B Cell-Mediated Antigen Presentation through MHC Class II Is Dispensable for Atherosclerosis Progression

T-Cell/B-Cell Interactions in Atherosclerosis

Atherosclerosis is a complex inflammatory disease in which the adaptive immune response plays an important role. While the overall impact of T and B cells in atherosclerosis is relatively well established, we are only beginning to understand how bidirectional T-cell/B-cell interactions can exert prominent atheroprotective and proatherogenic functions. In this review, we will focus on these T-cell/B-cell interactions and how we could use them to therapeutically target the adaptive immune response in atherosclerosis.

How the immune system shapes atherosclerosis: roles of innate and adaptive immunity

Atherosclerosis is the root cause of many cardiovascular diseases. Extensive research in preclinical models and emerging evidence in humans have established the crucial roles of the innate and adaptive immune systems in driving atherosclerosis-associated chronic inflammation in arterial blood vessels. New techniques have highlighted the enormous heterogeneity of leukocyte subsets in the arterial wall that have pro-inflammatory or regulatory roles in atherogenesis. Understanding the homing and activation pathways of these immune cells, their disease-associated dynamics and their regulation by microbial and metabolic factors will be crucial for the development of clinical interventions for atherosclerosis, including potentially vaccination-based therapeutic strategies. Here, we review key molecular mechanisms of immune cell activation implicated in modulating atherogenesis and provide an update on the contributions of innate and adaptive immune cell subsets in atherosclerosis.

Humoral immunity in atherosclerosis and myocardial infarction: from B cells to antibodies

Immune mechanisms are critically involved in the pathogenesis of atherosclerosis and its clinical manifestations. Associations of specific antibody levels and defined B cell subsets with cardiovascular disease activity in humans as well as mounting evidence from preclinical models demonstrate a role of B cells and humoral immunity in atherosclerotic cardiovascular disease. These include all aspects of B cell immunity, the generation of antigen-specific antibodies, antigen presentation and co-stimulation of T cells, as well as production of cytokines. Through their impact on adaptive and innate immune responses and the regulation of many other immune cells, B cells mediate both protective and detrimental effects in cardiovascular disease. Several antigens derived from (oxidised) lipoproteins, the vascular wall and classical autoantigens have been identified. The unique antibody responses they trigger and their relationship with atherosclerotic cardiovascular disease are reviewed. In particular, we focus on the different effector functions of specific IgM, IgG, and IgE antibodies and the cellular responses they trigger and highlight potential strategies to target B cell functions for therapy.

Atherosclerosis: immunopathogenesis and strategies for immunotherapy

Atherosclerosis, a chronic inflammatory condition in which atheroma accumulates within the intima of the arterial wall, is a life-threatening manifestation of cardiovascular disease, due to atheroma rupture, chronic luminal narrowing and thrombosis. Current knowledge of the role of a protective immune response in atherosclerotic lesions has provided promising opportunities to develop new immunotherapeutic strategies. In particular, Tregs exert an atheroprotective role by releasing anti-inflammatory cytokines (IL-10/TGF-β) and suppressing autoreactive T lymphocytes. In vivo animal experiments have shown that this can be achieved by developing vaccines that stimulate immunological tolerance to atheroma antigens. Here, we present an overview of the current knowledge of the proatherogenic immune response, and we discuss the strategies currently used as immunoregulatory therapy.

Functional Role of B Cells in Atherosclerosis

Atherosclerosis is a lipid-driven inflammatory disease of blood vessels, and both innate and adaptive immune responses are involved in its development. The impact of B cells on atherosclerosis has been demonstrated in numerous studies and B cells have been found in close proximity to atherosclerotic plaques in humans and mice. B cells exert both atheroprotective and pro-atherogenic functions, which have been associated with their B cell subset attribution. While B1 cells and marginal zone B cells are considered to protect against atherosclerosis, follicular B cells and innate response activator B cells have been shown to promote atherosclerosis. In this review, we shed light on the role of B cells from a different, functional perspective and focus on the three major B cell functions: antibody production, antigen presentation/T cell interaction, and the release of cytokines. All of these functions have the potential to affect atherosclerosis by multiple ways and are dependent on the cellular milieu and the activation status of the B cell. Moreover, we discuss B cell receptor signaling and the mechanism of B cell activation under atherosclerosis-prone conditions. By summarizing current knowledge of B cells in and beyond atherosclerosis, we are pointing out open questions and enabling new perspectives.

Autoantibody Responses to Apolipoprotein A-I Are Not Diet- or Sex-Linked in C57BL/6 Mice

Atherosclerosis is responsible for a large percentage of all-cause mortality worldwide, but it is only now beginning to be understood as a complex disease process involving metabolic insult, chronic inflammation, and multiple immune mechanisms. Abs targeting apolipoprotein A-I (ApoA-I) have been found in patients with cardiovascular disease, autoimmune conditions, as well as those with no documented history of either. However, relatively little is known about how these Abs are generated and their relationship to diet and sex. In the current study, we modeled this aspect of autoimmunity using anti–ApoA-I immunization of male and female C57BL/6 mice. Unexpectedly, we found that autoantibodies directed against a single, previously unknown, epitope within the ApoA-I protein developed irrespective of immunization status or dyslipidemia in mice. When total IgG subclasses were analyzed over the course of time, we observed that rather than driving an increase in inflammatory IgG subclasses, consumption of Western diet suppressed age-dependent increases in IgG2b and IgG2c in male mice only. The lack of change observed in female mice suggested that diet and sex might play a combined role in Th1/Th2 balance and, ultimately, in immunity to pathogen challenge. This report demonstrates the need for inclusion of both sexes in studies pertaining to diet and aging and suggests that further study of immunogenic epitopes present in ApoA-I is warranted.

B Cell and CD4 T Cell Interactions Promote Development of Atherosclerosis

Interaction between B and CD4 T cells is crucial for their optimal responses in adaptive immunity. Immune responses augmented by their partnership promote chronic inflammation. Here we report that interaction between B and CD4 T cells augments their atherogenicity to promote lipid-induced atherosclerosis. Genetic deletion of the gene encoding immunoglobulin mu (μ) heavy chain (μMT) in ApoE^−/− mice resulted in global loss of B cells including those in atherosclerotic plaques, undetectable immunoglobulins and impaired germinal center formation. Despite unaffected numbers in the circulation and peripheral lymph nodes, CD4 T cells were also reduced in spleens as were activated and memory CD4 T cells. In hyperlipidemic μMT^−/− ApoE^−/− mice, B cell deficiency decreased atherosclerotic lesions, accompanied by absence of immunoglobulins and reduced CD4 T cell accumulation in lesions. Adoptive transfer of B cells deficient in either MHCII or co-stimulatory molecule CD40, molecules required for B and CD4 T cell interaction, into B cell-deficient μMT^−/− ApoE^−/− mice failed to increase atherosclerosis. In contrast, wildtype B cells transferred into μMT^−/− ApoE^−/− mice increased atherosclerosis and increased CD4 T cells in lesions including activated and memory CD4 T cells. Transferred B cells also increased their expression of atherogenic cytokines IL-1β, TGF-β, MCP-1, M-CSF, and MIF, with partial restoration of germinal centers and plasma immunoglobulins. Our study demonstrates that interaction between B and CD4 T cells utilizing MHCII and CD40 is essential to augment their function to increase atherosclerosis in hyperlipidemic mice. These findings suggest that targeting B cell and CD4 T cell interaction may be a therapeutic strategy to limit atherosclerosis progression.

Immune-Mediated Inflammation in Vulnerable Atherosclerotic Plaques

Atherosclerosis is a chronic long-lasting vascular disease leading to myocardial infarction and stroke. Vulnerable atherosclerotic (AS) plaques are responsible for these life-threatening clinical endpoints. To more successfully work against atherosclerosis, improvements in early diagnosis and treatment of AS plaque lesions are required. Vulnerable AS plaques are frequently undetectable by conventional imaging because they are non-stenotic. Although blood biomarkers like lipids, C-reactive protein, interleukin-6, troponins, and natriuretic peptides are in pathological ranges, these markers are insufficient in detecting the critical perpetuation of AS anteceding endpoints. Thus, chances to treat the patient in a preventive way are wasted. It is now time to solve this dilemma because clear results indicate a benefit of anti-inflammatory therapy per se without modification of blood lipids (CANTOS Trial, NCT01327846). This fact identifies modulation of immune-mediated inflammation as a new promising point of action for the eradication of fatal atherosclerotic endpoints.