INFECTION AND IMMUNITY, July 2011, p. 2865–2870
Copyright © 2011, American Society for Microbiology. All Rights Reserved.
Vol. 79, No. 7
LOX-1 Deletion Improves Neutrophil Responses, Enhances Bacterial
Clearance, and Reduces Lung Injury in a Murine
Polymicrobial Sepsis Model?
Zhuang Wu,1Tatsuya Sawamura,2Anna K. Kurdowska,1Hong-Long Ji,1Steven Idell,1and Jian Fu1*
Texas Lung Injury Institute, Center for Biomedical Research, University of Texas Health Science Center at Tyler, Tyler, Texas 75708,1
and Department of Vascular Physiology, National Cardiovascular Center Research Institute, Osaka, Japan2
Received 15 December 2010/Returned for modification 3 February 2011/Accepted 3 May 2011
Inflammatory tissue injury and immunosuppression are the major causes of death in sepsis. Novel thera-
peutic targets that can prevent excessive inflammation and improve immune responses during sepsis could be
critical for treatment of this devastating disease. LOX-1 (lectin-like oxidized low-density lipoprotein receptor-
1), a membrane protein expressed in endothelial cells, has been known to mediate vascular inflammation. In
the present study, we demonstrated that LOX-1 deletion markedly improved the survival rate in a murine
model of polymicrobial sepsis. Wild-type (LOX-1?/?) and LOX-1 knockout (LOX-1?/?) mice were subjected
to cecal ligation and puncture (CLP) to induce sepsis. LOX-1 deletion significantly reduced systemic inflam-
mation and inflammatory lung injury during sepsis, together with decreased production of proinflammatory
cytokines and reduced lung edema formation. Furthermore, LOX-1 deletion improved host immune responses
after the induction of sepsis, as indicated by enhanced bacterial clearance. Interestingly, we were able to
demonstrate that LOX-1 is expressed in neutrophils. LOX-1 deletion prevented neutrophil overreaction and
increased neutrophil recruitment to infection sites after sepsis induction, contributing at least partly to
increased immune responses in LOX-1 knockout mice. Our study results indicate that LOX-1 is an important
mediator of inflammation and neutrophil dysfunction in sepsis.
Sepsis often leads to multiorgan failure and death due to
systemic inflammatory injury and immune suppression (7, 9).
To launch efficient defense without causing inflammatory tis-
sue damage or immune suppression, host responses must be
tightly controlled. Molecular mechanisms of sepsis-induced in-
flammation and immune dysfunction remain largely unknown,
hampering the development of therapeutics to treat this dev-
LOX-1 (lectin-like oxidized low-density lipoprotein [oxLDL]
receptor-1) is a membrane protein previously identified as an
oxLDL receptor in endothelial cells (12). Further studies have
demonstrated that LOX-1 is a multiligand receptor that also
binds to C-reactive protein, apoptotic cells, bacteria, and acti-
vated platelets (12, 32, 36–38). LOX-1 has been reported to
play diverse roles in proinflammatory signaling and endothelial
dysfunction (12). LOX-1 expression and signaling contribute to
the pathogenesis of vascular inflammation (12). NF-?B is a
transcriptional regulator of proinflammatory mediators, in-
cluding cytokines, chemokines, and adhesion molecules (13,
21). LOX-1 activation has been reported to induce NF-?B
activation and increase the production of proinflammatory me-
diators TNF-? and IL-1 (5, 26, 39). The expression and func-
tion of LOX-1 in sepsis-induced inflammation have not been
studied. In the present study, we employed LOX-1 knockout
mice to assess the role of LOX-1 in modulating inflammatory
responses in a murine model of polymicrobial sepsis.
Neutrophils are central components of the innate immune
system that play a critical role in eliminating infectious agents
(8, 23). However, overreactions of neutrophils may lead to
tissue injury, exhaustion of neutrophil responses, and immune
suppression in sepsis (1–3). Clinically, dysregulation of neutro-
phil function has been associated with increased mortality in
sepsis patients (31, 40). Toll-like receptor (TLR) signaling is
critical in neutrophil activation (4, 15). Nevertheless, neutro-
phil overreaction via TLR signaling is associated with inhibi-
tion of neutrophil function in sepsis (1–3). Indeed, TLR2 and
TLR4 deletion has been shown to improve neutrophil che-
motaxis and increase mouse survival in sepsis (1–3). Therefore,
new therapeutic targets that help control neutrophil reaction
could be beneficial for maintaining neutrophil function during
sepsis. In the present report, we demonstrate that LOX-1 is
also expressed in neutrophils and mediates sepsis-induced neu-
MATERIALS AND METHODS
Reagents. Ultrapure lipopolysaccharide (LPS) (Escherichia coli 0111:B4) was
obtained from InvivoGen (San Diego, CA). Mouse CXCL-2/MIP-2, phycoeryth-
rin (PE)-conjugated rat anti-mouse LOX-1, and goat anti-mouse LOX-1 mono-
clonal antibodies were obtained from R&D Systems (Minneapolis, MN). Fluo-
rescein isothiocyanate (FITC)–Gr-1 monoclonal antibody was obtained from
eBioscience (San Diego, CA). Allophycocyanin (APC)-Ly6G, PE-Ly6G, Alexa
Fluor 647-CXCR2, and FITC-CD11b antibodies were purchased from Biolegend
(San Diego, CA). Phospo-p38 mitogen-activated protein kinase (MAPK) anti-
body was obtained from Cell Signaling (Danvers, MA).
Mice. Mice were housed in cages with access to food and water in a temper-
ature-controlled room with a 12-h dark/12-h light cycle. All experiments and
animal care procedures were approved by the Institutional Animal Care and Use
Committee of the University of Texas Health Science Center at Tyler. Genera-
tion of LOX-1 knockout mice has been described previously (29). Age- and
sex-matched littermate mice were used in the studies. TLR4 knockout mice
* Corresponding author. Mailing address: Texas Lung Injury Insti-
tute, Center for Biomedical Research, University of Texas Health
Science Center at Tyler, Tyler, TX 75708. Phone: (903) 877-7942. Fax:
(903) 877-5914. E-mail: firstname.lastname@example.org.
?Published ahead of print on 16 May 2011.
(TLR4?/?) and TLR2 knockout mice (TLR2?/?) were obtained from Jackson
Laboratory (Bar Harbor, ME).
Sepsis model. The cecal ligation and puncture (CLP) sepsis model was per-
formed as described previously (30). Briefly, CLP surgery was performed on 7-
to 10-week-old mice to induce sepsis. After the ligation, the cecum was punc-
tured twice with a 20-gauge needle and gently squeezed to extrude a small
amount of feces. For control animals, the same procedure was performed but
without ligation and puncture of the cecum. For survival studies, mice were
monitored every 12 h for 7 days after CLP surgery.
Neutrophil isolation, FACS analysis, bacterial CFU determinations, and
ELISA. Isolation of mouse neutrophils was performed as described previously (6,
14, 16). Neutrophils were ?95% pure as determined by Hema-3 staining (Fisher
Scientific, Pittsburgh, PA) and by Gr-1highexpression analysis using flow cytom-
etry. Fluorescence-activated cell sorter (FACS) analysis was conducted as de-
scribed previously (14, 16). Bacterial CFU in the lung, blood, and peritoneal
cavity were determined as described previously (11, 35). Concentrations of tumor
necrosis factor alpha (TNF-?) and interleukin-6 (IL-6) in plasma or lung ho-
mogenates were determined using enzyme-linked immunosorbent assay
(ELISA) kits (Biolegend).
In vitro neutrophil phagocytosis, bactericidal, and chemotaxis assays. In vitro
neutrophil phagocytosis assays were performed as described previously (27, 44)
using a Vybrant phagocytosis assay kit (Invitrogen). Briefly, neutrophils from
LOX-1?/?or LOX-1?/?littermates were isolated by density gradient centrifu-
gation. Fluorescein-labeled E. coli particles were opsonized and incubated with
neutrophils at 37°C for 1 h with gentle rotation in darkness. Cells were then
washed with cold Hanks’ balanced salt solution and incubated with 0.4% trypan
blue to quench extracellular fluorescein-labeled bacteria. The amount of phago-
cytosed E. coli is expressed as the mean fluorescence intensity (MFI) of the
intracellular fluorescein detected.
Neutrophil bactericidal assays were performed using E. coli as described pre-
viously (27, 44). Briefly, bacteria were opsonized with 50% pooled mouse serum
and incubated with neutrophils isolated from LOX-1?/?or LOX-1?/?litter-
mates at a preoptimized ratio (1:1 [106/ml]) at 37°C for 1 h. Neutrophils were
then lysed with water (pH 11). Serial dilutions were spread on LB agar plates and
incubated at 37°C overnight. Data are expressed as numbers of E. coli CFU per
Chemotaxis analysis was conducted using a 24-well plate with a transwell
membrane (Corning) (5-?m pore size) as described previously (3). Briefly, neu-
trophils (1 ? 105cells in 100 ?l of medium) were loaded on upper chamber and
allowed to migrate toward medium alone or CXCL2 (30 ng/ml) in the lower
chamber at 37°C with 5% CO2for 1 h. The membrane was then fixed and stained.
Neutrophils that migrated through the membrane were counted in at least 10
randomly selected fields under a light microscope.
Statistic analysis. Data are expressed as means ? standard errors of the
means (SEM). Student’s t test was used for all studies unless otherwise stated.
Survival curves were analyzed by log-rank tests using GraphPad Prism (Graph-
Pad Software, San Diego, CA). Statistical significance was assigned to data with
P values smaller than 0.05.
Increased survival of LOX-1 knockout mice in sepsis. To
investigate the potential role of LOX-1 in sepsis, we first ex-
amined the effect of LOX-1 deletion on sepsis-induced mor-
tality after CLP surgery. LOX-1?/?mice exhibited a substan-
tially higher survival rate than wild-type littermates (71%
versus 27%) at 7 days after CLP (Fig. 1), suggesting that
LOX-1 may modulate host responses and lead to increased
death rates during sepsis.
LOX-1 knockout mice exhibited decreased inflammatory re-
sponses and lung injury after sepsis induction. To assess the
role of LOX-1 in systemic inflammatory responses in sepsis, we
examined proinflammatory cytokine production in the blood of
wild-type and LOX-1 knockout mice after CLP. Blood TNF-?
and IL-6 production was significantly reduced in LOX-1
knockout mice after CLP compared with the results seen with
wild-type littermates (Fig. 2). Inflammatory tissue injury is a
major cause of death in sepsis (43). We examined lung inflam-
mation and injury in wild-type and LOX-1 knockout mice after
CLP. Our data showed that CLP-induced TNF-? and IL-6
production and lung wet/dry ratios were significantly lower for
LOX-1 knockout mice than for wild-type littermates (Fig. 3).
FIG. 1. Increased survival in LOX-1 knockout mice after the in-
duction of sepsis. Mortality rates of wild-type mice (WT) and LOX-1
knockout mice (LOX-1?/?) were monitored after the induction of
sepsis by CLP. Survival curves were analyzed by log-rank test. ?, P ?
0.02 versus WT (n ? 15 mice per group).
FIG. 2. LOX-1 deletion reduces inflammatory responses after the
induction of sepsis. At 24 h after CLP, blood TNF-? and IL-1? levels
in wild-type mice (WT) and LOX-1 knockout mice (LOX-1?/?) were
assessed by ELISA. ?, P ? 0.01 (n ? 7).
FIG. 3. LOX-1 deletion attenuates lung inflammation and injury in
sepsis. (A) At 24 h after CLP, lungs from LOX-1 knockout (LOX-
1?/?) mice and wild-type (WT) mice were collected to examine wet/dry
ratios. (B) Lung TNF-? and IL-6 levels were assessed by ELISA. ?,
P ? 0.01 (n ? 7).
2866 WU ET AL.INFECT. IMMUN.
LOX-1 deletion enhances bacterial clearance in sepsis. We
also conducted experiments to investigate whether LOX-1 is
involved in the regulation of host defense. We examined the
effect of LOX-1 deletion on bacterial infection during sepsis.
LOX-1 knockout mice had cecal flora similar to those seen
with wild-type littermates when no surgery was performed
(data not shown). Interestingly, LOX-1 knockout mice exhib-
ited decreased bacterial CFU levels in the peritoneal cavity,
blood, and lung after CLP (Fig. 4) compared with wild-type
littermates, suggesting that LOX-1 deletion enhances bacterial
clearance during sepsis.
LOX-1 deletion increases neutrophil migration in sepsis.
Neutrophils are the most abundant leukocytes in circulation
and play a critical role in host defense (2). Severe sepsis has
been known to cause impaired neutrophil migration to the
infectious focus (1–3), which leads to immune suppression and
increased mortality. To assess whether LOX-1 modulates neu-
trophil responses during sepsis, we examined the effects of
LOX-1 deletion on neutrophil function. Blood neutrophil
counts in wild-type and LOX-1 knockout mice before the sur-
gery were not significantly different (data not shown). We
found that neutrophil counts in the peritoneal cavity of LOX-
1?/?mice after CLP were significantly higher (Fig. 5A) than in
that of wild-type littermates. We then conducted in vitro ex-
periments to further assess the effects of LOX-1 deletion on
neutrophil chemotaxis, phagocytosis, and bactericidal activity.
We demonstrated that neutrophils isolated from wild-type
mice after CLP exhibited impaired chemotaxis with respect to
CXCL2 (Fig. 5B), whereas neutrophils isolated from LOX-1
knockout mice were less affected. Neutrophil phagocytosis and
bactericidal activity were not significantly altered by LOX-1
deletion (Fig. 5C and D). Our data suggest that LOX-1 signal-
ing is involved in impaired neutrophil chemotaxis during sep-
LOX-1 is expressed in neutrophils. LOX-1 is a membrane
protein with a single transmembrane domain (12). To explore
the direct role of LOX-1 in neutrophil function, we examined
whether LOX-1 is expressed in neutrophils. We detected high
basal LOX-1 protein expression in neutrophils in wild-type
mice (Fig. 6A and B). Interestingly, LPS challenge did not
increase LOX-1 total protein expression (Fig. 6A). However,
LOX-1 surface (membrane) expression, which was shown by
FACS to label only cell surface LOX-1, was quickly and mark-
edly increased after the challenge by LPS (Fig. 6B and C).
LOX-1 surface expression was also upregulated by the TLR2
agonist LTA (lipoteichoic acid), but the effect of LTA was less
potent than that of LPS (data not shown). Our data indicate
that LOX-1 is presynthesized and stored in neutrophils and
can be quickly released from the intracellular store to the cell
surface after LPS challenge. Neutrophil LOX-1 expression and
the specificity of LOX-1 antibodies were further confirmed
using LOX-1 knockout mice. No LOX-1 protein or surface
expression was observed in LOX-1?/?neutrophils (Fig. 6A
and B). We also assessed whether LOX-1 surface expression is
modulated during sepsis. FACS analysis showed that LOX-1
surface expression was increased in neutrophils of wild-type
mice after CLP surgery (Fig. 6D). TLR2?/?and TLR4?/?
neutrophils exhibited lower LOX-1 surface expression than
wild-type neutrophils after the induction of sepsis (Fig. 6B),
suggesting that both TLR2 and TLR4 activation can mediate
neutrophil LOX-1 surface expression during sepsis.
FIG. 4. LOX-1 deletion increases bacterial clearance in the peritoneal cavity, blood, and lung after the induction of sepsis. Bacterial CFU in
mouse peritoneal cavity (A), blood (B), and lung (C) were examined in LOX-1 knockout (LOX-1?/?) mice and wild-type (WT) mice 24 h after
CLP. ?, P ? 0.05 versus WT. The horizontal lines represent means.
FIG. 5. Neutrophils in LOX-1 knockout mice exhibit increased
peritoneal migration in sepsis. (A) At 6 h after CLP, peritoneal cells
were collected. Neutrophil counts were determined by FACS analysis.
Neutrophil numbers in the peritoneal cavity were significantly higher
in LOX-1 knockout (LOX-1?/?) mice. ?, P ? 0.02 versus wild-type
(WT)/CLP (n ? 6). (B) Neutrophils were isolated from whole blood
2 h after CLP. Chemotaxis of neutrophils for CXCL2 was examined. ?,
P ? 0.01. (C) For phagocytosis assays, FITC-labeled and heat-killed E.
coli bacteria were incubated with blood neutrophils isolated from
LOX-1 knockout (LOX-1?/?) or WT littermates. Following quenching
of extracellular E. coli with trypan blue, uptake of E. coli was analyzed
by FACS. NS, not significant (n ? 8). (D) Bacterial killing by neutro-
phils isolated from LOX-1 knockout (LOX-1?/?) or WT littermates
was determined. NS, not significant (n ? 8). PMN, polymorphonuclear
VOL. 79, 2011ROLE OF LOX-1 IN SEPSIS 2867
LOX-1 deletion inhibits neutrophil overreaction and
CXCR2 downregulation in sepsis. Our finding of LOX-1 ex-
pression in neutrophils implies that LOX-1 may possess a novel
function in neutrophils. To test this possibility, we examined
whether LOX-1 deletion could affect neutrophil activation after
the induction of sepsis. CLP induced neutrophil activation in
wild-type mice, as demonstrated by increased neutrophil p38
MAPK phosphorylation and CD11b surface expression. Interest-
ingly, neutrophil p38 MAPK phosphorylation and CD11b surface
expression during sepsis were significantly lower in LOX-1 knock-
out mice (Fig. 7A and B), indicating that LOX-1 deletion may
prevent neutrophil overreaction in sepsis.
Chemokine receptor CXCR2 is a key mediator of neutrophil
migration (2, 3). Neutrophil overreaction during severe sepsis has
been known to induce CXCR2 downregulation and impaired
neutrophil migration (1–3). We examined whether reduced neu-
FIG. 6. LOX-1 is expressed in neutrophils. (A) Wild-type mice were challenged with LPS (2.5 mg/kg of body weight by intraperitoneal
injection). Immunoblotting analysis was performed to examine LOX-1 protein expression in isolated neutrophils (PMN) at 2 h after LPS challenge.
No LOX-1 expression was detected in neutrophils of LOX-1?/?mice with or without LPS challenge. The data also showed that LOX-1 total
protein expression in wild-type neutrophils was not affected by LPS challenge. (B) FACS was conducted to examine LOX-1 surface expression in
blood neutrophils of LOX-1 knockout (LOX-1?/?) or wild-type littermate mice at 2 h after LPS challenge (2.5 mg/kg by intraperitoneal injection).
Representative histograms of isotype IgG staining (filled gray) and LOX-1 staining of neutrophils from control mice (Con) and LPS-challenged
mice (LPS) are shown. (C) Representative histograms of LOX-1 staining in neutrophils (Gr-1highblood cells) from control mice (Con [challenged
by phosphate-buffered saline]), mice challenged by LPS for 1 h (1 h LPS) and 3 h (3 h LPS), or PE-isotype IgG control mice (filled gray). The bar
graph shows neutrophil LOX-1 surface expression at different time points after LPS challenge. Data represent mean fluorescence intensity (MFI).
MFI values were calculated by subtracting isotype control antibody staining from LOX-1 antibody staining. ?, P ? 0.05 versus control. (D) At 6 h
after CLP, LOX-1 surface expression in blood neutrophils (Ly6G?) of wild-type (WT), TLR2 knockout (TLR2?/?), and TLR4 knockout
(TLR4?/?) mice was analyzed by FACS. MFI values were calculated by subtracting isotype control antibody staining from LOX-1 antibody
staining. ?, P ? 0.03 versus WT (n ? 6).
FIG. 7. LOX-1 deletion prevents p38 MAPK phosphorylation, CD11b surface expression, and CXCR2 downregulation after the induction of
sepsis. At 2 h after CLP-induced sepsis, p38 MAPK phosphorylation (A) and CD11b (B) and CXCR2 (C) surface expression in blood neutrophils
(Ly6G?) of wild-type (WT) and LOX-1?/?mice were examined by FACS. Data represent mean fluorescence intensity (MFI). MFI values were
calculated by subtracting isotype control antibody staining from phosphorylated p38, CD11b, or CXCR2 antibody staining. ?, P ? 0.03 (n ? 4).
2868WU ET AL.INFECT. IMMUN.
trophil activation after the induction of sepsis in LOX-1 knockout
mice could lead to improved CXCR2 surface expression. LOX-1
deletion prevented the downregulation of neutrophil CXCR2
the increased neutrophil migration and chemotaxis seen both in
vivo and in vitro (Fig. 5), suggesting that LOX-1 deletion helps
maintain a better CXCR2-mediated neutrophil response.
The results from our study indicate that LOX-1 signaling
aggravates inflammatory responses and mediates immune sup-
pression after the induction of sepsis (Fig. 8), likely contribut-
ing to the increased mortality seen in a mouse model of poly-
microbial sepsis. Sepsis contains two pathophysiological
phases, systemic inflammation and immune suppression (2, 10,
33). An ideal treatment for sepsis-induced multiorgan dysfunc-
tion would be able to prevent inflammatory tissue injury but
maintain immune responses (2, 10, 33). Therefore, the finding
that LOX-1 signaling modulates both the inflammatory and
immune responses during sepsis is very intriguing.
Production and upregulation of proinflammatory mediators
such as TNF-? and IL-6 contribute to sepsis-induced inflam-
matory tissue injury (17, 20). In our studies, LOX-1 deletion
prevented TNF-? and IL-6 production and lung injury in sep-
sis. The role of LOX-1 in the development of inflammation-
driving cardiovascular diseases has been well established (24).
LOX-1 blockade was able to prevent proinflammatory and
prooxidant responses in endothelial cells and reduce athero-
genesis in mouse atherosclerosis models (18, 45). LOX-1 sig-
naling has been shown to mediate the production of proinflam-
matory cytokines TNF-? and IL-1 (39). Our studies suggest
that LOX-1 also plays a role in sepsis-induced inflammatory
Immune suppression has been a major problem for sepsis
patients (2, 25, 33). It hampers bacterial clearance and causes
further exaggeration of tissue damage. Immune suppression
also makes sepsis patients more susceptible to secondary in-
fection. A high mortality rate has been reported for sepsis
patients with secondary bacterial pneumonia (2, 25, 33). Sepsis
has been known to suppress neutrophil function, including
chemotaxis and signaling (2, 3), likely due to overreaction and
exhaustion of neutrophils (1–3). Our studies showed that
LOX-1 deletion prevented neutrophil overreaction in sepsis
and allowed better control of neutrophil responses during sep-
sis, leading to increased neutrophil recruitment at the sites of
infection and increased bacterial clearance.
CXCR2 surface expression in neutrophils can regulate a
wide variety of cellular responses, including respiratory burst,
degranulation, integrin activation, and migration (2, 3, 41).
Downregulation of CXCR2 surface expression due to neutro-
phil overreaction during sepsis leads to impaired neutrophil
migration (1–3), which is a major factor in sepsis-induced neu-
trophil suppression. p38 MAPK activation has been reported
to mediate CXCR2 downregulation in neutrophils (19, 22, 42).
LOX-1 deletion prevented p38 MAPK activation in neutro-
phils, possibly contributing to the inhibition of CXCR2 down-
regulation and improved neutrophil migration in LOX-1
knockout mice during sepsis.
Neutrophil modulation appears to be involved in both in-
flammatory and immune dysfunction in sepsis (7, 25, 33, 34).
Proinflammatory cytokines and chemokines induce neutrophil
activation and migration to the sites of infection (34). On the
other hand, neutrophil overreaction, impaired neutrophil che-
motaxis, and altered Toll-like receptor (TLR) signaling are
associated with immunosuppression in sepsis (1–3). Our pres-
ent study specifically examined LOX-1 expression and function
in neutrophils. However, since LOX-1 is also expressed in
endothelial cells and macrophages (12, 28), we cannot exclude
the possibility that LOX-1 deletion may also provide the ob-
served beneficial effects for those cells. Apparently, more stud-
ies are required to characterize the function of LOX-1 in other
cell types during sepsis. Nevertheless, our studies indicate that
better-controlled neutrophil responses in LOX-1 knockout
mice contribute, at least in part, to the improved immune
responses in sepsis. Further studies are warranted to explore
the underlying mechanisms. For example, identifying potential
ligands and receptors that may interact with LOX-1 in neutro-
phils could advance our current knowledge of neutrophil func-
tion and the pathogenesis of sepsis.
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