Natural antibodies against phosphorylcholine as potential protective factors in atherosclerosis, cardiovascular disease and systemic lupus erythematosus

Abstract

Atherosclerosis is the most important contributor to cardiovascular disease (CVD). Although it was initially considered as a bland lipid storage disease, processes of inflammation and autoimmunity are now considered to play a major role in atherosclerosis.

Oxidation of low density lipoprotein (OxLDL) has been implicated in atherogenesis and much of the lipid content in the atherosclerotic lesions consists of OxLDL. OxLDL binds to and is taken up by macrophages through scavenger receptors (SRs) and subsequently these macrophages develop into foam cells, which marks the initiation of atherosclerosis.

The important role of autoimmunity and inflammation in atherosclerosis and CVD is illustrated by Systemic lupus erythematosus (SLE), an autoimmune disease, where the risk of CVD is very high and the prevalence of atherosclerotic plaques is increased. This association is only partially linked to the traditional risk factors for CVD. In addition to these, systemic inflammation and autoantibodies against phospholipids among others can be considered SLE-related risk factors to CVD. CVD in SLE is not only an important clinical problem, but may also shed light on the role of immunity in atherosclerosis.

The purpose of this thesis was 1) to develop an assay for anti-phosphorylcholine (anti-PC) antibodies and define the correlation between anti-PC levels and atherosclerosis, CVD and SLE, 2) to determine whether PC-epitopes on OxLDL are involved in the uptake of OxLDL by human macrophages, and 3) to investigate if anti-PC antibodies have any anti-inflammatory role in CVD.

An ELISA-based method was developed to measure anti-PC antibodies levels in clinical cohorts. We used sepharose columns to purify anti-PC antibodies from human IgM and IgG. Next, we determined the affinity and specificity of such purified anti-PC antibodies by competition experiments. We established a method to study different properties of anti-PC, including macrophage uptake of OxLDL and effects of anti-PC on inflammatory reactions relevant in atherosclerosis, by use of flow cytometry and confocal microscopy.

In the first study, we developed an assay for and determined anti-PC levels in hypertensive subjects. We found that high levels of IgM anti-PC antibodies and IgM anti-OxLDL antibodies predict a favorable outcome in the development of carotid atherosclerosis.

In the second study on SLE and SLE related CVD, we determined, surprisingly, that low levels of IgG and IgM anti-PC are more prevalent in SLE and SLE-related CVD and that levels of IgG anti-PC are decreased in these groups. Further, we found that anti-PC antibodies have anti-inflammatory effects on adhesion molecule expression stimulated by an inflammatory phospholipid.

In the third, prospective study, we found that low levels of IgM anti-PC antibodies predict CVD in 60-year old men, and determined that human IgM anti-PC antibodies block the uptake of OxLDL in macrophages which may be one explanation as to why anti-PC may protect against atherosclerosis development in humans.

In the fourth paper, which is a follow-up study of the 60-year old cohort, we determined, that in contrast to IgM anti-PC, IgG anti-PC did not predict risk for CVD. Specific IgG anti-PC antibodies extracted from total IgG only had minimal effects in preventing uptake of OxLDL. Howerer, upon pre-incubation with antibodies against FC-receptors, IgG anti-PC were able to induce a reduction in uptake of OxLDL, indicating that FC-receptors may contribute to OxLDL uptake in macrophages.

In conclusion, we report for the first time that high levels of IgM anti-PC antibodies is a novel protective factor for atherosclerosis, and low IgM anti-PC antibodies is an independent risk marker for development of CVD in humans. Further, we demonstrate that anti-PC can inhibit the uptake of OxLDL in macrophages and exert anti-inflammatory effects relevant in the atherosclerotic lesions. It is thus conceivable that raising anti-PC levels through active immunization or passive transfer of anti-PC could be a novel therapeutic possibility.