Bone marrow-derived cells as progenitors of lung alveolar epithelium

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.

Hypoxia-induced pulmonary vascular remodeling requires recruitment of circulating mesenchymal precursors of a monocyte/macrophage lineage

Vascular remodeling in chronic hypoxic pulmonary hypertension includes marked fibroproliferative changes in the pulmonary artery (PA) adventitia. Although resident PA fibroblasts have long been considered the primary contributors to these processes, we tested the hypothesis that hypoxia-induced pulmonary vascular remodeling requires recruitment of circulating mesenchymal precursors of a monocyte/macrophage lineage, termed fibrocytes. Using two neonatal animal models (rats and calves) of chronic hypoxic pulmonary hypertension, we demonstrated a dramatic perivascular accumulation of mononuclear cells of a monocyte/macrophage lineage (expressing CD45, CD11b, CD14, CD68, ED1, ED2). Many of these cells produced type I collagen, expressed alpha-smooth muscle actin, and proliferated, thus exhibiting mesenchymal cell characteristics attributed to fibrocytes. The blood-borne origin of these cells was confirmed in experiments wherein circulating monocytes/macrophages of chronically hypoxic rats were in vivo-labeled with DiI fluorochrome via liposome delivery and subsequently identified in the remodeled pulmonary, but not systemic, arterial adventitia. The DiI-labeled cells that appeared in the vessel wall expressed monocyte/macrophage markers and procollagen. Selective depletion of this monocytic cell population, using either clodronate-liposomes or gadolinium chloride, prevented pulmonary adventitial remodeling (ie, production of collagen, fibronectin, and tenascin-C and accumulation of myofibroblasts). We conclude that circulating mesenchymal precursors of a monocyte/macrophage lineage, including fibrocytes, are essential contributors to hypoxia-induced pulmonary vascular remodeling.

Stem cells and cystic fibrosis

Although cystic fibrosis at first sight appears to be one of the most obvious human diseases to treat with gene therapy, since it is caused by a single-gene defect and the main affected organ is the lung which is relatively easily accessible, clinical results have thus far been disappointingly limited. At least one cause for this lack of success is the failure to permanently correct the gene defect in addition to the rapid turnover of lung epithelial cells. Alternative approaches therefore involve the search for and use of stem cell populations. This review presents an overview of recent attempts to identify lung- or bone marrow-derived populations of stem cells or progenitor cells and to apply such cells, heterologous or gene-corrected autologous, to colonize the airways while differentiating into functional respiratory columnar epithelial cells. The most successful approaches thus far appear to be obtained with bone marrow-derived cells such as mesenchymal stem cells, although the transdifferentiation rate thus far has been limited to below the 1% level. As an alternative the proven multipotent nature of bronchioalveolar stem cells isolated from lung tissue may provide another promising approach for successful stem cell therapy.

Differentiation of bone marrow mesenchymal cells to neural cells

To explore the possibility and condition of differentiation of bone marrow mesenchymal cells (BMSCs) to neural cells in vitro, BMSCs from whole bone marrow of rats were cultured. The BMSCs of passage 3 were identified with immunocytochemical staining of CD44 (+), CD71 (+) and CD45 (-). There were type I and type II cells in BMSCs. Type I BMSCs were spindle-shaped and strong positive in immunocytochemical staining of CD44 and CD71, whereas flat and big type II BMSCs were lightly stained. The BMSCs of same passage were induced to differentiate into neural cells by beta-mercaptoethanol (BME). After induction by BME, the type I BMSCs withdrew to form neuron-like round soma and axon-like and dendrite-like processes, and were stained positively for neurofilament (NF). The type II BMSCs did not change in the BME medium and were negatively or slightly stained of NF.

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.

Circulating Progenitor Epithelial Cells Traffic via CXCR4/CXCL12 in Response to Airway Injury

Recipient airway epithelial cells are found in human sex-mismatched lung transplants, implying that circulating progenitor epithelial cells contribute to the repair of the airway epithelium. Markers of circulating progenitor epithelial cells and mechanisms for their trafficking remain to be elucidated. We demonstrate that a population of progenitor epithelial cells exists in the bone marrow and the circulation of mice that is positive for the early epithelial marker cytokeratin 5 (CK5) and the chemokine receptor CXCR4. We used a mouse model of sex-mismatched tracheal transplantation and found that CK5+ circulating progenitor epithelial cells contribute to re-epithelialization of the airway and re-establishment of the pseudostratified epithelium. The presence of CXCL12 in tracheal transplants provided a mechanism for CXCR4+ circulating progenitor epithelial cell recruitment to the airway. Depletion of CXCL12 resulted in the epithelium defaulting to squamous metaplasia, which was derived solely from the resident tissue progenitor epithelial cells. Our findings demonstrate that CK5+CXCR4+ cells are markers of circulating progenitor epithelial cells in the bone marrow and circulation and that CXCR4/CXCL12-mediated recruitment of circulating progenitor epithelial cells is necessary for the re-establishment of a normal pseudostratified epithelium after airway injury. These findings support a novel paradigm for the development of squamous metaplasia of the airway epithelium and for developing therapeutic strategies for circulating progenitor epithelial cells in airway diseases.

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.

Quo vadis haemapheresis. Current developments in haemapheresis

The techniques of haemapheresis originated in the development of centrifugal devices separating cells from plasma and later on plasma from cells. Subsequently membrane filtration was developed allowing for plasma-cell separation. The unspecificity of therapeutic plasma exchange led to the development of secondary plasma separation technologies being specific, semi-selective or selective such as adsorption, filtration or precipitation. In contrast on-line differential separation of cells is still under development. Whereas erythrocytapheresis, granulocytapheresis, lymphocytapheresis and stem cell apheresis are technically advanced, monocytapheresis may need further improvement. Also, indications such as erythrocytapheresis for the treatment of polycythaemia vera or photopheresis though being clinically effective and of considerable importance for an appropriate disease control are to some extent under debate as being either too costly or without sufficient understanding of the mechanism. Other forms of cell therapy are under development. Rheohaemapheresis as the most advanced technology of extracorporeal haemorheotherapy is a rapidly developing approach contributing to the treatment of microcirculatory diseases and tissue repair. Whereas the control of a considerable number of (auto-) antibody mediated diseases is beyond discussion, the indication of apheresis therapy for immune complex mediated diseases is quite often still under debate. Detoxification for artificial liver support advanced considerably during the last years, whereas conclusions on the efficacy of septicaemia treatment are debatable indeed. LDL-apheresis initiated in 1981 as immune apheresis is well established since 24 years, other semi-selective or unspecific procedures, allowing for the elimination of LDL-cholesterol among other plasma components are also being used. Correspondingly Lp(a) apheresis is available as a specific, highly efficient elimination procedure superior to techniques which also eliminate Lp(a). Quality control systems, more economical technologies as for instance by increasing automation, influencing the over-interpretation of evidence based medicine especially in patients with rare diseases without treatment alternative, more insight into the need of controlled clinical trials or alternatively improved diagnostic procedures are among others tools ways to expand the application of haemapheresis so far applied in cardiology, dermatology, haematology, immunology, nephrology, neurology, ophthalmology, otology, paediatrics, rheumatology, surgery and transfusion medicine.

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.

Hemoglobin is expressed by alveolar epithelial cells

Hemoglobin gene expression in non-erythroid cells has been previously reported in activated macrophages from adult mice and lens cells, and recent studies indicate that alveolar epithelial cells can be derived from hematopoietic stem cells. Our laboratory has now produced strong evidence that hemoglobin is expressed by alveolar type II (ATII) cells and Clara cells, the primary producers of pulmonary surfactant. ATII cells are also closely involved in innate immunity within the lung and are stem cells that differentiate into alveolar type I cells. Reverse transcriptase-PCR was used to measure the expression of transcripts from the alpha- and beta-globin gene clusters in several human and rodent pulmonary epithelial cells. Surprisingly, the two major globin mRNAs characteristic of adult erythroid precursor cells were clearly expressed in human A549 and H441 cell lines, mouse MLE-15 cells, and primary ATII cells isolated from normal rat and mouse lungs. DNA sequencing verified that these PCR products were indeed the result of specific amplification of globin gene cDNAs. These alveolar epithelial cells also expressed the corresponding hemoglobin protein subunits as determined by Western blotting, and tandem mass spectrometry sequencing was used to verify the presence of both alpha- and beta-globin polypeptides in rat primary ATII cells. The function of hemoglobin expression by cells of the pulmonary epithelium will be determined by future studies, but this novel finding could potentially have important implications for the physiology and pathology of the lung.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

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.

Alveolar epithelial cells: differentiation and lung injury

Re-epithelialization of alveolar epithelial cells is one of the important repair processes in many types of lung injury. The major functions of alveolar type II cells are synthesis and secretion of surfactant, hyperplasia in reaction to alveolar epithelial injury, and serving as progenitor cells for alveolar type I cells. The authors have examined the effects of several soluble factors on cultured alveolar type II cells in vitro, and also examined the histopathology and gene expression of surfactant proteins in the rat lungs with LPS, bleomycin and/or treated with keratinocyte growth factor. The authors next examined the effects of bone marrow stromal cells (BMSC) implanted transvenously into bleomycin-induced lungs. The authors found that keratinocyte growth factor (KGF) is a strong growth factor for alveolar type II cells, and that KGF instillation prevents bleomycin-induced lung injury. Furthermore, the authors showed the possibility of differentiation of implanted BMSC into alveolar epithelial cells. KGF and BMSC may play an important role in maintaining the alveolar epithelium and repairing the damaged epithelium after injury, and may well provide potential therapeutic alternatives.

Adipose Tissue: Stem Cells and Beyond

This article highlights potential uses for harvested fat and describes the current state of the art regarding adipose stem cells.

Idiopathic pulmonary fibrosis : new concepts in pathogenesis and implications for drug therapy

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and usually fatal pulmonary disease for which there are no proven drug therapies. Anti-inflammatory and immunosuppressive agents have been largely ineffective. The precise relationship of IPF to other idiopathic interstitial pneumonias (IIPs) is not known, despite the observation that different histopathologic patterns of IIP may coexist in the same patient. We propose that these different histopathologic 'reaction' patterns may be determined by complex interactions between host and environmental factors that alter the local alveolar milieu. Recent paradigms in IPF pathogenesis have focused on dysregulated epithelial-mesenchymal interactions, an imbalance in T(H)1/T(H)2 cytokine profile and potential roles for aberrant angiogenesis. In this review, we discuss these evolving concepts in disease pathogenesis and emerging therapies designed to target pro-fibrogenic pathways in IPF.

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.

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.

Differentiation potential of murine neural stem cells in vitro and after transplantation

We examined the differentiation potential of murine neural stem cells (NSCs) grown in vitro and transplanted into intact and irradiated recipients. NSCs were isolated from neonatal Balb/c mice using the neurosphere assay. On in vitro differentiation assays, NSCs produced beta-III tubulin(+) neurons, glial fibrillary acidic protein (GFAP(+)) astrocytes, and O4(+) oligodendrocytes. After neural grafting to histocompatible adult mice, NSCs gave rise to neuronal and glial cells. When cells were transplanted in the form of solid neurospheres, they reached terminal differentiation and spatial arrangements that mimicked the three-dimensional organization of nervous tissue. To create conditions that would allow us to assess the potential for generation of nonneural cells, NSCs were intravenously injected into irradiated mice. Transplantation of NSCs stimulated hematopoiesis because the number of colony-forming units of granulocyte-monocyte lineage (CFU-GM) colonies isolated from the spleen and bone marrow of transplanted mice was greater than that from irradiated, nontransplanted animals. Moreover, transplanted cells tagged with beta-galactosidase were identified in the thymus of animals grafted with labelled NSCs. NSCs harvested from the neurosphere assay produced viable and transplantable cells. In vitro differentiation assays and neural grafting confirmed the multipotency of NSCs and their commitment to generate neuronal and glial cells. Following intravenous injection of NSCs, the transplanted cells colonized hematopoietic and lymphatic organs, facilitating hematopoiesis in irradiated animals.

Fibroblast growth factor 2 facilitates the differentiation of transplanted bone marrow cells into hepatocytes

We have developed an in vivo mouse model, the green fluorescent protein (GFP)/carbon tetrachloride (CCl(4)) model, and have previously reported that transplanted GFP-positive bone marrow cells (BMCs) differentiate into hepatocytes via hepatoblast intermediates. Here, we have investigated the growth factors that are closely related to the differentiation of transplanted BMCs into hepatocytes, and the way that a specific growth factor affects the differentiation process in the GFP/CCl(4) model. We performed immunohistochemical analysis to identify an important growth factor in our model, viz., fibroblast growth factor (FGF). In liver samples, the expression of FGF1 and FGF2 and of FGF receptors (FGFRs; FGFR1, FGFR2) was significantly elevated with time after bone marrow transplantation (BMT) compared with other factors, and co-expression of GFP and FGFs or FGFRs could be detected. We then analyzed the effect and molecular mechanism of FGF signaling on the enhancement of BMC differentiation into hepatocytes by immunohistochemistry, immunoblotting, and microarray analysis. Treatment with recombinant FGF (rFGF), especially rFGF2, elevated the repopulation rate of GFP-positive cells in the liver and significantly increased the expression of both Liv2 (hepatoblast marker) and albumin (hepatocyte marker). Administration of rFGF2 at BMT also raised serum albumin levels and improved the survival rate. Transplantation of BMCs with rFGF2 specifically activated tumor necrosis factor-alpha (TNF-alpha) signaling. Thus, FGF2 facilitates the differentiation of transplanted BMCs into albumin-producing hepatocytes via Liv2-positive hepatoblast intermediates through the activation of TNF-alpha signaling. Administration of FGF2 in combination with BMT improves the liver function and prognosis of mice with CCl(4)-induced liver damage.

Identification of bronchioalveolar stem cells in normal lung and lung cancer

Injury models have suggested that the lung contains anatomically and functionally distinct epithelial stem cell populations. We have isolated such a regional pulmonary stem cell population, termed bronchioalveolar stem cells (BASCs). Identified at the bronchioalveolar duct junction, BASCs were resistant to bronchiolar and alveolar damage and proliferated during epithelial cell renewal in vivo. BASCs exhibited self-renewal and were multipotent in clonal assays, highlighting their stem cell properties. Furthermore, BASCs expanded in response to oncogenic K-ras in culture and in precursors of lung tumors in vivo. These data support the hypothesis that BASCs are a stem cell population that maintains the bronchiolar Clara cells and alveolar cells of the distal lung and that their transformed counterparts give rise to adenocarcinoma. Although bronchiolar cells and alveolar cells are proposed to be the precursor cells of adenocarcinoma, this work points to BASCs as the putative cells of origin for this subtype of lung cancer.

Engraftment of neonatal lung fibroblasts into the normal and elastase-injured lung

Interstitial fibroblasts are an integral component of the alveolar wall. These cells produce matrix proteins that maintain the extracellular scaffold of alveolar structures. Emphysema is characterized by airspace enlargement resulting from the loss of alveolar cellularity and matrix. In this study, we explored the endotracheal delivery of fibroblasts to the lung parenchyma as a means of repairing damaged alveolar structures directly or indirectly for the delivery of transgenes. Fibroblasts were isolated from the lungs of neonatal transgenic mice expressing GFP during the period of rapid alveolarization. These GFP+ cells maintained their myofibroblast phenotype in culture and expressed elastin and alpha-smooth muscle actin mRNA. We administered GFP+ fibroblasts to saline- and elastase-treated mice by endotracheal instillation. We detected more GFP+ fibroblasts in the alveolar walls and in the interstitial areas of elastase-injured lungs than in normal lungs as assessed by immunohistochemistry and fluorescent imaging. The presence of GFP+ fibroblasts in the interstitium demonstrated transepithelial migration of these cells. Expression of GFP+ fibroblasts in recipient lungs was maintained for at least 20 d after endotracheal administration. These cells synthesize matrix components including elastin in vitro and could contribute to restoring the structural integrity of the alveolar wall.

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.

Coculture of Embryonic Stem Cells with Pulmonary Mesenchyme: A Microenvironment That Promotes Differentiation of Pulmonary Epithelium

Coculture of stem/progenitor cells with mature cells or tissues can drive their differentiation toward required lineages. Thus, we hypothesized that coculture of murine embryonic stem (ES) cells with embryonic mesenchyme from distal lung promotes the differentiation of pneumocytes. Murine ES cells were differentiated to embryoid bodies (EBs) and cultured for 5 or 12 days with pulmonary mesenchyme from embryonic day 11.5 or 13.5 murine embryos, in direct contact or separated by a membrane. Controls included EBs cultured alone or with embryonic gut mesenchyme. Histology revealed epithelium-lined channels in directly cocultured EBs, whereas EBs grown alone showed little structural organization. The lining cells expressed cytokeratin and thyroid transcription factor 1, an early developmental marker in pulmonary epithelium. Differentiation of type II pneumocytes specifically was demonstrated by the presence of surfactant protein C (SP-C) in some of the epithelial cells. None of these markers was seen in EBs cultured alone or with embryonic gut mesenchyme. Indirect coculture of EBs with lung mesenchyme resulted in a 14-fold increase in SP-C gene expression. Thus, provision of an appropriate microenvironment, in the form of pulmonary mesenchyme, appears to promote the differentiation of ES cells toward lung epithelium. Our findings may have applications in regenerative medicine strategies and the engineering of lung tissue.

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.

Increase of bone marrow-derived secretory lineage epithelial cells during regeneration in the human intestine

BACKGROUND & AIMS

We have previously reported that bone marrow (BM)-derived cells contribute to the regeneration of the human intestinal epithelium. To analyze further how these cells arise, proliferate, and differentiate as epithelial cells, histologic analysis was conducted using endoscopic specimens.

METHODS

Thirty biopsy specimens from 14 female, sex-mismatched BM-transplantation recipients were examined. BM-derived cells were identified by fluorescent in situ hybridization (FISH) for the Y chromosome and immunohistochemistry. Multicolor FISH was used to exclude cell fusion. These cells were further analyzed for various differentiation or proliferation markers.

RESULTS

No evidence of cell fusion was detected. BM-derived cells did not distribute within the crypt as stem cells and rarely expressed Musashi-1. However, BM-derived epithelial cells frequently expressed Ki-67, and some of these cells appeared as pairs of adjacent cells. These cells also expressed markers of all 4 lineages of terminally differentiated cells. During regeneration following graft-vs-host disease, the number of BM-derived cells was substantially increased within Ki-67-positive cells. Interestingly, the number of cells expressing markers for secretory lineage cells was significantly increased within BM-derived cells. This change was unique for BM-derived cells, resulting in a significantly increased proportion of BM-derived cells among secretory lineage cells.

CONCLUSIONS

BM-derived epithelial cells arise via a mechanism other than cell fusion and rarely give rise to stem cells. However, a small proportion of these cells express proliferation markers, and a majority reside as terminally differentiated cells. During regeneration BM-derived cells increase as secretory lineage cells, thereby contributing to restore epithelial functions.

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.

Critical variables in the conversion of marrow cells to skeletal muscle

We have studied conversion of marrow cells to skeletal muscle in cardiotoxin-injured anterior tibialis muscle in a green fluorescent protein (GFP) to C57BL/6 transplantation model and ascertained that total body irradiation (TBI) with establishment of chimerism is a critical factor. Local irradiation has little effect in lower doses and was detrimental at higher doses. Whole body (1000 cGy) with shielding of the leg or a combination of 500 cGy TBI and 500 cGy local radiations was found to give the best results. In non-obese diabetic-severe combined immunodeficient (NOD-SCID) recipients, we were able to show that conversion could occur without radiation, albeit at relatively lower levels. Within 3 days of cardiotoxin injury, GFP-positive mononuclear cells were seen in the muscle, and within 2 weeks GFP-positive muscle fibers were identified. Conversion rates were increased by increasing donor-cell dose. Timing of the cardiotoxin injury relative to the transplantation was critical. These studies show that variables in transplantation and injury are critical features of marrow-to-muscle conversions. Irradiation primarily effects conversion by promoting chimerism. These data may explain the differences in the literature for the frequency of marrow-to-skeletal muscle conversion and can set a platform for future models and perhaps clinical protocols.

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.

Embryonic lung side population cells are hematopoietic and vascular precursors

Side population (SP) cells are a select cell population identified by a capacity to efflux Hoechst dye that are highly enriched for stem/progenitor cell activity. In this study, we found that SP cells comprised of CD45(+) and CD45(-) subtypes are present in the embryonic lung (E-SP) at levels varying with gestational age. Long-term in vivo competitive blood reconstitution studies demonstrated that hematopoeitic stem cell capacity resided within the CD45(+) E-SP cell subset. Immunophenotyping of CD45(-) E-SP cells determined that this population consists of two subtypes: CD31(-) and CD31(+). Limited gene expression profiling indicated that CD45(-)/CD31(-) E-SP cells have features of smooth muscle precursors, and give rise to smooth muscle in culture. On the other hand, CD45(-)/CD31(+) E-SP cells express genes characteristic of endothelium, but by themselves do not grow or differentiate in culture. Co-culture of CD45(-)/CD31(+) and CD45(-)/CD31(-) E-SP cells, however, resulted in the formation of complex tubular networks that express markers of endothelium. Together, these findings illustrate that embryonic lung SP cells are heterogeneous, composed of hematopoeitic and nonhematopoeitic progenitors, and may play a key role in the formation of the lung vasculature.

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.

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.

Stem cell origin of cancer and differentiation therapy

Our forefathers in pathology, on observing cancer tissue under the microscope in the mid-19th century, noticed the similarity between embryonic tissue and cancer, and suggested that tumors arise from embryo-like cells [Recherches dur le Traitement du Cancer, etc. Paris. (1829); Editoral Archiv fuer pathologische Anatomie und Physiologie und fuer klinische Medizin 8 (1855) 23]. The concept that adult tissues contain embryonic remnants that generally lie dormant, but that could be activated to become cancer was later formalized by Cohnheim [Path. Anat. Physiol. Klin. Med. 40 (1867) 1-79; Virchows Arch. 65 (1875) 64] and Durante [Arch. Memori ed Osservazioni di Chirugia Practica 11 (1874) 217-226], as the "embryonal rest" theory of cancer. An updated version of the embryonal rest theory of cancer is that cancers arise from tissue stem cells in adults. Analysis of the cellular origin of carcinomas of different organs indicates that there is, in each instance, a determined stem cell required for normal tissue renewal that is the most likely cell of origin of carcinomas [Lab. Investig. 70 (1994) 6-22]. In the present review, the nature of normal stem cells (embryonal, germinal and somatic) is presented and their relationships to cancer are further expanded. Cell signaling pathways shared by embryonic cells and cancer cells suggest a possible link between embryonic cells and cancer cells. Wilm's tumors (nephroblastomas) and neuroblastomas are presented as possible tumors of embryonic rests in children. Teratocarcinoma is used as the classic example of the totipotent cancer stem cell which can be influenced by its environment to differentiate into a mature adult cell. The observation that "promotion" of an epidermal cancer may be accomplished months or even years after the initial exposure to carcinogen ("initiation"), implies that the original carcinogenic event occurs in a long-lived epithelial stem cell population. The cellular events during hepatocarcinogenesis illustrate that cancers may arise from cells at various stages of differentiation in the hepatocyte lineage. Examples of genetic mutations in epithelial and hematopoietic cancers show how specific alterations in gene expression may be manifested as maturation arrest of a cell lineage at a specific stage of differentiation. Understanding the signals that control normal development may eventually lead us to insights in treating cancer by inducing its differentiation (differentiation therapy). Retinoid acid (RA) induced differentiation therapy has acquired a therapeutic niche in treatment of acute promyelocytic leukemia and the ability of RA to prevent cancer is currently under examination.

Chimerism of metanephric adenoma but not of carcinoma in kidney transplants

Recipient-derived cells integrate into renal allografts inducing organ-specific microchimerism. Circulating pluripotent progenitor cells with high plasticity for differentiation were suggested as a potential source of allograft chimerism. Whether or not these cells also contribute to tumor formation in renal transplants is unknown. We analyzed six histologically different tumors in renal allografts for the presence of recipient-derived cells. To circumvent dependency on gender mismatch, a polymerase chain reaction assay for highly polymorphic short tandem repeat marker (DNA fingerprinting) in combination with laser microdissection was applied. Pure tumor cell populations were harvested by laser microdissection after immunohistochemical (CD45/CD68) marking of contaminating leukocytes. In cases of gender mismatch (n = 2), results were confirmed by sex chromosome in situ hybridization. Two metanephric adenomas demonstrated microchimerism comprising both donor- and recipient-derived tumor cells. Two clear cell carcinomas, one transitional cell carcinoma, and one renal cortical adenoma were all of donor origin without chimerism. We conclude that except for metanephric adenomas, tumors arising in renal transplants originate completely from graft cells. The mixed derivation of metanephric adenomas indicates an incorporation of recipient-derived progenitor cells. This finding suggests that adult stem cells can assume neoplastic phenotypes.

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.

Internalized antigens must be removed to prepare hypoimmunogenic mesenchymal stem cells for cell and gene therapy

Adult stem cells from human bone marrow stroma, referred to as mesenchymal stem cells or marrow stromal cells (hMSCs), are attractive candidates for clinical use. The optimal conditions for hMSC expansion require medium supplemented with fetal calf serum (FCS). Some forms of cell therapy will involve multiple doses, raising a concern over immunological reactions caused by medium-derived FCS proteins. By a sensitive fluorescence-based assay we determined that 7 to 30 mg of FCS proteins are associated with a standard preparation of 100 million hMSCs, a dosage that probably will be needed for clinical therapies. Here we present ex vivo growth conditions for hMSCs that reduce the FCS proteins to less than 100 ng per 100 million hMSCs, approximately a 100,000-fold reduction. The cells maintain their proliferative capacity and sustain their ability for multilineage differentiation. Experiments in rats demonstrate that rat MSCs grown in 20% FCS induce a substantial humoral response after repeated administrations, whereas cells grown under the conditions described in this study reduce the immunogenicity in terms of IgG response over 1000-fold to barely detectable levels. Our results have the potential to dramatically improve cellular and genetic therapies using hMSCs and perhaps other cells.

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.

Receptor for advanced glycation end-products is a marker of type I lung alveolar cells

Lung alveolar epithelial cells are comprised of type I (ATI) and type II (ATII) cells. ATI cells are polarized, although they have very flat morphology. The identification of marker proteins for apical and basolateral membranes of ATI cells is important to investigate into the differentiation of ATI cells. In this paper, we characterized receptor for advanced glycation end-products (RAGE) as a marker for ATI cells. RAGE was localized on basolateral membranes of ATI cells in the immunoelectron microscopy and its expression was enhanced in a parallel manner to the differentiation of ATI cells in vivo and in primary cultures of ATII cells. RAGE and T1 alpha, a well-known ATI marker protein, were targeted to basolateral and apical membranes, respectively, when expressed in polarized Madine Darby canine kidney cells. Moreover, RAGE was expressed in ATI cells after T1 alpha in vivo and in ex in vivo organ cultures. In conclusion, RAGE is a marker for basolateral membranes of well-differentiated ATI cells. ATI cells require some signal provided by the in vivo environment to express RAGE.

Lentivirus-mediated gene transfer to the respiratory epithelium: a promising approach to gene therapy of cystic fibrosis

Gene therapy of cystic fibrosis (CF) lung disease needs highly efficient delivery and long-lasting complementation of the CFTR (cystic fibrosis transmembrane conductance regulator) gene into the respiratory epithelium. The development of lentiviral vectors has been a recent advance in the field of gene transfer and therapy. These integrating vectors appear to be promising vehicles for gene delivery into respiratory epithelial cells by virtue of their ability to infect nondividing cells and mediate long-term persistence of transgene expression. Studies in human airway tissues and animal models have highlighted the possibility of achieving gene expression by lentiviral vectors, which outlasted the normal lifespan of the respiratory epithelium, indicating targeting of a 'stem cell' compartment. Modification of the paracellular permeability and pseudotyping with heterologous envelopes are the strategies currently used to overcome the paucity of specific viral receptors on the apical surface of airway epithelial cells and to reach the basolateral surface receptors. Preclinical studies on CF mice, demonstrating complementation of the CF defect, offer hope that lentivirus gene therapy can be translated into an effective treatment of CF lung disease. Besides a direct targeting of the stem/progenitor niche(s) in the CF airways, an alternative approach may envision homing of hematopoietic stem cells engineered to express the CFTR gene by lentiviral vectors. In the context of lentivirus-mediated CFTR gene transfer to the CF airways, biosafety aspects should be of primary concern.

Retinoic acid in alveolar development, maintenance and regeneration

Recent data suggest that exogenous retinoic acid (RA), the biologically active derivative of vitamin A, can induce alveolar regeneration in a rat model of experimental emphysema. Here, we describe a mouse model of disrupted alveolar development using dexamethasone administered postnatally. We show that the effects of dexamethasone are concentration dependent, dose dependent, long lasting and result in a severe loss of alveolar surface area. When RA is administered to these animals as adults, lung architecture and the surface area per unit of body weight are completely restored to normal. This remarkable effect may be because RA is required during normal alveolar development and administering RA re-awakens gene cascades used during development. We provide evidence that RA is required during alveologenesis in the mouse by showing that the levels of the retinoid binding proteins, the RA receptors and two RA synthesizing enzymes peak postnatally. Furthermore, an inhibitor of RA synthesis, disulphiram, disrupts alveologenesis. We also show that RA is required throughout life for the maintenance of lung alveoli because when rats are deprived of dietary retinol they lose alveoli and show the features of emphysema. Alveolar regeneration with RA may therefore be an important novel therapeutic approach to the treatment of respiratory diseases characterized by a reduced gas-exchanging surface area such as bronchopulmonary dysplasia and emphysema for which there are currently no treatments.

Transplanted BM and BM side population cells contribute progeny to the lung and liver in irradiated mice

BACKGROUND

BM cells have been shown to give rise to progeny of various cell lineages, including cells in lung and liver. This investigation evaluated whether purified BM mononuclear cells and side population (SP) cells that have hematopoietic stem-cell activity also had this property; whether a TBI preparative regimen was necessary for engraftment; and where BM-derived cells were engrafted.

METHODS

Either 1-3 million BM mononuclear cells or 2000 BM SP cells from transgenic enhanced green fluorescent protein-expressing (EGFP) mice were transplanted i.v. to unirradiated or 7-9.5 Gy irradiated recipients.

RESULTS

Flow cytometric analysis showed that lung cells (mean 45%, range 4-70%) and liver cells (mean 4%, range 0.4-8.3%) from irradiated, but not unirradiated recipients, were EGFP donor-derived. Similar results were obtained transplanting BM mononuclear cells or SP cells. Morphologically, donor-derived cells in the lung were primarily monocytes and macrophages. Additionally, lung fibroblasts and Type I, but not Type II, alveolar cells and rare cells in the bronchial epithelium were donor BM derived. In the liver, Kupffer cells, inflammatory cells and small clusters of hepatocytes, but not bile duct cells, were donor-derived.

DISCUSSION

BM mononuclear and SP cells generated progeny in some compartments of the lung and liver, but only in TBI recipients. Stem cells in BM can contribute to repair of tissue injury in some compartments, but not to the same extent in the lung and liver.

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.

Robust conversion of marrow cells to skeletal muscle with formation of marrow-derived muscle cell colonies: a multifactorial process

OBJECTIVE

Murine marrow cells are capable of repopulating skeletal muscle fibers. A point of concern has been the "robustness" of such conversions. We have investigated the impact of type of cell delivery, muscle injury, nature of delivered cell, and stem cell mobilizations on marrow-to-muscle conversion.

METHODS

We transplanted green fluorescence protein (GFP)-transgenic marrow into irradiated C57BL/6 mice and then injured anterior tibialis muscle by cardiotoxin. One month after injury, sections were analyzed by standard and deconvolutional microscopy for expression of muscle and hematopoietic markers.

RESULTS

Irradiation was essential to conversion, although whether by injury or induction of chimerism is not clear. Cardiotoxin- and, to a lesser extent, PBS-injected muscles showed significant number of GFP(+) muscle fibers, while uninjected muscles showed only rare GFP(+) cells. Marrow conversion to muscle was increased by two cycles of G-CSF mobilization and to a lesser extent by G-CSF and steel or GM-CSF. Transplantation of female GFP to male C57BL/6 and GFP to ROSA26 mice showed fusion of donor cells to recipient muscle. High numbers of donor-derived muscle colonies and up to 12% GFP(+) muscle cells were seen after mobilization or direct injection. These levels of donor muscle chimerism approach levels that could be clinically significant in developing strategies for the treatment of muscular dystrophies.

CONCLUSION

In summary, the conversion of marrow to skeletal muscle cells is based on cell fusion and is critically dependent on injury. This conversion is also numerically significant and increases with mobilization.

Cell Type-specific Occurrence of Caveolin-1α and -1β in the Lung Caused by Expression of Distinct mRNAs

Two isoforms of caveolin-1, alpha and beta, had been thought to be generated by alternative translation initiation of an mRNA (FL mRNA), but we showed previously that a variant mRNA (5'V mRNA) encodes the beta isoform specifically. In the present study, we demonstrated strong correlation between the expression of the caveolin-1 protein isoforms and mRNA variants in culture cells and the developing mouse lung. The alpha isoform protein and FL mRNA were expressed constantly during the lung development, whereas expression of the beta isoform protein and 5'V mRNA was negligible in the fetal lung before 17.5 days post coitum, and markedly increased simultaneously at 18.5 days post coitum, when the alveolar type I cells started to differentiate. Immunohistochemical analysis revealed the cell type-specific expression of the two isoforms; the alveolar type I cell expresses the beta isoform predominantly, while the endothelium harbors the alpha isoform chiefly. The mutually exclusive expression of caveolin-1 isoforms was verified by Western blotting of the selective plasma membrane preparation obtained from the endothelial and alveolar epithelial cells. The present result indicates that the two caveolin-1 isoforms are generated from distinct mRNAs in vivo and that their production is regulated independently at the transcriptional level. The result also suggests that the alpha and beta isoforms of caveolin-1 may have unique physiological functions.