Disruption of TGF-β signaling in T cells accelerates atherosclerosis

Increasing evidence suggests that atherosclerosis is an inflammatory disease promoted by hypercholesterolemia. The role of adaptive immunity has been controversial, however. We hypothesized that proatherogenic T cells are controlled by immunoregulatory cytokines. Among them, TGF-β has been implied in atherosclerosis, but its mechanism of action remains unclear. We crossed atherosclerosis-prone ApoE-knockout mice with transgenic mice carrying a dominant negative TGF-β receptor II in T cells. The ApoE-knockout mice with disrupted TGF-β signaling in T cells exhibited a sixfold increase in aortic lesion surface area, a threefold increase in aortic root lesion size, and a 125-fold increase in aortic IFN-γ mRNA when compared with age-matched ApoE-knockout littermates. When comparing size-matched lesions, those of mice with T cell–specific blockade of TGF-β signaling displayed increased T cells, activated macrophages, and reduced collagen, consistent with a more vulnerable phenotype. Ab’s to oxidized LDL, circulating T cell cytokines, and spleen T cell activity were all increased in ApoE-knockout mice with dominant negative TGF-β receptors in T cells. Taken together, these results show that abrogation of TGF-β signaling in T cells increases atherosclerosis and suggest that TGF-β reduces atherosclerosis by dampening T cell activation. Inhibition of T cell activation may therefore represent a strategy for antiatherosclerotic therapy.

Atherosclerosis is an inflammatory disease in which blood-borne immune cells infiltrate the artery wall. While monocyte macrophages and macrophage-derived foam cells constitute the major cellular component of early atherosclerotic lesions, T cells represent an important additional component. Activation of both cell types is observed at all stages of atherosclerosis and, in particular, at sites of plaque activation and rupture in patients with unstable angina and myocardial infarction (1–4). Cell culture studies show that activated macrophages and T cells secrete factors that may destabilize plaques (5, 6). These factors include proinflammatory cytokines that inhibit smooth muscle cell proliferation, differentiation, and collagen secretion and promote endothelial expression of tissue factor and adhesion molecules. Activated macrophages may also secrete MMPs, which can directly degrade collagen of the plaque’s fibrous cap and may release reactive oxygen and nitrogen species, which can damage cells and modify extracellular components of the lesion.

T cells are key inducers and regulators of immune activity. They recognize antigens in a clonally specific manner, provide help for B cell activation, and activate macrophages by expressing cytokines and costimulatory molecules. Candidate antigens involved in atherosclerosis include oxidized LDL, heat shock protein-60, and microbial components (5). T cells specific for these antigens have been isolated from human atherosclerotic lesions (7–9). The role of T cells has been addressed in several experimental models of atherosclerosis. SCID × ApoE^–/– (E⁰) mice, which lack T and B cells, develop less atherosclerosis than immunocompetent E⁰ mice (10). Similarly, E⁰ mice deficient in IFN-γ signaling, IL-18, or CD40L (CD154) exhibit reduced disease development (11–13). Surprisingly, data from RAG^–/– × E⁰ mice have been contradictory (14–16), and immunization with candidate antigens may result in protection as well as increased disease, depending on the experimental conditions (for review see refs. 4, 5). This suggests that immunoregulatory mechanisms may counteract the disease-aggravating ones.

T cell activation is controlled by cytokines produced by neighboring cells. IL-10, secreted by macrophages and the Th2 subset of T cells, inhibits secretion of several proinflammatory T cell cytokines by T cells. Another immunoregulatory cytokine, TGF-β, is produced by several different cell types, including endothelial and smooth muscle cells, macrophages, platelets, and regulatory T cells (T_reg’s) (17). Both IL-10 and TGF-β appear to dampen atherosclerosis in mouse models (18–22), and the latter has also been implicated in human disease (23–25). Since TGF-β has multiple targets, its mode of action, however, is not known. For instance, it modulates smooth muscle proliferation and collagen formation, endothelial proliferation, and macrophage activity (26). It has therefore been proposed that the main target for the antiatherosclerotic action of TGF-β is the vascular smooth muscle cell or the macrophage.

We hypothesized that proatherogenic T cells are controlled by immunoregulatory TGF-β, which maintains the T cell population in an inhibited state. To test this, we crossed atherosclerosis-prone E⁰ mice with mice carrying a dominant negative TGF-β receptor II in T cells. Our data show that E⁰ mice with abrogated TGF-β signaling in T cells develop dramatically increased atherosclerosis. This indicates that TGF-β exerts important antiatherosclerotic activity by inhibiting T cell activation.

The T cell response to TGF-β is abrogated in the CD4-dnTGF-βRII transgenic mouse (CD4dnTβRII), which expresses a dominant negative TGF-β receptor under the CD4 promoter (27). TGF-β signaling in all T cells is blocked because the CD8 silencer has been removed from the CD4 promoter. We crossed this mouse with the E⁰ mouse, which spontaneously develops hypercholesterolemia and atherosclerosis. At 12 weeks of age, no significant differences were observed between E⁰ × CD4dnTβRII and E⁰ mice with regard to body weight (20.1 ± 0.4 versus 20.0 ± 0.4 g). Histopathological examination of the large intestine showed modest infiltration of inflammatory cells in three of nine 12-week-old E⁰ × CD4dnTβRII mice, but there were no clinical signs of inflammatory bowel disease at this age. From 18 weeks of age, body weight was lower than for E⁰ mice, and the large intestine showed signs of colitis.

Disrupted TGF-β signaling in T cells leads to increased atherosclerosis. E⁰ × CD4dnTβRII mice exhibited a dramatic increase in atherosclerotic lesion size in the aortic root, visualized by oil red-O staining (Figure 1, a–c). Lesions were substantially thicker and more advanced in 12-week-old E⁰ × CD4dnTβRII mice when compared with E⁰ mice of the same age with intact TGF-β signaling in T cells. The extent of atherosclerosis in the entire aorta was quantified in Sudan IV–stained en face preparations of aortas. This analysis revealed a sixfold increase in the lipid lesion area in E⁰ × CD4dnTβRII mice (Figure 1, d–f).

Figure 1

Abrogation of TGF-β signaling in T cells increases atherosclerosis and IFN-γ expression in age-matched mice. (a) Lesion size in the aortic root (× 10⁵ μm²) in 12-week-old mice. Individual values are displayed by dots, and medians for each group are indicated by horizontal lines. Representative micrographs showing hematoxylin/oil red-O–stained lesions in E⁰ × CD4dnTβRII (b) and E⁰ (c) mice. Original magnification ×50. (d) Lipid lesion area in the entire aorta (percentage of total aortic surface area); dot plots as above. Sudan IV–stained en face preparations of aortas from E⁰ × CD4dnTβRII (e) and E⁰ (f) mice. (g) IFN-γ mRNA levels in lesions of E⁰ × CD4dnTβRII and E⁰ mice. Data are expressed as arbitrary units as described in Methods; dot plots as above.

Increased IFN-γ expression in lesions of E⁰ × CD4dnTβRII mice. Activated Th1 cells promote macrophage activation and inflammation by secreting IFN-γ. To directly test the hypothesis that the expression of this cytokine is increased in lesions of E⁰ × CD4dnTβRII mice, we performed real-time RT-PCR on RNA from such atherosclerotic aortas. This analysis revealed a dramatic, 125-fold increase in IFN-γ mRNA in mice with disrupted TGF-β signaling in T cells (Figure 1g).

Vulnerable and inflammatory lesion phenotype in E⁰ × CD4dnTβRII mice. To evaluate whether disrupted TGF-β signaling in T cells affected the composition as well as size of lesions, lesions from E⁰ × CD4dnTβRII and E⁰ mice were characterized. Since lesions were much larger and more advanced in the E⁰ × CD4dnTβRII mice and could not be compared with lesions of the same size in age-matched E⁰ mice, we compared lesions of 12-week-old E⁰ × CD4dnTβRII mice with those of 18-week-old E⁰ mice. The latter contain aortic root lesions of approximately the same size as those found in 12-week-old E⁰ × CD4dnTβRII mice. Disrupted TGF-β signaling in T cells was associated with reduced collagen staining (Figure 2, a and e). Importantly, reduced collagen in lesions has been linked to plaque vulnerability and a propensity for thrombosis (31). In contrast, the proportion of lesion area stained by the macrophage marker CD68 was higher in E⁰ × CD4dnTβRII mice (Figure 2, b and e). Similarly, the number of CD3⁺ T cells was increased in lesions of these mice (Figure 2, d and e). The macrophage-activating cytokine IFN-γ is secreted by activated T cells and induces I-A expression in target cells. A higher number of cells expressed I-A in lesions of E⁰ × CD4dnTβRII mice (Figure 2, c and e). Taken together, these data suggest that the lack of TGF-β signaling in T cells leads to accelerated atherosclerosis with the formation of large, vulnerable plaques.

Figure 2

Lesion composition in 12-week-old E⁰ mice with abrogated TGF-β signaling in T cells. Comparison with size-matched lesions of 18-week-old E⁰ mice. (a) Collagen content (collagen-stained cross-section surface/total lesion surface). (b) Area occupied by CD68⁺ macrophages (CD68-stained surface/total lesion surface). (c) I-A⁺ cells per lesion cross-section area. (d) CD3⁺ T cells per lesion cross-section area. (e) Representative micrographs showing Masson′s trichrome stain (collagen in green; original magnification ×400), immunohistochemical staining for CD68, I-A, and CD3 (all ×400), and immunohistochemical staining for α-smooth muscle actin (α actin) and VCAM-1 (×200). Lesion contours are outlined in the CD3-stained micrograph.

The leukocyte adhesion molecule, VCAM-1, which can mediate T cell recruitment, was expressed in lesions of both E⁰ × CD4dnTβRII and E⁰ mice (Figure 2c). Its distribution was compatible with expression by smooth muscle cells and macrophages, as previously reported (32, 33). This pattern did not differ between lesions of E⁰ × CD4dnTβRII mice and those with similar size from E⁰ mice. Similarly, the distribution of α-smooth muscle actin did not differ between groups (Figure 2c).

Effects on lipids and lipoproteins. To assess whether the effects on atherosclerosis were due to changes in lipid metabolism, we analyzed serum lipids and lipoproteins. Surprisingly, total serum cholesterol was significantly lower in E⁰ × CD4dnTβRII mice (13.0 ± 0.6 versus 17.7 ± 0.9 mmol/l in E⁰ mice), while serum triglycerides did not differ between groups (1.0 ± 0.1 versus 1.1 ± 0.1 mmol/l). Separation of serum lipoproteins by FPLC showed a reduced chylomicron/VLDL fraction, whereas LDL and HDL remained unchanged (Figure 3a). Cholesterol-7α-hydroxylase activity in the liver did not differ between groups (data not shown). It is therefore unlikely that the reduced serum cholesterol levels were due to hepatic or ileal dysfunction with concomitant effects on cholesterol clearance.

Figure 3

Effects of abrogated TGF-β signaling on serum lipoproteins and Ab’s to modified LDL. (a) FPLC analysis of lipoprotein profiles from sera of 12-week-old mice. The cholesterol concentration in each fraction (y axis) is plotted against retention time (x axis); curves show mean ± SD for E⁰ × CD4dnTβRII (gray line) and E⁰ (black line) sera (n = 9 per group). The lipoprotein profile of a human serum is plotted for reference. CR, chylomicron remnants. (b) Titers of IgM, IgG1, IgG2a, and IgG2b Ab’s to MDA-LDL (OD values of ELISA) from 12-week-old E⁰ × CD4dnTβRII and E⁰ mice; individual values for each mouse and medians for each group are displayed.

Increased Ab response to modified LDL in E⁰ × CD4dnTβRII mice. Atherosclerosis is associated with Ab formation to oxidatively modified LDL. Such modifications include MDA conjugation of the LDL protein (34). Anti–MDA-LDL Ab’s of all isotypes were increased in E⁰ × CD4dnTβRII mice (Figure 3b), with no apparent bias toward any T helper–dependent isotype. The finding of elevated anti–MDA-LDL is compatible with an increased antigen-specific immune response.

Increased systemic immune activity in E⁰ × CD4dnTβRII mice. Analysis of spleen cells revealed increased T cell activation and cytokine secretion in E⁰ × CD4dnTβRII mice. Thus, a higher proportion of T cells expressed the activation marker CD69 (Figure 4, a and b), and mRNA was increased for the T cell cytokines, IFN-γ and IL-4 (data not shown). Similarly, larger amounts of IFN-γ, IL-4, and IL-10 were secreted after mitogen stimulation of T cells from E⁰ × CD4dnTβRII compared with E⁰ mice (Figure 4, d–f). In contrast, proliferation and IL-2 secretion of activated T cells was reduced in the former group, probably because the enhanced activation recruited a larger proportion of T cells from the naive, highly proliferative, and IL-2–secreting population into the memory effector subsets (Figure 4c and data not shown).

Figure 4

Effects of abrogated TGF-β signaling on T cell activation and cytokine secretion. (a,b) FACS analysis of the activation marker CD69 on CD4⁺ (a) and CD8⁺ (b) spleen T cells from 12-week-old E⁰ × CD4dnTβRII (n = 6, gray boxes) and E⁰ mice (n = 5, white boxes). Box plots (median, quartiles, tenth, and ninetieth percentiles) show percentage of CD69⁺ among all CD4⁺ or CD8⁺ T cells; staining for isotype control is subtracted. Experiments were repeated twice. (c–f) Cell proliferation and cytokine secretion after in vitro stimulation of spleen cells from 12-week-old E⁰ × CD4dnT (n = 6) and E⁰ (n = 5) mice. (c) Stimulation index after ³H-thymidine incorporation. Concentrations of (d) IFN-γ, (e) IL-10, and (f) IL-4 in supernatants 48 hours after stimulation of spleen cells with the T cell mitogen, concavalin A. Box plots as above. (g–j) Concentrations of IFN-γ (g), TNF-α (h), IL-5 (i), and IL-2 (j) were determined in sera from 12-week-old E⁰ × CD4dnTβRII and E⁰ mice by using cytofluorometric bead assays. Dot plots show values for individual mice and medians for each group.

Serum levels of cytokines were determined by cytometric bead arrays. As shown in Figure 4, g–i, IFN-γ, TNF-α, and IL-5 were increased in E⁰ × CD4dnTβRII mice when compared with E⁰ mice. This indicates that the increased cytokine expression in immune cells resulted in elevated systemic steady-state levels of these molecules. No significant changes were registered, however, for IL-2 or IL-4 (Figure 4j and data not shown). Similarly, plasma levels of TGF-β did not differ between groups (7.63 ± 1.66 versus 5.62 ± 0.82 ng/ml).

Disruption of the ApoE gene leads to increased disease-related immune response but does not affect cytokine levels in CD4dnTβRII mice. In the CD4dnTβRII mice, T cell activation is antigen driven rather than spontaneous (17). To evaluate the disease-related immune response in E⁰ × CD4dnTβRII mice, Ab titers to MDA-LDL were compared with those in CD4dnTβRII mice with intact ApoE genes. Sera were harvested from 24-week-old mice, permitting the development of autoimmune disease and severe atherosclerosis, respectively. Anti–MDA-LDL titers were significantly increased in E⁰ × CD4dnTβRII mice when compared with those in CD4dnTβRII mice with intact ApoE genes and with those in E⁰ mice with functional TGF-β receptors in T cells (Figure 5a), implying an increased antigen-specific, disease-related immune response in the E⁰ × CD4dnTβRII mice. It remains to be determined to what extent the effects on lesion formation are due to immune responses to modified lipoproteins.

Figure 5

Serum anti–MDA-LDL Ab’s and cytokines in mice with ApoE deficiency and/or abrogated TGF-β signaling in T cells. (a) Titers of IgG Ab’s to MDA-LDL in 24-week-old male mice carrying intact or targeted ApoE genes, dominant negative CD4dnTβRII genes, or both. OD values of ELISA, dot plots with values for individual mice, and medians per group. (b–e) Cytokine concentrations of IFN-γ, TNF-α, IL-5, and IL-2 in sera from mice with intact or targeted ApoE genes, dominant negative CD4dnTβRII, or both, as determined by cytofluorometric bead assay. Dot plots as above.

To assess whether the changes in circulating cytokines were related to the increased immune activity due to disruption of TGF-β signaling in T cells or to the atherosclerotic disease, we compared serum levels between 24-week-old E⁰ × CD4dnTβRII mice, CD4dnTβRII mice with intact ApoE genes, and E⁰ mice with functional TGF-β receptors in T cells. While the levels of IFN-γ, TNF-α, and IL-5 were significantly higher in E⁰ × CD4dnTβRII than in E⁰ mice, they did not differ between E⁰ × CD4dnTβRII and ApoE-competent CD4dnTβRII mice (Figure 5, b–d). IL-2 did not differ between any of the groups (Figure 5e), and IL-4 was undetectable (data not shown). Therefore, the increase in circulating cytokines was due to the lack of TGF-β regulation of T cells rather than the atherosclerotic process.

The present findings point to an important role for TGF-β as an immunomodulating cytokine that dampens atherosclerosis by regulating T cell activation. Abrogation of TGF-β signaling in T cells in E⁰ mice led to (a) increased atherosclerosis, (b) increased IFN-γ expression in lesions, (c) reduced plaque collagen and a plaque morphology compatible with increased vulnerability, (d) increased Ab’s to oxidized LDL, and (e) increased T cell activation and cytokine secretion. These data lend new support to the notion that activated T cells are proatherogenic and strongly suggest that TGF-β inhibits atherosclerosis by dampening proatherogenic T cells. This hypothesis is illustrated in Figure 6. Downstream pathogenic mechanisms induced by activated T cells may include increased plaque inflammation, increased lipoprotein uptake via scavenger and Fc receptors, and increased plaque vulnerability due to protease secretion, tissue factor expression, reduced smooth muscle proliferation, and inhibited collagen synthesis (5, 31).

Figure 6

Proposed mechanism by which T cell–dependent inflammation promotes atherosclerosis but is dampened by TGF-β (a). The lack of functional TGF-β receptors on T cells abrogates the modulatory action of this cytokine, leading to aggravated atherosclerosis in E⁰ × CD4dnTβRII mice (b). TGF-βR, TGF-β receptor.

Several studies have provided evidence for a role of TGF-β in atherosclerosis. Plaque-infiltrating lymphocytes display low or undetectable levels of TGF-β receptors (25). Smooth muscle cells derived from human plaques exhibit resistance to the antiproliferative and apoptotic effects of TGF-β and may contain acquired mutations in TGF-β receptor-II (24, 35). Mice heterozygous for a targeted TGFβ1 allele have an increased tendency to fatty streak formation upon challenge with a fatty diet (36). In line with this, the estrogen receptor agonist, tamoxifen, which has TGF-β–inducing effects, suppressed fatty streak formation in fat-fed mice (18). Administration of soluble TGF-β receptors to E⁰ mice reduced lesion size and increased the number of inflammatory cells (21) while neutralizing TGF-β Ab’s increased plaque size and intraplaque inflammation (22). This was thought to be due to effects on lesion cells by TGF-β, which modulates vascular cell proliferation, stimulates collagen production, suppresses macrophage activation, and modulates scavenger receptor expression (37, 38). While all these effects of TGF-β may be significant, the present results show that inhibition of T cell activation is an important pathway through which TGF-β inhibits atherosclerosis.

Immunomodulating TGF-β may be derived from several sources. TGF-β expressed by T_reg’s is suggested to play an important role in controlling T cell activation, but TGF-β secreted from other cells is also likely to contribute (17, 39). In addition, T_reg’s produce other immunomodulating cytokines in addition to TGF-β, in particular IL-10, which is also atheroprotective (19–20). Since T_reg activation is antigen specific, presentation of disease-associated antigens to T_reg’s might dampen activation of proinflammatory Th1 cells. In addition, bystander suppression by T_reg’s specific for other antigens could also play a role, and it is likely that TGF-β production by cells other than T_reg’s are as important for modulating proatherogenic T cell activation.

Mediators produced by Th1 cells are present in human atherosclerosis and exacerbate disease in animal models (5). Gene-targeted deletion of IFN-γ receptors, IL-18, or CD154, as well as blocking Ab’s to CD154, attenuate disease, while injection of recombinant IFN-γ or IL-18 aggravates it (11–13, 40–43). Similarly, treatment with pentoxifyllin, which inhibits Th1 development, reduces atherosclerosis in E⁰ mice (44). The pathway(s) causing immune-dependent reduction of disease have been unclear, however, and recent studies suggest that the Th2 cytokine, IL-4, may also have proatherogenic properties (45). In the present study, both Th1 and Th2 activity were increased in E⁰ cells × CD4dnTβRII mice when compared with E⁰ mice with intact TGF-β signaling. The effect on atherosclerosis was therefore not due to a switch of the Th1/Th2 balance. The net effect of the increased T cell activity was a dramatic acceleration of atherosclerosis despite a concomitant reduction of serum cholesterol levels. Switching the balance from Th1 to Th2 activity may therefore not be sufficient to attenuate atherosclerosis. Instead, it may be desirable to suppress T cell activation.

A comparison between atherosclerotic lesions from E⁰ × CD4dnTβRII and E⁰ mice revealed increased amounts of macrophages and T cells as well as increased I-A expression and reduced collagen content. Furthermore, IFN-γ expression was dramatically increased in E⁰ × CD4dnTβRII mice. Taken together, these findings point to a more vulnerable lesion phenotype in the absence of TGF-β signaling in T cells. Since IFN-γ is produced by activated T cells, induces I-A expression and macrophage activation, and reduces collagen production, this cytokine may play a key role for the aggravation of atherosclerosis in this model. The dramatic increase in T cells could reflect increased recruitment as well as local clonal expansions, which have been detected in E⁰ mice (46).

The importance of specific antigens in atherosclerosis remains controversial. Although several studies have demonstrated that lesions contain antigen-specific T cells and that immunization with specific antigens can protect against disease (4, 5, 7–9), others have shown that antigen-independent pathways may also cause inflammatory activation in lesions. The latter include signaling through Toll-like receptor ligation (47, 48) and production of inflammatory mediators by nonimmune cells (49, 50). In the CD4dnTβRII model employed in the present study, T cell activation is antigen driven rather than spontaneous, since no activation of T cells takes place in the absence of antigen (17). Therefore, disease-related antigens may be responsible for the increased atherosclerosis seen in the E⁰ × CD4dnTβRII mouse. In support of this, Ab titers to the disease-associated antigen MDA-LDL were significantly increased in the E⁰ × CD4dnTβRII animals when compared with ApoE^+/+ CD4dnTβRII mice as well as TGFβRII-competent E⁰ mice. Further studies will be needed to determine the role of antigen-specific T cells and Ab’s in this model. Our data imply that loss of TGF-β regulation of T cells, however, aggravates the immune process in atherosclerotic vascular disease.

To summarize, we have shown that abrogation of TGF-β signaling in T cells increases atherosclerosis in hypercholesterolemic E⁰ mice. These findings imply that activated T cells are strongly proatherogenic, but controlled by TGF-β. They point toward a new strategy for development of antiatherosclerotic therapy by targeting TGF-β signaling in T cells.

Note added in proof. At the time that this paper was accepted for publication, a report by Gojova et al. appeared in preliminary form on the webpage of Blood (51). The report shows that the transfer of bone marrow containing T cells with defective TGRβ receptors increases lesion vulnerability in irradiated ldlr^–/– mice.

We thank I. Bodin, L. Larsson, and I. Törnberg for technical assistance; B. Rozell for expert histopathological analysis; and A. Nicoletti and G. Paulsson Berne for helpful advice. This work was supported by the Swedish Research Council (project 6816 and 14053), the Swedish Heart-Lung Foundation, the Grönberg Fund, and the Torsten and Ragnar Söderberg Foundation. R.A. Flavell is an investigator and L. Gorelik was an associate of the Howard Hughes Medical Institute.

Conflict of interest: The authors have declared that no conflict of interest exists.

Nonstandard abbreviations used: ApoE knockout (E⁰); regulatory T cell (T_reg); CD4-dnTGFβRIITg+ (CD4dnTβRII); fast-performance liquid chromatography (FPLC); malondialdehyde (MDA).

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