Derivation of type II alveolar epithelial cells from murine embryonic stem cells

Human Amnion Epithelial Cells Prevent Bleomycin-Induced Lung Injury and Preserve Lung Function

Human amnion epithelial cells (hAECs) have attracted recent attention as a promising source of cells for regenerative therapies, with reports that cells derived from human term amnion possess multipotent differentiation ability, low immunogenicity, and anti-inflammatory properties. Specifically, in animal models of lung disease characterized by significant loss of lung tissue secondary to chronic inflammation and fibrosis, the transplantation of hAECs has been shown to reduce both inflammation and subsequent fibrosis. To further explore the mechanisms by which hAECs reduce pulmonary fibrosis and enhance lung regeneration, we utilized a bleomycin-induced model of pulmonary fibrosis and investigated the ability of hAECs to reduce fibrosis and thereby improve pulmonary function. We aimed to determine if hAECs, injected into the peritoneal cavity could migrate to the lung, engraft, and form functional lung epithelium, and whether hAECs could modulate the inflammatory environment in the bleomycin-injured lung. We demonstrated that, compared to bleomycin alone, IP administration of hAECs 24 h after bleomcyin, decreased gene expression of the proinflammatory cytokines TNF-α, TGF-β, IFN-γ, and IL-6 and decreased subsequent pulmonary fibrosis with less pulmonary collagen deposition, reduced levels of α-smooth muscle actin and decreased inflammatory cell infiltrate. We also showed that hAECs are able to prevent a decline in pulmonary function associated with bleomycin-induced lung damage. We were unable to detect any significant engraftment of hAECs in injured, or uninjured, lung after administration. The findings from this study support the further investigation of hAECs as a potential cell therapy for inflammatory and fibrogenic diseases.

Therapeutic potential of lung epithelial progenitor cells derived from embryonic and induced pluripotent stem cells

Embryonic stem (ES) cells derived from preimplantation blastocysts and induced pluripotent stem (iPS) cells generated from somatic cell sources are pluripotent and capable of indefinite expansion in vitro. They provide a possible unlimited source of cells that could be differentiated into lung progenitor cells for potential clinical use in pulmonary regenerative medicine. Because of inherent difficulties in deriving endodermal cells from undifferentiated cell cultures, applications using lung epithelial cells derived from ES and iPS cells have lagged behind similar efforts devoted to other tissues, such as the heart and spinal cord. However, during the past several years, significant advances in culture, differentiation, and purification protocols, as well as in bioengineering methodologies, have fueled enthusiasm for the development of stem cell-based lung therapeutics. This article provides an overview of recent research achievements and discusses future technical challenges that must be met before the promise of stem cell applications for lung disease can be realized.

Mammalian artificial chromosomes and clinical applications for genetic modification of stem cells: an overview

Modifying multipotent, self-renewing human stem cells with mammalian artificial chromosomes (MACs), present a promising clinical strategy for numerous diseases, especially ex vivo cell therapies that can benefit from constitutive or overexpression of therapeutic gene(s). MACs are nonintegrating, autonomously replicating, with the capacity to carry large cDNA or genomic sequences, which in turn enable potentially prolonged, safe, and regulated therapeutic transgene expression, and render MACs as attractive genetic vectors for "gene replacement" or for controlling differentiation pathways in progenitor cells. The status quo is that the most versatile target cell would be one that was pluripotent and self-renewing to address multiple disease target cell types, thus making multilineage stem cells, such as adult derived early progenitor cells and embryonic stem cells, as attractive universal host cells. We will describe the progress of MAC technologies, the subsequent modifications of stem cells, and discuss the establishment of MAC platform stem cell lines to facilitate proof-of-principle studies and preclinical development.

Effects of fibroblast growth factors on the differentiation of the pulmonary progenitors from murine embryonic stem cells

The fibroblast growth factors (FGFs) play an important role in the development of embryonic lung. In this study, we investigated the effects of mainly FGF 1, 2, and 10 at concentrations selected on the basis of data obtained from previous in vitro culture on the derivation of the pulmonary progenitors from murine embryonic stem cells cultured on gelatin or Matrigel-coated plates. For cells cultured on a gelatin-coated plate, high concentrations of FGF1 were found to enhance the expression of mRNAs for SPC and CC10, markers of distal airway epithelium, while high levels of FGF2 decreased the expression of RNAs for not only SPC, CC10 but also for the additional markers SPD and aquaporin 5. FGF10 at all tested concentrations was found to have no effect on the differentiation of pneumocytes when ESCs were grown on gelatin-coated plates. However, when differentiation was performed on Matrigel-coated plates, the addition of 60 ng/ml FGF10 enhanced the expression of pneumocyte markers, suggesting a synergic effect of FGF10 and extracellular matrix. In conclusion, growth factors were proven to be effective in the differentiation of pulmonary progenitors from mESCs. The need of signals from extracellular matrix proteins depends on the growth factors supplemented.

Human amnion epithelial cell transplantation abrogates lung fibrosis and augments repair

RATIONALE

Chronic lung disease characterized by loss of lung tissue, inflammation, and fibrosis represents a major global health burden. Cellular therapies that could restore pneumocytes and reduce inflammation and fibrosis would be a major advance in management.

OBJECTIVES

To determine whether human amnion epithelial cells (hAECs), isolated from term placenta and having stem cell-like and antiinflammatory properties, could adopt an alveolar epithelial phenotype and repair a murine model of bleomycin-induced lung injury.

METHODS

Primary hAECs were cultured in small airway growth medium to determine whether the cells could adopt an alveolar epithelial phenotype. Undifferentiated primary hAECs were also injected parenterally into SCID mice after bleomycin-induced lung injury and analyzed for production of surfactant protein (SP)-A, SP-B, SP-C, and SP-D. Mouse lungs were also analyzed for inflammation and collagen deposition.

MEASUREMENTS AND MAIN RESULTS

hAECs grown in small airway growth medium developed an alveolar epithelial phenotype with lamellar body formation, production of SPs A-D, and SP-D secretion. Although hAECs injected into mice lacked SPs, hAECs recovered from mouse lungs 2 weeks post-transplantation produced SPs. hAECs remained engrafted over the 4-week test period. hAEC administration reduced inflammation in association with decreased monocyte chemoattractant protein-1, tumor necrosis factor-alpha, IL-1 and -6, and profibrotic transforming growth factor-beta in mouse lungs. In addition, lung collagen content was significantly reduced by hAEC treatment as a possible consequence of increased degradation by matrix metalloproteinase-2 and down-regulation of the tissue inhibitors of matrix metalloproteinase-1 and 2.

CONCLUSIONS

hAECs offer promise as a cellular therapy for alveolar restitution and to reduce lung inflammation and fibrosis.

Generation of thymic epithelial cell progenitors by mouse embryonic stem cells

Thymopoiesisis regulated by the thymic microenvironment, of which epithelial cells are the major components. Both cortical and medullary thymic epithelial cells (TECs) have been shown to arise from a common progenitor cell. Here we show for the first time that mouse embryonic stem cells (mESCs) can be selectively induced in vitro to differentiate into cells that have the phenotype of thymic epithelial progenitors (TEPs). When placed in vivo, these mESC-derived TEPs self-renew, develop into TECs, and reconstitute the normal thymic architecture. Functionally, these ESC-derived TEPs enhanced thymocyte regeneration after bone marrow transplantation and increased the number of functional naive splenic T cells. In addition to providing a model to study the molecular events underlying thymic epithelial cell development, the ability to selectively induce the development of TEPs in vitro from mESCs has important implications regarding the prevention and/or treatment of primary and secondary T-cell immunodeficiencies.

Cell-based therapy improves cell viability and increases airway size in an explant model

Cell-based therapy is a promising treatment option for lung disease, but no studies have demonstrated its benefit in promoting perinatal lung growth. Embryonic day 18 (E18) fetal lungs treated with vascular inhibitors were grown as explant organ cultures to inhibit endothelial growth in the explant cultures. Disruption of pulmonary vasculature decreased explant mean cord length and viability, whereas coculture with fetal pulmonary or predifferentiated embryonic stem cells rescued both parameters. These results demonstrate in a model of perinatal lung growth, exogenous addition of fetal pulmonary cells or differentiated embryonic stem (ES) cells promotes survival and alveolar morphogenesis. These experiments represent the first evidence of the benefits of cell-based therapy for perinatal lung growth.

Pulmonary tissue engineering using dual-compartment polymer scaffolds with integrated vascular tree

OBJECTIVES

The persistent shortage of donor organs for lung transplantation illustrates the need for new strategies in organ replacement therapy. Pulmonary tissue engineering aims at developing viable hybrid tissue for patients with chronic respiratory failure.

METHODS

Dual-chamber polymer constructs that mimic the characteristics of the pulmonary air-blood interface were fabricated by microfabrication techniques using the biocompatible polymer polydimethylsiloxane. One compartment ("vascular chamber") was designed as a capillary network to mimic the pulmonary microvasculature. The other compartment ("parenchymal chamber") was designed to permit gas exchange. Immortalized mouse lung epithelium cells (MLE-12) were cultured on the surface of polystyrene microcarrier beads. These beads were subsequently injected into the parenchymal chamber of the dual-chamber microsystems. The vascular compartment was perfused with cell culture medium in a bioreactor and the construct was maintained in culture for 1 week.

RESULTS

The microcarriers evenly distributed MLE-12 cells on the parenchymal compartment surface. Confluent cell layers were confirmed by fluorescent and electron microscopy. Adequate proliferation of MLE-12 cells within the construct was monitored via the DNA content. Viability of the cells was maintained over 1 week. Finally, cellular specificity and functional capacity in situ were demonstrated by immunostaining for proSP-B and proSP-C (alveolar epithelium), and by using MLE-12 cells transfected to overexpress green fluorescent protein.

CONCLUSION

We conclude that functional hybrid microsystems mimicking the basic building plan of alveolar tissue can be engineered in vitro.

Transplantation of human embryonic stem cell-derived alveolar epithelial type II cells abrogates acute lung injury in mice

Respiratory diseases are a major cause of mortality and morbidity worldwide. Current treatments offer no prospect of cure or disease reversal. Transplantation of pulmonary progenitor cells derived from human embryonic stem cells (hESCs) may provide a novel approach to regenerate endogenous lung cells destroyed by injury and disease. Here, we examine the therapeutic potential of alveolar type II epithelial cells derived from hESCs (hES-ATIICs) in a mouse model of acute lung injury. When transplanted into lungs of mice subjected to bleomycin (BLM)-induced acute lung injury, hES-ATIICs behaved as normal primary ATIICs, differentiating into cells expressing phenotypic markers of alveolar type I epithelial cells. Without experiencing tumorigenic side effects, lung injury was abrogated in mice transplanted with hES-ATIICs, demonstrated by recovery of body weight and arterial blood oxygen saturation, decreased collagen deposition, and increased survival. Therefore, transplantation of hES-ATIICs shows promise as an effective therapeutic to treat acute lung injury.

Efficient derivation of alveolar type II cells from embryonic stem cells for in vivo application

In the present study, mouse embryonic stem cells (ESCs) were differentiated into alveolar epithelial type II (AEII) cells for endotracheal injection. These enriched lung-like populations expressed lung epithelial markers SP-A, SP-B, SP-C, and CC10. First we show that rapid differentiation of ESCs requires a dissociated seeding method instead of an embryoid body culture method. We then investigated a two-step differentiation of ESCs into definitive endoderm by activin or A549-conditioned medium as a precursor to lung epithelial cells. When conditioned medium from A549 cells was used to derive endoderm, yield was increased above that of activin alone. Further studies showed that Wnt3a may be one of the secreted factors produced by A549 cells and promotes definitive endoderm differentiation, in part, through suppression of primitive endoderm. Activin and Wnt3a together at appropriate doses with dissociated cell seeding promoted greater endoderm yield than activin alone. Next, fibroblast growth factor 2 was shown to induce a dose-dependent expression of SPC, and these cells contained lamellar bodies characteristic of mature AEII cells from ESC-derived endoderm. Finally, ES-derived lung cells were endotracheally injected into preterm mice with evidence of AEII distribution within the lung parenchyma. This study concludes that a recapitulation of development may enhance derivation of an enriched population of lung-like cells for use in cell-based therapy.

Human umbilical cord mesenchymal stem cells reduce fibrosis of bleomycin-induced lung injury

Acute respiratory distress syndrome is characterized by loss of lung tissue as a result of inflammation and fibrosis. Augmenting tissue repair by the use of mesenchymal stem cells may be an important advance in treating this condition. We evaluated the role of term human umbilical cord cells derived from Wharton's jelly with a phenotype consistent with mesenchymal stem cells (uMSCs) in the treatment of a bleomycin-induced mouse model of lung injury. uMSCs were administered systemically, and lungs were harvested at 7, 14, and 28 days post-bleomycin. Injected uMSCs were located in the lung 2 weeks later only in areas of inflammation and fibrosis but not in healthy lung tissue. The administration of uMSCs reduced inflammation and inhibited the expression of transforming growth factor-beta, interferon-gamma, and the proinflammatory cytokines macrophage migratory inhibitory factor and tumor necrosis factor-alpha. Collagen concentration in the lung was significantly reduced by uMSC treatment, which may have been a consequence of the simultaneous reduction in Smad2 phosphorylation (transforming growth factor-beta activity). uMSCs also increased matrix metalloproteinase-2 levels and reduced their endogenous inhibitors, tissue inhibitors of matrix metalloproteinases, favoring a pro-degradative milieu following collagen deposition. Notably, injected human lung fibroblasts did not influence either collagen or matrix metalloproteinase levels in the lung. The results of this study suggest that uMSCs have antifibrotic properties and may augment lung repair if used to treat acute respiratory distress syndrome.

Integration of functional bacterial artificial chromosomes into human cord blood-derived multipotent stem cells

Stem cells from a patient with a genetic disease could be used for cell therapy if it were possible to insert a functional copy of the defective gene. In this study, we investigate the transfection and subsequent integration of large genomic fragments into human cord blood-derived multipotent stem cells. We describe for the first time the creation of clonal stem cells carrying a human bacterial artificial chromosome (BAC) containing the Friedreich ataxia locus with an enhanced green fluorescent protein (EGFP) reporter gene fused to exon 5a of the frataxin (FXN) gene. Integration of the BAC into the host cell genome was confirmed by PCR, Southern blot and fluorescent in situ hybridization analysis. Reverse transcription-PCR and flow cytometry confirmed expression of FXN-EGFP. Correct mitochondrial localization of the protein was confirmed using fluorescent microscopy. The transfected stem cells also retained the ability to differentiate into cells from all three germline layers, as demonstrated by the capacity to form neuron-specific beta-tubulin-expressing cells, Alizarin Red S-positive bone-like cells, and epithelial-like cells expressing surfactant protein C. This is the first study to demonstrate that cord blood-derived multipotent stem cells may be useful targets for gene therapy applications using large genomic loci.

Identification of a bone marrow-derived epithelial-like population capable of repopulating injured mouse airway epithelium

The bone marrow compartment is enriched in stem and progenitor cells, and an unidentified subpopulation of these cells can contribute to lung epithelial repair. Here we identify this subpopulation and quantitate its relative contribution to injured airway epithelium. A subpopulation of adherent human and murine bone marrow cells that expresses Clara cell secretory protein (CCSP) was identified using flow cytometry. When cultured at the air-liquid interface in ex vivo cultures, Ccsp+ cells expressed type I and type II alveolar markers as well as basal cell markers and active epithelial sodium channels. Ccsp+ cells preferentially homed to naphthalene-damaged airways when delivered transtracheally or intravenously, with the former being more efficient than the latter. Interestingly, naphthalene-induced lung damage transiently increased Ccsp expression in bone marrow and peripheral circulation. Furthermore, lethally irradiated Ccsp-null mice that received tagged wild-type bone marrow contained donor-derived epithelium in both normal and naphthalene-damaged airways. This study therefore identifies what we believe to be a newly discovered cell in the bone marrow that might have airway reconstitution potential in the context of cell-based therapies for lung disease. Additionally, these data could reconcile previous controversies regarding the contribution of bone marrow to lung regeneration.

The differentiation of distal lung epithelium from embryonic stem cells

The potential for embryonic stem (ES) cells to differentiate into cells with a distal lung epithelial phenotype has been demonstrated using different in vitro culture methods. Three separate protocols are described here that utilize both murine and human ES cells. The distal lung epithelial phenotype is induced through the use of embryonic distal lung mesenchyme in coculture systems with differentiating embryoid bodies or the use of soluble factors in defined media to maximize definitive endoderm formation and select and maintain the desired phenotype. Phenotypic analysis is demonstrated using immunocytochemistry and SP-C promoter-eGFP reporter gene expression in transgenic ES cells. These methods provide an increased efficiency of distal lung epithelial derivation from ES cells and, therefore, they provide the foundation for the development of a cell replacement product to treat chronic lung disease or a useful in vitro model for the study of lung disease and development.

Human embryonic stem cells and lung regeneration

Human embryonic stem cells are pluripotent cells derived from the inner cell mass of preimplantation stage embryos. Their unique potential to give rise to all differentiated cell types has generated great interest in stem cell research and the potential that it may have in developmental biology, medicine and pharmacology. The main focus of stem cell research has been on cell therapy for pathological conditions with no current methods of treatment, such as neurodegenerative diseases, cardiac pathology, retinal dysfunction and lung and liver disease. The overall aim is to develop methods of application either of pure cell populations or of whole tissue parts to the diseased organ under investigation. In the field of pulmonary research, studies using human embryonic stem cells have succeeded in generating enriched cultures of type II pneumocytes in vitro. On account of their potential of indefinite proliferation in vitro, embryonic stem cells could be a source of an unlimited supply of cells available for transplantation and for use in gene therapy. Uncovering the ability to generate such cell types will expand our understanding of biological processes to such a degree that disease understanding and management could change dramatically.

Human amniotic fluid stem cells can integrate and differentiate into epithelial lung lineages

A new source of stem cells has recently been isolated from amniotic fluid; these amniotic fluid stem cells have significant potential for regenerative medicine. These cells are multipotent, showing the ability to differentiate into cell types from each embryonic germ layer. We investigated the ability of human amniotic fluid stem cells (hAFSC) to integrate into murine lung and to differentiate into pulmonary lineages after injury. Using microinjection into cultured mouse embryonic lungs, hAFSC can integrate into the epithelium and express the early human differentiation marker thyroid transcription factor 1 (TTF1). In adult nude mice, following hyperoxia injury, tail vein-injected hAFSC localized in the distal lung and expressed both TTF1 and the type II pneumocyte marker surfactant protein C. Specific damage of Clara cells through naphthalene injury produced integration and differentiation of hAFSC at the bronchioalveolar and bronchial positions with expression of the specific Clara cell 10-kDa protein. These results illustrate the plasticity of hAFSC to respond in different ways to different types of lung damage by expressing specific alveolar versus bronchiolar epithelial cell lineage markers, depending on the type of injury to recipient lung. Disclosure of potential conflicts of interest is found at the end of this article.

Embryonic stem cells as a source of pulmonary epithelium in vitro and in vivo

Embryonic stem cells (ESCs) derived from the preimplantation blastocyst are pluripotent and capable of indefinite expansion in vitro. As such, they present a cell source to derive a potentially inexhaustible supply of pulmonary cells and tissue. ESC-derived pulmonary epithelium could be used for in vitro cell or tissue models or, in the future, implanted into the damaged or diseased lung to effect repair. Efforts to date have largely focused on obtaining distal lung epithelial phenotypes from ESCs, notably alveolar epithelium. Several disparate methods have been developed to enhance differentiation of ESCs into pulmonary epithelial lineages; these are broadly based on recapitulating developmental signaling events, mimicking the physical environment, or forcibly reprogramming the ESC nucleus. Early findings of our preclinical experiments implanting differentiated ESCs into the injured lung are also described here. Future efforts will focus on maximizing ESC differentiation efficiency and yield of the target phenotype, as well as characterizing the function of derived cells in vivo and in vitro.

The benefit of human embryonic stem cell encapsulation for prolonged feeder-free maintenance

The majority of methodologies for maintaining human embryonic stem cell (hESC) pluripotency require the use of human or animal feeder cell layers, the most common being murine embryonic fibroblasts. In this study, we applied a protocol aimed at maintaining hESCs in culture without exposure to animal cells or proteins. hESCs were encapsulated in 1.1% (w/v) calcium alginate hydrogels and grown in basic maintenance medium for a period of up to 260 days. Investigation of the cell aggregates formed within the hydrogels yielded no evidence of the formation of any of the three germ layers, although the hESCs retained their pluripotency and could differentiate when they were subsequently cultured in a conditioned environment. Immunohistochemistry and RT-PCR showed that the hESC aggregates expressed protein and gene markers characteristic of pluripotency including Oct-4, Nanog, SSEA-4, TRA-1-60 and TRA-1-81. At the ultrastructural level, the cells were arranged in closely packed clusters and showed no cytoplasmic organelles, suggesting an undifferentiated state. These data show that it is possible to maintain hESCs in an undifferentiated state, without passaging or embryoid body formation, and without animal contamination.

Serum-free differentiation of murine embryonic stem cells into alveolar type II epithelial cells

Alveolar type II (AT2) epithelial cells have important functions including the production of surfactant and regeneration of lost alveolar type I epithelial cells. The ability of in vitro production of AT2 cells would offer new therapeutic options in treating pulmonary injuries and disorders including genetically based surfactant deficiencies. Aiming at the generation of AT2-like cells, the differentiation of murine embryonic stem cells (mESCs) toward mesendodermal progenitors (MEPs) was optimized using a "Brachyury-eGFP-knock in" mESC line. eGFP expression demonstrated generation of up to 65% MEPs at day 4 after formation of embryoid bodies (EBs) under serum-free conditions. Plated EBs were further differentiated into AT2-like cells for a total of 25 days in serum-free media resulting in the expression of endodermal marker genes (FoxA2, Sox17, TTR, TTF-1) and of markers for distal lung epithelium (surfactant proteins (SP-) A, B, C, and D, CCSP, aquaporin 5). Notably, expression of SP-C as the only known AT2 cell specific marker could be detected after serum-induction as well as under serum-free conditions. Cytoplasmic localization of SP-C was demonstrated by confocal microscopy. The presence of AT2-like cells was confirmed by electron microscopy providing evidence for polarized cells with apical microvilli and lamellar body-like structures. Our results demonstrate the differentiation of AT2-like cells from mESCs after serum-induction and under serum-free conditions. The established serum-free differentiation protocol will facilitate the identification of key differentiation factors leading to a more specific and effective generation of AT2-like cells from ESCs.

Nuclear proteomics and directed differentiation of embryonic stem cells

During the past decade, regenerative medicine has been the subject of intense interest due, in large part, to our growing knowledge of embryonic stem (ES) cell biology. ES cells give rise to cell lineages from the three primordial germ layers--endoderm, mesoderm, and ectoderm. This process needs to be channeled if these cells are to be differentiated efficiently and used subsequently for therapeutic purposes. Indeed, an important area of investigation involves directed differentiation to influence the lineage commitment of these pluripotent cells in vitro. Various strategies involving timely growth factor supplementation, cell co-cultures, and gene transfection are used to drive lineage specific emergence. The underlying goal is to control directly the center of gene expression and cellular programming--the nucleus. Gene expression is enabled, managed, and sustained by the collective actions and interactions of proteins found in the nucleus--the nuclear proteome--in response to extracellular signaling. Nuclear proteomics can inventory these nuclear proteins in differentiating cells and decipher their dynamics during cellular phenotypic commitment. This review details what is currently known about nuclear effectors of stem cell differentiation and describes emerging techniques in the discovery of nuclear proteomics that will illuminate new transcription factors and modulators of gene expression.

Therapeutic potential of stem cells in lung disease: progress and pitfalls

There has been increasing excitement over the last few years with the suggestion that exogenous stem cells may offer new treatment options for a wide range of diseases. Within respiratory medicine, these cells have been shown to have the ability to differentiate and function as both airway and lung parenchyma epithelial cells in both in vitro and increasingly in vivo experiments. The hypothesis is that these cells may actively seek out damaged tissue to assist in the local repair, and the hope is that their use will open up new cellular and genetic treatment modalities. Such is the promise of these cells that they are being rushed from the benchside to the bedside with the commencement of early clinical trials. However, important questions over their use remain and the field is presently littered with controversy and uncertainty. This review evaluates the progress made and the pitfalls encountered to date, and critically assesses the evidence for the use of stem cells in lung disease.

Stem cell differentiation and expansion for clinical applications of tissue engineering

This invited review discusses the latest advances stem cell biology, tissue engineering and the transition from bench to bedside. An overview is presented as to which the best cell source might be for cell therapy and tissue engineering applications, best biomaterials currently available and the challenges the field faces to translate basic research into therapies for a large number of human diseases.

Treatment of COPD: a matrix perspective

Fundamental physical properties, such as the intrinsic recoil of the lung, are governed by the extracellular matrix. The prototypical roles of the matrix proteins, collagen and elastin, in pulmonary fibrosis and emphysema have long been recognized, and much research effort has been devoted to understanding mechanisms of extracellular matrix synthesis and turnover in the lung. Yet, despite extensive knowledge of the biochemical properties of collagen and elastin, none of the present clinical strategies for treating COPD directly target the extracellular matrix. From a matrix perspective, therapeutic interventions that limit elastic fiber destruction and/or restore function to damaged alveolar units merit particular consideration as clinical strategies for treating the emphysema component of COPD. Effective treatment of the bronchiolar component of COPD requires a better understanding of the relationship between airway fibrosis and airflow obstruction. Translating basic knowledge of extracellular matrix biology into the clinical venue will be essential in the development of new approaches to COPD treatment.

Derivation of lung epithelium from human cord blood-derived mesenchymal stem cells

RATIONALE

Recent studies have suggested that both embryonic stem cells and adult bone marrow stem cells can participate in the regeneration and repair of diseased adult organs, including the lungs. However, the extent of airway epithelial remodeling with adult marrow stem cells is low, and there are no available in vivo data with embryonic stem cells. Human umbilical cord blood contains both hematopoietic and nonhematopoietic stem cells, which have been used clinically as an alternative to bone marrow transplantation for hematologic malignancies and other diseases.

OBJECTIVES

We hypothesized that human umbilical cord blood stem cells might be an effective alternative to adult bone marrow and embryonic stem cells for regeneration and repair of injured airway epithelium.

METHODS

Human cord blood was obtained from normal deliveries at the University of Vermont. Cultured plastic adherent cells were characterized as mesenchymal stem cells (MSCs) by flow cytometry and differentiation assays. Cord blood-derived MSCs (CB-MSCs) were cultured in specialized airway growth media or with specific growth factors, including keratinocyte growth factor and retinoic acid. mRNA and protein expression were analyzed with PCR and immunofluorescent staining. CB-MSCs were systematically administered to immunotolerant, nonobese diabetic/severe combined immunodeficiency (NOD-SCID) mice. Lungs were analyzed for presence of human cells.

MEASUREMENTS AND MAIN RESULTS

When cultured in specialized airway growth media or with specific growth factors, CB-MSCs differentially expressed Clara cell secretory protein (CCSP), cystic fibrosis transmembrane conductance regulator (CFTR), surfactant protein C, and thyroid transcription factor-1 mRNA, and CCSP and CFTR protein. Furthermore, CB-MSCs were easily transduced with recombinant lentiviral vectors to express human CFTR. After systemic administration to immunotolerant, NOD-SCID, mice, rare cells were found in the airway epithelium that had acquired cytokeratin and human CFTR expression.

CONCLUSIONS

CB-MSCs appear to be comparable to MSCs obtained from adult bone marrow in ability to express phenotypic markers of airway epithelium and to participate in airway remodeling in vivo.

Lung stem cells

The lung is a relatively quiescent tissue comprised of infrequently proliferating epithelial, endothelial, and interstitial cell populations. No classical stem cell hierarchy has yet been described for the maintenance of this essential tissue; however, after injury, a number of lung cell types are able to proliferate and reconstitute the lung epithelium. Differentiated mature epithelial cells and newly recognized local epithelial progenitors residing in specialized niches may participate in this repair process. This review summarizes recent discoveries and controversies, in the field of stem cell biology, that are not only challenging, but also advancing an understanding of lung injury and repair. Evidence supporting a role for the numerous cell types believed to contribute to lung epithelial homeostasis is reviewed, and initial studies employing cell-based therapies for lung disease are presented. As a detailed understanding of stem cell biology, lung development, lineage commitment, and epithelial differentiation emerges, an ability to modulate lung injury and repair is likely to follow.

Effects of Growth Factors on the Differentiation of Murine ESC into Type II Pneumocytes

We have previously shown that embryonic stem cells (ESC) can be directed to differentiate into alveolar type II cells by provision of a serum-free medium designed for in vitro maintenance of mature alveolar epithelial cells (small airway growth medium: SAGM), although the target cell yield was low. SAGM comprises a basal serum-free medium (SABM) plus a series of defined supplements. In order to try increase the proportion of pneumocytes in differentiated cultures, we aimed in this study to determine the effects on murine ESC of each of the individual growth factors in SAGM. In accordance with our previous reports, expression of surfactant protein C (SPC) and its mRNA was used to monitor differentiation of type II pneumocytes. Surprisingly, we found that addition of each factor separately to SABM decreased the expression of SPC mRNA when compared with the effect of SABM alone. Thus, it seems that the observed enhancement by SAGM of pneumocyte differentiation from murine ESC can, in fact, be attributed to the provision of a serum-free environment.

Stem Cell Therapy: A Hope for Dying Hearts

The restoration of functional myocardium following heart failure still remains a formidable challenge among researchers. Irreversible damage caused by myocardial infarction is followed by left ventricular remodeling. The current pharmacologic and interventional strategies fail to regenerate dead myocardium and are usually insufficient to meet the challenge caused by necrotic cardiac myocytes. There is growing evidence, suggesting that the heart has the ability to regenerate through the activation of resident cardiac stem cells or through the recruitment of a stem cell population from other tissues such as bone marrow. These new findings belie the earlier conception about the poor regenerating ability of myocardial tissue. Stem cell therapy is a promising new approach for myocardial repair. However, it has been limited by the paucity of cell sources for functional human cardiomyocytes. Moreover, cells isolated from different sources exhibit idiosyncratic characteristics including modes of isolation, ease of expansion in culture, proliferative ability, characteristic markers, etc., which are the basis for several technical manipulations to achieve successful engraftment. Clinical trials show some evidence for the successful integration of stem cells of extracardiac origin in adult human heart with an improved functional outcome. This may be attributed to the discrepancies in the methods of detection, study subject selection (early or late post transplantation), presence of inflammation, and false identification of infiltrating leukocytes. This review discusses these issues in a comprehensive manner so that their physiological significance in animal as well as in human studies can be better understood.

Directed engineering of umbilical cord blood stem cells to produce C-peptide and insulin

OBJECTIVES

In this study, we investigated the potential of umbilical cord blood stem cell lineages to produce C-peptide and insulin.

MATERIALS AND METHODS

Lineage negative, CD133+ and CD34+ cells were analyzed by flow cytometry to assess expression of cell division antigens. These lineages were expanded in culture and subjected to an established protocol to differentiate mouse embryonic stem cells (ESCs) toward the pancreatic phenotype. Phase contrast and fluorescence immunocytochemistry were used to characterize differentiation markers with particular emphasis on insulin and C-peptide.

RESULTS

All 3 lineages expressed SSEA-4, a marker previously reported to be restricted to the ESC compartment. Phase contrast microscopy showed all three lineages recapitulated the treatment-dependent morphological changes of ESCs as well as the temporally restricted expression of nestin and vimentin during differentiation. After engineering, each isolate contained both C-peptide and insulin, a result also obtained following a much shorter protocol for ESCs.

CONCLUSIONS

Since C-peptide can only be derived from de novo synthesis and processing of pre-proinsulin mRNA and protein, we conclude that these results are the first demonstration that human umbilical cord blood-derived stem cells can be engineered to engage in de novo synthesis of insulin.

Stem cells in lung repair and regeneration

Repair or regeneration of defective lung tissue would be of great clinical use. Potential cellular sources for the regeneration of lung tissue in vivo or lung tissue engineering in vitro include endogenous pulmonary stem cells, extrapulmonary circulating stem cells and embryonic stem cells. This review summarizes the recent research on each of these stem cell types and their potential for use in the treatment of lung injury and disease.

A pure population of lung alveolar epithelial type II cells derived from human embryonic stem cells

Alveolar epithelial type II (ATII) cells are small, cuboidal cells that constitute approximately 60% of the pulmonary alveolar epithelium. These cells are crucial for repair of the injured alveolus by differentiating into alveolar epithelial type I cells. ATII cells derived from human ES (hES) cells are a promising source of cells that could be used therapeutically to treat distal lung diseases. We have developed a reliable transfection and culture procedure, which facilitates, via genetic selection, the differentiation of hES cells into an essentially pure (>99%) population of ATII cells (hES-ATII). Purity, as well as biological features and morphological characteristics of normal ATII cells, was demonstrated for the hES-ATII cells, including lamellar body formation, expression of surfactant proteins A, B, and C, alpha-1-antitrypsin, and the cystic fibrosis transmembrane conductance receptor, as well as the synthesis and secretion of complement proteins C3 and C5. Collectively, these data document the successful generation of a pure population of ATII cells derived from hES cells, providing a practical source of ATII cells to explore in disease models their potential in the regeneration and repair of the injured alveolus and in the therapeutic treatment of genetic diseases affecting the lung.

Acute lung injury/acute respiratory distress syndrome (ALI/ARDS): the mechanism, present strategies and future perspectives of therapies

Acute lung injury/acute respiratory distress syndrome (ALI/ARDS), which manifests as non-cardiogenic pulmonary edema, respiratory distress and hypoxemia, could be resulted from various processes that directly or indirectly injure the lung. Extensive investigations in experimental models and humans with ALI/ARDS have revealed many molecular mechanisms that offer therapeutic opportunities for cell or gene therapy. Herein the present strategies and future perspectives of the treatment for ALI/ARDS, include the ventilatory, pharmacological, as well as cell therapies.

Viral Vector–mediated and Cell-based Therapies for Treatment of Cystic Fibrosis

Gene and cell-based therapies are considered to be potentially powerful new approaches for the management of cystic fibrosis (CF) lung disease. Despite tremendous efforts that have been made, especially in studies to understand the obstacles to gene delivery, major challenges to the application of these approaches remain to be solved. This article will review the advancements made and challenges remaining in the development of viral vector-mediated and cell-based approaches to treat patients with CF.

A murine respiratory-inducing niche displays variable efficiency across human and mouse embryonic stem cell species

Human embryonic stemlike cells (hESCs) are pluripotent cells derived from blastocysts. Differentiating hESCs into respiratory lineages may benefit respiratory therapeutic programs. We previously demonstrated that 24% of all mouse embryonic stem cell (mESC) derivatives cocultured with embryonic day 11.5 (E11.5) mouse lung rudiments display immunoreactivity to the pneumonocyte II specific marker surfactant-associated protein C (Sftpc). Here we further investigate the effects of this inductive niche in terms of its competence to induce hESC derivative SFTPC immunoreactivity and the expression of other markers of terminal lung secretory units. When hESCs were cocultured as single cells, clumps of approximately 10 cells or embryoid bodies (EBs), hESC derivatives formed pan-keratin-positive epithelial tubules at high frequency (>30% of all hESC derivatives). However, human-specific SFTPC immunoreactivity associated with tubule formation only at low frequency (<0.1% of all hESC derivatives). Human-specific SFTPD and secretoglobin family 1A member 1 (SCGB1A1, also known as CC10) transcripts were detected by PCR after prolonged culture. Expression of other terminal lung secretory unit markers (TITF1, SFTPA, and SFTPB) was not detected at any time point analyzed. On the other hand, hESC derivatives cultured as plated EBs in media previously demonstrated to induce Sftpc expression in isolated mouse fetal tracheal epithelium expressed all terminal lung secretory unit markers examined. mESCs and hESCs thus display fundamental differences in their response to the E11.5 mouse lung inductive niche, and these data provide an important step in the delineation of signaling mechanisms capable of efficiently inducing hESC differentiation into terminal secretory units of the lung.

Antibiotics reduce the growth rate and differentiation of embryonic stem cell cultures

Embryonic stem cells (ESCs) are being investigated increasingly for their potential as a cell source for tissue engineering. Antibiotics are regularly used in ESC culture media to control contamination, although they can be cytotoxic and interfere with protein synthesis. Our aim was to examine the effects of the frequently used antibiotics gentamicin and combined penicillin and streptomycin on ESC culture using differentiation of murine ESC into type II pneumocytes as a model. Antibiotics reduced the expression of the specific marker for type II pneumocytes, SPC mRNA, by up to 60%. We also identified an adverse effect on the growth rate of differentiating embryoid bodies, causing a significant ( p < 0.05) reduction of up to 40%, and an increase in population doubling time of up to 48%. No contamination was seen in any of the cultures. Our findings suggest that the routine use of antibiotics in ESC culture should be avoided as it may reduce the efficiency of the culture system.

Stem cells – potential for repairing damaged lungs and growing human lungs for transplant

Repair or regeneration of defective lung epithelium would be of great therapeutic potential. It is estimated by the British Lung Foundation that 1 in 7 people in the UK is affected by a lung disease and that 1 in 4 admissions to children's wards are as a result of respiratory problems. Potential cellular sources for the regeneration of lung tissue in vivo or lung tissue engineering in vitro include endogenous pulmonary epithelial stem cells, extrapulmonary circulating stem cells and embryonic stem cells. This article discusses the potential role of each of these stem cell types in future approaches to the treatment of lung injury and disease.

Derivation of distal airway epithelium from human embryonic stem cells

The pluripotency of embryonic stem cells (ESC) is offering new opportunities in tissue engineering and cell therapy. We have shown previously that alveolar epithelial cells, specifically type II pneumocytes, can be derived from murine ESC and hypothesized that a similar protocol could be used successfully on human ESC. Undifferentiated human ESC were induced to form embryoid bodies that were transferred into adherent culture conditions and grown in a medium designed for the maintenance of mature small airway epithelium. On inverted microscopy, the generated cells showed the cobblestone-like morphology of epithelium. The presence of surfactant protein C, a specific marker of type II pneumocytes, and its corresponding RNA were demonstrated by immunostaining and reverse transcription polymerase chain reaction, respectively. Electron microscopy revealed frequent cells with the typical ultrastructure of type II pneumocytes. This study provides evidence for in vitro induction of the differentiation from human ESC of alveolar type II cells, which have the potential for therapeutic use or construction of an in vitro model of human lung.

Embryonic stem cells form glandular structures and express surfactant protein C following culture with dissociated fetal respiratory tissue

Mouse embryonic stem cells (MESCs) are pluripotent, theoretically immortal cells derived from the inner cell mass of developing blastocysts. The respiratory epithelium develops from the primitive foregut endoderm as a result of inductive morphogenetic interactions with the surrounding visceral mesoderm. After dissociation of the explanted fetal lung into single cells, these morphogenetic signaling pathways instruct reconstitution of the developing lung according to a process known as organotypic regeneration. Data presented here demonstrate that such fetal lung morphogenetic cues induce MESC derivatives to incorporate into the reforming pseudoglandular-like tubular ducts, display pan-keratin and surfactant protein C (Sftpc) immunoreactivity, and express Sftpc transcripts while displaying a normal diploid karyotype in coculture. The Sftpc inductive capacity of dissociated fetal lung tissue shows stage specificity with 24% of all MESC derivatives displaying Sftpc immunoreactivity after coculture with embryonic day 11.5 (E11.5) lung buds compared with 6% and 0.02% following coculture with E12.5 and E13.5 lung buds, respectively. MESC derivative Sftpc immunoreactivity follows a spatial and temporal specific maturation profile with an initially ubiquitous cellular Sftpc immunostaining pattern becoming apically polarized with time. Directing differentiation of MESCs into respiratory lineages has important implications for cell replacement therapeutics aimed at treating respiratory-specific diseases such as cystic fibrosis and idiopathic pulmonary fibrosis.

[Repair and regeneration of the airway epithelium]

Despite an efficient defence system, the airway surface epithelium, in permanent contact with the external milieu, is frequently injured by inhaled pollutants, microorganisms and viruses. The response of the airway surface epithelium to an acute injury includes a succession of cellular events varying from the loss of the surface epithelium integrity to partial shedding of the epithelium or even to complete denudation of the basement membrane. The epithelium has then to repair and regenerate to restore its functions, through several mechanisms including basal cell spreading and migration, followed by proliferation and differentiation of epithelial cells. The cellular and molecular factors involved in wound repair and epithelial regeneration are closely interacting and imply extracellular matrix proteins, matrix metalloproteinases (MMPs) and their inhibitors as well as cytokines and growth factors secreted by airway epithelial and mesenchymal cells. The development of in vitro and in vivo models of airway epithelium wound repair allowed the study of the spatio-temporal modulation of these factors during the different steps of epithelial repair and regeneration. In this context, several studies have demonstrated that the matrix and secretory environment are markedly involved in these mechanisms and that their dysregulation may induce remodelling of the airway mucosa. A better knowledge of the mechanisms involved in airway epithelium regeneration may pave the way to regenerative therapeutics allowing the reconstitution of a functional airway epithelium in numerous respiratory diseases such as asthma, chronic obstructive pulmonary diseases, cystic fibrosis and bronchiolitis.

Derivation of Distal Lung Epithelial Progenitors from Murine Embryonic Stem Cells Using a Novel Three-Step Differentiation Protocol

Embryonic stem cells (ESCs) are a potential source for the cell-based therapy of a wide variety of lung diseases for which the only current treatment is transplantation. However, distal lung epithelium, like many other endodermally derived somatic cell lineages, is proving difficult to obtain from both murine and human ESCs. We have previously obtained alveolar epithelium from ESCs, although final cell yield remained extremely low. Here, we present an optimized three-step protocol for the derivation of distal lung epithelial cells from murine ESCs. This protocol incorporates (a) treatment of early differentiating embryoid bodies with activin A to enhance the specification of the endodermal germ layer, followed by (b) adherent culture in serum-free medium and (c) the final application of a commercial, lung-specific medium. As well as enhancing the specification of distal lung epithelium, this protocol was found to yield cells with a phenotype most closely resembling that of lung-committed progenitor cells present in the foregut endoderm and the early lung buds during embryonic development. This is in contrast to our previous differentiation method, which drives differentiation through to mature type II alveolar epithelial cells. The derivation of a committed lung progenitor cell type from ESCs is particularly significant for regenerative medicine because the therapeutic implantation of progenitor cells has several clear advantages over the transplantation of mature, terminally differentiated somatic cells.

Tissue-Engineered Lung: An In Vivo and In Vitro Comparison of Polyglycolic Acid and Pluronic F-127 Hydrogel/Somatic Lung Progenitor Cell Constructs to Support Tissue Growth

In this study, we describe the isolation and characterization of a population of adult-derived or somatic lung progenitor cells (SLPC) from adult mammalian lung tissue and the promotion of alveolar tissue growth by these cells (both in vitro and in vivo) after seeding onto synthetic polymer scaffolds. After extended in vitro culture, differentiating cells expressed Clara cell 10kDa protein, surfactant protein-C, and cytokeratin but did not form organized structures. When cells were combined with synthetic scaffolds, polyglycolic acid (PGA) or Pluronic F-127 (PF-127), and maintained in vitro or implanted in vivo, they expressed lung-specific markers for Clara cells, pneumocytes, and respiratory epithelium and organized into identifiable pulmonary structures (including those similar to alveoli and terminal bronchi), with evidence of smooth muscle development. Although PGA has been shown to be an excellent polymer for culture of specific cell types in vitro, in vivo culture in an immunocompetent host induced a foreign body response that altered the integrity of the developing lung tissue. Use of PF-127/cell constructs resulted in the development of tissue with less inflammatory reaction. These data suggest that the therapeutic use of engineered tissues requires both the use of specific cell phenotypes, as well as the careful selection of synthetic polymers, to facilitate the assembly of functional tissue.