Circulating microparticles carry oxidation-specific epitopes and are recognized by natural IgM antibodies

Abstract

Oxidation-specific epitopes (OSEs) present on apoptotic cells and oxidized low density lipoprotein (OxLDL) represent danger-associated molecular patterns that are recognized by different arcs of innate immunity, including natural IgM antibodies. Here, we investigated whether circulating microparticles (MPs), which are small membrane vesicles released by apoptotic or activated cells, are physiological carriers of OSEs. OSEs on circulating MPs isolated from healthy donors and patients with ST-segment elevation myocardial infarction (STE-MI) were characterized by flow cytometry using a panel of OSE-specific monoclonal antibodies. We found that a subset of MPs carry OSEs on their surface, predominantly malondialdehyde (MDA) epitopes. Consistent with this, a majority of IgM antibodies bound on the surface of circulating MPs were found to have specificity for MDA-modified LDL. Moreover, we show that MPs can stimulate THP-1 (human acute monocytic leukemia cell line) and human primary monocytes to produce interleukin 8, which can be inhibited by a monoclonal IgM with specificity for MDA epitopes. Finally, we show that MDA+ MPs are elevated at the culprit lesion site of patients with STE-MI. Our results identify a subset of OSE+ MPs that are bound by OxLDL-specific IgM. These findings demonstrate a novel mechanism by which anti-OxLDL IgM antibodies could mediate protective functions in CVD.

Malondialdehyde epitopes as mediators of sterile inflammation

“…Indeed, several reports demonstrate that apoptotic cells can induce pro-inflammatory responses characterized by leukocyte influx, monocyte adherence to endothelial cells or chemokine secretion [43], [45]- [47]. Given that we showed that the MV-induced monocytic cytokine secretion is partially mediated by MDA epitopes as it was blocked with LR04 [39], we speculate that also the pro-inflammatory capacity of apoptotic cells could derive from the presence of MDA epitopes appearing at later stages of apoptosis. By binding to apoptotic cells, natural antibodies such as E014 and LR04 may facilitate a faster clearance of dying cells and cell debris, whose prolonged persistence in an organism is considered to be harmful and to result in chronic inflammatory and autoimmune responses [44], [48], [49].…”

“…Easily detectable due to their subsequent release into the circulation, MV could serve as biomarkers for inflammatory conditions as MV levels were found to be increased in patients suffering from diabetes [34], liver diseases [35], acute coronary syndrome [36], atherosclerosis [37], venous thromboembolism [38] or ST-segment elevation myocardial infarction (STE-MI) [39]. However, the contribution of MV content or their exposed membrane capable of inducing monocyte adhesion [40].…”

“…circulating MV isolated from healthy donors are recognized by the MDA-specific natural antibodies LR014 and NA17 [39], and about 50% of the isolated MDA-positive MV were covered with endogenous IgM found to be specific for MDA epitopes upon elution from lysed MV. In line with previous studies demonstrating pro-inflammatory properties of MDA epitopes, we could show that in vitro generated platelet-derived MV carrying MDA epitopes induce chemokine secretion in THP-1 cells and human peripheral blood mononuclear cells, an effect that was reduced by pre-treatment with the MDA-specific antibody LR04, indicating that MV may act as amplifiers of inflammatory responses in vitro that are at least partially mediated by MDA epitopes.…”

Senescent cells expose and secrete an oxidized form of membrane-bound vimentin as revealed by a natural polyreactive antibody

“…Consequently, naturally occurring IgMs are inherently polyreactive and among the diverse antigens of natural antibodies are synthetic haptens, nucleotides, polysaccharides, oxidized lipids, and advanced glycation end products (AGEs) (17,35,36). To determine if 9H4 exhibited polyreactivity, we performed direct ELISAs using a series of haptens and antigens previously used to characterize IgMs (35).…”

Increased Plasma IgE Accelerate Atherosclerosis in Secreted IgM Deficiency

“…Bone marrow-derived MCs (5×10 5 ) were incubated with sterile plasma (1:12 dilution in medium) from Ldlr −/− sIgM −/− mice of study 3 (Online Table I) for 2 hours at 37°C, after which supernatant was collected. IL-6 was determined by ELISA according to the manufacturer's protocol (BD Biosciences).…”