Dimitrios Tsiantoulas

B Cells and Humoral Immunity in Atherosclerosis

Insights into the important contribution of inflammation and immune functions in the development and progression of atherosclerosis have greatly improved our understanding of this disease. Although the role of T cells has been extensively studied for decades, only recently has the role of B cells gained more attention. Recent studies have identified differential effects of different B-cell subsets and helped to clarify the still poorly understood mechanisms by which these act. B1 cells have been shown to prevent lesion formation, whereas B2 cells have been suggested to promote it. Natural IgM antibodies, mainly derived from B1 cells, have been shown to mediate atheroprotective effects, but the functional role of other immunoglobulin classes, particularly IgG, still remains elusive. In this review, we will focus on recent insights on the role of B cells and various immunoglobulin classes and how these may mediate their effects in atherosclerotic lesion formation. Moreover, we will highlight potential therapeutic approaches focusing on B-cell depletion that could be used to translate experimental evidence to human disease.

BAFF Receptor Deficiency Reduces the Development of Atherosclerosis in Mice—Brief Report

Objective—The goal of this study was to assess the role of B-cell activating factor (BAFF) receptor in B-cell regulation of atherosclerosis. Methods and Results—Male LDL receptor-deficient mice (Ldlr−/−) were lethally irradiated and reconstituted with either wild type or BAFF receptor (BAFF-R)–deficient bone marrow. After 4 weeks of recovery, mice were put on a high-fat diet for 6 or 8 weeks. BAFF-R deficiency in bone marrow cells led to a marked reduction of conventional mature B2 cells but did not affect the B1a cell subtype. This was associated with a significant reduction of dendritic cell activation and T-cell proliferation along with a reduction of IgG antibodies against malondialdehyde-modified low-density lipoprotein. In contrast, serum IgM type antibodies were preserved. Interestingly, BAFF-R deficiency was associated with a significant reduction in atherosclerotic lesion development and reduced numbers of plaque T cells. Selective BAFF-R deficiency on B cells led to a similar reduction in lesion size and T-cell infiltration but in contrast did not affect dendritic cell activation. Conclusion—BAFF-R deficiency in mice selectively alters mature B2 cell-dependent cellular and humoral immune responses and limits the development of atherosclerosis.

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

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

Responsiveness of B cells is regulated by the hinge region of IgD

Mature B cells express immunoglobulin M (IgM)- and IgD-isotype B cell antigen receptors, but the importance of IgD for B cell function has been unclear. By using a cellular in vitro system and corresponding mouse models, we found that antigens with low valence activated IgM receptors but failed to trigger IgD signaling, whereas polyvalent antigens activated both receptor types. Investigations of the molecular mechanism showed that deletion of the IgD-specific hinge region rendered IgD responsive to monovalent antigen, whereas transferring the hinge to IgM resulted in responsiveness only to polyvalent antigen. Our data suggest that the increased IgD/IgM ratio on conventional B-2 cells is important for preferential immune responses to antigens in immune complexes, and that the increased IgM expression on B-1 cells is essential for B-1 cell homeostasis and function.

B-1 Cell Immunoglobulin Directed Against Oxidation-Specific Epitopes

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

Targeting B Cells in Atherosclerosis

Atherosclerotic plaque formation is strongly influenced by different arms of the immune system, including B lymphocytes. B cells are divided into 2 main families: the B1 and the B2 cells. B1 cells are atheroprotective mainly via the production of natural IgM antibodies that bind oxidized low-density lipoprotein and apoptotic cells. B2 cells, which include follicular and marginal zone B cells, are suggested to be proatherogenic. Antibody-mediated depletion of B cells has become a valuable treatment option for certain autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis that are also characterized by the development of premature atherosclerosis. Thus, B cells represent a novel interesting target for therapeutic modulation of the atherosclerotic disease process. Here, we discuss the effect of different of B-cell subsets in experimental atherosclerosis, their mechanism of action as well as potential ways to exploit these findings for the treatment of human disease. # Significance {#article-title-63}Atherosclerotic plaque formation is strongly influenced by different arms of the immune system, including B lymphocytes. B cells are divided into 2 main families: the B1 and the B2 cells. B1 cells are atheroprotective mainly via the production of natural IgM antibodies that bind oxidized low-density lipoprotein and apoptotic cells. B2 cells, which include follicular and marginal zone B cells, are suggested to be proatherogenic. Antibody-mediated depletion of B cells has become a valuable treatment option for certain autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis that are also characterized by the development of premature atherosclerosis. Thus, B cells represent a novel interesting target for therapeutic modulation of the atherosclerotic disease process. Here, we discuss the effect of different of B-cell subsets in experimental atherosclerosis, their mechanism of action as well as potential ways to exploit these findings for the treatment of human disease.

Targeting B Cells in Atherosclerosis Closing the Gap From Bench to Bedside

Atherosclerotic plaque formation is strongly influenced by different arms of the immune system, including B lymphocytes. B cells are divided into 2 main families: the B1 and the B2 cells. B1 cells are atheroprotective mainly via the production of natural IgM antibodies that bind oxidized low-density lipoprotein and apoptotic cells. B2 cells, which include follicular and marginal zone B cells, are suggested to be proatherogenic. Antibody-mediated depletion of B cells has become a valuable treatment option for certain autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis that are also characterized by the development of premature atherosclerosis. Thus, B cells represent a novel interesting target for therapeutic modulation of the atherosclerotic disease process. Here, we discuss the effect of different of B-cell subsets in experimental atherosclerosis, their mechanism of action as well as potential ways to exploit these findings for the treatment of human disease. # Significance {#article-title-63}Atherosclerotic plaque formation is strongly influenced by different arms of the immune system, including B lymphocytes. B cells are divided into 2 main families: the B1 and the B2 cells. B1 cells are atheroprotective mainly via the production of natural IgM antibodies that bind oxidized low-density lipoprotein and apoptotic cells. B2 cells, which include follicular and marginal zone B cells, are suggested to be proatherogenic. Antibody-mediated depletion of B cells has become a valuable treatment option for certain autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis that are also characterized by the development of premature atherosclerosis. Thus, B cells represent a novel interesting target for therapeutic modulation of the atherosclerotic disease process. Here, we discuss the effect of different of B-cell subsets in experimental atherosclerosis, their mechanism of action as well as potential ways to exploit these findings for the treatment of human disease.

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.

Sialic Acid-Binding Immunoglobulin-like Lectin G Promotes Atherosclerosis and Liver Inflammation by Suppressing the Protective Functions of B-1 Cells

Summary Atherosclerosis is initiated and sustained by hypercholesterolemia, which results in the generation of oxidized LDL (OxLDL) and other metabolic byproducts that trigger inflammation. Specific immune responses have been shown to modulate the inflammatory response during atherogenesis. The sialic acid-binding immunoglobulin-like lectin G (Siglec-G) is a negative regulator of the functions of several immune cells, including myeloid cells and B-1 cells. Here, we show that deficiency of Siglec-G in atherosclerosis-prone mice inhibits plaque formation and diet-induced hepatic inflammation. We further demonstrate that selective deficiency of Siglec-G in B cells alone is sufficient to mediate these effects. Levels of B-1 cell-derived natural IgM with specificity for OxLDL were significantly increased in the plasma and peritoneal cavity of Siglec-G-deficient mice. Consistent with the neutralizing functions of OxLDL-specific IgM, Siglec-G-deficient mice were protected from OxLDL-induced sterile inflammation. Thus, Siglec-G promotes atherosclerosis and hepatic inflammation by suppressing protective anti-inflammatory effector functions of B cells.

Marginal zone B cells control the response of follicular helper T cells to a high-cholesterol diet

Splenic marginal zone B (MZB) cells, positioned at the interface between circulating blood and lymphoid tissue, detect and respond to blood-borne antigens. Here we show that MZB cells in mice activate a homeostatic program in response to a high-cholesterol diet (HCD) and regulate both the differentiation and accumulation of T follicular helper (TFH) cells. Feeding mice an HCD resulted in upregulated MZB cell surface expression of the immunoregulatory ligand PDL1 in an ATF3-dependent manner and increased the interaction between MZB cells and pre-TFH cells, leading to PDL1-mediated suppression of TFH cell motility, alteration of TFH cell differentiation, reduced TFH abundance and suppression of the proatherogenic TFH response. Our findings reveal a previously unsuspected role for MZB cells in controlling the TFH–germinal center response to a cholesterol-rich diet and uncover a PDL1-dependent mechanism through which MZB cells use their innate immune properties to limit an exaggerated adaptive immune response.