Co-culture with mesenchymal stromal cells increases proliferation and maintenance of haematopoietic progenitor cells

Mesenchymal stem cells promote a primitive phenotype CD34+c-kit+ in human cord blood-derived hematopoietic stem cells during ex vivo expansion

The purpose of this study was to evaluate the influence of bone marrow-mesenchymal stem cells (BM-MSC) and exogenously added cytokines on the proliferation, primitive cell subpopulation maintenance (including the c-kit+ marker) and clonogenic capacity of hematopoietic stem cells (HSC). BM-MSC were collected from volunteer donors, isolated and characterized. Umbilical cord blood (UCB) samples were collected from healthy full-term deliveries. UCB-CD34+ cells were cultured in the presence or absence of BM-MSC and/or cytokines for 3 and 7 days. CD34+ cell proliferation was evaluated using the CSFE method and cell phenotype was determined by CD34, c-kit, CD33, CD38, HLA-DR, cyCD22 and cyCD3 detection. Cell clonogenic ability was also assessed. Exogenously added SCF, TPO and FLT3L increased CD34+ cell proliferation in the presence or absence of BM-MSC, but with concomitant cell differentiation. Without any added cytokines, BM-MSC are able to increase the percentage of primitive progenitors as evaluated by c-kit expression and CFU-GEMM increase. Interestingly, this latter effect was dependent on both cell-cell interactions and secreted factors. A 7-day co-culture period will be optimal for obtaining an increased primitive HSC level. Including c-kit as a marker for primitive phenotype evaluation has shown the relevance of BM-MSC and their secreted factors on UCB-HSC stemness function. This effect could be dissociated from that of the addition of exogenous cytokines, which induced cellular differentiation instead.

Identifying leukemia stem cells--is it feasible and does it matter?

Present evidence indicates that acute myeloid leukemia (AML) is a stem cell disease. Leukemia stem cells (LSC) might originate from malignant transformation of normal hematopoietic stem cells (HSC), or alternatively, from progenitors in which the acquired mutations have re-installed a dysregulated self-renewal program. Since LSC, similar to their normal counterparts, divide extreme slowly, this might account for the ineffectiveness of conventional chemotherapy in inducing long-term cure. The present review will focus on the detection of LSC, their cellular and molecular biology, their genetic heterogeneity and on correlative studies that have demonstrated the clinical significance of estimating LSC burden. For long-term cure of AML, it is of importance to define LSC candidates and to understand their biology compared to normal HSC. Finally, we will discuss the perspectives of developing treatment strategies for eradication of LSC.

Expression of the melanoma cell adhesion molecule in human mesenchymal stromal cells regulates proliferation, differentiation, and maintenance of hematopoietic stem and progenitor cells

The melanoma cell adhesion molecule defines mesenchymal stromal cells in the human bone marrow that regenerate bone and establish a hematopoietic microenvironment in vivo. The role of the melanoma cell adhesion molecule in primary human mesenchymal stromal cells and the maintenance of hematopoietic stem and progenitor cells during ex vivo culture has not yet been demonstrated. We applied RNA interference or ectopic overexpression of the melanoma cell adhesion molecule in human mesenchymal stromal cells to evaluate the effect of the melanoma cell adhesion molecule on their proliferation and differentiation as well as its influence on co-cultivated hematopoietic stem and progenitor cells. Knockdown and overexpression of the melanoma cell adhesion molecule affected several characteristics of human mesenchymal stromal cells related to osteogenic differentiation, proliferation, and migration. Furthermore, knockdown of the melanoma cell adhesion molecule in human mesenchymal stromal cells stimulated the proliferation of hematopoietic stem and progenitor cells, and strongly reduced the formation of long-term culture-initiating cells. In contrast, melanoma cell adhesion molecule-overexpressing human mesenchymal stromal cells provided a supportive microenvironment for hematopoietic stem and progenitor cells. Expression of the melanoma cell adhesion molecule increased the adhesion of hematopoietic stem and progenitor cells to human mesenchymal stromal cells and their migration beneath the monolayer of human mesenchymal stromal cells. Our results demonstrate that the expression of the melanoma cell adhesion molecule in human mesenchymal stromal cells determines their fate and regulates the maintenance of hematopoietic stem and progenitor cells through direct cell-cell contact.

Human platelet lysate gel provides a novel three dimensional-matrix for enhanced culture expansion of mesenchymal stromal cells

Cell culture in regenerative medicine needs to facilitate efficient expansion according to good manufacturing practice requirements. Human platelet lysate (HPL) can be used as a substitute for fetal calf serum without the risk of xenogeneic immune reactions or transmission of bovine pathogens. Heparin needs to be added as anticoagulant before addition of HPL to culture medium; otherwise, HPL-medium forms a gel within 1 h. Here, we demonstrated that such HPL-gels provide a suitable 3D-matrix for cell culture that-apart from heparin-consists of the same components as the over-layered culture medium. Mesenchymal stromal cells (MSCs) grew in several layers at the interface between HPL-gel and HPL-medium without contact with any artificial biomaterials. Notably, proliferation of MSCs was much higher on HPL-gel compared with tissue culture plastic. Further, the frequency of initial fibroblastoid colony forming units (CFU-f) increased on HPL-gel. The viscous consistency of HPL-gel enabled passaging with a convenient harvesting and reseeding procedure by pipetting cells together with their HPL-matrix-this method does not require washing steps and can easily be automated. The immunophenotype and in vitro differentiation potential toward adipogenic, osteogenic, and chondrogenic lineage were not affected by culture-isolation on HPL-gel. Taken together, HPL-gel has many advantages over conventional plastic surfaces: it facilitates enhanced CFU-f outgrowth, increased proliferation rates, higher cell densities, and nonenzymatic passaging procedures for culture expansion of MSCs.

Cord blood-hematopoietic stem cell expansion in 3D fibrin scaffolds with stromal support

Expansion of multipotent, undifferentiated and proliferating cord blood (CB)-hematopoietic stem cells (HSC) in vitro is limited and insufficient. Bone marrow (BM) engineering in vitro allows mimicking the main components of the hematopoietic niche compared to conventional expansion strategies. In this study, four different 3D biomaterial scaffolds (PCL, PLGA, fibrin and collagen) were tested for freshly isolated cord blood (CB)-CD34(+) cell expansion in presence of (i) efficient exogenous cytokine supplementation and (ii) umbilical cord (UC)-mesenchymal stem cells (MSC). Cell morphology, growth and proliferation were analyzed in vitro as well as multi-organ engraftment and multilineage differentiation in a murine transplantation model. All scaffolds, except 3D PLGA meshes, supported CB-CD34(+) cell expansion, which was additionally stimulated by UC-MSC support. CB-CD34(+) cells cultured on human-derived 3D fibrin scaffolds with UC-MSC support i) reached the highest overall growth (5 × 10(8)-fold expansion of total nuclear cells after fourteen days and 3 × 10(7)-fold expansion of CD34(+) cells after seven days, p < 0.001), ii) maintained a more primitive immunophenotype for more cell divisions, iii) exhibited superior morphological, migratory and adhesive properties, and iv) showed the significantly highest numbers of engraftment and multilineage differentiation (CD45, CD34, CD13, CD3 and CD19) in BM, spleen and peripheral blood in long-term transplanted NSG mice compared to the other 3D biomaterial scaffolds. Thus, the 3D fibrin scaffold based BM-mimicry strategy reveals optimal requirements for translation into clinical protocols for CB expansion and transplantation.

Identification of leukemia stem cells in acute myeloid leukemia and their clinical relevance

Acute myeloid leukemia (AML) is considered to be a disease of stem cells. A rare defective stem cell population is purported to drive tumor growth. Similarly to their normal counterparts, leukemic stem cells (LSC) divide extreme slowly. This may explain the ineffectiveness of conventional chemotherapy in combatting this disease. Novel treatment strategies aimed at disrupting the binding of LSC to stem cell niches within the bone marrow might render the LSC vulnerable to chemotherapy and thus improving treatment outcome. This review focuses on the detection of LSC, our current knowledge about their cellular and molecular biology, and LSC interaction with the niche. Finally, we discuss the clinical relevance of LSC and prospective targeted treatment strategies for patients with AML.

Combination of low O(2) concentration and mesenchymal stromal cells during culture of cord blood CD34(+) cells improves the maintenance and proliferative capacity of hematopoietic stem cells

The physiological approach suggests that an environment associating the mesenchymal stromal cells (MSC) and low O(2) concentration would be most favorable for the maintenance of hematopoietic stem cells (HSCs) in course of ex vivo expansion of hematopoietic grafts. To test this hypothesis, we performed a co-culture of cord blood CD34(+) cells with or without MSC in presence of cytokines for 10 days at 20%, 5%, and 1.5% O(2) and assessed the impact on total cells, CD34(+) cells, committed progenitors (colony-forming cells-CFC) and stem cells activity (pre-CFC and Scid repopulating cells-SRC). Not surprisingly, the expansion of total cells, CD34(+) cells, and CFC was higher in co-culture and at 20% O(2) compared to simple culture and low O(2) concentrations, respectively. However, co-culture at low O(2) concentrations provided CD34(+) cell and CFC amplification similar to classical culture at 20% O(2) . Interestingly, low O(2) concentrations ensured a better pre-CFC and SRC preservation/expansion in co-culture. Indeed, SRC activity in co-culture at 1.5% O(2) was higher than in freshly isolated CD34(+) cells. Interleukin-6 production by MSC at physiologically low O(2) concentrations might be one of the factors mediating this effect. Our data demonstrate that association of co-culture and low O(2) concentration not only induces sufficient expansion of committed progenitors (with respect to the classical culture), but also ensures a better maintenance/expansion of hematopoietic stem cells (HSCs), pointing to the oxygenation as a physiological regulatory factor but also as a cell engineering tool.

The establishment of a bank of stored clinical bone marrow stromal cell products

Background

Bone marrow stromal cells (BMSCs) are being used to treat a variety of conditions. For many applications a supply of cryopreserved products that can be used for acute therapy is needed. The establishment of a bank of BMSC products from healthy third party donors is described.

Methods

The recruitment of healthy subjects willing to donate marrow for BMSC production and the Good Manufacturing Practices (GMP) used for assessing potential donors, collecting marrow, culturing BMSCs and BMSC cryopreservation are described.

Results

Seventeen subjects were enrolled in our marrow collection protocol for BMSC production. Six of the 17 subjects were found to be ineligible during the donor screening process and one became ill and their donation was cancelled. Approximately 12 ml of marrow was aspirated from one posterior iliac crest of 10 donors; one donor donated twice. The BMSCs were initially cultured in T-75 flasks and then expanded for three passages in multilayer cell factories. The final BMSC product was packaged into units of 100 × 10⁶ viable cells, cryopreserved and stored in a vapor phase liquid nitrogen tank under continuous monitoring. BMSC products meeting all lot release criteria were obtained from 8 of the 11 marrow collections. The rate of growth of the primary cultures was similar for all products except those generated from the two oldest donors. One lot did not meet the criteria for final release; its CD34 antigen expression was greater than the cut off set at 5%. The mean number of BMSC units obtained from each donor was 17 and ranged from 3 to 40.

Conclusions

The production of large numbers of BMSCs from bone marrow aspirates of healthy donors is feasible, but is limited by the high number of donors that did not meet eligibility criteria and products that did not meet lot release criteria.

Mesenchymal stem cells and cardiovascular disease: a bench to bedside roadmap

In recent years, the incredible boost in stem cell research has kindled the expectations of both patients and physicians. Mesenchymal progenitors, owing to their availability, ease of manipulation, and therapeutic potential, have become one of the most attractive options for the treatment of a wide range of diseases, from cartilage defects to cardiac disorders. Moreover, their immunomodulatory capacity has opened up their allogenic use, consequently broadening the possibilities for their application. In this review, we will focus on their use in the therapy of myocardial infarction, looking at their characteristics, in vitro and in vivo mechanisms of action, as well as clinical trials.

MicroRNAs are shaping the hematopoietic landscape

Hematopoiesis is regulated by microRNAs (miRNAs). These small regulatory RNAs are master regulators of developmental processes that modulate expression of several target genes post-transcriptionally. Various miRNAs are up-regulated at specific stages during hematopoietic development and the functional relevance of miRNAs has been proven at many different stages of lineage specification. Knockout of specific miRNAs can produce dramatic phenotypes leading to severe hematopoietic defects. Furthermore, several studies demonstrated that specific miRNAs are differentially expressed in hematopoietic stem cells. However, the emerging picture is extremely complex due to differences between species, cell type dependent variation in miRNA expression and differential expression of diverse target genes that are involved in various regulatory networks. There is also evidence that miRNAs play a role in cellular aging or in the inter-cellular crosstalk between hematopoietic cells and their microenvironment. The field is rapidly evolving due to new profiling tools and deep sequencing technology. The expression profiles of miRNAs are of diagnostic relevance for classification of different diseases. Recent reports on the generation of induced pluripotent stem cells with miRNAs have fuelled the hope that specific miRNAs and culture conditions facilitate directed differentiation or culture expansion of the hematopoietic stem cell pool. This review summarizes our current knowledge about miRNA expression in hematopoietic stem and progenitor cells, and their role in the hematopoietic stem cell niche.

Combined characterization of microRNA and mRNA profiles delineates early differentiation pathways of CD133+ and CD34+ hematopoietic stem and progenitor cells

MicroRNAs (miRNAs) have been shown to play an important role in hematopoiesis. To elucidate the role of miRNAs in the early steps of hematopoiesis, we directly compared donor-matched CD133⁺ cells with the more differentiated CD34⁺CD133⁻ and CD34⁻CD133⁻ cells from bone marrow on the miRNA and mRNA level. Using quantitative whole genome miRNA microarray and sequencing-based profiling, we found that between 109 (CD133⁺) and 216 (CD34⁻CD133⁻) miRNAs were expressed. Quantification revealed that the 25 highest expressed miRNAs accounted for 73% of the total miRNA pool. miR-142-3p was the highest expressed miRNA with up to 2,000 copies per cell in CD34⁺CD133⁻ cells. Eighteen miRNAs were significantly differentially expressed between CD133⁺ and CD34⁺CD133⁻ cells. We analyzed their biological role by examining the coexpression of miRNAs and its bioinformatically predicted mRNA targets and luciferase-based reporter assays. We provide the first evidence for a direct regulation of CD133 by miR-142-3p as well as tropomyosin 1 and frizzled homolog 5 by miR-29a. Overexpression of miRNAs in CD133⁺ cells demonstrated that miR-142-3p has a negative influence on the overall colony-forming ability. In conclusion, the miRNAs expressed differentially between the CD133⁺ and CD34⁺CD133⁻ cells are involved in inhibition of differentiation, prevention of apoptosis, and cytoskeletal remodeling. These results are highly relevant for stem cell-based therapies with CD133⁺ cells and delineate for the first time how the stem cell character of CD133⁺ cells is defined by the expression of specific miRNAs. Stem Cells 2011;29:847–857

Leukemia stem cells

Leukemia stem cells (LSCs) might originate from malignant transformation of normal hematopoietic stem cells (HSCs), or alternatively, of progenitors in which the acquired mutations have re-installed a dysregulated self-renewal program. LSCs are on top of a hierarchy and generate leukemia cells with more differentiated characteristics. While most leukemia cells are initially sensitive to chemo- and radiotherapy, LSCs are resistant and are considered to be the basis for disease relapse after initial response. Albeit important knowledge on LSC biology has been gained from xenogeneic transplantation models introducing human leukemia cells into immune deficient mouse models, the prospective identification and isolation of human LSC candidates has remained elusive and their prognostic and therapeutic significance controversial. This review focuses on the identification, enrichment and characterization of human LSC derived from patients with acute myeloid leukemia (AML). Experimental data demonstrating the clinical significance of estimating LSC burden and strategies to eliminate LSC will be summarized. For long-term cure of AML, it is of importance to define LSC candidates and to understand their tumor biology compared to normal HSCs. Such comparative studies might provide novel markers for the identification of LSC and for the development of treatment strategies that might be able to eradicate them.

Inflammation and mesenchymal stem cell aging

In adults, mesenchymal stromal cells contain tissue-specific multipotent stem cells, MSC, which can be found throughout the body. With advancing age, tight controls of regulatory networks, which guide MSC biology, gradually deteriorate. Aberrations within the MSC microenvironment such as chronic inflammation eventually lead to adverse manifestations, such as the accumulation of fat deposits in bone and muscles, impaired healing and fibrosis after severe injury, or altered hematopoiesis and autoimmunity. MSC can also specifically interact with a large variety of immune cells, and in doing so, they secrete cytoprotective and immunoregulatory molecules, which together with intercellular contacts mediate immune modulatory processes. This review comprehends the current knowledge regarding molecular mechanisms and cellular interactions that occur in stem cell niches, which are jointly shared between MSC and hematopoietic stem and progenitor cells, as well as those intracellular interdependences taking place between mesenchymal and a wide variety of hematopoietic progeny in particular T lymphocytes, which eventually perturb tissue homeostasis and immunology at advanced age.

Serum after autologous transplantation stimulates proliferation and expansion of human hematopoietic progenitor cells

Regeneration after hematopoietic stem cell transplantation (HSCT) depends on enormous activation of the stem cell pool. So far, it is hardly understood how these cells are recruited into proliferation and self-renewal. In this study, we have addressed the question if systemically released factors are involved in activation of hematopoietic stem and progenitor cells (HPC) after autologous HSCT. Serum was taken from patients before chemotherapy, during neutropenia and after hematopoietic recovery. Subsequently, it was used as supplement for in vitro culture of CD34⁺ cord blood HPC. Serum taken under hematopoietic stress (4 to 11 days after HSCT) significantly enhanced proliferation, maintained primitive immunophenotype (CD34⁺, CD133⁺, CD45⁻) for more cell divisions and increased colony forming units (CFU) as well as the number of cobblestone area-forming cells (CAFC). The stimulatory effect decays to normal levels after hematopoietic recovery (more than 2 weeks after HSCT). Chemokine profiling revealed a decline of several growth-factors during neutropenia, including platelet-derived growth factors PDGF-AA, PDGF-AB and PDGF-BB, whereas expression of monocyte chemotactic protein-1 (MCP-1) increased. These results demonstrate that systemically released factors play an important role for stimulation of hematopoietic regeneration after autologous HSCT. This feedback mechanism opens new perspectives for in vivo stimulation of the stem cell pool.

Synergistic effects of growth factors and mesenchymal stromal cells for expansion of hematopoietic stem and progenitor cells

OBJECTIVE

The number of hematopoietic stem and progenitor cells (HPCs) per cord blood unit is limited, and this can result in delayed engraftment or graft failure. In vitro expansion of HPCs provides a perspective to overcome these limitations. Cytokines as well as mesenchymal stromal cells (MSCs) have been shown to support HPCs ex vivo expansion, but a systematic analysis of their interplay remains elusive.

MATERIALS AND METHODS

Twenty different combinations of growth factors (stem cell factor [SCF], thrombopoietin [TPO], fibroblast growth factor-1 [FGF-1], angiopoietin-like 5, and insulin-like growth factor-binding protein 2), either with or without MSC coculture were systematically compared for their ability to support HPC expansion. CD34(+) cells were stained with carboxyfluorescein diacetate N-succinimidyl ester to monitor cell division history in conjunction with immunophenotype. Colony-forming unit frequencies and hematopoietic reconstitution of nonobese diabetic severe combined immunodeficient mice were also assessed.

RESULTS

Proliferation of HPCs was stimulated by coculture with MSCs. This was further enhanced in combination with SCF, TPO, and FGF-1. Moreover, these conditions maintained expression of primitive surface markers for more than four cell divisions. Colony-forming unit-initiating cells were not expanded without stromal support, whereas an eightfold increase was reached by simultaneous cytokine-treatment and MSC coculture. Importantly, in comparison to expansion without stromal support, coculture with MSCs significantly enhanced hematopoietic chimerism in a murine transplantation model.

CONCLUSIONS

The supportive effect of MSCs on hematopoiesis can be significantly increased by addition of specific recombinant growth factors; especially in combination with SCF, TPO, and FGF-1.

Mesenchymal stem cells secreting angiopoietin-like-5 support efficient expansion of human hematopoietic stem cells without compromising their repopulating potential

Clinical and preclinical applications of human hematopoietic stem cells (HSCs) are often limited by scarcity of cells. Expanding human HSCs to increase their numbers while maintaining their stem cell properties has therefore become an important area of research. Here, we report a robust HSC coculture system wherein cord blood CD34(+) CD133(+) cells were cocultured with mesenchymal stem cells engineered to express angiopoietin-like-5 in a defined medium. After 11 days of culture, SCID repopulating cells were expanded ~60-fold by limiting dilution assay in NOD-scid Il2rg(-/-) (NSG) mice. The cultured CD34(+) CD133(+) cells had similar engraftment potential to uncultured CD34(+) CD133(+) cells in competitive repopulation assays and were capable of efficient secondary reconstitution. Further, the expanded cells supported a robust multilineage reconstitution of human blood cells in NSG recipient mice, including a more efficient T-cell reconstitution. These results demonstrate that the expanded CD34(+) CD133(+) cells maintain both short-term and long-term HSC activities. To our knowledge, this ~60-fold expansion of SCID repopulating cells is the best expansion of human HSCs reported to date. Further development of this coculture method for expanding human HSCs for clinical and preclinical applications is therefore warranted.

Replicative senescence-associated gene expression changes in mesenchymal stromal cells are similar under different culture conditions

BACKGROUND

Research on mesenchymal stromal cells has created high expectations for a variety of therapeutic applications. Extensive propagation to yield enough mesenchymal stromal cells for therapy may result in replicative senescence and thus hamper long-term functionality in vivo. Highly variable proliferation rates of mesenchymal stromal cells in the course of long-term expansions under varying culture conditions may already indicate different propensity for cellular senescence. We hypothesized that senescence-associated regulated genes differ in mesenchymal stromal cells propagated under different culture conditions.

DESIGN AND METHODS

Human bone marrow-derived mesenchymal stromal cells were cultured either by serial passaging or by a two-step protocol in three different growth conditions. Culture media were supplemented with either fetal bovine serum in varying concentrations or pooled human platelet lysate.

RESULTS

All mesenchymal stromal cell preparations revealed significant gene expression changes upon long-term culture. Especially genes involved in cell differentiation, apoptosis and cell death were up-regulated, whereas genes involved in mitosis and proliferation were down-regulated. Furthermore, overlapping senescence-associated gene expression changes were found in all mesenchymal stromal cell preparations.

CONCLUSIONS

Long-term cell growth induced similar gene expression changes in mesenchymal stromal cells independently of isolation and expansion conditions. In advance of therapeutic application, this panel of genes might offer a feasible approach to assessing mesenchymal stromal cell quality with regard to the state of replicative senescence.

Mesenchymal stromal cells derived from human umbilical cord tissues: primitive cells with potential for clinical and tissue engineering applications

Mesenchymal stem or stromal cells (MSCs) have a high potential for cell-based therapies as well as for tissue engineering applications. Since Friedenstein first isolated stem or precursor cells from the human bone marrow (BM) stroma that were capable of osteogenesis, BM is currently the most common source for MSCs. However, BM presents several disadvantages, namely low frequency of MSCs, high donor-dependent variations in quality, and painful invasive intervention. Thus, tremendous research efforts have been observed during recent years to find alternative sources for MSCs.In this context, the human umbilical cord (UC) has gained more and more attention. Since the UC is discarded after birth, the cells are easily accessible without ethical concerns. This postnatal organ was found to be rich in primitive stromal cells showing typical characteristics of bone-marrow MSCs (BMSCs), e.g., they grow as plastic-adherent cells with a fibroblastic morphology, express a set of typical surface markers, and can be directly differentiated at least along mesodermal lineages. Compared to BM, the UC tissue bears a higher frequency of stromal cells with a higher in vitro expansion potential. Furthermore, immune-privileged and immune-modulatory properties are reported for UC-derived cells, which open highly interesting perspectives for clinical applications.

Modeling of replicative senescence in hematopoietic development

Hematopoietic stem cells (HSC) give rise to an enormous number of blood cells throughout our life. In contrast their number of cell divisions preceding senescence is limited underin vitro culture conditions. Here we consider the question whether HSC can rejuvenate indefinitely or if the number of cell divisions is restricted. We have developed a multi-compartmental model for hematopoietic differentiation based on ordinary differential equations. The model is based on the hypothesis that in each step of maturation, the percentage of self-renewal versus differentiation is regulated by a single external feedback mechanism. We simulate the model under the assumption that hematopoietic differentiation precedes the six steps of maturation and the cells ultimately cease to proliferate after 50 divisions. Our results demonstrate that it is conceivable to maintain hematopoiesis over a life-time if HSC have a slow division rate and a high self-renewal rate. With age, the feedback signal increases and this enhances self-renewal, which results in the increase of the number of stem and progenitor cells. This study demonstrates that replicative senescence is compatible with life-long hematopoiesis and that model predictions are in line with experimental observations. Thus, HSC might not divide indefinitely with potentially important clinical implications.

Aging and replicative senescence have related effects on human stem and progenitor cells

The regenerative potential diminishes with age and this has been ascribed to functional impairments of adult stem cells. Cells in culture undergo senescence after a certain number of cell divisions whereby the cells enlarge and finally stop proliferation. This observation of replicative senescence has been extrapolated to somatic stem cells in vivo and might reflect the aging process of the whole organism. In this study we have analyzed the effect of aging on gene expression profiles of human mesenchymal stromal cells (MSC) and human hematopoietic progenitor cells (HPC). MSC were isolated from bone marrow of donors between 21 and 92 years old. 67 genes were age-induced and 60 were age-repressed. HPC were isolated from cord blood or from mobilized peripheral blood of donors between 27 and 73 years and 432 genes were age-induced and 495 were age-repressed. The overlap of age-associated differential gene expression in HPC and MSC was moderate. However, it was striking that several age-related gene expression changes in both MSC and HPC were also differentially expressed upon replicative senescence of MSC in vitro. Especially genes involved in genomic integrity and regulation of transcription were age-repressed. Although telomerase activity and telomere length varied in HPC particularly from older donors, an age-dependent decline was not significant arguing against telomere exhaustion as being causal for the aging phenotype. These studies have demonstrated that aging causes gene expression changes in human MSC and HPC that vary between the two different cell types. Changes upon aging of MSC and HPC are related to those of replicative senescence of MSC in vitro and this indicates that our stem and progenitor cells undergo a similar process also in vivo.