Targeted Injury of Type II Alveolar Epithelial Cells Induces Pulmonary Fibrosis

Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Medical Center, 1150 West Medical Center Drive, 6301 MSRB III, Ann Arbor, MI 48109-5642, USA.
American Journal of Respiratory and Critical Care Medicine (Impact Factor: 13). 10/2009; 181(3):254-63. DOI: 10.1164/rccm.200810-1615OC
Source: PubMed


Ineffective repair of a damaged alveolar epithelium has been postulated to cause pulmonary fibrosis. In support of this theory, epithelial cell abnormalities, including hyperplasia, apoptosis, and persistent denudation of the alveolar basement membrane, are found in the lungs of humans with idiopathic pulmonary fibrosis and in animal models of fibrotic lung disease. Furthermore, mutations in genes that affect regenerative capacity or that cause injury/apoptosis of type II alveolar epithelial cells have been identified in familial forms of pulmonary fibrosis. Although these findings are compelling, there are no studies that demonstrate a direct role for the alveolar epithelium or, more specifically, type II cells in the scarring process.
To determine if a targeted injury to type II cells would result in pulmonary fibrosis.
A transgenic mouse was generated to express the human diphtheria toxin receptor on type II alveolar epithelial cells. Diphtheria toxin was administered to these animals to specifically target the type II epithelium for injury. Lung fibrosis was assessed by histology and hydroxyproline measurement.
Transgenic mice treated with diphtheria toxin developed an approximately twofold increase in their lung hydroxyproline content on Days 21 and 28 after diphtheria toxin treatment. The fibrosis developed in conjunction with type II cell injury. Histological evaluation revealed diffuse collagen deposition with patchy areas of more confluent scarring and associated alveolar contraction.
The development of lung fibrosis in the setting of type II cell injury in our model provides evidence for a causal link between the epithelial defects seen in idiopathic pulmonary fibrosis and the corresponding areas of scarring.

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    • "In the literature, both SP-B and SP-C have been widely used as a marker of type II AECs in rodent lungs (Lin et al. 1999; Fehrenbach 2001; Sisson et al. 2010), but in this study, in comparison to antibodies directed against SP-C, the anti-mature SP-B antibodies stained the cells of the alveolar epithelium of fetal lung tissue more reliably, resulting in significantly higher positive cell counts. Furthermore, the specificity of the mature SP-B antibody used in this study (anti-human and anti-bovine mature SP-B) demonstrated no significant difference between the numerical densities obtained from positive cells present in the alveolar epithelium, thus indicating that either anti-human or antibovine mature SP-B antibodies are reliable markers to be used to evaluate surfactant system development in the fetal sheep lung in late gestation. "
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    ABSTRACT: Evaluation of the number of type II alveolar epithelial cells (AECs) is an important measure of the lung's ability to produce surfactant. Immunohistochemical staining of these cells in lung tissue commonly uses antibodies directed against mature surfactant protein (SP)-C, which is regarded as a reliable SP marker of type II AECs in rodents. There has been no study demonstrating reliable markers for surfactant system maturation by immunohistochemistry in the fetal sheep lung despite being widely used as a model to study lung development. Here we examine staining of a panel of surfactant pro-proteins (pro-SP-B and pro-SP-C) and mature proteins (SP-B and SP-C) in the fetal sheep lung during late gestation in the saccular/alveolar phase of development (120, 130, and 140 days), with term being 150 ± 3 days, to identify the most reliable marker of surfactant producing cells in this species. Results from this study indicate that during late gestation, use of anti-SP-B antibodies in the sheep lung yields significantly higher cell counts in the alveolar epithelium than SP-C antibodies. Furthermore, this study highlights that mature SP-B antibodies are more reliable markers than SP-C antibodies to evaluate surfactant maturation in the fetal sheep lung by immunohistochemistry. © The Author(s) 2015.
    No preview · Article · Aug 2015 · Journal of Histochemistry and Cytochemistry
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    • "Current animal models of tissue fibrosis suggest that repeated epithelial injury and defective repair lead to an aberrant fibrotic response within the lung (Hagimoto et al., 1997; Sisson et al., 2010; Lawson et al., 2011; Palmer et al., 2011; Hardie et al., 2010), liver (Hayashi and Sakai, 2011), and kidney (Grigic et al., 2012). While the cascade from epithelial cell injury to fibrosis is a complex subject of intensive research, airway fibrosis has been reported to occur as a result of altered crosstalk between epithelial cells and interstitial fibroblasts (Wynn, 2011). "
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    ABSTRACT: The airway epithelium serves as a biological barrier essential for host defense against inhaled pollutants. While chronic epithelial injury, commonly associated with chronic obstructive pulmonary disease and bronchiolitis obliterans syndrome, often results in airway fibrosis, limited animal models of airway fibrosis have been established. Club cells (Clara cells) in the small airways represent an important population of epithelial progenitor cells and also the principal site of localization of the cytochrome P-450 monooxygenase system, which metabolically activates xenobiotic chemicals such as naphthalene by converting them to toxic epoxide intermediates. We hypothesized that repeated exposure to naphthalene may cause prolonged loss of club cells, triggering aberrant local epithelial repair mechanisms that lead to peribronchial fibrosis. We administered intraperitoneal injections of naphthalene to C57/BL6J mice once a week for 14 consecutive weeks. Repeated club cell injury caused by naphthalene triggered regional hyperproliferation of epithelial progenitor cells, while other regions remained denuded or squamated, resulting in fibroblast proliferation and peribronchial collagen deposition associated with upregulation of the fibrogenic cytokines transforming growth factor-β and connective tissue growth factor. The total collagen content of the lung assessed by measurement of the hydroxyproline content was also increased after repeated exposure to naphthalene. These results lend support to the relevance of repeated injury of airway epithelial cells as a trigger for resting fibroblast proliferation and airway fibrosis. This model of airway fibrosis is simple and easy to reproduce, and may be expected to advance our understanding of the pathogenesis and potential treatment of airway fibrotic disorders.
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    • "As MUC5B, SFTPC, and SFTPA1 are expressed by alveolar type II cells, this raises the possibility that injury of these cells is a critical pathogenic mechanism in pulmonary fibrosis (Seibold et al., 2013). This hypothesis is supported by the observation that lung fibrosis occurs following ablation of alveolar type II in genetically modified mice using diphtheria toxin (DT) receptor transgene under the control of Sftpc promoter (Sisson et al., 2010). However, lung fibrosis was not observed in a different genetic mouse model where the DT expression is controlled by an inducible Cre recombinase knocked into the Sftpc locus (Barkauskas et al., 2013). "
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    ABSTRACT: Genetic studies of human diseases have identified multiple genetic risk loci for various fibrotic diseases. This has provided insights into the myriad of biological pathways potentially involved in disease pathogenesis. These discoveries suggest that alterations in immune responses, barrier function, metabolism and telomerase activity may be implicated in the genetic risks for fibrotic diseases. In addition to genetic disease-risks, the identification of genetic disease-modifiers associated with disease complications, severity or prognosis provides crucial insights into the biological processes implicated in disease progression. Understanding the biological processes driving disease progression may be critical to delineate more effective strategies for therapeutic interventions. This review provides an overview of current knowledge and gaps regarding genetic disease-risks and genetic disease-modifiers in human fibrotic diseases.
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