Complement and arteriosclerosis

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

Despite recent medical advances, atherosclerosis is a global burden accounting for numerous deaths and hospital admissions. Immune-mediated inflammation is a major component of the atherosclerotic process, but earlier research focus on adaptive immunity has gradually switched towards the role of innate immunity. The complement system and toll-like receptors (TLRs), and the crosstalk between them, may be of particular interest both with respect to pathogenesis and as therapeutic targets in atherosclerosis. Animal studies indicate that inhibition of C3a and C5a reduces atherosclerosis. In humans modified LDL-cholesterol activate complement and TLRs leading to downstream inflammation, and histopathological studies indicate that the innate immune system is present in atherosclerotic lesions. Moreover, clinical studies have demonstrated that both complement and TLRs are upregulated in atherosclerotic diseases, although interventional trials have this far been disappointing. However, based on recent research showing an intimate interplay between complement and TLRs we propose a model in which combined inhibition of both complement and TLRs may represent a potent anti-inflammatory therapeutic approach to reduce atherosclerosis.

Complement-mediated chronic inflammation is associated with diabetic microvascular complication

BACKGROUND

Chronic inflammation is characteristic of type 2 diabetes mellitus (T2DM). Obesity-activated adipocytes release adipocytokines, which induce the secretion of proinflammatory cytokines, resulting in vascular endothelial dysfunction and organ injury. C3a is a candidate to induce tissue inflammation.

METHODS

We investigated the association between diabetic microangiopathy and complement-mediated inflammation in 32 obese T2DM patients and 32 normal donors. Plasma levels of complement components and their activation intermediates were examined and related to the level of complication. An incubation study of post-prandial serum was carried out to measure the in vitro production of acylation stimulating protein (ASP/C3a desArg) by chylomicron.

RESULTS

Plasma levels of C3, C4, factor B, iC3b, Bb, and ASP were significantly increased in T2DM patients. Levels of C4d and membrane attack complex (C5b-9) were not significantly elevated. The activation rate of these factors indicated that only the early phase of alternative complement pathway was excessively activated. A statistical study revealed close correlation between ASP, body mass index, and highly sensitive C-reactive protein. Plasma ASP was significantly increased in the macroalbuminuric and proliferative retinopathy patient groups. An incubation study revealed that ASP was produced after the in vitro incubation of post-prandial serum from a T2DM patient with hyperchylomicronaemia.

CONCLUSIONS

Activation of the alternative complement pathway occurs in obese T2DM patients and is enhanced in the post-prandial hyperchylomicronic condition, which induces overproduction of ASP and C3a-mediated tissue inflammation. Therefore, complement-mediated inflammation may contribute to the acceleration of diabetic microangiopathy in addition to the development of macroangiopathy.

Noncalcified coronary plaque in systemic lupus erythematosus

OBJECTIVE

To study noncalcified coronary plaque (NCP) in systemic lupus erythematosus (SLE).

METHODS

Sixty-four-slice coronary multidetector computed tomography (MDCT) was performed in 39 consecutive patients with SLE. MDCT scans were evaluated semiquantitatively by a radiologist using dedicated software. The presence or absence of NCP in each coronary artery was assessed. Patients with mixed plaque (calcified and noncalcified portions) were included in the NCP group.

RESULTS

The patient group was 90% women, 64% Caucasian, 31% African American, 5% other; mean age 50.5 +/- 9.6 years. Fifty-four percent (21/39) had NCP. Seventy-six percent (16/21) of those with NCP also had coronary calcium (range 0.7 to 1264.1 Agatston units). In univariate analysis, NCP was associated with age (p = 0.01), current nonsteroidal antiinflammatory drug (NSAID) use (p = 0.04), hormone replacement therapy (p = 0.02), current use of immunosuppressive drugs (p = 0.02), current low serum C3 level (p = 0.07), current physician's global assessment of activity (PGA; p = 0.05), and low-density lipoprotein cholesterol (p = 0.04). NCP was not associated with other risk factors for atherosclerosis, including total serum cholesterol, high sensitivity C-reactive protein, and lipoprotein(a).

CONCLUSION

Unlike coronary calcium, which is not associated with SLE activity measures or with active serologies, NCP is more common in patients with SLE with current, 3-, and 6-month activity by PGA. NCP was also associated with the need for current NSAID or immunosuppressive therapy. NCP is an important part of the total atherosclerotic burden in SLE.

Effect of the extract of the tamarind (Tamarindus indica) fruit on the complement system: studies in vitro and in hamsters submitted to a cholesterol-enriched diet

This work evaluated a crude hydroalcoholic extract (ExT) from the pulp of the tamarind (Tamarindus indica) fruit as a source of compounds active on the complement system (CS) in vitro. ExT, previously characterized by other authors, had time and concentration dependent effects on the lytic activity of the CS. The activity of 0.8 mg/mL of the extract on the classical/lectin pathways (CP/LP) increased after 30 min of pre-incubation, while that of the alternative pathway (AP) decreased after 15 min at 1mg/mL. Since the CS is a mediator of inflammation, studies were also made in vivo, taking advantage of a model of hypercholesterolemia in hamsters to investigate the role of CS in the phase preceding the inflammatory process of atherosclerosis. Hamsters submitted to a diet rich in cholesterol showed increased lytic activity of the CP/LP and AP after 45 days. The activity levels of C2 and factor B components on respectively, classical/lectin and alternative pathways of the CS also increased. Early events cooperating to trigger the process of atherosclerotic lesions are not completely understood, and these alterations of complement may participate in these events. When treatment with a diet rich in cholesterol was associated to the furnishing of ExT, evaluation of complement components and complement lytic activity showed values similar to those of the controls, showing that treatment with ExT blocked the increase of complement activity caused by the cholesterol-rich diet. By itself, ExT had no effect on the complement system in vivo. ExT activity on the CS may be of interest for therapy and research purposes.

Immunomodulation and vaccination for atherosclerosis

During the last few decades, the pathogenesis of atherosclerosis has been related not only to cholesterol deposition and cell proliferation in the lesions, but also to infiltration of immune cells, which are involved in both systemic and local, innate as well as adaptive, immune responses. A number of antigen candidates, such as oxidised low-density lipoprotein and heat-shock proteins, have been associated with the disease process. As some inflammatory and autoimmune diseases could be treated by immunologically based therapy, it is of particular interest whether such principles can also be applied to prevent or treat atherosclerosis. Indeed, modification of immune reactions in animal models can greatly affect the development and progression of atherosclerosis. This review provides an overview of our current understanding of how immunomodulation changes the course of atherosclerosis and how vaccination may be used for preventing the disease.

Arterial wall chondroitin sulfate proteoglycans: diverse molecules with distinct roles in lipoprotein retention and atherogenesis

Chondroitin sulfate proteoglycans (CSPGs) of the arterial wall are generally considered to be atherogenic because of their ability to trap cholesterol-rich lipoproteins in vitro. Nevertheless, CSPGs are a diverse group of molecules with a long evolutionary history and distinct biologic functions. The three principal CSPGs in the arterial wall are versican, which is part of the hyalectan gene family; and decorin and biglycan, which are members of a separate gene family, the small leucine-rich proteoglycans. Importantly, there is now substantial evidence that the different molecular species of CSPGs participate unequally in lipoprotein retention, and that they exert unequal regulatory effects that are related to atherogenesis. Recently available murine models with genetic manipulations that affect CSPGs now allow causal studies of the roles of these molecules to be conducted in vivo, with occasionally surprising results. Moreover, tools are being developed to examine human genetic variations that are relevant to CSPGs, which may provide additional important insights into the human disease. The era in which proteoglycans are regarded as a nondescript backdrop, playing purely nonspecific structural roles, is over. Studies in manipulated animals and in human populations will continue to reveal precise, dynamic roles for these fascinating and ancient molecules.

Vascular stiffness in women with systemic lupus erythematosus

Large-vessel manifestations of systemic lupus erythematosus (SLE), a multisystem disease characterized by disturbances in the immune system, include higher than expected rates of hypertension and cardiovascular disease. Reductions in the elasticity of central arteries may act as a marker of early changes that predispose to the development of major vascular disease. This study evaluated risk factors associated with aortic stiffness measured by pulse wave velocity (PWV) in women with SLE. We expected SLE-specific factors, especially variables indicative of inflammation and active disease, to be associated with increasing PWV. The study population included 220 women currently enrolled in the Pittsburgh Lupus

REGISTRY

All risk factor data were collected on the day of the ultrasound examinations. PWV waveforms were collected from the right carotid and femoral arteries by Doppler probes. The mean age of the women was 45.5+/-10.8 years, the median SLE disease duration approximated 9 years, and the mean PWV was 6.1+/-1.7 m/s. Multiple regression models were stratified by menopausal status. Among postmenopausal women, PWV risk factors were primarily traditional factors and included age, systolic blood pressure, family history of vascular disease, carotid plaque, creatinine, obesity, glucose, white cell count, and cumulative SLE organ damage. Among premenopausal women, PWV risk factors consisted of a mix of SLE-related and traditional variables and included higher C3 levels, presence of ds-DNA antibodies, nonuse of hydroxychloroquine, lower leukocyte count, higher mean arterial pressure, and carotid plaque. SLE-specific variables appeared to be associated with increases in aortic PWV, indicating central artery stiffening. This was seen most clearly among premenopausal women. This finding may partially explain the higher rates of cardiovascular disease and hypertension observed in young women with SLE.

Complement activation and atherosclerosis

Atherosclerosis is an inflammatory disease mediated through the action of monocyte/macrophages, complement and T-lymphocytes. C5a and monocyte chemotactic factor released during complement activation in the arterial wall may participate in the initial monocyte recruitment. Assembly of C5b-9 on cells of the arterial wall may also induce cell lysis. On the other hand, sublytic assembly of C5b-9 on smooth muscle cells (SMC) and endothelial cells (EC) induces cell activation and proliferation. Analysis of mitogen activated protein kinases (MAPK) pathways induced by C5b-9 in aortic SMC revealed that extracellular signal regulated kinase (ERK) 1, c-jun NH2-terminal kinase (JNK) 1, and p38 MAPK are all activated by C5b-9. ERK1 activity was inhibited by wortmannin suggesting that ERK1 pathway is activated through phosphatidyl inositol -3 (PI 3-) kinase. Sublytic C5b-9 assembly on the plasma membrane was also able to activate Janus kinase (JAK) 1, signal transducer and activator (STAT) 3 and STAT4 in EC. JAK1 but not STAT3 activation induced by C5b-9 is dependent on Gi protein activation. New evidence accumulated during the last decade support the role of complement activation in both initiation and progression of the atherosclerotic lesions. Complement system activation is a major component of the chronic inflammatory process associated with atherosclerosis.

Processes in atherogenesis: complement activation

The complement system consists of a complex group of plasma proteins, which, on activation, lead to a cascade of interactions culminating in the production of a variety of pro-inflammatory molecules. The system also contains cellular receptors for complement fragments produced during activation and regulatory molecules. It is part of the innate immune system representing humoral defence, but in certain circumstances may itself contribute to disease. In the formation of atherosclerotic lesions, there are two outstanding cellular phenomena, monocyte recruitment, with subsequent development of lipid-filled foam cells and smooth muscle cell activation. Subendothelial deposition of low density lipoprotein appears to be an important stimulus in these events and substantial evidence suggests that complement activation may be a link between lipoprotein deposition and subsequent lesion development.

Association of serum C3 levels with the risk of myocardial infarction

PURPOSE

Serum complement and IgA levels have been found to be retrospectively associated with the presence of diffuse atherosclerosis. This study was performed to assess whether serum immunoglobulins and complement components are predictive of future ischemic events.

PATIENTS AND METHODS

The baseline values of IgG, IgA, IgM, C3, and C4 were measured in the sera from a cohort of 860 inhabitants of the town of Brisighella, Italy. They were 444 men and 416 women, mean age 53.9 years (SD 12.4, range 23 to 84), who had not had any ischemic events (myocardial infarction [MI], angina pectoris, stroke, transient ischemic attack, or intermittent claudication) at the time of blood sampling in 1984. Their baseline values for the main recognized risk factors for atherosclerosis were known at baseline and for 4 years of follow-up. Multiple logistic regression analysis was performed for associations between ischemic events and immunologic variables (including serum IgG, IgA, IgM, C3, and C4) and risk factors for atherosclerosis (including age, sex, diastolic blood pressure, cigarette consumption, Quetelet index, total cholesterol, HDL cholesterol, triglycerides and blood glucose).

RESULTS

During follow-up, 57 subjects experienced ischemic events, including 28 cases of coronary heart disease (17 MI and 11 angina pectoris). Of the immunologic variables studied, only serum C3 was found to be independently associated with ischemic events (P < 0.005 for any ischemic events, coronary heart disease, and MI). The population was divided into thirds according to C3 values. The cumulative incidence of MI was 7.1/1,000 in the low third, 10.6/1,000 in the middle third and 40.8/1,000 in the high third (risk ratio for high versus middle plus low = 4.2 after adjustment for age and sex; 95% CI 1.5 to 11.7). A separate analysis for the sexes showed that serum C3 was a particularly powerful predictor of MI in men. Men whose C3 levels were in the top third had a 72.6/1,000 incidence of MI while the incidence in the rest of the male population was 6.2/1,000 (risk ratio 10.7 after adjustment for age; 95% CI 2.3 to 49.0). When similar analyses were performed for angina pectoris, stroke, and intermittent claudication, no significant increase in risk was found to be associated with serum C3.

CONCLUSION

C3 levels measured in sera from male subjects without previous ischemic events are independently associated with the risk of MI.

Association of serum IgA and C4 with severe atherosclerosis

To assess the possible involvement of humoral immunity in diffuse atherosclerosis, IgG, IgA, IgM, C3 and C4 were measured in the sera of 23 atherosclerotic subjects (at least 3 stenoses greater than 75% in the arteries of the limbs and neck, as assessed by panangiography) and of 20 controls (possible stenoses less than 40% documented by arteriography of the aortic arch and epi-aortic branches and "normal" response to exercise stress testing and Doppler ultrasonography of the arteries of the lower limbs). Age (59-69) and sex distribution did not differ significantly in the 2 groups. The following serum concentrations were higher in the atherosclerotic subjects than in the controls: C4 (28.7 +/- 6.5 (1 SD) vs. 23.4 +/- 3.8 mg/dl; P = 0.0013); IgA (323.3 +/- 155.0 vs. 210.3 +/- 87.9 mg/dl; P = 0.0020); and C3 (126.3 +/- 16.9 vs. 111.0 +/- 18.9 mg/dl; P = 0.0109). To assess whether these parameters were independently associated with atherosclerosis, a multiple logistic regression was performed, also including other variables which differed between the atherosclerotic group and the control group with P values less than 0.20 (cigarette smoking, arterial hypertension, body mass index, serum HDL-cholesterol, HDL-cholesterol/total cholesterol ratio, serum triglycerides, IgG and IgM). In multivariate analysis only IgA (P = 0.0012), C4 (P = 0.0072), cigarette smoking (P = 0.0141) and serum triglycerides (P = 0.0177) were independently associated with atherosclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

The complement system in atherosclerosis

Complement is a term referring to a collection of plasma proteins, specific cellular receptors and cell surface regulatory molecules. Activation of the complement system to completion results in the formation of C5b-9 terminal complexes. These complexes have been observed in human atherosclerotic lesions by immunohistochemistry. Although the structure(s) which activate complement in lesions have not been defined, cholesterol and oxysterols exhibit this property in vitro. Endothelial cell damage leads to complement activation and endothelial cells overlying atherosclerotic lesions have been observed to contain C3 and C5b-9 antigens. Cardiac myocytes stain for complement proteins (C3, C4 and C5b-9) following myocardial infarction. Infarct size and extent of inflammatory cell infiltrates are diminished by decomplementation prior to experimentally-induced myocardial ischemia. Following myocardial infarction and ulceration of atherosclerotic lesions in human patients there is an increase in circulating complement activation products and a decrease in the level of native C1 through C4 proteins. Thus, it appears that complement plays a role in atherogenesis and its sequelae. Little is known however, about the pathophysiological effects complement activation products exert on lesion development, for example through modulation of macrophage functions, or how complement activation is regulated in lesions. Implications for complement in atherogenesis are discussed.

Immunogenetic markers in essential hypertension

In a series of 148 patients with essential hypertension, the presence of the atherosclerotic C3-F gene and HLA-antigen frequences were investigated, and the results were compared with those in 62 age and sex matched normotensive controls. The frequency of C3-F was significantly higher in patients (p less than 0.03), as was HLA-B15 in patients with a positive family history of hypertension (p less than 0.05). The presence of the C3-F gene was associated with an increased risk (= 10.2) for coronary heart disease, and B15 was associated with an increased risk (= 6.0) for cerebral events in both familial and non familial cases of hypertension. It is suggested that determination of the C3-F gene and HLA-antigens might be a tool in the identification of hypertensive patients at particular high risk for vascular events, irrespective of BP levels. The study support the suggestions that some genetic factors may act via immunological pathways.

Heparin and atherosclerosis. A review of old and recent findings

This article reviews the experimental and clinical evidence regarding heparin therapy in the prophylaxis of coronary heart disease. The actions of heparin take place at the vascular endothelium where injected heparin concentrates, and within the bloodstream. At the endothelium heparin acts to prevent endothelial injury, prevent thrombin generation, prevent platelet adhesion to endothelium, and to decrease uptake of serum lipoproteins. Within the bloodstream heparin increases lipoprotein lipase activity and reduces the concentration of atherogenic very low-density lipoproteins. The reduction in lipemia enhances oxygen transfer from blood to the tissues, and decreases thrombin or ADP-induced platelet aggregation. Heparin increases the concentration of high-density lipoproteins. It decreases hypercoagulability and inhibits overactivation of serum complement. Heparin reduced atherosclerosis in most studies in cholesterol-fed animals. In human subjects who had a myocardial infarct at least one year before the onset of treatment, long-term intermittent heparin therapy significantly decreased cardiovascular deaths as compared to control groups.

Selective phospholipid adsorption and atherosclerosis

Disaturated (fully saturated) lecithins adsorb onto solid surfaces more readily than lecithins in which one or both fatty acids are unsaturated. If saturated lecithins adsorb to arterial walls as they do to glass and polystyrene surfaces, there may be increased probability of atherosclerosis when the disaturated lecithin content of plasma is elevated. Analyses of lecithins in plasma samples from patients with myocardial infarction, and from patients with premature atherosclerosis but with low concentrations of plasma cholesterol and triglycerides, are consistent with the hypothesis that a high concentration of disaturated lecithin in plasma may be a significant risk factor for atherosclerosis, independent of triglyceride and cholesterol concentrations.

The effect of clofibrate and pyridinol carbamate on the circulating immune complexes and cellular immune response in experimental atherosclerosis

Cholesterol-fed rabbits were treated with clofibrate, pyridinol carbamate and with both drugs simultaneously. The quantity of circulating immune complexes in the sera of the animals was measured weekly and the migration inhibition test was carried out in the 12th week of the experiment. The trend of the changes in the concentration of the immune complexes was rather similar to that of the cellular immune response. Compared with the values obtained in the control animals, in the cholesterol-fed group a markedly higher level of immune complexes and a significant migration inhibition could be detected. The administration of clofibrate or pyridinol carbamate alone had no effect on the concentration of immune complexes. Pyridinol carbamate did not influence the migration inhibition; however, it became similar to the healthy controls in the clofibrate-treated group. Simultaneous treatment with both drugs resulted in a decrease in the quantity of immune complexes and a diminution of the migration inhibition.

Association between coronary heart disease and the C3F-gene in essential hypertension

The occurrence of the C3F allele was investigated in the following three groups: 69 consecutive referred patients with untreated essential hypertension, including borderline hypertension; 70 patients with established and treated essential hypertension, already attending the same outpatient clinic, and 62 age- and sex-matched normotensive healthy subjects without clinical signs of atherosclerosis or familial predisposition to hypertension. In the three groups the C3F allele was found in 38.2%, 29% and 20%, respectively. Among the treated hypertensive patients with C3F gene, 40% had coronary heart disease (CHD) compared to 6.1% among the C3F negative (P less than 0.005), and the relative risk of CHD among the treated hypertensive patients with this allele was found to be 10.2 (P less than 0.002). The C3F gene was present in 72.7% of the treated patients with CHD. In the untreated patients the occurrence of CHD was low, and no differences between C3F positive and negative patients could be demonstrated. No association of the C3F allele with familial predisposition to hypertension was found. This study provides further evidence of a positive association of the C3F allele with atherosclerosis, and it is concluded that this allele in a hypertensive patient might accelerate the atherosclerotic process, with subsequent premature development of vascular complications.

Studies on the occurrence of circulating immune complexes in vascular diseases

The presence of circulating immune complexes was studied in 347 samples of serum from 212 patients with various vascular diseases. Two quantitative methods (complement-consumption assay and C1q-solubility test) were used for the measurement of the concentration of the complexes. Immune complexes were detected in each group of patients tested (coronary arteriosclerosis, myocardial infarction, cerebral artery sclerosis, arteriosclerosis obliterans, phlebothrombosis, pulmonary infarction). A high proportion of positivity was recorded in myocardial infarction (in 43 patients out of the 94 tested) and in arteriosclerosis obliterans (7 out of 11 cases). The possible pathogenic role of the circulating immune complexes is discussed.

Serum glycoproteins in coronary artery disease

Serum glycoprotein levels were compared in two groups of age- and sex-matched patients, 15 with coronary artery disease and 14 normal controls. While total glycoprotein levels were increased in the coronary group, significantly higher levels were found in only five of 16 glycoproteins--C3c haptoglobin, GC-globulin, alpha1-acid glycoprotein, and C3 activator--with no change in 10 other glycoproteins and significant decrease in transferrin. This study demonstrates what appears to be a glycoprotein profile in coronary artery disease and reviews possible interactions of glycoproteins with known risk factors in atherogenesis.