Isolated allogeneic bone marrow-derived mesenchymal cells engraft and stimulate growth in children with osteogenesis imperfecta: Implications for cell therapy of bone

Mesenchymal progenitor cells derived from chorionic villi of human placenta for cartilage tissue engineering

Human mesenchymal stem cells are currently being studied extensively because of their capability for self-renewal and differentiation to various connective tissues, which makes them attractive as cell sources for regenerative medicine. Herein we report the isolation of human placenta-derived mesenchymal cells (hPDMCs) that have the potential to differentiate into various lineages to explore the possibility of using these cells for regeneration of cartilage. We first evaluated the chondrogenesis of hPDMCs in vitro and then embedded the hPDMCs into an atelocollagen gel to make a cartilage-like tissue with chondrogenic induction media. For in vivo assay, preinduced hPDMCs embedded in collagen sponges were subcutaneously implanted into nude mice and also into nude rats with osteochondral defect. The results of these in vivo and in vitro studies suggested that hPDMCs can be one of the possible allogeneic cell sources for tissue engineering of cartilage.

Multipotent mesenchymal stem cells with immunosuppressive activity can be easily isolated from dental pulp

BACKGROUND

Bone marrow mesenchymal stem cells (MSCs) are currently being investigated in preclinical and clinical settings because of their multipotent differentiative capacity or, alternatively, their immunosuppressive function. The aim of this study was to evaluate dental pulp (DP) as a potential source of MSCs instead of bone marrow (BM).

METHODS

Flow cytometric analysis showed that DP-MSCs and BM-MSCs were equally SH2, SH3, SH4, CD29 and CD 166 positive. The in vitro proliferative kinetics of MSCs were measured by 3H-thymidine incorporation uptake. The immunosuppressive function of MSCs was then tested by coculturing PHA-stimulated allogeneic T cells with or without MSCs for 3 days.

RESULTS

BM-MSCs could be differentiated in vitro into osteogenic, chondrogenic and adipogenic lineages. DP-MSCs showed osteogenic and adipocytic differentiation, but did not differentiate into chondrocytes. Although DP-MSCs grow rapidly in vitro between day 3 and day 8 of culture and then decrease their proliferation by day 15, BM-MSCs have a stable and continuous proliferation over the same period of time. The addition of DP-MSCs or BM-MSCs resulted in 91 +/- 4% and 75 +/- 3% inhibition of T cell response, respectively, assessed by a 3H-thymidine assay.

CONCLUSIONS

Dental pulp is an easily accessible and efficient source of MSCs, with different kinetics and differentiation potentialities from MSCs as isolated from the bone marrow. The rapid proliferative capacity together with the immunoregulatory characteristics of DP-MSCs may prompt future studies aimed at using these cells in the treatment or prevention of T-cell alloreactivity in hematopoietic or solid organ allogeneic transplantation.

The role of mesenchymal stem cells in haemopoiesis

The ontogeny of haemopoiesis during fetal life and the differentiation of blood cells in adult life depend upon a fully competent microenvironment to provide appropriate signals via production of soluble factors and cell contact interactions. The cellular constituents of the microenvironment, also defined as the haemopoietic niche, largely derive from a common progenitor of mesenchymal origin. Mesenchymal stem cells (MSC), initially identified in adult bone marrow, have also been described in fetal haemopoietic tissues where they accompany the migration of haemopoietic development. Their precise identity remains ill-defined because of the lack of specific markers. Their ability to self-renew and differentiate into tissues of mesodermal origin (osteocytes, adipocytes, chondrocytes) and their lack of expression of haemopoietic molecules are currently the main criteria for isolation. In the bone marrow the most important elements of the niche appear to be osteoblasts, whilst a less defined population of fibroblasts regulates the maturation of immature T cells in the thymus. Recently, MSC have been shown to exert a profound immunosuppressive effect on polyclonal as well as antigen-specific T cell responses by inducing a state of division arrest anergy. Thus, the multipotent capacity of MSC, their role in supporting haemopoiesis, and their immunoregulatory activity make MSC particularly attractive for therapeutic exploitation.

Human mesenchymal stem cell subpopulations express a variety of neuro-regulatory molecules and promote neuronal cell survival and neuritogenesis

Mesenchymal stem cells (MSCs) transplanted at sites of nerve injury are thought to promote functional recovery by producing trophic factors that induce survival and regeneration of host neurons. To evaluate this phenomenon further, we quantified in human MSCs neurotrophin expression levels and their effects on neuronal cell survival and neuritogenesis. Screening a human MSC cDNA library revealed expressed transcripts encoding BDNF and beta-NGF but not NT-3 and NT-4. Immunostaining demonstrated that BDNF and beta-NGF proteins were restricted to specific MSC subpopulations, which was confirmed by ELISA analysis of 56 separate subclones. Using a co-culture assay, we also demonstrated that BDNF expression levels correlated with the ability of MSC populations or subclones to induce survival and neurite outgrowth in the SH-SY5Y neuroblastoma cell line. However, these MSC-induced effects were only partially inhibited by a neutralizing anti-BDNF antibody. MSCs were also shown to promote neurite outgrowth within dorsal root ganglion explants despite secreting 25-fold lower level of beta-NGF required exogenously to produce a similar effect. Interrogation of the human MSC transcriptome identified expressed mRNAs encoding various neurite-inducing factors, axon guidance and neural cell adhesion molecules. Moreover, a subset of these transcripts was shown to correlate with BDNF expression in MSC subclones. Collectively, these studies reveal the existence of MSC subpopulations that co-express neurotrophins and other potent neuro-regulatory molecules, which contribute to MSC-induced effects on neuronal cell survival and nerve regeneration. These subpopulations may represent more potent vectors for treating a variety of neurological disorders.

Mesenchymal stem cells for bone repair: preclinical studies and potential orthopedic applications

Mesenchymal stem cells (MSCs), derived from adult bone marrow, are multi-potent stem cells capable of differentiating along several lineage pathways. From a small bone marrow aspirate, MSCs can be readily isolated and easily expanded. Therefore, MSCs are thought to be a readily available source of cells for many tissue engineering and regenerative medicine applications. This review covers preclinical models that evaluate the efficacy of MSC-loaded scaffolds in large bone defects as a potential substitute for autologous and allogeneic bone grafts. This review also covers new approaches to clinical use of MSC technology.

Adult mesenchymal stem cells: characterization, differentiation, and application in cell and gene therapy

A considerable amount of retrospective data is available that describes putative mesenchymal stem cells (MSCs). However, there is still very little knowledge available that documents the properties of a MSC in its native environment. Although the precise identity of MSCs remains a challenge, further understanding of their biological properties will be greatly advanced by analyzing the mechanisms that govern their self-renewal and differentiation potential. This review begins with the current state of knowledge on the biology of MSCs, specifically with respect to their existence in the adult organism and postulation of their biological niche. While MSCs are considered suitable candidates for cell-based strategies owing to their intrinsic capacity to self-renew and differentiate, there is currently little information available regarding the molecular mechanisms that govern their stem cell potential. We propose here a model for the regulation of MSC differentiation, and recent findings regarding the regulation of MSC differentiation are discussed. Current research efforts focused on elucidating the mechanisms regulating MSC differentiation should facilitate the design of optimal in vitro culture conditions to enhance their clinical utility cell and gene therapy.

Interferon-gamma-stimulated marrow stromal cells: a new type of nonhematopoietic antigen-presenting cell

Several studies have demonstrated that marrow stromal cells (MSCs) can suppress allogeneic T-cell responses. However, the effect of MSCs on syngeneic immune responses has been largely overlooked. We describe here that primary MSCs derived from C57BL/6 mice behave as conditional antigen-presenting cells (APCs) and can induce antigen-specific protective immunity. Interferon gamma (IFNgamma)-treated C57BL/6 MSCs, but not unstimulated MSCs, cocultured with ovalbumin-specific major histocompatibility (MHC) class II-restricted hybridomas in the presence of soluble ovalbumin-induced significant production of interleukin-2 (IL-2) in an antigen dose-dependent manner (P < .005). IFNgamma-treated MSCs could further activate in vitro ovalbumin-specific primary transgenic CD4+ T cells. C57BL/6 MSCs, however, were unable to induce antigen cross-presentation via the MHC class I pathway. When syngeneic mice were immunized intraperitoneally with ovalbumin-pulsed IFNgamma-treated MSCs, they developed antigen-specific cytotoxic CD8+ T cells and became fully protected (10 of 10 mice) against ovalbumin-expressing E.G7 tumors. Human MSCs were also studied for antigen-presenting functions. IFNgamma-treated DR1-positive human MSCs, but not unstimulated human MSCs, induced significant production of IL-2 when cocultured with DR1-restricted influenza-specific humanized T-cell hybridomas in the presence of purified influenza matrix protein 1. Taken together, our data strongly suggest that MSCs behave as conditional APCs in syngeneic immune responses.

Correction of a mineralization defect by overexpression of a wild-type cDNA for COL1A1 in marrow stromal cells (MSCs) from a patient with osteogenesis imperfecta: a strategy for rescuing mutations that produce dominant-negative protein defects

Gene therapy for dominant-negative disorders presents a more difficult challenge than gene therapy for recessive disorders, since even partial replacement of a protein for a recessive disorder can reverse symptoms. Osteogenesis imperfecta (OI) has frequently served as a model disorder for dominant-negative defects of structural proteins. The disease is caused by mutations in type I collagen (COL1A1), the major structural component of bone, skin and other connective tissues. The severity of the phenotype is largely dependent on the ratio of normal to mutant type I procollagen synthesized by cells. Recently, attempts have been made to develop strategies for cell and gene therapies using the adult stem cells from bone marrow referred to as mesenchymal stem cells or marrow stromal cells (MSCs). In this study, we used MSCs from a patient with type III OI who was heterozygous for an IVS 41A+4C mutation in COL1A1. A hybrid genomic / cDNA construct of COL1A1 was transfected into the MSCs and the transfectants were expanded over a 200-fold. Transfected MSCs showed increased expression of the wild-type mRNA and protein. In vitro assays demonstrated that the transfected cells more efficiently differentiated into mineralizing cells. The results indicated that it is possible to overexpress COL1A1 cDNA in OI MSCs and thereby to correct partially the dominant-negative protein defect.

Review: mesenchymal stem cells: cell-based reconstructive therapy in orthopedics

Adult stem cells provide replacement and repair descendants for normal turnover or injured tissues. These cells have been isolated and expanded in culture, and their use for therapeutic strategies requires technologies not yet perfected. In the 1970s, the embryonic chick limb bud mesenchymal cell culture system provided data on the differentiation of cartilage, bone, and muscle. In the 1980s, we used this limb bud cell system as an assay for the purification of inductive factors in bone. In the 1990s, we used the expertise gained with embryonic mesenchymal progenitor cells in culture to develop the technology for isolating, expanding, and preserving the stem cell capacity of adult bone marrow-derived mesenchymal stem cells (MSCs). The 1990s brought us into the new field of tissue engineering, where we used MSCs with site-specific delivery vehicles to repair cartilage, bone, tendon, marrow stroma, muscle, and other connective tissues. In the beginning of the 21st century, we have made substantial advances: the most important is the development of a cell-coating technology, called painting, that allows us to introduce informational proteins to the outer surface of cells. These paints can serve as targeting addresses to specifically dock MSCs or other reparative cells to unique tissue addresses. The scientific and clinical challenge remains: to perfect cell-based tissue-engineering protocols to utilize the body's own rejuvenation capabilities by managing surgical implantations of scaffolds, bioactive factors, and reparative cells to regenerate damaged or diseased skeletal tissues.

Interaction between Human NK Cells and Bone Marrow Stromal Cells Induces NK Cell Triggering: Role of NKp30 and NKG2D Receptors

In this study we have analyzed the interaction between in vitro cultured bone marrow stromal cells (BMSC) and NK cells. Ex vivo-isolated NK cells neoexpressed the activation Ag CD69 and released IFN-gamma and TNF-alpha upon binding with BMSC. Production of these proinflammatory cytokines was dependent on ligation of ICAM1 expressed on BMSC and its receptor LFA1 on NK cells. Furthermore, the NKp30, among natural cytotoxicity receptors, appeared to be primarily involved in triggering NK cells upon interaction with BMSC. Unexpectedly, autologous IL-2-activated NK cells killed BMSC. Again, LFA1/ICAM1 interaction plays a key role in NK/BMSC interaction; this interaction is followed by a strong intracellular calcium increase in NK cells. More importantly, NKG2D/MHC-I-related stress-inducible molecule A and/or NKG2D/UL-16 binding protein 3 engagement is responsible for the delivery of a lethal hit. It appears that HLA-I molecules do not protect BMSC from NK cell-mediated injury. Thus, NK cells, activated upon binding with BMSC, may regulate BMSC survival.

Mesenchymal stem cells induce apoptosis of activated T cells

Mesenchymal stem cells (MSC) have recently been used successfully in humans to control severe graft-versus-host disease. However, the mechanisms involved in their immunomodulatory effects remain a matter of debate. Here, we show that MSC are unable to activate allogeneic T cells even in the presence of T-cell growth factors. We then found that MSC inhibit T-cell proliferation triggered either by allogeneic, mitogenic or antigen-specific stimuli. Interestingly, MSC inhibit T-cell proliferation by inducing apoptosis of activated T cells, but have no effect on resting T cells. Furthermore, we show that this apoptosis could be related to the conversion of tryptophan into kynurenine by indoleamine 2,3-dioxygenase expressed by MSC in the presence of IFNgamma. Moreover, we show that the inhibitory effect of MSC is neither abrogated nor modified during expansion in culture or after irradiation. Together, these results bring new insight to the mechanisms of immunosuppression induced by MSC and might help to develop their clinical use controlling immune-related adverse effects in humans.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

Intervertebral disc cell-mediated mesenchymal stem cell differentiation

Low back pain is one of the largest health problems in the Western world today, and intervertebral disc degeneration has been identified as a main cause. Currently, treatments are symptomatic, but cell-based tissue engineering methods are realistic alternatives for tissue regeneration. However, the major problem for these strategies is the generation of a suitable population of cells. Adult bone marrow-derived mesenchymal stem cells (MSCs) are undifferentiated, multipotent cells that have the ability to differentiate into a number of cell types, including the chondrocyte-like cells found within the nucleus pulposus (NP) of the intervertebral disc; however, no method exists to differentiate these cells in an accessible monolayer environment. We have conducted coculture experiments to determine whether cells from the human NP can initiate the differentiation of human MSCs with or without cell-cell contact. Fluorescent labeling of the stem cell population and high-speed cell sorting after coculture with cell-cell contact allowed examination of individual cell populations. Real-time quantitative polymerase chain reaction showed significant increases in NP marker genes in stem cells when cells were cocultured with contact for 7 days, and this change was regulated by cell ratio. No significant change in NP marker gene expression in either NP cells or stem cells was observed when cells were cultured without contact, regardless of cell ratio. Thus, we have shown that human NP and MSC coculture with contact is a viable method for generating a large population of differentiated cells that could be used in cell-based tissue engineering therapies for regeneration of the degenerate intervertebral disc.

Mesenchymal stem cell therapy to rebuild cartilage

Disorders affecting cartilage touch almost the whole population and are one of the leading causes of invalidity in adults. To repair cartilage, therapeutic approaches initially focused on the implantation of autologous chondrocytes, but this technique proved unsatisfactory because of the limited number of chondrocytes obtained at harvest. The discovery that several adult human tissues contain mesenchymal stem cells (MSCs) capable of differentiating into chondrocytes raised the possibility of injecting MSCs to repair cartilages. The important data published recently on the factors controlling chondrocyte commitment must be thoroughly considered to make further progress towards this therapeutic approach. The potential application of MSC therapy provides new hope for the development of innovative treatments for the repair of cartilage disorders.

Human umbilical cord perivascular (HUCPV) cells: a source of mesenchymal progenitors

We describe the isolation of a nonhematopoietic (CD45-, CD34-, SH2+, SH3+, Thy-1+, CD44+) human umbilical cord perivascular (HUCPV) cell population. Each HUCPV cell harvest (2-5 x 10(6), depending on the length of cord available) gave rise to a morphologically homogeneous fibroblastic cell population, which expressed alpha-actin, desmin, vimentin, and 3G5 (a pericyte marker) in culture. We determined the colony-forming unit-fibro-blast (CFU-F) frequency of primary HUCPV cells to be 1:333 and the doubling time, which was 60 hours at passage 0 (P0), decreased to 20 hours at P2. This resulted in a significant cell expansion, producing over 10(10) HUCPV cells within 30 days of culture. Furthermore, HUCPV cells cultured in nonosteogenic conditions contained a subpopulation that exhibited a functional osteogenic phenotype and elaborated bone nodules. The frequency of this CFU-osteogenic subpopulation at P1 was 2.6/10(5) CFU-F, which increased to 7.5/10(5) CFU-F at P2. Addition of osteogenic supplements to the culture medium resulted in these frequencies increasing to 1.2/10(4) and 1.3/10(4) CFU-F, respectively, for P1 and P2. CFU-O were not seen at P0 in either osteogenic or non-osteogenic culture conditions, but P0 HUCPV cells did contain a 20% subpopulation that presented neither class I nor class II cell-surface major histocompatibility complexes (MHC-/-). This population increased to 95% following passage and cryopreservation (P5). We conclude that, due to their rapid doubling time, high frequencies of CFU-F and CFU-O, and high MHC-/- phenotype, HUCPV cells represent a significant source of cells for allogeneic mesenchymal cell-based therapies.

Human adult CD34- progenitor cells functionally express the chemokine receptors CCR1, CCR4, CCR7, CXCR5, and CCR10 but not CXCR4

The homing and tissue-specific recruitment of bone marrow-derived progenitor cells is a major issue in stem cell research and therapy. Chemokine biology plays a central role in the homing and trafficking of leukocytes. Here we show functional expression of the chemokine receptors CCR1, CCR4, CCR7, CCR10, and CXCR5 on primary isolates of CD34- mesenchymal progenitor cells as well as immortalized mesenchymal stem cell (MSC) lines. Although mRNA expression of CXCR4 was detected in both primary cells and immortalized clones, the receptor was not expressed on the cell surface. On the basis of this expression profile, the MSC could potentially home to secondary lymphatic organs (CCR7, CXCR5), skin (CCR4, CCR10), small intestine (CCR10), and salivary glands (CCR10). To study tissue-specific homing, murine CD34- MSC lines showing concordant chemokine receptor expression were either transiently labeled with CMFDA, or were stably transfected with green fluorescent protein (GFP) expression plasmids. The MSC were then injected into syngeneic healthy mice, and the distribution of the cells determined. The injected cells efficiently homed to spleen, thymus, and lymph nodes. In addition, cells were found in the mucosa of the small intestine, skin, and salivary gland. No significant recruitment to bone marrow, liver, or kidney was seen. Chemokine biology may play an important role in the homeostasis and potentially tissue recruitment of early adult progenitor cells.

Emerging therapeutic approaches for osteogenesis imperfecta

Osteogenesis imperfecta (OI) is an incurable genetic brittle-bone disease. Although drug therapy, surgery and physiotherapy represent current treatments for OI, the search is ongoing for effective and innovative new therapies targeting the underlying causes of the disease. In this regard, recent advances in the fields of gene and stem-cell therapies have been considerable. In spite of the many challenges that remain, potential new therapies for OI, which have been tested in cell culture systems, animal models and patients, offer hope for the future development of successful therapies. Recent progress in the field is reviewed here.

The efficacy of mesenchymal stem cells to regenerate and repair dental structures

OBJECTIVES

Identification, characterization, and potential application of mesenchymal stem cells (MSC) derived from human dental tissues.

METHODS

Dental pulp and periodontal ligament were obtained from normal human impacted third molars. The tissues were digested in collagenase/dispase to generate single cell suspensions. Cells were cultured in alpha-MEM supplemented with 20% fetal bovine serum, 2 mM l-glutamine, 100 microM l-ascorbate-2-phosphate. Magnetic and fluorescence activated cell sorting were employed to characterize the phenotype of freshly isolated and ex vivo expanded cell populations. The developmental potential of cultured cells was assessed following co-transplantation with hydroxyapetite/tricalcium phosphate (HA/TCP) particles into immunocompromised mice for 8 weeks.

RESULTS

MSC were identified in adult human dental pulp (dental pulp stem cells, DPSC), human primary teeth (stem cells from human exfoliated deciduous teeth, SHED), and periodontal ligament (periodontal ligament stem cells, PDLSC) by their capacity to generate clongenic cell clusters in culture. Ex vivo expanded DPSC, SHED, and PDLSC populations expressed a heterogeneous assortment of makers associated with MSC, dentin, bone, smooth muscle, neural tissue, and endothelium. PDLSC were also found to express the tendon specific marker, Scleraxis. Xenogeneic transplants containing HA/TCP with either DPSC or SHED generated donor-derived dentin-pulp-like tissues with distinct odontoblast layers lining the mineralized dentin-matrix. In parallel studies, PDLSC generated cementum-like structures associated with PDL-like connective tissue when transplanted with HA/TCP into immunocompromised mice.

CONCLUSION

Collectively, these data revealed the presence of distinct MSC populations associated with dental structures with the potential of stem cells to regenerate living human dental tissues in vivo.

Mesenchymal stem cells: progress toward promise

Despite having access to embryonic stem cells, many laboratories choose to study adult stem cells, not because of philosophical reasons but because of the practical aspects and day-to-day progress necessary for developing cellular therapeutics. There is certainly the ethical desire and responsibility to provide patients with therapies where few options exist. Multipotential cells have been isolated from adult tissues in many laboratories, characterized and their multipotentiality examined. Mesenchymal stem cells (MSC) can be isolated from several tissues but easily accessible BM seems to be the most common source. These adult stem cells may not be as 'powerful' or diverse as embryonic stem cells may one day become, but at present they offer many advantages for developing cellular therapeutics: ease of isolation, expansion potential, stable phenotype, shippability, and compatibility with different delivery methods and formulations. Their potential use as cellular therapeutics has prompted the investigation of interactions of allogeneic MSC with the immune response. The great importance of cardiovascular medicine has demanded that MSC also be tested in this discipline. We believe MSC continue to provide a substantial scientific and therapeutic opportunity, and have reviewed some of the recent developments in the field.

Allogeneic marrow stromal cells are immune rejected by MHC class I- and class II-mismatched recipient mice

It has been suggested that marrow stromal cells (MSCs) may be immunoprivileged and can engraft in allogeneic recipients with intact immune systems. We determined if the implantation of murine MSCs engineered to release erythropoietin (Epo) would be feasible in major histocompatibility complex (MHC)-mismatched allogeneic mice without immunosuppression, and we monitored hematocrit (Hct) as a reporter of MSC graft survival. MSCs from C57Bl/6 mice were engineered to release murine Epo (Epo+ MSCs) and implanted subcutaneously in either syngeneic C57Bl/6 mice or MHC-mismatched Balb/c mice. In syngeneic recipients, the Hct rapidly rose from baseline level and remained higher than .88 (88%) for more than 200 days. However, in MHC-mismatched recipient Balb/c mice, the Hct rose transiently and rapidly declined to baseline values. Repeat implantations in these same mice were associated with an acquired refractoriness in the Hct response consistent with alloimmunization to donor Epo+ MSCs. Allogeneic MSC implants had an increased proportion of host-derived lymphoid CD8+, natural killer T (NKT), and NK infiltrating cells compared with syngeneic controls, and splenocytes isolated from Balb/c mice that had received implants also displayed a significant interferon-gamma (IFNgamma) response to C57Bl/6 MSCs in vitro. These results strongly suggest that MSCs are not intrinsically immunoprivileged and cannot serve as a "universal donor" in immunocompetent MHC-mismatched recipients.

Characterization of the optimal culture conditions for clinical scale production of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent cells defined by multilineage potential, ease to gene modification, and immunosuppressive ability, thus holding promise for tissue engineering, gene therapy, and immunotherapy. They exhibit a unique in vitro expansion capacity, which, however, does not compensate for the very low percentage in their niches given the vast numbers of cells required for the relative studies. Taking into consideration the lack of a uniform approach for MSC isolation and expansion, we attempted in this study, by comparing various culture conditions, to identify the optimal protocol for the large-scale production of MSCs while maintaining their multilineage and immunosuppressive capacities. Our data indicate that, apart from the quality of fetal calf serum, other culture parameters, including basal medium, glucose concentration, stable glutamine, bone marrow mononuclear cell plating density, MSC passaging density, and plastic surface quality, affect the final outcome. Furthermore, the use of basic fibroblast growth factor (bFGF), the most common growth supplement in MSC culture media, greatly increases the proliferation rate but also upregulates HLA-class I and induces low HLA-DR expression. However, not only does this upregulation not elicit significant in vitro allogeneic T cell responses, but also bFGF-cultured MSCs exhibit enhanced in vivo immunosuppressive potential. Besides, addition of bFGF affects MSC multilineage differentiation capacity, favoring differentiation toward the osteogenic lineage and limiting neurogenic potential. In conclusion, in this report we define the optimal culture conditions for the successful isolation and expansion of human MSCs in high numbers for subsequent cellular therapeutic approaches.

Mesenchymal stem cells obtained after bone marrow transplantation or peripheral blood stem cell transplantation originate from host tissue

Mesenchymal stem cells (MSC) obtained from human bone marrow have been described as adult stem cells with the ability of extensive self-renewal and clonal expansion, as well as the capacity to differentiate into various tissue types and to modulate the immune system. Some data indicate that leukapheresis products may also contain non-hematopoietic stem cells, as they occur in whole bone marrow transplantation (BMT). However, there is still controversy whether MSC expand in the host after transplantation like blood progenitor cells do. Therefore, we were interested in finding out if graft MSC can be detected in leukapheresis products and in bone marrow after BMT and peripheral blood stem cell transplantation (PBSCT). Every sample from total bone marrow transplants exhibited growth of MSC after in vitro culture, but not one of nine leukapheresis products did. In addition, bone marrow aspirates of 9 patients receiving BMT and of 18 patients after PBSCT were examined for origin of MSC. Almost all MSC samples exhibited a complete host profile, whereas peripheral blood cells were of donor origin. We conclude that even if trace amounts of MSC are co-transplanted during PBSCT or BMT, they do not expand significantly in the host bone marrow.

Nucleofection is an efficient nonviral transfection technique for human bone marrow-derived mesenchymal stem cells

Viral-based techniques are the most efficient systems to deliver DNA into stem cells because they show high gene transduction and transgene expression in many cellular models. However, the use of viral vectors has several disadvantages mainly involving safety risks. Conversely, nonviral methods are rather inefficient for most primary cells. The Nucleofector technology, a new nonviral electroporation-based gene transfer technique, has proved to be an efficient tool for transfecting hard-to-transfect cell lines and primary cells. However, little is known about the capacity of this technique to transfect adult stem cells. In this study, we applied the Nucleofector technology to engineer human bone marrow- derived mesenchymal stem cells (hMSCs). Using a green fluorescent protein reporter vector, we demonstrated a high transgene expression level using U-23 and C-17 pulsing programs: 73.7%+/-2.9% and 42.5%+/-3.4%, respectively. Cell recoveries and viabilities were 38.7%+/-2.9%, 44.5%+/-3.9% and 91.4%+/-1.3%, 94.31%+/-0.9% for U-23 and C-17, respectively. Overall, the transfection efficiencies were 27.4%+/-2.9% (U-23) and 16.6%+/-1.4% (C-17) compared with 3.6%+/-2.4% and 5.4%+/-3.4% of other nonviral transfection systems, such as FUGENE6 and DOTAP, respectively (p<.005 for all comparisons). Nucleofection did not affect the immunophenotype of hM-SCs, their normal differentiation potential, or ability to inhibit T-cell alloreactivity. Moreover, the interleukin-12 gene could be successfully transfected into hMSCs, and the immunomodulatory cytokine was produced in great amount for at least 3 weeks without impairment of its biological activity. In conclusion, nucleofection is an efficient nonviral transfection technique for hMSCs, which then may be used as cellular vehicles for the delivery of biological agents.

In vitro expansion of human mesenchymal stem cells: choice of serum is a determinant of cell proliferation, differentiation, gene expression, and transcriptome stability

Human bone marrow mesenchymal stem cells (hMSCs) represent an appealing source of adult stem cells for cell therapy and tissue engineering, as they are easily obtained and expanded while maintaining their multilineage differentiation potential. All current protocols for in vitro culture of hMSCs include fetal bovine serum (FBS) as nutritional supplement. FBS is an undesirable additive to cells that are expanded for therapeutic purposes in humans because the use of FBS carries the risk of transmitting viral and prion diseases and proteins that may initiate xenogeneic immune responses. In the present study, we have therefore investigated if autologous serum (AS) or allogeneic human serum (alloHS) could replace FBS for the expansion of hMSCs in vitro. We discovered that the choice of serum affected hMSCs at several different levels. First, hMSCs in AS proliferated markedly faster than hMSCs in FBS, whereas use of alloHS resulted in hMSC growth arrest and death. Second, hMSCs in FBS differentiated more rapidly toward mesenchymal lineages compared with hMSCs in AS. Interestingly, genome-wide microarray analysis identified several transcripts involved in cell cycle and differentiation that were differentially regulated between hMSCs in FBS and AS. Finally, several transcripts, including some involved in cell cycle inhibition, were upregulated in hMSCs in FBS at a late passage, whereas the hMSC transcriptome in AS was remarkably stable. Thus, hMSCs may be expanded rapidly and with stable gene expression in AS in the absence of growth factors, whereas FBS induces a more differentiated and less stable transcriptional profile.

Heterogeneity of engrafted bone-lining cells after systemic and local transplantation

The outcome of various osteoprogenitor-cell transplantation protocols was assessed using Col1a1-GFP reporter transgenic mice. The model requires the recipient mice to undergo lethal total body irradiation (TBI) followed by rescue with whole bone marrow. When the mice are rescued with total bone marrow from a Col1a1-GFP transgenic mouse, green fluorescence protein (GFP)-positive donor cells can be observed on most endosteal and trabecular bone surfaces. Although the cells express an osteoblast-restricted GFP, they fail to progress to osteocytes, do not form a mineralized matrix, and do not generate bone nodules in vitro. However when calvarial progenitor cells derived from the same transgenic mice are injected into the bone marrow space, osteogenesis by the donor cells is observed. Using different GFP colors that distinguish the donor and recipient osteoblasts, commingling of the 2 cells types is observed along the mineralizing osteoblast surface as well as within the osteocyte population of the endosteal bone. Despite the ability of the injected progenitor cells to produce bone within the injected bone, they lack the ability to form mineralized bone nodules when explanted to primary osteoblast culture. These reagents and imaging protocols will be useful in evaluating other cells having a better progenitor potential than calvarial-derived stromal cells.

Suppression of human peripheral blood lymphocyte proliferation by immortalized mesenchymal stem cells derived from bone marrow of Banna Minipig inbred-line

This study sought to investigate whether mesenchymal stem cells (MSC) derived from Banna Minipig Inbred-line (BMI-MSC) suppressed human peripheral blood lymphocyte (hPBLs) proliferation in a one-way mixed lymphocyte reaction system. BMI-MSC failed to stimulate proliferative responses by hPBLs, which were activated by allogenic endothelial cells, BMI-PBLs and non-specific mitogenic stimuli. Furthermore, BMI-MSC also suppressed proliferation of hPBLs stimulated by mismatched allogenic, as well as xenogenic PBLs, and the mitogenic stimulus ConA. The suppression occurred in dose-dependent fashion when the ratio of hPBLs to BMI-MSC varied from 1 to 5 fold; fewer, BMI-MSC (0.001 to 0.01 times) showed no obvious suppression. When BMI-MSC were added to hPBLs stimulated for 72 hours, the proliferative suppression was still evident. Addition of anti-FasL or anti-TGF-beta1 antibody attenuated the proliferative suppression, while antibody against IL-10 had no effect on it. Further immunofluorescence analysis demonstrated that FasL and TGF-beta1 constitutively expressed BMI-MSC. These findings suggest that BMI-MSC suppress hPBLs proliferation relying on FasL and TGF-beta1 mediated pathways.

Mesenchymal stem cells remain of host origin even a long time after allogeneic peripheral blood stem cell or bone marrow transplantation

OBJECTIVE

Plasticity of hematopoietic stem cells (HSC) has gained major interest in stem cell research. In order to investigate whether HSC may differentiate into mesenchymal stem cells (MSC), we assessed chimerism in peripheral blood (PB), mononuclear cell fractions (MNC) of bone marrow, and MSC derived from bone marrow (BM) from 27 up to 4225 days after allogeneic transplantation.

PATIENTS AND METHODS

We applied fluorescence in situ hybridization using X/Y gene probes in sex-mismatched and STR-PCR in sex-matched patients. MSC could have been generated in 27 of 55 bone marrow samples derived from 20 patients. Fifteen patients received peripheral blood stem cell transplants (PBSCT), including CD34-selected PBSCT in two. Five patients received bone marrow.

RESULTS

While all patients had chimerism in PB and MNC of the BM, in all but one patient BM-derived MSC were of recipient origin. This single patient showed reproducibly MSC of donor origin in a frequency of 1% after having received a CD34-selected PBSCT. Looking at graft collections, MSCs were easily generated from BM specimens, while no MSC could be derived from PBSC samples.

CONCLUSION

Even though HSC have been found to differentiate into a variety of nonhematological cell types, they usually do not differentiate into MSC after allogeneic transplantation.

Fetal Mesenchymal Stem-Cell Engraftment in Bone after In Utero Transplantation in a Patient with Severe Osteogenesis Imperfecta

BACKGROUND

Mesenchymal stem cells (MSC) are progenitors of mesenchymal tissues such as bone, cartilage, and adipose. Adult human leukocyte antigen (HLA)-matched MSC have been used in cellular therapies of bone disorders such as osteogenesis imperfecta, with promising results.

METHODS

A female fetus with multiple intrauterine fractures, diagnosed as severe osteogenesis imperfecta, underwent transplantation with allogeneic HLA-mismatched male fetal MSC in the 32nd week of gestation. Engraftment analyses of donor cells, immunologic reaction against donor cells, and the well-being of the patient were assessed.

RESULTS

At 9 months of age, on slides stained for osteocalcin or osteopontin, a centromeric XY-specific probe revealed 0.3% of XY-positive cells in a bone biopsy specimen. Whole Y genome fluorescent in situ hybridization staining showed a median of 7.4% Y-positive cells (range, 6.8%-16.6%). Bone histology showed regularly arranged and configurated bone trabeculae. Patient lymphocyte proliferation against donor MSC was not observed in co-culture experiments performed in vitro after MSC injection. Complementary bisphosphonate treatment was begun at 4 months. During the first 2 years of life, three fractures were noted. At 2 years of corrected age, psychomotor development was normal and growth followed the same channel, -5 SD.

CONCLUSIONS

The authors' findings show that allogeneic fetal MSC can engraft and differentiate into bone in a human fetus even when the recipient is immunocompetent and HLA-incompatible.

Immortalization of Bone Marrow Mesenchymal Stem Cells from Inbred Pig for Regenerative Medicine

The aim of this experiment is to investigate whether MSCs from Banna Minipig Inbred Line (BMI) could be immortalized by introducing SV40 large T antigen gene. MSCs were isolated from BMI and transfected with pSV3neo plasmid. Normal BMI-MSCs would apoptosis and senescence during proliferation while its population doubling (PD) number exceeded 20. However, SV40-transfected cells were immortal. As same as that of normal BMI-MSCs, transfected MSCs were positive for stem cell markers and negative for differentiated osteoblast specific marker. After cultured in osteogenesis supplement media, cbfa1 and calcium deposit on stimulated cells were enhanced obviously. There were no data to prove the tumorigenicity potential of the immortalized cells. Furthermore, histological analysis demonstrated that bone formation was initiated in the pores of HA/TCP implants loaded immortalized BMI-MSCs 7 weeks postimplantation. BMI-MSCs were immortalized by introducing SV40 large T antigen into the cells and still kept the stem cell characters and might be used as seeding cells for tissue engineering as well as stable test cells for biocompatibility of bone biomaterials.

Bone marrow-derived mesenchymal stem cells already express specific neural proteins before any differentiation

Bone marrow mesenchymal stem cells (MSC) are multipotent cells. To explain their plasticity, we postulated that undifferentiated MSC may express proteins from other tissues such as neuronal tissues. MSC are obtained by two different approaches: plastic adhesion or negative depletion (RosetteSep and magnetic beads CD45/glycophorin A). MSC are evaluated through FACS analysis using a panel of antibodies (SH2, SH3, CD14, CD33, CD34, CD45, etc.). To confirm the multipotentiality in vitro, we have differentiated MSC into adipocytes, chondrocytes, osteocytes, and neuronal/glial cells using specific induction media. We have evaluated neuronal and glial proteins (Nestin, Tuj-I, betaIII Tubulin, tyrosine hydroxylase [TH], MAP-2, and GFAP) by using flow cytometry, Western blots, and RT-PCR. We found that MSC constituently express native immature neuronal proteins such as Nestin and Tuj-1. After only five passages, MSC can already express more mature neuronal or glial proteins, such as TH, MAP-2, and GFAP, without any specific induction. We noticed an increase in the expression of more mature neuronal/glial proteins (TH, MAP-2, and GFAP) after exposure to neural induction medium, thus confirming the differentiation of MSC into neurons and astrocytes. The constitutive expression of Nestin or Tuj-1 by MSC suggests that these cells are "multidifferentiated" cells and thus can retain the ability for neuronal differentiation, enhancing their potentiality to be employed in the treatment of neurological diseases.

Synergy between genetic and tissue engineering: Runx2 overexpression and in vitro construct development enhance in vivo mineralization

Tissue engineering has emerged as a promising strategy to generate bone-grafting substrates. These approaches, however, are limited by an insufficient supply of committed osteoprogenitor cells and dedifferentiation of osteogenic cells during in vitro culture. To address these limitations, we engineered bone marrow stromal cells to constitutively express the osteoblastic transcription factor Runx2/Cbfa1, using retroviral gene delivery. These Runx2-modified cells were integrated into three-dimensional polymeric scaffolds to create tissue-engineered constructs. Compared with control stromal cells, Runx2 overexpression significantly upregulated osteoblastic differentiation and mineralization in vitro and in vivo in an ectopic, nonosseous subcutaneous site. More importantly, in vitro construct development to create a mineralized template before implantation dramatically enhanced subsequent in vivo mineralized tissue formation, providing a novel templating tissue-engineering strategy to improve in vivo mineralization. Finally, Runx2 overexpression and in vitro construct development synergistically enhanced in vivo mineralization compared with in vitro construct development or genetic engineering alone. This work provides a novel integrated genetic and tissue-engineering strategy to create mineralized templates for generating robust bone-grafting material.

Immunobiology of Human Mesenchymal Stem Cells and Future Use in Hematopoietic Stem Cell Transplantation

Mesenchymal stem cells (MSCs) may be derived from adult bone marrow, fat, and several fetal tissues. In vitro, MSCs can be expanded and have the capacity to differentiate into several mesenchymal tissues, such as bone, cartilage, and fat. They escape the immune system in vitro, and this may make them candidates for cellular therapy in an allogeneic setting. They also have immunomodulatory effects, inhibit T-cell proliferation in mixed lymphocyte cultures, prolong skin allograft survival, and may decrease graft-versus-host disease (GVHD) when cotransplanted with hematopoietic stem cells. MSCs induce their immunosuppressive effect via a soluble factor. Some candidates have been suggested, and various mechanisms have also been suggested, although contradictory data exist; this may be due to differences in the cells and systems tested. A major problem has been that it has been difficult to identify and isolate MSCs after transplantation in vivo. However, MSCs seem to enhance hematopoietic engraftment in recipients of autologous and allogeneic grafts. Recently, they were found to reverse grade IV acute GVHD of the gut and liver. No tolerance was induced, however. Controlled studies are warranted. Thus, in allogeneic stem cell transplantation, MSCs may be used for hematopoiesis enhancement, as GVHD prophylaxis, and for the treatment of severe acute GVHD. They are also of potential use in the treatment of organ transplant rejection and in autoimmune inflammatory bowel disorders where immunomodulation and tissue repair are needed.

Mesenchymal stem cells: biological characteristics and potential clinical applications

Mesenchymal stem cells (MSC) are clonogenic, non-hematpoietic stem cells present in the bone marrow and are able to differentiate into multiple mesoderm-type cell lineages, for example, osteoblasts, chondrocytes, endothelial-cells and also non-mesoderm-type lineages, for example, neuronal-like cells. Several methods are currently available for isolation of the MSC based on their physical and physico-chemical characteristics, for example, adherence to plastics or other extracellular matrix components. Because of the ease of their isolation and their extensive differentiation potential, MSC are among the first stem cell types to be introduced in the clinic. Several studies have demonstrated the possible use of MSC in systemic transplantation for systemic diseases, local implantation for local tissue defects, as a vehicle for genes in gene therapy protocols or to generate transplantable tissues and organs in tissue engineering protocols. Before their widespread use in therapy, methods allowing the generation of large number of cells without affecting their differentiation potential as well as technologies that overcome immunological rejection (in case allogenic transplantation) must be developed.

Development of mammalian artificial chromosomes for the treatment of genetic diseases: Sandhoff and Krabbe diseases

Mammalian artificial chromosomes (MACs) are being developed as alternatives to viral vectors for gene therapy applications, as they allow for the introduction of large payloads of genetic information in a non-integrating, autonomously replicating format. One class of MACs, the satellite DNA-based artificial chromosome expression vehicle (ACE), is uniquely suited for gene therapy applications, in that it can be generated denovo in cells, along with being easily purified and readily transferred into a variety of recipient cell lines and primary cells. To facilitate the rapid engineering of ACEs, the ACE System was developed, permitting the efficient and reproducible loading of pre-existing ACEs with DNA sequences and/or target gene(s). As a result, the ACE System and ACEs are unique and versatile platforms for ex vivo gene therapy strategies that circumvent and alleviate existing safety and delivery limitations surrounding conventional gene therapy vectors. This review will focus on the status of MAC technologies and, in particular, the application of the ACE System towards an ex vivo gene therapy treatment of lysosomal storage diseases, specifically Sandhoff (MIM #268800) and Krabbe (MIM #245200) diseases.

Non-hematopoietic bone marrow stem cells: molecular control of expansion and differentiation

The first non-hematopoietic mesenchymal stem cells (MSCs) were discovered by Friedenstein in 1976, who described clonal, plastic adherent cells from bone marrow capable of differentiating into osteoblasts, adipocytes, and chondrocytes. More recently, investigators have now demonstrated that multi-potent MSCs can be recovered from a variety of other adult tissues and differentiate into numerous tissue lineages including myoblasts, hepatocytes and possibly even neural tissue. Because MSCs are multipotent and easily expanded in culture, there has been much interest in their clinical potential for tissue repair and gene therapy and as a result, numerous studies have been carried out demonstrating the migration and multi-organ engraftment potential of MSCs in animal models and in human clinical trials. This review describes the recent advances in the understanding of MSC biology.

Spontaneous human adult stem cell transformation

Human adult stem cells are being evaluated widely for various therapeutic approaches. Several recent clinical trials have reported their safety, showing them to be highly resistant to transformation. The clear similarities between stem cell and cancer stem cell genetic programs are nonetheless the basis of a recent proposal that some cancer stem cells could derive from human adult stem cells. Here we show that although they can be managed safely during the standard ex vivo expansion period (6-8 weeks), human mesenchymal stem cells can undergo spontaneous transformation following long-term in vitro culture (4-5 months). This is the first report of spontaneous transformation of human adult stem cells, supporting the hypothesis of cancer stem cell origin. Our findings indicate the importance of biosafety studies of mesenchymal stem cell biology to efficiently exploit their full clinical therapeutic potential.

Human adipose stromal cells expanded in human serum promote engraftment of human peripheral blood hematopoietic stem cells in NOD/SCID mice

Human mesenchymal stem cells (hMSC), that have been reported to be present in bone marrow, adipose tissues, dermis, muscles, and peripheral blood, have the potential to differentiate along different lineages including those forming bone, cartilage, fat, muscle, and neuron. Therefore, hMSC are attractive candidates for cell and gene therapy. The optimal conditions for hMSC expansion require medium supplemented with fetal bovine serum (FBS). Some forms of cell therapy will involve multiple doses, raising a concern over immunological reactions caused by medium-derived FBS proteins. In this study, we cultured human adipose stromal cells (hADSC) and bone marrow stroma cells (HBMSC) in human serum (HS) during their isolation and expansion, and demonstrated that they maintain their proliferative capacity and ability for multilineage differentiation and promote engraftment of peripheral blood-derived CD34(+) cells mobilized from bone marrow in NOD/SCID mice. Our results indicate that hADSC and hBMSC cultured in HS can be used for clinical trials of cell and gene therapies, including promotion of engraftment after allogeneic HSC transplantation.

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.

Tissue-specific engraftment after in utero transplantation of allogeneic mesenchymal stem cells into sheep fetuses

OBJECTIVE

Mesenchymal stem cells (MSC) have multiorgan differentiation capacity, providing the potential for prenatal treatment of genetic disorders. We address the question if in utero transplantation of MSC results in short-term organ-specific engraftment in the fetal sheep.

STUDY DESIGN

Sheep fetal liver-derived MSC selected by adherence culture (passage 1) were transplantated into the fetal peritoneal cavity with ultrasound-guidance (mean gestational age, 59 days). After 14 days recipient fetuses were analyzed by fluorescence-activated cell sorting (FACS), real-time polymerase chain reaction (PCR), and immunohistochemistry.

RESULTS

Fetuses (n = 11) were transplanted with 7.7 x 10(6) MSCs (mean). All surviving fetuses (n = 5) showed engraftment with mean levels of 3.2% (lung), 0.8% (spleen), 0.6% (liver, brain), 0.4% (bone marrow), 0.1% (blood, thymus), and <0.1% (kidneys) by flow cytometry. Immunohistochemistry showed organ-specific distribution.

CONCLUSION

In utero transplantation of allogeneic MSC results in low level, multiorgan engraftment at 14 days post transplant. This supports the potential of in utero MSC transplantation for the treatment of nonhematopoietic genetic disorders of the fetus.

Bone marrow mesenchymal stem cells induce division arrest anergy of activated T cells

It has been shown that mesenchymal stem cells (MSCs) induce T cells to become unresponsive. We characterized the phenotype of these T cells by dissecting the effect of MSCs on T-cell activation, proliferation, and effector function. For this purpose, an in vitro murine model was used in which T-cell responses were generated against the male HY minor histocompatibility antigen. In the presence of MSCs, the expression of early activation markers CD25 and CD69 was unaffected but interferon-γ (IFN-γ) production was reduced. The inhibitory effect of MSCs was directed mainly at the level of cell proliferation. Analysis of the cell cycle showed that T cells, stimulated in the presence of MSCs, were arrested at the G1 phase. At the molecular level, cyclin D2 expression was profoundly inhibited, whereas p27kip1 was up-regulated. When MSCs were removed from the cultures and restimulated with the cognate peptide, T cells produced IFN-γ but failed to proliferate. The addition of exogenous interleukin-2 (IL-2) did not restore proliferation. MSCs did not preferentially target any T-cell subset, and the inhibition was also extended to B cells. MSC-mediated inhibition induces an unresponsive T-cell profile that is fully consistent with that observed in division arrest anergy.

Runx2/Cbfa1-genetically engineered skeletal myoblasts mineralize collagen scaffolds in vitro

Genetic engineering of progenitor and stem cells is an attractive approach to address cell sourcing limitations associated with tissue engineering applications. Bone tissue engineering represents a promising strategy to repair large bone defects, but has been limited in part by the availability of a sustained, mineralizing cell source. This study examined the in vitro mineralization potential of primary skeletal myoblasts genetically engineered to overexpress Runx2/Cbfa1, an osteoblastic transcriptional regulator essential to bone formation. These cells were viable at the periphery of 3D fibrous collagen scaffolds for 6 weeks of static culture. Exogenous Runx2 expression induced osteogenic differentiation and repressed myogenesis in these constructs relative to controls. Runx2-modified cells deposited significant amounts of mineralized matrix and hydroxyapatite, as determined by microcomputed tomography, histological analysis, and Fourier transform infrared spectroscopy, whereas scaffolds seeded with control cells exhibited no mineralized regions. Although mineralization by Runx2-engineered cells was confined to the periphery of the construct, colocalizing with cell viability, it was sufficient to increase the compressive modulus of constructs 30-fold relative to controls. This work demonstrates that Runx2 overexpression in skeletal myoblasts may address current obstacles of bone tissue engineering by providing a potent cell source for in vitro mineralization and construct maturation. Additionally, the use of genetic engineering methods to express downstream control factors and transcriptional regulators, in contrast to soluble signaling molecules, represents a robust strategy to enhance cellular activities for tissue engineering applications.

Mesenchymal stem cells: cell biology and potential use in therapy

Mesenchymal stem cells are clonogenic, non-haematopoietic stem cells present in the bone marrow and are able to differentiate into multiple mesoderm-type cell lineages e.g. osteoblasts, chondrocytes, endothelial-cells and also non-mesoderm-type lineages e.g. neuronal-like cells. Several methods are currently available for isolation of the mesenchymal stem cells based on their physical and immunological characteristics. Because of the ease of their isolation and their extensive differentiation potential, mesenchymal stem cells are among the first stem cell types to be introduced in the clinic. Recent studies have demonstrated that the life span of mesenchymal stem cells in vitro can be extended by increasing the levels of telomerase expression in the cells and thus allowing culture of large number of cells needed for therapy. In addition, it has been shown that it is possible to culture the cells in xeno-free environment without affecting their growth or differentiation potential. Finally, the mesenchymal stem cells seems to be hypoimmunogenic and thus allogenic mesenchymal stem cells transplantation is possible. It is envisaged that mesenchymal stem cells can be used in systemic transplantation for generalized diseases, local implantation for local tissue defects, as a vehicle for genes in gene therapy protocols or to generate transplantable tissues and organs in tissue engineering protocols. The results of these initial trials are very encouraging and several clinical trials are under way to study the efficacy and long-term safety of therapeutics based on mesenchymal stem cells.

Human mesenchymal stem cells inhibit differentiation and function of monocyte-derived dendritic cells

Mesenchymal stem cells (MSCs), in addition to their multilineage differentiation, have a direct immunosuppressive effect on T-cell proliferation in vitro. However, it is unclear whether they also modulate the immune system by acting on the very first step. In this investigation, we addressed the effects of human MSCs on the differentiation, maturation, and function of dendritic cells (DCs) derived from CD14+ monocytes in vitro. Upon induction with granulocyte-macrophage colony-stimulating factor (GM-CSF) plus interleukin-4 (IL-4), MSC coculture could strongly inhibit the initial differentiation of monocytes to DCs, but this effect is reversible. In particular, such suppression could be recapitulated with no intercellular contact at a higher MSC/monocyte ratio (1:10). Furthermore, mature DCs treated with MSCs were significantly reduced in the expression of CD83, suggesting their skew to immature status. Meanwhile, decreased expression of presentation molecules (HLA-DR and CD1a) and costimulatory molecules (CD80 and CD86) and down-regulated IL-12 secretion were also observed. In consistence, the allostimulatory ability of MSC-treated mature DCs on allogeneic T cells was impaired. In conclusion, our data suggested for the first time that human MSCs could suppress monocyte differentiation into DCs, the most potent antigen-presenting cells (APCs), thus indicating the versatile regulation of MSCs on the ultimate specific immune response.

Autologous serum for isolation and expansion of human mesenchymal stem cells for clinical use

OBJECTIVE

Mesenchymal stem cells (MSC) are promising candidates for cell-based therapies. One major obstacle for their clinical use is the biosafety of fetal calf serum (FCS), which is a crucial part of all media currently used for the culture of MSC.

METHODS

Nine donors each contributed 5 mL of bone marrow aspirate. Isolation of MSC was conducted according to Caplan et al., although for expansion we used low-density seeding with 20 MSC/cm2. Four different media A, B, C, and D were tested, containing 1%, 3%, or 10% autologous serum (AS), or 10% selected FCS, respectively. MSC were cultured on 24-well plates until passage 2 and counted under the microscope at regular intervals. Osteogenic and adipogenic differentiation were induced in vitro by using a modified standard cocktail and were evaluated semi-quantitatively through a microscope.

RESULTS

Isolation of MSC after 3 days appeared best in media C with almost always C>D congruent with B>A. Proliferation was exponential with generally C>D>B>A. Morphologically, MSC isolated and expanded in medium C were indistinguishable from those in medium D. Phenotypic markers of MSC grown in medium C were: CD34-, CD45-, CD90+, CD105+, MHC class I+, MHC class II-, similar to MSC isolated and grown in medium D. Moreover, MSC grown in medium C showed more osteogenic potential than those from medium D in all cases: C+++, D++, B+, A 0. Cells retained their immaturity as shown by adipogenic differentiation and it always was: D+++, C++, B+, A 0.

CONCLUSIONS

Growth of MSC in a FCS-free medium is feasible without addition of growth factors. Ten percent AS appears at least as good as 10% FCS with regard to both isolation and expansion of human MSC, while 1% and 3% AS appear inferior. With respect to osteogenic differentiation, 10% AS proved superior to the other serum conditions.

Cotransplantation of marrow stromal cells may prevent lethal graft-versus-host disease in major histocompatibility complex mismatched murine hematopoietic stem cell transplantation

Marrow stromal cells (MSC) produce a microenvironment supporting hematopoiesis and may contribute immune tolerance because of low immunogenicity and the suppressive effect of alloreactivity. We investigated whether cotransplantation of MSC could prevent lethal graft-versus-host disease (GVHD) in major histocompatibility complex mismatched allogeneic murine hematopoietic stem cell transplantation (HSCT) using female BALB/c (H-2d, recipient) and C3H/He (H-2k, donor) mice. MSC were obtained from C3H/He bone marrow cells (BMC). MSC and irradiated BALB/c splenocytes (SP) were cocultured with C3H/He SP or BMC. Nonirradiated MSC did not inhibit the proliferation of alloantigen-stimulated BMC and SP. However, irradiated MSC suppressed the proliferation of alloantigen-stimulated SP at a level comparable with that of immunosuppressive agents, and the suppression by MSC was reversed to a significant degree by interleukin 2. Lethally irradiated BALB/c mice received transplants of donor cells according to the following experimental groups (group A, BMC only; group B, BMC and SP; group C, BMC, SP, and MSC; group D, BMC and MSC). The survival rate in group D was higher than in the other groups (P = .0057), and the clinical GVHD scores and serum levels of interferon-gamma were low in group D. Our results suggest that cotransplantation of MSC in HSCT prevents lethal GVHD, possibly by immune modulation.

Dkk-1-mediated expansion of adult stem cells

Despite the wealth of research on stem-cell biology and therapy, there have been few advances in the ex vivo expansion of adult stem cells. Fetal bovine serum, which can elicit an immune reaction in hosts receiving cellular products, remains essential to the culture methods. Recent work by Prockop et al. suggests that Dickkopf-1 (Dkk-1) could stimulate the proliferation of mesenchymal stem cells (MSCs) while maintaining an undifferentiated phenotype. The use of Dkk-1 in cell-processing laboratories might advance cell-expansion technology and greatly increase the clinical uses of MSCs.

Mesenchymal stem cells

About 40 years ago Friedenstein described stromal cells in the bone marrow that were spindle shaped and proliferate to form colonies. These cells attach to plastic and are able to differentiate under defined in vitro conditions into multiple cell types present in many different tissues, e.g. osteoblasts, chondroblasts, adipocytes, etc. Later on these cells, obtained from postnatal bone marrow, were called mesenchymal stem cells (MSC) or stromal stem cells. Recently the presence of somewhat similar cells has been demonstrated in many other tissues too. In spite of extensive attempts to characterize these cells we are still lacking definitive in vivo markers of MSC although retrospective functional data strongly support the existence of common adult stem cells that have the capacity to differentiate along various specific differentiation lineages. Since MSC can be rather easily isolated from the bone marrow and can also be expanded in vitro they have become a prime target for researchers of tissue regeneration. These cells have now been extensively used for transplantation experiments in animals and also for some therapeutic trials in humans. However, much new research is needed to learn enough on the molecular mechanisms of MSC differentiation to evaluate their full capacity for tissue regeneration.