A natural antibody to oxidized cardiolipin binds to oxidized low-density lipoprotein, apoptotic cells, and atherosclerotic lesions

Role of cardiolipin in regulating and treating atherosclerotic cardiovascular diseases

Cardiovascular diseases, mainly caused by atherosclerosis, are the leading causes of morbidity and mortality worldwide. Despite the discrepancies in clinical manifestations between different abnormalities, atherosclerosis shares similar pathophysiological processes, such as mitochondrial dysfunction. Cardiolipin (CL) is a conserved mitochondria-specific lipid that contributes to the cristae structure of the inner mitochondrial membrane (IMM). Alterations in the CL, including oxidative modification, reduced quantity, and abnormal localization, contribute to the onset and progression of atherosclerosis. In this review, we summarize the knowledge that CL is involved in the pathogenesis of atherosclerosis. On the one hand, CL and its oxidative modification promote the progression of atherosclerosis via several mechanisms, including oxidative stress, apoptosis, and inflammation in response to stress. On the other hand, CL externalizes to the outer mitochondrial membrane (OMM) and acts as the pivotal "eat-me" signal in mitophagy, removing dysfunctional mitochondria and safeguarding against the progression of atherosclerosis. Given the imbalance between proatherogenic and antiatherogenic effects, we provide our understanding of the roles of the CL and its oxidative modification in atherosclerotic cardiovascular diseases, in addition to potential therapeutic strategies aimed at restoring the CL. Briefly, CL is far more than a structural IMM lipid; broader significances of the evolutionarily conserved lipid need to be explored.

Systemic lupus erythematosus and cardiovascular disease

The prognosis in systemic lupus erythematosus (SLE) has improved due to better treatment and care, but cardiovascular disease (CVD) still remains an important clinical problem, since the risk of CVD in SLE is much higher than among controls. Atherosclerosis is the main cause of CVD in the general population, and in SLE, increased atherosclerosis, especially the prevalence of atherosclerotic plaques, has been demonstrated. Atherosclerosis is an inflammatory condition, where immunity plays an important role. Interestingly, oxidized low-density lipoprotein, defective clearance of dead cells, and inflammation, with a pro-inflammatory T-cell profile are characteristics of both atherosclerosis and SLE. In addition to atherosclerosis as an underlying cause of CVD in SLE, there are also other non-mutually exclusive mechanisms, and the most important of these are antiphospholipid antibodies (aPL) leading to the antiphospholipid antibody syndrome with both arterial and venous thrombosis. aPL can cause direct pro-inflammatory and prothrombotic effects on endothelial and other cells and also interfere with the coagulation, for example, by inhibiting annexin A5 from its antithrombotic and protective effects. Antibodies against phosphorylcholine (anti-PC) and other small lipid-related epitopes, sometimes called natural antibodies, are negatively associated with CVD and atherosclerosis in SLE. Taken together, a combination of traditional risk factors such as hypertension and dyslipidemia, and nontraditional ones, especially aPL, inflammation, and low anti-PC are implicated in the increased risk of CVD in SLE. Close monitoring of both traditional risk factors and nontraditional ones, including treatment of disease manifestations, not lest renal disease in SLE, is warranted.

The role of mitochondria in rheumatic diseases

The mitochondrion is an intracellular organelle thought to originate from endosymbiosis between an ancestral eukaryotic cell and an α-proteobacterium. Mitochondria are the powerhouses of the cell, and can control several important processes within the cell, such as cell death. Conversely, dysregulation of mitochondria possibly contributes to the pathophysiology of several autoimmune diseases. Defects in mitochondria can be caused by mutations in the mitochondrial genome or by chronic exposure to pro-inflammatory cytokines, including type I interferons. Following the release of intact mitochondria or mitochondrial components into the cytosol or the extracellular space, the bacteria-like molecular motifs of mitochondria can elicit pro-inflammatory responses by the innate immune system. Moreover, antibodies can target mitochondria in autoimmune diseases, suggesting an interplay between the adaptive immune system and mitochondria. In this Review, we discuss the roles of mitochondria in rheumatic diseases such as systemic lupus erythematosus, antiphospholipid syndrome and rheumatoid arthritis. An understanding of the different contributions of mitochondria to distinct rheumatic diseases or manifestations could permit the development of novel therapeutic strategies and the use of mitochondria-derived biomarkers to inform pathogenesis.

The OSE complotype and its clinical potential

Cellular death, aging, and tissue damage trigger inflammation that leads to enzymatic and non-enzymatic lipid peroxidation of polyunsaturated fatty acids present on cellular membranes and lipoproteins. This results in the generation of highly reactive degradation products, such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), that covalently modify free amino groups of proteins and lipids in their vicinity. These newly generated neoepitopes represent a unique set of damage-associated molecular patterns (DAMPs) associated with oxidative stress termed oxidation-specific epitopes (OSEs). OSEs are enriched on oxidized lipoproteins, microvesicles, and dying cells, and can trigger sterile inflammation. Therefore, prompt recognition and removal of OSEs is required to maintain the homeostatic balance. This is partially achieved by various humoral components of the innate immune system, such as natural IgM antibodies, pentraxins and complement components that not only bind OSEs but in some cases modulate their pro-inflammatory potential. Natural IgM antibodies are potent complement activators, and 30% of them recognize OSEs such as oxidized phosphocholine (OxPC-), 4-HNE-, and MDA-epitopes. Furthermore, OxPC-epitopes can bind the complement-activating pentraxin C-reactive protein, while MDA-epitopes are bound by C1q, C3a, complement factor H (CFH), and complement factor H-related proteins 1, 3, 5 (FHR-1, FHR-3, FHR-5). In addition, CFH and FHR-3 are recruited to 2-(ω-carboxyethyl)pyrrole (CEP), and full-length CFH also possesses the ability to attenuate 4-HNE-induced oxidative stress. Consequently, alterations in the innate humoral defense against OSEs predispose to the development of diseases associated with oxidative stress, as shown for the prototypical OSE, MDA-epitopes. In this mini-review, we focus on the mechanisms of the accumulation of OSEs, the pathophysiological consequences, and the interactions between different OSEs and complement components. Additionally, we will discuss the clinical potential of genetic variants in OSE-recognizing complement proteins – the OSE complotype - in the risk estimation of diseases associated with oxidative stress.

B-1b Cells Have Unique Functional Traits Compared to B-1a Cells at Homeostasis and in Aged Hyperlipidemic Mice With Atherosclerosis

Immunoglobulin M (IgM) to oxidation specific epitopes (OSE) are inversely associated with atherosclerosis in mice and humans. The B-1b subtype of B-1 cells secrete IgM to OSE, and unlike B-1a cells, are capable of long-lasting IgM memory. What attributes make B-1b cells different than B-1a cells is unknown. Our objectives were to determine how B-1b cells produce more IgM compared to B-1a cells at homeostatic condition and to see the differences in the B-1a and B-1b cell distribution and IgM CDR-H3 sequences in mice with advanced atherosclerosis. Here, in-vivo studies demonstrated greater migration to spleen, splenic production of IgM and plasma IgM levels in ApoE^-/-Rag1^-/- mice intraperitoneally injected with equal numbers of B-1b compared to B-1a cells. Bulk RNA seq analysis and flow cytometry of B-1a and B-1b cells identified CCR6 as a chemokine receptor more highly expressed on B-1b cells compared to B-1a. Knockout of CCR6 resulted in reduced B-1b cell migration to the spleen. Moreover, B-1b cell numbers were significantly higher in spleen of aged atherosclerotic ApoE^-/- mice compared to young ApoE^-/- mice. Single cell sequencing results of IgHM in B-1a and B-1b cells from peritoneal cavity and spleen of atherosclerotic aged ApoE^-/- mice revealed significantly more N additions at the V-D and D-J junctions, greater diversity in V region usage and CDR-H3 sequences in B-1b compared to B-1a cells. In summary, B-1b cells demonstrated enhanced CCR6-mediated splenic migration, IgM production, and IgM repertoire diversification compared to B-1a cells. These findings suggest that potential strategies to selectively augment B-1b cell numbers and splenic trafficking could lead to increased and more diverse IgM targeting OSE to limit atherosclerosis.

The Interplay of Hypoxia Signaling on Mitochondrial Dysfunction and Inflammation in Cardiovascular Diseases and Cancer: From Molecular Mechanisms to Therapeutic Approaches

Simple Summary

The regulation of hypoxia has recently emerged as having a central impact in mitochondrial function and dysfunction in various diseases, including the major disorders threatening worldwide: cardiovascular diseases and cancer. Despite the studies in this matter, its effective role in protection and disease progression even though its direct molecular mechanism in both disorders is still to be elucidated. This review aims to cover the current knowledge about the effect of hypoxia on mitochondrial function and dysfunction, and inflammation, in cardiovascular diseases and cancer, and reports further therapeutic strategies based on the modulation of hypoxic pathways.

Abstract

Cardiovascular diseases (CVDs) and cancer continue to be the primary cause of mortality worldwide and their pathomechanisms are a complex and multifactorial process. Insufficient oxygen availability (hypoxia) plays critical roles in the pathogenesis of both CVDs and cancer diseases, and hypoxia-inducible factor 1 (HIF-1), the main sensor of hypoxia, acts as a central regulator of multiple target genes in the human body. Accumulating evidence demonstrates that mitochondria are the major target of hypoxic injury, the most common source of reactive oxygen species during hypoxia and key elements for inflammation regulation during the development of both CVDs and cancer. Taken together, observations propose that hypoxia, mitochondrial abnormality, oxidative stress, inflammation in CVDs, and cancer are closely linked. Based upon these facts, this review aims to deeply discuss these intimate relationships and to summarize current significant findings corroborating the molecular mechanisms and potential therapies involved in hypoxia and mitochondrial dysfunction in CVDs and cancer.

Examining the Effect of Charged Lipids on Mitochondrial Outer Membrane Dynamics Using Atomistic Simulations

The outer mitochondrial membrane (OMM) is involved in multiple cellular functions such as apoptosis, inflammation and signaling via its membrane-associated and -embedded proteins. Despite the central role of the OMM in these vital phenomena, the structure and dynamics of the membrane have regularly been investigated in silico using simple two-component models. Accordingly, the aim was to generate the realistic multi-component model of the OMM and inspect its properties using atomistic molecular dynamics (MD) simulations. All major lipid components, phosphatidylinositol (PI), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS), were included in the probed OMM models. Because increased levels of anionic PS lipids have potential effects on schizophrenia and, more specifically, on monoamine oxidase B enzyme activity, the effect of varying the PS concentration was explored. The MD simulations indicate that the complex membrane lipid composition (MLC) behavior is notably different from the two-component PC-PE model. The MLC changes caused relatively minor effects on the membrane structural properties such as membrane thickness or area per lipid; however, notable effects could be seen with the dynamical parameters at the water-membrane interface. Increase of PS levels appears to slow down lateral diffusion of all lipids and, in general, the presence of anionic lipids reduced hydration and slowed down the PE headgroup rotation. In addition, sodium ions could neutralize the membrane surface, when PI was the main anionic component; however, a similar effect was not seen for high PS levels. Based on these results, it is advisable for future studies on the OMM and its protein or ligand partners, especially when wanting to replicate the correct properties on the water-membrane interface, to use models that are sufficiently complex, containing anionic lipid types, PI in particular.

Pathophysiology of the Antiphospholipid Antibody Syndrome

The antiphospholipid syndrome is characterized by antibodies directed against phospholipid-binding proteins and phospholipids attached to cell membrane receptors, mitochondria, oxidized lipoproteins, and activated complement components. When antibodies bind to these complex antigens, cells are activated and the coagulation and complement cascades are triggered, culminating in thrombotic events and pregnancy morbidity that further define the syndrome. The phospholipid-binding proteins most often involved are annexins II and V, β2-glycoprotein I, prothrombin, and cardiolipin. A distinguishing feature of the antiphospholipid syndrome is the "lupus anticoagulant". This is not a single entity but rather a family of antibodies directed against complex antigens consisting of β2-glycoprotein I and/or prothrombin bound to an anionic phospholipid. Although these antibodies prolong in vitro clotting times by competing with clotting factors for phospholipid binding sites, they are not associated with clinical bleeding. Rather, they are thrombogenic because they augment thrombin production in vivo by concentrating prothrombin on phospholipid surfaces. Other antiphospholipid antibodies decrease the clot-inhibitory properties of the endothelium and enhance platelet adherence and aggregation. Some are atherogenic because they increase lipid peroxidation by reducing paraoxonase activity, and others impair fetal nutrition by diminishing placental antithrombotic and fibrinolytic activity. This plethora of destructive autoantibodies is currently managed with immunomodulatory agents, but new approaches to treatment might include vaccines against specific autoantigens, blocking the antibodies generated by exposure to cytoplasmic DNA, and selective targeting of aberrant B-cells to reduce or eliminate autoantibody production.

In-silico-library-based method enables rapid and comprehensive annotation of cardiolipins and cardiolipin oxidation products using high resolution tandem mass spectrometer

Accumulated evidences suggest that cardiolipins (CLs) and cardiolipin oxidation products (oxCLs) are a class of essential molecules that play critical roles in many physiological functions. Diversity of four acyl chains leads to high structure complexity for cardiolipin species including CLs, monolysocardiolipins (MLCLs) and their oxCLs. The ability to rapidly identify CL species can be implemented by the match of mass spectrometry (MS)-based in-silico spectral database. In this study, after optimizing the chromatography conditions and MS detection, an in-silico library containing 377,754 simulated tandem mass spectra deducing from 31,578 CLs to 52,160 of MLCLs was successfully augmented based on LipidBlast templates. For the construction of the oxCLs' library, twenty-five fatty acyls oxidation products relating to nine oxidation types were permuted and combined. A total of 42,180 oxCL spectra were predicted based on the experimental measurements of oxCLs forming by artificially oxidation. Applying the in-silico database to murine mitochondria and cell samples enabled the sensitive and comprehensive annotation of 86 MLCLs, 307 CLs and 112 oxCLs with high annotation confidence. Compared to the conventional method, our proposed in-silico database provides a more comprehensive interpretation for CL species' characterization with high throughput and sensitivity in nontarget lipidomic study.

Lipoprotein Proteomics and Aortic Valve Transcriptomics Identify Biological Pathways Linking Lipoprotein(a) Levels to Aortic Stenosis

Lipoprotein(a) (Lp(a)) is one of the most important risk factors for the development of calcific aortic valve stenosis (CAVS). However, the mechanisms through which Lp(a) causes CAVS are currently unknown. Our objectives were to characterize the Lp(a) proteome and to identify proteins that may be differentially associated with Lp(a) in patients with versus without CAVS. Our second objective was to identify genes that may be differentially regulated by exposure to high versus low Lp(a) levels in explanted aortic valves from patients with CAVS. We isolated Lp(a) from the blood of 21 patients with CAVS and 22 volunteers and performed untargeted label-free analysis of the Lp(a) proteome. We also investigated the transcriptomic signature of calcified aortic valves from patients who underwent aortic valve replacement with high versus low Lp(a) levels (n = 118). Proteins involved in the protein activation cascade, platelet degranulation, leukocyte migration, and response to wounding may be associated with Lp(a) depending on CAVS status. The transcriptomic analysis identified genes involved in cardiac aging, chondrocyte development, and inflammation as potentially influenced by Lp(a). Our multi-omic analyses identified biological pathways through which Lp(a) may cause CAVS, as well as key molecular events that could be triggered by Lp(a) in CAVS development.

Mitochondrial DAMPs and altered mitochondrial dynamics in OxLDL burden in atherosclerosis

Atherosclerosis results in life-threatening cardiovascular pathologies, including ischemic heart disease, stroke, myocardial infarction, and peripheral arterial disease. The role of increased serum low-density lipoprotein (LDL) and resultant accumulation of oxidized-LDL (oxLDL) in atheroma formation is well established. Recent findings elucidate the significance of mitochondrial damage-associated molecular patterns (mtDAMPs) in triggering sterile inflammation in concert with oxLDL. The mtDAMPs including mitochondrial DNA (mtDNA), cytochrome C, cardiolipin, heat shock protein 60 (HSP60), mitochondrial transcription factor A (TFAM), and N-formyl peptides, are expected to possess proatherogenic roles. However, limited data are available in the literature. The mtDAMPs initiate sterile inflammation in atherosclerotic lesions via numerous signaling pathways, most of which converge to the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome. Priming the activation of the NLRP3 inflammasome, mtDAMPs promote secretion of proinflammatory cytokines, including interleukin-1β (IL-1β), implicated in atherosclerotic lesions through vascular smooth muscle and fibroblast proliferation, arterial wall thickening, and plaque formation. In this article we critically reviewed and discussed the central role of the NLRP3 inflammasome in mtDAMP-induced sterile inflammation in atherosclerosis with specific components including caspase-1, pregnane X receptor (PXR), adenosine monophosphate activated protein kinase (AMPK), protein phosphatase 2A (PP2A), thioredoxin-interacting protein (TXNIP), and downstream cytokines including IL-1β and IL-18 as potential mediators of atherosclerosis. Better understanding of the proinflammatory effects of mtDAMPs and its pathological association with oxLDL possess immense translational significance for novel therapeutic intervention.

Lauric acid alleviates insulin resistance by improving mitochondrial biogenesis in THP-1 macrophages

Mitochondrial dysfunction plays a crucial role in the central pathogenesis of insulin resistance and type 2 diabetes mellitus. Macrophages play important roles in the pathogenesis of insulin resistance. Lauric acid is a 12-carbon medium chain fatty acid (MCFA) found abundantly in coconut oil or palm kernel oil and it comes with multiple beneficial effects. This research objective was to uncover the effects of the lauric acid on glucose uptake, mitochondrial function and mitochondrial biogenesis in insulin-resistant macrophages. THP-1 monocytes were differentiated into macrophages and induce insulin resistance, before they were treated with increasing doses of lauric acid (5 μM, 10 μM, 20 μM, and 50 μM). Glucose uptake assay, cellular ROS and ATP production assays, mitochondrial content and membrane potential assay were carried out to analyse the effects of lauric acid on insulin resistance and mitochondrial biogenesis in the macrophages. Quantitative RT-PCR (qRT-PCR) and western blot analysis were also performed to determine the expression of the key regulators. Insulin-resistant macrophages showed lower glucose uptake, GLUT-1 and GLUT-3 expression, and increased hallmarks of mitochondrial dysfunction. Interestingly, lauric acid treatment upregulated glucose uptake, GLUT-1 and GLUT-3 expressions. The treatment also restored the mitochondrial biogenesis in the insulin-resistant macrophages by improving ATP production, oxygen consumption, mitochondrial content and potential, while it promoted the expression of mitochondrial biogenesis regulator genes such as TFAM, PGC-1α and PPAR-γ. We show here that lauric acid has the potential to improve insulin sensitivity and mitochondrial dysregulation in insulin-resistant macrophages.

Pilot study of anti-mitochondrial antibodies in antiphospholipid syndrome

BACKGROUND

Mitochondria are intracellular organelles of bacterial origin capable of stimulating the immune system when released into the extracellular milieu. We previously reported the expression of anti-mitochondrial antibodies (AMA) targeting whole organelles (AwMA), mitochondrial DNA (AmtDNA) or mitochondrial RNA (AmtRNA) in patients with systemic lupus erythematosus (SLE). Antiphospholipid syndrome (APS) is an autoimmune condition that may be independent of, or associated with, other diseases, usually SLE. This study aimed to detect AMA in patients with APS and to explore the association with clinical features of APS.

METHODS

AwMA-, AmtDNA- and AmtRNA-IgG and -IgM were detected in a pilot study (healthy controls n = 30 and APS patients n = 24) by direct ELISA, and their levels were associated with demographic and disease characteristics.

RESULTS

AmtDNA-IgM and AmtRNA-IgG and IgM were elevated in APS compared to healthy controls (p = 0.009, p = 0.0005 and p = 0.01, respectively). AwMA-IgG were increased in patients positive for lupus anticoagulant (median ± interquartile range = 0.36 ± 0.31 vs. 0.14 ± 0.08, p = 0.008), and optical density values for AwMA-IgM were correlated with titres of IgM against cardiolipin (r_s = 0.51, p = 0.01). An increment of 0.1 unit of AmtDNA-IgM levels was associated with reduced prior reporting of arterial events (odds ratio = 0.86; 95% confidence interval 0.74-1.00; p = 0.047).

CONCLUSION

Our pilot study suggests that AMA are represented within the autoantibody repertoire in APS and may display different associations with the clinical manifestations of the disease. Further studies should focus on reproducing these preliminary results by following AMA levels through time in larger prospective cohorts.

Mitochondria and Inflammation: Cell Death Heats Up

Mitochondrial outer membrane permeabilization (MOMP) is essential to initiate mitochondrial apoptosis. Due to the disruption of mitochondrial outer membrane integrity, intermembrane space proteins, notably cytochrome c, are released into the cytosol whereupon they activate caspase proteases and apoptosis. Beyond its well-established apoptotic role, MOMP has recently been shown to display potent pro-inflammatory effects. These include mitochondrial DNA dependent activation of cGAS-STING signaling leading to a type I interferon response. Secondly, via an IAP-regulated mechanism, MOMP can engage pro-inflammatory NF-κB signaling. During cell death, apoptotic caspase activity inhibits mitochondrial dependent inflammation. Importantly, by engaging an immunogenic form of cell death, inhibiting caspase function can effectively inhibit tumorigenesis. Unexpectedly, these studies reveal mitochondria as inflammatory signaling hubs during cell death and demonstrate its potential for therapeutic exploitation.

Post-Transplant Natural Antibodies Associate with Kidney Allograft Injury and Reduced Long-Term Survival

Background The development of antibodies specific to HLA expressed on donor tissue (donor-specific antibodies [DSAs]) is a prominent risk factor for kidney graft loss. Non-HLA antibodies with pathogenic potential have also been described, including natural antibodies (Nabs). These IgG Nabs bind to immunogenic self-determinants, including oxidation-related antigens.Methods To examine the relationship of Nabs with graft outcomes, we assessed Nabs in blinded serum specimens collected from a retrospective cohort of 635 patients who received a transplant between 2005 and 2010 at Necker Hospital in Paris, France. Serum samples were obtained immediately before transplant and at the time of biopsy-proven rejection within the first year or 1 year after transplant. Nabs were detected by ELISA through reactivity to the generic oxidized epitope malondialdehyde.Results Univariate Cox regression analysis identified the development of post-transplant Nabs (defined as 50% increase in reactivity to malondialdehyde) as a significant risk factor for graft loss (hazard ratio, 2.68; 95% confidence interval, 1.49 to 4.82; P=0.001). Post-transplant Nabs also correlated with increased mean Banff scores for histologic signs of graft injury in post-transplant biopsy specimens. Multivariable Cox analyses confirmed Nabs development as a risk factor independent from anti-HLA DSAs (hazard ratio, 2.07; 95% confidence interval, 1.03 to 4.17; P=0.04). Moreover, patients with Nabs and DSAs had a further increased risk of kidney graft loss.Conclusions These findings reveal an association between Nabs, kidney graft injury, and eventual graft failure, suggesting the involvement of Nabs in immune mechanisms of rejection.

Phosphatidylglycerol Incorporates into Cardiolipin to Improve Mitochondrial Activity and Inhibits Inflammation

Chronic inflammation and concomitant oxidative stress can induce mitochondrial dysfunction due to cardiolipin (CL) abnormalities in the mitochondrial inner membrane. To examine the responses of mitochondria to inflammation, macrophage-like RAW264.7 cells were activated by Kdo2-Lipid A (KLA) in our inflammation model, and then the mitochondrial CL profile, mitochondrial activity, and the mRNA expression of CL metabolism-related genes were examined. The results demonstrated that KLA activation caused CL desaturation and the partial loss of mitochondrial activity. KLA activation also induced the gene upregulation of cyclooxygenase (COX)-2 and phospholipid scramblase 3, and the gene downregulation of COX-1, lipoxygenase 5, and Δ-6 desaturase. We further examined the phophatidylglycerol (PG) inhibition effects on inflammation. PG supplementation resulted in a 358-fold inhibition of COX-2 mRNA expression. PG(18:1)₂ and PG(18:2)₂ were incorporated into CLs to considerably alter the CL profile. The decreased CL and increased monolysocardiolipin (MLCL) quantity resulted in a reduced CL/MLCL ratio. KLA-activated macrophages responded differentially to PG(18:1)₂ and PG(18:2)₂ supplementation. Specifically, PG(18:1)₂ induced less changes in the CL/MLCL ratio than did PG(18:2)₂, which resulted in a 50% reduction in the CL/MLCL ratio. However, both PG types rescued 20–30% of the mitochondrial activity that had been affected by KLA activation.

Linking autoimmunity to the origin of the adaptive immune system

In jawed vertebrates, the adaptive immune system (AIS) cooperates with the innate immune system (IIS) to protect hosts from infections. Although targeting non-self-components, the AIS also generates self-reactive antibodies which, when inadequately counter-selected, can give rise to autoimmune diseases (ADs). ADs are on the rise in western countries. Why haven’t ADs been eliminated during the evolution of a ∼500 million-year old system? And why have they become more frequent in recent decades? Self-recognition is an attribute of the phylogenetically more ancient IIS and empirical data compellingly show that some self-reactive antibodies, which are classifiable as elements of the IIS rather then the AIS, may protect from (rather than cause) ADs. Here, we propose that the IIS’s self-recognition system originally fathered the AIS and, as a consequence of this relationship, its activity is dampened in hygienic environments. Rather than a mere breakdown or failure of the mechanisms of self-tolerance, ADs might thus arise from architectural constraints.

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

Studying the phenomenon of cellular senescence has been hindered by the lack of senescence-specific markers. As such, detection of proteins informally associated with senescence accompanies the use of senescence-associated β-galactosidase as a collection of semiselective markers to monitor the presence of senescent cells. To identify novel biomarkers of senescence, we immunized BALB/c mice with senescent mouse lung fibroblasts and screened for antibodies that recognized senescence-associated cell-surface antigens by FACS analysis and a newly developed cell-based ELISA. The majority of antibodies that we isolated, cloned, and sequenced belonged to the IgM isotype of the innate immune system. In-depth characterization of one of these monoclonal, polyreactive natural antibodies, the IgM clone 9H4, revealed its ability to recognize the intermediate filament vimentin. By using 9H4, we observed that senescent primary human fibroblasts express vimentin on their cell surface, and MS analysis revealed a posttranslational modification on cysteine 328 (C328) by the oxidative adduct malondialdehyde (MDA). Moreover, elevated levels of secreted MDA-modified vimentin were detected in the plasma of aged senescence-accelerated mouse prone 8 mice, which are known to have deregulated reactive oxygen species metabolism and accelerated aging. Based on these findings, we hypothesize that humoral innate immunity may recognize senescent cells by the presence of membrane-bound MDA-vimentin, presumably as part of a senescence eradication mechanism that may become impaired with age and result in senescent cell accumulation.

A Special Connection between γδ T Cells and Natural Antibodies?

Natural antibodies (NAbs) play an important role in early host defense, autophagy and tissue remodeling, and in immune regulation. They arise spontaneously (without specific immunization), and are already present at birth. NAbs are produced by B1 B cells, MZ B cells and other B cell types. They include all major Ig subclasses but IgM antibodies are prevalent, especially early in development. NAbs may be poly-specific, recognize particular auto-antigens, or detect neo-determinants such as those exposed during apoptosis or generated by oxidation. NAbs do not require cognate T cell help but depend on soluble mediators produced by T cells. Our recent studies suggest that γδ T cells may have a special relationship with NAbs, and play a prominent role in their regulation, in part through the fine-tuning of IL-4 levels. The spontaneously activated state of these cells likely enables their cytokine production and other functions in the absence of external stimulation. Ontogenetically, the earlier arising γδ T cells are better positioned than αβ T cells to shape the developing repertoire of NAbs. Intriguingly, ligand specificities of NAbs and γδ T cell receptors appear to be overlapping, perhaps allowing γδ cognate help for certain NAb specificities. Via NAbs, γδ T cells could exert a regulatory influence on numerous processes in health and disease.

Toll-like Receptors in the Vascular System: Sensing the Dangers Within

Toll-like receptors (TLRs) are components of the innate immune system that respond to exogenous infectious ligands (pathogen-associated molecular patterns, PAMPs) and endogenous molecules that are released during host tissue injury/death (damage-associated molecular patterns, DAMPs). Interaction of TLRs with their ligands leads to activation of downstream signaling pathways that induce an immune response by producing inflammatory cytokines, type I interferons (IFN), and other inflammatory mediators. TLR activation affects vascular function and remodeling, and these molecular events prime antigen-specific adaptive immune responses. Despite the presence of TLRs in vascular cells, the exact mechanisms whereby TLR signaling affects the function of vascular tissues are largely unknown. Cardiovascular diseases are considered chronic inflammatory conditions, and accumulating data show that TLRs and the innate immune system play a determinant role in the initiation and development of cardiovascular diseases. This evidence unfolds a possibility that targeting TLRs and the innate immune system may be a novel therapeutic goal for these conditions. TLR inhibitors and agonists are already in clinical trials for inflammatory conditions such as asthma, cancer, and autoimmune diseases, but their study in the context of cardiovascular diseases is in its infancy. In this article, we review the current knowledge of TLR signaling in the cardiovascular system with an emphasis on atherosclerosis, hypertension, and cerebrovascular injury. Furthermore, we address the therapeutic potential of TLR as pharmacological targets in cardiovascular disease and consider intriguing research questions for future study.

Innate sensing of oxidation-specific epitopes in health and disease

Ageing, infections and inflammation result in oxidative stress that can irreversibly damage cellular structures. The oxidative damage of lipids in membranes or lipoproteins is one of these deleterious consequences that not only alters lipid function but also leads to the formation of neo-self epitopes - oxidation-specific epitopes (OSEs) - which are present on dying cells and damaged proteins. OSEs represent endogenous damage-associated molecular patterns that are recognized by pattern recognition receptors and the proteins of the innate immune system, and thereby enable the host to sense and remove dangerous biological waste and to maintain homeostasis. If this system is dysfunctional or overwhelmed, the accumulation of OSEs can trigger chronic inflammation and the development of diseases, such as atherosclerosis and age-related macular degeneration. Understanding the molecular components and mechanisms that are involved in this process will help to identify individuals with an increased risk of developing chronic inflammation, and will also help to indicate novel modes of therapeutic intervention.

Mitochondria-targeted esculetin alleviates mitochondrial dysfunction by AMPK-mediated nitric oxide and SIRT3 regulation in endothelial cells: potential implications in atherosclerosis

Mitochondria-targeted compounds are emerging as a new class of drugs that can potentially alter the pathophysiology of those diseases where mitochondrial dysfunction plays a critical role. We have synthesized a novel mitochondria-targeted esculetin (Mito-Esc) with an aim to investigate its effect during oxidative stress-induced endothelial cell death and angiotensin (Ang)-II-induced atherosclerosis in ApoE^−/− mice. Mito-Esc but not natural esculetin treatment significantly inhibited H2O2- and Ang-II-induced cell death in human aortic endothelial cells by enhancing NO production via AMPK-mediated eNOS phosphorylation. While L-NAME (NOS inhibitor) significantly abrogated Mito-Esc-mediated protective effects, Compound c (inhibitor of AMPK) significantly decreased Mito-Esc-mediated increase in NO production. Notably, Mito-Esc promoted mitochondrial biogenesis by enhancing SIRT3 expression through AMPK activation; and restored H2O2-induced inhibition of mitochondrial respiration. siSIRT3 treatment not only completely reversed Mito-Esc-mediated mitochondrial biogenetic marker expressions but also caused endothelial cell death. Furthermore, Mito-Esc administration to ApoE^−/− mice greatly alleviated Ang-II-induced atheromatous plaque formation, monocyte infiltration and serum pro-inflammatory cytokines levels. We conclude that Mito-Esc is preferentially taken up by the mitochondria and preserves endothelial cell survival during oxidative stress by modulating NO generation via AMPK. Also, Mito-Esc-induced SIRT3 plays a pivotal role in mediating mitochondrial biogenesis and perhaps contributes to its anti-atherogenic effects.

Malondialdehyde Epitopes as Targets of Immunity and the Implications for Atherosclerosis

Accumulating evidence suggests that oxidation-specific epitopes (OSEs) constitute a novel class of damage-associated molecular patterns (DAMPs) generated during high oxidative stress but also in the physiological process of apoptosis. To deal with the potentially harmful consequences of such epitopes, the immune system has developed several mechanisms to protect from OSEs and to orchestrate their clearance, including IgM natural antibodies and both cellular- and membrane-bound receptors. Here, we focus on malondialdehyde (MDA) epitopes as prominent examples of OSEs that trigger both innate and adaptive immune responses. First, we review the mechanisms of MDA generation, the different types of adducts on various biomolecules and provide relevant examples for physiological carriers of MDA such as apoptotic cells, microvesicles, or oxidized low-density lipoproteins. Based on recent insights, we argue that MDA epitopes contribute to the maintenance of homeostatic functions by acting as markers of elevated oxidative stress and tissue damage. We discuss multiple lines of evidence that MDA epitopes are proinflammatory and thus important targets of innate and adaptive immune responses. Finally, we illustrate the relevance of MDA epitopes in human pathologies by describing their capacity to drive inflammatory processes in atherosclerosis and highlighting protective mechanisms of immunity that could be exploited for therapeutic purposes.

Natural antibodies bridge innate and adaptive immunity

Natural Abs, belonging to isotypes IgM, IgG3, and IgA, were discovered nearly half a century ago. Despite knowledge about the role of the polyreactive natural IgM in pathogen elimination, B cell survival and homeostasis, inflammatory diseases, and autoimmunity, there is a lack of clarity about the physiological role of natural IgG and natural IgA because they appear incapable of recognizing Ags on their own and are perceived as nonreactive. However, recent research revealed exciting functions of natural IgG in innate immunity. Natural IgG:lectin collaboration swiftly and effectively kills invading pathogens. These advances prompt further examination of natural Abs in immune defense and homeostasis, with the potential for developing novel therapeutics. This review provides new insights into the interaction between natural Abs and lectins, with implications on how interactions between molecules of the innate and adaptive immune systems bridge these two arms of immunity.

The Role of Cardiolipin in Cardiovascular Health

Cardiolipin (CL), the signature phospholipid of mitochondrial membranes, is crucial for both mitochondrial function and cellular processes outside of the mitochondria. The importance of CL in cardiovascular health is underscored by the life-threatening genetic disorder Barth syndrome (BTHS), which manifests clinically as cardiomyopathy, skeletal myopathy, neutropenia, and growth retardation. BTHS is caused by mutations in the gene encoding tafazzin, the transacylase that carries out the second CL remodeling step. In addition to BTHS, CL is linked to other cardiovascular diseases (CVDs), including cardiomyopathy, atherosclerosis, myocardial ischemia-reperfusion injury, heart failure, and Tangier disease. The link between CL and CVD may possibly be explained by the physiological roles of CL in pathways that are cardioprotective, including mitochondrial bioenergetics, autophagy/mitophagy, and mitogen activated protein kinase (MAPK) pathways. In this review, we focus on the role of CL in the pathogenesis of CVD as well as the molecular mechanisms that may link CL functions to cardiovascular health.

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

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.

Antibodies against native and oxidized cardiolipin and phosphatidylserine and phosphorylcholine in atherosclerosis development

Background

Antibodies against cardiolipin and phosphatidylserine (anti-CL and anti-PS) are associated with thrombosis. In contrast, we determined that IgM antibodies against oxidized CL and PS (OxCL and OxPS) and phosphorylcholine (anti-PC) could be protection markers for cardiovascular disease (CVD).

Methods

226 individuals with established hypertension (diastolic pressure>95 mmHg) from the European Lacidipine Study on Atherosclerosis. Antibodies were tested by ELISA. As a surrogate measure of atherosclerosis, the mean of the maximum intima-media thicknesses (IMT) in the far walls of common carotids and bifurcations was determined by ultrasonography at the time of inclusion and 4 years following inclusion.

Results

Increases in IMT measures at follow-up were significantly less common in subjects which at baseline had high IgM anti-OxPS and anti-PC at above 75^th percentile: OR 0,45, CI (0,23–0,86) and OR 0.37, CI (0,19–0,71), p = 0.0137 respectively and above 90^th percentile: OR 0.32, CI (0,12–0,84) and OR 0.39, CI (0,15–1.00), p = 0.050 and OR 0,22, CI (0,08–0,59) p = 0,0029. IgM anti-OxCL was negatively associated with IMT increases (OR, 0.32, CI (0,12–0,84), p = 0231). There were no associations for IgM anti-PS or anti-CL. Anti-PC, as determined herein by a commercial ELISA, was strongly associated with data from our previously published in house ELISA (R = 0,87; p<0,0001).) Anti-PC was also a risk marker at low levels (below 25^th percentile; OR = 2,37 (1,16–4,82), p = 0,0177).

Conclusions

High levels of IgM anti-OxPS and anti-OxCL, but not traditional anti-phospholipid antibodies (anti-PS and anti-CL), are associated with protection against atherosclerosis development. In addition, low IgM anti-PC was a risk marker but high a protection marker.

ABCG1 is required for pulmonary B-1 B cell and natural antibody homeostasis

Many metabolic diseases, including atherosclerosis, type 2 diabetes, pulmonary alveolar proteinosis, and obesity, have a chronic inflammatory component involving both innate and adaptive immunity. Mice lacking the ATP-binding cassette transporter G1 (ABCG1) develop chronic inflammation in the lungs, which is associated with the lipid accumulation (cholesterol, cholesterol ester, and phospholipid) and cholesterol crystal deposition that are characteristic of atherosclerotic lesions and pulmonary alveolar proteinosis. In this article, we demonstrate that specific lipids, likely oxidized phospholipids and/or sterols, elicit a lung-specific immune response in Abcg1(-/-) mice. Loss of ABCG1 results in increased levels of specific oxysterols, phosphatidylcholines, and oxidized phospholipids, including 1-palmitoyl-2-(5'-oxovaleroyl)-sn-glycero-3-phosphocholine, in the lungs. Further, we identify a niche-specific increase in natural Ab (NAb)-secreting B-1 B cells in response to this lipid accumulation that is paralleled by increased titers of IgM, IgA, and IgG against oxidation-specific epitopes, such as those on oxidized low-density lipoprotein and malondialdehyde-modified low-density lipoprotein. Finally, we identify a cytokine/chemokine signature that is reflective of increased B cell activation, Ab secretion, and homing. Collectively, these data demonstrate that the accumulation of lipids in Abcg1(-/-) mice induces the specific expansion and localization of B-1 B cells, which secrete NAbs that may help to protect against the development of atherosclerosis. Indeed, despite chronic lipid accumulation and inflammation, hyperlipidemic mice lacking ABCG1 develop smaller atherosclerotic lesions compared with controls. These data also suggest that Abcg1(-/-) mice may represent a new model in which to study the protective functions of B-1 B cells/NAbs and suggest novel targets for pharmacologic intervention and treatment of disease.

Oxidized but not native cardiolipin has pro-inflammatory effects, which are inhibited by Annexin A5

OBJECTIVE

Cardiolipin (CL) is a phospholipid with an unusual dimeric structure containing four double-bonds and is easily oxidized. CL is present in mitochondria. Here we explored potential pro-inflammatory properties implicated in cardiovascular disease (CVD): activation of endothelial cells, 5-lipoxygenase (5-LOX) and leukotriene B4 (LTB4), by oxidized CL (oxCL) and inhibitory effects of Annexin A5, an antithrombotic and antiinflammatory plasma protein.

METHODS

In monocytes/macrophages and neutrophils, calcium mobilization was monitored spectrophotometrically with Fura-2 and synthesis of LTB4 was analyzed by EIA. Expression of adhesion molecules on endothelial cells was studied by FACScan. Binding of Annexin A5 were analyzed by ELISA. The mRNA expression of 5-LOX and cyclooxygenase-2 was assessed by Real-Time PCR.

RESULTS

We demonstrate that oxCL but not its non-oxidized counterpart CL induces biosynthesis of LTB4 and increases intracellular concentrations of calcium in monocytes/macrophages and neutrophils. oxCL rather than CL selectively elevates gene expression of 5-LOX but not COX-2 in human macrophages. Furthermore, oxCL but not CL raises levels of adhesion molecules ICAM-1 and VCAM-1 in endothelial cells. Annexin A5 can bind oxCL to abolish all these oxCL-induced effects.

CONCLUSIONS

oxCL may promote inflammation and related diseases especially in conditions involving unresolved apoptosis and necrosis, such as atherosclerosis, where free oxCL is likely to be released from liberated mitochondria. Increased intracellular calcium could activate 5-LOX to produce Leukotriene B4 (LTB4). Annexin A5 inhibits the pro-inflammatory effects of oxCL and its potential therapeutic use when oxCL is implicated in inflammation could be of interest.

Human monoclonal Fab and human plasma antibodies to carbamyl-epitopes cross-react with malondialdehyde-adducts

Oxidized low-density lipoprotein (OxLDL) plays a crucial role in the development of atherosclerosis. Carbamylated LDL has been suggested to promote atherogenesis in patients with chronic kidney disease. Here we observed that plasma IgG and IgM antibodies to carbamylated epitopes were associated with IgG and IgM antibodies to oxidation-specific epitopes (ρ = 0·65-0·86, P < 0·001) in healthy adults, suggesting a cross-reaction between antibodies recognizing carbamyl-epitopes and malondialdehyde (MDA)/malondialdehyde acetaldehyde (MAA) -adducts. We used a phage display technique to clone a human Fab antibody that bound to carbamylated LDL and other carbamylated proteins. Anti-carbamyl-Fab (Fab106) cross-reacted with oxidation-specific epitopes, especially with MDA-LDL and MAA-LDL. We showed that Fab106 bound to apoptotic Jurkat cells known to contain these oxidation-specific epitopes, and the binding was competed with soluble carbamylated and MDA-/MAA-modified LDL and BSA. In addition, Fab106 was able to block the uptake of carbamyl-LDL and MDA-LDL by macrophages and stained mouse atherosclerotic lesions. The observed cross-reaction between carbamylated and MDA-/MAA-modified LDL and its contribution to enhanced atherogenesis in uraemic patients require further investigation.

Mitochondrial damage-associated molecular patterns and vascular function

Immune system activation occurs not only due to foreign stimuli, but also due to endogenous molecules. As such, endogenous molecules that are released into the circulation due to cell death and/or injury alarm the immune system that something has disturbed homeostasis and a response is needed. Collectively, these molecules are known as damage-associated molecular patterns (DAMPs). Mitochondrial DAMPs (mtDAMPs) are potent immunological activators due to the bacterial ancestry of mitochondria. Mitochondrial DAMPs are recognized by specific pattern recognition receptors of the innate immune system, some of which are expressed in the cardiovascular system. Cell death leads to release of mtDAMPs that may induce vascular changes by mechanisms that are currently not well understood. This review will focus on recently published evidence linking mtDAMPs and immune system activation to vascular dysfunction and cardiovascular disease.

Emerging roles of mitochondria ROS in atherosclerotic lesions: causation or association?

Mitochondrial-derived reactive oxygen species (mtROS) is one of the major sources of cellular ROS, and excessive mtROS is associated with atherosclerosis progression in both human and mouse models. This review aims to summarize the most recent studies showing the existence, the causes and pathological consequences of excessive mtROS in atherosclerosis. Despite numerous association and causation studies demonstrating the importance of mtROS in atherosclerosis progression, the failure of antioxidant therapy in human randomized clinical trials demands more definitive, cell-type specific investigations. Better mechanistic understanding of mtROS in atherosclerosis may lead to more effective therapeutic strategies.

Autoantibodies to posttranslational modifications in rheumatoid arthritis

Autoantibodies have been associated with human pathologies for a long time, particularly with autoimmune diseases (AIDs). Rheumatoid factor (RF) is known since the late 1930s to be associated with rheumatoid arthritis (RA). The discovery of anticitrullinated protein antibodies in the last century has changed this and other posttranslational modifications (PTM) relevant to RA have since been described. Such PTM introduce neoepitopes in proteins that can generate novel autoantibody specificities. The recent recognition of these novel specificities in RA provides a unique opportunity to understand human B-cell development in vivo. In this paper, we will review the three of the main classes of PTMs already associated with RA: citrullination, carbamylation, and oxidation. With the advancement of research methodologies it should be expected that other autoantibodies against PTM proteins could be discovered in patients with autoimmune diseases. Many of such autoantibodies may provide significant biomarker potential.

Low Levels of Antibodies Against Oxidized but not Nonoxidized Cardiolipin and Phosphatidylserine Are Associated with Atherosclerotic Plaques in Systemic Lupus Erythematosus

Objective.

We have reported that the prevalence of atherosclerotic plaques and their echolucency was increased in systemic lupus erythematosus (SLE). We here study antibodies against oxidized cardiolipin (anti-OxCL) and phosphatidylserine (anti-OxPS) in SLE and in relation to atherosclerosis measures.

Methods.

Patients with SLE (n = 114) were compared with age- and sex-matched population-based controls (n = 122). Common carotid intima-media thickness and plaque occurrence were determined by B-mode ultrasonography. Plaques were graded according to echogenicity as 1–4, with 1 being echolucent. Antibodies were determined by ELISA.

Results.

In the SLE group, the prevalence of low IgM anti-OxPS and low total IgM levels (below 33rd percentile) was increased compared to controls (p = 0.045 and p = 0.0079, respectively). Among SLE patients with atherosclerotic plaques, the prevalence of low IgM anti-OxPS (p = 0.0019) and anti-OxCL (p = 0.031) was increased. Only IgM anti-OxPS remained significant (p = 0.019) after adjusting for other significant factors. Echolucent plaques (total, or left side) were more prevalent among SLE patients with low IgM anti-OxCL and anti-OxPS when controlled for other significant factors (p < 0.05). Low total IgM was independently associated with echolucent plaque on left side (p < 0.05), but not other atherosclerosis measures. IgM anticardiolipin antibodies (aCL) and antiphosphatidylserine antibodies (anti-PS) were higher among SLE patients with cardiopulmonary disease, including arterial, valvular, and venous disease (p < 0.05). There were no associations between antibodies and other disease manifestations or activity. Both anti-OxCL and anti-OxPS, in contrast to aCL, and anti-PS, were cofactor−β2-glycoprotein I (β2-GPI)-independent.

Conclusion.

The prevalence of low levels of IgM anti-OxCL and anti-OxPS (both cofactor-β2-GPI-independent) is associated with the presence of plaques and echolucent plaques in SLE.

Natural IgG autoantibodies are abundant and ubiquitous in human sera, and their number is influenced by age, gender, and disease

The presence of self-reactive IgG autoantibodies in human sera is largely thought to represent a breakdown in central tolerance and is typically regarded as a harbinger of autoimmune pathology. In the present study, immune-response profiling of human serum from 166 individuals via human protein microarrays demonstrates that IgG autoantibodies are abundant in all human serum, usually numbering in the thousands. These IgG autoantibodies bind to human antigens from organs and tissues all over the body and their serum diversity is strongly influenced by age, gender, and the presence of specific diseases. We also found that serum IgG autoantibody profiles are unique to an individual and remarkably stable over time. Similar profiles exist in rat and swine, suggesting conservation of this immunological feature among mammals. The number, diversity, and apparent evolutionary conservation of autoantibody profiles suggest that IgG autoantibodies have some important, as yet unrecognized, physiological function. We propose that IgG autoantibodies have evolved as an adaptive mechanism for debris-clearance, a function consistent with their apparent utility as diagnostic indicators of disease as already established for Alzheimer’s and Parkinson’s diseases.

B-1 cell immunoglobulin directed against oxidation-specific epitopes

Natural antibodies (NAbs) are pre-existing antibodies with germline origin that arise in the absence of previous exposure to foreign antigens. NAbs are produced by B-1 lymphocytes and are primarily of the IgM isotype. There is accumulating evidence that - in addition to their role in antimicrobial host defense - NAbs exhibit important housekeeping functions by facilitating the non-immunogenic clearance of apoptotic cells as well as the removal of (neo-)self antigens. These properties are largely mediated by the ability of NAbs to recognize highly conserved and endogenously generated structures, which are exemplified by so-called oxidation-specific epitopes (OSEs) that are products of lipid peroxidation. The generation of OSEs as well as their interaction with the immune system have been studied extensively in the context of atherosclerosis, a chronic inflammatory disease of the vascular wall that is characterized by the accumulation of cellular debris and oxidized low-density lipoproteins (OxLDL). Both apoptotic cells as well as OxLDL carry OSEs that are targeted by NAbs. Therefore, OSEs represent stress-induced neo self-structures that mediate recognition of metabolic waste (e.g., cellular debris) by NAbs, allowing its safe disposal, which has fundamental implications in health and disease.

Low levels of IgM antibodies to oxidized cardiolipin increase and high levels decrease risk of cardiovascular disease among 60-year olds: a prospective study

Background

Antibodies against cardiolipin (aCL) are associated with increased risk of cardiovascular disease (CVD). We here determine the role of antibodies against oxidized CL (aOxCL).

Methods

One third of sixty-year olds from the Stockholm County were screened (2039 men, 2193 women), where 211 incident CVD-cases and 633 age- and sex-matched controls were identified (5–7 year follow-up). Antibodies were determined by ELISA and uptake of oxLDL in macrophages by FACScan.

Results

IgM aOxCL was lower among CVD cases than controls (p=0.024). aOxCL-levels were divided in quartiles with the highest quartile set as the reference group. After adjustment for smoking, BMI, type II diabetes, hypercholesterolaemia and hypertension, an increased risk was determined in the lowest quartile of IgM aOxCL (OR: 1.80, CI: 1.12–2.91, p=0.0159); OR for men in the lowest quartile was 2.46 (CI 1.34–4.53, p=0.0037) for CVD and for stroke: 12.28 (CI: 1.48-101.77, p=0.02). IgG aOxCL levels did not differ between quartiles in CVD-risk. High levels of IgM aOxCL (reaching significance above 86th) and IgG aOxCL (above 95th percentile) were associated with decreased risk of CVD (OR: 0.485, CI: 0.283-0.829; p=0.0082 and OR: 0.23, CI: 0.07-0.69; p=0.0091). aCL were not associated with CVD. oxCL but not CL competed out uptake of OxLDL in macrophages, and aOxLDL recognized oxCL but not CL. In contrast to aCL, aOxCL was not dependent on co-factor Beta2-glycoprotein-I.

Conclusions

aOxCL is a novel risk/protection marker for CVD, with therapeutic implications. OxCL competes with oxLDL for uptake in macrophages and the possibility that aOxCL inhibits such uptake by interfering with same or similar epitopes in oxCL and oxLDL should be further studied.

High-density lipoprotein and 4F peptide reduce systemic inflammation by modulating intestinal oxidized lipid metabolism: novel hypotheses and review of literature

Oxidized phospholipids are found in the vasculature of animal models of atherosclerosis, in human atherosclerotic lesions, and in other inflammatory diseases. Oxidized phospholipids cause vascular and nonvascular cells to initiate an inflammatory reaction. Metabolites of arachidonic acid, such as 12-hydroxyeicosatetraenoic acid, can mimic some of the inflammatory properties of oxidized phospholipids. In vitro and in vivo normal high-density lipoprotein (HDL), normal apolipoprotein A-I, and apolipoprotein A-I mimetic peptides, each likely acting in a different manner, prevent the inflammatory reaction characteristic of atherosclerosis, and this is associated with decreased levels of oxidized lipids in tissues and cells. HDL from animal models of atherosclerosis or from humans with atherosclerosis or from humans or animals with other chronic inflammatory diseases does not prevent the inflammatory reaction characteristic of atherosclerosis and may even enhance the inflammatory reaction. In mice and perhaps humans, ≈30% of the steady-state plasma HDL-cholesterol pool is derived from the small intestine. The metabolism of phospholipids by gut bacteria has been recently implicated in atherosclerosis in both mice and humans. Studies with apolipoprotein A-I mimetic peptides suggest that the small intestine is a major tissue regulating systemic inflammation in mouse models of atherosclerosis and may be important for determining the functionality of HDL.

Emerging applications for zebrafish as a model organism to study oxidative mechanisms and their roles in inflammation and vascular accumulation of oxidized lipids

With the advent of genetic engineering, zebrafish (Danio rerio) were recognized as an attractive model organism to study many biological processes. Remarkably, the small size and optical transparency of zebrafish larvae enable high-resolution imaging of live animals. Zebrafish respond to various environmental and pathological factors with robust oxidative stress. In this article, we provide an overview of the molecular mechanisms involved in oxidative stress and antioxidant response in zebrafish. Existing applications of genetically encoded fluorescent sensors allow imaging, in real time, of the production of H(2)O(2) and studying its involvement in inflammatory responses, as well as activation of the oxidation-sensitive transcription factors HIF and NRF2. Oxidative stress, combined with hyperlipidemia, leads to oxidation of lipoproteins, the process that contributes significantly to the development of atherosclerosis in humans. Recent work found that feeding zebrafish a high-cholesterol diet results in hypercholesterolemia, vascular lipid accumulation, and extreme lipoprotein oxidation. Generation of a transgenic zebrafish expressing a green fluorescent protein-tagged human antibody to malondialdehyde (MDA)-modified LDL makes possible the in vivo visualization of MDA epitopes in the vascular wall and testing of the efficacy of antioxidants and dietary interventions. Thus, using zebrafish as a model organism provides important advantages in studying the roles of reactive oxygen species and lipid oxidation in basic biologic and pathologic processes.

Peptide mimotopes of malondialdehyde epitopes for clinical applications in cardiovascular disease

Autoantibodies specific for malondialdehyde-modified LDL (MDA-LDL) represent potential biomarkers to predict cardiovascular risk. However, MDA-LDL is a high variability antigen with limited reproducibility. To identify peptide mimotopes of MDA-LDL, phage display libraries were screened with the MDA-LDL-specific IgM monoclonal Ab LRO4, and the specificity and antigenic properties of MDA mimotopes were assessed in vitro and in vivo. We identified one 12-mer linear (P1) and one 7-mer cyclic (P2) peptide carrying a consensus sequence, which bound specifically to murine and human anti-MDA monoclonal Abs. Furthermore, MDA mimotopes were found to mimic MDA epitopes on the surface of apoptotic cells. Immunization of mice with P2 resulted in the induction of MDA-LDL-specific Abs, which strongly immunostained human atherosclerotic lesions. We detected IgG and IgM autoAbs to both MDA mimotopes in sera of healthy subjects and patients with myocardial infarction and stable angina pectoris undergoing percutaneous coronary intervention, and the titers of autoAbs correlated significantly with respective Ab titers against MDA-LDL. In conclusion, we identified specific peptides that are immunological mimotopes of MDA. These mimotopes can serve as standardized and reproducible antigens that will be useful for diagnostic and therapeutic applications in cardiovascular disease.

Recognition of Porphyromonas gingivalis gingipain epitopes by natural IgM binding to malondialdehyde modified low-density lipoprotein

Objective

Increased risk for atherosclerosis is associated with infectious diseases including periodontitis. Natural IgM antibodies recognize pathogen-associated molecular patterns on bacteria, and oxidized lipid and protein epitopes on low-density lipoprotein (LDL) and apoptotic cells. We aimed to identify epitopes on periodontal pathogen Porphyromonas gingivalis recognized by natural IgM binding to malondialdehyde (MDA) modified LDL.

Methods and Results

Mouse monoclonal IgM (MDmAb) specific for MDA-LDL recognized epitopes on P. gingivalis on flow cytometry and chemiluminescence immunoassays. Immunization of C57BL/6 mice with P. gingivalis induced IgM, but not IgG, immune response to MDA-LDL and apoptotic cells. Immunization of LDLR^−/− mice with P. gingivalis induced IgM, but not IgG, immune response to MDA-LDL and diminished aortic lipid deposition. On Western blot MDmAb bound to P. gingivalis fragments identified as arginine-specific gingipain (Rgp) by mass spectrometry. Recombinant domains of Rgp produced in E. coli were devoid of phosphocholine epitopes but contained epitopes recognized by MDmAb and human serum IgM. Serum IgM levels to P. gingivalis were associated with anti-MDA-LDL levels in humans.

Conclusion

Gingipain of P. gingivalis is recognized by natural IgM and shares molecular identity with epitopes on MDA-LDL. These findings suggest a role for natural antibodies in the pathogenesis of two related inflammatory diseases, atherosclerosis and periodontitis.

Protective Roles of Natural IgM Antibodies

Antibodies are a vital part of the armamentarium of the adaptive immune system for the fine-tuning of the recognition and response to foreign threats. However, in health there are some types of antibodies that instead recognize self-antigens and these contribute to the enhancement of primitive innate functions. This repertoire of natural IgM antibodies is postulated to have been selected during immune evolution for their contributions to critical immunoregulatory and housekeeping properties. The clearance of dying cells is one of the most essential responsibilities of the immune system, which is required to prevent uncontrolled inflammation and autoimmunity. In the murine immune system, natural IgM antibodies that recognize apoptotic cells have been shown to enhance the phagocytic clearance of dead and dying cells and to suppress innate immune signaling pathways. In the mouse, natural IgM are often the products of B-1 cell clones that arise during immune development without an absolute requirement for exogenous antigenic stimulation. In patients with systemic lupus erythematosus, IgM autoantibodies, which bind to neo-epitopes on apoptotic cells, have been demonstrated to be present at significantly higher levels in patients with lower disease activity and with less severe organ damage. While certain specificities of IgM autoantibodies correlate with protection from lupus renal disease, others may convey protective properties from lupus-associated atherosclerotic cardiovascular disease. New and unexpected insights into the functional roles of IgM antibodies are still emerging, especially regarding the functions of natural antibodies. Herein, we review recent progress in our understanding of the potential roles of natural IgM autoantibodies in the regulation of immune homeostasis and for protection from autoimmune and inflammatory diseases.

The innate immune response to products of phospholipid peroxidation

Lipid peroxidation occurs in the context of many physiological processes but is greatly increased in various pathological situations. A consequence of phospholipid peroxidation is the generation of oxidation-specific epitopes, such as phosphocholine of oxidized phospholipids and malondialdehyde, which form neo-self determinants on dying cells and oxidized low-density lipoproteins. In this review we discuss evidence demonstrating that pattern recognition receptors of the innate immune system recognize oxidation-specific epitopes as endogenous damage-associated molecular patterns, allowing the host to identify dangerous biological waste. Oxidation-specific epitopes are important targets of both cellular and soluble pattern recognition receptors, including toll-like and scavenger receptors, C-reactive protein, complement factor H, and innate natural IgM antibodies. This recognition allows the innate immune system to mediate important physiological house keeping functions, for example by promoting the removal of dying cells and oxidized molecules. Once this system is malfunctional or overwhelmed the development of diseases, such as atherosclerosis and age-related macular degeneration is favored. Understanding the molecular components and mechanisms involved in this process, will help the identification of individuals with increased risk of developing chronic inflammation, and indicate novel points for therapeutic intervention. This article is part of a Special Issue entitled: Oxidized phospholipids—their properties and interactions with proteins.

Naturally occurring IgM antibodies to oxidation-specific epitopes

Naturally occurring antibodies (NAbs) have specificity for both microbial and self antigens, which allows them to act in the first line defense against invading pathogens, as well as in tissue homeostasis by mediating the clearance of cellular debris. This latter recognition of self by NAbs was often thought to reflect the polyreactivity of low affinity antibodies. The finding that oxidation-specific epitopes are dominant targets of naturally occurring IgM antibodies shed light on this and provided novel insights into the understanding of the house keeping functions of NAbs. Oxidation-specific epitopes represent stress-induced or altered self structures that are generated as a consequence of lipidperoxidation during many physiological and pathological situations. Importantly, the same structures have been found in the membranes of dying cells. Only oxidized lipids and dying cells-but not native membrane lipids or viable cells-are recognized by this set of NAbs. Thus, oxidation-specific epitopes represent ideal marks that identify biological waste for its clearance and the neutralization of its pro-inflammatory properties. Furthermore, this binding property of NAbs has also important implications for various chronic inflammatory diseases, including atherosclerosis.

Oxidation-specific epitopes are danger-associated molecular patterns recognized by pattern recognition receptors of innate immunity

Oxidation reactions are vital parts of metabolism and signal transduction. However, they also produce reactive oxygen species, which damage lipids, proteins and DNA, generating "oxidation-specific" epitopes. In this review, we discuss the hypothesis that such common oxidation-specific epitopes are a major target of innate immunity, recognized by a variety of "pattern recognition receptors" (PRRs). By analogy with microbial "pathogen-associated molecular patterns" (PAMPs), we postulate that host-derived, oxidation-specific epitopes can be considered to represent "danger (or damage)-associated molecular patterns" (DAMPs). We also argue that oxidation-specific epitopes present on apoptotic cells and their cellular debris provided the primary evolutionary pressure for the selection of such PRRs. Furthermore, because many PAMPs on microbes share molecular identity and/or mimicry with oxidation-specific epitopes, such PAMPs provide a strong secondary selecting pressure for the same set of oxidation-specific PRRs as well. Because lipid peroxidation is ubiquitous and a major component of the inflammatory state associated with atherosclerosis, the understanding that oxidation-specific epitopes are DAMPs, and thus the target of multiple arcs of innate immunity, provides novel insights into the pathogenesis of atherosclerosis. As examples, we show that both cellular and soluble PRRs, such as CD36, toll-like receptor-4, natural antibodies, and C-reactive protein recognize common oxidation-specific DAMPs, such as oxidized phospholipids and oxidized cholesteryl esters, and mediate a variety of immune responses, from expression of proinflammatory genes to excessive intracellular lipoprotein accumulation to atheroprotective humoral immunity. These insights may lead to improved understanding of inflammation and atherogenesis and suggest new approaches to diagnosis and therapy.

Natural IgM antibodies against oxidation-specific epitopes

INTRODUCTION

Natural antibodies (NAbs) are mostly antibodies of the IgM isotype with germline or close to germline encoded variable regions that provide them with specificity for both microbial and altered self antigens. Thus, natural IgM possess an important function in the first line defense against invading pathogens and in tissue homeostasis by regulating the clearance of cellular debris.

RESULTS

Oxidation-specific epitopes represent prominent examples of stress-induced altered self, as they are generated ubiquitously as a consequence of lipid peroxidation during many physiological and pathological situations, and are found on the membranes of apoptotic cells. Importantly, oxidation-specific epitopes are dominant targets of natural IgM antibodies, indicating an important role for natural IgM in their clearance and the neutralization of their pro-inflammatory properties.

SUMMARY AND CONCLUSION

These findings and the insights they provide into the ability of NAbs in mediating host homeostasis in health and diseases, including atherosclerosis and other chronic inflammatory diseases, will be discussed.

The importance of natural IgM: scavenger, protector and regulator

The existence of IgM has been known for more than a century, but its importance in immunity and autoimmunity continues to emerge. Studies of mice deficient in secreted IgM have provided unexpected insights into its role in several diverse processes, from B cell survival to atherosclerosis, as well as in autoimmunity and protection against infection. Among the various distinct properties that underlie the functions of IgM, two stand out: its polyreactivity and its ability to facilitate the removal of apoptotic cells. In addition, new B cell-targeted therapies for the treatment of autoimmunity have been shown to cause a reduction in serum IgM, potentially disrupting the functions of this immunoregulatory molecule and increasing susceptibility to infection.

Generation and biological activities of oxidized phospholipids

Glycerophospholipids represent a common class of lipids critically important for integrity of cellular membranes. Oxidation of esterified unsaturated fatty acids dramatically changes biological activities of phospholipids. Apart from impairment of their structural function, oxidation makes oxidized phospholipids (OxPLs) markers of "modified-self" type that are recognized by soluble and cell-associated receptors of innate immunity, including scavenger receptors, natural (germ line-encoded) antibodies, and C-reactive protein, thus directing removal of senescent and apoptotic cells or oxidized lipoproteins. In addition, OxPLs acquire novel biological activities not characteristic of their unoxidized precursors, including the ability to regulate innate and adaptive immune responses. Effects of OxPLs described in vitro and in vivo suggest their potential relevance in different pathologies, including atherosclerosis, acute inflammation, lung injury, and many other conditions. This review summarizes current knowledge on the mechanisms of formation, structures, and biological activities of OxPLs. Furthermore, potential applications of OxPLs as disease biomarkers, as well as experimental therapies targeting OxPLs, are described, providing a broad overview of an emerging class of lipid mediators.

Platelet activation by low concentrations of intact oxidized LDL particles involves the PAF receptor

OBJECTIVE

Mitochondrial depolarization aids platelet activation. Oxidized LDL (oxLDL) contains the medium length oxidatively truncated phospholipid hexadecyl azelaoyl-lysoPAF (HAz-LPAF) that disrupts mitochondrial function in nucleated cells, so oxLDL may augment platelet activation.

METHODS AND RESULTS

Flow cytometry showed intact oxLDL particles synergized with subthreshold amounts of soluble agonists to increase intracellular Ca2+, and initiate platelet aggregation and surface expression of activated gpIIb/IIIa and P-selectin. oxLDL also induced aggregation and increased intracellular Ca2+ in FURA2-labeled cells by itself at low, although not higher, concentrations. HAz-LPAF, alone and in combination with substimulatory amounts of thrombin, rapidly increased cytoplasmic Ca2+ and initiated aggregation. HAz-LPAF depolarized mitochondria in intact platelets, but this required concentrations beyond those that directly activated platelets. An unexpectedly large series of chemically pure truncated phospholipids generated by oxidative fragmentation of arachidonoyl-, docosahexaneoyl-, or linoleoyl alkyl phospholipids were platelet agonists. The PAF receptor, thought to effectively recognize only phospholipids with very short sn-2 residues, was essential for platelet activation because PAF receptor agonists blocked signaling by all these medium length phospholipids and oxLDL.

CONCLUSIONS

Intact oxLDL particles activate platelets through the PAF receptor, and the PAF receptor responds to a far wider range of oxidized phospholipids in oxLDL than anticipated.