This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Stripp, B. R. Articles by Shapiro, S. D. Search for Related Content PubMed PubMed Citation Articles by Stripp, B. R. Articles by Shapiro, S. D.

Stem Cells in Lung Disease, Repair, and the Potential for Therapeutic Interventions

State-of-the-Art and Future Challenges

University of Pittsburgh School of Public Health, Pittsburgh, Pennsylvania

Steven D. Shapiro, Editor

American Journal of Respiratory Cell and Molecular Biology

Articles that are being published concurrently with this editorial in companion American Thoracic Society journals, the American Journal of Respiratory and Critical Care Medicine and the Proceedings of the American Thoracic Society, have focused attention on lung tissue repair and regeneration and cell types that may be of importance in considering therapeutic interventions. The purpose of this editorial is to put these articles in perspective with the growing literature describing basic science studies investigating cell types contributing to lung tissue repair, regeneration, and remodeling.

What Are Stem Cells?

Even though many cells with proliferative capacity exist within the lung and other organs, only a subset of these may be considered stem cells. Stem cells are relatively undifferentiated cells that can be distinguished from their more differentiated counterparts by virtue of their ability for unlimited self-renewal. Their undifferentiated character and capacity for unlimited self-renewal are typically conferred through interaction with supportive cells that are organized into a unique microenvironment referred to as a niche. A third defining characteristic of adult tissue stem cells is their infrequent proliferation relative to that of other cells with proliferative capacity within the tissue. Proliferation of the stem cell results from the depletion of other proliferative cells within the tissue and leads to replenishment of these transiently amplifying (TA) cells (commonly referred to as progenitor cells) through the differentiation of one daughter cell. For long-term maintenance of the stem cell, its proliferation must be accompanied by at least one of the progeny retaining the stem cell character of its parent. The differentiation potential of a tissue stem cell and the range of TA cells that may be generated are largely governed by the cellular and anatomic complexity of the tissue in which it resides.

Sources of Stem Cells for Lung Repair, Regeneration, and Remodeling

Endogenous Lung Stem Cells
Progenitor cells that participate in the maintenance and repair of injured lung epithelium have been described for tracheobronchial, bronchiolar, and alveolar compartments (1–3). The existence of putative lung tissue stem cells has only been suggested relatively recently through the use of rodent injury models in which abundant progenitor cells are depleted through either chemical or physical means (4–7). At least three distinct regions have been described that support populations of lung tissue stem cells: intercartellagenous regions of tracheobronchial airways (4), neuroepithelial bodies (NEB) in bronchioles (6), and the bronchoalveolar duct junction (BADJ) (5, 7). Each region harbors putative tissue stem cells that share the common properties of a relatively undifferentiated phenotype, infrequent proliferation (demonstrated through use of DNA label retention assays), and a differentiation potential that is appropriate for the compartment in which they reside. A combination of immunophenotypic and cell ablative strategies has been employed to demonstrate that bronchiolar stem cells residing within both the NEB and BADJ microenvironments represent a Clara cell secretory protein (CCSP)-expressing variant Clara cell, and that the BADJ-associated population are dual positive for both CCSP and pro–surfactant protein C. Recent studies by Kim and colleagues suggest that CCSP/SP-C dual positive cells can be enriched based upon their unique cell surface phenotype (Sca1⁺, CD34⁺, CD31^–, CD45^–) and maintained long-term in vitro (7). This report provides an initial step toward establishment of much-needed in vitro culture models to further explore the molecular phenotype and regulation of bronchiolar stem cells, in addition to making inroads toward development of cell-based therapeutic approaches.

Recruited Cells in Lung Repair and Remodeling
Much excitement over the past five years has been generated over the possibility that nonresident cells, such as those derived from the bone marrow, may have the potential to graft within the lung and assume phenotypic properties of lung epithelial cells. This concept evolved from initial studies that used either reconstitution of the hematopoietic lineage within myeloablated recipient mice (8) or introduced crude preparations of marrow stromal cells into the venous circulation (9). These initial reports suggesting that engraftment results in the efficient generation of lung epithelium have not been reproduced in more recent studies, some of which suggest that this is a very inefficient process (10), others which suggest that it is an experimental artifact (11, 12). Despite this, there is strong evidence that marrow-derived cells do have the capacity to engraft and contribute to the mesenchymal compartment of the lung, the consequences of which may be either beneficial (13) or harmful (14, 15), depending upon the lineage of engrafting cells.

Specialized Cells Derived from Embryonic Stem Cells
Differentiating cultures of embryonic stem cells have been proposed as an alternative source of committed lung progenitor or stem cells. A number of reports have now been published describing conditions that favor differentiation of mouse embryonic stem cells into heterogeneous populations that include subsets of cells capable of expressing lung epithelial cell-specific markers such as surfactant protein C (SP-C) and Clara cell secretory protein (CCSP), in addition to cell types with morphologic characteristics of bronchial or alveolar epithelial cell types (16, 17). Further manipulation of culture conditions used for the directed differentiation of ES cells may have potential to enhance the efficiency with which lung progenitor cells are generated in vitro (18). Extrapolation of these studies using murine ES cells to the limited repertoire of available human ES cell lines has not yet been attempted.

Future Needs and Anticipated Difficulties

1. How are stem cells maintained within the niche and what regulates their differentiation?
   
2. With improvements (current and future) in our understanding of molecular signaling pathways regulating stem cell behavior, is it possible to exploit this new knowledge for pharmacologic manipulation of stem cells and their progeny?
   
3. How is the stem cell and its progeny impacted by changes in the cytokine and growth factor milieu of the chronically diseased lung?
   
4. What opportunities exist for the generation and expansion of autologous cells for transplantation?
   
5. Basic studies are needed to develop and optimize methods for the isolation, purification, in vitro expansion, and in vivo validation of lung stem cells.
   
6. How might cell-based therapeutic interventions be considered in light of known differences between airway regions in the cellular organization and regulation of region-specific stem cells?
   
7. What mechanisms govern the observed phenotypic plasticity of engrafting marrow-derived cells and the efficiency of their engraftment?
   
8. How might the beneficial effects of engrafting cells be harnessed while avoiding the potential harmful effects associated with engraftment of exogenously supplied cells within lung tissue?
   

Translation of Stem Cell Therapy to the Clinical Setting

As we learn more about endogenous sources of stem cells and their properties, we will be in a position to determine the role of stem cells in disease pathogenesis and be in a position to augment their function and repair the lung. In addition, as we develop means to isolate and manipulate stem cells in vitro, we will be able to re-introduce stem cells to the injured lung.

Will fixing/addition of stem cells lead to formation of new airway and alveoli with restoration of lung function? Unfortunately, the lung is composed of a complicated three-dimensional structure requiring precise alignment of airspace and vasculature to exchange gas. The extracellular matrix plays a major role in both this structural organization, and not surprisingly, in many of the diseases that afflict humans. In pulmonary fibrosis there is excess accumulation of collagen and other matrix proteins. Remodeling of this matrix would be required to restore lung function to the affected airspaces. Emphysema, on the other hand, is characterized by loss of alveolar units as a result of matrix proteolysis (followed by cell death). Restoration of functional alveolar units may require the addition of "scaffolding" to allow migration of stem cells and formation of alveoli. Overall, these are daunting tasks, yet the achievement of these goals would have a dramatic effect on the health of our patients.

Footnotes

Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

References

1. Evans MJ, Cabral-Anderson LJ, Freeman G. Role of the Clara cell in renewal of the bronchiolar epithelium. Lab Invest 1978;38:648–653.
2. Evans MJ, Dekker NP, Cabral-Anderson LJ, Freeman G. Quantitation of damage to the alveolar epithelium by means of type 2 cell proliferation. Am Rev Respir Dis 1978;118:787–790.[Medline]
3. Evans MJ, Shami SG, Cabral-Anderson LJ, Dekker NP. Role of nonciliated cells in renewal of the bronchial epithelium of rats exposed to NO2. Am J Pathol 1986;123:126–133.[Abstract]
4. Borthwick DW, Shahbazian M, Krantz QT, Dorin JR, Randell SH. Evidence for stem-cell niches in the tracheal epithelium. Am J Respir Cell Mol Biol 2001;24:662–670.[Abstract/Free Full Text]
5. Giangreco A, Reynolds SD, Stripp BR. Terminal bronchioles harbor a unique airway stem cell population that localizes to the bronchoalveolar duct junction. Am J Pathol 2002;161:173–182.[Abstract/Free Full Text]
6. Hong KU, Reynolds SD, Giangreco A, Hurley CM, Stripp BR. Clara cell secretory protein-expressing cells of the airway neuroepithelial body microenvironment include a label-retaining subset and are critical for epithelial renewal after progenitor cell depletion. Am J Respir Cell Mol Biol 2001;24:671–681.[Abstract/Free Full Text]
7. Kim CF, Jackson EL, Woolfenden AE, Lawrence S, Babar I, Vogel S, Crowley D, Bronson RT, Jacks T. Identification of bronchioalveolar stem cells in normal lung and lung cancer. Cell 2005;121:823–835.[CrossRef][Medline]
8. Krause DS, Theise ND, Collector MI, Henegariu O, Hwang S, Gardner R, Neutzel S, Sharkis SJ. Multi-organ, multi-lineage engraftment by a single bone marrow-derived stem cell. Cell 2001;105:369–377.[CrossRef][Medline]
9. Kotton DN, Ma BY, Cardoso WV, Sanderson EA, Summer RS, Williams MC, Fine A. Bone marrow-derived cells as progenitors of lung alveolar epithelium. Development 2001;128:5181–5188.[Medline]
10. Loi R, Beckett T, Goncz KK, Suratt BT, Weiss DJ. Limited restoration of cystic fibrosis lung epithelium in vivo with adult bone marrow–derived cells. Am J Respir Crit Care Med 2006;173:171–179.[Abstract/Free Full Text]
11. Chang JC, Summer R, Sun X, Fitzsimmons K, Fine A. Evidence that bone marrow cells do not contribute to the alveolar epithelium. Am J Respir Cell Mol Biol 2005;33:335–342.[Abstract/Free Full Text]
12. Kotton DN, Fabian AJ, Mulligan RC. Failure of bone marrow to reconstitute lung epithelium. Am J Respir Cell Mol Biol 2005;33:328–334.[Abstract/Free Full Text]
13. Ortiz LA, Gambelli F, McBride C, Gaupp D, Baddoo M, Kaminski N, Phinney DG. Mesenchymal stem cell engraftment in lung is enhanced in response to bleomycin exposure and ameliorates its fibrotic effects. Proc Natl Acad Sci USA 2003;100:8407–8411.[Abstract/Free Full Text]
14. Hashimoto N, Jin H, Liu T, Chensue SW, Phan SH. Bone marrow-derived progenitor cells in pulmonary fibrosis. J Clin Invest 2004;113:243–252.[CrossRef][Medline]
15. Phillips RJ, Burdick MD, Hong K, Lutz MA, Murray LA, Xue YY, Belperio JA, Keane MP, Strieter RM. Circulating fibrocytes traffic to the lungs in response to CXCL12 and mediate fibrosis. J Clin Invest 2004;114:438–446.[CrossRef][Medline]
16. Ali NN, Edgar AJ, Samadikuchaksaraei A, Timson CM, Romanska HM, Polak JM, Bishop AE. Derivation of type II alveolar epithelial cells from murine embryonic stem cells. Tissue Eng 2002;8:541–550.[CrossRef][Medline]
17. Coraux C, Nawrocki-Raby B, Hinnrasky J, Kileztky C, Gaillard D, Dani C, Puchelle E. Embryonic stem cells generate airway epithelial tissue. Am J Respir Cell Mol Biol 2005;32:87–92.[Abstract/Free Full Text]
18. Rippon HJ, Polak JM, Qin M, Bishop AE. Derivation of distal lung epithelial progenitors from murine embryonic stem cells using a novel 3-step differentiation protocol. Stem Cells 2006; (Feb):2. (Epub ahead of print).

This article has been cited by other articles:

				V. Sueblinvong, R. Loi, P. L. Eisenhauer, I. M. Bernstein, B. T. Suratt, J. L. Spees, and D. J. Weiss Derivation of Lung Epithelium from Human Cord Blood-derived Mesenchymal Stem Cells Am. J. Respir. Crit. Care Med., April 1, 2008; 177(7): 701 - 711. [Abstract] [Full Text] [PDF]

				V. Dave, S. E. Wert, T. Tanner, A. R. Thitoff, D. E. Loudy, and J. A. Whitsett Conditional Deletion of Pten Causes Bronchiolar Hyperplasia Am. J. Respir. Cell Mol. Biol., March 1, 2008; 38(3): 337 - 345. [Abstract] [Full Text] [PDF]

				L-P. Boulet and P. J. Sterk A new series on airway remodelling Eur. Respir. J., February 1, 2007; 29(2): 231 - 232. [Full Text] [PDF]

				G. P. Fadini, A. Avogaro, and C. Agostini Pathophysiology of circulating progenitor cells in pulmonary disease and parallels with cardiovascular disease. Am. J. Respir. Cell Mol. Biol., September 1, 2006; 35(3): 403 - 404. [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Stripp, B. R. Articles by Shapiro, S. D. Search for Related Content PubMed PubMed Citation Articles by Stripp, B. R. Articles by Shapiro, S. D.