TREM2 protects from atherosclerosis by limiting necrotic core formation.

Atherosclerosis is a chronic disease of the vascular wall driven by lipid accumulation and inflammation in the intimal layer of arteries, and its main complications, myocardial infarction and stroke, are the leading cause of mortality worldwide [1], [2]. Recent studies have identified Triggering receptor expressed on myeloid cells 2 (TREM2), a lipid-sensing receptor regulating myeloid cell functions [3], to be highly expressed in macrophage foam cells in experimental and human atherosclerosis [4]. However, the role of TREM2 in atherosclerosis is not fully known. Here, we show that hematopoietic or global TREM2 deficiency increased, whereas TREM2 agonism decreased necrotic core formation in early atherosclerosis. We demonstrate that TREM2 is essential for the efferocytosis capacities of macrophages, and to the survival of lipid-laden macrophages, indicating a crucial role of TREM2 in maintaining the balance between foam cell death and clearance of dead cells in atherosclerotic lesions, thereby controlling plaque necrosis.

Advancements in risk stratification and management strategies in primary cardiovascular prevention.

Atherosclerotic cardiovascular disease (ASCVD) remains a leading cause of morbidity and mortality worldwide, highlighting the urgent need for advancements in risk assessment and management strategies. Although significant progress has been made recently, identifying and managing apparently healthy individuals at a higher risk of developing atherosclerosis and those with subclinical atherosclerosis still poses significant challenges. Traditional risk assessment tools have limitations in accurately predicting future events and fail to encompass the complexity of the atherosclerosis trajectory. In this review, we describe novel approaches in biomarkers, genetics, advanced imaging techniques, and artificial intelligence that have emerged to address this gap. Moreover, polygenic risk scores and imaging modalities such as coronary artery calcium scoring, and coronary computed tomography angiography offer promising avenues for enhancing primary cardiovascular risk stratification and personalised intervention strategies. On the other hand, interventions aiming against atherosclerosis development or promoting plaque regression have gained attention in primary ASCVD prevention. Therefore, the potential role of drugs like statins, ezetimibe, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, omega-3 fatty acids, antihypertensive agents, as well as glucose-lowering and anti-inflammatory drugs are also discussed. Since findings regarding the efficacy of these interventions vary, further research is still required to elucidate their mechanisms of action, optimize treatment regimens, and determine their long-term effects on ASCVD outcomes. In conclusion, advancements in strategies addressing atherosclerosis prevention and plaque regression present promising avenues for enhancing primary ASCVD prevention through personalised approaches tailored to individual risk profiles. Nevertheless, ongoing research efforts are imperative to refine these strategies further and maximise their effectiveness in safeguarding cardiovascular health.

Autoimmune diseases and atherosclerotic cardiovascular disease.

Autoimmune diseases are associated with a dramatically increased risk of atherosclerotic cardiovascular disease and its clinical manifestations. The increased risk is consistent with the notion that atherogenesis is modulated by both protective and disease-promoting immune mechanisms. Notably, traditional cardiovascular risk factors such as dyslipidaemia and hypertension alone do not explain the increased risk of cardiovascular disease associated with autoimmune diseases. Several mechanisms have been implicated in mediating the autoimmunity-associated cardiovascular risk, either directly or by modulating the effect of other risk factors in a complex interplay. Aberrant leukocyte function and pro-inflammatory cytokines are central to both disease entities, resulting in vascular dysfunction, impaired resolution of inflammation and promotion of chronic inflammation. Similarly, loss of tolerance to self-antigens and the generation of autoantibodies are key features of autoimmunity but are also implicated in the maladaptive inflammatory response during atherosclerotic cardiovascular disease. Therefore, immunomodulatory therapies are potential efficacious interventions to directly reduce the risk of cardiovascular disease, and biomarkers of autoimmune disease activity could be relevant tools to stratify patients with autoimmunity according to their cardiovascular risk. In this Review, we discuss the pathophysiological aspects of the increased cardiovascular risk associated with autoimmunity and highlight the many open questions that need to be answered to develop novel therapies that specifically address this unmet clinical need.

Cell-autonomous regulation of complement C3 by factor H limits macrophage efferocytosis and exacerbates atherosclerosis.

Complement factor H (CFH) negatively regulates consumption of complement component 3 (C3), thereby restricting complement activation. Genetic variants in CFH predispose to chronic inflammatory disease. Here, we examined the impact of CFH on atherosclerosis development. In a mouse model of atherosclerosis, CFH deficiency limited plaque necrosis in a C3-dependent manner. Deletion of CFH in monocyte-derived inflammatory macrophages propagated uncontrolled cell-autonomous C3 consumption without downstream C5 activation and heightened efferocytotic capacity. Among leukocytes, Cfh expression was restricted to monocytes and macrophages, increased during inflammation, and coincided with the accumulation of intracellular C3. Macrophage-derived CFH was sufficient to dampen resolution of inflammation, and hematopoietic deletion of CFH in atherosclerosis-prone mice promoted lesional efferocytosis and reduced plaque size. Furthermore, we identified monocyte-derived inflammatory macrophages expressing C3 and CFH in human atherosclerotic plaques. Our findings reveal a regulatory axis wherein CFH controls intracellular C3 levels of macrophages in a cell-autonomous manner, evidencing the importance of on-site complement regulation in the pathogenesis of inflammatory diseases.

Soluble TREM2 levels reflect the recruitment and expansion of TREM2+ macrophages that localize to fibrotic areas and limit NASH.

BACKGROUND & AIMS: Previous single-cell RNA-sequencing analyses have shown that Trem2-expressing macrophages are present in the liver during obesity, non-alcoholic steatohepatitis (NASH) and cirrhosis. Herein, we aimed to functionally characterize the role of bone marrow-derived TREM2-expressing macrophage populations in NASH. METHODS: We used bulk RNA sequencing to assess the hepatic molecular response to lipid-dependent dietary intervention in mice. Spatial mapping, bone marrow transplantation in two complementary murine models and single-cell sequencing were applied to functionally characterize the role of TREM2+ macrophage populations in NASH. RESULTS: We found that the hepatic transcriptomic profile during steatohepatitis mirrors the dynamics of recruited bone marrow-derived monocytes that already acquire increased expression of Trem2 in the circulation. Increased Trem2 expression was reflected by elevated levels of systemic soluble TREM2 in mice and humans with NASH. In addition, soluble TREM2 levels were superior to traditionally used laboratory parameters for distinguishing between different fatty liver disease stages in two separate clinical cohorts. Spatial transcriptomics revealed that TREM2+ macrophages localize to sites of hepatocellular damage, inflammation and fibrosis in the steatotic liver. Finally, using multiple murine models and in vitro experiments, we demonstrate that hematopoietic Trem2 deficiency causes defective lipid handling and extracellular matrix remodeling, resulting in exacerbated steatohepatitis, cell death and fibrosis. CONCLUSIONS: Our study highlights the functional properties of bone marrow-derived TREM2+ macrophages and implies the clinical relevance of systemic soluble TREM2 levels in the context of NASH. LAY SUMMARY: Our study defines the origin and function of macrophages (a type of immune cell) that are present in the liver and express a specific protein called TREM2. We find that these cells have an important role in protecting against non-alcoholic steatohepatitis (a progressive form of fatty liver disease). We also show that the levels of soluble TREM2 in the blood could serve as a circulating marker of non-alcoholic fatty liver disease.

Anti-inflammatory and Immunomodulatory Therapies in Atherosclerosis.

Hypercholesterolemia is a major risk factor in atherosclerosis development and lipid-lowering drugs (i.e., statins) remain the treatment of choice. Despite effective reduction of LDL cholesterol in patients, a residual cardiovascular risk persists in some individuals, highlighting the need for further therapeutic intervention. Recently, the CANTOS trial paved the way toward the development of specific therapies targeting inflammation, a key feature in atherosclerosis progression. The pre-existence of multiple drugs modulating both innate and adaptive immune responses has significantly accelerated the number of translational studies applying these drugs to atherosclerosis. Additional preclinical research has led to the discovery of new therapeutic targets, offering promising perspectives for the treatment and prevention of atherosclerosis. Currently, both drugs with selective targeting and broad unspecific anti-inflammatory effects have been tested. In this chapter, we aim to give an overview of current advances in immunomodulatory treatment approaches for atherosclerotic cardiovascular diseases.

APRIL limits atherosclerosis by binding to heparan sulfate proteoglycans.

Atherosclerotic cardiovascular disease causes heart attacks and strokes, which are the leading causes of mortality worldwide1. The formation of atherosclerotic plaques is initiated when low-density lipoproteins bind to heparan-sulfate proteoglycans (HSPGs)2 and become trapped in the subendothelial space of large and medium size arteries, which leads to chronic inflammation and remodelling of the artery wall2. A proliferation-inducing ligand (APRIL) is a cytokine that binds to HSPGs3, but the physiology of this interaction is largely unknown. Here we show that genetic ablation or antibody-mediated depletion of APRIL aggravates atherosclerosis in mice. Mechanistically, we demonstrate that APRIL confers atheroprotection by binding to heparan sulfate chains of heparan-sulfate proteoglycan 2 (HSPG2), which limits the retention of low-density lipoproteins, accumulation of macrophages and formation of necrotic cores. Indeed, antibody-mediated depletion of APRIL in mice expressing heparan sulfate-deficient HSPG2 had no effect on the development of atherosclerosis. Treatment with a specific anti-APRIL antibody that promotes the binding of APRIL to HSPGs reduced experimental atherosclerosis. Furthermore, the serum levels of a form of human APRIL protein that binds to HSPGs, which we termed non-canonical APRIL (nc-APRIL), are associated independently of traditional risk factors with long-term cardiovascular mortality in patients with atherosclerosis. Our data reveal properties of APRIL that have broad pathophysiological implications for vascular homeostasis.

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 (oxidized) 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.

Impact of B-Cell-Targeted Therapies on Cardiovascular Disease.

Atherosclerosis is a lipid-driven chronic inflammatory disease that is modulated by many immune cell subsets, including B cells. Therefore, targeting the inflammatory component of cardiovascular disease represents a promising therapeutic strategy. In the past years, immunotherapy has revolutionized the treatment of autoimmunity and cancer. Many of these clinically used strategies target B cells. Given the multifaceted role of B cells in atherogenesis, it is conceivable that B-cell-directed therapies can modulate disease development. Here, we review clinically available B-cell-targeted therapies and the possible benefits or detrimental effects on cardiovascular disease.

Complement Factor H Modulates Splenic B Cell Development and Limits Autoantibody Production.

Complement factor H (CFH) has a pivotal role in regulating alternative complement activation through its ability to inhibit the cleavage of the central complement component C3, which links innate and humoral immunity. However, insights into the role of CFH in B cell biology are limited. Here, we demonstrate that deficiency of CFH in mice leads to altered splenic B cell development characterized by the accumulation of marginal zone (MZ) B cells. Furthermore, B cells in Cfh-/- mice exhibit enhanced B cell receptor (BCR) signaling as evaluated by increased levels of phosphorylated Bruton's tyrosine kinase (pBTK) and phosphorylated spleen tyrosine kinase (pSYK). We show that enhanced BCR activation is associated with uncontrolled C3 consumption in the spleen and elevated complement receptor 2 (CR2, also known as CD21) levels on the surface of mature splenic B cells. Moreover, aged Cfh-/- mice developed splenomegaly with distorted spleen architecture and spontaneous B cell-dependent autoimmunity characterized by germinal center hyperactivity and a marked increase in anti-double stranded DNA (dsDNA) antibodies. Taken together, our data indicate that CFH, through its function as a complement repressor, acts as a negative regulator of BCR signaling and limits autoimmunity.

B Cell-Activating Factor Neutralization Aggravates Atherosclerosis.

BACKGROUND: Atherosclerotic cardiovascular disease (heart attacks and strokes) is the major cause of death globally and is caused by the buildup of a plaque in the arterial wall. Genomic data showed that the B cell-activating factor (BAFF) receptor pathway, which is specifically essential for the survival of conventional B lymphocytes (B-2 cells), is a key driver of coronary heart disease. Deletion or antibody-mediated blockade of BAFF receptor ablates B-2 cells and decreases experimental atherosclerosis. Anti-BAFF immunotherapy is approved for treatment of autoimmune systemic lupus erythematosus, and can therefore be expected to limit their associated cardiovascular risk. However, direct effects of anti-BAFF immunotherapy on atherosclerosis remain unknown. METHODS: To investigate the effect of BAFF neutralization in atherosclerosis, the authors treated Apoe-/- and Ldlr-/- mice with a well-characterized blocking anti-BAFF antibody. Moreover, to investigate the mechanism by which BAFF impacts atherosclerosis, the authors studied atherosclerosis-prone mice that lack the alternative receptor for BAFF: transmembrane activator and calcium modulator and cyclophilin ligand interactor. RESULTS: The authors demonstrate here that anti-BAFF antibody treatment increased atherosclerosis in mice, despite efficient depletion of mature B-2 cells, suggesting a unique mechanism of action. Indeed, myeloid cell-specific deletion of transmembrane activator and calcium modulator and cyclophilin ligand interactor also results in increased atherosclerosis, while B cell-specific transmembrane activator and calcium modulator and cyclophilin ligand interactor deletion had no effect. Mechanistically, BAFF-transmembrane activator and calcium modulator and cyclophilin ligand interactor signaling represses macrophage IRF7-dependent (but not NF-κB-dependent) Toll-like receptor 9 responses including proatherogenic CXCL10 production. CONCLUSIONS: These data identify a novel B cell-independent anti-inflammatory role for BAFF in atherosclerosis and may have important clinical implications.