COMMENTARIES

Autoantibodies Recognizing Specific Antigens in Oxidized LDL - Do They Have a Protective Role?

Gunilla Nordin Fredrikson and Jan Nilsson, Department of Clinical Sciences,
Malmö University Hospital, Lund University, S-20502 Malmö,
Sweden
E-mail: Gunilla.Nordin_Fredrikson@med.lu.se

Gunilla Nordin Fredrikson Jan Nilsson

During the last ten to fifteen years there has been a dramatic shift in the way we look at mechanisms involved in the pathogenesis of atherosclerosis. From being regarded as a result of passive lipid accumulation and activation of smooth muscle cell proliferation, atherosclerosis is now seen as a degenerative inflammatory disease involving multiple aspects of both innate and adaptive immunity. This has focused attention on the immune system as a possible novel target in prevention and treatment of ischemic heart disease and stroke.

Several lines of evidence indicate that adaptive immune responses are involved in the development of atherosclerosis and promote inflammation and plaque growth [1,2]. However, immunization of hypercholesterolemic animals with native or oxidized LDL have been shown to result in a significant reduction of atherosclerosis development [3-7]. Moreover, the existence of an athero-protective humoral immunity has been supported by studies demonstrating that treatment with polyclonal immunoglobulins of apo E-/- mice inhibited atherosclerosis development as well as B cell substitution in splenectomized apo E-/-mice [8,9]. In addition, it has been reported that B cell transfer to Rag-1 mice reduce the injury-induced plaque formation [10]. Taken together, this may reflect that a specific activation of the immune system results in either protective immunity or down-regulation of pre-existing, pro-atherogenic immune responses.

Immune Responses against Peptide Antigens in Oxidized LDL

We have previously characterized a large number of different malondialdehyde (MDA)-modified apolipoprotein (apo) B-100 amino acid epitopes specifically recognized by antibodies present in human plasma [11]. These observations have made it possible to develop ELISAs for measurements of autoantibodies against specific antigens in oxidized LDL. The levels of several of these antibodies show inverse association with plasma-oxidized LDL, suggesting that antibodies are involved in the clearance of these particles [11]. Immunization of apo E-/-mice with some of these native and aldehyde-modified apo B-100 peptides induces an immunoglobulin switch from IgM to IgG that is accompanied by an inhibition of atherosclerosis [12-14]. In order to study the possible atheroprotective effects of this IgG we produced human IgG1 specific for MDA-P45 (the amino acids between 661 and 680 in apo B-100) by recombinant technique. Subcutaneous immunization with MDA-P45 peptide has previously been shown to inhibit atherosclerosis by about 50% in apo E-/- mice [13]. A similar inhibition of atherosclerosis together with decreased plaque macrophage immunoreactivity and MDA apo B-100 epitopes was observed in apo E-/- mice following injections of recombinant anti-MDA-P45 IgG1 [15]. Moreover, recombinant antibody treatment prevented constrictive remodeling after injury in the carotid arteries and potently reduced lesion extent in the uninjured carotid arteries in mice [16]. Taken together these results suggest that IgG recognizing the MDA-modified peptide sequence in apo B-100 may protect against atherosclerosis.

Autoantibodies to Specific Antigens in Oxidized LDL

Autoantibodies, both IgM and IgG, against oxidized LDL are common in human plasma. The pathophysiological role of these autoantibodies remains to be elucidated. Measurements of autoantibodies are used in the clinical routine for several chronic inflammatory diseases with autoimmune components, such as type I diabetes, SLE, and rheumatoid arthritis. However, the use of this approach has not been extensively studied when it comes to cardiovascular disease. Salonen et al. [17] first reported increased antibody binding to MDA-LDL in subjects with accelerated 2-year progression of carotid disease as compared to controls without progression. The results of several subsequent studies have been inconsistent. Some have reported that antibodies against MDA- or oxidized LDL are increased in patients with coronary artery disease [18,19] predict risk for development of myocardial infarction [20] and increase with severity of carotid disease [21]. In contrast, other studies have reported that high levels of primarily oxidized LDL-specific IgG are associated with less atherosclerosis as assessed by carotid intima-media thickness [22,23]. To some extent these inconsistencies may be due to technical difficulties in developing standardized ELISA assays because of the poor characterization of oxidized LDL antigens.

We have recently presented an independent association between high levels of IgG antibodies against a specific oxidized LDL antigen, a MDA-modified peptide corresponding to the sequence between amino acids 661 and 680 in apo B-100 (P45) and a low degree of carotid stenosis [24]. The population selected for this prospective study was part of the larger Malmö Diet and Cancer study and consisted of 76 subjects who later developed acute myocardial infarction or death due to coronary artery disease, and 148 healthy controls matched for age, sex, smoking, and hypertension. We have previously shown that IgM against this aldehyde-modified peptide correlated positively with carotid intima-media thickness in the same population [11]. Thus, IgG and IgM presented opposite trends of associations with the same parameters in this population, implying that they may have different roles in atherosclerosis or reflect different aspects of the disease. In our recently published study, we also observed a correlation between high levels of oxidized LDL in plasma and metabolic factors characteristic for the metabolic syndrome, including low HDL cholesterol, high triglycerides, high insulin, and increased BMI. Additionally, a weak association was observed between high levels of MDA-p45 IgG and decreased levels of oxidized LDL in plasma, suggesting that these antibodies may help to clear circulating oxidized LDL from plasma. The approach used in this study to analyze antibodies against a single clearly defined antigen present in oxidized LDL may offer the advantage of a higher specificity and reproducibility. A limitation of this technique is that only a fraction of all antibodies against epitopes in oxidized LDL are measured. However, by studying autoantibodies against single antigens in oxidized LDL it may be possible to identify more specific markers of disease severity and risk.

Concluding Remarks

In conclusion, this suggests that subjects with low levels of IgG autoantibodies against the p45 sequence of apo B-100 have more atherosclerosis and a higher risk for development of acute cardiovascular events. These results are in line with experimental studies demonstrating that immunization of apo E-/- mice with MDA-p45 peptide results in an increase in specific antibodies and inhibition of atherosclerosis [13]. Treatment of apo E-/- mice with human recombinant IgG specific for the MDA-p45 sequences also reduced aortic plaque area and decreased plaque inflammation [15]. Taken together these observations support the notion that the p45 sequence of apo B-100 (amino acids 661-680) is a potential target for immunomodulatory treatment of atherosclerosis. Moreover, in the future autoantibodies against specific oxidized LDL antigens might become new diagnostic tools to identify individuals that are in need of the treatment.

References

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