Mesenchymal stem cell engraftment in lung is enhanced in response to bleomycin exposure and ameliorates its fibrotic effects

Transplanted mesenchymal stem cells accelerate glomerular healing in experimental glomerulonephritis

Bone marrow-derived cells contribute to glomerular cell turnover and repair, but the cell types involved are unknown. Whether rat mesenchymal stem cells (MSC) can accelerate recovery from damage in rat mesangioproliferative anti-Thy1.1 glomerulonephritis was studied. After injection into the left renal artery on day 2 after disease induction, fluorescently labeled MSC were detected in 20 to 50% of glomeruli and rare intrarenal vessels but not in the tubulointerstitium, in contralateral kidneys, or in medium controls. In control experiments, injected mesangial cells were detected less frequently in glomeruli in comparison with injected MSC. In nephritic outbred Wistar rats, MSC injection led to an approximately 50% reduction of mesangiolysis on days 4 and 6 after disease induction, accompanied by three- to four-fold higher intraglomerular cell proliferation on day 4 and more rapid mesangial reconstitution as detected by alpha-smooth muscle actin expression. Injection of MSC into tail veins or intra-arterial injection of mesangial cells instead of MSC failed to reproduce any of these findings. In inbred Lewis rats, anti-Thy1.1 nephritis followed an aggravated course with transient acute renal failure. Acute renal failure was ameliorated by MSC injection into the left renal artery on day 2 after disease induction. Again, MSC led to more rapid recovery from mesangiolysis, increased glomerular cell proliferation, and reduction of proteinuria by 28%. Double immunostaining of 5-bromo-2'-deoxyuridine-labeled MSC for endothelial, mesangial, or monocyte/macrophage antigens showed that 85 to 95% of MSC that localized in glomeruli on day 6 failed to express these markers. In vitro, MSC secreted high amounts of vascular endothelial growth factor and TGF-beta1 but not PDGF-BB. In conclusion, even low numbers of MSC can markedly accelerate glomerular recovery from mesangiolytic damage possibly related to paracrine growth factor release and not to differentiation into resident glomerular cell types or monocytes/macrophages.

Hepatocyte growth factor gene transfer to alveolar septa for effective suppression of lung fibrosis

We examined therapeutic gene transfer of human hepatocyte growth factor (hHGF) to alveolar septa in mouse bleomycin-induced lung fibrosis using macroaggregated albumin-polyethylenimine complex (MAA-PEI). Intravenous administration of MAA-PEI along with 1 microg pCAG.hHGF to C57BL/6 mice increased the uptake of plasmids into alveolar capillary endothelial cells and epithelial cells, prolonged hHGF expression in the lung, and induced a level of hHGF expression equal to that seen with 10 microg of hHGF-expression plasmids alone. The exogenous source of hHGF gene expression increased the endogenous mouse HGF in the lungs and significantly decreased TNF-alpha, IL-6, and collagen synthesis after bleomycin injury. Because GFP-labeled bone marrow-derived stem cells after bleomycin injury were reduced in number by HGF, the primary mechanism of HGF is likely to be the prevention of apoptosis, as has been suggested by in vitro experiments. This novel HGF gene transfer method to alveolar septa with nonstimulatory MAA-PEI conjugates may have promising clinical applications.

Stem cells in the lung parenchyma and prospects for lung injury therapy

Until recently, it was thought that only embryonic stem cells were pluripotent and that adult stem cells were restricted in their differentiative and regenerative potential to become the tissues in which they reside. However, the discovery that adult stem cells in one tissue can contribute to the formation of other tissues, especially after injury or cell damage, implies that stem cells have developmental plasticity. For example, haematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) from bone marrow can be used to regenerate diverse tissues at distant sites, including the lung. This article reviews the character of stem cells in the lung parenchyma and focuses on the potential uses of adult stem cells in research of lung injury and lung disease therapies.

Adult bone marrow-derived stem cells for the lung: implications for pediatric lung diseases

Bronchopulmonary dysplasia (BPD) and cystic fibrosis (CF) are two common serious chronic respiratory disorders without specific treatments affecting children. BPD is characterized by an arrest in alveolar growth in premature infants requiring respiratory support. CF is the most common fatal inherited genetic disorder characterized by abnormally thick mucus secretions, recurrent infection and ultimately lung destruction. One commonality between these two diseases is the promise of utilizing stem cells therapeutically. Indeed, the use of exogenous cells to supplement the natural repair mechanisms or the possibility of genetic manipulation in vitro before administration are appealing therapeutic options for these diseases. Increasing attention has been focused on the use of adult bone marrow-derived stem cells (BMSC) to regenerate damaged organs such as the heart, the brain, and the liver. However, due to the lung's complexity as well as the low rate of cellular turnover within the lung, progress has been slower in this area compared with the skin or liver. Initial work suggests that BMSC can engraft and differentiate into a variety of lung cells, but these findings have been challenged recently. This article critically reviews the current advances on the therapeutic use of stem cells for lung regeneration.

Focal interstitial CC chemokine receptor 7 (CCR7) expression in idiopathic interstitial pneumonia

BACKGROUND/AIMS

Idiopathic interstitial pneumonias (IIPs) are a diverse grouping of chronic pulmonary diseases characterised by varying degrees of pulmonary fibrosis. The triggers of the fibroproliferative process in IIP remain enigmatic but recent attention has been directed towards chemokine involvement in this process.

METHODS

The expression of two chemokine receptors, CCR7 and CXCR4, and their respective ligands, CCL19, CCL21, and CXCL12, were examined in surgical lung biopsies (SLBs) from patients with IIP. Transcript and protein expression of these receptors and their ligands was compared with that detected in histologically normal margin SLBs.

RESULTS

CCR7 and CXCR4 were detected by gene array and real time polymerase chain reaction analysis and CCR7, but not CXCR4, expression was significantly raised in usual interstitial pneumonia (UIP) relative to biopsies from patients diagnosed with non-specific interstitial pneumonia (NSIP) or respiratory bronchiolitis/interstitial lung disease (RBILD). CCR7 protein was expressed in interstitial areas of all upper and lower lobe UIP SLBs analysed. CCR7 expression was present in 50% of NSIP SLBs, and CCR7 was restricted to blood vessels and mononuclear cells in 75% of RBILD SLBs. Immune cell specific CXCR4 expression was seen in IIP and normal margin biopsies. CCR7 positive areas in UIP biopsies were concomitantly positive for CD45 (the leucocyte common antigen) but CCR7 positive areas in all IIP SLBs lacked the haemopoietic stem cell antigen CD34, collagen 1, and alpha smooth muscle actin.

CONCLUSION

This molecular and immunohistochemical analysis showed that IIPs are associated with abnormal CCR7 transcript and protein expression.

Potential application for mesenchymal stem cells in the treatment of cardiovascular diseases

Stem cells isolated from various sources have been shown to vary in their differentiation capacity or pluripotentiality. Two groups of stem cells, embryonic and adult stem cells, may be capable of differentiating into any desired tissue or cell type, which offers hope for the development of therapeutic applications for a large number of disorders. However, major limitations with the use of embryonic stem cells for human disease have led researchers to focus on adult stem cells as therapeutic agents. Investigators have begun to examine postnatal sources of pluripotent stem cells, such as bone marrow stroma or adipose tissue, as sources of mesenchymal stem cells. The following review focuses on recent research on the use of stem cells for the treatment of cardiovascular and pulmonary diseases and the future application of mesenchymal stem cells for the treatment of a variety of cardiovascular disorders.

Bone marrow production of lung cells: the impact of G-CSF, cardiotoxin, graded doses of irradiation, and subpopulation phenotype

OBJECTIVE

Previous studies have demonstrated the production of various types of lung cells from marrow cells under diverse experimental conditions. Our aim was to identify some of the variables that influence conversion in the lung.

METHODS

In separate experiments, mice received various doses of total-body irradiation followed by transplantation with whole bone marrow or various subpopulations of marrow cells (Lin(-/+), c-kit(-/+), Sca-1(-/+)) from GFP(+) (C57BL/6-TgN[ACTbEGFP]1Osb) mice. Some were given intramuscular cardiotoxin and/or mobilized with granulocyte colony-stimulating factor (G-CSF).

RESULTS

The production of pulmonary epithelial cells from engrafted bone marrow was established utilizing green fluorescent protein (GFP) antibody labeling to rule out autofluorescence and deconvolution microscopy to establish the colocaliztion of GFP and cytokeratin and the absence of CD45 in lung samples after transplantation. More donor-derived lung cells (GFP(+)/CD45(-)) were seen with increasing doses of radiation (5.43% of all lung cells, 1200 cGy). In the 900-cGy group, 61.43% of GFP(+)/CD45(-) cells were also cytokeratin(+). Mobilization further increased GFP(+)/CD45(-) cells to 7.88% in radiation-injured mice. Up to 1.67% of lung cells were GFP(+)/CD45(-) in radiation-injured mice transplanted with Lin(-), c-kit(+), or Sca-1(+) marrow cells. Lin(+), c-kit(-), and Sca-1(-) subpopulations did not significantly engraft the lung.

CONCLUSIONS

We have established that marrow cells are capable of producing pulmonary epithelial cells and identified radiation dose and G-CSF mobilization as variables influencing the production of lung cells from marrow cells. Furthermore, the putative lung cell-producing marrow cell has the phenotype of a hematopoietic stem cell.

Limited restoration of cystic fibrosis lung epithelium in vivo with adult bone marrow-derived cells

RATIONALE

Recent literature suggests that adult bone marrow-derived cells can localize to lung and acquire immunophenotypic characteristics of lung epithelial cells. We speculated this might be a potential therapeutic approach for correcting defective lung epithelium in cystic fibrosis.

OBJECTIVE

To determine whether adult bone marrow-derived cells containing normal cystic fibrosis transmembrane conductance regulator protein (CFTR) could repopulate lung epithelium in transgenic mice deficient in that protein.

METHODS

Stromal marrow cells or total marrow obtained from adult male wild-type mice were transplanted into adult female Cftr knockout mice. To increase marrow cell recruitment naphthalene was used to induce airway epithelial injury in recipient mice.

MEASUREMENTS AND MAIN RESULTS

At 1 wk, 1 mo, and 3 mo after transplantation, Cftr mRNA was detected in lung homogenates of recipient mice by reverse transcription-polymerase chain reaction. Cftr mRNA was not found in either donor marrow cells or mature circulating leukocytes. In situ examination of recipient mouse lungs demonstrated rare (0.025%) chimeric airway epithelial cells, some of which (0.01%) expressed CFTR protein. Naphthalene-induced airway remodeling nonsignificantly increased the number of chimeric airway epithelial cells expressing Cftr.

CONCLUSIONS

These results demonstrate that adult marrow cells can be recruited to airway epithelium and induced to express Cftr in mice otherwise lacking this protein. However, the number of observed chimeric epithelial cells is small and new strategies for enhancing airway epithelial remodeling by adult bone marrow-derived cells will be necessary for correction of defective CFTR-dependent chloride transport.

Engraftment of Bone Marrow-Derived Epithelial Cells

Discoveries of the ability of bone marrow-derived cells (BMDCs) to differentiate into nonhematopoietic cells have opened up a new field of inquiry in adult stem cell plasticity. There are far more questions than there are answers to date. We and others have investigated whether differentiation occurs in response to tissue damage, what the underlying mechanisms might be, and whether this plasticity may be useful clinically. BMDC have been shown to differentiate into mature-appearing epithelial cells in the lung, liver, gastrointestinal tract, skin, buccal mucosa, and kidney. The mechanism(s) by which cells transition to these nonhematopoietic phenotypes is not yet clear, but possibilities include cell-to-cell fusion, direct differentiation of a nonhematopoietic precursor cell from the BM, and transdifferentiation of a BM cell that had previously been committed to a different phenotype. Data obtained to date support the first two possibilities, and there are no data proving that transdifferentiation is responsible for the engraftment of marrow-derived epithelial cells. Theoretically, the engraftment of marrow-derived cells as nonhematopoietic cell types could be used in either the autologous or the allogeneic setting to restore functional epithelial cells to a diseased organ. For example, a marrow-derived cell that has been transduced to express a specific transgene can continue to express this transgene after it engrafts as a nonhematopoietic epithelial cell in the lung. Analyses of the kinetics of this engraftment suggest that it can be increased within days to weeks following certain types of injury, depending on the tissue examined. Most reports of adult stem cell plasticity show relatively low frequencies of marrow-derived nonhematopoietic cells, on the order of 1 in 10(3) to 1 in 10(4) epithelial cells in many organs being marrow derived. This frequency is likely to be too low to be of therapeutic relevance. Therefore, future efforts will need to be focused on enhancing levels of engraftment.

Biological Activities Encoded by the Murine Mesenchymal Stem Cell Transcriptome Provide a Basis for Their Developmental Potential and Broad Therapeutic Efficacy

We used serial analysis of gene expression to catalog the transcriptome of murine mesenchymal stem cells (MSCs) enriched from bone marrow by immunodepletion. Interrogation of this database, results of which are delineated in the appended databases, revealed that immunodepleted murine MSCs (IDmMSCs) highly express transcripts encoding connective tissue proteins and factors modulating T-cell proliferation, inflammation, and bone turnover. Categorizing the transcriptome based on gene ontologies revealed the cells also expressed mRNAs encoding proteins that regulate mesoderm development or that are characteristic of determined mesenchymal cell lineages, thereby reflecting both their stem cell nature and differentiation potential. Additionally, IDmMSCs also expressed transcripts encoding proteins regulating angiogenesis, cell motility and communication, hematopoiesis, immunity and defense as well as neural activities. Immunostaining and fluorescence-activated cell sorting analysis revealed that expression of various regulatory proteins was restricted to distinct subpopulations of IDmMSCs. Moreover, in some cases, these proteins were absent or expressed at reduced levels in other murine MSC preparations or cell lines. Lastly, by comparing their transcriptome to that of 17 other murine cell types, we also identified 43 IDmMSC-specific transcripts, the nature of which reflects their varied functions in bone and marrow. Collectively, these results demonstrate that IDmMSC express a diverse repertoire of regulatory proteins, which likely accounts for their demonstrated efficacy in treating a wide variety of diseases. The restricted expression pattern of these proteins within populations suggests that the cellular composition of marrow stroma and its associated functions are more complex than previously envisioned.

Mesenchymal stem cells: isolation, in vitro expansion and characterization

Mesenchymal stem cells (MSC), one type of adult stem cell, are easy to isolate, culture, and manipulate in ex vivo culture. These cells have great plasticity and the potential for therapeutic applications, but their properties are poorly understood. MSCs can be found in bone marrow and in many other tissues, and these cells are generally identified through a combination of poorly defined physical, phenotypic, and functional properties; consequently, multiple names have been given to these cell populations. Murine MSCs have been directly applied to a wide range of murine models of diseases, where they can act as therapeutic agents per se, or as vehicles for the delivery of therapeutic genes. In addition to their systemic engraftment capabilities, MSCs show great potential for the replacement of damaged tissues such as bone, cartilage, tendon, and ligament. Their pharmacological importance is related to four points: MSCs secrete biologically important molecules, express specific receptors, can be genetically manipulated, and are susceptible to molecules that modify their natural behavior. Due to their low frequency and the lack of knowledge on cell surface markers and their location of origin, most information concerning MSCs is derived from in vitro studies. The search for the identity of the mesenchymal stem cell has depended mainly on three culture systems: the CFU-F assay, the analysis of bone marrow stroma, and the cultivation of mesenchymal stem cell lines. Other cell populations, more or less related to the MSC, have also been described. Isolation and culture conditions used to expand these cells rely on the ability of MSCs, although variable, to adhere to plastic surfaces. Whether these conditions selectively favor the expansion of different bone marrow precursors or cause similar cell populations to acquire different phenotypes is not clear. The cell populations could also represent different points of a hierarchy or a continuum of differentiation. These issues reinforce the urgent need for a more comprehensive view of the mesenchymal stem cell identity and characteristics.

Pulmonary epithelium

Repair or regeneration of defective lung epithelium would be of great therapeutic potential. Cellular sources for such repair have long been searched for within the lung, but the identification and characterization of stem or progenitor cells have been hampered by the complexity and cellular heterogeneity of the organ. In recent years, various pulmonary cells have been identified that meet the criteria for stem cells but it remains to be seen how far manipulation of these tissue-specific cell pools can upregulate epithelial repair. The initial excitement that greeted the results of animal experiments showing cells of bone marrow origin in murine lung has been tempered by more recent data suggesting that the cells do not repair pulmonary epithelium. However, there are reports of engraftment of bone marrow-derived cells in human lung, albeit at a low level, so the administration of cell therapy via the circulation, for repair and/or gene delivery, needs further investigation. The potential of human embryonic stem cells to generate any cell, tissue, or organ on demand for tissue repair or replacement is promising to revolutionize the treatment of human disease. Although some headway has been made into making pulmonary epithelium from these stem cells, human embryonic stem cell technology is still in its infancy and many technical, safety, and ethical hurdles must be cleared before clinical trials can begin. This chapter focuses on the potential role of stem cells in future approaches to lung repair and regeneration.

Administered mesenchymal stem cells enhance recovery from ischemia/reperfusion-induced acute renal failure in rats

BACKGROUND

Adult stem cells are promising for the development of novel therapies in regenerative medicine. Acute renal failure (ARF) remains a frequent clinical complication, associated with an unacceptably high mortality rate, in large part due to the ineffectiveness of currently available therapies. The aim of this study was, therefore, to evaluate the therapeutic effectiveness of bone marrow-derived mesenchymal stem cells in a rat model of ischemia/reperfusion (I/R) ARF.

METHODS

We used a common I/R model in rats to induce ARF by clamping both renal pedicles for 40 minutes. Mesenchymal stem cells were iron-dextran-labeled for in vivo tracking studies by magnetic resonance imaging (MRI) and kidneys were imaged for mesenchymal stem cells immediately after infusion and at day 3 after ARF. Renal injury was scored on day 3 and cells were additionally tracked by Prussian blue staining.

RESULTS

We show in I/R-induced ARF in rats, modeling the most common form of clinical ARF, that infusion of mesenchymal stem cells enhances recovery of renal function. Mesenchymal stem cells were found to be located in the kidney cortex after injection, as demonstrated by MRI. Mesenchymal stem cells-treated animals had both significantly better renal function on days 2 and 3 and better injury scores at day 3 after ARF. Histologically, mesenchymal stem cells were predominantly located in glomerular capillaries, while tubules showed no iron labeling, indicating absent tubular transdifferentiation.

CONCLUSION

We conclude that the highly renoprotective capacity of mesenchymal stem cells opens the possibility for a cell-based paradigm shift in the treatment of I/R ARF.

Stem cells: classifications, controversies, and clinical applications

The application of stem cells in regenerative and reparative therapies is emerging in surgery. Published information can lead to an over simplified view of stem cells with respect to their definitions, tissues of origin, abilities to differentiate into tissue lineages, and their capacity for functional tissue regeneration. The goals of this review article are to define embryonic and adult stem cells, compare differences between them, and summarize their potential clinical applications.

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.

A subset of human rapidly self-renewing marrow stromal cells preferentially engraft in mice

Controversies have arisen as to whether adult stem cells or progenitor cells from bone marrow can engraft into nonhematopoietic tissues in vivo. To resolve some of the controversies, we developed a highly sensitive polymerase chain reaction-based single nucleotide polymorphism (PCR-SNP) assay for competitive engraftment of mixtures of stem/progenitor cells. We used the assay to follow engraftment in immunodeficient mice of subpopulations of the stem/progenitor cells from human bone marrow referred to as either mesenchymal stem cells or marrow stromal cells (MSCs). The engraftment into adult mice without induced tissue injury was low and variable, but there was preferential engraftment of a subpopulation of rapidly self-renewing MSCs (RS-MSCs) compared with a subpopulation of slowly renewing MSCs (SR-MSCs). After intravenous infusion, there was a tendency for the cells to engraft into the hippocampal region that was previously designated a "vascular niche." Migration assays suggested that preferential engraftment of RS-MSCs was in part explained by their expression of CXCR4 and CX3R1, the receptors for SDF-1 and fractalkine.

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.

Stem cells of the alveolar epithelium

Elucidation of the biology of stem cells of the lung parenchyma could revolutionise treatment of patients with lung disorders such as cancer, acute respiratory distress syndrome, emphysema, and fibrotic lung disease. How close is this goal? Despite remarkable observations and ensuing advances, more questions than answers have been generated. Progenitors of the alveolar epithelium remain largely mysterious, so the prospect of isolating enough of these cells and delivering them effectively to cure disease remains remote. Similarly, the bone-marrow-derived cell that might most effectively engraft the lung remains unknown. If this mechanism is an important process for lung repair, why will the administration of additional cells be more effective? Finally, there is an issue of control of multipotent cells to avoid the generation of multiple teratomas, longevity of the graft, and possible immunological reactions to gene products inserted to replace a deficiency. The biology is exciting but not yet well enough understood to support therapeutic advances.

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.

Increased circulating endothelial progenitor cells are associated with survival in acute lung injury

RATIONALE

Repair of damaged endothelium is important in recovery from acute lung injury. In animal models, bone marrow-derived endothelial progenitor cells differentiate into mature endothelium and assist in repairing damaged vasculature.

OBJECTIVES

The quantity of endothelial progenitor cells in patients with acute lung injury is unknown. We hypothesize that increased numbers of circulating endothelial progenitor cells will be associated with an improved outcome in acute lung injury and the acute respiratory distress syndrome.

METHODS

Peripheral blood mononuclear cells from the buffy coat of patients with early acute lung injury (n=45), intubated control subjects (n=10), and healthy volunteers (n=7) were isolated using Ficoll density gradient centrifugation, and plated on fibronectin-coated cellware. After 24 hours, nonadherent cells were removed and replated on fibronectin-coated cellware at a concentration of 1x10(6) cells/well. Colony-forming units were counted after 7 days' incubation.

MEASUREMENTS/MAIN RESULTS

Endothelial progenitor cell colony numbers were significantly higher in patients with acute lung injury compared with healthy control subjects (p<0.05), but did not differ between patients with acute lung injury and intubated control subjects. However, in the 45 patients with acute lung injury, improved survival correlated with a higher colony count (p<0.04). Patients with acute lung injury with a colony count of >or= 35 had a mortality of 30%, compared with 61% in those with colony counts <35 (p<0.03), results that persisted in a multivariable analysis correcting for age, sex, and severity of illness.

CONCLUSIONS

An increased number of circulating endothelial progenitor cells in acute lung injury is associated with improved survival.

Failure of bone marrow to reconstitute lung epithelium

A new paradigm of epithelial tissue reconstitution has been suggested whereby circulating cells derived from bone marrow contribute to a variety of epithelial cell types. With regard to the lung, several recent reports have used immunofluorescence microscopy to demonstrate engraftment of bone marrow-derived cells as type II pneumocytes, the endogenous progenitors of the lung alveolus. We show here that immunofluorescence microscopy, as has been used in previous reports, cannot reliably identify rare engrafted cells in lung tissue sections after transplantation of bone marrow cells or purified hematopoietic stem cells tracked with ubiquitous labels. We have employed a lineage-specific reporter system based on transgenic mice that express the GFP reporter gene only in lung epithelial cells (surfactant protein C-GFP) to assay for engrafted cells by flow cytometry, histology, and molecular methods. Using this approach to evaluate transplant recipients, including those subjected to bleomycin-induced lung injury, we demonstrate that when autofluorescence, dead cells, and contaminating blood cells are excluded from analysis, there is no detectable reconstitution of lung alveolar epithelial cells by unfractionated bone marrow cells or purified hematopoietic stem cells.

Tissue distribution and engraftment of human mesenchymal stem cells immortalized by human telomerase reverse transcriptase gene

Engraftment of mesenchymal stem cells (MSC) in peripheral tissues for replenishing of local stem cell function has been proposed as a therapeutic approach to degenerative diseases. We have previously reported the development of an immortalized human telomerase reverse transcriptase transduced MSC line (hMSC-TERT). In the present study, we co-transduced hMSC-TERT with enhanced green fluorescent protein gene, and studied tissue distribution, engraftment, and cell survival after intracardiac and intravenous injections in immunodeficient mice. The pattern of organ distribution suggested that infused cells were efficiently arrested in microvasculature during first-pass, but only for a fraction of the infused cells was arrest followed by vascular emigration and tissue engraftment. Few engrafted cells in lungs, heart, and kidney glomeruli remained after 4 weeks. These observations are consistent with several reports on limited systemic transplantability of primary MSC. HMSC-TERT may constitute a valuable tool for mechanistic studies on how to control MSC homing and engraftment.

Bone marrow-derived mesenchymal stem cells protect against experimental liver fibrosis in rats

AIM: Recent reports have shown the capacity of mesenchymal stem cells (MSCs) to differentiate into hepatocytes in vitro and in vivo. MSCs administration could repair injured liver, lung, or heart through reducing inflammation, collagen deposition, and remodeling. These results provide a clue to treatment of liver fibrosis. The aim of this study was to investigate the effect of infusion of bone marrow (BM)-derived MSCs on the experimental liver fibrosis in rats.

METHODS: MSCs isolated from BM in male Fischer 344 rats were infused to female Wistar rats induced with carbon tetrachloride (CCl₄) or dimethylnitrosamine (DMN). There were two random groups on the 42^nd d of CCl₄:CCl₄/MSCs, to infuse a dose of MSCs alone; CCl₄/saline, to infuse the same volume of saline as control. There were another three random groups after exposure to DMN: DMN10/MSCs, to infuse the same dose of MSCs on d 10; DMN10/saline, to infuse the same volume of saline on d 10; DMN20/MSCs, to infuse the same dose of MSCs on d 20. The morphological and behavioral changes of rats were monitored everyday. After 4-6 wk of MSCs administration, all rats were killed and fibrosis index were assessed by histopathology and radioimmunoassay. Smooth muscle alpha-actin (alpha-SMA) of liver were tested by immunohistochemistry and quantified by IBAS 2.5 software. Male rats sex determination region on the Y chromosome (sry) gene were explored by PCR.

RESULTS: Compared to controls, infusion of MSCs reduced the mortality rates of incidence in CCl₄-induced model (10% vs 20%) and in DMN-induced model (20-40% vs 90%).The amount of collagen deposition and alpha-SMA staining was about 40-50% lower in liver of rats with MSCs than that of rats without MSCs. The similar results were observed in fibrosis index. And the effect of the inhibition of fibrogenesis was greater in DMN10/MSCs than in DMN20/MSCs. The sry gene was positive in the liver of rats with MSCs treatment by PCR.

CONCLUSION: MSCs treatment can protect against experimental liver fibrosis in CCl₄-induced or DMN-induced rats and the mechanisms of the anti-fibrosis by MSCs will be studied further.

Mesenchymal stem cells ameliorate experimental autoimmune encephalomyelitis inducing T-cell anergy

We studied the immunoregulatory features of murine mesenchymal stem cells (MSCs) in vitro and in vivo. MSCs inhibited T-cell receptor (TCR)-dependent and -independent proliferation but did not induce apoptosis on T cells. Such inhibition was paired with a decreased interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha production and was partially reversed by interleukin-2 (IL-2). Thus, we used MSCs to treat myelin oligodendrocyte glycoprotein (MOG)35-55-induced experimental autoimmune encephalomyelitis (EAE) in C57BL/6J mice. We injected intravenously 1 x 10(6) MSCs before disease onset (preventive protocol) and at different time points after disease occurrence (therapeutic protocol). MSC administration before disease onset strikingly ameliorated EAE. The therapeutic scheme was effective when MSCs were administered at disease onset and at the peak of disease but not after disease stabilization. Central nervous system (CNS) pathology showed decreased inflammatory infiltrates and demyelination in mice that received transplants of MSCs. T-cell response to MOG and mitogens from MSC-treated mice was inhibited and restored by IL-2 administration. Upon MSC transfection with the enhanced green fluorescent protein (eGFP), eGFP(+) cells were detected in the lymphoid organs of treated mice. These data suggest that the immunoregulatory properties of MSCs effectively interfere with the autoimmune attack in the course of EAE inducing an in vivo state of T-cell unresponsiveness occurring within secondary lymphoid organs.

Bone marrow-derived mesenchymal stem cells in repair of the injured lung

We sought to determine whether an intact bone marrow is essential to lung repair following bleomycin-induced lung injury in mice, and the mechanisms of any protective effects conferred by bone marrow-derived mesenchymal stem cell (BMDMSC) transfer. We found that myelosupression increased susceptibility to bleomycin injury and that BMDMSC transfer was protective. Protection was associated with the differentiation of engrafted BMDMSC into specific and distinct lung cell phenotypes, with an increase in circulating levels of G-CSF and GM-CSF (known for their ability to promote the mobilization of endogenous stem cells) and with a decrease in inflammatory cytokines. In vitro, cells from injured, but not from normal, mouse lung produced soluble factors that caused BMDMSC to proliferate and migrate toward the injured lung. We conclude that bone marrow stem cells are important in the repair of bleomycin-injured lung and that transfer of mesenchymal stem cells protects against the injury. BMDMSC localize to the injured lung and assume lung cell phenotypes, but protection from injury and fibrosis also involves suppression of inflammation and triggering production of reparative growth factors.

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.

Engraftment potential of human amnion and chorion cells derived from term placenta

BACKGROUND

Fetal membranes are tissues of particular interest for several reasons, including their role in preventing rejection of the fetus and their early embryologic origin. which may entail progenitor potential. The immunologic reactivity and the transplantation potential of amnion and chorion cells, however, remain to be elucidated.

METHODS

Amnion and chorion cells were isolated from human term placenta and characterized by immunohistochemistry, flow cytometric analysis, and expression profile of relevant genes. The immunomodulatory characteristics of these cells were studied in allogeneic and xenogeneic mixed lymphocyte reactions and their engraftment potential analyzed by transplantation into neonatal swine and rats. Posttransplant chimerism was determined by polymerase chain reaction analysis with probes specific for human DNA.

RESULTS

Phenotypic and gene expression studies indicated mesenchymal stem cell-like profiles in both amnion and chorion cells that were positive for neuronal, pulmonary, adhesion, and migration markers. In addition, cells isolated both from amnion and chorion did not induce allogeneic nor xenogeneic lymphocyte proliferation responses and were able to actively suppress lymphocyte responsiveness. Transplantation in neonatal swine and rats resulted in human microchimerism in various organs and tissues.

CONCLUSIONS

Human amnion and chorion cells from term placenta can successfully engraft neonatal swine and rats. These results may be explained by the peculiar immunologic characteristics and mesenchymal stem cell-like phenotype of these cells. These findings suggest that amnion and chorion cells may represent an advantageous source of progenitor cells with potential applications in a variety of cell therapy and transplantation procedures.

Cellular vehicles for cancer gene therapy: current status and future potential

Cancer is a difficult target for any therapeutic strategy; therefore, there is a continuous search for new therapeutic modalities, for application either alone or in combination. In this regard, gene-based therapy is a new approach that offers hope of improved control of tumors. Intensive research to apply gene therapy for cancer treatment has led to identification of the most important technical and theoretical barriers that need to be overcome for clinical success. One of the central unresolved challenges remains the issue of specific and efficient delivery of genes to target cells or tissues, emphasizing the importance of the gene carrier. Along with different viral and non-viral vector systems, mammalian cells have also been considered as vehicles for delivery of anti-cancer therapeutics. The cell-based delivery approach was introduced as the first attempt to apply gene therapy to cancer treatment, and in general, has followed most of the ups and downs of gene therapy applications, progressing alongside new knowledge gained in this field. As a result, significant progress has been made in some aspects of the cell-based approach, while the development of other essential issues is only just gaining speed. It appears that the initial phase of development of cell-based protocols - the achievement of efficient ex vivo cell loading with therapeutics - has largely been fulfilled. However, the desired efficacy of cell-based strategies in general has not yet been reached, and specificity of tumor homing needs to be improved considerably. There is hope that advances in related scientific fields will promote the utilization of cells as powerful and versatile vehicles for cancer gene therapy.

Plasticity of bone marrow-derived stem cells

Stem cell plasticity refers to the ability of adult stem cells to acquire mature phenotypes that are different from their tissue of origin. Adult bone marrow cells (BMCs) include two populations of bone marrow stem cells (BMCs): hematopoietic stem cells (HSCs), which give rise to all mature lineages of blood, and mesenchymal stem cells (MSCs), which can differentiate into bone, cartilage, and fat. In this article, we review the literature that lends credibility to the theory that highly plastic BMCs have a role in maintenance and repair of nonhematopoietic tissue. We discuss the possible mechanisms by which this may occur. Also reviewed is the possibility that adult BMCs can change their gene expression profile after fusion with a mature cell, which has brought into question whether this stem cell plasticity is real.

Bone Marrow-Derived Cells in the Pathogenesis of Lung Fibrosis

Progressive pulmonary fibrosis is characterized by failed alveolar reepithelialization and fibroblast/myofibroblast accumulation, with deposition of extracellular matrix. This results in loss of lung elasticity, alveolar collapse and fibrosis, impaired gas exchange and progressive decline in pulmonary function. Myofibroblasts represent an activated, contractile cellular phenotype that are potent producers of collagen and other extracellular matrix proteins. It is generally thought that myofibroblasts derive from local tissue fibroblasts. However, recent evidence suggests a portion of the progenitors for these cells may arise from the bone marrow. Fibrocytes, which share both leukocyte and mesenchymal markers, are found in increased numbers in bone marrow and lung of injured mice. Fibrocytes circulate in blood and are recruited to injured sites via chemotactic signals. Studies with bone marrow chimeric and parabiotic mice suggest that fibroblasts (and in some cases myofibroblasts) arise from circulating bone marrow precursors. Chemokine and chemokine receptor interactions are critical for the recruitment of bone marrow-derived progenitors. Once fibrocytes arrive in injured tissues, local factors induce their differentiation into fibroblasts/myofibroblasts. This review will summarize the experimental findings, supporting a role for the participation of bone marrow-derived cells in animal models of lung fibrosis, and potential implications for the pathogenesis of fibrotic lung diseases.

Stem cells and pulmonary metamorphosis: New concepts in repair and regeneration

Adult stem cells are likely to have much more versatile differentiation capabilities than once believed. Numerous studies have appeared over the past decade demonstrating the ability of adult stem cells to differentiate into a variety of cells from non-hematopoietic organs, including the lung. The goal of this review is to provide an overview of the growth factors which are thought to be involved in lung development and disease, describe the cells within the lung that are believed to replace cells that have been injured, review the studies that have demonstrated the transformation of bone marrow-derived stem cells into lung cells, and describe potential clinical applications with respect to human pulmonary disease.

BM-derived cells restore expression of peroxiredoxin V in the airways following acute naphthalene injury in mice

BACKGROUND

Naphthalene-induced respiratory tract toxicity in mice is characterized by specific and rapid loss of the Clara cell population, which is restored only after several days. The sources of restoration of this cell population remain unclear. We investigated whether BM-derived cells participated in the process of epithelial restoration following naphthalene toxicity compared with bacterial infection. We further investigated the role of BM-derived cells in restoration of expression of peroxiredoxin V (PRXV), one of the major proteins of antioxidant defense, specifically expressed in the bronchial epithelium.

METHODS

We transplanted GFP-tagged BM cells into 5 Gy-irradiated C57BL/6 recipients. Following 1 month of recovery, experimental animals were subjected to 250 mg/kg naphthalene i.p. An additional group of animals received intratracheal instillation of Escherichia coli to induce acute bacterial inflammation. Animals were killed at 1-12 days after naphthalene and analyzed immunohistochemically.

RESULTS

Recipients' cells of bronchial epithelium demonstrated significantly reduced levels of PRXV expression following naphthalene. In animals with acute bacterial inflammation, PRXV levels were not reduced in epithelium and participation of BM-derived cells in epithelial restoration was minimal. Following naphthalene, GFP(+) cells were present in large numbers in lung parenchyma and epithelium of conducting airways starting at 1 day following injury. GFP(+) progeny of BM cells was the major source of PRXV in the epithelium.

DISCUSSION

These data suggest that BM-derived cells may provide a source of antioxidant protection of airways by expression of PRXV in a model of acute epithelial respiratory tract toxicity.

Adult stem cells from bone marrow stroma differentiate into airway epithelial cells: potential therapy for cystic fibrosis

Cystic fibrosis (CF), the most prevalent, fatal genetic disorder in the Caucasian population, is caused by mutations of CF transmembrane conductance regulator (CFTR). The mutations of this chloride channel alter the transport of chloride and associated liquid and thereby impair lung defenses. Patients typically succumb to chronic bacterial infections and respiratory failure. Restoration of the abnormal CFTR function to CF airway epithelium is considered the most direct way to treat the disease. In this report, we explore the potential of adult stem cells from bone marrow, referred to as mesenchymal or marrow stromal stem cells (MSCs), to provide a therapy for CF. We found that MSCs possess the capacity of differentiating into airway epithelia. MSCs from CF patients are amenable to CFTR gene correction, and expression of CFTR does not influence the pluripotency of MSCs. Moreover, the CFTR-corrected MSCs from CF patients are able to contribute to apical Cl(-) secretion in response to cAMP agonist stimulation, suggesting the possibility of developing cell-based therapy for CF. The ex vivo coculture system established in this report offers an invaluable approach for selection of stem-cell populations that may have greater potency in lung differentiation.

Development of Respiratory Stem Cells and Progenitor Cells

Research of stem cells has caught much attention in the past few years with its promise for therapeutic and regenerative applications in a variety of diseases and organ systems. The latest studies have also urged us to understand further the somatic stem cell plasticity or transdifferentiation capability. More vigorous research is urgently required to verify whether or not bone marrow stem cells can differentiate into a variety of cell types in different organs including heart, liver, lung, and so forth. The lung employs a myriad of cell phenotypes in its unique function of inhaling and expiring air. Due to this structural complexity, transdifferentiation of stem cells into the lung is particularly complicated. In addition, assessing the stem cells and lung progenitor cells in the respiratory system is technically difficult. Despite these difficulties, recent studies have advanced our understanding of bone marrow stem cells differentiating into lung progenitors as well as characteristics of the local progenitor cells. This review will briefly discuss the current state of research of stem cell transdifferentiation and development, with a focus on the obstacles that limit use of stem cells in lung regeneration.

Cells Derived from the Circulation Contribute to the Repair of Lung Injury

Bone marrow (stem/progenitor) cells have been shown to "differentiate" into cells in multiple tissues, including lung. A low number of hematopoietic stem/progenitor cells also circulate in peripheral blood. The physiologic roles of these cells are still uncertain. This study was designed to test, using parabiotic mice that were joined surgically, whether stem/progenitor cells in blood contributed to the regeneration of lung after injury. Parabiotic mice were generated surgically by joining green fluorescent protein transgenic mice and wild-type littermates. These mice developed a common circulation (approximately 50% green cells in blood) by 2 weeks after surgery. The wild-type mouse was either uninjured or lethally irradiated or received intratracheal elastase or the combination of radiation with intratracheal elastase injection. Radiation or the combination of radiation with elastase significantly increased the proportion of bright green cells in the lungs of the wild-type mice. Morphologically, interstitial monocytes/macrophages, subepithelial fibroblast-like interstitial cells, and additionally type I alveolar epithelial cells immunostained for green fluorescent protein in wild-type mice. Approximately 5 to 20% of lung fibroblasts primary cultured from injured wild-type mice were green fluorescent protein expressing cells, indicating their blood derivation. This study demonstrates that stem/progenitor cells in blood contribute to the repair of lung injury in irradiated mice.

In vivo contribution of murine mesenchymal stem cells into multiple cell-types under minimal damage conditions

Murine mesenchymal stem cells are capable of differentiating in vitro into different lineages under stimulation with certain cytokines, growth factors and chemicals. However, the true capacitiy of these cells to contribute to different cell-types in vivo is still unclear, especially under minimal injury conditions. In this study, we describe a method of purifying murine mesenchymal stem cells from bone marrow and efficiently transducing them using a lentivirus vector expressing the eGFP reporter gene. Lentivirus-transduced mesenchymal stem cells retained their in vitro ability to differentiate into adipocytes, osteocytes and chondrocytes as well as into myocyte- and astrocyte-like cells. eGFP-mesenchymal stem cells were delivered systemically into minimally injured syngeneic mice. Tracking and tissue-specific differentiation were determined by PCR and immunohistochemistry, respectively. We found donor-derived hepatocytes, lung epithelial cells, myofibroblasts, myofibers and renal tubular cells in some of the recipient mice. Our data indicate that even in the absence of substantial injury, phenotypically defined murine mesenchymal stem cells could acquire tissue specific morphology and antigen expression and thus contribute to different tissue cell-types in vivo.

Mesenchymal stem cells: clinical applications and biological characterization

Mesenchymal stem cells (MSCs) have been isolated from bone marrow, periosteum, trabecular bone, adipose tissue, synovium, skeletal muscle and deciduous teeth. These cells have the capacity to differentiate into cells of connective tissue lineages, including bone, fat, cartilage and muscle. A great deal has been learned in recent years about the isolation and characterization of MSCs, and control of their differentiation. These cells have generated a great deal of interest because of their potential use in regenerative medicine and tissue engineering and there are some dramatic examples, derived from both pre-clinical and clinical studies, that illustrate their therapeutic value. This review summarizes recent findings regarding the potential clinical use of MSCs in cardiovascular, neural and orthopaedic applications. As new methods are developed, there are several aspects to the implanted cell-host interaction that need to be addressed before we can fully understand the underlying mechanisms. These include the host immune response to implanted cells, the homing mechanisms that guide delivered cells to a site of injury and the differentiation in vivo of implanted cells under the influence of local signals.

Systemic infusion of FLK1(+) mesenchymal stem cells ameliorate carbon tetrachloride-induced liver fibrosis in mice

BACKGROUND

Fibrosis is the common end stage of most liver diseases, for which, unfortunately, there is no effective treatment available currently. It has been shown that mesenchymal stem cells (MSCs) from bone marrow (BM) could engraft in the lung after bleomycin exposure and ameliorate its fibrotic effects. This study was designed to evaluate the effect of Flk1 MSCs from murine BM (termed here Flk1 mMSCs) on fibrosis formation induced by carbon tetrachloride (CCl4).

METHODS

A CCl(4)-induced hepatic fibrosis model was used. Flk1 mMSCs were systemically infused immediately or 1 week after mice were challenged with CCl(4). Control mice received only saline infusion. Fibrosis index and donor-cell engraftment were assessed 2 or 5 weeks after CCl(4) challenge.

RESULTS

We found that Flk1 mMSCs transplantation immediately, but not 1 week after exposure to CCl(4), significantly reduced CCl(4)-induced liver damage and collagen deposition. In addition, levels of hepatic hydroxyproline and serum fibrosis markers in mice receiving immediate Flk1 mMSCs transplantation after CCl(4) challenge were significantly lower compared with those of control mice. More importantly, histologic examination suggested that hepatic damage recovery was much better in these immediately Flk1 mMSCs-treated mice. Immunofluorescence, polymerase chain reaction, and fluorescence in situ hybridization analysis revealed that donor cells engrafted into host liver, had epithelium-like morphology, and expressed albumin, although at low frequency.

CONCLUSION

These results suggest that Flk1 mMSCs might initiate endogenous hepatic tissue regeneration, engraft into host liver in response to CCl(4) injury, and ameliorate its fibrogenic effects.

Stem cells and repair of lung injuries

Fueled by the promise of regenerative medicine, currently there is unprecedented interest in stem cells. Furthermore, there have been revolutionary, but somewhat controversial, advances in our understanding of stem cell biology. Stem cells likely play key roles in the repair of diverse lung injuries. However, due to very low rates of cellular proliferation in vivo in the normal steady state, cellular and architectural complexity of the respiratory tract, and the lack of an intensive research effort, lung stem cells remain poorly understood compared to those in other major organ systems. In the present review, we concisely explore the conceptual framework of stem cell biology and recent advances pertinent to the lungs. We illustrate lung diseases in which manipulation of stem cells may be physiologically significant and highlight the challenges facing stem cell-related therapy in the lung.

Mesenchymal stem cells: paradoxes of passaging

The field of stem cell biology continues to evolve with the ongoing characterization of multiple types of stem cells with their inherent potential for experimental and clinical application. Mesenchymal stem cells (MSC) are one of the most promising stem cell types due to their availability and the relatively simple requirements for in vitro expansion and genetic manipulation. Multiple populations described as "MSCs" have now been isolated from various tissues in humans and other species using a variety of culture techniques. Despite extensive in vitro characterization, relatively little has been demonstrated regarding their in vivo biology and therapeutic potential. Nevertheless, clinical trials utilizing MSCs are currently underway. The aim of this review is to critically analyze the field of MSC biology, particularly with respect to the current paradox between in vitro promise and in vivo efficacy. It is the authors' opinion that until this paradox is better understood, therapeutic applications will remain limited.

The fate of mesenchymal stem cells transplanted into immunocompetent neonatal mice: implications for skeletal gene therapy via stem cells

To explore the feasibility of skeletal gene and cell therapies, we transduced murine bone marrow-derived mesenchymal stem cells (MSCs) with a retrovirus carrying the enhanced green fluorescent protein and zeocin-resistance genes prior to transplantation into 2-day-old immunocompetent neonatal mice. Whole-body imaging of the recipient mice at 7 days post-systemic cell injection demonstrated a wide distribution of the cells in vivo. Twenty-five days posttransplantation, most of the infused cells were present in the lung as assessed by examination of the cells cultured from the lungs of the recipient mice. The cells persisted in lung and maintained a high level of gene expression and could be recovered from the recipient mice at 150 days after cell transplantation. A significant number of GFP-positive cells were also present in the bones of the recipient mice at 35 days post-cell transplantation. Recycling of the cells recovered from femurs of the recipient mice at 25 days posttransplantation by repeated injections into different neonatal mice resulted in the isolation of a clone of cells that was detected in bone and cartilage, but not in lung and liver after systemic injection. These data demonstrate that MSCs persist in immunocompetent neonatal mice, maintain a high level of gene expression, and may participate in skeletal growth and development of the recipient animals.