No abstract

SummaryThe major characteristics of human atherosclerotic lesions are similar to those of a chronic inflammatory reaction, namely fibrosis, mesenchymal cell proliferation, the presence of resident macrophages, and cell necrosis. Atherosclerosis exhibits in addition the feature of lipid (mainly cholesterol) accumulation . The results ofthe present report demonstrate that a specific cholesterolcontaining lipid particle present in human atherosclerotic lesions activates the complement system to completion. Thus, lipid could represent a stimulatory factor for the inflammatory reaction, whose underlying mechanistic basis may be, at least in part, complement activation . The complement-activating lipid was purified from saline extracts of aortic atherosclerotic lesions by sucrose density gradient centrifugation followed by molecular sieve chromatography on Sepharose 2B. It contained little protein other than albumin, was 100-500 nm in size, exhibited an unesterified to total cholesterol ratio of 0.58 and an unesterified cholesterol to phospholipid ratio of 1.2. The lipid, termed lesion lipid complement (LCA), activated the alternative pathway ofcomplement in a dose-dependent manner. Lesion-extracted low density lipoprotein (LDL) obtained during the purification procedure failed to activate complement. Specific generation of C3a desArg and C5b-9 by LCA indicated C3/C5 convertase formation with activation proceeding to completion. Biochemical and electron microscopic evaluations revealed that much of the C5b-9 present in atherosclerotic lesions is membraneous, rather than fluid phase SC5b-9 . The observations reported herein establish a link between lipid insudation and inflammation in atherosclerotic lesions via the mechanism of complement activation. therosclerosis shares in common with chronic inflamma-1A tion the features ofleukocyte infiltration, fibrosis, mesenchymal cell proliferation, and tissue necrosis . Complement plays a central role in many inflammatory and immune diseases, and most of the complement components have now been identified in human atherosclerotic lesions (1) . Of particular importance is the presence of C5b-9 terminal complement complexes since their assembly indicates that complement activation with C3/C5 convertase formation has taken place. The respective convertases cleave C3 and C5 generating the anaphylatoxins C3a and C5a, which in turn mediate proinflammatory leukocyte functions (2) . The C5b-9 complex, in addition to its potential cytolytic effect, can influence cell physiology in sublytic doses via activation of various calcium-dependent pathways (3) .In a previous report, we demonstrated a temporal and spatial colocalization of C5b-9 complexes and lipid in the aortic tunica intima of cholesterol-fed rabbits (4) . Lipid vesicles rich in unesterified cholesterol are an early and consistent feature of human and animal atherosclerotic lesions (5-11) . In vitro, liposomes containing a >50 mole percent of unesterified cholesterol activate complement to completion (12, 13) . We therefore su...

The contribution of Escherichia coli hemolysin (ECH) to bacterial virulence has been considered mainly in context with its hemolytic properties. We here report that this prevalent bacterial cytolysin is the most potent leukocidin known to date. Very low concentrations (approximately 1 ng/ml) of ECH evoke membrane permeability defects in PMN (2-10 x 10(6) cells/ml) leading to an efflux of cellular ATP and influx of propidium iodide. The attacked cells do not appear to repair the membrane lesions. Human serum albumin, high density and low density lipoprotein, and IgG together protect erythrocytes and platelets against attack by even high doses (5-25 micrograms/ml) of ECH. In contrast, PMN are still permeabilized by ECH at low doses (50-250 ng/ml) in the presence of these plasma inactivators. Thus, PMN become preferred targets for attack by ECH in human blood and protein-rich body fluids. Kinetic studies demonstrate that membrane permeabilization is a rapid process, ATP-release commencing within seconds after application of toxin to leukocytes. It is estimated that membrane permeabilization ensues upon binding of approximately 300 molecules ECH/PMN. This process is paralleled by granule exocytosis, and by loss of phagocytic killing capacity of the cells. The recognition that ECH directly counteracts a major immune defence mechanism of the human organism through its attack on granulocytes under physiological conditions sheds new light on its possible role and potential importance as a virulence factor of E. coli.