Understanding Atherosclerotic Plaque Cellular Composition: Recent Advances Driven by Single Cell Omics

Abstract

Atherosclerosis, a chronic inflammatory condition of the arterial cell wall, is the leading cause of cardiovascular disease (CVD) and death. It is characterised by the accumulation of lipid and immune cell-rich plaques in the arterial intima, which is driven by a dysregulated immune response to cholesterol-containing lipoproteins at the lesion site. Initially thought to be driven by passive lipid accumulation, atherosclerosis is now recognised as a complex process encompassing a multitude of inflammatory and remodelling mechanisms, driven by both immune cells (e.g., macrophages, T-cells, B-cells, antigen-presenting cells) and non-immune cells (smooth muscle cells, endothelial cells). With the development of single-cell transcriptomic and proteomic technologies, a previously inconceivable wealth of data has been generated in an attempt to better understand the pathophysiology of the disease and identify novel avenues for the development of targeted therapeutic interventions. This review provides an overview of the latest findings in the field obtained using single-cell technologies, with a focus on the major cell types present at the atherosclerotic plaque site, their suggested repartition in subsets, as well as their predicted function(s) within the complex processes that interplay to drive atherosclerotic disease. We conclude by highlighting the discrepancies and areas of consensus brought about by these studies and briefly discuss the likely future advances that will come from the continuous development and improvement of single-cell omics technologies.

Keywords: atherosclerosis; single-cell omics.

Figure 1

Notable points of consensus in atherosclerotic lesional macrophage clustering between mouse (Cochain) and human (Depuydt) plaques. A summary of converging conclusions regarding macrophage transcriptomic and functional heterogeneity at the atherosclerotic lesion, between scRNAseq analysis of mouse (LdlR^−/− mice on HFD) and human (Carotid endartectomy patients) plaque [5,10].

Figure 2

T-cell exhaustion at the plaque site is likely a consequence, rather than a cause, of increased inflammation and disease progression. Schematic representation of the findings by Fernandez et al. showing that plaque T cells are more activated (yellow) than their blood counterparts (green) and display signs of exhaustion (red), suggesting that exposure to chronic inflammation at the plaque may drive exhaustion reprogramming in activated T-cells [9]. Given the prior observations that T-cell exhaustion is athero-protective [27,28,29], it is unlikely that increased T-cell exhaustion in symptomatic plaque is a driver of disease progression. Instead, acute systemic inflammation as a result of MI or stroke likely contributed to further inflammation and T-cell exhaustion at the plaque site. Specifically elevated genes indicative of resting (green), activated (yellow), and exhausted (red) T-cells are indicated.

Figure 3

Schematic summary of cellular trans-differentiation at the atherosclerotic lesion site. The differentiation of plaque macrophages into smooth muscle cell (SMC)-like cells (orange); plaque SMCs into macrophage-like cells (orange), osteoblast-like cells (grey), and potentially endothelial cells (EC)-like cells (green); and plaque ECs into SMC-like cells (green) has been supported by sRNAseq findings. The supporting publications, specific cell subsets affected, and cell-type-specific marker genes used are indicated in the framed text box. Gene markers are colour-coded as follows: blue, EC, yellow, SMC, red, macrophage, grey, osteoblast. A potential positive feedback loop, suggested by Zhao et al., in which EndMT drives an inflammatory profile in ECs, which in turn drives EndMT, is also indicated [10,11,13].