We explored the role of the classic complement pathway in atherogenesis by intercrossing C1q-deficient mice (C1qa ؊/؊ ) with low-density lipoprotein receptor knockout mice (Ldlr ؊/؊ ). Mice were fed a normal rodent diet until 22 weeks of age. Aortic root lesions were threefold larger in C1qa ؊/؊ /Ldlr ؊/؊ mice compared with Ldlr ؊/؊ mice (3.72 ؎ 1.0% aortic root versus 1.1 ؎ 0.4%; mean ؎ SEM , P < 0.001). Furthermore , the cellular composition of lesions in C1qa ؊/؊ / Ldlr ؊/؊ was more complex, with an increase in vascular smooth muscle cells. The greater aortic root lesion size in C1qa ؊/؊ /Ldlr ؊/؊ mice occurred despite a significant reduction in C5b-9 deposition per lesion unit area, suggesting the critical importance of proximal pathway activity. Apoptotic cells were readily detectable by cleaved caspase-3 staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and electron microscopy in C1qa ؊/؊ /Ldlr ؊/؊ , whereas apoptotic cells were not detected in Ldlr ؊/؊ mice. This is the first direct demonstration of a role for the classic complement pathway in atherogenesis. The greater lesion size in C1qa ؊/؊ /Ldlr ؊/؊ mice is consistent with the emerging homeostatic role for C1q in the disposal of dying cells. This study suggests the importance of effective apoptotic cell removal for containing the size and complexity of early lesions in atherosclerosis.

Objective-Whereas studies in humans and animal models have suggested a role for complement activation in atherosclerosis, there has been little analysis of the importance of complement regulators. We tested the hypothesis that the terminal pathway inhibitor CD59 plays an essential role in limiting the proinflammatory effects of complement activation. Methods and Results-CD59 gene targeted mice (CD59a Ϫ/Ϫ ) mice were crossed with low-density lipoprotein receptordeficient (Ldlr Ϫ/Ϫ ) mice. CD59-deficient Ldlr Ϫ/Ϫ mice had significantly more extensive en face Sudan IV staining of thoracoabdominal aorta than Ldlr Ϫ/Ϫ single knock-outs, both after a low-fat diet (6.51Ϯ0.36% versus 2.63Ϯ0.56%, PϽ0.001) or a high-fat diet (17.05Ϯ2.15% versus 7.69Ϯ1.17%, PϽ0.004). Accelerated lesion formation in CD59a Ϫ/Ϫ /Ldlr Ϫ/Ϫ mice on a high-fat diet was associated with increased lesional vascular smooth muscle cell (VSMC) number and fibrous cap formation. Conclusion-Our data show that CD59 deficiency accelerates the development of lesions and increases plaque VSMC composition. Assuming that the main function of CD59 is to prevent the development of C5b-9 membrane attack complexes, our observations are consistent with the terminal complement pathway having proatherogenic potential in the Ldlr Ϫ/Ϫ mouse model, and highlight the importance of complement regulation.

Decay-accelerating factor (DAF; CD55) is a membrane protein that regulates complement pathway activity at the level of C3. To test the hypothesis that DAF plays an essential role in limiting complement activation in the arterial wall and protecting from atherosclerosis, we crossed DAF gene targeted mice (daf-1 ؊/؊ ) with low-density lipoprotein-receptor deficient mice (Ldlr ؊/؊ ). Daf-1 ؊/؊ Ldlr ؊/؊ mice had more extensive en face Sudan IV staining of the thoracoabdominal aorta than Ldlr ؊/؊ mice, both following a 12-week period of low-fat diet or a high-fat diet. Aortic root lesions in daf-1 ؊/؊ Ldlr ؊/؊ mice on a low-fat diet showed increased size and complexity. DAF deficiency increased deposition of C3d and C5b-9 , indicating the importance of DAF for downstream complement regulation in the arterial wall. The acceleration of lesion development in the absence of DAF provides confirmation of the proinflammatory and proatherosclerotic potential of complement activation in the Ldlr ؊/؊ mouse model. Because upstream complement activation is potentially protective, this study underlines the importance of DAF in shielding the arterial wall from the atherogenic effects of complement.