Macrophages in atherosclerosis: a dynamic balance

Abstract

Atherosclerosis is a chronic inflammatory disease that arises from an imbalance in lipid metabolism and a maladaptive immune response driven by the accumulation of cholesterol-laden macrophages in the artery wall. Through the analysis of the progression and regression of atherosclerosis in animal models, there is a growing understanding that the balance of macrophages in the plaque is dynamic and that both macrophage numbers and the inflammatory phenotype influence plaque fate. In this Review, we summarize recently identified pro- and anti-inflammatory pathways that link lipid and inflammation biology with the retention of macrophages in plaques, as well as factors that have the potential to promote their egress from these sites.

Figure 1. Mechanisms regulating monocyte recruitment and accumulation in plaques

Hyperlipidemia increases the number of GR1⁺LY6C^hi monocytes, which constitute 80% of the monocytes recruited to mouse atherosclerotic plaques, with the remainder being the GR1^–LY6C^low patrolling monocytes. These monocyte subsets use different chemokine—chemokine receptor pairs to infiltrate the intima, which is facilitated by endothelial adhesion molecules, including selectin, intercellular adhesion molecule 1 (ICAM1) and vascular adhesion molecule 1 (VCAM1). The recruited monocytes differentiate into macrophages or dendritic cells (DCs) in the intima, where they interact with atherogenic lipoproteins. Macrophages avidly take up native and modified low-density lipoprotein (LDL) via macropinocytosis or scavenger receptor-mediated pathways (including scavenger receptor A (SRA and CD36), resulting in the formation of the foam cells that are a hallmark of the atherosclerotic plaque. These foam cells secrete pro-inflammatory cytokines (including interleukin-1 (IL-1), IL-6, and tumor necrosis factor (TNF)) and chemokines (such as CC-chemokine ligand 2 (CCL2), CCL5 and CXC-chemokine ligand 1 (CXCL1)), as well as macrophage retention factors (such as netrin 1 and semaphorin 3E) that amplify the inflammatory response. CX3CL1, CX3C–chemokine ligand 1; CX3CR1, CX3C–chemokine receptor 1; LFA1, lymphocyte function-associated antigen 1; PSGL1, P-selectin glycoprotein ligand 1;

Figure 2. Mechanisms governing macrophage lipoprotein uptake and efflux

Macrophages internalize native (low density lipoprotein, LDL; very low density lipoprotein, VLDL) and oxidized lipoproteins in the plaque via macropinocytosis, phagocytosis of aggregated LDL and scavenger receptor-mediated uptake (including by scavenger receptor A (SRA), LOX1, SRB1 and CD36). The internalized lipoproteins and their associated lipids are digested in the lysosome resulting in the release of free cholesterol that can travel to the plasma membrane and be effluxed from the cell or to the endoplasmic reticulum (ER) membrane and be esterified by acyl-coA cholesterol acyltransferase (ACAT) and ultimately stored in this form in cytosolic lipid droplets. These stored lipids can be mobilized for efflux via either lipolysis by neutral cholesterol ester hydrolases (nCEH) or lipophagy, a form of autophagy, which results in delivery of lipid droplets to lysosomes. The accumulation of cellular cholesterol activates the liver-X-receptor (LXR)/retinoid X receptor (RXR) heterodimeric transcription factor that upregulates expression of the ABC transporters ABCA1 and ABCG1, that mediate the transfer of free cholesterol to lipid poor APOA-I to form nascent high-density lipoprotein (HDL) or more lipidated HDL particles in which free cholesterol has been esterified and stored the core of the particle (mature HDL),. Excessive free cholesterol accumulation can induce cholesterol crystal formation in the lysosome to activate the NLRP3 inflammasome, and may also interfere with the function of the ER (ER stress), which if prolonged results in cell death by apoptosis, and Lipid rafts are enriched in sphingomyelin, which forms a complex with free cholesterol. As the cholesterol content of lipid rafts increases, pro-inflammatory Toll-like receptor 4 (TLR4) signalling is promoted, which can also be induced by oxidized low-density lipoprotein (LDL) through a heterotrimeric complex composed of CD36–TLR4–TLR6. This signalling resulted in the activation of nuclear factor-κB (NF-κB) and production of pro-inflammatory cytokines and chemokines.

Figure 3. Pathways regulating macrophage retention and emigration in plaques

Imbalances in macrophage lipid metabolism in the progressing plaque lead to the retention of macrophages and chronic inflammation. The accumulating lipid-laden macrophages express retention molecules (such as netrin 1 and its receptor Unc5b, semaphorin 3E and cadherins) that promote macrophage chemostasis. In this inflammatory milieu, these accumulating macrophages experience endoplasmic reticulum (ER) stress, which if prolonged, results in apoptosis. This cell death, coupled with defective efferocytosis, results in the formation of the necrotic core characteristic of advance plaques. Mechanisms that promote lipid unloading of the foam cell, including factors that upregulate ABCA1 expression on plaque macrophages and cholesterol efflux, reverse the accumulation of these foam cells. This plaque regression is characterized by an upregulation of CC-chemokine receptor 7 (CCR7) on myeloid-derived cells and a decrease in the expression of retention factors. Accumulating evidence summarized in this review supports that the regulation of these macrophage migration factors contributes to macrophage emigration from the plaque through reverse transmigration to the lumen or trafficking to the adventitial lymphatics.