Myeloid cells in liver and bone marrow acquire a functionally distinct inflammatory phenotype during obesity-related steatohepatitis

Dynamic Shifts in the Composition of Resident and Recruited Macrophages Influence Tissue Remodeling in NASH

Macrophage-mediated inflammation is critical in the pathogenesis of non-alcoholic steatohepatitis (NASH). Here, we describe that, with high-fat, high-sucrose-diet feeding, mature TIM4^pos Kupffer cells (KCs) decrease in number, while monocyte-derived Tim4^neg macrophages accumulate. In concert, monocyte-derived infiltrating macrophages enter the liver and consist of a transitional subset that expresses Cx3cr1/Ccr2 and a second subset characterized by expression of Trem2, Cd63, Cd9, and Gpmnb; markers ascribed to lipid-associated macrophages (LAMs). The Cx3cr1/Ccr2-expressing macrophages, referred to as C-LAMs, localize to macrophage aggregates and hepatic crown-like structures (hCLSs) in the steatotic liver. In C-motif chemokine receptor 2 (Ccr2)-deficient mice, C-LAMs fail to appear in the liver, and this prevents hCLS formation, reduces LAM numbers, and increases liver fibrosis. Taken together, our data reveal dynamic changes in liver macrophage subsets during the pathogenesis of NASH and link these shifts to pathologic tissue remodeling.

Current perspectives on the pathophysiology of metabolic associated fatty liver disease: are macrophages a viable target for therapy?

INTRODUCTION

Metabolic associated fatty liver disease (MAFLD) is a new nomenclature for fatty liver replacing nonalcoholic fatty liver disease (NAFLD). MAFLD has emerged as the leading cause of liver-related morbidity and mortality with increasing incidence due to its close association with the global epidemic of obesity and type 2 diabetes mellitus. Macrophages play a key role in MAFLD development and progression of steatohepatitis and fibrosis. Therefore, targeting macrophages may be a new therapeutic approach for MAFLD and MAFLD with steatohepatitis.

AREAS COVERED

We provide a comprehensive review of the significant role of macrophages in MAFLD. Further, we evaluate the current status of lifestyle interventions and pharmacological treatments with a focus on effects mediated through direct or indirect targeting of macrophages.

EXPERT OPINION

Targeting macrophages holds promise as a treatment option for the management of MAFLD and steatohepatitis. Improved stratification of patients according to MAFLD phenotype would contribute to more adequate design enhancing the yield of clinical trials ultimately leading to personalized medicine for patients with MAFLD. Furthermore, reflecting the multifactorial pathogenesis of MAFLD, combination therapies based on the various pathophysiological driver events including as pertinent to this review, macrophage recruitment, polarization and action, present an intriguing target for future investigation.

The Power of Single-Cell Analysis for the Study of Liver Pathobiology

Single cell transcriptomics has emerged as a powerful lens through which to study the molecular diversity of complex tissues such as the liver, during health and disease, both in animal models and in humans. The earliest gene expression methods measured bulk tissue RNA, but the results were often confusing because they derived from the combined transcriptomes of many different cell types in unknown proportions. To better delineate cell-type-specific expression, investigators developed cell isolation, purification, and sorting protocols, yet still, the RNA derived from ensembles of cells obscured recognition of cellular heterogeneity. Profiling transcriptomes at the single-cell level has opened the door to analyses that were not possible in the past. In this review, we discuss the evolution of single cell transcriptomics and how it has been applied for the study of liver physiology and pathobiology to date.

Nonalcoholic steatohepatitis: the role of peroxisome proliferator-activated receptors

The increasing epidemic of obesity worldwide is linked to serious health effects, including increased prevalence of type 2 diabetes mellitus, cardiovascular disease and nonalcoholic fatty liver disease (NAFLD). NAFLD is the liver manifestation of the metabolic syndrome and includes the spectrum of liver steatosis (known as nonalcoholic fatty liver) and steatohepatitis (known as nonalcoholic steatohepatitis), which can evolve into progressive liver fibrosis and eventually cause cirrhosis. Although NAFLD is becoming the number one cause of chronic liver diseases, it is part of a systemic disease that affects many other parts of the body, including adipose tissue, pancreatic β-cells and the cardiovascular system. The pathomechanism of NAFLD is multifactorial across a spectrum of metabolic derangements and changes in the host microbiome that trigger low-grade inflammation in the liver and other organs. Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear regulatory factors that provide fine tuning for key elements of glucose and fat metabolism and regulate inflammatory cell activation and fibrotic processes. This Review summarizes and discusses the current literature on NAFLD as the liver manifestation of the systemic metabolic syndrome and focuses on the role of PPARs in the pathomechanisms as well as in the potential targeting of disease.

Can Walnut Serve as a Magic Bullet for the Management of Non-Alcoholic Fatty Liver Disease?

Walnut contains many nutrients and bioactive components such as essential fatty acids, polyphenols, fiber, ɤ-tocopherol, folate, minerals, and vegetable protein, and has therefore been regarded as a natural functional food. Walnut-enriched diets have been demonstrated to be useful for heart health, cancer prevention, and metabolic disorders owing to their anti-oxidative and anti-inflammatory properties as well as for the maintenance of a healthy metabolism and immune function. Walnut extracts, either phenolic or lipid, also demonstrated the health effects in animal and cultured cell studies. More recently, the beneficial effects of walnut consumption on non-alcoholic fatty liver disease, which is a hepatic manifestation of obesity, hyperlipidemia, type 2 diabetes mellitus, and metabolic syndrome with substantial hepatic accumulation of triglyceride, have been proposed because walnut and a walnut-containing diet can modulate the etiologic mechanism such as ameliorating systemic and hepatic dyslipidemia, reducing lipotoxicity and inflammation, enhancing immune function, and maintaining gut microbiota balance. Through the extensive literature review we discuss the preventive roles of walnut in the development and progression of non-alcoholic fatty liver disease (NAFLD) and provide mechanistic insights into these effects.

Immunological mechanisms and therapeutic targets of fatty liver diseases

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) are the two major types of chronic liver disease worldwide. Inflammatory processes play key roles in the pathogeneses of fatty liver diseases, and continuous inflammation promotes the progression of alcoholic steatohepatitis (ASH) and nonalcoholic steatohepatitis (NASH). Although both ALD and NAFLD are closely related to inflammation, their respective developmental mechanisms differ to some extent. Here, we review the roles of multiple immunological mechanisms and therapeutic targets related to the inflammation associated with fatty liver diseases and the differences in the progression of ASH and NASH. Multiple cell types in the liver, including macrophages, neutrophils, other immune cell types and hepatocytes, are involved in fatty liver disease inflammation. In addition, microRNAs (miRNAs), extracellular vesicles (EVs), and complement also contribute to the inflammatory process, as does intertissue crosstalk between the liver and the intestine, adipose tissue, and the nervous system. We point out that inflammation also plays important roles in promoting liver repair and controlling bacterial infections. Understanding the complex regulatory process of disrupted homeostasis during the development of fatty liver diseases may lead to the development of improved targeted therapeutic intervention strategies.

Single-cell genomics and spatial transcriptomics: Discovery of novel cell states and cellular interactions in liver physiology and disease biology

Transcriptome analysis enables the study of gene expression in human tissues and is a valuable tool to characterise liver function and gene expression dynamics during liver disease, as well as to identify prognostic markers or signatures, and to facilitate discovery of new therapeutic targets. In contrast to whole tissue RNA sequencing analysis, single-cell RNA-sequencing (scRNA-seq) and spatial transcriptomics enables the study of transcriptional activity at the single cell or spatial level. ScRNA-seq has paved the way for the discovery of previously unknown cell types and subtypes in normal and diseased liver, facilitating the study of rare cells (such as liver progenitor cells) and the functional roles of non-parenchymal cells in chronic liver disease and cancer. By adding spatial information to scRNA-seq data, spatial transcriptomics has transformed our understanding of tissue functional organisation and cell-to-cell interactions in situ. These approaches have recently been applied to investigate liver regeneration, organisation and function of hepatocytes and non-parenchymal cells, and to profile the single cell landscape of chronic liver diseases and cancer. Herein, we review the principles and technologies behind scRNA-seq and spatial transcriptomic approaches, highlighting the recent discoveries and novel insights these methodologies have yielded in both liver physiology and disease biology.

Differential effects of selective- and pan-PPAR agonists on experimental steatohepatitis and hepatic macrophages☆

BACKGROUND & AIMS

Peroxisome proliferator-activated receptors (PPARs) are essential regulators of whole-body metabolism, but also modulate inflammation in immune cells, notably macrophages. We compared the effects of selective PPAR agonists to those of the pan-PPAR agonist lanifibranor in non-alcoholic fatty liver disease (NAFLD), and studied isoform-specific effects on hepatic macrophage biology.

METHODS

Lanifibranor or selective PPARα (fenofibrate), PPARγ (pioglitazone) and PPARδ (GW501516) agonists were therapeutically administered in choline-deficient, amino acid-defined high-fat diet (CDAA-HFD)- and Western diet (WD)-fed mouse models of NAFLD. Acute liver injury was induced by carbon tetrachloride (CCl4). The role of PPARs on macrophage functionality was studied in isolated hepatic macrophages, bone marrow-derived macrophages stimulated with palmitic acid, and circulating monocytes from patients with NAFLD.

RESULTS

Lanifibranor improved all histological features of steatohepatitis in CDAA-HFD-fed mice, including liver fibrosis, thereby combining and exceeding specific effects of the single PPAR agonists. Its potent anti-steatotic efficacy was confirmed in a 3D liver biochip model with primary cells. Infiltrating hepatic monocyte-derived macrophages were reduced following PPAR agonist administration, especially with lanifibranor, even after short-term treatment, paralleling improved steatosis and hepatitis. Lanifibranor similarly decreased steatosis, liver injury and monocyte infiltration in the WD model. In the acute CCl4 model, neither single nor pan-PPAR agonists directly affected monocyte recruitment. Hepatic macrophages isolated from WD-fed mice displayed a metabolically activated phenotype. Lanifibranor attenuated the accompanying inflammatory activation in both murine palmitic acid-stimulated bone marrow-derived macrophages, as well as patient-derived circulating monocytes, in a PPARδ-dependent fashion.

CONCLUSION

Pan-PPAR agonists combine the beneficial effects of selective PPAR agonists and may counteract inflammation and disease progression more potently. PPARδ agonism and lanifibranor directly modulate macrophage activation, but not infiltration, thereby synergizing with beneficial metabolic effects of PPARα/γ agonists.

LAY SUMMARY

Peroxisome proliferated-activated receptors (PPARs) are essential regulators of metabolism and inflammation. We demonstrated that the pan-PPAR agonist lanifibranor ameliorated all aspects of non-alcoholic fatty liver disease in independent experimental mouse models. Non-alcoholic fatty liver disease and fatty acids induce a specific polarization status in macrophages, which was altered by lanifibranor to increase expression of lipid handling genes, thereby decreasing inflammation. PPAR isoforms have differential therapeutic effects on fat-laden hepatocytes, activated hepatic stellate cells and inflammatory macrophages, supporting the clinical development of pan-PPAR agonists.

Functionally Diverse Inflammatory Responses in Peripheral and Liver Monocytes in Alcohol-Associated Hepatitis

Alcohol-associated hepatitis (AH) is an acute inflammatory disease in which gut-microbial byproducts enter circulation and peripheral immune cells infiltrate the liver, leading to nonresolving inflammation and injury. Single-cell RNA sequencing of peripheral blood mononuclear cells isolated from patients with AH and healthy controls paired with lipopolysaccharide (LPS) challenge revealed how diverse monocyte responses are divided among individual cells and change in disease. After LPS challenge, one monocyte subtype expressed pro-inflammatory genes in both disease and healthy controls, while another monocyte subtype was anti-inflammatory in healthy controls but switched to pro-inflammatory in AH. Numerous immune genes are clustered within genomic cassettes, including chemokines and C-type lectin receptors (CTRs). CTRs sense byproducts of diverse microbial and host origin. Single-cell data revealed correlated expression of genes within cassettes, thus further diversifying different monocyte responses to individual cells. Monocyte up-regulation of CTRs in response to LPS caused hypersensitivity to diverse microbial and host-derived byproducts, indicating a secondary immune surveillance pathway up-regulated in a subset of cells by a closely associated genomic cassette. Finally, expression of CTR genes was higher in livers of patients with severe AH, but not other chronic liver diseases, implicating secondary immune surveillance in nonresolving inflammation in severe AH.

Nutrition Regulates Innate Immunity in Health and Disease

Nutrient content and nutrient timing are considered key regulators of human health and a variety of diseases and involve complex interactions with the mucosal immune system. In particular, the innate immune system is emerging as an important signaling hub that modulates the response to nutritional signals, in part via signaling through the gut microbiota. In this review we elucidate emerging evidence that interactions between innate immunity and diet affect human metabolic health and disease, including cardiometabolic disorders, allergic diseases, autoimmune disorders, infections, and cancers. Furthermore, we discuss the potential modulatory effects of the gut microbiota on interactions between the immune system and nutrition in health and disease, namely how it relays nutritional signals to the innate immune system under specific physiological contexts. Finally, we identify key open questions and challenges to comprehensively understanding the intersection between nutrition and innate immunity and how potential nutritional, immune, and microbial therapeutics may be developed into promising future avenues of precision treatment.

Deciphering the Immune Microenvironment on A Single Archival Formalin-Fixed Paraffin-Embedded Tissue Section by An Immediately Implementable Multiplex Fluorescence Immunostaining Protocol

Simple Summary

Technological breakthroughs have fundamentally changed our understanding on the complexity of tissue organization, both in healthy and diseased conditions. Characterizing the immune cell composition in relation to spatial distribution and histological changes may provide important diagnostic and therapeutic information. Immunohistochemistry remains a method of choice for these purposes, with crucial implications in clinics or in medical research. Nowadays, the widespread use of fluorophore-conjugated antibodies enables simultaneous visualization of an increasing number of proteins on a single tissue slice, with unprecedented resolution. However, advanced methods usually require modern and specific equipment, a significant amount of time for assay optimization, and highly specialized skills. This work reports on the use of a multiplex immunostaining method based on sequential immunostaining and antibody stripping, combined with digital image processing and analysis. Our aim is to provide the medical and research communities with a simple, cost-effective workflow to encompass some current limitations of multiplex immunohistochemistry.

Abstract

Technological breakthroughs have fundamentally changed our understanding on the complexity of the tumor microenvironment at the single-cell level. Characterizing the immune cell composition in relation to spatial distribution and histological changes may provide important diagnostic and therapeutic information. Immunostaining on formalin-fixed paraffin-embedded (FFPE) tissue samples represents a widespread and simple procedure, allowing the visualization of cellular distribution and processes, on preserved tissue structure. Recent advances in microscopy and molecular biology have made multiplexing accessible, yet technically challenging. We herein describe a novel, simple and cost-effective method for a reproducible and highly flexible multiplex immunostaining on archived FFPE tissue samples, which we optimized for solid organs (e.g., liver, intestine, lung, kidney) from mice and humans. Our protocol requires limited specific equipment and reagents, making multiplexing (>12 antibodies) immediately implementable to any histology laboratory routinely performing immunostaining. Using this method on single sections and combining it with automated whole-slide image analysis, we characterize the hepatic immune microenvironment in preclinical mouse models of liver fibrosis, steatohepatitis and hepatocellular carcinoma (HCC) and on human-patient samples with chronic liver diseases. The data provide useful insights into tissue organization and immune–parenchymal cell-to-cell interactions. It also highlights the profound macrophage heterogeneity in liver across premalignant conditions and HCC.

Systemically-delivered biodegradable PLGA alters gut microbiota and induces transcriptomic reprogramming in the liver in an obesity mouse model

Biodegradable materials, including the widely used poly (lactic-co-glycolic acid) (PLGA) nanoparticles contained in slow-release drug formulations, scaffolds and implants, are ubiquitous in modern biomedicine and are considered inert or capable of being metabolized through intermediates such as lactate. However, in the presence of metabolic stress, such as in obesity, the resulting degradation products may play a detrimental role, which is still not well understood. We evaluated the effect of intravenously-administered PLGA nanoparticles on the gut-liver axis under conditions of caloric excess in C57BL/6 mice. Our results show that PLGA nanoparticles accumulate and cause gut acidification in the cecum, accompanied by significant changes in the microbiome, with a marked decrease of Firmicutes and Bacteroidetes. This was associated with transcriptomic reprogramming in the liver, with a downregulation of mitochondrial function, and an increase in key enzymatic, inflammation and cell activation pathways. No changes were observed in systemic inflammation. Metagenome analysis coupled with publicly available microarray data suggested a mechanism of impaired PLGA degradation and intestinal acidification confirming an important enterohepatic axis of metabolite-microbiome interaction resulting in maintenance of metabolic homeostasis. Thus, our results have important implications for the investigation of PLGA use in metabolically-compromised clinical and experimental settings.

Single-cell technologies in hepatology: new insights into liver biology and disease pathogenesis

Liver disease is a major global health-care problem, affecting an estimated 844 million people worldwide. Despite this substantial burden, therapeutic options for liver disease remain limited, in part owing to a paucity of detailed analyses defining the cellular and molecular mechanisms that drive these conditions in humans. Single-cell transcriptomic technologies are transforming our understanding of cellular diversity and function in health and disease. In this Review, we discuss how these technologies have been applied in hepatology, advancing our understanding of cellular heterogeneity and providing novel insights into fundamental liver biology such as the metabolic zonation of hepatocytes, endothelial cells and hepatic stellate cells, and the cellular mechanisms underpinning liver regeneration. Application of these methodologies is also uncovering critical pathophysiological changes driving disease states such as hepatic fibrosis, where distinct populations of macrophages, endothelial cells and mesenchymal cells reside within a spatially distinct fibrotic niche and interact to promote scar formation. In addition, single-cell approaches are starting to dissect key cellular and molecular functions in liver cancer. In the near future, new techniques such as spatial transcriptomics and multiomic approaches will further deepen our understanding of disease pathogenesis, enabling the identification of novel therapeutic targets for patients across the spectrum of liver diseases.

Crosstalk Between Liver Macrophages and Surrounding Cells in Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH), the advanced stage of nonalcoholic fatty liver disease (NAFLD), is emerging as a leading cause of progressive liver fibrosis and end-stage liver disease. Liver macrophages, mainly composed of Kupffer cells (KCs) and monocyte-derived macrophages (MoMFs), play a vital role in NASH progression and regression. Recent advances suggest that cell–cell communication is a fundamental feature of hepatic microenvironment. The reprogramming of cell–cell signaling between macrophages and surrounding cells contributes to the pathogenesis of NASH. In this review, we summarize the current knowledge of NASH regarding the composition of liver macrophages and their communication with surrounding cells, which are composed of hepatocytes, hepatic stellate cells (HSCs), liver sinusoidal endothelial cells (LSECs) and other immune cells. We also discuss the potential therapeutic strategies based on the level of macrophages.

Advances of single-cell genomics and epigenomics in human disease: where are we now?

Cellular heterogeneity is revolutionizing the way to study, monitor and dissect complex diseases. This has been possible with the technological and computational advances associated to single-cell genomics and epigenomics. Deeper understanding of cell-to-cell variation and its impact on tissue function will open new avenues for early disease detection, accurate diagnosis and personalized treatments, all together leading to the next generation of health care. This review focuses on the recent discoveries that single-cell genomics and epigenomics have facilitated in the context of human health. It highlights the potential of single-cell omics to further advance the development of personalized treatments and precision medicine in cancer, diabetes and chronic age-related diseases. The promise of single-cell technologies to generate new insights about the differences in function between individual cells is just emerging, and it is paving the way for identifying biomarkers and novel therapeutic targets to tackle age, complex diseases and understand the effect of life style interventions and environmental factors.

The liver as an immunological barrier redefined by single-cell analysis

The liver is a front-line immune tissue that plays a major role in the detection, capture and clearance of pathogens and foreign antigens entering the bloodstream, especially from the gut. Our largest internal organ maintains this immune barrier in the face of constant exposure to external but harmless antigens through a highly specialized network of liver-adapted immune cells. Mapping the immune resident compartment in the liver has been challenging because it requires multimodal single-cell deep phenotyping approaches of often rare cell populations in difficult to access samples. We can now measure the RNA transcripts present in a single cell (scRNA-seq), which is revolutionizing the way we characterize cell types. scRNA-seq has been applied to the diverse array of immune cells present in murine and human livers in health and disease. Here, we summarize how emerging single-cell technologies have advanced or redefined our understanding of the immunological barrier provided by the liver.

Macrophage Subsets in Obesity, Aligning the Liver and Adipose Tissue

The increasing prevalence of obesity is accompanied by a rising incidence in metabolic syndrome and related pathologies such as non-alcoholic fatty liver disease. Macrophages are hypothesized to play central roles in these diseases, through their role as inflammatory mediators and as such are thought to be potential targets for future therapies. Recently, single cell technologies have revealed significant heterogeneity within the macrophage pool in both liver and adipose tissue in obesity. Thus current efforts are focused on dissecting this heterogeneity and understanding the distinct functions of the individual subsets. In this review, we discuss the current knowledge regarding macrophage heterogeneity, ontogeny and functions in the context of obese adipose tissue and fatty liver disease and attempt to align the distinct populations described to date.

Macrophages in metabolic associated fatty liver disease

Metabolic associated fatty liver disease (MAFLD), formerly named non-alcoholic fatty liver disease is the most common liver disorder in many countries. The inflammatory subtype termed steatohepatitis is a driver of disease progression to cirrhosis, hepatocellular carcinoma, liver transplantation, and death, but also to extrahepatic complications including cardiovascular disease, diabetes and chronic kidney disease. The plasticity of macrophages in response to various environmental cues and the fact that they can orchestrate cross talk between different cellular players during disease development and progression render them an ideal target for drug development. This report reviews recent advances in our understanding of macrophage biology during the entire spectrum of MAFLD including steatosis, inflammation, fibrosis, and hepatocellular carcinoma, as well as for the extra-hepatic manifestations of MAFLD. We discuss the underlying molecular mechanisms of macrophage activation and polarization as well as cross talk with other cell types such as hepatocytes, hepatic stellate cells, and adipose tissue. We conclude with a discussion on the potential translational implications and challenges for macrophage based therapeutics for MAFLD.

The Medium-Chain Fatty Acid Receptor GPR84 Mediates Myeloid Cell Infiltration Promoting Steatohepatitis and Fibrosis

Medium-chain fatty acids (MCFAs) have been associated with anti-steatotic effects in hepatocytes. Expression of the MCFA receptor GPR84 (G protein-coupled receptor 84) is induced in immune cells under inflammatory conditions and can promote fibrogenesis. We aimed at deciphering the role of GPR84 in the pathogenesis of non-alcoholic steatohepatitis (NASH), exploring its potential as a therapeutic target. GPR84 expression is upregulated in liver from patients with non-alcoholic fatty liver disease (NAFLD), correlating with the histological degree of inflammation and fibrosis. In mouse and human, activated monocytes and neutrophils upregulate GPR84 expression. Chemotaxis of these myeloid cells by GPR84 stimulation is inhibited by two novel, small molecule GPR84 antagonists. Upon acute liver injury in mice, treatment with GPR84 antagonists significantly reduced the hepatic recruitment of neutrophils, monocytes, and monocyte-derived macrophages (MoMF). We, therefore, evaluated the therapeutic inhibition of GPR84 by these two novel antagonists in comparison to selonsertib, an apoptosis signal-regulating kinase 1 (ASK1) inhibitor, in three NASH mouse models. Pharmacological inhibition of GPR84 significantly reduced macrophage accumulation and ameliorated inflammation and fibrosis, to an extent similar to selonsertib. In conclusion, our findings support that GPR84 mediates myeloid cell infiltration in liver injury and is a promising therapeutic target in steatohepatitis and fibrosis.

A Single-Cell Perspective of the Mammalian Liver in Health and Disease

The liver contains heterogeneous cell types that exhibit distinct spatial, molecular, and functional properties. The cells in the liver communicate with each other via a network of ligands and receptors and undergo cell type-specific transcriptomic reprogramming in disease. The advance of single-cell genomics has provided a powerful tool for unraveling the complexity of liver cells in health and disease with unprecedented resolution. In this review, we discuss insights gained from the recent single-cell RNA sequencing studies that shed new light on the molecular nature of liver cell heterogeneity, intercellular crosstalk, and disease-associated reprogramming.

Recent Advances in Practical Methods for Liver Cell Biology: A Short Overview

Molecular and cellular research modalities for the study of liver pathologies have been tremendously improved over the recent decades. Advanced technologies offer novel opportunities to establish cell isolation techniques with excellent purity, paving the path for 2D and 3D microscopy and high-throughput assays (e.g., bulk or single-cell RNA sequencing). The use of stem cell and organoid research will help to decipher the pathophysiology of liver diseases and the interaction between various parenchymal and non-parenchymal liver cells. Furthermore, sophisticated animal models of liver disease allow for the in vivo assessment of fibrogenesis, portal hypertension and hepatocellular carcinoma (HCC) and for the preclinical testing of therapeutic strategies. The purpose of this review is to portray in detail novel in vitro and in vivo methods for the study of liver cell biology that had been presented at the workshop of the 8th meeting of the European Club for Liver Cell Biology (ECLCB-8) in October of 2018 in Bonn, Germany.

The role of the innate immune system in the development and treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver cancer. Most patients present with advanced or metastatic HCC at diagnosis and face a dismal prognosis. Tyrosine kinases are the gold standard treatment for this disease but yield limited survival benefits. Immune checkpoint inhibitors that augment adaptive immunity have been tested in HCC. Complex interactions between tumor cells, lymphocytes and the tumor environment determine the efficacy of such immunotherapies. Innate immune mechanisms – known drivers of liver disease progression in pre-HCC conditions such as fibrosis or cirrhosis – may either support or counteract tumor-related immune activation. In this review, we will highlight current concepts of the role of the innate immune system in hepatocarcinogenesis and discuss their relevance for translation into clinics.

Macrophage Phenotype and Function in Liver Disorder

Hepatic macrophages are a remarkably heterogeneous population consisting of self-renewing tissue-resident phagocytes, termed Kupffer cells (KCs), and recruited macrophages derived from peritoneal cavity as well as the bone marrow. KCs are located in the liver sinusoid where they scavenge the microbe from the portal vein to maintain liver homeostasis. Liver injury may trigger hepatic recruitment of peritoneal macrophages and monocyte-derived macrophages. Studies describing macrophage accumulation have shown that hepatic macrophages are involved in the initiation and progression of various liver diseases. They act as tolerogenic antigen-presenting cells to inhibit T-cell activation by producing distinct sets of cytokines, chemokines, and mediators to maintain or resolve inflammation. Furthermore, by releasing regenerative growth factors, matrix metalloproteinase arginase, they promote tissue repair. Recent experiments found that KCs and recruited macrophages may play different roles in the development of liver disease. Given that hepatic macrophages are considerably plastic populations, their phenotypes and functions are likely switching along disease progression. In this review, we summarize current knowledge about the role of tissue-resident macrophages and recruited macrophages in pathogenesis of alcoholic liver disease (ALD), non-alcoholic steatohepatitis (NASH), viral hepatitis, and hepatocellular carcinoma (HCC).

Pathogenesis of Nonalcoholic Steatohepatitis: An Overview

Nonalcoholic fatty liver disease (NAFLD) is a heterogeneous group of liver diseases characterized by the accumulation of fat in the liver. The heterogeneity of NAFLD is reflected in a clinical and histologic spectrum where some patients develop isolated steatosis of the liver, termed nonalcoholic fatty liver, whereas others develop hepatocyte injury, ballooning, inflammation, and consequent fibrosis, termed nonalcoholic steatohepatitis (NASH). Systemic insulin resistance is a major driver of hepatic steatosis in NAFLD. Lipotoxicity of accumulated lipids along with activation of the innate immune system are major drivers of NASH. Lipid‐induced sublethal and lethal stress culminates in the activation of inflammatory processes, such as the release of proinflammatory extracellular vesicles and cell death. Innate and adaptive immune mechanisms involving macrophages, dendritic cells, and lymphocytes are central drivers of inflammation that recognize damage‐ and pathogen‐associated molecular patterns and contribute to the progression of the inflammatory cascade. While the activation of the innate immune system and the recruitment of proinflammatory monocytes into the liver in NASH are well known, the exact signals that lead to this remain less well defined. Further, the contribution of other immune cell types, such as neutrophils and B cells, is an area of intense research. Many host factors, such as the microbiome and gut–liver axis, modify individual susceptibility to NASH. In this review, we discuss lipotoxicity, inflammation, and the contribution of interorgan crosstalk in NASH pathogenesis.

The immunobiology of hepatocellular carcinoma in humans and mice: Basic concepts and therapeutic implications

Basic and clinical studies have demonstrated the efficacy of immunotherapy, a technical and conceptual breakthrough that has revolutionised cancer treatment. Hepatocellular carcinoma (HCC), a deadly malignancy with aetiologic diversity and a chronic course, is strongly influenced by the immune system, and was recently found to partially benefit from immune-checkpoint inhibitor therapy. Notably, HCC onco-immunology depends on diverse genetic and environmental factors that together shape cancer-promoting inflammation and immune dysfunction - critical processes that control HCC malignant progression and response to therapy. Herein, we summarise the current understanding of liver and HCC onco-immunology obtained through basic studies with mouse models and clinical practice in humans. In particular, we discuss preclinical and clinical findings that implicate immunomodulation as a major factor in HCC development and explain the basis for HCC-targeting immunotherapy.

Chronic Inflammation in Non-Alcoholic Steatohepatitis: Molecular Mechanisms and Therapeutic Strategies

Non-Alcoholic Steatohepatitis (NASH) is the progressive form of Non-Alcoholic Fatty Liver Disease (NAFLD), the main cause of chronic liver complications. The development of NASH is the consequence of aberrant activation of hepatic conventional immune, parenchymal, and endothelial cells in response to inflammatory mediators from the liver, adipose tissue, and gut. Hepatocytes, Kupffer cells and liver sinusoidal endothelial cells contribute to the significant accumulation of bone-marrow derived-macrophages and neutrophils in the liver, a hallmark of NASH. The aberrant activation of these immune cells elicits harmful inflammation and liver injury, leading to NASH progression. In this review, we highlight the processes triggering the recruitment and/or activation of hepatic innate immune cells, with a focus on macrophages, neutrophils, and innate lymphoid cells as well as the contribution of hepatocytes and endothelial cells in driving liver inflammation/fibrosis. On-going studies and preliminary results from global and specific therapeutic strategies to manage this NASH-related inflammation will also be discussed.

Therapeutic Targeting of Hepatic Macrophages for the Treatment of Liver Diseases

Hepatic macrophages play a central role in maintaining homeostasis in the liver, as well as in the initiation and progression of liver diseases. Hepatic macrophages are mainly derived from resident hepatic macrophages called Kupffer cells or circulating bone marrow-derived monocytes. Kupffer cells are self-renewing and typically non-migrating macrophages in the liver and are stationed in the liver sinusoids in contrast to macrophages originating from circulating monocytes. Kupffer cells regulate liver homeostasis by mediating immunity against non-pathogenic blood-borne molecules, while participating in coordinated immune responses leading to pathogen clearance, leukocyte recruitment and antigen presentation to lymphocytes present in the vasculature. Monocyte-derived macrophages infiltrate into the liver tissue when metabolic or toxic damage instigates and are likely dispensable for replenishing the macrophage population in homeostasis. In recent years, different populations of hepatic macrophages have been identified with distinct phenotypes with discrete functions, far beyond the central dogma of M1 and M2 macrophages. Hepatic macrophages play a central role in the pathogenesis of acute and chronic liver failure, liver fibrosis, non-alcoholic fatty liver disease, alcoholic liver disease, viral hepatitis, and hepatocellular carcinoma, as well as in disease resolution. The understanding of the role of hepatic macrophages in liver diseases provides opportunities for the development of targeted therapeutics for respective malignancies. This review will summarize the current knowledge of the hepatic macrophages, their origin, functions, their critical role in maintaining homeostasis and in the progression or resolution of liver diseases. Furthermore, we will provide a comprehensive overview of the therapeutic targeting strategies against hepatic macrophages developed for the treatment of liver diseases.

The application of single-cell sequencing technology in the diagnosis and treatment of hepatocellular carcinoma

Single-cell sequencing technology refers to the sequencing of the genome, transcriptome and epigenome in one single cell. Comparing to traditional histology, single-cell sequencing can reveal the genetic heterogeneity among different cells. Due to the complex pathogenesis and various pathological types of hepatocellular carcinoma (HCC), studies on the heterogeneity of tumor cells confer improvement for its clinical diagnosis, treatment and prognosis. This article summarizes the principal basis and development of single-cell sequencing technology, as well as its increasing application in the field of HCC.

Chronic aerobic exercise improves insulin sensitivity and modulates Nrf2 and NF‑κB/IκBα pathways in the skeletal muscle of rats fed with a high fat diet

The present study aimed to investigate the molecular mechanisms of the ameliorative effects of chronic aerobic exercise on non-alcoholic steatohepatitis (NASH) in rat skeletal muscle. Female Sprague-Dawley rats (n=6–9 per group) were divided into four groups: i) Rats fed with normal chow; ii) exercise rats fed with normal chow + exercise (run on a rotarod for 30 min per day from 9–12 weeks); iii) rats fed with a high-fat diet (HFD); iv) rats fed with an HFD + exercise. All HFD rats were fed with an HFD consisting of 30% fat from fish oil throughout the study for 12 weeks. Exercise decreased the levels of hepatic lipogenic markers carbohydrate-responsive element-binding protein, fat-specific protein 27 and liver X receptor and improved systemic glucose and insulin intolerance in the NASH animal model. The beneficial effects may have been mediated partly via the tripartite motif-containing family protein 72 (TRIM72)/PI3K/Akt/mTOR pathway, accompanied with an upregulation of glucose transporter 4 in the skeletal muscle. The exercise regimen activated the master regulator of antioxidant enzymes, nuclear factor erythroid 2-related factor 2, with upregulation of superoxide dismutase [Cu-Zn] expression and a corresponding decrease in kelch-like ECH-associated protein 1 expression, but failed to decrease the levels of the oxidative marker malondialdehyde in the HFD rat skeletal muscle. Chronic exercise decreased the expression of the inflammation marker NF-κB, followed by a decrease in interleukin-6 and tumor necrosis factor-α levels, as verified by a corresponding increase in the level of NF-κB inhibitor α expression. Exercise may exert its beneficial effects by improving muscle insulin sensitivity via the TRIM72/PI3K/Akt/mTOR pathway, contributing to the improvement of systemic insulin intolerance, and finally leading to decreased hepatic lipogenesis during NASH. The attenuation of insulin resistance by exercise may be partly achieved through a decrease in the level of inflammation and an increased antioxidant response.

Unravelling fibrosis using single-cell transcriptomics

Fibrosis, the excessive accumulation of extracellular matrix, is a major global healthcare burden. Despite major advances in our understanding of the mechanisms regulating fibrosis, treatment options for patients with fibrosis remain very limited. However, recent developments in the rapidly evolving field of single-cell transcriptomics are enabling the interrogation of individual pathogenic cell populations in the context of fibrosis at unprecedented resolution. In this review, we will discuss how single-cell transcriptomics is driving this step change in our understanding of fibrotic disease pathogenesis, and how these cutting-edge approaches should accelerate the precise identification of novel, relevant and potentially druggable therapeutic targets to treat patients with fibrosis.

Liver X Receptors Regulate Cholesterol Metabolism and Immunity in Hepatic Nonparenchymal Cells

Excess dietary cholesterol intake and the dysregulation of cholesterol metabolism are associated with the pathogenesis and progression of nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, and fibrosis. Hepatic accumulation of free cholesterol induces activation of nonparenchymal cells, including Kupffer cells, macrophages, and hepatic stellate cells, which leads to persistent inflammation and fibrosis. The nuclear receptors liver X receptor α (LXRα) and LXRβ act as negative regulators of cholesterol metabolism through the induction of hepatocyte cholesterol catabolism, excretion, and the reverse cholesterol transport pathway. Additionally, LXRs exert an anti-inflammatory effect in immune cell types, such as macrophages. LXR activation suppresses acute hepatic inflammation that is mediated by Kupffer cells/macrophages. Acute liver injury, diet-induced steatohepatitis, and fibrosis are exacerbated by significant hepatic cholesterol accumulation and inflammation in LXR-deficient mice. Therefore, LXRs regulate hepatic lipid metabolism and immunity and they are potential therapeutic targets in the treatment of hepatic inflammation that is associated with cholesterol accumulation.

Liver DCs in health and disease

Hepatic dendritic cells represent a unique and multifaceted subset of antigen-presenting leukocytes that orchestrate specified immune responses in the liver. They are constantly exposed to antigens and signals derived not only from the hepatic microenvironment and the systemic circulation but also from the portal vein draining the gut and conveying food antigens as well as microbial compounds. Modulated by these various factors they shape intrahepatic immune responses during acute and chronic liver diseases, hepatocellular carcinoma and allograft tolerance as well as systemic responses to gut-derived components. Hence, hepatic DC are central targets to decipher and fine-tune innate and adaptive hepatic immune responses as well as tolerance. This review focuses on the origin of hepatic DC, the different DC subsets present in the liver and their functionality during different acute and chronic liver diseases in mice and men and will discuss potential DC directed therapeutic interventions in liver disease.

Phenotypical and Functional Polymorphism of Liver Resident Macrophages

Liver diseases are one of the main causes of mortality. In this regard, the development of new ways of reparative processes stimulation is relevant. Macrophages play a leading role in the regulation of liver homeostasis in physiological conditions and in pathology. In this regard, the development of new liver treatment methods is impossible without taking into account this cell population. Resident macrophages of the liver, Kupffer cells, represent a unique cell population, first of all, due to their development. Most of the liver macrophages belong to the self-sustaining macrophage cell population, whose origin is not bone marrow. In addition, Kupffer cells are involved in such processes as regulation of hepatocyte proliferation and apoptosis, remodeling of the intercellular matrix, lipid metabolism, protective function, etc. Such a broad spectrum of liver macrophage functions indicates their high functional plasticity. The review summarizes recent data on the development, phenotypic and functional plasticity, and participation in the reparative processes of liver macrophages: resident macrophages (Kupffer cells) and bone marrow-derived macrophages.

Immune Regulation of Metabolic Homeostasis by Helminths and Their Molecules

Since time immemorial, humans have coevolved with a wide variety of parasitic helminths that have contributed to shape their immune system. The recent eradication of helminth infections in modern societies has coincided with a spectacular rise in inflammatory metabolic diseases, such as obesity, nonalcoholic steatohepatitis, and type 2 diabetes. Landmark studies in the emerging field of immunometabolism have highlighted the central role of the immune system in regulating metabolic functions, notably in adipose tissue, liver, and the gut. In this review we discuss how helminths, which are among the strongest natural inducers of type 2 immunity, and some of their unique immunomodulatory molecules, may contribute to the maintenance of tissue-specific and whole-body metabolic homeostasis and protection against obesity-associated meta-inflammation.

Single Cell RNA Sequencing Identifies Subsets of Hepatic Stellate Cells and Myofibroblasts in Liver Fibrosis

Activation of hepatic stellate cells (HSCs) and their trans-differentiation towards collagen-secreting myofibroblasts (MFB) promote liver fibrosis progression. During chronic liver disease, resting HSCs become activated by inflammatory and injury signals. However, HSCs/MFB not only produce collagen, but also secrete cytokines, participate in metabolism, and have biomechanical properties. We herein aimed to characterize the heterogeneity of these liver mesenchymal cells by single cell RNA sequencing. In vivo resting HSCs or activated MFB were isolated from C57BL6/J mice challenged by carbon tetrachloride (CCl₄) intraperitoneally for 3 weeks to induce liver fibrosis and compared to in vitro cultivated MFB. While resting HSCs formed a homogenous population characterized by high platelet derived growth factor receptor β (PDGFRβ) expression, in vivo and in vitro activated MFB split into heterogeneous populations, characterized by α-smooth muscle actin (α-SMA), collagens, or immunological markers. S100 calcium binding protein A6 (S100A6) was a universal marker of activated MFB on both the gene and protein expression level. Compared to the heterogeneity of in vivo MFB, MFB in vitro sequentially and only transiently expressed marker genes, such as chemokines, during culture activation. Taken together, our data demonstrate the heterogeneity of HSCs and MFB, indicating the existence of functionally relevant subsets in hepatic fibrosis.