Stem cells in management of Chronic Obstructive Pulmonary Disease (COPD)

Article Summary, from International Journal of COPD 2010:5 81-88. Tillie L Hackett, Darryl A Knight, Don D Sin, Heart and Lung Institute, St Paul’s Hospital, Vancouver, BC, Canada


Chronic obstructive pulmonary disease (COPD) is a worldwide epidemic affecting an estimated 210 million people and accounting for more than three million deaths annually. (1). Over the next 20 years, the World Health Organization expects total COPD mortality to more than double.

In China alone, three million men and women are expected to die from COPD by the year 2030 if current cigarette and biomass exposure trends continue. (2)

Unfortunately, aside from supplemental domiciliary oxygen for the small number of patients who demonstrate resting arterial hypoxemia and smoking cessation for continued smokers, there are no interventions that have been unequivocally shown to prolong survival in patients with COPD (3) and no therapies that can fully restore the lost lung function associated with COPD.

As COPD is characterized by loss of lung tissue and remodeling of the airways, there is growing enthusiasm for using stem and progenitor cells to regenerate healthy parenchymal and airway cells and restore lung function in patients with COPD.

Morphologically, COPD is characterized by two distinct but related pathologic features, ie, bronchiolitis involving predominantly the small airways (airways less than 2 mm in diameter) and emphysema. (4). Emphysema is characterized by dilatation and destruction of lung tissue beyond the terminal bronchioles. The centriacinar form of emphysema is characterized by tissue destruction of the respiratory bronchioles and occurs most frequently in cigarette smokers. Panacinar emphysema, on the other hand, is characterized by destruction of the entire acinar unit and occurs largely in the setting of alpha-1 antitrypsin deficiency. (4)

The key pathophysiologic changes underlying airflow limitation in COPD include the loss of elastic lung recoil pressure due to the destruction of alveolar septa and terminal bronchioles (emphysema), increased airflow resistance due to airway wall remodeling (ie, thickening of small airway wall) and mucoid impaction of the airway lumen. (4) In addition, there is growing evidence that endothelial dysfunction and vascular remodeling initiated by vascular endothelial growth factor-mediated apoptosis of endothelial cells may also contribute to disease progression in COPD. (5)

Although there have been tremendous advances in our understanding of COPD pathobiology over the past two decades, the exact pathologic mechanisms by which emphysema and airway remodeling occur in COPD remain largely a mystery. One of the leading theories is the “inflammatory hypothesis”. Proponents argue that in certain (genetically) susceptible individuals, lung inflammation, which occurs in response to environmental triggers such as air pollution and cigarette smoke, changes from a “normal” response to an abnormal one, characterized by excess innate and adaptive immunity, at some point during the exposure.

Interestingly, once the inflammatory changes are firmly established in the lungs, the removal of the environmental trigger such as cigarette smoke does not fully abrogate the abnormal inflammatory response observed in the airways. Indeed, smokers who discontinue smoking continue to demonstrate airway inflammation. (10)

Another emerging hypothesis relates to accelerated cellular aging, or senescence that results in a series of perturbations in cell morphology and function, ultimately culminating in cell cycle arrest. (11,12)  Collectively, there appears to be sufficient evidence to suggest that resident structural cells within the lungs and lymphocytes within the systemic circulation of COPD patients exhibit markers of cellular senescence and accelerated aging, which is potentially detrimental to normal lung repair.

Mechanisms of lung repair

Theoretically, if the reparative and regenerative processes in the lungs can keep up with the destructive, inflammatory, or apoptotic processes, then lung homeostasis can be maintained, leading to the preservation of normal tissue and function. Following epithelial injury, the airway epithelium begins to repair almost immediately. At the wound edge, the (undamaged) epithelial cells de-differentiate and migrate to “cover up” the wounded area and release a variety of proinflammatory cytokines and growth factors to attract proteins and cells needed for restoration of the extracellular matrix, which is crucial for normal wound repair. Once this occurs, re-epithelialization can proceed.

The adult human lung comprises various trophic units which are each lined by specialized types of airway epithelia. (26) The ability of the lung to repair itself in the setting of injury is determined by the molecular events that mobilize the resident stem and progenitor cells within each of the trophic units. Stem cells and progenitors are similar in that they both proliferate and give rise to differentiated cells but only stem cells are capable of self-renewal. (27) The reader is referred to Figure 1 for the putative stem and progenitor cell niches within the human lung.

Resident stem cells within the lung

In contrast to dermal and intestinal epithelia, which are highly proliferative and rapidly renewing, the turnover of the airway epithelium is extremely slow unless injured. (28)  Each of the tracheal, bronchial, and alveolar regions within the lung has a distinct resident stem or progenitor cell population which possesses unique cellular physiologic properties. To date, several cells within the trachea and bronchial tissue have been reported to be enriched for stem/progenitor cell activity including cytokeratin 5/14-expressing basal cells, secretory (Clara) cells, cells residing in submucosal glands, and neuroepithelial bodies (NEB). (29–33).

A more recent study indicates that cytokeratin 5/14 expressing basal cells can self-renew and also give rise to new ciliated Clara and secretory cells following epithelial injury. (34)

We have also demonstrated that within human airways the basal cell population contains a side population (SP) of cells, which are characterized by the ability to efflux actively the DNA binding dye, Hoescht 33342. (35)

As with other epithelial tissues such as in mammary glands, (36,37) the eye, (38) and the skin, (39) SP cells within the airways are rare, making up less than 0.1%of the total epithelial cell population.

Both embryonic and adult stem cells in vitro can be induced to differentiate into airway and alveolar epithelial cells.

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MSCs isolated from amniotic fluid, umbilical cord blood, adipose tissue, or bone marrow have been used to generate tracheal cartilage using biosynthetic scaffolds in order to repair congenital tracheal defects in rodent models and more recently in human clinical trials. (56–58)

Macchiarini et al were able to use a patient’s epithelial cells and MSC-derived chondrocytes to generate a bioengineered trachea, which after engraftment provided a functional airway without the requirement for immunosuppressive drugs. (59)

Mesenchymal stem cells (MSCs) are nonhematopoietic stem cells of mesodermal origin, with the capacity to differentiate into both mesenchymal and nonmesenchymal lineages.

MSCs are found primarily in the bone marrow of adults and give rise to blood, skeletal muscle, vascular, and connective tissues throughout the body. Postnatally, bone marrow MSC can be isolated from adipose tissue, liver, synovial membrane, teeth, and tendons. In particular, MSCs are easily isolated from a small aspirate of bone marrow and can be expanded with high efficiency. MSCs have great potential in clinical therapy because they express intermediate to low levels of HLA Class I, low levels of HLA Class II, and low levels of costimulatory molecules to avoid self-recognition by the immune system. (69)

In immunocompetent patients, MSCs have also been demonstrated to suppress allogeneic T-cell proliferation and evade alloreactive recognition. (70)

The immunomodulatory properties of MSCs are thought to involve the secretion of soluble mediators and cell-cell contact inhibition; however, the exact mechanisms of action are unclear. (71).

Autologous and allogeneic MSCs are currently being tested in clinical trials for a variety of  diseases including Crohn’s disease, multiple sclerosis, diabetes mellitus and end-stage liver disease, and to prevent transplant rejection and restore left ventricular function in patients with congestive heart failure. An open-label Phase II trial utilizing Prochymal ®, an allogenic MSC infusion in patients with severe Crohn’s disease, who were unresponsive to corticosteroids, infliximab (anti-TNF antibody), and other immunosuppressive therapies, has recently been completed. The study reported significant mprovements in symptoms as assessed by the Crohn’s disease activity index (CDAI). This has led to the approval by the Food and Drug Administration for a Phase III doubleblind, placebo-controlled trial of this therapy for the treatment of Crohn’s disease. (77)


Despite significant progress in our understanding of lung stem cells and their functional capacities over the past decade, much remains unknown about the processes involved in lung repair. Accumulating data from both animal models and clinical trials suggest that adult-derived stem cells may provide potential therapeutic strategies for lung repair in COPD.

Bone Marrow-Derived Progenitor Cells Promote Lung Tissue Repair In COPD And Lung Fibrosis.

The following article is a resume of the paper published by CHEST magazine (Bone Marrow-Derived Stem Cells and Respiratory Disease CHEST 2011; 140(1): 205 – 211) and written by Carla P. Jones, PhD; and Sara M. Rankin, PhD; from the Leukocyte Biology Section, National Heart and Lung Institute, Imperial College London, London, England.

Abbreviations: EOG-EPC= early outgrowth endothelial progenitor cell; EPC = endothelial progenitor cell; LOG-EPC = late outgrowth endothelial progenitor cell; MSC = mesenchymal stem cell; PDGFR = platelet-derived growth factor receptor; PH =pulmonary hypertension; VEGF = vascular endothelial growth factor


In recent years it has been discovered that in addition to the well-characterized hematopoietic stem cells, adult bone marrow contains nonhematopoietic stem cells, such as endothelial progenitor cells (EPCs), fibrocytes, and mesenchymal stem cells (MSCs). It is believed that under homeostatic conditions these cells are released from the bone marrow, circulate in the blood, and contribute to the repair of tissues in response to general wear and tear.

Endothelial Progenitor Cells

EPCs were first described in 1997 when Asahara et al. identified a population of mononuclear cells in the blood capable of differentiating into endothelial cells in vitro. It was proposed that these circulating progenitor cells may contribute to angiogenesis and vasculogenesis (formation of new blood vessels). Currently there are no specific markers to identify EPCs in mice or humans, but as EPCs proliferate clonally and produce colonies in vitro, specific colony assays can be generally performed to quantify numbers of EPCs in blood and tissues.

In these assays, mononuclear cells are plated onto fibronectin or gelatin and grown in the presence of endothelial growth factors (including vascular endothelial growth factor [VEGF]). It is now clear that there are two distinct subsets of EPCs: the first derived from hematopoietic lineage (also known as early outgrowth EPCs [EOG-EPCs]) and the second from endothelial lineage (late outgrowth EPCs [LOG-EPCs]).

EOG-EPCs are not believed to directly form new vessels, but they have been shown to secrete key proangiogenic factors, and they promote angiogenesis/vasculogenesis through a paracrine mechanism.

The other subset of EPCs is the LOG-EPCs that are observed in colony assays after 21 days in culture and exhibit cobblestone morphology that is a characteristic of mature endothelial cells. In contrast to the EOG-EPCs, LOG-EPCs possess the ability to form vessels in vitro and in vivo in lung and heart models.

It is well documented that the ELR 1 CXC chemokines can stimulate angiogenesis in vivo, and it is, therefore, of interest that EPCs have been reported to express the chemokine receptor CXCR2.

 CD34 is a cell surface marker expressed by hematopoietic progenitor cells and EPCs. In patients with COPD, levels of circulating CD34 cells are reduced compared with normal control subjects, and it has been suggested, that this reflect the increased trafficking of progenitor cells into the lungs. Levels of circulating CD34 cells appear to increase during exacerbations of COPD, which has been correlated with an elevation in the plasma levels of VEGF-A from these patients.

Finally, two small clinical trials carried out by a group in China have reported positive results examining the efficacy of EPC transplantation in patients with idiopathic pulmonary arterial hypertension, when given in addition to conventional therapy.

There is considerable evidence that bone marrow derived fibroblast progenitor cells, called fibrocytes, may also contribute to this process. It is believed that fibrocytes are recruited to sites of inflammation to stimulate repair by producing extracellular matrix molecules such as collagen I and III, vimentin, and fibronectin. These data suggest that blocking fibrocyte recruitment to the lungs may represent a potential therapeutic target to reduce lung fibrosis.

Mesenchymal Stem Cells (MCSs)

MSCs are multipotent stromal cells that can be isolated from numerous tissues, including the bone marrow, skeletal muscle, amniotic fluid, and adipose tissue. They are plastic adherent cells that exhibit trilineage differentiation into adipocytes, chondrocytes, and osteoblasts. Some studies have shown that these stem cells also have the ability to differentiate into neurons, myocytes, and skeletal muscle.

A unique feature of MSCs is their ability to produce a potent immunosuppressive effect both in vitro and in vivo.  Mechanistically it has been suggested that this may be due to their ability to secrete a range of immunomodulators,

MSCs can be readily harvested from bone marrow or adipose tissue and expanded ex vivo for use as a cell therapy. Impressive immunosuppressive effects of these cells have been reported in a wide range of models of disease, including acute graft-vs-host disease and multiple sclerosis.  With respect to respiratory disease, there are now a number of publications that report a reduction in disease pathology.

MSCs are currently being evaluated in several clinical trials for a variety of diseases, including Crohn’s disease, multiple sclerosis, diabetes mellitus, and acute graft-vs-host disease, with promising results reported from some of these trials.

Epithelial Progenitor Cells

Although there are endogenous epithelial progenitor cells in the adult lung. These studies suggest that bone marrow-derived progenitor cells may incorporate and differentiate into lung epithelium and thereby promote tissue repair.

Pharmacologic activation of endogenous stem cells represents an alternative approach to stem cell therapies. One strategy is to stimulate the mobilization of stem cells from the bone marrow in order to induce regeneration or immune modulation during disease development. We have recently shown that at a molecular level the mobilization of progenitor cell subsets (hematopoietic progenitor cells, vs MSCs and EPCs) is differentially regulated, suggesting that pharmacologic therapies could be developed to selectively mobilize a specific subset of stem cells from the bone marrow.

In our Hospital we harvest and concentrated the stem cells from bone marrow and peripheral blood and after these procedures they are injected into the lung in order to regenerate the lung tissue.

Mechanisms of cellular therapy in Respiratory Diseases

The present publication is a resume of the review made by Soraia C. Abreu et al. on October 2011 and published at Intensive Care Med (2011) 37:1421–1431.




Stem cells are undifferentiated cell groups that have varying degrees of self-renewal and differentiation capacity.

According to their origin, they can be divided into embryonic stem cells and adult stem cells. Despite having the ability to generate any terminally differentiated cell in the body, embryonic stem cells present important issues that limit their use; thus, most studies have focused on therapy with adult stem cells.

Several experimental studies have demonstrated that both recruited endogenous and delivered exogenous stem cells can home to and/or participate in rebuilding of missing or damaged lung tissue. Thus, there is a great deal of interest and research into stem cell therapies for lung diseases with no current effective treatment.

This article presents a critical review of advances in the field of stem cell biology, as well as highlighting the effects of mesenchymal stem cell (MSC) therapy in acute lung injury (ALI) /acute respiratory distress syndrome (ARDS), pulmonary fibrosis, and chronic obstructive pulmonary disease (COPD).

Adult stem cells

Adult stem cells (ASC) are the term used to describe postnatal stem cells that remain in body tissues throughout life. These cells can be found in well-protected, innervated, and vascularized niches, from which they are recruited to maintain tissue homeostasis.

If required, stem cells may undergo asymmetric cell division, generating one stem cell and a committed progenitor.

A number of specialized ASC niches have recently been identified in lung tissue in addition to Clara cells and type II pneumocytes (precursors of airway and alveolar epithelial cells, respectively): intercartilaginous regions of the tracheobronchial tree and neuroepithelial bodies in the bronchioles and bronchoalveolar duct junctions, which may significantly contribute to natural turnover or rebuilding of injured lung tissue. The angiogenic factor and fibroblast growth factor-2 (FGF-2) seems to be essential for resident stem cell activation in the lung.

ASC from various organs may be recruited for lung repair. Bone marrow is the main source of adult hematopoietic stem cells and mesenchymal stem cells.

Bone marrow cells

The term bone marrow-derived mononuclear cells (BMDMC) includes both hematopoietic and mesenchymal (nonhematopoietic) stem cell types. Hematopoietic stem cells (HSC) are nonadherent cells that have the ability to proliferate and differentiate into blood cells.

These are rare cells that represent only 1 in 104 to 1 in 105 of total blood cells in bone marrow, identified by CD34 and CD45 surface markers. During postnatal life, a steady state is established, in which the HSC pool size is maintained by regulation of self-renewal and differentiation.

This is possible because the bone marrow contains specialized niches in which the multipotency of HSC is preserved through cell division, while their progeny are directed towards lineage differentiation.

To define MSC (mesenchymal stem cells), minimum consensus criteria should be adopted:

(1)   Selection for a plastic-adherent cell population in standard culture conditions;

(2)   Expression of CD105, CD73, and CD90 and no expression of CD45, CD34, CD14, CD11b, CD79-, CD19, or HLA-DR surface molecules; and

(3)   Ability to differentiate into adipocytes, osteocytes, and chondrocytes in vitro

MSC are found in the stromal fraction of bone marrow, and provide support to hematopoiesis. Mesenchymal stemlike cells have also been recently identified in different tissues: adult peripheral blood, adipose tissue, skin, and lung.

MSC are capable of adopting the morphology and phenotype of parenchymal cells of many nonhematopoietic tissues, including the lung, where they can increase the number of fibroblast-like cells  and differentiate into bronchial epithelial cells and alveolar type I and II pneumocytes.

Mechanism of action

MSC may promote lung tissue repair through plasticity (the ability of ASC to cross lineage barriers and to adopt the expression profiles and functional phenotypes of cells unique to other tissues). The acquisition of a new phenotype by MSC may occur in different ways:

  1.  Differentiation: process by which an undifferentiated cell becomes structurally and functionally more complex and specialized. In the lung, MSC differentiation into more specialized type II pneumocytes may contribute to repair of disrupted alveolar surfaces, characteristic of many respiratory diseases.
  2. Transdifferentiation: refers to the ability of a committed cell to change its gene expression pattern without cell fusion. So far, no data have been published to support this theory.
  3. Cell fusion: MSC fusion with other cell types to form a heterokaryon, converting its gene expression pattern to that of the fusion partner. A recent experimental study has demonstrated that 20–50% of lung epithelial cells derived from MSC result from cell fusion.
  4. Lateral transfer of RNA: uptake of messenger RNA (mRNA) microvesicles derived from other cell types, with expression of protein translated from the mRNA that was taken up.

Since several studies have demonstrated that the effects of these cells on organ systems are currently attributed to a paracrine effect—an ability to secrete soluble factors that modulate immune responses of different diseases. This mechanism was first identified by observing that systemic administration of MSC was able to inhibit expression of several proinflammatory and profibrogenic cytokines in models of ALI and pulmonary fibrosis. In short, MSC or BMDMC did not act only through plasticity, but also through paracrine effects, interfering significantly with the pathophysiological processes of lung diseases.

Stem cell therapy

Stem cell therapy appears to be a promising strategy for treatment of many respiratory diseases with no effective treatment, such as ALI and its severe form ARDS, pulmonary fibrosis, and COPD. These lung disorders differ substantially in their time course and pathophysiology, but have one feature in common, namely the ability of the alveolar epithelium to recover after injury.

Stem cells in pulmonary fibrosis

Pulmonary fibrosis is a devastating disease with high mortality rate and no effective therapy to reverse or delay the natural course of disease. After disappointment with the effects of anti-inflammatory treatment, evidence has shown a role of abnormal alveolar repair and remodeling in the pathophysiology of fibrosis. Therefore, since MSC can yield alveolar epithelial repair and inhibit fibrogenesis, MSC therapy is a possible therapeutic tool for pulmonary fibrosis.

Stem cells in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease is characterized by repeated repair and destruction processes, with subsequent tissue remodeling as well as sustained and irreversible airflow limitation. No effective therapy is available for COPD.

A reduced number of progenitor cells has been observed in end-stage COPD patients and in experimental emphysema induced by elastase . These progenitor cells differentiate into many types of lung cells, mainly mature endothelial cells. Other studies have demonstrated that nonsmoker COPD patients present an increase in the number of progenitor cells that is proportional to bronchial obstruction and arterial oxygen tension, suggesting an intriguing compensatory effect of hypoxia on progenitor cell mobilization.

Different routes of administration have been described: intravenous, intra-arterial, and intratracheal.

A recent study has demonstrated that intravenous administration of MSC results in higher pulmonary engraftment compared with BMDMC and multipotent adult progenitor cells (MAPC). This occurs because MSC are larger and express many adhesion molecules, such as VCAM-1 and P-selectin, which facilitates their retention in the lung.

Once delivered, MSC are recruited to the injured tissue, where they promote release of cytokines and growth factors, contributing to a positive therapeutic outcome. In this line, the future of MSC therapy may lie in stimulation of specific mediators, such as growth factors and anti-inflammatory cytokines that participate in cell recruitment as well as endothelium and epithelium repair.

Some ongoing clinical trials are testing the safety and feasibility of cell-based therapy in respiratory diseases.

Conclusion: There is evidence for beneficial effects of MSC on lung development, repair, and remodeling. The engraftment in the injured lung does not occur easily, but several studies report that paracrine factors can be effective in reducing inflammation and promoting tissue repair.

Harvard Confirms the Stem Cell Existence in the Human Lungs


On May 12, 2011 the New England Journal of Medicine published a very exciting and interesting study of the Brigham and Women’s Hospital, Harvard Medical School, Boston; and the University of Parma, Italy; confirming strong and convincing evidence about the finding and isolation of Human Lung Stem Cells.

Researchers of those Universities found that human lungs contain undifferentiated human lung stem cells nested in niches in the distal airways.

This is a very important scientific finding because confirm the fact that stem cells exist in the human lung with the capacity to regenerate the pulmonary tissues and improve some diseases like chronic obstructive pulmonary disease (COPD), emphysema and fibrosis.

Many series of exciting reports over the last 5 to 10 years have demonstrated that adult bone marrow-derived stem cells may have more plasticity and are able to differentiate into bronchial and alveolar epithelium, vascular endothelium, and interstitial cell types, making them prime candidates for lung repair.

The above mentioned studies provide direct evidence that mesenquimal stem cells (MSCs) and other bone marrow-derived cells can be used for COPD, or other lung disease, but the precise mechanism underlying this process needs to be completely understood.

For example, Lobinger (London, UK) have shown that bone marrow-derived progenitors have the ability to differentiate and function as airway and parenchyma lung cells, but the medical community do not know what is the biology role between the resident lung stem cells and bone marrow-derived cells.

The hypothesis that bone marrow-derived cells may be used for regenerative purposes in lung diseases is confirmed by the fact that these cells engraft in the lung, acquire characteristics of certain respiratory cell populations and reduce the degree of respiratory fibrosis in the patient.

Due to the relevance of this fact, hereafter you will find a short overview of the main concepts of the Harvard and Parma Universities scientific paper related with the lung stem cells evidence. (Evidence for Human Lung Stem Cells, J. Kajstura, M. Rota, el al.; N Engl J Med 2011;364:1795-806.)

Various cell populations with some of the stem cells features have been described in distinct anatomical regions of the lung, but the demonstration of a noncommitted cell without specialized functions remains elusive.

In order to demonstrate the stem cell exist in the human lungs, researchers obtained samples of normal human lung tissue from unused donor organs, fetal lungs after cases of fetal death and samples of normal lung tissue from the Brigham and Women’s Hospital Thoracic Surgery Tissue Bank.

To establish whether the human lung possesses a stem-cell pool, they used the stem-cell antigen c-kit as a marker of identification and characterization. Their criteria for human lung stem cells were self-renewal, clonogenicity, and multipotentiality in vitro and in vivo. All samples were prepared for transplant.

After cells recovery, anesthetized female mice immunosuppressed with the use of cyclosporine underwent thoracotomy, and 2 to 3 mm2 of lung tissue was damaged with a steel probe that had been cooled in liquid nitrogen. The area adjacent to the damaged tissue then received six injections (with about 20,000 cells in each injection) of human lung stem cells mixed with labeled polystyrene microspheres. The mice chests were then closed and the animals were killed at 12 hours, 2 days, or 10 to 14 days after surgery. In some mice, lung tissue that had regenerated was excised, prepared and and injected into another mouse with the same cool damaged lung.

After 2 days, approximately 30% of the delivered cells were present within the damaged tissue and the bordering region, and after 10 to 14 days, clonal human lung stem cells had formed human bronchioles, alveoli, and pulmonary vessels, partly restoring the structural integrity of the mice lung tissue.  Thus, the clonal human lung stem cells showed self-renewal and multipotentiality in vivo.

Human lung stem cells generated bronchioles, approximately 30 to 250 μm in diameter, as well as small and intermediate-sized pulmonary arterioles approximately 20 to 70 μm in diameter and the newly formed human lung parenchyma replaced more than 30% of the original damaged tissue.

Ten to 14 days after cryoinjury and cell implantation, undifferentiated, human lung stem cells were identified within the regenerated human lung parenchyma and in the adjacent, intact recipient mouse lung. Approximately 20,000 human lung stem cells were present in each treated mouse.

After enzymatic digestion of the damaged lung, cells were recovered and delivered immediately to the cryoinjured portion of the lung of another new recipient mouse. Ten days after cell treatment, all treated mice were killed, and human bronchi, alveoli, and vessels were identified, documenting that the newly formed human lung structures derived from the serially transplanted human lung stem cells.

Those human lung stem cells that were detected in these mice, providing further evidence in support of the self-renewal and long-term proliferation of human lung stem cells in vivo. Furthermore, direct connections were found between preexisting pulmonary vessels and regenerated pulmonary vessels documenting the integration of temporally distinct preexisting (mouse) and new (human) segments of the pulmonary vasculature.

These results suggest that the human lung possesses a pool cells that have the fundamental properties of stem cells: they are selfrenewing, clonogenic, and multipotent in vitro and in vivo. The ability of human lung stem cells to create human bronchioles, alveoli, and pulmonary vessels in the mouse provides evidence in favor of the crucial role that human lung stem cells may have in lung homeostasis and tissue regeneration after injury. cells.

These findings do not rule out or challenge the notion that basal epithelial cells, bronchoalveolar stem cells, Clara cells, side population cells, and type II alveolar epithelial cells are involved in the epithelial-cell response to inflammation or injury.

Similarly, the presence of a human lung stem cell does not rule out the possibility that mature cells within the adult lung dedifferentiate or reprogram themselves to form a committed progeny, a phenomenon shown to be operative in other organ.

These findings, together with the results of our in vitro studies, provide evidence of a resident multipotent stem cell in the human lung.

In conclusion, this study provide several lines of evidence of the existence of human lung stem cells. Clonal human lung stem cells divided asymmetrically and generated bronchioles, alveoli, and pulmonary vessels of various dimensions, including capillaries, in vivo in a mouse model.

Furthermore, human lung stem cells obtained from regenerated lung tissue were able to selfrenew and create lung parenchyma in vivo in another mouse with lung damage. The immunohistochemical identification of newly regenerated pulmonary structures is strengthened by the recognition of human sex chromosomes and human transcripts of epithelial and vascular genes within the regenerated mouse lung.

Thus, human lung stem cells show self-renewal, clonogenicity, and multipotentiality and could be used in lung regeneration.