Autoimmunity, oxidized LDL and cardiovascular disease

Autoimmunity Reviews 1 (2002) 233–237

Autoimmunity, oxidized LDL and cardiovascular disease Johan Frostegard* ˚ Department of Rheumatology and Centre for Molecular Medicine, Karolinska Hospital, Karolinska Institute, P.O. Box 30, 17176 Stockholm, Sweden Accepted 25 June 2002

Abstract Atherosclerosis is the major cause of cardiovascular disease (CVD) and in addition to established risk factors as smoking, hypertension, diabetes and dyslipidemia, inflammation and autoimmune reactions have been much discussed recently. Several lines of evidence indicate that also inflammation and autoimmune reactions are highly relevant in atherosclerosis and CVD. Inflammatory cells and cytokines are present in lesions, already at an early stage; animal experiments suggest that immune reactions, though not necessary for development of atherosclerosis, can modulate disease development and systemic inflammation is associated with an enhanced risk of CVD. The enhanced risk of CVD in a major autoimmune disease, systemic lupus erythematosus (SLE), is therefore highly relevant, and in addition to being an important clinical problem, SLE-related CVD could give insights into the nature of autoimmunity in atherosclerosis and CVD in general. We recently defined traditional and non-traditional risk factors for CVD in SLE. These include increased atherosclerosis (as determined by intima-media thickness of carotid artery); raised oxidized low density lipoprotein (OxLDL) and autoantibodies to OxLDL; dyslipidemia with raised triglycerides and Lp(a) and decreased HDL-cholesterol concentrations; raised systemic inflammation; presence of anti-phospholipid antibodies including lupus anticoagulant, homocysteine-levels and more frequent osteoporosis. Disease duration, smoking, blood pressure or diabetes mellitus did not differ significantly between the groups. Taken together, immune reactions are highly relevant in atherosclerosis, and patients with autoimmune disease like SLE are at high-risk of CVD. If confirmed prospectively, non-traditional risk factors like OxLDL in the circulation, autoantibodies against OxLDL and phospholipids and inflammation could lead to new therapeutic strategies and insight into disease mechanisms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cardiovascular disease; Autoimmunity; Lipoprotein

1. Introduction During recent years several lines of evidence indicate that inflammation and immune reactions *Tel.: q46-8-517-76086; fax: q46-8-517-73080. E-mail address: [email protected] (J. Frostegard). ˚

are implicated in atherosclerosis and cardiovascular disease (CVD). Firstly, inflammatory cells and cytokines are present in lesions, already at an early stage. Secondly, animal experiments suggest that immune reactions, though not necessary for development of atherosclerosis, can modulate disease

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development. Thirdly, systemic inflammation, as determined by levels of C-reactive protein, is associated with an enhanced risk of CVD. Furthermore, a role for autoimmune and other specific immune reactions in atherosclerosis is suggested by recent findings indicating that a typical autoimmune disease, systemic lupus erythematosus (SLE), is associated with a strongly enhanced risk of CVD. These three lines of evidence will be discussed with references below, and also factors, mainly oxidized low density lipoprotein (OxLDL) that may cause andyor contribute to the immune reaction present in atherosclerosis. 2. Inflammatory cells and cytokines in atherosclerotic lesions Early studies indicate that monocytesymacrophages are present in the atherosclerotic lesions w1–3x. Many of the foam cells in the lesions are derived from these cells, which express specific scavenger receptors that take up modified forms of lipoproteins w4,5x. These findings provided an explanation for abundance of lipid in atherosclerotic plaques. In the mid-80th, Hansson et al. identified T cell in the atherosclerotic lesions and later demonstrated that many of these were activated w6,7x. Monocytesymacrophages and dendritic cells of the same lineage are major participants in immune reactions, with several properties that are pivotal for T and B cell activation and function. Antigenic peptides bound to major histocompatibility comples on cells of macrophage lineage can activate T cells which also provide help for antibody formation to B cells. In addition, cytokines and other components of an immune reactions are produced by all these cells. Proinflammatory cytokines are abundant in late human atherosclerotic lesions from patients undergoing carotid operation due to ischaemic symptoms, and may also contribute to plaque rupture w8,9x. 3. Immune modulation of atherosclerosis The role of immune reactions in atherosclerosis is unclear in general, since cell-mediated immune

reactions have been reported to be related both to an increase and a decrease in the development of disease, w10–15x. Though it appears that most animal experiments favour the notion that proinflammatory Th1 cytokines like IFN-gamma are atherogenic, the situation in humans is less clear, and in SLE an up to 50 times increased risk of CVD and also accelerated atherosclerosis has been reported in spite of this disease in general being considered a disease with a Th2 cytokine pattern w16–18x. These apparently conflicting data may be related to the different animal models used, but it is also possible that the role of the cell-mediated immune system in atherogenesis may depend on the disease stage, type and on the presence of other risk factors. However, available data clearly indicate that immune reactions may modulate atherosclerosis. Several studies indicates that immunization with b2 glycoprotein I, a co-factor for anti-phospholipid antibodies (aPL), increased atherosclerosis in a mouse model w19x. Likewise, when heat shock protein 60y65 was used as antigen, in similar immunization experiments, atherosclerosis was enhanced w11,20,21x. On the other hand, immunization with OxLDL generating enhanced autoantibodies to OxLDL (aOxLDL) levels decreases atherosclerosis both in rabbit and mouse models w22–24x. These observations are, in line with recent observation that aOxLDL were decreased but aHSP65 and aß2GPI were raised in early CVD w25–28x. 4. Systemic lupus erythematosus, inflammation, and cardiovascular disease Systemic inflammation, as determined by raised levels of C-reactive protein, is associated with an increased risk of CVD w29x and it has been suggested that CRP levels therefore could be used as a novel risk factor. An inherent problem with this is that CRP levels could be up to 100 times higher in infections as pneumonia and CRP as a risk factor at the individual level is therefore problematic. SLE is a major autoimmune disease mainly affecting women, and it has become increasingly

J. Frostegard ˚ / Autoimmunity Reviews 1 (2002) 233–237

clear that the risk of CVD in SLE is very high, up to 50 times in some age groups w18,30x. We recently studied traditional and non-traditional risk factors for CVD in 26 women with SLE and CVD (SLE cases), 26 women with SLE but without manifest CVD (SLE controls) and 26 populationbased control women (controls). SLE cases had increased carotid intima-media thickness (IMT), while the IMT of SLE controls did not differ from that of controls. Raised plasma concentrations of aOxLDL, dyslipidemia, raised acute phase reactants, especially CRP and alfa-1-antitrypsin; aPL including lupus anticoagulant and homocysteinelevels discriminated between SLE cases and SLE controls. Among traditional risk factors, hypertension, diabetes, BMI and smoking did not differ between SLE and controls. SLE-related disease activity and disease duration did not differ between the groups. Disease activity or duration did not differ between SLE groups but an interesting observation is that osteoporosis was more frequent among SLE cases w31x. We also reported the novel observation that oxidized-phospholipid epitopes detected by the monoclonal antibody E06 are significantly more frequent on apo B particles in SLE cases as compared to SLE controls, indicating that OxLDL levels are raised in SLE-related CVD. aOxLDL cross reacts with aPL w32x and many aPL recognize oxidized phospholipids. Furthermore, E06 recognized apoptotic cells and is similar to a monoclonal antibody recognizing a phospholipid-related pneumococcal antigen w33,34x. OxLDL stimulates endothelial adhesiveness w35–37x and promotes T cell and monocyte activation w38–40x. OxLDL is taken up by macrophages in the artery wall, which develop into foam cells w4,5x. OxLDL is therefore generally believed to be atherogenic and could therefore be a common pathogenic factor in SLE and atherosclerosis. 5. Summary Recent findings indicate that there is an important immunological and inflammatory aspect of atherosclerosis. This is supported by several lines of evidence: inflammatory cells and cytokines are present in lesions, already at an early stage; animal

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experiments suggest that immune reactions can modulate disease development; a typical autoimmune disease, SLE, is associated with a strongly enhanced risk of CVD. SLE cases with CVD had increased atherosclerosis, raised plasma concentrations of circulating oxidized LDL (OxLDL), aOxLDL, dyslipidemia, enhanced inflammation and presence of aPL. OxLDL may be an important common factor that could contribute to autoimmune and inflammatory factors in atherosclerosis and its main consequence, CVD. It will be important to determine in prospective studies whether OxLDL and other non-traditional risk factors can predict future CVD, especially in relation to autoimmune mechanisms. If so, they can be used to identify a high-risk group that would be eligible for intense intervention, for example with potent antioxidants and anti-inflammatory agents. Take-home messages ● Atherosclerosis is an inflammatory disease where cytokines and inflammatory and immune competent cells are common in the lesions. Inflammation, as determined by levels of CRP and other markers, is associated with cardiovascular disease. ● Experimental evidence indicates that immune reactions can modulate the development of atherosclerosis. ● The risk of cardiovascular disease is very high in a major autoimmune disease, SLE.

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The World of Autoimmunity; Literature Synopsis More about Genetic Modifiers of Lupus Genetic factors have a major influence on susceptibility to autoimmune disease development. Bolland et al. (J Exp Med 2002;196:1167) report on the association between several lupus susceptibility genes and lupus expression. Mice deficient in FcgammaRIIB develop spontaneous ANA and fatal glomerulonephritis when on the C57BLy6 background. In order to determine mechanisms underlying the epistasis displayed by this gene they have constructed hybrids between FcgammaRIIB(-y-) and lupus modifiers yaa and lpr and the susceptibility locus Sle1. Compound heterozygotes of Sle1 and B6.RIIB(-y-) developed significant disease, while single heterozygotes displayed no evidence of autoimmunity or disease. They have also suggested that the generation of ANA in itself is insufficient to account for the severity of autoimmune disease in this model. Whereas B6.RIIB(-y-)ylpr mice are protected from disease progression despite equivalent titers of ANA, B6.RIIB(-y-)yyaa mice have significantly enhanced disease despite reduced ANA titers. This occurs because Yaa modifies the specificity and thus the pathogenicity of the B6.RIIB(-y-) ANA. Could eye muscle autoantibodies be related to subtype of thyroid-associated ophthalmopathy? A recent study by Kaspar et al. (Thyroid 2002;12:187) attempted to test the prevalence of several autoantibodies in thyroid-associated ophthalmopathy (TAO), and whether they can be related to the subtype of TAO. They tested for serum antibodies against purified human recombinant flavoprotein, flavine adenine nucleotide (FAD), and a G2s fusion protein (thyroid and eye muscle shared protein of unknown function), in patients with TAO and control patients. These autoantibodies were found in different percentages of patients with TAO, Graves’ hyperthyroidism, Hashimoto’s thyroiditis, multi-nodular goiter and in normal subjects. In patients having TAO, anti-flavoprotein antibodies were found in 67%, and anti-G2s in 89%, compared with only 11% and 22% in normal subjects, respectively. These authors suggest that flavoprotein which is found within the mitochondrial membrane leads to antibody production following eye muscle necrosis, and thus these antibodies do not play a role in its pathogenesis. The mechanism for the production of autoantibodies in subjects who do not have TAO is unclear as is the significance of such antibodies in control subjects.