Niche-Specific Reprogramming of Epigenetic Landscapes Drives Myeloid Cell Diversity in Nonalcoholic Steatohepatitis

Epigenetics in metabolic dysfunction-associated steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) is a complex disease involving genetics, environment, and lifestyle, with the potential to progress to liver fibrosis, cirrhosis, and even hepatocellular carcinoma (HCC). Although the pathogenesis of MASH is not fully clear, increasing evidence has indicated that epigenetics plays an important role in the genesis and progression of MASH, during which, as drastic changes in metabolites, epigenetics undergo drastic changes. Roles of chromatin structure, chromatin accessibility, DNA methylation, histone modification, and non-coding RNAs were considered as bridges of pathogenic factors and MASH. In this review, the research progress on the epigenetics of MASH was summarized, and indepth research and therapeutic strategies based on epigenetics is expected to bring new hope to MASH patients.

Reviewing the function of macrophages in liver disease

INTRODUCTION

The liver is a central metabolic organ, but is also hosting a unique immune microenvironment to sustain homeostasis and proper defense measures against injury threats in healthy individuals. Liver macrophages, mostly represented by the tissue-resident Kupffer cells and bone marrow- or monocyte-derived macrophages, are intricately involved in various aspects of liver homeostasis and disease, including tissue injury, inflammation, fibrogenesis and repair mechanisms.

AREAS COVERED

We review recent findings on defining the liver macrophage landscape and their functions in liver diseases with the aim of highlighting potential targets for therapeutic interventions. A comprehensive literature search in PubMed and Google Scholar was conducted to identify relevant literature up to date.

EXPERT OPINION

Liver macrophages orchestrate key homeostatic and pathogenic processes in the liver. Thus, targeting liver macrophages represents an attractive strategy for drug development, e.g. to ameliorate liver inflammation, steatohepatitis or fibrosis. However, translation from fundamental research to therapies remains challenging due to the versatile nature of the liver macrophage compartment. Recent and major technical advances such as single-cell and spatially-resolved omics approaches deepened our understanding of macrophage biology at a molecular level. Yet, further studies are needed to identify suitable, etiology- and stage-dependent strategies for the treatment of liver diseases.

Sequential activation of transcription factors promotes liver regeneration through specific and developmental enhancers

The mammalian liver exhibits remarkable regenerative capabilities after injury or resection. Central to this process is the precise modulation of gene expression, driven by changes in chromatin structure and the temporal activation of distal regulatory elements. In this study, we integrated chromatin accessibility and transcriptomic data after partial hepatectomy in mice. We show that the expression of crucial regeneration genes is orchestrated by a diverse array of cis-regulatory elements, including regeneration-specific enhancers and enhancers repurposed from various developmental stages. These enhancers collaborate to activate the transcriptional programs required for hepatocyte priming and proliferation, with their activity initially regulated by the activator protein-1 (AP-1) complex and ATF3, and subsequently by nuclear factor erythroid 2 (NFE2)-related factor 2 (NRF2) during proliferation. Our results also indicate that hepatic regeneration involves the repression of enhancers regulating liver-specific metabolic functions, particularly those involved in lipid metabolism. This study provides a genome-wide atlas of enhancer-gene interactions, offering new insights into the regulatory mechanisms underlying liver regeneration.

Dynamic changes in macrophage populations and resulting alterations in Prostaglandin E2 sensitivity in mice with diet-induced MASH

BACKGROUND

The transition from metabolic dysfunction-associated steatotic liver disease (MASLD) to steatohepatitis (MASH) is characterized by a chronic low-grade inflammation, involving activation of resident macrophages (Kupffer cells; KC) and recruitment of infiltrating macrophages. Macrophages produce cytokines and, after induction of Cyclooxygenase 2 (COX-2), the key enzyme of prostanoid synthesis, prostaglandin E2 (PGE2). PGE2 modulates cytokine production in an autocrine and paracrine manner, therefore playing a pivotal role in regulating inflammatory processes. Changes in the hepatic macrophage pool during MASLD progression to MASH could influence PGE2- and cytokine-mediated signaling processes. The aim of this study was to characterize these changes in mice with diet-induced MASH and further elucidate the role of COX-2-dependently formed PGE2 on the inflammatory response in different macrophage populations of mice with a macrophage-specific COX-2-deletion.

METHODS

Male, 6-7-week-old wildtype mice were fed either a Standard or high-fat, high-cholesterol MASH-inducing diet for 4, 12 and 20 weeks. Liver macrophages were isolated and analyzed by flow cytometry. For in vitro experiments primary KC, peritoneal macrophages (PM) and Bone-marrow-derived macrophages (BMDM) were isolated from macrophage-specific COX-2-deficient and wildtype mice and treated with lipopolysaccharide (LPS) and/or PGE2.

RESULTS

During MASH-development, the proportion of KC (Clec4F+Tim4+) decreased, while the proportion of monocyte-derived macrophages (Clec4F-Tim4-) and monocyte-derived cells exhibiting a phenotype similar to KC (Clec4F+Tim4-) significantly increased over time. In vitro experiments showed that exogenous PGE2 completely abrogated the LPS-induced mRNA expression and secretion of tumor necrosis factor-alpha (TNF-α) in primary KC, PM and BMDM from wildtype mice. PM and BMDM, as in vitro models for infiltrating macrophages, were more sensitive to PGE2 compared to KC. Deletion of COX-2 in all macrophage populations led to an impaired PGE2-dependent feedback inhibition of TNF-α production. LPSinduced TNF-α mRNA expression was higher compared to the respective wildtype macrophage population.

CONCLUSION

The current study, using a murine MASH model, indicates that PGE2 may have a protective, anti-inflammatory effect, especially by inhibiting the expression of pro-inflammatory cytokines such as TNFα in infiltrating monocyte-derived macrophages. An inhibition of endogenous PGE2 synthesis in macrophages by pharmacological inhibition of COX-2 could potentially increase inflammation and promote the progression of MASH.

TREM2-expressing macrophages in liver diseases

Metabolic dysfunction-associated steatotic liver disease (MASLD) affects over 30% of the global population and spans a spectrum of liver abnormalities, including simple steatosis, inflammation, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Recent studies have identified triggering receptors expressed on myeloid cells 2 (TREM2)-expressing macrophages as key regulators of MASLD progression. TREM2 plays a pivotal role in regulating macrophage-mediated processes such as efferocytosis, inflammatory control, and fibrosis resolution. Additionally, soluble TREM2 (sTREM2) was proposed as a noninvasive biomarker for diagnosing and monitoring MASLD progression. However, the molecular mechanisms through which TREM2 influences MASLD pathogenesis remain incompletely understood. This review summarizes the current understanding of TREM2-expressing macrophages in MASLD, with the goal of illuminating future research and guiding the development of innovative therapeutic strategies targeting TREM2 signaling pathways.

GPNMB regulates the differentiation and transformation of monocyte-derived macrophages during MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD) is an increasingly concerning global health issue characterized by pronounced hepatic steatosis and liver fibrosis. Hepatic monocyte-derived macrophages (MDMs) are crucial in the pathogenesis of liver fibrosis under MASLD. Nevertheless, the precise functions of MDMs and the underlying mechanisms governing their differentiation remain inadequately elucidated. In this study, we revealed an orchestrator of this process: Glycoprotein Non-Metastatic Melanoma Protein B (GPNMB), one of the characteristic genes of MDMs. Notably, myeloid-specific Gpnmb-knockout contributed to the retention of resident Kupffer cells (KCs) and rerouted monocyte differentiation towards a monocyte-derived macrophage subset that occupies the Kupffer cell niche (MoKC subset, resembling resident KCs), thereby impeding the formation of hepatic lipid-associated macrophages (LAMs). This transition has a profound impact, manifested in significantly reduced steatosis and modestly decreased liver fibrosis in myeloid-specific Gpnmb-knockout mice. In conclusion, our research clarifies the complex interactions between Gpnmb and MDMs and underscores the therapeutic potential of targeting Gpnmb within MDMs to manage MASLD.

Cannabigerol Alleviates Liver Damage in Metabolic Dysfunction-Associated Steatohepatitis Female Mice via Inhibition of Transforming Growth Factor Beta 1

Background and Aims: Metabolic dysfunction-associated steatohepatitis (MASH), a progressive form of metabolic dysfunction-associated steatotic liver disease (MASLD), involves inflammation, fibrosis, steatosis, and oxidative stress. Previous research from our lab shows that cannabigerol (CBG) reduces inflammation and fibrosis in male MASH mice, but its effects in females remain unknown. Given immune cell population changes in MASLD patients, this study examines CBG's impact on methionine-choline deficient (MCD) diet-induced MASH in female mice. Methods: MCD-fed female mice are supplemented with two different doses for three weeks. Liver fibrosis, steatosis, oxidative stress, ductular reaction, and inflammation are assessed via Sirius Red, Oil Red O, immunohistochemistry, and immunofluorescence staining. Immune cell changes in non-parenchymal cells (NPCs) are analyzed via flow cytometry. Results: CBG treatment improves liver health by reducing leukocyte infiltration. Both CBG doses significantly decrease fibrosis, oxidative stress, ductular proliferation, and inflammation in MCD-fed mice, including monocyte and T lymphocyte reductions. Additionally, CBG downregulates mast cell activation, inhibiting transforming growth factor (TGF)-β1 release, thereby suppressing hepatic stellate cell activation. This reduces collagen deposition, fibrosis, and ductular proliferation. Conclusions: Our findings provide insights for pre-clinical and clinical research, highlighting CBG's potential therapeutic role and dosage considerations in mitigating liver fibrosis and inflammation in female patients.

Gold Nanoparticles (AuNPs) Coadministered with a β-Blocker Prevent Liver Fibrosis Caused by Ethanol and Methamphetamine in Rats by Downregulating the Expression of M2 Macrophages

Simultaneous abuse of ethanol and methamphetamine (METH) causes severe liver damage through oxidative stress and inflammation. This study evaluated the antifibrotic effects of gold nanoparticles (AuNPs) coadministered with the β-blocker carvedilol (CARV) against liver damage in rats. Male Wistar rats received 30% ethanol (7 g/kg) daily for 28 days, with METH (10 mg/kg) administered on the 22nd and 28th days. Liver damage was assessed using serum hepatic enzymes, glutathione (GSH) levels, malondialdehyde (MDA) formation, myeloperoxidase (MPO) inhibition, and histopathological analysis, including H&E, Picrosirius Red staining, immunofluorescence, and transmission electron microscopy. Cytokine levels were measured in liver tissue samples. In vitro, RAW 264.7 macrophages were induced to polarize into M1 and M2 phenotypes and cocultured with AuNPs + CARV-treated 3T3 cells, analyzed by rtPCR. AuNPs + CARV effectively protected the liver by modulating interactions between hepatic stellate cells (HSCs) and Kupffer cells, promoting an antifibrotic immune response driven by M1 macrophages. This was indicated by downregulation of profibrotic M2 macrophages and upregulation of M1 macrophages, shown by an increased CD86/CD163 ratio and reduced levels of IL-1β, TNF-α, TGFβ, AKT, and PI3K., pointing an attenuated liver inflammation. These results suggest that AuNPs combined with CARV could potentially serve as a therapy for alcohol and METH-induced liver fibrosis by targeting M2 macrophages.

Exploring the Roles of Liver X Receptors in Lipid Metabolism and Immunity in Atherosclerosis

Hypercholesterolemia causes atherosclerosis by inducing immune cell migration and chronic inflammation in arterial walls. Recent single-cell analyses reveal the presence of lipid-enriched foamy macrophages, as well as other macrophage subtypes, neutrophils, T cells, and B cells, in atherosclerotic plaques in both animal models and humans. These cells interact with each other and other cells, including non-immune cells such as endothelial cells and smooth muscle cells. They thereby regulate metabolic, inflammatory, phagocytic, and cell death processes, thus affecting the progression and stability of atherosclerotic plaques. The nuclear receptors liver X receptor (LXR)α and LXRβ are transcription factors that are activated by oxysterols and regulate lipid metabolism and immune responses. LXRs regulate cholesterol homeostasis by controlling cholesterol's transport, absorption, synthesis, and breakdown in the liver and intestine. LXRs are also highly expressed in tissue-resident and monocyte-derived macrophages and other immune cells, including both myeloid cells and lymphocytes, and they regulate both innate and adaptive immune responses. Interestingly, LXRs have immunosuppressive and immunoregulatory functions that are cell-type-dependent. In animal models of atherosclerosis, LXRs have been shown to be involved in both progression and regression phases. The pharmacological activation of LXR enhances cholesterol efflux from macrophages and promotes atherosclerosis progression. Deleting LXR in immune cells, especially myeloid cells, accelerates atherosclerosis by increasing monocyte migration, macrophage proliferation and activation, and neutrophil extracellular traps (NETs); furthermore, the deletion of hematopoietic LXRs impairs the regression of atherosclerotic plaques. Therefore, LXRs in immune cells may be a potent therapeutic target for atherosclerosis.

Particle uptake by macrophages triggers bifurcated transcriptional pathways that differentially regulate inflammation and lysosomal gene expression

Exposure to particles is a driver of several inflammatory diseases. Here, we investigated macrophage responses to monosodium urate crystals, calcium pyrophosphate crystals, aluminum salts, and silica nanoparticles. While each particle induced a distinct gene expression pattern, we identified a common inflammatory signature and acute activation of lysosomal acidification genes. Using monosodium urate crystals as a model, we demonstrated that this lysosomal gene program is regulated by a 5'-prime-AMP-activated protein kinase (AMPK)-dependent transcriptional network, including TFEB, TFE3, and the epigenetic regulators DNA methyl transferase 3a (DNMT3A) and DOT1L. This lysosomal acidification program operates in parallel with, but largely independently of, a JNK-AP-1-dependent network driving crystal-induced chemokine and cytokine expression. These findings reveal a bifurcation in pathways governing inflammatory and lysosomal responses, offering insights for treating particle-associated diseases.

The liver as a central "hub" of the immune system: pathophysiological implications

The purpose of this review is to describe the immune function of the liver, guiding the reader from the homeostatic tolerogenic status to the aberrant activation demonstrated in chronic liver disease. An extensive description of the pathways behind the inflammatory modulation of the healthy liver will be provided focusing on the complex immune cell network residing within the liver. The limit of tolerance will be presented in the context of organ transplantation, seizing the limits of homeostatic mechanisms that fail in accepting the graft, progressing eventually toward rejection. The triggers and mechanisms behind chronic activation in metabolic liver conditions and viral hepatitis will be discussed. The last part of the review will be dedicated to one of the greatest paradoxes for a tolerogenic organ, developing autoimmunity. Through the description of the three most common autoimmune liver diseases, the autoimmune reaction against hepatocytes and biliary epithelial cells will be dissected.

Kupffer cell and recruited macrophage heterogeneity orchestrate granuloma maturation and hepatic immunity in visceral leishmaniasis

In murine models of visceral leishmaniasis (VL), the parasitization of resident Kupffer cells (resKCs) drives early Leishmania infantum growth in the liver, leading to granuloma formation and subsequent parasite control. Using the chronic VL model, we demonstrate that polyclonal resKCs redistributed to form granulomas outside the sinusoids, creating an open sinusoidal niche that was gradually repopulated by monocyte-derived KCs (moKCs) acquiring a tissue specific, homeostatic profile. Early-stage granulomas predominantly consisted of CLEC4F+KCs. In contrast, late-stage granulomas led to remodeling of the sinusoidal network and contained monocyte-derived macrophages (momacs) along with KCs that downregulated CLEC4F, with both populations expressing iNOS and pro-inflammatory chemokines. During late-stage infection, parasites were largely confined to CLEC4F-KCs. Reduced monocyte recruitment and increased resKCs proliferation in infected Ccr2-/- mice impaired parasite control. These findings show that the ontogenic heterogeneity of granuloma macrophages is closely linked to granuloma maturation and the development of hepatic immunity in VL.

Single-cell RNA sequencing advances in revealing the development and progression of MASH: the identifications and interactions of non-parenchymal cells

Developing drugs for the treatment of Metabolic Associated Steatohepatitis (MASH) has always been a significant challenge. Researchers have been dedicated to exploring drugs and therapeutic strategies to alleviate disease progression, but treatments remain limited. This is partly due to the complexity of the pathophysiological processes, and inadequate knowledge of the cellular and molecular mechanisms in MASH. Especially, the liver non-parenchymal cells (NPCs) like Kupffer cells, hepatic stellate cells and sinusoidal endothelial cells which play critical roles in live function, immune responses, fibrosis and disease progression. Deciphering how these cells function in MASH, would help understand the pathophysiological processes and find potential drug targets. In recent years, new technologies have been developed for single-cell transcriptomic sequencing, making cell-specific transcriptome profiling a reality in healthy and diseased livers. In this review, we discussed how the use of single-cell transcriptomic sequencing provided us with an in-depth understanding of the heterogeneous, cellular interactions among non-parenchymal cells and tried to highlight recent discoveries in MASH by this technology. It is hoped that the summarized features and markers of various subclusters in this review could provide a technical reference for further experiments and a theoretical basis for clinical applications.

Myeloid cell path to malignancy: insights into liver cancer

Clinically approved treatments for advanced liver cancer often lack potency because of the heterogeneous characteristics of hepatocellular carcinoma (HCC). This complexity is largely driven by context-dependent inflammatory responses brought on by diverse etiologies, such as metabolic dysfunction-associated steatohepatitis (MASH), the genetic makeup of cancer cells, and the versatile adaptability of immune cells, such as myeloid cells. In this review, we discuss the evolutionary dynamics of the immune landscape, particularly that of liver-resident Kupffer cells (KCs), TREM2+, and SPP1+ macrophages with an active role during liver disease progression, which eventually fuels hepatocarcinogenesis. We highlight exploitable immunomodulatory avenues amenable to mitigate both the inherent pathological characteristics of liver cancers and the associated external factors that favor malignancy, paving a roadmap toward improving the management and therapeutic outcome for patients with HCC.

Metabolic-Dysfunction-Associated Steatotic Liver Disease: Molecular Mechanisms, Clinical Implications, and Emerging Therapeutic Strategies

Metabolic-dysfunction-associated steatotic liver disease (MASLD), previously known as non-alcoholic fatty liver disease (NAFLD), is a highly prevalent metabolic disorder characterized by hepatic steatosis in conjunction with at least one cardiometabolic risk factor, such as obesity, type 2 diabetes, hypertension, or dyslipidemia. As global rates of obesity and metabolic syndrome continue to rise, MASLD is becoming a major public health concern, with projections indicating a substantial increase in prevalence over the coming decades. The disease spectrum ranges from simple steatosis to metabolic-dysfunction-associated steatohepatitis (MASH), fibrosis, cirrhosis, and hepatocellular carcinoma, contributing to significant morbidity and mortality worldwide. This review delves into the molecular mechanisms driving MASLD pathogenesis, including dysregulation of lipid metabolism, chronic inflammation, oxidative stress, mitochondrial dysfunction, and gut microbiota alterations. Recent advances in research have highlighted the role of genetic and epigenetic factors in disease progression, as well as novel therapeutic targets such as peroxisome proliferator-activated receptors (PPARs), fibroblast growth factors, and thyroid hormone receptor beta agonists. Given the multifaceted nature of MASLD, a multidisciplinary approach integrating early diagnosis, molecular insights, lifestyle interventions, and personalized therapies is critical. This review underscores the urgent need for continued research into innovative treatment strategies and precision medicine approaches to halt MASLD progression and improve patient outcomes.

Regulation of Hepatic Stellate Cell Phenotypes in Metabolic Dysfunction-Associated Steatohepatitis

Hepatic stellate cells (HSCs) play a crucial role in the pathogenesis of liver fibrosis in metabolic dysfunction-associated steatohepatitis (MASH), a condition characterized by excessive fat accumulation in the hepatocytes, unrelated to alcohol consumption. In a healthy liver, HSCs are quiescent, store vitamin A, and function as pericytes. However, in response to liver injury and inflammation, HSCs become activated. In MASH, HSC activation is driven by metabolic stress, lipotoxicity, and chronic inflammation. Injured hepatocytes, recruited macrophage, capillarized sinusoidal endothelial cells, and permeable intestinal epithelium may each contribute to activating HSCS. This leads to a unique inflammatory environment that promotes fibrosis. MASH HSCs change their metabolism to favor glycolysis, glutaminolysis, and lactate generation. Activated HSCs transform into myofibroblast-like cells, producing excessive extracellular matrix components that result in fibrosis. In addition, HSCs in MASH have inflammatory and intermediate activated phenotypes. This fibrotic process is a key feature of MASH, which can lead to cirrhosis and liver cancer. Understanding the mechanisms of HSC activation and their role in MASH progression is essential for developing targeted therapies to treat and prevent liver fibrosis in affected individuals.

RNase T2 deficiency promotes TLR13-dependent replenishment of tissue-protective Kupffer cells

Lysosomal stress due to the accumulation of nucleic acids (NAs) activates endosomal TLRs in macrophages. Here, we show that lysosomal RNA stress, caused by the lack of RNase T2, induces macrophage accumulation in multiple organs such as the spleen and liver through TLR13 activation by microbiota-derived ribosomal RNAs. TLR13 triggered emergency myelopoiesis, increasing the number of myeloid progenitors in the bone marrow and spleen. Splenic macrophages continued to proliferate and mature into macrophages expressing the anti-inflammatory cytokine IL-10. In the liver, TLR13 activated monocytes/macrophages to proliferate and mature into monocyte-derived KCs (moKCs), in which, the liver X receptor (LXR) was activated. In accumulated moKCs, tissue clearance genes such as MerTK, AXL, and apoptosis inhibitor of macrophage (AIM) were highly expressed, while TLR-dependent production of proinflammatory cytokines was impaired. Consequently, Rnaset2-/- mice were resistant to acute liver injuries elicited by acetaminophen (APAP) and LPS with D-galactosamine. These findings suggest that TLR13 activated by lysosomal RNA stress promotes the replenishment of tissue-protective Kupffer cells.

Kupffer cell diversity maintains liver function in alcohol-associated liver disease

BACKGROUND AND AIMS

Liver macrophages are heterogeneous and play an important role in alcohol-associated liver disease (ALD) but there is limited understanding of the functions of specific macrophage subsets in the disease. We used a Western diet alcohol (WDA) mouse model of ALD to examine the hepatic myeloid cell compartment by single cell RNAseq and targeted KC ablation to understand the diversity and function of liver macrophages in ALD.

APPROACH AND RESULTS

In the WDA liver, KCs and infiltrating monocytes/macrophages each represented about 50% of the myeloid pool. Five major KC clusters all expressed genes associated with receptor-mediated endocytosis and lipid metabolism, but most were predicted to be noninflammatory and antifibrotic with 1 minor KC cluster having a proinflammatory and extracellular matrix degradation gene signature. Infiltrating monocyte/macrophage clusters, in contrast, were predicted to be proinflammatory and profibrotic. In vivo, diphtheria toxin-based selective KC ablation during alcohol exposure resulted in a liver failure phenotype with increases in PT/INR and bilirubin, loss of differentiated hepatocyte gene expression, and an increase in expression of hepatocyte progenitor markers such as EpCAM, CK7, and Igf2bp3. Gene set enrichment analysis of whole-liver RNAseq from the KC-ablated WDA mice showed a similar pattern as seen in human alcoholic hepatitis.

CONCLUSIONS

In this ALD model, KCs are anti-inflammatory and are critical for the maintenance of hepatocyte differentiation. Infiltrating monocytes/macrophages are largely proinflammatory and contribute more to liver fibrosis. Future targeting of specific macrophage subsets may provide new approaches to the treatment of liver failure and fibrosis in ALD.

Lipid droplet efferocytosis attenuates proinflammatory signaling in macrophages via TREM2- and MS4A7-dependent mechanisms

Metabolic dysfunction-associated steatohepatitis (MASH) is characterized by injury to steatotic hepatocytes that triggers the release of endogenous danger-associated molecular patterns. Recent work demonstrated that exposed lipid droplets (LDs) serve as a pathogenic signal that promotes monocyte infiltration and its maturation into triggering receptor expressed in myeloid cells 2 (TREM2+) macrophages in MASH liver. Here we explore the role of LD exposure in modulating inflammatory signaling in macrophages. We found that LD efferocytosis triggers a global transcriptional response and dampens pro-inflammatory signaling in macrophages. LD treatment attenuated NLRP3 inflammasome activation via mechanisms independent of lysosomal LD hydrolysis. While TREM2 was dispensable for LD efferocytosis by macrophages, it was required for the attenuation of proinflammatory signaling upon LD exposure. Additionally, MS4A7 downregulation contributes to LD efferocytosis-mediated dampening of inflammatory response. These results underscore the dual role of LD exposure in MASH liver by promoting monocyte infiltration and TREM2+ macrophage induction, while restraining proinflammatory response in macrophages.

Spatially restricted and ontogenically distinct hepatic macrophages are required for tissue repair

Our understanding of the functional heterogeneity of resident versus recruited macrophages in the diseased liver is limited. A population of recruited lipid-associated macrophages (LAMs) has been reported to populate the diseased liver alongside resident Kupffer cells (KCs). However, the precise roles of these distinct macrophage subsets remain elusive. Here, using proteogenomics, we have identified LAMs in multiple models of liver injury. Moreover, we found that this phenotype is not specific to recruited macrophages, as a subset of resident KCs can also adopt a LAM-like phenotype in the mouse and human liver. By combining genetic mouse models targeting the distinct populations, we determined that both recruited LAMs and resident LAM-like KCs play crucial roles in tissue repair. Specifically, triggering receptor expressed on myeloid cells 2 (TREM2) expression on either resident or recruited macrophages is required for the efficient clearance of dying cells, enhancing repair and preventing exacerbated fibrosis.

Transcriptional analysis of murine biliary atresia identifies macrophage heterogeneity and subset-specific macrophage functions

Introduction

Macrophages play an important role in disease progression of pediatric cholestatic liver disease, particularly biliary atresia (BA); however, the restorative versus pathogenic role for precise macrophage subsets remains poorly defined. We aimed to distinguish the transcriptional profiles and roles of defined macrophage subset(s) in murine BA.

Methods

We used multiparameter flow cytometry and RNA-sequencing analysis to profile recruited CD11bhiCD64+ hepatic macrophages by cell surface expression of MHCII and Ly6c in the Rhesus rotavirus (RRV)-induced murine model of BA versus saline controls. Modulation of macrophage numbers via intra-peritoneal injections of clodronate-loaded liposomes was performed to determine the association between macrophage numbers and histologic injury (Ishak score).

Results

Ly6c+ macrophages demonstrated the greatest increase in numbers and percent of total macrophages in murine BA versus saline controls whereas MHCII+ macrophages decreased. Transcriptional changes in murine BA MHCII+ macrophages included reduced expression of the Kupffer cell gene signature, lower expression of genes involved in homeostatic processes, and increased expression of genes involved in inflammatory processes. Ly6c+ macrophages in murine BA showed increased expression for Hif1a and other genes involved in the cellular response to hypoxia. Among all subsets, the number of Ly6c+ macrophages exhibited the strongest correlation with severity of histologic liver injury by Ishak score.

Conclusions

Our data identify specific pathways upregulated in Ly6c vs MHCII+ macrophage subsets in murine BA. Transcriptional similarities between murine BA and human cholestatic macrophages may enable translation of future mechanistic studies to new macrophage subset-specific therapies.

A mutation in LXRα uncovers a role for cholesterol sensing in limiting metabolic dysfunction-associated steatohepatitis

Liver x receptor alpha (LXRα) functions as an intracellular cholesterol sensor that regulates lipid metabolism at the transcriptional level in response to the direct binding of cholesterol derivatives. We have generated mice with a mutation in LXRα that reduces activity in response to endogenous cholesterol derived LXR ligands while still allowing transcriptional activation by synthetic agonists. The mutant LXRα functions as a dominant negative that shuts down cholesterol sensing. When fed a high fat, high cholesterol diet LXRα mutant mice rapidly develop pathologies associated with Metabolic Dysfunction-Associated Steatohepatitis (MASH) including ballooning hepatocytes, liver inflammation, and fibrosis. Strikingly LXRα mutant mice have decreased liver triglycerides but increased liver cholesterol. Therefore, elevated cholesterol in the liver may play a critical role in the development of MASH. Reengaging LXR signaling by treatment with synthetic agonist reverses MASH in LXRα mutant mice suggesting that LXRα normally functions to impede the development of liver disease.

Single-Cell RNA Sequencing Reveals Macrophage Dynamics During MASH in Leptin-Deficient Rats

Macrophages play important roles in metabolic dysfunction-associated steatohepatitis (MASH), an advanced and inflammatory stage of metabolic dysfunction-associated steatotic liver disease (MASLD). In humans and mice, the cellular heterogeneity and diverse function of hepatic macrophages in MASH have been investigated by single cell RNA sequencing (scRNA-seq). However, little is known about their roles in rats. Here, we collected liver tissues at the postnatal week 16, when our previously characterized Lep∆I14/∆I14 rats developed MASH phenotypes. By scRNA-seq, we found an increase in the number of macrophages and endothelial cells and a decrease in that of NK and B cells. Hepatic macrophages in rats underwent a unique M1 to M2 transition without expression of the classical markers such as Arg1 and Nos2, except for Cd163. Lipid-associated macrophages (LAMs) were increased, which could be detected by the antibody against Cd63. In the microenvironment, macrophages had an increased number of interactions with hepatocytes, myofibroblasts, T cells, neutrophils, and dendritic cells, while their interaction strengths remained unchanged. Finally, the macrophage migration inhibitory factor (MIF) pathway was identified as the top upregulated cell-communication pathway in MASH. In conclusion, we dissected hepatic macrophage dynamics during MASH at single cell resolution and provided fundamental tools for the investigation of MASH in rat models.

Innate immune cells link dietary cues to normal and abnormal metabolic regulation

A slew of common metabolic disorders, including type 2 diabetes, metabolic dysfunction-associated steatotic liver disease and steatohepatitis, are exponentially increasing in our sedentary and overfed society. While macronutrients directly impact metabolism and bioenergetics, new evidence implicates immune cells as critical sensors of nutritional cues and important regulators of metabolic homeostasis. A deeper interrogation of the intricate and multipartite interactions between dietary components, immune cells and metabolically active tissues is needed for a better understanding of metabolic regulation and development of new treatments for common metabolic diseases. Responding to macronutrients and micronutrients, immune cells play pivotal roles in interorgan communication between the microbiota, small intestine, metabolically active cells including hepatocytes and adipocytes, and the brain, which controls feeding behavior and energy expenditure. This Review focuses on the response of myeloid cells and innate lymphocytes to dietary cues, their cross-regulatory interactions and roles in normal and aberrant metabolic control.

Immunoregulation of Liver Fibrosis: New Opportunities for Antifibrotic Therapy

Liver fibrosis develops in response to chronic liver injury and is characterized by a sustained inflammatory response that leads to excessive collagen deposition by myofibroblasts. The fibrogenic response is governed by the release of inflammatory mediators from innate, adaptive, and innate-like lymphoid cells and from nonprofessional immune cells (i.e., epithelial cells, hepatic myofibroblasts, and liver sinusoidal endothelial cells). Upon removal of the underlying cause, liver fibrosis can resolve via activation of specific immune cell subsets. Despite major advances in the understanding of fibrosis pathogenesis, there is still no approved antifibrotic therapy. This review summarizes our current knowledge of the immune cell landscape and the inflammatory mechanisms underlying liver fibrosis progression and regression. We discuss how reprogramming immune cell phenotype, in particular through targeting selective inflammatory pathways or modulating cell-intrinsic metabolism, may be translated into antifibrogenic therapies.

Roles of Cellular Neighborhoods in Hepatocellular Carcinoma Pathogenesis

The development of hepatocellular carcinoma (HCC) involves an intricate interplay among various cell types within the liver. Unraveling the orchestration of these cells, particularly in the context of various etiologies, may hold the key to deciphering the underlying mechanisms of this complex disease. The advancement of single-cell and spatial technologies has revolutionized our ability to determine cellular neighborhoods and understand their crucial roles in disease pathogenesis. In this review, we highlight the current research landscape on cellular neighborhoods in chronic liver disease and HCC, as well as the emerging computational approaches applicable to delineate disease-associated cellular neighborhoods, which may offer insights into the molecular mechanisms underlying HCC pathogenesis and pave the way for effective disease interventions.

Immunologic role of macrophages in sepsis-induced acute liver injury

Sepsis-induced acute liver injury (SALI), a manifestation of sepsis multi-organ dysfunction syndrome, is associated with poor prognosis and high mortality. The diversity and plasticity of liver macrophage subpopulations explain their different functional responses in different liver diseases. Kupffer macrophages, liver capsular macrophages, and monocyte-derived macrophages are involved in pathogen recognition and clearance and in the regulation of inflammatory responses, exacerbating the progression of SALI through different pathways of pyroptosis, ferroptosis, and autophagy. Concurrently, they play an important role in maintaining hepatic homeostasis and in the injury and repair processes of SALI. Other macrophages are recruited to diseased tissues under pathological conditions and are polarized into various phenotypes (mainly M1 and M2 types) under the influence of signaling molecules, transcription factors, and metabolic reprogramming, thereby exerting different roles and functions. This review provides an overview of the immune role of macrophages in SALI and discusses the multiple roles of macrophages in liver injury and repair to provide a reference for future studies.

Hepatic Inactivation of Carnitine Palmitoyltransferase 1a Lowers Apolipoprotein B Containing Lipoproteins in Mice

Genome- and epigenome-wide association studies have associated variants and methylation status of carnitine palmitoyltransferase 1a (CPT1a) to reductions in very low-density lipoprotein (VLDL) cholesterol and triglyceride levels. We report significant associations between the presence of CPT1a SNPs and reductions in plasma cholesterol, as well as positive associations between hepatic Cpt1a expression and plasma cholesterol levels across inbred mouse strains. Mechanistic studies show that both wild type and human apolipoprotein B100 (apoB)-transgenic mice with liver-specific deletion of Cpt1a (LKO) display lower circulating apoB levels consistent with reduced LDL-cholesterol (LDL-C) and LDL particle number. Despite a reduction in steady-state plasma lipids, VLDL-triglyceride (VLDL-TG) and cholesterol (VLDL-C) secretion rates are increased, suggesting accelerated clearance of apoB-containing lipoproteins (apoB-LPs) in LKO mice. Mechanistic approaches show greater peroxisome proliferator activated receptor α (PPARα) signaling which favors enhanced lipoprotein lipase-mediated metabolism of apoB-LPs, including increases in ApoCII and ApoAIV and reductions in ApoCIII & Angptl3. These studies provide mechanistic insight linking genetic variants and methylation status of CPT1a to reductions in circulating apoB-LPs in humans.

HIGHLIGHTS

Loss-of-function SNPs in CPT1a associate with reductions in plasma cholesterol in humans Hepatic Cpt1a expression positively associates with plasma cholesterol levels across inbred strains of miceLiver-specific Cpt1a deficiency lowers circulating apoB, plasma cholesterol, LDL-C, and LDL particle numberCpt1a ablation activates PPARα and favors clearance of apoB-containing lipoproteins.

Chromatin accessibility: biological functions, molecular mechanisms and therapeutic application

The dynamic regulation of chromatin accessibility is one of the prominent characteristics of eukaryotic genome. The inaccessible regions are mainly located in heterochromatin, which is multilevel compressed and access restricted. The remaining accessible loci are generally located in the euchromatin, which have less nucleosome occupancy and higher regulatory activity. The opening of chromatin is the most important prerequisite for DNA transcription, replication, and damage repair, which is regulated by genetic, epigenetic, environmental, and other factors, playing a vital role in multiple biological progresses. Currently, based on the susceptibility difference of occupied or free DNA to enzymatic cleavage, solubility, methylation, and transposition, there are many methods to detect chromatin accessibility both in bulk and single-cell level. Through combining with high-throughput sequencing, the genome-wide chromatin accessibility landscape of many tissues and cells types also have been constructed. The chromatin accessibility feature is distinct in different tissues and biological states. Research on the regulation network of chromatin accessibility is crucial for uncovering the secret of various biological processes. In this review, we comprehensively introduced the major functions and mechanisms of chromatin accessibility variation in different physiological and pathological processes, meanwhile, the targeted therapies based on chromatin dynamics regulation are also summarized.

Immunological dynamics in MASH: from landscape analysis to therapeutic intervention

Metabolic dysfunction-associated steatohepatitis (MASH), previously known as nonalcoholic steatohepatitis (NASH), is a multifaceted liver disease characterized by inflammation and fibrosis that develops from simple steatosis. Immune and inflammatory pathways have a central role in the pathogenesis of MASH, yet, how to target immune pathways to treat MASH remains perplexed. This review emphasizes the intricate role that immune cells play in the etiology and pathophysiology of MASH and highlights their significance as targets for therapeutic approaches. It discusses both current strategies and novel therapies aimed at modulating the immune response in MASH. It also highlights challenges in liver-specific drug delivery, potential off-target effects, and difficulties in targeting diverse immune cell populations within the liver. This review is a comprehensive resource that integrates current knowledge with future perspectives in the evolving field of MASH, with the goal of driving forward progress in medical therapies designed to treat this complex liver disease.

Infection history imprints prolonged changes to the epigenome, transcriptome and function of Kupffer cells

BACKGROUND & AIMS

Liver macrophages fulfill various homeostatic functions and represent an essential line of defense against pathogenic insults. However, it remains unclear whether a history of infectious disease in the liver leads to long-term alterations to the liver macrophage compartment.

METHODS

We utilized a curable model of parasitic infection invoked by the protozoan parasite Trypanosoma brucei brucei to investigate whether infection history can durably reshape hepatic macrophage identity and function. Employing a combination of fate mapping, single-cell CITE-sequencing, single-nuclei multiome analysis, epigenomic analysis, and functional assays, we studied the alterations to the liver macrophage compartment during and after the resolution of infection.

RESULTS

We show that T. brucei brucei infection alters the composition of liver-resident macrophages, leading to the infiltration of monocytes that differentiate into various infection-associated macrophage populations with divergent transcriptomic profiles. Whereas infection-associated macrophages disappear post-resolution of infection, monocyte-derived macrophages engraft in the liver, assume a Kupffer cell (KC)-like profile and co-exist with embryonic KCs in the long-term. Remarkably, the prior exposure to infection imprinted an altered transcriptional program on post-resolution KCs that was underpinned by an epigenetic remodeling of KC chromatin landscapes and a shift in KC ontogeny, along with transcriptional and epigenetic alterations in their niche cells. This reprogramming altered KC functions and was associated with increased resilience to a subsequent bacterial infection.

CONCLUSION

Our study demonstrates that a prior exposure to a parasitic infection induces trained immunity in KCs, reshaping their identity and function in the long-term.

IMPACT AND IMPLICATIONS

Although the liver is frequently affected during infections, and despite housing a major population of resident macrophages known as Kupffer cells (KCs), it is currently unclear whether infections can durably alter KCs and their niche cells. Our study provides a comprehensive investigation into the long-term impact of a prior, cured parasitic infection, unveiling long-lasting ontogenic, epigenetic, transcriptomic and functional changes to KCs as well as KC niche cells, which may contribute to KC remodeling. Our data suggest that infection history may continuously reprogram KCs throughout life with potential implications for subsequent disease susceptibility in the liver, influencing preventive and therapeutic approaches.

Mouse and human macrophages and their roles in cardiovascular health and disease

The past 15 years have witnessed a leap in understanding the life cycle, gene expression profiles, origins and functions of mouse macrophages in many tissues, including macrophages of the artery wall and heart that have critical roles in cardiovascular health. Here, we review the phenotypical and functional diversity of macrophage populations in multiple organs and discuss the roles that proliferation, survival, and recruitment and replenishment from monocytes have in maintaining macrophages in homeostasis and inflammatory states such as atherosclerosis and myocardial infarction. We also introduce emerging data that better characterize the life cycle and phenotypic profiles of human macrophages. We discuss the similarities and differences between murine and human macrophages, raising the possibility that tissue-resident macrophages in humans may rely more on bone marrow-derived monocytes than in mouse.

Biological and clinical role of TREM2 in liver diseases

Liver diseases constitute a major health burden worldwide, accounting for more than 4% of all disease-related mortalities. While the incidence of viral hepatitis is expected to decrease, metabolic liver disorders are increasingly diagnosed. Liver pathology is diverse, with functional and molecular alterations in both parenchymal and mesenchymal cells, including immune cells. Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor of the immunoglobulin superfamily and mainly expressed on myeloid cells. Several studies have demonstrated that TREM2 plays a critical role in tissue physiology and various pathological conditions. TREM2 is recognized as being associated with the development of liver diseases by regulating tissue homeostasis and the immune microenvironment. The biological and clinical impact of TREM2 is complex, given its diverse context-dependent functions. This review aims to summarize recent progress in understanding the association between TREM2 and different liver disorders and shed light on the clinical significance of targeting TREM2.

Eosinophil specialization is regulated by exposure to the esophageal epithelial microenvironment

Distinct subsets of eosinophils are reported in inflammatory and healthy tissues, yet the functions of uniquely specialized eosinophils and the signals that elicit them, particularly in eosinophilic esophagitis, are not well understood. Herein, we report an ex vivo system wherein freshly isolated human eosinophils were cocultured with esophageal epithelial cells and disease-relevant proinflammatory (IL-13) or profibrotic (TGF-β) cytokines. Compared with untreated cocultures, IL-13 increased expression of CD69 on eosinophils, whereas TGF-β increased expression of CD81, CD62L, and CD25. Eosinophils from IL-13-treated cocultures demonstrated increased secretion of GRO-α, IL-8, and macrophage colony-stimulating factor and also generated increased extracellular peroxidase activity following activation. Eosinophils from TGF-β-treated cocultures secreted increased IL-6 and exhibited increased chemotactic response to CCL11 compared with eosinophils from untreated or IL-13-treated coculture conditions. When eosinophils from TGF-β-treated cocultures were cultured with fibroblasts, they upregulated SERPINE1 expression and fibronectin secretion by fibroblasts compared with eosinophils that were cultured with granulocyte macrophage colony-stimulating factor alone. Translational studies revealed that CD62L was heterogeneously expressed by eosinophils in patient biopsy specimens. Our results demonstrate that disease-relevant proinflammatory and profibrotic signals present in the esophagus of patients with eosinophilic esophagitis cause distinct profiles of eosinophil activation and gene expression.

Understanding the complex macrophage landscape in MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD) represents a spectrum of disease states ranging from simple steatosis to metabolic dysfunction-associated steatohepatitis (MASH), which can eventually lead to the development of cirrhosis and hepatocellular carcinoma. Macrophages have long been implicated in driving the progression from steatosis to end-stage disease, yet we still know relatively little about the precise involvement of these cells in MASLD progression and/or regression. Rather, there are a considerable number of conflicting reports regarding the precise roles of these cells. This confusion stems from the fact that, until recently, macrophages in the liver were considered a homogenous population. However, thanks to recent technological advances including multi-parameter flow cytometry, single-cell RNA sequencing and spatial proteogenomics, we now know that this is not the case. Rather hepatic macrophages, even in the healthy liver, are heterogenous, existing in multiple subsets with distinct transcriptional profiles and hence likely functions. This heterogeneity is even more prominent in MASLD, where the macrophage pool consists of multiple different subsets of resident and recruited cells. To probe the unique functions of these cells and determine if targeting macrophages may be a viable therapeutic strategy in MASLD, we first need to unravel this complexity and decipher which populations and/or activation states are present and what functions each of these may play in driving MASLD progression. In this review, we summarise recent advances in the field, highlighting what is currently known about the hepatic macrophage landscape in MASLD and the questions that remain to be tackled.

In-Depth Proteomic Analysis Reveals Phenotypic Diversity of Macrophages in Liver Fibrosis

Macrophages make up a heterogeneous population of immune cells that exhibit diverse phenotypes and functions in health and disease. Although macrophage epigenomic and transcriptomic profiles have been reported, the proteomes of distinct macrophage populations under various pathological conditions remain largely elusive. Here, we employed a label-free proteomic approach to characterize the diversity of the hepatic macrophage pool in an experimental model of CCl4-induced liver fibrosis. We found a decrease in the proportion of liver resident embryo-derived KCs (EmKCs), and a drastic increase in the proportion of monocyte-derived KCs (MoKCs) and CLEC2-Macs. Proteomic profiling revealed that MoKCs largely resembled EmKCs, whereas CLEC2-Macs exhibited greater proteomic alternations compared with EmKCs, suggesting two distinct destinations for monocyte differentiation during liver fibrosis. Furthermore, CLEC2-Macs were characterized by increased expression of proteins associated with inflammatory response, antigen processing and presentation processes, which may be involved in the pathogenesis of liver fibrosis. Collectively, our study provides insights into the considerable heterogeneity within the hepatic macrophage pool during liver fibrosis.

Lipid-associated macrophages between aggravation and alleviation of metabolic diseases

Lipid-associated macrophages (LAMs) are phagocytic cells with lipid-handling capacity identified in various metabolic derangements. During disease development, they locate to atherosclerotic plaques, adipose tissue (AT) of individuals with obesity, liver lesions in steatosis and steatohepatitis, and the intestinal lamina propria. LAMs can also emerge in the metabolically demanding microenvironment of certain tumors. In this review, we discuss major questions regarding LAM recruitment, differentiation, and self-renewal, and, ultimately, their acute and chronic functional impact on the development of metabolic diseases. Further studies need to clarify whether and under which circumstances LAMs drive disease progression or resolution and how their phenotype can be modulated to ameliorate metabolic disorders.

Identification and validation of efferocytosis-related biomarkers for the diagnosis of metabolic dysfunction-associated steatohepatitis based on bioinformatics analysis and machine learning

Background

Metabolic dysfunction-associated steatohepatitis (MASH) is a highly prevalent liver disease globally, with a significant risk of progressing to cirrhosis and even liver cancer. Efferocytosis, a process implicated in a broad spectrum of chronic inflammatory disorders, has been reported to be associated with the pathogenesis of MASH; however, its precise role remains obscure. Thus, we aimed to identify and validate efferocytosis linked signatures for detection of MASH.

Methods

We retrieved gene expression patterns of MASH from the GEO database and then focused on assessing the differential expression of efferocytosis-related genes (EFRGs) between MASH and control groups. This analysis was followed by a series of in-depth investigations, including protein-protein interaction (PPI), correlation analysis, and functional enrichment analysis, to uncover the molecular interactions and pathways at play. To screen for biomarkers for diagnosis, we applied machine learning algorithm to identify hub genes and constructed a clinical predictive model. Additionally, we conducted immune infiltration and single-cell transcriptome analyses in both MASH and control samples, providing insights into the immune cell landscape and cellular heterogeneity in these conditions.

Results

This research pinpointed 39 genes exhibiting a robust correlation with efferocytosis in MASH. Among these, five potential diagnostic biomarkers-TREM2, TIMD4, STAB1, C1QC, and DYNLT1-were screened using two distinct machine learning models. Subsequent external validation and animal experimentation validated the upregulation of TREM2 and downregulation of TIMD4 in MASH samples. Notably, both TREM2 and TIMD4 demonstrated area under the curve (AUC) values exceeding 0.9, underscoring their significant potential in facilitating the diagnosis of MASH.

Conclusion

Our study comprehensively elucidated the relationship between MASH and efferocytosis, constructing a favorable diagnostic model. Furthermore, we identified potential therapeutic targets for MASH treatment and offered novel insights into unraveling the underlying mechanisms of this disease.

Notch signaling regulates macrophage-mediated inflammation in metabolic dysfunction-associated steatotic liver disease

The liver macrophage population comprises resident Kupffer cells (KCs) and monocyte-derived macrophages with distinct pro- or anti-inflammatory properties that affect the severity and course of liver diseases. The mechanisms underlying macrophage differentiation and functions in metabolic dysfunction-associated steatotic liver disease and/or steatohepatitis (MASLD/MASH) remain mostly unknown. Using single-cell RNA sequencing (scRNA-seq) and fate mapping of hepatic macrophage subpopulations, we unraveled the temporal and spatial dynamics of distinct monocyte and monocyte-derived macrophage subsets in MASH. We revealed a crucial role for the Notch-Recombination signal binding protein for immunoglobulin kappa J region (RBPJ) signaling pathway in controlling the monocyte-to-macrophage transition, with Rbpj deficiency blunting inflammatory macrophages and monocyte-derived KC differentiation and conversely promoting the emergence of protective Ly6Clo monocytes. Mechanistically, Rbpj deficiency promoted lipid uptake driven by elevated CD36 expression in Ly6Clo monocytes, enhancing their protective interactions with endothelial cells. Our findings uncover the crucial role of Notch-RBPJ signaling in monocyte-to-macrophage transition and will aid in the design of therapeutic strategies for MASH treatment.

Immunological mechanisms in steatotic liver diseases: An overview and clinical perspectives

Steatotic liver diseases (SLD) are the principal worldwide cause of cirrhosis and end-stage liver cancer, affecting nearly a quarter of the global population. SLD includes metabolic dysfunction-associated alcoholic liver disease (MetALD) and metabolic dysfunction-associated steatotic liver disease (MASLD), resulting in asymptomatic liver steatosis, fibrosis, cirrhosis and associated complications. The immune processes include gut dysbiosis, adiposeliver organ crosstalk, hepatocyte death and immune cell-mediated inflammatory processes. Notably, various immune cells such as B cells, plasma cells, dendritic cells, conventional CD4+ and CD8+ T cells, innate-like T cells, platelets, neutrophils and macrophages play vital roles in the development of MetALD and MASLD. Immunological modulations targeting hepatocyte death, inflammatory reactions and gut microbiome include N-acetylcysteine, selonsertib, F-652, prednisone, pentoxifylline, anakinra, JKB-121, HA35, obeticholic acid, probiotics, prebiotics, antibiotics and fecal microbiota transplantation. Understanding the immunological mechanisms underlying SLD is crucial for advancing clinical therapeutic strategies.

Loss of embryonically-derived Kupffer cells during hypercholesterolemia accelerates atherosclerosis development

Hypercholesterolemia is a major risk factor for atherosclerosis and associated cardiovascular diseases. The liver plays a key role in the regulation of plasma cholesterol levels and hosts a large population of tissue-resident macrophages known as Kupffer cells (KCs). KCs are located in the hepatic sinusoids where they ensure key functions including blood immune surveillance. However, how KCs homeostasis is affected by the build-up of cholesterol-rich lipoproteins that occurs in the circulation during hypercholesterolemia remains unknown. Here, we show that embryo-derived KCs (EmKCs) accumulate large amounts of lipoprotein-derived cholesterol, in part through the scavenger receptor CD36, and massively expand early after the induction of hypercholesterolemia. After this rapid adaptive response, EmKCs exhibit mitochondrial oxidative stress and their numbers gradually diminish while monocyte-derived KCs (MoKCs) with reduced cholesterol-loading capacities seed the KC pool. Decreased proportion of EmKCs in the KC pool enhances liver cholesterol content and exacerbates hypercholesterolemia, leading to accelerated atherosclerotic plaque development. Together, our data reveal that KC homeostasis is perturbed during hypercholesterolemia, which in turn alters the control of plasma cholesterol levels and increases atherosclerosis.

HIF-2α drives hepatic Kupffer cell death and proinflammatory recruited macrophage activation in nonalcoholic steatohepatitis

Proinflammatory hepatic macrophage activation plays a key role in the development of nonalcoholic steatohepatitis (NASH). This involves increased embryonic hepatic Kupffer cell (KC) death, facilitating the replacement of KCs with bone marrow-derived recruited hepatic macrophages (RHMs) that highly express proinflammatory genes. Moreover, phago/efferocytic activity of KCs is diminished in NASH, enhancing liver inflammation. However, the molecular mechanisms underlying these changes in KCs are not known. Here, we show that hypoxia-inducible factor 2α (HIF-2α) mediates NASH-associated decreased KC growth and efferocytosis by enhancing lysosomal stress. At the molecular level, HIF-2α stimulated mammalian target of rapamycin (mTOR)- and extracellular signal-regulated kinase-dependent inhibitory transcription factor EB (TFEB) phosphorylation, leading to decreased lysosomal and phagocytic gene expression. With increased metabolic stress and phago/efferocytic burden in NASH, these changes were sufficient to increase lysosomal stress, causing decreased efferocytosis and lysosomal cell death. Of interest, HIF-2α-dependent TFEB regulation only occurred in KCs but not RHMs. Instead, in RHMs, HIF-2α promoted mitochondrial reactive oxygen species production and proinflammatory activation by increasing ANT2 expression and mitochondrial permeability transition. Consequently, myeloid lineage-specific or KC-specific HIF-2α depletion or the inhibition of mTOR-dependent TFEB inhibition using antisense oligonucleotide treatment protected against the development of NASH in mice. Moreover, treatment with an HIF-2α-specific inhibitor reduced inflammatory and fibrogenic gene expression in human liver spheroids cultured under a NASH-like condition. Together, our results suggest that macrophage subtype-specific effects of HIF-2α collectively contribute to the proinflammatory activation of liver macrophages, leading to the development of NASH.

Identification of neutrophil extracellular trap-related biomarkers in non-alcoholic fatty liver disease through machine learning and single-cell analysis

Non-alcoholic Fatty Liver Disease (NAFLD), noted for its widespread prevalence among adults, has become the leading chronic liver condition globally. Simultaneously, the annual disease burden, particularly liver cirrhosis caused by NAFLD, has increased significantly. Neutrophil Extracellular Traps (NETs) play a crucial role in the progression of this disease and are key to the pathogenesis of NAFLD. However, research into the specific roles of NETs-related genes in NAFLD is still a field requiring thorough investigation. Utilizing techniques like AddModuleScore, ssGSEA, and WGCNA, our team conducted gene screening to identify the genes linked to NETs in both single-cell and bulk transcriptomics. Using algorithms including Random Forest, Support Vector Machine, Least Absolute Shrinkage, and Selection Operator, we identified ZFP36L2 and PHLDA1 as key hub genes. The pivotal role of these genes in NAFLD diagnosis was confirmed using the training dataset GSE164760. This study identified 116 genes linked to NETs across single-cell and bulk transcriptomic analyses. These genes demonstrated enrichment in immune and metabolic pathways. Additionally, two NETs-related hub genes, PHLDA1 and ZFP36L2, were selected through machine learning for integration into a prognostic model. These hub genes play roles in inflammatory and metabolic processes. scRNA-seq results showed variations in cellular communication among cells with different expression patterns of these key genes. In conclusion, this study explored the molecular characteristics of NETs-associated genes in NAFLD. It identified two potential biomarkers and analyzed their roles in the hepatic microenvironment. These discoveries could aid in NAFLD diagnosis and management, with the ultimate goal of enhancing patient outcomes.

Targeting hepatic macrophages for non-alcoholic fatty liver disease therapy

Non-alcoholic fatty liver disease (NAFLD) and its more advanced form, non-alcoholic steatohepatitis (NASH), have become global health challenges with significant morbidity and mortality rates. NAFLD encompasses several liver diseases, ranging from simple steatosis to more severe inflammatory and fibrotic forms. Ultimately, this can lead to liver cirrhosis and hepatocellular carcinoma. The intricate role of hepatic macrophages, particularly Kupffer cells (KCs) and monocyte-derived macrophages (MoMFs), in the pathogenesis of NAFLD and NASH, has received increasing attention. Hepatic macrophages can interact with hepatocytes, hepatic stellate cells, and endothelial cells, playing a crucial role in maintaining homeostasis. Paradoxically, they also participate in the pathogenesis of some liver diseases. This review highlights the fundamental role of hepatic macrophages in the pathogenesis of NAFLD and NASH, emphasizing their plasticity and contribution to inflammation and fibrosis, and hopes to provide ideas for subsequent experimental research and clinical treatment.

Unravelling monocyte functions: from the guardians of health to the regulators of disease

Monocytes are a key component of the innate immune system. They undergo intricate developmental processes within the bone marrow, leading to diverse monocyte subsets in the circulation. In a state of healthy homeostasis, monocytes are continuously released into the bloodstream, destined to repopulate specific tissue-resident macrophage pools where they fulfil tissue-specific functions. However, under pathological conditions monocytes adopt various phenotypes to resolve inflammation and return to a healthy physiological state. This review explores the nuanced developmental pathways and functional roles that monocytes perform, shedding light on their significance in both physiological and pathological contexts.

Lipid-associated macrophages' promotion of fibrosis resolution during MASH regression requires TREM2

While macrophage heterogeneity during metabolic dysfunction-associated steatohepatitis (MASH) has been described, the fate of these macrophages during MASH regression is poorly understood. Comparing macrophage heterogeneity during MASH progression vs regression, we identified specific macrophage subpopulations that are critical for MASH/fibrosis resolution. We elucidated the restorative pathways and gene signatures that define regression-associated macrophages and establish the importance of TREM2+ macrophages during MASH regression. Liver-resident Kupffer cells are lost during MASH and are replaced by four distinct monocyte-derived macrophage subpopulations. Trem2 is expressed in two macrophage subpopulations: i) monocyte-derived macrophages occupying the Kupffer cell niche (MoKC) and ii) lipid-associated macrophages (LAM). In regression livers, no new transcriptionally distinct macrophage subpopulation emerged. However, the relative macrophage composition changed during regression compared to MASH. While MoKC was the major macrophage subpopulation during MASH, they decreased during regression. LAM was the dominant macrophage subtype during MASH regression and maintained Trem2 expression. Both MoKC and LAM were enriched in disease-resolving pathways. Absence of TREM2 restricted the emergence of LAMs and formation of hepatic crown-like structures. TREM2+ macrophages are functionally important not only for restricting MASH-fibrosis progression but also for effective regression of inflammation and fibrosis. TREM2+ macrophages are superior collagen degraders. Lack of TREM2+ macrophages also prevented elimination of hepatic steatosis and inactivation of HSC during regression, indicating their significance in metabolic coordination with other cell types in the liver. TREM2 imparts this protective effect through multifactorial mechanisms, including improved phagocytosis, lipid handling, and collagen degradation.

Reactive oxygen species regulation by NCF1 governs ferroptosis susceptibility of Kupffer cells to MASH

Impaired self-renewal of Kupffer cells (KCs) leads to inflammation in metabolic dysfunction-associated steatohepatitis (MASH). Here, we identify neutrophil cytosolic factor 1 (NCF1) as a critical regulator of iron homeostasis in KCs. NCF1 is upregulated in liver macrophages and dendritic cells in humans with metabolic dysfunction-associated steatotic liver disease and in MASH mice. Macrophage NCF1, but not dendritic cell NCF1, triggers KC iron overload, ferroptosis, and monocyte-derived macrophage infiltration, thus aggravating MASH progression. Mechanistically, elevated oxidized phospholipids induced by macrophage NCF1 promote Toll-like receptor (TLR4)-dependent hepatocyte hepcidin production, leading to increased KC iron deposition and subsequent KC ferroptosis. Importantly, the human low-functional polymorphic variant NCF190H alleviates KC ferroptosis and MASH in mice. In conclusion, macrophage NCF1 impairs iron homeostasis in KCs by oxidizing phospholipids, triggering hepatocyte hepcidin release and KC ferroptosis in MASH, highlighting NCF1 as a therapeutic target for improving KC fate and limiting MASH progression.

Role of the type 3 cytokines IL-17 and IL-22 in modulating metabolic dysfunction-associated steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) comprises a spectrum of liver diseases that span simple steatosis, metabolic dysfunction-associated steatohepatitis (MASH) and fibrosis and may progress to cirrhosis and cancer. The pathogenesis of MASLD is multifactorial and is driven by environmental, genetic, metabolic and immune factors. This review will focus on the role of the type 3 cytokines IL-17 and IL-22 in MASLD pathogenesis and progression. IL-17 and IL-22 are produced by similar adaptive and innate immune cells such as Th17 and innate lymphoid cells, respectively. IL-17-related signaling is upregulated during MASLD resulting in increased chemokines and proinflammatory cytokines in the liver microenvironment, enhanced recruitment of myeloid cells and T cells leading to exacerbation of inflammation and liver disease progression. IL-17 may also act directly by activating hepatic stellate cells resulting in increased fibrosis. In contrast, IL-22 is a pleiotropic cytokine with a dominantly protective signature in MASLD and is currently being tested as a therapeutic strategy. IL-22 also exhibits beneficial metabolic effects and abrogates MASH-related inflammation and fibrosis development via inducing the production of anti-oxidants and anti-apoptotic factors. A sex-dependent effect has been attributed to both cytokines, most importantly to IL-22 in MASLD or related conditions. Altogether, IL-17 and IL-22 are key effectors in MASLD pathogenesis and progression. We will review the role of these two cytokines and cells that produce them in the development of MASLD, their interaction with host factors driving MASLD including sexual dimorphism, and their potential therapeutic benefits.

Bardoxolone methyl prevents metabolic dysfunction-associated steatohepatitis by inhibiting macrophage infiltration

BACKGROUND AND PURPOSE

Bardoxolone methyl (2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid methyl ester, CDDO-Me) is a potent activator of nuclear factor erythroid 2-related factor 2 (Nrf2), which induces the expression of antioxidative-associated genes. CDDO-Me exerts protective effects against chronic inflammatory diseases in the kidneys and lungs. However, its pharmacological effects on metabolic dysfunction-associated steatohepatitis (MASH) caused by fat accumulation remain unknown. In this study, we examined the hepatoprotective effects of CDDO-Me in a diet-induced MASH mouse model and elucidated its pharmacological mechanisms using RNA-seq analysis.

EXPERIMENTAL APPROACH

CDDO-Me was orally administered to mice fed a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD), and histological, biochemical, and transcriptomic analyses were performed on livers of mice that developed MASH.

KEY RESULTS

CDDO-Me administration induced the expression of antioxidant genes and cholesterol transporters downstream of Nrf2 and significantly prevented the symptoms of MASH. Whole-transcriptome analysis revealed that CDDO-Me inhibited the inflammatory pathway that led to phagocyte recruitment, in addition to activating the Nrf2-dependent pathway. Among inflammatory pathways, CC chemokine ligands (CCL)3 and CCL4, which are downstream of NF-κB and are associated with the recruitment of macrophages expressing CC chemokine receptors (CCR)1 and CCR5, were released into the blood in MASH mice. However, CDDO-Me directly inhibited the expression of CCL3-CCR1 and CCL4-CCR5 in macrophages.

CONCLUSIONS AND IMPLICATIONS

Overall, we revealed the potent hepatoprotective effect of CDDO-Me in a MASH mouse model and demonstrated that its pharmacological effects were closely associated with a reduction of macrophage infiltration, through CCL3-CCR1 and CCL4-CCR5 inhibition, in addition to Nrf2-mediated hepatoprotective effects.

Emerging concepts in alcohol, infection & immunity: A summary of the 2023 alcohol and immunology research interest group (AIRIG) meeting

On December 8th 2023, the annual Alcohol and Immunology Research Interest Group (AIRIG) meeting was held at the University of Colorado Anschutz Medical Campus in Aurora, Colorado. The 2023 meeting focused broadly on how acute and chronic alcohol exposure leads to immune dysregulation, and how this contributes to damage in multiple tissues and organs. These include impaired lung immunity, intestinal dysfunction, autoimmunity, the gut-Central Nervous System (CNS) axis, and end-organ damage. In addition, diverse areas of alcohol research covered multiple pathways behind alcohol-induced cellular dysfunction, including inflammasome activation, changes in miRNA expression, mitochondrial metabolism, gene regulation, and transcriptomics. Finally, the work presented at this meeting highlighted novel biomarkers and therapeutic interventions for patients suffering from alcohol-induced organ damage.