Immune cell census in murine atherosclerosis: cytometry by time of flight illuminates vascular myeloid cell diversity

Macrophage subsets in atherosclerosis as defined by single-cell technologies

Macrophages play a major role in the pathogenesis of atherosclerosis. Many studies have shone light on the different phenotypes and functions that macrophages can acquire upon exposure to local cues. The microenvironment of the atherosclerotic plaque contains a plethora of macrophage-controlling factors, such as cytokines, oxidised low-density lipoproteins and cell debris. Previous research has determined macrophage function within the plaque mainly by using immunohistochemistry and bulk analysis. The recent development and rapid progress of single-cell technologies, such as cytometry by time of flight and single-cell RNA sequencing, now enable comprehensive mapping of the wide range of cell types and their phenotypes present in atherosclerotic plaques. In this review we discuss recent advances applying these technologies in defining macrophage subsets residing in the atherosclerotic arterial wall of mice and men. Resulting from these studies, we describe three main macrophage subsets: resident-like, pro-inflammatory and anti-inflammatory foamy TREM2hi macrophages, which are found in both mouse and human atherosclerotic plaques. Furthermore, we discuss macrophage subset-specific markers and functions. More insights into the characteristics and phenotype of immune cells within the atherosclerotic plaque may guide future clinical approaches to treat disease. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The haematopoietic stem cell niche: a new player in cardiovascular disease?

Haematopoiesis, the process of blood production, can be altered during the initiation or progression of many diseases. Cardiovascular disease (CVD) has been shown to be heavily influenced by changes to the haematopoietic system, including the types and abundance of immune cells produced. It is now well established that innate immune cells are increased in people with CVD, and the mechanisms contributing to this can be vastly different depending on the risk factors or comorbidities present. Many of these changes begin at the level of the haematopoietic stem and progenitor cells (HSPCs) that reside in the bone marrow (BM). In general, the HSPCs and downstream myeloid progenitors are expanded via increased proliferation in the setting of atherosclerotic CVD. However, HSPCs can also be encouraged to leave the BM and colonise extramedullary sites (i.e. the spleen). Within the BM, HSPCs reside in specialized microenvironments, often referred to as a niche. To date in depth studies assessing the damage or dysregulation that occurs in the BM niche in varying CVDs are scarce. In this review, we provide a general overview of the complex components and interactions within the BM niche and how they influence the function of HSPCs. Additionally, we discuss the main findings regarding changes in the HSPC niche that influence the progression of CVD. We hypothesize that understanding the influence of the BM niche in CVD will aid in delineating new pathways for therapeutic interventions.

Immunity and Inflammation in Atherosclerosis

There is now overwhelming experimental and clinical evidence that atherosclerosis is a chronic inflammatory disease. Lessons from genome-wide association studies, advanced in vivo imaging techniques, transgenic lineage tracing mice, and clinical interventional studies have shown that both innate and adaptive immune mechanisms can accelerate or curb atherosclerosis. Here, we summarize and discuss the pathogenesis of atherosclerosis with a focus on adaptive immunity. We discuss some limitations of animal models and the need for models that are tailored to better translate to human atherosclerosis and ultimately progress in prevention and treatment.

Macrophages in Atherosclerosis Regression

Macrophages play a central role in the development of atherosclerotic cardiovascular disease (ASCVD), which encompasses coronary artery disease, peripheral artery disease, cerebrovascular disease, and aortic atherosclerosis. In each vascular bed, macrophages contribute to the maintenance of the local inflammatory response, propagate plaque development, and promote thrombosis. These central roles, coupled with their plasticity, makes macrophages attractive therapeutic targets in stemming the development of and stabilizing existing atherosclerosis. In the context of ASCVD, classically activated M1 macrophages initiate and sustain inflammation, and alternatively activated M2 macrophages resolve inflammation. However, this classification is now considered an oversimplification, and a greater understanding of plaque macrophage physiology in ASCVD is required to aid in the development of therapeutics to promote ASCVD regression. Reviewed herein are the macrophage phenotypes and molecular regulators characteristic of ASCVD regression, and the current murine models of ASCVD regression.

Single-cell immune landscape of human atherosclerotic plaques

Atherosclerosis is driven by multifaceted contributions of the immune system within the circulation and at vascular focal sites. However, specific characteristics of dysregulated immune cells within atherosclerotic lesions that lead to clinical events such as ischemic stroke or myocardial infarction are poorly understood. Here, using single-cell proteomic and transcriptomic analyses, we uncovered distinct features of both T cells and macrophages in carotid artery plaques of patients with clinically symptomatic disease (recent stroke or transient ischemic attack) compared to asymptomatic disease (no recent stroke). Plaques from symptomatic patients were characterized by a distinct subset of CD4+ T cells and by T cells that were activated and differentiated. Moreover, some T cell subsets in these plaques presented markers of T cell exhaustion. Additionally, macrophages from these plaques contained alternatively activated phenotypes, including subsets associated with plaque vulnerability. In plaques from asymptomatic patients, T cells and macrophages were activated and displayed evidence of interleukin-1β signaling. The identification of specific features of innate and adaptive immune cells in plaques that are associated with cerebrovascular events may enable the design of more precisely tailored cardiovascular immunotherapies.

Impact of B-Cell–Targeted Therapies on Cardiovascular Disease

Atherosclerosis is a lipid-driven chronic inflammatory disease that is modulated by many immune cell subsets, including B cells. Therefore, targeting the inflammatory component of cardiovascular disease represents a promising therapeutic strategy. In the past years, immunotherapy has revolutionized the treatment of autoimmunity and cancer. Many of these clinically used strategies target B cells. Given the multifaceted role of B cells in atherogenesis, it is conceivable that B-cell–directed therapies can modulate disease development. Here, we review clinically available B-cell–targeted therapies and the possible benefits or detrimental effects on cardiovascular disease.

Structure-preserving visualisation of high dimensional single-cell datasets

Single-cell technologies offer an unprecedented opportunity to effectively characterize cellular heterogeneity in health and disease. Nevertheless, visualisation and interpretation of these multi-dimensional datasets remains a challenge. We present a novel framework, ivis, for dimensionality reduction of single-cell expression data. ivis utilizes a siamese neural network architecture that is trained using a novel triplet loss function. Results on simulated and real datasets demonstrate that ivis preserves global data structures in a low-dimensional space, adds new data points to existing embeddings using a parametric mapping function, and scales linearly to hundreds of thousands of cells. ivis is made publicly available through Python and R interfaces on https://github.com/beringresearch/ivis .

A Novel Triple-Cell Two-Dimensional Model to Study Immune-Vascular Interplay in Atherosclerosis

Atherosclerosis is a complex inflammatory pathology underpinning cardiovascular diseases (CVD), which are the leading cause of death worldwide. The interplay between vascular stromal cells and immune cells is fundamental to the progression and outcome of atherosclerotic disease, however, the majority of in vitro studies do not consider the implications of these interactions and predominantly use mono-culture approaches. Here we present a simple and robust methodology involving the co-culture of vascular endothelial (ECs) and smooth muscle cells (SMCs) alongside an inflammatory compartment, in our study containing THP-1 macrophages, for studying these complex interactions. Using this approach, we demonstrate that the interaction between vascular stromal and immune cells produces unique cellular phenotypes and soluble mediator profiles not observed in double-cell 2D cultures. Our results highlight the importance of cellular communication and support the growing idea that in vitro research must evolve from mono-culture systems to provide data more representative of the multi-cellular environment found in vivo. The methodology presented, in comparison with established approaches, has the advantage of being technically simple whilst enabling the isolation of pure populations of ECs, SMCs and immune cells directly from the co-culture without cell sorting. The approach described within would be applicable to those studying mechanisms of vascular inflammation, particularly in relation to understanding the impact cellular interaction has on the cumulative immune-vascular response to atherogenic or inflammatory stimuli.

[Immunity and inflammation in atherosclerosis]

There is now overwhelming experimental and clinical evidence that arteriosclerosis is a chronic inflammatory disease. Lessons learned from genome-wide association studies, advanced in vivo imaging techniques, transgenic lineage tracing mice models and clinical interventional studies have shown that both innate and adaptive immune mechanisms can accelerate or curb arteriosclerosis. This article summarizes and discusses the pathogenesis of arteriosclerosis with a focus on the role of the adaptive immune system. Some limitations of animal models are discussed and the need for models that are tailored to better translate to human atherosclerosis and ultimately progress in prevention and treatment are emphasized.

Macrophage Biology, Classification, and Phenotype in Cardiovascular Disease: JACC Macrophage in CVD Series (Part 1)

Macrophages represent one of the most numerous and diverse leukocyte types in the body. Furthermore, they are important regulators and promoters of many cardiovascular disease programs. Their functions range from sensing pathogens to digesting cell debris, modulating inflammation, and producing key cytokines and other regulatory factors throughout the body. Macrophage research has undergone a renaissance in recent years, which has propelled a newfound interest in their heterogeneity as well as a new understanding of ontological differences in their development. In addition, recent technological advances such as single-cell mass-cytometry by time-of-flight have enabled phenotype and functional analyses of individual immune myeloid cells, including macrophages, at unprecedented resolution. In this Part 1 of a 4-part review series covering the macrophage in cardiovascular disease, we focus on the basic principles of macrophage development, heterogeneity, phenotype, tissue-specific differentiation, and functionality as a basis to understand their role in cardiovascular disease.

Inflammation, Immunity, and Infection in Atherothrombosis: JACC Review Topic of the Week

Observations on human and experimental atherosclerosis, biomarker studies, and now a large-scale clinical trial support the operation of immune and inflammatory pathways in this disease. The factors that incite innate and adaptive immune responses implicated in atherogenesis and in lesion complication include traditional risk factors such as protein and lipid components of native and modified low-density lipoprotein, angiotensin II, smoking, visceral adipose tissue, and dysmetabolism. Infectious processes and products of the endogenous microbiome might also modulate atherosclerosis and its complications either directly, or indirectly by eliciting local and systemic responses that potentiate disease expression. Trials with antibiotics have not reduced recurrent cardiovascular events, nor have vaccination strategies yet achieved clinical translation. However, anti-inflammatory interventions such as anticytokine therapy and colchicine have begun to show efficacy in this regard. Thus, inflammatory and immune mechanisms can link traditional and emerging risk factors to atherosclerosis, and offer novel avenues for therapeutic intervention.

Atherosclerosis in the single-cell era

PURPOSE OF REVIEW

The immune system plays a critical role in the development and modulation of atherosclerosis. New high-parameter technologies, including mass cytometry (CyTOF) and single-cell RNA sequencing (scRNAseq), allow for an encompassing analysis of immune cells. Unexplored marker combinations and transcriptomes can define new immune cell subsets and suggest their functions. Here, we review recent advances describing the immune cells in the artery wall of mice with and without atherosclerosis. We compare technologies and discuss limitations and advantages.

RECENT FINDINGS

Both CyTOF and scRNAseq on leukocytes from digested aortae show 10-30 immune cell subsets. Myeloid, T, B and natural killer cells were confirmed. Although cellular functions can be inferred from RNA-Seq data, some subsets cannot be identified based on current knowledge, suggesting they may be new cell types. CyTOF and scRNAseq each identified four B-cell subsets and three macrophage subsets in the atherosclerotic aorta. Limitations include cell death caused by enzymatic digestion and the limited depth of the scRNAseq transcriptomes.

SUMMARY

High-parameter methods are powerful tools for uncovering leukocyte diversity. CyTOF is currently more powerful at discerning leukocyte subsets in the atherosclerotic aorta, whereas scRNAseq provides more insight into their likely functions.