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(Chest. 2004;125:2278-2288.)
© 2004 American College of Chest Physicians

Identification of Spontaneous Feline Idiopathic Pulmonary Fibrosis*

Morphology and Ultrastructural Evidence for a Type II Pneumocyte Defect

Kurt Williams, DVM, PhD; David Malarkey, DVM, PhD; Leah Cohn, DVM, PhD; Daniel Patrick, DVM; Janice Dye, DVM, PhD and Galen Toews, MD, FCCP

* From the Department of Pathobiology and Diagnostic Investigation (Drs. Williams and Patrick), College of Veterinary Medicine, Michigan State University, East Lansing, MI; Department of Microbiology, Pathology, and Parasitology (Dr. Malarkey), College of Veterinary Medicine, North Carolina State University, Raleigh, NC; Department of Veterinary Medicine and Surgery (Dr. Cohn), College of Veterinary Medicine, University of Missouri, Columbia, MO; Experimental Toxicology Division (Dr. Dye), National Health and Environmental Effects Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC; and University of Michigan Department of Internal Medicine (Dr. Toews), Pulmonary and Critical Care Medicine, University of Michigan, Ann Arbor, MI.

Correspondence to: Kurt J. Williams, DVM, PhD, Department of Pathobiology and Diagnostic Investigation, 210 Food Safety and Toxicology Building, Michigan State University, East Lansing, MI 48824; e-mail: williamsk{at}dcpah.msu.edu


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: Idiopathic pulmonary fibrosis (IPF) is a poorly understood chronic respiratory disease of humans, which has no correlate in other animals. Understanding the role that inflammation, alveolar epithelial cells, and myofibroblasts play in the progression of the disease is controversial, and hampered by the lack of an animal model. We have identified spontaneous IPF in domestic cats and hypothesized that this newly identified disease shares the pathology of human IPF; further, this work provides data suggesting that the disease is related to a defect in type II pneumocyte biology.

Setting and subjects: Chronic respiratory disease with pathology consistent with usual interstitial pneumonia (UIP) spontaneously developed in 16 domestic cats.

Results: The histopathology of feline IPF consisted of the following: (1) interstitial fibrosis with fibroblast/myofibroblast foci, (2) honeycombing with alveolar epithelial metaplasia and type II pneumocyte hyperplasia, and (3) alveolar interstitial smooth-muscle metaplasia. Interstitial inflammation was not a prominent feature of the disease. {alpha}-Smooth muscle actin-positive myofibroblasts were prominent in myofibroblast foci, beneath honeycomb and hyperplastic epithelium, and in alveolar septa away from the remodeling. Feline IPF type II pneumocyte ultrastructure is similar to a heritable form of human IPF, with abnormal cytoplasmic lamellar body-like inclusions.

Conclusions: We conclude the following: (1) chronic respiratory disease with clinical and pathology features of UIP/IPF occurs in the domestic cat; (2) as in human IPF, the type II pneumocyte and myofibroblasts are important cellular constituents of feline IPF; and (3) type II cell ultrastructure suggests feline IPF is a defect in the type II pneumocyte.

Key Words: animal model • cats • fibrosis • idiopathic pulmonary fibrosis • lung • type II pneumocytes


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Idiopathic pulmonary fibrosis (IPF), also known as cryptogenic fibrosing alveolitis, is a poorly understood respiratory disease of humans. Affecting approximately 11 male and 8 female patients, respectively, per 100,000 individuals per year, it is one of the more prevalent interstitial lung diseases.1 Confounding these many cases is the lack of efficacy of most therapeutics for the disease; this lack of therapeutic options is associated with a 5-year mortality between 50% and 70%.2 The indolent nature of the disease, with the high mortality, unknown cause(s), and poorly understood pathogenesis makes the identification of appropriate animal models extremely important and challenging. The National Heart, Lung, and Blood Institute of the National Institutes of Health convened a workshop to identify critical future research areas directed toward a better understanding of IPF.2 Identification/development of an animal model of IPF was deemed critical toward any future progress in filling gaps in understanding of the disease and the testing of future therapeutic modalities.2 Unfortunately, there is not currently an animal model that recapitulates the progression of the disease, nor develops remodeling within the lungs typical of IPF.

Historically, there has been confusion as to the variety of morphologic presentations of IPF; no less than four distinct entities were classified under the rubric of IPF, including acute interstitial pneumonia (Hamman-Rich syndrome), usual interstitial pneumonia (UIP), desquamative interstitial pneumonia, and nonspecific interstitial pneumonia (NSIP).234 In 2000, an international consensus statement was issued by the American Thoracic Society and the European Respiratory Society that defines the criteria for diagnosis of IPF in humans.4 This statement eliminated all but UIP from the definition of IPF, leaving the other three as separate, distinct entities.4 The salient histologic features of UIP lungs are as follows: temporal heterogeneity of lung remodeling, with the primary changes involved being interstitial fibrosis and ongoing fibroblast/myofibroblast proliferation, "honeycomb" change (enlarged airspaces lined by prominent variable epithelium), and scant inflammation.3 The histologic complexity is complicated by the finding of Flaherty et al5; data presented show that there is abundant interlobar variability in patients with idiopathic interstitial pneumonia, with significant numbers of patients having changes consistent with both UIP and NSIP.5

Animal models currently used to study IPF do not appropriately mimic the morphologic changes of IPF. As stated in the National Heart, Lung, and Blood Institute workshop summary,2 persistent progressive fibrosis with evidence of temporal heterogeneity is a hallmark of IPF; these features are lacking in the contemporary models of lung fibrosis. Currently, the primary model for the study of IPF is the bleomycin-treated rodent; however, neither the acute nor chronic pulmonary changes resemble UIP.67 Administration of bleomycin to mice, rats, and hamsters results in pulmonary inflammation and fibrogenesis.67 Neither the acute or chronic morphology of the lungs of bleomycin-treated rats resembles the changes of IPF.8 Indeed, there is evidence of spontaneous resolution of the lung injury 4 months after bleomycin treatment, a phenomenon that is not seen in IPF.8

Because the cause(s) and pathogenesis of IPF are not well understood in humans, the identification of an animal in which pulmonary disease spontaneously develops that mimics both the clinical progression and morphologic features of human IPF has important implications in the study of the human disease. Spontaneous IPF in another species provides an opportunity to investigate common cellular and biochemical features between the two species. Confirming the presence, morphology, and distribution of cellular effectors presumed to be important in the human disease (ie, myofibroblasts and type II pneumocytes) in another species with IPF further suggests their importance in maintaining the lung phenotype. In light of the limitations of the currently utilized animal models of IPF, identification of spontaneous IPF in an animal that shares the morphologic fidelity with the human disease is the first step in establishing a true model of the disease.

We report herein a novel spontaneous chronic, progressive respiratory disease in domestic cats with the morphologic features of UIP; these features include the temporal heterogeneity, persistent, progressive proliferation of myofibroblasts/fibroblasts, and an association between IPF and the development of primary pulmonary neoplasia. The light microscopic and ultrastructure characteristics of the type II pneumocytes in spontaneous IPF of cats is similar to a familial form of IPF in humans,9 suggesting that the disease in cats may be genetically based, and providing an opportunity to develop the cat as a model to study the human disease. Based on these finding we conclude the following: (1) spontaneous chronic respiratory disease with both the clinical and pathology findings consistent with UIP/IPF occurs in the domestic cat; (2) as with the human disease, hyperplastic type II pneumocytes and myofibroblasts are cellular constituents in feline IPF; (3) the changes in type II pneumocyte ultrastructure in feline IPF are similar to a familial form of human IPF associated with a mutation in the surfactant protein C gene; (4) the altered type II cell ultrastructure suggests spontaneous feline IPF is primarily a defect in the type II pneumocyte; and (5) understanding the cause(s) and pathogenesis of IPF in the cat holds promise for advancing our understanding of the disease in humans.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Tissue Collection and Preparation
Tissues were collected from either postmortem samples or open-lung biopsies of affected cats; normal feline lung was obtained from archived tissues at Michigan State University. Lung tissue samples from cats 1 to 4 and cats 13 to 15 were acquired through case material submitted to the Diagnostic Center for Population and Animal Health at Michigan State University. Cats 5 to 8 and cat 16 were submitted to the necropsy service at the North Carolina State University College of Veterinary Medicine; cats 9 to 12 were initially examined at the College of Veterinary Medicine, University of Missouri. All of the described tissues for histopathology were fixed in 10% neutral-buffered formalin and embedded in paraffin. The tissues were processed routinely for histopathology, with 6 µm sections placed on glass slides for hematoxylin-eosin (HE), Alcian blue/periodic acid Schiff (AB-PAS), and Masson trichrome staining. Unstained 6 µm sections were placed on glass slides for immunohistochemistry.

Electron Microscopy:
Tissues from three affected cats were prepared for electron microscopy. The samples were rinsed in 0.1 mol/L phosphate buffer and placed in osmium tetroxide (Electron Microscopy Sciences; Fort Washington, PA) for 3 h. The tissues were then rerinsed in 0.1M phosphate buffer, followed by three 10-min rinses in 30% ethanol. The tissues were transferred into 2% uranyl acetate (Electron Microscopy Sciences) for 1 h, rinsed in 30% ethanol, and dehydrated in a graded series of ethanol. The tissues were placed in propylene oxide, before being embedded in DMP-30 and araldite 501 (Electron Microscopy Sciences); 1 µm sections were cut on a LKB ultramicrotome (LKB; Bromma, Sweden), and stained with toluidine blue. Tissues of interest were sectioned at 600 angstroms, stained with uranyl acetate and lead citrate (Electron Microscopy Sciences), and examined on a Phillips 301 electron microscope (Phillips; Atlanta, GA).

Immunohistochemistry
Smooth-muscle actin (SMA) was detected using mouse monoclonal antibody and biotinylated anti-mouse antibody utilized at the dilution recommended by the manufacturer (Dako Corporation; Carpinteria, CA). These were followed by avidin-biotin conjugated horseradish peroxidase, used per instructions of the manufacturer (Vector Laboratories; Burlingame, CA). Diaminobenzidine (Sigma Chemical; St. Louis, MO) was used as the substrate for the peroxidase.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Clincal Findings
A summary of the signalment and clinical signs is found in Table 1 . The average age of the affected cats was 8.7 years, with an equal number of male and female animals. In nine of the cats that lived a period of time after onset of the clinical signs, the average duration of signs before death/euthanasia was 5.5 months. In the remaining cats, death occurred suddenly or after a period of days.


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  		Table 1. Signalment and Clinical Findings in Cats With IPF

 
Necropsy Findings
Grossly, the distribution of lesions involved large regions of the lungs. The pleural surface of the lungs was often irregular and cobblestoned in appearance (Fig 1 , top, A). The areas of fibrosis and remodeling formed plaque-like depositions that were discrete from the more normal parenchyma, and extended from the subpleural regions to deep within the organ (Fig 1, middle, B). Grossly discernable honeycombing of the lung was uncommon in the cats, but was a prominent feature in a single cat (Fig 1, bottom, C).



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  		Figure 1. Gross pathology of feline IPF. Areas of fibrosis are scattered through the parenchyma, giving the pleural surface a roughened, cobblestone appearance (arrows, top, A). In 3 of the 16 cats, there were primary lung carcinomas in addition to the fibrotic foci (asterisks, top, A). The areas of fibrosis are restricted to the parenchyma, extending from the subpleural region to the deep lung parenchyma (arrows, middle, B), and are interposed with more normal parenchyma (asterisks, middle, B). Grossly apparent honeycomb lung occurs rarely in feline IPF (arrows, bottom, C). Li = liver; Hrt = heart.

 
Histopathology
A summary of the relative abundance of the four predominant histologic changes in feline IPF, along with the presence or absence of pulmonary neoplasia, is found in Table 2 . Histologically, the disease process in cats, as in human IPF, is multifocal, with relatively normal parenchyma interspersed with the affected tissue. The remodeled lung often was most prominent subpleurally (Fig 2 , top left, A, and top right, B). The primary histologic changes in cats, as with humans, included interstitial fibrosis with fibroblast/myofibroblast foci, metaplasia of the alveolar epithelium (honeycomb lung), and interstitial smooth-muscle metaplasia/hyperplasia. Interstitial inflammation was variable but usually not prominent (Table 2). In the most severely affected regions of the lungs, there was extension of the histologic alterations into the deep parenchyma without distinction between subpleural and deeper microenvironments (Fig 2, top left, A, and top right, B).


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  		Table 2. Occurrence and Relative Proportions of the Major Histologic Features in IPF in Cats*

 


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  		Figure 2. Comparative histopathology of feline and human IPF (HE), showing honeycomb lung in human (top left, A) and feline (top right, B) IPF with adjacent relatively normal lung; columnar epithelial cells lining the honeycomb lung of human IPF adjacent to a terminal bronchiole (middle left, C); honeycomb lung in feline (middle right, D) IPF. Bottom right, F: AB-PAS staining of mucous cell metaplasia in honeycomb lung of feline IPF. The mucus is apically oriented in the mucous cells (arrows). Bottom left, E: Mucin in bronchiolar epithelium (BE) of human IPF (AB-PAS stain). Mucosubstances in the human IPF are restricted to the bronchiolar epithelium. Note the turquoise interstitial mast cells in the feline lung. Inset shows immunohistochemistry for mast cell tryptase. Bars in top left, A, and top right, B indicate 400 µm; all other bars indicate 50 µm.

 
Honeycombing was present in all cats, and in these areas the epithelium was composed of lowto-tall columnar cells that often formed welldifferentiated mucous cells (Fig 2, middle left, C [human], and middle right, D [feline]); mucous cell metaplasia was the predominant phenotype, being present in 69% of the lung samples analyzed (Fig 2, bottom left, E [human], and bottom right, F [feline]). In the cats without mucous cell metaplasia, the lining cells were well-differentiated type II pneumocytes or columnar cells of unknown phenotype; small foci of squamous metaplasia were less commonly a feature of the epithelium. AB-PAS staining of the lung revealed numerous turquoise interstitial mast cells in the fibrotic and honeycomb lung (Fig 2, bottom right, F). The identity of the mast cells was confirmed using immunohistochemistry against mast cell tryptase (inset, Fig 2, bottom right, F).

Fibroblast foci, small foci of ongoing mesenchymal cell proliferation with fibroblasts/myofibroblasts and collagen, similar to the foci seen in UIP of humans were observed at the periphery of the honeycomb lung (Fig 3 , top left, A [human], and top right, B [feline]). As with human IPF, the epithelial cells overlying the fibroblast foci in feline IPF were often attenuated or cuboidal type II pneumocytes (Fig 3, top left, A [human], and top right, B [feline]).



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  		Figure 3. Comparative histopathology of feline and human IPF (HE), showing fibroblast/myofibroblast foci in human (top left, A) and feline (top right, B) IPF. The foci are comprised of proliferating mesenchymal cells (arrows) overlain by epithelial cells (EC) that vary from attenuated cells to type II pneumocytes and columnar epithelial cells. Smooth-muscle metaplasia (arrows) is shown in areas of fibrosis in human (bottom left, C) and feline (bottom right, D) IPF. Bars indicate 50 µm.

 
Interstitial smooth-muscle metaplasia/hyperplasia was present in all of the cats. The smooth-muscle cells were interspersed with the metaplastic epithelium and fibrous connective tissue, forming discrete, thick bundles; similar smooth-muscle changes are found in human IPF (Fig 3, bottom left, C [human], and bottom right, D [feline]).

In addition to the above changes, scattered large foci of alveolar macrophages were found in all of the cats. These regions of alveolar histiocytes were often associated with hyperplastic type II pneumocytes; the macrophages were not as abundant in areas of chronic remodeling and honeycomb lung. In addition to the chronic lung remodeling of IPF, three of the cats had primary pulmonary neoplasms consistent with bronchioloalveolar carcinomas.

Immunohistochemistry
{alpha}-SMA:
In normal cat lung, {alpha}-SMA was restricted to the smooth muscle of the pulmonary vasculature, airway walls, and openings to the respiratory bronchioles; there was little SMA evident in the alveolar septa (Fig 4 , bottom left, E). In IPF cats, there was abundant SMA in the bundles of well-differentiated metaplastic smooth muscle. SMA-positive myofibroblasts were found subjacent to the metaplastic epithelium of the honeycomb lung, as well as in the myofibroblast foci; a similar distribution of SMA was seen in the human IPF lung (Fig 4, top left, A [human], and top right, B [feline]). Many myofibroblasts were localized to the areas of acute, ongoing alveolar septal injury, as well as septa in the areas of minimal injury and remodeling (Fig 4, middle left, C, and middle right, D).



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  		Figure 4. Immunohistochemical localization of {alpha}-SMA in human and feline IPF, showing {alpha}-SMA immunohistochemistry for myofibroblast metaplasia in human (top left, A) and feline (top right, B) honeycomb lung (arrows indicate subepithelial myofibroblasts; arrowhead indicates smooth-muscle bundle); early myofibroblast metaplasia (arrow) in the alveolar septa of active epithelial injury (middle left, C) [note the sloughed cells, morphologically consistent with hypertrophied type II pneumocytes, shown by asterisks]; myofibroblast metaplasia in histologically normal alveolar septa (middle right, D); normal feline lung (bottom left, E) [SMA signal in smooth muscle around terminal bronchiole (TB) and respiratory bronchiole (RB)]. Bars indicate 50 µm.

 
Ultrastructure:
In the toluidine blue-stained, plastic-embedded lung prepared for electron microscopy, the interstitium of the pulmonary parenchyma was markedly thickened with abundant collagen. In the areas of type II pneumocyte hyperplasia and differentiation, the individual pneumocytes contained many cytoplasmic, variably sized dark inclusions; similar bodies were not seen in normal feline lung (Fig 5 , top left, A, and top right, B). By ultrastructure, the type II pneumocytes of normal cats are as described for other species, with surface microvilli and lamellar bodies with stacks of phospholipid membranes (Fig 5, middle left, C). The type II pneumocytes in the IPF cat lung were markedly enlarged (Fig 5, middle right, D), with numerous large condensed lamellar body-like structures (Fig 5, bottom left, E, and bottom right, F). These cells were often free within the lumen of the airspace. Abnormal lamellar bodies were seen within alveolar macrophages as well as occasionally free within the interstitium of the fibrotic lungs. Similar type II pneumocytes were found distant from the foci of remodeling, in the more normal lung parenchyma (data not shown).



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  		Figure 5. Feline IPF type II pneumocyte morphology, showing toluidine-blue stained, plastic-embedded normal (top left, A) and feline IPF (top right, B) lung. Type II pneumocytes in feline IPF contain dense cytoplasmic inclusions that are not visible in normal type II cells (arrowheads). Also shown are ultrastructure of normal (middle left, C) and feline IPF (middle right, D) type II pneumocytes. In feline IPF, the type II cells are hypertrophied and contain many dense cytoplasmic, lamellar body-like structures. Also shown are detailed ultrastructure of normal feline lamellar bodies (bottom left, E) and abnormal lamellar body-like inclusions in feline IPF (bottom right, F). Bars in top left, A, and top right, B indicate 50 µm; middle left, C, and middle right, D are original x 5,700; bottom left, E, and bottom right, F are original x 25,000.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
IPF is a poorly understood chronic respiratory disease of humans. Complicating advancements in understanding the pathogenesis of the disease and development of therapies is the lack of animal models of the disease that follow the progressive clinical course, and which develop the persistent, progressive lung fibrosis characteristic of the disease. Identification of new animal models of IPF is a stated priority of the National Heart, Lung, and Blood Institute of the National Institutes of Health.2 An ideal animal model of IPF should recapitulate the complex lung morphology and clinical progression of the human disease; descriptive features of both the lung morphology and clinical disease should emphasize the persistent, progressive fibrosis characteristic of IPF.2 This study describes spontaneous feline IPF, a newly identified chronic lung disease of domestic cats that shares critical features with human IPF. The important gross pathology, histopathology, cell differentiation markers, and ultrastructural features are compared to the well-described features of the disease in humans. IPF in humans is a chronic respiratory disease whose pathology is characterized by temporally heterogeneous, persistent, progressive fibrosis of the lung, usually without significant inflammation.10 The characteristic morphology consists of patchy remodeling in the lungs leading to honeycomb lung late in the disease, with the characteristic histopathology. This histopathology shows evidence of temporal heterogeneity with fibrosis, fibroblast foci, and evidence of honeycombing in the parenchyma.210 Each of these features is found in spontaneous feline IPF. Additionally, ultrastructural features of the type II pneumocytes in feline IPF share morphologic features with the type II cells in a familial form of human UIP, suggesting that the disease in cats may be due to an abnormality in the type II cell. Genetic analysis of surfactant protein genes in affected and normal cats is currently underway, providing hope that the domestic cat may be developed as a new model of IPF.

The gross lesions within the lungs of feline IPF share similarities with human IPF. The lungs of human patients at autopsy are typical of end-stage interstitial lung disease with fibrosis and honeycombing of the alveolar parenchyma.11 In both feline and human IPF, the distribution of the lesions is patchy, with normal areas of lung interspersed with adjacent foci of fibrosis and honeycomb change; this distribution is considered important in the diagnosis of IPF.4 Unlike humans, where honeycomb lung is apparent as greatly dilated peripheral air spaces, the honeycomb change in the feline lung is comprised of smaller microhoneycombing, and only occasionally forms the typical lesions of human IPF. There are no detailed reports of the gross pathology in rodent models of IPF that share the features of the human and feline disease. Because of the size of the feline lung relative to rodents, and the discrete nature of the diseased vs more normal lung, feline IPF can facilitate investigations into microenvironmental changes within the IPF lung that are important in the pathogenesis of the disease. Work is currently underway to investigate differences in the fibroblast/myofibroblasts populations from fibrotic and nonfibrotic feline lung; similar work has been accomplished in human IPF lung.12

Cats with IPF acquire lung histopathology similar to human IPF. Previous to the 2000 statement designating UIP as the pathologic manifestation of IPF, acute interstitial pneumonia, desquamative interstitial pneumonia, and NSIP were also considered variations of IPF.10 These other pulmonary diseases lack the temporal heterogeneity and evidence of ongoing fibrogenesis of IPF. Complicating this classification system is the finding of Flaherty et al,5 who found considerable variation between lobes of individual patients with IPF; many of the patients had histologic features consistent with fibrotic NSIP in lobes away from the UIP changes. We are unable at this time to discuss the uniformity of changes between lobes in individual cats because the tissues examined represent material collected from individual lobes, prior to knowledge of the nature of the disease process and the potential for interlobar variability. Examination of entire lung fields from affected cats, with careful sampling of a variety of lobes, will be important to address this question.

Microscopic honeycomb change is very common, and characteristic in human and feline IPF. The identification of this morphologic feature, which is not a feature of rodent models of lung fibrosis, has important implications for studying the disease. Numerous important growth factors, including many with profibrogenic activity such as transforming growth factor-ß, can be immunohistochemically localized to this population of cells in humans.131415 Studies are ongoing to immunohistochemically characterize the presence of similar proteins in the feline lung. Mucous cell metaplasia in feline IPF is similar to reports in honeycomb lung of humans.1617 The mucous metaplasia in feline and human IPF may be an adaptive response to the chronic, progressive injury to the lung. Alternatively, this phenotype may reflect local production of cytokines, such as interleukin-13, which are known to induce pulmonary mucous cell metaplasia.18 The role of mucous cell metaplasia in the progression of the disease is unclear, although excess mucus production has been associated with a poorer prognosis in human patients with IPF.19

The distribution of myofibroblasts in feline IPF are similar to that of the human disease. While myofibroblasts are found in rodent models of pulmonary fibrosis, because of the lack of honeycombing, they lack the relationship with the metaplastic epithelium that is important in the progressive fibrosis of IPF.20 Uhal et al21 showed that in human IPF lung there is an increase in the apoptosis of the metaplastic epithelial cells overlying foci of myofibroblast metaplasia. This apoptosis may be mediated by local conversion of native angiotensinogen into angiotensin II, which goes on to mediate apoptosis through the angiotensin receptor subtype AT-1.2223 Induction of myofibroblasts subjacent to the metaplastic epithelial cells in cats with IPF may create a similar environment of epithelial cell loss. The finding of attenuated epithelial cells overlying the sites of myofibroblast metaplasia implies repair after previous cell loss, the pathogenesis of which may involve the above process.

The ultrastructure of the type II pneumocyte in spontaneous feline IPF suggests that the type II cell may be integral in the pathogenesis of the disease. The morphology of the lamellar body-like structures is abnormal and similar to those reported by Thomas et al9 in a kindred of people with UIP and cellular NSIP associated with a mutation in the prosurfactant protein C gene. The authors suggest that misfolding of the pro-surfactant protein C may lead to type II cell injury and loss.9 Recently, the role of abnormalities in surfactant genes and interstitial lung disease in humans has been reviewed.2425 Exfoliation of the type II cells in the lungs of feline IPF suggests that the cells are being lost as part of the aberrant alveolar epithelialization. This process has been suggested to be important in the development of pulmonary fibrosis.26272829 An abnormality in alveolar repair would also preclude the necessity of continued superimposed inflammation to drive alveolar remodeling; the lack of an essential role for inflammation in the pathogenesis has been proposed by Selman et al.27 The ultrastructural findings in feline IPF suggest that the propagation of the disease may be the result of an underlying defect in type II pneumocyte biology. Whether this is due to inherent genetic defect(s) in the affected feline type II cells is currently under investigation.

In summary, this study has shown that the domestic cat acquires a chronic respiratory disease that is pathologically very similar to IPF of humans. As with human IPF, the affected cats are older (mean age, 8.7 years) and poorly responsive to corticosteroid therapy. Both the gross pulmonary lesions and the histopathology share most of the features of the human disease. These features include multifocal distribution within the lung with subpleural orientation, honeycomb formation with abundant metaplasia of the lining epithelium, alveolar septal myofibroblast metaplasia with smooth-muscle formation, and fibrosis with fibroblast/myofibroblast foci formation. The ultrastructure of the type II pneumocyte in feline IPF suggests that abnormalities in type II cell biology are important in the abnormal alveolar repair of the disease, and that this drives the progressive fibrosis of feline IPF. These features are important in identifying animal models of IPF that faithfully recapitulate the pathogenesis and progression of the human disease. Based on the findings of this study, we conclude the following: (1) spontaneous chronic respiratory disease with pathology findings consistent with IPF/UIP occurs in the domestic cat; (2) as with the human disease, the type II pneumocyte and myofibroblasts are important cellular constituents in feline IPF; (3) the changes in type II pneumocyte ultrastructure in feline IPF are similar to a familial form of human IPF associated with a mutation in the surfactant protein C gene; (4) spontaneous feline IPF may be primarily a defect in type II pneumocyte biology; and (5) understanding the cause and pathogenesis of IPF in the cat holds promise for advancing our understanding of the disease in people.


   Acknowledgements
 
The authors thank Dr. Andrew Flint for providing the human IPF samples, Dr. Beth Moore for reviewing the manuscript, and Scot Marsh for performing the {alpha}-SMA immunohistochemistry.


   Footnotes
 
Abbreviations: AB-PAS = Alcian blue/periodic acid Schiff; HE = hematoxylin-eosin; IPF = idiopathic pulmonary fibrosis; NSIP = nonspecific interstitial pneumonia; SMA = smooth-muscle actin; UIP = usual interstitial pneumonia

This work was performed at the Department of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, East Lansing, MI.

Support was provided by the Companion Animal Fund at Michigan State University.

Received for publication August 6, 2003. Accepted for publication December 3, 2003.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Coultas, DB, Zumwalt, RE, Black, WC, et al (1994) The epidemiology of interstitial lung diseases. Am J Respir Crit Care Med 150,967-972[Abstract]
  2. Crystal, RG, Bitterman, PB, Mossman, B, et al Future research directions in idiopathic pulmonary fibrosis: summary of a National Heart, Lung, and Blood Institute working group. Am J Respir Crit Care Med 2002;166,236-246[Abstract/Free Full Text]
  3. Katzenstein, AL, Myers, JL Idiopathic pulmonary fibrosis: clinical relevance of pathologic classification. Am J Respir Crit Care Med 1998;157,1301-1315[Free Full Text]
  4. American Thoracic Society. Idiopathic pulmonary fibrosis: diagnosis and treatment; international consensus statement. American Thoracic Society (ATS), and the European Respiratory Society (ERS). Am J Respir Crit Care Med 2000;161,646-664[Free Full Text]
  5. Flaherty, KR, Travis, WD, Colby, TV, et al Histopathologic variability in usual and nonspecific interstitial pneumonias. Am J Respir Crit Care Med 2001;164,1722-1727[Abstract/Free Full Text]
  6. Zia, S, Hyde, DM, Giri, SN Development of a bleomycin hamster model of subchronic lung fibrosis. Pathology 1992;24,155-163[ISI][Medline]
  7. Thrall, RS, McCormick, JR, Jack, RM, et al Bleomycin-induced pulmonary fibrosis in the rat: inhibition by indomethacin. Am J Pathol 1979;95,117-130[Abstract]
  8. Borzone, G, Moreno, R, Urrea, R, et al Bleomycin-induced chronic lung damage does not resemble human idiopathic pulmonary fibrosis. Am J Respir Crit Care Med 2001;163,1648-1653[Abstract/Free Full Text]
  9. Thomas, AQ, Lane, K, Phillips, J, III, et al Heterozygosity for a surfactant protein C gene mutation associated with usual interstitial pneumonitis and cellular nonspecific interstitial pneumonitis in one kindred. Am J Respir Crit Care Med 2002;165,1322-1328[Abstract/Free Full Text]
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M. Selman and A. Pardo
Role of epithelial cells in idiopathic pulmonary fibrosis: from innocent targets to serial killers.
Proceedings of the ATS, January 1, 2006; 3(4): 364 - 372.
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