Soluble TREM2 levels reflect the recruitment and expansion of TREM2+ macrophages that localize to fibrotic areas and limit NASH

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.

Blockade of TREM2 ameliorates pulmonary inflammation and fibrosis by modulating sphingolipid metabolism

Pulmonary fibrosis is a chronic interstitial lung disease involving systemic inflammation and abnormal collagen deposition. Dysregulations in lipid metabolism, such as macrophage-dependent lipid catabolism, have been recognized as critical factors for the development of pulmonary fibrosis. However, little is known about the signaling pathways involved and the key regulators. Here we found that triggering receptor expressed on myeloid cells 2 (TREM2) plays a pivotal role in regulating the lipid handling capacities of pulmonary macrophages and triggering fibrosis. By integrating analysis of single-cell and bulk RNA sequencing data from patients and mice with pulmonary fibrosis, we revealed that pulmonary macrophages consist of heterogeneous populations with distinct pro-fibrotic properties, and found that both sphingolipid metabolism and the expression of chemotaxis-related genes are elevated in fibrotic lungs. TREM2, a sensor recognizing multiple lipid species, is specifically upregulated in a subset of monocyte-derived macrophages. Blockade of TREM2 by conventional/conditional knock-out or soluble TREM2 administration can attenuate bleomycin-induced pulmonary fibrosis. By utilizing scRNA Seq and lipidomics, we found that Trem2 deficiency downregulates the synthesis of various sphingomyelins, and inhibits the expression of chemokines such as Ccl2. Together, our findings not only reveal the alterations in lipidomic profiles and the atlas of pulmonary macrophages during pulmonary fibrosis, but also suggest that targeting TREM2, the crucial regulator affecting both pulmonary sphingolipid metabolism and the chemokines secretion, can benefit pulmonary fibrosis patients in the future.

Extracellular Vesicles from Mesenchymal Stem Cells: Potential as Therapeutics in Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD)

Background/Objectives: Metabolic dysfunction-associated steatotic liver disease (MASLD) is characterized by the accumulation of triglycerides within hepatocytes, which can progress to more severe conditions, such as metabolic dysfunction-associated steatohepatitis (MASH), which may include progressive fibrosis, leading to cirrhosis, cancer, and death. This goal of this review is to highlight recent research showing the potential of mesenchymal stem cell-derived extracellular vesicles (MSC-EVs) in reducing the key pathogenic pathways of MASLD or MASH. Methods: Relevant published studies were identified using PubMed with one or more of the following search terms: MASLD, MASH, NAFLD, NASH, exosome, extracellular vesicle (EV), therapy, and/or mesenchymal stem cells (MSC). The primary literature were subsequently downloaded and summarized. Results: Using in vitro or in vivo models, MSC-EVs have been found to counteract oxidative stress, a significant contributor to liver injury in MASH, and to suppress disease progression, including steatosis, inflammation, and, in a few instances, fibrosis. Some of these outcomes have been attributed to specific EV cargo components including microRNAs and proteins. Thus, MSC-EVs enriched with these types of molecules may have improved the therapeutic efficacy for MASLD/MASH and represent a novel approach to potentially halt or reverse the disease process. Conclusions: MSC-EVs are attractive therapeutic agents for treating MASLD/MASH. Further studies are necessary to validate the clinical applicability and efficacy of MSC-EVs in human MASH patients, focusing on optimizing delivery strategies and identifying the pathogenic pathways that are targeted by specific EV components.

A Strategy Involving Microporous Microneedles Integrated with CAR-TREM2-Macrophages for Scar Management by Regulating Fibrotic Microenvironment

Dipeptidyl peptidase 4 (DPP4) positive fibroblasts play a pivotal role in scar development following skin injury. Heterogeneous vascular endothelial cells (ECs) within scarred areas retain the capacity to drive tissue regeneration and repair. Simultaneously, TREM2 macrophages play a crucial role in the progression and resolution of fibrosis by engaging in mutual regulation with ECs. However, effective strategies to inhibit scar formation through multi-factor regulation of the scar microenvironment remain a challenge. Here, CAR-TREM2-macrophages (CAR-TREM2-Ms) capable of targeting DPP4+ fibroblasts and modulating ECs subtype within the scar microenvironment are engineered to effectively prevent scarring. Hydrogel microporous microneedles (mMNs) are employed to deliver CAR-TREM2-Ms, which can effectively alleviate scar. Single-cell transcriptome sequencing (scRNA-seq) analysis reveals that CAR-TREM2-Ms can modify ECs fibrotic phenotype and regulate fibrosis by suppressing the profibrotic gene leucine-rich-alpha-2-glycoprotein 1 (Lrg1). In vitro experiments further demonstrate that CAR-TREM2-Ms improve the scar microenvironment by phagocytosing DPP4+ fibroblasts and suppressing TGFβ secretion. This, in turn, inhibits the phenotypic conversion of LRG1 ECs and provides multifactorial way of alleviating scars. This study uncovers the evidence that mMNs attached to CAR-TREM2-Ms may exert vital influences on skin scarring through the regulation of the skin scar microenvironment, providing a promising approach for treating posttraumatic scarring.

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.

Association of Sympathovagal Imbalance with Increased Inflammation and Impaired Adaptive Immunity in Bladder Cancer Patients

Stress responses can impact bladder cancer (BC) outcomes via immune-inflammatory pathway modulation. This study explores heart rate variability (HRV) associations with serum immune-inflammatory biomarkers, blood count inflammatory markers, and psychosocial self-report measures in patients versus healthy controls. The TREM-1 and TREM-2 expressions on peripheral blood monocytes were analysed via flow cytometry; serum inflammatory biomarkers by ELISA; HRV (5-min ECG) pre-tumour resection; blood counts by haematology analyser; and psychosocial factors by validated questionnaires. Patients exhibited altered immune-inflammatory profiles with increased TREM-1/TREM-2, sTREM-1, sTREM-1/sTREM-2 ratio, BDNF, MCP-1, and NLR, and reduced IFN-γ, IL-10, LMR, and PMR. HRV analysis indicated sympathetic dominance (SNS, Stress indices, ACmod) and reduced parasympathetic modulation (PNS index, SDNN, RMSSD, 2UV%, DCmod, SD1). Sympathetic HRV indices correlated positively with sTREM-1, sTREM-1/sTREM-2 ratio, fractalkine, and inflammatory markers (SII, NLR, PLR) and negatively with parasympathetic HRV indices-correlations absent in controls. Only in patients, reduced physical function and social support, and higher anxiety, depression, and fatigue, associated positively with sympathetic HRV indices and inflammatory markers. This study links immune-inflammatory markers, HRV parameters, and psychosocial factors in BC, suggesting that immune and autonomic variations may relate to unfavourable outcomes. Incorporating these assessments could help tailor more personalised treatment strategies for BC patients.

The role of macrophages in liver fibrosis: composition, heterogeneity, and therapeutic strategies

Macrophages, the predominant immune cells in the liver, are essential for maintaining hepatic homeostasis and responding to liver injury caused by external stressors. The hepatic macrophage population is highly heterogeneous and plastic, mainly comprised of hepatic resident kuffer cells (KCs), monocyte-derived macrophages (MoMφs), lipid-associated macrophages (LAMs), and liver capsular macrophages (LCMs). KCs, a population of resident macrophages, are localized in the liver and can self-renew through in situ proliferation. However, MoMφs in the liver are recruited from the periphery circulation. LAMs are a self-renewing subgroup of liver macrophages near the bile duct. While LCMs are located in the liver capsule and derived from peripheral monocytes. LAMs and LCMs are also involved in liver damage induced by various factors. Hepatic macrophages exhibit distinct phenotypes and functions depending on the specific microenvironment in the liver. KCs are critical for initiating inflammatory responses after sensing tissue damage, while the MoMφs infiltrated in the liver are implicated in both the progression and resolution of chronic hepatic inflammation and fibrosis. The regulatory function of liver macrophages in hepatic fibrosis has attracted significant interest in current research. Numerous literatures have documented that the MoMφs in the liver have a dual impact on the progression and resolution of liver fibrosis. The MoMφs in the liver can be categorized into two subtypes based on their Ly-6C expression level: inflammatory macrophages with high Ly-6C expression (referred to as Ly-6Chi subgroup macrophages) and reparative macrophages with low Ly-6C expression (referred to as Ly-6Clo subgroup macrophages). Ly-6Chi subgroup macrophages are conducive to the occurrence and progression of liver fibrosis, while Ly-6Clo subgroup macrophages are associated with the degradation of extracellular matrix (ECM) and regression of liver fibrosis. Given this, liver macrophages play a pivotal role in the occurrence, progression, and regression of liver fibrosis. Based on these studies, treatment therapies targeting liver macrophages are also being studied gradually. This review aims to summarize researches on the composition and origin of liver macrophages, the macrophage heterogeneity in the progression and regression of liver fibrosis, and anti-fibrosis therapeutic strategies targeting macrophages in the liver.

Metabolic dysfunction-associated steatotic liver disease: a key factor in hepatocellular carcinoma therapy response

The conceptual evolution of non-alcoholic fatty liver disease (NAFLD) to what, since 2023, is called metabolic dysfunction-associated steatotic liver disease (MASLD) not only represents a change in the classification and definition of the disease but also reflects a broader understanding of this heterogeneous condition, which still with many aspects to refine. Although the definition of NAFLD can be interchanged to a high percentage with the new MASLD concept in different aspects, MASLD has been proposed as a relevant factor that influences the response to new immunotherapeutic treatments in the management of MASLD-related hepatocellular carcinoma (HCC), compared to HCC of other etiologies. This indicates that the etiology of HCC plays a relevant role in the prognosis, highlighting the urgency of evaluating treatment regimens for this subgroup of patients in upcoming clinical trials. A better understanding of the pathophysiology of MASLD generates strategies that not only aid in its management but also provide strategies to directly intervene in the carcinogenesis of HCC.

The double-edged role and therapeutic potential of TREM2 in atherosclerosis

Atherosclerosis is a chronic lipid-driven inflammatory disease characterized by infiltration of large numbers of macrophages. The progression of the disease is closely related to the status of macrophages in atherosclerotic plaques. Recent advances in plaque analysis have revealed a subpopulation of macrophages that express high levels of triggering receptor expressed on myeloid cells 2 (TREM2). Although TREM2 is known to play a critical role in inflammation, lipid metabolism, and tissue repair, its role in atherosclerosis is still not fully understood. Recent studies have shown that TREM2 promotes macrophage cholesterol uptake and efflux, enhances efferocytosis function, regulates inflammation and metabolism, and promotes cell survival, all of which are significant functions in atherosclerosis. In early plaques TREM2 promotes lipid uptake and increases lesion size. In advanced plaques TREM2 promotes macrophage survival and increases plaque stability. The dualistic nature of TREM2 in atherosclerosis, where it can exert both protective effect and a side effect of increased lesion size, presents a complex but crucial area of study. Understanding these dual roles could help in the development of new therapeutic strategies to modulate TREM2 activity and utilize its atheroprotective function while mitigating its deleterious effects. In this review, we discuss the roles and mechanisms of TREM2 during different stages of atherosclerotic plaques, as well as the potential applications of TREM2 in the diagnosis and treatment of atherosclerosis.

Unveiling the impact of TREM-2+ Macrophages in metabolic disorders

The Triggering Receptor Expressed on Myeloid cells 2 (TREM-2) has been widely known by its anti-inflammatory activity. It can be activated in response to microbes and tissue damage, leading to phagocytosis, autophagy, cell polarization and migration, counter inflammation, and tissue repair. So far, the receptor has been largely explored in neurodegenerative disorders, however, a growing number of studies have been investigating its contribution in different pathological conditions, including metabolic diseases, in which (resident) macrophages play a crucial role. In this regard, TREM-2 + macrophages have been implicated in the onset and development of obesity, atherosclerosis, and fibrotic liver disease. These macrophages can be detected in the brain, white adipose tissue, liver, and vascular endothelium. In this review we discuss how different murine models have been demonstrating the ability of such cells to contribute to tissue and body homeostasis by phagocytosing cellular debris and lipid structures, besides contributing to lipid homeostasis in metabolic diseases. Therefore, understanding the role of TREM-2 in metabolic disorders is crucial to expand our current knowledge concerning their immunopathology as well as to foster the development of more targeted therapies to treat such conditions.

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.

Amplified response of drug-induced liver fibrosis via immune cell co-culture in a 3D in vitro hepatic fibrosis model

Liver fibrosis, a critical consequence of chronic liver diseases, is characterized by excessive extracellular matrix (ECM) deposition driven by inflammation. This process involves complex interactions among hepatocytes, hepatic stellate cells (HSCs), and Kupffer cells, the liver's resident macrophages. Kupffer cells are essential in initiating fibrosis through the release of pro-inflammatory cytokines that activate HSCs. Although various in vitro liver fibrosis models have been developed, there is a lack of models that include the immune environment of the liver to clarify the influence of immune cells on the progression of liver fibrosis. We developed an in vitro liver fibrosis model by co-culturing hepatocytes (HepaRG), a hepatic stellate cell line (LX-2), and macrophages (differentiated THP-1). The effects of liver fibrosis inducers, transforming growth factor-beta1 (TGF-β1) and methotrexate (MTX), on the inflammatory response and stellate cell activation were evaluated in this triple co-culture model. A triple co-culture condition was developed as a 3D in vitro model using gelatin methacrylate (GelMA), offering a more biomimetic environment and achieving liver fibrosis via immune cell activation associated ECM deposition. In this study, the developed triple co-culture model has the potential to elucidate cell progression roles in liver fibrosis and can be applied in drug screening and toxicity assessments targeting liver fibrosis.

Macrophages and T cells in metabolic disorder-associated cancers

Cancer and metabolic disorders have emerged as major global health challenges, reaching epidemic levels in recent decades. Often viewed as separate issues, metabolic disorders are shown by mounting evidence to heighten cancer risk and incidence. The intricacies underlying this connection are still being unraveled and encompass a complex interplay between metabolites, cancer cells and immune cells within the tumour microenvironment (TME). Here, we outline the interplay between metabolic and immune cell dysfunction in the context of three highly prevalent metabolic disorders, namely obesity; two associated liver diseases, metabolic dysfunction-associated steatotic liver disease (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH); and type 2 diabetes. We focus primarily on macrophages and T cells, the critical roles of which in dictating inflammatory response and immune surveillance in metabolic disorder-associated cancers are widely reported. Moreover, considering the ever-increasing number of patients prescribed with metabolism disorder-altering drugs and diets in recent years, we discuss how these therapies modulate systemic and local immune phenotypes, consequently impacting cancer malignancy. Collectively, unraveling the determinants of metabolic disorder-associated immune landscape and their role in fuelling cancer malignancy will provide a framework essential to therapeutically address these highly prevalent diseases.

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.

Macrophage-Hepatocyte Circuits Mediated by Grancalcin Aggravate the Progression of Metabolic Dysfunction Associated Steatohepatitis

The dynamic interplay between parenchymal hepatocytes and non-parenchymal cells (NPCs), such as macrophages, is an important mechanism for liver metabolic homeostasis. Although numerous endeavors have been made to identify the mediators of metabolic dysfunction associated steatohepatitis (MASH), the molecular underpinnings of MASH progression remain poorly understood, and therapies to arrest MASH progression remain elusive. Herein, it is revealed that the expression of grancalcin (GCA) is upregulated in the macrophages of patients and rodents with MASH and correlates with MASH progression. Notably, the administration of recombinant GCA aggravates the development of MASH, whereas, Gca deletion in myeloid cells blunts liver steatosis and inflammation in multiple MASH murine models. Mechanistically, GCA activates macrophages via TLR9-NF-κB signaling, and the activated macrophages promote hepatocyte lipid accumulation and apoptosis via secretion of Interleukin-6(IL-6), Tumor Necrosis Factor α (TNFα), and Interleukin-1β(IL-1β), thereby leading to hepatic steatosis and inflammation. Finally, the therapeutic administration of antibody blocking GCA effectively halts the progression of MASH. Collectively, these findings implicate GCA as a crucial mediator of MASH and clarify a new metabolic signaling axis between the hepatocytes and macrophages, implying that GCA can emerge as a particularly interesting putative therapeutic target for reversing MASH progression.

TREM2 macrophages mediate the beneficial effects of bariatric surgery against MASH

BACKGROUND AND AIMS

For patients with obesity and metabolic syndrome, bariatric procedures such as vertical sleeve gastrectomy (VSG) have a clear benefit in ameliorating metabolic dysfunction-associated steatohepatitis (MASH). While the effects of bariatric surgeries have been mainly attributed to nutrient restriction and malabsorption, whether immuno-modulatory mechanisms are involved remains unclear.

APPROACH AND RESULT

Using murine models, we report that VSG ameliorates MASH progression in a weight loss-independent manner. Single-cell RNA sequencing revealed that hepatic lipid-associated macrophages (LAMs) expressing the triggering receptor expressed on myeloid cells 2 (TREM2) repress inflammation and increase their lysosomal activity in response to VSG. Remarkably, TREM2 deficiency in mice ablates the reparative effects of VSG, suggesting that TREM2 is required for MASH resolution. Mechanistically, TREM2 prevents the inflammatory activation of macrophages and is required for their efferocytic function.

CONCLUSIONS

Overall, our findings indicate that bariatric surgery improves MASH through a reparative process driven by TREM2+ macrophages, providing insights into the mechanisms of disease reversal that may result in new therapies and improved surgical interventions.

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.

Spatial genomics: mapping human steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD, formerly known as non-alcoholic fatty liver disease) is a leading cause of chronic liver disease worldwide. MASLD can progress to metabolic dysfunction-associated steatohepatitis (MASH, formerly known as non-alcoholic steatohepatitis) with subsequent liver cirrhosis and hepatocellular carcinoma formation. The advent of current technologies such as single-cell and single-nuclei RNA sequencing have transformed our understanding of the liver in homeostasis and disease. The next frontier is contextualizing this single-cell information in its native spatial orientation. This understanding will markedly accelerate discovery science in hepatology, resulting in a further step-change in our knowledge of liver biology and pathobiology. In this Review, we discuss up-to-date knowledge of MASLD development and progression and how the burgeoning field of spatial genomics is driving exciting new developments in our understanding of human liver disease pathogenesis and therapeutic target identification.

Single-nucleus transcriptome reveals cell dynamic response of liver during the late chick embryonic development

The late embryonic development of the liver, a major metabolic organ, remains poorly characterized at single cell resolution. Here, we used single-nucleus RNA-sequencing (snRNA-seq) to characterize the chicken liver cells at 2 embryonic development time points (E14 and D1). We uncovered 8 cell types including hepatocytes, endothelial cells, hepatic stellate cells, erythrocytes, cholangiocytes, kupffer cells, mesothelial cells, and lymphocytes. And we discovered significant differences in the abundance of different cell types between E14 and D1. Moreover, we characterized the heterogeneity of hepatocytes, endothelial cells, and mesenchymal cells based on the gene regulatory networks of each clusters. Trajectory analyses revealed 128 genes associated with hepatocyte development and function, including apolipoprotein genes involved hepatic lipid metabolism and NADH dehydrogenase subunits involved hepatic oxidative phosphorylation. Furthermore, we identified the differentially expressed genes (DEGs) between E14 and D1 at the cellular levels, which contribute to changes in liver development and function. These DEGs were significantly enriched in PPAR signaling pathways and lipid metabolism related pathways. Our results presented the single-cell mapping of chick embryonic liver at late stages of development and demonstrated the metabolic changes across the 2 age stages at the cellular level, which can help to further study the molecular development mechanism of embryonic liver.

Understanding Macrophage Complexity in Metabolic Dysfunction-Associated Steatotic Liver Disease: Transitioning from the M1/M2 Paradigm to Spatial Dynamics

Metabolic dysfunction-associated steatotic liver disease (MASLD) encompasses metabolic dysfunction-associated fatty liver (MASL) and metabolic dysfunction-associated steatohepatitis (MASH), with MASH posing a risk of progression to cirrhosis and hepatocellular carcinoma (HCC). The global prevalence of MASLD is estimated at approximately a quarter of the population, with significant healthcare costs and implications for liver transplantation. The pathogenesis of MASLD involves intrahepatic liver cells, extrahepatic components, and immunological aspects, particularly the involvement of macrophages. Hepatic macrophages are a crucial cellular component of the liver and play important roles in liver function, contributing significantly to tissue homeostasis and swift responses during pathophysiological conditions. Recent advancements in technology have revealed the remarkable heterogeneity and plasticity of hepatic macrophage populations and their activation states in MASLD, challenging traditional classification methods like the M1/M2 paradigm and highlighting the coexistence of harmful and beneficial macrophage phenotypes that are dynamically regulated during MASLD progression. This complexity underscores the importance of considering macrophage heterogeneity in therapeutic targeting strategies, including their distinct ontogeny and functional phenotypes. This review provides an overview of macrophage involvement in MASLD progression, combining traditional paradigms with recent insights from single-cell analysis and spatial dynamics. It also addresses unresolved questions and challenges in this area.

Cathepsin D is essential for the degradomic shift of macrophages required to resolve liver fibrosis

BACKGROUND AND OBJECTIVES

Fibrosis contributes to 45% of deaths in industrialized nations and is characterized by an abnormal accumulation of extracellular matrix (ECM). There are no specific anti-fibrotic treatments for liver fibrosis, and previous unsuccessful attempts at drug development have focused on preventing ECM deposition. Because liver fibrosis is largely acknowledged to be reversible, regulating fibrosis resolution could offer novel therapeutical options. However, little is known about the mechanisms controlling ECM remodeling during resolution. Changes in proteolytic activity are essential for ECM homeostasis and macrophages are an important source of proteases. Herein, in this study we evaluate the role of macrophage-derived cathepsin D (CtsD) during liver fibrosis.

METHODS

CtsD expression and associated pathways were characterized in single-cell RNA sequencing and transcriptomic datasets in human cirrhosis. Liver fibrosis progression, reversion and functional characterization were assessed in novel myeloid-CtsD and hepatocyte-CtsD knock-out mice.

RESULTS

Analysis of single-cell RNA sequencing datasets demonstrated CtsD was expressed in macrophages and hepatocytes in human cirrhosis. Liver fibrosis progression, reversion and functional characterization were assessed in novel myeloid-CtsD (CtsDΔMyel) and hepatocyte-CtsD knock-out mice. CtsD deletion in macrophages, but not in hepatocytes, resulted in enhanced liver fibrosis. Both inflammatory and matrisome proteomic signatures were enriched in fibrotic CtsDΔMyel livers. Besides, CtsDΔMyel liver macrophages displayed functional, phenotypical and secretomic changes, which resulted in a degradomic phenotypical shift, responsible for the defective proteolytic processing of collagen I in vitro and impaired collagen remodeling during fibrosis resolution in vivo. Finally, CtsD-expressing mononuclear phagocytes of cirrhotic human livers were enriched in lysosomal and ECM degradative signaling pathways.

CONCLUSIONS

Our work describes for the first-time CtsD-driven lysosomal activity as a central hub for restorative macrophage function during fibrosis resolution and opens new avenues to explore their degradome landscape to inform drug development.

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.

Galectin-3 induces pathogenic immunosuppressive macrophages through interaction with TREM2 in lung cancer

BACKGROUND

High infiltration of tumor-associated macrophages (TAMs) is associated with tumor promotion and immunosuppression. The triggering receptor expressed on myeloid cells 2 (TREM2) is emerged as a key immunosuppressive regulator for TAMs, however, how TREM2-expressing TAMs are recruited and what ligands TREM2 interacts with to mediate immunosuppression is unknown.

METHODS

Flow cytometry and single-cell RNA sequencing were used to analyze TREM2 expression. Mechanistically, mass spectrometry and immunoprecipitation were employed to identify proteins binding to TREM2. Phagocytosis and co-culture experiments were used to explore the in vitro functions of galectin3-TREM2 pair. Establishment of TREM2f/f-Lyz2-cre mice to validate the role of TREM2 signaling pathway in lung carcinogenesis. GB1107 were further supplemented to validate the therapeutic effect of Galectin3 based on TREM2 signaling regulation.

RESULTS

This study identified that abundant TREM2+ macrophages were recruited at the intra-tumor site through the CCL2-CCR2 chemotactic axis. Galectin-3 impaired TREM2-mediated phagocytosis and promoted the conversion of TREM2+ macrophages to immunosuppressive TAMs with attenuated antigen presentation and co-stimulatory functions both in vitro both in vivo, and galectin-3 is a potential ligand for TREM2. Genetic and pharmacological blockade of TREM2 and galectin-3 significantly inhibited lung cancer progression in subcutaneous and orthotopic cancer models by remodeling the tumor immune microenvironment.

CONCLUSION

Our findings revealed a previously unknown association between galectin-3 and TREM2 in TAMs of lung cancer, and suggested simultaneous inhibition of galectin3 and TREM2 as potent therapeutic approach for lung cancer therapy.

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.

Cutting Edge: Hepatic Stellate Cells Drive the Phenotype of Monocyte-derived Macrophages to Regulate Liver Fibrosis in Metabolic Dysfunction-associated Steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) is characterized by infiltration of monocyte-derived macrophages (MdMs) into the liver; however, the function of these macrophages is largely unknown. We previously demonstrated that a population of MdMs, referred to as hepatic lipid-associated macrophages (LAMs), assemble into aggregates termed hepatic crown-like structures in areas of liver fibrosis. Intriguingly, decreasing MdM recruitment resulted in increased liver fibrosis, suggesting that LAMs contribute to antifibrotic pathways in MASH. In this study, we determined that hepatic crown-like structures are characterized by intimate interactions between activated hepatic stellate cells (HSCs) and macrophages in a collagen matrix in a mouse model of MASH. MASH macrophages displayed collagen-degrading capacities, and HSCs derived from MASH livers promoted expression of LAM marker genes and acquisition of a collagen-degrading phenotype in naive macrophages. These data suggest that crosstalk between HSCs and macrophages may contribute to collagen degradation MASH.

Inflammation in Steatotic Liver Diseases: Pathogenesis and Therapeutic Targets

Alcohol-related liver disease (ALD) and metabolic dysfunction-associated steatotic liver disease (MASLD), two main types of steatotic liver disease (SLDs), are characterized by a wide spectrum of several different liver disorders, including simple steatosis, steatohepatitis, cirrhosis, and hepatocellular carcinoma. Multiple immune cell-mediated inflammatory responses not only orchestrate the killing and removal of infected/damaged cells but also exacerbate the development of SLDs when excessive or persistent inflammation occurs. In recent years, single-cell and spatial transcriptome analyses have revealed the heterogeneity of liver-infiltrated immune cells in ALD and MASLD, revealing a new immunopathological picture of SLDs. In this review, we will emphasize the roles of several key immune cells in the pathogenesis of ALD and MASLD and discuss inflammation-based approaches for effective SLD intervention. In conclusion, the study of immunological mechanisms, especially highly specific immune cell population functions, may provide novel therapeutic opportunities for this life-threatening disease.

Natural product/diet-based regulation of macrophage polarization: Implications in treatment of inflammatory-related diseases and cancer

Macrophages are phagocytic cells with important physiological functions, including the digestion of cellular debris, foreign substances, and microbes, as well as tissue development and homeostasis. The tumor microenvironment (TME) shapes the aggressiveness of cancer, and the biological and cellular interactions in this complicated space can determine carcinogenesis. TME can determine the progression, biological behavior, and therapy resistance of human cancers. The macrophages are among the most abundant cells in the TME, and their functions and secretions can determine tumor progression. The education of macrophages to M2 polarization can accelerate cancer progression, and therefore, the re-education and reprogramming of these cells is promising. Moreover, macrophages can cause inflammation in aggravating pathological events, including cardiovascular diseases, diabetes, and neurological disorders. The natural products are pleiotropic and broad-spectrum functional compounds that have been deployed as ideal alternatives to conventional drugs in the treatment of cancer. The biological and cellular interactions in the TME can be regulated by natural products, and for this purpose, they enhance the M1 polarization of macrophages, and in addition to inhibiting proliferation and invasion, they impair the chemoresistance. Moreover, since macrophages and changes in the molecular pathways in these cells can cause inflammation, the natural products impair the pro-inflammatory function of macrophages to prevent the pathogenesis and progression of diseases. Even a reduction in macrophage-mediated inflammation can prevent organ fibrosis. Therefore, natural product-mediated macrophage targeting can alleviate both cancerous and non-cancerous diseases.

Human iPSC-liver organoid transplantation reduces fibrosis through immunomodulation

Donor organ shortages for transplantation remain a serious global concern, and alternative treatment is in high demand. Fetal cells and tissues have considerable therapeutic potential as, for example, organoid technology that uses human induced pluripotent stem cells (hiPSCs) to generate unlimited human fetal-like cells and tissues. We previously reported the in vivo vascularization of early fetal liver-like hiPSC-derived liver buds (LBs) and subsquent improved survival of recipient mice with subacute liver failure. Here, we show hiPSC-liver organoids (LOs) that recapitulate midgestational fetal liver promote de novo liver generation when grafted onto the surface of host livers in chemical fibrosis models, thereby recovering liver function. We found that fetal liver, a hematopoietic tissue, highly expressed macrophage-recruiting factors and antifibrotic M2 macrophage polarization factors compared with the adult liver, resulting in fibrosis reduction because of CD163+ M2-macrophage polarization. Next, we created midgestational fetal liver-like hiPSC-LOs by fusion of hiPSC-LBs to induce static cell-cell interactions and found that these contained complex structures such as hepatocytes, vasculature, and bile ducts after transplantation. This fusion allowed the generation of a large human tissue suitable for transplantation into immunodeficient rodent models of liver fibrosis. hiPSC-LOs showed superior liver function compared with hiPSC-LBs and improved survival and liver function upon transplantation. In addition, hiPSC-LO transplantation ameliorated chemically induced liver fibrosis, a symptom of liver cirrhosis that leads to organ dysfunction, through immunomodulatory effects, particularly on CD163+ phagocytic M2-macrophage polarization. Together, our results suggest hiPSC-LO transplantation as a promising therapeutic option for liver fibrosis.

Multiplexed spatial transcriptomics methods and the application of expansion microscopy

While spatial transcriptomics has undeniably revolutionized our ability to study cellular organization, it has driven the development of a great number of innovative transcriptomics methods, which can be classified into in situ sequencing (ISS) methods, in situ hybridization (ISH) techniques, and next-generation sequencing (NGS)-based sequencing with region capture. These technologies not only refine our understanding of cellular processes, but also open up new possibilities for breakthroughs in various research domains. One challenge of spatial transcriptomics experiments is the limitation of RNA detection due to optical crowding of RNA in the cells. Expansion microscopy (ExM), characterized by the controlled enlargement of biological specimens, offers a means to achieve super-resolution imaging, overcoming the diffraction limit inherent in conventional microscopy and enabling precise visualization of RNA in spatial transcriptomics methods. In this review, we elaborate on ISS, ISH and NGS-based spatial transcriptomic protocols and on how performance of these techniques can be extended by the combination of these protocols with ExM. Moving beyond the techniques and procedures, we highlight the broader implications of transcriptomics in biology and medicine. These include valuable insight into the spatial organization of gene expression in cells within tissues, aid in the identification and the distinction of cell types and subpopulations and understanding of molecular mechanisms and intercellular changes driving disease development.

Macrophage metabolism impacts metabolic dysfunction-associated steatotic liver disease and its progression

Metabolic dysfunction-associated steatotic liver disease (MASLD), with a progressive form of metabolic dysfunction-associated steatohepatitis (MASH), is the leading chronic liver disease worldwide, which can progress to advanced liver disease and hepatocellular carcinoma. MASLD is tightly associated with metabolic disorders such as obesity, insulin resistance, and type 2 diabetes. Macrophages, as an innate immune component and a linker of adaptive immune response, play important roles in the pathogenesis and treatment of MASLD or MASH. Metabolic reprogramming can regulate macrophage activation and polarization to inhibit MASLD or MASH progression to advanced liver disease. Here, we summarize the underlying mechanisms of how different metabolites such as amino acids, glucose, and fatty acids can regulate macrophage function and phenotype, the factors that regulate macrophage metabolism, and potential treatment options to regulate macrophage function in MASLD or MASH, as well as other associated metabolic disorders.

Detection of early-stage NASH using non-invasive hyperpolarized 13C metabolic imaging

Non-alcoholic steatohepatitis (NASH) is characterized from its early stages by a profound remodeling of the liver microenvironment, encompassing changes in the composition and activities of multiple cell types and associated gene expression patterns. Hyperpolarized (HP) 13C MRI provides a unique view of the metabolic microenvironment, with potential relevance for early diagnosis of liver disease. Previous studies have detected changes in HP 13C pyruvate to lactate conversion, catalyzed by lactate dehydrogenase (LDH), with experimental liver injury. HP ∝ -ketobutyrate ( ∝ KB) is a close molecular analog of pyruvate with modified specificity for LDH isoforms, specifically attenuated activity with their LDHA-expressed subunits that dominate liver parenchyma. Building on recent results with pyruvate, we investigated HP ∝ KB in methionine-choline deficient (MCD) diet as a model of early-stage NASH. Similarity of results between this new agent and pyruvate (~ 50% drop in cytoplasmic reducing capacity), interpreted together with gene expression data from the model, suggests that changes are mediated through broad effects on intermediary metabolism. Plausible mechanisms are depletion of the lactate pool by upregulation of gluconeogenesis (GNG) and pentose phosphate pathway (PPP) flux, and a possible shift toward increased lactate oxidation. These changes may reflect high levels of oxidative stress and/or shifting macrophage populations in NASH.

Macrophage and fibroblast trajectory inference and crosstalk analysis during myocardial infarction using integrated single-cell transcriptomic datasets

BACKGROUND

Cardiac fibrosis after myocardial infarction (MI) has been considered an important part of cardiac pathological remodeling. Immune cells, especially macrophages, are thought to be involved in the process of fibrosis and constitute a niche with fibroblasts to promote fibrosis. However, the diversity and variability of fibroblasts and macrophages make it difficult to accurately depict interconnections.

METHODS

We collected and reanalyzed scRNA-seq and snRNA-seq datasets from 12 different studies. Differentiation trajectories of these subpopulations after MI injury were analyzed by using scVelo, PAGA and Slingshot. We used CellphoneDB and NicheNet to infer fibroblast-macrophage interactions. Tissue immunofluorescence staining and in vitro experiments were used to validate our findings.

RESULTS

We discovered two subsets of ECM-producing fibroblasts, reparative cardiac fibroblasts (RCFs) and matrifibrocytes, which appeared at different times after MI and exhibited different transcriptional profiles. We also observed that CTHRC1+ fibroblasts represent an activated fibroblast in chronic disease states. We identified a macrophage subset expressing the genes signature of SAMs conserved in both human and mouse hearts. Meanwhile, the SPP1hi macrophages were predominantly found in the early stages after MI, and cell communication analysis indicated that SPP1hi macrophage-RCFs interactions are mainly involved in collagen deposition and scar formation.

CONCLUSIONS

Overall, this study comprehensively analyzed the dynamics of fibroblast and macrophage subsets after MI and identified specific subsets of fibroblasts and macrophages involved in scar formation and collagen deposition.

Myeloid Trem2 ameliorates the progression of metabolic dysfunction-associated steatotic liver disease by regulating macrophage pyroptosis and inflammation resolution

BACKGROUND

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD) is increasing year by year and has become one of the leading causes of end-stage liver disease worldwide. Triggering Receptor Expressed on Myeloid Cells 2 (Trem2) has been confirmed to play an essential role in the progression of MASLD, but its specific mechanism still needs to be clarified. This study aims to explore the role and mechanism of Trem2 in MASLD.

METHODS

Human liver tissues were obtained from patients with MASLD and controls. Myeloid-specific knockout mice (Trem2mKO) and myeloid-specific overexpression mice (Trem2TdT) were fed a high-fat diet, either AMLN or CDAHFD, to establish the MASLD model. Relevant signaling molecules were assessed through lipidomics and RNA-seq analyses after that.

RESULTS

Trem2 is upregulated in human MASLD/MASH-associated macrophages and is associated with hepatic steatosis and inflammation progression. Hepatic steatosis and inflammatory responses are exacerbated with the knockout of myeloid Trem2 in MASLD mice, while mice overexpressing Trem2 exhibit the opposite phenomenon. Mechanistically, Trem2mKO can aggravate macrophage pyroptosis through the PI3K/AKT signaling pathway and amplify the resulting inflammatory response. At the same time, Trem2 promotes the inflammation resolution phenotype transformation of macrophages through TGFβ1, thereby promoting tissue repair.

CONCLUSIONS

Myeloid Trem2 ameliorates the progression of Metabolic dysfunction-associated steatotic liver disease by regulating macrophage pyroptosis and inflammation resolution. We believe targeting myeloid Trem2 could represent a potential avenue for treating MASLD.

From NAFLD to NASH: Understanding the spectrum of non-alcoholic liver diseases and their consequences

Non-alcoholic fatty liver disease (NAFLD) has become one of the most frequent chronic liver diseases worldwide in recent decades. Metabolic diseases like excessive blood glucose, central obesity, dyslipidemia, hypertension, and liver function abnormalities cause NAFLD. NAFLD significantly increases the likelihood of liver cancer, heart disease, and mortality, making it a leading cause of liver transplants. Non-alcoholic steatohepatitis (NASH) is a more advanced form of the disease that causes scarring and inflammation of the liver over time and can ultimately result in cirrhosis and hepatocellular carcinoma. In this review, we briefly discuss NAFLD's pathogenic mechanisms, their progression into NASH and afterward to NASH-related cirrhosis. It also covers disease epidemiology, metabolic mechanisms, glucose and lipid metabolism in the liver, macrophage dysfunction, bile acid toxicity, and liver stellate cell stimulation. Additionally, we consider the contribution of intestinal microbiota, genetics, epigenetics, and ecological factors to fibrosis progression and hepatocellular carcinoma risk in NAFLD and NASH patients.

Emerging Roles of Spatial Transcriptomics in Liver Research

Spatial transcriptomics, leveraging sequencing- and imaging-based techniques, has emerged as a groundbreaking technology for mapping gene expression within the complex architectures of tissues. This approach provides an in-depth understanding of cellular and molecular dynamics across various states of healthy and diseased livers. Through the integration of sophisticated bioinformatics strategies, it enables detailed exploration of cellular heterogeneity, transitions in cell states, and intricate cell-cell interactions with remarkable precision. In liver research, spatial transcriptomics has been particularly revelatory, identifying distinct zonated functions of hepatocytes that are crucial for understanding the metabolic and detoxification processes of the liver. Moreover, this technology has unveiled new insights into the pathogenesis of liver diseases, such as the role of lipid-associated macrophages in steatosis and endothelial cell signals in liver regeneration and repair. In the domain of liver cancer, spatial transcriptomics has proven instrumental in delineating intratumor heterogeneity, identifying supportive microenvironmental niches and revealing the complex interplay between tumor cells and the immune system as well as susceptibility to immune checkpoint inhibitors. In conclusion, spatial transcriptomics represents a significant advance in hepatology, promising to enhance our understanding and treatment of liver diseases.

Inflammation in Alcohol-Associated Hepatitis: Pathogenesis and Therapeutic Targets

Alcohol-associated hepatitis (AH) is an acute-on-chronic liver injury that occurs in patients with chronic alcohol-associated liver disease (ALD). Patients with severe AH have high short-term mortality and lack effective pharmacologic therapies. Inflammation is believed to be one of the key factors promoting AH progression and has been actively investigated as therapeutic targets over the last several decades, but no effective inflammatory targets have been identified so far. In this review, we discuss how inflammatory cells and the inflammatory mediators produced by these cells contribute to the development and progression of AH, with focus on neutrophils and macrophages. The crosstalk between inflammatory cells and liver nonparenchymal cells in the pathogenesis of AH is elaborated. We also deliberate the application of recent cutting-edge technologies in characterizing liver inflammation in AH. Finally, the potential therapeutic targets of inflammatory mediators for AH are briefly summarized.

Innate Immunity and MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD) has emerged as the most common liver disease worldwide in recent years. MASLD commonly presents as simple hepatic steatosis, but ~25% of patients develop liver inflammation, progressive fibrosis, liver cirrhosis and related hepatocellular carcinoma. Liver inflammation and the degree of fibrosis are key determinants of the prognosis. The pathophysiology of liver inflammation is incompletely understood and involves diverse factors and specifically innate and adaptive immune responses. More specifically, diverse mediators of innate immunity such as proinflammatory cytokines, adipokines, inflammasomes and various cell types like mononuclear cells, macrophages and natural killer cells are involved in directing the inflammatory process in MASLD. The activation of innate immunity is driven by various factors including excess lipids and lipotoxicity, insulin resistance and molecular patterns derived from gut commensals. Targeting pathways of innate immunity might therefore appear as an attractive therapeutic strategy in the future management of MASLD and possibly its complications.

Regional Differences in the Small Intestinal Proteome of Control Mice and of Mice Lacking Lysosomal Acid Lipase

The metabolic contribution of the small intestine (SI) is still unclear despite recent studies investigating the involvement of single cells in regional differences. Using untargeted proteomics, we identified regional characteristics of the three intestinal tracts of C57BL/6J mice and found that proteins abundant in the mouse ileum correlated with the high ileal expression of the corresponding genes in humans. In the SI of C57BL/6J mice, we also detected an increasing abundance of lysosomal acid lipase (LAL), which is responsible for degrading triacylglycerols and cholesteryl esters within the lysosome. LAL deficiency in patients and mice leads to lipid accumulation, gastrointestinal disturbances, and malabsorption. We previously demonstrated that macrophages massively infiltrated the SI of Lal-deficient (KO) mice, especially in the duodenum. Using untargeted proteomics (ProteomeXchange repository, data identifier PXD048378), we revealed a general inflammatory response and a common lipid-associated macrophage phenotype in all three intestinal segments of Lal KO mice, accompanied by a higher expression of GPNMB and concentrations of circulating sTREM2. However, only duodenal macrophages activated a metabolic switch from lipids to other pathways, which were downregulated in the jejunum and ileum of Lal KO mice. Our results provide new insights into the process of absorption in control mice and possible novel markers of LAL-D and/or systemic inflammation in LAL-D.

Diverse potential of secretome from natural killer cells and monocyte-derived macrophages in activating stellate cells

Chronic liver diseases, such as non-alcoholic steatohepatitis (NASH)-induced cirrhosis, are characterized by an increasing accumulation of stressed, damaged, or dying hepatocytes. Hepatocyte damage triggers the activation of resident immune cells, such as Kupffer cells (KC), as well as the recruitment of immune cells from the circulation toward areas of inflammation. After infiltration, monocytes differentiate into monocyte-derived macrophages (MoMF) which are functionally distinct from resident KC. We herein aim to compare the in vitro signatures of polarized macrophages and activated hepatic stellate cells (HSC) with ex vivo-derived disease signatures from human NASH. Furthermore, to shed more light on HSC activation and liver fibrosis progression, we investigate the effects of the secretome from primary human monocytes, macrophages, and NK cells on HSC activation. Interleukin (IL)-4 and IL-13 treatment induced transforming growth factor beta 1 (TGF-β1) secretion by macrophages. However, the supernatant transfer did not induce HSC activation. Interestingly, PMA-activated macrophages showed strong induction of the fibrosis response genes COL10A1 and CTGF, while the supernatant of IL-4/IL-13-treated monocytes induced the upregulation of COL3A1 in HSC. The supernatant of PMA-activated NK cells had the strongest effect on COL10A1 induction in HSC, while IL-15-stimulated NK cells reduced the expression of COL1A1 and CTGF. These data indicate that other factors, aside from the well-known cytokines and chemokines, might potentially be stronger contributors to the activation of HSCs and induction of a fibrotic response, indicating a more diverse and complex role of monocytes, macrophages, and NK cells in liver fibrosis progression.

TREM2 protects from atherosclerosis by limiting necrotic core formation

Atherosclerosis is a chronic disease of the vascular wall driven by lipid accumulation and inflammation in the intimal layer of arteries, and its main complications, myocardial infarction and stroke, are the leading cause of mortality worldwide [1], [2]. Recent studies have identified Triggering receptor expressed on myeloid cells 2 (TREM2), a lipid-sensing receptor regulating myeloid cell functions [3], to be highly expressed in macrophage foam cells in experimental and human atherosclerosis [4]. However, the role of TREM2 in atherosclerosis is not fully known. Here, we show that hematopoietic or global TREM2 deficiency increased, whereas TREM2 agonism decreased necrotic core formation in early atherosclerosis. We demonstrate that TREM2 is essential for the efferocytosis capacities of macrophages, and to the survival of lipid-laden macrophages, indicating a crucial role of TREM2 in maintaining the balance between foam cell death and clearance of dead cells in atherosclerotic lesions, thereby controlling plaque necrosis.

Drug targets regulate systemic metabolism and provide new horizons to treat nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH), is the advanced stage of nonalcoholic fatty liver disease (NAFLD) with rapidly rising global prevalence. It is featured with severe hepatocyte apoptosis, inflammation and hepatic lipogenesis. The drugs directly targeting the processes of steatosis, inflammation and fibrosis are currently under clinical investigation. Nevertheless, the long-term ineffectiveness and remarkable adverse effects are well documented, and new concepts are required to tackle with the root causes of NASH progression. We critically assess the recently validated drug targets that regulate the systemic metabolism to ameliorate NASH. Thermogenesis promoted by mitochondrial uncouplers restores systemic energy expenditure. Furthermore, regulation of mitochondrial proteases and proteins that are pivotal for intracellular metabolic homeostasis normalize mitochondrial function. Secreted proteins also improve systemic metabolism, and NASH is ameliorated by agonizing receptors of secreted proteins with small molecules. We analyze the drug design, the advantages and shortcomings of these novel drug candidates. Meanwhile, the structural modification of current NASH therapeutics significantly increased their selectivity, efficacy and safety. Furthermore, the arising CRISPR-Cas9 screen strategy on liver organoids has enabled the identification of new genes that mediate lipid metabolism, which may serve as promising drug targets. In summary, this article discusses the in-depth novel mechanisms and the multidisciplinary approaches, and they provide new horizons to treat NASH.

Retreatment with HBV siRNA Results in Additional Reduction in HBV Antigenemia and Immune Stimulation in the AAV-HBV Mouse Model

BACKGROUND AND AIMS

Treatment with siRNAs that target HBV has demonstrated robust declines in HBV antigens. This effect is also observed in the AAV-HBV mouse model, which was used to investigate if two cycles of GalNAc-HBV-siRNA treatment could induce deeper declines in HBsAg levels or prevent rebound, and to provide insights into the liver immune microenvironment.

METHODS

C57Bl/6 mice were transduced with one of two different titers of AAV-HBV for 28 days, resulting in stable levels of HBsAg of about 103 or 105 IU/mL. Mice were treated for 12 weeks (four doses q3wk) per cycle with 3 mg/kg of siRNA-targeting HBV or an irrelevant sequence either once (single treatment) or twice (retreatment) with an 8-week treatment pause in between. Blood was collected to evaluate viral parameters. Nine weeks after the last treatment, liver samples were collected to perform phenotyping, bulk RNA-sequencing, and immunohistochemistry.

RESULTS

Independent of HBsAg baseline levels, treatment with HBV-siRNA induced a rapid decline in HBsAg levels, which then plateaued before gradually rebounding 12 weeks after treatment stopped. A second cycle of HBV-siRNA treatment induced a further decline in HBsAg levels in serum and the liver, reaching undetectable levels and preventing rebound when baseline levels were 103 IU/mL. This was accompanied with a significant increase in inflammatory macrophages in the liver and significant upregulation of regulatory T-cells and T-cells expressing immune checkpoint receptors.

CONCLUSIONS

Retreatment induced an additional decline in HBsAg levels, reaching undetectable levels when baseline HBsAg levels were 3log10 or less. This correlated with T-cell activation and upregulation of Trem2.

Adipocyte p53 coordinates the response to intermittent fasting by regulating adipose tissue immune cell landscape

In obesity, sustained adipose tissue (AT) inflammation constitutes a cellular memory that limits the effectiveness of weight loss interventions. Yet, the impact of fasting regimens on the regulation of AT immune infiltration is still elusive. Here we show that intermittent fasting (IF) exacerbates the lipid-associated macrophage (LAM) inflammatory phenotype of visceral AT in obese mice. Importantly, this increase in LAM abundance is strongly p53 dependent and partly mediated by p53-driven adipocyte apoptosis. Adipocyte-specific deletion of p53 prevents LAM accumulation during IF, increases the catabolic state of adipocytes, and enhances systemic metabolic flexibility and insulin sensitivity. Finally, in cohorts of obese/diabetic patients, we describe a p53 polymorphism that links to efficacy of a fasting-mimicking diet and that the expression of p53 and TREM2 in AT negatively correlates with maintaining weight loss after bariatric surgery. Overall, our results demonstrate that p53 signalling in adipocytes dictates LAM accumulation in AT under IF and modulates fasting effectiveness in mice and humans.

Molecular mechanisms in MASLD/MASH-related HCC

Liver cancer is the third leading cause of cancer-related deaths and ranks as the sixth most prevalent cancer type globally. NAFLD or metabolic dysfunction-associated steatotic liver disease, and its more severe manifestation, NASH or metabolic dysfunction-associated steatohepatitis (MASH), pose a significant global health concern, affecting approximately 20%-25% of the population. The increased prevalence of metabolic dysfunction-associated steatotic liver disease and MASH is parallel to the increasing rates of obesity-associated metabolic diseases, including type 2 diabetes, insulin resistance, and fatty liver diseases. MASH can progress to MASH-related HCC (MASH-HCC) in about 2% of cases each year, influenced by various factors such as genetic mutations, carcinogen exposure, immune microenvironment, and microbiome. MASH-HCC exhibits distinct molecular and immune characteristics compared to other causes of HCC and affects both men and women equally. The management of early to intermediate-stage MASH-HCC typically involves surgery and locoregional therapies, while advanced HCC is treated with systemic therapies, including anti-angiogenic therapies and immune checkpoint inhibitors. In this comprehensive review, we consolidate previous research findings while also providing the most current insights into the intricate molecular processes underlying MASH-HCC development. We delve into MASH-HCC-associated genetic variations and somatic mutations, disease progression and research models, multiomics analysis, immunological and microenvironmental impacts, and discuss targeted/combined therapies to overcome immune evasion and the biomarkers to recognize treatment responders. By furthering our comprehension of the molecular mechanisms underlying MASH-HCC, our goal is to catalyze the advancement of more potent treatment strategies, ultimately leading to enhanced patient outcomes.

sTREM2 in the prognostic evaluation of acute lung injury after cardiac surgery in infants

BACKGROUND

Previous studies have shown that TREM2 plays a protective role in acute lung injury (ALI). This prospective study aimed to investigate the role of sTREM2 as a forecasting factor for ALI in infants after pediatric cardiac surgery undergoing cardiopulmonary bypass (CPB).

METHODS

Seventy-five consecutive patients younger than 1 year who underwent cardiac surgery were enrolled in this study. Sixty-one fulfilled the inclusion criteria and had been divided into ALI and non-ALI groups. Children's demographic characteristics and clinical data were collected. Perioperative sTREM2 levels were analyzed at five timepoints.

RESULTS

In this study, children in the ALI group were younger, lighter, with higher RACHS-1 scores and underwent significantly longer CPB time. Post-CPB ALI had an impact on clinical outcomes, which contributed to a longer duration of mechanical ventilation, ICU and hospital stay than non-ALI group. Significant differences were manifested off-CPB, 1 h/6 h after CPB, and day 1 after surgery between the two groups. Binary logistic models revealed that off-CPB sTREM2 was significantly associated with the incidence of post-CPB ALI after adjustment. ROC analysis showed that the AUC of off-CPB sTREM2 level was 0.791, and the optimal cutoff value was 788.6 pg/ml.

CONCLUSIONS

The off-CPB sTREM2 level was an independent prognostic factor for post-CPB ALI in infants.

IMPACT

Plasma sTREM2 works together with downstream TREM2 to regulate inflammation response by binding the receptor to other cells. Previous studies have shown that TREM2 plays a protective role in ischemia-reperfusion and has anti-inflammatory effects on acute lung injury (ALI). This study analyzed the risk factors of post-cardiopulmonary bypass (CPB) ALI. We found that weight and off-CPB sTREM2 level were independent prognostic factors for post-CPB ALI. Plasma sTREM2 may serve as an early biomarker in the prognostic evaluation of acute lung injury after cardiac surgery in infants.

Effect of triggering receptor expressed on myeloid cells 2-associated alterations on lipid metabolism in macrophages in the development of non-alcoholic fatty liver disease

BACKGROUND AND AIM

Triggering receptor expressed on myeloid cells 2 (TREM2) plays crucial roles in metabolic homeostasis and inflammatory response. Altered metabolic function in macrophages could modulate their activation and immune phenotype. The present study aimed to investigate the expression of TREM2 in non-alcoholic fatty liver disease (NAFLD) and to clarify the underlying mechanism of TREM2 on macrophages lipid metabolism and oxidative stress.

METHODS

Hepatic TREM2 expression and its relationship with NAFLD progression were analyzed in patients with NAFLD and mice fed a high-fat diet. Lipid metabolism and oxidative stress were investigated in macrophages from NAFLD mice or stimulated with saturated fatty acids. Knockdown and overexpression of TREM2 were further explored.

RESULTS

Triggering receptor expressed on myeloid cells 2+ macrophages were increased along with NAFLD development, characterized by aggravated steatosis and liver damage in humans and mice. TREM2 expression was upregulated and lipid metabolism was changed in macrophages from NAFLD mice or metabolically activated by saturated fatty acid in vitro, as demonstrated by increased lipid uptake and catabolism, but reduced de novo synthesis of fatty acids (FAs). Regulation of TREM2 expression in lipid-laden macrophages reprogrammed lipid metabolism, especially the fatty acid oxidation capacity of mitochondria. TREM2 knockdown promoted oxidative stress by aggravating FAs deposition in mitochondria. Intervention of mitochondrial FAs transport in lipid-laden macrophages alleviated FA deposition and reactive oxygen species production induced by TREM2 knockdown.

CONCLUSIONS

Triggering receptor expressed on myeloid cells 2 expression was associated with the lipid metabolic profile and reactive oxygen species production in macrophages. High expression of TREM2 in macrophages may protect the liver from oxidative stress in NAFLD.

Immunomodulation in non-alcoholic fatty liver disease: exploring mechanisms and applications

Non-alcoholic fatty liver disease (NAFLD) exhibits increased lipid enrichment in hepatocytes. The spectrum of this disease includes stages such as nonalcoholic simple fatty liver (NAFL), nonalcoholic steatohepatitis (NASH), and liver fibrosis. Changes in lifestyle behaviors have been a major factor contributing to the increased cases of NAFLD patients globally. Therefore, it is imperative to explore the pathogenesis of NAFLD, identify therapeutic targets, and develop new strategies to improve the clinical management of the disease. Immunoregulation is a strategy through which the organism recognizes and eliminates antigenic foreign bodies to maintain physiological homeostasis. In this process, multiple factors, including immune cells, signaling molecules, and cytokines, play a role in governing the evolution of NAFLD. This review seeks to encapsulate the advancements in research regarding immune regulation in NAFLD, spanning from underlying mechanisms to practical applications.

Function and mechanism of TREM2 in bacterial infection

Triggering receptor expressed on myeloid cells 2 (TREM2), which is a lipid sensing and phagocytosis receptor, plays a key role in immunity and inflammation in response to pathogens. Here, we review the function and signaling of TREM2 in microbial binding, engulfment and removal, and describe TREM2-mediated inhibition of inflammation by negatively regulating the Toll-like receptor (TLR) response. We further illustrate the role of TREM2 in restoring organ homeostasis in sepsis and soluble TREM2 (sTREM2) as a diagnostic marker for sepsis-associated encephalopathy (SAE). Finally, we discuss the prospect of TREM2 as an interesting therapeutic target for sepsis.

Nutrient-sensing growth hormone secretagogue receptor in macrophage programming and meta-inflammation

OBJECTIVE

Obesity-associated chronic inflammation, aka meta-inflammation, is a key pathogenic driver for obesity-associated comorbidity. Growth hormone secretagogue receptor (GHSR) is known to mediate the effects of nutrient-sensing hormone ghrelin in food intake and fat deposition. We previously reported that global Ghsr ablation protects against diet-induced inflammation and insulin resistance, but the site(s) of action and mechanism are unknown. Macrophages are key drivers of meta-inflammation. To unravel the role of GHSR in macrophages, we generated myeloid-specific Ghsr knockout mice (LysM-Cre;Ghsrf/f).

METHODS

LysM-Cre;Ghsrf/f and control Ghsrf/f mice were subjected to 5 months of high-fat diet (HFD) feeding to induce obesity. In vivo, metabolic profiling of food intake, physical activity, and energy expenditure, as well as glucose and insulin tolerance tests (GTT and ITT) were performed. At termination, peritoneal macrophages (PMs), epididymal white adipose tissue (eWAT), and liver were analyzed by flow cytometry and histology. For ex vivo studies, bone marrow-derived macrophages (BMDMs) were generated from the mice and treated with palmitic acid (PA) or lipopolysaccharide (LPS). For in vitro studies, macrophage RAW264.7 cells with Ghsr overexpression or Insulin receptor substrate 2 (Irs2) knockdown were studied.

RESULTS

We found that Ghsr expression in PMs was increased under HFD feeding. In vivo, HFD-fed LysM-Cre;Ghsrf/f mice exhibited significantly attenuated systemic inflammation and insulin resistance without affecting food intake or body weight. Tissue analysis showed that HFD-fed LysM-Cre;Ghsrf/f mice have significantly decreased monocyte/macrophage infiltration, pro-inflammatory activation, and lipid accumulation, showing elevated lipid-associated macrophages (LAMs) in eWAT and liver. Ex vivo, Ghsr-deficient macrophages protected against PA- or LPS-induced pro-inflammatory polarization, showing reduced glycolysis, increased fatty acid oxidation, and decreased NF-κB nuclear translocation. At molecular level, GHSR metabolically programs macrophage polarization through PKA-CREB-IRS2-AKT2 signaling pathway.

CONCLUSIONS

These novel results demonstrate that macrophage GHSR plays a key role in the pathogenesis of meta-inflammation, and macrophage GHSR promotes macrophage infiltration and induces pro-inflammatory polarization. These exciting findings suggest that GHSR may serve as a novel immunotherapeutic target for the treatment of obesity and its associated comorbidity.

Efferocytosis in liver disease

The process of dead cell clearance by phagocytic cells, called efferocytosis, prevents inflammatory cell necrosis and promotes resolution and repair. Defective efferocytosis contributes to the progression of numerous diseases in which cell death is prominent, including liver disease. Many gaps remain in our understanding of how hepatic macrophages carry out efferocytosis and how this process goes awry in various types of liver diseases. Thus far, studies have suggested that, upon liver injury, liver-resident Kupffer cells and infiltrating monocyte-derived macrophages clear dead cells, limit inflammation, and, through macrophage reprogramming, repair liver damage. However, in unusual settings, efferocytosis can promote liver disease. In this review, we will focus on efferocytosis in various types of acute and chronic liver diseases, including metabolic dysfunction-associated steatohepatitis. Understanding the mechanisms and consequences of efferocytosis by hepatic macrophages has the potential to shed new light on liver disease pathophysiology and to guide new treatment strategies to prevent disease progression.