Macrophage polarization and metabolism in atherosclerosis

Abstract

Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of fatty deposits in the inner walls of vessels. These plaques restrict blood flow and lead to complications such as heart attack or stroke. The development of atherosclerosis is influenced by a variety of factors, including age, genetics, lifestyle, and underlying health conditions such as high blood pressure or diabetes. Atherosclerotic plaques in stable form are characterized by slow growth, which leads to luminal stenosis, with low embolic potential or in unstable form, which contributes to high risk for thrombotic and embolic complications with rapid clinical onset. In this complex scenario of atherosclerosis, macrophages participate in the whole process, including the initiation, growth and eventually rupture and wound healing stages of artery plaque formation. Macrophages in plaques exhibit high heterogeneity and plasticity, which affect the evolving plaque microenvironment, e.g., leading to excessive lipid accumulation, cytokine hyperactivation, hypoxia, apoptosis and necroptosis. The metabolic and functional transitions of plaque macrophages in response to plaque microenvironmental factors not only influence ongoing and imminent inflammatory responses within the lesions but also directly dictate atherosclerotic progression or regression. In this review, we discuss the origin of macrophages within plaques, their phenotypic diversity, metabolic shifts, and fate and the roles they play in the dynamic progression of atherosclerosis. It also describes how macrophages interact with other plaque cells, particularly T cells. Ultimately, targeting pathways involved in macrophage polarization may lead to innovative and promising approaches for precision medicine. Further insights into the landscape and biological features of macrophages within atherosclerotic plaques may offer valuable information for optimizing future clinical treatment for atherosclerosis by targeting macrophages.

Fig. 1. The pathological mechanisms underlying atherosclerosis with autoimmune-like features.

A normal artery consists of three layers, the tunica adventitia, tunica media (containing abundant SMCs), and tunica intima (located in the subendothelial space). Under homeostatic conditions, almost no blood leukocytes accumulate in the endothelial layer. In contrast, when activated by proinflammatory cytokines or other cardiovascular risk factors, endothelial cells express leukocyte adhesion molecules (such as ICAM-1 and VCAM-1) and thereby cause the rolling and attachment of circulating monocytes, neutrophils and lymphocytes. In parallel, increased endothelium permeability promotes the entry of lipoproteins into the intima. Monocytes are recruited to the intima by chemokines (such as CCL2) and differentiate into mature macrophages mediated by M-CSF. Within the intima, macrophages take up excessive lipoproteins and form foam cells, which is a crucial step in initiating atherosclerosis. Recent evidence has demonstrated that foam cells can also be formed from macrophage-like cells that are produced via the transdifferentiation of SMCs. Furthermore, excessive lipid metabolism in foam cells results in inflammatory cytokine production and induces cell death (apoptosis). During the development of atherosclerosis, abundant apoptotic macrophages or neutrophil extracellular traps (NETs) within plaques cannot be removed by surrounding macrophages because of defective efferocytosis, leading to secondary necrosis accompanied by the release of cellular contents such as lipids, cell debris and DAMPs, which ultimately contribute to a lipid-rich necrotic core and unresolved inflammation. Notably, macrophages can be classified into five main subsets based on several scRNA-Seq studies on atherosclerotic plaques in mice; these subsets are namely res-like macrophages, TREM2^hi macrophages, inflammatory macrophages, proliferating macrophages and IFNIC macrophages. Among these macrophages, res-like macrophages, with an M2-like phenotype, are distributed in the adventitia. TREM2^hi macrophages, with powerful lipid catabolism capabilities, are regarded as foam cells and are likely derived from SMCs. Inflammatory macrophages, with upregulated inflammatory genes and pathway activation, are key contributors to plaque inflammation. Generated by BioRender.

Fig. 2. Uncovering the systematic plaque cell formation via experimental and human atherosclerosis studies for the discovery of precision drugs by using single-cell and multiomics approaches.

Tissues at different stage of plaque development from atherosclerotic model mice or patients with atherosclerosis-related cardiovascular diseases were processed into tissue sections and single-cell suspensions for single-cell and multiomics analysis. a, b Using imaging mass cytometry and spactial transcriptomics to map in situ plaque microenvironment information such as cell type, metabolic characteristics and spatial distribution. c–f Single-cell transcriptome sequencing (scRNA-seq), cytometry by time of flight (CyTOF), cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) were performed to comprehensively map the metabolic, functional and epigenetic landscapes of plaque cells. Generated by BioRender.

Fig. 3. Subpopulation classification of macrophage within plaque based on scRNA-seq.

Plaque macrophages can be classified into five main subsets based on several scRNA-seq studies in humans and mice, including Res-like macrophages, Foamy/TREM2hi macrophages, Inflammatory macrophages, Proliferating macrophages, and IFNIC macrophages (Type I IFN induced response signature). These macrophage subsets exhibit distinct gene characteristic, biological functions, and spatial distributions.

Fig. 4. Macrophage metabolism and functions in response to plaque microenvironment development.

a In atherosclerotic plaques, macrophages are exposed to various lipids (including LDLs, ox-LDLs, oxPAPCs, and cholesterol crystals) that either promote (mostly) or attenuate a proatherogenic microenvironment. Production of intracellular cholesterol crystals and accumulation of free cholesterol following excessive lipid uptake by macrophages via receptor pathways and phagocytosis directly or indirectly lead to metabolic changes (i.e., increased glycolysis and OXPHOS rates), the production of proinflammatory cytokines and the activation of death pathways, ultimately resulting in atherosclerotic progression, which is blunted by effective cholesterol efflux. For example, β-cyclodextrin promotes atherosclerosis regression by activating LXR-targeting genes (such as ABCA1 and ABCG1) that mediate cholesterol metabolism and inflammatory responses in macrophages. b Moreover, hypoxic microenvironments jointly created by increased oxygen demand in inflammatory cells and vigorous metabolism (such as that in activated macrophages and lipid-loaded foam cells) inversely potentiate glycolysis, inflammatory gene expression, and necroptotic death in macrophages. c Cholesterol crystals can also trigger NETosis and the release of danger signals (MPO, MMPs, ROS, dsDNA) that prime macrophages for cytokine release (IL-1β). Recent works have shown that metabolites (including DNA, arginine, ornithine, and methionine) released from apoptotic cells during efferocytosis enhance continuous efferocytosis and injury resolution through different signaling pathways. d Additionally, macrophages in the plaque undergo distinct death modes (i.e., apoptosis, necroptosis, and pyroptosis), which influence atherosclerotic progression and plaque stability. The elimination of apoptotic cells via efficient efferocytosis induces immune suppression, while defective efferocytosis induces immune responses triggered by danger signals from necroptotic or pyroptotic cells. Generated by BioRender.