Rationale: Atherosclerotic lesions are known for their cellular heterogeneity, yet the molecular complexity within the cells of human plaques have not been fully assessed. Objective: Using single-cell transcriptomics and chromatin accessibility we gained a better understanding of the pathophysiology underlying human atherosclerosis. Methods and Results: We performed single-cell RNA and single-cell ATAC sequencing on human carotid atherosclerotic plaques to define the cells at play and determine their transcriptomic and epigenomic characteristics. We identified 14 distinct cell populations including endothelial cells, smooth muscle cells, mast cells, B cells, myeloid cells, and T cells and identified multiple cellular activation states and suggested cellular interconversions. Within the endothelial cell population we defined subsets with angiogenic capacity plus clear signs of endothelial to mesenchymal transition. CD4 + and CD8 + T cells showed activation-based subclasses, each with a gradual decline from a cytotoxic to a more quiescent phenotype. Myeloid cells included two populations of pro-inflammatory macrophages showing IL1B or TNF expression as well as a foam cell-like population expressing TREM2 and displaying a fibrosis-promoting phenotype. ATACseq data identified specific transcription factors associated with the myeloid subpopulation and T cell cytokine profiles underlying mutual activation between both cell types. Finally, cardiovascular disease susceptibility genes identified using public GWAS data were particularly enriched in lesional macrophages, endothelial and smooth muscle cells. Conclusions: This study provides a transcriptome-based cellular landscape of human atherosclerotic plaques and highlights cellular plasticity and intercellular communication at the site of disease. This detailed definition of cell communities at play in atherosclerosis will facilitate cell-based mapping of novel interventional targets with direct functional relevance for the treatment of human disease.

These findings show that specific inhibition of the NLRP3 inflammasome using MCC950 can be a promising therapeutic approach to inhibit atherosclerotic lesion development.

The TGF-β family member GDF-15 promotes lesion formation and plaque instability in atherosclerosis-prone LDLr-deficient mice.

No abstract

Mast cells, like many other types of inflammatory cell, perform pleiotropic roles in cardiometabolic diseases such as atherosclerosis, abdominal aortic aneurysms, obesity, and diabetes mellitus, as well as complications associated with these diseases. Low numbers of mast cells are present in the heart, aorta, and adipose tissue of healthy humans, but patients with cardiometabolic diseases and animals with experimentally-induced cardiometabolic pathologies have high numbers of mast cells with increased activity in the affected tissues. Mediators released by the activated mast cells, such as chemokines, cytokines, growth factors, heparin, histamine, and proteases, not only function as biomarkers of cardiometabolic diseases, but might also directly contribute to the pathogenesis of such diseases. Mast-cell mediators impede the functions of vascular cells, the integrity of the extracellular matrix, and the activity of other inflammatory cells, thereby contributing to the pathobiology of the conditions at multiple levels. In mouse models, mast-cell activation aggravates the progression of various cardiometabolic pathologies, whereas a genetic deficiency or pharmacological stabilization of mast cells, or depletion or inhibition of specific mast-cell mediators, tends to delay the progression of such conditions. Pharmacological inhibition of mast-cell activation or their targeted effector functions offers potential novel therapeutic strategies for patients with cardiometabolic disorders.

Atherosclerotic plaque rupture is a leading cause of acute coronary syndromes, such as unstable angina and myocardial infarction ( 1 ). Infl ammatory cells are regarded as key players in the pathogenesis of plaque rupture ( 2, 3 ), and the mast cell, a potent infl ammatory cell, has been shown to accumulate in the rupture-prone shoulder region of human atheromas ( 4 ). Activated mast cells have been identifi ed in the adventitia of vulnerable and ruptured lesions in patients with myocardial infarction, and more importantly, their numbers and degree of activation were found to correlate with the incidence of plaque rupture and erosion ( 5 ). We previously demonstrated that systemic mast cell activation during atherogenesis leads to increased plaque progression in apoE defi cient mice ( 6 ), while others show that the absence of mast cells, and in particular mast cell-derived interleukin (IL)-6 and interferon (IFN)-␥ , attenuated atherosclerotic lesion development in low-density lipoprotein receptor-deficient (LDLr Ϫ / Ϫ ) mice ( 7 ). Moreover, focal activation of mast cells in the adventitia of advanced carotid artery plaques promoted macrophage apoptosis, microvascular leakage, de novo leukocyte infl ux, and the incidence of intraplaque hemorrhage. Mast cell stabilization by cromolyn was seen to prevent these pathophysiological events ( 6 ).Various pathways of mast cell activation have been demonstrated such as cross-linking of the high-affi nity Abstract Lysophosphatidic acid (LPA), a bioactive lysophospholipid, accumulates in the atherosclerotic plaque. It has the capacity to activate mast cells, which potentially exacerbates plaque progression. In this study, we thus aimed to investigate whether LPA contributes to plaque destabilization by modulating mast cell function. We here show by an imaging mass spectrometry approach that several LPA species are present in atherosclerotic plaques. Subsequently, we demonstrate that LPA is a potent mast cell activator which, unlike other triggers, favors release of tryptase. Local perivascular administration of LPA to an atherosclerotic carotid artery segment increases the activation status of perivascular mast cells and promotes intraplaque hemorrhage and macrophage recruitment without impacting plaque cell apoptosis. The mast cell stabilizer cromolyn could prevent intraplaque hemorrhage elicited by LPAmediated mast cell activation. Finally, the involvement of mast cells in these events was further emphasized by the lack of effect of perivascular LPA administration in mast cell defi cient animals. We demonstrate that increased accumulation of LPA in plaques induces perivascular mast cell activation and in this way contributes to plaque destabilization in vivo. This study points to local LPA availability as an important factor in atherosclerotic plaque stability. Press, February 10, 2013 DOI 10.1194 Lysophosphatidic acid triggers mast cell-driven atherosclerotic plaque destabilization by increasing vascular infl ammation Abbreviations: apoE Ϫ / Ϫ , apolipoprotein E-defi...

Lysophosphatidic acid (LPA) accumulates in the central atheroma of human atherosclerotic plaques and is the primary platelet-activating lipid constituent of plaques. Here, we investigated the enzymatic regulation of LPA homeostasis in atherosclerotic lesions at various stages of disease progression. Atherosclerotic lesions were induced in carotid arteries of low-density lipoprotein receptor-deficient mice by semiconstrictive collar placement. At 2-week intervals after collar placement, lipids and RNA were extracted from the vessel segments carrying the plaque. Enzymaticand liquid chromatography-mass spectrometry-based lipid profiling revealed progressive accumulation of LPA species in atherosclerotic tissue preceded by an increase in lysophosphatidylcholine, a precursor in LPA synthesis. Plaque expression of LPA-generating enzymes cytoplasmic phospholipase A 2 IVA (cPLA 2 IVA) and calcium-independent PLA 2 VIA (iPLA 2 VIA) was gradually increased, whereas that of the LPA-hydrolyzing enzyme LPA acyltransferase ␣ was quenched. Increased expression of cPLA 2 IVA and iPLA 2 VIA in advanced lesions was confirmed by immunohistochemistry. Moreover, LPA receptors 1 and 2 were 50% decreased and sevenfold upregulated, respectively. Therefore, key proteins in LPA homeostasis are increasingly dysregulated in the plaque during atherogenesis, favoring intracellular LPA production. This might at least partly explain the observed progressive accumulation of this thrombogenic proinflammatory lipid in human and mouse plaques. Thus, intervention in the enzymatic LPA production may be an attractive measure to lower intraplaque LPA content, thereby reducing plaque progression and thrombogenicity. (Am J

Mesenchymal stem cells (MSCs) have regenerative properties, but recently they were also found to have immunomodulatory capacities. We therefore investigated whether MSCs could reduce atherosclerosis, which is determined by dyslipidaemia and chronic inflammation. We adoptively transferred MSCs into low-density lipoprotein-receptor knockout mice and put these on a Western-type diet to induce atherosclerosis. Initially after treatment, we found higher levels of circulating regulatory T cells. In the long-term, overall numbers of effector T cells were reduced by MSC treatment. Moreover, MSC-treated mice displayed a significant 33% reduction in circulating monocytes and a 77% reduction of serum CCL2 levels. Most strikingly, we found a previously unappreciated effect on lipid metabolism. Serum cholesterol was reduced by 33%, due to reduced very low-density lipoprotein levels, likely a result of reduced de novo hepatic lipogenesis as determined by a reduced expression of Stearoyl-CoA desaturase-1 and lipoprotein lipase. MSCs significantly affected lesion development, which was reduced by 33% in the aortic root. These lesions contained 56% less macrophages and showed a 61% reduction in T cell numbers. We show here for the first time that MSC treatment affects not only inflammatory responses but also significantly reduces dyslipidaemia in mice. This makes MSCs a potent candidate for atherosclerosis therapies.