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CUTTING EDGE |



*
Institute of Molecular Medicine for the Prevention of Human Diseases,
Department of Integrative Biology, Physiology, and Pharmacology, and
Department of Biochemistry and Molecular Biology, University of Texas-Houston, Houston, TX 77030
| Abstract |
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was
significantly elevated in the C3aR-/- mice compared with
their littermates following LPS challenge. These findings demonstrate
an important protective role for the C3aR in endotoxin shock and
indicate that, in addition to its traditionally accepted functions in
mediating inflammation, the C3aR also acts in vivo as an
anti-inflammatory receptor by attenuating LPS-induced
proinflammatory cytokine production. | Introduction |
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, IL-1
, and IL-6
secretion from isolated PBMC (5, 6), and attenuating
TNF-
and IL-6 secretion from B-cells stimulated with
Staphylococcus aureus Cowan strain I and IL-2
(7). Activation of complement during sepsis results in the generation of C3a and inactivated C3a (C3adesArg; Ref. 8). Several investigators have found a close relationship between elevated plasma levels of C3a/C3adesArg in patients with septic shock, and the risk of the development of adult respiratory distress syndrome or multisystem organ failure (9, 10, 11, 12). Others, however, have not been able to demonstrate any positive correlation between these observations (13). Therefore, it has remained uncertain whether C3a contributes to the pathogenesis of septic shock.
To investigate the role of C3a in septic shock,
C3aR-/- mice were produced by gene targeting
and studied in an in vivo model of shock generated by i.v. injection of
LPS. LPS challenge revealed increased susceptibility to shock in the
C3aR-/- mice compared with their wild-type
littermates. The increased sensitivity to LPS-induced shock was
accompanied by significantly increased plasma concentrations of IL-1
and a trend toward increased plasma concentrations of TNF-
and IL-6
in the C3aR-/- mice as compared with wild-type
mice. Collectively, these results indicate that the C3aR plays a
significant role in tempering the pathological effects of
endotoxin-induced shock and supports the in vitro data indicating that
C3a functions as an anti-inflammatory molecule, as well as a
proinflammatory mediator.
| Materials and Methods |
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A targeting vector was designed from a 129/Svj genomic clone (14) by replacing exon 2, containing the entire open reading frame of the C3aR gene, with the neomycin resistance gene from pKO SelectNeo V800 (Lexicon Genetics, The Woodlands, TX). RW-4 embryonic stem cells were transfected with the linearized targeting vector and the generation of chimeric and mutated mice was conducted as described previously (15).
Southern blot analysis
DNA was isolated from G418-resistant embryonic stem cells or from mouse tail biopsies and digested with HindIII. Standard Southern blotting was performed with a 32P-radiolabeled 5' probe external to the targeting vector.
PCR genotyping
DNA was isolated from mouse tail biopsies. The isolated DNA was used as a template in PCR with primers C1 (TACAATATAGTCAGTTGGAAGTCAGCC), NeoA (TGGGCTCTATGGCTTCTGAGGCGGAAAG), and A201+ (GAGAATCAGGTGAGCCAAGGAGAAG). The primers C1 and NeoA yield a fragment of 537 bp for targeted DNA. Primers C1 and A201+ yield a fragment of 726 bp for wild-type DNA.
Antibodies
Polyclonal rabbit anti-mouse C3aR Abs were generated to a peptide corresponding to amino acid residues 240258 of the mouse C3aR (CSPEDSFSLDSANQQPHYGG). By ELISA, it was determined that the rabbit anti-serum raised specifically recognizes the C3aR peptide.
Immunofluorescence and flow cytometry
Thioglycollate-stimulated peritoneal macrophages were obtained from wild-type and C3aR-/- mice as described (16). Microscope slides containing 1.5 x 105 cells were prepared using a cytocentrifuge. After blocking nonspecific binding sites, the slides were incubated with rabbit anti-mouse C3aR antiserum diluted 1:1000. Bound Abs were detected with FITC-conjugated goat anti-rabbit F(ab')2 antiserum (Sigma, St. Louis, MO). Cells were analyzed on a FACScalibur flow cytometer (Becton Dickinson, Franklin Lakes, NJ).
Endotoxin induced shock and cell counts
Six to ten-wk old C3aR-/-, C3aR+/-, and wild-type littermates (on a C57BL/6 and 129SvJ mixed background) were injected (i.v.) with 48 mg/kg with LPS from E. coli serotype 0127:B8 (Sigma). The mice were injected at time 0 and monitored for survival at indicated times and at least twice daily for 7 days.
Cytokine and nitrate/nitrite measurements
Serum levels of TNF-
, IL-1
, IL-10, and IL-6 were
determined using ELISA kits obtained from R&D Systems (Minneapolis,
MN). Briefly, LPS-challenged mice were anesthetized and blood was
collected by cardiac puncture. Plasma from three mice was pooled within
each group. Three to four pools from each group were measured by ELISA.
NO3- in duplicate samples of
plasma was converted to NO2-
with reductase (Sigma N-7265) and NADPH (ICN 101167).
NO2- was quantified as
described (17).
Statistical analysis
Statistical analysis was performed using the Graphpad (San Diego, CA) Prism program package. Survival curves were compared using the logrank test and ELISA results using the two-tailed, unpaired t test.
| Results |
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The mouse C3aR gene was disrupted by replacing the entire open
reading frame, contained in exon 2, with a neor
cassette (Fig. 1
). To confirm that the
gene disruption created a null mutation, immunofluorescence and flow
cytometry of thioglycollate-stimulated peritoneal macrophages were used
to demonstrate the absence of the C3aR in the
C3aR-/- mice (Fig. 2
). Furthermore, RT-PCR was performed
using total RNA isolated from brain, kidney, liver, lung, testis, and
thioglycollate-stimulated peritoneal macrophages of wild-type and
C3aR-/- mice. RNA from wild-type mice yielded a
PCR fragment corresponding to the C3aR, whereas RNA from homozygous
knockout mice did not produce any fragment, confirming the absence of
C3aR gene expression (data not shown).
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C3aR-/- mice display increased mortality to endotoxin-induced shock
To analyze whether the role of the C3aR serves dual functions
(pro- and anti-inflammatory) in endotoxemia, 6- to 10-wk-old
C3aR-/- mice and their wild-type littermates
were injected i.v. with increasing doses of LPS. Following challenge,
the C3aR-/- mice experienced increased
susceptibility to the lethal effects of endotoxin compared with their
wild-type littermates. At 48 mg/kg, 85% of the
C3aR-/- mice died within 3 days, compared with
only 17% of the wild-type (Fig. 3
). The
C3aR heterozygous deficient mice (C3aR+/-) were
also more sensitive to LPS challenge than their wild-type littermates,
but less sensitive than homozygous C3aR-/-
mice, indicating a gene-dosage effect (Fig. 3
).
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after
LPS challenge
To determine whether the increased sensitivity of the
C3aR-/- mice to LPS-challenge is reflected in
alterations of cytokine and NO levels, TNF-
, IL-1
, IL-6, IL-10,
and nitrite (NO2-) and nitrate
(NO3-) levels were determined
in plasma samples obtained from C3aR-/- mice
and their wild-type littermates injected with LPS. Compared with their
wild-type littermates, the C3aR-/- animals
contained elevated plasma levels of all the proinflammatory cytokines
(IL-1
, IL-6, and TNF-
) after LPS treatment (Fig. 4
). IL-1
levels were significantly
elevated, with an approximate 3-fold increase in the samples from
C3aR-/- mice, 0.15 ng/ml ± 0.04 (SE) and
0.43 ng/ml ± 0.12 (SE) for wild-type and
C3aR-/- mice, respectively. In contrast, there
were no differences in the plasma levels of IL-10 and nitrite/nitrate
in the C3aR-/- and wild-type mice after LPS
challenge.
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| Discussion |
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Measurements of IL-1
indicated a significant elevation in plasma
concentration of this proinflammatory cytokine in the
C3aR-/- mice, which may explain, at least in
part, the increased LPS sensitivity. That high levels of IL-1
may be
a major contributor to the increased LPS sensitivity is supported by
the observation that deficiency of the IL-1
converting enzyme
protects mice from LPS-induced shock (18).
The increased sensitivity of C3aR-/- mice to
i.v. injection of LPS might be considered paradoxical, because the C3a
ligand is generally viewed as a mediator of proinflammatory functions.
However, recent reports have demonstrated in vitro that C3a may act as
an anti-inflammatory molecule by suppressing LPS induced secretion
of TNF-
, IL-1
, and IL-6 from isolated PBMC (5, 6),
as well as TNF-
and IL-1
production from S. aureus
Cowan strain I/IL-2-stimulated B-cells (7). Our
observations of increased mortality and elevated IL-1
plasma
concentrations in LPS-treated C3aR-/- mice
support these in vitro data. Moreover, they suggest that the
anti-inflammatory effects of C3a are significant in vivo, and can
occur through specific interactions with the C3aR.
The genetically C3aR-/- mice will be useful models to further investigate the yet unknown functions of C3a, and its participation in numerous diseases in which complement anaphylatoxin peptides have been implicated, including asthma, myocardial ischemia injury, rheumatoid arthritis, inflammatory bowel disease, and psoriasis.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Rick A. Wetsel, Institute of Molecular Medicine for the Prevention of Human Diseases, 2121 West Holcombe Boulevard, Houston, TX 77030. ![]()
Received for publication August 17, 2000. Accepted for publication September 13, 2000.
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