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(Circulation Research. 2006;99:1188.)
© 2006 American Heart Association, Inc.

Molecular Medicine

Fc Receptor Deficiency Confers Protection Against Atherosclerosis in Apolipoprotein E Knockout Mice

Purificación Hernández-Vargas, Guadalupe Ortiz-Muñoz, Oscar López-Franco, Yusuke Suzuki, Julio Gallego-Delgado, Guillermo Sanjuán, Alberto Lázaro, Virginia López-Parra, Luis Ortega, Jesús Egido, Carmen Gómez-Guerrero

From the Renal and Vascular Research Laboratory (P.H.-V., G.O.-M., O.L.-F., J.G.-D., G.S., A.L., V.L.-P., J.E., C.G.-G.), Fundación Jiménez Díaz, Autónoma University, Madrid, Spain; Division of Nephrology (Y.S.), Department of Internal Medicine, Juntendo University, Tokyo, Japan; and Pathology Department (L.O.), Hospital Clínico, Complutense University, Madrid, Spain.

Correspondence to Carmen Gómez-Guerrero, PhD, Renal and Vascular Research Laboratory, Fundación Jiménez Díaz, Autónoma University, Avda. Reyes Católicos, 2, Madrid 28040, Spain. E-mail cgomez{at}fjd.es

	Abstract

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IgG Fc receptors (FcRs) play a role in activating the immune system and in maintaining peripheral tolerance, but their role in atherosclerosis is unknown. We generated double-knockout (DKO) mice by crossing apolipoprotein E–deficient mice (apoE^–/–) with FcR chain–deficient mice (^–/–). The size of atherosclerotic lesions along the aorta was approximately 50% lower in DKO compared with apoE^–/– control mice, without differences in serum lipid levels. The macrophage and T-cell content of lesions in the DKO were reduced by 49±6% and 56±8%, respectively, compared with the content in apoE^–/– lesions. Furthermore, the expression of monocyte chemoattractant protein-1 (MCP-1), RANTES (Regulated on Activated Normal T-cell Expressed and Secreted), and intercellular adhesion molecule-1 (ICAM-1) and the activation of nuclear factor-B (NF-B) were significantly reduced in aortic lesions from DKO mice. In vitro, vascular smooth muscle cells (VSMCs) from both ^–/– and DKO mice failed to respond to immune complexes, as shown by impaired chemokine expression and NF-B activation. ApoE^–/– mice have higher levels of activating FcRI and FcRIIIA, and inhibitory FcRIIB, compared with wild-type mice. The DKO mice express only the inhibitory FcRIIB receptor. We conclude that FcR deficiency limits development and progression of atherosclerosis. In addition to leukocytes, FcR activation in VSMCs contributes to the inflammatory process, in part, by regulating chemokine expression and leukocyte invasion of the vessel wall. These results underscore the critical role of FcRs in atherogenesis and support the use of immunotherapy in the treatment of this disease.

Key Words: Fc receptors • atherosclerosis • double-knockout mice • immune complexes

	Introduction

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Atherosclerosis is a chronic inflammatory disease of the arterial wall characterized by progressive accumulation of lipids, cells, and extracellular matrix. In recent years, the immune processes associated to atherogenesis have received considerable attention. Studies in both humans and animals have demonstrated that atheromatous lesions at all stage of development contain a wide variety of cells and molecules characteristic of immune system, such as macrophages, lymphocytes, CD40, interferon-, major histocompatibility complex-II, complement, and antibodies (Abs).^1–4 In addition, congenital deficiency of macrophages, T and B lymphocytes, or even the inhibition of their mediators has resulted in the reduction of atherosclerotic lesion.^1,3 One of the critical steps in atherogenesis is the accumulation within the arterial intima of cholesteryl ester-laden foam cells, many of them derived from macrophages, whose formation is ultimately dependent on the uptake of various forms of low-density lipoproteins (LDLs).^1,2 Although emphasis has been placed on scavenger receptors, foam-cell development may also be influenced by lipoprotein interaction with other receptors, such as LDL receptors, very-low-density lipoprotein (VLDL) receptors, and IgG Fc (FcRs) receptors.⁵ Different studies have demonstrated that FcR is the most important macrophage receptor involved in the clearance of immune complexes (ICs) containing native or oxidized LDL.^6,7 Indeed, modified LDL present in atherosclerotic lesions can trigger an autoimmune response in vivo, and auto-Abs (mainly IgG1 and IgG3) to oxidized LDL have been found in blood and plaques.^4,8,9 These results suggest that the clearance of LDL-containing ICs by macrophage FcRs contributes to foam-cell development in vivo.

Previous studies found functional evidence of FcRs in monocytic cells from atherosclerotic patients, and differential expression of FcRs in the proliferative areas of human lesions.^10–12 Moreover, the beneficial effect of immunomodulation in atherosclerosis has recently been reported.¹³ Therapy with intact immunoglobulins (Igs) or Fc fragments has been tried in a wide range of immune and inflammatory disorders,^14,15 and its potential use in chronic heart failure and cardiomyopathy has recently been suggested.^16,17 Although different mechanisms may participate in the beneficial effects of Igs,^14,18 FcR blockade in macrophages and other effector cells is probably implicated in the protection from cardiovascular damage.^13,18

FcRs are well-characterized cell surface glycoproteins that mediate phagocytosis or Ab-dependent cell cytotoxicity in immune and resident cells of many tissues.¹⁹ Two general classes of FcRs are known, the activating and the inhibitory receptors, which transmit their signals via immunoreceptor tyrosine-based activation (ITAM) or inhibitory motifs (ITIM). In activating receptors, the ITAM motif can be either intrinsic to the receptor, as in FcRIIA (a receptor not found in the mouse), or as part of an associated subunit, mainly the chain, as in FcRI, FcRIIIA, and FcRIV (receptors conserved between mouse and human). The inhibitory receptor (FcRIIB) contains the ITIM sequence in its cytoplasmic domain and binds IgG and ICs with low affinity.^19,20 In the last years, studies in genetically modified mice have documented that FcR activation is involved in several inflammatory and immune diseases.^19,21–23 However, the in vivo role of FcRs in atherosclerotic lesion formation has not been well examined. In this report, we analyze the contribution of FcRs present in vascular and infiltrating cells to the inflammatory process during atherosclerosis. We have generated double-knockout (DKO) mice by crossing apolipoprotein E–deficient mice (apoE^–/–) with chain–deficient mice (^–/–), the FcRI and FcRIII of which are not functional. The study of lesion development in these DKO mice will be useful for determining the involvement of immune system in the modulation of atherosclerosis.

	Materials and Methods

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For more details, see the expanded Material and Methods section in the online data supplement, available at http://circres.ahajournals.org.

Mice and Atherosclerotic Model
Female apoE^–/– mice²⁴ were crossed with male ^–/– mice (C57BL/6 background both),^21,25 and the progeny was bred back to apoE^–/– mice for 7 generations to obtain the DKO lineage (apoE^{–/– –/–}) and the littermate controls apoE^–/– (apoE^{–/– +/+}) and -heterozygous (apoE^{–/– –/+}). The apoE and genotypes were determined by genomic PCR, using specific primers.^21,24,25 Mice (8 weeks) with apoE^–/–, -heterozygous, and DKO genotypes (males, n=11; females, n=8 per group) were fed a Western-type high cholesterol (HC) diet for 16 weeks. To compare the atherogenic diet effect, male apoE^–/–, DKO, and wild-type (WT) were placed on standard low-cholesterol (LC) diet for 16 weeks (n=8 per group). These animals were processed for histological examination. In other set of experiments, male apoE^–/–, DKO, and WT were fed a HC diet and used for mRNA analysis (n=10 per group). These studies were performed in accordance with the European Union normative and were approved by the ethical committee of our institution.

Histological and Immunohistochemical Analysis
Aortas were dissected out from euthanized mice and frozen. Serial 8-µm sections of the aortic root were stained with Oil red O/hematoxylin,²⁶ and intimal lesion areas were quantified by morphometry. Infiltrating cells and RANTES (Regulated on Activated Normal T-cell Expressed and Secreted) were analyzed by indirect immunoperoxidase with MOMA-2 (macrophages), CD4 and CD8 (T lymphocytes), CD22 (B lymphocytes), and RANTES Abs. Activated nuclear factor B (NF-B) was detected by Southwestern histochemistry with digoxigenin-labeled probes, using competition and mutant probe as specificity controls.²⁷ Immunostaining was expressed as percentage of positive staining per millimeter squared (macrophages and RANTES) and positive nuclear staining per millimeter squared (NF-B).^28,29 Cells positive for CD4, CD8, and CD22 were recorded per millimeter squared.

Cell Culture
Murine VSMCs were cultured in DMEM containing 10% FBS and characterized by immunostaining (-actin positive, factor VIII negative).³⁰ Cells (second to seventh passage) were stimulated with ICs (heat-aggregated mouse IgG)³¹ and lipopolysaccharide (LPS).

mRNA and Protein Expression Analysis
Total RNA from aorta and VSMCs was isolated with TRIzol.^30,31 Gene expression of murine monocyte chemoattractant protein-1 (MCP-1), adhesion molecule-1 (ICAM-1), RANTES, FcRI, FcRIIB, and FcRIIIA was analyzed by real-time PCR. Chemokine expression was also analyzed by RNAse protection assay. Target gene expression was normalized to housekeeping gene (GAPDH).

Cytosolic proteins (25 µg) were resolved on SDS-PAGE gels and immunoblotted with RANTES and macrophage inflammatory protein-1ß (MIP-1ß) Abs.³¹ MCP-1 concentration in VSMC supernatants was measured with a commercial ELISA.

NF-B Activity
For electrophoretic mobility shift assay (EMSA), nuclear proteins (10 µg) were incubated in buffer containing 0.035 pmol of [³²P]NF-B oligonucleotide, then electrophoresed and autoradiographed. Specificity was confirmed using a 100-fold excess of unlabeled probe.³² Nuclear detection of NF-B subunits was detected in fixed, permeabilized cells with p50 and p65 Abs, followed by fluorescein isothiocyanate (FITC)-labeled Ab, and counterstained with diamidino-2-phenylindole (DAPI).

Statistical Analysis
Results are given as mean±SD. Differences between the groups were considered significant at P<0.05 using the ANOVA and Tukey–Kramer tests.

	Results

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FcR Deficiency Reduces Atherosclerotic Lesions
To evaluate the contribution of FcRs to atherogenesis, we generated DKO mice with targeted deletion of apoE and -chain genes. Aortic roots were investigated in DKO mice and their littermate controls (apoE^–/– and -heterozygous) after feeding with either atherogenic or standard diet. At 16 weeks of HC diet, female apoE^–/– mice exhibited significantly greater lesions than diet-matched males and, as expected, than LC diet-fed males (Figure 1A and 1C). Compared with their respective apoE^–/– controls, -heterozygous mice showed smaller lesions, whereas a drastic decrease was observed in DKO (Figure 1A). Plaque quantification revealed significant reduction in the extension (Figure 1B) and size (Figure 1C) of the lesions in both male and female DKO (percentage decrease versus apoE^–/–: 53±10 and 57±4). The atheroprotective effect of FcR deficiency was also seen in the LC diet model (54±6% decrease). Animals from the 3 genotypes showed no differences in cholesterol and triglycerides, suggesting that FcR deficiency did not affect the lipid metabolism (Table 1). Body weights were also unaltered. Moreover, DKO survived longer than apoE^–/– mice (survival rate at 10 months: 90% versus 50%).

View larger version (112K): [in this window] [in a new window]   Figure 1. FcR gene deficiency diminished the extension and size of atherosclerotic lesions. A, Oil red O/hematoxylin staining in aortic sections from mice fed a HC or LC diet (sex and number of animals are indicated). Magnification, x40. B, Lesion area throughout the studied region in male apoE–/– (black), -heterozygous (gray), and DKO (white) mice fed a HC diet. C, Average of maximal lesion areas from each group of apoE–/– (black), -heterozygous (gray), and DKO (white) mice. *P<0.01 vs apoE–/–, #P<0.05 vs heterozygous, P<0.01 vs HC diet (female).

View this table: [in this window] [in a new window]   Table 1. Body Weights and Serum Concentrations of Cholesterol and Triglycerides in Mice With the Different Genotypes

The inflammatory infiltrate was analyzed by immunohistochemistry with MOMA-2 (macrophages), CD4/CD8 (T cells), and CD22 (B cells) Abs. Numerous macrophages infiltrate the lesion in apoE^–/– controls (Figure 2A). Lipid accumulation (Oil red O) coincided with MOMA staining, indicating the presence of foam cell–rich areas. CD4⁺ T cells were also detected within lesions, with the CD8⁺ T cells and CD22⁺ B cells being somewhat less abundant (not shown). Quantification revealed significant reduction of macrophages and T lymphocytes in -heterozygous and DKO mice fed either a LC or HC diet. However, FcR deficiency was not able to decrease the B-cell content of lesions (Table 2).

View larger version (79K): [in this window] [in a new window]   Figure 2. FcR absence reduces inflammatory responses in aorta from apoE–/– mice. Histological examination of macrophages (A and B), RANTES (C and D), and NF-B (E through H) in aorta from male mice fed a HC diet. Aortic roots from apoE–/– contain a substantial number of macrophages in the intima (A) and abundant RANTES (C) and NF-B (E) in both intima and media. These parameters were decreased in DKO mice (B, D, and F, respectively). G, NF-B–specific binding in apoE–/– sample was verified with 200-fold excess of unlabeled probe. H, Colocalization of NF-B (nuclear staining in blue) and RANTES (cytoplasmic staining in brown) in apoE–/–. Arrows denote representative cells with double staining. Original magnification, x200. Atheroma (a), media (m), and lumen (l) are indicated. I, Quantification of area with positive staining. J, MCP-1 and ICAM-1 expression in aorta from apoE–/– and DKO mice (n=10 per group) were determined by real-time PCR and expressed in arbitrary units. *P<0.05 vs apoE–/–. Black bars indicate apoE–/–; gray bars, -heterozygous; white bars, DKO.

View this table: [in this window] [in a new window]   Table 2. Inflammatory Cell Infiltrate in the Atherosclerotic Lesions

Reduced Expression of Proinflammatory Genes in FcR-Deficient Mice
We next evaluated the expression of genes involved in the leukocyte recruitment to the intima at the earliest stages of atherosclerosis.^33–35 In male apoE^–/– fed the HC diet, RANTES protein was detected in the intima and media, and its expression diminished in -heterozygous and DKO mice (Figure 2C, 2D, and 2I). Likewise, mRNA expressions of MCP-1 and ICAM-1 were significantly higher in apoE^–/– than in DKO (Figure 2J).

Among the transcription factors regulating proinflammatory genes during atherogenesis,^28–30,36 we analyzed NF-B activation. Southwestern histochemistry in apoE^–/– sections revealed an intense nuclear staining in intimal and medial cells (Figure 2E), which colocalized with RANTES (Figure 2H), suggesting that NF-B regulates RANTES expression in the lesions. Number of NF-B–positive cells were decreased in aortas from -heterozygous and DKO mice (Figure 2F and 2I).

ICs Induce Chemokine Expression and NF-B Activation in VSMCs
Because RANTES and NF-B were detected not only in the intima but also in the media of vessels, mainly constituted by VSMCs, we next examined whether cultured VSMCs are sensitive to ICs. In VSMCs from WT mice, ICs enhanced the expression of chemokines known to be implicated in atherogenesis, mainly RANTES, MIP-1ß, and MCP-1 (Figure 3A), as evaluated by RNAse protection assay. Time-dependent mRNA expression of MCP-1 and RANTES, determined by real-time PCR (Figure 3B), was maximal at 8 and 24 hours, respectively. Protein expression profile followed similar kinetics, as determined by Western blot (MIP-1ß and RANTES; Figure 3C) and ELISA (MCP-1; Figure 3D). The involvement of FcRs in chemokine expression was demonstrated by using VSMCs from ^–/– and DKO mice. Both cell types identically responded to IC stimulation, exhibiting a reduced mRNA (Figure 3A and 3B) and protein (Figure 3C and 3D) expression of chemokines, when compared with WT cells.

View larger version (40K): [in this window] [in a new window]   Figure 3. Chemokine expression in VSMCs stimulated with ICs. VSMCs from WT (black bars), –/– (gray bars), and DKO (white bars) mice were incubated with 150 µg/mL ICs for different periods of time. A, The mRNA expression of the indicated chemokines was measured by RNAse protection assay. B, Real-time PCR for RANTES and MCP-1. Insets, Fluorescence intensity vs cycle number plots in WT and DKO cells at 0 and 8 hours of stimulation. C, Western blot for MIP-1ß (triangles) and RANTES (circles) protein in VSMCs from WT (black) and –/– (gray) mice. D, Secreted MCP-1 protein was measured by ELISA. Expression data were normalized to GAPDH (A and B), tubulin (C), and cell protein content (D) and are mean±SD of 4 to 5 experiments. *P<0.05 vs basal, #P<0.05 vs WT.

NF-B activity was analyzed by EMSA (Figure 4A) and immunofluorescence with p65 and p50 Abs (Figure 4B and not shown). Consistent with chemokine expression, IC-induced NF-B activity was attenuated in ^–/– when compared with WT cells. Nevertheless, the response to the positive control (LPS) was similar in both cell types.

View larger version (58K): [in this window] [in a new window]   Figure 4. IC-induced NF-B activation is reduced in FcR-deficient cells. VSMCs from WT (black) and –/– (gray) mice were stimulated with ICs (150 µg/mL) or LPS (1 µg/mL) for 60 minutes. A, NF-B activity in nuclear extracts was analyzed by EMSA. com indicates competition assay in sample from IC-stimulated cells. Arrows indicate specific bands. Densitometric data are mean±SD of 4 experiments (*P<0.05 vs basal, #P<0.05 vs WT). B, Nuclear translocation of activated complex was visualized by immunofluorescence with p65 Ab (green) in combination with DAPI (blue). Representative of 3 independent experiments.

Regulation of FcR Expression in Mice and Cells
Next, we analyzed the FcRI, FcRIIB, and FcRIIIA expression in aorta from HC diet-fed mice. In WT mice, FcRIIB and FcRIIIA were found modestly increased, whereas apoE^–/– displayed markedly upregulation of the 3 receptor types, mainly FcRIIIA. As expected, lack of detectable expression of chain–dependent receptors (FcRI and FcRIIIA) was observed in DKO, but, interestingly, FcRIIB was found to be clearly upregulated (Figure 5A). Based on the mRNA expression profile, the activating/inhibitory ratio of WT, apoE^–/–, and DKO (1.4, 2.5, and 0.4, respectively) revealed a change from activation to inhibitory state in FcR-deficient mice.

View larger version (22K): [in this window] [in a new window]   Figure 5. Expression levels of FcRs in mice and cultured VSMCs. A, FcR expression in aorta from male WT, apoE–/–, and DKO mice fed a HC diet, analyzed by real-time PCR. Data were normalized to GADPH and are mean±SD of 10 animals per group (*P<0.05 vs WT, #P<0.05 vs apoE–/–). B and C, Analysis of FcR expression in cultured VSMCs from WT and DKO stimulated with ICs (150 µg/mL). B, Fold changes (relative to basal) of mRNA levels are mean±SD of 3 to 5 experiments (*P<0.05 vs basal). C, Fluorescence intensity vs cycle number plots in WT and DKO at 0 and 8 hours of stimulation.

Next, we analyzed whether ICs influence FcR expression in vitro. In basal conditions, VSMCs from WT mice expressed low levels of FcRs, which were time-dependently increased by ICs. FcRI and FcRIIIA signals did not temporally vary in DKO cells treated for up to 24 hours with ICs. In contrast, the FcRIIB mRNA expression, although detectable in control conditions, was clearly induced after IC incubation (Figure 5B and 5C).

	Discussion

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There is accumulating evidence that immunologic mechanisms contribute to atherogenesis.^1,2 Several studies support the idea that, in addition to scavenger receptors, FcR could be a second mechanism for phagocytosis and clearance of LDL-containing ICs.^6,7 FcR expression has been described in blood cells and plaques of cardiovascular patients¹¹ and in cultured vascular and endothelial cells.¹¹ However, whether FcR activation influences atheroma formation in vivo has hardly been studied. In this work, we demonstrate that FcR gene deficiency protects against atherosclerosis development. We studied for the first time DKO mice lacking genes for apoE and chain, the common activation subunit required for surface assembly and signaling of FcRI, FcRIII, and FcRI.¹⁹ Independently of sex and diet, DKO mice exhibited a significant reduction in the lesion area compared with apoE^–/– controls, thus underscoring the critical role of FcRs in atherogenesis. These data confirm previous studies describing that FcR blockade is among the underlying mechanisms of the beneficial effects of Ig therapy in cardiovascular diseases.^13,16–18

In the present study, we observed that FcR deficiency led to a reduction in macrophage accumulation in the lesions, a key step in atherosclerosis.² In monocytes, FcR activation triggers the production of cytokines, proteolytic enzymes, and reactive oxygen intermediates.^37–39 In addition to the direct clearance of LDL-ICs,⁴⁰ FcRs are involved in the upregulation of LDL and scavenger receptors,^39,41 affecting the ability to take up modified LDL through a non–FcR-mediated pathway. Moreover, LDL-ICs promote survival of monocytes through FcRI, then constituting a novel mechanism by which immune reaction regulates lesion formation.⁴² Based on these findings, we postulate that FcR deficiency may suppress the accumulation, activation, and maintenance of intimal macrophages in atherosclerotic lesions. We have also observed a lower infiltration rate of CD4⁺ and CD8⁺ T lymphocytes in lesions of DKO mice, whereas the number of B lymphocytes was not affected. It has been reported that most plaque T cells are proinflammatory CD4⁺ Th1 effector cells.^3,43 Moreover, downregulation of Th1 responses reduces atherogenesis, whereas transfer of CD4⁺ T cells aggravates atherosclerosis.³ By contrast, B-cell infusion reduces lesion size and T-cell infiltration,^1,44 suggesting that protective immunity may also occur in atherosclerosis. Then, a conceivable consequence of FcR deficiency would be the reduction of proatherogenic immune activation in the artery, which may explain the decrease in the severity of atherosclerosis. The recent finding that chain deficiency decreases leukocyte attachment to the vascular wall and neointimal hyperplasia after balloon vascular injury is in agreement with such interpretation.⁴⁵

In parallel with the reduction in leukocyte infiltration, DKO exhibited a blunted expression of chemokines and adhesion molecules. These genes were found increased in cardiovascular diseases,^{2,29,30,33,46} and inhibition of their function markedly decreases plaque formation.^33,34 In vitro, ICs trigger the expression of chemokines in immune and tissue cells.^32,47 Moreover, it has recently been demonstrated that Ig therapy has direct effects on leukocyte recruitment through inhibition of adhesion molecules.⁴⁸ Our data are compatible with those suggesting that the atheroprotective effect of FcR deficiency in apoE^–/– mice is mediated, in part, by the reduced generation of proinflammatory molecules.

NF-B is an important transcription factor required for the cooperative induction of proinflammatory genes^47,49 that has been found activated in atherosclerotic plaques.^28,30,36 However, the benefit of inhibiting NF-B pathway in atherosclerosis is a current controversy, because it has been proposed that NF-B could be either proinflammatory regulator in early inflammation²⁹ or antiinflammatory regulator in the resolution phases.^36,50 In this work, we observed a decreased NF-B activation in DKO mice. Moreover, NF-B colocalized with RANTES in cells of the intima and media, suggesting that, in addition to infiltrating cells, the resident vascular cells are involved in the inflammatory responses mediated by FcR in vivo. This is in line with recent reports describing the active role that VSMCs play in vascular injury, mediating both early inflammatory and thrombotic responses, such as extracellular matrix synthesis, LDL uptake, and cytokine secretion.^51,52 Here, we reveal that VSMCs are sensitive to ICs. Then, FcR engagement in VSMCs may result in NF-B activation and expression of chemokines, responsible for leukocyte recruitment, thereby accelerating lesion formation. However, we cannot rule out the idea that FcR gene deficiency in inflammation may be nonspecific for atherosclerosis and may be a general antiinflammatory pathway. Therefore, the possibility that FcR deficiency may prevent the activation of other intracellular pathways regulating proinflammatory genes needs to be evaluated in the future. In this sense, it has recently been reported that FcRIIB, via protein phosphatase 2A, mediates the inhibition of endothelial NO synthase by C reactive protein.⁵³

In this study, we describe that FcRI, FcRIIB, and FcRIIIA were upregulated in atherosclerotic mice and were also time-dependently increased by ICs in cultured VSMCs, thus providing evidence that FcR expression on infiltrating and/or intrinsic vascular cells is associated with vascular inflammation. It has been reported that the chain is required for FcRI, FcRIII, and FcR expression, and for effector functions such as IC phagocytosis.^19,20,54 Consistent with this, our DKO mice were shown not to express FcRI and FcRIIIA, but, interestingly, the FcRIIB was potentiated. Thus, it is very likely that activating FcRI and FcRIIIA are required for initiating IC-mediated inflammation in aorta, but limited in their response by inhibitory FcRIIB expression. This compelling concept is supported by the recent observation that FcRIIB plays an immunoregulatory role in the development of immune diseases.²³ Moreover, the antiinflammatory activity of intravenous Igs in experimental myocarditis and atherosclerosis is linked to the FcRIIB expression on effector cells.^18,20,53 However, we cannot rule out that FcRIIIA may be functioning even in the absence of chain if it associates with 2 chains, as previously reported.^19,20 It has been described that the ratio of activating to inhibitory FcRs on immune cells may be regulated by different factors. Thus, inflammatory mediators upregulate activating receptors, whereas antiinflammatory cytokines increase FcRIIB expression, thereby setting high thresholds for cell activation.⁵⁵ Although other factors, such as affinity and isotype regulation, can influence the interaction of ICs with FcRs in cells of the lesions, modulation of expression levels and activating/inhibitory ratios may represent another link between immune system and atherosclerosis progression.

In summary, these data provide evidence that FcRI and FcRIII activation in leukocytes and VSMCs are involved in the IC-mediated responses during atherogenesis. Our findings contribute to the understanding of the immune mechanisms associated to atherosclerosis and support the use of immunotherapy in the treatment of this disease.

	Acknowledgments

 
We thank the technical assistance of the Pathology Department (Fundación Jiménez Díaz).

Sources of Funding

This work was supported by grants from the Fondo de Investigacion Sanitaria (PI02/0539), Comunidad Autónoma de Madrid (GR/SAL/0411/2004), Ministerio de Educación y Cultura (2005/05857; and Ramon y Cajal Program to C.G.-G.), and Mutua Madrileña. G.O.-M. and V.L.-P. are fellows from Fondo de Investigacion Sanitaria and Comunidad Autónoma de Madrid, respectively.

Disclosures

None.

	Footnotes

 
Original received December 12, 2005; resubmission received July 17, 2006; revised resubmission received October 1, 2006; accepted October 10, 2006.

	References

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