Animal Models of Fibrotic Lung Disease
Bethany B. Moore1, William E. Lawson4,5, Tim D. Oury6, Thomas H. Sisson1, Krishnan Raghavendran2,
and Cory M. Hogaboam3
1Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine,2Department of Surgery, and3Department of Pathology,
University of Michigan, Ann Arbor, Michigan;4Division of Allergy, Pulmonary, and Critical Care Medicine, Department of Medicine, Vanderbilt
University School of Medicine, Nashville, Tennessee;5Department of Veterans Affairs Medical Center, Nashville, Tennessee; and6Department of
Pathology, University of Pittsburgh Medical School, Pittsburgh, Pennsylvania
Interstitial lung fibrosis can develop as a consequence of occupa-
tional or medical exposure, as a result of genetic defects, and after
trauma or acute lung injury leading to fibroproliferative acute
respiratory distress syndrome, or it can develop in an idiopathic
manner. The pathogenesis of each form of lung fibrosis remains
poorly understood. They each result in a progressive loss of lung
function with increasing dyspnea, and most forms ultimately result
in mortality. To better understand the pathogenesis of lung fibrotic
summarizes the common and emerging models of lung fibrosis to
highlight their usefulness in understanding the cell–cell and soluble
have allowed for the development of models to study targeted inju-
ries of Type II alveolar epithelial cells, fibroblastic autonomous ef-
has more closely mimicked the pathology of human fibrotic lung dis-
ease. We also have a much better understanding of the fact that the
aged lung has increased susceptibility to fibrosis. Each of the models
of fibrotic lung disease.
Keywords: fibrosis; collagen; fibroblast; aging; cytokines
Interstitial lung disease is often associated with the development
of chronic fibrosis. These diseases are characterized clinically by
progressive dyspnea, cough, restrictive physiology, and impaired
gas exchange. Humans manifest many types of fibrotic lung
disease (1). Among the diffuse parenchymal lung disorders
(DPLDs) are diseases of known cause (e.g., drug-related, envi-
ronmental exposures, or those associated with collagen vascular
disease), the idiopathic interstitial pneumonias (IIPs), the gran-
ulomatous DPLDs (e.g., sarcoidosis), and rare noncategorized
diseases, such as lymphangioleiomyomatosis. Idiopathic pulmo-
nary fibrosis (IPF) is the most common disease within the cat-
egory of IIPs, and is histopathologically identified as usual
interstitial pneumonia (UIP). Additional diseases within the
IIP category include desquamative interstitial pneumonia,
respiratory bronchiolitis interstitial lung disease, acute intersti-
tial pneumonia, cryptogenic organizing pneumonia, lymphocytic
interstitial pneumonia, and nonspecific interstitial pneumonia
(NSIP). IPF carries a poor prognosis, with a mean survival time
of less than 5 years after diagnosis (2–5). Biopsies from a single
patient can show heterogenous patterns consistent with both UIP
and NSIP (4, 6, 7), suggesting that NSIP shares common patho-
genic mechanisms with UIP.
Diagnoses of patients with IPF who do not exhibit classic
high-resolution computed tomography scan changes are con-
firmed by histopathologic evaluations of surgical lung biopsies,
which demonstrate the pattern of UIP. Hallmark features of
UIP include epithelial cell hyperplasia, basement membrane de-
that is spatially and temporally heterogeneous (8). Attempts to
understand the pathogenesis of IPF and other fibrotic lung dis-
orders have relied on animal models. Unfortunately, no animal
model exhibits progressive disease, and most animal models do
not fully recapitulate the histologic pattern of UIP or other
interstitial lung diseases. This, however, does not mean animal
models are dispensable for research in pulmonary fibrosis. Tra-
ditional animal models of lung fibrosis have generated impor-
tant insights into the pathobiology of lung injury, inflammation,
and fibroproliferation (9). In addition, animal models have re-
cently been refined to reflect better the known pathogenic
mechanisms in IPF. Although it is appreciated that the sponta-
neous development of fibrosis in other species (e.g., cats, horses,
and dogs) (10–12) can be instructive for comparative medicine,
the most tractable models for studies of pathogenesis involve
rodents. Many traditional and newly developed models of ex-
perimental lung fibrosis offer opportunities to study cell–cell
interactions and the soluble mediators driving pathologic fi-
brotic remodeling. A better understanding of these pathways
in experimental models provides a critical approach to identi-
fying novel targets to assess and validate in human studies. We
will review several established and newer models of lung fibro-
sis, and highlight particularly useful aspects of each. We include
environmental and genetic models because these offer insights
into the development of fibrosis by known causes, and also shed
(Received in original form February 28, 2013 and in final form March 12, 2013)
This research was supported by National Institutes of Health grants HL087846
and HL091745 (B.B.M.), National Institutes of Health grant HL105479 (W.E.L.),
the Department of Veterans Affairs (W.E.L.), National Institutes of Health grant
R21HL09549 (T.D.O.), National Institutes of Health grant RO1 HL-102013 (K.R.), and
National Institutes of Health grant RC2 HL101740 (C.M.H.)
Correspondence and requests for reprints should be addressed to Bethany
Moore, Ph.D., Division of Pulmonary and Critical Care Medicine, Department
of Internal Medicine, University of Michigan, 109 Zina Pitcher Place, 4053 BSRB,
Ann Arbor, MI 48109-2200. E-mail: email@example.com
Am J Respir Cell Mol Biol
Copyright ª 2013 by the American Thoracic Society
Originally Published in Press as DOI: 10.1165/rcmb.2013-0094TR on March 22, 2013
Internet address: www.atsjournals.org
Vol 49, Iss. 2, pp 167–179, Aug 2013
This review summarizes common and recently developed
animal models of lung fibrosis. It highlights the pathologic
features of these models, and will aid basic science and
translational science investigators in choosing the most
appropriate model system to be used for their own fibrosis
light on the pathogenesis of intractable fibrotic diseases such as
IPF. In addition, we include discussions on animal models of
interstitial fibrosis secondary to specific risk factors for acute
lung injury (ALI) and acute respiratory distress syndrome
ANIMAL MODELS REPLICATING IPF
Asbestosis continues to be an important fibrotic lung disease in
exposed humans. Human asbestosis and IPF are similar in their
gross distribution, and share a common UIP histopathology (13,
14). In terms of their main difference, asbestosis involves fewer
myofibroblastic foci than does IPF, and this may explain the
slower progression of asbestosis compared with IPF (14). As-
bestosis is distinguished from IPF by histologic findings of as-
bestos bodies in the lungs, indicating the cause of disease. These
features are largely recapitulated in inhalation models in ani-
mals, and these models are useful for understanding the patho-
genesis of both asbestosis and UIP. An intratracheal instillation
of asbestos fibers promotes the rapid development of fibrosis
with a single administration. One caveat about intratracheal
models involves the distribution between lung lobes, which
can be uneven, and the pattern of fibrosis tends to be central
rather than subpleural, as is more prominent in the inhalation
model. However, the ability to visualize asbestos fibers histo-
logically can confirm an appropriate deposition, and can be used
to locate areas where fibrosis is likely to develop. As indicated,
inhalation models develop a more peripheral pattern of disease,
but they require special inhalation chambers and the time for
disease development can be long, especially if using nonamphibole
(chrysotile) fibers. All asbestos should be baked before in vivo
instillation to destroy common contamination by lipopolysac-
charide. The intratracheal models with amphibole fibers show
fibrosis by Day 7, which is mature by Day 14 (Figure 1). Inha-
lation models may take over a month to develop fibrosis. The
fibrosis that does develop is persistent and may be progressive
with large enough dosing, especially when using amphibole
fibers. The deposition of asbestos fibers specifically induces
oxidative stress and alveolar epithelial cell injury (15).
Macrophages, lymphocytes, eosinophils, and neutrophils have
also been implicated in promoting injury in these models. Mu-
rine asbestosis comprises one of the few animal models to de-
velop fibrotic foci, and therefore is particularly useful for
understanding the development of this pathologic lesion. Table
1 summarizes the pathogenic features that are especially well
studied, using the asbestos model and other models of lung
The instillation of silica into murine lungs results in the develop-
ment of fibrotic nodules that resemble simple silicotic nodular
fibrosis, which develops in humans after some occupational
exposures (16), but are generally more cellular than the fibrotic
nodules seen in humans. Silica can be delivered to rodents via
aerosolization (17), intratracheal administration (18), or oro-
pharyngeal aspiration (19). The fibrotic response to silica instil-
lation is strain-dependent. C3H/HeN, MRL/MpJ, and NZB
mice are all susceptible in the aerosolized model, whereas Balb/c
mice show little response (17). Similarly, C57Bl/6 mice are more
susceptible than CBA/J mice after intratracheal silica (20). Silica is
retained in the lung, and the response is characterized by a persis-
tent, toxic, and inflammatory response. Fibrotic nodules develop
around silica deposits, and silica particles are easily identified both
histologically and by polarization microscopy. The intratracheal
models are easier and more cost-efficient, whereas inhalation mod-
els more closely mimic human exposure but take longer to develop
(40–120 d) (18) than the intratracheal models (14–28 d) (19). Care
must be exercised when choosing formulations of silica particles,
because different formulations vary widely in their potency of
stimulating fibrotic responses. Silica should be baked before in vivo
instillations, to destroy contamination by lipopolysaccharide. One
important feature of silicosis involves the strong influence of mac-
rophage NACHT, LRR, and PYD domains-containing protein 3
(NALP3) inflammasome activation (21), making it an important
model for studying the innate immune regulation of lung fibrotic
responses. Figure 2 demonstrates the histopathologic findings in
the silica model.
Among currently applied models of experimentally induced pul-
monary fibrosis, the administration of bleomycin is used most
frequently (9, 22). The recognition that bleomycin could result
in pulmonary fibrosis in humans led to its use in experimental
models, and for four decades it has been the most commonly
applied model of experimental lung fibrosis. It has been used in
multiple animals, including mice, rats, guinea pigs, hamsters, rab-
bits, dogs, and primates, but mice are most common (9, 23). Fur-
thermore, it has been delivered by multiple methods, including
intratracheal, intraperitoneal, subcutaneous, intravenous, and inha-
lational (9, 23). Although each method of delivery has its own
strengths and weaknesses, the intratracheal delivery of bleomycin
has emerged as the most frequent route (22). Whatever the route
of administration, the delivery of bleomycin results in direct cell
Figure 1. Asbestos induced pulmonary fibrosis. (A) Control lung shows
normal terminal bronchi and alveolar parenchyma. (B) Intratracheal
instillation of crocidolite asbestos induces robust peribronchial fibrosis
with extension into the adjacent alveolar parenchyma (14 d after
exposure). Scale bars ¼ 50 mm.
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