Transcriptional programming of human macrophages: on the way to systems immunology

Intraplaque Neovascularization, CD68+ and iNOS2+ Macrophage Infiltrate Intensity Are Associated with Atherothrombosis and Intraplaque Hemorrhage in Severe Carotid Atherosclerosis

BACKGROUND

Atherosclerosis is a progressive disease that results from endothelial dysfunction, inflammatory arterial wall disorder and the formation of the atheromatous plaque. This results in carotid artery stenosis and is responsible for atherothrombotic stroke and ischemic injury. Low-grade plaque inflammation determines biological stability and lesion progression.

METHODS

Sixty-seven cases with active perilesional inflammatory cell infiltrate were selected from a larger cohort of patients undergoing carotid endarterectomy. CD68+, iNOS2+ and Arg1+ macrophages and CD31+ endothelial cells were quantified around the atheroma lipid core using digital morphometry, and expression levels were correlated with determinants of instability: ulceration, thrombosis, plaque hemorrhage, calcification patterns and neovessel formation.

RESULTS

Patients with intraplaque hemorrhage had greater CD68+ macrophage infiltration (p = 0.003). In 12 cases where iNOS2 predominated over Arg1 positivity, the occurrence of atherothrombotic events was significantly more frequent (p = 0.046). CD31 expression, representing neovessel formation, correlated positively with atherothrombosis (p = 0.020).

CONCLUSIONS

Intraplaque hemorrhage is often described against the background of an intense inflammatory cell infiltrate. Atherothrombosis is associated with the presence of neovessels and pro-inflammatory macrophages expressing iNOS2. Modulating macrophage polarization may be a successful therapeutic approach to prevent plaque destabilization.

Multistability in Macrophage Activation Pathways and Metabolic Implications

Macrophages are innate immune cells with a dynamic range of reversible activation states including the classical pro-inflammatory (M1) and alternative anti-inflammatory (M2) states. Deciphering how macrophages regulate their transition from one state to the other is key for a deeper understanding of inflammatory diseases and relevant therapies. Common regulatory motifs reported for macrophage transitions, such as positive or double-negative feedback loops, exhibit a switchlike behavior, suggesting the bistability of the system. In this review, we explore the evidence for multistability (including bistability) in macrophage activation pathways at four molecular levels. First, a decision-making module in signal transduction includes mutual inhibitory interactions between M1 (STAT1, NF-KB/p50-p65) and M2 (STAT3, NF-KB/p50-p50) signaling pathways. Second, a switchlike behavior at the gene expression level includes complex network motifs of transcription factors and miRNAs. Third, these changes impact metabolic gene expression, leading to switches in energy production, NADPH and ROS production, TCA cycle functionality, biosynthesis, and nitrogen metabolism. Fourth, metabolic changes are monitored by metabolic sensors coupled to AMPK and mTOR activity to provide stability by maintaining signals promoting M1 or M2 activation. In conclusion, we identify bistability hubs as promising therapeutic targets for reverting or blocking macrophage transitions through modulation of the metabolic environment.

Classical Dichotomy of Macrophages and Alternative Activation Models Proposed with Technological Progress

Macrophages are important immune cells that participate in the regulation of inflammation in implant dentistry, and their activation/polarization state is considered to be the basis for their functions. The classic dichotomy activation model is commonly accepted, however, due to the discovery of macrophage heterogeneity and more functional and iconic exploration at different technologies; some studies have discovered the shortcomings of the dichotomy model and have put forward the concept of alternative activation models through the application of advanced technologies such as cytometry by time-of-flight (CyTOF), single-cell RNA-seq (scRNA-seq), and hyperspectral image (HSI). These alternative models have great potential to help macrophages divide phenotypes and functional genes.

The equine mononuclear phagocyte system: The relevance of the horse as a model for understanding human innate immunity

The mononuclear phagocyte system (MPS) is a family of cells of related function that includes bone marrow progenitors, blood monocytes and resident tissue macrophages. Macrophages are effector cells in both innate and acquired immunity. They are a major resident cell population in every organ and their numbers increase in response to proinflammatory stimuli. Their function is highly regulated by a wide range of agonists, including lymphokines, cytokines and products of microorganisms. Macrophage biology has been studied most extensively in mice, yet direct comparisons of rodent and human macrophages have revealed many functional differences. In this review, we provide an overview of the equine MPS, describing the variation in the function and phenotype of macrophages depending on their location and the similarities and differences between the rodent, human and equine immune response. We discuss the use of the horse as a large animal model in which to study macrophage biology and pathological processes shared with humans. Finally, following the recent update to the horse genome, facilitating further comparative analysis of regulated gene expression between the species, we highlight the importance of future transcriptomic macrophage studies in the horse, the findings of which may also be applicable to human as well as veterinary research.

Macrophages Derived From Human Induced Pluripotent Stem Cells Are Low-Activated "Naïve-Like" Cells Capable of Restricting Mycobacteria Growth

In peripheral tissues, immune protection critically depends on the activity of tissue resident macrophages, which makes our understanding of the biology of these cells of great significance. Until recently, human macrophage studies were largely based on the analysis of monocyte-derived macrophages that differ from tissue resident macrophages by many characteristics. To model tissue resident macrophages, methods of generating macrophages from pluripotent stem cells have been developed. However, the immunological properties of macrophages derived from pluripotent stem cells remain under-investigated. In this study, we aimed to perform the multifarious immunological characteristics of macrophages generated from human induced pluripotent stem cells (iMϕs), including an analysis of their phenotype, secretory and antibacterial activities, as well as their comparison with macrophages derived from blood monocytes and infected lung tissue. We report that iMϕs displayed the morphology and the CD11b⁺CD45⁺CD14⁺ phenotype typical for mononuclear phagocytes. The cells co-expressed markers known to be associated with classically (CD80, CD86, CCR5) and alternatively (CD163 and CD206) activated macrophages, with a bias toward a higher expression of the latter. iMϕs secreted pro-inflammatory (IL-6, CXCL8, CCL2, CCL4, CXCL1, CXCL10) and anti-inflammatory (IL-10, IL-1RA, CCL22) cytokines with a high IL-10/IL-12p70 index (>20). iMϕs were phagocytic and restricted Mycobacterium tuberculosis growth in vitro by >75%. iMϕs differed from blood monocytes/macrophages by a lower expression level of HLA-DR and the CD14⁺CD16^int phenotype and shared several phenotypic characteristics with lung macrophages. In response to LPS, iMϕs up-regulated HLA-DR and produced TNF-α. IFN-γ increased iMϕ reactivity to LPS, but did not increase iMϕ mycobactericidal capacity. The results characterize iMϕs as differentiated but low-activated/low-polarized “naïve-like” macrophages that are capable of mounting inflammatory and antibacterial responses when exposed to inflammatory stimuli or pathogens. iMϕs represent a valuable model for studying antibacterial responses of tissue resident macrophages and for developing approaches to modulating macrophage activity.

The neuroprotective role of microglial cells against amyloid beta-mediated toxicity in organotypic hippocampal slice cultures

During Alzheimer's disease (AD) progression, microglial cells play complex roles and have potentially detrimental as well as beneficial effects. The use of appropriate model systems is essential for characterizing and understanding the roles of microglia in AD pathology. Here, we used organotypic hippocampal slice cultures (OHSCs) to investigate the impact of microglia on amyloid beta (Aβ)-mediated toxicity. Neurons in OHSCs containing microglia were not vulnerable to cell death after 7 days of repeated treatment with Aβ_1-42 oligomer-enriched preparations. However, when clodronate was used to remove microglia, treatment with Aβ_1-42 resulted in significant neuronal death. Further investigations indicated signs of endoplasmic reticulum stress and caspase activation after Aβ_1-42 challenge only when microglia were absent. Interestingly, microglia provided protection without displaying any classic signs of activation, such as an amoeboid morphology or the release of pro-inflammatory mediators (e.g., IL-6, TNF-α, NO). Furthermore, depleting microglia or inhibiting microglial uptake mechanisms resulted in significant more Aβ deposition compared to that observed in OHSCs containing functional microglia, suggesting that microglia efficiently cleared Aβ. Because inhibiting microglial uptake increased neuronal cell death, the ability of microglia to engulf Aβ is thought to contribute to its protective properties. Our study argues for a beneficial role of functional ramified microglia whereby they act against the accumulation of neurotoxic forms of Aβ and support neuronal resilience in an in situ model of AD pathology.

Modified LDL Particles Activate Inflammatory Pathways in Monocyte-derived Macrophages: Transcriptome Analysis

BACKGROUND

A hallmark of atherosclerosis is its complex pathogenesis, which is dependent on altered cholesterol metabolism and inflammation. Both arms of pathogenesis involve myeloid cells. Monocytes migrating into the arterial walls interact with modified low-density lipoprotein (LDL) particles, accumulate cholesterol and convert into foam cells, which promote plaque formation and also contribute to inflammation by producing proinflammatory cytokines. A number of studies characterized transcriptomics of macrophages following interaction with modified LDL, and revealed alteration of the expression of genes responsible for inflammatory response and cholesterol metabolism. However, it is still unclear how these two processes are related to each other to contribute to atherosclerotic lesion formation.

METHODS

We attempted to identify the main mater regulator genes in macrophages treated with atherogenic modified LDL using a bioinformatics approach.

RESULTS

We found that most of the identified genes were involved in inflammation, and none of them was implicated in cholesterol metabolism. Among the key identified genes were interleukin (IL)-7, IL-7 receptor, IL- 15 and CXCL8.

CONCLUSION

Our results indicate that activation of the inflammatory pathway is the primary response of the immune cells to modified LDL, while the lipid metabolism genes may be a secondary response triggered by inflammatory signalling.

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.

Chromatin Remodeling in Monocyte and Macrophage Activation

Increasing evidence collected during the last years supports the idea that monocyte and macrophage activation is not only associated with transcriptional changes but also changes in the chromatin landscape. Moreover, the introduction of a multidimensional model of macrophage activation allows a more precise description of monocytes and macrophages under homeostatic and pathophysiological conditions. Monocytes and macrophages are masters of integrating microenvironmental signals, thereby reshaping their chromatin landscape and as a consequence their transcriptional and functional programs. Albeit these cells share a large number of epigenetic landmarks, their chromatin is significantly shaped by environmental signals. The chromatin landscape of any given tissue macrophage is a rather specific fingerprint of these cells, which is directly linked to tissue-specific functions of these cells. Moreover, chromatin remodeling in response to stress signals also seems to be an important mechanism of these cells to increase their readiness for future stressors. Understanding this sophisticated epigenetic regulatory network in monocytes and macrophages will open up new avenues toward tissue- and disease-specific therapeutic strategies in many of the chronic inflammatory diseases our societies are currently facing.

Adipose tissue infiltration in normal-weight subjects and its impact on metabolic function

Discordant phenotypes, metabolically healthy obese and unhealthy normal-weight individuals, are always interesting to provide important insights into the mechanistic link between adipose tissue dysfunction and associated metabolic alterations. Macrophages can release factors that impair the proper activity of the adipose tissue. Thus, studying subcutaneous and visceral adipose tissues, we investigated for the first time the differences in monocyte/macrophage infiltration, inflammation, and adipogenesis of normal-weight subjects who differed in their degree of metabolic syndrome. The study included 92 normal-weight subjects who differed in their degree of metabolic syndrome. Their anthropometric and biochemical parameters were measured. RNA from subcutaneous and visceral adipose tissues was isolated, and mRNA expression of monocyte/macrophage infiltration (CD68, CD33, ITGAM, CD163, EMR-1, CD206, MerTK, CD64, ITGAX), inflammation (IL-6, tumor necrosis factor alpha [TNFα], IL-10, IL-1b, CCL2, CCL3), and adipogenic and lipogenic capacity markers (PPARgamma, FABP4) were measured. Taken together, our data provide evidence of a different degree of macrophage infiltration between the adipose tissues, with a higher monocyte/macrophage infiltration in subcutaneous adipose tissue in metabolically unhealthy normal-weight subjects, whereas visceral adipose tissue remained almost unaffected. An increased macrophage infiltration of adipose tissue and its consequences, such as a decrease in adipogenesis function, may explain why both the obese and normal-weight subjects can develop metabolic diseases or remain healthy.

Exploiting the Immunomodulatory Properties of Chemotherapeutic Drugs to Improve the Success of Cancer Immunotherapy

Cancer immunotherapy is gaining momentum in the clinic. The current challenge is to understand why a proportion of cancer patients do not respond to cancer immunotherapy, and how this can be translated into the rational design of combinatorial cancer immunotherapy strategies aimed at maximizing success of immunotherapy. Here, we discuss how tumors orchestrate an immunosuppressive microenvironment, which contributes to their escape from immune attack. Relieving the immunosuppressive networks in cancer patients is an attractive strategy to extend the clinical success of cancer immunotherapy. Since the clinical availability of drugs specifically targeting immunosuppressive cells or mediators is still limited, an alternative strategy is to use conventional chemotherapy drugs with immunomodulatory properties to improve cancer immunotherapy. We summarize the preclinical and clinical studies that illustrate how the anti-tumor T cell response can be enhanced by chemotherapy-induced relief of immunosuppressive networks. Treatment strategies aimed at combining chemotherapy-induced relief of immunosuppression and T cell-boosting checkpoint inhibitors provide an attractive and clinically feasible approach to overcome intrinsic and acquired resistance to cancer immunotherapy, and to extend the clinical success of cancer immunotherapy.