Macrophage subsets in atherosclerosis

Nature Reviews Cardiology

Atherosclerosis is characterized by increased accumulation of macrophages within the vessel wall. In response to stimuli such as modified lipids, cytokines, and senescent erythrocytes present in the atherosclerotic lesion, these macrophages can alter their functional phenotypes. Different macrophage subsets can influence the growth and composition of the atherosclerotic plaque in distinct ways. In this Review, Chinetti-Gbaguidi et al. highlight the diverse range of macrophage phenotypes present in atherosclerotic lesions, and their roles in both plaque progression and stability.

 

Giulia Chinetti-Gbaguidi,^1, Sophie Colin^1, & Bart Staels^1,

Abstract:

Macrophage accumulation within the vascular wall is a hallmark of atherosclerosis. In atherosclerotic lesions, macrophages respond to various environmental stimuli, such as modified lipids, cytokines, and senescent erythrocytes, which can modify their functional phenotypes. The results of studies on human atherosclerotic plaques demonstrate that the relative proportions of macrophage subsets within a plaque might be a better indicator of plaque phenotype and stability than the total number of macrophages. Understanding the function of specific macrophage subsets and their contribution to the composition and growth of atherosclerotic plaques would aid the identification of novel strategies to delay or halt the development of the disease and its associated pathophysiological consequences. However, most studies aimed at characterizing the phenotypes of human macrophages are performed in vitro and, therefore, their functional relevance to human pathology remains uncertain. In this Review, the diverse range of macrophage phenotypes in atherosclerotic lesions and their potential roles in both plaque progression and stability are discussed, with an emphasis on human pathology.

 Key points

• Only M1 proinflammatory and M2 anti-inflammatory macrophages have been described in vitro—however, a wide spectrum of intermediate phenotypes has been identified in in vivo studies

• Various stimuli (cytokines, lipids and their derivatives, senescent cells, iron) can influence macrophage phenotypes in atherosclerotic lesions

• Macrophages with different functional phenotypes are likely to perform different roles in the development of atherosclerosis

• M1 macrophages are associated with symptomatic and unstable plaques, whereas M2 macrophages are particularly abundant in stable zones of the plaque and asymptomatic lesions

• Modulation of macrophage phenotypes might be a novel strategy for the pharmacological treatment of atherosclerosis

Introduction

The development of atherosclerosis involves activation of various cell types (including endothelial cells, smooth muscle cells, lymphocytes, monocytes, and macrophages) in the intima of the arteries, which results in a local inflammatory response.¹ An increase in circulating LDL-cholesterol levels and the subsequent accumulation of oxidized LDL in the subendothelial space triggers the recruitment and retention of monocytes and lymphocytes in the arterial wall. In the intima, monocytes differentiate into macrophages, which scavenge lipoprotein particles, and eventually become foam cells.² These macrophage-derived foam cells secrete inflammatory molecules and factors that further promote lipoprotein retention, degrade the extracellular matrix, and sustain inflammation.^{3, 4}

 

Progression of atherosclerosis is characterized by apoptosis of these resident macrophages in the lipid core of the lesion. The clearance of apoptotic cells is mediated by phagocytes, mostly macrophages, which recognize and internalize dead cells in a process termed efferocytosis.⁵ In early lesions, phagocytes readily clear apoptotic cells, avoiding further progression of atherosclerosis. In chronic, advanced lesions, however, efferocytosis is no longer sufficient to engulf all dead cells, and the gradual accumulation of apoptotic debris results in formation of a necrotic core, which triggers further inflammation, necrosis, and thrombosis.⁵ Macrophages are crucial in the maintenance of efficient efferocytosis, and thereby contribute to both resolving inflammation and preventing the formation of a necrotic core within the plaque (Figure 1).⁶

 

Figure 1: Potential role of M2 macrophages in efferocytosis within atherosclerotic plaques.

M2 macrophages localized in areas of neovascularization or outside the lipid core can phagocytose apoptotic M1 macrophages, contributing to the resolution of inflammation. If efferocytosis is insufficient, dead M1 macrophages accumulate and undergo postapoptotic necrosis, leading to the formation of a necrotic core, which contributes to plaque instability and rupture.

Novel observations have challenged these previously well-established concepts. Whereas atherosclerosis was initially considered a type 1 T helper cell (T_H1)-driven inflammatory process, the concept of heterogeneity of macrophages resident within lesions has gradually emerged over the past decade.^{7, 8} Firstly, several studies revealed that both monocytes and macrophages comprise heterogeneous cell populations that adapt their functional phenotype in response to specific microenvironmental signals and molecules.^{9, 10} These different monocyte and macrophage subtypes can be identified based on their differential expression of surface markers and chemokine receptors.^{9, 10} Secondly, the general dogma that tissue-resident macrophages are incapable of proliferating was challenged by investigators who observed macrophages in mouse lungs that proliferated independently of monocyte recruitment.¹¹ Both resident and recruited macrophages can be induced to proliferate by IL-4.¹¹ Moreover, in mice, macrophages in early atherosclerotic lesions are predominantly derived from recruited monocytes, whereas macrophage proliferation is a preponderant feature of advanced plaques and is influenced by microenvironment signals.¹² Accordingly, differences in resident macrophage phenotypes can also influence their capacity to proliferate locally, which can alter the abundance of macrophages with a given phenotype.^{11, 13}

 

In this Review, we describe and discuss the reported functional phenotypes of macrophages in atherosclerotic plaques, focusing mainly on human pathology. The correlative studies suggesting a link between such macrophage phenotypes and the structure or progression of atherosclerotic lesions will also be discussed.

Figure 2: Main macrophage subtypes found in atherosclerotic lesions.

Stimuli present in atherosclerotic lesions drive the differentiation of monocytes towards different macrophage phenotypes. a | M1 macrophages release proinflammatory cytokines. b | M(Hb), Mhem, and M2 macrophages are resistant to lipid accumulation, possess iron-handling capacities, and have anti-inflammatory effects. c | Mox macrophages display an antioxidant gene expression profile. d | M4 macrophages, like M1 macrophages, are proinflammatory, but lack the capacity for phagocytosis. Abbreviations: COX-2, cyclooxygenase; CXCL4, C-X-C motif chemokine 4; HMOX-1, haem oxygenase (decycling) 1; LDL, low-density lipoprotein; LXR, liver X receptor; MMP-7, matrix metalloproteinase-7; NFE2L2, nuclear factor (erythroid-derived 2)-like 2; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; TLR, toll-like receptor; TNF, tumour necrosis factor.

Figure 3: Localization of macrophage subsets in human atherosclerotic lesions.

M1 macrophages are predominantly found in the plaque shoulder and lipid core, whereas M2 macrophages are most abundant in the adventitia and areas of neovascularization, which also contain iron deposits. Fibrous caps contain similar amounts of both macrophage subsets.

Conclusions

 Macrophages are important in the development of atherosclerotic lesions because they participate in all stages of plaque formation and progression.⁷⁷ During their lifetime, macrophages are exposed to a plethora of microenvironmental signals and stimuli (including cytokines modified lipids, senescent erythorocytes, and iron) that influence their transcriptional programme and functional phenotype. The intensity of these signals changes during plaque progression, and varies between plaque regions. Therefore, macrophages adapt their phenotype both over time and in response to their physical location, contributing to and modulating plaque progression and composition. The local accumulation of macrophage subpopulations that are highly competent in clearing apoptotic cells (such as M2 macrophages) can sustain efferocytosis, thereby contributing to the resolution of inflammation and prevention of necrotic core formation within plaques.

 

Whether the distinct macrophage phenotypes represent different stages of differentiation of a single population of phenotypically and functionally plastic resident macrophages, or are consequences of the recruitment and differentiation of specific monocyte subpopulations in the lesion, is still a matter of debate. However, despite rapid advances in this field, novel studies and research methods need to be developed to determine whether monocyte subpopulations give rise to specific macrophage phenotypes. Our current knowledge suggests that M2 macrophages, owing to their localization within human plaques and their intrinsic anti-inflammatory properties, are primarily associated with plaque stability. However, one of the most important challenges in this field will be to demonstrate a direct causative link between macrophage phenotype and the structure or progression of atherosclerotic lesions. Given that only correlation studies are feasible in humans, specific mouse models need to be developed.

 

Further identification of novel phenotypic and functional markers, and the use of novel large-scale expression profiling approaches and imaging technology, is expected to lead to an improved definition and understanding of the specific functions of the different macrophage subtypes identified within the plaque. The identification of biological stimuli that can modulate macrophage phenotypes could lead to the development of novel therapeutic approaches for the treatment of atherosclerosis.

Published online 04 November 2014