Dual role of B7 costimulation in obesity-related nonalcoholic steatohepatitis and metabolic dysregulation

Itaconate alleviates diet-induced obesity via activation of brown adipocyte thermogenesis

Despite medical advances, there remains an unmet need for better treatment of obesity. Itaconate, a product of the decarboxylation of the tricarboxylic acid cycle intermediate cis-aconitate, plays a regulatory role in both metabolism and immunity. Here, we show that itaconate, as an endogenous compound, counteracts high-fat-diet (HFD)-induced obesity through leptin-independent mechanisms in three mouse models. Specifically, itaconate reduces weight gain, reverses hyperlipidemia, and improves glucose tolerance in HFD-fed mice. Additionally, itaconate enhances energy expenditure and the thermogenic capacity of brown adipose tissue (BAT). Unbiased proteomic analysis reveals that itaconate upregulates key proteins involved in fatty acid oxidation and represses the expression of lipogenic genes. Itaconate may provoke a major metabolic reprogramming by inducing fatty acid oxidation and suppression of fatty acid synthesis in BAT. These findings highlight itaconate as a potential activator of BAT-mediated thermogenesis and a promising candidate for anti-obesity therapy.

ER stress signaling at the interphase between MASH and HCC

HCC is the most frequent primary liver cancer with an extremely poor prognosis and often develops on preset of chronic liver diseases. Major risk factors for HCC include metabolic dysfunction-associated steatohepatitis, a complex multifactorial condition associated with abnormal endoplasmic reticulum (ER) proteostasis. To cope with ER stress, the unfolded protein response engages adaptive reactions to restore the secretory capacity of the cell. Recent advances revealed that ER stress signaling plays a critical role in HCC progression. Here, we propose that chronic ER stress is a common transversal factor contributing to the transition from liver disease (risk factor) to HCC. Interventional strategies to target the unfolded protein response in HCC, such as cancer therapy, are also discussed.

The double roles of T cell-mediated immune response in the progression of MASLD

Metabolic dysfunction-associated steatotic liver disease(MASLD), formerly known as non-alcoholic fatty liver disease(NAFLD), has become a major cause of chronic liver disease and a significant risk factor for hepatocellular carcinoma, which poses a huge burden on global public health and economy. MASLD includes steatotic liver disease, steatohepatitis, and cirrhosis, and the latter two cause great harm to human health and life, even complicated with liver cancer. Immunologic mechanism plays a major role in promoting its development into hepatitis and cirrhosis. Now more and more evidences show that T cells play an important role in the progression of MASLD. In this review, we discuss the double roles of T cells in MASLD from the perspective of T cell response pathways, as well as new evidences regarding the possible application of immunomodulatory therapy in MASH.

The PD-1/PD-L1 Axis in the Biology of MASLD

Metabolic Dysfunction-Associated Steatotic Liver (MASL), previously named nonalcoholic fatty liver (NAFL), is a multifactorial disease in which metabolic, genetic, and environmental risk factors play a predominant role. Obesity and type 2 diabetes act as triggers of the inflammatory response, which contributes to the progression of MASL to Metabolic Dysfunction-Associated Steatohepatitis and the development of hepatocellular carcinoma. In the liver, several parenchymal, nonparenchymal, and immune cells maintain immunological homeostasis, and different regulatory pathways balance the activation of the innate and adaptative immune system. PD-1/PD-L1 signaling acts, in the maintenance of the balance between the immune responses and the tissue immune homeostasis, promoting self-tolerance through the modulation of activated T cells. Recently, PD-1 has received much attention for its roles in inducing an exhausted T cells phenotype, promoting the tumor escape from immune responses. Indeed, in MASLD, the excessive fat accumulation dysregulates the immune system, increasing cytotoxic lymphocytes and decreasing their cytolytic activity. In this context, T cells exacerbate liver damage and promote tumor progression. The aim of this review is to illustrate the main pathogenetic mechanisms by which the immune system promotes the progression of MASLD and the transition to HCC, as well as to discuss the possible therapeutic applications of PD-1/PD-L1 target therapy to activate T cells and reinvigorate immune surveillance against cancer.

Regulatory T cell: a double-edged sword from metabolic-dysfunction-associated steatohepatitis to hepatocellular carcinoma

Metabolic-dysfunction-associated steatotic liver disease (MASLD) is becoming a leading cause of end-stage liver disease globally. Metabolic-dysfunction-associated steatohepatitis (MASH) represents a progressive inflammatory manifestation of MASLD. MASH underlies a versatile and dynamic inflammatory microenvironment, accompanied by aberrant metabolism and ongoing liver regeneration, establishing itself as a significant risk factor for hepatocellular carcinoma (HCC). The mechanisms underlying the escape and survival of malignant cells within the extensive inflammatory microenvironment of MASH remain elusive. Regulatory T cells (Tregs) play a crucial role in maintaining homeostasis and preventing excessive immune responses in the liver. Paradoxically, Tregs have been implicated in inhibiting tumour-promoting inflammation and facilitating the evasion of cancer cells. Recent studies have unveiled distinct behaviours of Tregs at different stages of MASLD, suggesting a dual role in the pathogenesis. In this review, we explore the fate of Tregs from MASLD to HCC, offering recent insights into potential targets for clinical intervention.

Liver fibrosis in NAFLD/NASH: from pathophysiology towards diagnostic and therapeutic strategies

Liver fibrosis, as an excess deposition of extracellular matrix (ECM) components, results from chronic liver injury as well as persistent activation of inflammatory response and of fibrogenesis. Liver fibrosis is a major determinant for chronic liver disease (CLD) progression and in the last two decades our understanding on the major molecular and cellular mechanisms underlying the fibrogenic progression of CLD has dramatically improved, boosting pre-clinical studies and clinical trials designed to find novel therapeutic approaches. From these studies several critical concepts have emerged, starting to reveal the complexity of the pro-fibrotic microenvironment which involves very complex, dynamic and interrelated interactions between different hepatic and extrahepatic cell populations. This review will offer first a recapitulation of established and novel pathophysiological basic principles and concepts by intentionally focus the attention on NAFLD/NASH, a metabolic-related form of CLD with a high impact on the general population and emerging as a leading cause of CLD worldwide. NAFLD/NASH-related pro-inflammatory and profibrogenic mechanisms will be analysed as well as novel information on cells, mediators and signalling pathways which have taken advantage from novel methodological approaches and techniques (single cell genomics, imaging mass cytometry, novel in vitro two- and three-dimensional models, etc.). We will next offer an overview on recent advancement in diagnostic and prognostic tools, including serum biomarkers and polygenic scores, to support the analysis of liver biopsies. Finally, this review will provide an analysis of current and emerging therapies for the treatment of NAFLD/NASH patients.

The bidirectional immune crosstalk in metabolic dysfunction-associated steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) is an unabated risk factor for end-stage liver diseases with no available therapies. Dysregulated immune responses are critical culprits of MASLD pathogenesis. Independent contributions from either the innate or adaptive arms of the immune system or their unidirectional interplay are commonly studied in MASLD. However, the bidirectional communication between innate and adaptive immune systems and its impact on MASLD remain insufficiently understood. Given that both innate and adaptive immune cells are indispensable for the development and progression of inflammation in MASLD, elucidating pathogenic contributions stemming from the bidirectional interplay between these two arms holds potential for development of novel therapeutics for MASLD. Here, we review the immune cell types and bidirectional pathways that influence the pathogenesis of MASLD and highlight potential pharmacologic approaches to combat MASLD based on current knowledge of this bidirectional crosstalk.

From Reductionistic Approach to Systems Immunology Approach for the Understanding of Tumor Microenvironment

The tumor microenvironment (TME) is a complex and dynamic ecosystem that includes a variety of immune cells mutually interacting with tumor cells, structural/stromal cells, and each other. The immune cells in the TME can have dual functions as pro-tumorigenic and anti-tumorigenic. To understand such paradoxical functions, the reductionistic approach classifies the immune cells into pro- and anti-tumor cells and suggests the therapeutic blockade of the pro-tumor and induction of the anti-tumor immune cells. This strategy has proven to be partially effective in prolonging patients' survival only in a fraction of patients without offering a cancer cure. Recent advances in multi-omics allow taking systems immunology approach. This essay discusses how a systems immunology approach could revolutionize our understanding of the TME by suggesting that internetwork interactions of the immune cell types create distinct collective functions independent of the function of each cellular constituent. Such collective function can be understood by the discovery of the immunological patterns in the TME and may be modulated as a therapeutic means for immunotherapy of cancer.

Predictive plasma biomarkers of long-term increase in hepatic steatosis index after HCV eradication in HIV/HCV-coinfected patients

Hepatic steatosis is a common condition found in the liver of hepatitis C virus (HCV)-infected patients, contributing to more severe forms of liver disease. In addition, the human immunodeficiency virus (HIV) may accelerate this process. Alternatively, several immune checkpoint proteins have been reported to be upregulated and correlated with disease progression during HCV and HIV infections. In steatosis, a detrimental immune system activation has been established; however, the role of the immune checkpoints has not been addressed so far. Thus, this study aimed to evaluate the association between plasma immune checkpoint proteins at baseline (before antiviral therapy) with hepatic steatosis index (HSI) increase at the end of follow-up (∼ five years after sustained virologic response (SVR)). We performed a multicenter retrospective study in 62 patients coinfected with HIV/HCV who started antiviral therapy. Immune checkpoint proteins were analyzed at baseline using a Luminex 200TM analyzer. The statistical association analysis was carried out using Generalized Linear Models (GLM) and Partial Least Squares Discriminant Analysis (PLS-DA). Fifty-three percent of the patients showed HSI increase from baseline to the end of follow-up. Higher immune checkpoint protein levels of BTLA, CD137(4-1BB), CD80, GITR, LAG-3, and PD-L1 before HCV therapy were associated with a long-term increase in HSI after successful HCV therapy, suggesting a potential predictive role for early detection of progression towards steatosis in HIV/HCV-coinfected patients.

Integrated omics analysis for characterization of the contribution of high fructose corn syrup to non-alcoholic fatty liver disease in obesity

BACKGROUND

High-Fructose Corn Syrup (HFCS), a sweetener rich in glucose and fructose, is nowadays widely used in beverages and processed foods; its consumption has been correlated to the emergence and progression of Non-Alcoholic Fatty Liver Disease (NAFLD). Nevertheless, the molecular mechanisms by which HFCS impacts hepatic metabolism remain scarce, especially in the context of obesity. Besides, the majority of current studies focuses either on the detrimental role of fructose in hepatic steatosis or compare separately the additive impact of fructose versus glucose in high fat diet-induced NAFLD.

AIM

By engaging combined omics approaches, we sought to characterize the role of HFCS in obesity-associated NAFLD and reveal molecular processes, which mediate the exaggeration of steatosis under these conditions.

METHODS

Herein, C57BL/6 mice were fed a normal-fat-diet (ND), a high-fat-diet (HFD) or a HFD supplemented with HFCS (HFD-HFCS) and upon examination of their metabolic and NAFLD phenotype, proteomic, lipidomic and metabolomic analyses were conducted to identify HFCS-related molecular alterations of the hepatic metabolic landscape in obesity.

RESULTS

Although HFD and HFD-HFCS mice displayed comparable obesity, HFD-HFCS mice showed aggravation of hepatic steatosis, as analysis of the lipid droplet area in liver sections revealed (12,15 % of total section area in HFD vs 22,35 % in HFD-HFCS), increased NAFLD activity score (3,29 in HFD vs 4,86 in HFD-HFCS) and deteriorated hepatic insulin resistance, as compared to the HFD mice. Besides, the hepatic proteome of HFD-HFCS mice was characterized by a marked upregulation of 5 core proteins implicated in de novo lipogenesis (DNL), while an increased phosphatidyl-cholines(PC)/phosphatidyl-ethanolamines(PE) ratio (2.01 in HFD vs 3.04 in HFD-HFCS) was observed in the livers of HFD-HFCS versus HFD mice. Integrated analysis of the omics datasets indicated that Tricarboxylic Acid (TCA) cycle overactivation is likely contributing towards the intensification of steatosis during HFD-HFCS-induced NAFLD.

CONCLUSION

Our results imply that HFCS significantly contributes to steatosis aggravation during obesity-related NAFLD, likely deriving from DNL upregulation, accompanied by TCA cycle overactivation and deteriorated hepatic insulin resistance.

Decoding the role of immune T cells: A new territory for improvement of metabolic-associated fatty liver disease

Metabolic-associated fatty liver disease (MAFLD) is a new emerging concept and is associated with metabolic dysfunction, generally replacing the name of nonalcoholic fatty liver disease (NAFLD) due to heterogeneous liver condition and inaccuracies in definition. The prevalence of MAFLD is rising by year due to dietary changes, metabolic disorders, and no approved therapy, affecting a quarter of the global population and representing a major economic problem that burdens healthcare systems. Currently, in addition to the common causative factors like insulin resistance, oxidative stress, and lipotoxicity, the role of immune cells, especially T cells, played in MAFLD is increasingly being emphasized by global scholars. Based on the diverse classification and pathophysiological effects of immune T cells, we comprehensively analyzed their bidirectional regulatory effects on the hepatic inflammatory microenvironment and MAFLD progression. This interaction between MAFLD and T cells was also associated with hepatic-intestinal immune crosstalk and gut microbiota homeostasis. Moreover, we pointed out several T-cell-based therapeutic approaches including but not limited to adoptive transfer of T cells, fecal microbiota transplantation, and drug therapy, especially for natural products and Chinese herbal prescriptions. Overall, this study contributes to a better understanding of the important role of T cells played in MAFLD progression and corresponding therapeutic options and provides a potential reference for further drug development.

Tissue adaptation of regulatory T cells in adipose tissue

Foxp3⁺ regulatory T (Treg) cells critically suppress over-activated immune responses and therefore maintain immune homeostasis. Adipose tissue-resident Treg (AT Treg) cells are known for modulating immunity and metabolism in adipose tissue microenvironment through various physiological signals, as well as their heterogeneous subsets, which potentially play disparate roles in aging and obesity. Recent single-cell studies of Treg cells have revealed specialized trajectories of their tissue adaptation and development in lymphoid tissues and at barrier sites. Here, we reviewed a T Cell Receptor (TCR)-primed environmental cue-boosted model of adipose Treg cells' tissue adaptation, especially in response to IL-33, IFN-α, insulin, and androgen signals, which trigger sophisticated transcriptional cascades and ultimately establish unique transcriptional modules in adipose Treg cell subsets. In addition, we further discuss potential therapeutic strategies against aging and obesity by blocking detrimental environmental cues, strengthening the functions of specific AT Treg subsets and modifying the communications between AT Treg subsets and adipocytes.

Fibrogenic Pathways in Metabolic Dysfunction Associated Fatty Liver Disease (MAFLD)

The prevalence of nonalcoholic fatty liver disease (NAFLD), recently also re-defined as metabolic dysfunction associated fatty liver disease (MAFLD), is rapidly increasing, affecting ~25% of the world population. MALFD/NAFLD represents a spectrum of liver pathologies including the more benign hepatic steatosis and the more advanced non-alcoholic steatohepatitis (NASH). NASH is associated with enhanced risk for liver fibrosis and progression to cirrhosis and hepatocellular carcinoma. Hepatic stellate cells (HSC) activation underlies NASH-related fibrosis. Here, we discuss the profibrogenic pathways, which lead to HSC activation and fibrogenesis, with a particular focus on the intercellular hepatocyte–HSC and macrophage–HSC crosstalk.

Immune-related pathogenesis and therapeutic strategies of nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome and has become prevalent in the adult population worldwide, given the ongoing obesity pandemic. NAFLD comprises several hepatic disorders, ranging from fatty liver to nonalcoholic steatohepatitis (NASH), cirrhosis, and carcinoma. Excessive fat accumulation in the liver can induce the development of fatty liver, whereas the progression of fatty liver to NASH involves various complex factors. The crucial difference between fatty liver and NASH is the presence of inflammation and fibrosis, the emergence of which is closely associated with the action of immune cells and immunological factors, such as chemokines and cytokines. Thus, expanding our understanding of immunological mechanisms contributing to NASH pathogenesis will lead to the identification of therapeutic targets and the development of viable therapeutics against NASH.

Hepatic Senescence Accompanies the Development of NAFLD in Non-Aged Mice Independently of Obesity

Senescence is considered to be a cardinal player in several chronic inflammatory and metabolic pathologies. The two dominant mechanisms of senescence include replicative senescence, predominantly depending on age-induced telomere shortening, and stress-induced senescence, triggered by external or intracellular harmful stimuli. Recent data indicate that hepatocyte senescence is involved in the development of nonalcoholic fatty liver disease (NAFLD). However, previous studies have mainly focused on age-related senescence during NAFLD, in the presence or absence of obesity, while information about whether the phenomenon is characterized by replicative or stress-induced senescence, especially in non-aged organisms, is scarce. Herein, we subjected young mice to two different diet-induced NAFLD models which differed in the presence of obesity. In both models, liver fat accumulation and increased hepatic mRNA expression of steatosis-related genes were accompanied by hepatic senescence, indicated by the increased expression of senescence-associated genes and the presence of a robust hybrid histo-/immunochemical senescence-specific staining in the liver. Surprisingly, telomere length and global DNA methylation did not differ between the steatotic and the control livers, while malondialdehyde, a marker of oxidative stress, was upregulated in the mouse NAFLD livers. These findings suggest that senescence accompanies NAFLD emergence, even in non-aged organisms, and highlight the role of stress-induced senescence during steatosis development independently of obesity.

Metabolic tissue-resident CD8+ T cells: A key player in obesity-related diseases

Obesity-induced low-grade chronic inflammation in the metabolic tissues, such as adipose tissue (AT) and liver tissue, in individuals with obesity is a major etiological factor for several diseases, such as insulin resistance, type 2 diabetes, fatty liver disease, atherosclerosis and cardiovascular problems, as well as cancer and autoimmune diseases. Previous studies have revealed that tissue-resident macrophages play a crucial role in this process. However, the mechanisms responsible for recruiting and activating macrophages and initiating chronic inflammation in the metabolic tissues have not yet been clearly elucidated. In the most recent decade, there has been a growing emphasis on the critical role of the adaptive CD8⁺ T cells in obesity-induced chronic inflammation and related metabolic diseases. In this review, we will summarize the relevant studies in both mice and human regarding the role of metabolic tissue-resident CD8⁺ T cells in obesity-related inflammation and diseases, as well as the possible mechanisms underlying the regulation of CD8⁺ T cell recruitment, activation and function in the metabolic tissues, and discuss their potential as therapeutic targets for obesity-related diseases.

Inflammatory Mechanisms Underlying Nonalcoholic Steatohepatitis and the Transition to Hepatocellular Carcinoma

Nonalcoholic fatty liver disease (NAFLD) is a rising chronic liver disease and comprises a spectrum from simple steatosis to nonalcoholic steatohepatitis (NASH) to end-stage cirrhosis and risk of hepatocellular carcinoma (HCC). The pathogenesis of NAFLD is multifactorial, but inflammation is considered the key element of disease progression. The liver harbors an abundance of resident immune cells, that in concert with recruited immune cells, orchestrate steatohepatitis. While inflammatory processes drive fibrosis and disease progression in NASH, fueling the ground for HCC development, immunity also exerts antitumor activities. Furthermore, immunotherapy is a promising new treatment of HCC, warranting a more detailed understanding of inflammatory mechanisms underlying the progression of NASH and transition to HCC. Novel methodologies such as single-cell sequencing, genetic fate mapping, and intravital microscopy have unraveled complex mechanisms behind immune-mediated liver injury. In this review, we highlight some of the emerging paradigms, including macrophage heterogeneity, contributions of nonclassical immune cells, the role of the adaptive immune system, interorgan crosstalk with adipose tissue and gut microbiota. Furthermore, we summarize recent advances in preclinical and clinical studies aimed at modulating the inflammatory cascade and discuss how these novel therapeutic avenues may help in preventing or combating NAFLD-associated HCC.

The Adipocyte and Adaptive Immunity

Not only do Adipocytes have energy storage and endocrine functions, but they also play an immunological role. Adipocytes are involved in adaptive immunity to mediate the pathological processes of a variety of chronic inflammatory diseases and autoimmune syndromes. The adaptive immune response consists of T cell-mediated cellular immunity and B cell-mediated humoral immunity. Obese adipocytes overexpress MHC class II molecules and costimulators to act as antigen-presenting cells (APCs) and promote the activation of CD4⁺ T cells. In addition, various adipokines secreted by adipocytes regulate the proliferation and differentiation of T cells. Adipokines are also involved in B cell generation, development, activation, and antibody production. Therefore, adipocytes play an important role in B cell-mediated adaptive immunity. This review describes how adipocytes participate in adaptive immunity from the perspective of T cells and B cells, and discusses their role in the pathogenesis of various diseases.

Metabolic inflammation as an instigator of fibrosis during non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is characterized by excessive storage of fatty acids in the form of triglycerides in hepatocytes. It is most prevalent in western countries and includes a wide range of clinical and histopathological findings, namely from simple steatosis to steatohepatitis and fibrosis, which may lead to cirrhosis and hepatocellular cancer. The key event for the transition from steatosis to fibrosis is the activation of quiescent hepatic stellate cells (qHSC) and their differentiation to myofibroblasts. Pattern recognition receptors (PRRs), expressed by a plethora of immune cells, serve as essential components of the innate immune system whose function is to stimulate phagocytosis and mediate inflammation upon binding to them of various molecules released from damaged, apoptotic and necrotic cells. The activation of PRRs on hepatocytes, Kupffer cells, the resident macrophages of the liver, and other immune cells results in the production of proinflammatory cytokines and chemokines, as well as profibrotic factors in the liver microenvironment leading to qHSC activation and subsequent fibrogenesis. Thus, elucidation of the inflammatory pathways associated with the pathogenesis and progression of NAFLD may lead to a better understanding of its pathophysiology and new therapeutic approaches.

Tolerogenic properties of liver macrophages in non-alcoholic steatohepatitis

BACKGROUND & AIMS

Our understanding of non-alcoholic fatty liver disease (NAFLD) pathogenesis is improving, but there is still limited data on the function of resident liver macrophages in this context, especially when considering their contribution in dampening liver inflammation.

METHODS

Liver macrophages were studied in mouse models of prolonged diet-induced liver steatohepatitis and carbon tetrachloride-induced liver injury. We assessed liver macrophages phenotype and costimulatory/inhibitory properties upon exposure to lipopolysaccharide or interleukin 4. We did phagocytosis and antigen presentation assays to investigate liver macrophages function as scavengers and immune response initiators. Using immunofluorescence staining, we further determined, in human liver tissue of patients with simple steatosis, non-alcoholic steatohepatitis and chronic hepatitis B infection, the expression of the co-inhibitory protein CD274 (Programmed-death ligand 1) and major histocompatibility complex (MHC) class II.

RESULTS

Both in humans and mice, within chronically inflamed fatty livers, liver macrophages acquired immunomodulatory properties by reducing the expression of MHC class II, and by enhancing co-inhibitory signalling. Liver macrophages circumscribed endotoxin-mediated inflammatory response by upregulating anti-inflammatory genes arginase 1 and interleukin-10. While hepatic macrophages isolated from mice with normal livers were capable of achieving endotoxin tolerance, our results indicated an impairment of this protective mechanism in the presence NASH-like parenchymal abnormalities.

CONCLUSIONS

Liver macrophages can achieve endotoxin tolerance, but in the chronically inflamed fatty liver, while they acquire an immunomodulatory phenotype, liver macrophages fail to dampen immune-mediated damage. Therefore, loss of tolerogenicity induced by ongoing liver insult may be a mechanism contributing to the worsening of NAFLD.

Adaptive immunity: an emerging player in the progression of NAFLD

In the past decade, nonalcoholic fatty liver disease (NAFLD) has become a leading cause of chronic liver disease and cirrhosis, as well as an important risk factor for hepatocellular carcinoma (HCC). NAFLD encompasses a spectrum of liver lesions, including simple steatosis, steatohepatitis and fibrosis. Although steatosis is often harmless, the lobular inflammation that characterizes nonalcoholic steatohepatitis (NASH) is considered a driving force in the progression of NAFLD. The current view is that innate immune mechanisms represent a key element in supporting hepatic inflammation in NASH. However, increasing evidence points to the role of adaptive immunity as an additional factor promoting liver inflammation. This Review discusses data regarding the role of B cells and T cells in sustaining the progression of NASH to fibrosis and HCC, along with the findings that antigens originating from oxidative stress act as a trigger for immune responses. We also highlight the mechanisms affecting liver immune tolerance in the setting of steatohepatitis that favour lymphocyte activation. Finally, we analyse emerging evidence concerning the possible application of immune modulating treatments in NASH therapy.

OX40 Regulates Both Innate and Adaptive Immunity and Promotes Nonalcoholic Steatohepatitis

Both innate and adaptive immune cells are involved in the pathogenesis of nonalcoholic steatohepatitis (NASH), but the crosstalk between innate and adaptive immunity is largely unknown. Here we show that compared with WT mice, OX40^-/- mice exhibit decreased liver fat accumulation, lobular inflammation, and focal necrosis after feeding with diets that induce NASH. Mechanistically, OX40 deficiency suppresses Th1 and Th17 differentiation, and OX40 deficiency in T cells inhibits monocyte migration, antigen presentation, and M1 polarization. Soluble OX40 stimulation alone upregulates antigen presentation, chemokine receptor expression, and proinflammatory cytokine secretion by liver monocytes. Furthermore, plasma soluble OX40 levels are positively associated with NASH in humans, suggesting clinical relevance of the findings. In conclusion, we show a mechanism for T cell regulation of innate immune cells. OX40 is a key regulator of both intrahepatic innate and adaptive immunity, generates two-way signals, and promotes both proinflammatory monocyte and macrophage and T cell function, resulting in NASH development.

Depletion of CD40 on CD11c+ cells worsens the metabolic syndrome and ameliorates hepatic inflammation during NASH

The co-stimulatory CD40-CD40L dyad plays a central role in fine-tuning immune reactions, including obesity-induced inflammation. Genetic ablation of CD40L reduced adipose tissue inflammation, while absence of CD40 resulted in aggravated metabolic dysfunction in mice. During obesity, CD40 expressing CD11c⁺ dendritic cells (DC) and macrophages accumulate in adipose tissue and liver. We investigated the role of CD40⁺CD11c⁺ cells in the metabolic syndrome and nonalcoholic steatohepatitis (NASH). DC-CD40-ko mice (CD40^fl/flCD11c^cre) mice were subjected to obesity or NASH. Obesity and insulin resistance were induced by feeding mice a 54% high fat diet (HFD). NASH was induced by feeding mice a diet containing 40% fat, 20% fructose and 2% cholesterol. CD40^fl/flCD11c^cre mice fed a HFD displayed increased weight gain, increased adipocyte size, and worsened insulin resistance. Moreover, CD40^fl/flCD11c^cre mice had higher plasma and hepatic cholesterol levels and developed profound liver steatosis. Overall, regulatory T cell numbers were decreased in these mice. In NASH, absence of CD40 on CD11c⁺ cells slightly decreased liver inflammation but did not affect liver lipid accumulation. Our experiments suggest that CD40 expressing CD11c⁺ cells can act as a double-edged sword: CD40 expressing CD11c⁺ cells contribute to liver inflammation during NASH but are protective against the metabolic syndrome via induction of regulatory T cells.

Tissue Immune Cells Fuel Obesity-Associated Inflammation in Adipose Tissue and Beyond

Obesity-associated inflammation stems from a combination of cell-intrinsic changes of individual immune cell subsets and the dynamic crosstalk amongst a broad array of immune cells. Although much of the focus of immune cell contributions to metabolic disease has focused on adipose tissue-associated cells, these potent sources of inflammation inhabit other metabolic regulatory tissues, including liver and gut, and recirculate to promote systemic inflammation and thus obesity comorbidities. Tissue-associated immune cells, especially T cell subpopulations, have become a hotspot of inquiry based on their contributions to obesity, type 2 diabetes, non-alcoholic fatty liver diseases and certain types of cancers. The cell-cell interactions that take place under the stress of obesity are mediated by intracellular contact and cytokine production, and constitute a complicated network that drives the phenotypic alterations of immune cells and perpetuates a feed-forward loop of metabolic decline. Herein we discuss immune cell functions in various tissues and obesity-associated cancers from the viewpoint of inflammation. We also emphasize recent advances in the understanding of crosstalk amongst immune cell subsets under obese conditions, and suggest future directions for focused investigations with clinical relevance.

Comprehensive and quantitative analysis of white and brown adipose tissue by shotgun lipidomics

Objective

Shotgun lipidomics enables an extensive analysis of lipids from tissues and fluids. Each specimen requires appropriate extraction and processing procedures to ensure good coverage and reproducible quantification of the lipidome. Adipose tissue (AT) has become a research focus with regard to its involvement in obesity-related pathologies. However, the quantification of the AT lipidome is particularly challenging due to the predominance of triacylglycerides, which elicit high ion suppression of the remaining lipid classes.

Methods

We present a new and validated method for shotgun lipidomics of AT, which tailors the lipid extraction procedure to the target specimen and features high reproducibility with a linear dynamic range of at least 4 orders of magnitude for all lipid classes.

Results

Utilizing this method, we observed tissue-specific and diet-related differences in three AT types (brown, gonadal, inguinal subcutaneous) from lean and obese mice. Brown AT exhibited a distinct lipidomic profile with the greatest lipid class diversity and responded to high-fat diet by altering its lipid composition, which shifted towards that of white AT. Moreover, diet-induced obesity promoted an overall remodeling of the lipidome, where all three AT types featured a significant increase in longer and more unsaturated triacylglyceride and phospholipid species.

Conclusions

The here presented method facilitates reproducible systematic lipidomic profiling of AT and could be integrated with further –omics approaches used in (pre-) clinical research, in order to advance the understanding of the molecular metabolic dynamics involved in the pathogenesis of obesity-associated disorders.

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Validated shotgun lipidomics method of AT covering 300 lipids of 20 classes and linear dynamic range of 4 orders of magnitude.

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Increase of longer and more unsaturated triacylglycerides and phospholipids in brown and white AT under high-fat diet.

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Differences in the lipidomes of gonadal, subcutaneous and brown AT.

Ascorbic acid inhibits visceral obesity and nonalcoholic fatty liver disease by activating peroxisome proliferator-activated receptor α in high-fat-diet-fed C57BL/6J mice

BACKGROUND/OBJECTIVES

Ascorbic acid is a known cofactor in the biosynthesis of carnitine, a molecule that has an obligatory role in fatty acid oxidation. Our previous studies have demonstrated that obesity is regulated effectively through peroxisome proliferator-activated receptor α (PPARα)-mediated fatty acid β-oxidation. Thus, this study aimed to determine whether ascorbic acid can inhibit obesity and nonalcoholic fatty liver disease (NAFLD) in part through the actions of PPARα.

DESIGN

After C57BL/6J mice received a low-fat diet (LFD, 10% kcal fat), a high-fat diet (HFD, 45% kcal fat), or the same HFD supplemented with ascorbic acid (1% w/w) (HFD-AA) for 15 weeks, variables and determinants of visceral obesity and NAFLD were examined using metabolic measurements, histology, and gene expression.

RESULTS

Compared to HFD-fed obese mice, administration of HFD-AA to obese mice reduced body weight gain, visceral adipose tissue mass, and visceral adipocyte size without affecting food consumption profiles. Concomitantly, circulating ascorbic acid concentrations were significantly higher in HFD-AA mice than in HFD mice. Ascorbic acid supplementation increased the mRNA levels of PPARα and its target enzymes involved in fatty acid β-oxidation in visceral adipose tissues. Consistent with the effects of ascorbic acid on visceral obesity, ascorbic acid not only inhibited hepatic steatosis but also increased the mRNA levels of PPARα-dependent fatty acid β-oxidation genes in livers. Similarly, hepatic inflammation, fibrosis, and apoptosis were also decreased during ascorbic acid-induced inhibition of visceral obesity. In addition, serum levels of alanine aminotransferase, aspartate aminotransferase, total cholesterol, and LDL cholesterol were lower in HFD-AA-fed mice than in those of HFD-fed mice.

CONCLUSIONS

These results suggest that ascorbic acid seems to suppress HFD-induced visceral obesity and NAFLD in part through the activation of PPARα.

A Unique Population: Adipose-Resident Regulatory T Cells

Regulatory T (Treg) cell is well known for its anti-inflammatory function in a variety of tissues in health and disease. Accordingly, Treg cells that reside in adipose tissue exhibit specific phenotypes. Their numbers are regulated by age, gender and environmental factors, such as diet and cold stimulation. Adipose-resident Treg cells have been suggested to be critical regulators of immune and metabolic microenvironment in adipose tissue, as well as involved in pathogenesis of obesity-related metabolic disorders. This review surveys existing information on adipose-resident Treg cells. We first describe the origin, phenotype and function of adipose-resident Treg cells. We then describe the major regulators of adipose-resident Treg cells, and discuss how the adipose-resident Treg cells are regulated in lean and obese conditions, especially in humans. Finally, we highlight their therapeutic potential in obesity-related disorders.

Effect of Gangjihwan on hepatic steatosis and inflammation in high fat diet-fed mice

ETHNOPHARMACOLOGICAL RELEVANCE

Gangjihwan (DF), a polyherbal drug composed of Ephedra intermedia Schrenk et C. A. Mayer (Ephedraceae), Lithospermum erythrorhizon Siebold et Zuccarini (Borraginaceae), and Rheum palmatum L. (Polygonaceae), is used to treat obesity in local Korean clinics. The constituents of DF have traditionally been reported to exert anti-obesity and anti-nonalcoholic fatty liver disease (NAFLD) effects. Thus, we investigated the effects of DF on obesity and NAFLD and the underlying mechanisms.

MATERIALS AND METHODS

DF was extracted with water (DF-FW), 30% ethyl alcohol (DF-GA30), or 70% ethyl alcohol (DF-GA70). The chemical profile of DF was monitored using high performance liquid chromatography (HPLC)-ultraviolet analysis. The effects of DF on indices of obesity and NAFLD in high fat diet (HFD)-fed C57BL/6J mice and HepG2 cells were examined using quantitative real-time polymerase chain reaction, Oil red O staining, hematoxylin-eosin staining, toluidine blue staining, and immunohistochemistry.

RESULTS

The presence of ephedrine, pseudoephedrine, aloe-emodin, and emodin in DF was determined by 3D chromatography using HPLC. Administration of DF-GA70 to HFD-fed obese mice decreased body weight, epididymal adipose tissue mass, and epididymal adipocyte size. DF-GA70 reduced serum levels of free fatty acids and triglycerides. All three DF extracts lowered serum alanine transaminase levels, hepatic lipid accumulation, and infiltration of macrophages, with the largest effects observed for DF-GA70. DF-GA70 increased mRNA levels of fatty acid oxidation genes and decreased mRNA levels of genes for lipogenesis and inflammation in the liver of obese mice. Treatment of HepG2 cells with a mixture of oleic acid and palmitoleic acid induced significant lipid accumulation, whereas all three DF extracts inhibited lipid accumulation. DF-GA70 also altered the expression of lipolytic and lipogenic genes in HepG2 cells.

CONCLUSIONS

These results indicate that DF inhibits obesity and obesity-induced severe hepatic steatosis and inflammation without any adverse effects and that these effects may be mediated by regulation of the hepatic expression of lipid metabolism and inflammatory genes. These findings suggest that DF is a safe and efficient anti-obesity and anti-nonalcoholic steatohepatosis drug.

A self-sustained loop of inflammation-driven inhibition of beige adipogenesis in obesity

In obesity, white adipose tissue (AT) inflammation is associated with reduced beige adipogenesis, a thermogenic and energy-dissipating function mediated by uncoupling protein-1 (UCP1)-expressing beige adipocytes. Here, we dissected an inflammation-driven inhibitory mechanism of beige adipogenesis in obesity that required direct adhesive interactions between macrophages and adipocytes mediated, respectively, by α₄ integrin and its counter-receptor VCAM-1, the expression of which was upregulated in obesity. This adhesive interaction reciprocally and concomitantly modulated inflammatory activation in macrophages and Erk–dependent downregulation of UCP1 in adipocytes. Genetic or pharmacologic inactivation of α₄ integrin in mice resulted in elevated UCP1 expression and beige adipogenesis of the subcutaneous AT in obesity. Our findings, established in both mouse and human systems, reveal a self-sustained cycle of inflammation-driven impairment of beige adipogenesis in obesity.

Modulation of T Cell Activation in Obesity

SIGNIFICANCE

Immune T cells are present in adipose tissues (AT), and the stoichiometry of the different T cell subsets is altered during diet-induced obesity (DIO). T cells contribute to the early steps of AT inflammation during DIO. Recent Advances: Many factors could potentially be responsible for this altered pro-inflammatory versus anti-inflammatory T cell balance.

CRITICAL ISSUES

T cells are potentially activated in AT, which vitamin D might contribute to, as will be discussed in this article. In addition, we will review the different possible contributors to T cell activation in AT, such as the CD28 and CD154 T cell costimulatory molecules in AT.

FUTURE DIRECTIONS

The potential antigen presentation capacities of adipocytes should be further investigated. Moreover, the properties of these AT resident (or migrating to AT) T cells must be further assessed. Antioxid. Redox Signal. 26, 489-500.

The effects of herbal composition Gambigyeongsinhwan (4) on hepatic steatosis and inflammation in Otsuka Long-Evans Tokushima fatty rats and HepG2 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Hepatic steatosis has risen rapidly in parallel with a dramatic increase in obesity. The aim of this study was to determine whether the herbal composition Gambigyeongsinhwan (4) (GGH(4)), composed of Curcuma longa L. (Zingiberaceae), Alnus japonica (Thunb.) Steud. (Betulaceae), and the fermented traditional Korean medicine Massa Medicata Fermentata, regulates hepatic steatosis and inflammation.

MATERIALS AND METHODS

The effects of GGH(4) on hepatic steatosis and inflammation in Otsuka Long-Evans Tokushima fatty (OLETF) rats and HepG2 cells were examined using Oil red O, hematoxylin and eosin, and toluidine blue staining, immunohistochemistry, quantitative real-time polymerase chain reaction, and peroxisome proliferator-activated receptor α (PPARα) transactivation assay.

RESULTS

Administration of GGH(4) to OLETF rats improved hepatic steatosis and lowered serum levels of alanine transaminase, total cholesterol, triglycerides, and free fatty acids. GGH(4) increased mRNA levels of fatty acid oxidation enzymes (ACOX, HD, CPT-1, and MCAD) and decreased mRNA levels of lipogenesis genes (FAS, ACC1, C/EBPα, and SREBP-1c) in the liver of OLETF rats. In addition, infiltration of inflammatory cells and expression of inflammatory cytokines (CD68, TNFα, and MCP-1) in liver tissue were reduced by GGH(4). Treatment of HepG2 cells with a mixture of oleic acid and palmitoleic acid induced significant lipid accumulation, but GGH(4) inhibited lipid accumulation by regulating the expression of hepatic fatty acid oxidation and lipogenic genes. GGH(4) also increased PPARα reporter gene expression. These effects of GGH(4) were similar to those of the PPARα activator fenofibrate, whereas the PPARα antagonist GW6471 reversed the inhibitory effects of GGH(4) on lipid accumulation in HepG2 cells.

CONCLUSIONS

These results suggest that GGH(4) inhibits obesity-induced hepatic steatosis and that this process may be mediated by regulation of the expression of PPARα target genes and lipogenic genes. GGH(4) also suppressed obesity-related hepatic inflammation. Thus, GGH(4) may be a promising drug for the treatment of obesity-related liver diseases.

Liver inflammation and fibrosis

Chronic liver inflammation leads to fibrosis and cirrhosis, which is the 12th leading cause of death in the United States. Hepatocyte steatosis is a component of metabolic syndrome and insulin resistance. Hepatic steatosis may be benign or progress to hepatocyte injury and the initiation of inflammation, which activates immune cells. While Kupffer cells are the resident macrophage in the liver, inflammatory cells such as infiltrating macrophages, T lymphocytes, neutrophils, and DCs all contribute to liver inflammation. The inflammatory cells activate hepatic stellate cells, which are the major source of myofibroblasts in the liver. Here we review the initiation of inflammation in the liver, the liver inflammatory cells, and their crosstalk with myofibroblasts.

No Role for Mast Cells in Obesity-Related Metabolic Dysregulation

Obesity-related adipose tissue (AT) inflammation that promotes metabolic dysregulation is associated with increased AT mast cell numbers. Mast cells are potent inducers of inflammatory responses and could potentially contribute to obesity-induced AT inflammation and metabolic dysregulation. Conflicting findings were reported on obesity-related metabolic dysfunction in mast cell-deficient mice, thus creating a controversy that has not been resolved to date. Whereas traditional Kit hypomorphic mast cell-deficient strains featured reduced diet-induced obesity and diabetes, a Kit-independent model of mast cell deficiency, Cpa3^Cre/+ mice, displayed no alterations in obesity and insulin sensitivity. Herein, we analyzed diet-induced obesity in Mcpt5-Cre R-DTA mice, in which the lack of mast cells is caused by a principle different from mast cell deficiency in Cpa3^Cre/+ mice or Kit mutations. We observed no difference between mast cell-deficient and -proficient mice in diet-induced obesity with regards to weight gain, glucose tolerance, insulin resistance, metabolic parameters, hepatic steatosis, and AT or liver inflammation. We conclude that mast cells play no essential role in obesity and related pathologies.

An inflammatory link in atherosclerosis and obesity. Co-stimulatory molecules

Atherosclerosis and obesity-induced metabolic dysfunction are lipid-driven inflammatory pathologies responsible for a major part of cardiovascular complications. Immune cell activation as well as interactions between the different immune cells is dependent on and controlled by a variety of co-stimulatory signals. These co-stimulatory signals can either aggravate or ameliorate the disease depending on the stage of the disease, the cell-types involved and the signal transduction cascades initiated. This review focuses on the diverse roles of the most established co-stimulatory molecules of the B7 and Tumor Necrosis Factor Receptor (TNFR) families, ie the CD28/CTLA4-CD80/CD86 and CD40L/CD40 dyads in the pathogenesis of atherosclerosis and obesity. In addition, we will explore their potential as therapeutic targets in both atherosclerosis and obesity.

The role of immune mechanisms in alcoholic and nonalcoholic steatohepatitis: a 2015 update

So far, innate immune mechanisms have been recognized as the main responsible for the evolution of both alcoholic steatohepatitis (ASH) and nonalcoholic steatohepatitis (NASH). However, increasing evidence points toward the possible role of adaptive immune responses, as an additional factor in promoting hepatic inflammation in steatohepatitis. In this article, we discuss recent data involving circulating antibodies and lymphocyte-mediated responses in sustaining the progression of ASH and NASH to fibrosis, as well as the possible mechanisms implicated in favoring the onset of adaptive immunity in the setting of steatohepatitis.

The Immune Landscape in Nonalcoholic Steatohepatitis

The liver lies at the intersection of multiple metabolic pathways and consequently plays a central role in lipid metabolism. Pathological disturbances in hepatic lipid metabolism are characteristic of chronic metabolic diseases, such as obesity-mediated insulin resistance, which can result in nonalcoholic fatty liver disease (NAFLD). Tissue damage induced in NAFLD activates and recruits liver-resident and non-resident immune cells, resulting in nonalcoholic steatohepatitis (NASH). Importantly, NASH is associated with an increased risk of significant clinical sequelae such as cirrhosis, cardiovascular diseases, and malignancies. In this review, we describe the immunopathogenesis of NASH by defining the known functions of immune cells in the progression and resolution of disease.

Antigen Presentation and T-Cell Activation Are Critical for RBP4-Induced Insulin Resistance

Adipose tissue (AT) inflammation contributes to impaired insulin action, which is a major cause of type 2 diabetes. RBP4 is an adipocyte- and liver-derived protein with an important role in insulin resistance, metabolic syndrome, and AT inflammation. RBP4 elevation causes AT inflammation by activating innate immunity, which elicits an adaptive immune response. RBP4-overexpressing mice (RBP4-Ox) are insulin resistant and glucose intolerant and have increased AT macrophages and T-helper 1 cells. We show that high-fat diet-fed RBP4(-/-) mice have reduced AT inflammation and improved insulin sensitivity versus wild type. We also elucidate the mechanism for RBP4-induced macrophage antigen presentation and subsequent T-cell activation. In RBP4-Ox, AT macrophages display enhanced c-Jun N-terminal kinase, extracellular signal-related kinase, and p38 phosphorylation. Inhibition of these pathways and of NF-κB reduces activation of macrophages and CD4 T cells. MyD88 is an adaptor protein involved in proinflammatory signaling. In macrophages from MyD88(-/-) mice, RBP4 fails to stimulate secretion of tumor necrosis factor, IL-12, and IL-6 and CD4 T-cell activation. In vivo blockade of antigen presentation by treating RBP4-Ox mice with CTLA4-Ig, which blocks costimulation of T cells, is sufficient to reduce AT inflammation and improve insulin resistance. Thus, MyD88 and downstream mitogen-activated protein kinase and NF-κB pathways are necessary for RBP4-induced macrophage antigen presentation and subsequent T-cell activation. Also, blocking antigen presentation with CTLA4-Ig improves RBP4-induced insulin resistance and macrophage-induced T-cell activation.

Developmental endothelial locus-1 modulates platelet-monocyte interactions and instant blood-mediated inflammatory reaction in islet transplantation

Platelet-monocyte interactions are strongly implicated in thrombo-inflammatory injury by actively contributing to intravascular inflammation, leukocyte recruitment to inflamed sites, and the amplification of the procoagulant response. Instant blood-mediated inflammatory reaction (IBMIR) represents thrombo-inflammatory injury elicited upon pancreatic islet transplantation (islet-Tx), thereby dramatically affecting transplant survival and function. Developmental endothelial locus-1 (Del-1) is a functionally versatile endothelial cell-derived homeostatic factor with anti-inflammatory properties, but its potential role in IBMIR has not been previously addressed. Here, we establish Del-1 as a novel inhibitor of IBMIR using a whole blood-islet model and a syngeneic murine transplantation model. Indeed, Del-1 pre-treatment of blood before addition of islets diminished coagulation activation and islet damage as assessed by C-peptide release. Consistently, intraportal islet-Tx in transgenic mice with endothelial cell-specific overexpression of Del-1 resulted in a marked decrease of monocytes and platelet-monocyte aggregates in the transplanted tissues, relative to those in wild-type recipients. Mechanistically, Del-1 decreased platelet-monocyte aggregate formation, by specifically blocking the interaction between monocyte Mac-1-integrin and platelet GPIb. Our findings reveal a hitherto unknown role of Del-1 in the regulation of platelet-monocyte interplay and the subsequent heterotypic aggregate formation in the context of IBMIR. Therefore, Del-1 may represent a novel approach to prevent or mitigate the adverse reactions mediated through thrombo-inflammatory pathways in islet-Tx and perhaps other inflammatory disorders involving platelet-leukocyte aggregate formation.

The role of innate immunity in the regulation of brown and beige adipogenesis

The adipose tissue (AT) is multifunctional, acting as an endocrine tissue and participating in the regulation of the organism's homeostasis. Metabolic, endocrine and inflammatory mechanisms are tightly intertwined within the AT, regulating its function. Disruption of the equilibrium among these mechanisms leads to pathologies, the most common being obesity-related insulin resistance. Two types of AT exist, the white and the brown AT. Traditionally the white AT (WAT) was thought to store energy in the form of lipids, while the brown AT (BAT) was known to mediate heat generation. Recently, the 'brite' or 'beige' AT was identified, which is localized predominantly in subcutaneous WAT, but shares functional features with the BAT and is capable of heat production. The major stimulus triggering beige and brown adipogenesis is cold exposure and catecholamine signalling. However, several further signals and mechanisms exist, which can orchestrate and fine-tune beige and brown AT function. Immune cells and inflammation have emerged as regulators of beige and brown AT function. The present review will focus on the recently identified crosstalk between innate immunity and the regulation of beige and brown adipogenesis.

The role of immune cells in metabolism-related liver inflammation and development of non-alcoholic steatohepatitis (NASH)

The low grade inflammatory state present in obesity promotes the progression of Non-Alcoholic Fatty Liver Disease (NAFLD). In Non-Alcoholic Steatohepatitis (NASH), augmented hepatic steatosis is accompanied by aberrant intrahepatic inflammation and exacerbated hepatocellular injury. NASH is an important disorder and can lead to fibrosis, cirrhosis and even neoplasia. The pathology of NASH involves a complex network of mechanisms, including increased infiltration of different subsets of immune cells, such as monocytes, T-lymphocytes and neutrophils, to the liver, as well as activation and in situ expansion of liver resident cells such as Kupffer cells or stellate cells. In this review, we summarize recent advances regarding understanding the role of the various cells of the innate and adaptive immunity in NASH development and progression, and discuss possible future therapeutic options and tools to interfere with disease progression.

Immunology in the liver--from homeostasis to disease

The liver is a central immunological organ with a high exposure to circulating antigens and endotoxins from the gut microbiota, particularly enriched for innate immune cells (macrophages, innate lymphoid cells, mucosal-associated invariant T (MAIT) cells). In homeostasis, many mechanisms ensure suppression of immune responses, resulting in tolerance. Tolerance is also relevant for chronic persistence of hepatotropic viruses or allograft acceptance after liver transplantation. The liver can rapidly activate immunity in response to infections or tissue damage. Depending on the underlying liver disease, such as viral hepatitis, cholestasis or NASH, different triggers mediate immune-cell activation. Conserved mechanisms such as molecular danger patterns (alarmins), Toll-like receptor signalling or inflammasome activation initiate inflammatory responses in the liver. The inflammatory activation of hepatic stellate and Kupffer cells results in the chemokine-mediated infiltration of neutrophils, monocytes, natural killer (NK) and natural killer T (NKT) cells. The ultimate outcome of the intrahepatic immune response (for example, fibrosis or resolution) depends on the functional diversity of macrophages and dendritic cells, but also on the balance between pro-inflammatory and anti-inflammatory T-cell populations. As reviewed here, tremendous progress has helped to understand the fine-tuning of immune responses in the liver from homeostasis to disease, indicating promising targets for future therapies in acute and chronic liver diseases.

The Cellular and Molecular Basis of Translational Immunometabolism

The immune response requires major changes to metabolic processes, and indeed, energy metabolism and functional activation are fully integrated in immune cells to determine their ability to divide, differentiate, and carry out effector functions. Immune cell metabolism has therefore become an attractive target area for therapeutic purposes. A neglected aspect in the translation of immunometabolism is the critical connection between systemic and cellular metabolism. Here, we discuss the importance of understanding and manipulating the integration of systemic and immune cell metabolism through in-depth analysis of immune cell phenotype and function in human metabolic diseases and, in parallel, of the effects of conventional metabolic drugs on immune cell differentiation and function. We examine how the recent identification of selective metabolic programs operating in distinct immune cell subsets and functions has the potential to deliver tools for cell- and function-specific immunometabolic targeting.

Immune Cells and Metabolism

Low-grade inflammation in the obese AT (AT) and the liver is a critical player in the development of obesity-related metabolic dysregulation, including insulin resistance, type 2 diabetes and non-alcoholic steatohepatitis (NASH). Myeloid as well as lymphoid cells infiltrate the AT and the liver and expand within these metabolic organs as a result of excessive nutrient intake, thereby exacerbating tissue inflammation. Macrophages are the paramount cell population in the field of metabolism-related inflammation; as obesity progresses, a switch takes place within the AT environment from an M2-alternatively activated macrophage state to an M1-inflammatory macrophage-dominated milieu. M1-polarized macrophages secrete inflammatory cytokines like TNF in the obese AT; such cytokines contribute to insulin resistance in adipocytes. Besides macrophages, also CD8+ T cells promote inflammation in the AT and the liver and thereby the deterioration of the metabolic balance in adipocytes and hepatocytes. Other cells of the innate immunity, such as neutrophils or mast cells, interfere with metabolic homeostasis as well. On the other hand, eosinophils or T-regulatory cells, the number of which in the AT decreases in the course of obesity, function to maintain metabolic balance by ameliorating inflammatory processes. In addition, eosinophils and M2-polarized macrophages may contribute to "beige" adipogenesis under lean conditions; beige adipocytes are located predominantly in the subcutaneous AT and have thermogenic and optimal energy-dispensing properties like brown adipocytes. This chapter will summarize the different aspects of the regulation of homeostasis of metabolic tissues by immune cells.

CD40 promotes MHC class II expression on adipose tissue macrophages and regulates adipose tissue CD4+ T cells with obesity

Obesity activates both innate and adaptive immune responses in adipose tissue, but the mechanisms critical for regulating these responses remain unknown. CD40/CD40L signaling provides bidirectional costimulatory signals between antigen-presenting cells and CD4(+) T cells, and CD40L expression is increased in obese humans. Therefore, we examined the contribution of CD40 to the progression of obesity-induced inflammation in mice. CD40 was highly expressed on adipose tissue macrophages in mice, and CD40/CD40L signaling promoted the expression of antigen-presenting cell markers in adipose tissue macrophages. When fed a high fat diet, Cd40-deficient mice had reduced accumulation of conventional CD4(+) T cells (Tconv: CD3(+)CD4(+)Foxp3(-)) in visceral fat compared with wild-type mice. By contrast, the number of regulatory CD4(+) T cells (Treg: CD3(+)CD4(+)Foxp3(+)) in lean and obese fat was similar between wild-type and knockout mice. Adipose tissue macrophage content and inflammatory gene expression in fat did not differ between obese wild-type and knockout mice; however, major histocompatibility complex class II and CD86 expression on adipose tissue macrophages was reduced in visceral fat from knockout mice. Similar results were observed in chimeric mice with hematopoietic Cd40-deficiency. Nonetheless, neither whole body nor hematopoietic disruption of CD40 ameliorated obesity-induced insulin resistance in mice. In human adipose tissue, CD40 expression was positively correlated with CD80 and CD86 expression in obese patients with type 2 diabetes. These findings indicate that CD40 signaling in adipose tissue macrophages regulates major histocompatibility complex class II and CD86 expression to control the expansion of CD4(+) T cells; however, this is largely dispensable for the development of obesity-induced inflammation and insulin resistance in mice.

Adipocyte-Specific Hypoxia-Inducible Factor 2α Deficiency Exacerbates Obesity-Induced Brown Adipose Tissue Dysfunction and Metabolic Dysregulation

Angiogenesis is a central regulator for white (WAT) and brown (BAT) adipose tissue adaptation in the course of obesity. Here we show that deletion of hypoxia-inducible factor 2α (HIF2α) in adipocytes (by using Fabp4-Cre transgenic mice) but not in myeloid or endothelial cells negatively impacted WAT angiogenesis and promoted WAT inflammation, WAT dysfunction, hepatosteatosis, and systemic insulin resistance in obesity. Importantly, adipocyte HIF2α regulated vascular endothelial growth factor (VEGF) expression and angiogenesis of obese BAT as well as its thermogenic function. Consistently, obese adipocyte-specific HIF2α-deficient mice displayed BAT dysregulation, associated with reduced levels of uncoupling protein 1 (UCP1) and a dysfunctional thermogenic response to cold exposure. VEGF administration reversed WAT and BAT inflammation and BAT dysfunction in adipocyte HIF2α-deficient mice. Together, our findings show that adipocyte HIF2α is protective against maladaptation to obesity and metabolic dysregulation by promoting angiogenesis in both WAT and BAT and by counteracting obesity-mediated BAT dysfunction.

Endothelial-specific deficiency of Junctional Adhesion Molecule-C promotes vessel normalisation in proliferative retinopathy

In proliferative retinopathies, like proliferative diabetic retinopathy and retinopathy of prematurity (ROP), the hypoxia response is sustained by the failure of the retina to revascularise its ischaemic areas. Non-resolving retina ischaemia/hypoxia results in upregulation of pro-angiogenic factors and pathologic neovascularisation with ectopic, fragile neovessels. Promoting revascularisation of the retinal avascular area could interfere with this vicious cycle and lead to vessel normalisation. Here, we examined the function of endothelial junctional adhesion molecule-C (JAM-C) in the context of ROP. Endothelial-specific JAM-C-deficient (EC-JAM-C KO) mice and littermate JAM-C-proficient (EC-JAM-C WT) mice were subjected to the ROP model. An increase in total retinal vascularisation was found at p17 owing to endothelial JAM-C deficiency, which was the result of enhanced revascularisation and vessel normalisation, thereby leading to significantly reduced avascular area in EC-JAM-C KO mice. In contrast, pathologic neovessel formation was not affected by endothelial JAM-C deficiency. Consistent with improved vessel normalisation, tip cell formation at the interface between vascular and avascular area was higher in EC-JAM-C KO mice, as compared to their littermate controls. Consistently, JAM-C inactivation in endothelial cells resulted in increased spreading on fibronectin and enhanced sprouting in vitro in a manner dependent on β1-integrin and on the activation of the small GTPase RAP1. Together, endothelial deletion of JAM-C promoted endothelial cell sprouting, and consequently vessel normalisation and revascularisation of the hypoxic retina without altering pathologic neovascularisation. Thus, targeting endothelial JAM-C may provide a novel therapeutic strategy for promoting revascularisation and vessel normalisation in the treatment of proliferative retinopathies.

Adaptive Immunity and Antigen-Specific Activation in Obesity-Associated Insulin Resistance

Type 2 diabetes mellitus (T2D) is a metabolic disease that is strongly tied to obesity and often preceded by insulin resistance (IR). It has been established that chronic inflammation of hypertrophic adipose tissue depots in obese individuals leads to obesity-associated IR and is mediated by cells of the innate immune system, particularly macrophages. More recently, cells of the adaptive immune system, B and T lymphocytes, have also emerged as important regulators of glucose homeostasis, raising the intriguing possibility that antigen-driven immune responses play a role in disease. In this review, we critically evaluate the roles that various B and T cell subsets play in IR, and then we examine the data suggesting that antigen-driven mechanisms, such as antigen presentation and costimulation, may drive the activity of these lymphocytes.

Are obesity-related insulin resistance and type 2 diabetes autoimmune diseases?

Obesity and associated insulin resistance predispose individuals to develop chronic metabolic diseases, such as type 2 diabetes and cardiovascular disease. Although these disorders affect a significant proportion of the global population, the underlying mechanisms of disease remain poorly understood. The discovery of elevated tumor necrosis factor-α in adipose tissue as an inducer of obesity-associated insulin resistance marked a new era of understanding that a subclinical inflammatory process underlies the insulin resistance and metabolic dysfunction that precedes type 2 diabetes. Advances in the field identified components of both the innate and adaptive immune response as key players in regulating such inflammatory processes. As antigen specificity is a hallmark of an adaptive immune response, its role in modulating the chronic inflammation that accompanies obesity and type 2 diabetes begs the question of whether insulin resistance and type 2 diabetes can have autoimmune components. In this Perspective, we summarize current data that pertain to the activation and perpetuation of adaptive immune responses during obesity and discuss key missing links and potential mechanisms for obesity-related insulin resistance and type 2 diabetes to be considered as potential autoimmune diseases.