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First published online June 27, 2005
Stem Cells Vol. 23 No. 8 September 2005, pp. 1073 -1081
doi:10.1634/stemcells.2005-0039; www.StemCells.com
© 2005 AlphaMed Press

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CONCISE REVIEWS

Identification of Novel Resident Pulmonary Stem Cells: Form and Function of the Lung Side Population

Susan M. Majkaa, Michelle A. Beutzb, Moira Hagenb, Angelo A. Izzod, Norbert Voelkelb, Karen M. Helmc

a Department of Medicine, Cardiovascular Pulmonary Research Section,
b Department of Medicine, Pulmonary Hypertension Center, and
c Cancer Center, Flow Cytometry Core, University of Colorado Health Sciences Center, Denver, Colorado, USA;
d Colorado State University Department of Microbiology, Immunology & Pathology, Ft. Collins, Colorado, USA

Key Words. Side population • Lung side population stem cells • Adult stem cells

Correspondence: Susan Majka, Ph.D., Department of Medicine, Cardiovascular Pulmonary Research Section, University of Colorado Health Sciences Center, 4200 East 9th Avenue, SOM 3811, mail stop B-133, Denver, Colorado 80262, USA. Telephone: 303-883-8786; Fax: 303-315-4871; e-mail: Susan.majka{at}uchsc.edu


   ABSTRACT
Top
 Abstract
Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
Resident lung stem cells function to replace all lineages of pulmonary tissue, including mesenchyme, epithelium, and vasculature. The phenotype of the lung side population (SP) cells is currently under investigation; their function is currently unknown. Recent data suggest lung SP cells are an enriched tissue-specific source of organ-specific pulmonary precursors and, therefore, a source of adult stem cells. The adult lung SP cell population has been isolated and characterized for expression of markers indicative of stem cell, epithelial, and mesenchymal lineages. These studies determined that the adult mouse lung SP has epithelial and mesenchymal potential that resides within a CD45– mesenchymal subpopulation, as well as limited hematopoietic ability, which resides in the bone marrow–derived CD45+ subpopulation. The ability to identify these adult lung precursor cells allows us to further study the potential of these cells and their role in the regulation of tissue homeostasis and response to injury. The identification of this target population will potentially allow earlier treatment and, long term, a functional restoration of injured pulmonary tissue and lung health.


   INTRODUCTION
Top
 Abstract
 Introduction
Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
Resident stem cell populations have been identified in a variety of adult tissues, including lung. These cells likely contribute to local tissue regeneration throughout life and as such represent a target population that may be studied and manipulated to functionally restore injured pulmonary tissue. The lung stem cell potential may be compromised in various diseases, such as pulmonary hypertension (PH), pulmonary fibrosis (PF), and emphysema. A delineation of the mechanisms by which stem cells fail to regenerate local tissue, whether due to inappropriate terminal differentiation or apoptosis, is vital to understanding the pathology of lung diseases to identify appropriate targets for therapy. Studying how the microenvironment under these circumstances influences stem cell differentiation may also aid in the development of stem cell–based regeneration strategies, i.e., autologous bone marrow (BM)–based therapies, stimulation of local precursors, and gene therapy.

Hematopoietic stem cells (HSCs) and multipotent adult progenitor cells (MAPCs) derived from BM are well-studied populations of pulmonary precursor cells and may be important for autologous therapies [1]. However, the phenotypes and origin of true resident stem cells within the lung remain largely undefined. Resident lung stem cells may function to replace pulmonary tissue, including epithelium, mesenchyme, and vasculature. Tissue-specific stem cells are typically located at a specialized site, proximal to the cell type they will regenerate. The epithelial precursor cell niches in the lung are well studied and have been extensively reviewed (Table 1Go). In the lung, epithelial stem cells include the type II pneumocyte in the alveolus and the Clara cell in the bronchiole. Pulmonary epithelial precursors have also been identified in the tracheal submucosal gland ducts. The microenvironment, such as underlying mesenchyme, in these instances is believed to influence stem cell commitment to specific lineages [2–5]. Recently, a more primitive putative adult stem cell population has been identified in the lung, the lung side population (SP) of cells, which seems to have both mesenchymal and epithelial potential (Fig. 1Go) [6–9]. To date, no resident pulmonary vascular stem cell has been described.


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  		Table 1. Potential pulmonary stem cells
 


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  		Figure 1. Lung side population (SP) profile. (A): SP cells may be isolated from adult lung via Hoechst 33342 staining and fluorescence-activated cell sorting analysis. (B): SP cells are defined by the presence of epithelial and mesenchymal markers. The SP cells represent less than 1% of the total lung cells. The viability of cells after staining and sorting is typically greater than 85%. (C): Upon isolation, the cells appear round and bright, similar to bone marrow SP. Subpopulations of lung SP may represent epithelial precursors and in vitro begin to express (D) cytokeratin and (E) epidermal growth factor receptor. Magnification x 100.

 

   STEM CELL CRITERIA
Top
 Abstract
 Introduction
 Stem Cell Criteria
Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
Over the last few years, criteria have evolved for the identification of true stem cells. These criteria include the ability to self-renew as well as the ability of a single cell to functionally differentiate into cells of the tissue of origin both in vivo and in vitro [10–13]. Self-renewal may be defined as the ability to generate progeny with the same characteristics as the parent cell without undergoing senescence. BM HSCs are the best-studied adult stem cells that meet these criteria. However, many adult precursor cells have been termed stem cells but lack rigorous characterization with regard to identification of changes in the expression of markers of lineage differentiation for a specific cell/tissue type, functional properties, and the ability to fill an appropriate host tissue niche in vivo. Stem cells may be categorized into three subpopulations based on potential for plasticity: totipotent, or having the capacity to regenerate all germ layers and form an organism; pluripotent, or having the capacity to regenerate all germ layers but unable to reconstitute the organism; and multipotent, or having the capacity to give rise to several cell types. There is mounting evidence that resident pulmonary stem cells exist and may arise both from local lung tissue and BM. The cells isolated and studied have vascular, epithelial, and limited hematopoietic potential. However, the rigorous application of stem cell criteria to further evaluate these populations is necessary.


   CHARACTERISTICS OF THE SP
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
SP cells, regardless of tissue origin, are identified by their unique fluorescence-activated cell sorting (FACS) profile. When separated by a flow cytometer with a UV laser, SP cells are distinct from cells that take up the Hoechst 33342 dye. When stained with the vital DNA dye, Hoechst 33342, and excited by a UV (351- to 364-nm) laser, the SP cells exhibit a low blue (440- to 460-nm) and low red (>675-nm) fluorescent staining pattern. This pattern presents as an SP tailing off the main G0-G1 population (Fig. 1Go) and is created by an efflux of the Hoechst 33342 dye from the SP cells. This efflux is the result of the presence of a multidrug resistance-like (MDR) transporter in the SP cells (Figs. 1A, 1BGo). Strict adherence to the staining protocol as to timing, temperature, cell concentration, and Hoechst dye concentration is necessary to accurately identify the SP. It is helpful to include propidium iodide in the stain mixture as a dead cell discriminator. After staining, the sample must be maintained at 4°C to prevent additional efflux of the Hoechst 33342 dye. Additionally, the flow cytometer must be properly aligned for linear signal collection, and a high number of events (at least 100,000) must be collected to ensure statistical validity of the data.

SP cells are negative for all hematopoietic lineage (lin–) markers, and BM-derived SP cells reconstitute lethally irradiated mice in smaller numbers compared with the whole BM HSC compartment [14–16]. These SP cells were initially identified in BM by Goodell and colleagues [14, 15] and functionally characterized in transplantation analyses as enriched primitive hematopoietic precursors, lacking all markers of differentiated blood lineages [14, 15]. Further functional analyses have shown this population as well as single-cell derivatives to have multipotent stem cell ability; they can engraft into cardiac myocytes/fibers, vascular endothelium, liver, and skeletal muscle [16–19]. These studies along with novel technology to identify an HSC population using flow cytometry formed the basis for the identification of putative resident adult tissue-specific stem cell populations (Table 1Go) [20, 21]. SP cells have been identified in various tissues, such as skeletal muscle, liver, lung, brain, kidney, heart, intestine, mammary, and spleen, and in tumors associated with these organs [6, 14, 15, 19, 22–24].


   RESIDENT LUNG SP CELLS
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
The HSC potential of the lung SP cells has been evaluated initially in vitro using methocellulose assays, subfractionating the SP cells into CD45+ and CD45– populations [6], and subsequently in vivo using BM transplantation analyses [25–27]. Asakura and Rudnicki [6] demonstrated that whole-lung tissue gave rise to hematopoietic colonies and followed with the study of the lung SP cells subfractionated by CD45 reactivity. Typically, the CD45+ fraction of cells includes HSCs, whereas the CD45– fraction is associated with local mesenchymal-type precursors [16, 28–30]. The hematopoietic ability resided within the CD45+ fraction, but only granulocytic and monocytic colonies were detected. Therefore, unlike BM and skeletal muscle, the lung cells did not have HSC activity or the ability to generate all hematopoietic lineages. The specific hematopoietic precursor ability is maintained by local endothelial cells and differs from tissue to tissue [31]. This idea is further supported by studies by Abe and colleagues [22–27], who evaluated the hematopoietic reconstitution potential of lung cells in vivo after sublethal irradiation (7 Gy). Whole lung cells (1 x 106) engrafted into peripheral blood lineages (B220, Mac-1, GR-1, CD4/8 positive) at a rate of 30%, undetectable until 5 weeks after transplant, whereas lung SP cells (2 x 103) did not stably engraft and were not detectable. The authors suggested that the small number of SP cells compared with the whole lung cells was a potential cause of lack of engraftment. However, competitive transplants and serial transplantation may have been more appropriate than sublethal irradiation, in which the host BM may have had an advantage over the lower numbers of lung SP cells, as also evidenced by the lack of detection of donor-derived progeny in periphery until 5 weeks. In further independent experiments, CD45+ lung cells (2.5 x 106) also engrafted at a rate of 30%, whereas CD45– lung cells (2.5 x 106) were detected at less than 5% in peripheral blood, and by 24 days after transplant, the survival rate was 0.5 and 0, respectively. The survival in this instance was much lower than the whole-lung transplantation, in which engraftment was evaluated up to 9 weeks. However, despite these data, the authors have concluded that HSC activity is resident within the lung. Interestingly, the lung SP cells were found to be negative for cytokeratin upon isolation but upregulated the expression of cytokeratin when cultured under epithelial differentiation conditions, illustrating the potential for epithelial lineage-committed differentiation.

To date, Summer and colleagues [8, 9] have begun the initial characterization of the origin and evolution of lung SP cells and their epithelial differentiation potential. The cells were found to be lin–, bcrp1+; CD45+ (60%–70%) Sca-1+, CD34+; and CD45– (30%–40%) Sca-1+ and CD34–. RNA for the transcription factor HNF3b was detected in the lung SP cells, whereas TTF-1, a marker of a more differentiated epithelial cell, was not detected. Studies to define the localization in vivo using bcrp1, the MDR transporter, were unsuccessful because the functional protein did not correlate with message expression. The hunt to localize these cells in the lung is ongoing. To define the origin of lung SP cells, BM-SP cells were transplanted into lethally irradiated recipients and the lung SP analyzed for donor cells 8 months later. After irradiation, there is lung injury and fibrosis in animal models, and myofibroblasts in these instances have been identified as having a BM origin [32–37]. Although this is not an ideal model to study the normal lung SP, Summer has demonstrated BM-SP as the origin of the CD45+ fraction and a small portion of the CD45– cells, suggesting that the CD45– cells are derived from the host pulmonary tissue. The small fraction of the CD45– population may be the result of BM fibrocyte recruitment after the nonspecific insult to the lung. Also, CD45 membrane expression is downregulated on hematopoietic cells during apoptosis, even when there is no detectable annexin V expression [38]. Transcription factors indicative of hematopoietic cells, GATA-1, GATA-2, and PU.1, were detected in CD45+ cells, and they were absent in CD45– cells. The CD45+ and CD45– cells were also heterogeneous for the mesodermal and ectodermal markers smooth muscle alpha actin, keratin, and cytokeratin. The CD45– population expressed a significantly higher level of the mesenchymal marker smooth muscle alpha actin. These results suggested that subpopulations exist within the lung SP with both ectodermal and mesodermal potential, which likely have distinct functions. Giangreco et al. [7] have also begun to characterize the airway CD45– lung SP, although whole lung tissue was the source of cells. The CD45– cells expressed both vimentin and Clara cell secretory protein message. In contrast to the aforementioned studies, Giangreco et al. [7] concluded that the lung SP is a source of lung stem cells enriched for the presence of airway epithelial stem cells; morphological or functional data would further support this conclusion. A limitation raised by the authors of this study was toxicity of H33342 [GenBank] staining.

Based on marker expression, there exists phenotypic heterogeneity within lung SP cells. The cells lack markers of differentiated cell types while expressing markers of some early lineage commitment (Table 1Go). This variation may be explained by the presence of multiple cell lineages in the lung, to which this population contributes. Because the CD45– population lacks markers of differentiated cell types and lacks hematopoietic potential, this population likely represents a lung niche for mesenchymal stem cell (MSC) activity. MSCs were traditionally considered stem cells residing in the BM, capable of giving rise to cells of the adipocyte, fibroblast muscle, and bone lineages [28, 39]. Recent evidence of stem cell plasticity is consistent with the presence of MAPCs, which copurify with MSCs in the BM and subsequently also in the dorsal aorta, muscle, and brain [28, 39–41]. SP cells derived from tissues other than BM may represent a multipotent tissue-specific population of cells. The presence of telomerase activity also serves as a criterion to identify these cells [42–44]. However, to truly define a stem cell as resident within the SP, clonal analysis must be performed.

No uniform protocol has been established for the isolation of lung SP cells, which raises the issue as to the true comparative nature of the cell populations being studied. Variability lies in many steps along this process, including the efficiency and viability of cell isolation from the lung tissue, the Hoechst 33342 dye concentration and staining conditions, and, lastly, the calibration of instrumentation available for collection and analysis of the cells. Viability issues with the protocol have been raised by Giangreco et al. [7], and consistency in the FACS profile, percentage of SP cells, and viability have not yet been established among groups. The absolute numbers and distribution of the SP profile remain problematic. These variations may be attributed to altered concentrations in Hoechst dye: A higher dye concentration relative to optimal cell concentration or prolonged incubation can result in a stunted SP arm closer to the non-SP population in the FACS profile and decreased percent cell number and viability, whereas the converse is true for too-low dye concentration and incubation times. Also, because the SP represents such a small fraction of the total cell population isolated from the lung, the cells are considered rare events. The percentage of SP cells will change depending on how many total lung SP cells are collected from the total whole lung cell population under analysis; for example, 500 to 5,000 SP cells can offer a significantly different result as far as the percentage of total cells. The more SP cells collected (>1,000) in the gate for analysis of percentage of total lung cells, the less variability. Less variability in the quantitation of SP as a percentage of total cells also allows the distinction between subtle changes in this group of rare events. Issues of overcompensation to detect multiple fluorescent markers can also lead to artifacts in data interpretation. Therefore, it is impor-tanttoconsiderthattheisolation/characterizationoflungSPcells (as lung stem cells) is in a very early stage [15]. A multi-investigator consensus of data must be established as to the population of cells under study. The detailed characterization and separation of lung SP subpopulations will allow the differentiation between cell origin and the ability to isolate a more homogeneous group. The characterization of stem cell, epithelial, mesenchymal, and vascular-specific factors expressed by lung SP will allow us to identify these cells as true precursors. The more readily these cells are identifiable in vitro, the more easily we may detect them and assess their potential in vivo.

We may then begin to assess the functional significance for the CD45+ BM-derived cells, which are resident within the lung as well as the CD45– lung-derived putative stem cell population (Fig. 2Go). We can at present only speculate that the hematopoietic CD45+ BM-derived cells may be part of the normal lung immune surveillance as well as serving the function to replenish depleted local stem cell niches. They may be early responders to local infection or inflammation, leaving the BM entering the circulation as immature hematopoietic cells. These cells may play an important role in the maintenance of homeostasis in the lung. There is differential expression of multiple CD45 isoforms via the alternative splicing of exons, which encode sections of the extracellular domain [45, 46]. On mouse T cells, there is variable CD45RB as the T cell encounters antigen on a dendritic cell and becomes activated; i.e., CD45RB is a lineage marker for the state of differentiation of CD4 T cells. Perhaps CD45 variants on these resident lung BM-derived/hematopoietic cells may be exploited to identify their location as well as differentiation state. Other than the lack of hematopoietic ability and potential for epithelial and mesenchymal differentiation, little is known about the CD45– lung SP cells.



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  		Figure 2. Potential of the lung side population (SP) cell population. CD45+ lung SP cells are bone marrow–derived and retain hematpoietic function. The CD45– fraction is a resident lung population that has been characterized to have both epithelial and mesenchymal lineage potential.

 

   PULMONARY TRANSDIFFERENTIATION
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
The Potential for Lung...
 Conclusion and Future Goals
 References
 
Transdifferentiation involves the conversion of a lineage-determined cell into another phenotypically distinct cell type. Although transdifferentiation of many cell types has been reported, single-cell or clonal analyses have not been performed to rule out the possibility that the population under investigation did not contain multiple cells types, or "uncommitted" cells, which then differentiate rather than "trans" differentiate. The most well-characterized lung epithelial is the alveolar type II cell, which gives rise to the alveolar epithelial type I cell, both of which maintain alveolar homeostasis [47, 48]. Danto et al. [47], using isolated type II cells, demonstrated that the commitment of type II cell to the type I lineage required continuous regulation via external signals and that, in the absence of these stimuli, the cell types were capable of reversible transdifferentiation between the two phenotypes [47]. The evidence for transdifferentiation between these two epithelial cell types is the focus of extensive study; generally accepted functional criteria have been established and summarized by Fehrenbach [48]. As far as the occurrence of additional transdifferentiation events in the lung, there are data suggesting that vascular smooth muscle cells and fibroblasts may be derived from pulmonary artery (PA) endothelial cells [49], alveolar type II cells may be derived from tracheal epithelium [3, 4], and myofibroblasts may be derived from lipofibroblasts [50, 51]. Most recent reports of mature PA vascular endothelial cells transdifferentiating into smooth muscle suggest that such a process may contribute to both tissue regeneration and pathological vascular remodeling in response to injury. Shannon et al. [4] isolated embryonic tracheal epithelium and demonstrated that in the absence of appropriate inductive mesenchymal signals, these cells had the potential to assume an alveolar type II cell phenotype with surfactant protein C expression; however, this ability is developmentally restricted. Boros et al. [50] demonstrated the influence of increased oxygen tension on the transition from a fetal lipofibroblast to a more contractile myofibroblast. These studies suggest that nonconventional pathways of cell differentiation may contribute to pulmonary regeneration in response to injury. Thus, true transdifferentiation of a committed cell type has not been rigorously demonstrated. Although these studies provide a convincingly detailed characterization of both vascular and epithelial transdifferentiation, they require the evaluation of differentiation on a clonal level.


   THE POTENTIAL FOR LUNG STEM CELLS TO CONTRIBUTE TO PULMONARY DISEASE
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
Conclusion and Future Goals
 References
 
Stem cells may contribute to the pathogenesis of several adult pulmonary diseases either through their contribution to a hyperproliferative response or negatively via an impairment or depletion of local regenerative cells. Examples of hyperproliferative disease characteristics that may be attributed to pulmonary stem cells include adenocarcinoma and the plexiform lesions in primary pulmonary hypertension. Both of these processes are thought to arise from the monoclonal expansion of cells [52]. Adenocarcinoma, a common type of lung cancer, is the leading cause of cancer deaths to date in the U.S. and Japan. Most tumors occur at the periphery of the lung and may be asymptomatic until late in their progression. Due to their diagnosis during relatively late stages of progression, their exact cellular origin is unclear and subsequent transformation remains uncharacterized. However, adenocarcinoma has been defined as malignancy that begins in cells derived from lung glandular epithelium. Resident adult stem cell populations are a reservoir for precancerous cells; many quiescent or slow-to-divide cells harbor mutations that arose during the exponential growth phase during development [53, 54]. MSCs transduced with telomerase exhibit characteristics of transformed cells [43, 55]. These cells contribute to tissue remodeling during normal turnover and following injury or an insult, resulting in a second mutation. Although the frequency of these mutated adult stem cells is low, they substantially contribute to the total risk of cancer [53, 56]. There is evidence that tumors are clonal, i.e., representing the offspring of a single cell [55, 56]. The recent studies by Al-Hajj et al. [56] using mammary stem cells demonstrated that characterizing the phenotype as well as the function of the isolated cell populations was pivotal in the identification of an adult stem cell population with 50-fold enriched ability to form cancer cells. Hirschmann-Jax et al. [23] identified SP cells in human cancers such as lung, breast, neurblastoma, and glioblastoma. These cells have an increased ability to externalize chemotherapeutic agents. If we can identify these adult lung stem cells, it will allow us to further study their tumorigenic potential as well as facilitate the understanding of pathways that regulate neoplastic cell growth and survival. Thus, a target population for therapeutic intervention is likely to emerge.

Primary pulmonary arterial hypertension is characterized by endothelial cell dysfunction, which causes constriction and remodeling of the pulmonary vasculature [57]. The onset of primary PH is characterized by elevated mean arterial pulmonary blood pressure, with an increase in pulmonary vascular resistance, resulting in end-stage right ventricular failure. In addition to endothelial cell dysfunction, sequelae include vessel remodeling involving intimal thickening, medial hypertrophy, smooth muscle hyperplasia, and plexiform lesions of intraluminal monoclonal endothelial cell proliferation that occlude precapillary pulmonary arteries [46]. Stem cell contribution to monoclonal plexiform lesions obstructing pulmonary vasculature during primary PH is unknown and under study [58]. Pulmonary mesenchyme in the developing embryo is a source of vascular cells, and, therefore, in addition to circulating endothelial precursors, the adult mesenchyme provides a resource for local candidate endothelial cell precursors [2, 59].

Idiopathic pulmonary fibrosis (IPF) and emphysema are two examples of the depletion of local regenerative cells, in part attributed to abnormal pulmonary stem cell localization and differentiation. IPF may be the result of an initial injury, which provokes either an unresolved inflammatory response or alveolar epithelial cell activation resulting in elevated cytokine levels, inappropriate epithelial-mesenchymal signaling, and a disordered wound-healing response [60–63]. There is an inability of alveolar type II cells to replenish the alveolar epithelium [61, 63] that results in fibroblast proliferation, the formation of fibroblast foci, and the appearance of transitional phenotypic cells, possibly through transdifferentiation [61, 62].

In emphysematous lungs, we find destruction of lung parenchyma, enlargement of airspaces, with fibrosis of small airways [64]. Tobacco smoke is the most common cause of chronic obstructive lung disease and emphysema. Features of emphysema include decreased vascularization and increased matrix deposition and turnover. In both these diseases, there is a progressive decrease in pulmonary function and often a lethal outcome. Current treatment is largely ineffective. The origin of the cells, which contribute to the terminal fibrosis, is controversial. Recent reports suggest they are of both lung and BM origin [25–27, 32, 34, 35, 65]. Therapy for IPF would involve stimulating the formation of intact epithelium and suppression of fibroblast proliferation. An understanding of the mechanisms by which the fibroblast phenotype is promoted, including increased elastin deposition, at the expense of appropriate stem cell differentiation is vital to understanding the pathogenesis to identify appropriate targets for treatment and therapy. In addition, very little is known about lung stem cells during pulmonary infection and whether they contribute to the resolution of tissue damage. Given the number of bacteria, viruses, and fungal pathogens whose port of entry is the lung, it would be of great interest to determine whether these cells play a role during the infectious disease process or the immune response. Stem cell deficiency and functional impairment could be playing a role in end-stage obstructive lung disease (emphysema). Alternatively, or in addition, these patients may have a degree of BM failure that prevents repopulation of the injured lung.


   CONCLUSION AND FUTURE GOALS
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
References
 
Recent evidence suggests the potential for the lung SP cells to be enriched for epithelial precursors as well as hematopoietic cells. The studies reviewed are in their early stages and provide a foundation for future in vitro and in vivo functional studies as well as the obvious necessity for the stem cell criteria to be used as a guideline. The mounting evidence as to the potential for BM-derived cells to participate and differentiate into pulmonary tissues during various disease processes has been previously summarized (Table 1Go). However, BM-derived precursors have not been shown to restore tissue function during disease processes and seem to offer little hope for the regeneration of functional tissue without manipulation. With more uniform isolation and rigorous characterization of the local lung-specific stem cell population, we will begin to understand how the local environment influences these changes. This knowledge will allow the manipulation of the local microenvironment to facilitate a more normal regenerative process via both the resident and BM-derived cells.


   ACKNOWLEDGMENTS
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 
We appreciate Drs. Ivan McMurtry, and Joseph Crossno Jr. for valuable comments and critical reading of the manuscript. This work was funded in part by grants to S.M.M. from the American Heart Association (SDG-0335052N); to A.A.I. from the ALA of Illinois, NIH/NIAID (AI052040) and the University of Colorado Department of Medicine, Cardiovascular Pulmonary Research Section; and to K.H. from NIH grant 5 P30 CA 46934-15.


   REFERENCES
Top
 Abstract
 Introduction
 Stem Cell Criteria
 Characteristics of the SP
 Resident Lung SP Cells
 Pulmonary Transdifferentiation
 The Potential for Lung...
 Conclusion and Future Goals
 References
 

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Received January 28, 2005; accepted for publication April 4, 2005.



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