Anti-modified LDL antibodies and LDL-containing immune complexes in IDDM patients and healthy controls

An immunohistochemical analysis to validate the rationale behind the enhanced immunogenicity of D-ribosylated low density lipo-protein

Advanced glycation end products (AGEs) are thought to contribute to the abnormal lipoprotein profiles and increased risk of cardiovascular disease in patients with diabetes and renal failure. D-ribose is one of the naturally occurring pentose monosaccharide present in all living cells and is a key component of numerous biomolecules involved in many important metabolic pathways. Formation of D-ribose derived glycated low density lipoprotein (LDL) has been previously demonstrated but no studies have been performed to assess the immune complex deposition in the kidney of rabbits immunized with glycated LDL. In this study, LDL was glycated with D-ribose, and it was further used as an immunogen for immunizing NZW female rabbits. The results showed that female rabbits immunized with D-ribose modified LDL induced antibodies as detected by direct binding and competitive ELISA. The modified LDL was found to be highly immunogenic eliciting high titer immunogen-specific antibodies, while the native forms were moderately immunogenic. The induced antibodies from modified LDL exhibited wide range of heterogeneity in recognizing various proteins and amino acids conformers. Furthermore, our histopathological results illustrated the deposits of immune complex in glomerular basement membrane in rabbits immunized with D-ribose-LDL.

Modified low density lipoprotein and lipoprotein-containing circulating immune complexes as diagnostic and prognostic biomarkers of atherosclerosis and type 1 diabetes macrovascular disease

In atherosclerosis; blood low-density lipoproteins (LDL) are subjected to multiple enzymatic and non-enzymatic modifications that increase their atherogenicity and induce immunogenicity. Modified LDL are capable of inducing vascular inflammation through activation of innate immunity; thus, contributing to the progression of atherogenesis. The immunogenicity of modified LDL results in induction of self-antibodies specific to a certain type of modified LDL. The antibodies react with modified LDL forming circulating immune complexes. Circulating immune complexes exhibit prominent immunomodulatory properties that influence atherosclerotic inflammation. Compared to freely circulating modified LDL; modified LDL associated with the immune complexes have a more robust atherogenic and proinflammatory potential. Various lipid components of the immune complexes may serve not only as diagnostic but also as essential predictive markers of cardiovascular events in atherosclerosis. Accumulating evidence indicates that LDL-containing immune complexes can also serve as biomarker for macrovascular disease in type 1 diabetes.

Immune mechanisms in atherosclerosis, especially in diabetes type 2

Atherosclerosis and ensuing cardiovascular disease (CVD) are major complications of diabetes type 2. Atherosclerosis is a chronic inflammatory condition involving immunocompetent cells of different types present in the lesions. Even though inflammation and immune activation may be more pronounced in atherosclerosis in diabetes type 2, there does not appear to be any major differences between diabetics and non-diabetics. Similar factors are thus implicated in atherosclerosis-associated immune activation in both groups. The cause of immune activation is not known and different mutually non-exclusive possibilities exist. Oxidized and/or enzymatically modified forms of low-density lipoprotein (OxLDL) and dead cells are present in atherosclerotic plaques. OxLDL could play a role, being pro-inflammatory and immunostimulatory as it activates T-cells and is cytotoxic at higher concentrations. Inflammatory phospholipids in OxLDL are implicated, with phosphorylcholine (PC) as one of the exposed antigens. Antibodies against PC (anti-PC) are anti-atherogenic in mouse studies, and anti-PC is negatively associated with development of atherosclerosis and CVD in humans. Bacteria and virus have been discussed as potential causes of immune activation, but it has been difficult to find direct evidence supporting this hypothesis, and antibiotic trials in humans have been negative or inconclusive. Heat shock proteins (HSP) could be one major target for atherogenic immune reactions. More direct causes of plaque rupture include cytokines such as interleukin 1β (IL-1β), tumor necrosis factor (TNF), and also lipid mediators as leukotrienes. In addition, in diabetes, hyperglycemia and oxidative stress appear to accelerate the development of atherosclerosis, one mechanism could be via promotion of immune reactions. To prove that immune reactions are causative of atherosclerosis and CVD, further studies with immune-modulatory treatments are needed.

Pathogenic role of modified LDL antibodies and immune complexes in atherosclerosis

There is strong evidence supporting a key role of the adaptive immune response in atherosclerosis, given that both activated Th cells producing predominantly interferon-γ and oxidized LDL (oxLDL) and the corresponding antibodies have been isolated from atheromatous plaques. Studies carried out using immune complexes (IC) prepared with human LDL and rabbit antibodies have demonstrated proatherogenic and pro-inflammatory properties, mostly dependent on the engagement of Fcγ receptors Ⅰ and Ⅱ in macrophages and macrophage-like cell lines. Following the development of a methodology for isolating modified LDL (mLDL) antibodies from serum and isolated IC, it was confirmed that antibodies reacting with oxLDL and advanced glycation end product-modified LDL are predominantly IgG of subtypes 1 and 3 and that mLDL IC prepared with human reagents possesses pro-inflammatory and proatherogenic properties. In previous studies, LDL separated from isolated IC has been analyzed for its modifications, and the reactivity of antibodies isolated from the same IC with different LDL modifications has been tested. Recently, we obtained strong evidence suggesting that the effects of mLDL IC on phagocytic cells are modulated by the composition of the mLDL. Clinical studies have shown that the level of mLDL in circulating IC is a strong predictor of cardiovascular disease (CVD) and, in diabetic patients, other significant complications, such as nephropathy and retinopathy. In conclusion, there is convincing ex vivo and clinical data supporting the hypothesis that, in humans, the humoral immune response to mLDL is pathogenic rather than protective.

Immunoassay of modified forms of human low density lipoprotein in isolated circulating immune complexes

Modified lipoproteins are able to induce inflammatory reactions through innate immunity pathways and are immunogenic, leading to an autoimmune response that results in the formation of proinflammatory immune complexes. The measurement of circulating oxidized lipoproteins and corresponding antibodies has, therefore, been proposed as an approach to assess the risk for complications in patients with diabetes and for the risk of cardiovascular disease in the general population. However, the majority of modified low density lipoprotein (LDL) in the peripheral circulation exists in the form of immune complexes, and this is a significant obstacle for the measurement of modified LDL and the corresponding antibodies. In this manuscript, we describe in detail the methodology developed by our group for isolation and fractionation of circulating immune complexes (IC), allowing the accurate assay of different LDL modifications. This approach has resulted in several studies showing that the levels of modified LDL are risk factors with a stronger association to diabetic retinopathy, nephropathy, and macrovascular disease. Ongoing research is focused on evaluating the predictive power of modified LDL levels for the development or progression of atherosclerotic cardiovascular disease in other patient populations and on the simplification of the assay to make it more applicable to diagnostic laboratories.

High levels of oxidized LDL in circulating immune complexes are associated with increased odds of developing abnormal albuminuria in Type 1 diabetes

BACKGROUND

Modified low-density lipoprotein (LDL) immune complexes (IC) have proinflammatory properties and play a role in albuminuria development.

METHODS

We measured oxidized LDL (oxLDL) and advanced glycation end-product (AGE)-LDL in IC isolated from sera of Type 1 diabetic subjects followed for 14-20 years and studied their association with abnormal albuminuria. Patients with albumin excretion rates (AER)<40 mg/24 h at baseline and follow-up (n=302) were deemed resistant to developing abnormal albuminuria. Patients with AER<40 mg/24 h at baseline whose AER levels progressed to >40 mg/24 h were considered prone to abnormal albuminuria (n=185), those who progress to AER>299 mg/24 h were considered as having macroalbuminuria (n=57). The odds of developing abnormal albuminuria were estimated by logistic regression based on natural log-transformed levels of oxLDL and AGE-LDL in IC and stratified by baseline AER decile.

RESULTS

OxLDL and AGE-LDL were significantly higher in IC isolated from patients progressing to abnormal albuminuria. In unadjusted conditional logistic analysis, an increase of 1 SD in oxLDL and AGE-LDL levels in IC significantly increased the odds ratio (OR) for development of macroalbuminuria, respectively, by a factor of 2.5 and 1.8 (P<0.001, P=0.008). The increased odds of developing macroalbuminuria remained significant when adjusted for treatment group, diabetes duration, retinopathy, baseline hemoglobin A1c and LDL (OR=2.5 and 1.8, respectively, P<0.01).

CONCLUSION

Higher levels of oxLDL and AGE-LDL in circulating IC were associated with increased odds to develop abnormal albuminuria.

LDL-containing immune complexes in the DCCT/EDIC cohort: associations with lipoprotein subclasses

Immune complexes containing modified LDL (LDL-IC) and NMR-determined Total LDL particle concentrations are significantly associated with intima-media thickness (IMT). We analyzed the associations between concentrations of NMR-determined lipoprotein subclasses and LDL-IC in the DCCT/EDIC cohort. LDL-IC concentrations in women and men of the DCCT/EDIC cohort did not differ significantly and were positively associated with Total LDL particle concentrations in men and women (r=0.34, r=0.32, respectively; P<.01) and with Small LDL concentration (r=0.22, r=0.13, respectively; P<.01). In women, Large LDL concentrations were also associated with LDL-IC (r=0.20, P<.01) while in men, the association was more modest (r=0.11, P<.05). Thus, both Small and Large LDL are associated with LDL-IC formation. Based on the results from statistical mediation analyses, we concluded that plasma concentrations of LDL-IC may provide a physiological link between the statistically significant association of Total LDL particle concentration with carotid artery IMT in subjects with Type 1 diabetes. Furthermore, after adjusting for conventional risk factors, there was a decrease in LDL-IC concentration even in the presence of high Total LDL particle concentrations in those women with high concentrations of Large HDL, but the association was not evident in men. This suggests that the associations between Large HDL and Total LDL particle concentrations, and their associations with LDL-IC levels, differ by gender and suggest that LDL-IC partially mediate the contribution of Total LDL particle concentration to increased carotid IMT in diabetic men.

Levels of oxidized LDL and advanced glycation end products-modified LDL in circulating immune complexes are strongly associated with increased levels of carotid intima-media thickness and its progression in type 1 diabetes

OBJECTIVE

High cholesterol levels in circulating immune complexes (IC), surrogate markers of modified LDL, are associated with increased carotid intima-media thickness (IMT) and cardiovascular events in type 1 diabetes. Different modifications of LDL are involved in IC formation, but which of these are predictive of vascular events is not known. Therefore, we measured oxidized LDL (oxLDL), advanced glycation end products-modified LDL (AGE-LDL), and malondialdehyde-modified LDL (MDA-LDL) in IC and determined their relationship with increased carotid IMT and compared the strength of the association with that observed with conventional risk factors.

RESEARCH DESIGN AND METHODS

Levels of oxLDL, AGE-LDL, and MDA-LDL were measured in circulating IC isolated from sera of 479 patients of the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) cohort, collected at baseline. Internal and common carotid IMT were measured 8 and 14 years later by DCCT/EDIC.

RESULTS

OxLDL, AGE-LDL, and MDA-LDL levels in circulating IC were significantly correlated with diabetes duration, BMI, and lipid and blood pressure, but not with age. Multivariate logistic regression models indicated that individuals in the highest versus lowest quartile of oxLDL and AGE-LDL in IC had a 6.11-fold [confidence interval (CI) 2.51-14.8] and a 6.4-fold (CI 2.53-16.2) increase in the odds of having high carotid IMT, respectively, after adjusting for conventional risk factors. Parallel analyses resulted in odds ratios of 2.62 (CI 1.24, 5.55) for LDL-C, 1.45 (CI 0.69, 3.03) for diastolic blood pressure, and 2.33 (CI 1.09, 4.99) for A1C.

CONCLUSIONS

OxLDL and AGE-LDL in circulating IC were significantly associated with progression and increased levels of carotid IMT in type 1 diabetes.

Oxidized LDL immune complexes and coronary artery calcification in type 1 diabetes

OBJECTIVE

Oxidized LDL (oxLDL) and oxLDL antibodies form immune complexes (IC) that reflect essential components in the development of atherosclerosis: dyslipidemia, oxidative stress and induction of a pro-inflammatory humoral immune response. We measured oxLDL in IC (oxLDL-IC) isolated from patients with type 1 diabetes to assess the relationship between oxLDL-IC and coronary artery calcification (CAC).

METHODS

OxLDL was measured in IC isolated from baseline samples from a subgroup of 476 patients of the Diabetes Control and Complications Trial (DCCT). CAC was determined by computed tomography (CT) 11-20 years later. Multivariable log-binomial regression models were used to estimate the risk ratios associated with having a high CAC score with an increase of 1 standard deviation (SD) of the natural logarithm of oxLDL-IC.

RESULTS

Multivariable regression models indicate that a 1 SD increase in the levels of oxLDL-IC was associated with a 37% increase in the risk of having high CAC score (RR=1.36; 95% CI: 1.12-1.67) at follow-up after adjustment for DCCT treatment group, retinopathy/AER groups, gender and CT scanning site as well as baseline age, diabetes duration and HbA1C %. Further adjustment for smoking status, blood pressure and LDL resulted in a risk ratio of 1.23 (95% CI: 1.01-1.50) which remained statistically significant indicating that baseline oxLDL-IC is independently associated with the development of CAC.

DISCUSSION

Increased levels of oxLDL-IC are associated with the development of coronary calcification. This observation reinforces previously published clinical and experimental data demonstrating that oxLD-IC has pro-inflammatory and proatherogenic properties.

The Role of Immune Complexes in Atherogenesis

Atherosclerosis is now recognized as a chronic inflammatory disease and is characterized by features of inflammation at all stages of its development. It also appears to display elements of autoimmunity, and several autoantibodies including those directed against oxidized low-density lipoprotein (ox-LDL) and heat shock proteins (Hsps) have been identified in atherosclerosis. Immune complexes (ICs) may form between these antigens and autoantibodies and via Fc receptor signaling and complement activation may modulate the inflammation in atherosclerosis. Antibody isotype may direct the role that ICs play in atherogenesis, immunoglobulin G (IgG) being potentially pro-atherogenic and immunoglobulin M (IgM) playing a protective role. Therapeutic options targeting complement activation and those which are potentially Fc-receptor mediated have been investigated in animal models, though targeting Fc receptor signaling is an area that needs further investigation.

Clinical significance of the humoral immune response to modified LDL

Human low density lipoprotein (LDL) undergoes oxidation and glycation in vivo. By themselves, oxidized LDL (oxLDL) and AGE-LDL have proinflammatory properties and are considered atherogenic. But the atherogenicity of these lipoproteins are significantly increased as a consequence of the formation of immune complexes (IC) involving specific autoantibodies. OxLDL and AGE antibodies have been shown to be predominantly of the IgG1 and IgG3 isotypes. OxLDL antibodies are able to activate the complement system by the classical pathway and to induce FcR-mediated phagocytosis. In vitro and ex vivo studies performed with modified LDL-IC have proven their pro-inflammatory and atherogenic properties. Clinical studies have demonstrated that the levels of circulating modified LDL-IC correlate with parameters indicative of cardiovascular and renal disease in diabetic patients and other patient populations. The possibility that spontaneously formed or induced modified LDL antibodies (particularly IgM oxLDL antibodies) may have a protective effect has been suggested, but the data is unclear and needs to be further investigated.

Distribution of IgM and IgG antibodies to oxidized LDL in immune complexes isolated from patients with type 1 diabetes and its relationship with nephropathy

Modified lipoproteins are immunogenic and play a key pathogenic role in vascular disease. Antibodies to oxidized LDL (oxLDL) are mostly of the pro-inflammatory IgG1 and IgG3 isotypes. We measured IgG and IgM oxLDL antibodies in immune complexes (IC) isolated from 36 patients with type 1 diabetes using a nested case control design. IgG antibodies predominated over IgM antibodies by an 8:1 ratio. IgG antibody concentrations were higher in the nephropathy cases compared to controls (p = 0.09), but no significant difference was observed because of two patients included in the study who had end-stage renal disease (creatinine > 5 mg/dL and glomerular filtration rate (GFR) less than 17 mL/min). After eliminating these patients from the analysis, significant positive associations of IgG antibody concentration with serum creatinine and albumin excretion rate were observed. Similarly, a negative correlation with estimated glomerular filtration rate was observed in this subsample of 34 patients. Differences in IgM antibody concentrations by nephropathy classification were not supported by the data. In conclusion, the predominance of pro-inflammatory IgG oxLDL antibodies is associated with existence of diabetic nephropathy, and a protective role of IgM antibodies could not be demonstrated.

Atherogenesis and the humoral immune response to modified lipoproteins

Modified forms of LDL are immunogenic and activate both cell-mediated and humoral immune responses. Both types of responses are pro-inflammatory and are probably primary players in the perpetuation of the chronic inflammatory reaction characteristic of atherosclerosis. The immunologic response to modified LDL can be directed to MHC-II-associated peptides in the case of T helper cells, and to a variety of epitopes-modified lysine groups, modified phospholipids, proteins that become associated with oxidized LDL (such as beta2GP1)--in the case of B cell responses. T cell activation is likely to play a major role through cross-activation of macrophages. Humoral responses to modified LDL are pathogenic as a consequence of the formation of antigen-antibody complexes containing modified LDL and IgG antibodies. Those immune complexes induce cholesterol ester accumulation in macrophages and macrophage-like cells, and induce the release of pro-inflammatory cytokines, chemokines, oxygen active radicals, and matrix metalloproteinases from those cells. There is no conclusive evidence supporting a protective role for IgM antibodies in humans, possibly because autoantibodies to modified lipoproteins are predominantly of the IgG isotype.

Immune complexes containing modified lipoproteins are related to the progression of internal carotid intima-media thickness in patients with type 1 diabetes

Modified lipoproteins induce autoimmune responses including the synthesis of autoantibodies with pro-inflammatory characteristics. Circulating modified lipoprotein autoantibodies combine with circulating antigens and form immune complexes (IC). We now report the results of a study investigating the role of circulating IC containing modified lipoproteins in the progression of carotid intima-media thickness (IMT) in patients enrolled in the Epidemiology of Diabetes Interventions and Complications (EDIC) Trial, a follow-up study of the Diabetes Control and Complications Trial (DCCT). This cohort includes 1229 patients with type 1 diabetes in whom B-mode ultrasonography of internal and common carotid arteries was performed in 1994-1996 and in 1998-2000. Conventional CHD risk factors, antibodies against modified forms of LDL and modified lipoprotein IC were determined in 1050 of these patients from blood collected in 1996-1998. Cholesterol and apolipoprotein B content of IC (surrogate markers of modified ApoB-rich lipoproteins) were significantly higher in patients who showed progression of the internal carotid IMT than in those showing no progression, regression or mild progression. Multivariate linear and logistic regression modeling using conventional and non-conventional risk factors showed that the cholesterol content of IC was a significant positive predictor of internal carotid IMT progression. In conclusion these data demonstrate that increased levels of modified ApoB-rich IC are associated with increased progression of internal carotid IMT in the DCCT/EDIC cohort of type 1 diabetes.

Alteration of the copy number of mitochondrial DNA in leukocytes of patients with hyperlipidemia

Lipid metabolism in leukocytes may be disturbed by mitochondrial dysfunction caused by depletion of mitochondrial DNA (mtDNA) in response to an increase of oxidative stress in blood circulation. It is possible that alteration in mtDNA copy number of the leukocyte is involved in the impairment of the scavenging of oxidatively modified plasma proteins such as oxidized low-density lipoprotein (oxLDL). To test this hypothesis, we recruited 91 healthy subjects and 63 patients with hyperlipidemia (LDL >130 mg/dL) for this study. The copy number of mtDNA in the leukocyte and the titer of oxLDL IgG autoantibody (oLAB) were determined as indices of the oxidative stress response of immune cells. The results revealed a significant higher level of plasma oxLDL, lower titer of oLAB, and decreased copy number of mtDNA in patients with hyperlipidemia (P <0.05). In the analysis of partial correlations under age control, we found that an increase in the copy number of mtDNA was positively correlated with an increase in the level of oLAB (P <0.005, r = 0.3002) and a decrease in the oxLDL level (P <0.05, r = -0.2654) in healthy subjects but not in patients. Based on the results obtained from this case-control study, we conclude that the increase of mtDNA copy number might provide the leukocyte an increased capability of scavenging oxLDL, possibly by enhanced generation of oLAB in healthy subjects, but not in hyperlipidemic patients who had lower mtDNA copy numbers in their leukocytes. Taken together, these findings suggest that an alteration of mtDNA copy number in the leukocyte may be one of the risk factors for hyperlipidemia.

The immunogenicity of modified lipoproteins

The immunogenicity of modified low-density lipoprotein (mLDL) has been demonstrated both in laboratory animals and humans. Circulating human mLDL antibodies, purified by affinity chromatography, are predominantly of the IgG isotype, subclasses 1 and 3. The purified antibodies react with malondialdehyde-lysine and carboxymethyl-lysine epitopes, but also recognize minimally modified forms of LDL that do not contain significant amounts of those two epitopes. The quantitative assays of mLDL and mLDL antibodies in serum samples by enzymoimmunoassay (EIA) are unreliable owing to the interference of preformed circulating immune complexes (CICs). Isolation of CICs by precipitation with low concentrations of polyethylene glycol followed by analysis of antigens and antibodies contained in the precipitates is a technically complex approach, but one that yields valuable data. With this approach we have confirmed that the IgG antibodies involved in IC formation belong to the proinflammatory IgG1 and IgG3 isotypes, have a higher avidity than those that remain unbound in the supernatant after CIC precipitation, and are of higher avidity in diabetic patients with macroalbuminuria than in those with normal albuminuria. We have also developed capture assays for different forms of mLDL. These assays have shown a significant enrichment in mLDL of the precipitated ICs. The enrichment is also more pronounced in the CICs obtained from diabetic patients with macroalbuminuria. In conclusion, isolation and characterization of LDL-ICs appears to yield information of significant value that is not derived from other approaches to measure LDL modifications and their corresponding antibodies in humans.

Markers of endothelial dysfunction in the prediction of coronary artery disease in type 1 diabetes. The Pittsburgh Epidemiology of Diabetes Complications Study

Low-density lipoprotein (LDL) oxidation, the immune response it provokes, and lipoprotein subclasses measured by nuclear magnetic resonance (NMR) spectroscopy have explained some of the enhanced coronary artery disease (CAD) risks in Type 1 diabetes. We examined whether cellular adhesion molecules further improve CAD prediction. Participants were identified from the Epidemiology of Diabetes Complications (EDC) cohort, a 10-year prospective study of childhood-onset Type 1 diabetes. Mean age at baseline was 28 years, and diabetes duration was 19 years. CAD incidence was determined by EDC physician-diagnosed angina, confirmed myocardial infarction (MI), stenosis > or =50%, ischemic ECG, or revascularization. Cases were gender, age, and diabetes duration (+/-3 years) matched with the controls. The samples and risk factors used in the analyses were identified from the earliest exam prior to incidence in the cases. Sixty cases and 72 controls (including 43 pairs) had complete information on all covariates. Cox proportional hazard models with backward elimination and conditional logistic regression (for paired analyses) were conducted. Separate analyses were conducted to examine whether E-selectin related differently to soft (ischemic ECG and angina; n=68) or hard (revascularization, MI, and fatal events; n=37) CAD endpoints. Mean E-selectin concentration was elevated among cases (P=.0009) compared to controls. Adjusting for previously established CAD risk factors, E-selectin remained an independent predictor of CAD (HR=1.07, 95% Cl=1.01-1.15). Multivariable models confirmed the importance of E-selectin as a risk factor of soft (HR=1.13, 95% Cl=1.03-1.24; HRs are per standard deviation increase) but not hard CAD. Study results suggest that E-selectin may enhance CAD prediction beyond traditional risk factors or markers of oxidative stress in Type 1 diabetes.

Development of capture assays for different modifications of human low-density lipoprotein

Antibodies to malondialdehyde (MDA)-modified low-density lipoprotein (LDL), copper-oxidized LDL (oxLDL), Nepsilon(carboxymethyl) lysine (CML)-modified LDL, and advanced glycosylation end product (AGE)-modified LDL were obtained by immunization of rabbits with in vitro-modified human LDL preparations. After absorption of apolipoprotein B (ApoB) antibodies, we obtained antibodies specific for each modified lipoprotein with unique patterns of reactivity. MDA-LDL antibodies reacted strongly with MDA-LDL and also with oxLDL. CML-LDL antibodies reacted strongly with CML-LDL and also AGE-LDL. oxLDL antibodies reacted with oxLDL but not with MDA-LDL, and AGE-LDL antibodies reacted with AGE-LDL but not with CML-LDL. Capture assays were set with each antiserum, and we tested their ability to capture ApoB-containing lipoproteins isolated from precipitated immune complexes (IC) and from the supernatants remaining after IC precipitation (free lipoproteins). All antibodies captured lipoproteins contained in IC more effectively than free lipoproteins. Analysis of lipoproteins in IC by gas chromatography-mass spectrometry showed that they contained MDA-LDL and CML-LDL in significantly higher concentrations than free lipoproteins. A significant correlation (r=0.706, P<0.019) was obtained between the MDA concentrations determined by chemical analysis and by the capture assay of lipoproteins present in IC. In conclusion, we have developed capture assays for different LDL modifications in human ApoB/E lipoprotein-rich fractions isolated from precipitated IC. This approach obviates the interference of IC in previously reported modified LDL assays and allows determination of the degree of modification of LDL with greater accuracy.

Autoantibodies against oxidized LDLs and atherosclerosis in type 2 diabetes

OBJECTIVE

The aim of our study was to examine, in type 2 diabetic patients, the relationship between autoantibodies against oxidatively modified LDL (oxLDL Ab) and two indexes of atherosclerosis, intimal-medial thickness of the common carotid artery (CCA-IMT), which reflects early atherosclerosis, and the ankle-brachial index (ABI), which reflects advanced atherosclerosis.

RESEARCH DESIGN AND METHODS

Thirty newly diagnosed type 2 diabetic patients, 30 type 2 diabetic patients with long duration of disease, and 56 control subjects were studied. To detect oxLDL Ab, the ImmunoLisa Anti-oxLDL Antibody ELISA was used. ABI was estimated at rest by strain-gauge plethysmography. Carotid B-mode imaging was performed on a high-resolution imaging system (ATL HDI 5000).

RESULTS

In patients with long duration of disease, IgG oxLDL Ab were significantly higher and ABI significantly lower compared with the other two groups. We found a correlation between IgG oxLDL Ab and CCA-IMT in all diabetic patients. A significant inverse correlation between IgG oxLDL Ab and ABI only in patients with long duration of disease was seen, demonstrating a close relationship between these autoantibodies and advanced atherosclerosis.

CONCLUSIONS

IgG OxLDL Ab may be markers of the advanced phase of the atherosclerotic process and the response of the immunological system to the oxLDL, which are present within atherosclerotic lesions.

Role of simvastatin as an immunomodulator in type 2 diabetes

OBJECTIVE

To test the hypothesis that simvastatin reduces the levels of circulating immune complexes (ICs) containing modified lipoproteins (mLDLs; mLDL-ICs), which may represent an additional mechanism for the reduced incidence of cardiovascular events in patients treated with simvastatin.

RESEARCH DESIGN AND METHODS

A total of 26 patients with type 2 diabetes and triglyceride levels <400 mg/dl who were not receiving lipid-lowering medications or CYP 3A4 inhibitors were enrolled in the study. After 2 weeks on a lipid-lowering diet and exercise, the patients were started on simvastatin 20 mg/day. The dose of simvastatin was adjusted until the levels of LDL cholesterol were < or =100 mg/dl. Blood was collected at baseline, 3 and 6 months after LDL cholesterol levels reached target, and 3 months after stopping simvastatin to measure advanced glycation end product LDL and oxidized LDL antibodies, mLDL-IC, intracellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), E-selectin, metalloproteinase-1 (MMP-1), lipid profile, liver function tests, creatinine kinase, glucose, and HbA(1c).

RESULTS

Twenty-one patients completed the study. Their HbA(1c) remained within 1% of baseline levels. There was a highly significant decrease in mLDL-IC levels after 3 and 6 months of treatment with simvastatin, with a return to near baseline levels after discontinuation.

CONCLUSIONS

Simvastatin significantly reduced the concentration of mLDL-IC, probably as a consequence of both a decrease in the formation of mLDL and to a reduction in the titers of mLDL antibodies. This effect is likely to have a beneficial impact in the inflammatory reaction associated with atherosclerosis.

Circulating oxidized LDL forms complexes with beta2-glycoprotein I: implication as an atherogenic autoantigen

Beta2-glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies (Abs, aPL) present in patients with antiphospholipid syndrome (APS). We recently reported (J. Lipid Res., 42: 697, 2001; J. Lipid Res., 43: 1486, 2002) that beta2-GPI specifically binds to Cu2+-oxidized LDL (oxLDL) and that the beta2-GPI ligands are omega-carboxylated 7-ketocholesteryl esters. In the present study, we demonstrate that oxLDL forms stable and nondissociable complexes with beta2-GPI in serum, and that high serum levels of the complexes are associated with arterial thrombosis in APS. A conjugated ketone function at the 7-position of cholesterol as well as the omega-carboxyl function of the beta2-GPI ligands was necessary for beta2-GPI binding. The ligand-mediated noncovalent interaction of beta2-GPI and oxLDL undergoes a temperature- and time-dependent conversion to much more stable but readily dissociable complexes in vitro at neutral pH. In contrast, stable and nondissociable beta2-GPI-oxLDL complexes were frequently detected in sera from patients with APS and/or systemic lupus erythematodes. Both the presence of beta2-GPI-oxLDL complexes and IgG Abs recognizing these complexes were strongly associated with arterial thrombosis. Further, these same Abs correlated with IgG immune complexes containing beta2-GPI or LDL. Thus, the beta2-GPI-oxLDL complexes acting as an autoantigen are closely associated with autoimmune-mediated atherogenesis.

Autoimmune response to advanced glycosylation end-products of human LDL

Advanced glycosylation end-products (AGEs) are believed to play a significant role in the development of vascular complications in diabetic patients. One such product, AGE-LDL, has been shown to be immunogenic. In this report, we describe the isolation and characterization of human AGE-LDL antibodies from the sera of seven patients with Type 1 diabetes by affinity chromatography using an immobilized AGE-LDL preparation that contained primarily the AGE N epsilon (carboxymethyl)lysine (CML, 14.6 mmol/mol lysine), and smaller amounts of N epsilon (carboxyethyl)lysine (CEL, 2.7 mmol/mol lysine). The isolated antibodies were predominantly IgG of subclasses 1 and 3, and considered proinflammatory because of their ability to promote Fc gamma R-mediated phagocytosis and to activate complement. We determined dissociation constants (Kd) for the purified antibodies. The average Kd values (4.76 +/- 2.52 x 10(-9) mol/l) indicated that AGE-LDL antibodies are of higher avidity than oxidized LDL antibodies measured previously (Kd = 1.53 +/- 07 x 10(-8) ml/l), but of lower avidity than rabbit polyclonal LDL antibodies (Kd = 9.34 x 10(-11)). Analysis of the apolipoprotein B-rich lipoproteins isolated with polyethylene glycol-precipitated antigen-antibody complexes from the same patients showed the presence of both CML and CEL, thus confirming that these two modifications are recognized by human autoantibodies. A comparative study of the reactivity of purified AGE-LDL antibodies with CML-LDL and CML-serum albumin showed no cross-reactivity.

Proatherogenic and proinflammatory properties of immune complexes prepared with purified human oxLDL antibodies and human oxLDL

Immune complexes (IC) prepared with human low density lipoprotein (LDL) and rabbit LDL antibodies induce foam cell transformation of human macrophages and activate the release of proinflammatory mediators by human macrophages and THP-1 cells. Because the affinity of human oxidized LDL (oxLDL) antibodies is lower than that of rabbit antibodies, IC formed with human antibodies could have limited pathogenic potential. Immune complexes prepared with human oxidized LDL (oxLDL) and purified human oxLDL antibodies (predominantly of the IgG1 and IgG3 isotypes) were presented to THP-1 cells using two protocols previously described in studies of the properties of LDL-IC prepared with rabbit antibodies. OxLDL/human oxLDL antibody IC immobilized by adsorption to red blood cells (RBC) induced the release of significantly higher levels of TNF from THP-1 cells (872-313 pg/ml) than oxLDL adsorbed to RBC (461-75.6 pg/ml) and caused a higher degree of cholesterol ester accumulation in the same cells (5.4-0.77 in cells incubated with IC-coated RBC vs 1.99-1.16 in oxLDL-coated RBC). Insoluble IC prepared with oxLDL/human oxLDL antibody were even more effective in promoting intracellular accumulation of cholesterol in THP-1 cells (total cholesterol = 53.8-13.5 and cholesterol esters = 24.0-7.2 mg/l in THP-1 cells incubated with insoluble IC (200 micrograms) vs total cholesterol = 32.4-8.2 and cholesterol esters = 7.7 +/- 2.8 micrograms/l in THP-1 cells incubated with an identical concentration of oxLDL) and also induced the release of TNF. Thus we have demonstrated that IC prepared with human oxLDL and human oxLDL antibodies have the same atherogenic and proinflammatory properties as IC prepared with human LDL and rabbit LDL antibodies. This strongly supports the concept that modified LDL-IC present in circulation and/or tissues play an important pathogenic role in arteriosclerosis.

[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.

Serum heat shock protein and anti-heat shock protein antibody levels in aging

We have previously reported the presence of Hsp60 and Hsp70 in the peripheral circulation of normal individuals. Given that the capacity to generate stress proteins declines with age, this study measured Hsp60 and Hsp70 levels in the sera of 60 individuals aged between 20 and 96 years. Levels of anti-human Hsp60, anti-human Hsp70 and anti-mycobacterial Hsp65 antibody were also measured. Senieur-approximated elderly subjects were well and randomly selected from the Belfast Elderly Longitudinal Free-living Aging STudy (BELFAST). Samples from younger individuals were obtained from the Northern Ireland Blood Transfusion Service. Hsp60, anti-Hsp60, anti-Hsp70 and anti-mycobacterial Hsp65 antibodies were detected in all samples, whereas Hsp70 was detectable in only 46 of the samples analysed (77%). Regression analysis revealed a progressive decline in Hsp60 (759ng/ml < 40 years; 294ng/ml > or = 90 years) and Hsp70 (400ng/ml < 40 years; 20ng/ml > or = 90 years) levels with age whereas no relationship was apparent for anti-Hsp60 and Hsp65 antibody levels. Hsp70 antibody levels tended to increase with age (115U/ml < 40 years; 191U/ml > or = 90 years). This study in Senieur-approximated subjects demonstrates an apparent decrease in Hsp60 and Hsp70 with increasing age that does not appear to be related to anti-heat shock protein antibody status. These findings support in vitro work that demonstrates an age-related reduced ability to respond to stress. Further studies are required to understand the basis for declining serum Hsp60 and Hsp70 levels in aging and to elucidate their origin and role in the maintenance of homeostasis and resistance to environmental challenges.

Fc-gamma receptor cross-linking by immune complexes induces matrix metalloproteinase-1 in U937 cells via mitogen-activated protein kinase

Matrix metalloproteinase-1 (MMP-1) secreted by macrophages potentially contributes to plaque rupture. Because large quantities of immunoglobulin G and ICs (ICs), including low density lipoprotein-containing ICs (LDL-ICs), are present in atherosclerotic lesions, we examined the effect of LDL-ICs on macrophage MMP-1 expression. With the use of ICs prepared with human LDL and rabbit anti-LDL antiserum, our enzyme-linked immunosorbent assays and Northern blots showed that MMP-1 secretion and expression by U937 histiocytes were induced by LDL-ICs. Furthermore, our results showed that blocking of Fc-gamma receptor I and II (FcgammaRI and FcgammaRII) inhibited 70% and 55%, respectively, of the LDL-IC-induced secretion of MMP-1. Finally, our data showed that both PD98059, an inhibitor of the mitogen-activated protein kinase pathway, and Ro-31-2880, an inhibitor of protein kinase C, inhibited LDL-IC-stimulated MMP-1 secretion in a dose-dependent manner, with 96% and 95% inhibition, respectively, at the respective doses of 50 micromol/L and 80 nmol/L. In conclusion, this study demonstrated for the first time that LDL-ICs induce macrophage MMP-1 secretion by cocross-linking FcgammaRI and FcgammaRII and triggering a protein kinase C-dependent mitogen-activated protein kinase pathway. These results suggest that LDL-ICs and other ICs localized in atherosclerotic plaques may be potent stimulators for macrophage MMP-1 expression and may contribute to plaque rupture and acute coronary events.

The preparation of copper-oxidized LDL for the measurement of oxidized LDL antibodies by EIA

In the present study we try to define the optimal conditions for preparation of copper-oxidized low-density lipoprotein (oxLDL) to be used for the assay of oxLDL antibodies by enzyme immunoassay (EIA). Oxidation of LDL was monitored by measuring the formation of conjugated dienes at 234 nm and the generation of fluorescent products with emission at 430 nm when excitation is performed at 360 nm. The generation of immunogenic epitopes was evaluated by testing the reactivity of aliquots collected at different times during the oxidation process with human sera with high oxLDL antibody levels and with a purified human oxLDL antibody. The values of fluorescence emission at 430 nm correlated best with reactivity with oxLDL antibodies; strong reactivity was usually associated with values greater than 1.1 U. The time needed for fluorescence emission to reach maximum levels varied between 6 and 14 h for most LDL, but it was considerably longer in a few LDL preparations. The maximal reactivity of oxLDL with oxLDL antibodies was observed when the LDL oxidation reaction was stopped 4 or more hours after the fluorescence readings reached their peak. At this stage of the oxidation reaction, apolipoprotein B fragmentation and aggregation were observed as shown by Western blot analysis. The CV for 13 EIA runs of two reference oxLDL antibodies reacting with four different pools of standardized oxLDL prepared according to the stated guidelines was 14.5 and 3.9%, confirming the reproducibility of our oxidation conditions.

Immunochemical characterization of purified human oxidized low-density lipoprotein antibodies

The goal of this study was to characterize the isotypes and reactivity of human autoantibodies to copper oxidized LDL (oxLDL). Forty-six purified oxLDL antibodies contained immunoglobulins of the three major isotypes, with a predominance of IgG, subclasses 1 and 3. These IgG isotypes are known to interact with FcRgammaI and to activate the complement system and thus are potentially able to activate macrophages and cause foam cell formation. The same purified antibodies were tested for cross-reactivity with malondialdehyde (MDA)-, glycated (Glyc)-, and native (n)LDL and cardiolipin. Absorption with oxLDL resulted in a decrease of reactivity of 77.2 +/- 4.7%. Absorption with MDA-LDL resulted in a wider range of reduction of reactivity values, ranging from 50 to 87%, possibly reflecting differences in the degree of MDA modification. Absorption with Glyc- and nLDL caused a minor decrease in the reactivity of antibodies to oxLDL (5.9 +/- 7.1 and 6.8 +/- 6. 4%, respectively), comparable to the reduction of reactivity (2.1 +/- 4.0%) measured after absorption with transferrin, an irrelevant protein used as a negative control. These results suggest that oxLDL antibodies recognize primarily MDA epitopes. To determine whether purified oxLDL antibodies also recognize other epitopes known to be generated during copper oxidation of LDL, such as 4-hydroxynonenal (HNE)- and N(epsilon)(carboxymethyl)-lysine (CML), two additional sets of experiments were carried out. First, we monitored the formation of CML-, MDA-lysine, and HNE-lysine at different times during copper oxidation of two LDL pools. Both pools showed simultaneous increases in protein modification, as indicated by increasing fluorescence emission at 430 nm, and in immunoreactivity with oxLDL antibodies, coinciding closely with MDA modification of lysine groups. Second, we assessed whether the reactivity of oxLDL antibodies could be blocked by absorption with CML- or HNE-LDL. HNE-LDL did not react with isolated oxLDL antibodies. Highly modified CML-LDL (>90% of lysine residues modified) reduced the reactivity of oxLDL antibodies, but only by 25.5%. Finally, we investigated the possible cross-reactivity of oxLDL antibodies with cardiolipin. Seventeen purified oxLDL antibodies were used in this study, which showed that absorption with oxLDL or nLDL did not affect their reactivity with immobilized cardiolipin.

Atherosclerosis-related markers in systemic lupus erythematosus patients: the role of humoral immunity in enhanced atherogenesis

Systemic Lupus Erythematosus (SLE) patients experience premature atherosclerosis. A deranged lipid metabolism and use of immunosuppressive medications accounts partially for the accelerated process. The role of autoimmunity in atherosclerosis has recently been highlighted. Autoantigenic determinants thought to play a role in the development of atherosclerosis include: modified lipoproteins, heat shock proteins and beta2-glycoprotein I (a target of 'autoimmune' anticardiolipin antibodies). In this present work we determined autoimmune markers which may be associated with premature atherosclerotic process found in SLE patients. We have found that antibodies to oxLDL were raised in the sera of lupus patients and cross-reacted with cardiolipin and with beta2GPI. OxLDL containing immune-complexes of the IgG and IgM isotypes were both elevated in the SLE patients as compared with healthy controls. Patients with high Lipoprotein (a) concentrations (>30 mg/dl) had higher levels of IgM oxLDL-containing immune-complexes. IgM but not IgG anti-HSP-65 antibodies were elevated in the lupus patients and levels of oxLDL containing immune-complexes correlated positively with the presence of anti-HSP 65 antibodies. Lysophosphatidylcholine (LPC) is a peroxide-derivative formed during LDL oxidation, was shown to evoke a humoral response in healthy subjects. Antibodies to lysophosphatidylcholine of the IgG but not the IgM isotype were reduced in SLE patients compared with controls, suggesting it may be 'consumed' into oxLDL containing immune complexes. Therefore, SLE patients exhibit a humoral autoimmune response towards the antigenic candidates incriminated in the progression of atherosclerosis. These findings may help identify factors that are involved in accelerating atherogenesis in SLE patients.

Antibodies to oxidized LDL and LDL-containing immune complexes as risk factors for coronary artery disease in diabetes mellitus

Several groups have published results from clinical studies supporting the involvement of anti-modified LDL antibodies as risk factors for the initiation or progression of cardiovascular disease. However, the data published so far are judged inconclusive because of several contradictory observations concerning the correlation between clinical evidence of arteriosclerosis and the levels of antibodies to oxidized LDL (oxLDL Ab). We have previously reported that oxLDL Ab exist both in free form and as antigen-antibody complexes (LDL-IC) in patients with insulin-dependent diabetes mellitus (IDDM). The presence of LDL-IC in IDDM patients has important implications: it may interfere with the assay of oxLDL antibodies and the levels of LDL-IC may correlate better with the development of arteriosclerosis than the levels of free oxLDL antibodies. To clarify these questions baseline samples collected from 49 IDDM patients, who subsequently developed coronary artery disease (CAD) during an 8-year follow-up period, were compared to baseline samples from 49 age-, sex-, and duration-matched control IDDM subjects who remained free of clinical CAD during an identical follow-up period. The levels of free oxLDL antibody were significantly lower in the patients who developed CAD. The same patients had significantly higher concentrations of total cholesterol, apolipoprotein B, and IgA in immune complex-enriched polyethylene glycol (PEG) precipitates. The concentration of IgG was also higher in PEG precipitates from patients who developed CAD, but did not reach statistical significance. This indicates that patients who develop CAD had higher levels of circulating LDL-IC, a fact that could not be deduced from the measurement of free oxLDL antibody concentrations. A linear regression analysis of the correlation between the concentrations of total cholesterol in PEG precipitates, taken as a surrogate measurement of PEG-precipitated oxLDL-IC, and the concentration of free oxLDL antibody in serum showed a statistically significant negative correlation (r = -0.229, P = 0. 024). Our results support the conclusion that oxLDL-IC may be a risk factor for the development of macrovascular disease in IDDM patients. We also have demonstrated that circulating oxLDL-IC interfere with the assay of free oxLDL antibodies.