Heterogeneity of Macrophages in Atherosclerosis

Abstract

Atherosclerosis is a prevalent inflammatory condition and a frequent cause of morbidity and mortality worldwide. Macrophages are among the key immune cells driving lesion formation in the arterial wall. They have therefore evolved as potential targets for therapeutic strategies. Understanding of the different macrophage phenotypes and functions seems to be of pivotal importance for the development of treatments to target these immune cells. This review highlights the complexity of the mononuclear phagocyte system and summarizes important features of macrophage biology contributing to atherosclerosis.

Keywords

atherosclerosis - macrophage - monocyte - inflammation - therapy

Introduction

Atherosclerosis is a chronic inflammatory disease of the arteries resulting from metabolic dysregulation and a maladaptive immune response of the vessel wall. This leads to atherosclerotic plaque formation with narrowing of the arterial lumen and plaque rupture causing myocardial infarction and stroke. Due to their high prevalence, atherosclerosis-related pathologies are nowadays among the most important diseases worldwide.[1] This review focuses on the biology of macrophages therein.

Origin and Phenotype of Lesional Macrophages—Moving Targets

Among all leukocytes in the arterial wall, macrophages account for about 30 to 50% of total CD45+ cells. Their high abundance already suggests some relevance in the context of atherosclerosis.[2] [3] [4] Where do they originate? It has been text book knowledge for decades that bone marrow (BM)-derived monocytes extravasate at sites of atherosclerotic lesions where they differentiate to macrophages and phagocytose-modified low-density lipoprotein (LDL) particles. However, recent publications have illustrated a vast heterogeneity within macrophage populations, which is at least in part associated with differences in cell ontogeny, resulting in potentially distinct functions in atherosclerosis.[4] [5] [6] Due to comparable morphology and similarities in the expression of common phagocyte markers, an unequivocal identification of these subpopulations has remained challenging and so far required specific lineage tracing and fate mapping analysis[7] ([Fig. 1]). By these means, several studies over the past years have shown that tissue-resident macrophages originate in relevant amounts from erythro-myeloid progenitors that arise in the yolk sac (YS)—an early endodermal embryonic structure.[8] [9] [10] Shortly after initiation of YS hematopoiesis, hematopoietic stem cells (HSCs) are also generated in the dorsal aorta and seed the fetal liver. At later stages of fetal development, hematopoiesis shifts to the BM compartment, giving rise to circulating monocytes and eventually tissue macrophages.[6] [11] [12] In mouse arteries, macrophages have a mixed origin from fetal and BM hematopoiesis.[6] However, the quantitative contribution in the arterial wall as well as their response to inflammatory cues, particularly in the process of atherosclerosis, has remained unclear. Further, potential functional consequences of this observation have remained unknown.

Fig. 1 Diversity of tissue-resident macrophages. Depicted are cellular origins and functional markers as well as stimulatory and secreted molecules of different macrophage subpopulations.

Due to early lineage specification, macrophages are provided with tissue-specific signatures already during fetal development.[13] These findings support the notion that ontogeny contributes not only to macrophage heterogeneity but also to functional differences of tissue (arterial) macrophages. However, the tissue environment continuously shapes the cellular identity of resident macrophages.[14] [15] Thus, it is unknown to date whether cell ontogeny has functional consequences for macrophage biology in later life.

Besides these differences in macrophage origin, also the phenotypic diversity of macrophages within arteries has been greater than expected. Several pioneering studies recently identified three to five subsets of macrophages by ribonucleic acid (RNA) sequencing or mass cytometry.[2] [3] [4] We will discuss some of the molecular markers in the course of this review.

Monocyte-Derived Macrophage Recruitment from the Circulation

The process of monocyte recruitment to inflamed arteries is facilitated by multiple mechanisms and is fundamental to the process of atherosclerosis.[16] [17] On the one hand, oxidized LDL (oxLDL) stimulates the endothelium to express adhesion molecules, secrete macrophage colony stimulation factor (M-CSF), as well as granulocyte macrophage (GM)-CSF, and reduce the synthesis of nitric oxide (NO).[18] [19] Monocytes, on the other hand, upregulate the expression of adhesion molecules, mainly involving different heterodimers of β1 integrins, like very late antigen 4, and β2 integrins, like lymphocyte function-associated antigen 1 and macrophage-1 antigen (Mac1)[20] ([Fig. 2]).

Fig. 2 Macrophages in the context of atherosclerosis. Depicted are major steps during the pathogenesis of atheroscerlosis which involve macrophages. Key aspects of this review are highlighted in red.

The process of monocyte extravasation starts with P- and E-selectin-mediated rolling along the endothelium followed by intercellular cell adhesion molecule-, vascular cell adhesion molecule (VCAM)-, and integrin-dependent firm adhesion.[21] [22] [23] Subsequent transendothelial migration requires C-C motif chemokine receptor (CCR) 2 which interacts with monocyte chemoattractant protein 1 (MCP1/CCL2). Additionally, CCR5 and CX3C chemokine receptor (CX3CR) 1 are involved. Therefore, it is not surprising that deficiency of these receptors has been associated with a significant reduction in atherosclerosis.[24] Within the intimal layer, BM-derived monocytes differentiate to macrophages under the influence of M-CSF/GM-CSF, where they proliferate and sustain the sterile inflammation.[25] An additional recruitment of monocytes through plaque-infiltrating blood vessels from the adventitial side is currently under investigation. Notably, especially human atherosclerotic plaques are characterized by a dense vascularization[26] [27] ([Fig. 2]).

It is of fundamental importance that not only monocyte-derived but also resident macrophages in the vascular wall, which include macrophages of fetal origin, are recruited to sites of intimal oxLDL accumulation. Being present in the vessel wall already in steady state, they can proliferate upon activation to provide an effective machinery for cholesterol phagocytosis.[28] [29] As foam cells become increasingly immotile, the inflammatory process is locally enhanced.[30]

Unfortunately, cellular markers of mononuclear phagocytes are often overlapping and make an unequivocal identification of different macrophage subclasses challenging. This becomes increasingly difficult as vascular smooth muscle cells (VSMCs) and pericyte-like cells display various degrees of plasticity and can upregulate markers on their surface that are commonly related to the macrophage lineage.[31] There is significant evidence of VSMC-to-macrophage transdifferentiation in vivo, which is associated with clonal expansion and downregulation of VSMC markers.[32] Further, VSMCs and also endothelial cells are capable of phagocytosing some modified LDL particles.[33] Recent data on the transcriptional landscape of aortic macrophages supports this heterogeneity and points to yet undefined subpopulations.[4] A detailed understanding of the phenotype and function of all cells contributing to the mononuclear phagocyte lineage in the arterial wall will be essential for a precise understanding of atherogenesis and the development of targeted therapies.

Adhesion Receptors as Potential Therapeutic Targets

An obvious approach to limit foam cell formation and development of atherosclerotic plaques would be the inhibition of monocyte/macrophage adhesion and extravasation. In animal studies, an inhibition of the adhesion receptor VCAM reduced atherosclerosis.[34] [35] However, this did not translate well into humans as demonstrated in the Aggressive Reduction of Inflammation Stops Events study which aimed to reduce major cardiac events by a VCAM inhibitor.[36] [37] In a similar vein, targeting selectins or integrins was not applicable to humans as selective therapy for atherosclerosis. Interestingly, therapeutic approaches aiming at cytokine-mediated pathways which have been linked to certain macrophage subpopulations have shown some promising results. For example, CCR2 is typically expressed in BM-derived monocytes and is essential for monocyte extravasation in atherosclerotic lesions. In this regard, small interfering RNA-mediated silencing of CCR2 reduced monocyte infiltration in atherosclerotic plaques.[38] Potentially, a specific treatment of macrophage subpopulations might represent a useful approach to limit atherosclerosis while maintaining other macrophage functions.

Macrophage Polarization in the Context of Atherosclerosis

Macrophages can not only be distinguished by their origin, but also by their functional differentiation pattern, which is also referred to as macrophage polarization. In analogy to T1 and T2 helper cells, macrophages have been classified in M1 and M2 subpopulations. Today, this classification seems to be oversimplified, since macrophage characteristics vary on a continuum between the classical M1 and M2 phenotypes and they are significantly influenced by the local microenvironment. Still, this conventional classification helps to summarize different macrophage characteristics and macrophage subclasses which are nowadays referred to as M1- or M2-like[39] ([Fig. 1]).

M1-like macrophages are classically activated by tumor necrosis factor α (TNFα), interferon γ, and lipopolysaccharides (LPS) as well as other pathogen-associated molecular pattern (PAMP) and danger-associated molecular pattern (DAMP) signaling cascades. Upon activation, these cells secrete TNFα, interleukin (IL) 1β, IL6, IL12, IL23, NO, and reactive oxygen species (ROS) as well as multiple chemokines.[40] M1-like macrophages are characterized by the expression of inducible NO synthase, Toll-like receptor (TLR) 2 and 4, as well as CD68, CD80, and CD86. These characteristics of M1-like macrophages are in line with their predominant role in pathogen defense including phagocytosis and destruction of foreign bodies as well as antigen presentation to T cells, triggering an adaptive immune response[41] [42] [43] ([Fig. 1]).

M2-like alternatively activated macrophages are stimulated mainly by IL4, IL10, and IL13. They are characterized by the expression of arginase 1 (Arg1), mannose receptor C type 1 (CD206), chitinase-like protein 3 (Chil3, Ym1), Chil4 (Ym2), and resistin-like alpha (Fizz1) as well as CD86. Arg1 and Fizz1 expression are dependent on signal transducer and transcription activator 6 (STAT6) signaling.[44] Major secreted molecules are IL1 receptor antagonist, IL10, transforming growth factor β (TGFβ), as well as multiple chemokines including C-C motif chemokine 17 (CCL17), CCL18, CCL22, and CCL24.[43] M2-like macrophages can be further subdivided in several subclasses with partially overlapping functions. In principle, all M2-like macrophages are characterized by a rather immunosuppressive phenotype as well as predominant functions in tissue repair and homeostasis.[41] [42] [43] M2 polarization has been associated with expression of the triggering receptor expressed on myeloid cells-2 (TREM-2).[45] In recent studies, using single-cell RNA sequencing, TREM-2^high macrophages were linked to the population of foamy macrophages[4] [46] ([Fig. 1]).

M1-like macrophages are rather inflammatory and their appearance is associated with high-risk plaques, while M2-like macrophages seem to be predominantly present in stable plaques and the adventitia.[40] [47] Expression of STAT6 has been described to cause a shift from macrophages with an inflammatory M1-like to a more stable M2-like phenotype.[44] Also, environmental circumstances are capable of influencing the equilibrium between M1- and M2-like activated macrophages since incubation with LDL causes a proinflammatory shift in macrophage subpopulations.[48] [49] Kadl et al even described an entirely new macrophage subpopulation upon oxLDL exposure in mice—the so-called Mox macrophages. These cells are proatherogenic and characterized by a decreased phagocytic and chemotactic capacity in a nuclear factor like 2-dependent manner.[50] Moreover, also MHb macrophages have been described in atherosclerotic lesions as they develop upon exposure to the heme products emerging from intraplaque hemorrhage of infiltrating blood vessels.[51] [52] They express high levels of CD163 as well as CD206 and they are resistant to cholesterol accumulation due to increased expression of cholesterol efflux receptors like the adenosine triphosphate (ATP)-binding cassette transporters (ABC), ABCA1 and ABCG1.[53] Additionally, heme-dependent macrophage subpopulations have also been associated with efficient phagocytosis of extravasated erythrocytes (erythrophagocytosis).[54]

In the context of atherosclerosis, another new subpopulation was named M4-like macrophages as their phenotype is somewhere in between the continuum of M1 and M2 characteristics and since they are stimulated by platelet-derived CXCL4. They express TNFα, IL6, matrix metallopeptidase (MMP) 7, and MMP12 as well as S100 calcium-binding protein A8.[55] [56] Interestingly, the expression of CD163 is irreversibly lost in these cells leading to a proatherogenic phenotype and advanced plaque morphology.[40] [55] [57]

Novel Approaches to Characterize Macrophage Subpopulations

The enormous complexity in today's macrophage populations have been summarized by specialists in the field in order to provide a clear overview with all different subclasses.[58] Several studies now try to specifically address macrophages subpopulations.[59] Moreover, novel transcriptome-based experiments strive to further characterize and classify these diverse populations[2] [4] [60] ([Fig. 1]). Cole et al identified five different macrophage subpopulations in atherosclerotic aortas by mass cytometry,[2] whereas Cochain et al distinguish three subpopulations identified by single-cell RNA sequencing.[4] Both groups reported a shift towards inflammatory monocyte-macrophage populations under high fat diet.[2] [4] In a similar approach, using single-cell RNA sequencing and mass cytometry Winkels et al identified one macrophage population by RNA sequencing and three populations by mass cytometry. Interestingly, lymphocyte antigen 6 complex positive inflammatory monocytes seemed to be decreased in disease, while the populations of macrophages increased by 110%.[3] It is still unclear if infiltrating monocytes, which differentiate to macrophages, or local proliferation of tissue resident macrophages, is the major trigger for cell pool expansion.[61]

All three studies identified a predominant group of macrophage clusters, which is found under physiological conditions and usually referred to as resident macrophages. These cells express high levels of lymphatic vessel endothelial hyaluronic acid receptor 1 (LYVE1)[3] [4] and CD206[2] [4] when compared to macrophage clusters in atherosclerotic lesions. Interestingly, also several previous studies have attributed these markers to tissue-resident macrophages.[6] [62] [63] M-CSF 1 receptor (CSF1R) has been shown to interact with LYVE1 to regulate arterial stiffness by controlling collagen expression in VSMCs.[63] Moreover, an inhibition of CSF1R significantly reduced the number of resident macrophages without affecting monocytes in the blood or aorta.[63] [64] M-CSF and CSF1R seem to selectively influence and control tissue-resident macrophages without affecting inflammatory reactions mediated by monocytes. This therapeutic approach might therefore be useful for targeted interventions and is even discussed for cancer therapies.[65]

In contrast, macrophage populations in atherosclerotic lesions are characterized by a shift towards inflammatory characteristics, which have previously been described as M1-like.[2] [4] Interesting candidates characterizing these clusters are CD14, nucleotide oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3), and IL1β.[2] [4] However, the quantitative contribution of expanding resident macrophages versus infiltrating inflammatory cells remains unclear, and is even contradictory in the three latest phenotypical screens as Winkels et al observed a decrease of inflammatory monocytes in response to western diet, while the two other studies described an increase of this respective immune cell population.[2] [3] [4]

Future therapeutic approaches might specifically target macrophage plasticity and polarization. Markers identified by phenotypical clustering using RNA sequencing and mass cytometry show clear analogies to conventional M1- and M2-like macrophages.[2] [3] [4] Potential interventions could be designed to modulate the shift between M1- and M2-like polarization states. Further, targeting of specific macrophage subpopulations may offer to intervene with lesion formation and avoiding adverse effects on immunological defense mechanisms (e.g. against invading pathogens).

Macrophage Immunometabolism—An Emerging Target

Recent evidence supports the notion that macrophage functions are closely intertwined with their metabolic features.[66] [67] In a simplified perspective, M1-like macrophages utilize glycolytic metabolism while M2-like macrophages feature mitochondrial oxidative phosphorylation and fatty acid metabolism. Consequently, glycolysis inhibition blocks classical activation of primary monocytes and monocytic cell lines.[68] Thus, specific macrophage effector functions could be regulated through distinct metabolic pathways.[69] However, drug targeting of metabolic pathways has been challenging, even in vitro, due to off-target effects and challenges of dosing.[70] Thus, the overlay of immunometabolism with macrophage polarization and function requires further research.

Initiation of Lesion Formation: Lipid Uptake Alters Macrophage Functions

Based on the basic concept of plaque development by Ross and Glomset[71] and Goldstein and Brown,[72] LDL preferentially enters the endothelium in areas of vascular branches where endothelial cells partially lose their definite orientation and barrier structure. Along with turbulent flow and endothelial dysfunction, these areas are predisposed to LDL extravasation by passive diffusion through endothelial cell junctions.[73] Apolipoprotein B (ApoB)—the major component of LDL particles—is linked to matrix proteoglycans in the intima and mediates the local accumulation of lipids which are subsequently phagozytosed by macrophages[74] ([Fig. 2]). Lipid uptake together with alterations of the tissue microenvironment imposes significant changes in macrophage phenotype and functions, representing a major driver of lesion formation.[75] In the following, we will summarize the process of lipid uptake and its consequences in more detail.

Macrophages Phagocytose Modified LDL

LDL modifications in the vascular intima most importantly involve oxidation processes by ROS, metal ions, and local enzymes like lipoxygenases.[76] OxLDL is engulfed by macrophages where it accumulates and leads to changes in macrophage morphology (foam cells—a central component of atherosclerotic plaques) and polarization towards an inflammatory phenotype[75] ([Fig. 2]). Therapeutic interventions addressing LDL oxidation might be promising to reduce atherosclerosis in the future. For example, lipoxygenase-12 is an enzyme that generates bioactive lipids from different polyunsaturated fatty acids. Mice deficient in this enzyme develop significantly less atherosclerotic lesions.[77]

Beyond its important function in removing LDL from peripheral tissues, high-density lipoprotein (HDL) inhibits the process of LDL oxidation via esterases contained in HDL particles.[78] [79] Moreover, HDL weakens the proinflammatory function of mononuclear phagocytes.[80] Yet, also proinflammatory effects of HDL have been identified as it leads to lipid raft disruption by passive cholesterol depletion and increased inflammatory cytokine expression.[81]

An important defense mechanism conserved between humans and other species is characterized by the recognition of distinct molecular structures—so-called pattern recognition receptors (PRRs). These PRRs are primarily known for their ability to recognize pathogens. However, under certain circumstances like tissue damage, organisms are also required to phagocytose their own molecular structures—so-called DAMPs. Heat shock proteins, ATP, high mobility group box 1, and also oxLDL are important DAMPs triggering macrophage responses. The uptake of oxLDL particles is mediated by scavenger receptors with their most prominent representatives being the class B receptor CD36, CD68, lectin-like oxLDL receptor 1 (LOX1), as well as different class A scavenger receptors (SR-A).[82] [83] [84] [85] Interestingly, downregulation of CD36 by the activation of Mac1—an integrin adhesion receptor—prevents the formation of inflammatory macrophages and foam cells.[86] Beyond macrophages, also endothelial cells and VSMCs are capable of oxLDL phagocytosis in atherosclerotic lesions.[33] Upon receptor-mediated endocytosis, TLR complexes assemble and enhance nuclear factor κ light chain enhancer of activated B cells (NFκB) signaling.[87] [88]

Intracellular lipid turnover involves hydrolysis, esterification, and efflux. The major mediators of lipid efflux are ABCA1 and ABCG1.[89] Interestingly, large genome-wide association studies have shown that the isoform ABCG8 is associated with a decreased risk for atherosclerosis.[90] Similar associations have been identified for ApoE, especially its isoform ApoE ε2, which is involved in cholesterol removal and thereby reduces plaque burden.[90] [91] In macrophages, accumulating lipids are sensed by liver X receptors (LXRs), which can reduce scavenger receptor-mediated endocytosis and enhance cholesterol efflux. By these and others processes, LXRs mediate anti-inflammatory effects in mouse models of atherosclerosis.[92] [93] In a similar vein, lipophagy—a special form of autophagy—also reduces lipid accumulation and prevents the local inflammatory response.[94] [95] However, intracellular lipid accumulation leads to activation of the inflammasome complex involving NFκB signaling.[95] [96] Associated proteins like NLRP3 and caspase 1 are upregulated with subsequent release of major inflammatory mediators like IL1β and IL18, which in turn induced IL6 and TNFα production.[97] Thus, excessive phagocytosis of modified LDL particles in atherosclerotic lesions sustains the vascular inflammatory response in the absence of pathogens—a so-called sterile inflammation[5] [98] ([Fig. 2]). In addition to oxLDL, interactions with cholesterol crystals induce an inflammatory macrophage phenotype by activation of the NLRP3 inflammasome, which enhances atherosclerotic lesion formation.[99]

It should be noted that lipid metabolism exerts a multitude of effects on the organism, including the hematopoietic system. Disturbance of cholesterol homeostasis results in a hematopoietic bias towards proinflammatory monocytes, which are known to promote atherosclerosis. These effects can be direct, for example, by affecting cholesterol metabolism in HSCs,[100] or indirect by disturbing lipid metabolism in tissues that signal to the BM compartment (e.g. adipose tissue).[101] Regarding this exciting topic, please refer to excellent review articles by Murphy and colleagues.[102] [103]

To reduce the atherosclerotic burden, several steps in the process of cholesterol extravasation, oxidation, and subsequent uptake by macrophages could potentially be harnessed as therapeutic targets. Today's prevention strategies primarily address factors that minimize endothelial dysfunction as a prerequisite for cholesterol accumulation. Going one step further in the pathophysiological cascade, oxLDL uptake could potentially be targeted by the inhibition of scavenger receptors. However, eight different classes of receptors are summarized under the term scavenger receptors and partially overlapping functions as well as reciprocal compensation make targeted therapies very challenging.[104] [105] Modification and genetic deletion of SR-A mostly resulted in reduction of atherosclerotic lesions[83] [105] however, also opposite results have been reported.[106] Similarly, studies on CD36 showed controversial results, especially with respect to sex-dependent differences in mice.[85] [107] Finally, knockout studies of LOX1 have provided evidence for a reduction in atherosclerosis. However, genetic deletion was not limited to macrophages and effected also the endothelium, limiting conclusions regarding the specific mechanism involved.[108] [109] Beyond these partially controversial results, uptake of modified LDL particles is not limited to scavenger receptors since also acetyl-CoA acetyltransferase-1, a mitochondrial enzyme that catalyzes acetoacetyl-CoA formation, is involved in this process.[110] [111] [112] Further studies, including parallel targeting of different scavenger receptors, should enlighten this field in the future and might eventually provide new therapeutic approaches.

Resolution of Inflammation: Efferocytosis Controls the Inflammatory Response

In order to maintain tissue homeostasis, the organism is capable of removing dead cells in an immunologically silent form—the so-called efferocytosis (Latin “efferre”: “to bury” or “to take to the grave”) or programmed cell removal.[113] [114] Mononuclear phagocytes as well as nonspecialized cells with phagocytic capabilities like VSMCs or endothelial cells are capable of efferocytosis.[115] [116] However, it was reported that the capacity of efferocytosis in atherosclerotic plaques is 20-fold less in comparison to other tissues.[117] Therefore, it is not surprising that atherosclerosis is a highly active immunological process in which multiple proinflammatory mediators including DAMPs are released during the necrotic lysis of dying cells.[118] Efferocytosis can be divided in four major steps: First, “find me” signals help to attract cells capable of efferocytosis by chemotaxis (1). Most important representatives of these “find me” signals are lysophosphatidylcholine, CX3CL1, sphingosine 1 phosphate, and ATP or uridine triphosphate nucleotides[119] [120] as well as different alarmins as secreted chemotactic agents.[121] Afterwards, different “eat me” and “don't eat me” signals trigger the decision of phagocytosis (2) which requires cellular reorganization for proper engulfment—a process mainly regulated by small GTPases like Ras homolog gene family member A, cell division control protein 42, and Ras-related C3 botulinum toxin substrate (3).[122] Defective “eat me” signals are highly associated with atherosclerotic progression and represent the most important players in the process of efferocytosis. Prominent representatives are LDL receptor-related protein 1, MER proto-oncogene tyrosine kinase (MerTK), and protein S, among others.[123] Finally, engulfed cells are digested and processed including the release of anti-inflammatory cytokines (4).

Defects in any of these steps lead to a significant exacerbation of atherosclerosis.[124] [125] Interestingly, also clinically relevant molecules such as ApoE or HDL turned out to be positively associated with active efferocytosis and therefore prevent excessive atherosclerosis.[126] In case of insufficient efferocytosis, a necrotic core within the plaques is formed which maintains an inflammatory response and favors lesion progression.

A therapeutic approach to modulate efferocytosis could target the relation between “eat-me” and “don't eat me” signals in order to control the inflammatory response by effective efferocytosis. In this sense, stimulation of MerTK signaling might prevent plaque progression. For example, in LDL receptor deficient mice additional deletion of MerTK causes plaque progression by reduced efferocytosis.[127] [128] Very similar findings were found in ApoE^−/− MerTK^−/− mice.[125] MerTK can be cleaved to produce inactive MerTK and soluble Mer which are both associated with advanced atherosclerosis.[127] [129]

Current and Future Clinical Interventions

Until today, numerous strategies have been attempted to reduce and prevent atherosclerosis. Therapeutic approaches targeted key pathophysiological mechanisms underlying different steps of lesion formation ranging from LDL regulation and prevention of oxLDL formation, to platelet inhibition, reduction of monocyte/macrophage recruitment, and immunosuppressive strategies. However, the large diversity of mononuclear phagocytes makes targeted interventions very challenging and we have only begun to unravel the enormous cellular complexity with respect to origin, polarization, and metabolism in a population which has so far been regarded as uniform. Moreover, the cellular identity of macrophages continuously adapts to the tissue of residence and can be reprogrammed by an altered (inflammatory) microenvironment.

Most successful in humans was the introduction of statins and related drugs leading to significant LDL reduction as well as potential pleiotropic effects which are currently under investigation. This treatment limits the development of atherosclerosis and even reduces all-cause mortality.[130] Other important therapeutic approaches address smoking, the optimization of blood pressure, or glucose levels which contribute to endothelial dysfunction.

In an era of increasingly controlled cardiovascular risk factors, the histological features of vascular injuries are changing as superficial erosions become more and more relevant, while plaque rupture events decrease. This pathophysiological difference will need to be considered in the development of treatment strategies as discussed in a recent review by Libby et al.[131]

Novel therapeutic developments have increasingly focused on the inflammatory aspect of atherosclerosis. The CANTOS (Canakinumab Anti-Inflammatory Thrombosis Outcomes Study) trial investigated canakinumab as a monoclonal antibody that targets IL1β in the setting of cardiovascular disease. Interestingly, this medical intervention in humans significantly reduced the rate of cardiovascular events including myocardial infarction.[132] A potential mechanism and the idea behind this treatment concept was the reduction of inflammatory activity in atherosclerosis. Similarly, an anti-inflammatory effect of statins beyond their effect on lowering cholesterol has been described.[133] [134] [135] The LoDoCo (Low Dose Colchicine for Secondary Prevention of Cardiovascular Disease) trial also provided some evidence that colchicine reduces cardiovascular events through its anti-inflammatory properties.[136] Interestingly, methotrexate treatment did not reduce the rate of cardiovascular events.[137] Ongoing studies now address the protective effect of colchicine and tocilizumab—an antibody against the IL6 receptor—in the context of atherosclerosis.[138]

Conclusion

The complexity in the inflammatory process underlying atherosclerosis has made therapeutic interventions very challenging. The heterogeneity of macrophage populations, their differential contribution to distinct stages of lesion formation, as well as their precise targeting requires further research. Selective inhibition of proatherosclerotic macrophage subsets or a reduction in inflammatory monocyte recruitment, could potentially reduce the atherosclerotic burden without affecting basic protective macrophage functions. Beyond the conventional targeting of surface receptors, immunometabolism might serve as an effective switch to selectively address certain macrophage subpopulations in dependence of their predominant form of energy supply. These approaches are likely to provide new perspectives for the treatment and prevention of lesion formation through macrophage-directed immunotherapies.

Conflict of Interest

None declared.

• References

• 1 Global Health Estimates 2016: Deaths by Cause, Age, Sex, by Country and by Region, 2000–2016; 2018
• 2 Cole JE, Park I, Ahern DJ. , et al. Immune cell census in murine atherosclerosis: cytometry by time of flight illuminates vascular myeloid cell diversity. Cardiovasc Res 2018; 114 (10) 1360-1371
• 3 Winkels H, Ehinger E, Vassallo M. , et al. Atlas of the immune cell repertoire in mouse atherosclerosis defined by single-cell RNA-sequencing and mass cytometry. Circ Res 2018; 122 (12) 1675-1688
• 4 Cochain C, Vafadarnejad E, Arampatzi P. , et al. Single-cell RNA-Seq reveals the transcriptional landscape and heterogeneity of aortic macrophages in murine atherosclerosis. Circ Res 2018; 122 (12) 1661-1674
• 5 Bobryshev YV, Ivanova EA, Chistiakov DA, Nikiforov NG, Orekhov AN. Macrophages and their role in atherosclerosis: pathophysiology and transcriptome analysis. BioMed Res Int 2016; 2016: 9582430
• 6 Ensan S, Li A, Besla R. , et al. Self-renewing resident arterial macrophages arise from embryonic CX3CR1(+) precursors and circulating monocytes immediately after birth. Nat Immunol 2016; 17 (02) 159-168
• 7 Schulz C, Massberg S. Atherosclerosis--multiple pathways to lesional macrophages. Sci Transl Med 2014; 6 (239) 239ps2
• 8 Ginhoux F, Greter M, Leboeuf M. , et al. Fate mapping analysis reveals that adult microglia derive from primitive macrophages. Science 2010; 330 (6005): 841-845
• 9 Schulz C, Gomez Perdiguero E, Chorro L. , et al. A lineage of myeloid cells independent of Myb and hematopoietic stem cells. Science 2012; 336 (6077): 86-90
• 10 Stremmel C, Schuchert R, Wagner F. , et al. Yolk sac macrophage progenitors traffic to the embryo during defined stages of development. Nat Commun 2018; 9 (01) 75
• 11 Yona S, Kim KW, Wolf Y. , et al. Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis. Immunity 2013; 38 (01) 79-91
• 12 Gomez Perdiguero E, Klapproth K, Schulz C. , et al. Tissue-resident macrophages originate from yolk-sac-derived erythro-myeloid progenitors. Nature 2015; 518 (7540): 547-551
• 13 Mass E, Ballesteros I, Farlik M. , et al. Specification of tissue-resident macrophages during organogenesis. Science 2016; 353 (6304): aaf4238
• 14 Gosselin D, Link VM, Romanoski CE. , et al. Environment drives selection and function of enhancers controlling tissue-specific macrophage identities. Cell 2014; 159 (06) 1327-1340
• 15 Lavin Y, Winter D, Blecher-Gonen R. , et al. Tissue-resident macrophage enhancer landscapes are shaped by the local microenvironment. Cell 2014; 159 (06) 1312-1326
• 16 Moore KJ, Sheedy FJ, Fisher EA. Macrophages in atherosclerosis: a dynamic balance. Nat Rev Immunol 2013; 13 (10) 709-721
• 17 Moore KJ, Tabas I. Macrophages in the pathogenesis of atherosclerosis. Cell 2011; 145 (03) 341-355
• 18 Lin J, Kakkar V, Lu X. Impact of MCP-1 in atherosclerosis. Curr Pharm Des 2014; 20 (28) 4580-4588
• 19 Smith JD, Trogan E, Ginsberg M, Grigaux C, Tian J, Miyata M. Decreased atherosclerosis in mice deficient in both macrophage colony-stimulating factor (op) and apolipoprotein E. Proc Natl Acad Sci U S A 1995; 92 (18) 8264-8268
• 20 Finney AC, Stokes KY, Pattillo CB, Orr AW. Integrin signaling in atherosclerosis. Cell Mol Life Sci 2017; 74 (12) 2263-2282
• 21 Dong ZM, Chapman SM, Brown AA, Frenette PS, Hynes RO, Wagner DD. The combined role of P- and E-selectins in atherosclerosis. J Clin Invest 1998; 102 (01) 145-152
• 22 Galkina E, Ley K. Vascular adhesion molecules in atherosclerosis. Arterioscler Thromb Vasc Biol 2007; 27 (11) 2292-2301
• 23 Collins RG, Velji R, Guevara NV, Hicks MJ, Chan L, Beaudet AL. P-selectin or intercellular adhesion molecule (ICAM)-1 deficiency substantially protects against atherosclerosis in apolipoprotein E-deficient mice. J Exp Med 2000; 191 (01) 189-194
• 24 Combadière C, Potteaux S, Rodero M. , et al. Combined inhibition of CCL2, CX3CR1, and CCR5 abrogates Ly6C(hi) and Ly6C(lo) monocytosis and almost abolishes atherosclerosis in hypercholesterolemic mice. Circulation 2008; 117 (13) 1649-1657
• 25 Chen GY, Nuñez G. Sterile inflammation: sensing and reacting to damage. Nat Rev Immunol 2010; 10 (12) 826-837
• 26 Maiellaro K, Taylor WR. The role of the adventitia in vascular inflammation. Cardiovasc Res 2007; 75 (04) 640-648
• 27 Scott NA, Cipolla GD, Ross CE. , et al. Identification of a potential role for the adventitia in vascular lesion formation after balloon overstretch injury of porcine coronary arteries. Circulation 1996; 93 (12) 2178-2187
• 28 Goldstein JL, Ho YK, Basu SK, Brown MS. Binding site on macrophages that mediates uptake and degradation of acetylated low density lipoprotein, producing massive cholesterol deposition. Proc Natl Acad Sci U S A 1979; 76 (01) 333-337
• 29 Ross R. Atherosclerosis--an inflammatory disease. N Engl J Med 1999; 340 (02) 115-126
• 30 Randolph GJ. Mechanisms that regulate macrophage burden in atherosclerosis. Circ Res 2014; 114 (11) 1757-1771
• 31 Shankman LS, Gomez D, Cherepanova OA. , et al. KLF4-dependent phenotypic modulation of smooth muscle cells has a key role in atherosclerotic plaque pathogenesis. Nat Med 2015; 21 (06) 628-637
• 32 Feil S, Fehrenbacher B, Lukowski R. , et al. Transdifferentiation of vascular smooth muscle cells to macrophage-like cells during atherogenesis. Circ Res 2014; 115 (07) 662-667
• 33 Orekhov AN, Bobryshev YV, Chistiakov DA. The complexity of cell composition of the intima of large arteries: focus on pericyte-like cells. Cardiovasc Res 2014; 103 (04) 438-451
• 34 Cybulsky MI, Iiyama K, Li H. , et al. A major role for VCAM-1, but not ICAM-1, in early atherosclerosis. J Clin Invest 2001; 107 (10) 1255-1262
• 35 Sundell CL, Somers PK, Meng CQ. , et al. AGI-1067: a multifunctional phenolic antioxidant, lipid modulator, anti-inflammatory and antiatherosclerotic agent. J Pharmacol Exp Ther 2003; 305 (03) 1116-1123
• 36 Tardif JC, McMurray JJ, Klug E. , et al; Aggressive Reduction of Inflammation Stops Events (ARISE) Trial Investigators. Effects of succinobucol (AGI-1067) after an acute coronary syndrome: a randomised, double-blind, placebo-controlled trial. Lancet 2008; 371 (9626): 1761-1768
• 37 Besemer J, Harant H, Wang S. , et al. Selective inhibition of cotranslational translocation of vascular cell adhesion molecule 1. Nature 2005; 436 (7048): 290-293
• 38 Leuschner F, Dutta P, Gorbatov R. , et al. Therapeutic siRNA silencing in inflammatory monocytes in mice. Nat Biotechnol 2011; 29 (11) 1005-1010
• 39 Murray PJ, Wynn TA. Protective and pathogenic functions of macrophage subsets. Nat Rev Immunol 2011; 11 (11) 723-737
• 40 Chistiakov DA, Bobryshev YV, Nikiforov NG, Elizova NV, Sobenin IA, Orekhov AN. Macrophage phenotypic plasticity in atherosclerosis: the associated features and the peculiarities of the expression of inflammatory genes. Int J Cardiol 2015; 184: 436-445
• 41 Ruytinx P, Proost P, Van Damme J, Struyf S. Chemokine-induced macrophage polarization in inflammatory conditions. Front Immunol 2018; 9: 1930
• 42 Ginhoux F, Jung S. Monocytes and macrophages: developmental pathways and tissue homeostasis. Nat Rev Immunol 2014; 14 (06) 392-404
• 43 Shapouri-Moghaddam A, Mohammadian S, Vazini H. , et al. Macrophage plasticity, polarization, and function in health and disease. J Cell Physiol 2018; 233 (09) 6425-6440
• 44 Sanson M, Distel E, Fisher EA. HDL induces the expression of the M2 macrophage markers arginase 1 and Fizz-1 in a STAT6-dependent process. PLoS One 2013; 8 (08) e74676
• 45 Turnbull IR, Gilfillan S, Cella M. , et al. Cutting edge: TREM-2 attenuates macrophage activation. J Immunol 2006; 177 (06) 3520-3524
• 46 Kim K, Shim D, Lee JS. , et al. Transcriptome analysis reveals nonfoamy rather than foamy plaque macrophages are proinflammatory in atherosclerotic murine models. Circ Res 2018; 123 (10) 1127-1142
• 47 Cochain C, Zernecke A. Macrophages and immune cells in atherosclerosis: recent advances and novel concepts. Basic Res Cardiol 2015; 110 (04) 34
• 48 Al-Sharea A, Lee MK, Moore XL. , et al. Native LDL promotes differentiation of human monocytes to macrophages with an inflammatory phenotype. Thromb Haemost 2016; 115 (04) 762-772
• 49 van Tits LJ, Stienstra R, van Lent PL, Netea MG, Joosten LA, Stalenhoef AF. Oxidized LDL enhances pro-inflammatory responses of alternatively activated M2 macrophages: a crucial role for Krüppel-like factor 2. Atherosclerosis 2011; 214 (02) 345-349
• 50 Kadl A, Meher AK, Sharma PR. , et al. Identification of a novel macrophage phenotype that develops in response to atherogenic phospholipids via Nrf2. Circ Res 2010; 107 (06) 737-746
• 51 Finn AV, Nakano M, Polavarapu R. , et al. Hemoglobin directs macrophage differentiation and prevents foam cell formation in human atherosclerotic plaques. J Am Coll Cardiol 2012; 59 (02) 166-177
• 52 Boyle JJ, Harrington HA, Piper E. , et al. Coronary intraplaque hemorrhage evokes a novel atheroprotective macrophage phenotype. Am J Pathol 2009; 174 (03) 1097-1108
• 53 Habib A, Finn AV. The role of iron metabolism as a mediator of macrophage inflammation and lipid handling in atherosclerosis. Front Pharmacol 2014; 5: 195
• 54 Boyle JJ. Heme and haemoglobin direct macrophage Mhem phenotype and counter foam cell formation in areas of intraplaque haemorrhage. Curr Opin Lipidol 2012; 23 (05) 453-461
• 55 Gleissner CA, Shaked I, Little KM, Ley K. CXC chemokine ligand 4 induces a unique transcriptome in monocyte-derived macrophages. J Immunol 2010; 184 (09) 4810-4818
• 56 Erbel C, Tyka M, Helmes CM. , et al. CXCL4-induced plaque macrophages can be specifically identified by co-expression of MMP7+S100A8+ in vitro and in vivo. Innate Immun 2015; 21 (03) 255-265
• 57 Erbel C, Wolf A, Lasitschka F. , et al. Prevalence of M4 macrophages within human coronary atherosclerotic plaques is associated with features of plaque instability. Int J Cardiol 2015; 186: 219-225
• 58 Murray PJ, Allen JE, Biswas SK. , et al. Macrophage activation and polarization: nomenclature and experimental guidelines. Immunity 2014; 41 (01) 14-20
• 59 De Paoli F, Staels B, Chinetti-Gbaguidi G. Macrophage phenotypes and their modulation in atherosclerosis. Circ J 2014; 78 (08) 1775-1781
• 60 Schmidt SV, Krebs W, Ulas T. , et al. The transcriptional regulator network of human inflammatory macrophages is defined by open chromatin. Cell Res 2016; 26 (02) 151-170
• 61 Hilgendorf I, Swirski FK, Robbins CS. Monocyte fate in atherosclerosis. Arterioscler Thromb Vasc Biol 2015; 35 (02) 272-279
• 62 Colin S, Chinetti-Gbaguidi G, Staels B. Macrophage phenotypes in atherosclerosis. Immunol Rev 2014; 262 (01) 153-166
• 63 Lim HY, Lim SY, Tan CK. , et al. Hyaluronan receptor LYVE-1-expressing macrophages maintain arterial tone through hyaluronan-mediated regulation of smooth muscle cell collagen. Immunity 2018; 49 (06) 1191
• 64 Kubota Y, Takubo K, Shimizu T. , et al. M-CSF inhibition selectively targets pathological angiogenesis and lymphangiogenesis. J Exp Med 2009; 206 (05) 1089-1102
• 65 Hume DA, MacDonald KP. Therapeutic applications of macrophage colony-stimulating factor-1 (CSF-1) and antagonists of CSF-1 receptor (CSF-1R) signaling. Blood 2012; 119 (08) 1810-1820
• 66 Buck MD, Sowell RT, Kaech SM, Pearce EL. Metabolic instruction of immunity. Cell 2017; 169 (04) 570-586
• 67 O'Neill LA, Pearce EJ. Immunometabolism governs dendritic cell and macrophage function. J Exp Med 2016; 213 (01) 15-23
• 68 Wang F, Zhang S, Jeon R. , et al. Interferon gamma induces reversible metabolic reprogramming of M1 macrophages to sustain cell viability and pro-inflammatory activity. EBioMedicine 2018; 30: 303-316
• 69 Artyomov MN, Sergushichev A, Schilling JD. Integrating immunometabolism and macrophage diversity. Semin Immunol 2016; 28 (05) 417-424
• 70 Divakaruni AS, Hsieh WY, Minarrieta L. , et al. Etomoxir inhibits macrophage polarization by disrupting CoA homeostasis. Cell Metab 2018; 28 (03) 490-503
• 71 Ross R, Glomset JA. Atherosclerosis and the arterial smooth muscle cell: proliferation of smooth muscle is a key event in the genesis of the lesions of atherosclerosis. Science 1973; 180 (4093): 1332-1339
• 72 Goldstein JL, Brown MS. The low-density lipoprotein pathway and its relation to atherosclerosis. Annu Rev Biochem 1977; 46: 897-930
• 73 Gimbrone Jr MA. Endothelial dysfunction, hemodynamic forces, and atherosclerosis. Thromb Haemost 1999; 82 (02) 722-726
• 74 Borén J, Olin K, Lee I, Chait A, Wight TN, Innerarity TL. Identification of the principal proteoglycan-binding site in LDL. A single-point mutation in apo-B100 severely affects proteoglycan interaction without affecting LDL receptor binding. J Clin Invest 1998; 101 (12) 2658-2664
• 75 Adamson S, Leitinger N. Phenotypic modulation of macrophages in response to plaque lipids. Curr Opin Lipidol 2011; 22 (05) 335-342
• 76 Yoshida H, Kisugi R. Mechanisms of LDL oxidation. Clin Chim Acta 2010; 411 (23-24): 1875-1882
• 77 Cyrus T, Witztum JL, Rader DJ. , et al. Disruption of the 12/15-lipoxygenase gene diminishes atherosclerosis in apo E-deficient mice. J Clin Invest 1999; 103 (11) 1597-1604
• 78 Shih DM, Xia YR, Wang XP. , et al. Combined serum paraoxonase knockout/apolipoprotein E knockout mice exhibit increased lipoprotein oxidation and atherosclerosis. J Biol Chem 2000; 275 (23) 17527-17535
• 79 Hegele RA. Paraoxonase genes and disease. Ann Med 1999; 31 (03) 217-224
• 80 Lee MK, Moore XL, Fu Y. , et al. High-density lipoprotein inhibits human M1 macrophage polarization through redistribution of caveolin-1. Br J Pharmacol 2016; 173 (04) 741-751
• 81 van der Vorst EPC, Theodorou K, Wu Y. , et al. High-density lipoproteins exert pro-inflammatory effects on macrophages via passive cholesterol depletion and PKC-NF-κB/STAT1-IRF1 signaling. Cell Metab 2017; 25 (01) 197-207
• 82 Podrez EA, Febbraio M, Sheibani N. , et al. Macrophage scavenger receptor CD36 is the major receptor for LDL modified by monocyte-generated reactive nitrogen species. J Clin Invest 2000; 105 (08) 1095-1108
• 83 Suzuki H, Kurihara Y, Takeya M. , et al. A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection. Nature 1997; 386 (6622): 292-296
• 84 Stewart CR, Stuart LM, Wilkinson K. , et al. CD36 ligands promote sterile inflammation through assembly of a Toll-like receptor 4 and 6 heterodimer. Nat Immunol 2010; 11 (02) 155-161
• 85 Febbraio M, Podrez EA, Smith JD. , et al. Targeted disruption of the class B scavenger receptor CD36 protects against atherosclerotic lesion development in mice. J Clin Invest 2000; 105 (08) 1049-1056
• 86 Yakubenko VP, Bhattacharjee A, Pluskota E, Cathcart MK. αMβ₂ integrin activation prevents alternative activation of human and murine macrophages and impedes foam cell formation. Circ Res 2011; 108 (05) 544-554
• 87 Xu XH, Shah PK, Faure E. , et al. Toll-like receptor-4 is expressed by macrophages in murine and human lipid-rich atherosclerotic plaques and upregulated by oxidized LDL. Circulation 2001; 104 (25) 3103-3108
• 88 Kawai T, Akira S. Signaling to NF-kappaB by Toll-like receptors. Trends Mol Med 2007; 13 (11) 460-469
• 89 Yvan-Charvet L, Wang N, Tall AR. Role of HDL, ABCA1, and ABCG1 transporters in cholesterol efflux and immune responses. Arterioscler Thromb Vasc Biol 2010; 30 (02) 139-143
• 90 Maguire EM, Pearce SWA, Xiao Q. Foam cell formation: a new target for fighting atherosclerosis and cardiovascular disease. Vascul Pharmacol 2019; 112: 54-71
• 91 Fazio S, Babaev VR, Murray AB. , et al. Increased atherosclerosis in mice reconstituted with apolipoprotein E null macrophages. Proc Natl Acad Sci U S A 1997; 94 (09) 4647-4652
• 92 Lee SD, Tontonoz P. Liver X receptors at the intersection of lipid metabolism and atherogenesis. Atherosclerosis 2015; 242 (01) 29-36
• 93 Kappus MS, Murphy AJ, Abramowicz S. , et al. Activation of liver X receptor decreases atherosclerosis in Ldlr⁻/⁻ mice in the absence of ATP-binding cassette transporters A1 and G1 in myeloid cells. Arterioscler Thromb Vasc Biol 2014; 34 (02) 279-284
• 94 Singh R, Kaushik S, Wang Y. , et al. Autophagy regulates lipid metabolism. Nature 2009; 458 (7242): 1131-1135
• 95 Razani B, Feng C, Coleman T. , et al. Autophagy links inflammasomes to atherosclerotic progression. Cell Metab 2012; 15 (04) 534-544
• 96 Jiang Y, Wang M, Huang K. , et al. Oxidized low-density lipoprotein induces secretion of interleukin-1β by macrophages via reactive oxygen species-dependent NLRP3 inflammasome activation. Biochem Biophys Res Commun 2012; 425 (02) 121-126
• 97 de Torre-Minguela C, Mesa Del Castillo P, Pelegrín P. The NLRP3 and pyrin inflammasomes: implications in the pathophysiology of autoinflammatory diseases. Front Immunol 2017; 8: 43
• 98 Woollard KJ, Geissmann F. Monocytes in atherosclerosis: subsets and functions. Nat Rev Cardiol 2010; 7 (02) 77-86
• 99 Duewell P, Kono H, Rayner KJ. , et al. NLRP3 inflammasomes are required for atherogenesis and activated by cholesterol crystals. Nature 2010; 464 (7293): 1357-1361
• 100 Dragoljevic D, Kraakman MJ, Nagareddy PR. , et al. Defective cholesterol metabolism in haematopoietic stem cells promotes monocyte-driven atherosclerosis in rheumatoid arthritis. Eur Heart J 2018; 39 (23) 2158-2167
• 101 Nagareddy PR, Kraakman M, Masters SL. , et al. Adipose tissue macrophages promote myelopoiesis and monocytosis in obesity. Cell Metab 2014; 19 (05) 821-835
• 102 Murphy AJ, Dragoljevic D, Tall AR. Cholesterol efflux pathways regulate myelopoiesis: a potential link to altered macrophage function in atherosclerosis. Front Immunol 2014; 5: 490
• 103 Lee MKS, Al-Sharea A, Dragoljevic D, Murphy AJ. Hand of FATe: lipid metabolism in hematopoietic stem cells. Curr Opin Lipidol 2018; 29 (03) 240-245
• 104 Krieger M. The other side of scavenger receptors: pattern recognition for host defense. Curr Opin Lipidol 1997; 8 (05) 275-280
• 105 Mäkinen PI, Lappalainen JP, Heinonen SE. , et al. Silencing of either SR-A or CD36 reduces atherosclerosis in hyperlipidaemic mice and reveals reciprocal upregulation of these receptors. Cardiovasc Res 2010; 88 (03) 530-538
• 106 de Winther MP, Gijbels MJ, van Dijk KW. , et al. Scavenger receptor deficiency leads to more complex atherosclerotic lesions in APOE3Leiden transgenic mice. Atherosclerosis 1999; 144 (02) 315-321
• 107 Moore KJ, Kunjathoor VV, Koehn SL. , et al. Loss of receptor-mediated lipid uptake via scavenger receptor A or CD36 pathways does not ameliorate atherosclerosis in hyperlipidemic mice. J Clin Invest 2005; 115 (08) 2192-2201
• 108 Mehta JL, Sanada N, Hu CP. , et al. Deletion of LOX-1 reduces atherogenesis in LDLR knockout mice fed high cholesterol diet. Circ Res 2007; 100 (11) 1634-1642
• 109 Xu S, Ogura S, Chen J, Little PJ, Moss J, Liu P. LOX-1 in atherosclerosis: biological functions and pharmacological modifiers. Cell Mol Life Sci 2013; 70 (16) 2859-2872
• 110 Dove DE, Su YR, Swift LL, Linton MF, Fazio S. ACAT1 deficiency increases cholesterol synthesis in mouse peritoneal macrophages. Atherosclerosis 2006; 186 (02) 267-274
• 111 Fazio S, Major AS, Swift LL. , et al. Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages. J Clin Invest 2001; 107 (02) 163-171
• 112 Yang L, Yang JB, Chen J. , et al. Enhancement of human ACAT1 gene expression to promote the macrophage-derived foam cell formation by dexamethasone. Cell Res 2004; 14 (04) 315-323
• 113 Hoffmann PR, deCathelineau AM, Ogden CA. , et al. Phosphatidylserine (PS) induces PS receptor-mediated macropinocytosis and promotes clearance of apoptotic cells. J Cell Biol 2001; 155 (04) 649-659
• 114 Ravichandran KS, Lorenz U. Engulfment of apoptotic cells: signals for a good meal. Nat Rev Immunol 2007; 7 (12) 964-974
• 115 Tabas I. Apoptosis and plaque destabilization in atherosclerosis: the role of macrophage apoptosis induced by cholesterol. Cell Death Differ 2004; 11 (Suppl. 01) S12-S16
• 116 Liao X, Sluimer JC, Wang Y. , et al. Macrophage autophagy plays a protective role in advanced atherosclerosis. Cell Metab 2012; 15 (04) 545-553
• 117 Schrijvers DM, De Meyer GR, Kockx MM, Herman AG, Martinet W. Phagocytosis of apoptotic cells by macrophages is impaired in atherosclerosis. Arterioscler Thromb Vasc Biol 2005; 25 (06) 1256-1261
• 118 Seimon T, Tabas I. Mechanisms and consequences of macrophage apoptosis in atherosclerosis. J Lipid Res 2009; 50 (Suppl): S382-S387
• 119 Elliott MR, Chekeni FB, Trampont PC. , et al. Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance. Nature 2009; 461 (7261): 282-286
• 120 Szondy Z, Garabuczi E, Joós G, Tsay GJ, Sarang Z. Impaired clearance of apoptotic cells in chronic inflammatory diseases: therapeutic implications. Front Immunol 2014; 5: 354
• 121 Bianchi ME. DAMPs, PAMPs and alarmins: all we need to know about danger. J Leukoc Biol 2007; 81 (01) 1-5
• 122 Mao Y, Finnemann SC. Regulation of phagocytosis by Rho GTPases. Small GTPases 2015; 6 (02) 89-99
• 123 Tajbakhsh A, Rezaee M, Kovanen PT, Sahebkar A. Efferocytosis in atherosclerotic lesions: malfunctioning regulatory pathways and control mechanisms. Pharmacol Ther 2018; 188: 12-25
• 124 Ait-Oufella H, Kinugawa K, Zoll J. , et al. Lactadherin deficiency leads to apoptotic cell accumulation and accelerated atherosclerosis in mice. Circulation 2007; 115 (16) 2168-2177
• 125 Thorp E, Cui D, Schrijvers DM, Kuriakose G, Tabas I. Mertk receptor mutation reduces efferocytosis efficiency and promotes apoptotic cell accumulation and plaque necrosis in atherosclerotic lesions of apoe-/- mice. Arterioscler Thromb Vasc Biol 2008; 28 (08) 1421-1428
• 126 Tao H, Yancey PG, Babaev VR. , et al. Macrophage SR-BI mediates efferocytosis via Src/PI3K/Rac1 signaling and reduces atherosclerotic lesion necrosis. J Lipid Res 2015; 56 (08) 1449-1460
• 127 Cai B, Thorp EB, Doran AC. , et al. MerTK receptor cleavage promotes plaque necrosis and defective resolution in atherosclerosis. J Clin Invest 2017; 127 (02) 564-568
• 128 Ait-Oufella H, Pouresmail V, Simon T. , et al. Defective mer receptor tyrosine kinase signaling in bone marrow cells promotes apoptotic cell accumulation and accelerates atherosclerosis. Arterioscler Thromb Vasc Biol 2008; 28 (08) 1429-1431
• 129 Wan E, Yeap XY, Dehn S. , et al. Enhanced efferocytosis of apoptotic cardiomyocytes through myeloid-epithelial-reproductive tyrosine kinase links acute inflammation resolution to cardiac repair after infarction. Circ Res 2013; 113 (08) 1004-1012
• 130 Long-Term Intervention with Pravastatin in Ischaemic Disease (LIPID) Study Group. Prevention of cardiovascular events and death with pravastatin in patients with coronary heart disease and a broad range of initial cholesterol levels. N Engl J Med 1998; 339 (19) 1349-1357
• 131 Libby P, Pasterkamp G, Crea F, Jang IK. Reassessing the mechanisms of acute coronary syndromes. Circ Res 2019; 124 (01) 150-160
• 132 Ridker PM, Everett BM, Thuren T. , et al; CANTOS Trial Group. Antiinflammatory therapy with canakinumab for atherosclerotic disease. N Engl J Med 2017; 377 (12) 1119-1131
• 133 Ridker PM, Cannon CP, Morrow D. , et al; Pravastatin or Atorvastatin Evaluation and Infection Therapy-Thrombolysis in Myocardial Infarction 22 (PROVE IT-TIMI 22) Investigators. C-reactive protein levels and outcomes after statin therapy. N Engl J Med 2005; 352 (01) 20-28
• 134 Ridker PM, Pradhan A, MacFadyen JG, Libby P, Glynn RJ. Cardiovascular benefits and diabetes risks of statin therapy in primary prevention: an analysis from the JUPITER trial. Lancet 2012; 380 (9841): 565-571
• 135 Bohula EA, Giugliano RP, Cannon CP. , et al. Achievement of dual low-density lipoprotein cholesterol and high-sensitivity C-reactive protein targets more frequent with the addition of ezetimibe to simvastatin and associated with better outcomes in IMPROVE-IT. Circulation 2015; 132 (13) 1224-1233
• 136 Nidorf SM, Eikelboom JW, Budgeon CA, Thompson PL. Low-dose colchicine for secondary prevention of cardiovascular disease. J Am Coll Cardiol 2013; 61 (04) 404-410
• 137 Ridker PM, Everett BM, Pradhan A. , et al. Low-dose methotrexate for the prevention of atherosclerotic events. N Engl J Med 2019; 380 (08) 752-762
• 138 Everett BM, Pradhan AD, Solomon DH. , et al. Rationale and design of the Cardiovascular Inflammation Reduction Trial: a test of the inflammatory hypothesis of atherothrombosis. Am Heart J 2013; 166 (02) 199-207