Complicated abdominal aortic aneurysm (AAA) is a major cause of mortality in elderly men. Ang II–dependent TGF-β activity promotes aortic aneurysm progression in experimental Marfan syndrome. However, the role of TGF-β in experimental models of AAA has not been comprehensively assessed. Here, we show that systemic neutralization of TGF-β activity breaks the resistance of normocholesterolemic C57BL/6 mice to Ang II–induced AAA formation and markedly increases their susceptibility to the disease. These aneurysms displayed a large spectrum of complications on echography, including fissuration, double channel formation, and rupture, leading to death from aneurysm complications. The disease was refractory to inhibition of IFN-γ, IL-4, IL-6, or TNF-α signaling. Genetic deletion of T and B cells or inhibition of the CX3CR1 pathway resulted in partial protection. Interestingly, neutralization of TGF-β activity enhanced monocyte invasiveness, and monocyte depletion markedly inhibited aneurysm progression and complications. Finally, TGF-β neutralization increased MMP-12 activity, and MMP-12 deficiency prevented aneurysm rupture. These results clearly identify a critical role for TGF-β in the taming of the innate immune response and the preservation of vessel integrity in C57BL/6 mice, which contrasts with its reported pathogenic role in Marfan syndrome.

Matrix metalloproteinases (MMPs) are thought to be involved in the growth, destabilization, and eventual rupture of atherosclerotic lesions. Using the mouse brachiocephalic artery model of plaque instability, we compared apolipoprotein E (apoE)/MMP-3, apoE/MMP-7, apoE/MMP-9, and apoE/MMP-12 double knockouts with their age-, strain-, and sex-matched apoE single knockout controls. Brachiocephalic artery plaques were significantly larger in apoE/MMP-3 and apoE/MMP-9 double knockouts than in controls. The number of buried fibrous layers was also significantly higher in the double knockouts, and both knockouts exhibited cellular compositional changes indicative of an unstable plaque phenotype. Conversely, lesion size and buried fibrous layers were reduced in apoE/MMP-12 double knockouts compared with controls, and double knockouts had increased smooth muscle cell and reduced macrophage content in the plaque, indicative of a stable plaque phenotype. ApoE/MMP-7 double knockout plaques contained significantly more smooth muscle cells than controls, but neither lesion size nor features of stability were altered in these animals. Hence, MMP-3 and MMP-9 appear normally to play protective roles, limiting plaque growth and promoting a stable plaque phenotype. MMP-12 supports lesion expansion and destabilization. MMP-7 has no effect on plaque growth or stability, although it is associated with reduced smooth muscle cell content in plaques. These data demonstrate that MMPs are directly involved in atherosclerotic plaque destabilization and clearly show that members of the MMP family have widely differing effects on atherogenesis.

Background— These studies examined the early time course of plaque development and destabilization in the brachiocephalic artery of the apolipoprotein E–knockout mouse, the effects of pravastatin thereon, and the effects of pravastatin on established unstable plaques. Methods and Results— Male apolipoprotein E–knockout mice were fed a high-fat, cholesterol-enriched diet from the age of 8 weeks. Animals were euthanized at 1-week intervals between 4 and 9 weeks of fat feeding. Acutely ruptured plaques were observed in the brachiocephalic arteries of 3% of animals up to and including 7 weeks of fat feeding but in 62% of animals after 8 weeks, which suggests that there is a sharp increase in the number of plaque ruptures at 8 weeks. These acute plaque ruptures then appear to heal and form buried fibrous caps; after 9 weeks of fat feeding, mice had 1.05±0.15 buried fibrous caps at a single site in the brachiocephalic artery. Pravastatin (40 mg/kg of body weight per day for 9 weeks; resultant plasma concentration 16±4 nmol/L) had no effect on plasma cholesterol concentration in fat-fed apolipoprotein E–knockout mice but reduced the number of buried fibrous caps by 43% ( P <0.0001). In longer-term experiments, the delay of pravastatin treatment until unstable plaques had developed reduced the incidence of acute plaque rupture by 36% ( P <0.0001). Conclusions— Plaque rupture occurs at high frequency in the brachiocephalic arteries of male apolipoprotein E–knockout mice after 8 weeks of fat feeding. Pravastatin treatment inhibits early plaque rupture and is also effective when begun after unstable plaques have developed.

Clinical complications of atherosclerosis arise primarily as a result of luminal obstruction due to atherosclerotic plaque growth, with inadequate outward vessel remodeling and plaque destabilization leading to rupture. IL-1 is a proinflammatory cytokine that promotes atherogenesis in animal models, but its role in plaque destabilization and outward vessel remodeling is unclear. The studies presented herein show that advanced atherosclerotic plaques in mice lacking both IL-1 receptor type I and apolipoprotein E (Il1r1^–/–Apoe^–/– mice) unexpectedly exhibited multiple features of plaque instability as compared with those of Il1r1^+/+Apoe^–/– mice. These features included reduced plaque SMC content and coverage, reduced plaque collagen content, and increased intraplaque hemorrhage. In addition, the brachiocephalic arteries of Il1r1^–/–Apoe^–/– mice exhibited no difference in plaque size, but reduced vessel area and lumen size relative to controls, demonstrating a reduction in outward vessel remodeling. Interestingly, expression of MMP3 was dramatically reduced within the plaque and vessel wall of Il1r1^–/–Apoe^–/– mice, and Mmp3^–/–Apoe^–/– mice showed defective outward vessel remodeling compared with controls. In addition, MMP3 was required for IL-1–induced SMC invasion of Matrigel in vitro. Taken together, these results show that IL-1 signaling plays a surprising dual protective role in advanced atherosclerosis by promoting outward vessel remodeling and enhancing features of plaque stability, at least in part through MMP3-dependent mechanisms.

—Recent studies suggest that mast cell–derived neutral proteases can activate matrix-degrading metalloproteinases (MMPs). We have investigated the role of the mast cell proteases tryptase and chymase in the activation of MMPs in human carotid endarterectomy specimens (atherosclerotic, n=32) and postmortem carotid arteries (control, n=17). In vitro degranulation of mast cells in atherosclerotic carotid arteries by compound 48/80 caused a significant increase in MMP activity. Addition of the nonselective tryptase inhibitor antipain, the specific trypsinlike protease inhibitor 4-amidinophenylmethanesulfonyl fluoride, and the chymase inhibitor chymostatin reduced this increase in MMP activity by 30±6%, 23±6%, and 9±2%, respectively. Immunocytochemistry identified significantly higher numbers of tryptase-containing cells (mast cells) and cells expressing MMP-1 and MMP-3 in the “shoulder” regions of atherosclerotic artery lesions compared with the tunica media of control arteries. Dual immunocytochemistry showed collocation of MMP-1 and MMP-3 with mast cells in the shoulder regions. Degranulation was observed in 78±5% (mean±SEM) of mast cells in this area, whereas nonactivated mast cells were observed in all other areas. In situ zymography revealed caseinolytic and gelatinolytic activity in these areas. In conclusion, in vitro mast cell degranulation, which releases mast cell proteases, in carotid arteries increases MMP activity. Furthermore, elevated MMP-1 and MMP-3 expression is collocated with increased numbers of degranulated mast cells and with greater MMP activity in the shoulder regions of atherosclerotic plaques. Activation of MMPs by mast cell–derived proteases may be an important mechanism in atherosclerotic plaque destabilization.

Atherosclerosis underlies most cardiovascular diseases, and is accepted as a primary cause of mortality worldwide. Proteases have been implicated in the development and progression of atherosclerosis, due to their ability to provoke focal destruction of the vascular extracellular matrix. Members of the metalloproteinase family, especially matrix metalloproteinases (MMPs), and their endogenous tissue inhibitors (TIMPs) have been suggested to perform complex dual roles during late-stage progression and rupture of atherosclerotic plaques. Proposed favourable actions of metalloproteinases include the promotion of vascular smooth muscle growth and survival which stabilises plaques, while conversely extracellular matrix destruction alongside interminable monocyte/macrophage accumulation can encourage plaque rupture. This review provides a summary of the cogent evidence connecting the contribution of individual metalloproteinases to atherosclerotic plaque development, progression, and instability. Topics discussed include structural, functional and cell-specific diversity of MMP members; evidence from animal models of atherosclerosis and comparisons with findings in humans; the dual role of MMPs and the requirement to selectively target individual MMPs; and the need for efficient surrogate markers of MMP inhibition. Accordingly, as our knowledge of the complex roles individual MMPs play especially during the progression and rupture of atherosclerotic plaques expands, new impetus is required for clinical trials evaluating the therapeutic potential of selective MMP inhibition, which could limit cardiovascular morbidity and mortality worldwide.