Publisher Correction: Oxidized phospholipids are proinflammatory and proatherogenic in hypercholesterolaemic mice.

In this Letter, affiliation number 1 was originally missing from the HTML; the affiliations were missing for author Ming-Yow Hung in the HTML; and the Fig. 4 legend erroneously referred to panels a-h, instead of a-g. These errors have been corrected online.

Oxidized phospholipids are proinflammatory and proatherogenic in hypercholesterolaemic mice.

Oxidized phospholipids (OxPL) are ubiquitous, are formed in many inflammatory tissues, including atherosclerotic lesions, and frequently mediate proinflammatory changes 1 . Because OxPL are mostly the products of non-enzymatic lipid peroxidation, mechanisms to specifically neutralize them are unavailable and their roles in vivo are largely unknown. We previously cloned the IgM natural antibody E06, which binds to the phosphocholine headgroup of OxPL, and blocks the uptake of oxidized low-density lipoprotein (OxLDL) by macrophages and inhibits the proinflammatory properties of OxPL2-4. Here, to determine the role of OxPL in vivo in the context of atherogenesis, we generated transgenic mice in the Ldlr-/- background that expressed a single-chain variable fragment of E06 (E06-scFv) using the Apoe promoter. E06-scFv was secreted into the plasma from the liver and macrophages, and achieved sufficient plasma levels to inhibit in vivo macrophage uptake of OxLDL and to prevent OxPL-induced inflammatory signalling. Compared to Ldlr-/- mice, Ldlr -/- E06-scFv mice had 57-28% less atherosclerosis after 4, 7 and even 12 months of 1% high-cholesterol diet. Echocardiographic and histologic evaluation of the aortic valves demonstrated that E06-scFv ameliorated the development of aortic valve gradients and decreased aortic valve calcification. Both cholesterol accumulation and in vivo uptake of OxLDL were decreased in peritoneal macrophages, and both peritoneal and aortic macrophages had a decreased inflammatory phenotype. Serum amyloid A was decreased by 32%, indicating decreased systemic inflammation, and hepatic steatosis and inflammation were also decreased. Finally, the E06-scFv prolonged life as measured over 15 months. Because the E06-scFv lacks the functional effects of an intact antibody other than the ability to bind OxPL and inhibit OxLDL uptake in macrophages, these data support a major proatherogenic role of OxLDL and demonstrate that OxPL are proinflammatory and proatherogenic, which E06 counteracts in vivo. These studies suggest that therapies inactivating OxPL may be beneficial for reducing generalized inflammation, including the progression of atherosclerosis, aortic stenosis and hepatic steatosis.

Development and validation of an isoform-independent monoclonal antibody-based ELISA for measurement of lipoprotein(a).

The study aims were to develop a new isoform-independent enzyme-linked immunoassay (ELISA) for the measurement of lipoprotein(a) [Lp(a)], validate its performance characteristics, and demonstrate its accuracy by comparison with the gold-standard ELISA method and an LC-MS/MS candidate reference method, both developed at the University of Washington. The principle of the new assay is the capture of Lp(a) with monoclonal antibody LPA4 primarily directed to an epitope in apolipoprotein(a) KIV2 and its detection with monoclonal antibody LPA-KIV9 directed to a single antigenic site present on KIV9. Validation studies were performed following the guidelines of the Clinical Laboratory Improvement Amendments and the College of American Pathologists. The analytical measuring range of the LPA4/LPA-KIV9 ELISA is 0.27-1,402 nmol/L, and the method meets stringent criteria for precision, linearity, spike and recovery, dilutability, comparison of plasma versus serum, and accuracy. Method comparison with both the gold-standard ELISA and the LC-MS/MS method performed in 64 samples with known apolipoprotein(a) isoforms resulted in excellent correlation with both methods (r=0.987 and r=0.976, respectively). Additionally, the variation in apolipoprotein(a) size accounted for only 0.2% and 2.2% of the bias variation, respectively, indicating that the LPA4/LPA-KIV9 ELISA is not affected by apolipoprotein(a) size polymorphism. Peptide mapping and competition experiments demonstrated that the measuring monoclonal antibodies used in the gold-standard ELISA (a-40) and in the newly developed ELISA (LPA-KIV9) are directed to the same epitope, 4076LETPTVV4082, on KIV9. In conclusion, no statistically or clinically significant bias was observed between Lp(a) measurements obtained by the LPA4/LPA-KIV9 ELISA and those obtained by the gold-standard ELISA or LC-MS/MS, and therefore, the methods are considered equivalent.

Intracellular AIBP (Apolipoprotein A-I Binding Protein) Regulates Oxidized LDL (Low-Density Lipoprotein)-Induced Mitophagy in Macrophages.

OBJECTIVE: Atherosclerotic lesions are often characterized by accumulation of OxLDL (oxidized low-density lipoprotein), which is associated with vascular inflammation and lesion vulnerability to rupture. Extracellular AIBP (apolipoprotein A-I binding protein; encoded by APOA1BP gene), when secreted, promotes cholesterol efflux and regulates lipid rafts dynamics, but its role as an intracellular protein in mammalian cells remains unknown. The aim of this work was to determine the function of intracellular AIBP in macrophages exposed to OxLDL and in atherosclerotic lesions. Approach and Results: Using a novel monoclonal antibody against human and mouse AIBP, which are highly homologous, we demonstrated robust AIBP expression in human and mouse atherosclerotic lesions. We observed significantly reduced autophagy in bone marrow-derived macrophages, isolated from Apoa1bp-/- compared with wild-type mice, which were exposed to OxLDL. In atherosclerotic lesions from Apoa1bp-/- mice subjected to Ldlr knockdown and fed a Western diet, autophagy was reduced, whereas apoptosis was increased, when compared with that in wild-type mice. AIBP expression was necessary for efficient control of reactive oxygen species and cell death and for mitochondria quality control in macrophages exposed to OxLDL. Mitochondria-localized AIBP, via its N-terminal domain, associated with E3 ubiquitin-protein ligase PARK2 (Parkin), MFN (mitofusin)1, and MFN2, but not BNIP3 (Bcl2/adenovirus E1B 19-kDa-interacting protein-3), and regulated ubiquitination of MFN1 and MFN2, key components of mitophagy. CONCLUSIONS: These data suggest that intracellular AIBP is a new regulator of autophagy in macrophages. Mitochondria-localized AIBP augments mitophagy and participates in mitochondria quality control, protecting macrophages against cell death in the context of atherosclerosis.

Diversification and CXCR4-Dependent Establishment of the Bone Marrow B-1a Cell Pool Governs Atheroprotective IgM Production Linked to Human Coronary Atherosclerosis.

RATIONALE: B-1 cell-derived natural IgM antibodies against oxidation-specific epitopes on low-density lipoprotein are anti-inflammatory and atheroprotective. Bone marrow (BM) B-1a cells contribute abundantly to IgM production, yet the unique repertoire of IgM antibodies generated by BM B-1a and the factors maintaining the BM B-1a population remain unexplored. CXCR4 (C-X-C motif chemokine receptor 4) has been implicated in human cardiovascular disease and B-cell homeostasis, yet the role of B-1 cell CXCR4 in regulating atheroprotective IgM levels and human cardiovascular disease is unknown. OBJECTIVE: To characterize the BM B-1a IgM repertoire and to determine whether CXCR4 regulates B-1 production of atheroprotective IgM in mice and humans. METHODS AND RESULTS: Single-cell sequencing demonstrated that BM B-1a cells from aged ApoE-/- mice with established atherosclerosis express a unique repertoire of IgM antibodies containing increased nontemplate-encoded nucleotide additions and a greater frequency of unique heavy chain complementarity determining region 3 sequences compared with peritoneal cavity B-1a cells. Some complementarity determining region 3 sequences were common to both compartments suggesting B-1a migration between compartments. Indeed, mature peritoneal cavity B-1a cells migrated to BM in a CXCR4-dependent manner. Furthermore, BM IgM production and plasma IgM levels were reduced in ApoE-/- mice with B-cell-specific knockout of CXCR4, and overexpression of CXCR4 on B-1a cells increased BM localization and plasma IgM against oxidation specific epitopes, including IgM specific for malondialdehyde-modified LDL (low-density lipoprotein). Finally, in a 50-subject human cohort, we find that CXCR4 expression on circulating human B-1 cells positively associates with plasma levels of IgM antibodies specific for malondialdehyde-modified LDL and inversely associates with human coronary artery plaque burden and necrosis. CONCLUSIONS: These data provide the first report of a unique BM B-1a cell IgM repertoire and identifies CXCR4 expression as a critical factor selectively governing BM B-1a localization and production of IgM against oxidation specific epitopes. That CXCR4 expression on human B-1 cells was greater in humans with low coronary artery plaque burden suggests a potential targeted approach for immune modulation to limit atherosclerosis.

Generation and characterization of LPA-KIV9, a murine monoclonal antibody binding a single site on apolipoprotein (a).

Lipoprotein (a) [Lp(a)] is a risk factor for CVD and a target of therapy, but Lp(a) measurements are not globally standardized. Commercially available assays generally use polyclonal antibodies that detect multiple sites within the kringle (K)IV2 repeat region of Lp(a) and may lead to inaccurate assessments of plasma levels. With increasing awareness of Lp(a) as a cardiovascular risk factor and the active clinical development of new potential therapeutic approaches, the broad availability of reagents capable of providing isoform independence of Lp(a) measurements is paramount. To address this issue, we generated a murine monoclonal antibody that binds to only one site on apo(a). A BALB/C mouse was immunized with a truncated version of apo(a) that contained eight total KIV repeats, including only one copy of KIV2 We generated hybridomas, screened them, and successfully produced a KIV2-independent monoclonal antibody, named LPA-KIV9. Using a variety of truncated apo(a) constructs to map its binding site, we found that LPA-KIV9 binds to KIV9 without binding to plasminogen. Fine peptide mapping revealed that LPA-KIV9 bound to the sequence 4076LETPTVV4082 on KIV9 In conclusion, the generation of monoclonal antibody LPA-KIV9 may be a useful reagent in basic research studies and in the clinical application of Lp(a) measurements.

A monoclonal antibody to assess oxidized cholesteryl esters associated with apoAI and apoB-100 lipoproteins in human plasma.

Atherosclerosis is associated with increased lipid peroxidation, leading to generation of multiple oxidation-specific epitopes (OSEs), contributing to the pathogenesis of atherosclerosis and its clinical manifestation. Oxidized cholesteryl esters (OxCEs) are a major class of OSEs found in human plasma and atherosclerotic tissue. To evaluate OxCEs as a candidate biomarker, we generated a novel mouse monoclonal Ab (mAb) specific to an OxCE modification of proteins. The mAb AG23 (IgG1) was raised in C57BL6 mice immunized with OxCE-modified keyhole limpet hemocyanin, and hybridomas were screened against OxCE-modified BSA. This method ensures mAb specificity to the OxCE modification, independent of a carrier protein. AG23 specifically stained human carotid artery atherosclerotic lesions. An ELISA method, with AG23 as a capture and either anti-apoAI or anti-apoB-100 as the detection Abs, was developed to assay apoAI and apoB-100 lipoproteins that have one or more OxCE epitopes. OxCE-apoA or OxCE-apoB did not correlate with the well-established oxidized phospholipid-apoB biomarker. In a cohort of subjects treated with atorvastatin, OxCE-apoA was significantly lower than in the placebo group, independent of the apoAI levels. These results suggest the potential diagnostic utility of a new biomarker assay to measure OxCE-modified lipoproteins in patients with CVD.

ABCG1 regulates pulmonary surfactant metabolism in mice and men.

Idiopathic pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by accumulation of surfactant. Surfactant synthesis and secretion are restricted to epithelial type 2 (T2) pneumocytes (also called T2 cells). Clearance of surfactant is dependent upon T2 cells and macrophages. ABCG1 is highly expressed in both T2 cells and macrophages. ABCG1-deficient mice accumulate surfactant, lamellar body-loaded T2 cells, lipid-loaded macrophages, B-1 lymphocytes, and immunoglobulins, clearly demonstrating that ABCG1 has a critical role in pulmonary homeostasis. We identify a variant in the ABCG1 promoter in patients with PAP that results in impaired activation of ABCG1 by the liver X receptor α, suggesting that ABCG1 basal expression and/or induction in response to sterol/lipid loading is essential for normal lung function. We generated mice lacking ABCG1 specifically in either T2 cells or macrophages to determine the relative contribution of these cell types on surfactant lipid homeostasis. These results establish a critical role for T2 cell ABCG1 in controlling surfactant and overall lipid homeostasis in the lung and in the pathogenesis of human lung disease.

B-1b Cells Secrete Atheroprotective IgM and Attenuate Atherosclerosis.

RATIONALE: B cells contribute to atherosclerosis through subset-specific mechanisms. Whereas some controversy exists about the role of B-2 cells, B-1a cells are atheroprotective because of secretion of atheroprotective IgM antibodies independent of antigen. B-1b cells, a unique subset of B-1 cells that respond specifically to T-cell-independent antigens, have not been studied within the context of atherosclerosis. OBJECTIVE: To determine whether B-1b cells produce atheroprotective IgM antibodies and function to protect against diet-induced atherosclerosis. METHODS AND RESULTS: We demonstrate that B-1b cells are sufficient to produce IgM antibodies against oxidation-specific epitopes on low-density lipoprotein both in vitro and in vivo. In addition, we demonstrate that B-1b cells provide atheroprotection after adoptive transfer into B- and T-cell deficient (Rag1(-/-)Apoe(-/-)) hosts. We implicate inhibitor of differentiation 3 (Id3) in the regulation of B-1b cells as B-cell-specific Id3 knockout mice (Id3(BKO)Apoe(-/-)) have increased numbers of B-1b cells systemically, increased titers of oxidation-specific epitope-reactive IgM antibodies, and significantly reduced diet-induced atherosclerosis when compared with Id3(WT)Apoe(-/-) controls. Finally, we report that the presence of a homozygous single nucleotide polymorphism in ID3 in humans that attenuates Id3 function is associated with an increased percentage of circulating B-1 cells and anti-malondialdehyde-low-density lipoprotein IgM suggesting clinical relevance. CONCLUSIONS: These results provide novel evidence that B-1b cells produce atheroprotective oxidation-specific epitope-reactive IgM antibodies and protect against atherosclerosis in mice and suggest that similar mechanisms may occur in humans.

Blockade of Tim-1 and Tim-4 Enhances Atherosclerosis in Low-Density Lipoprotein Receptor-Deficient Mice.

OBJECTIVE: T cell immunoglobulin and mucin domain (Tim) proteins are expressed by numerous immune cells, recognize phosphatidylserine on apoptotic cells, and function as costimulators or coinhibitors. Tim-1 is expressed by activated T cells but is also found on dendritic cells and B cells. Tim-4, present on macrophages and dendritic cells, plays a critical role in apoptotic cell clearance, regulates the number of phosphatidylserine-expressing activated T cells, and is genetically associated with low low-density lipoprotein and triglyceride levels. Because these functions of Tim-1 and Tim-4 could affect atherosclerosis, their modulation has potential therapeutic value in cardiovascular disease. APPROACH AND RESULTS: ldlr(-/-) mice were fed a high-fat diet for 4 weeks while being treated with control (rat immunoglobulin G1) or anti-Tim-1 (3D10) or -Tim-4 (21H12) monoclonal antibodies that block phosphatidylserine recognition and phagocytosis. Both anti-Tim-1 and anti-Tim-4 treatments enhanced atherosclerosis by 45% compared with controls by impairment of efferocytosis and increasing aortic CD4(+)T cells. Consistently, anti-Tim-4-treated mice showed increased percentages of activated T cells and late apoptotic cells in the circulation. Moreover, in vitro blockade of Tim-4 inhibited efferocytosis of oxidized low-density lipoprotein-induced apoptotic macrophages. Although anti-Tim-4 treatment increased T helper cell (Th)1 and Th2 responses, anti-Tim-1 induced Th2 responses but dramatically reduced the percentage of regulatory T cells. Finally, combined blockade of Tim-1 and Tim-4 increased atherosclerotic lesion size by 59%. CONCLUSIONS: Blockade of Tim-4 aggravates atherosclerosis likely by prevention of phagocytosis of phosphatidylserine-expressing apoptotic cells and activated T cells by Tim-4-expressing cells, whereas Tim-1-associated effects on atherosclerosis are related to changes in Th1/Th2 balance and reduced circulating regulatory T cells.

Protective Role for B-1b B Cells and IgM in Obesity-Associated Inflammation, Glucose Intolerance, and Insulin Resistance.

OBJECTIVE: Little is known about the role(s) B cells play in obesity-induced metabolic dysfunction. This study used a mouse with B-cell-specific deletion of Id3 (Id3(Bcell KO)) to identify B-cell functions involved in the metabolic consequences of obesity. APPROACH AND RESULTS: Diet-induced obese Id3(Bcell KO) mice demonstrated attenuated inflammation and insulin resistance in visceral adipose tissue (VAT), and improved systemic glucose tolerance. VAT in Id3(Bcell KO) mice had increased B-1b B cells and elevated IgM natural antibodies to oxidation-specific epitopes. B-1b B cells reduced cytokine production in VAT M1 macrophages, and adoptively transferred B-1b B cells trafficked to VAT and produced natural antibodies for the duration of 13-week studies. B-1b B cells null for Id3 demonstrated increased proliferation, established larger populations in Rag1(-/-) VAT, and attenuated diet-induced glucose intolerance and VAT insulin resistance in Rag1(-/-) hosts. However, transfer of B-1b B cells unable to secrete IgM had no effect on glucose tolerance. In an obese human population, results provided the first evidence that B-1 cells are enriched in human VAT and IgM antibodies to oxidation-specific epitopes inversely correlated with inflammation and insulin resistance. CONCLUSIONS: NAb-producing B-1b B cells are increased in Id3(Bcell KO) mice and attenuate adipose tissue inflammation and glucose intolerance in diet-induced obese mice. Additional findings are the first to identify VAT as a reservoir for human B-1 cells and to link anti-inflammatory IgM antibodies with reduced inflammation and improved metabolic phenotype in obese humans.

MD-2 binds cholesterol.

Cholesterol is a structural component of cellular membranes, which is transported from liver to peripheral cells in the form of cholesterol esters (CE), residing in the hydrophobic core of low-density lipoprotein. Oxidized CE (OxCE) is often found in plasma and in atherosclerotic lesions of subjects with cardiovascular disease. Our earlier studies have demonstrated that OxCE activates inflammatory responses in macrophages via toll-like receptor-4 (TLR4). Here we demonstrate that cholesterol binds to myeloid differentiation-2 (MD-2), a TLR4 ancillary molecule, which is a binding receptor for bacterial lipopolysaccharide (LPS) and is indispensable for LPS-induced TLR4 dimerization and signaling. Cholesterol binding to MD-2 was competed by LPS and by OxCE-modified BSA. Furthermore, soluble MD-2 in human plasma and MD-2 in mouse atherosclerotic lesions carried cholesterol, the finding supporting the biological significance of MD-2 cholesterol binding. These results help understand the molecular basis of TLR4 activation by OxCE and mechanisms of chronic inflammation in atherosclerosis.

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.

Innate response activator B cells aggravate atherosclerosis by stimulating T helper-1 adaptive immunity.

BACKGROUND: Atherosclerotic lesions grow via the accumulation of leukocytes and oxidized lipoproteins in the vessel wall. Leukocytes can attenuate or augment atherosclerosis through the release of cytokines, chemokines, and other mediators. Deciphering how leukocytes develop, oppose, and complement each other's function and shape the course of disease can illuminate our understanding of atherosclerosis. Innate response activator (IRA) B cells are a recently described population of granulocyte macrophage colony-stimulating factor-secreting cells of hitherto unknown function in atherosclerosis. METHODS AND RESULTS: Here, we show that IRA B cells arise during atherosclerosis in mice and humans. In response to a high-cholesterol diet, IRA B cell numbers increase preferentially in secondary lymphoid organs via Myd88-dependent signaling. Mixed chimeric mice lacking B cell-derived granulocyte macrophage colony-stimulating factor develop smaller lesions with fewer macrophages and effector T cells. Mechanistically, IRA B cells promote the expansion of classic dendritic cells, which then generate interferon γ-producing T helper-1 cells. This IRA B cell-dependent T helper-1 skewing manifests in an IgG1-to-IgG2c isotype switch in the immunoglobulin response against oxidized lipoproteins. CONCLUSIONS: Granulocyte macrophage colony-stimulating factor-producing IRA B cells alter adaptive immune processes and shift the leukocyte response toward a T helper-1-associated milieu that aggravates atherosclerosis.

Interleukin-3/granulocyte macrophage colony-stimulating factor receptor promotes stem cell expansion, monocytosis, and atheroma macrophage burden in mice with hematopoietic ApoE deficiency.

OBJECTIVE: Coronary heart disease is associated with monocytosis. Studies using animal models of monocytosis and atherosclerosis such as ApoE(-/-) mice have shown bone marrow (BM) hematopoietic stem and multipotential progenitor cell (HSPC) expansion, associated with increased cell surface expression of the common ß subunit of the granulocyte macrophage colony-stimulating factor/interleukin-3 receptor (CBS) on HSPCs. ApoE(-/-) mice also display increased granulocyte macrophage colony-stimulating factor-dependent monocyte production in the spleen. We investigated the role of the CBS in cholesterol-driven HSPC expansion, monocytosis, and atherosclerosis. APPROACH AND RESULTS: Ldlr(-/-) mice were transplanted with ApoE(-/-)Cbs(-/-) or ApoE(-/-) BM followed by Western-type diet feeding. Compared with ApoE(-/-) BM-transplanted controls, ApoE(-/-)Cbs(-/-) BM-transplanted mice had reduced BM and splenic HSPC proliferation, fewer blood monocytes and neutrophils, and reduced macrophage content and area of early atherosclerotic lesions. More advanced lesions showed diminished macrophage and collagen content; however, lesion size was unchanged, reflecting an increase in necrotic core area, associated with a marked decrease in Abcg1 expression and increased macrophage apoptosis. Compared with wild-type mice, Western-type diet-fed ApoE(-/-) mice showed increased CBS expression on granulocyte macrophage colony-stimulating factor-producing innate response activator B cells and expansion of this population. ApoE(-/-)Cbs(-/-) BM-transplanted Ldlr(-/-) mice showed a marked decrease in innate response activator B cells compared with ApoE(-/-) BM-transplanted Ldlr(-/-) controls. CONCLUSIONS: Increased levels of CBS on HSPCs and splenic innate response activator B cells lead to expansion of these populations in ApoE(-/-) BM-transplanted Ldlr(-/-) mice, contributing to monocytosis and increased lesional macrophage content. However, in more advanced lesions, the CBS also has a role in atherosclerotic plaque stabilization.

Atheroprotective immunization with malondialdehyde-modified LDL is hapten specific and dependent on advanced MDA adducts: implications for development of an atheroprotective vaccine.

Immunization with homologous malondialdehyde (MDA)-modified LDL (MDA-LDL) leads to atheroprotection in experimental models supporting the concept that a vaccine to oxidation-specific epitopes (OSEs) of oxidized LDL could limit atherogenesis. However, modification of human LDL with OSE to use as an immunogen would be impractical for generalized use. Furthermore, when MDA is used to modify LDL, a wide variety of related MDA adducts are formed, both simple and more complex. To define the relevant epitopes that would reproduce the atheroprotective effects of immunization with MDA-LDL, we sought to determine the responsible immunodominant and atheroprotective adducts. We now demonstrate that fluorescent adducts of MDA involving the condensation of two or more MDA molecules with lysine to form malondialdehyde-acetaldehyde (MAA)-type adducts generate immunodominant epitopes that lead to atheroprotective responses. We further demonstrate that a T helper (Th) 2-biased hapten-specific humoral and cellular response is sufficient, and thus, MAA-modified homologous albumin is an equally effective immunogen. We further show that such Th2-biased humoral responses per se are not atheroprotective if they do not target relevant antigens. These data demonstrate the feasibility of development of a small-molecule immunogen that could stimulate MAA-specific immune responses, which could be used to develop a vaccine approach to retard or prevent atherogenesis.

Helix-loop-helix factor inhibitor of differentiation 3 regulates interleukin-5 expression and B-1a B cell proliferation.

OBJECTIVE: Natural immunity is emerging as an important mediator of protection from atherogenesis. Natural IgM antibodies that recognize oxidation-specific epitopes on low-density lipoprotein or phospholipids and the B-1a B cells that produce them attenuate atherosclerosis. We previously demonstrated that Apoe(-/-) mice globally deficient in the helix-loop-helix protein inhibitor of differentiation 3 (Id3) develop early diet-induced atherosclerosis. Furthermore, B cell-mediated attenuation of atherosclerosis in B cell-deficient mice was dependent on Id3. Here, we sought to determine whether Id3 regulates B-1a B cells and the natural antibodies that they produce and identify mechanisms mediating these effects. APPROACH AND RESULTS: Mice lacking Id3 had significantly fewer B-1a B cells in the spleen and peritoneal cavity and reduced serum levels of the natural antibody E06. B cell-specific deletion of Id3 revealed that this effect was not because of the loss of Id3 in B cells. Interleukin (IL)-33 induced abundant, Id3-dependent IL-5 production in the recently identified innate lymphoid cell, the natural helper (NH) cell, but not Th2 or mast cells. In addition, delivery of IL-5 to Id3-deficient mice restored B-1a B cell proliferation. B-1a B cells were present in aortic samples also containing NH cells. Aortic NH cells produced IL-5, a B-1a B cell mitogen in response to IL-33 stimulation. CONCLUSIONS: These studies are the first to identify NH and B-1a B cells in the aorta and provide evidence that Id3 is a key regulator of NH cell IL-5 production and B-1a B cell homeostasis.

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.