Oxidised low-density lipoprotein and oxLDL auto-antibodies: immunity modulates atherosclerosis
Oxidised LDL and antibodies to oxLDL are pathogenetically significant contributors in animal models of atherosclerosis, but the pathophysiological role of anti-oxLDL in humans, discussed in this article, remains to be clarified.
by Prof. Dr Thomas Dschietzig
It is currently generally accepted that oxidised low-density lipoprotein (oxLDL) plays a major pathogenetic role in initiating and fueling the process of atherosclerosis , [Figure 1]. In the sub-endothelial space, it is taken up via different scavenger receptors (SR-A1, SR-A2, and LOX-1) on the surface of macrophages, which induces foam cell formation and the appearance of fatty streaks, the first histological signs of atherosclerosis. Moreover, oxLDL leads to endothelial dysfunction, chronic vascular inflammation and transformation of vascular smooth muscle cells into the so-called synthetic phenotype typical of vascular remodeling.
OxLDL is measured in plasma using ELISA techniques . As oxidation of lipoproteins is a complex process generating hundreds of unique epitopes, the different antibodies used may vary significantly in their readings. This currently poses a major limitation since these ELISAs are not necessarily comparable, either in terms of absolute values or, more importantly, in terms of pathophysiological meaning . On the other hand, the largest database, which was hitherto collected with the antibody E06 detecting the amount of oxidised phospholipid epitopes on apolipoprotein B-100 (oxPL/apoB), clearly reveals the potential clinical utility of measuring oxLDL: in several studies  including the Bruneck  and the EPIC-Norfolk study , oxPL/apoB was demonstrated to correlate strongly with atherosclerosis and to predict future death, myocardial infarction, stroke and need for revascularisation. In those analyses, the parameter was independent of all traditional and non-traditional risk factors, including inflammatory and thrombotic risk factors, with occasional exceptions for Lp(a). Even more importantly, in the EPIC-Norfolk study, there was evidence of increasing c-statistic values (a measure of added value of new parameters in logistic regression models) when a panel of oxidative biomarkers was added to oxPL/apoB, including Lp(a), CRP, myeloperoxidase, Lp-PLA2 (phospholipase A2) activity and soluble PLA2 mass and activity.
Auto-antibodies against oxLDL
The rate of LDL oxidation is increased when cardiovascular risk factors such as smoking, diabetes mellitus, dyslipidaemia and hypertension induce oxidative stress in the vessel wall . OxLDL, in turn, represents a variety of differently modified lipid and protein components of LDL, the most abundant of which are malonyldialdehyde-LDL (MDA-LDL) and copper-oxidised LDL (Cu-LDL) . This modification renders oxLDL highly immunogenic; correspondingly, auto-antibodies of the IgM and IgG classes are commonly found. Natural IgM auto-antibodies form immune complexes with oxLDL that cannot bind to Fcγ receptors on macrophages and, therefore, do not activate these key players in atherosclerosis. Hence, IgM auto-antibodies may serve to clear oxLDL particles from circulation in a non-inflammatory, protective manner. In contrast, IgG auto-antibodies obviously promote atherosclerosis because they bind and activate macrophages via Fcγ receptors  [Figure 2].
In animal studies, the circulating levels of free oxLDL auto-antibodies reflected the general activity of the atherosclerotic process . Natural IgM antibodies – i. e. antibodies pertaining to innate immunity – recognising oxLDL were shown to be protective in different mouse models of atherosclerosis [7,8].
In clinical studies, an inverse relationship between circulating IgM anti-oxLDL and the occurrence of cardiovascular atherosclerosis (carotid artery disease, coronary artery disease) was observed while the opposite, a positive correlation, held true for IgG antibodies [9-11]. Additionally, an unstable phenotype of coronary plaque has been linked to high levels of IgG anti-oxLDL; in contrast, high levels of IgM anti-oxLDL are associated with stable plaques . In these epidemiological studies, however, all described associations were not independent: after correction for other known risk factors in multivariate analyses, anti-oxLDL levels were no longer predictive of atherosclerotic burden. It remains therefore a matter of debate whether oxLDL antibodies in humans represent mere markers of disease or causal players, albeit that the above-mentioned animal studies provided remarkable evidence in favour of the latter hypothesis.
For anti-oxLDL detection by ELISA, oxidation-specific epitopes (‘model oxLDL’), mostly MDA-LDL or Cu-LDL epitopes, are generated in vitro and coupled onto micro-titre plates. Free oxLDL antibodies in diluted plasma samples bind to these epitopes and are then detected with secondary antibodies specific to IgG or IgM [see Figure 1].
Oxidised LDL and antibodies to oxLDL are pathogenetically significant contributors in animal models of atherosclerosis. As opposed to oxLDL itself, the pathophysiological role of anti-oxLDL in humans (marker or player?) remains to be clarified. Both parameters can be measured using ELISA techniques. For clinical risk assessment in patients with metabolic syndrome and atherosclerosis, circulating oxLDL appears to offer added value to traditional risk factors. It allows significant readjustment of the Framingham Risk Score [3;4] which will help determine how aggressively other risk factors should be treated. Also, combining oxLDL measurement with other parameters of oxidative damage may be useful, with the general caveat that new oxLDL tests be validated thoroughly with regard to their pathophysiological meaning.
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Prof. Dr med. Thomas Dschietzig
Charité Berlin, Germany