrations of these cytokines. Quantitative assays were performed to determine the influences of the cytokine combinations tested on changes in long-term repopulating hematopoietic stem cells (HSCs), in vitro colony-forming cells (CFCs), and total cell numbers. A two-level factorial design was first used to screen the effects of TPO, SF, FL, and IL-11 as well as two different incubation temperatures. IL-11 and SF were found to be the most significant stimulators of murine HSC expansion. More detailed analyses of the effects on c-kit(+)Sca-1(+)lin(-) cells of IL-11, SF, and FL concentrations and their interactions using response surface methodology showed IL-11 to have a maximal stimulatory effect on HSC expansion at 20 ng/mL with higher concentrations being inhibitory. In contrast, not even high concentration saturation of the effects of either SF or FL was observed as the stimulatory effect of both SF and FL increased beyond 300 ng/mL. A negative interaction between SF and FL on HSCs was discovered. Interestingly, a generally similar pattern of cytokine effects was found to influence the 14-day output of CFCs and total cells from the same c-kit(+)Sca-1(+)lin(-) starting cell population. However, compared with HSCs, the cytokine requirements for maximizing the generation of CFCs and total cells were at much lower cytokine doses. From the information provided by the factorial analysis, mathematical models based on Monod kinetics for inhibitory substrates were developed that allow total cell, CFC, and HSC expansion to be predicted as a function of the IL-11, SF, and FL concentrations in terms of more widely recognized parameters. Overall, these methods should also serve as a guide for the future design and testing of other ex vivo stem cell expansion systems.

Haematopoietic stem cells

Considerable efforts have been made in recent years in determining the composition of the cell types that constitute the human haematopoietic stem cell (HSC) compartment. These studies have emphasized the heterogeneity of the human HSC in terms of proliferative and self-renewal capacities. Recent studies have indicated that CD34 is not the universal marker of all human HSCs. New markers for purifying HSCs have been described. A number of genes that regulate the formation, self-renewal, or differentiation of HSCs has been identified. The elucidation of the molecular phenotype of the HSC has just begun. Finally, an unexpected degree of developmental or differentiation plasticity of HSC has emerged. This review summarizes all the recent advances made in the human HSC field and examines the impacts that these discoveries may have both clinically and in understanding the organization of the human haematopoietic system.

Stem cell bioengineering

Tissue engineering and cellular therapies, either on their own or in combination with therapeutic gene delivery, have the potential to significantly impact medicine. Implementation of technologies based on these approaches requires a readily available source of cells for the generation of cells and tissues outside a living body. Because of their unique capacity to regenerate functional tissue for the lifetime of an organism, stem cells are an attractive "raw material" for multiple biotechnological applications. By definition they are self-renewing because on cell division they can generate daughter stem cells. They are also multipotent because they can differentiate into numerous specialized, functional cells. Recent findings have shown that stem cells exist in most, if not all, tissues, and that stem cell tissue specificity may be more flexible than originally thought. Although the potential for producing novel cell-based products from stem cells is large, currently there are no effective technologically relevant methodologies for culturing stem cells outside the body, or for reproducibly stimulating them to differentiate into functional cells. A mechanistic understanding of the parameters important in the control of stem cell self-renewal and lineage commitment is thus necessary to guide the development of bioprocesses for the ex vivo culture of stem cells and their derivates.

Stromal-derived factor 1 inhibits the cycling of very primitive human hematopoietic cells in vitro and in NOD/SCID mice

Stromal-derived factor 1 (SDF-1) is a -CXC- chemokine that plays a critical role in embryonic and adult hematopoiesis, and its specific receptor, CXCR4, has been implicated in stem cell homing. In this study, it is shown that the addition of SDF-1 to long-term cultures (LTCs) of normal human marrow can selectively, reversibly, and specifically block the S-phase entry of primitive quiescent erythroid and granulopoietic colony-forming cells (CFCs) present in the adherent layer. Conversely, addition of anti-SDF-1 antibody or SDF-1(G2), a specific CXCR4 antagonist, to preactivated human LTCs prevented both types of primitive CFCs from re-entering a quiescent state, demonstrating that endogenous SDF-1 contributes to the control of primitive CFC proliferation in the LTC system. Interestingly, SDF-1 failed to arrest the proliferation of primitive chronic myeloid leukemia CFCs in the adherent layer of LTCs containing normal marrow stromal cells. In vivo, injection of SDF-1 arrested the cycling of normal human LTC-initiating cells as well as primitive CFCs in the marrow of nonobese diabetic/severe combined immunodeficient mice engrafted with human cord blood cells. Conversely, injection of the antagonist, SDF-1(G2), reactivated the cycling of quiescent primitive human CFCs present in the marrow of mice engrafted with human marrow cells. These studies are the first to demonstrate a potential physiological role of SDF-1 in regulating the cell-cycle status of primitive hematopoietic cells and suggest that the deregulated cycling activity of primitive chronic myeloid leukemia (CML) cells is due to the BCR-ABL-mediated disruption of a pathway shared by multiple chemokine receptors.

Generation of mesoscopic patterns of viable Escherichia coli by ambient laser transfer

We have generated mesoscopic patterns of viable Escherichia coli on Si(1 1 1), glass, and nutrient agar plates by using a novel laser-based transfer process termed matrix assisted pulsed laser evaporation direct write (MAPLE DW). We observe no alterations to the E. coli induced by the laser-material interaction or the shear forces during the transfer. Transferred E. coli patterns were observed by optical and electron microscopes, and cell viability was shown through green fluorescent protein (GFP) expression and cell culturing experiments. The transfer mechanism for our approach appears remarkably gentle and suggests that active biomaterials such as proteins, DNA and antibodies could be serially deposited adjacent to viable cells. Furthermore, this technique is a direct write technology and therefore does not involve the use of masks, etching, or other lithographic tools.

Cell separation mediated by differential rolling adhesion

Recently, we showed a correlation between the maturity of hematopoietic stem and progenitor cells during development and rolling efficiency on selectins. These findings motivated us to explore a novel separation that exploits differences in selectin-mediated rolling adhesion between populations of cells. We extend the use of a previously developed cell-free system to study the separation of populations of sialyl Lewis x (sLe(x))-coated microspheres designed to roll with different average velocities on L-selectin chimeric substrates under well-defined flow. Results show that a separation that exploits differences in average rolling velocities between cell or microsphere populations is attainable. Excellent recovery and purity values for the slower rolling, or more desirable, populations are obtained and can be estimated from rolling velocity measurements. We also assess the feasibility of a selectin-mediated separation of adult bone marrow cell populations using previously obtained rolling velocity and rolling flux data for CD34+ and CD34- adult bone marrow cells on L-selectin substrates. We believe that a cell separation mediated by differential rolling adhesion can be used to enrich populations of hematopoietic stem and progenitor cells from an adult bone marrow cell preparation and that this method possesses several major advantages over existing antibody-mediated cell-affinity chromatography technologies.

Normal and leukemic CD34-negative human hematopoietic stem cells

Considerable progress has been made in recent years in purifying human and murine hematopoietic stem cells. The essential marker identified is the sialomucin CD34, which is expressed on primitive cells and downregulated as they differentiate into more abundant mature cells. CD34 is not unique to stem cells, however, as it is also expressed on clonogenic progenitors and some endothelial cells. Nevertheless, all clinical and experimental protocols are targeted to CD34+ cells enriched by a variety of selection methods. Recent studies in both the murine and human systems have indicated that some stem cells capable of multilineage repopulation do not express detectable levels of cell surface CD34. These studies challenge the dogma that all human repopulating cells are found in the CD34+ subset. However, the precise relationship between CD34- and CD34+ stem cells is still not well understood. In this review, the results on the discovery of the CD34- repopulating cell are summarized and the impacts this discovery may have, both clinically and in our understanding of the organization of the human hematopoietic system, are examined.

Distinct role of gp130 activation in promoting self-renewal divisions by mitogenically stimulated murine hematopoietic stem cells

Previous studies have demonstrated hematopoietic stem cell amplification in vitro after the activation of three cell-surface receptors: flt3/flk2, c-kit, and gp130. We now show flt3-ligand and Steel factor alone will stimulate >85% of c-kit(+)Sca-1(+)lin(-) adult mouse bone marrow cells to proliferate in single-cell serum-free cultures, but concomitant retention of their stem cell activity requires additional exposure to a ligand that will activate gp130. Moreover, this response is restricted to a narrow range of gp130-activating ligand concentrations, above and below which hematopoietic stem cell activity is lost. These findings indicate a unique contribution of gp130 signaling to the maintenance of hematopoietic stem cell function when these cells are stimulated to divide with additional differential effects dictated by the intensity of gp130 activation.

Primitive quiescent leukemic cells from patients with chronic myeloid leukemia spontaneously initiate factor-independent growth in vitro in association with up-regulation of expression of interleukin-3

It was previously shown that patients with chronic myeloid leukemia (CML) have a rare but consistently detectable population of quiescent (G0) leukemic (Philadelphia chromosome-positive and BCR-ABL-positive [BCR-ABL+]) CD34+ cells. In the study described here, most such cells expressed a primitive phenotype (CD38-, CD45RA-, CD71-, and HLA-DR(lo)) and cultures of these cells containing growth factors produced ultimately larger, but initially more slowly growing clones than do cultures of initially cycling CD34+ leukemic cells. Initially quiescent leukemic cells expressing BCR-ABL proliferated in single-cell cultures in the absence of added growth factors, thereby demonstrating their ability to spontaneously exit G0 and enter a continuously cycling state. Interestingly, on isolation, few of these quiescent BCR-ABL+ cells contained either interleukin-3 (IL-3) or granulocyte colony-stimulating factor (G-CSF) transcripts, whereas both were present in most cycling BCR-ABL+ CD34+ cells. However, after 4 days of culture in the absence of added growth factors and in association with their entry into the cell cycle (as indicated by up-regulation of Ki-67 and cdc25 transcripts), IL-3 transcripts became detectable. These findings show that entry of leukemic (BCR-ABL-expressing) progenitors into a quiescent (G0) state in vivo is highest among the most primitive leukemic cell populations, associated with a down-regulation of IL-3 and G-CSF gene expression, and spontaneously reversible in association with up-regulation of IL-3 expression. These results highlight the potential physiologic relevance of quiescent CML progenitors, even in treated patients, in whom these cells would be predicted to have a proliferative advantage over their quiescent normal counterparts when cytokine concentrations are low.

Issues in the manufacture and transplantation of genetically modified hematopoietic stem cells

The advent of safe and practical means to correct, enhance or protect blood cells at the genetic level offers tantalizing therapeutic perspectives. At present, gene delivery using a replication-defective retrovirus is the most efficient method to stably transduce hematopoietic cells. The successful adaptation of retroviral infection to hematopoietic stem cells requires optimized transduction conditions that maximize gene transfer while preserving the cells' potential for engraftment and longterm hematopoiesis. The successful establishment of effective transduction protocols hinges on retrovirus biology as well as stem cell and transplantation biology. Interestingly, the genetic approach could permit novel strategies to promote host repopulation by transplanted stem cells. However, regulated and predictable expression of any transgene integrated at random chromosomal locations cannot be taken for granted. Investigation of the control of transgene expression and prevention of vector silencing will become increasingly important.

Production of soluble CD34 by human myeloid cells

CD34, a glycophosphoprotein present in lymphohaematopoietic stem and progenitor cells, as well as in other cell types, exists in both transmembrane and intracytoplasmic forms. Transmembrane CD34 expression, which is high in the earliest haematopoietic precursors, decreases as cells mature. However, to our knowledge, there is no information on whether a decrease in transmembrane CD34 can also predict a release of the molecule from the cell membrane into the extracellular fluid. To investigate the above possibility, we studied conditions (incubation time, cell density and proliferative status) in human myeloid cells (lines KG-1a, KG-1 and cord blood-derived cells) that may cause a decrease in surface CD34 and the generation of a soluble form of the molecule. The latter, as demonstrated by Western blot analysis, adds more complexity to the proposed structural features and functional properties of CD34 in myeloid cells.

Efficient retrovirus-mediated gene transfer to transplantable human bone marrow cells in the absence of fibronectin

The low frequency of transplantable hematopoietic stem cells in adult human bone marrow (BM) and other differences from cord blood stem cells have impeded studies to optimize the retroviral transduction of stem cells from adult sources. To address this problem, first a cytokine combination was defined that would both maximize the kinetics of adult BM CD34+CD38− cell mitogenesis and minimize the period of prestimulation required for the transduction of these cells by a MSCV-GFP/neor virus in tissue culture dishes in the absence of fibronectin. Three days of stimulation with flt3-ligand, Steel factor, interleukin (IL)-3, and hyper-IL-6 proved both necessary and sufficient to obtain 83% ± 2% GFP+ CD34+CD38− cells, 75% ± 10% G418-resistant clonogenic progenitors, and 50% ± 20% transduced long-term culture-initiating cells as recovered 48 hours after a single exposure to virus. Moreover, this was accompanied by a several-fold increase in viral receptor (pit-1) messenger RNA transcripts in the target cells. Using this prestimulation protocol, repeated daily exposure to new virus (3×) did not alter the proportion of transduced cells over that obtained with a single exposure. Adult human BM cells able to engraft immunodeficient (NOD/SCID-β2M−/−) mice were also efficiently transduced (10%-20% GFP+ human lymphoid and myeloid cells present 6-8 weeks after transplant) using a 6-day prestimulation and infection protocol. A clinically useful efficiency of retrovirus-mediated gene transfer to transplantable adult human BM stem cells can thus be obtained with a protocol that allows their semisynchronous activation into cycle and concomitant increased expression of virus receptor transcripts before virus exposure.

Efficient retrovirus-mediated gene transfer to transplantable human bone marrow cells in the absence of fibronectin

The low frequency of transplantable hematopoietic stem cells in adult human bone marrow (BM) and other differences from cord blood stem cells have impeded studies to optimize the retroviral transduction of stem cells from adult sources. To address this problem, first a cytokine combination was defined that would both maximize the kinetics of adult BM CD34+CD38− cell mitogenesis and minimize the period of prestimulation required for the transduction of these cells by a MSCV-GFP/neor virus in tissue culture dishes in the absence of fibronectin. Three days of stimulation with flt3-ligand, Steel factor, interleukin (IL)-3, and hyper-IL-6 proved both necessary and sufficient to obtain 83% ± 2% GFP+ CD34+CD38− cells, 75% ± 10% G418-resistant clonogenic progenitors, and 50% ± 20% transduced long-term culture-initiating cells as recovered 48 hours after a single exposure to virus. Moreover, this was accompanied by a several-fold increase in viral receptor (pit-1) messenger RNA transcripts in the target cells. Using this prestimulation protocol, repeated daily exposure to new virus (3×) did not alter the proportion of transduced cells over that obtained with a single exposure. Adult human BM cells able to engraft immunodeficient (NOD/SCID-β2M−/−) mice were also efficiently transduced (10%-20% GFP+ human lymphoid and myeloid cells present 6-8 weeks after transplant) using a 6-day prestimulation and infection protocol. A clinically useful efficiency of retrovirus-mediated gene transfer to transplantable adult human BM stem cells can thus be obtained with a protocol that allows their semisynchronous activation into cycle and concomitant increased expression of virus receptor transcripts before virus exposure.