The Alternative Pathway Is Critical for Pathogenic Complement Activation in Endotoxin- and Diet-Induced Atherosclerosis in Low-Density Lipoprotein Receptor–Deficient Mice

Abstract

Background The early components of the classical and lectin complement pathways have been shown to protect low-density lipoprotein receptor deficient mice (Ldlr−/−) from early atherogenesis. However, the role of the alternative pathway remained unknown and that was investigated in this study. Methods and Results Mice lacking factor B (Bf−/−), the initiator of the alternative pathway, were crossed with Ldlr−/− mice and studied under different pro-atherogenic conditions. There was no statistically significant difference in lipid profiles or atherosclerotic lesion development between Bf−/−.Ldlr−/− and Ldlr−/− mice fed a low-fat diet. However, in these groups administration of bacterial lipopolysaccharide (LPS) led to a significant increase in atherosclerosis only in Ldlr−/− and not in Bf−/−.Ldlr−/− mice, indicating that the alternative pathway is necessary for endotoxin-mediated atherogenesis. Bf−/−.Ldlr−/− mice also had significantly decreased cross-sectional aortic root lesion fraction area and reduced lesion complexity compared to Ldlr−/− animals after a 12-week period of high-fat diet, although this was also accompanied by reduced levels of serum cholesterol. Under both experimental conditions, the atherosclerotic changes in the Bf−/−.Ldlr−/− mice were accompanied by a marked reduction in complement activation in the circulation and in atherosclerotic plaques, with no statistical significant differences in IgG deposition or in the serum antibody response to oxidised LDL. Conclusions These data demonstrate that amplification of complement activation by the alternative pathway in response to LPS or high fat diet plays a pro-atherogenic role.

Molecular Biology of Atherosclerosis

“…During a high-fat diet, LDL-C was much lower in CFB KO mice which likely explained the modestly decreased atherosclerosis. However, CFB KO mice were almost entirely protected from an accelerated atherosclerosis that was induced by chronic intraperitoneal LPS administration (1124). These results demonstrate that enhancement of atherosclerosis by LPS or chronic infection need not be mediated solely by Toll-like receptors but that alternative complement pathway activation can also contribute.…”

The Role of Complement in the Development and Manifestation of Murine Atherogenic Inflammation: Novel Avenues

“…Overall, triglycerides and total cholesterol in complement-deficient mice appear not to be different from controls on either an ApoE -/-or LDLR -/-background alone (which already present with hyperlipidaemia) with two exceptions; Bf-deficient (Bf -/-)/LDLR -/-mice have significantly reduced cholesterol levels on an HFD compared with Bf-sufficient LDLR -/-mice [32] , and C3 -/-/ApoE -/-/ LDLR -/-mice on normal chow show significantly increased levels of serum triglycerides and cholesterol compared to C3-sufficient ApoE -/-/LDLR -/-controls [33] . Although the postprandial clearance of triglycerides was the same between C3…”

“…While the membrane-bound down-regulator of complement activation CD55 (decay accelerating factor, dissociates C3 and C5 convertases) regulates lesional lipid deposition in LDLR -/-mice on an HFD [29] , development of lesions is attenuated in ApoE -/-mice on an HFD in the absence of CD55 [30] . By contrast, C6 (part of MAC) [23] , C3a receptor C3aR [31] and factor B (Bf; of the AP) [32] appear to exacerbate disease during HFD. In studies analysing an early and later time point, the effect of complement deficiency on an atherosclerosis-prone genetic background manifests within the first 10-16 weeks of an HFD.…”

The complement system and toll-like receptors as integrated players in the pathophysiology of atherosclerosis

“…[185] Ldlr.C1qa À/À mice Normal or high-fat diet Aortic atheroma size and apoptotic cells Increased aortic atheroma and reduced apoptotic cell clearance in Ldlr.C1qa À/À mice. Lewis, 2009 [186] C1qa.sIgM.Ldlr À/À mice Low-and high-fat diet Aortic atheroma size Increased aortic atheroma in sIgM.Ldlr À/À mice, indicating IgM protection, independent of classical pathway Matthijsen, 2009 [187] Ldlr À/À mice High-fat diet Presence of MBL MBL is present in early but not late atherosclerotic lesions Orsini, 2012 [188] Mice/rats Cerebral artery occlusion and MBL inhibition Presence of MBL cerebral infarct MBL is present in ischemic areas and MBL À/À and MBL inhibited animals are protected Malik, 2010 [189] Bf.Ldlr À/À mice Low-fat diet LPS Aortic atheroma size Aortic root atheromas were larger in Ldlr À/À compared to Bf.Ldlr À/À Shagdarsuren, 2010 [176] ApoE À/À mice Femoral artery injury and C5aR inhibition Neo-intima formation and inflammatory cells.…”