Platelet-activating factor and lysophospatidylcholine in oxidized low density lipoprotein-mediated immune activation

Inflammation is a risk factor for development of cardiovascular disease, as in addition to known risk factors such as hypertension, hyperlipidemia, smoking and diabetes. Patients with inflammatory diseases including SLE have an enhanced risk of cardiovascular disease. The main cause of cardiovascular diseases is atherosclerosis, which can be characterized as a chronic inflammation in the artery wall, where proinflammatory cytokines as IFN-[gamma], TNF-[alpha] and IL-6 are produced. Oxidized low density lipoprotein(oxLDL) has been implicated in atherosclerosis. The aim of this study was to understand how oxLDL might be involved in atherosclerotic inflammation.

By using the highly sensitive ELISPOT-technique, we here demonstrate that oxLDL has the capacity to induce production of IFN-[gamma] in peripheral blood mononuclear cells (PBMC), an effect was inhibited by antibodies to MHC class II. Furthermore, oxLDL induced enhanced antibody formation.

Platelet activating factor (PAF) is a phospholipid with a wide range of proinflammatory properties. PAF is synthesized by several different cell types, including monocytes and endothelial cells, and PAF-like lipids but not PAF itself are present in oxLDL. Like oxLDL, PAF induced production of IFN-[gamma] in PBMC and TNF-a in diluted whole blood. A specific PAF receptor antagonist WEB 2170, could inhibit both PAF- and oxLDL-induced IFN-r and TNF-a production. This finding indicates that oxLDL induces these proinflammatory cytokines by a PAF-receptor dependent mechanism. PAF also production of a Th2 cytokine, IL-4, as determined by ELISPOT. However, oxLDL had a much weaker capacity to induce IL-4 indicating differences between the immune-stimulatory effects of PAF and oxLDL.

During LDL-oxidation, a large number of lipid compounds are formed, including Iysophosphatidylcholine (LPC), oxysterols, arachidonic acid (AA) and 1 5-lipoxygenase products of AA. Of the substances tested, only LPC could mimic the effects of oxLDL and PAF on cytokine-production. LPC-induced IFN-[gamma], TNF-[alpha] and immunoglobulin were all inhibited by WEB 2170. This finding indicates that LPC is one PAF-like lipid in oxLDL that has pro-inflammatory properties, and may promote Iymphocyte and monocyte activation. LPC is produced also by enzymes such as phospholipase A2, which we recently detected in arteries.

We then hypothesized that LPC was involved in antibody binding to oxLDL and demonstrated antibodies against LPC of IgG and IgM type, present in healthy individuals. Antibody titers to phosphatidylcholine (PC) were very low as compared to LPC, and hydrolysis of phosphatidylcholine at the sn-2 position is thus essential for immune reactivity. LPC competitively inhibited anti-oxLDL reactivity, which indicates that LPC is one important antigen in oxLDL.

Taken together, the mechanism by which oxLDL induces immune-activation may be related to nonspecific, co-stimulatory effects due to proinflammatory cytokines induced by PAF-like lipids such as LPC. On the other hand, the inhibitory effects of MHC class II antibodies may indicate that a conventional antigen recognized by T cells is also involved. The two possibilities are not mutually exclusive. OxLDL is also recognized by specific antibodies of both IgG and IgM type, many of which are specific for LPC. The findings indicate that PAF-like lipids, and especially LPC, may play an important role in the development of cardiovascular disease by inducing inflammatory reactions in the artery wall.