Endothelial Overexpression of Fas Ligand Decreases Atherosclerosis in Apolipoprotein E–Deficient Mice

From the Department of Molecular Cardiology (J.Y., K.S., T.A., A.B., K.W.), Whitaker Cardiovascular Institute, Boston University School of Medicine, and the Program in Cellular, Molecular, and Developmental Biology (J.Y., T.A., K.W.), Tufts University, Sackler School of Graduate Biomedical Sciences, Boston, Mass; and the Department of Molecular Microbiology and Immunology (N.J.B., J.H., H.P.), St. Louis University, St. Louis, Mo.

ABSTRACT

Objective— Fas ligand (FasL) can induce apoptosis in cells bearing the Fas receptor. The role of FasL in the vasculature with regard to atherosclerosis is controversial. This study examined the function of endothelial FasL during atherosclerosis.

Methods and Results— Transgenic (Tg) mice that specifically overexpress different levels of FasL on vascular endothelial cells were crossed into the apolipoprotein E–knockout background (ApoE-KO) to generate ApoE-KO/FasL–Tg mice. Although plasma cholesterol and triglyceride levels were not different between ApoE-KO/FasL–Tg mice and ApoE-KO mice after 12 weeks of a high-fat diet, overexpression of the FasL transgene significantly reduced atherosclerotic lesion area in aortae by 49%. The reduction of atherosclerotic lesion area was more pronounced in thoracic and abdominal aortae than in the aortic arch, and a 34% reduction in lesion area was observed in aortic root sections from the ApoE-KO/FasL–Tg group compared with the ApoE-KO group. Immunostaining revealed significant decreases in both macrophage and CD8 T-cell accumulation in lesions of ApoE-KO/FasL–Tg mice. ApoE-KO/FasL–Tg mice that express lower levels of endothelial FasL also displayed reduced lesion size, but this reduction was statistically significant at the aortic arch only.

Conclusions— Overexpression of endothelial FasL is antiinflammatory and inhibits atherosclerosis under hypercholesterolemic conditions.

Fas ligand (FasL) can induce apoptosis in cells bearing the Fas receptor. This study examined the function of endothelial FasL during atherosclerosis. Overexpression of endothelial FasL transgene significantly reduced atherosclerotic lesion areas in aortae. Overexpression of endothelial FasL is antiinflammatory and inhibits atherosclerosis under hypercholesterolemic conditions.

Key Words: atherosclerosis ? inflammation ? endothelium ? Fas ligand ? transgene

Introduction

Fas ligand (FasL) is a type II membrane protein that triggers apoptosis in Fas-bearing cells.1 FasL is expressed on immune cells and tissues that encounter immune cells.2,3 In this context, Fas-mediated apoptosis generally functions to minimize inflammation by eliminating inflammatory cells. However, overexpression of FasL has been observed to exert both anti- and proinflammatory effects. For example, FasL overexpression in thyroid and vessel allografts attenuates the immune response and prolongs the survival of these grafts.4,5 In contrast, accelerated immune system rejection and massive neutrophil infiltration were reported with heart and pancreas allografts that overexpress FasL.6,7

Chronic inflammatory responses contribute to the pathogenesis of atherosclerosis (reviewed in Ross8 and Libby9). Atherogenesis involves the interplay among 4 cell types: endothelial cells, smooth muscle cells, macrophages, and T cells. Under hypercholesterolemic conditions, oxidized LDL and other modified forms of lipid accumulate in the vessel intima. Inflammatory cells such as monocytes and T lymphocytes migrate through the endothelium, infiltrate the vessel wall, and release cytokines and growth factors, causing medial smooth muscle cells to proliferate and migrate into the intima. Smooth muscle cells are also recruited to the growing lesion from circulating bone marrow–derived precursor cells under these conditions.10 Experimental manipulations that inhibit monocyte transmigration or differentiation into macrophages result in reduced atherosclerotic lesion formation, indicating that monocytes/macrophages play an important role in the disease process.11 Furthermore, depletion of CD4 T cells leads to reduced lesion formation12,13 that may be mediated by a reduction in the levels of interferon-.14

Fas and FasL are implicated in cellular apoptosis in atherosclerotic lesions. Fas has been detected on smooth muscle cells, as well as on macrophages in human atherosclerotic lesions,15,16 and has been shown to mediate apoptosis in these cells.17–19 Human vascular lesions express FasL on T cells, macrophages, and endothelial cells.16,20,21 It has been reported that macrophage-derived cytokines upregulate Fas on smooth muscle cells.15,22 FasL is transported to the macrophage cell surface after exposure to oxidized LDL.18 Human blood-derived macrophages have also been shown to be capable of inducing Fas-mediated apoptosis in human plaque-derived smooth muscle cells.23 Thus, the role of Fas-mediated apoptosis in atherogenesis is complex and controversial.24 On one hand, FasL-mediated apoptosis may eliminate cells from the lesion and thereby attenuate the progression of atherosclerosis. Consistent with this hypothesis, it has been reported that ectopic expression of FasL inhibits intimal hyperplasia after acute vascular injury.22,25–27 However, because smooth muscle cells synthesize extracellular matrix molecules that stabilize plaque, loss of smooth muscle cells through Fas-mediated apoptosis could also promote plaque rupture.28

FasL is also expressed on the surface of vascular endothelial cells, which are normally resistant to Fas-mediated apoptosis.24 Overexpression of FasL on the vascular endothelium by way of adenovirus delivery has been shown to inhibit tumor necrosis factor-–induced leukocyte extravasation.29 These results indicate that FasL expressed on vascular endothelial cells performs an antiinflammatory function. In support of this notion, heart or carotid artery allografts displaying FasL on vascular endothelium show delayed infiltration of inflammatory cells and diminished intimal hyperplasia associated with transplantation.5,30 However, in a study by Schneider et al,31 adenovirus-mediated delivery of FasL to the vascular endothelium was shown to accelerate atherosclerotic lesion formation and promote smooth muscle cell proliferation in a hypercholesterolemic rabbit model. Furthermore, it has been shown that endothelial cells may become sensitive to Fas-mediated death under hypercholesterolemic conditions.32 Therefore, Fas activation could promote endothelial cell loss under these conditions, leading to endothelial dysfunction and accelerated atherogenesis.

In this study, we explored the function of endothelial FasL during hypercholesterolemia-induced atherosclerosis in a transgenic (Tg) mouse model. Vascular endothelial cadherin (VEcad)/FasL-Tg mice selectively overexpress functional FasL in vascular endothelial cells.33 These mice were crossed into an apolipoprotein E (ApoE)–knockout (KO) background to produce ApoE-KO/FasL–Tg mice. The extent of atherosclerosis was compared between ApoE-KO/FasL–Tg mice and ApoE-KO mice after 12 weeks of a high-fat diet. Here we report that FasL overexpression on the vascular endothelium protects against atherogenesis.

Methods

Mice

Human FasL cDNA was expressed from the mouse VEcad promoter in VEcad/FasL-Tg mice.33 Two lines of heterozygous VEcad/FasL mice (B6C3H, hybrid of C57/B6 and C3H), lines 17 and 12, were crossed to ApoE-KO mice (C57BL/6, Jackson Laboratory, Bar Harbor, Me). The heterozygous littermates (F1) were intercrossed to obtain the 4 experimental groups in this study: ApoE-KO/FasL–Tg, ApoE-KO, FasL-Tg, and wild type (WT). Polymerase chain reaction with tail snip DNA was used to identify FasL-Tg mice33 and ApoE-KO mice.34 All manipulations performed on the mice were approved by the Institutional Animal Care and Use Committee. At 7 weeks of age, mice were fed with an adjusted-calorie diet (21% fat, wt/wt; 0.15% cholesterol, wt/wt; Harlan Teklad). The high-fat diet was maintained for 12 weeks. On the day of analysis, mice were subjected to an 8-hour fast and weighed before euthanization. Mice were euthanized by exsanguination by cardiac puncture while under anesthesia with 2.5% 2,2,2-tribromoethanol 10 ml/kg BW ip (Aldrich). Blood was drawn by cardiac puncture and sent to a clinical laboratory for lipid analysis. The aorta was perfused with 0.9% NaCl and was dissected out from the aortic arch to the iliac bifurcation. The surrounding adipose tissue was thoroughly cleaned. The aortic root was dissected separately from the heart and embedded in OCT compound (Tissue-Tek, Sakura Finetechnical Co).

Atherosclerotic Lesion Analysis

The extent of atherosclerosis was examined in both opened aortae and serial sections from the aortic root. The aorta was cut open longitudinally and pinned flat on a silicone-coated dissecting dish. The aorta was fixed with 10% neutral buffered formalin for 24 hours. After fixation, the aorta was washed with phosphate-buffered saline (PBS) for 1 hour and stained with oil red O solution (0.3% in isopropyl alcohol and then diluted with water, 3:2, vol/vol) for 50 minutes. Excess stain was washed off with 60% isopropyl alcohol. Images were captured with an Olympus digital camera mounted on an Olympus SZX9 dissecting microscope. The aortic root samples were cut into 10-μm sections. Five consecutive sections were taken at 120-μm intervals from each mouse. These 5 sections were centered at the root of the aortic valves. Sections were fixed with 10% neutral buffered formalin for 30 seconds, washed with PBS, and then treated with 60% isopropyl alcohol for 1 minute. The sections were then stained with oil red O solution (0.03% in isopropyl alcohol and then diluted with water, 3:2, vol/vol) for 15 minutes and washed with 60% isopropyl alcohol. After the oil red O staining procedure, the sections were counterstained with hematoxylin. Images were captured with an Olympus BX417 microscope and an Olympus digital camera. The lipid-stained areas of total aorta and aortic root sections were analyzed with Adobe Photoshop 6 software and Scion Image software. The lipid-stained area of aortic root is reported as the mean area from the 5 sections of each mouse.

Immunohistochemistry

Frozen sections of aortic root were fixed in 4% paraformaldehyde for 15 minutes. Endogenous peroxidase activity was neutralized with 0.3% H2O2/PBS for 15 minutes. Sections were incubated with anti-mouse macrophage/monocyte antibody (MOMA-2, Serotec) or anti-CD4, anti-CD8 antibodies (BD Pharmingen). The color was developed with use of an AEC kit (Vector) according to the manufacturer’s directions. Sections were counterstained with hematoxylin. Four or 5 sections from each mouse were analyzed for macrophage and T-cell content.

Immunofluorescent Staining for Flow Cytometry Analysis

Peripheral blood samples were first lysed in BD FACSlyse (BD Biosciences). Antibodies were prepared as cocktails in flow wash buffer (PBS with 1% fetal bovine serum and 0.1% NaN3). Cells were incubated with antibodies for 1 hour at 4°C. Flow cytometry was performed on a BD FACSCalibur. Conjugated anti-mouse CD3, CD4, CD8, CD19, CD11b, and CD45 antibodies were purchased from BD Pharmingen.

Localization of Apoptotic Cells in Atherosclerotic Lesions

Frozen sections of aortic root were fixed in 4% paraformaldehyde for 15 minutes. After fixation, sections were incubated with anti-mouse macrophage/monocyte antibody (MOMA-2). Antibody binding was detected with fluorescein streptavidin (Vector). After fluorescent immunostaining, apoptotic cells in sections were identified by the terminal deoxynucleotidyl transferase–mediated dUTP nick end-labeling (TUNEL) method with an in situ cell death detection kit with tetramethylrhodamine red (Roche) according to the manufacturer’s instructions. Slides were mounted with Vectashield mounting medium with DAPI (4', 6 diamidino-2-phenylindole; Vector). Fluorescence microscopy was performed with a Nikon Eclipse TE300. To quantify TUNEL-positive cells, 5 consecutive sections centered at the root of the aortic valves were counted for each mouse.

Statistical Analysis

All values are presented as mean±SE. Statistical analysis was done with StatView 5.0.1 software (SAS Institute Inc). Differences between 2 groups were analyzed with an unpaired Student’s t test. Differences between 3 or 4 groups were analyzed by ANOVA. Probability values <0.05 were considered statistically significant.

Results

Generation of ApoE-KO/FasL–Tg mice

Previously, we constructed and characterized VEcad/FasL-Tg mice that express human FasL cDNA under control of the mouse VEcad promoter.33 The FasL transgene is expressed specifically on the surface of vascular endothelial cells and is capable of inducing apoptosis in cocultured, FasL-sensitive cells. VEcad/FasL-Tg mice did not show any detectable developmental and pathological abnormalities. To assess the function of endothelial FasL during atherosclerosis, 2 VEcad/FasL-Tg lines (17 and 12) were crossed into the ApoE-KO background, which develops hypercholesterolemia and atherosclerosis.35 All 4 experimental groups, ApoE-KO/FasL–Tg, ApoE-KO, FasL-Tg, and WT, were derived from the F2 generation (see Methods).

All experimental groups were maintained on a high-fat, Western-type diet (21% fat; 0.15% cholesterol) for 12 weeks starting at 7 weeks of age. At the time of analysis, ApoE-KO/FasL–Tg and ApoE-KO mice weighed slightly less than FasL-Tg and WT mice (Table 1). The cholesterol levels of ApoE-KO/FasL–Tg and ApoE-KO mice were increased nearly 10-fold compared with those of FasL-Tg and WT littermates. However, cholesterol levels were not different between ApoE-KO/FasL–Tg and ApoE-KO mice or between FasL-Tg and WT mice (for line 17, see Table 1; line 12 data are not shown). The levels of HDL were not changed by overexpression of endothelial FasL (data not shown). Similarly, plasma triglyceride levels were elevated in ApoE-KO/FasL–Tg and ApoE-KO mice but were not different between these 2 strains.

TABLE 1. Characteristics of Mice Derived From L17

Atherosclerotic Lesion Formation

The extent of atherosclerosis was assessed by measuring lipid-stained areas in both aortae and aortic root sections. After a 12-week, Western-type diet, atherosclerotic lesions could be seen on the aortae from ApoE-KO/FasL–Tg mice and ApoE-KO mice, whereas FasL-Tg and WT littermates were free of detectable lesions. Representative photographs of oil red O staining of aortae are shown in Figure 1A. Lipid-stained areas were distributed mostly at the aortic curvature or branching points for other vessels. The lipid-stained areas in ApoE-KO/(line 17) FasL–Tg mice were significantly reduced by 49% compared with those of ApoE-KO mice (1.37±0.13 vs 2.66±0.29 mm2; Figure 1B). ApoE-KO/FasL–Tg mice derived from line 12, which express a lower level of the transgene,33 did not show a significant difference compared with ApoE-KO mice, although there was a trend toward fewer lesions in these animals. A significant difference in lipid-stained area was also observed between ApoE-KO/(line 17) FasL–Tg mice and ApoE-KO/(line 12) FasL-Tg mice. These data indicate that the inhibitory effect of FasL on atherosclerosis is proportional to its level of expression.

Figure 1. Overexpression of endothelial FasL reduces atherosclerotic lesion formation in aortae. A, Representative photographs of oil red O staining of aortae from 3 groups: ApoE-KO/(line 17) FasL–Tg, ApoE-KO/(line 12) FasL–Tg, and ApoE-KO after 12 weeks of a high-fat diet. B, Quantification of lipid-stained areas in aortae. ApoE-KO/(line 17) FasL–Tg, n=28; ApoE-KO/(line 12) FasL–Tg, n=21; ApoE-KO, n=25.

The most striking difference in lipid-stained area was observed in the thoracic and abdominal aortae. For line 17 mice, an 82% reduction was observed in the thoracic aorta and a 57% reduction was observed in the abdominal aorta, whereas only a 29% reduction occurred at the aortic arch (Figure 2). For ApoE-KO/(line 12) FasL–Tg mice, the lipid-stained area at the aortic arch was significantly different from that in ApoE-KO mice.

Figure 2. Overexpression of endothelial FasL reduces lesion formation in the aortic arch, thoracic aorta, and abdominal aorta. Quantification of lipid-stained areas in aortic arch (A), thoracic aorta (B), and abdominal aorta (C) is shown. ApoE-KO/(line 17) FasL–Tg, n=28; ApoE-KO/(line 12) FasL–Tg, n=21; ApoE-KO, n=25.

Five consecutive sections centered at the root of the aortic valves were taken from each mouse and stained with oil red O to assess lesion formation in that region. Representative sections are shown in Figure 3A. Consistently, a significant reduction in mean lipid-stained area was found with aortic root sections from ApoE-KO/(line 17) FasL–Tg mice when compared with sections from ApoE-KO mice. Taken together, ApoE-KO/(line 17) FasL–Tg mice showed reductions in both lesion size and lesion thickness compared with ApoE-KO mice. Thus, overexpression of endothelial FasL reduces atherogenesis under hypercholesterolemic conditions.

Figure 3. Overexpression of endothelial FasL reduces lesion area in the aortic root. A, Representative photographs of oil red O staining of aortic root sections from ApoE-KO/(line 17) FasL–Tg and ApoE-KO groups. Original magnification x40. B, Quantification of lipid-stained areas in aortic root. ApoE-KO/(line 17) FasL–Tg, n=13; ApoE-KO, n=11. *P<0.01.

Vascular Lesion Characteristics

Masson’s trichrome staining was used to examine the histologic features of the atherosclerotic lesions (Figure 4A). Lesions from both groups showed similar histologic features, including the presence of foam cells and a necrotic core that was covered by a fibrous cap. It has been shown that endothelial FasL inhibits inflammation by attenuating leukocyte accumulation in the vessel wall.29,33 Therefore, we investigated whether the antiatherosclerotic function of FasL was correlated with a reduced level of leukocytes within the vessel wall. Aortic root sections were stained for macrophages (Figure 4B) and CD4, CD8 T cells (Figure 5A and 5C). The mean macrophage content within the atherosclerotic lesion was 29% less in ApoE-KO/(line 17) FasL–Tg mice than in ApoE-KO mice (Figure 4C), which corresponds to the 34% reduction in lipid-stained area in aortic root sections. Although the number of CD4 T cells within lesions was not different (Figure 5B), CD8 T cells were reduced in ApoE-KO/(line 17) FasL–Tg mice compared with ApoE-KO mice (Figure 5D). Circulating monocytes and T-cell populations were not significantly different between the ApoE-KO/FasL–Tg and ApoE-KO mice (Table 2). Furthermore, no differences in T-cell populations in the thymus and spleen could be detected between the 2 mouse strains (data not shown).

Figure 4. Macrophage infiltration into atherosclerotic lesions was reduced by overexpression of endothelial FasL. A, Photographs of Masson’s trichrome staining of aortic root sections from ApoE-KO/(line 17) FasL–Tg and ApoE-KO groups. The fibrous cap is stained blue. Original magnification x200. B, Photographs of MOMA-2 immunostaining of aortic root sections. Right panel shows negative staining with isotype control. Original magnification x100. C, Quantification of MOMA-2–positive areas in aortic root sections. ApoE-KO/(line 17) FasL–Tg, n=7; ApoE-KO, n=8. *P<0.05. D, Colocalization of TUNEL staining with macrophages. Macrophages were stained with MOMA-2 antibody (green). Apoptotic cells are stained red. Lower panel shows negative staining with isotype control for MOMA-2. Original magnification x200. E, Quantification of TUNEL-positive cells. ApoE-KO/(line 17) FasL–Tg, n=5; ApoE-KO, n=5.

Figure 5. T-cell accumulation within lesions was reduced by overexpression of endothelial FasL. Representative photographs of CD4 staining (A) and of CD8 staining (C) in aortic root sections. Original magnification x400. Quantification of CD4 cells (B) and CD8 cells (D) in sections. ApoE-KO/(line 17) FasL–Tg, n=6; ApoE-KO, n=6. *P<0.05.

TABLE 2. Evaluation of Circulating Leukocytes in Mice Derived From L17

Because the Fas/FasL system has been suggested to control cell viability in lesions,15 we examined whether overexpression of endothelial FasL affects the level of apoptosis within the vessel wall. Apoptotic cells in sections from ApoE-KO/(line 17) FasL–Tg and ApoE-KO aortic roots were detected by the TUNEL method. Essentially no apoptosis was found in endothelial or smooth muscle cells from any strain (data not shown). Apoptosis staining predominantly colocalized with MOMA-2–positive macrophages (Figure 4D). The total apoptotic cell number per section was not significantly different between ApoE-KO mice and ApoE-KO/(line 17) FasL–Tg mice, although there was a trend toward fewer TUNEL-positive cells in the vessels of the ApoE-KO/(line 17) FasL–Tg mice (Figure 4E).

Discussion

In this study, we created mice that overexpress FasL on the endothelium and are deficient for ApoE (ie, ApoE-KO/FasL–Tg). Although ApoE-KO/FasL–Tg mice and ApoE-KO mice showed similar plasma cholesterol and triglyceride levels, significantly fewer atherosclerotic lesions were observed in the aortae of ApoE-KO/FasL–Tg mice. Correspondingly, vessels from ApoE-KO/FasL–Tg mice contained fewer macrophages and CD8 T cells than did vessels from ApoE-KO mice. Therefore, these data demonstrate that FasL overexpression on the endothelium protects against atherosclerosis.

Our findings with ApoE-KO/FasL–Tg mice contradict the conclusions reached in a study of FasL overexpression in hypercholesterolemic rabbit vessels.31 In the rabbit study, increased atherosclerosis was observed in arteries that were infected with high titers of a FasL-expressing adenovirus. The reasons for the discrepancies between the 2 studies are not clear. One possibility is that the adenovirus vector produces a confounding proliferative effect on the lesion when FasL is overexpressed under atherogenic conditions. Alternatively, the difference may be due to the possibility that higher FasL expression levels were achieved by adenoviral transduction than in the Tg mouse study. In this regard, it has been shown that low levels of FasL protect liver grafts from rejection and induce death in infiltrating T cells, whereas higher levels of FasL cause fulminant liver degeneration.36

Vascular inflammation plays an important role in promoting atherogenesis. In this regard, a number of studies have shown that FasL expression on the endothelium is anti-inflammatory. Adenovirus-mediated overexpression of FasL on the endothelium of carotid artery allografts inhibits T-cell and macrophage infiltration after transplantation.5 Furthermore, display of FasL on the vascular endothelium of heart allografts by biotin-streptavidin interaction delays infiltration of inflammatory cells and prolongs the survival of these grafts in the allogenic host.30 Endothelial FasL expression has also been shown to be protective in the context of ischemia/reperfusion injury.33 Collectively, these data show that endothelial expression of FasL is protective against acute inflammatory responses in instances where tissue damage is initiated by the rapid influx of neutrophils.

In the current study, endothelial FasL was assessed in a model of chronic inflammation that primarily involved the participation of monocytes and lymphocytes.8,9 Here, it was found that FasL overexpression led to a reduction in both macrophages and CD8 T cells within lesions. Although CD4 T cells have been shown to promote atherosclerosis,12,13 the role of CD8 T cells in atherosclerosis is not well understood. However, CD8 T cells are capable of secreting proatherosclerotic cytokines, such as interferon- and tumor necrosis factor-, as well as growth factors, such as basic fibroblast growth factor.37 Therefore, inhibition of both monocyte and CD8 T-cell infiltration may contribute to the reduction in lesion area. The reason why CD4 T-cell levels in the vessel wall were not affected in our study is not clear. However, the sensitivity of both CD4 and CD8 T cells to FasL-induced apoptosis can vary depending on their stage of activation,38,39 and this may explain the differential effect of the FasL transgene on lymphocyte infiltration.

There are at least 2 mechanisms that could account for the antiatherogenic effect of endothelial FasL in the Tg model. One possibility is that endothelial cells overexpressing FasL on the surface induce apoptosis in cells that compose the lesion. For example, endothelial cells may shed FasL, which then diffuses into the lesion and triggers apoptosis in smooth muscle cells and macrophages, thereby reducing the size of the atherosclerotic lesion. However, we do not favor this hypothesis because we did not observe an increase in apoptosis within the lesions of the FasL-Tg mice (Figure 4E). In this regard, it is reported that shed FasL is a very weak agonist and that it can inhibit the activity of membrane-bound FasL.40 Alternatively, FasL overexpressed on the luminal side of the endothelium may induce apoptosis in Fas-bearing leukocytes before they can invade the lesion. In support of this hypothesis, endothelial cells isolated from VEcad/FasL-Tg mice have been shown to be capable of inducing apoptosis in cocultured monocytic cells.33 Furthermore, in an acute model of vascular inflammation, leukocytes (monocytes and T cells) that attached to the endothelium were found to undergo apoptosis instead of extravasation when endothelial cells overexpressed FasL.29 This mechanism was also put forth to explain estrogen’s protective effect on the vasculature. In a rabbit atherosclerosis model, estrogen treatment was found to promote FasL expression on the endothelium, which reduced macrophage infiltration.41

Although FasL overexpression can promote tissue destruction and inflammation in other models, endothelial overexpression of FasL attenuates both acute33 and chronic inflammatory responses in the vasculature. A number of features may contribute to the antiinflammatory nature of FasL in the vasculature. First, endothelial cells are relatively resistant to FasL-induced apoptosis.19,42 This resistance is due to high levels of FLIP expression, which blocks Fas signaling at the receptor level.43–45 In contrast, overexpression of FasL on sensitive cell types can lead to apoptosis and the release of inflammatory cytokines (eg, interleukin-1?) that are activated by the caspases in the apoptotic pathway.46,47 Furthermore, the microenvironment of the vessel wall may minimize the inflammatory activity of FasL. For example, the immunosuppressive cytokine, transforming growth factor-?, is expressed in the vessel wall,48–50 where it may function to facilitate the antiinflammatory activity of endothelial FasL.51 Collectively, these data are supportive of the hypothesis that FasL serves an antiinflammatory role on the vascular endothelium in the context of atherogenesis in a murine model.

Acknowledgments

This work was supported by National Institutes of Health grants AG15052, HD23681, AG17241, AR40197, and HL66957 to K.W.

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