Complement C6 Deficiency Protects Against Diet-Induced Atherosclerosis in Rabbits

Abstract

Abstract-Low-density lipoprotein (LDL) can be transformed to an atherogenic moiety by nonoxidative, enzymatic degradation. Enzymatically degraded LDL induces macrophage foam cell formation, provokes release of cytokines, and also activates complement. To determine whether complement activation may contribute to atherogenesis, 6 pairs of homozygous C6-deficient rabbits and their non-C6-deficient heterozygous siblings were fed a cholesterol-rich diet for 14 weeks. Cholesterol levels and plasma lipoprotein profiles of the animals in the C6-competent and C6-deficient groups did not significantly differ, and the high density lipoprotein and LDL cholesterol ratios at the end of the experiment were 0.07Ϯ0.01 and 0.08Ϯ0.01 (SEM), respectively. However, differences in atherosclerotic plaque formation were discernible macroscopically, with extensive aortic lesions being visible in all C6-competent animals and absent in all C6-deficient animals. Aortas were sectioned from thorax to abdomen, and 10 sections were stained from each aorta. Quantification of atherosclerotic lesions and lumen stenosis with the use of computer-based morphometry documented a dramatic protective effect of C6 deficiency on the development of diet-induced atherosclerosis. We conclude that the terminal complement sequence is centrally involved in atherosclerotic lesion progression. (Arterioscler Thromb Vasc Biol. 1998;18:1790-1795.)Key Words: complement activation Ⅲ atherosclerosis T he possible relevance of complement activation in atherogenesis has not received much attention to date, and only a few reviews are available on the topic. 1,2 The first immunohistochemical studies on complement deposition in atherosclerotic lesions appeared in 1985 to 1987, 3-5 and C5b-9 complexes were subsequently quantified by ELISA in detergent extracts of lesion homogenates. 6 In those studies, the stages of lesion development were not defined, so it could not be excluded that complement activation might have occurred subsequent to tissue damage. An experimental study was then performed in rabbits, leading to the clear demonstration that diet-induced deposition of lipids in the subendothelium was temporally associated with complement activation, which occurred before lesion infiltration by monocytes. 7 A directed search led to tentative identification of the complement-activating entity. Heterogeneously sized lipid droplets containing high amounts of free cholesterol were isolated from early lesions and were shown to be capable of spontaneously activating the alternative complement pathway. 8 The origin of this lipid, termed the lesion complement activator (LCA), was unknown, but the possibility that it represented an LDL derivative was obvious. To corroborate this assumption, attempts were undertaken to transform LDL in vitro into a complement-activating moiety. This was found to be possible by combined treatment of the lipoprotein with a protease, cholesterol-esterase, and neuraminidase, 9 enzymes that occur ubiquitously in lysosomes of mammalian cells and that are...

Influence of C3 Deficiency on Atherosclerosis

“…C6-deficient rabbits seemed to be protected from lesion development, 19 whereas aortic root lesion size in C5-deficient ApoE-null mice was not different from control ApoE mice. 20 The latter mouse study did not examine more distal regions of the aorta, nor did it examine lesion phenotype.…”

“…In rabbits, there is an apparent protective effect of C6 deficiency on diet-induced atherosclerosis. 19 In contrast, a recent study reported that C5 deficiency does not reduce the development of atherosclerotic lesions in apolipoprotein E (ApoE) deficient mice. 20 Because C3 is the central molecule in both classical and alternative pathways, we reasoned that studying the effect of C3 deficiency on atherosclerosis would be a sensitive way of detecting a relationship between the complement system and atherogenesis.…”

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

“…In hyperlipidemic rabbits, C5b-9 deposition occurs at an early stage of atherogenesis in conjunction with cholesterol accumulation and, judging from protection in C6-deficient rabbit, seems to accelerate lesion formation. 40,42,43 Although C5b-9 is clearly generated in Ldlr Ϫ/Ϫ mice, it is important to note that fluid phase and cell surface inhibitors may act to reduce the amount of terminal pathway activation in relation to proximal pathway activity. Cell surface inhibitory factors, such as decay accelerating factor (CD55), CD59, and, in mice, complement receptor 1 (CR1)-related gene y (Crry) may be particularly important, because access of plasma inhibitors to the subendothelial space may be limited.…”

Endothelial Nuclear Factor-κB Translocation and Vascular Cell Adhesion Molecule-1 Induction by Complement

“…1,[42][43][44] In addition to causing direct tissue injury, complement may also indirectly mediate vascular injury by stimulating leukocyte activation and chemotaxis and by increasing endothelial leukocyte adhesion molecule expression. 16,17,45 However, the intracellular mechanisms by which the terminal complement components induce endothelial leukocyte adhesion molecule expression are not completely characterized.…”

“…I ncreasing evidence suggests that the terminal complement complex (C5b-9) plays an integral role in the pathogenesis of atherosclerosis [1][2][3][4][5] and vascular injury after ischemiareperfusion, cardiopulmonary bypass, and acute myocardial infarction. 6 -11 In addition to amplifying the local inflammatory response by inducing endothelial interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) secretion, 12 C5b-9 also influences endothelial vascular tone [13][14][15] and expression of leukocyte adhesion molecules (eg, vascular cell adhesion molecule-1 [VCAM-1], 16 intercellular adhesion molecule-1 [ICAM-1], 17 and endothelial leukocyte adhesion molecule-1 [E-selectin] 16 ).…”