Single-cell profiling of CD11c+ B cells in atherosclerosis

Circulating CD11c+ B cells, a novel subset of activated B cells, have been linked to autoimmunity and shown to expand with age. Atherosclerosis is an age-associated disease that involves innate and adaptive immune responses to modified self-antigens. Yet, the expression of CD11c on specific B-cell subtypes and its link to atherosclerosis are poorly understood. In this study, we characterized the frequency of CD11c+ B cells in tissues in mice with aging. We observed an age-associated increase in CD11c+ B cells in the spleen and bone marrow of ApoE-/- mice, and this was associated with an increase in aortic plaque. In addition, we also utilized single-cell multi-omics profiling of 60 human subjects undergoing advanced imaging for coronary artery disease (CAD) to subtype CD11c+ B cells and determine their frequency in subjects with high and low severity of CAD. Using unsupervised clustering, we identified four distinct clusters of CD11c+ B cells, which include CD27 and IgD double negative 2 (DN2), age-associated (ABC), CD11c+ unswitched memory (USWM), and activated Naïve (aNav) B cells. We observed an increase in the frequency of both ABC B cells and DN2 B cells in patients with high CAD severity. Pathway analysis further demonstrated augmentation of autophagy, IFNg signaling, and TLR signaling in DN2 cells in high-severity CAD patients. On the other hand, an increase in the negative regulator of BCR signaling through CD72 was found in ABC cells in low-severity CAD patients. Through investigating scRNAseq of atheroma, these DN2 cells were also found to infiltrate human coronary atheroma.

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.

Age Associated B Cells positively associate with atherosclerosis in mice and Humans

Age-Associated B cells (ABCs: CD19+CD11c+) have recently been reported as a new subset of B cells with unique cell surface and transcriptional signatures. ABCs are mostly comprised of antigen experienced memory B cells that arise in response to microbial infections. ABCs have been reported to be elevated in autoimmune diseases, where in, these cells have characteristics of auto-antibody producing memory B cells. However, the role of ABCs during atherosclerosis has not be studied. Atherosclerosis is a chronic inflammatory disease and the major underlying cause for cardiovascular diseases (CVD). We for the first time report the role of ABCs in both atherosclerotic mice and in human CVD. Chow fed 50- and 100- week old, male, ApoEKO mice were used to study atherosclerosis. Lesion area was measured following Sudan-IV enface staining of aorta. 100-week-old mice developed significantly more disease than the 50-week-old mice. Further, through flow cytometric analysis ABCs were detected in spleen, bone marrow (BM) and blood of these mice. The frequency of ABCs from total B cells was significantly higher in all these tissue compartments in 100-week-old mice compared to 50-week-old mice. Additionally, most ABCs (>80 %) were actively proliferating cells (Ki67+), as compared to normal CD11c− B cells (~35 %). In mice, we observed a significant correlation of atherosclerosis disease levels with the frequency of ABCs in spleen (p=0.004, R2=0.61), BM (p=0.01, R2=0.53), and blood (p=0.0005, R2=0.75). In humans as well, the frequency of circulating ABCs was significantly higher in CVD subjects compared to non-CVD subjects. Results from this study suggest a strong association of ABCs with aggravated atherosclerosis in both murine models and humans with CVD.

Abstract 429: Loss Of Ten-Eleven Translocation 2 (TET2) Reduces RNA Expression Of Chemokine Receptor CCR6 And Increases Peritoneal B1 B Cell Number And Total Plasma IgM Level

TET2 is an epigenetic regulator with an emerging role in regulating CVD severity. TET2 deficiency in mice (TET2 -/- ) was shown to increase atherosclerosis via upregulation of proinflammatory pathways in macrophages. While TET2 has been reported to regulate germinal center formation and class switch recombination in atherogenic B2 cells, its role in atheroprotective B1 cell subsets remains largely unexplored. To investigate the role of TET2 in B1 subsets, flow cytometry of cells from the peritoneal cavity (PEC), spleen and bone marrow of TET2 -/- mice and wildtype littermate controls (n = 14/group) was performed. Results demonstrated increased B1a (p = 0.0162) and B1b (p = 0.0056) cells in TET2 -/- mice in the PEC, their primary niche, but not in the spleen, while in the bone marrow only TET2 -/- B1a cells were increased (p = 0.0021). An enzyme linked immunosorbent assay using the plasma from these mice demonstrated a significant increase in total IgM in the TET2 -/- mice compared to the wildtype mice (p = 0.0287). In addition, sort purified PEC B1a, B1b and B2 cells from TET2 -/- mice and wildtype littermate controls were analyzed for methylation status and RNA expression (n = 24). RNAseq analysis of TET2 -/- PEC B1a, B1b, and B2 cells revealed significantly reduced chemokine receptor CCR6 expression in B1a (-4.88 fold change, 8.03E-08 padj.) and a trending reduction of CCR6 in B1b cells (-2.17 fold change, 0.3477 padj.) compared to wildtype. Further, methylation analysis showed significant hypermethylation of CCR6 in B1a and B1b TET2 -/- cells compared to wildtype. This was corroborated by gene pathway analysis of the RNAseq data which showed significant downregulation of chemotaxis and migration pathways in B1a and B1b TET2 -/- cells. We conclude that loss of TET2 increases hypermethylation of CCR6 and suppresses CCR6 RNA expression in B1 cells which may impair trafficking out of the PEC to the spleen, but not necessarily the bone marrow, leading to PEC accumulation of B1 subtypes and an increase in the total plasma level of atheroprotective IgM. Further study is needed to identify mechanisms of potential B1 cell-mediated atheroprotection in the case of TET2 loss.

Loss of Ten-Eleven Translocation 2 (TET2) reduces CCR6 expression and increases B1 B cell number in the peritoneal cavity

TET2 is an evolutionarily conserved dioxygenase that catalyzes the conversion of 5-methyl-cytosine to 5- hydroxymethyl-cytosine, promotes DNA demethylation and regulates transcription. TET2 has been reported to regulate germinal center formation and class switch recombination in B2 B cells, yet its role in B1 B cell subsets remains largely unexplored. To investigate the role of TET2 in B1 subsets, flow cytometry of cells from the peritoneal cavity (PEC), spleen and bone marrow of TET2−/− mice and wildtype littermate controls (n = 14/group) was performed. In addition, sort purified PEC B1a, B1b and B2 cells were analyzed for methylation status (n = 24) and RNA expression (n = 24). Results demonstrated increased B1a (p = 0.0162) and B1b (p = 0.0056) cells in TET2−/− mice in the PEC, their primary niche, but not in the spleen, while in the bone marrow only TET2−/− B1a cells were increased (p = 0.0021). RNAseq analysis of PEC B1a, B1b, and B2 cells from TET2−/− and wildtype mice revealed reduced chemokine receptor CCR6 expression in B1a (−4.879068141 fold change, 1. 8.03E-08 padj.) and B1b cells (−2.17044776 fold change, 0.34771255 padj.) from the TET2−/− mice. Further, methylation analysis showed significant hypermethylation of CCR6 in B1a and B1b TET2−/− cells compared to wildtype B1a and B1b cells. This was corroborated by gene pathway analysis of the RNAseq data which showed significant downregulation of chemotaxis and migration pathways in B1a and B1b TET2−/− cells. We conclude that loss of TET2 increases hypermethylation of CCR6 and suppresses CCR6 RNA expression in B1 cells which may impair trafficking out of the PEC to the spleen, but not necessarily the bone marrow, leading to PEC accumulation of B1 subtypes.

Supported by grants from the NIH: 1R01HL 136098-01 (McNamara, C PI), R01 HL141123 (McNamara, C PI), T32 HL007284 (Dennis, E)

B-1b Cells Possess Unique bHLH-Driven P62-Dependent Self-Renewal and Atheroprotection

BACKGROUND

B1a and B1b lymphocytes produce IgM that inactivates oxidation-specific epitopes (IgM^OSE) on LDL (low-density lipoprotein) and protects against atherosclerosis. Loss of ID3 (inhibitor of differentiation 3) in B cells selectively promotes B1b but not B1a cell numbers, leading to higher IgM^OSE production and reduction in atherosclerotic plaque formation. Yet, the mechanism underlying this regulation remains unexplored.

METHODS

Bulk RNA sequencing was utilized to identify differentially expressed genes in B1a and B1b cells from Id3KO and Id3WT mice. CRISPR/Cas9 and lentiviral genome editing coupled with adoptive transfer were used to identify key Id3-dependent signaling pathways regulating B1b cell proliferation and the impact on atherosclerosis. Biospecimens from humans with advanced coronary artery disease imaging were analyzed to translate murine findings to human subjects with coronary artery disease.

RESULTS

Through RNA sequencing, P62 was found to be enriched in Id3KO B1b cells. Further in vitro characterization reveals a novel role for P62 in mediating BAFF (B-cell activating factor)-induced B1b cell proliferation through interacting with TRAF6 and activating NF-κB (nuclear factor kappa B), leading to subsequent C-MYC upregulation. Promoter-reporter assays reveal that Id3 inhibits the E2A protein from activating the P62 promoter. Mice adoptively transferred with B1 cells overexpressing P62 exhibited an increase in B1b cell number and IgM^OSE levels and were protected against atherosclerosis. Consistent with murine mechanistic findings, P62 expression in human B1 cells was significantly higher in subjects harboring a function-impairing SNP (rs11574) in the ID3 gene and directly correlated with plasma IgM^OSE levels.

CONCLUSIONS

This study unveils a novel role for P62 in driving BAFF-induced B1b cell proliferation and IgM^OSE production to attenuate diet-induced atherosclerosis. Results identify a direct role for Id3 in antagonizing E2A from activating the p62 promoter. Moreover, analysis of putative human B1 cells also implicates these pathways in coronary artery disease subjects, suggesting P62 as a new immunomodulatory target for treating atherosclerosis.

Loss of Id3 (Inhibitor of Differentiation 3) Increases the Number of IgM-Producing B-1b Cells in Ischemic Skeletal Muscle Impairing Blood Flow Recovery During Hindlimb Ischemia

OBJECTIVE

Neovascularization can maintain and even improve tissue perfusion in the setting of limb ischemia during peripheral artery disease. The molecular and cellular mechanisms mediating this process are incompletely understood. We investigate the potential role(s) for Id3 (inhibitor of differentiation 3) in regulating blood flow in a murine model of hindlimb ischemia (HLI). Approach and Results: HLI was modeled through femoral artery ligation and resection and blood flow recovery was quantified by laser Doppler perfusion imaging. Mice with global Id3 deletion had significantly impaired perfusion recovery at 14 and 21 days of HLI. Endothelial- or myeloid cell-specific deletion of Id3 revealed no effect on perfusion recovery while B-cell-specific knockout of Id3 (Id3BKO) revealed a significant attenuation of perfusion recovery. Flow cytometry revealed no differences in ischemia-induced T cells or myeloid cell numbers at 7 days of HLI, yet there was a significant increase in B-1b cells in Id3BKO. Consistent with these findings, ELISA (enzyme-linked immunoassay) demonstrated increases in skeletal muscle and plasma IgM. In vitro experiments demonstrated reduced proliferation and increased cell death when endothelial cells were treated with conditioned media from IgM-producing B-1b cells and tibialis anterior muscles in Id3BKO mice showed reduced density of total CD31+ and αSMA+CD31+ vessels.

CONCLUSIONS

This study is the first to demonstrate a role for B-cell-specific Id3 in maintaining blood flow recovery during HLI. Results suggest a role for Id3 in promoting blood flow during HLI and limiting IgM-expressing B-1b cell expansion. These findings present new mechanisms to investigate in peripheral artery disease pathogenesis.

B Cell-Activating Factor Antagonism Attenuates the Growth of Experimental Abdominal Aortic Aneurysm

B cell-activating factor (BAFF), a tumor necrosis factor family of cytokine, was recently identified as a regulator of atherosclerosis; however, its role in aortic aneurysm has not been determined. Here, we examined the effect of selective BAFF antagonism using an anti-BAFF antibody (blocks binding of BAFF to receptors BAFF receptor 3, transmembrane activator and CAML interactor, and B-cell maturation antigen) and mBaffR-mFc (blocks binding of BAFF to BAFF receptor 3) on a murine model of abdominal aortic aneurysm (AAA). In a prevention strategy, the antagonists were injected before the induction of AAA, and in an intervention strategy, the antagonists were injected after the induction of AAA. Both strategies attenuated the formation of AAA. In the intervention group, BAFF antagonism depleted most of the mature B-cell subsets in spleen and circulation, leading to enhanced resolution of inflammation in AAA as indicated by decreased infiltration of B cells and proinflammatory macrophages and a reduced number of apoptotic cells. In AAA tissues, B cells and macrophages were found in close contact. In vitro, B cells, irrespective of treatment with BAFF, impaired the efferocytosis activity of macrophages, suggesting a direct innate role of B cells on macrophage function. Altogether, BAFF antagonism affects survival of the mature B cells, promotes resolution of inflammation in the aorta, and attenuates the growth of AAA in mice.

Splenic B-1 Cells Shows Diversified IgM Repertoire in Aged Atherosclerotic Mice

B-1 cell derived IgM attenuates chronic inflammatory diseases like atherosclerosis. The spleen is a major lymphoid tissue where B-1 cells (both B-1a and B-1b) produce natural IgM. B-1 derived natural IgM has long been thought to contain restricted predominantly germline encoded V-D-J gene segments with low number of non-template encoded nucleotide (N)-additions, which are added at the junctions between V-D and D-J segments. However, differences in VDJ gene usage and N-addition frequency between peritoneal cavity (PerC) and spleen B-1 cells in aged atherosclerotic mice are unknown. B-1a and B-1b cells were single-cell sorted from PerC and spleen of 100-week-old, chow-fed ApoE−/− mice. Single cell sequencing of the immunoglobulin heavy chain variable region was performed. Splenic B-1 cells displayed increased diversity at the V-D and D-J junctions, as evidenced by increased average number of N-additions. Sequences containing >1 N-additions at both V-D and D-J junctions were 2% and 27% for PerC B-1a and B-1b sequences as compared to 44% and 44% of sequences in splenic B-1a and B-1b respectively. B-1 cells from PerC and spleen showed marked differences in VH, D and JH gene segment usage. Importantly, the most commonly expressed heavy chain complementarity determining region 3 (CDR-H3) amino acid sequence in spleen B-1a (AREVTTMYYFDY) and B-1b (AREDYYGSSYYFDY) cells was different from PerC B-1a (AGDYDGYWYFDV) and B-1b (AGDRDGYWYFDV) cells. Results provide clear evidence of a diversified IgM repertoire expressed by spleen B-1 cells compared to PerC B-1 cells during advanced atherosclerosis. The CDR-H3 sequences of splenic B-1 cells suggest underlying differences in antigen specificity which could further regulate disease progression.

Chemokine Receptor-6 Promotes B-1 Cell Trafficking to Perivascular Adipose Tissue, Local IgM Production and Atheroprotection

Chemokine receptor-6 (CCR6) mediates immune cell recruitment to inflammatory sites and has cell type-specific effects on diet-induced atherosclerosis in mice. Previously we showed that loss of CCR6 in B cells resulted in loss of B cell-mediated atheroprotection, although the B cell subtype mediating this effect was unknown. Perivascular adipose tissue (PVAT) harbors high numbers of B cells including atheroprotective IgM secreting B-1 cells. Production of IgM antibodies is a major mechanism whereby B-1 cells limit atherosclerosis development. Yet whether CCR6 regulates B-1 cell number and production of IgM in the PVAT is unknown. In this present study, flow cytometry experiments demonstrated that both B-1 and B-2 cells express CCR6, albeit at a higher frequency in B-2 cells in both humans and mice. Nevertheless, B-2 cell numbers in peritoneal cavity (PerC), spleen, bone marrow and PVAT were no different in ApoE ^-/- CCR6 ^-/- compared to ApoE ^-/- CCR6 ^+/+ mice. In contrast, the numbers of atheroprotective IgM secreting B-1 cells were significantly lower in the PVAT of ApoE ^-/- CCR6 ^-/- compared to ApoE ^-/- CCR6 ^+/+ mice. Surprisingly, adoptive transfer (AT) of CD43^- splenic B cells into B cell-deficient μMT ^-/- ApoE ^-/- mice repopulated the PerC with B-1 and B-2 cells and reduced atherosclerosis when transferred into ApoE ^-/- CCR6 ^+/+ sIgM ^-/- mice only when those cells expressed both CCR6 and sIgM. CCR6 expression on circulating human B cells in subjects with a high level of atherosclerosis in their coronary arteries was lower only in the putative human B-1 cells. These results provide evidence that B-1 cell CCR6 expression enhances B-1 cell number and IgM secretion in PVAT to provide atheroprotection in mice and suggest potential human relevance to our murine findings.

B Lymphocytes and Adipose Tissue Inflammation

The immune system plays an important role in obesity-induced adipose tissue inflammation and the resultant metabolic dysfunction, which can lead to hypertension, dyslipidemia, and insulin resistance and their downstream sequelae of type 2 diabetes mellitus and cardiovascular disease. While macrophages are the most abundant immune cell type in adipose tissue, other immune cells are also present, such as B cells, which play important roles in regulating adipose tissue inflammation. This brief review will overview B-cell subsets, describe their localization in various adipose depots and summarize our knowledge about the function of these B-cell subsets in regulating adipose tissue inflammation, obesity-induced metabolic dysfunction and atherosclerosis.

Novel Autoimmune IgM Antibody Attenuates Atherosclerosis in IgM Deficient Low-Fat Diet-Fed, but Not Western Diet-Fed Apoe-/- Mice

OBJECTIVE

Oxidized phospholipids (OxPL), such as the oxidized derivatives of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine, 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine, and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine, have been shown to be the principal biologically active components of minimally oxidized LDL (low-density lipoprotein). The role of OxPL in cardiovascular diseases is well recognized, including activation of inflammation within vascular cells. Atherosclerotic Apoe-/- mice fed a high-fat diet develop antibodies to OxPL, and hybridoma B-cell lines producing natural anti-OxPL autoantibodies have been successfully generated and characterized. However, as yet, no studies have been reported demonstrating that treatment with OxPL neutralizing antibodies can be used to prevent or reverse advanced atherosclerosis. Approach and Results: Here, using a screening against 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine/1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine, we generated a novel IgM autoantibody, 10C12, from the spleens of Apoe-/- mice fed a long-term Western diet, that demonstrated potent OxPL neutralizing activity in vitro and the ability to inhibit macrophage accumulation within arteries of Apoe-/- mice fed a Western diet for 4 weeks. Of interest, 10C12 failed to inhibit atherosclerosis progression in Apoe-/- mice treated between 18 and 26 weeks of Western diet feeding likely due at least in part to high levels of endogenous anti-OxPL antibodies. However, 10C12 treatment caused a 40% decrease in lipid accumulation within aortas of secreted IgM deficient, sIgM-/-Apoe-/-, mice fed a low-fat diet, when the antibody was administrated between 32-40 weeks of age.

CONCLUSIONS

Taken together, these results provide direct evidence showing that treatment with a single autoimmune anti-OxPL IgM antibody during advanced disease stages can have an atheroprotective outcome.

In vivo liposomal delivery of PPARα/γ dual agonist tesaglitazar in a model of obesity enriches macrophage targeting and limits liver and kidney drug effects

Macrophages are important regulators of obesity-associated inflammation and PPARα and -γ agonism in macrophages has anti-inflammatory effects. In this study, we tested the efficacy with which liposomal delivery could target the PPARα/γ dual agonist tesaglitazar to macrophages while reducing drug action in common sites of drug toxicity: the liver and kidney, and whether tesaglitazar had anti-inflammatory effects in an in vivo model of obesity-associated dysmetabolism. Methods: Male leptin-deficient (ob/ob) mice were administered tesaglitazar or vehicle for one week in a standard oral formulation or encapsulated in liposomes. Following the end of treatment, circulating metabolic parameters were measured and pro-inflammatory adipose tissue macrophage populations were quantified by flow cytometry. Cellular uptake of liposomes in tissues was assessed using immunofluorescence and a broad panel of cell subset markers by flow cytometry. Finally, PPARα/γ gene target expression levels in the liver, kidney, and sorted macrophages were quantified to determine levels of drug targeting to and drug action in these tissues and cells. Results: Administration of a standard oral formulation of tesaglitazar effectively treated symptoms of obesity-associated dysmetabolism and reduced the number of pro-inflammatory adipose tissue macrophages. Macrophages are the major cell type that took up liposomes with many other immune and stromal cell types taking up liposomes to a lesser extent. Liposome delivery of tesaglitazar did not have effects on inflammatory macrophages nor did it improve metabolic parameters to the extent of a standard oral formulation. Liposomal delivery did, however, attenuate effects on liver weight and liver and kidney expression of PPARα and -γ gene targets compared to oral delivery. Conclusions: These findings reveal for the first time that tesaglitazar has anti-inflammatory effects on adipose tissue macrophage populations in vivo. These data also suggest that while nanoparticle delivery reduced off-target effects, yet the lack of tesaglitazar actions in non-targeted cells such (as hepatocytes and adipocytes) and the uptake of drug-loaded liposomes in many other cell types, albeit to a lesser extent, may have impacted overall therapeutic efficacy. This fulsome analysis of cellular uptake of tesaglitazar-loaded liposomes provides important lessons for future studies of liposome drug delivery.

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.

ApoE attenuates unresolvable inflammation by complex formation with activated C1q

ApoE has been implicated in Alzheimer´s disease, atherosclerosis, and other unresolvable inflammatory conditions but a common mechanism of action remains elusive. We found in ApoE-deficient mice that oxidized lipids activated the classical complement cascade (CCC) resulting in leukocyte infiltration of the choroid plexus (ChP). All human ApoE isoforms attenuated CCC activity via high-affinity binding to the activated CCC-initiating C1q protein (K_D~140-580 pM) in vitro; and C1q-ApoE complexes emerged as markers for ongoing complement activity of diseased ChPs, Aβ plaques, and atherosclerosis in vivo. C1q-ApoE complexes in human ChPs, Aβ plaques, and arteries correlated with cognitive decline and atherosclerosis, respectively. Treatment with siRNA against C5 which is formed by all complement pathways, attenuated murine ChP inflammation, Aβ-associated microglia accumulation, and atherosclerosis. Thus, ApoE is a direct checkpoint inhibitor of unresolvable inflammation and reducing C5 attenuates disease burden.

Abstract 260: BAFF 60mer is Critical for B Cell Activation and BAFF Depletion Suppresses AAA Formation

Marginal zone and follicular B cells together constitute the B2 cell population, which is known to promote cardiovascular diseases by secretion of pathogenic antibodies. However, it is not completely understood how B2 cells are activated. Here, we tested the hypothesis that B cell activating factor (BAFF) activates B2 cells and promote abdominal aortic aneurysm (AAA) formation. Since BAFF can either exist as a 3mer or multimerize to a highly active 60mer, we further examined if the 60mer is critical for B2 cell-mediated pathogenicity. Anti-BAFF antibody (Ab) Sandy-2 was injected to C57BL/6 male mice at 1 mg/kg once in every 14 days. AAA was induced by topical elastase model after 14 days of Sandy-2 injection. Native PAGE and ELISA methods were used to determine binding of Abs to recombinant BAFF 3mer and 60mer. For in vitro experiments, B cells were isolated from murine spleens. Activation of B cells was examined by Western blotting and RNA sequencing and by surface expression of CD23 and MHC II by flow cytometry. Metabolic reprogramming of B cells by BAFF was determined by extracellular flux analysis using a Seahorse XF24 Flux Analyzer. Sandy-2 bound to both 3mer and 60mer, resulting in suppressed AAA formation (n=8, p<0.05) with (1) marked depletion of B2 cells, transitional 2, germinal center, plasma and memory B cells, but not transitional 1 and B1 cells, (2) a lower level of IgG1 and IgG2, and (3) a lack of immunoglobulin deposition in AAA sections. In vitro , the 60mer, but not the 3mer, significantly activated both NF-kB1 and -kB2 signaling, and induced expression of B2 cell activation markers and anti-apoptotic genes in B cells. Inhibitors of NF-kB signaling decreased B cell activation in response to 60mer. The 60mer treatment significantly increased mitochondrial respiration and glycolysis in B cells, supporting an activated status. An antibody against multimerization site of BAFF (anti-multiBAFF) significantly suppressed B cell activation relative to a control Ab, in a neutrophil:B cell co-culture model. The effect of the anti-multiBAFF Ab on AAA formation is currently being tested in our laboratory. Altogether, our results suggest a critical role for BAFF 60mer in skewing B cells to an activated B2 cell phenotype, supporting a pathogenic role of B2 cells in AAA.

Abstract 417: CXCR4 Distinguishes and Maintains Atheroprotective IgM-producing B-1 cells

B1 cells exert protective effects in atherosclerosis through production of anti-inflammatory IgM antibodies recognizing oxidation-specific epitopes, such as MDA-LDL, present in diseased arteries. However, factors mediating B1 IgM production are currently unclear. We evaluated MDA-LDL binding and chemokine receptor expression on human B1 cells in a cohort of subjects undergoing intravascular ultrasound (IVUS) for coronary artery assessment. Results demonstrate that a subset of human B1 cells (~35%) is able to bind MDA-LDL. Moreover, expression of the chemokine receptor CXCR4 on circulating B1 cells associates with increased plasma levels of anti-MDA-LDL IgM antibodies (p=0.0009), and decreased plaque burden in coronary arteries (p=0.0002). Mice with B cell-specific loss of CXCR4 on the atherogenic ApoE -/- background (CXCR4 BKO ) demonstrate fewer B1a cells (n=6-8,p<0.0001) and IgM antibody-secreting cells (n=6,p<0.01) in the bone marrow, and reduced plasma IgM levels (n=6-8,p<0.05), relative to littermate controls (CXCR4 WT ). Furthermore, retroviral-mediated overexpression of CXCR4 on B1a cells in vivo is associated with increased B1a localization to the bone marrow (p<0.01) and increased circulating levels of anti-MDA-LDL IgM antibodies (p<0.05). To determine the atheroprotective role of CXCR4 on the B1a cell subset, we adoptively transferred CXCR4 WT or CXCR4 BKO B1a cells into lymphocyte-deficient Rag1 -/- ApoE -/- mice. After 16 weeks of Western diet feeding, recipients given CXCR4 BKO B1a cells demonstrate reduced plasma IgM levels (n=7,p<0.001), and fewer donor B1a cells in the bone marrow and spleen (n=7,p<0.05) compared to recipients given CXCR4 WT B1a cells. Intriguingly, B1a transfer reduces plasma cholesterol levels in mice regardless of CXCR4 expression (n=7,p<0.05). However, CXCR4 further strengthens the atheroprotective ability of B1a cells, as recipients given CXCR4 WT B1a cells have reduced aortic lesion area compared to PBS controls (n=7,p<0.01) while recipients given CXCR4 BKO B1a cells did not attain the same level of protection. Overall, these data suggest that CXCR4 is an important regulator of IgM production and B1a-mediated atheroprotection.

Abstract 665: Perivascular Adipose Tissue near Aortic Arch is a Major Site for Atheroprotective IgM Producing B-1 Cells

Background: Perivascular adipose tissue (PVAT) regulates artery physiology and pathology, such as promoting atherosclerosis development through local production of inflammatory cytokines. The phenotype of PVAT is region-specific and PVAT composed of brown adipose tissue (near the aortic arch and thoracic aorta) and white adipose tissue (near the abdominal aorta). B-1 cells limit adipose tissue inflammation and atherosclerosis through the production of IgM antibodies. PVAT harbors high numbers of B cells. Id3 is a basic helix-loop-helix protein and important for B cell development. B cell-specific Id3 deficiency (Id3 BKO ) increases B-1b cell numbers and provides atheroprotection. However, the location and regulation of IgM producing B-1 cells in the PVAT are unknown.

Methods and Results: Flow cytometry analysis of PVAT of normal chow diet fed young ApoE -/- mice (n=5) demonstrated that the abundant CD19 + B cells (per gram fat) harbored in PVAT around aortic arch (2.7 + 0.69 x10 5 cells) compared to PVAT near the thoracic (0.9 + 0.29 x10 5 cells) and abdominal aorta (1.1 + 0.51 x10 5 cells). Interestingly, a large proportion (40%) of these B cells are belong to the CD19 hi B220 low B-1 subset in PVAT near aortic arch. CXCL13 is a chemokine, which is important for B-1 cell recruitment to omental fat. Real time-PCR data confirmed that high numbers of B-1 cell recruitment to PVAT near aortic arch is due to high expression of CXCL13 in the PVAT near the aortic arch compared to PVAT near thoracic aorta and abdominal aorta. Moreover, Flow cytometry and ELISPOT data in normal chow fed young ApoE -/- mice with Id3 BKO demonstrated that Id3 BKO increased B-1b cells (Id3 BKO : 1.2 + 0.16 x10 3 ; Id3 WT : 0.4 + 0.06 x10 3 ; p-value: <0.01; n=6 mice/group) and IgM secreting cells (Id3 BKO : 2.0 + 0.42 x10 3 ; Id3 WT : 0.78 + 0.23 x10 3 ; p-value: <0.05; n=6 mice/group) respectively in PVAT near aortic arch compared to Id3 WT control mice.

Conclusion: Results provide the first evidence that atheroprotective B-1 cell profile in PVAT is region-specific and identify Id3 and CXCL13 as regulators of aortic arch PVAT B-1b cell numbers and IgM production.

Perivascular Adipose Tissue Harbors Atheroprotective IgM-Producing B Cells

Adipose tissue surrounding major arteries (Perivascular adipose tissue or PVAT) has long been thought to exist to provide vessel support and insulation. Emerging evidence suggests that PVAT regulates artery physiology and pathology, such as, promoting atherosclerosis development through local production of inflammatory cytokines. Yet the immune subtypes in PVAT that regulate inflammation are poorly characterized. B cells have emerged as important immune cells in the regulation of visceral adipose tissue inflammation and atherosclerosis. B cell-mediated effects on atherosclerosis are subset-dependent with B-1 cells attenuating and B-2 cells aggravating atherosclerosis. While mechanisms whereby B-2 cells aggravate atherosclerosis are less clear, production of immunoglobulin type M (IgM) antibodies is thought to be a major mechanism whereby B-1 cells limit atherosclerosis development. B-1 cell-derived IgM to oxidation specific epitopes (OSE) on low density lipoproteins (LDL) blocks oxidized LDL-induced inflammatory cytokine production and foam cell formation. However, whether PVAT contains B-1 cells and whether atheroprotective IgM is produced in PVAT is unknown. Results of the present study provide clear evidence that the majority of B cells in and around the aorta are derived from PVAT. Interestingly, a large proportion of these B cells belong to the B-1 subset with the B-1/B-2 ratio being 10-fold higher in PVAT relative to spleen and bone marrow. Moreover, PVAT contains significantly greater numbers of IgM secreting cells than the aorta. ApoE^−/− mice with B cell-specific knockout of the gene encoding the helix-loop-helix factor Id3, known to have attenuated diet-induced atherosclerosis, have increased numbers of B-1b cells and increased IgM secreting cells in PVAT relative to littermate controls. Immunostaining of PVAT on human coronary arteries identified fat associated lymphoid clusters (FALCs) harboring high numbers of B cells, and flow cytometry demonstrated the presence of T cells and B cells including B-1 cells. Taken together, these results provide evidence that murine and human PVAT harbor B-1 cells and suggest that local IgM production may serve to provide atheroprotection.

Abstract 484: Multimerization of BAFF Regulates B Cell Function and Growth of Aortic Aneurysms

B cell activating factor (BAFF) regulates differentiation and survival of B cells by binding to the surface receptors BAFF receptor (BR3), transmembrane activator and CAML interactor (TACI) and B cell maturation antigen (BCMA). During differentiation, intracellular metabolic reprogramming is crucial, such as, naïve B cells are metabolically quiescent, whereas, antibody producing plasma cells are metabolically active. We have reported that depletion of B cells protects mice from abdominal aortic aneurysm (AA), however it is not clear how B cells promote AA growth. BAFF exists as a 3mer (binds only to BR3) or as a 60mer (binds to BR3, TACI and BCMA). Therefore, we hypothesize that BAFF multimerization regulates the immune and metabolic phenotype of B cells by binding to BAFF receptors and modulate AA growth. Immunohistology was performed on AA tissues collected from patients undergoing open AA repair. Experimental AA was induced by elastase perfusion of abdominal aorta or angiotensin II infusion (1000 ng/kg/min) method in 8 weeks old male C57BL/6 or apolipoprotein E knockout mice, respectively. Western blotting, flow cytometry and Seahorse extracellular flux assays were used to determine immune and metabolic changes in B cells in response to recombinant BAFF 3mer and 60mer. BR3+ B cells were detected in the milieu of BAFF in human AAs. Mouse AAs demonstrated significant infiltration (>50/section) of CD138+ plasma B cells, but few (4-10/section) CD20+ B cells. In vitro, BAFF 3mer induced canonical NF-kB, whereas, 60mer induced both canonical and non-canonical NF-kB signaling. Moreover, the 3mer significantly decreased mitochondrial density, oxygen consumption rate, and surface expression of IgD and IgM indicating a metabolically quiescent state of B cells. However, these parameters were significantly increased by the 60mer similar to plasma cells. Anti-BR3 IgG1, but not a control IgG1 antibody decreased BAFF 60mer-induced oxygen consumption rate by 50%. In a pilot study (n=10/group), anti-BR3 IgG1, but not the control IgG1 aggravated angiotensin II-induced AA growth. Altogether, our results suggest that BAFF 3mer and 60mer oppositely regulate immune and metabolic phenotype of B cells and inhibition of BAFF-BR3 signaling is detrimental for AA growth.

Abstract 221: B-1b Cells Produce IgM to Malondialdehyde-Modified Low Density Lipoprotein in Perivascular Adipose Tissue in Response to Immunization And Attenuate Diet Induced Atherosclerosis

Background: B-1b cells are capable of long-lasting IgM memory and secrete more IgM, particularly malondialdehyde-modified low density lipoprotein (MDA-LDL) specific IgM than B-1a cells after transfer into hyperlipidemic Rag1 -/- mice. Id3 is a basic helix-loop-helix protein and dominant negative inhibitor of E proteins. B cell specific Id3 deficiency (Id3 BKO ) increases B-1b cell numbers systemically and provides atheroprotection. Adipose tissue is a source of B-1b-derived IgM and regulates inflammatory cytokine production from M1 macrophages locally. Perivascular adipose tissue (PVAT) has been implicated in regulation of atherosclerosis. However, the effect of B cell specific Id3 deficiency on B-1 cell responses in PVAT is not yet known. Also, whether these B-1b cells respond to MDA-LDL immunization is unknown.

Hypothesis: B-1b cells are present in PVAT and produce MDA-LDL specific IgM in PVAT, and immunization with MDA-LDL can enhance B-1b-mediated atheroprotection.

Methods and Results: Flow Cytometry and Enzyme-Linked ImmunoSpot (ELISPOT) analysis of PVAT of normal chow diet fed young mice demonstrated that ApoE.Id3 BKO mice have significantly higher numbers of B-1b cells and IgM secreting cells but not B-2 and B-1a cell numbers in PVAT compared to ApoE.Id3 WT mice. ELISPOT demonstrated that the % of MDA-LDL specific IgM of total IgM secreting cells was significantly greater in PVAT, but not in spleen or bone marrow, of ApoE.Id3 BKO mice compared to ApoE.Id3 WT mice, suggesting that modified lipids such as MDA-LDL in plaques and PVAT may stimulate local B-1b cells to secrete MDA-LDL specific IgM. Adoptive transfer of B-1b cells into ApoE.Rag1 -/- mice followed by MDA-LDL+PPS3 (pneumococcal polysaccharide) immunization, increased plasma IgM to MDA-LDL after 2 weeks and significantly attenuated atherosclerosis after 16 weeks of Western diet compared to PBS injected mice and B-1b cells transferred mice with PBS immunization.

Conclusion: B-1b cells produce IgM to MDA-LDL in PVAT. MDA-LDL immunization increased IgM to MDA-LDL after two weeks and attenuated diet-induced atherosclerosis. Taken together, results suggest that B-1b cells may regulate atherosclerosis through both local and systemic IgM production.

Abstract 223: CXCR4 Regulates B1 Cell Localization, Proliferation, Survival, and Atheroprotective IgM Production

Background: B1 cells exert protective effects in atherosclerosis through production of anti-inflammatory natural IgM antibodies that recognize oxidation-specific epitopes, such as MDA-LDL, present in diseased arteries. The bone marrow, spleen, and omental fat are known niches for B1 antibody production. However, the mechanisms underlying B1 localization to these sites and B1 antibody production are currently unclear.

Methods and Results: To identify key immune mediators that may be relevant to atherosclerosis, our lab has correlated surface marker expression on peripheral blood mononuclear cells with clinical markers of atherosclerosis in a human cohort. Expression of the chemokine receptor CXCR4 on circulating B1 cells associates with decreased plaque burden in coronary arteries and increased plasma levels of anti-MDA-LDL IgM antibodies. To study the role of CXCR4 in modulating B1 cell function, we generated mice with B cell-specific loss of CXCR4 on the atherogenic ApoE -/- background (CXCR4 BKO ). Chow-fed, 8-week-old CXCR4 BKO mice demonstrate fewer B1 cells (N=6-8, p<0.0001) and IgM antibody-secreting cells (N=6, p=0.004) in the bone marrow, and reduced plasma total IgM levels (N=6-8, p=0.04), relative to littermate controls (CXCR4 WT ). To determine the role of CXCR4 on the B1 cell subset specifically, we adoptively transferred CXCR4 WT or CXCR4 BKO B1 cells into lymphocyte-deficient Rag1 -/- ApoE -/- mice. After 16 weeks of Western diet feeding, recipient mice given CXCR4 BKO B1 cells demonstrate significantly reduced plasma IgM levels (n=5-7, p=0.02), and fewer donor B1 cells in the spleen, peritoneal cavity, and omental fat compared to recipients given CXCR4 WT B1 cells. Few to no donor cells were detected in the bone marrow. To determine whether CXCR4 has a role in B1 cell proliferation or survival, BrdU incorporation and expression of the death receptor FasR were assayed in CXCR4 WT and CXCR4 BKO B1 cells. B1 cells from CXCR4 BKO mice display heightened BrdU incorporation (n=4, p=0.005), yet have increased expression of FasR (n=4, p=0.01) relative to CXCR4 WT B1 cells.

Conclusion: Our data demonstrate novel roles for CXCR4 in regulating several processes in B1 cells, including proliferation and survival, consequently impacting production of IgM.

B cells and atherosclerosis

B cells have emerged as important immune cells in cardiovascular disease. Initial studies have suggested that B cells protect against atherosclerosis development. However, subsequent studies demonstrating aggravation of atherosclerosis by B-2 cells have shed light on the subset-dependent effects of B cells. Here, we review the literature that has led to our current understanding of B cell regulation of atherosclerosis, touching on the importance of subsets, local regulation, human translation, and therapeutic potential.

Artery Tertiary Lymphoid Organs: Powerhouses of Atherosclerosis Immunity

Artery tertiary lymphoid organs (ATLOs) are atherosclerosis-associated lymphoid aggregates with varying degrees of complexity ranging from small T/B-cell clusters to well-structured lymph node-like though unencapsulated lymphoid tissues. ATLOs arise in the connective tissue that surrounds diseased arteries, i.e., the adventitia. ATLOs have been identified in aged atherosclerosis-prone hyperlipidemic apolipoprotein E-deficient (ApoE-/-) mice: they are organized into distinct immune cell compartments, including separate T-cell areas, activated B-cell follicles, and plasma cell niches. Analyses of ATLO immune cell subsets indicate antigen-specific T- and B-cell immune reactions within the atherosclerotic arterial wall adventitia. Moreover, ATLOs harbor innate immune cells, including a large component of inflammatory macrophages, B-1 cells, and an aberrant set of antigen-presenting cells. There is marked neoangiogenesis, irregular lymphangiogenesis, neoformation of high endothelial venules, and de novo synthesis of lymph node-like conduits. Molecular mechanisms of ATLO formation remain to be identified though media vascular smooth muscle cells may adopt features of lymphoid tissue organizer-like cells by expressing lymphorganogenic chemokines, i.e., CXCL13 and CCL21. Although these data are consistent with the view that ATLOs participate in primary T- and B-cell responses against elusive atherosclerosis-specific autoantigens, their specific protective or disease-promoting roles remain to be identified. In this review, we discuss what is currently known about ATLOs and their potential impact on atherosclerosis and make attempts to define challenges ahead.

B-Cell Depletion Promotes Aortic Infiltration of Immunosuppressive Cells and Is Protective of Experimental Aortic Aneurysm

OBJECTIVE

B-cell depletion therapy is widely used for treatment of cancers and autoimmune diseases. B cells are abundant in abdominal aortic aneurysms (AAA); however, it is unknown whether B-cell depletion therapy affects AAA growth. Using experimental models of murine AAA, we aim to examine the effect of B-cell depletion on AAA formation.

APPROACH AND RESULTS

Wild-type or apolipoprotein E-knockout mice were treated with mouse monoclonal anti-CD20 or control antibodies and subjected to an elastase perfusion or angiotensin II infusion model to induce AAA, respectively. Anti-CD20 antibody treatment significantly depleted B1 and B2 cells, and strikingly suppressed AAA growth in both models. B-cell depletion resulted in lower circulating IgM levels, but did not affect the levels of IgG or cytokine/chemokine levels. Although the total number of leukocyte remained unchanged in elastase-perfused aortas after anti-CD20 antibody treatment, the number of B-cell subtypes was significantly lower. Interestingly, plasmacytoid dendritic cells expressing the immunomodulatory enzyme indole 2,3-dioxygenase were detected in the aortas of B-cell-depleted mice. In accordance with an increase in indole 2,3-dioxygenase+ plasmacytoid dendritic cells, the number of regulatory T cells was higher, whereas the expression of proinflammatory genes was lower in aortas of B-cell-depleted mice. In a coculture model, the presence of B cells significantly lowered the number of indole 2,3-dioxygenase+ plasmacytoid dendritic cells without affecting total plasmacytoid dendritic cell number.

CONCLUSIONS

The present results demonstrate that B-cell depletion protects mice from experimental AAA formation and promotes emergence of an immunosuppressive environment in aorta.

Abstract 464: Chemokine Receptor CCR6 Expression on B Cells Augments Local IgM Production and Atheroprotection

Background: CCR6 mediates immune cell recruitment to inflammatory sites and has cell type-specific effects on diet-induced atherosclerosis in mice. Recent studies implicate the local immune responses in the adventitia/perivascular adipose tissue (PVAT) in atherosclerosis development. We have previously demonstrated that adoptive transfer of CD43 - splenocytes (B cells) into B cell deficient μMT -/- ApoE -/- mice results in reduced diet-induced atherosclerosis in a CCR6-dependent manner. Notably, there were significantly greater numbers of B cells in the aorta including PVAT of μMT -/- ApoE -/- mice which received splenic B cells from CCR6 +/+ mice compared to CCR6 -/- mice, despite no difference in B cell numbers in blood, spleen and peritoneal cavity, suggesting that CCR6 expression on B cells is important in B cell aortic homing. Production of IgM antibodies is thought to be a major mechanism whereby B cells limit atherosclerosis development. Yet whether B cells produce IgM locally in the PVAT and whether this is regulated by chemokine receptors such as CCR6 is unknown.

Methods and Results: FACS experiments demonstrated high numbers of B cells available in the PVAT than aorta of young ApoE -/- (49121±11190 and 80±11; p<0.001, n=7) mice. ELISPOT experiments demonstrated significantly fewer IgM secreting cells were in the PVAT of ApoE -/- CCR6 -/- mice compared to ApoE -/- CCR6 +/+ mice (100±25 vs 850±150, p<0.05, n=5), despite no differences in IgM secreting cell numbers in spleen and bone marrow. Adoptive transfer of CD43 - splenic B cells from ApoE -/- CCR6 -/- and ApoE -/- CCR6 +/+ mice into secretory IgM deficient ApoE -/- sIgM -/- mice demonstrated significantly reduced atherosclerosis in mice that received B cells from ApoE -/- CCR6 +/+ mice compared to those that received B cells from ApoE -/- CCR6 -/- mice. Moreover, the B cells from ApoE -/- CCR6 +/+ mice attenuated atherosclerosis only when they were capable of secreting IgM. FACS data from human blood demonstrated that circulating B and T cells but not monocytes express CCR6, suggesting potential human relevance to our murine findings.

Conclusion: Results provide evidence that CCR6 expression on B cells mediates B cell recruitment into aorta and/or PVAT to provide atheroprotection via IgM secretion.

Artery Tertiary Lymphoid Organs Control Multilayered Territorialized Atherosclerosis B-Cell Responses in Aged ApoE-/- Mice

Supplemental Digital Content is available in the text.

Objective—

Explore aorta B-cell immunity in aged apolipoprotein E-deficient (ApoE^−/−) mice.

Approach and Results—

Transcript maps, fluorescence-activated cell sorting, immunofluorescence analyses, cell transfers, and Ig-ELISPOT (enzyme-linked immunospot) assays showed multilayered atherosclerosis B-cell responses in artery tertiary lymphoid organs (ATLOs). Aging-associated aorta B-cell–related transcriptomes were identified, and transcript atlases revealed highly territorialized B-cell responses in ATLOs versus atherosclerotic lesions: ATLOs showed upregulation of bona fide B-cell genes, including Cd19, Ms4a1 (Cd20), Cd79a/b, and Ighm although intima plaques preferentially expressed molecules involved in non–B effector responses toward B-cell–derived mediators, that is, Fcgr3 (Cd16), Fcer1g (Cd23), and the C1q family. ATLOs promoted B-cell recruitment. ATLO B-2 B cells included naive, transitional, follicular, germinal center, switched IgG1⁺, IgA⁺, and IgE⁺ memory cells, plasmablasts, and long-lived plasma cells. ATLOs recruited large numbers of B-1 cells whose subtypes were skewed toward interleukin-10⁺ B-1b cells versus interleukin-10⁻ B-1a cells. ATLO B-1 cells and plasma cells constitutively produced IgM and IgG and a fraction of plasma cells expressed interleukin-10. Moreover, ApoE^−/− mice showed increased germinal center B cells in renal lymph nodes, IgM-producing plasma cells in the bone marrow, and higher IgM and anti–MDA-LDL (malondialdehyde-modified low-density lipoprotein) IgG serum titers.

Conclusions—

ATLOs orchestrate dichotomic, territorialized, and multilayered B-cell responses in the diseased aorta; germinal center reactions indicate generation of autoimmune B cells within the diseased arterial wall during aging.

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.

Artery Tertiary Lymphoid Organs Control Aorta Immunity and Protect against Atherosclerosis via Vascular Smooth Muscle Cell Lymphotoxin β Receptors

Tertiary lymphoid organs (TLOs) emerge during nonresolving peripheral inflammation, but their impact on disease progression remains unknown. We have found in aged Apoe^−/− mice that artery TLOs (ATLOs) controlled highly territorialized aorta T cell responses. ATLOs promoted T cell recruitment, primed CD4⁺ T cells, generated CD4⁺, CD8⁺, T regulatory (Treg) effector and central memory cells, converted naive CD4⁺ T cells into induced Treg cells, and presented antigen by an unusual set of dendritic cells and B cells. Meanwhile, vascular smooth muscle cell lymphotoxin β receptors (VSMC-LTβRs) protected against atherosclerosis by maintaining structure, cellularity, and size of ATLOs though VSMC-LTβRs did not affect secondary lymphoid organs: Atherosclerosis was markedly exacerbated in Apoe^−/−Ltbr^−/− and to a similar extent in aged Apoe^−/−Ltbr^fl/flTagln-cre mice. These data support the conclusion that the immune system employs ATLOs to organize aorta T cell homeostasis during aging and that VSMC-LTβRs participate in atherosclerosis protection via ATLOs.

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Artery tertiary lymphoid organs control atherosclerosis T cell immunity

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Artery tertiary lymphoid organs generate effector memory T cells

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Artery tertiary lymphoid organs convert naive CD4⁺ T cells into induced Treg cells

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Artery tertiary lymphoid organs protect from atherosclerosis

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.

Abstract 21: 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 B cells, B-1a B cells are atheroprotective. The function of B-1b B cells, a unique subset of B-1 cells that can produce T cell-independent (TI) memory, has 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 (OSE) on LDL in vitro in response to activating stimulation. Additionally, we demonstrate that B-1b cells provide direct atheroprotection after adoptive transfer into B and T cell deficient (Rag1-/-Apoe-/-) hosts. To support these findings, we utilize a B cell specific Id3 knockout mouse model (Id3BKO), which develops increased numbers of B-1b cells systemically, to demonstrate that these mice develop attenuated atherosclerosis and increased OSE IgM antibodies compared to wild-type controls (Id3WT) after being fed a Western diet for 16 weeks. Finally, we report that the presence of a functionally relevant homozygous SNP in ID3 in humans associates with increased proportion of circulating B-1 cells and anti-MDA-LDL IgM and that the percentage of circulating B-1 cells is directly associated with the amount of anti-MDA-LDL IgM suggesting clinical relevance.

Conclusions: These results provide novel evidence that B-1b B cells produce atheroprotective OSE IgM antibodies and protect against atherosclerosis in mice, and suggest that similar mechanisms may occur in humans.

Abstract 633: B Cells in Artery Tertiary Lymphoid Organs of Aged Apolipoprotein-e Deficient Mice

Background: Atherosclerosis is recognized as a chronic in[[Unable to Display Character: &#64258;]]ammatory disease of arteries characterized by accumulation of in[[Unable to Display Character: &#64258;]]ammatory cells in the inner layer of the arterial wall, i.e. the intima. Though adaptive immune responses play important roles in all stages of atherosclerosis, the role of B cells is controversial as both atheroprotective and atherogenic effects have been reported. Moreover, information on B cells in the arterial wall is limited. We observed well-structured T and B cell aggregates in the adventitia of aorta segments adjacent to atherosclerotic lesions of aged Apolipoprotein-E Deficient (ApoE-/-) mice and referred to these structures as artery tertiary lymphoid organs (ATLOs).

Methods and Results: To understand atherosclerosis-associated B cell responses better, we undertook to delineate the B cell response in ATLOs versus secondary lymphoid organs. In ATLOs, marked numbers of germinal centre (GC) B cells, mantle zone B cells, and plasma cells were observed. Surprisingly, high numbers of B-1 B cells (20% per total B cells in ATLOs) were identified with B-1b cells making up 80% per total B-1 cells. The accumulation of B-1b cells in the diseased aorta was atherosclerosis-specific as the B-1b/B-1a ratio in ATLOs was increased when compared with the ratio in the peritoneal cavity (the major location of B-1 cells in mice), the spleen, and renal lymph nodes of Wt and ApoE-/- mice. ATLOs contained ~10% of GL-7+, PNA+ GC B cells per total IgD- B cells. The IgG1+ memory B cell percentage among total IgD- B cells was significantly increased, the IgM+/IgD+ (naïve B cells) percentage was significantly decreased, and the IgM-/IgD- percentage was significantly increased in ATLOs when compared with spleen, renal lymph nodes, and blood of aged Wt and ApoE-/- mice. Adoptive B-2 B cell transfer experiments revealed that B cell recruitment into ATLOs was specific, rapid, and highly regulated. Constitutively IgM and IgG antibody secreting cells were observed in ATLOs by ELISPOT.

Conclusion: Adventitial B cell aggregates were associated with marked expression of the B lymphocyte chemoattractant CXCL13. Our data indicate that ATLOs are a major site where adaptive and innate B cell immune responses are carried out in atherosclerosis.

Artery Tertiary Lymphoid Organs Contribute to Innate and Adaptive Immune Responses in Advanced Mouse Atherosclerosis

Tertiary lymphoid organs emerge in tissues in response to nonresolving inflammation. Recent research characterized artery tertiary lymphoid organs in the aorta adventitia of aged apolipoprotein E–deficient mice. The atherosclerosis-associated lymphocyte aggregates are organized into distinct compartments, including separate T-cell areas harboring conventional, monocyte-derived, lymphoid, and plasmacytoid dendritic cells, as well as activated T-cell effectors and memory cells; B-cell follicles containing follicular dendritic cells in activated germinal centers; and peripheral niches of plasma cells. Artery tertiary lymphoid organs show marked neoangiogenesis, aberrant lymphangiogenesis, and extensive induction of high endothelial venules. Moreover, newly formed lymph node–like conduits connect the external lamina with high endothelial venules in T-cell areas and also extend into germinal centers. Mouse artery tertiary lymphoid organs recruit large numbers of naïve T cells and harbor lymphocyte subsets with opposing activities, including CD4 + and CD8 + effector and memory T cells, natural and induced CD4 + regulatory T cells, and memory B cells at different stages of differentiation. These data suggest that artery tertiary lymphoid organs participate in primary immune responses and organize T- and B-cell autoimmune responses in advanced atherosclerosis. In this review, we discuss the novel concept that pro- and antiatherogenic immune responses toward unknown arterial wall–derived autoantigens may be organized by artery tertiary lymphoid organs and that disruption of the balance between pro- and antiatherogenic immune cell subsets may trigger clinically overt atherosclerosis.