Journal of Toxicology and Environmental Health, Part B, 14:153–178, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1093-7404 print / 1521-6950 online
NON-NEOPLASTIC AND NEOPLASTIC PLEURAL ENDPOINTS FOLLOWING
V. Courtney Broaddus1∗#, Jeffrey I. Everitt2, Brad Black3, Agnes B. Kane4#
1University of California, San Francisco, San Francisco, California
2GlaxoSmithKline, Research Triangle Park, North Carolina
3Center for Asbestos Related Disease, Libby, Montana
4Department of Pathology and Laboratory Medicine, Brown University, Providence, Rhode
Exposure to asbestos fibers is associated with non-neoplastic pleural diseases including
plaques, fibrosis, and benign effusions, as well as with diffuse malignant pleural mesothe-
lioma. Translocation and retention of fibers are fundamental processes in understanding the
interactions between the dose and dimensions of fibers retained at this anatomic site and
the subsequent pathological reactions. The initial interaction of fibers with target cells in
the pleura has been studied in cellular models in vitro and in experimental studies in vivo.
The proposed biological mechanisms responsible for non-neoplastic and neoplastic pleural
diseases and the physical and chemical properties of asbestos fibers relevant to these mecha-
nisms are critically reviewed. Understanding mechanisms of asbestos fiber toxicity may help
us anticipate the problems from future exposures both to asbestos and to novel fibrous mate-
rials such as nanotubes. Gaps in our understanding have been outlined as guides for future
TRANSLOCATION AND RETENTION OF
FIBERS IN THE PLEURA
Anatomy and Physiology of the Pleura
The parietal pleura lines the chest wall
and the superior surface of the diaphragm and
the visceral pleura covers the lungs (Figure 1).
The pleural space in humans contains a small
amount of fluid (0.1–0.2 ml/kg body weight)
that is a filtrate from the underlying sys-
temic circulation (Owens & Milligan, 1995;
Broaddus, 2008). This space (10–20 μm wide)
is lined by a single layer of mesothelial cells
*This author has disclosed a potential conflict of interest as described by one or more of the following: He/She has acted and/or
is currently acting as an expert witness or consultant for law firms representing plaintiffs and/or defendants in asbestos litigation and
compensation board proceedings, and has been a paid or unpaid consultant to regulatory and medical agencies and compensation
boards in North America, including but not limited to NIEHS, EPA, ATSDR, ATS, NGOs and individual and collective citizen groups
concerned with asbestos exposure and disease.
#This author has disclosed a potential conflict of interest as described by one or more of the following: He/She may have also
received (and may also apply in future for) competitive-funding research grants from U.S. publicly financed, peer-reviewed grant approval
process agencies concerning asbestos exposure and disease, including topics covered in all aspects of the workshop, including but not
limited to research support from NIEHS.
Dr. Broaddus acknowledges research support from the Department of Defense (PR 080717) and Dr. Kane acknowledges research
support from the National Institute of Environmental Health Sciences (RO1 ES016178 and P42 ES013660).
Address correspondence to Agnes B. Kane, Brown University, Department of Pathology and Laboratory Medicine, Box G-E5, 70
Ship Street, Providence, RI 02912, USA. E-mail: firstname.lastname@example.org
resting on a basement membrane and under-
lying connective tissue and blood vessels.
The major routes of drainage of fluid, pro-
tein, particulates, and cells from the pleural
space are through the lymphatic stomata that
open between mesothelial cells on the parietal
pleural lining (Hammar, 1994; Wang, 1975;
Broaddus et al., 1988).
Effusions, an accumulation of excess liq-
uid in the pleural space, are common features
of a multitude of diseases. Transudative effu-
sions, those not associated with inflammation
or injury, usually develop due to increased
154V. C. BROADDUS ET AL.
FIGURE 1. Fluid turnover and lymphatic drainage from the pleural space. In the normal pleural space (shown here), as in other interstitial
spaces of the body, liquid slowly filters from systemic capillaries and is absorbed via lymphatics (solid arrows). In the pleural space, the
capillary filtrate from systemic capillaries moves across a permeable pleural membrane toward the lower pressure pleural space and is
absorbed via the parietal pleural lymphatics. From there, liquid moves via lymphatic propulsion to the central veins. When interstitial
edema forms in the adjacent lung, some of that excess liquid moves across the visceral pleura into the pleural space. Asbestos fibers
may follow similar routes from the lung to the pleura and are thought to lodge in the parietal pleura preferentially at sites of lymphatic
hydrostatic pressure. In congestive heart fail-
ure, the most common cause of transudative
effusions, increased pulmonary venous pres-
sure leads to fluid accumulation in the inter-
stitial spaces of the lung; the fluid then moves
toward the lower pressure pleural space and
leaks across the visceral pleura into the pleu-
ral space (Broaddus et al., 1990; Owens &
Milligan, 1995). In the setting of inflamma-
tion or injury of the lung, pleura, or other
organs, exudative effusions may form; these
effusions contain elevated levels of protein due
to the increased leakage across capillaries with
increased permeability (Mutsaers et al., 2004).
Excess fluid in any part of the body may find
its way to the pleural space via the interstitial
tissues along pressure gradients and by moving
across the permeable pleural membranes. The
normal and pathological paths by which liquid,
cells, and particles enter and exit the pleu-
ral space suggest pathways by which asbestos
fibers may also enter and exit or fail to exit
the pleural space. The study of the physiology
of the pleural space is challenging; even when
using laboratory animal studies, analyses of the
pleural space are limited by the narrowness of
the space and the difficulty in sampling without
inducing inflammation or injury.
Pathways Leading to Translocation of
Fibers to the Pleura
The route of translocation of fibers from
the lungs to the visceral pleura, into the
pleural space, and to the parietal pleura is
unknown. It is postulated that asbestos fibers
may migrate to the lung interstitium and vis-
ceral pleura by a paracellular route or by direct
penetration across injured alveolar epithelial
cells (Miserocchi et al., 2008). Fibers may be
transported to the pleural space via the lym-
phatics and bloodstream (Oberdörster et al.,
1983). Fibers may translocate by themselves
or within macrophages. Although studies of
asbestos fiber movement have not been pos-
sible due to technical limitations, it is likely that
asbestos fibers translocate to the pleural space
passively in the same manner as interstitial
fluid. This process may be enhanced by lung
inflammation induced by asbestos fibers or by
mixed dust exposures that increase interstitial
fluid accumulation and thus fluid movement
along the interstitial spaces to the pleural space
(Miserocchi et al., 2008).
There are thus few studies that investigated
the translocation of fibers from the lung into
the pleural space. Even in the few existing stud-
ies, data from animal studies may have limited
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE 155
relevance for humans because of the differ-
ent visceral pleural anatomy in rodents. In the
rodent, the visceral pleura is “thin,” consist-
ing mostly of a mesothelial layer and basement
membrane lying directly over the alveoli. There
is little submesothelial connective tissue and
no pleural vasculature. In sheep and humans
and other large mammals, the visceral pleura
is “thick” and has a significant submesothelial
connective tissue space, containing nerves and
systemic blood vessels (Figure 1). In contrast to
the visceral pleura, the parietal pleura in differ-
ent species has a constant and similar anatomy
(Figure 1; Light & Broaddus, 2010). Thus, due
to differences in the visceral pleura, one might
postulate a difference between rodents and
humans in the movement of the fibers into the
pleural space. Due to similarities in the parietal
pleura, one might suggest that the localiza-
tion, accumulation, and actions of fibers in the
parietal pleura might be similar.
These questions have been almost impossi-
ble to address using current technology but it is
hoped that new tools and imaging techniques
such as nuclear magnetic resonance (NMR)
spectroscopy or two-photon microscopy can
be developed to provide data on (1) how fibers
distribute in the lungs and pleura, (2) the ulti-
mate destination of fibers, (3) how fiber move-
ment is enhanced, and (4) whether fibers are
translocated and retained differently in animals
and humans. Such techniques could be inva-
sive, using labeled fibers that could be traced,
for use in animal studies, and noninvasive for
clinical studies of those exposed to asbestos.
This imaging information on fiber localiza-
tion would enhance diagnosis and follow-up
of subjects exposed to asbestos, such as in
directing where to sample tissues to assess
fiber dosimetry, and how to determine pre-
neoplastic biomarkers (Greillier et al., 2008)
that might lead to intervention and preven-
tion of non-neoplastic and neoplastic pleural
Pleural Fiber Dosimetry in Rodents
Translocation and retention of fibrous
particulates from initial sites of pulmonary
deposition to extrapulmonary sites are believed
to be important aspects of their potential
toxicity (Dodson et al., 2003; Suzuki &
Kohyama, 1991). Pathologic tissue responses
such as edema, inflammation, or fibrosis might
potentially affect translocation and retention of
particulates in the body, as well as properties
of particles themselves including dose, dimen-
sions and biopersistence. Although similarities
exist between animal models and humans
concerning physiological processes such as
interstitial fluid dynamics and lymphatic flow,
there are also anatomical differences such as
in visceral pleural thickness (Tyler, 1983), as
well as physiological differences such as of
macrophage size and function, that need to be
taken into account when comparing across ani-
mal species and when extrapolating from ani-
mals to humans (Jarabek et al., 2005; Maxim &
McConnell, 2001). Rodents and humans also
differ in particle respirability (Mossman et al.,
2011) and this limits the use of rodent mod-
els for human risk assessment based on fiber
dimensions (Lippmann & Schlesinger, 1984;
Lippmann et al., 1980).
parenchyma also influences the fiber dose
that is ultimately translocated to the pleura.
Biopersistence in the lung is dependent on
(1) site and rate of deposition, (2) pulmonary
clearance parameters, (3) solubility in lung flu-
ids, (4) breakage rate and patterns, and (5) rates
of fiber translocation and retention. Surface
chemistry and diameter are important determi-
nants of solubility. Much of the knowledge base
concerning the role of biopersistence is actually
derived from studies of synthetic vitreous fibers
(Bernstein, 2007; Oberdörster, 2000).
Fiber characteristics also affect clearance
from the lung and translocation to the pleura.
Macrophage-mediated particle clearance in
the lung is likely to influence translocation of
particles to interstitial sites. There are important
interspecies differences in particle clearance, as
well as in biological effects of high pulmonary
concentrations of particles, in humans and
in different animal species (Bermudez et al.,
2002; Oberdörster, 2002). In addition, the
method of dose administration in experimental
fibers inthe lung
156 V. C. BROADDUS ET AL.
animals is shown to influence pleural pathol-
ogy outcomes following particle exposure. In
silica-exposed rats, pleural granulomas devel-
oped in animals following inhalation, but not
after instillation, and the different response was
likely due to differences in kinetics of particle
delivery and lymphatic clearance (Henderson
et al., 1995).
The effects of asbestos may be altered
when asbestos is mixed with other partic-
ulates, a situation common in occupational
and environmental exposures. Studies by Davis
and colleagues (1991) showed that coex-
posure of rats to amosite asbestos and to
quartz increased the incidence of amosite-
induced pleural mesothelioma, presumably
by elevation in fiber dosimetry and translo-
cation through the visceral pleura. Recent
studies by Bernstein and colleagues (2008)
demonstrated that coexposure of chrysotile
asbestos together with nonfibrous particulates
decreased fiber retention in the lungs of rats,
perhaps by increasing macrophage recruitment
and macrophage-mediated clearance or by
inducing more inflammatory fluid movement
to the pleura.
Fiber translocation in rodents appears to be
rapid and may be responsible in some cases
in particular for pathologic outcomes. In rats,
short chrysotile asbestos fibers are found in the
pleural space within a week following intratra-
cheal instillation (Viallat et al., 1986). Similarly,
crocidolite fibers were detected in the pleural
space 1 wk following inhalation (Choe et al.,
1997). In another study in rats, short fibers
(<5 μm length) were found 5 d after inhalation
exposure to a synthetic vitreous fiber (refrac-
tory ceramic fiber) aerosol (Gelzleichter et al.,
1996). In studies in rats and hamsters involving
chronic inhalation of synthetic vitreous fibers
as well as of amosite and chrysotile asbestos
used as reference materials, significant inter-
species differences in pleural pathology were
seen (Mast et al., 1994; McConnell, 1994).
Subsequent short-term mechanistic studies of
translocation showed that the Syrian golden
hamster, a species prone to development of
pleural fibrosis and mesothelioma following
synthetic vitreous fiber exposure, displayed
greater translocation of fibers to the pleura than
did similarly exposed rats (Gelzleichter et al.,
1999). The greater translocation in the Syrian
golden hamster may thus have accounted
for its greater susceptibility to fiber-induced
Pleural Fiber Dosimetry in Humans
There is virtually no knowledge of the
kinetics of fiber translocation and retention in
the human pleura and there are few pleural
fiber burden studies in occupationally exposed
workers. In addition, due to loss of anatomical
orientation after ashing or digestion of target
tissues, it is not known where fibers reside
intracellularly or extracellularly. Fibers are iden-
tified within mesothelial cells (Davis, 1974;
Fasske, 1986; Lee et al., 1993) but, due to
technical limitations, no comprehensive stud-
ies have quantified intracellular fiber burden
at the microscopic level. Although analytical
transmission electron microscopy (TEM) with
x-ray energy-dispersive analysis is the gold
standard for quantitation and identification of
asbestos fibers in tissue, in vivo studies would
be enhanced greatly by nondestructive imaging
approaches that could detect the presence of
fibers, their chemical composition, or even the
cellular response without destroying anatomi-
A major data gap in understanding mecha-
nisms of asbestos-related pleural disease is the
paucity of information available to determine
the dose of asbestos fibers that is deposited
and retained in the pleural membranes. This
overview describes the technical complexity
and limitations associated with quantitation of
human lung and pleural fiber burdens, as well
as summarizing available data.
Roggli (1990, 1992), Roggli and Sharma
(2004), and Dodson and Atkinson (2006)
reviewed the numerous variables and techni-
cal considerations associated with quantitation
of tissue fiber burdens in general. Their major
conclusions and caveats include:
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE157
1. The source of tissue samples ranged from
pleural biopsies obtained during diagnos-
tic thoracoscopy (Boutin & Rey, 1993); to
surgical specimens including needle biop-
sies, wedge biopsies, or pneumonectomy
or pleural decortication samples; and to
pleural and lung tissues obtained during
autopsy examination (Roggli & Sharma,
2. Regardless of the source of tissue, sampling
is a potential source of error since there
is significant variation in anatomical distri-
bution of fibers, especially in the parietal
pleura (Roggli, 1992; Boutin et al., 1996;
Mitchev, et al., 2002).
3. Tissues may be contaminated during surgical
resection or at autopsy due to fibers present
in fixatives, in specimen containers, on sur-
gical gloves, or on dissecting instruments
(Roggli & Sharma, 2004).
4. Light microscopy is inadequate for iden-
tification and counting of asbestos fibers.
Dodson and Atkinson (2006) recommend
analytical transmission electron microscopy
in combination with x-ray energy-dispersive
analysis and selected area diffraction tech-
niques for specific mineralogical identifi-
cation. Both coated and uncoated fibers,
as well as particulates, should be ana-
lyzed and quantitated (Dodson & Atkinson,
5. A systematic approach to counting fibers
of all dimensions and analysis of lung fiber
burdens needs to be used, as described by
the European Respiratory Society (DeVuyst
et al., 1988).
6. Appropriate control populations need to be
used because there is significant variability
in human lung fiber burdens (Roggli, 1990).
A systematic analysis of lung asbestos fiber
burdens in workers with asbestos-related
disease, people with asbestos exposure in
households or in buildings, and control
cases revealed a wide range of counts
with considerable overlap between workers,
other asbestos-exposed cases, and controls
(Roggli & Sharma, 2004).
7. The criteria used to define and count
asbestos fibers need to be stated explicitly.
Some investigators only count fibers longer
than 5 μm; however, the majority of
asbestos fibers in human tissue samples are
shorter than 5 μm (Dodson & Atkinson,
8. Tissue preparation techniques may intro-
duce artifacts due to tissue drying or trau-
matic disruption of fiber bundles (Dodson &
Finally, although quantitation of human
lung and pleural asbestos fiber burden is
the only technique available to assess the
dose delivered to and retained at the target
tissue, there are additional considerations
in interpretation of these data. Tissue fiber
burden depends on the time since cessation of
exposure. In addition, the fiber burden and the
types of fibers in the lung may not reflect the
fiber burden in the pleura. For example, shorter
uncoated fibers are more readily cleared from
the lungs; however, while these fibers may be
decreasing in the lungs, they may be accumu-
lating in the pleura and extrapulmonary sites
(Holt, 1981) and be associated with develop-
ment of disease at these sites (Dodson &
It is important to note that the lungs of nor-
mal control cases evaluated at autopsy contain
significant numbers of commercial and non-
commercial asbestos fibers, as well as other
particulate and fibrous minerals. This is note-
worthy especially in lungs from those who
resided in urban settings (Table 1). By com-
paring lung fiber burdens between those with
pleural mesothelioma and those without, inves-
tigators showed that, although there is overlap,
there is an increased risk for mesothelioma,
with an elevated lung burden of certain fibers,
such as crocidolite, amosite, and tremolite;
due to its lower biopersistence, chrysotile may
not be reliably analyzed by autopsy studies
In contrast to these and other studies of
fiber burdens in the lung, only a few stud-
ies have reported asbestos fiber burdens in
the pleura. In those few studies that analyzed
pleural fiber burden, the results from lung and
158 V. C. BROADDUS ET AL.
TABLE 1. Asbestos Fiber Content in Lung Tissue of an Urban
Fiber type Fiber number/g wet lung
Chrysotile asbestos fibers
Amosite and crocidolite
130.0 × 103
2.5 × 103
15.0 × 103
5.1 × 103
3.7 × 103
1.1 × 103
Note. Analysis of 21 urban cases using analytical transmis-
sion electron microscopy with analysis of all fibers longer than
1 μm revealed these average fiber numbers/g wet lung. (Churg &
TABLE 2. Lung Fiber Burdens in Malignant Mesothelioma
with fibers Fiber type
Note. In a study of young persons (age 50 yr or less at the time
of diagnosis), the lungs of 69 patients who had died with malig-
nant pleural mesothelioma and 57 controls selected from the
national work-related disease surveillance system in the United
Kingdom were analyzed by electron microscopy for fiber distri-
bution. Increased odds ratios for mesothelioma were found for
crocidolite, amosite, and tremolite; the contribution of chrysotile
was less clear due to low biopersistence. Nonasbestos fibers
probably made no contribution to mesothelioma in this study
(McDonald et al., 2001).
pleura differed, perhaps due to the technical
problems described earlier, and appeared to
indicate that pleura has a predominance of
short chrysotile fibers. Sebastien et al. (1980)
concluded that lung fiber burden could not
be used as an accurate reflection of pleural
fiber burden. In their parietal pleural samples,
most of the asbestos fibers were short chrysotile
fibers. Gibbs et al. (1991) also reported lower
asbestos counts in the visceral pleura than in
matched lung samples from the same patients
and found mostly short chrysotile asbestos
Dodson et al. (1990) analyzed lung tissue,
lymph nodes, and pleural plaques obtained at
autopsy from eight shipyard workers in Italy.
Data showed both chrysotile and amphibole
asbestos fibers in the lungs; however, chrysotile
asbestos fibers were the most frequent type
of asbestos found in pleural plaques. Most
fibers in the lymph nodes and pleural plaques
were shorter than 5 μm, although some fibers
longer than 8 μm were present at these sites.
More recently, Suzuki and his coworkers (2005)
compared asbestos fiber burdens of human
mesothelioma tissues obtained following bulk
tissue digestion or ashing of 25-μm tissue
sections using high-resolution analytical elec-
tron microscopy. The majority of fibers were
≤5 μm long and 92.7% were ≤0.25 μm wide.
Chrysotile asbestos fibers were identified most
frequently in a total of 168 cases of human
malignant mesotheliomas obtained from biopsy
or autopsy specimens (Suzuki et al., 2005).
In an earlier study, Suzuki and Yuen (2001)
detected only short, thin chrysotile asbestos
fibers in 25.7% of the lungs and in 77.4% of
the mesothelial tissues of patients with malig-
nant mesothelioma. These tissue samples were
obtained from cases throughout the United
States that were sent to Dr. Suzuki for patho-
logical review and were systematically ana-
lyzed using histology, immunohistochemistry,
and electron microscopy, in some cases, over
a 15-yr period. As summarized succinctly by
Dumortier et al. (1998) in a letter to the edi-
tor in 1998, the size and type of asbestos
fibers associated with development of diffuse
malignant pleural mesothelioma remain con-
troversial (Mossman et al., 2011; Case et al.,
2011; Aust et al., 2011).
One possible explanation for the confusion
in pleural sampling came from a pioneering
study carried out by Boutin et al. (1996).
Using video-assisted fiber-optic thoracoscopy
in eight asbestos-exposed patients and six
unexposed cases, Dr. Boutin and colleagues
(1996) sampled specific anatomic regions of
the parietal pleura identified as collecting
spots for inorganic particulates and fibers that
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE159
translocate to the pleural spaces. These regions
are called “black spots” due to localized
accumulation of carbon particles and are sites
of lymphatic drainage located in the lower
coastal regions of the parietal pleura and on
the superior dome of the diaphragm. Using
transmission electron microscopy, Boutin et al.
(1996) identified numerous amphibole as
well as chrysotile fibers at black spots, and
22.5% were ≥5 μm long. The mean asbestos
fiber concentration in the 8 exposed cases
was 12.4 ± 9.8 × 106fibers/g dry lung tissue,
4.1 ± 1.9 × 106fibers/g black spots on the
parietal pleura, and 0.5 ± 0.2 × 106fibers/g
normal parietal pleura, using bleach digestion
of lung tissue and low-temperature ashing of
pleural tissue samples. Evidence indicated that
asbestos fibers accumulate in focal areas of the
parietal pleura and that these “black spots”
are the most likely anatomic origin of diffuse
malignant mesothelioma. In a subsequent
study of black spots analyzed from 150 consec-
utive autopsies of urban residents in Brussels,
Belgium, the histopathological appearance of
black spots showed chronic inflammation with
lymphocytes, plasma cells, and macrophages
with a variety of particulates and fibers both
et al., 2002). Of note, there was no anatomic
relationship between black spots and parietal
pleural plaques. Black spots were present
in 92.7% of these cases; these lesions were
more numerous in older cases and in males.
Evidence indicated that these cases may have
had greater exposure to coal dust used for
home heating and in industry. In this case
series, asbestos bodies >1000/g dry lung were
found in 15 of 97 cases studied; unfortunately,
pleural samples were not analyzed for the
presence of asbestos fibers (Mitchev et al.,
2002). The discrepancy between studies of
pleural fiber burden and distribution may thus
be explained by the inhomogeneity of fiber
deposition in the parietal pleura. Since this
important observation of the localization of
pleural fibers in black spots, almost no studies
addressed pleural fiber burden to clarify which
fibers are present and which fibers are asso-
ciated most closely with asbestos-induced
Knowledge and Data Gaps in Fiber
Translocation and Dosimetry
In considering the data existing on the sub-
ject of fiber translocation and dosimetry, there
are numerous gaps in the knowledge base that
may be amenable to newer methods.
(a) There is a significant lack of understanding
of the contributions of the various potential
routesof fiber translocation,
direct interstitial transport, macrophage-
mediated transport, lymphatic transport,
and hematogenous transport. There is lit-
tle known about which fibers move out
from the lung to the pleura and at what
rate, and which accumulate in the pleura.
There is a need to improve our under-
standing of the kinetics of fiber translocation
and pathogenic pleural responses follow-
ing mixed asbestos fiber exposure and with
coexposure to other particulates.
(b) There are gaps in understanding the role
of fiber dose, dimension, and type in the
induction of pleural lesions. There is a
lack of understanding of the relationship
between pleural fiber burden and disease in
mixed fiber dust exposure. To date, exper-
imental animal studies only examined rel-
atively limited size fractions of fibers, due
to limited respirability in rodent studies
(Lippmann & Schlesinger, 1984; Lippmann
et al., 1980).
(c) There is a need to study the role of short,
thin fibers in the induction of pleural lesions.
Most pleural disease is believed to be
due to amphibole exposure (Churg, 1982;
Roggli et al., 2002), and most disease was
ascribed to long, thin fibers, but there is
still much uncertainty concerning the con-
tributions to disease of short, thin fibers
that predominate in pleural fiber burden
studies (Dodson et al., 2003; Suzuki et al.,
2005; Mossman et al., 2011; Aust et al.,
2011; Case et al., 2011). While the pre-
ponderance of evidence shows that long,
thin fibers are the most pathogenic, there
160 V. C. BROADDUS ET AL.
is little understanding of how dose, sur-
face properties, and biopersistence of short,
thin fibers affect the exposure-response
relationship with respect to non-neoplastic
pleural outcomes in mixed exposures. This
need is made more urgent with recent
findings concerning the pleural effects of
engineered fibrous nanomaterials such as
instilled (Poland et al., 2008) and inhaled
(Ryman-Rasmussen et al., 2009) carbon
nanotubes in mice.
(d) There is a significant lack of information cor-
relating kinetics with pathological outcomes
in the pleura following experimental fiber
exposures in laboratory animals. Maxim and
McConnell (2001) suggested that humans
and rats are similar in pathological responses
to fibers with respect to pulmonary fibrosis
outcomes; as yet, there are no comparable
data for pleural fibrosis.
(e) There is need to develop fiber size separa-
tion methods to enable mechanistic studies
of characterized fiber preparations. This will
allow an understanding of the role of fiber
size and dimension on cellular targets of
(f) In general, it is not understood how inhala-
tion of fibers leads ultimately to pleural
disease. To date there have been few inhala-
tion studies with well-characterized aerosols
of different asbestos fiber types in exper-
imental animals. Most inhalation bioassays
have been long-term hazard assessment
studies in animal models; otherwise, studies
have relied on short-term instillation stud-
ies in rodents or in vitro studies. The cost,
complexity, and specialized requirements
of inhalation studies with fibers have not
allowed routine state-of-the-art fiber inhala-
tion exposures in support of mechanistic
INTERACTION OF FIBERS WITH
TARGET CELLS IN THE PLEURA
For reasons yet to be fully elucidated,
fibrous particulates have an unusual affinity
for the visceral and parietal pleura, and these
tissues are sites for inflammatory, fibroprolif-
erative, and neoplastic diseases in humans
and in experimental animals. Non-neoplastic
asbestos-associated diseases of the pleura in
humans include benign asbestos-related pleu-
ral effusion, pleural plaques, diffuse pleural
thickening, and rounded atelectasis (Chapman
et al., 2003; Nishimura & Broaddus, 1998).
develop in rodents in response to inhaled
fibers, but these have not been categorized
into separate lesion types as is the case in
humans and often have not been described
separately from pulmonary parenchymal fibro-
sis by toxicologic pathologists. It is noteworthy
that significant pleural lesions similar to human
pleural fibrotic lesions were found in chronic
rodent inhalation bioassays with synthetic vit-
reous fibers as well as with asbestos fibers
(McConnell et al., 1999).
Mesothelial cells, resident and elicited
inflammatory cells, and pleural fibroblasts are
believed to be important effector cells in
the pathogenesis of asbestos-induced non-
neoplastic pleural diseases. Mesothelial pro-
genitor cells may also participate in pleural
repair and disease (Herrick & Mutsaers, 2004).
There have been numerous studies both in
vivo in experimental animals and in vitro using
human and animal cell culture systems that
review the cellular interactions in pleural tis-
sues and in the pleural space (Chapman et al.,
2003; Mutsaers et al., 2004, 2006; Robledo &
Mossman, 1999). It is known that following
inhalation and instillation of asbestos fibers
into the lung, there are rapid alterations in
both resident and elicited populations of pleu-
ral inflammatory cells and mesothelial cells.
Pleural inflammatory cell changes were pro-
duced in rats in association with translocation
of asbestos fibers (Choe et al., 1997) or fol-
lowing particulate-induced pulmonary inflam-
mation itself (Lehnert et al., 1985). Changes
were noted in mesothelial cells of the vis-
ceral pleura at early time points following
asbestos instillation and inhalation (Adamson,
1997; Dodson & Ford, 1985), and interac-
tions between pleural inflammatory cells and
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE161
mesothelial cells are believed to be important
in the development of fiber-induced pleu-
ral injury and disease. Rat and rabbit pleu-
ral mesothelial cells are known to release
chemoattractants for inflammatory cells follow-
ing exposure to asbestos (Boylan et al., 1992;
Hill et al., 2003; Tanaka et al., 2000), and pleu-
ral macrophage-derived mediators can modu-
late mesothelial cell function (Baumann et al.,
Mesothelial Cell Biology and Fiber
During the past two decades, there has
been a great increase in our knowledge of
the importance of the mesothelial cell in fiber-
induced pleural disease. It has become appar-
ent that these cells play a central dynamic role
in the control of injury and repair processes that
take place in the pleural and other serosal tis-
sues (Mutsaers et al., 2004). Mesothelial cells
are a unique cell type originating from meso-
derm and vested with a number of impor-
tant specialized functions including release
of pro- and anti-inflammatory and other
immunomodulatory mediators; secretion of
factors that promote deposition and clear-
ance of fibrin; and synthesis of growth factors
and extracellular matrix proteins that aid in
serosal repair (Jantz & Antony, 2008). Asbestos
may injure pleural mesothelial cells either
by direct or indirect mechanisms, including
(1) injury by free radicals, (2) inflammasome
activation, (3) alterations of intracellular sig-
naling pathways, (4) release of cytokines and
chemokines, (5) physical disruption of chro-
mosomes, (6) alterations in growth factors, and
(7) changes in coagulation and fibrinolysis
pathways (Manning et al., 2002; Mutsaers
et al., 2004; Robledo & Mossman, 1999).
Mesothelial cells internalize the fibers via inte-
grins or other receptors, and uptake of the
fibers was found in some studies to be nec-
essary for adverse effects of the fibers such
as reactive oxygen species (ROS) generation,
DNA damage, and apoptosis (Liu et al., 2000).
Reactive oxygen species derived directly from
the surface chemistry of fibers themselves
(Fubini, 1997) as well as from cellular responses
are believed to be important in both neo-
plastic and non-neoplastic asbestos-associated
pleural disease (Janssen-Heininger et al., 2008;
Shukla et al., 2003a). Although the limitations
of cell culture systems for particulate studies
have been well described, it is recognized that
much of the mechanistic understanding of how
fibers interact with mesothelial cells and pro-
duce fiber-induced effects derives from in vitro
experiments (Donaldson et al., 2009).
Knowledge and Data Gaps Concerning
Pleural Cell Biology and Asbestos Fiber
(a) Fibers may translocate to the pleural space
and are postulated to induce pleural dis-
ease by direct interaction with pleural cells.
However, some pleural conditions such as
inflammation or fibrosis may be influenced
by fibers and their actions in the neighboring
lung. The relative contribution of direct fiber
exposure versus indirect signaling effects on
mesothelial cells is not known.
(b) There is still much to know about how
fiber mineralogy, dimensions, physicochem-
ical properties, and biopersistence con-
tribute to induction and progression of
(c) There is need for better development
assessment of pleural changes in exper-
imental animal models. Although bron-
choalveolar lavage fluid (BALF) analysis
has been routinely utilized in experimental
studies of the lung, pleural lavage has not
been routinely used to assess changes in
the pleural space. Advancing understanding
of pleural disease will require better use
of pleural endpoints in acute and chronic
studies of fiber exposure.
(d) The major target of asbestos in the pleural
space is thought to be the mesothelial
cell. The contribution of the inflamma-
tory pleural cells including macrophages is
less well understood. In addition, there is
a need to develop additional understand-
ing of possible mesothelial progenitor cells
162 V. C. BROADDUS ET AL.
in asbestos-associated injury and disease
(Herrick & Mutsaers, 2004).
(e) The thoracoscopic study by Boutin et al.
(1996) demonstrating the focal accumula-
tion of fibers in “black spots” of the pari-
etal pleura provided important new insights.
Similar studies might advance the under-
standing of pleural fiber burden and disease.
By obtaining biopsy samples from those
undergoing thoracoscopy or thoracotomy,
one could determine the locations of fibers
and the genetic changes at sites of asbestos
deposition, and investigate biomarkers of
asbestos toxicity. A systematic analysis using
analytical transmission electron microscopy
could quantitate the dimensions and types
of mineral particles and fibers that are
translocated to and retained in the pleura
(both visceral and parietal) of control indi-
viduals and patients with asbestos-related
pleural diseases. Such studies may also help
identify whether fibers are located predom-
inately intracellularly or extracellularly, and
identify the target cells.
(f) The role of the specific arms of the inflam-
matory response can now be studied using
mice with genetically engineered deletion of
specific cell types or inflammatory cytokines.
Such studies can be used to indicate the role
of inflammation in producing the various
fiber-induced pleural diseases and whether
particular inflammatory mechanisms might
be a therapeutic target.
(g) Noninvasive techniques for assessing fiber
burden or the tissue reaction to fibers would
be of inestimable value in investigating the
natural history of pleural reactions in ani-
mals and in humans over the decades of
tumor development. Novel imaging tech-
niques could ultimately serve as a tool for
following those at risk and testing strategies
Biological Mechanisms Responsible for
Non-Neoplastic Pleural Disease
It has generally been accepted from stud-
ies of animal models of asbestos fiber exposure
that inflammatory changes in the lung and
pleura precede subsequent fibroproliferative
and mesothelial cell proliferative responses.
The early pioneering intracavitary instillation
and implantation studies of Freidrich Pott and
coworkers (Pott, 1980; Pott et al., 1974) and
Merle Stanton and colleagues (Stanton et al.,
1969, 1977, 1981) revealed that the phys-
ical properties of fibers such as dimension
are important in the pathogenesis of asbestos-
associated disease of the serosal tissues (Case
et al., 2011; Aust et al., 2011).
Since those initial studies, much has been
learned about other physicochemical proper-
ties of inhaled particles believed to be impor-
tant in their disease-inducing abilities such as
the surface properties relevant for oxidant gen-
eration (Fubini, 1997) and the chemical proper-
ties relevant for biopersistence (Bernstein et al.,
2001, 2005; Bernstein, 2007; Mossman et al.,
It is noteworthy
biopersistence studies focused on the lung
parenchyma. There are few pleural fiber bur-
den or pleural biopersistence investigations in
either experimental animals or humans.
Fiber Type and Potency
In rodent studies in which high concentra-
tions of fibers were instilled or implanted in
the pleural space, all mineralogical forms of
asbestos fibers were produced pleural fibrosis
and malignant mesothelioma. In an inhalation
study in rats using well-characterized aerosols
and state-of-the-art methods to assess retained
lung burdens, Bernstein et al. (1995) found
that chrysotile exposure failed to induce pleu-
ral lesions despite producing severe pulmonary
fibrosis (asbestosis) and lung tumors (Mast
et al., 1994). This appears to correlate with
human epidemiologic studies because recent
analysis suggests that most asbestos-associated
mesotheliomas are due to amphibole expo-
sure (Berman & Crump, 2008; Mossman et al.,
While there is a general lack of under-
standing of comparative pleural potency of
different asbestos fiber types, a reanalysis of
previous asbestos fiber inhalation studies in
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE163
rats compared size, shape, and mineralogy
with lung tumor and mesothelioma outcomes
(Berman et al., 1995). In this study, multivari-
ate measures of exposure were identified that
described the lung tumor responses in 13 pre-
vious asbestos (chrysotile, amosite, crocidolite,
tremolite) inhalation experiments in AF/HAN
rats. Due to limitations in the characteriza-
tion of asbestos fiber dimensions in the original
studies, new exposure measures were devel-
oped from samples of the original dusts that
were regenerated and analyzed by transmis-
sion electron microscopy using a direct transfer
technique. Structures contributing to lung can-
cer risk appeared to be long (≥20 μm) and
thin (≤0.4 μm) fibers. The analysis did not find
significant mineralogical differences in potency
across asbestos types for pulmonary tumors
but noted that amphibole asbestos was more
potent than chrysotile in induction of malignant
Proposed Mechanisms for
Pleural Lesions in Rodents
Although the rodent visceral pleura differs
markedly from that of humans in thickness and
and the resident pleural inflammatory cells are
similar to those of humans (Everitt et al., 1997;
Gelzleichter et al., 1996). In rodents, asbestos
and synthetic vitreous fiber exposure by inhala-
tion or instillation resulted in pleural inflam-
matory and fibrotic changes (McConnell et al.,
1999). In animal models of asbestos-induced
lung cancer and mesothelioma, inflammation
and fibrosis always preceded the development
of oncogenic outcomes (Greim et al., 2001),
and these processes may share some mech-
anistic underpinnings. Similarly, it is worthy of
conducted to date, pleural inflammatory and
fibroproliferative lesions were accompanied
by pulmonary parenchymal changes. Recently,
there have been a number of chronic studies
that suggested that the Syrian golden hamster
may be particularly susceptible to the devel-
opment of pleural mesothelioma as well as
of pleural fibroproliferative changes following
asbestos and synthetic vitreous fiber inhalation
(Everitt et al., 1997; Gelzleichter et al., 1999;
Mast et al., 1994; McConnell et al., 1999).
responses to inflammatory mediators released
from lung parenchymal and pleural cells are
believed to be important in the pathogenesis of
pleural fibrosis and asbestos-associated pleurisy
(Mutsaers et al., 2004; Robledo & Mossman,
1994). Mesothelial cells are known to phagocy-
tize asbestos fibers (Boylan et al., 1995), a step
that may then lead to oxidant-induced signaling
pathways, altered cell proliferation, apoptosis
(Liu et al., 2000; Shukla et al., 2003b), necrosis
(Yang et al., 2010), and release of chemokines
and cytokines that mediate pleural inflamma-
tion (Jantz & Antony, 2008). A variety of growth
factors are associated with pleural fibrosis,
especially transforming growth factor (TGF)-β1
(Decologne et al., 2007; Mutsaers et al.,
Proposed Mechanisms for
Pleural Disease in Humans
asbestos in the pleura include benign asbestos
pleurisy, pleural plaques, diffuse pleural fibro-
sis, and rounded atelectasis (ATS Official
Statement, 2004; Nishimura
1998). Although these nonmalignant pleural
diseases may themselves produce symptoms,
especially diffuse pleural fibrosis, these are
important clinically because the symptoms
identify people who have had significant expo-
sure to asbestos and often mimic and require
diagnostic workups to exclude malignancy.
These diseases are also important for research
by giving insight into fiber toxicology and
pathogenesis and by identifying groups for
which development of biomarkers and early
intervention for diagnosis or treatment are
Benign asbestos pleurisy may develop as
early as 10 yr after exposure (ATS, 2004).
Because it usually produces no apparent
164 V. C. BROADDUS ET AL.
symptoms and is often detected incidentally,
the incidence is unclear. The benign pleural
effusion may be bloody, thus leading to con-
cern for underlying malignancy. The effusion
may last for months, may be unilateral or bilat-
eral, and may recur. The effusion may ante-
date diffuse pleural fibrosis (Lillis et al., 1988),
although the reason for this association is not
Pleural plaques are the most common
pleural manifestations of asbestos exposure
and represent evidence of clinically significant
exposure, retention, and biologic response to
fibers. In general, plaques develop 20–30 yr
after initial exposure (Nishimura & Broaddus,
1998; ATS, 2004). Plaques are usually located
on the parietal pleura or on the dome of the
diaphragm and appear as circumscribed areas
of collagen deposition without inflammation.
Plaques may be associated with decreases in
lung function and symptoms of dyspnea, but
most individuals with pleural plaques alone dis-
play no apparent symptoms and no obvious
impaired lung function. Although localized and
unilateral pleural thickening may have other
causes such as prior tuberculosis, trauma, or
talc instillation, multiple and bilateral pleu-
ral plaques, particularly when calcified, are
considered to be pathognomonic for asbestos
or erionite exposure (Nishimura & Broaddus,
1998). Of note, those subjects without plaques
may also have significant asbestos exposure; it
is not known why some exposed individuals
form plaques and others do not.
Plaques are biomarkers for asbestos or eri-
onite exposure and of elevated fiber burden in
the lung (Churg, 1982; Kishimoto et al., 1989;
Roggli & Sanders, 2000). It is not known how
pleural plaques correlate with pleural fiber bur-
den. Different fiber types may play a role in
plaque formation: Plaques have been associ-
ated with the presence of high aspect ratio
amphiboles in the lung (Churg, 1982, 1983,
1994), but in at least one study, only chrysotile
fibers were found in the plaques themselves
The biologic response to asbestos fibers in
individuals with pleural plaques may differ from
those without plaques. In animal studies pleural
plaques were found to be a consequence of
the cellular inflammatory response to asbestos
(Sahn & Antony, 1984). In any case, pleu-
ral plaques represent a marker of exposure to
asbestos and therefore a marker of increased
risk for asbestos-related disease, and perhaps
may be used to select patients for focused clin-
ical trials to assess other biomarkers of risk
and to identify prevention strategies. In gen-
eral, understanding more about the formation
of pleural plaques and their significance as a
biomarker of exposure and of enhanced risk
would provide insight into pathological mech-
anisms and suggest possible interventions in
Rounded atelectasis is thought to be a
consequence of any type of pleuritis and pleu-
ral fibrosis and represents a folding of the
lung within a region of pleural thickening. It
may resemble a mass and thus raise concern
for lung cancer. Little is known regarding the
pathogenesis of this entity (Hillerdal, 1989).
Diffuse pleural thickening is a diffuse,
not circumscribed, thickening of the pleura
that develops approximately 30 yr following
exposure (Nishimura & Broaddus, 1998; ATS,
2004). Unlike pleural plaques, diffuse thicken-
ing mostly affects the visceral pleura. Diffuse
pleural thickening is associated with clinically
significant ventilatory impairment, pulmonary
restriction, and low lung volumes. Diffuse pleu-
ral thickening may coexist with pleural plaques,
and may be associated with a higher fiber bur-
den than is found with pleural plaques alone
(Stephens et al., 1987). The relationship of
diffuse pleural thickening to asbestos-induced
fibrosis of the lungs is not known. The types of
asbestos fibers likely to produce diffuse pleu-
ral thickening are not known. In one study, the
fibers found in the pleura were short chrysotile
fibers, while the fibers in the lungs were longer
and thinner amphiboles (Gibbs et al., 1991).
As with other non-neoplastic asbestos-induced
pleural disease, diffuse pleural thickening raises
concerns for underlying mesothelioma. If lung
function is severely compromised, the patient
may undergo decortication or removal of the
pleura; nevertheless, removal of the thick-
ened pleura may not improve lung function
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE165
due to accompanying fibrosis of the underly-
These non-neoplastic pleural pathologies
are particularly common in those exposed to
amphibole fibers in Libby, MT (Peipins et al.,
2003), suggesting that these fibers may exert
unique toxicity for the pleura. Libby amphi-
bole is a mixture of winchite, richterite, and
tremolite, in decreasing order of abundance
(Meeker et al., 2003). In 1980, a morbidity
study was carried out on workers who had
used Libby vermiculite as an inert carrier for
various types of lawn-care products (Lockey
et al., 1984). Libby vermiculite was found
to contain asbestiform minerals. In the work-
ers exposed to Libby vermiculite, workplace
exposures were associated with bloody pleu-
ral effusions and localized pleural thickening. A
follow-up study of the worker cohort 25 years
after discontinuation of Libby vermiculite min-
ing in 1980 demonstrated an elevated preva-
lence of pleural changes, increasing from 2%
in 1980 to 29% (80 of 280 workers) in 2005
(Rohs et al., 2005). Of workers with a low life-
time cumulative fiber exposure (CFE) of only
<2.2 fibers/cc-yr, as many as 20% displayed
pleural changes. A significant CFE response
relationship was demonstrated between per-
cent pleural changes, which ranged from 7
to 54%, and the lowest to the highest CFE
quartile. The mean CFE (SD) related to local-
ized pleural thickening, diffuse pleural thick-
ening, and interstitial fibrosis in vermiculite
workers with no historical exposure to com-
mercial asbestos was 3.45 (4.95), 8 (5.32),
and 11.37 (6.82) fiber-cc/yr, respectively (Rohs
et al., 2005). This relationship was confirmed
by Whitehouse (2004), who demonstrated
progressive loss of lung function in Libby
residents with and without reported occupa-
tional exposure who had predominantly pleu-
ral changes. Studies of Libby miners and millers
demonstrated an association between Libby
amphibole exposure and increased incidence
of nonmalignant respiratory disease mortality
at a CFE of less than 4.5 fiber/cc-yr (Sullivan,
2007). In summary, studies of workers exposed
to the Libby amphibole indicate the propensity
for these amphiboles to induce pleural disease
and nonmalignant respiratory morbidity and
mortality at relatively low lifetime cumulative
fiber exposure levels.
Knowledge and Data Gaps
in Nonmalignant Pleural Disease
Unanswered questions regarding nonma-
lignant pleural disease include:
(a) Pleural plaques have been associated with
long amphibole fibers in the lung and with
short chrysotile fibers in the pleura. Thus,
it is not known which types of asbestos
fibers induce pleural plaques and how pleu-
ral plaques correlate with pleural (not lung)
(b) The fiber burden and fiber types in the
pleura of those with pleural disease have not
been documented. Fiber burden in the lung
may not correlate with that in the pleura:
Low counts in the lung may be associated
with high counts in the pleura if fibers have
translocated to the pleura; fiber types found
in the lung may be the ones that are retained
at this site, whereas different fibers may
translocate to the pleura and induce dis-
ease there. Autopsy studies might be used to
compare lung and pleural fiber burdens and
relative distribution of different fiber types
and sizes in these different locations.
(c) Most studies of pleural fiber burden reported
the presence of short chrysotile fibers, and
yet the role of these short chrysotile fibers
in pleural disease has not been established.
Because most pleural disease has been
attributed to high aspect ratio amphibole
fibers, it is not known whether the short
chrysotile fibers are pathogenic, either alone
or by enhancingthe toxicityof longer amphi-
bole fibers, or whether they are acting as
bystanders. These fibers may be located out-
side the area of interest, corresponding to
the “black spots” where pathogenic fibers
are located. Further animal studies using
well-characterized short chrysotile fibers in
the pleural space would be valuable in
addressing this important issue.
166 V. C. BROADDUS ET AL.
(d) Although the incidence and severity of pleu-
ral disease following exposure to Libby
amphibole is high, it is not yet known
whether it is actually higher than after expo-
sure to other asbestos types. If Libby amphi-
bole is particularly toxic for the human
pleura, the mechanism is not known; per-
haps Libby fibers are more readily translo-
cated and retained in the pleura or the
Libby fibers that reach the pleura are par-
ticularly toxic. In vitro and in vivo studies
using Libby amphibole can address these
(e) The genetic determinants of the individ-
ual responses to asbestos are not known–
genetic factors may determine susceptibility
to non-neoplastic and neoplastic pleural dis-
ease following asbestos exposure. Similarly,
it is not known whether genetic differences
explain why some individuals develop a
fibrotic response while others have a neo-
plastic response. Genetic studies of affected
individuals and their families would be valu-
able to address this issue.
(f) New mechanistically oriented, short-term
testing strategies need to be developed to
assess pathogenicity of fiber and particulate
preparations, not only with respect to car-
cinogenicity but also for fibrotic and inflam-
matory changes in the pleura. The reason
for the propensity of the Syrian golden ham-
ster to develop pleural disease needs further
(g) There is a need to compare nanoparticle-
induced lung and pleural changes with
asbestos-associated pleural diseases to iden-
tify specific physicochemical determinants
(h) The role of inflammation in the develop-
ment of pleural fibrosis is not understood.
Studies are needed in the role of inflamma-
tory cells and of profibrotic cytokines such
as TGF-beta. The role of specific recep-
tors such as the Nalp3 inflammasome and
its contribution to chronic inflammatory
states (Dostert et al., 2008) need additional
RESPONSIBLE FOR NEOPLASTIC
Fiber Type and Potency
The most potent risk factors for diffuse
malignant mesothelioma are environmental or
occupational exposure to erionite, asbestos
fibers, and vermiculite that contains noncom-
mercial amphiboles (Institute of Medicine,
2006). Based on a recent meta-analysis of the
epidemiologic evidence (Berman & Crump,
2008), amphibole asbestos is more potent than
chrysotile asbestos in inducing diffuse malig-
nant mesothelioma. This difference in potency
was attributed to the greater biopersistence
of amphibole asbestos in lungs in comparison
with chrysotile asbestos (Bernstein & Hoskins,
2006). Development of diffuse malignant
mesothelioma following exposure to chrysotile
asbestos is attributed to contamination of some
chrysotile deposits with tremolite, a naturally
occurring amphibole (Institute of Medicine,
Biopersistence in the lungs is a key physic-
ochemical property of crystalline mineral fibers
and is associated with induction of fibrosis,
lung cancer, and malignant mesothelioma in
rodent models (ILSI, 2005). Biopersistence in
the pleura has not been studied extensively.
Boutin and Rey (1993) recovered asbestos
fibers in parietal pleural samples of asbestos
workers during thoracoscopy. It is likely that
long asbestos fibers accumulate at the pari-
etal pleural membrane because they cannot be
efficiently cleared through lymphatic stomata.
Earlier studies reported low pleural asbestos
fiber burdens in asbestos workers (Gibbs et al.,
1991). More recent studies recovered large
numbers of short asbestos fibers from the
lungs and pleural tissues of asbestos-exposed
patients (Dodson et al., 2003, 2005, 2007;
Suzuki et al., 2005). Mechanistic studies con-
ducted in cell culture associated exposure to
long asbestos fibers with activation of the EGF
receptor and intracellular signaling pathways
leading to cell proliferation (Pache et al., 1998;
Mossman et al., 2011). Asbestos fibers were
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE167
also found to interfere physically with the
mitotic apparatus (Hei et al., 2000; Huang
et al., 2011).
Fibers may be altered secondarily in the
lungs or pleura. Depending on their chemi-
cal composition, surface area, and crystalline
structure, asbestos fibers may leach, split, or
break (ILSI, 2005). The kinetics of fiber alter-
ation and clearance from the pleural space
has not been investigated. Secondary modifi-
cation of surface properties including binding
of phospholipids, acquisition or depletion of
cations, and protein adsorption in the pleura
may also modify toxicity (Fubini & Mollo,
Proposed Mechanisms for
Asbestos and erionite fibers were shown
to induce genotoxicity directly. Chronic rodent
studies established an association between
persistent inflammation and carcinogenicity
induced by inhalation of crystalline mineral
fibers (ILSI, 2005). Chronic inflammation trig-
gered in response to biopersistent fibers may
amplify the genotoxicity of asbestos fibers in
pleural target cells (Figure 2). Following inter-
nalization by phagocytosis, asbestos fibers trig-
ger macrophage activation and generation of
reactive oxygen and nitrogen species, leading
Translocation of asbestos fibers to the pleural space
Asbestos fibers + macrophages
recruitment and activation
Release of ROS, RNS, cytokines, chemokines, growth factors
DNA damage, disruption of mitosis, apoptosis/necrosis
Activation of intracellular signaling pathways
Resistance to apoptosis
Sustained cell proliferation
Impaired DNA repair
Chromosomal and epigenetic alterations
Inactivation of tumor suppressor genes omit;activation of oncogenes;
FIGURE 2. Proposed mechanisms for asbestos-induced mesothelioma. Asbestos fibers are thought to lead to mesothelioma via
mechanisms as outlined in this algorithm. Asbestos fibers enter the pleural space, where they interact with pleural macrophages and
mesothelial cells and induce an influx of inflammatory cells. These early interactions result in release of reactive oxygen and nitrogen
species (ROS, RNS), cytokines, and growth factors that may mediate indirect effects on mesothelial cells. The fibers may also act directly
on mesothelial cells by inducing DNA damage, interrupting chromosomal segregation, or inducing apoptosis or necrosis. Such direct and
indirect actions lead to chronic stimulation and injury of the mesothelium that may proceed over decades by a multistep path to cancer.
Key steps in the development of cancer include genetic and epigenetic alterations leading to sustained cell proliferation, resistance to
apoptosis, and inactivation of tumor suppressor genes.
168 V. C. BROADDUS ET AL.
to tissue injury. Recent studies in genetically
engineered mice suggest a central role for
the NALP3 inflammasome in rapid release of
active IL-1β (Cassel et al., 2008; Dostert et al.,
2008), a cytokine that triggers recruitment of
additional inflammatory cells and release of
cytokines (tumor necrosis factor [TNF]-α, inter-
leukin [IL]-6, IL-8) that perpetuate inflamma-
tion in response to biopersistent asbestos fibers
(Shukla et al., 2003a). TNF-α also activates the
nuclear factor (NF)-κB pathway in mesothelial
cells, allowing these cells to survive and prolif-
erate in the presence of asbestos-induced DNA
damage (Yang et al., 2006).
Amphibole asbestos fibers contain surface
redox-active iron (Fe) that generates ROS lead-
ing to lipid peroxidation, protein oxidation,
and DNA damage in lung and pleural target
cells (Manning et al., 2002). Erionite fibers may
secondarily acquire Fe that catalyzes genera-
tion of ROS (Hardy & Aust, 1995; Aust et al.,
2011). Secondary deposition of endogenous
Fe may enhance redox activity or disrupt Fe
homeostasis in the lungs or pleura producing
oxidative stress (Ghio et al., 2008). In response
to chronic oxidative stress, intracellular signal-
ing pathways trigger activation of transcription
factors, stimulation of cell proliferation, and
resistance to apoptosis (Albrecht et al., 2004;
Mossman et al., 2011).
Asbestos in cell culture (Broaddus et al.,
1996; Berube et al., 1996) and in some ani-
mal studies (Marchi et al., 2000) was found
to induce apoptosis in mesothelial cells, and
ROS may contribute to this early apoptosis
(Broaddus et al., 1996); at later times, DNA or
chromosomal damage may also trigger apopto-
sis. Presumably, it is the cells that have inherent
resistance to apoptosis or acquire resistance
that will survive the initial and ongoing dam-
age to initiate multistep acquisition of genetic
abnormalities that characterize tumor develop-
ment (Broaddus, 1997). Mesothelial cells (or
other progenitor cells) may acquire resistance
by inherent overexpression of anti-apoptotic
molecules or more likely by upregulation of
these molecules, e.g., the Bcl-2 or the IAP
family. Mesothelial cells with other preexisting
abnormalities in DNA damage-induced sig-
naling or in mitochondrial function may not
undergo apoptosis and may persist despite
asbestos-induced toxicity (Upadhyay & Kamp,
Mesothelial cells may have inherent or
acquired activation of prosurvival pathways,
from either the exposure to asbestos with
upregulation of growth factor receptors (Pache
et al., 1998) or downstream pathways (MAPK,
ERK, Akt/mTOR) or another survival mecha-
nism (Jimenez et al., 1997; Altomare et al.,
2005), as proposed with SV40 infection
(Kroczynska et al., 2006). Inflammation may
also initiate an environment that itself pro-
motes prosurvival mechanisms. Inflammation
may also be induced by the fibers themselves,
by an influx of cells of the innate immune
system or by asbestos-induced necrosis (Yang
et al., 2010). The microenvironment including
the presence of inflammatory cells, endothelial
cells, and fibroblasts, along with the formation
of a three-dimensional shape itself, supports
the resistance to apoptosis (Barbone et al.,
2008; Daubriac et al., 2009). Critically impor-
tant survival mechanisms that inhibit apoptosis,
if understood, might be a target for intervention
(Heintz et al., 2010).
This chronic inflammatory environment
may contribute to acquired, heritable genetic,
or epigenetic alterations leading to inactiva-
tion of tumor suppressor genes, activation
of oncogenes, and altered regulation of cell
cycle and DNA repair pathways (Kratzke and
Gazdar, 2005). Specific genetic, epigenetic,
and chromosomal alterations are characteristic
of diffuse malignant mesothelioma (Murthy and
Testa, 1999; Apostolou et al., 2005). Oxidants
generated directly by redox-active asbestos
fibers or indirectly following phagocytosis may
also induce DNA and chromosomal dam-
age (Jaurand, 1996; Hei et al., 2000; Huang
et al., 2011). An indirect mechanism associated
with persistent inflammation was proposed for
altered gene methylation profiles characteris-
tic of human malignant pleural mesotheliomas
(Christensen et al., 2009). These genetic and
epigenetic alterations may select for mesothe-
lial cells that are able to survive and proliferate
PLEURAL ENDPOINTS FOLLOWING FIBER EXPOSURE169
in a chronic inflammatory environment (Huang
et al., 2011).
KNOWLEDGE AND DATA GAPS FOR
THE BIOLOGIC MECHANISMS FOR
NEOPLASTIC PLEURAL DISEASE
(a) Workers are usually exposed to mixed
dusts contaminated with asbestos fibers. It
is unknown whether asbestos-related pleu-
ral diseases are potentiated by exposure
to other dusts such as vermiculite, crys-
talline silica, or metals in the occupational
(b) The role of SV40 virus as a cofactor with
asbestos fibers in the development of dif-
(Gazdar et al., 2002). Mechanistic studies
in cell cultures and in rodents suggest that
SV40 viral oncoproteins induce mesothe-
lial cell transformation and diffuse malignant
mesothelioma, although human epidemi-
ological studies do not support a causal
association (Weiner & Neragi-Miandoab,
2009). Additional epidemiological studies
using specific serological markers for SV40
virus infections are needed (Kean et al.,
(c) The physical and chemical properties of
mineral fibers associated with carcinogenic-
ity include surface chemistry and reactivity,
surface area, fiber dimensions, and bioper-
sistence. The relative importance of these
different properties with respect to car-
cinogenic potency is uncertain and may
depend on the geological source of the
fibrous mineral and its associated con-
taminants. Commercial asbestos fibers and
erionite fibers have been most widely
studied. Noncommercial amphibole fibers,
other naturally occurring asbestiform fibers,
and newly engineered fibrous nanomaterials
(Jaurand et al., 2009; Sanchez et al., 2009)
need to be well characterized and their
potential for translocation and persistence in
the pleura must be determined.
(d) The potential for any natural or engineered
fibrous materialwith physicochemical
properties similar to asbestos fibers to
induce persistent inflammation and pro-
mote the development of diffuse malignant
mesothelioma needs to be investigated
before the fibers are widely used.
(e) Various direct and indirect mechanisms
were proposed for the induction of diffuse
malignant mesothelioma by asbestos fibers.
These mechanisms may interact at multiple
stages during the long latent period asso-
ciated with this malignancy. The relative
importance of these different mechanisms
in tumor development and progression is
unknown. The ability of different fiber types
to induce specific genetic and epigenetic
alterations characteristic of diffuse malig-
nant mesothelioma needs to be determined
(Andujar et al., 2007).
(f) Chronic rodent inhalation assays are expen-
sive, technically demanding, and not suit-
able for mechanistic studies because only
a minority of rats develop diffuse malignant
mesothelioma following inhalation. Current
screening assays for fiber toxicity use short-
term in vitro or in vivo assays; however, it is
difficult to extrapolate from acute, high-dose
exposures to chronic or repeated, low-dose
exposures in vivo. A new toxicologic screen-
ing strategy needs to be developed and
validated to assess potential carcinogenic-
ity of naturally occurring mineral fibers and
engineered fibrous nanomaterials.
(g) Mesothelial cells are mobile and appear to
be able to detach and relocate at other sites
in the pleural space (Foley-Comer et al.,
2002). Mesothelioma is also associated with
mobile spheroids, clumps of malignant cells
floating in the pleural fluid; such spheroids
appear to remain viable and resistant to
apoptosis (Barbone et al., 2008; Daubriac
et al., 2009). It is not known whether this
mobility may allow preneoplastic cells to
move from areas of asbestos accumulation
to other areas where mesothelioma may
develop. If so, and if new technologies are
developed that will allow one to distinguish
malignant mesothelial cells from reactive
benign mesothelial cells, pleural fluid could
be sampled for preneoplastic mesothelial
170 V. C. BROADDUS ET AL.
cells in order to identify individuals at risk for
developing diffuse malignant mesothelioma.
(h) New immunohistochemical and molecu-
lar markers of preneoplastic and neoplastic
lesions would improve early diagnosis and
therapy of diffuse malignant mesothelioma
(Husain et al., 2009).
(i) Populations exposed to asbestos or asbesti-
form fibers from Libby MT, workers in
certain trades, those exposed on 9/11 at
Ground Zero, or those with known high
exposure (e.g., those with bilateral pleu-
ral plaques) need to be followed in epi-
demiologic studies that include noninvasive
studies of biomarkers and imaging with the
potential for more invasive studies using
pleuroscopy or pleural lavage. In this way,
knowledge can be gained about the natu-
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