Monocytes in atherosclerosis: subsets and functions

Abstract

Chronic inflammation drives atherosclerosis, the leading cause of cardiovascular disease. Over the past two decades, data have emerged showing that immune cells are involved in the pathogenesis of atherosclerotic plaques. The accumulation and continued recruitment of leukocytes are associated with the development of 'vulnerable' plaques. These plaques are prone to rupture, leading to thrombosis, myocardial infarction or stroke, all of which are frequent causes of death. Plaque macrophages account for the majority of leukocytes in plaques, and are believed to differentiate from monocytes recruited from circulating blood. However, monocytes represent a heterogenous circulating population of cells. Experiments are needed to address whether monocyte recruitment to plaques and effector functions, such as the formation of foam cells, the production of nitric oxide and reactive oxygen species, and proteolysis are critical for the development and rupture of plaques, and thus for the pathophysiology of atherosclerosis, as well as elucidate the precise mechanisms involved.

Figure 1

Inflammation and the role of blood monocytes. Two distinct subsets are evident in the blood circulation of mice. Gr1⁺/Ly6C^high monocytes that can extravasate into tissue, differentiate into Tip-DC, M1-type or classically activated macrophages, and phagocytose pathogens, produce antibacterial products, mediate inflammation and proteolysis. Gr1⁻/Ly6C^low monocytes can crawl or patrol the vasculature under the steady state, but in response to inflammation, extravasate into tissue, differentiate into M2-type or alternatively activated macrophages and phagoctose pathogens and are involved in wound repair, tissue remodeling and expression of chemokines. Whether Gr1⁻ or Gr1⁺ differentiated monocytes mediate foam cell formation in response to lipids requires further investigation. Abbreviations: CXCL, chemokine (C-X-C motif) ligand; iNOS, inducible nitric oxide synthase; ROS, reactive oxygen species; Tip-DC, TNF and iNOS producing dendritic cells; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor.

Figure 2

Blood monocytes in atherosclerosis. a | Atherogenesis and the formation of fatty streaks. Monocytes are recruited (influx) into the intima and may differentiate into macrophages that accumulate lipids and cholesterol derivatives, form foam cells in the subintima, and make up fatty streaks. However, little is known as to which monocyte subset differentiates into a specific macrophage phenotype mediating foam cell formation. Monocytes (mainly Gr1⁺/Ly-6C^high) may also give rise to dendritic cells in the plaque. The role of Gr1⁻/Ly-6C^low monocytes in this process has not been described. Gr1⁻/Ly-6C^low monocytes may be involved in scavenging the endothelium of lipid derivatives, dead and/or dying cells. There is evidence to suggest that interplaque cells may efflux back into the luminal blood flow. b | Maturation into atherosclerotic plaques and rupture. Continued monocyte influx is reportedly due to Gr1⁺/Ly-6C^high monocytes. Macrophages and dendritic cells accumulate. Whether they differentiate into a specific phenotype from recruited monocytes and/or proliferate from local precursors is unknown. The role and influx of Gr1⁻/Ly-6C^low have not been characterized. Vasa vasorum, and neovascularization within the plaque lead to increased trafficking of leukocytes. Deposition of matrix components and recruitment of smooth muscle cells give rise to the fibroproliferative progression of the plaques. Apoptosis of macrophages and/or foam cells creates a necrotic core. Thinning and erosion of the fibrous cap in unstable plaques, through matrix degradation by proteases, ultimately results in plaque rupture and thrombosis of the artery. Egress of these monocyte/macrophages or dendritic cells from plaques has been described.

Figure 3

Heterogeneity of mouse monocyte subsets. a | Flow cytometry assessment of mouse whole blood shows the heterogeneity of mouse monocyte subsets. A diverse pattern of Ly6C expression can be seen on Lin⁻ (CD3, NK1.1, CD19) CD115⁺ CD11b⁺ cells in mouse whole blood (gate 3; monocytes). These populations have been color back gated as Gr1⁺/Ly-6C^high (inflammatory; red) and Gr1⁻/Ly-6C^low subsets (patrolling; blue). Note the close CD11b and Ly6C expression profile of both NK cells (gate 1) and neutrophils (gate 4; green), which without an appropriate gating strategy can easily be interpreted as monocytes. b | Differentiation of monocyte subsets. Monocytes differentiate from HSCs through a common proliferating MDP that gives rise to CD115⁺ CD11b⁺ Gr1⁺/Ly-6C^high and CD115⁺ CD11b⁺ Gr1⁻/Ly-6C^low subsets in the bone marrow., These subsets then exit into the blood circulation as mature CD115⁺ CD11b⁺ monocytes. Gr1⁺/Ly-6C^high population can shuttle between the bone marrow and blood circulation. The developmental relationship between Gr1⁺/Ly-6C^high and Gr1⁻/Ly-6C^low monocytes (putative CD14⁺ and CD16⁺ homolog in humans respectively) is not yet established. Abbreviations: HSC, hematopoietic stem cell; MDP, macrophage and dendritic cell precursor; NK, natural killer.

Figure 4

Monocyte recruitment. a | Gr1⁺/Ly6C^high monocytes tether and roll on the endothelium through the classic adhesion cascade, involving endothelial AMs and their ligands expressed by monocytes (examples provided). After activation they can extravasate into the tissue, through slow rolling, chemokine activation (firm adhesion) and emigration. The kinetics of this movement is rapid (μm/min). b | Little is known about Gr1⁻/Ly6C^low monocytes. Crawling (patrolling) occurs in a random behavior and involves LFA1, CX3CR1 and the chemokine fractalkine; however, in response to inflammation, these cells can also extravasate through uncharacterized mechanisms. The kinetics of crawling in these cells is relatively slow (mm/h). Note the difference in scale for rolling versus crawling on the endothelium, describing increased distance of slow crawling (patrolling). Abbreviations: AMs, adhesion molecules; CX3CR1, CX3C chemokine receptor 1; ICAM, intercellular adhesion molecule; JAMs, junctional adhesion molecules; LFA1, Leukocyte function-associated molecule 1 (also known as β₂-integrin); PECAM-1, platelet endothelial cell adhesion molecule 1; PSGL-1, P-selectin glycoprotein ligand-1; VCAM, vascular cell-adhesion molecule; VLA4, very late antigen-4 (also known as α₄-integrin).