Occurrence of anti-low density lipoprotein antibodies and circulating immune complexes in aged subjects

Chronic exposure to schistosome eggs reduces serum cholesterol but has no effect on atherosclerotic lesion development

Previous studies have shown that people infected with schistosomiasis have lower levels of serum cholesterol than uninfected controls. To better understand the impact of this parasitic infection on serum cholesterol levels and on atherosclerotic lesion development induced by hypercholesterolemia, apolipoprotein E (ApoE)-deficient mice were chronically exposed to the eggs of Schistosoma mansoni over a period of 16 weeks. Total serum cholesterol and low-density lipoprotein (LDL) were reduced in egg-exposed ApoE-deficient mice fed a diet high in cholesterol compared to unexposed controls. However, exposure to eggs had no effect on atherosclerotic lesion size or progression in ApoE-deficient mice. Macrophages isolated from egg-exposed mice had an enhanced ability to take up LDL but not acetylated LDL (acLDL). This study suggests that schistosome eggs alone may alter serum lipid profiles through enhancing LDL uptake by macrophages, but these changes do not ultimately affect atherosclerotic lesion development.

Effect of plasma lipoproteins on natural killer cell activity in the elderly population

BACKGROUND

Natural killer (NK) cells possess spontaneous cytotoxicity against tumors and virus-infected cells to play a major role in immunosurveillance and defense against the development of cancer, as well as bacterial and viral infections. The role of plasma lipoproteins in atherogenesis is well recognized, but the physiological relevance of their immunoregulatory properties is still questioned. In particular, it is unknown whether hypercholesterolemia should be considered a risk factor for diminished immunity in old age.

METHODS

To evaluate effects of plasma lipoprotein levels on immune function, we assessed the relation between plasma lipoprotein profiles and NK cell activity. NK cell activity was assayed by release of (51)Cr from K562 target cells, and concurrent plasma lipoprotein levels were measured in 47 samples of elderly males (mean age +/- SD, 66.6 +/- 1.7 years).

RESULTS

Univariate regression analyses revealed direct relations between NK cell activity and high-density lipoprotein cholesterol (r=.46, p<.001), apolipoprotein (Apo) A-1 (r=.48, p<.001), and Apo A-2 (r=.46, p<.005). In addition, multiple regression analyses showed a direct independent relation between NK cell activity and Apo A-1 (b=0.32+/-0.09 mg/dl, beta=0.48, and p <.001).

CONCLUSION

NK cell activity is related directly to plasma Apo A-1 levels in elderly subjects. The mechanisms of this interaction are unknown, but Apo A-1 contributes to the composition of the antiatherogenic fraction of high-density lipoprotein and could also defend against infectious and malignant disease through a potential for NK cell activity.

Supplementation of diets with the black rice pigment fraction attenuates atherosclerotic plaque formation in apolipoprotein e deficient mice

Apolipoprotein (apo)E-deficient mice were used to study the antiatherogenic effect of black rice pigment fraction (BRF) and the possible mechanisms by which it inhibits atherogenesis. The apoE-deficient mice (n = 45) were randomly divided into three groups and received AIN-93G diet (positive group), AIN-93G with 5 g of black rice pigment fraction/100 g (BRF group) and AIN-93G with 5 g of white rice outer layer fraction/100 g (WRF group) for 16 wk. C57BL/6J mice (n = 15) received AIN-93G and were used as a control group. Blood samples were collected for measurement of lipid concentration, antioxidized LDL antibody and nitric oxide concentration. Livers were extracted for determination of cholesterol concentrations, and aortas were used to determine cholesterol concentrations and inducible nitric oxide synthase protein and mRNA expression. Hearts were used to assess atherosclerotic plaque formation. The apoE-deficient mice fed the black rice pigment fraction diet had 48% (P < 0.01) less atherosclerotic lesion area compared with apoE-deficient mice fed only the AIN-93G diet and 46% (P < 0.01) less lesion area compared with mice fed the white rice outer layer fraction diet. This observation corresponded with significantly (P < 0.05) lower total serum cholesterol, lower liver and aorta cholesterol (P < 0.01) and higher HDL cholesterol (P < 0.05) concentrations and lower (P < 0.05) antioxidized LDL antibody titer in apoE-deficient mice fed the black rice pigment fraction diet compared with positive and WRF groups. Notwithstanding this, mice fed the black rice pigment fraction diet also had lower CD4(+) T lymphocyte expression (P < 0.05) and weaker inducible nitric oxide synthase expression (P < 0.05) compared with mice fed the AIN-93G diet and the white rice outer layer fraction diet, respectively. We concluded that the inhibition of atherosclerotic lesions of the black rice pigment fraction is attributed to the improvement in cholesterol accumulation and reduction in oxidative stress and inflammation.

[Atherosclerosis and anti-oxidized low density lipoprotein antibodies in an elderly population]

BACKGROUND

Antibodies to oxidized LDL (anti-OxLDL) have been found to be associated with atherosclerosis. The aim of the study was to evaluate the relationship between anti-OxLDL antibodies and atherosclerosis in the elderly.

PATIENTS AND METHOD

We studied several risk factors and different clinical manifestations of atherosclerosis in 100 people older than 65 years and in 48 healthy blood donor controls (age range: 20 to 55 years). Anti-OxLDL antibodies were measured by ELISA.

RESULTS

Thirty-one percent of the patients tested positive for anti-OxLDL. This percentage increased in the group of subjects without any risk factor or clinical manifestation (66.6%) as it was the case of the mean optical density (O.D.) value (O.D. = 0.671 vs. 0.357 in our general geriatric population). Most of those with a proven atherosclerotic event tested negative for anti-OxLDL antibodies and a statistically significant difference was shown for those with a calcified aortic arch (P = 0.041, 95% CI 0.15-0.97). Mean value of risk factors and clinical manifestations was 3.50 among anti-OxLDL-negative patients, whereas it was 2.51 in the anti-OxLDL-positive group (P = 0.035, 95% CI 0.07-1.91). Actually, when more adverse effects were present, patients' trend to test negative for anti-OxLDL antibodies was higher. An inverse correlation was observed between anti-OxLDL titers and the relative risk of coronary heart disease (P = 0.020).

CONCLUSIONS

The cause of the decrease of free anti-OxLDL antibodies in situations that lead to an oxidative stress is unknown but it may be explained by the formation of immunocomplexes in an effort to ease the clearance of oxidized substrates.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Scavenger receptors and oxidized low density lipoproteins

Oxidized LDL has been shown to exhibit a number of potentially proatherogenic actions and properties, including receptor-mediated uptake and lipid accumulation within macrophages. It has been postulated that rapid, unregulated uptake of oxidatively modified LDL could account for the transformation of monocyte-derived macrophages to foam cells in atherosclerotic lesions. In support of this hypothesis, oxidized LDL and lipid peroxidation products have been shown to exist in atheromas in vivo. Furthermore, a number of cell membrane proteins that can bind oxidized LDL with high affinity have been identified on the surface of macrophages, endothelial cells and smooth muscle cells. One characteristic that almost all of these 'scavenger receptors' share is the ability to bind with high affinity to a broad spectrum of structurally unrelated ligands. Of all of the different classes of scavenger receptors that have been identified, the scavenger receptor class A type I/II (SR-AI/II) has received the most attention. Studies with macrophages from mice deficient in the gene for SR-AI/II provide direct evidence that a receptor other than the SR-AI/II is responsible for most of the uptake of oxidized LDL in murine macrophages. This article provides an overview of the characterization and functions of the scavenger receptors that have been shown to interact with oxidized LDL, including SR-AI/II, CD36, SR-BI, macrosialin/CD68, LOX-1, and SREC. Isolation and characterization of these and other scavenger receptors has increased our understanding of their role in the uptake of oxidized LDL and the pathogenesis of atherosclerosis.

Oxidized LDL-containing immune complexes induce Fc gamma receptor I-mediated mitogen-activated protein kinase activation in THP-1 macrophages

Our previous studies have shown that Fc gamma receptor (FcgammaR)-mediated uptake of LDL-containing immune complexes (oxLDL-ICs) by human monocyte-derived macrophages leads to not only transformation of macrophages into foam cells but also macrophage activation and release of cytokines. It has been shown that cross-linking of FcgammaR triggers activation of signal transduction pathways that alter gene expression in macrophages. In this study, we determined whether engagement of FcgammaR by oxLDL-ICs leads to activation of mitogen-activated protein (MAP) kinase pathway, a signaling cascade serving many important functions, including the regulation of gene expression, in THP-1 macrophage-like cells. Our results from immunoblotting, using specific anti-phosphorylated MAP kinase antibodies, showed that oxLDL-ICs induced extracellular signal regulated kinase 2 (ERK2) MAP kinase phosphorylation in THP-1 macrophage-like cells in time- and dose-dependent manners. Cholesterol loading before stimulation led to a longer phosphorylation of ERK2. Nuclear translocation of phosphorylated ERK was markedly increased after the stimulation. Moreover, our data showed that oxLDL-IC induction of MAP kinase was prevented by human monomeric IgG1, suggesting that the specific engagement of type I FcgammaR by oxLDL-IC is responsible for the MAP kinase activation. Finally, we showed that human anti-oxLDL autoantibody-containing immune complexes immobilized on type I collagen induced MAP kinase activation in THP-1 cells. These results strongly suggest that oxLDL-IC, which has been detected in atherosclerotic plaques, may play an important role in macrophage activation and atherogenesis.

Receptors for oxidized low density lipoprotein

An increasing body of evidence indicates that oxidized low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis. One of the first biologic actions of oxidized LDL to be identified in vitro was its ability to interact with the 'acetyl LDL receptor' discovered by Goldstein and Brown. Over the past decade, considerable progress has been made in identifying and characterizing cell-surface receptors for oxidized LDL. Most of these receptors are thought to be multifunctional because they interact with several structurally different ligands, and accordingly have been termed 'scavenger receptors'. The objective of this article is to review the most important publications dealing with structure, ligand specificity, regulation, and function of scavenger receptors.

Anti-oxidized low-density lipoprotein antibodies in patients with coronary heart disease and normal healthy volunteers

We have developed a solid-phase enzyme immunoassay for anti-oxidized low-density lipoprotein antibodies. Most sera showed some degree of non-specific binding to plates coated with oxidized low-density lipoprotein and the autoantibodies to oxidized low-density lipoprotein often appeared to have a relatively low affinity. To differentiate between specific and non-specific binding each sample was tested untreated and after absorption with oxidized low-density lipoprotein. The optical densities obtained with dilutions of the absorbed sample were considered to reflect non-specific binding and were subtracted from values obtained with identical dilutions of the unabsorbed sample, to yield corrected values from which the concentrations of anti-oxidized low-density lipoprotein antibody were calculated. Similar absorptions with native low-density lipoprotein and oxidized human serum albumin failed to induce a significant reduction in binding to immobilized oxidized low-density lipoprotein proving that the antibodies measured by this assay are primarily specific for oxidized low-density lipoprotein. We studied sera from two groups of individuals: (1) 33 subjects submitted to coronary angiography and split into two subgroups depending on the degree of coronary stenosis and (2) 64 healthy individuals also split into two subgroups according to lipid levels. Anti-oxidized low-density lipoprotein antibodies were detected both in patients and healthy individuals. Higher levels were detected in patients with moderate coronary disease and hyperlipemic healthy individuals, but the differences between patients and healthy volunteers or between their respective subgroups did not reach statistical significance. Our results suggest that autoantibodies to oxidized low-density lipoprotein are relatively frequent in both symptomatic and asymptomatic individuals.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of lipid peroxidation and antioxidants in oxidative modification of LDL

The purpose of this study is to provide a comprehensive survey on the compositional properties of LDL (e.g., lipid classes, fatty acids, antioxidants) relevant for its susceptibility to oxidation, on the mechanism and kinetics of LDL oxidation, and on the chemical and physico-chemical properties of LDL oxidized by exposure to copper ions. Studies on the occurrence of oxidized LDL in plasma, arteries, and plaques of humans and experimental animals are discussed with particular focus on the use of poly- and monoclonal antibodies for immunochemical demonstration of apolipoprotein B modifications characteristic for lipid peroxidation. Apart from uptake of oxidized LDL by macrophages, studies describing biological effects of heavily or minimally oxidized LDL are only briefly addressed, since several reviews dealing with this subject were recently published. This article is concluded with a section on the role of natural and synthetic antioxidants in protecting LDL against oxidation, as well as some previously unpublished material from our laboratories.

Enhanced uptake and impaired intracellular metabolism of low density lipoprotein complexed with anti-low density lipoprotein antibodies

We have previously shown that incubation of human macrophages with antigen-antibody complexes prepared with native human low density lipoprotein (LDL) and rabbit anti-LDL antibodies (LDL-ICs) results in an increased intracellular accumulation of cholesteryl esters (CEs) and induces a marked increase in the number of LDL receptors. To determine whether the increased CE accumulation in these cells occurred during incubation of the cells with LDL-ICs or whether it was secondary to the uptake of LDL by overexpressed LDL receptors, we incubated human macrophages with LDL-ICs for 22 hours, followed by incubation with native LDL for another 20 hours. We found that about 90% of the accumulated CEs could be accounted for by the first incubation with LDL-ICs. We then proceeded to show that the CEs accumulated during incubation of cells with LDL-ICs was secondary to enhanced uptake and impaired degradation of the LDL complexed with immunoglobulin G (IgG) (LDL-IC), which led to a marked intracellular accumulation of undergraded LDL (levels 199-fold higher than those obtained when the cells were incubated with the same concentration of native LDL not complexed with IgG). We have also shown that not all CEs accumulated in these cells were derived from accumulation of undegraded LDL and that some of them were derived from the reesterification of free cholesterol released during hydrolysis of LDL. LDL-ICs promoted increased CE accumulation and foam cell formation at concentrations as low as 25 micrograms/ml. To determine which receptors were involved in the uptake of LDL-ICs, we performed experiments in which the uptake of LDL-ICs was competitively inhibited with heat-aggregated gamma globulin, native LDL, beta-very low density lipoprotein, or acetylated LDL. Our results demonstrated that LDL-IC uptake was most effectively inhibited by heat-aggregated gamma globulin, partially inhibited by native LDL or by a monoclonal antibody to the LDL receptor, and not inhibited by acetylated LDL or beta-very low density lipoprotein. Thus, we conclude that the majority of LDL-ICs are taken up through Fc gamma receptors. Finally, we investigated whether the increase in LDL receptor expression was dependent on the receptor pathway used by the LDL-ICs, and we were able to demonstrate that when macrophages were incubated with LDL-ICs prepared with F(ab')2 fragments of the anti-LDL antibody, LDL receptor expression was not enhanced. Therefore, we postulate that the uptake of LDL-ICs through Fc gamma receptors results in an uncoupling of the normal regulation of the LDL receptor expression.

Erythrocyte-bound low-density lipoprotein immune complexes lead to cholesteryl ester accumulation in human monocyte-derived macrophages

We have recently shown that incubation of macrophages with insoluble immune complexes (IC) containing low-density lipoproteins (LDL) leads to intracellular accumulation of esterified cholesterol (CE). This accumulation is associated with morphological transformation of the macrophages into "foam cells." In order to better characterize the conditions that lead to the uptake of LDL-IC and CE accumulation on macrophages, we studied the effects of soluble, insoluble, and red blood cell (RBC)-bound LDL-IC on macrophage lipid and lipoprotein metabolism. Using both apoB or IgG [anti-LDL] as the labeled moieties, we observed that the uptake of LDL-IC by human monocyte-derived macrophages (HMM) was markedly enhanced when the IC were adsorbed to RBC. Competition studies with unlabeled heat-aggregated IgG, native LDL, and acetylated LDL demonstrated that LDL-IC were ingested via the Fc receptor of HMM. The uptake of RBC-bound LDL-IC led to a marked intracellular accumulation of cholesteryl esters in HMM (78.4 +/- 1.7 vs 5.5 +/- 0.6 micrograms/mg cell protein; P less than 0.01) which apparently resulted from delayed degradation of the ingested LDL. Thus, it appears that the metabolism of LDL is altered when it is ingested as part of an antigen-antibody complex. These findings suggest that the formation of LDL-IC and their adsorption to red cells may play a significant role in the onset or in the evolution of human atherosclerosis.

Lipoprotein interactions with T cells: an update

The role of plasma lipoproteins in atherogenesis is well recognized but the physiological relevance of their immunoregulatory properties is still questioned. Here Karine Traill and colleagues outline the recent advances that have been made towards unravelling the mechanisms of immunoregulation by lipoproteins in vitro and consider whether any of these mechanisms are operative in vivo. In particular they address the possible detrimental effects of high serum lipoprotein levels on immune function and the question of whether hyperlipidemia (or hypercholesterolemia) should be considered a risk factor for diminished immunity, for example in old age.

Low density lipoprotein metabolism by human macrophages activated with low density lipoprotein immune complexes. A possible mechanism of foam cell formation

Human macrophages play a key role in atherogenesis and are believed to be the progenitors of the cholesteryl ester (CE)-laden foam cells present in early atherosclerotic lesions. Several mechanisms by which macrophages accumulate CE have been recently described. One involves a perturbation in LDL metabolism subsequent to macrophage activation. Thus, we decided to study the effect of macrophage activation by immune complexes on N-LDL metabolism. Initially, LDL-containing immune complexes (LDL-IC) were chosen, since increased plasma levels of these IC have been reported in patients with coronary heart disease. Human macrophages stimulated for 22 h with LDL-IC (250 micrograms/ml) and incubated afterwards for 20 h with 10 micrograms/ml 125I-N-LDL showed a six- and fourfold increase in the accumulation and degradation, respectively, of 125I-N-LDL over the values observed in nonstimulated cells. Scatchard analysis of 125I-N-LDL-specific binding suggests an increase (20-fold) in the number of LDL receptors in macrophages stimulated with LDL-IC. We studied other immune complexes varying in size and antigen composition. Some of the IC were able to stimulate, although to a lesser degree, the uptake of N-LDL by macrophages. Lipoprotein IC are more efficient and have the greatest capacity to increase N-LDL uptake and CE accumulation. We conclude that human macrophage activation by LDL-IC leads to an increase in LDL receptor activity and promotes in vitro foam cell formation.

Analysis of fluorescent low density lipoprotein uptake by lymphocytes. Paradoxical increase in the elderly

Uptake of dioctadecylindocarbocyanine (DiI)-labelled low density lipoproteins (LDL) by peripheral blood lymphocytes (PBL) from young healthy donors has been characterized by flow cytometric analysis. The receptor positive cells were primarily (greater than 70%) T cells. Saturation and competition studies were performed with freshly isolated PBL as well as after a 2-3-day incubation in cholesterol-free medium. In both cases uptake was specific for LDL and not high density lipoprotein. It was also abrogated by chemical modification of apo B, and was not shown by PBL from a patient with familial hypercholesterolemia. DiI-LDL-uptake by cells from elderly donors was compared with that of PBL from young donors. There was a clear increase in uptake by freshly isolated PBL from aged donors which was shown not to stem from underlying "sickness". In contrast, uptake by pre-incubated cells was very variable, in terms of percentage receptor-positive cells and the level of uptake by those cells. However, LDL rescued mevinolin-suppressed mitogen responses from both old and young donors indicating that there is no impairment of uptake or degradation of LDL by PBL from the elderly.

Immunological and microbiological factors in the pathogenesis of atherosclerosis

Among the several pathological events that lead to the formation of an atheromatous lesion, endothelial cell damage, smooth muscle cell proliferation, and foam cell formation, are considered as particularly significant. In this review we summarize data suggesting that immunological and microbial factors may cause, directly or indirectly, these pathological events. Binding of immunocomplexes to endothelial cells, phagocytic cells, platelets, or erythrocytes could be the starting point for a variety of circuits leading to endothelial cell cytotoxicity and to the release of a variety of mediators, including cell proliferative factors. Endothelial cell toxicity could also be induced, directly or indirectly, by endotoxin; however, the possibility that endotoxin and other microbial factors may induce abnormalities in lipid metabolism at the monocyte/macrophage level which eventually result in intracellular accumulation of cholesterol (particularly if cholesterol levels are elevated) is specially attractive as a potential pathogenic mechanism. The various pathologic pathways discussed in this review appear plausible on the basis of our current knowledge and point to the need to investigate the potential role of infectious processes, autoimmune reactions, and administration of antigenic compounds as possible risk factors for the development of atherosclerosis.