REVIEW ARTICLES
Biomarkers in acute lung injury
MANEESH BHARGAVA, and CHRIS H. WENDT
MINNEAPOLIS, MINN
Acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) result in high
permeability pulmonary edema causing hypoxic respiratory failure with high morbidity and mortality. As the population ages, the incidence of ALI is expected to
rise. Over the last decade, several studies have identified biomarkers in plasma
and bronchoalveolar lavage fluid providing important insights into the mechanisms
involved in the pathophysiology of ALI. Several biomarkers have been validated
in subjects from the large, multicenter ARDS clinical trials network. Despite these
studies, no single or group of biomarkers has made it into routine clinical practice.
New high throughput ‘‘omics’’ techniques promise improved understanding of the
biologic processes in the pathogenesis in ALI and possibly new biomarkers that
predict disease and outcomes. In this article, we review the current knowledge on
biomarkers in ALI. (Translational Research 2012;159:205–217)
Abbreviations: ARDS ¼ acute respiratory distress syndrome; ALI ¼ acute lung injury; ROC ¼
receiver-operating characteristic; AUROCC ¼ area under the ROC curve; BALF ¼ bronchoalveolar lavage fluid; IL ¼ interleukin; TNF ¼ tumor necrosis factor; sTNFR-I and II ¼ soluble TNF receptors I and II; HMGB ¼ high mobility group box nuclear protein 1; LBP ¼ lipopolysaccharide
binding protein; NO ¼ nitric oxide; SP ¼ surfactant proteins; RAGE ¼ receptor for advanced
glycation end products; CCSP ¼ clara cell secretory protein; vWF ¼ vonWillebrand factor;
s, sICAM-1 ¼ soluble intercellular adhesion molecule-1; Ang-1 and -2 ¼ angiopoietin-1 and -2;
PAI-1 ¼ plasminogen activator inhibitor-1; KGF ¼ keratinocyte growth factor; HGF ¼ hepatocyte growth factor; FGF ¼ fibroblast growth factor; VEGF ¼ vascular endothelial growth factor;
N-PCP-III ¼ N-terminal procollagen peptide-III; LIPS ¼ lung injury prediction score; LC-MS/MS ¼
liquid chromatography combined with mass spectrometry; IGFBP- ¼ insulin like growth factor
binding protein-3; NMR ¼ nuclear magnetic resonance; NFKB ¼ nuclear factor kappa beta
cute respiratory distress in adults was first described by Ashbaugh and Petty in 19671 in
a case series of 12 subjects with acute onset
of tachypnea, hypoxia, and loss of compliance after
a variety of stimuli. Subsequent research has increased
our understanding of this disease’s pathophysiology,2
A
epidemiology,3 treatment options,4-11 and outcomes.3,12
A uniform definition of this syndrome has been
adopted for research, epidemiology, and clinical care
based on a report of the American-European consensus
conference on acute respiratory distress syndrome
(ARDS).13 The incidence of ARDS, and its less severe
From the Pulmonary and Critical Care Medicine, Department of
Medicine, University of Minnesota, Minneapolis, Minn; and Veterans
Affairs Medical Center, Minneapolis, Minn.
Reprint requests: Chris H. Wendt, Pulmonary and Critical Care, Department of Medicine, Veterans Affairs Medical Center, Minneapolis,
MN 55417; e-mail: wendt005@umn.edu.
Submitted for publication November 4, 2011; revision submitted
January 6, 2012; accepted for publication January 8, 2012.
1931-5244/$ - see front matter
Published by Mosby, Inc.
doi:10.1016/j.trsl.2012.01.007
205
206
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April 2012
Bhargava and Wendt
form, acute lung injury (ALI), is believed to be 58.7 and
78.9 cases per 100,000 person-years, respectively,3 with
an estimated 74,500 deaths and 2.2 million ICU days annually. As the US population ages, it is expected that ALI
will become an even greater health problem.14
Over the last 2 decades, biologic markers have revealed novel information about the pathophysiology
of lung injury/repair and identified cells and putative
mediators involved in ALI. However, despite this new
knowledge biomarkers in ALI remain primarily a research tool. The focus of this review is to outline the current state of biomarkers in ALI and ARDS.
BIOMARKERS
Biomarkers are broadly defined as markers of a biologic process or state. A commonly used definition of
a biomarker is ‘‘a characteristic that is objectively
measured and evaluated as an indicator of normal
biological process, pathogenic processes, or pharmacologic responses to a therapeutic intervention’’.15 Thus
clinical parameters such as vital signs, physiologic measurements, biochemical, or molecular markers could be
used as biomarkers to determine its relationship with
an endpoint.
ENDPOINTS IN BIOMARKER RESEARCH IN ACUTE
LUNG INJURY
Several clinical endpoints for biomarker research
have been investigated in critically ill patients with hypoxic respiratory failure from ALI. These end points
have focused on the ability to diagnose ALI in highrisk patients or discriminate patients with hydrostatic
from high permeability pulmonary edema. Also of interest are identifying subgroups of patients with different outcomes or response to treatment in patients at
risk of or with established ALI. As these are surrogate
endpoints, the most clinically relevant outcome is mortality and therefore biomarker research has concentrated on prediction of short- and long-term mortality
in ALI. Besides a potential utility in the clinical arena
for diagnosis, stratification, and prediction of mortality,
biomarkers in ALI could also be used in clinical trials
for selection of homogenous patients and as end points.
STATISTICAL BASIS FOR USE OF BIOMARKERS
The rationale of when to measure laboratory parameters, which marker may be useful, and how to interpret
the results are not well defined. It is vital that validation
and confirmation of candidate biomarkers by robust statistical methods are performed during biomarker discovery. Sensitivity and specificity are common quality
parameters for biomarkers. Sensitivity describes the
probability of a positive test in cases and specificity
describes probability of negative test in controls. An
association between sensitivity and specificity is represented in the receiver-operating characteristic (ROC) by
graphing sensitivity vs 100-specificity. Area under the
ROC curve (AUROCC) is a measure of performance
of a marker. There is no absolute cutoff value of
AUROCC for robustness of a marker though a minimum
of 0.7 is required and values greater than 0.8 are good
particularly in a heterogeneous patient population seen
with critical illnesses.16
An ideal biomarker in ALI should have a clear relationship between the biomarker and the pathophysiologic
events. The markers would need to be reliable and
reproducible, relatively inexpensive, measure changes
in response to interventions, have little or no diurnal
variation, be sensitive, disease specific with high positive
and negative predictive values, and be sampled by simple
methods. Exhaled breath condensate,17,18 urine,19,20
undiluted pulmonary edema fluid,21-23 bronchoalveolar
lavage fluid (BALF), and plasma/serum have been
studied for biomarker discovery in ALI.
BIOMARKERS OF ARDS/ALI STAGES
The pathologic states of ARDS consist of 3 discrete
stages that overlap both temporally and spatially
(Fig).24 Histologically, the initial exudative phase is
characterized by diffuse alveolar damage. In this early
phase, the epithelial and endothelial cells release factors
reacting to injury and death. The loss of cellular integrity results in flooding of the alveolus with a proteinaceous exudate that results in the impairment of gas
exchange. The subsequent dilution of surfactant proteins leads to alveolar collapse and decreased lung
compliance. Over the ensuing days, the pulmonary
edema fluid is cleared and a proliferative stage develops.
Histologically, this is marked by proliferation and phenotypic changes in type II alveolar cells and fibroblasts.
In the absence of recovery, some patients progress to
a fibrotic stage that is characterized by diffuse fibrosis
and the obliteration of normal lung architecture. Various
observational and clinical studies identify biomarkers
that correlate with these stages, some of which have
been associated with clinical outcomes. To put the biomarkers in the context of the physiologic stages of ALI,
we have segregated them to correspond to the exudative
(Table I) and proliferative phase (Table II) of ALI.
EXUDATIVE PHASE
A hallmark of ARDS is diffuse alveolar damage consisting of widespread epithelial and endothelial injury
and death accompanied by a proteinaceous exudate.
With this histologic finding in mind many investigators
have sought to determine if specific cellular proteins
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Volume 159, Number 4
Fig. Time course in acute lung injury. Early in the course the alveoli
are filled with protein rich permeability pulmonary edema. By day 5
to 7, there is proliferation of type II alveolar epithelial cells, leading
to re-epithelialization and restoration of the alveolar structure or progressive fibrosis and irreversible hypoxic respiratory failure (Reprinted
with permission from Elsevier Publications. Katzenstein AA and Askin
FB. Acute lung injury patterns: diffuse alveolar damage and bronchiolitis obliterans-organizing pneumonia in surgical pathology of
non-neoplastic lung (ed 4), 2006). (Color version of figure is available
online.)
released during injury could represent biomarkers for
the diagnosis or prognosis of ARDS.
Inflammation. In ALI, a complex network of cytokines
mediates the inflammatory response to a primary infection in the lungs or systemic inflammation such as seen
in sepsis or pancreatitis.25 Greater prominence in
(BALF) of certain cytokines suggests that inflammatory
mediators have a pulmonary origin. Levels of both
proinflammatory (interleukin (IL) 1b, tumor necrosis
factor (TNF)-a, IL-6, and IL-8) and anti-inflammatory
cytokines (IL- 1ra, IL-10, IL-13) are elevated in plasma
or BALF in ALI indicating a balance of these mediators
governs the development of ALI.26 Both pro- and antiinflammatory biomarkers have been studied to establish
their role in predicting the development, diagnosis, and
in prognosticating ALI but only a few have been
validated in multicenter studies.20,27-29
TNF is an important mediator in ALI.30 Higher
plasma levels of TNF-a have been reported in at-risk
patients with sepsis.31 Though elevated plasma TNFa levels were seen in patients with ARDS, they were
not different from patients at-risk of developing
ARDS.32 Similarly, other studies have demonstrated
no significant difference in serum TNF-a levels in patients at-risk of ARDS compared with patients with
ARDS,33 though in this study mean BAL levels of
TNF-a were significantly higher in patients with
ARDS in comparison to normal subjects. Parsons
et al29 measured plasma levels of TNF-a and soluble
Bhargava and Wendt
207
TNF receptors I and II (sTNFR-I and II) from patients
enrolled in the ARDS network low tidal volume study.
Plasma TNF-a levels at the time of enrollment were
detectable only in 9% of the subjects and were not different at baseline or in 3 days in those who did or did not
survive. In contrast, sTNFR-I and sTNFR-II were detectable and were strongly related to an increased risk
of death, fewer nonpulmonary organ failure free days
and fewer ventilator free days. Calfee et al27 have reported lower levels of sTNFR- I in trauma related ALI
patients in the ARDS network low tidal volume study
and the ALVEOLI study.4
IL-6 is one of the most important mediators of fever
and in ARDS high plasma and BALF levels are predictive of poor outcomes. Though IL-6 activates both proand anti-inflammatory pathways, early in ARDS it
correlates with a proinflammatory profile with increased
levels seen in response to LPS in experimental
models.34 Elevated plasma IL-6 were seen in patients
at risk for developing ARDS who met criteria within
48 hours.35 In that study, the higher BALF IL-6 levels
seen in at-risk patients who developed ARDS were similar to patients with ARDS, and there was a strong negative predictive value of serum and BALF IL-6 levels
for the development of ARDS. Other investigators
have also reported high plasma IL-6 levels in patients
with risk factors who developed ALI in comparison to
those who did not develop ALI.36 High baseline (day
1) levels were seen in patients with ALI who died and
a persistent elevation predicted mortality.37 In 593 patients from the ARDS network study,5 baseline levels
of IL-6 were higher in nonsurvivors28 even after controlling for ventilation strategy, severity of illness, vasopressor use, platelet count, and severity of impairment
in gas exchange in a multivariate analysis. In addition,
higher IL-6 levels were independently associated with
fewer ventilator free days and organ failure free days.
IL-8 is a proinflammatory cytokine with high plasma
and BALF levels found early in ALI35-38 that predicts
outcomes. In early studies the predictive power of IL-8
in identifying cases of ALI varied.35,36,38-40 However, in
the larger ARDS network low tidal volume study,5 higher
baseline levels of plasma IL-8 were associated with
increased risk of death and in a multivariate analysis controlling for ventilation strategy, severity of illness, vasopressor use, platelet count, and severity of impairment
in gas exchange, were associated with fewer ventilator
free and organ failure free days. Similar to IL-6, ventilation using low tidal volume was associated with a faster
decline in IL-8 levels.
IL-1b is a potent cytokine secreted by activated macrophages41 resulting in elevated levels in plasma, BALF
and edema fluid42 early in ARDS43,44 and is an
important bioactive cytokine in the BALF in the early
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Table I. Summary of biomarkers reflective of the Exudative Phase of ALI/ARDS
System
Subtype
Molecule
Biologic Source
Altered in ALI/ARDS
Predicts Outcome
Validated
References
Surfactant A, B
Surfactant A
Surfactant D
KL-6
RAGE
Laminin
Desmosine
vWF
sICAM
PAI-1
Protein C
IL-1b
IL6
IL-8
CRP
sTNFR-I and II
IL-10
BALF
Plasma
Plasma
ELF, plasma
Plasma
Plasma, ELF
Urine
Plasma
Plasma, ELF
Plasma, ELF
Plasma, ELF
Plasma, BALF Plasma, BALF
Plasma, BALF
Serum
Plasma
Plasma
Decreased
Increased
Increased
Increased
Increased
Increased
Increased
Increased
Increased
Increased
Decreased
Increased
Increased
Increased
Increased
Increased
Increased
Yes
No
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
No
Yes
No
Yes
Yes
Yes
Greene50
Eisner52
Eisner31
Ishizaka55
Uchida57, Calfee58, Fremont59
Katayama63
McClintock19
Ware68
Agouridakis70, Conner21, Calfee69
Prabhakaran23
Ware87, McClintock88
Pugin43, Suter44, Park45
Bouros35, Meduri37, Parsons28
Donnely38, Takala36, Parsons28
Bajwa48
Parsons29, Calfee27
Parsons28
Lung injury
Alveolar type II
Alveolar type I
Lung matrix
Endothelial cell
Coagulation
Inflammation
Proinflammatory
Anti-infllammatory
No
No
Yes
Yes
No
Yes
Yes
Abbreviations: ARDS, acute respiratory distress syndrome; ALI, acute lung injury; BALF, bronchoalveolar lavage fluid; ELF, epithelial lining fluid; CCSP, clara cell secretory protein; vWF, von
Willebrand factor; sICAM, soluble intercellular adhesion molecule-1; PAI-1, plasminogen activator inhibitor-1; sTNFR 1 and II, soluble TNF receptor 1 and II.
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Bhargava and Wendt
209
Table II. Summary of biomarkers reflective of the Proliferative Phase of ALI/ARDS
Type of
proliferation
Growth
factor
Biologic
source
Altered in
ALI/ARDS
Predicts
outcome
Validated
References
Epithelial
KGF
HGF
VEGF
Ang-2
BALF
BALF
Plasma
Plasma
Increased
Increased
Increased
Increased
Yes
No
No
Yes
No
No
No
Yes
Stern94
Stern94
Thickett96,97
Gallagher72, Ong74
Endothelial
Abbreviations: ARDS, acute respiratory distress syndrome; ALI, acute lung injury; KGF, keratinocyte growth factor; HGF, hepatocyte growth
factor; VEGF, vascular endothelial growth factor; BALF, bronchoalveolar lavage fluid.
phase of ALI. Large multicenter studies looking into the
role of IL-1b are lacking, but small studies show
persistent elevation of plasma IL-1b37 and elevated
BALF IL-1b43 are associated with worse outcomes.
IL-1Ra levels peak between day 1 and 3,45 suggesting
a balance between these molecules contributing to the
overall proinflammatory state in the lung.
In addition to IL-1Ra, a number of other antiinflammatory mediators contribute to the overall inflammatory balance in the lung. Park et al identified
anti-inflammatory responses that peaked after the onset
of ARDS and these mediators included IL-1Ra, IL1RII, sTNF-I, siL-6R, and IL-10.45 Only IL-1028 and
sTNFR-I and II29 have been studied in multicenter trials. In the ARDS network low tidal volume study,
high baseline IL-10 levels were associated with higher
mortality but were less strongly associated with morbidity as measured by organ failure and ventilator free
days. Similarly, higher baseline sTNFR-I and II levels
have been associated with higher mortality in patients
from the ARDS network low tidal volume ventilation
study.29
Overall, the current evidence indicates that cytokine
levels are characteristic and may have utility in prognostication but are only weakly predictive for the development of ALI. Other mediators of inflammation
have been studied to identify biomarkers to predict the
development of ARDS in at-risk patients. High mobility
group box nuclear protein 1 (HMGB1), which is a DNA
nuclear binding protein, was increased within 30 min
after severe trauma and correlated with severity of injury, tissue hypoperfusion, early posttraumatic coagulopathy, systemic inflammatory response, acute kidney
injury, and subsequent development of respiratory failure. In addition, higher levels are correlated with higher
mortality.46 Villar et al47 investigated the role of lipopolysaccharide binding protein (LBP), an acute phase
protein that mediates inflammation, in 180 patients
with sepsis. Though the baseline LBP serum levels
were similar in survivors and nonsurvivors at study entry, at 48 h and 7 days, the levels were higher in ARDS
patients than ALI patients. An increase in serum LBP at
48 h was also associated with high mortality. Nitric
oxide (NO), a marker of oxidative stress, was investigated in patients from ARDS Network low tidal volume
ventilation study20 with a working hypothesis that
peroxynitrites would oxidize proteins such as a1- antitrypsin and surfactant protein A and promote an inflammatory state. It was thus hypothesized that the lower
tidal volume ventilation group would have lower urine
NO levels. Surprisingly, higher urine NO levels were
strongly associated with better clinical outcomes including mortality, organ failure free days and ventilator
free days. Mechanism that could be responsible for
these findings will need further evaluation. Similarly,
higher levels of CRP48 within 48 h of onset of ARDS
was found to be associated with better survival, lower
number of organ failure free day and days on mechanical ventilation, a finding that contradicts long held
views.
Alveolar type II cell injury. In the acute phase of ARDS,
the alveolar epithelial cell is injured, a key component to
the clinical presentation. An important function of the
type II cell is the production of surfactant. These
surface-active lipoprotein complexes decrease surface
tension keeping the alveolus open and increasing
compliance. Surfactant proteins are important in normal
lung physiology and host defenses. Surfactant proteins
(SP) A and D are also involved in innate immunity.
Early observations in ARDS revealed a loss in surface
tension suggesting a functional loss of the surfactant
proteins.1 In 1999 Greene and colleagues described
complex changes in various surfactant proteins in ARDS
both prior to its onset and throughout the exudative
period.49 They observed that early in the exudative
phase SP-A and SP-B decreased in BALF. They felt it
was not due to dilution since SP-D concentration
remained stable. This implies that surfactant proteins are
either consumed and/or there is a concomitant decrease
in production due to cellular injury. Interestingly, serum
levels did not correlate with BALF levels for either
SP-A or SP-D, with both increasing during the first 7
days after diagnosis. This report suggested that
surfactant proteins in the BALF are markers for survival.
A subsequent study by the same group found serum
SP-A to be a predictor of developing ARDS in 51
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individuals at risk.50 Subgroup analysis revealed this to be
a good predictor for ARDS associated with sepsis or
aspiration, but not trauma. In addition to SP-A, a single
center study of 54 patients found plasma levels of SP-B
to be predictive of those that develop ARDS.51 A
subsequent larger and longitudinal study of 565 clinical
trial participants used a multivariate analysis to
determine clinical outcomes based on the plasma levels
of SP-A and SP-D.52 Baseline plasma SP-A levels did
not correlate to any clinical outcome in this large,
multicenter study. However, higher baseline SP-D
plasma levels were associated with a higher mortality
and comorbidities such as the number of ventilator and
organ-failure days. A finding that a polymorphism in
SP-B is associated with an increase risk of developing
ARDS in women further suggests the importance of
surfactant proteins in lung homeostasis and its role in
ARDS.53
Another marker for alveolar epithelial type II cells is
the membrane glycoprotein KL-6 that belongs to the mucin family of proteins. Type II cells that are injured or
proliferating have increased expression and measurable
levels of KL-6 are present in both the BALF and plasma.
This marker of cellular injury appears nonspecific since
elevated levels of KL-6 have been found in patients
with interstitial lung disease.54 In ALI/ARDS Ishizaka
and colleagues found higher concentrations in epithelial
lining fluid and plasma correlated with higher mortality55
suggesting these higher levels represent a higher degree
of epithelial cell injury.
Alveolar type I cell injury. Highly susceptible to injury,
the thin and fragile type I cell covers the majority of the
alveolus. Present predominantly on the basal surface of
type I cells is the receptor for advanced glycation end
products (RAGE). RAGE belongs to the immunoglobulin
superfamily and functions as a multi-ligand receptor that
propagates the inflammatory response via nuclear factor
kappa beta.56,57 Elevated levels of RAGE have been
reported in ALI57 and in the ARDS network low tidal
volume trial5 higher baseline plasma levels of RAGE
were associated with increased mortality.58 These
findings persisted when adjusted for multiple
confounders such as age, gender, severity of illness and
sepsis. However, this finding was limited to the high
tidal volume group that had a higher mortality and
presumably higher injury. In a separate retrospective
nested case control study of 192 patients, RAGE was 1
of 7 biomarkers out of 21 measured that had a high
diagnostic accuracy in distinguishing ALI from nonALI in trauma patients.59
Bronchiolar cell injury. Although the alveolar epithelium plays a central role in the pathophysiology of
ARDS, the injury extends beyond the alveolus to the distal
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April 2012
airways. Present in the small airways are bronchial
epithelial cells that produce Clara cell secretory protein
(CCSP). Its role is unclear, but CCSP has been implicated
in regulating the inflammatory response.60 In a single
center study, elevated serum levels of CCSP were
associated with an increased risk of mortality, 61
however, this association with mortality was not found
in a similar small study.22 In yet another small, single
center study, elevated plasma levels of CCSP in patients
with ventilator-associated pneumonia identified those
with ALI/ARDS.62 Although one could postulate that
CCSP production would increase in the presence of
injury, its association with higher mortality suggests the
elevated levels actually reflect clara cell injury. Until
larger studies are performed, the role of CCSP as
a biomarker for ALI/ARDS remains unclear.
Lung matrix injury. The extracellular matrix of the lung
functions as the scaffold that supports the epithelium and
vascular structures. It consists of collagens, glycoproteins,
and proteoglycans. Laminin is an extracellular protein
deposited in the basement membrane that is important
for cellular adhesion, growth, and differentiation; therefore, important for re-populating and repairing the
epithelium. In a small, single center study plasma and
lung edema fluid levels of laminin Y2 fragments, an
amino terminal fragment of the gamma 2 chain of
laminin-5, were elevated in those with ALI/ARDS
compared with controls.63 Interestingly, elevated levels
at day 5 of lung injury correlated with an increase in
mortality, presumably reflecting ongoing injury. Elastin
is another critical protein in the extracellular matrix
that gives the lung its elastic recoil ability. When
damaged it releases small fragments containing
desmosine and iso-desmosine that can be measured in
extracellular fluids, including serum, BALF, and urine.
In the same ARDS network trial of low tidal volume, the
investigators measured urinary desmosine levels by
radioimmunoassay. Individuals ventilated with high tidal
volumes had higher urinary desmosine levels,
presumably a reflection of structural lung damage.
However, there were no correlations to clinical
outcomes, such as mortality.19
Endothelial cell injury. In addition to the epithelium,
the endothelium is also a site of injury in ALI/ARDS.
The endothelial cell produces a number of compounds
important in vasoregulation and hemostasis. Although
responsive vasoconstriction in ALI/ARDS has been recognized, it has not been a target to define biomarkers.
Several endothelial derived hemostasis factors are elevated in ARDS presumably as a response to cellular injury, although it is not clear what stimulus accounts for
the elevated levels. The endothelial cell product von Willebrand factor (vWF) forms a complex with factor VIII
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that is essential for platelet adhesion to damaged endothelium and platelet aggregation. These factors help
maintain vascular integrity, however, in excess the balance could shift toward in situ thrombosis and extension
of vascular injury. In a sentinel paper in 1982 Carvalho
et al reported 100 patients with ALI/ARDS that demonstrated a 5-fold increase in vWF levels in ARDS.64 In
a prospective study of 45 patients with nonpulmonary
sepsis, Rubins and colleagues found elevations in vWF
to be predictive of developing ALI with a sensitivity of
87% and specificity of 77%.65 However, these findings
were not confirmed in subsequent trials, including one
by Bajaj and coworkers where vWF along with other
endothelial markers, tissue factor pathway inhibitor
and thrombomodulin, were not found to be predictive
of developing ARDS.66 This study was limited due to
its small size; where only 8 of the 15 patients at risk
actually developed ARDS. In a larger study of 96
patients with sepsis and nonsepsis risk factors for
developing ARDS, Moss and colleagues did not find
vWF levels helpful in predicting the progression to
ARDS in either group.67 Although levels were elevated
in ARDS, the sensitivity in detecting ARDS was 70%
or less. In a definitive study of 559 subjects of the
ARDS network trial for low tidal volume, Ware and
coworkers measured vWF plasma levels and reported
similar baseline levels comparing sepsis to nonsepsis
patients, however, significantly higher levels were
found in nonsurvivors. Higher levels were also
significantly associated with fewer organ failure free
days suggesting the degree of endothelial activation
and injury is strongly associated with outcomes in
ALI/ARDS.68
The soluble intercellular adhesion molecule-1
(sICAM-1) is a low-molecular weight adhesion molecule. It is present in both epithelial and endothelial
cells and is released in the setting of injury where elevated levels have been found in both lung edema fluid
and plasma.21,69,70 In a prospective cohort study of
pediatric patients with ALI, elevated plasma levels of
sICAM-1 had increased risk of death or prolonged
mechanical ventilation.71 In an observational study by
Calfee and colleagues, they found edema fluid levels
of sICAM-1 to be elevated in ALI in 67 patients from
their center. In a large study of patients (778 individuals)
from the ARDS network low tidal volume trial they confirmed elevated levels of sICAM-1 were associated with
ALI and found that elevated levels over the first 3 days
portended a higher risk of death.69
Angiopoietin-1 and -2 (Ang-1 and -2) are vascular
growth factors that have been proposed as biomarkers
for ALI/ARDS. Both function through the endothelial
tyrosine kinase receptor; however, they have opposite
effects. Ang-1 stabilizes the endothelium by decreasing
Bhargava and Wendt
211
apoptosis and inflammation. Whereas, Ang-2 is proinflammatory, promotes both endothelial and epithelial
apoptosis, increases neutrophil adhesion, and induces
permeability by altering the cellular cytoskeleton.
Hypothesizing that Ang-2 may be associated with
a poor outcome, Gallagher and colleagues measured
Ang-2 levels in critically ill patients. They found elevated plasma levels in those with ALI and in nonsurvivors.72 This finding is supported by 2 single nucleotide
polymorphisms in Ang-2 associating with a risk in developing ALI.73 Ong and colleagues found the concentration of angiopoietin-2 relative to angiopoietin-1 was
an independent predictor of death in an observational
cohort study of ALI/ARDS patients.74 These findings
not only identify a potential biomarker for ALI/ARDS
and survival, but also suggest a possible therapeutic
target to prevent vascular leak.
E-selectin belongs to a family of adhesion molecules
only expressed on endothelial cells that is involved in
leukocyte-endothelial adhesion. It has been shown to
be released in the presence of TNF and elevated levels
of this molecule have been associated with sepsis and
signal a higher mortality.75 Since sepsis and ALI can
co-exist and have similar vascular injuries, Okajima
et al measured E-selectin plasma levels in 55 individuals
at risk for developing ALI/ARDS and found that higher
E-selectin levels were associated both with ALI and
a higher mortality.76
These studies highlight that endothelial cell activation
and/or injury is present in ALI/ARDS and release of
endothelial specific proteins strongly associate with
outcomes, such as survival. This suggests that the endothelial cell and its components are potential targets for
therapeutic intervention.
Coagulation. A hallmark of ARDS is the formation of
hyaline membranes from intra-alveolar fibrin
deposition due to an imbalance in coagulation and
fibrinolysis during the exudative phase.77,78 This fibrin
can serve as a provisional matrix for epithelial cells to
repopulate the damaged alveolus.79,80 However, fibrin
deposition can also be detrimental if excessive and
occurs in the absence of re-epithelialization. In
addition, fibrin can act as a sump for certain antiinflammatory proteins such as surfactant and thereby
activate the inflammatory process.81-85 Therefore,
a balance between pro-coagulant and fibrinolytic
processes is necessary to effectively close the
damaged alveolus and allow effective epithelial
repopulation without excessive inflammation or
persistent obstruction of the alveolar space.
Given the presence of fibrin and fibronectin deposits
in the exudative phase of ARDS, several observational
studies were performed to determine if pro-coagulant
and anti-fibrinolytic molecules are biomarkers for
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Bhargava and Wendt
ARDS. Bertozzi and colleagues reported a decrease in
urokinase activity in BALF in a small observational
study of ARDS patients.82 This decrease in activity occurred despite normal levels of urokinase suggesting
urokinase inactivation and possible urokinase inhibitor
presence. This was supported by the presence of increased levels of the urokinase inhibitor plasminogen
activator inhibitor-1 (PAI-1) in the BALF. In a larger,
more complex population of patients, it was observed
that ARDS was associated with both increased procoagulant and decreased fibrinolytic activities.86 This
alteration in coagulation state was in part due to decreased levels of urokinase-type plasminogen activator
and increased PAI-1 and a2-antiplasmin levels, which
favored a pro-coagulant environment. These observational studies were followed by studies to determine if
alterations in the procoagulant/fibrinolytic pathways
determined outcomes. In 2003, Prabhakaran and colleagues found that elevated levels of PAI-1 in plasma
and edema fluid were associated with a higher mortality.23 That same year a study by Ware identified lower
levels of the anti-coagulant protein C were associated
with a higher mortality in ARDS/ALI patients.87 Interestingly, thrombomodulin levels were 10-fold higher in
edema fluid compared with healthy controls and 2-fold
higher compared with ARDS plasma. This implied
thrombomodulin is produced locally in the lung.
Thrombomodulin is an activator of the anti-coagulant
protein C. Protein C activity was not measured so the
significance of these levels is not known. In a separate
study, multivariate analysis that included protein C
and thrombomodulin, decreased levels of protein C
along with elevated levels of IL-8 and intercellular adhesion molecule were predictors of survival.88
PROLIFERATIVE PHASE
Recovery from ALI/ARDS requires a wellorchestrated repair of the damaged alveolus and vascular structures. The exudative phase of lung injury results
in a rich, proteinaceous environment that can function as
a provisional matrix for subsequent cellular repopulation. As early as a few days into acute lung injury, type
II cells begin to regenerate along the alveolar septa
and signal the onset of the proliferative phase.89 Presumably, the exudative phase subsides due to improved vascular integrity and subsequently by 7th to 10th day
a fibroproliferative process is underway (Fig). Various
stages of proliferation can occur throughout the lung
simultaneously. With increasing duration of ALI/ARDS
the fibroproliferative phase can predominate and recovery occurs in those able to remodel the lung. If well
orchestrated, this process results in repair of the alveolus
and vascular structures. If the process is incomplete or
Translational Research
April 2012
the fibroproliferative phase is over-exuberant in the
absence of remodeling, then too often death ensues.
Epithelial proliferation. Certain growth factors, such as
keratinocyte growth factor (KGF) and hepatocyte growth
factor (HGF) are known to be potent mitogens for type II
alveolar epithelial cells. Important in the development of
the fetal lung, KGF is a member of the fibroblast growth
factor (FGF) family and is expressed by mesenchymal
cells. However, KGF’s receptors only occur on epithelial
cells thereby conferring its epithelial cell specificity.90-92
Numerous in vitro and in vivo studies have demonstrated
beneficial effects on the proliferating epithelium
including enhancing motility, resistance to injury,
surfactant production, decreased apoptosis and release
of autocrine factors.93 However, few human studies
have been done to identify KGF’s role in ALI/ARDS. In
a small study, Stern and colleagues measured KGF in
BALF from 32 patients (17 ARDS, 8 hydrostatic
edema, 7 non-ARDS) compared with 10 nonventilated
controls. KGF was detected in 13 of the 17 ARDS
patients and was associated with detectable type III
procollagen, a biomarker of fibroproliferation.94 Only 1
of the hydrostatic edema patients and none of the nonARDS patients or controls had detectable KGF levels.
Although this was a small study, measurable KGF was
associated with death (P 5 0.02). In one other study by
Verghese, KGF was detected in low levels in the edema
fluid in patients with ALI and hydrostatic pulmonary
edema but no statistical difference in the KGF
concentration was observed.95
HGF is a nonspecific mitogen that is produced by
a variety of cells including neutrophils, macrophages,
endothelial cells, and fibroblasts. HGF has several effects including protecting cells from DNA damage
and inducing motility. Unlike KGF, there is a paucity
of animal lung injury studies for HGF. In addition to
measuring KGF, Stern and coworkers also measured
HGF in the same cohort described above. They found
HGF levels to be less specific for ARDS. HGF was
not detected in any controls; however, it was measurable
in 15 of the 17 ARDS patients, 7 of the 8 patients with
hydrostatic edema and 6 of the 10 non-ARDS patients.94
When all groups were pooled, HGF concentrations
were higher in nonsurvivors compared with survivors.
Although KGF and HGF have favorable effects on epithelial cell protection and proliferation, elevated levels
portend a poor outcome in this small study. This finding
may reflect an exaggerated response to severe, ongoing
injury. Verghese et al also observed high HGF levels in
edema fluid in patients with ALI in comparison to hydrostatic edema and higher levels were associated
with worse outcomes.95 As these studies have small
number of subjects, these findings need to be validated
in a larger, multicenter study.
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Volume 159, Number 4
Endothelial proliferation. Vascular endothelial growth
factor (VEGF) has a complex role in the lung that not
only includes mitogen activity but it is also a key player
in inducing vascular permeability. Many cells in the
lung including alveolar type II cells, alveolar macrophages and neutrophils release VEGF. Acute overexpression of VEGF leads to pulmonary edema in
animal models. Therefore, its role in the pathophysiology and as a biomarker of ALI/ARDS has been sought.
The first report of VEGF in ALI/ARDS was by Thickett
and colleagues in 2001, where they observed elevated
plasma VEGF in ARDS patients compared with those
at risk for ARDS in both ventilated and nonventilated
controls.96 Subsequently, they and others found decreased levels of VEGF in both BALF and epithelial lining fluid in ARDS patients compared with controls or
those at risk for ARDS.97,98 It was not clear why there
was a difference in plasma levels vs BALF. In 2005,
Ware and coworkers measured VEGF in plasma and
undiluted pulmonary edema fluid comparing ARDS/
ALI to severe hydrostatic pulmonary edema and
epithelial lining fluid in normal lungs. They found that
pulmonary edema fluid VEGF levels were lower in
both ARDS/ALI and hydrostatic edema compared
with normal lungs.99 They concluded that dilution
might be a factor in decreased pulmonary levels. Therefore, the physiologic role and biomarker utility VEGF
plays in ARDS/ALI remains unclear.
Fibroblast proliferation. The fibroproliferative phase of
ALI is felt to occur late, however the underpinnings for
fibroproliferation may start as early as 24 h of the diagnosis of ALI. A number of observational studies have
demonstrated a marker of collagen turnover, N-terminal
procollagen peptide-III (N-PCP-III), is elevated within
24 h of the diagnosis of ARDS100-102 suggesting an
early up-regulation of the fibroproliferative process.
Marshall and colleagues measured N-PCP-III levels in
BALF and serum along with BALF activity, ie, ability
of BALF to stimulate human lung fibroblasts in vitro.
They found at 24 h serum N-PCP-III levels were elevated in ARDS compared with controls and were
significantly elevated in nonsurvivors of ARDS compared with survivors. This corresponded to an elevated
mitogen activity of the BALF. This mitogenic activity
remained elevated in ARDS at 7 days and was also
significantly higher in nonsurvivors. This indicates
that fibroproliferation can occur early in ARDS and
may signal a poor outcome.103
COMBINING BIOMARKERS IN ALI
Despite evidence that individual biomarkers might
identify patients with ALI and also assist in classifying
patients with worse outcomes, no single biomarker diagnoses or prognosticates ALI with high accuracy. To iden-
Bhargava and Wendt
213
tify if a panel of markers will perform better than any
individual biomarker, Freemont and colleagues59 conducted a retrospective nested study in a trauma intensive
care unit. From the 21 biomarkers studied, a panel of 7
biomarkers that included RAGE, Angiopoietin-2, PCP
III, BNP, IL-10, TNF-a, and IL-8, discriminated ALI/
ARDS cases from critically ill trauma control patients
with clear chest radiographs or hydrostatic pulmonary
edema. AUCROCC analysis showed an AUC of 0.86
(95% confidence interval [CI] 0.82–0.92). McClintock
et al88 studied plasma biomarkers of inflammation
(IL-6, IL-8, ICAM-1), coagulation (Thrombomodulin,
protein C) and fibrinolysis (PAI-1) in 50 patients with
early ALI ventilated by low tidal volume to determine
if these markers remained predictive of outcomes with
lung protective ventilation. All markers except IL-6
were significantly different between survivors and
nonsurvivors. After multivariate analysis that included
clinical and demographic variables, 3 markers, IL-8,
ICAM-1, and protein C were independently associated
with a higher risk of death. In another study, Gajic104 investigated clinical and demographic parameters for the
prediction of death and prolonged mechanical ventilation in ALI. A model based on age, oxygen index, and
cardiovascular failure at day 3 was identified in a derivation cohort and it performed better in the clinical trial
validation cohort with a AUCROCC of 0.81 (95% CI
0.77–0.84) than population-based validation cohort
(0.71, 95% CI 0.65–0.76). A lung injury prediction score
(LIPS) has recently been described and validated for prediction of development of ALI in a multicenter observational cohort study. 105,106 LIPS discriminated patients
who developed ALI from those who did not with an
AUC of 0.80 (95% CI, 0.78–0.82). Combining clinical
risk factors with biologic markers in plasma were also
investigated in subjects enrolled in the ARDS Network
higher vs lower positive end expiratory pressure trial.4
Six clinical parameters and 8 biologic markers were
studied to predict mortality at 60 days.107 Clinical
predictors that included APACHE III, organ failure,
age, underlying cause, alveolar-arterial oxygen gradient
and plateau pressure, and predicted mortality with
AUCROCC of 0.82. When the clinical parameters
were used with the 8 biologic markers that included
vFW, SP-D, TNFR I, IL-6, IL-8, ICAM-1, protein-C,
and PAI-1, the discrimination improved to AUCROCC
of 0.85. The best performing biomarkers were IL-8
and SP-D suggesting the key role of inflammation and
alveolar epithelial injury in ALI/ARDS.
NEW APPROACHES FOR BIOMARKER DISCOVERY IN
ALI/ARDS
Biologic systems are complex with a large number of
functionally diverse and frequently multifunctional sets
214
Bhargava and Wendt
of elements interacting selectively and nonlinearly. Because of the intrinsic complexity of these biologic
systems, a combination of experimental and systems
level approaches are expected to improve our understanding of heterogeneous conditions like ARDS/ALI.
Genomics tools have been used both with candidate
gene approach53,108-111 and genome wide analysis.108
Gene expression profiling at the level of the proteome
have also been utilized in ARDS/ALI using DIGE and
mass spectrometric studies. Chang et al have found
complex protein interactions in the BALF protein expression in patients with ARDS. These changes were
dynamic over the course of injury and network analysis
demonstrated unexpected ‘‘central components’’ in the
protein interaction networks.112 Proteomic studies in
the BALF from 3 patients using liquid chromatography
combined with tandem MS (LC-MS/MS) demonstrated
higher levels of insulin like growth factor binding
protein-3 (IGFBP-3) in ARDS patients in comparison
to controls113 and that IGFBP-3/IGF pathway was involved in pathogenesis of ALI by repressing apoptosis
in fibroblasts but not epithelial cells. In a pilot study
of nuclear magnetic resonance (NMR) based plasma
metabolomics, Stringer et al have observed distinct metabolic pathways that distinguished sepsis induced ALI
from healthy controls.114
CONCLUSION
Biomarkers in acute lung injury have provided valuable knowledge into the pathogenesis. In the last 10
years, a number of biomarkers have been tested in large
studies. A single biomarker or panels of markers that are
easily available and predict either the development of
ALI or diagnose ALI for routine clinical use remain elusive. With improvement in high through put ‘‘omics’’
platforms and availability of increasingly sophisticated
bioinformatics tools, there is great hope of identifying
new gene signatures and protein or small molecules
that would serve as biomarkers for prediction, prognostication and diagnosis of ALI. These findings will hopefully provide insight into the biology of the disease and
identify novel targets for therapeutic interventions.
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