Human marrow stromal precursors are alpha 1 integrin subunit-positive

Unveiling heterogeneity in MSCs: exploring marker-based strategies for defining MSC subpopulations

Mesenchymal stem/stromal cells (MSCs) represent a heterogeneous cell population distributed throughout various tissues, demonstrating remarkable adaptability to microenvironmental cues and holding immense promise for disease treatment. However, the inherent diversity within MSCs often leads to variability in therapeutic outcomes, posing challenges for clinical applications. To address this heterogeneity, purification of MSC subpopulations through marker-based isolation has emerged as a promising approach to ensure consistent therapeutic efficacy. In this review, we discussed the reported markers of MSCs, encompassing those developed through candidate marker strategies and high-throughput approaches, with the aim of explore viable strategies for addressing the heterogeneity of MSCs and illuminate prospective research directions in this field.

Single-cell RNA sequencing of human non-hematopoietic bone marrow cells reveals a unique set of inter-species conserved biomarkers for native mesenchymal stromal cells

BACKGROUND

Native bone marrow (BM) mesenchymal stem/stromal cells (BM-MSCs) participate in generating and shaping the skeleton and BM throughout the lifespan. Moreover, BM-MSCs regulate hematopoiesis by contributing to the hematopoietic stem cell niche in providing critical cytokines, chemokines and extracellular matrix components. However, BM-MSCs contain a heterogeneous cell population that remains ill-defined. Although studies on the taxonomy of native BM-MSCs in mice have just started to emerge, the taxonomy of native human BM-MSCs remains unelucidated.

METHODS

By using single-cell RNA sequencing (scRNA-seq), we aimed to define a proper taxonomy for native human BM non-hematopoietic subsets including endothelial cells (ECs) and mural cells (MCs) but with a focal point on MSCs. To this end, transcriptomic scRNA-seq data were generated from 5 distinct BM donors and were analyzed together with other transcriptomic data and with computational biology analyses at different levels to identify, characterize and classify distinct native cell subsets with relevant biomarkers.

RESULTS

We could ascribe novel specific biomarkers to ECs, MCs and MSCs. Unlike ECs and MCs, MSCs exhibited an adipogenic transcriptomic pattern while co-expressing genes related to hematopoiesis support and multilineage commitment potential. Furthermore, by a comparative analysis of scRNA-seq of BM cells from humans and mice, we identified core genes conserved in both species. Notably, we identified MARCKS, CXCL12, PDGFRA, and LEPR together with adipogenic factors as archetypal biomarkers of native MSCs within BM. In addition, our data suggest some complex gene nodes regulating critical biological functions of native BM-MSCs together with a preferential commitment toward an adipocyte lineage.

CONCLUSIONS

Overall, our taxonomy for native BM non-hematopoietic compartment provides an explicit depiction of gene expression in human ECs, MCs and MSCs at single-cell resolution. This analysis helps enhance our understanding of the phenotype and the complexity of biological functions of native human BM-MSCs.

Cadherins, Selectins, and Integrins in CAM-DR in Leukemia

The interaction between leukemia cells and the bone microenvironment is known to provide drug resistance in leukemia cells. This phenomenon, called cell adhesion-mediated drug resistance (CAM-DR), has been demonstrated in many subsets of leukemia including B- and T-acute lymphoblastic leukemia (B- and T-ALL) and acute myeloid leukemia (AML). Cell adhesion molecules (CAMs) are surface molecules that allow cell-cell or cell-extracellular matrix (ECM) adhesion. CAMs not only recognize ligands for binding but also initiate the intracellular signaling pathways that are associated with cell proliferation, survival, and drug resistance upon binding to their ligands. Cadherins, selectins, and integrins are well-known cell adhesion molecules that allow binding to neighboring cells, ECM proteins, and soluble factors. The expression of cadherin, selectin, and integrin correlates with the increased drug resistance of leukemia cells. This paper will review the role of cadherins, selectins, and integrins in CAM-DR and the results of clinical trials targeting these molecules.

ALCAM (CD166) as a gene expression marker for human mesenchymal stromal cell characterisation

Background

Human mesenchymal stromal cells (MSCs) phenotypically share their positive expression of the International Society for Cell and Gene Therapy (ISCT) markers CD73, CD90 and CD105 with fibroblasts. Fibroblasts are often co-isolated as an unwanted by-product from biopsy and they can rapidly overgrow the MSCs in culture. Indeed, many other surface markers have been proposed, though no unique MSC specific marker has been identified yet. Quantitative PCR (qPCR) is a precise, efficient and rapid method for gene expression analysis. To identify a marker suitable for accurate MSC characterisation, qPCR was exploited.

Methods and results

Two commercially obtained bone marrow (BM) derived MSCs and an hTERT immortalised BM-MSC line (MSC-TERT) have been cultured for different days and at different oxygen levels before RNA extraction. Together with RNA samples previous extracted from umbilical cord derived MSCs and MSC-TERT cells cultured in 2D or 3D, this heterogeneous sample set was quantitatively analysed for the expression levels of 18 candidate MSC marker genes. The expression levels in MSCs were compared with the expression levels in fibroblasts to verify the differentiation capability of these genes between MSCs and fibroblasts. None of the ISCT markers could differentiate between fibroblasts and MSCs. A total of six other genes (ALCAM, CLIC1, EDIL3, EPHA2, NECTIN2, and TMEM47) were identified as possible biomarkers for accurate identification of MSCs.

Conclusion

Justified by considerations on expression level, reliability and specificity, Activated-Leukocyte Cell Adhesion Molecule (ALCAM) was the best candidate for improving the biomarker set of MSC identification.

CD34 and CD49f Double-Positive and Lineage Marker-Negative Cells Isolated from Human Myometrium Exhibit Stem Cell-Like Properties Involved in Pregnancy-Induced Uterine Remodeling

Repeated and dramatic pregnancy-induced uterine enlargement and remodeling throughout reproductive life suggests the existence of uterine smooth muscle stem/progenitor cells. The aim of this study was to isolate and characterize stem/progenitor-like cells from human myometrium through identification of specific surface markers. We here identify CD49f and CD34 as markers to permit selection of the stem/progenitor cell-like population from human myometrium and show that human CD45(-) CD31(-) glycophorin A(-) and CD49f(+) CD34(+) myometrial cells exhibit stem cell-like properties. These include side population phenotypes, an undifferentiated status, high colony-forming ability, multilineage differentiation into smooth muscle cells, osteoblasts, adipocytes, and chondrocytes, and in vivo myometrial tissue reconstitution following xenotransplantation. Furthermore, CD45(-) CD31(-) glycophorin A(-) and CD49f(+) CD34(+) myometrial cells proliferate under hypoxic conditions in vitro and, compared with the untreated nonpregnant myometrium, show greater expansion in the estrogen-treated nonpregnant myometrium and further in the pregnant myometrium in mice upon xenotransplantation. These results suggest that the newly identified myometrial stem/progenitor-like cells influenced by hypoxia and sex steroids may participate in pregnancy-induced uterine enlargement and remodeling, providing novel insights into human myometrial physiology.

Human uterine leiomyoma stem/progenitor cells expressing CD34 and CD49b initiate tumors in vivo

CONTEXT

Uterine leiomyoma is the most common benign tumor in reproductive-age women. Using a dye-exclusion technique, we previously identified a side population of leiomyoma cells exhibiting stem cell characteristics. However, unless mixed with mature myometrial cells, these leiomyoma side population cells did not survive or grow well in vitro or in vivo.

OBJECTIVE

The objective of this study was to identify cell surface markers to isolate leiomyoma stem/progenitor cells.

DESIGN

Real-time PCR screening was used to identify cell surface markers preferentially expressed in leiomyoma side population cells. In vitro colony-formation assay and in vivo tumor-regeneration assay were used to demonstrate functions of leiomyoma stem/progenitor cells.

RESULTS

We found significantly elevated CD49b and CD34 gene expression in side population cells compared with main population cells. Leiomyoma cells were sorted into three populations based on the expression of CD34 and CD49b: CD34(+)/CD49b(+), CD34(+)/CD49b(-), and CD34(-)/CD49b(-) cells, with the majority of the side population cells residing in the CD34(+)/CD49b(+) fraction. Of these populations, CD34(+)/CD49b(+) cells expressed the lowest levels of estrogen receptor-α, progesterone receptor, and α-smooth muscle actin, but the highest levels of KLF4, NANOG, SOX2, and OCT4, confirming their more undifferentiated status. The stemness of CD34(+)/CD49b(+) cells was also demonstrated by their strongest in vitro colony-formation capacity and in vivo tumor-regeneration ability.

CONCLUSIONS

CD34 and CD49b are cell surface markers that can be used to enrich a subpopulation of leiomyoma cells possessing stem/progenitor cell properties; this technique will accelerate efforts to develop new therapies for uterine leiomyoma.

Concise review: combining human leukocyte antigen G and mesenchymal stem cells for immunosuppressant biotherapy

Both human leukocyte antigen G (HLA-G) and multipotential mesenchymal stem/stromal cells (MSCs) exhibit immunomodulatory functions. In allogeneic tranplantation, the risks of acute and chronic rejection are still high despite improvement in immunosuppressive treatments, and the induction of a state of tolerance to alloantigens is not achieved. Immunomodulatory properties of MSCs and HLA-G in human allogeneic tranplantation to induce tolerance appears attractive and promising. Interestingly, we and others have demonstrated that MSCs can express HLA-G. In this review, we focus on the expression of HLA-G by MSCs and discuss how to ensure and improve the immunomodulatory properties of MSCs by selectively targeting MSCs expressing HLA-G (MSCs(HLA-G+)). We also discuss the possible uses of MSCs(HLA-G+) for therapeutic purposes, notably, to overcome acute and chronic immune rejection in solid-organ allogeneic transplantation in humans. Since MSCs are phenotypically and functionally heterogeneous, it is of primary interest to have specific markers ensuring that they have strong immunosuppressive potential and HLA-G may be a valuable candidate.

"Mesenchymal" stem cells

Two opposing descriptions of so-called mesenchymal stem cells (MSCs) exist at this time. One sees MSCs as the postnatal, self-renewing, and multipotent stem cells for the skeleton. This cell coincides with a specific type of bone marrow perivascular cell. In skeletal physiology, this skeletal stem cell is pivotal to the growth and lifelong turnover of bone and to its native regeneration capacity. In hematopoietic physiology, its role as a key player in maintaining hematopoietic stem cells in their niche and in regulating the hematopoietic microenvironment is emerging. In the alternative description, MSCs are ubiquitous in connective tissues and are defined by in vitro characteristics and by their use in therapy, which rests on their ability to modulate the function of host tissues rather than on stem cell properties. Here, I discuss how the two views developed, conceptually and experimentally, and attempt to clarify the confusion arising from their collision.

Cell therapy for bone repair

When natural bone repair mechanisms fail, autologous bone grafting is the current standard of care. The osteogenic cells and bone matrix in the graft provide the osteo-inductive and osteo-conductive properties required for successful bone repair. Bone marrow (BM) mesenchymal stem cells (MSCs) can differentiate into osteogenic cells. MSC-based cell therapy holds promise for promoting bone repair. The amount of MSCs available from iliac-crest aspirates is too small to be clinically useful, and either concentration or culture must therefore be used to expand the MSC population. MSCs can be administered alone via percutaneous injection or implanted during open surgery with a biomaterial, usually biphasic hydroxyapatite/β-calcium-triphosphate granules. Encouraging preliminary results have been obtained in patients with delayed healing of long bone fractures or avascular necrosis of the femoral head. Bone tissue engineering involves in vitro MSC culturing on biomaterials to obtain colonisation of the biomaterial and differentiation of the cells. The biomaterial-cell construct is then implanted into the zone to be treated. Few published data are available on bone tissue engineering. Much work remains to be done before determining whether this method is suitable for the routine filling of bone tissue defects. Increasing cell survival and promoting implant vascularisation are major challenges. Improved expertise with culturing techniques, together with the incorporation of regulatory requirements, will open the way to high-quality clinical trials investigating the usefulness of cell therapy as a method for achieving bone repair. Cell therapy avoids the drawbacks of autologous bone grafting, preserving the bone stock and diminishing treatment invasiveness.

Integrin α1β1

Integrin α1β1 is widely expressed in mesenchyme and the immune system, as well as a minority of epithelial tissues. Signaling through α1 contributes to the regulation of extracellular matrix composition, in addition to supplying in some tissues a proliferative and survival signal that appears to be unique among the collagen binding integrins. α1 provides a tissue retention function for cells of the immune system including monocytes and T cells, where it also contributes to their long-term survival, providing for peripheral T cell memory, and contributing to diseases of autoimmunity. The viability of α1 null mice, as well as the generation of therapeutic monoclonal antibodies against this molecule, have enabled studies of the role of α1 in a wide range of pathophysiological circumstances. The immune functions of α1 make it a rational therapeutic target.

Production of mesenchymal stromal/stem cells according to good manufacturing practices: a review

Because of their multi/pluripotency and immunosuppressive properties, mesenchymal stem/stromal cells (MSCs) are important tools for treating immune disorders and for tissue repair. The increasing use of MSCs, their definition as advanced-therapy medicinal products in European regulations, and the US Food and Drug Administration requirements for their production and use imply the use of production processes that should be in accordance with Good Manufacturing Practices (GMPs). Complying with GMPs requires precisely defining the production process (es) as well as the multiple criteria required for a quality final product. Such variables include the environment, staff training and qualification, and controls. Developing processes based on well-defined or completely defined media and operating in closed systems or bioreactors is important and will increase safety and reproducibility. One of the most challenging issues remains implementation of relevant and reproducible controls for safety and efficacy. A linking of researchers, research and development teams, producers, and clinicians is mandatory to achieve GMP-compliant processes with relevant controls for producing well-defined, safe, and efficient MSCs.

Chondrogenic potentials of human synovium-derived cells sorted by specific surface markers

OBJECTIVE

We aimed to evaluate expression levels of nine candidate surface markers for chondrogenic potential in human synovial cells and to determine whether cell pellets positively sorted by each specific marker would have valuable chondrogenic potential.

METHODS

The expression levels of the selected nine leading surface markers in synovial cells from knee joints in 15 patients with primary knee osteoarthritis were evaluated at the stage of isolation and after cultivation using flow cytometry. We obtained positive and negative cells for each surface marker using a magnetically activated cell sorting method and compared chondrogenic potentials between the positive and the negative cell pellets.

RESULTS

CD29, CD44, CD73, and CD90 were expressed on the most synovial cells at the isolation stage and on almost all cells at stage of P0 and P1. CD133 was rarely expressed at any stages of the evaluated cells. CD166 was expressed in 7.1% of cells at the isolation stage on average, but this expression increased after cell passages. The expressions of CD10 and CD105 also increased after cell passages while the expression of CD49a made no significant difference at progressive stages of isolation and passage. Comparison of chondrogenic potentials between positive and negative cell pellets for each marker revealed that only CD105- and CD166-positive cell pellets showed better chondrogenic potentials (type II collagen gene expression, cartilage matrix formation, and GAG expression) than the corresponding negative cell pellets.

CONCLUSION

Our study suggests that CD105 and CD166 would be valuable surface markers associated with chondrogenic potential; thus, CD105- and CD166-enriched cells derived from human synovium would be practical and valuable sources for cartilage regeneration.

Primary marrow-derived stromal cells: isolation and manipulation

Marrow stromal cells (MSCs) are relatively rare cells difficult to visualize in marrow biopsies or detect in aspirated marrow. Under specific conditions MSC can be expanded in vitro and the population can give rise to several mesenchymal lineages. "MSC" also refers to mesenchymal stem cells which implies that all cells in the population are multipotent. It is generally agreed that while there may be a few multipotent stem cells in an MSC population the majority are not stem cells. In either case MSCs do not produce hematopoietic cells. Although MSCs have been isolated and characterized from several tissues, bone marrow is their most common source for research and clinical use. Primary MSC populations can be derived from bone marrow mononuclear cells with relative ease, but it is important to recognize the cellular heterogeneity within a culture and how this may vary from donor to donor. In this chapter, we describe methodology to derive primary MSCs from bone marrow screens, an otherwise discarded by-product of bone marrow harvests used for clinical transplantation. We also describe some useful techniques to characterize and manipulate MSCs-both primary and immortalized cell lines.

In vivo transplantation of autogenous marrow-derived cells following rapid intraoperative magnetic separation based on hyaluronan to augment bone regeneration

INTRODUCTION

This project was designed to test the hypothesis that rapid intraoperative processing of bone marrow based on hyaluronan (HA) could be used to improve the outcome of local bone regeneration if the concentration and prevalence of marrow-derived connective tissue progenitors (CTPs) could be increased and nonprogenitors depleted before implantation.

METHODS

HA was used as a marker for positive selection of marrow-derived CTPs using magnetic separation (MS) to obtain a population of HA-positive cells with an increased CTP prevalence. Mineralized cancellous allograft (MCA) was used as an osteoconductive carrier scaffold for loading of HA-positive cells. The canine femoral multidefect model was used and four cylindrical defects measuring 10 mm in diameter and 15 mm in length were grafted with MCA combined with unprocessed marrow or with MS processed marrow that was enriched in HA(+) CTPs and depleted in red blood cells and nonprogenitors. Outcome was assessed at 4 weeks using quantitative 3D microcomputed tomography (micro-CT) analysis of bone formation and histomorphological assessment.

RESULTS

Histomorphological assessment showed a significant increase in new bone formation and in the vascular sinus area in the MS-processed defects. Robust bone formation was found throughout the defect area in both groups (defects grafted with unprocessed marrow or with MS processed marrow.) Percent bone volume in the defects, as assessed by micro-CT, was greater in defects engrafted with MS processed cells, but the difference was not statistically significant.

CONCLUSION

Rapid intraoperative MS processing to enrich CTPs based on HA as a surface marker can be used to increase the concentration and prevalence of CTPs. MCA grafts supplemented with heparinized bone marrow or MS processed cells resulted in a robust and advanced stage of bone regeneration at 4 weeks. A greater new bone formation and vascular sinus area was found in defects grafted with MS processed cells. These data suggest that MS processing may be used to enhance the performance of marrow-derived CTPs in clinical bone regeneration procedures. Further assessment in a more stringent bone defect model is proposed.

Human endometrial stem cells as a new source for programming to neural cells

Human EnSC (endometrial-derived stem cell) is an abundant and easily available source for cell replacement therapy. Many investigations have shown the potency of the cells to differentiate into several mesoderm-derived cell lineages, including osteocytes and adipocytes. Here, the potency of EnSC in neural differentiation has been investigated. Flow cytometric analysis showed that they were positive for CD90, CD105, OCT4, CD44 and negative for CD31, CD34, CD133. The characterized cells were induced into neural differentiation by bFGF (basic fibroblast growth factor), PDGF (platelet-derived growth factor) and EGF (epidermal growth factor) signalling molecules, respectively in a sequential protocol, and differentiated cells were analysed for expression of neuronal markers by RT-PCR (reverse transcription-PCR) and immunocytochemistry, including Nestin, GABA (γ-aminobutyric acid), MAP2 (microtubule-associated protein 2), β3-tub (class III β-tubulin) and NF-L (neurofilament-light) at the level of their mRNAs. The expression of MAP2, β3-tub and NF-L proteins in EnSC was confirmed 28 days PT (post-treatment) by immunocytochemistry. In conclusion, EnSC can respond to signalling molecules that are usually used as standards in neural differentiation and can programme neuronal cells, making these cells worth considering as a unique source for cell therapy in neurodegenerative disease.

Heparan sulfate enhances the self-renewal and therapeutic potential of mesenchymal stem cells from human adult bone marrow

Insufficient cell number hampers therapies utilizing adult human mesenchymal stem cells (hMSCs) and current ex vivo expansion strategies lead to a loss of multipotentiality. Here we show that supplementation with an embryonic form of heparan sulfate (HS-2) can both increase the initial recovery of hMSCs from bone marrow aspirates and increase their ex vivo expansion by up to 13-fold. HS-2 acts to amplify a subpopulation of hMSCs harboring longer telomeres and increased expression of the MSC surface marker stromal precursor antigen-1. Gene expression profiling revealed that hMSCs cultured in HS-2 possess a distinct signature that reflects their enhanced multipotentiality and improved bone-forming ability when transplanted into critical-sized bone defects. Thus, HS-2 offers a novel means for decreasing the expansion time necessary for obtaining therapeutic numbers of multipotent hMSCs without the addition of exogenous growth factors that compromise stem cell fate.

Bone marrow mesenchymal stem cells: biological properties and their role in hematopoiesis and hematopoietic stem cell transplantation

Mesenchymal stem cells (MSCs) are multipotent adult stem cells that are present in practically all tissues as a specialized population of mural cells/pericytes that lie on the abluminal side of blood vessels. Originally identified within the bone marrow (BM) stroma, not only do they provide microenvironmental support for hematopoietic stem cells (HSCs), but can also differentiate into various mesodermal lineages. MSCs can easily be isolated from the BM and subsequently expand in vitro and in addition they exhibit intriguing immunomodulatory properties, thereby emerging as attractive candidates for various therapeutic applications. This review addresses the concept of BM MSCs via a hematologist's point of view. In this context it discusses the stem cell properties that have been attributed to BM MSCs, as compared to those of the prototypic hematopoietic stem cell model and then gives a brief overview of the in vitro and vivo features of the former, emphasizing on their immunoregulatory properties and their hematopoiesis-supporting role. In addition, the qualitative and quantitative characteristics of BM MSCs within the context of a defective microenvironment, such as the one characterizing Myelodysplastic Syndromes are described and the potential involvement of these cells in the pathophysiology of the disease is discussed. Finally, emerging clinical applications of BM MSCs in the field of hematopoietic stem cell transplantation are reviewed and potential hazards from MSC use are outlined.

"Mesenchymal" stem cells in human bone marrow (skeletal stem cells): a critical discussion of their nature, identity, and significance in incurable skeletal disease

At the turn of a decade of intensive wishful thinking, "mesenchymal stem cells" are changing their profile, while retaining their charm. As hopes to turn bone into brain or vice versa seem on the wane, we learn (1) that the archetypal "mesenchymal stem cell," the skeletal stem cell found in the bone marrow, can be directly identified as a specialized type of mural cell/pericyte, found in the wall of sinusoids and long known as adventitial reticular cells; (2) that bone marrow skeletal stem cells are also defined by expression of CD146, and can self-renew in vivo, while giving rise to skeletal tissues, and therefore earn consideration as bona fide stem cells; (3) that a broader class of microvascular mural cells endowed with clonogenicity and progenitor properties may exist in other tissues, although their true potency needs to be firmly established by stringent assays and thorough comparisons across tissues; (4) that bone marrow skeletal stem cells display unique angiopoietic and hematopoietic niche-related functions, consisting in their ability to transfer the hematopoietic microenvironment and to guide the assembly of microvascular networks, which seem to define their inherent biology; and (5) that use of skeletal stem cells as disease models, and as models of high-risk strategies for cell and gene therapy specifically in incurable skeletal diseases, may provide new challenges for the next decade, and perhaps reward for medicine in the one that follows.

Good manufacturing practices production of mesenchymal stem/stromal cells

Because of their multi/pluripotency and immunosuppressive properties mesenchymal stem/stromal cells (MSCs) are important tools for treating immune disorders and for tissue repair. The increasing use of MSCs has led to production processes that need to be in accordance with Good Manufacturing Practice (GMP). In cellular therapy, safety remains one of the main concerns and refers to donor validation, choice of starting material, processes, and the controls used, not only at the batch release level but also during the development of processes. The culture processes should be reproducible, robust, and efficient. Moreover, they should be adapted to closed systems that are easy to use. Implementing controls during the manufacturing of clinical-grade MSCs is essential. The controls should ensure microbiological safety but also avoid potential side effects linked to genomic instability driving transformation and senescence or decrease of cell functions (immunoregulation, differentiation potential). In this rapidly evolving field, a new approach to controls is needed.

Skeletal stem cells and bone regeneration: Translational strategies from bench to clinic

Clinical imperatives for new bone to replace or restore the function of traumatized or bone lost as a consequence of age or disease has led to the need for therapies or procedures to generate bone for skeletal applications. Tissue regeneration promises to deliver specifiable replacement tissues and the prospect of efficacious alternative therapies for orthopaedic applications such as non-union fractures, healing of critical sized segmental defects and regeneration of articular cartilage in degenerative joint diseases. In this paper we review the current understanding of the continuum of cell development from skeletal stem cells, osteoprogenitors through to mature osteoblasts and the role of the matrix microenvironment, vasculature and factors that control their fate and plasticity in skeletal regeneration. Critically, this review addresses in vitro and in vivo models to investigate laboratory and clinical based strategies for the development of new technologies for skeletal repair and the key translational points to clinical success. The application of developmental paradigms of musculoskeletal tissue formation specifically, understanding developmental biology of bone formation particularly in the adult context of injury and disease will, we propose, offer new insights into skeletal cell biology and tissue regeneration allowing for the critical integration of stem cell science, tissue engineering and clinical applications. Such interdisciplinary, iterative approaches will be critical in taking patient aspirations to clinical reality.

Bone regeneration: the stem/progenitor cells point of view

After bone injuries, several molecular mechanisms establish bone repair from stem/progenitor cells. Inflammation factors attract regenerative cells which expand and differentiate in order to build up a bone highly similar to that before injury. Bone marrow (BM) mesenchymal stem cells (MSCs) as skeletal stem cells and endothelial progenitors (EPCs) are at the origin of such reparation mechanisms. However, discrepancies exist about their identities. Although cultured MSCs are extensively described, their in vivo native forms are poorly known. In addition, recent experiments show that several types of EPC exist. We therefore review up-to-date data on the characterization of such stem/progenitor cells and propose a new point of view of their function in bone regeneration.

Mesenchymal Stem Cell Therapy for Nonmusculoskeletal Diseases: Emerging Applications

Mesenchymal stem cells are stem/progenitor cells originated from the mesoderm and can different into multiple cell types of the musculoskeletal system. The vast differentiation potential and the relative ease for culture expansion have established mesenchymal stem cells as the building blocks in cell therapy and tissue engineering applications for a variety of musculoskeletal diseases, including repair of fractures and bone defects, cartilage regeneration, treatment of osteonecrosis of the femoral head, and correction of genetic diseases such as osteogenesis imperfect. However, research in the past decade has revealed differentiation potentials of mesenchymal stem cells beyond lineages of the mesoderm, suggesting broader applications than originally perceived. In this article, we review the recent developments in mesenchymal stem cell research with respect to their emerging properties and applications in nonmusculoskeletal diseases.

Mesenchymal stem cells for therapeutic purposes

Mesenchymal stem cells (MSC) are multipotent adult stem cells harboring a wide range of differentiations and non-human leukocyte antigen-restricted immunosuppressive properties that lead to an increasing use of MSC in immunomodulation and in regenerative medicine. To produce MSC, definitive standards are still lacking. Whatever the starting material used (e.g., bone marrow, adipose tissue, or cord blood), numerous parameters including cell plating density, number of passages, and culture medium, play a major role in the culture process and have to be determined. To date, the different production processes have been effective, and based on phenotypic analysis and differentiation potential, a first set of simple controls have been defined. However, controls of the final product should provide precise data on efficacy and safety. The next challenge will be to develop production processes that reach good manufacturing practices goals and to define more accurate control methods of cultivated MSC.

Use of an alpha-smooth muscle actin GFP reporter to identify an osteoprogenitor population

Identification of a reliable marker of skeletal precursor cells within calcified and soft tissues remains a major challenge for the field. To address this, we used a transgenic model in which osteoblasts can be eliminated by pharmacological treatment. Following osteoblast ablation a dramatic increase in a population of alpha-smooth muscle actin (alpha-SMA) positive cells was observed. During early recovery phase from ablation we have detected cells with the simultaneous expression of alpha-SMA and a preosteoblastic 3.6GFP marker, indicating the potential for transition of alpha-SMA+ cells towards osteoprogenitor lineage. Utilizing alpha-SMAGFP transgene, alpha-SMAGFP+ positive cells were detected in the microvasculature and in the osteoprogenitor population within bone marrow stromal cells. Osteogenic and adipogenic induction stimulated expression of bone and fat markers in the alpha-SMAGFP+ population derived from bone marrow or adipose tissue. In adipose tissue, alpha-SMA+ cells were localized within the smooth muscle cell layer and in pericytes. After in vitro expansion, alpha-SMA+/CD45-/Sca1+ progenitors were highly enriched. Following cell sorting and transplantation of expanded pericyte/myofibroblast populations, donor-derived differentiated osteoblasts and new bone formation was detected. Our results show that cells with a pericyte/myofibroblast phenotype have the potential to differentiate into functional osteoblasts.

Skeletal stem cells: phenotype, biology and environmental niches informing tissue regeneration

Advances in our knowledge of the biology of skeletal stem cells, together with an increased understanding of the regeneration of normal tissue offer exciting new therapeutic approaches in musculoskeletal repair. Skeletal stem cells from various adult tissues such as bone marrow can be identified and isolated based on their expression of a panel of markers associated with smooth muscle cells, pericytes and endothelial cells. Thus, skeletal stem cell-like populations within bone marrow may share a common perivascular stem cell niche within the microvascular network. To date, the environmental niche that nurtures and maintains the stromal stem cell at different anatomical sites remains poorly understood. However, an understanding of the osteogenic and perivascular niches will inform identification of the key growth factors, matrix constituents and physiological conditions that will enhance the ex vivo amplification and differentiation of osteogenic stem cells to mimic native tissue critical for tissue repair. This review will examine skeletal stem cell biology, the advances in our understanding of the skeletal and perivascular niche and interactions therein and the opportunities to harness that knowledge for musculoskeletal regeneration.

Alpha10 integrin expression is up-regulated on fibroblast growth factor-2-treated mesenchymal stem cells with improved chondrogenic differentiation potential

Mesenchymal stem cells (MSCs) are multipotent cells that have the capacity to differentiate into various different cell lineages and can generate bone, cartilage and adipose tissue. MSCs are presently characterized using a broad range of different cell-surface markers that are not exclusive to MSCs and not sensitive to culture conditions or differentiation capacity. We show that the integrin subunits alpha10 and alpha11 of the collagen binding integrins alpha10beta1 and alpha11beta1 are expressed by human MSCs in monolayer cultures. We also demonstrate that the expression of alpha10 increases, while alpha1 and alpha11 decrease, during aggregate culture of MSCs in chondrogenic medium. Alpha10beta1 is expressed by chondrocytes in cartilage, whereas alpha11beta1 integrins are predominantly expressed by subsets of the fibroblastic lineage. In extensive monolayer cultures of MSCs, alpha10 expression is down-regulated. We show that this down-regulation is reversed by fibroblast growth factor-2 (FGF-2) treatment. Addition of FGF-2 to MSCs not only results in increased alpha10 expression, but also in decreased alpha11 expression. FGF-2 treatment of MSCs has been shown to keep the cells more multipotent and also induces cell proliferation and Sox-9 up-regulation. We demonstrate improved chondrogenecity as well as increased collagen-dependant migratory potential of FGF-2-treated MSCs having a high alpha10 expression. We also demonstrate expression of alpha10 and alpha11 integrin subunits in the endosteum and periosteum of mice, but very low or not detectable expression levels in freshly aspired human or mouse BM. We show that MSCs with high chondrogenic differentiation potential are highly alpha10 positive and propose alpha10 as a potential marker to predict the differentiation state of MSCs.

Specific plasma membrane protein phenotype of culture-amplified and native human bone marrow mesenchymal stem cells

We have studied the plasma membrane protein phenotype of human culture-amplified and native bone marrow mesenchymal stem cells (BM MSCs). We have found, using microarrays and flow cytometry, that cultured cells express specifically 113 transcripts and 17 proteins that were not detected in hematopoietic cells. These antigens define a lineage-homogenous cell population of mesenchymal cells, clearly distinct from the hematopoietic lineages, and distinguishable from other cultured skeletal mesenchymal cells (periosteal cells and synovial fibroblasts). Among the specific membrane proteins present on cultured MSCs, 9 allowed the isolation from BM mononuclear cells of a minute population of native MSCs. The enrichment in colony-forming units-fibroblasts was low for CD49b, CD90, and CD105, but high for CD73, CD130, CD146, CD200, and integrin alphaV/beta5. In addition, the expression of CD73, CD146, and CD200 was down-regulated in differentiated cells. The new marker CD200, because of its specificity and immunomodulatory properties, deserves further in-depth studies.

Neuronal conditioning medium and nerve growth factor induce neuronal differentiation of collagen-adherent progenitors derived from human umbilical cord blood

The aim of the study was to isolate and characterize a population of neuronal progenitors in the human umbilical cord blood (HUCB) mononuclear cell (MNC) fraction, for in vitro manipulation towards neuronal differentiation. Selection of the HUCB neuronal progenitors (HUCBNPs) was based on the neuronal prerequisite for adherence to collagen. Populations of collagen-adherent, nestin-positive (94.8+/-2.9%) progenitors expressing alpha1/2 integrin receptors, as revealed by Western blot and adhesion assay using selective antagonists, were isolated and survived for more than 14 days. In vitro differentiation of the HUCBNPs was achieved by treatment with 10% human SH-SY5Y neuroblastoma cell-conditioning media (CM) supplemented with 10 ng/ml nerve growth factor (NGF). Some 83+/-8.2% of the surviving progenitors acquired a neuronal-like morphology, expressed by cellular outgrowths of different lengths. About 35+/-6% of the HUCBNPs had long outgrowths with a length/cell diameter ratio greater than 2, typical of developing neurons. The majority of these progenitors, analyzed by immunocytochemistry and/or RT-PCR, expressed common neuronal markers such as microtubule-associated protein 2 (MAP-2; 98.5+/-2%), neurotrophin receptor (TrkA; 98.5+/-0.06%), neurofillament-160 (NF-160; 94.2+/-1%), beta-tubulin III (89.8+/-4.2%) and neuron specific enolase (NSE). Combined CM and NGF treatment induced constitutive activation of the mitogen-activated protein kinases ERK2 (36-fold vs control), p38alpha (nine-fold vs control) and p38beta (23-fold vs control), most likely related to survival and/or differentiation. The results point to operationally defined conditions for activating neuronal differentiation of HUCBNPs ex vivo and emphasize the crucial role of neuronal CM and NGF in this process.

Postnatal skeletal stem cells

Postnatal skeletal stem cells are a subpopulation of the bone marrow stromal cell network. To date, the most straightforward way of assessing the activity of skeletal stem cells within the bone marrow stromal cell (BMSC) population is via analysis of the rapidly adherent, colony-forming unit-fibroblast (CFU-F), and their progeny, BMSCs. Several in vitro methods are employed to determine the differentiation capacity of BMSCs, using osteogenic and adipogenic "cocktails" and staining protocols, and pellet cell culture for chondrogenic differentiation. However, true differentiation potential is best determined by in vivo transplantation in either closed or open systems. By in vivo transplantation, approximately 10% of the clonal strains are able to form bone, stroma, and marrow adipocytes, and are true skeletal stem cells. Furthermore, when derived from patients or animal models with abnormalities in gene expression, they recapitulate the disease phenotype on in vivo transplantation. Although ex vivo expansion of BMSCs inevitably dilutes the skeletal stem cells, when used en masse, they are attractive candidates for reconstruction of segmental bone defects, and as targets for gene therapy.

Human leukocyte antigen-G5 secretion by human mesenchymal stem cells is required to suppress T lymphocyte and natural killer function and to induce CD4+CD25highFOXP3+ regulatory T cells

Adult bone marrow-derived mesenchymal stem cells (MSCs) are multipotent cells that are the subject of intense investigation in regenerative medicine. In addition, MSCs possess immunomodulatory properties with therapeutic potential to prevent graft-versus-host disease (GvHD) in allogeneic hematopoietic cell transplantation. Indeed, MSCs can inhibit natural killer (NK) function, modulate dendritic cell maturation, and suppress allogeneic T-cell response. Here, we report that the nonclassic human leukocyte antigen (HLA) class I molecule HLA-G is responsible for the immunomodulatory properties of MSCs. Our data show that MSCs secrete the soluble isoform HLA-G5 and that such secretion is interleukin-10-dependent. Moreover, cell contact between MSCs and allostimulated T cells is required to obtain a full HLA-G5 secretion and, as consequence, a full immunomodulation from MSCs. Blocking experiments using neutralizing anti-HLA-G antibody demonstrate that HLA-G5 contributes first to the suppression of allogeneic T-cell proliferation and then to the expansion of CD4(+)CD25(high)FOXP3(+) regulatory T cells. Furthermore, we demonstrate that in addition to their action on the adaptive immune system, MSCs, through HLA-G5, affect innate immunity by inhibiting both NK cell-mediated cytolysis and interferon-gamma secretion. Our results provide evidence that HLA-G5 secreted by MSCs is critical to the suppressive functions of MSCs and should contribute to improving clinical therapeutic trials that use MSCs to prevent GvHD.

A sub-population of high proliferative potential-quiescent human mesenchymal stem cells is under the reversible control of interferon alpha/beta

Type I interferon (IFN) is shown to control the reversible quiescence of a primitive human bone marrow mesenchymal stem cell (MSC) subpopulation. A 24 h pre-treatment of Stro1+/GlycoA- or CD45-/GlycoA- subpopulations with a monoclonal antibody (mAb) against the IFNAR1 chain of the human type I IFN receptor (64G12), or with a polyclonal anti-IFNalpha antibody, resulted in a marked increase in the number of very large colonies (CFU-F >3000 cells) obtained in the presence of low, but necessary, concentrations of bFGF. Over a 2-month culture period, this short activation promoted a faster and greater amplification of mesenchymal progenitors for adipocytes and osteoblasts. Activation correlated with inhibition of STAT1 and STAT2 phosphorylation and of STAT1 nuclear translocation. A non-neutralizing anti-IFNAR1 mAb was ineffective. We demonstrate that control and activated MSCs express ST3GAL3, a sialyltransferase necessary to produce the embryonic antigens SSEA-3 and -4. Interestingly, activated MSC progeny expressed SSEA-3 and -4 at a higher level than control cultures, but this was not correlated with a significant expression of other embryonic markers. As MSCs represent an essential tool in tissue regeneration, the use of 64G12, which rapidly recruits a higher number of primitive cells, might increase amplification safety for cell therapy.

Co-expression of two perivascular cell markers isolates mesenchymal stem-like cells from human endometrium

BACKGROUND

Human endometrium has immense regenerative capacity, growing ~5 mm in 7 days every month. We have previously identified a small population of colony-forming endometrial stromal cells which we hypothesize are mesenchymal stem cells (MSC). The aim of this study was to determine if the co-expression of two perivascular cell markers, CD146 and platelet-derived growth factor-receptor beta (PDGF-Rbeta), will prospectively isolate endometrial stromal cells which exhibit MSC properties, and determine their location in human endometrium.

METHODS

Single cell suspensions of human endometrial stromal cells were fluorescence activated cell sorting (FACS) sorted into CD146(+)PDGF-Rbeta(+) and CD146(-)PDGF-Rbeta(-) populations and analysed for colony-forming ability, in vitro differentiation and expression of typical MSC markers. Full thickness human endometrial sections were co-stained for CD146 and PDGF-Rbeta.

RESULTS

FACS stromal CD146(+)PDGF-Rbeta(+) stromal cells (1.5% of sorted population) were enriched for colony-forming cells compared with CD146(-)PDGF-Rbeta(-) cells (7.7 +/- 1.7 versus 0.7 +/- 0.2% P <0.0001), and also underwent differentiation into adipogenic, osteogenic, myogenic and chondrogenic lineages. They expressed MSC phenotypic surface markers and were located near blood vessels.

CONCLUSION

This study shows that human endometrium contains a small population of MSC-like cells that may be responsible for its cyclical growth, and may provide a readily available source of MSC for tissue engineering applications.

Selection using the alpha-1 integrin (CD49a) enhances the multipotentiality of the mesenchymal stem cell population from heterogeneous bone marrow stromal cells

Bone marrow-derived mesenchymal stem cells consist of a developmentally heterogeneous population of cells obtained from colony forming progenitors. As these colonies express the alpha-1 integrin (CD49a), here we single-cell FACS sorted CD49a+ cells from bone marrow in order to create clones and then compared their colony forming efficiency and multilineage differentiation capacity to the unsorted cells. Following selection, 40% of the sorted CD49a+ cells formed colonies, whereas parental cells failed to form colonies following limited dilution plating at 1 cell/well. Following ex vivo expansion, clones shared a similar morphology to the parental cell line, and also demonstrated enhanced proliferation. Further analysis by flow cytometry using a panel of multilineage markers demonstrated that the CD49a+ clones had enhanced expression of CD90 and CD105 compared to unsorted cells. Culturing cells in adipogenic, osteogenic or chondrogenic medium for 7, 10 and 15 days respectively and then analysing them by quantitative PCR demonstrated that CD49a+ clones readily underwent multlineage differentiation into fat, bone and cartilage compared to unsorted cells. These results thus support the use of CD49a selection for the enrichment of mesenchymal stem cells, and describes a strategy for selecting the most multipotential cells from a heterogeneous pool of bone marrow mononuclear stem cells.

The concept of mesenchymal stem cells

In this chapter we examine whether criteria usually defining adult tissue stem cells apply to mesenchymal stem cells (MSCs) that give rise to cells of the skeletal connective tissues. MSCs appear to constitute a heterogeneous population of undifferentiated and committed, lineage-primed cells, capable of: homing upon engraftment to a number of growth microenvironments, extensive proliferation, producing large numbers of differentiated progeny, and functional tissue repair after injury. In addition, MSCs are extensively distributed throughout tissues, and bone marrow MSCs provide the stromal component of the niche of hematopoietic stem cells. The capacity of apparently differentiated mesenchymal cells to shift their differentiation pathway with changing microenvironmental conditions (known as differentiation plasticity) may be due to de-differentiation and reprogramming in MSCs. Because they present several features setting them apart from other stem cells, MSCs may constitute another paradigm for stem cell systems, where self-renewal and hierarchy are no longer essential, but where plasticity is the major characteristic.

[The hopes of the mesenchymal stem cells in regenerative medicine]

The mesenchymal stem cells are a cell population of bone marrow, which have the capacity to differentiate towards all the cells from the locomotor apparatus. They also have immunomodulatory properties and can contribute to tissue repair, thanks to the secretion of many growth factors. Such cells are also found in the cord blood. In the same way, very close stem cells exist in great quantity in fat tissue. These cells are very good candidates in regenerative medicine. Besides, several clinical trials were carried out in order to highlight their effectiveness mainly in osseous repair and also during hematopoietic stem cells graft or cardiac repair after infarction. However, these trials will be able to develop fully only with the condition that culture techniques meeting the conditions of good manufacturing practice are set-up. This presentation gives a progress report on the whole of these subjects.

Human mesenchymal stem cells at the single-cell level: simultaneous seven-colour immunofluorescence

Extracellular, intracellular or surface proteins can be used as putative markers to characterize human mesenchymal stem cells (hMSC). However, these markers are also expressed by other cell types and primary cell pools reveal considerable heterogeneity. Therefore, the simultaneous detection of several markers on a single cell appears to be an attractive approach to identify hMSC. Here we demonstrate the specific distinction of human MSC from human osteoblasts via seven-colour fluorescence on the single cell level with simultaneous marker detection of CD44, CD105/endoglin, CD106/VCAM-1, collagen-IV, fibronectin, actin and DAPI nuclear staining. We performed spectral image acquisition using a Sagnac-type interferometer. Subsequent linear unmixing allowed for decomposition of each pixel in its spectral components. Our approach reveals a typical expression profile of the adherent singular cells, allowing the specific distinction between hMSC and osteoblasts on the single cell level.

Formation of osteogenic colonies on well-defined adhesion peptides by freshly isolated human marrow cells

Bone graft performance can be enhanced by addition of connective tissue progenitors (CTPs) from fresh bone marrow in a manner that concentrates the CTP cell population within the graft. Here, we used small peptide adhesion ligands presented against an otherwise adhesion-resistant synthetic polymer background in order to illuminate the molecular basis for the attachment and colony formation by osteogenic CTPs from fresh human marrow, and contrast the behavior of fresh marrow to many commonly used osteogenic cell sources. The linear GRGDSPY ligand was as effective as tissue culture polystyrene in fostering attachment of culture-expanded porcine CTPs. Although this GRGDSPY peptide was more effective than control peptides in fostering alkaline phosphatase (AP)-positive colony formation from primary human marrow in 5 of the 7 patients tested, GRGDSPY was as effective as the control glass substrate in only one patient of 7. Thus, the peptide appears capable of enabling osteoblastic development from only a subpopulation of CTPs in marrow. The bone sialoprotein-derived peptide FHRRIKA was ineffective in fostering attachment of primary culture-expanded pig CTPs, although it was as effective as GRGDSPY in fostering AP-positive colonies from fresh human marrow. This study provides insights into integrin-mediated behaviors of CTPs and highlights differences between freshly isolated marrow and culture-expanded cells.

Differential expression profiling of membrane proteins by quantitative proteomics in a human mesenchymal stem cell line undergoing osteoblast differentiation

One of the major limitations for understanding the biology of human mesenchymal stem cells (hMSCs) is the absence of prospective markers needed for distinguishing them from other cells and for monitoring lineage-specific differentiation. Mass spectrometry (MS)-based proteomics has proven extremely useful for analyzing complex protein expression patterns and, when applied quantitatively, can be used to resolve subtle differences between samples. Thus, we used MS to characterize changes in expression of membrane protein markers before and after short-term induction of osteoblast (OB) differentiation in a cell model of hMSCs established by overexpression of human telomerase reverse-transcriptase gene. We identified 463 unique proteins with extremely high confidence, including all known markers of hMSCs (e.g., SH3 [CD71], SH2 [CD105], CD166, CD44, Thy1, CD29, and HOP26 [CD63]) among 148 integral membrane or membrane-anchored proteins and 159 membrane-associated proteins. Twenty-nine integrins and cell adhesion molecules, 20 receptors, and 18 Ras-related small GTPases were also identified. Upon OB differentiation, the expression levels of 83 proteins increased by at least twofold whereas the levels of another 21 decreased by at least twofold. For example, alkaline phosphatase (ALP), versican core protein, and tenascin increased 27-, 12-, and 4-fold, respectively, and fatty acid synthase decreased sixfold. The observed increases in veriscan and ALP were confirmed using immunocytochemistry and cytochemistry. Quantitative real-time reverse transcription-polymerase chain reaction confirmed the presence of mRNA of these membrane proteins. However, with the exception of ALP, no concordance was detected between the changes in levels of gene and protein expression during OB differentiation. In conclusion, MS-based proteomics can reveal novel markers for MSCs that can be used for their isolation and for monitoring OB differentiation.

Isolation of C15: A novel antibody generated by phage display against mesenchymal stem cell-enriched fractions of adult human marrow

Adult bone marrow stroma contains a source of mesenchymal stem cells (MSC) that have the capacity to self-renew and differentiate into multiple stromal lineages. These rare cells can be visualised indirectly by the formation of heterogeneous colonies, containing stem cells and their differentiated progeny in long-term culture. If MSC and their associated progenitor and precursor populations are to reach their full therapeutic potential, markers will be required to identify and characterize specific bone marrow stromal subsets. We sought to use phage display to generate antibodies against bone marrow mononuclear cells (BMMNC) enriched for colony forming cells. Initially, we identified our target cell population by comparing the colony forming efficiency (CFE) of CD49a-positive, STRO-1-positive and CD45-negative BMMNC subpopulations with unseparated BMMNC. Selection with anti-CD49a gave the greatest enrichment (19-fold) of colony forming cells and in light of these findings, we generated phage antibodies against CD49a-positive BMMNC by simultaneous positive/negative selection. A dominant clone (C15), generated after 3 rounds of selection, has been isolated and sequenced, then characterized for cell and tissue specificity. Sequence analysis showed that the V(H) and V(L) gene segments of C15 aligned most closely to the VH26/DP-47 and IGLV3S1/DPL16 germline V segments found in the synthetic repertoire. C15 bound to 4% of freshly isolated BMMNC and localized to osteoblastic cells and proximal marrow cells in areas of active bone formation in sections of osteophyte. C15 binding was upregulated in cultured bone marrow stromal cells (BMSC) and was also detected on bone-derived cell lines. This report demonstrates that phage display is a powerful tool for the isolation of antibodies against rare cell populations, and provides a platform for the future application of this technology in the search for antigens on MSC and other rare cell populations.

Optimization of a flow cytometry-based protocol for detection and phenotypic characterization of multipotent mesenchymal stromal cells from human bone marrow

BACKGROUND

To study the biology of rare bone marrow (BM) multipotent mesenchymal stromal cells (MSCs), recognized protocols are needed. Colony-forming unit-fibroblast (CFU-F) assays have historically been used for the enumeration of MSCs. However, the need to isolate and further analyze MSCs requires new strategies based on cell surface markers. The purpose of this work was to verify the phenotype of BM MSCs in vivo and to develop flow cytometry-based methods for their evaluation.

METHODS

Pre-enrichment with D7-FIB-conjugated microbeads, cell sorting for CD45low D7-FIB+ LNGFR+ cells, and CFU-F assay were used to confirm the phenotype of BM MSCs in vivo. Further phenotypic characterization of MSCs was performed using three-color flow cytometry following pre-enrichment or by direct four-color flow cytometry. The sensitivity of direct flow cytometry/rare event analysis for the accurate enumeration of MSCs was validated using 85 samples from patients with neoplastic BM diseases.

RESULTS

In normal BM, a significant correlation was found between the frequencies of CFU-Fs and CD45low D7-FIB+ LNGFR+ cells (n = 19, R = 0.719, P = 0.001). Following cell sorting, 15% of these cells were clonogenic. The same cells were enriched using LNGFR-based positive selection, CD45/Glycophorin A-based depletion, or plastic adherence. CD45low D7-FIB+ LNGFR+ cells expressed classic makers of cultured MSCs CD73/SH3 and CD105/SH2 and markers of stromal reticular cells CD106/VCAM and alkaline phosphatase. Novel markers were identified including leukemia inhibitory factor receptor and gp130. CD45low D7-FIB+ LNGFR+ cells were increased fourfold in the floating fat fraction of normal BM aspirates. Their frequency was decreased in chronic lymphocytic leukemia (threefold, n = 13, P = 0.049) and chronic myelogenous leukemia (ninefold, n = 11, P = 0.001) compared with that in age-matched controls (n = 26 and n = 31, respectively).

CONCLUSIONS

This study demonstrates the usefulness of flow cytometry-based methods for the detection, enumeration and further phenotypic analysis of BM MSCs. These findings have broad applications for the future evaluation of BM MSCs in health and disease.

Characterization of nonexpanded mesenchymal progenitor cells from normal adult human bone marrow

OBJECTIVE

Adult bone marrow (BM) mesenchymal stem/progenitor cells (MS/PC) are a potentially useful tool for cell therapy and tissue repair. However, the identification of cell subsets rich in MS/PC from fresh BM has not been described. We have developed a means of identifying such subsets from untouched bone marrow.

MATERIAL AND METHODS

First, MS/PC were enriched by short-time adherence (D(1-3)) before any cell division to evaluate the efficiency of CD73, CD105, CDw90, and CD49a antigens to select highly purified CD45(-)CD14(-) fluorescence-activated sorted subsets enriched in clonogenic mesenchymal cells. Then, we adapted this method to unmanipulated BM mononuclear cells (MNC).

RESULTS

Short-time (D(1-3)) adherent CD45(-)CD14(-) cells expressing CD73 or CD49a antigens contained all the CFU-F, even though the CD105(+) and CDw90(+) subsets comprised less than half the total. In fresh unmanipulated BM MNC, CD73 and CD49a were also highly discriminative and allowed up to a 3 log enrichment of CFU-F when compared to BM MNC. Normal culture conditions upregulated most of the tested antigens.

CONCLUSION

The CD45(-)CD14(-)/CD73(+) and CD45(-)CD14(-)/CD49a(+) phenotypes identified subsets containing all the CFU-F and sufficiently enriched to detect them in fresh BM, enabling evaluation of mesenchymal content of BM collections for cell therapy.

The role of very late antigen-1 in immune-mediated inflammation

The alpha1beta1 integrin, also known as "very late antigen" (VLA)-1, is normally expressed on mesenchymal cells, some epithelial cells, activated T cells, and macrophages, and interacts, via the I-domain of the extracellular domain of the alpha1 subunit, with collagen molecules in the extracellular matrix (ECM). By "outside-in" transmembranal signaling to the interior of the cell, it mediates adhesion, migration, proliferation, remodeling of the ECM, and cytokine secretion by endothelial cells, mesangial cells, fibroblasts, and immunocytes. Importantly, its expressions and functions are enhanced by inflammatory cytokines including interferon (IFN)gamma and tumor necrosis factor (TNF)alpha, thus augmenting angiogenesis and fibrosis linked, in particular, to inflammation. Moreover, within the immune system, VLA-1 marks effector memory CD4+ and CD8+ T cells that are retained in extralymphatic tissues by interactions of the integrin with collagen and produce high levels of IFNgamma. Thus, immune-mediated inflammation in vivo is inhibited by blockade of the VLA-1-collagen interaction in experimental animal models of arthritis, colitis, nephritis, and graft versus host disease (GVHD), suggesting that inhibiting the interaction of the alpha1 I-domain with its ligands or modulating "outside-in" signaling by VLA-1 would be a useful approach in the human diseases simulated by these experimental models.

The use of four-colour immunofluorescence techniques to identify mesenchymal stem cells

In stem-cell research a major difficulty is caused by the lack of distinctive features that allow the identification of human mesenchymal stem cells (hMSC). Until now, there has been no specific marker and the most common way to identify hMSC is by their characteristic stem-cell properties: self-replication and differentiation potential. However, these findings can only be revealed retrospectively, and, once differentiated, hMSC lose their stem-cell character. The aim of this study was to establish four-colour immunofluorescence of several markers simultaneously in order to address the problem of how to identify hMSC on the single-cell level. The four markers collagen-I, collagen-IV, fibronectin and CD44 are known to be expressed by hMSC. Antibody binding was detected using secondary antibodies conjugated to FITC, Alexa546, TexasRed and AMCA. Because the distinction between Alexa546 and TexasRed was not possible on conventional digital images using standard filter sets, we performed spectral image acquisition. The image was subsequently decomposed into its pure spectral components, which permitted linear unmixing. Using this procedure we were able to demonstrate four-colour immunofluorescence on hMSC. With the possibility of using more sophisticated marker profiles and/or additional markers, four-colour immunofluorescence offers the opportunity of identifying hMSC on the single-cell level without performing differentiation assays.