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From the Departments of Pathology* and Orthopaedic Surgery
and the School of Nursing,
Asahikawa Medical College, Asahikawa, Japan; and the Department of Pathology and Molecular Medicine,
McMaster University, Hamilton, Ontario, Canada
| Abstract |
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To study the role of DAP12/TREM-1 signaling during chronic inflammation, we constructed two adenoviral gene vectors (Figure 1B)
: Ad-FDAP12 (allowing increased expression of FLAG-DAP12) and Ad-TREM-1 Ig (allowing expression of an antagonist of the DAP12-signaling pathway-soluble form of extracellular domain of TREM-1), and investigated their respective modulatory effect in a mouse model of zymosan A-induced hepatic granuloma.18
Zymosan A (zymosan, zymocel, ß-glucans), which is composed of ß-1,3 polyglucose, causes very strong stimulation of macrophages, neutrophils, and natural killer cells.19
We hypothesized that DAP12 signaling could enhance granulomatous responses of monocytes/macrophages whereas TREM-1 Ig will suppress inflammatory response via its antagonistic effect on DAP12 signaling in vivo (Figure 1A)
. In this study, we demonstrated that zymosan A-induced granuloma formation was sustained and enhanced at later times by adenoviral-mediated DAP12 gene transfer. In contrast, gene transfer of a DAP12-signaling inhibitor, extracellular domain of TREM-1 markedly inhibited granuloma formation. Our results suggest that the DAP12-signaling pathway plays an important role in chronic inflammation and granuloma formation.
| Materials and Methods |
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C57BL/6NCrj female mice were purchased from Sankyo Labo Service Corp. (Sapporo, Japan). Experimental mice were used at 7 to 8 weeks of age. The murine myeloblastic leukemic cell line, M1, was obtained from Riken Gene Bank (Wakou, Japan). Cells were cultured in RPMI 1640 medium (Nissui Seiyaku Co., Tokyo, Japan) supplemented with 10% fetal calf serum and 5 x 10-5 mol/L 2-mercaptoethanol.
Antibodies
Rabbit anti-mouse DAP12 polyclonal antibody was generated by immunizing a rabbit (Japanese White) with GST-mouse DAP12 cytoplasmic domain fusion protein as described previously.16 Rat anti-murine F4/80 monoclonal antibody (mAb) (CI:A3-1) was purchased from BMA Biomedicals (August, Switzerland). Mouse anti-FLAG mAb (M2) was purchased from Sigma (St. Louis, MO). Control mouse IgG1 was purchased from Chemicon International Inc. (Temecula, CA).
Adenovirus Vectors and Zymosan A-Induced Hepatic Granuloma Formation
A fusion protein consisting of the extracellular domain of mouse TREM-1 and the Fc portion of human IgG1 was referred to as TREM-1 Ig (the human Ig Fc molecule was used both to prolong the half-life of soluble TREM-1 molecule and to serve as a tag). Recombinant adenovirus containing LacZ (Ad-LacZ), FLAG-DAP12 (Ad-FDAP12), or TREM-1 Ig (Ad-TREM-1 Ig) was generated by the COS-TPC method20
using the Adenovirus Expression Vector kit (Takara, Shiga, Japan) (see Figure 1B
for details). These viral vectors were deleted of E1A and E1B, as well as the E3 region. Encoded cDNA was expressed under the control of the CAG promoter.21
Mice were injected with 350 µl of phosphate-buffered saline (PBS) containing 350 µg of zymosan A from Saccharomyces cerevisiae (Nacaraitesque, Inc., Kyoto Japan) into right retro-orbital plexus. Twenty-four hours after zymosan A injection, mice were injected with 1 x 109 plaque-forming units of viral vector in 100 µl of PBS into the left retro-orbital plexus. Mice were killed by cervical dislocation under diethyl ether anesthesia at days 3, 5, 7, and 10 after zymosan A injection.
Localization of Transgene Expression in the Liver by LacZ Histochemical Staining
For X-gal staining, mouse liver was washed with 20 ml of PBS and fixed with 20 ml of fixation reagent (1% formalin, 0.2% glutaraldehyde, 0.002% Nonidet P-40 in PBS) using liver perfusion. Then, the liver was removed and soaked in fixation reagent for 30 minutes with gentle shaking, followed by three washes: 1) PBS for 10 minutes, 2) 1% Triton X in PBS for 10 minutes, and 3) PBS for 10 minutes. X-gal staining was performed with staining reagent (0.5 mmol/L MgCl2, 5 mmol/L K4[Fe(CN)6], 5 mmol/L K3[Fe(CN)6], 0.05% X-gal in PBS) at 37°C for 2 days. Stained liver was observed with a stereoscope.
Immunoprecipitation, Electrophoresis, and Blotting
Cells were lysed in lysis buffer (0.5% Triton X-100, 50 mmol/L Tris, pH 8.0, 140 mmol/L NaCl, 10 mmol/L ethylenediaminetetraacetic acid) containing the protease inhibitor cocktail Complete Mini (Roche, Mannheim, Germany). Lysates were clarified by centrifugation and immunoprecipitated with anti-DAP12 Ab bound to rProtein A Sepharose Fast Flow (Amersham Pharmacia Biotech AB, Uppsala Sweden) for 1 to 2 hours at 4°C. The resulting immunocomplexes were washed and run on 4 to 12% NuPage bis-Tris sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels (Novex, San Diego, CA) under reducing conditions. Proteins were then blotted onto Immobilon-P (Millipore, Bedford, MA), blocked in 5% skim milk, and probed with rabbit anti-DAP12 Ab or anti-FLAG mAb (M2) (Sigma) followed by donkey anti-rabbit IgG-horseradish peroxidase (Amersham Pharmacia Biotech AB) or sheep anti-mouse IgG-horseradish peroxidase (Amersham Pharmacia Biotech AB). The ECL system (Amersham Pharmacia Biotech AB) was used for detection.
Light Microscopy and Immunohistochemistry
Liver tissues fixed in 10% formaldehyde were embedded in paraffin. Paraffin sections were cut at 3-µm thick and slides were stained with hematoxylin and eosin (H&E) for light microscopy. Hepatic granulomas were defined as being composed of more than 10 cells according to a previous report.18 In an immunohistochemical study, after deparaffinization and inhibition of the endogenous peroxidase activity, the sections were stained by Histofine SAB-PO Kit (Nichirei, Tokyo, Japan) using the antibodies mentioned above. Hematoxylin was used for nuclear staining.
Assays for Differentiation by DAP12 Signaling
To immobilize antibodies, the SonicSeal slide wells (Nalge Nunc International Corp., Naperville, IL.) were incubated with anti-FLAG mAb (M2) (20 µg/ml in PBS) overnight at 4°C and washed with culture medium twice. M1 cells were incubated with LPS (10 µg/ml) from Escherichia coli serotype 0111:B4 (Sigma) overnight (15 to 17 hours) and washed with culture medium. Then, the cells were infected with a multiplicity of infection of 25 of Ad-LacZ and Ad-FDAP12 for 1 hour, and were transferred to anti-FLAG mAb (M2)-coated SonicSeal slide wells. The cells were cultured for 3 days and were stained with H&E.
Flow Cytometry for Quantitation of Macrophage Differentiation
Cultured M1 cells (1 x 106) were blocked with 50% goat serum for 1 hour at 4°C and then incubated with saturating amounts of fluorescein isothiocyanate-conjugated rat anti-mouse Mac-1 (M1/70) (PharMingen, San Diego, CA) and phycoerythrin-conjugated anti-mouse MHC Class II (M5/114.15.2) (PharMingen) for 30 minutes in staining buffer (PBS, 1% fetal calf serum, 0.1% sodium azide) at 4°C. Dead cells were gated out using 2 µg/ml of propidium iodide at the last step of staining. The fluorescence intensity was analyzed by FACScan (Becton Dickinson Immunocytometry Systems, San Jose, CA).
Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) Analysis
Total RNA was prepared using Sepasol-RNA I (Nacaraitesque, Inc.) according to the manufacturers protocol. cDNA was prepared from 1 µg of total RNA by using the first-strand cDNA synthesis kit for RT-PCR (AMV) (Roche). Primers used were as follows: FLAG-DAP12 forward (5'-GCG AAT TCC GCG TCA TGG CCT TAC CAG TGA-3'), FLAG-DAP12 reverse (5'-ACC CTG TGG ATC TGT ATT-3'), TREM-1 forward (5'-CGG AAT TCG AGC TTG AAG GAT GAG GAA GGC-3'), TREM-1 reverse (5'-AAT CCA GAG TCT GTC ACT TGA AGG TCA GTC-3'), ß-actin forward (5'-ACC CAC ACT GTG CCC ATG TA-3'), ß-actin reverse (5'-CGG AAC CGC TCA TTG CC-3'). PCR was performed under the conditions of 1 minute at 94°C, 30 cycles (5 seconds at 94°C, 30 seconds at 60°C, 90 seconds at 72°C), 7 minutes at 72°C.
Isolation of F4/80-Positive Cells
We performed collagenase perfusion using a buffer (140 mmol/L NaCl, 10 mmol/L HEPES, 5 mmol/L, CaCl2, 2H2O) including 400 U/ml of collagenase type 4 (Sigma). Hepatocytes were removed by centrifugation at 50 x g.22 Then cells containing Kupffer cells and monocytes were collected and labeled with F4/80 followed by goat anti-rat IgG microbeads (Miltenyi Biotec, Bergisch Gladbach, Germany). We purified F4/80-positive cells using the MACS system (Miltenyi Biotec).
Statistical Analysis
The results were analyzed using one-way analysis of variance, post hoc test, and the Mann-Whitney U-test. All data in this study are expressed as the mean ± SD and P < 0.05 is considered significant.
| Results |
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Mouse leukemic M1 cells infected with Ad-FDAP12 could transport FLAG-DAP12 to the cell surface during LPS stimulation because of the concomitant expression of associate molecule of DAP12 (data not shown). Without the associated molecule, the surface expression of DAP12 was not detectable in our system using flow cytometry analysis (data not shown). To investigate the effect of Ad-FDAP12 gene transfer on macrophage differentiation, we pretreated M1 cells with LPS to induce DAP12-associating molecules on the cell surface overnight before infection with Ad-FDAP12 and stimulation by immobilized anti-FLAG mAb. In contrast to Ad-LacZ, Ad-FDAP12-infected M1 cells showed morphologicalchanges suggestive of macrophage differentiation on stimulation via DAP12 (Figure 2A)
. To verify macrophage differentiation, we also performed fluorescence-activated cell sorting analysis by using mAbs against murine MHC class II and macrophage surface molecule Mac-1. Indeed, Ad-FDAP12-infected cell population contained at least 100% more macrophages that expressed bright Mac-1 or both Mac-1 and MHC class II (28%) whereas the control cell population contained
14% of macrophages (Figure 2B)
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Because signaling through DAP12 may affect monocyte differentiation and activation (Figure 2)
,16,17,23,24
we set out to investigate the role of DAP12 during inflammation in vivo using adenoviral vectors and zymosan A-induced hepatic granuloma formation system. Additionally, we also examined the effect of Ad-TREM-1 Ig that would block the signal thorough TREM-1/DAP12 in monocytes and neutrophils (Figure 1A)
.14
We first investigated transgene expression in the liver after the delivery of an adenoviral vector expressing a marker gene coding for LacZ via the retro-orbital plexus route. This route of gene transfer was previously shown to lead to transgene expression predominantly in the liver.25
Two days after intravenous gene transfer, the mouse liver injected with Ad-LacZ showed abundant LacZ staining (Figure 3)
. In contrast, the control mouse had little LacZ staining. LacZ expression in the liver lasted for at least 10 days after gene transfer (data not shown). These results suggest that transgene can be significantly expressed in the liver after intravenous adenoviral gene transfer.
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Because application of high-titer adenoviral vectors in mice may result in severe liver injury26-28 and thus confound granuloma formation by zymosan, we determined appropriate doses of zymosan A and adenoviral vectors in trial experiments. The peak of zymosan A-induced hepatic granuloma formation was observed at 7 days after a single injection with 350 µg of zymosan A and it vanished by 11 days after the administration (data not shown). Thus, this dose of zymosan was chosen for our following experiments (in contrast to larger doses often used in other studies18 ) to keep a relatively short course of granulomatous responses that would allow the observation of the effect of adenoviral-mediated transgene expression after a single delivery of viral vector (a dose of 1 x 109 plaque-forming units of adenoviral vector was chosen).
To investigate the role of DAP12 and TREM-1 in zymosan A-induced granuloma formation, we administered the control vector (Ad-LacZ), Ad-FDAP12, or Ad-TREM-1 Ig to groups of mice at day 1 after zymosan A injection. Under lower magnifications of light microscopy, at days 3 and 5, the number of granulomas was similar between control and Ad-FDAP12 groups whereas the number of granulomas was slightly less in Ad-TREM-1 Ig-treated mice (Figures 4 and 5)
. By day 7, although the number of granulomas in the control and Ad-FDAP12 groups markedly increased and remained similar, the number of granulomas in Ad-TREM-1 Ig-treated mice was minimal (Figure 6)
(the size of granuloma in Ad-TREM-1 Ig group also tended to be smaller; see Figure 9
for more details). By day 10, granulomas in the control group primarily disappeared, close to the constant low level of granuloma formation in Ad-TREM-1 Ig-treated mice (Figure 7)
. In sharp contrast, the number of granulomas in Ad-FDAP12-treated mice continued to increase (Figure 7)
. The number of granulomas was also enumerated at four different time points in three groups of mice (Figure 8)
and the results were in line with morphological observations. Under higher magnification of light microscopy, the discrete granuloma structure and kinetic influx of inflammatory cells were revealed (Figure 9)
. At day 3, the major cell types were polymorphonuclear leukocytes and monocytes/macrophages, regardless of treatment given (Figure 9
; A, E, I). By day 5, the main cell type within the granuloma was mononuclear cells including macrophages and lymphocytes (Figure 9
; B, F, J). At day 7, many macrophage-derived epithelioid cells were seen, suggestive of mature granuloma formation (Figure 9
; C, G, K). By day 10, many cells in the control group underwent apoptosis (Figure 9D)
, which was associated with diminishing granuloma whereas many epithelioid cells were still seen in the granuloma of Ad-FDAP12-treated mice (Figure 9H)
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As shown in Figure 2
, we demonstrated that DAP12 signaling using anti-FLAG mAb made M1 cells differentiated to macrophages vigorously. To induce potent signal via FLAG-DAP12 in Ad-FDAP12-infected monocytes and macrophages, we injected anti-FLAG mAb together with Ad-FDAP12. At day 7, a remarkable number of large and F4/80-positive mature granulomas were seen in anti-FLAG mAb-injected group (Figure 11, B and E)
. The number of granulomas in the anti-FLAG mAb-injected group was larger than that of isotype control Ig-injected group (Figure 12)
. Interestingly, the Ad-FDAP12, Ad-TREM-1 Ig, and anti-FLAG mAb-injected group revealed numerous inflammatory cells but the size of the granulomas remained very small (Figure 11, C and F)
. These granulomas looked different from the large mature granulomas observed in Ad-FDAP12 and the anti-FLAG mAb-injected group (Figure 11, B and E)
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| Discussion |
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It has been reported that Kupffer cells, together with monocytes, neutrophils, lymphocytes, and macrophages, have the critical role for mouse hepatic granuloma formation induced by zymosan.31
Zymosan A is composed of ß-1,3 glucan and causes a very strong stimulation of macrophages, neutrophils, and natural killer cells through its receptor, mouse CR3 (CD11b/CD18,
Mß2 integrin).19
Recently, it was reported that TLR2 and TLR6 were also involved in zymosan recognition by mouse macrophages.32
As shown in Figure 10B
, we demonstrated that F4/80-positive cells that include Kupffer cells and monocytes expressed FLAG-DAP12. The signaling through DAP12 molecule on these F4/80-positive cells during early inflammatory phase may affect the magnitude and sustenance of granuloma formation. Such influence, as a result of increased DAP12 expression, on local microenvironment may act through at least three potential mechanisms: first of all, the normal turnover of granuloma is perhaps affected by the rate of macrophage apoptosis and altered DAP12 expression may slow down granuloma turnover by decreasing macrophage apoptosis. Second, because it is believed that the maintenance of granuloma requires the continuous supply of newly arrived monocytes, increased DAP12 expression may enhance such supply from the peripheral blood and/or local Kupffer cell pool to sustain already formed granulomas. Third, Bouchon and colleagues23
showed that the TREM-2/DAP12-mediated pathway regulates maturation of human dendritic cells. Therefore compulsory expression of DAP12 on clustering immature dendritic cells may cause their maturation and they may sustain microenvironments suitable for granuloma formation. More recently, KARAP/DAP12 transgenic mice were generated and they suffered massive inflammatory syndrome associated with neutrophilia and lung infiltration by multinucleated macrophages.33
Hyperresponsiveness to experimental septic shock was also observed. These observations, together with our current findings, indicate that overexpression of DAP12 leads to myeloid cell activation and accumulation and may profoundly enhance an ongoing chronic tissue inflammatory response.
DAP12 alone is not sufficient for its expression and function at the cell surface.5,16
Thus, the combination of DAP12-associating molecule and DAP12 may account for transmitting a particular physiological signal via DAP12. It has been reported that the cellular distribution of TREM-1 and TREM-2 is limited to monocytes and granulocytes, and to macrophages and dendritic cells, respectively.9
It was also suggested that TREM-1 on monocytes and granulocytes plays a role in acute inflammation, contrasting the role of TREM-2 on macrophages and dendritic cells for chronic inflammation. Because, granulomas generated in early phase of inflammation after zymosan A injection consisted of monocytes and neutrophils (Figure 9)
, TREM-1 on these cells may have a critical role for initiation of granuloma formation through DAP12 signaling. In fact, granuloma formation was markedly inhibited throughout the entire course of observation (up to day 10) by TREM-1 Ig that blocked TREM-1/DAP12 signaling (Figure 1A)
. It was speculated that weakened TREM-1 Ig expression (Figure 10A)
caused accumulation of Kupffer cells and inflammatory cell infiltration (Figure 7, C and F)
at day 10. These results suggested that the signal through TREM-1/DAP12 on circulating monocytes and neutrophils was critical for Kupffer cell activation during granuloma formation.
Moreover, we demonstrated that the augmentation of DAP12 signaling using anti-FLAG mAb caused exaggeration of zymosan A-induced mouse hepatic granuloma formation. The result suggested that the direct signal from DAP12 activated Ad-FDAP12-infected cells, such as Kupffer cells and monocytes. In fact, when anti-FLAG mAb was not given as an agonist of DAP12-mediated signaling, we found no difference in the magnitude of granuloma formation between the control and Ad-FDAP12-treated groups up to day 7 (Figure 8)
, although there was enhanced earlier Kupffer cell accumulation with Ad-FDAP12 treatment (Figure 4)
. These findings, in conjunction with the finding that treatment with DAP12-signaling inhibitor, Ad-TREM-1 Ig, markedly inhibited zymosan A-induced granuloma formation throughout the entire course of observation, strongly suggest that at the early phase of granuloma formation, endogenous DAP12 expressed by monocytes/macrophages may be sufficient to trigger signals via its partner-associating receptors, such as the TREM receptor family. On the other hand, the result observed in the group administered with Ad-FDAP12, Ad-TREM-1 Ig, and anti-FLAG mAb was noteworthy. Although we detected much inflammatory cell infiltration in this group, these inflammatory cells remained scattered and they were unable to be assembled to form granulomas. Direct DAP12 signal augmented inflammatory cell infiltration but TREM-1 Ig inhibited the granuloma formation. Thus inflammatory cell infiltration and granuloma formation were separate events, and considered to require different signals.
As mentioned above, it has been reported that the blockade of TREM-1/DAP12 signal by TREM-1 Ig protects mice against LPS-induced shock and microbial sepsis.14
Here we show that TREM-1 Ig is also able to inhibit a chronic inflammatory response such as zymosan A-induced granuloma formation, although it does not seem to suppress the infiltration of inflammatory cells caused by augmentation of DAP12 signaling (Figure 11)
. Although this molecule represents an attractive therapeutic agent used to dampen inflammatory diseases, the precise mechanism of the anti-inflammatory role by TREM-1 Ig remains to be fully understood.
| Acknowledgements |
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| Footnotes |
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Supported by a grants-in-aid (grant no. 09470060) from the Ministry of Education, Science, Sports, and Culture of Japan.
H. N. and N. A. contributed equally to this work.
Accepted for publication January 6, 2003.
| References |
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