Transplantable hematopoietic stem cells in human fetal liver have a CD34(+) side population (SP)phenotype

Global transcriptional characterization of SP and MP cells from the myogenic C2C12 cell line: effect of FGF6

With the use of Hoechst staining techniques, we have previously shown that the C2C12 myogenic cell line contains a side population (SP) that is largely increased in the presence of fibroblast growth factor 6 (FGF6). Here, we compared transcriptional profiles from SP and main population (MP) cells from either C2C12 or FGF6-expressing C2C12. Expression profiles of SPs show that these cells are less differentiated than MPs and display some similarities to stem cells. Moreover, principal component analysis made it possible to distinguish specific contributions of either FGF6 or differentiation effects on gene expression profiles. This demonstrated that FGF6-expanded SPs were similar to parental C2C12-derived SPs. Conversely, FGF6-treated MPs differed from parental MPs and were more related to SP cells. These results show that FGF6 pushed committed myogenic cells toward a more immature phenotype resulting in the accumulation of cells with a SP phenotype. We propose that FGF6 conditioning could provide a way to expand the pool of immature cells by myoblast dedifferentiation.

Hematopoietic stem cells do not engraft with absolute efficiencies

Hematopoietic stem cells (HSCs) can be isolated from murine bone marrow by their ability to efflux the Hoechst 33342 dye. This method defines an extremely small and hematopoietically potent subset of cells known as the side population (SP). Recent studies suggest that transplanted single SP cells are capable of lymphohematopoietic repopulation at near absolute efficiencies. Here, we carefully reevaluate the hematopoietic potential of individual SP cells and find substantially lower rates of reconstitution. Our strategy involved the cotransplantation of single SP cells along with different populations of competitor cells that varied in their self-renewal capacity. Even with minimized HSC competition, SP cells were only able to reconstitute up to 35% of recipient mice. Furthermore, through immunophenotyping and clonal in vitro assays we find that SP cells are virtually homogeneous. Isolation of HSCs on the basis of Hoechst exclusion and a single cell-surface marker allows enrichment levels similar to that obtained with complex multicolor strategies. Altogether, our results indicate that even an extremely homogeneous HSC population, based on phenotype and dye efflux, cannot reconstitute mice at absolute efficiencies.

Side population cells derived from adult human liver generate hepatocyte-like cells in vitro

We sought to determine whether hepatic side population (SP) cells derived from adult human liver possess the potential of a novel candidate hepatic stem cell. Human cadaveric donor liver was subjected to collagenase perfusion and hepatocytes were separated from nonparenchymal cells by differential centrifugation. SP cells were isolated from the nonparenchymal portion after Hoechst 33342 staining. Since CD45 is a panleukocyte antigen, CD45-negative SP cells were separated from the vast majority of CD45-positive SP cells (90%), and hepatic growth medium was used to culture both groups. Both CD45-negative and CD45-positive hepatic SP cells generated colonies in the hepatic growth medium in 2-3 weeks. The colonies yielded large cells morphologically consistent with human hepatocytes, demonstrating granule-rich cytoplasm, dense, often double nuclei, and intracellular lipofuscin pigment. The cultured cells from both sources were positive for markers of human hepatocytes: HepPar, cytokeratin 8 (CK8), and human albumin. Reverse transcriptase-polymerase chain reaction (RT-PCR) performed on both groups demonstrated positivity for additional liver markers including human albumin, CK18, alpha-1 anti-trypsin, and the human cytochrome P450 enzyme CYP2B6. Double immunostaining (CD45 and HepPar) and RT-PCR confirmed that the hepatocyte-like cells derived from the CD45-negative SP cells acquired HepPar positivity but had no detectable CD45 antigen expression. In contrast, the cultured cells derived from the CD45-positive SP cells also acquired HepPar positivity, but only a minimal fraction expressed the CD45 antigen. We conclude that hepatic SP cells derived from the nonparenchymal portion of human liver are a potential source of human hepatocytes irrespective of their CD45 status, and further animal studies will be required to assess their regenerative potential.

Side population/ABCG2-positive cells represent a heterogeneous group of haemopoietic cells: implications for the use of adult stem cells in transplantation and plasticity protocols

Murine side population (SP) cells may have an increased ability to engraft lethally irradiated mice and lack CD34 expression. Strategies using CD34 as a primary marker of haemopoietic stem cells may therefore result in the exclusion of a primitive stem cell population. The molecular basis for the murine SP phenotype has been attributed to the multidrug-resistance transporter ABCG2. This study aimed to investigate ABCG2 expression from a variety of human sources and investigate the relationship between ABCG2 expression, the SP phenotype, and expression of markers such as CD34 and CD133. SP cells were observed in different haemopoietic sources, but a significant increase in the number of SP cells was observed in PB following granulocyte colony-stimulating factor mobilisation. No direct correlation between the frequency of SP cells and the expression of ABCG2 was observed. SP cells were identified in both lineage-positive and lineage-negative population and ABCG2 expression was enriched in lineage-negative SP cells. Lineage-negative SP cells were devoid of CD34 expression but enriched for CD133. Subsequent analysis revealed that ABCG2 and CD133 are coexpressed. Together, these data suggest that the ABCG2 transporter is neither required nor responsible for the SP phenotpye in many human blood cells.

Evidence for a qualitative hierarchy within the Hoechst-33342 'side population' (SP) of murine bone marrow cells

In vitro cobblestone area (CA)-forming cell (CAFC) and in vivo (short-term and competitive repopulation) assays demonstrate that a qualitative hierarchy exists within the Hoechst-33342-defined side population (SP) in murine bone marrow (BM). Consistent with and extending previous studies, we demonstrate that (i) hematopoietic activity found in whole BM (WBM) is concentrated within the SP, rather than the non-SP (NSP); and (ii) within the SP, those cells that more strongly efflux the dye (lower SP, LSP) are qualitatively different from those that less strongly efflux the dye (upper SP, USP). Qualitative differences are highlighted by evidence that (i) CA derived from LSP CAFC persist in culture significantly longer than CA derived from USP CAFC; (ii) short-term, multilineage repopulation of lethally irradiated mice by LSP cells is more rapid than that in mice receiving USP, NSP, whole SP (WSP), or WBM cells and (iii) LSP cells out-compete USP cells in the multilineage hematopoietic repopulation of lethally irradiated recipients. These data suggest that LSP cells are of higher quality than USP cells and potentially provide a means by which qualitative changes in primitive hematopoietic progenitors occurring naturally with aging, or clinically as a consequence of therapeutic manipulation, can be assessed.

Competitive equality of donor cells expressing a disparate MHC antigen following stem cell-enriched bone marrow transplantation

INTRODUCTION

Bone marrow cells expressing foreign MHC antigens survive poorly after transplantation. Stable mixed hematopoietic chimerism requires reconstitution with a relatively large number of foreign bone marrow cells and intensive depletion of host cells. In addition, when foreign MHC-transduced autologous bone marrow cells are transplanted, prolonged hematopoietic transgene expression requires extensive host conditioning. The competitive disadvantage associated with engraftment of donor cells expressing foreign MHC antigens is thought to result from a defect in engraftment secondary to donor-host incompatibility or immunologic resistance by the host.

METHODS

We used a limiting-dilution competitive repopulation assay with cells from HLA-A2.1 transgenic mice to determine whether and to what extent foreign MHC antigen expression impairs engraftment in C57BL/6 hosts. Transplants were performed with Hoechst 33342 fluorescence-sorted side population (SP) cells, a subset of bone marrow enriched for stem cells. RESULTS.: Transplantation with 250 stem cell-enriched HLA-A2.1-transgenic side population cells successfully competed with nearly 5000 host C57BL/6 side population cells to produce stable long-term mixed chimerism. There was a direct relationship between the number of transplanted donor HLA-A2-expressing cells and the percentage of HLA-A2-expressing cells in the peripheral blood of reconstituted C57BL/6 mice (r2=0.1799, P=0.031). This correlation was maintained in secondary transplant recipients.

CONCLUSIONS

HLA-A2-expressing hematopoietic cells do not have an engraftment defect when transplanted into C57BL/6 hosts and immunologic resistance did not limit chimerism following lethal irradiation. These results may have relevance to understanding long-term gene expression after hematopoietic stem cell based gene therapy.

Ponce de Leon's Fountain: stem cells and the regenerating heart

Despite current pharmacologic and whole organ transplantation strategies, advanced heart failure remains a common and deadly disease. Limited availability of donor organs for use in orthotopic heart transplantation has prompted the examination of alternative therapies, including cell transfer strategies. Stem cell populations have been identified in virtually all postnatal tissues with the exception of the heart, and these stem cells function in the maintenance and regeneration of the respective tissues. Recent studies challenge preexisting notions regarding cardiac repair and suggest that the heart is capable of limited regeneration through the activation of resident cardiac stem cells or the recruitment of stem cell populations from other tissues such as the bone marrow. This review highlights animal models that have the capacity for myocardial regeneration and examines potential sources of stem cell populations that may participate in tissue regeneration. While some authors view these cell-based strategies as a Fountain of Youth for the myopathic heart, future studies will decipher the regulatory mechanisms of stem cell populations and serve as a prelude to stem cell-based strategies.

A novel stem-cell population in adult liver with potent hematopoietic-reconstitution activity

A number of recent reports have documented that cells possessing hematopoietic-reconstitution ability can be identified and isolated from a variety of solid organs in the adult animal. In all studies to date, however, purified organ-derived stem cells demonstrate a diminished repopulating capacity relative to that of purified bone marrow-derived hematopoietic stem cells (BM HSCs). It has therefore been unclear whether organ-derived HSCs possess functional properties distinct from those of BM HSCs, or simply have not been purified to a comparable extent. Here we report the identification of a rare subset of cells in adult murine liver that possess potent blood-repopulating potential, approaching that of BM HSCs. The cells, isolated on the basis of dye-efflux activity and CD45 expression (termed CD45(+) liver side population [SP] tip cells), exhibit a surface phenotype similar to that of freshly isolated BM HSCs derived from normal adult animals, but are phenotypically distinct in that they do not express the stem-cell marker c-kit. Single-cell transplantation studies indicate that CD45(+) liver SP tip cells can be generated from BM HSCs, suggesting a relationship between stem-cell populations in the liver and bone marrow compartments. Overall, these studies have important implications for understanding extramedullary hematopoiesis, and may be relevant to current strategies aimed at inducing tolerance to transplanted organs.

Perivascular cells expressing annexin A5 define a novel mesenchymal stem cell-like population with the capacity to differentiate into multiple mesenchymal lineages

The annexin A5 gene (Anxa5) was recently found to be expressed in the developing and adult vascular system as well as the skeletal system. In this paper, the expression of an Anxa5-lacZ fusion gene was used to define the onset of expression in the vasculature and to characterize these Anxa5-lacZ-expressing vasculature-associated cells. After blastocyst implantation, Anxa5-lacZ-positive cells were first detected in extra-embryonic tissues and in angioblast progenitors forming the primary vascular plexus. Later, expression is highly restricted to perivascular cells in most blood vessels resembling pericytes or vascular smooth muscle cells. Viable Anxa5-lacZ+ perivascular cells were isolated from embryos as well as adult brain meninges by specific staining with fluorescent X-gal substrates and cell-sorting. These purified lacZ+ cells specifically express known markers of pericytes, but also markers characteristic for stem cell populations. In vitro and in vivo differentiation experiments show that this cell pool expresses early markers of chondrogenesis, is capable of forming a calcified matrix and differentiates into adipocytes. Hence, Anxa5 expression in perivascular cells from mouse defines a novel population of cells with a distinct developmental potential.

Analysis of primitive CD34- and CD34+ hematopoietic cells from adults: gain and loss of CD34 antigen by undifferentiated cells are closely linked to proliferative status in culture

There is limited understanding of CD34- hematopoietic cells and the linkage between CD34 antigen expression and cell proliferation. In this study, early CD34- CD38- LIN- (CD34-) cells were purified from mobilized adult peripheral blood and carefully analyzed in vitro for growth and modulation of CD34. Mobilized CD34+CD38- LIN- (CD34+) cells were used for comparison. Expression of CD34, CD38, and LIN antigens was determined, and proliferative responses were assessed with PKH tracking dye, expression of Ki67 antigen, and uptake of pyronin Y. Suspension cultures of adult CD34- cells generated CD34+ cells and progenitors for >8 weeks. Stromal cultures demonstrated the presence of long-term culture-initiating cells within the CD34- fraction. While CD34- cells were slower to initiate growth than the CD34+ cells were, no significant difference in hematopoietic cell output was found. Upon cultivation of CD34- cells, CD34 antigen appeared within 48 hours but was restricted to those cells that had initiated growth. Surprisingly, CD34+ precursors lost CD34 expression in culture if they remained in G0 for more than 2 days. Those cells later regained expression of CD34 antigen upon initiation of growth. Comparison of cells that did or did not rapidly modulate CD34 antigen revealed no differences in long-term growth potential. In conclusion, in vitro expression of CD34 by CD34- and CD34+ populations is tightly linked to cellular proliferation. In this culture system, expression of CD34 antigen by LIN- cells constitutes an early hallmark of growth. Measurement of CD34 expression by LIN- cells in expansion culture underestimates the total content of hematopoietic cells.

Characterization of a lineage-negative stem-progenitor cell population optimized for ex vivo expansion and enriched for LTC-IC

Current hematopoietic stem cell transplantation protocols rely heavily upon CD34+ cells to estimate hematopoietic stem and progenitor cell (HSPC) yield. We and others previously reported CD133+ cells to represent a more primitive cell population than their CD34+ counterparts. However, both CD34+ and CD133+ cells still encompass cells at various stages of maturation, possibly impairing long-term marrow engraftment. Recent studies demonstrated that cells lacking CD34 and hematopoietic lineage markers have the potential of reconstituting long-term in vivo hematopoiesis. We report here an optimized, rapid negative-isolation method that depletes umbilical cord blood (UCB) mononucleated cells (MNC) from cells expressing hematopoietic markers (CD45, glycophorin-A, CD38, CD7, CD33, CD56, CD16, CD3, and CD2) and isolates a discrete lineage-negative (Lin-) cell population (0.10% +/- 0.02% MNC, n=12). This primitive Lin- cell population encompassed CD34+/- and CD133+/- HSPC and was also enriched for surface markers involved in HSPC migration, adhesion, and homing to the bone marrow (CD164, CD162, and CXCR4). Moreover, our depletion method resulted in Lin- cells being highly enriched for long-term culture-initiating cells when compared with both CD133+ cells and MNC. Furthermore, over 8 weeks in liquid culture stimulated by a cytokine cocktail optimized for HSPC expansion, TPOFLK (thrombopoietin 10 ng/ml, Flt3 ligand 50 ng/ml, c-Kit ligand 20 ng/ml) Lin- cells underwent slow proliferation but maintained/expanded more primitive HSPC than CD133+ cells. Therefore, our Lin- stem cell offers a promising alternative to current HSPC selection methods. Additionally, this work provides an optimized and well-characterized cell population for expansion of UCB for a wider therapeutic potential, including adult 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.

ABC transporter activities of murine hematopoietic stem cells vary according to their developmental and activation status

Primitive hematopoietic cells from several species are known to efflux both Hoechst 33342 and Rhodamine-123. We now show that murine hematopoietic stem cells (HSCs) defined by long-term multilineage repopulation assays efflux both dyes variably according to their developmental or activation status. In day 14.5 murine fetal liver, very few HSCs efflux Hoechst 33342 efficiently, and they are thus not detected as “side population” (SP) cells. HSCs in mouse fetal liver also fail to efflux Rhodamine-123. Both of these features are retained by most of the HSCs present until 4 weeks after birth but are reversed by 8 weeks of age or after a new HSC population is regenerated in adult mice that receive transplants with murine fetal liver cells. Activation of adult HSCs in vivo following 5-fluorouracil treatment, or in vitro with cytokines, induces variable losses in Rhodamine-123 and Hoechst 33342 efflux activities, and HSCs from mdr-1a/1b-/- mice show a dramatic decrease in Rhodamine-123 efflux ability. Thus, the Rhodamine-123 and Hoechst 33342 efflux properties of murine HSCs fluctuate in the same fashion as a number of other HSC markers, suggesting these are regulated by a common control mechanism that operates independently of that regulating the regenerative function of HSCs. (Blood. 2004;103:4487-4495)

Presence of nonhematopoietic side population cells in the adult human and nonhuman primate pancreas

Side population (SP) cells defined by their ability to efflux Hoechst dye 33342 (Hst), demonstrate functional stem cell capabilities in adult murine tissues and may represent organ-specific stem cells. We examined adult human (Hu) and rhesus macaque (Rh) pancreatic tissue for the presence of SP cells.

METHODS

Hu cadaver (n = 4) and Rh donor (n = 5) pancreata were dispersed with collagenase and separated by density gradient centrifugation to relatively enrich fractions for islet, ductal, and acinar tissue in human and islet and nonislet tissue in Rh. Single cell suspensions were incubated with varying Hst concentrations to determine optimal conditions for SP cell analysis. Cellular heterogeneity was assessed using a panel of monoclonal antibodies positive for hematopoietic and/or endothelial cells.

RESULTS

Hu SP cells comprised approximately 0.12%, 0.08%, and 0.45% of the gated populations for Hu islet, ductal, and acinar fractions respectively. In Rh, 5.5% and 3.7% of the islet and nonislet fractions were identified as SP cells. FACS analysis of Hu pancreas-derived SP cells indicated that greater than 95% were CD45(-), and only 6% were CD34(+)CD45(-). A similar phenotype was detected in Rh pancreas-derived SP cell populations: greater than 70% were CD45(-) and less than 2% were endothelial lineage positive.

CONCLUSIONS

SP cells are found in both islet- and nonislet-enriched fractions of the adult Hu and Rh pancreas. The majority of pancreatic SPs are CD45(-) and CD34(-), suggesting nonhematopoietic lineage. Further preclinical study is needed to establish the phenotype and functional role of adult tissue-specific versus tissue-resident stem cells.

Multiparameter analysis of murine bone marrow side population cells

We describe the multiparameter flow cytometric analysis of the relationship between side population (SP) formation and well-characterized, antigen-defined stem cell subsets. We also compared the competitive repopulation ability of subsets defined by the SP profile to those identified by antigenic markers. The vast majority of SP cells possessed a primitive cell phenotype (c-kit+, SCA-1+, Thy-1+, CD31+, CD135neg, lineageneg), but only a minority of antigen-defined subsets were SP cells. Hence, although SP cells are identified independently of antigenic markers, they are not distinct from established stem cell phenotypes, but are a small subset of them. Approximately half of SP cells expressed CD34 at readily detectable levels, and one-third of SP cells possessed the primitive c-kit+, SCA-1+, lineageneg, CD34neg cell phenotype. Since most SP cells are a subset of c-kit+, Thy-1+, lineageneg, SCA-1+ cells (KTLS), we determined whether the SP cell subset represents a further enrichment in long-term repopulating cell content. The SP+ subset of KTLS+ cells was more enriched for competitive repopulation units than the SP- fraction of KTLS+ cells. Hence, the SP cell fraction highlights a subset of the long-term repopulating cells found within the already highly purified KTLS fraction.

A subpopulation of bone marrow cells depleted by a novel antibody, anti-Liv8, is useful for cell therapy to repair damaged liver

We previously reported a new in vivo model named as "GFP/CCl(4) model" for monitoring the transdifferentiation of green fluorescent protein (GFP) positive bone marrow cell (BMC) into albumin-positive hepatocyte under the specific "niche" made by CCl(4) induced persistent liver damage, but the subpopulation which BMCs transdifferentiate into hepatocytes remains unknown. Here we developed a new monoclonal antibody, anti-Liv8, using mouse E 11.5 fetal liver as an antigen. Anti-Liv8 recognized both hematopoietic progenitor cells in fetal liver at E 11.5 and CD45-positive hematopoietic cells in adult bone marrow. We separated Liv8-positive and Liv8-negative cells and then transplanted these cells into a continuous liver damaged model. At 4 weeks after BMC transplantation, more efficient repopulation and transdifferentiation of BMC into hepatocytes were seen with Liv8-negative cells. These findings suggest that the subpopulation of Liv8-negative cells includes useful cells to perform cell therapy on repair damaged liver.

Establishment, characterization, and long-term maintenance of cultures of human fetal hepatocytes

Cultured human hepatocytes have broad research and clinical applications; however, the difficulties in culturing rodent and human hepatocytes are well known. These problems include the rapid loss of the hepatocytic phenotype in primary culture and the limited replicating capacity of the cultured cells. We describe the establishment of serum-free primary cultures of human fetal hepatocytes (HFHs) that retain hepatocytic morphology and gene expression patterns for several months and maintain sufficient proliferative activity to permit subculturing for at least 2 passages. Initially, HFH cultures contained 2 main cell types that morphologically resembled large and small hepatocytes. The fetal hepatocytes expressed alpha-fetoprotein (AFP), cytokeratin (CK) 19, albumin, and other hepatic proteins. Treatment of the cultures with oncostatin M (OSM) increased cell size and enhanced cell differentiation and formation of bile canaliculi, probably through an effect on hepatocyte nuclear factor (HNF) 4alpha. Approximately 1 month after plating, multiple clusters of very small cells became apparent in the cultures. These cells had very few organelles and are referred to as blast-like cells. Flow cytometric analysis of these cells showed that they express oval cell/stem cell markers such as CD90 (Thy-1), CD34, and OV-6 but do not stain with antibodies to beta(2)-microglobulin. HFH cultures maintained for 9 to 12 months produced grossly visible organoids containing ductular structures that stained for CK18, CK19, and AFP. In conclusion, HFH cultures, which might contain a population of hepatic stem cells, constitute an excellent tool for a variety of studies with human hepatocytes, including the mechanisms of viral infection.

CD34- hematopoietic stem cells: current concepts and controversies

Recent data have suggested that human CD34(-) hematopoietic stem cells (HSCs) exist, challenging the concept that HSCs necessarily and exclusively express the CD34 antigen. In mice, quiescent HSCs have been shown to be mostly CD34(-), but as a consequence of 5-fluorouracil treatment or cytokine stimulation, differentiate into CD34(+) cells. Of particular interest is a novel, specific marker to identify HSCs, namely the Hoechst dye efflux property, with which a distinct side population (SP) is identified. These SP cells are mostly CD34(-), highly enriched for long-term repopulating cells, and durably engraft in sublethally irradiated non-obese diabetic/severe combined immunodeficient mice. Using a semiquantitative reverse transcription-polymerase chain reaction, one of the ATP-binding cassette (ABC) transporters, the breast cancer resistance protein (Bcrp) or ABC transporter G2 (ABCG2), was found to be highly expressed in SP cells as well as other primitive HSCs and to sharply drop with hematopoietic differentiation. Enforced expression of the ABCG2 cDNA resulted in a robust SP phenotype and a reduction in hematopoietic maturation. These data suggest that the Bcrp/ABCG2 gene contributes importantly to the generation of the SP phenotype, which allows for the selection of immature, pluripotent HSCs. The isolation of Bcrp/ABCG2(+) cells appears to be an attractive tool to analyze and characterize HSCs, and may eventually allow for the purification of these cells for clinical purposes. In this review, current concepts on murine and human CD34(-) HSCs and their relationship with CD34(+) HSCs are discussed.

Hematopoietic stem cells

Considerable effort has been made in recent years in defining the embryonic origin of the hematopoietic stem cell (HSC). Using transgenic mouse models, a number of genes that regulate the formation, self-renewal, or differentiation of HSCs have been identified. Of particular interest, it has recently been shown that key regulators of definitive blood formation played a crucial role in adult HSC development. Specifically, the use of some of these regulatory molecules has dramatically improved the potential of adult HSC expansion. Furthermore, the elucidation of the molecular phenotype of the HSC has just begun. Finally, unexpected degrees of HSC developmental or differentiation plasticity have emerged. In this review, we will summarize the recent advances made in the human HSC field, and we will examine the impacts these discoveries may have clinically and on our understanding of the organization of the human hematopoietic system.

Stem cell biology and therapeutic applications

PURPOSE OF REVIEW

Chronic diseases are common and deadly. Stem cell therapies have received intense interest for the repopulation of damaged or diseased tissues. A detailed understanding of the similarities and differences between embryonic stem cells and somatic stem cells will enhance our understanding of mechanisms of tissue repair or cellular augmentation. In addition, emerging technologies will be useful in the definition of the molecular regulation of the respective stem cell populations.

RECENT FINDINGS

A number of postnatal tissues have a population of somatic stem cells, which function in the maintenance and repair of tissues. Using molecular technologies these somatic stem cell populations have been shown to be pluripotent when placed in a permissive environment. Recent studies have utilized emerging technologies to define a molecular signature of embryonic stem cells and selected somatic stem cell populations. These strategies will be useful for the definition of a molecular program that promotes a stem cell phenotype (i.e. stemness phenotype).

SUMMARY

Recent studies suggest that embryonic and somatic stem cell populations hold promise as sources for tissue engineering. The use of cell biological and molecular technologies will enhance our understanding of embryonic and somatic stem cell populations and their molecular regulatory events that promote multipotentiation.

Spontaneous circulation of myeloid-lymphoid-initiating cells and SCID-repopulating cells in sickle cell crisis

The only curative therapy for sickle cell disease (SCD) is allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy approaches for autologous HSC transplantation are being developed. Although earlier engraftment is seen when cells from GCSF-mobilized blood are transplanted than when bone marrow is transplanted, administration of GCSF to patients with SCD can cause significant morbidity. We tested whether primitive hematopoietic progenitors are spontaneously mobilized in the blood of patients with SCD during acute crisis (AC-SCD patients). The frequency of myeloid-lymphoid-initiating cells (ML-ICs) and SCID-repopulating cells (SRCs) was significantly higher in blood from AC-SCD patients than in blood from patients with steady-state SCD or from normal donors. The presence of SRCs in peripheral blood was not associated with detection of long-term culture-initiating cells, consistent with the notion that SRCs are more primitive than long-term culture-initiating cells. As ML-ICs and SRCs were both detected in blood of AC-SCD patients only, these assays may both measure primitive progenitors. The frequency of ML-ICs also correlated with increases in stem cell factor, GCSF, and IL-8 levels in AC-SCD compared with steady-state SCD and normal-donor sera. Because significant numbers of ML-ICs and SRCs are mobilized in the blood without exogenous cytokine treatment during acute crisis of SCD, collection of peripheral blood progenitors during crisis may yield a source of autologous HSCs suitable for ex-vivo correction by gene therapy approaches and subsequent transplantation.

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.

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.

Adult bone marrow stem cells for cell and gene therapies: implications for greater use

There is excitement generated almost daily about the possible uses of stem cells to treat human disease. Much of the interest of late is generated by embryonic stem cells (ESCs). As exciting as ESCs may be, they are quite controversial for moral reasons, given their source. They are also scientifically controversial since they are much less well understood than the original, long-standing, and clinically successful hematopoietic stem cell (HSC). HSCs have the distinct advantage of being reasonably well characterized and have been proven in the clinic. They can be isolated by simple procedures directly from the bone marrow or from peripheral blood after being stimulated (mobilized). They can then be manipulated and delivered to a patient, often producing a cure. Their biology provides the paradigm by which all other stem cells are judged, and they have little in the way of moral controversy surrounding them given they are isolated from adults who have consented to the procedure. Another putative stem cell has gained momentum in the last few years; the mesenchymal stem cell (MSC). MSCs appear to have much in common with HSCs. They were originally characterized from bone marrow, are capable of differentiating along multiple lineages and, at least in vitro, have significant expansion capability. Unlike HSCs, they have not yet been definitively shown to function as stem cells, despite their ability to differentiate into various mesenchymal cell types under the right culture conditions. Still, there is mounting evidence these cells may be useful, if not as true stem cells then at least as vehicles for emerging cell and gene therapies, especially in the field of tissue engineering. While this is an important endpoint, it is more important to thoroughly understand stem cell biology. That understanding can then be applied toward the ultimate goal of using these cells not just for various forms of therapy, but rather as a tool to discover the mechanisms and means to bring about directed repair and regeneration of damaged or diseased tissues and organs. The excitement of HSCs and MSCs has been muted somewhat by the excitement surrounding ESCs, primarily due to the fact HSCs and MSCs are viewed as limited to specific cell types while ESCs could potentially be applied to any cell type. Recent information indicates HSCs, MSCs, and other cells in general may have more universal differentiation abilities than previously thought.

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.

Side population cells from diverse adult tissues are capable of in vitro hematopoietic differentiation

OBJECTIVE

Pluripotent hematopoietic stem cells and muscle-derived hematopoietic potential cells isolated by Hoechst 33342 dye-mediated fluorescein-activated cell sorting (FACS) as side population (SP) cells, give rise to hematopoietic cells as well as skeletal muscle cells following intravenous transplantation. However, besides bone marrow and skeletal muscle, it has remained unclear whether other adult tissues also contain SP cells that are enriched for cells that exhibit hematopoietic potential.

METHODS

To test whether adult tissues contain SP cells with hematopoietic potential, Hoechst-FACS analysis and hematopoietic colony formation assays were performed with cells isolated from a variety of adult tissues, skeletal muscle, heart, brain, spleen, liver, kidney, lung, and small intestine and compared with peripheral blood and bone marrow cells.

RESULTS

In addition to hematopoietic tissues, cell preparations from nonhematopoietic tissues, such as skeletal muscle, kidney, lung, and small intestine, displayed markedly higher hematopoietic colony formation activity compared to peripheral blood cells. Moreover, the hematopoietic progenitors in these adult tissues expressed the hematopoietic cell marker CD45. Hoechst-FACS analysis demonstrated that all adult tissues examined contained SP cells. In addition, these SP fractions were enriched for cells that efficiently formed hematopoietic colonies in vitro.

CONCLUSION

These results indicate that hematopoietic progenitors are present in significant numbers in all adult tissues examined.

Bcrp1 gene expression is required for normal numbers of side population stem cells in mice, and confers relative protection to mitoxantrone in hematopoietic cells in vivo

Hematopoietic stem cells (HSCs) can be identified by a "side population" (SP) phenotype. Previous studies have implicated the ATP binding cassette transporter genes Mdr1a/1b and/or Bcrp1 in the SP phenotype. To define the relative role of these transporters, we generated Bcrp1 null mice and evaluated HSCs both functionally and phenotypically. Loss of Bcrp1 gene expression, but not Mdr1a/1b, led to a significant reduction in the number of SP cells in the bone marrow and in skeletal muscle. In the bone marrow, there was a nearly absolute loss of lineage negative, c-Kit-positive, Sca-1-positive SP cells, and the residual SP cells were depleted of repopulating cells in a transplant assay, demonstrating that Bcrp1 expression is necessary for the SP phenotype in HSCs. Furthermore, Bcrp1 null hematopoietic cells were significantly more sensitive to mitoxantrone in drug-treated transplanted mice. These results show that Bcrp1 gene expression alone defines the SP stem cell phenotype, and suggest that the physiological function of Bcrp1 expression in HSCs is to provide protection from cytotoxic substrates.

CD34(+) or CD34(-): which is the more primitive?

Remarkable progress has been achieved in the characterization and isolation of primitive hematopoietic stem cells (HSC). HSC represent a very small subset of hematopoietic cells and provide self-renewal, possess differentiation capacity and allow a constant supply of the entire hematopoietic cell spectrum. Until recently, CD34 has been used as a convenient marker for HSC, since CD34(+) cells have been shown to possess colony-forming potential in short-term assays, maintain long-term colony-forming potential in in vitro cultures and allow the expression and differentiation of blood cells from different hematopoietic lineages in in vivo models. Clinical and experimental protocols have targeted CD34(+) cells enriched by a variety of selection models and have readily used these for transplantation, purging and gene therapies and targets for future organ replacement. Recent studies in murine and human models, however, have indicated that CD34(-) HSC exist as well, which possess engraftment potential and distinct HSC characteristics. These studies challenge the dogma that HSC are uniformly found in the CD34(+) subset, and question whether primitive HSC are CD34(+) or CD34(-). In this review, results on murine and human CD34(+) and CD34(-) HSC, differences between them and their possible interactions are examined.

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.