Prevalence of M4 macrophages within human coronary atherosclerotic plaques is associated with features of plaque instability

The role of immune system in atherosclerosis: Molecular mechanisms, controversies, and future possibilities

Numerous cardiovascular disorders have atherosclerosis as their pathological underpinning. Numerous studies have demonstrated that, with the aid of pattern recognition receptors, cytokines, and immunoglobulins, innate immunity, represented by monocytes/macrophages, and adaptive immunity, primarily T/B cells, play a critical role in controlling inflammation and abnormal lipid metabolism in atherosclerosis. Additionally, the finding of numerous complement components in atherosclerotic plaques suggests yet again how heavily the immune system controls atherosclerosis. Therefore, it is essential to have a thorough grasp of how the immune system contributes to atherosclerosis. The specific molecular mechanisms involved in the activation of immune cells and immune molecules in atherosclerosis, the controversy surrounding some immune cells in atherosclerosis, and the limitations of extrapolating from relevant animal models to humans were all carefully reviewed in this review from the three perspectives of innate immunity, adaptive immunity, and complement system. This could provide fresh possibilities for atherosclerosis research and treatment in the future.

Cholinergic Polarization of Human Macrophages

Macrophages serve as vital defenders, protecting the body by exhibiting remarkable cellular adaptability in response to invading pathogens and various stimuli. These cells express nicotinic acetylcholine receptors, with the α7-nAChR being extensively studied due to its involvement in activating the cholinergic anti-inflammatory pathway. Activation of this pathway plays a crucial role in suppressing macrophages' production of proinflammatory cytokines, thus mitigating excessive inflammation and maintaining host homeostasis. Macrophage polarization, which occurs in response to specific pathogens or insults, is a process that has received limited attention concerning the activation of the cholinergic anti-inflammatory pathway and the contributions of the α7-nAChR in this context. This review aims to present evidence highlighting how the cholinergic constituents in macrophages, led by the α7-nAChR, facilitate the polarization of macrophages towards anti-inflammatory phenotypes. Additionally, we explore the influence of viral infections on macrophage inflammatory phenotypes, taking into account cholinergic mechanisms. We also review the current understanding of macrophage polarization in response to these infections. Finally, we provide insights into the relatively unexplored partial duplication of the α7-nAChR, known as dup α7, which is emerging as a significant factor in macrophage polarization and inflammation scenarios.

Molecular and cellular mechanisms of inflammation in atherosclerosis

Atherosclerosis and its complications are a major cause of morbidity and mortality worldwide in spite of the improved medical and invasive treatment in terms of revascularization. Atherosclerosis is a dynamic, multi-step process in which inflammation is a ubiquitous component participating in the initiation, development, and entanglements of the atherosclerotic plaque. After activation, the immune system, either native or acquired, is part of the atherosclerotic dynamics enhancing the pro-atherogenic function of immune or non-immune cells, such as endothelial cells, smooth muscle cells, or platelets, through mediators such as cytokines or directly by cell-to-cell interaction. Cytokines are molecules secreted by the activated cells mentioned above that mediate the inflammatory component of atherosclerosis whose function is to stimulate the immune cells and the production of further cytokines. This review provides insights of the cell axis activation and specific mechanisms and pathways through which inflammation actuates atherosclerosis.

OxLDL sensitizes platelets for increased formation of extracellular vesicles capable of finetuning macrophage gene expression

Platelet extracellular vesicles (PEVs) generated upon platelet activation may play a role in inflammatory pathologies such as atherosclerosis. Oxidized low-density lipoprotein (oxLDL), a well-known contributor to atherogenesis, activates platelets and presensitizes them for activation by other agonists. We studied the effect of oxLDL on the secretion, composition, and inflammatory functions of PEVs using contemporary EV analytics. Platelets were activated by co-stimulation with thrombin (T) and collagen (C) ± oxLDL and characterized by high-resolution flow cytometry, nanoparticle tracking analysis, proximity extension assay, western blot, and electron microscopy. The effect of PEVs on macrophage differentiation and functionality was examined by analyzing macrophage surface markers, cytokine secretion, and transcriptome. OxLDL upregulated TC-induced formation of CD61⁺, P-selectin⁺ and phosphatidylserine⁺ PEVs. Blocking the scavenger receptor CD36 significantly suppressed the oxLDL+TC-induced PEV formation, and HDL caused a slight but detectable suppression. The inflammatory protein cargo differed between the PEVs from stimulated and unstimulated platelets. Both oxLDL+TC- and TC-induced PEVs enhanced macrophage HLA-DR and CD86 expression and decreased CD11c expression as well as secretion of several cytokines. Pathways related to cell cycle and regulation of gene expression, and immune system signaling were overrepresented in the differentially expressed genes between TC PEV -treated vs. control macrophages and oxLDL+TC PEV -treated vs. control macrophages, respectively. In conclusion, we speculate that oxLDL and activated platelets contribute to proatherogenic processes by increasing the number of PEVs that provide an adhesive and procoagulant surface, contain inflammatory mediators, and subtly finetune the macrophage gene expression.

Inflammation in coronary artery disease-clinical implications of novel HDL-cholesterol-related inflammatory parameters as predictors

Coronary artery disease (CAD) is the leading cause of death worldwide. Inflammation and atherosclerotic plaques are the primary pathological mechanisms of CAD. Upon stimulation by deposited lipids and damaged endothelium, innate and adaptive immune cells are activated and recruited to initiate plaque development. Therefore, inflammatory cells and mediators are used to identify inflammatory risk in CAD patients. HDL-cholesterol (HDL-C) is demonstrated to have anti-inflammatory roles in atherosclerosis by interfering with plasma membrane lipid rafts of immune cells. Based on this, novel inflammatory parameters such as monocyte to HDL-C ratio are explored to improve the risk estimation of CAD prognosis. Moreover, with the advance in treatment strategies targeting the inflammatory process in atherosclerosis, identifying CAD patients with increased inflammatory risk by novel inflammatory parameters is of great importance in guiding CAD management. Therefore, this review aims to summarize the current information regarding inflammatory activation and HDL-C in atherosclerosis with a particular focus on the clinical implication of the novel HDL-C-related inflammatory parameters in CAD.

Origin, production and molecular determinants of macrophages for their therapeutic targeting

Macrophages, the most heterogeneous cells of the hematopoietic system and the giant eaters of the immune system that present either as tissue-resident cells or infiltrated immune cells, eliminate foreign pathogens and microbes and also play different physiological roles to maintain the body's immune response. In this review, we basically provide a broad overview of macrophages from their origin, functional diversity to M1-M2 polarization, specialized markers, and their role as important therapeutic targets in different diseases based on the current research and evidence. Apart from this, we have precisely discussed about tumor-associated macrophages (TAMs) and their role in tumor progression and newly discovered lesser-known markers of TAMs that could be used as potential therapeutic targets to treat life-threatening diseases. It is really very important to understand the diversity of macrophages to develop TAM-modulating strategies to activate our own immune system against diseases and to overcome immune resistance.

Reducing residual cardiovascular risk in Europe: Therapeutic implications of European medicines agency approval of icosapent ethyl/eicosapentaenoic acid

Atherosclerotic cardiovascular disease (ASCVD) and its atherothrombotic complications impose a substantial disease burden in Europe, representing a cost of €210 billion per year for the European Union. Hypertriglyceridemia, a major risk factor for premature ASCVD, is present in more than 20% of the European population, and is a key feature of atherogenic dyslipidemia. Recent findings from the Progression of Early Subclinical Atherosclerosis (PESA) cohort in Spain showed that even in apparently healthy, middle-aged individuals without a history of cardiovascular (CV) risk, elevated triglyceride levels are associated with subclinical atherosclerosis and arterial inflammation. Emerging evidence from epidemiologic and genetic studies supports an independent causative role of triglycerides, triglyceride-rich lipoproteins, and their remnants in this pathology. Icosapent ethyl (IPE) is a highly purified, stable ethyl ester of eicosapentaenoic acid (EPA) that was initially approved by the United States Food and Drug Administration to treat severe hypertriglyceridemia, and subsequently received an expanded indication to reduce the risk of CV events in adult statin-treated patients. Approval was based on the pivotal, randomized, placebo-controlled, double-blind Reduction of Cardiovascular Events with Icosapent Ethyl-Intervention Trial (REDUCE-IT), which showed that high-dose IPE (4 g/day) significantly reduced the risk of primary and secondary composite endpoints comprising major CV events and CV death relative to placebo. In 2021, the European Medicines Agency (EMA) approved IPE to reduce the risk of CV events in adult statin-treated patients at high CV risk with elevated triglyceride levels (≥1.7 mmol/L [≥150 mg/dL]) and established CV disease, or diabetes and at least one other CV risk factor. Clinical studies in Europe, which included patients with acute myocardial infarction, coronary artery disease, and those undergoing cardiac rehabilitation, established that 12.5% to 23.3% of these high-risk populations may benefit from treatment with IPE. Such clinical benefit may in part result from the moderate triglyceride-lowering properties of IPE/EPA; equally however, concentrations of atherogenic remnant particle-cholesterol are markedly reduced. Furthermore, IPE/EPA exerts pleiotropic actions beyond its lipid-lowering properties, which include modulation of endothelial function, attenuation of intra-plaque inflammation and oxidative stress, and reduction in macrophage accumulation. Plasma phospholipids, into which EPA is primarily incorporated and transported, appear to serve as precursors for a series of anti-inflammatory metabolites involving the resolvins RvE1 to RvE3, a pathway which may confer cardioprotective benefits. In addition, plaque imaging data from the Effect of Icosapent Ethyl on Progression of Coronary Atherosclerosis in Patients With Elevated Triglycerides on Statin Therapy (EVAPORATE) and the Combination Therapy of Eicosapentaenoic Acid and Pitavastatin for Coronary Plaque Regression Evaluated by Integrated Backscatter Intravascular Ultrasonography (CHERRY) trials show that plaque stabilization may be favorably affected. These factors may act synergistically to stabilize atherosclerotic plaques and reduce CV risk. In addition to robust efficacy data, multiple cost-utility studies across several countries indicate that IPE/EPA is a cost-effective treatment option that is favorably situated relative to some common willingness-to-pay thresholds. This review will evaluate the relevance of hypertriglyceridemia to residual ASCVD burden in statin-treated dyslipidemic patients, the potential of IPE/EPA to reduce the risk of ASCVD and cardiovascular mortality in high-risk patient populations, and the mechanisms which may underlie these effects. Finally, the clinical implications of the EMA label for IPE will be critically appraised in light of the updated 2019 European Society of Cardiology/European Atherosclerosis Society guidelines on the management of dyslipidemia and the recent European Atherosclerosis Society consensus statement on triglyceride-rich lipoproteins and their remnants, together with considerations of its cost-effectiveness across several countries.

Macrophage polarization by potential nutraceutical compounds: A strategic approach to counteract inflammation in atherosclerosis

Chronic inflammation represents a main event in the onset and progression of atherosclerosis and is closely associated with oxidative stress in a sort of vicious circle that amplifies and sustains all stages of the disease. Key players of atherosclerosis are monocytes/macrophages. According to their pro- or anti-inflammatory phenotype and biological functions, lesional macrophages can release various mediators and enzymes, which in turn contribute to plaque progression and destabilization or, alternatively, lead to its resolution. Among the factors connected to atherosclerotic disease, lipid species carried by low density lipoproteins and pro-oxidant stimuli strongly promote inflammatory events in the vasculature, also by modulating the macrophage phenotyping. Therapies specifically aimed to balance macrophage inflammatory state are increasingly considered as powerful tools to counteract plaque formation and destabilization. In this connection, several molecules of natural origin have been recognized to be active mediators of diverse metabolic and signaling pathways regulating lipid homeostasis, redox state, and inflammation; they are, thus, considered as promising candidates to modulate macrophage responsiveness to pro-atherogenic stimuli. The current knowledge of the capability of nutraceuticals to target macrophage polarization and to counteract atherosclerotic lesion progression, based mainly on in vitro investigation, is summarized in the present review.

The Role of Endothelial Progenitor Cells in Atherosclerosis and Impact of Anti-Lipemic Treatments on Endothelial Repair

Cardiovascular complications are associated with advanced atherosclerosis. Although atherosclerosis is still regarded as an incurable disease, at least in its more advanced stages, the discovery of endothelial progenitor cells (EPCs), with their ability to replace old and injured cells and differentiate into healthy and functional mature endothelial cells, has shifted our view of atherosclerosis as an incurable disease, and merged traditional theories of atherosclerosis pathogenesis with evolving concepts of vascular biology. EPC alterations are involved in the pathogenesis of vascular abnormalities in atherosclerosis, but many questions remain unanswered. Many currently available drugs that impact cardiovascular morbidity and mortality have shown a positive effect on EPC biology. This review examines the role of endothelial progenitor cells in atherosclerosis development, and the impact standard antilipemic drugs, including statins, fibrates, and ezetimibe, as well as more novel treatments such as proprotein convertase subtilisin/kexin type 9 (PCSK9) modulating agents and angiopoietin-like proteins (Angtpl3) inhibitors have on EPC biology.

Contribution to the peripheral vasculopathy and endothelial cell dysfunction by CXCL4 in Systemic Sclerosis

BACKGROUND

CXCL4, a chemokine with anti-angiogenic property, is involved in systemic sclerosis (SSc) related pulmonary arterial hypertension (PAH).

OBJECTIVE

To investigated the contribution of CXCL4 to SSc development by focusing on the correlation of circulatory CXCL4 levels with their peripheral vasculopathy, and the effect of CXCL4 on endothelial cell dysfunction and the potential signaling.

METHODS

We measured the plasma CXCL4 levels in 58 patients with SSc, 10 patients with the very early diagnosis of SSc (VEDOSS), and 80 healthy controls (HCs). Then, CXCL4 concentrations were correlated with clinical features, especially the peripheral vasculopathy. These observations were further validated in an additional cohort. Moreover, we studied the anti-angiogenic effects of CXCL4 and the underlying downstream signaling in human umbilical vein endothelial cells (HUVECs) in vitro.

RESULTS

Circulating CXCL4 levels were 103.62 % higher in patients with SSc and 201.51 % higher in patients with VEDOSS than matched HCs, which were confirmed in two independent cohorts. CXCL4 levels were associated with digital ulcers (DU) and nailfold videocapillaroscopy (NVC) abnormalities in SSc. The proliferation, migration, and tube formation of HUVECs were inhibited by CXCL4 or SSc derived plasma, which reversed by CXCL4 neutralizing antibody, but failed by CXCR3 inhibitor. CXCL4 downregulated the transcription factor Friend leukaemia integration factor-1 (Fli-1) via c-Abl signaling. Furthermore, CXCL4 blocked the transforming growth factor (TGF) -β or platelet-derived growth factor (PDGF) induced cell proliferation of HUVECs.

CONCLUSIONS

CXCL4 may contribute to peripheral vasculopathy in SSc by downregulating Fli-1 via c-Abl signaling in endothelial cells and interfering angiogenesis.

Molecular Crosstalk Between Macrophages and Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSCs) have been widely investigated for regenerative medicine applications, from treating various inflammatory diseases as a cell therapy to generating engineered tissue constructs. Numerous studies have evaluated the potential effects of MSCs following therapeutic administration. By responding to their surrounding microenvironment, MSCs may mediate immunomodulatory effects through various mechanisms that directly (i.e., contact-dependent) or indirectly (i.e., paracrine activity) alter the physiology of endogenous cells in various disease pathologies. More specifically, a pivotal crosstalk between MSCs and tissue-resident macrophages and monocytes (TMφ) has been elucidated using in vitro and in vivo preclinical studies. An improved understanding of this crosstalk could help elucidate potential mechanisms of action (MOAs) of therapeutically administered MSCs. TMφ, by nature of their remarkable functional plasticity and prevalence within the body, are uniquely positioned as critical modulators of the immune system – not only in maintaining homeostasis but also during pathogenesis. This has prompted further exploration into the cellular and molecular alterations to TMφ mediated by MSCs. In vitro assays and in vivo preclinical trials have identified key interactions mediated by MSCs that polarize the responses of TMφ from a pro-inflammatory (i.e., classical activation) to a more anti-inflammatory/reparative (i.e., alternative activation) phenotype and function. In this review, we describe physiological and pathological TMφ functions in response to various stimuli and discuss the evidence that suggest specific mechanisms through which MSCs may modulate TMφ phenotypes and functions, including paracrine interactions (e.g., secretome and extracellular vesicles), nanotube-mediated intercellular exchange, bioenergetics, and engulfment by macrophages. Continued efforts to elucidate this pivotal crosstalk may offer an improved understanding of the immunomodulatory capacity of MSCs and inform the development and testing of potential MOAs to support the therapeutic use of MSCs and MSC-derived products in various diseases.

New Classification of Macrophages in Plaques: a Revolution

PURPOSE

Macrophages play vital roles in the development of atherosclerosis in responding to lipid accumulation and inflammation. Macrophages were classified as inflammatory (M1) and alternatively activated (M2) macrophage types based on results of in vitro experiments. On the other hand, the composition of macrophages in vivo is more complex and remains unresolved. This review summarizes the transcriptional variations of macrophages in atherosclerosis plaques that were discovered by single-cell RNA sequencing (scRNA-seq) to better understand their contribution to atherosclerosis.

RECENT FINDINGS

ScRNA-seq provides a more detailed transcriptional landscape of macrophages in atherosclerosis, which challenges the traditional view. By mining the data of GSE97310, we discovered the transcriptional variations of macrophages in LDLR^-/- mice that were fed with high-fat diet (HFD) for 11 and 20 weeks. Cells were represented in a two-dimensional tSNE plane and clusters were identified and annotated via Seurat and SingleR respectively, which were R toolkits for single-cell genomics. The results showed that in healthy conditions, Trem2^hi (high expression of triggering receptors expressed on myeloid cells 2)-positive, inflammatory, and resident-like macrophages make up 68%, 18%, and 6% of total macrophages respectively. When mice were fed with HFD for 11 weeks, Trem2^hi, monocytes, and monocyte-derived dendritic cells take possession of 40%, 18%, and 17% of total macrophages respectively. After 20 weeks of HFD feeding, Trem2^hi, inflammatory, and resident-like macrophages occupied 12%, 37%, and 35% of total macrophages respectively. The phenotypes of macrophages are very different from the previous studies. In general, Trem2^hi macrophages are the most abundant population in healthy mice, while the proportion of monocytes increases after 11 weeks of HFD. Most importantly, inflammatory and resident-like macrophages make up 70% of the macrophage populations after 20 weeks of HFD. These strongly indicate that inflammatory and resident-like macrophages promote the progression of atherosclerosis plaques.

Pathogenesis of atherosclerosis in the tunica intima, media, and adventitia of coronary arteries: An updated review

Atherosclerosis is a chronic inflammatory disease of arteries and it affects the structure and function of all three layers of the coronary artery wall. Current theories suggest that the dysfunction of endothelial cells is one of the initial steps in the development of atherosclerosis. The view that the tunica intima normally consists of a single layer of endothelial cells attached to the subendothelial layer and internal elastic membrane has been questioned in recent years. The structure of intima changes with age and it becomes multilayered due to migration of smooth muscle cells from the media to intima. At this stage, the migration and proliferation of smooth muscle cells do not cause pathological changes in the intima. The multilayering of intima is classically considered to be an important stage in the development of atherosclerosis, but in fact atherosclerotic plaques develop only focally due to the interplay of various processes that involve the resident and invading inflammatory cells. The tunica media consists of multiple layers of smooth muscle cells that produce the extracellular matrix, and this layer normally does not contain microvessels. During the development of atherosclerosis, the microvessels from the tunica adventitia or from the lumen may penetrate thickened media to provide nutrition and oxygenation. According to some theories, the endothelial dysfunction of these nutritive vessels may significantly contribute to the atherosclerosis of coronary arteries. The adventitia contains fibroblasts, progenitor cells, immune cells, microvessels, and adrenergic nerves. The degree of inflammatory cell infiltration into the adventitia, which can lead to the formation of tertiary lymphoid organs, correlates with the severity of atherosclerotic plaques. Coronary arteries are surrounded by perivascular adipose tissue that also participates in the atherosclerotic process.

Macrophage polarization in atherosclerosis

Atherosclerosis is a chronic inflammatory response that increases the risk of cardiovascular diseases. An in-depth study of the pathogenesis of atherosclerosis is critical for the treatment of atherosclerotic cardiovascular disease. The development of atherosclerosis involves many cells, such as endothelial cells, vascular smooth muscle cells, macrophages, and others. The considerable effects of macrophages in atherosclerosis are inextricably linked to macrophage polarization and the resulting phenotype. Moreover, the significant impact of macrophages on atherosclerosis depend not only on the function of the different macrophage phenotypes but also on the relative ratio of different phenotypes in the plaque. Research on atherosclerosis therapy indicates that the reduced plaque size and enhanced stability are partly due to modulating macrophage polarization. Therefore, regulating macrophage polarization and changing the proportion of macrophage phenotypes in plaques is a new therapeutic approach for atherosclerosis. This review provides a new perspective for atherosclerosis therapy by summarizing the relationship between macrophage polarization and atherosclerosis, as well as treatment targeting macrophage polarization.

Functional aspects, phenotypic heterogeneity, and tissue immune response of macrophages in infectious diseases

Macrophages are a functionally heterogeneous group of cells with specialized functions depending not only on their subgroup but also on the function of the organ or tissue in which the cells are located. The concept of macrophage phenotypic heterogeneity has been investigated since the 1980s, and more recent studies have identified a diverse spectrum of phenotypic subpopulations. Several types of macrophages play a central role in the response to infectious agents and, along with other components of the immune system, determine the clinical outcome of major infectious diseases. Here, we review the functions of various macrophage phenotypic subpopulations, the concept of macrophage polarization, and the influence of these cells on the evolution of infections. In addition, we emphasize their role in the immune response in vivo and in situ, as well as the molecular effectors and signaling mechanisms used by these cells. Furthermore, we highlight the mechanisms of immune evasion triggered by infectious agents to counter the actions of macrophages and their consequences. Our aim here is to provide an overview of the role of macrophages in the pathogenesis of critical transmissible diseases and discuss how elucidation of this relationship could enhance our understanding of the host-pathogen association in organ-specific immune responses.

Macrophages and T cells in atherosclerosis: a translational perspective

Atherosclerosis is now considered a chronic maladaptive inflammatory disease. The hallmark feature in both human and murine disease is atherosclerotic plaques. Macrophages and various T-cell lineages play a crucial role in atherosclerotic plaque establishment and disease progression. Humans and mice share many of the same processes that occur within atherogenesis. The various similarities enable considerable insight into disease mechanisms and those which contribute to cardiovascular complications. The apolipoprotein E-null and low-density lipoprotein receptor-null mice have served as the foundation for further immunological pathway manipulation to identify pro- and antiatherogenic pathways in attempt to reveal more novel therapeutic targets. In this review, we provide a translational perspective and discuss the roles of macrophages and various T-cell lineages in contrasting proatherosclerotic and atheroprotective settings.

Organization of the Skin Immune System and Compartmentalized Immune Responses in Infectious Diseases

The skin is an organ harboring several types of immune cells that participate in innate and adaptive immune responses. The immune system of the skin comprises both skin cells and professional immune cells that together constitute what is designated skin-associated lymphoid tissue (SALT). In this review, I extensively discuss the organization of SALT and the mechanisms involved in its responses to infectious diseases of the skin and mucosa. The nature of these SALT responses, and the cellular mediators involved, often determines the clinical course of such infections. I list and describe the components of innate immunity, such as the roles of the keratinocyte barrier and of inflammatory and natural killer cells. I also examine the mechanisms involved in adaptive immune responses, with emphasis on new cytokine profiles, and the role of cell death phenomena in host-pathogen interactions and control of the immune responses to infectious agents. Finally, I highlight the importance of studying SALT in order to better understand host-pathogen relationships involving the skin and detail future directions in the immunological investigation of this organ, especially in light of recent findings regarding the skin immune system.

Role of Macrophages in Cardioprotection

Cardiovascular diseases are the leading cause of mortality worldwide. It is widely known that non-resolving inflammation results in atherosclerotic conditions, which are responsible for a host of downstream pathologies including thrombosis, myocardial infarction (MI), and neurovascular events. Macrophages, as part of the innate immune response, are among the most important cell types in every stage of atherosclerosis. In this review we discuss the principles governing macrophage function in the healthy and infarcted heart. More specifically, how cardiac macrophages participate in myocardial infarction as well as cardiac repair and remodeling. The intricate balance between phenotypically heterogeneous populations of macrophages in the heart have profound and highly orchestrated effects during different phases of myocardial infarction. In the early "inflammatory" stage of MI, resident cardiac macrophages are replaced by classically activated macrophages derived from the bone marrow and spleen. And while the macrophage population shifts towards an alternatively activated phenotype, the inflammatory response subsides giving way to the "reparative/proliferative" phase. Lastly, we describe the therapeutic potential of cardiac macrophages in the context of cell-mediated cardio-protection. Promising results demonstrate innovative concepts; one employing a subset of yolk sac-derived, cardiac macrophages that have complete restorative capacity in the injured myocardium of neonatal mice, and in another example, post-conditioning of cardiac macrophages with cardiosphere-derived cells significantly improved patient's post-MI diagnoses.

Shrunken Pore Syndrome Is Associated With Increased Levels of Atherosclerosis-Promoting Proteins

INTRODUCTION

Shrunken pore syndrome (SPS), originally defined by cystatin C-based estimated glomerular filtration rate (eGFRcystatin C) being less than 60% of creatinine-based estimated glomerular filtration rate (eGFRcreatinine) in the absence of extrarenal influences on the plasma levels of cystatin C or creatinine, is associated with a high increase in mortality, even in the absence of reduced glomerular filtration rate (GFR). The objective of the present study was to determine whether the proteome of patients with SPS shows differences from that of patients with normal or reduced measured GFR (mGFR) without SPS.

METHODS

Four patient cohorts were included: 1 cohort with normal mGFR without SPS, 1 with normal mGFR with SPS, 1 with reduced mGFR without SPS, and 1 with reduced mGFR with SPS. The plasma levels of 177 selected proteins were analyzed.

RESULTS

Differences in the levels of 30 proteins were specific for SPS; 31 differences were specific for patients with both SPS and reduced mGFR; and 27 were specific for reduced mGFR. Eighteen of the differences specific for SPS concerned proteins described as promoting, or being associated with, atherosclerosis. Twelve of the differences specific for patients with both SPS and reduced mGFR and 10 of the differences specific for reduced mGFR also concerned proteins described as promoting, or being associated with, atherosclerosis. Almost all (82 of 88) of the concentration differences represented increased levels. For SPS, but not for reduced mGFR, a correlation between protein size and increase in level was observed, with smaller proteins being associated with higher levels.

CONCLUSION

The high mortality in shrunken pore syndrome might be caused by the accumulation of atherosclerosis-promoting proteins in this condition.

Immunohistochemical characterization of the M4 macrophage population in leprosy skin lesions

Background

Since macrophages are one of the major cell types involved in the Mycobacterium leprae immune response, roles of the M1 and M2 macrophage subpopulations have been well defined. However, the role of M4 macrophages in leprosy or other infectious diseases caused by mycobacteria has not yet been clearly characterized. This study aimed to investigate the presence and potential role of M4 macrophages in the immunopathology of leprosy.

Methods

We analyzed the presence of M4 macrophage markers (CD68, MRP8, MMP7, IL-6, and TNF-α) in 33 leprosy skin lesion samples from 18 patients with tuberculoid leprosy and 15 with lepromatous leprosy by immunohistochemistry.

Results

The M4 phenotype was more strongly expressed in patients with the lepromatous form of the disease, indicating that this subpopulation is less effective in the elimination of the bacillus and consequently is associated with the evolution to one of the multibacillary clinical forms of infection.

Conclusion

M4 macrophages are one of the cell types involved in the microbial response to M. leprae and probably are less effective in controlling bacillus replication, contributing to the evolution to the lepromatous form of the disease.

Chemokine-Induced Macrophage Polarization in Inflammatory Conditions

Macrophages represent a heterogeneous cell population and are known to display a remarkable plasticity. In response to distinct micro-environmental stimuli, e.g., tumor stroma vs. infected tissue, they polarize into different cell subtypes. Originally, two subpopulations were defined: classically activated macrophages or M1, and alternatively activated macrophages or M2. Nowadays, the M1/M2 classification is considered as an oversimplified approach that does not adequately cover the total spectrum of macrophage phenotypes observed in vivo. Especially in pathological circumstances, macrophages behave as plastic cells modifying their expression and transcription profile along a continuous spectrum with M1 and M2 phenotypes as extremes. Here, we focus on the effect of chemokines on macrophage differentiation and polarization in physiological and pathological conditions. In particular, we discuss chemokine-induced macrophage polarization in inflammatory diseases, including obesity, cancer, and atherosclerosis.

Ox-LDL induces endothelial cell apoptosis and macrophage migration by regulating caveolin-1 phosphorylation

Oxidative low-density lipoprotein (ox-LDL) is a risk factor for atherosclerosis. Ox-LDL leads to endothelial injury in the initial stage of atherosclerosis. In this study, we investigated the role of ox-LDL in endothelial injury and macrophage recruitment. We demonstrated that ox-LDL promoted a dose-dependent phosphorylation of caveolin-1 in human umbilical vein endothelial cells. Phosphorylated caveolin-1 increased ox-LDL uptake. Intracellular accumulation of ox-LDL induced NF-κB p65 phosphorylation, promoted HMGB1 translocation from nucleus to cytoplasm and cytochrome C release from mitochondria to cytoplasm, and activated caspase 3, resulting in cell apoptosis. NF-κB activation also facilitated cavolin-1 phosphorylation and HMGB1 expression. In addition, caveolin-1 phosphorylation favored HMGB1 release and nuclear translocation of EGR1. Nuclear translocation of EGR1 contributed to cytoplasmic translocation of HMGB1. The extracellular HMGB1 induced the migration of PMBC-derived macrophages toward HUVECs in a TLR4-dependent manner. Our results suggested that ox-LDL promoted HUVECs apoptosis and macrophage recruitment by regulating caveolin-1 phosphorylation.

New Insights into the Immunobiology of Mononuclear Phagocytic Cells and Their Relevance to the Pathogenesis of Cardiovascular Diseases

Macrophages are the primary immune cells that reside within the myocardium, suggesting that these mononuclear phagocytes are essential in the orchestration of cardiac immunity and homeostasis. Independent of the nature of the injury, the heart triggers leukocyte activation and recruitment. However, inflammation is harmful to this vital terminally differentiated organ with extremely poor regenerative capacity. As such, cardiac tissue has evolved particular strategies to increase the stress tolerance and minimize the impact of inflammation. In this sense, growing evidences show that mononuclear phagocytic cells are particularly dynamic during cardiac inflammation or infection and would actively participate in tissue repair and functional recovery. They respond to soluble mediators such as metabolites or cytokines, which play central roles in the timing of the intrinsic cardiac stress response. During myocardial infarction two distinct phases of monocyte influx have been identified. Upon infarction, the heart modulates its chemokine expression profile that sequentially and actively recruits inflammatory monocytes, first, and healing monocytes, later. In the same way, a sudden switch from inflammatory macrophages (with microbicidal effectors) toward anti-inflammatory macrophages occurs within the myocardium very shortly after infection with Trypanosoma cruzi, the causal agent of Chagas cardiomyopathy. While in sterile injury, healing response is necessary to stop tissue damage; during an intracellular infection, the anti-inflammatory milieu in infected hearts would promote microbial persistence. The balance of mononuclear phagocytic cells seems to be also dynamic in atherosclerosis influencing plaque initiation and fate. This review summarizes the participation of mononuclear phagocyte system in cardiovascular diseases, keeping in mind that the immune system evolved to promote the reestablishment of tissue homeostasis following infection/injury, and that the effects of different mediators could modulate the magnitude and quality of the immune response. The knowledge of the effects triggered by diverse mediators would serve to identify new therapeutic targets in different cardiovascular pathologies.

Pathophysiological relevance of macrophage subsets in atherogenesis

Macrophages are highly heterogeneous and plastic cells. They were shown to play a critical role in all stages of atherogenesis, from the initiation to the necrotic core formation and plaque rupture. Lesional macrophages primarily derive from blood monocyte, but local macrophage proliferation as well as differentiation from smooth muscle cells have also been described. Within atherosclerotic plaques, macrophages rapidly respond to changes in the microenvironment, shifting between pro- (M1) or anti-inflammatory (M2) functional phenotypes. Furthermore, different stimuli have been associated with differentiation of newly discovered M2 subtypes: IL-4/IL-13 (M2a), immunecomplex (M2b), IL-10/glucocorticoids (M2c), and adenosine receptor agonist (M2d). More recently, additional intraplaque macrophage phenotypes were also recognized in response to CXCL4 (M4), oxidized phospholipids (Mox), haemoglobin/haptoglobin complexes (HAmac/M(Hb)), and heme (Mhem). Such macrophage polarization was described as a progression among multiple phenotypes, which reflect the activity of different transcriptional factors and the cross-talk between intracellular signalling. Finally, the distribution of macrophage subsets within different plaque areas was markedly associated with cardiovascular (CV) vulnerability. The aim of this review is to update the current knowledge on the role of macrophage subsets in atherogenesis. In addition, the molecular mechanisms underlying macrophage phenotypic shift will be summarised and discussed. Finally, the role of intraplaque macrophages as predictors of CV events and the therapeutic potential of these cells will be discussed.

Functional diversity of macrophages in vascular biology and disease

Atherosclerosis is a multifactorial chronic inflammatory disease and is largely responsible for cardiovascular disease, the most common cause of global mortality. The hallmark of atherogenesis is immune activation following lipid accumulation in the arterial wall. In particular, macrophages play a non-redundant role in both the progression and regression of inflammation in the atherosclerotic lesion. Macrophages are remarkably heterogeneous phagocytes that perform versatile functions in health and disease. Their functional diversity in vascular biology is only partially mapped. Targeting macrophages is often highlighted as a therapeutic approach for cancer, metabolic and inflammatory diseases. Future strategies for therapeutic intervention in atherosclerosis may benefit from attempts to reduce local proliferation of pro-inflammatory macrophage subsets or enhance resolution of inflammation. Thus, characterisation of macrophage subsets during atherosclerosis would empower clinical interventions. Therefore, it would be of fundamental importance to understand how pathological factors modulate macrophage activity in order to exploit their use in the treatment of atherosclerosis and other diseases.

CXCL4-induced macrophages in human atherosclerosis

Atherosclerosis is considered an inflammatory disease of the arterial wall. Monocytes and monocyte-derived cells (most often termed macrophages) play an essential role in the formation of atherosclerotic lesions, as they take up lipids leading to subsequent foam cell formation accompanied by release of pro-inflammatory cytokines. Similarly, platelets have been discovered to represent an important cell type mediating inflammatory and immune processes in atherogenesis, mainly by secreting chemokines, which are stored in the platelets' alpha granules, upon platelet activation. Therefore, the interaction between monocyte-derived cells and platelets is of exceptional importance. In this review, we specifically focus on the chemokine (platelet factor-4, PF4) and its effects on monocytes and monocyte-derived cells. By formation of heterodimers dimers and -oligomers with CCL5, CXCL4 induces binding of monocytes cells to endothelial cell and thereby promotes diapedesis of monocytes into the subendothelial space. CXCL4 also affects the differentiation of monocytes as it induces a specific macrophage phenotype, which we suggested to term "M4". For example, CXCL4-induced macrophages irreversibly lose the hemoglobin-haptoglobin scavenger receptor CD163. The combination of CD68, S100A8, and MMP7 turned out to reliably identify M4 macrophages both in vitro and in vivo within atherosclerotic lesions. In human atherosclerotic plaques, M4 macrophages are predominantly present in the adventitia and the intima and their prevalence is associated with plaque instability suggesting that they are a marker of pro-inflammatory activity. Overall, CXCL4-induced M4 macrophages may represent a target for diagnostic and therapeutic interventions in human atherosclerotic disease.

Protein Thiol Redox Signaling in Monocytes and Macrophages

SIGNIFICANCE

Monocyte and macrophage dysfunction plays a critical role in a wide range of inflammatory disease processes, including obesity, impaired wound healing diabetic complications, and atherosclerosis. Emerging evidence suggests that the earliest events in monocyte or macrophage dysregulation include elevated reactive oxygen species production, thiol modifications, and disruption of redox-sensitive signaling pathways. This review focuses on the current state of research in thiol redox signaling in monocytes and macrophages, including (i) the molecular mechanisms by which reversible protein-S-glutathionylation occurs, (ii) the identification of bona fide S-glutathionylated proteins that occur under physiological conditions, and (iii) how disruptions of thiol redox signaling affect monocyte and macrophage functions and contribute to atherosclerosis. Recent Advances: Recent advances in redox biochemistry and biology as well as redox proteomic techniques have led to the identification of many new thiol redox-regulated proteins and pathways. In addition, major advances have been made in expanding the list of S-glutathionylated proteins and assessing the role that protein-S-glutathionylation and S-glutathionylation-regulating enzymes play in monocyte and macrophage functions, including monocyte transmigration, macrophage polarization, foam cell formation, and macrophage cell death.

CRITICAL ISSUES

Protein-S-glutathionylation/deglutathionylation in monocytes and macrophages has emerged as a new and important signaling paradigm, which provides a molecular basis for the well-established relationship between metabolic disorders, oxidative stress, and cardiovascular diseases.

FUTURE DIRECTIONS

The identification of specific S-glutathionylated proteins as well as the mechanisms that control this post-translational protein modification in monocytes and macrophages will facilitate the development of new preventive and therapeutic strategies to combat atherosclerosis and other metabolic diseases. Antioxid. Redox Signal. 25, 816-835.

Atorvastatin attenuates p‑cresyl sulfate‑induced atherogenesis and plaque instability in ApoE knockout mice

p-cresyl sulfate (PCS) is a protein-bound uremic toxin retained in the blood of patients with chronic kidney disease (CKD) As atherosclerosis is a primary cardiovascular complication for patients with CKD, the aim of the present study was to investigate the mechanisms underlying the aggravation of atherosclerosis by PCS. In addition, the effect of atorvastatin was assessed in reversing the effects of PCS. PCS was revealed to promote the initiation and progression of atherosclerosis. Following treatment with atorvastatin, apolipoprotein E knockout mice demonstrated a reduction in PCS-induced atherogenesis and plaque vulnerability. In addition, atorvastatin decreased the protein expression levels of vascular cell adhesion molecule-1 and intercellular cell adhesion molecule-1, and the interaction between leukocytes and endothelia. The plasma lipid profiles of mice were not significantly affected by gavage of low-dose atorvastatin. The results of the present study indicate that PCS promotes plaque growth and instability by enhancing leukocyte-endothelium interaction, and that these effects may be attenuated by atorvastatin treatment.

Interaction between allergic asthma and atherosclerosis

Prior studies have established an essential role of mast cells in allergic asthma and atherosclerosis. Mast cell deficiency or inactivation protects mice from allergen-induced airway hyper-responsiveness and diet-induced atherosclerosis, suggesting that mast cells share pathologic activities in both diseases. Allergic asthma and atherosclerosis are inflammatory diseases that contain similar sets of elevated numbers of inflammatory cells in addition to mast cells in the airway and arterial wall, such as macrophages, monocytes, T cells, eosinophils, and smooth muscle cells. Emerging evidence from experimental models and human studies points to a potential interaction between the 2 seemingly unrelated diseases. Patients or mice with allergic asthma have a high risk of developing atherosclerosis or vice versa, despite the fact that asthma is a T-helper (Th)2-oriented disease, whereas Th1 immunity promotes atherosclerosis. In addition to the preferred Th1/Th2 responses that may differentiate the 2 diseases, mast cells and many other inflammatory cells also contribute to their pathogenesis by more than just T cell immunity. Here, we summarize the different roles of airway and arterial wall inflammatory cells and vascular cells in asthma and atherosclerosis and propose an interaction between the 2 diseases, although limited investigations are available to delineate the molecular and cellular mechanisms by which 1 disease increases the risk of the other. Results from mouse allergic asthma and atherosclerosis models and from human population studies lead to the hypothesis that patients with atherosclerosis may benefit from antiasthmatic medications or that the therapeutic regimens targeting atherosclerosis may also alleviate allergic asthma.

Differential regulation of aldose reductase expression during macrophage polarization depends on hyperglycemia

Aldose reductase (AR; gene AKR1B1) is the rate-limiting enzyme of the polyol pathway and has been associated with diabetes and atherosclerosis. Here, we sought to identify the mechanisms underlying differential AR expression in human atherosclerotic plaque macrophages. In vitro, M1-polarized human monocyte-derived macrophages expressed significantly higher levels of AKR1B1 mRNA and AR protein compared with M2-polarized macrophages. AR activity was significantly higher in M1 macrophages. AKR1B1 mRNA expression correlated positively with the M1 marker TNF(r = 0.430,P = 0.006) and negatively with the M2 marker MRC1 (r = -0.443,P = 0.044). Increased AR expression in M1 macrophages depended on hyperglycemia. Concomitantly, expression of SLC2A1 (coding for the Glc transporter GLUT-1) was significantly higher in M1 than in M2 macrophages. Pharmacological inhibition of GLUT-1 using STF-32 completely abrogated Glc-induced AR up-regulation in M1 macrophages. When analyzing AR expression in post-mortem coronary artery plaque macrophages, a history of diabetes was associated with a significantly increased proportion of CD68(+)AR(++)macrophages, supporting the in vivo relevance of our in vitro findings. We demonstrate that the phenotype of atherosclerotic plaque macrophages may be affected by cardiovascular risk factors such as hyperglycemia. Our data illustrate the complex interplay between systemic and local factors in atherogenesis.

Mitochondrial regulation of macrophage cholesterol homeostasis

This review explores the relationship between mitochondrial structure and function in the regulation of macrophage cholesterol metabolism and proposes that mitochondrial dysfunction contributes to loss of the elegant homeostatic mechanisms which normally maintain cellular sterol levels within defined limits. Mitochondrial sterol 27-hydroxylase (CYP27A1) can generate oxysterol activators of liver X receptors which heterodimerise with retinoid X receptors, enhancing the transcription of ATP binding cassette transporters (ABCA1, ABCG1, and ABCG4), that can remove excess cholesterol via efflux to apolipoproteins A-1, E, and high density lipoprotein, and inhibit inflammation. The activity of CYP27A1 is regulated by the rate of supply of cholesterol substrate to the inner mitochondrial membrane, mediated by a complex of proteins. The precise identity of this dynamic complex remains controversial, even in steroidogenic tissues, but may include steroidogenic acute regulatory protein and the 18 kDa translocator protein, together with voltage-dependent anion channels, ATPase AAA domain containing protein 3A, and optic atrophy type 1 proteins. Certainly, overexpression of StAR and TSPO proteins can enhance macrophage cholesterol efflux to apoA-I and/or HDL, while perturbations in mitochondrial function, or changes in the expression of mitochondrial fusion proteins, alter the efficiency of cholesterol efflux. Molecules which can sustain or improve mitochondrial function or increase the activity of the protein complex involved in cholesterol transfer may have utility in resolving the problem of dysregulated macrophage cholesterol homeostasis, a condition which may contribute to inflammation, atherosclerosis, nonalcoholic steatohepatitis, osteoblastic bone resorption, and some disorders of the central nervous system.

CXCL4 Plasma Levels Are Not Associated with the Extent of Coronary Artery Disease or with Coronary Plaque Morphology

BACKGROUND

CXCL4 is a platelet chemokine released at micromolar concentrations upon platelet activation. CXCL4 has been shown to promote atherogenesis by various mechanisms. However, data on CXCL4 plasma levels in patients with coronary artery disease are largely inconclusive. Computed coronary artery angiography (CCTA) represents an excellent tool to quantify and characterize coronary atherosclerotic plaques. We hypothesized that increased CXCL4 plasma levels may be associated with features of plaque instability resulting in adverse cardiovascular events. Specifically, we sought to determine whether CXCL4 levels are correlated with specific features of coronary artery disease including (1) plaque volume, (2) calcium score, (3) degree of stenosis, or (4) vascular remodeling.

METHODS AND RESULTS

CXCL4 plasma levels were measured by ELISA in 217 patients undergoing CCTA for suspected CAD (mean age 64.2 ± 9.4 years, 107 (49.3%) male). Mean CXCL4 plasma levels were 12.5 ± 4.6 ng/mL. There was no significant correlation between CXCL4 levels and any clinical or demographic parameters including cardiovascular risk factors. CXCL4 plasma levels did not differ between patient with or without coronary artery disease (CAD: 12.5 ± 4.5 ng/ml, no CAD: 12.5 ± 4.8 ng/ml). Neither univariate nor multivariate analysis showed an association between CXCL4 levels and plaque volume, total calcium score, degree of stenosis, or vascular remodeling. Subgroup analysis of patients with CAD as confirmed by CCTA did not show any association of CXCL4 levels with the extent of CAD.

CONCLUSIONS

While CXCL4 may be present and active within the arterial wall, local increase of CXCL4 may not translate into systemically elevated CXCL4 levels. Further studies will have to test whether CXCL4 may still represent a suitable therapeutic target in human atherosclerosis.

Macrophages and immune cells in atherosclerosis: recent advances and novel concepts

Atherosclerotic lesion-related thrombosis is the major cause of myocardial infarction and stroke, which together constitute the leading cause of mortality worldwide. The inflammatory response is considered as a predominant driving force in atherosclerotic plaque formation, growth and progression towards instability and rupture. Notably, accumulation of macrophages in the intima and emergence of a pro-inflammatory milieu are a characteristic feature of plaque progression, and these processes can be modulated by adaptive immune responses. Recently, novel evidences of onsite proliferation of macrophages in lesions and transdifferentiation of smooth muscle cells to macrophages have challenged the prevalent paradigm that macrophage accumulation mostly relies on recruitment of circulating monocytes to plaques. Furthermore, previously unrecognized roles of inflammatory cell subsets such as plasmacytoid dendritic cells, innate response activator B cells or CD8(+) T cells in atherosclerosis have emerged, as well as novel mechanisms by which regulatory T cells or natural killer T cells contribute to lesion formation. Here, we review and discuss these recent advances in our understanding of inflammatory processes in atherosclerosis.