Macrophage Trafficking, Inflammatory Resolution, and Genomics in Atherosclerosis: JACC Macrophage in CVD Series (Part 2)

Abstract

Atherosclerosis is characterized by the retention of modified lipoproteins in the arterial wall. These modified lipoproteins activate resident macrophages and the recruitment of monocyte-derived cells, which differentiate into mononuclear phagocytes that ingest the deposited lipoproteins to become "foam cells": a hallmark of this disease. In this Part 2 of a 4-part review series covering the macrophage in cardiovascular disease, we critically review the contributions and relevant pathobiology of monocytes, macrophages, and foam cells as relevant to atherosclerosis. We also review evidence that via various pathways, a failure of the resolution of inflammation is an additional key aspect of this disease process. Finally, we consider the likely role played by genomics and biological networks in controlling the macrophage phenotype in atherosclerosis. Collectively, these data provide substantial insights on the atherosclerotic process, while concurrently offering numerous molecular and genomic candidates that appear to hold great promise for selective targeting as clinical therapies.

Keywords: atherosclerosis; cardiovascular; inflammation; macrophage; resolution.

Central Illustration:

Macrophage trafficking, Role of resolution of inflammation and genetic/genomic factors in atherosclerosis.

Figure 1:. Macrophage dynamics during atherosclerotic plaque progression and regression.

Major kinetic processes dictating macrophage burden are recruitment of monocytes and the proliferation, retention, death, and egress of monocyte-derived macrophages. During hypercholesterolemia there is an increase in monocyte precursors in bone marrow and spleen, resulting in more circulating Ly6C^hi monocytes. Some become patrolling Ly6C^low monocytes, but the majority of monocytes recruited to plaques are Ly6C^hi, which transmigrate into the subendothelial space. In progression, these monocytes take up modified and retained lipoproteins transforming them into inflammatory macrophage foam cells. VSMCs can also become macrophage-appearing foam cells, but their properties and fates are largely undefined. In regression, recruited monocytes become M2, inflammation resolving, macrophages. In advanced plaques, macrophages can proliferate, and death by apoptosis and necroptosis can contribute to necrotic core formation, with falling levels of efferocytosis promoting core growth. In early plaques, reverse transmigration of macrophages may occur. This abates with progressing disease, but in regression, reverse macrophage transmigration can be restored by reduced retention and increased emigration factors.

Figure 2.. Inflammation and Resolution.

A generalized, healthy inflammatory response consists of three phases: inflammation, resolution and post-resolution. (A) The acute inflammatory response is characterized edema and a predominantly neutrophilic infiltrate in response to pro-inflammatory factors. These features typify the response to an acute inflammatory stimulus like trauma or infection, but are less relevant for chronic conditions like atherosclerosis. Nevertheless, this initial response activates the resolution program, which leads to lipid mediator class switching toward the production of pro-resolving factors and catabolism of pro-inflammatory factors. This response, along with increased production of additional mediators of resolution, leads to tissue repair by halting the influx of leukocytes, promoting their egress and clearing dead cells. A proper resolution response leads to post-resolution adaptive immunity, which prepares the organism for future insults. (B) If an inflammatory stimulus persists (e.g. atherosclerosis) or resolution fails, the initial inflammatory response may become chronic. This leads to a maladaptive post-resolution adaptive immune phase and impaired ability to respond to subsequent challenges. MDSC, myeloid-derived suppressor cell; Mφ, macrophage; TGF-β, transforming growth factor; TNF-α, tumor necrosis factor; Treg, T regulatory cell.

Figure 3.. Efferocytosis and apoptotic cell clearance drive pro-resolving macrophage functions.

In resolving macrophages (left), MerTK signaling drives 5-LOX translocation from the nuclear membrane to the cytoplasm, where it is brought into proximity with 12/15-LOX. The coordinated activity of these enzymes favors pro-resolving LXA₄ synthesis. In addition, signaling through MerTK and other efferocytosis receptors upregulates pro-resolving cytokines including IL-10 and TGF-β while inhibiting pro-inflammatory cytokine production. In inflammatory macrophages (right), as in advanced atherosclerosis, efferocytosis becomes defective. With diminished MerTK signaling, 5-LOX is phosphorylated and confined to the nuclear membrane, bringing it into proximity with LTA4 hydrolase and favoring the synthesis of the pro-inflammatory mediator LTB₄. Further, in the absence of MerTK and other efferocytosis receptor signaling, there is a failure to suppress the production of pro-inflammatory cytokines or to upregulate pro-resolving cytokines. The overall effect of failed efferocytosis in advanced plaques is the accumulation of apoptotic cells that become secondarily necrotic; increased necrotic core size; increased inflammation and impaired resolution; and decreased collagen cap thickness.

Figure 4.. Macrophage-specific association between CAD risk loci and gene expression.

We leveraged the STARNET datasets to perform a pilot analysis on the associations between CAD risk loci (single nucleotide polymorphisms [SNPs]) and gene expression in macrophages and foam cells. Thus, we investigated the 98 CAD-associated GWAS SNPs (129,130), asking whether these SNPs predict macrophage or foam cell gene expression within 1Mb; so-called cis-eQTL. We required that the eQTL had the strongest effect in gene expression from macrophages (n=95) or foam cells (n=80) compared to other tissues in STARNET. This approach identified 29 SNPs that are associated with the expression levels of 52 genes (p<0.01). Thus, 29/98 (~30%) of CAD risk loci are most strongly associated with gene expression in macrophages and/or foam cells. In particular, association with IL6R, NME7, TRIP4, PROCR, and EIF6 was the most significant and detected in both macrophages and foam cells. Candidate genes are color-coded and organized based on Gene Ontology terms according to biological processes of potential relevance to macrophage biology. Further details are provided in the Online Appendix.