Complement C1q Reduces Early Atherosclerosis in Low-Density Lipoprotein Receptor-Deficient Mice

Abstract

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.

Decay-Accelerating Factor Suppresses Complement C3 Activation and Retards Atherosclerosis in Low-Density Lipoprotein Receptor-Deficient Mice

“…As in our previous studies with C1q and CD59 deficiencies, these changes were most marked in mice fed a low-fat diet. 24,41 Taking the three studies together, the data are consistent with a homeostatic role for the classical complement pathway in the arterial wall on a low-fat diet, which is overridden by the greater inflammatory milieu caused by high fat feeding.…”

“…Relevant studies are as follows: (1) rabbits with natural deficiency of C6 have been shown to be protected from diet-induced atherosclerosis 18,19 ; (2) although C5 deficiency has been found to have no effect on lesion development in high fat diet-fed ApoE Ϫ/Ϫ mice, 20 a recent study has shown protective effects of an anti-C5 antibody in ApoE Ϫ/Ϫ mice deficient in both Cd59a and ) show accelerated atherosclerosis with increased lesion complexity. 24 The increased lesion size and complexity in low fat diet-fed C1qa Ϫ/Ϫ Ldlr Ϫ/Ϫ mice was associated with an increase in lesional apoptotic cells, consistent with previous studies that have demonstrated a direct role for C1q in apoptotic cell clearance, independent of terminal pathway activation. 25,26 Recently, the role of the lectin pathway has also been shown to have atheroprotective functions in mice, 27 in line with the involvement of mannosebinding lectin in apoptotic cell clearance and also with the association of mannose-binding lectin deficiency with accelerated atherosclerosis in humans.…”

“…40 Observations that the classical pathway exerts atheroprotective effects without terminal pathway activation suggest the importance of a strong complement regulatory system in the arterial wall. 14,24 Consistent with this, we and others have recently published evidence that CD59 deficiency leads to an acceleration of atherosclerosis in Ldlr…”

“…En face staining of thoracoabdominal aorta and quantification of the aortic root lesion area were conducted as described. 24,41 Immunohistochemistry was performed by standard procedures on residual sections not required for analysis of lesion size, as described. 24,41 Results of immunocytochemistry are presented as a percentage area fraction of the aortic root or as the percentage of lesional cells, as analyzed by Image ProPlus software above.…”

“…Images were then captured with Olympus DP50 (Olympus, Southend-on-Sea, UK) digital microscopy camera. Thin collagen fibers (green) and thick collagen fibers (orange/red) were measured by using the National Institutes of Health open access generic image analysis software Image J (http:// rsbweb.nih.gov/ij 24 ). Images were first converted into three greyscale images (red, green, and blue).…”