Liver Sinusoidal Endothelial Cells Contribute to Hepatic Antigen-Presenting Cell Function and Th17 Expansion in Cirrhosis

Cirrhosis-downregulated LSECtin can be retrieved by cytokines, shifts the TLR-induced LSECs secretome and correlates with the hepatic Th response

BACKGROUND AND AIMS

We evaluated tolerogenic C-type lectin LSECtin loss in cirrhosis and its potential regulation by cytokines.

METHODS

Liver tissue from patients with cirrhosis and healthy controls, immortalised and generated LSECtin-CRISPR immortalised LSECs, and murine primary LSECs from the CCl4 model were handled.

RESULTS

LSECtin expression was reduced in liver tissue from cirrhotic patients, and it decreased from compensated to decompensated disease. Increased phosphorylation of MAPK, Akt and NFkB was observed upon LSECtin stimulation in LSEC murine cell line, showing a pattern of inflammatory and chemotactic cytokines either restrained (IL-10, CCL4) or unrestrained (TNF-α, IL-1β, IL-6, CCL2). CD44 attenuated whereas LAG-3 increased all substrates phosphorylation in combination with TLR4 and TLR2 ligands except for NFkB. TNF-α, IL-1 β, IL-6 and CCL2 were restrained by LSECtin crosslinking on TLRs studied. Conversely, IL-10 and CCL4 were upregulated, suggesting a LSECtin-TLRs synergistic effect. Also, LSECtin was significantly induced after IL-13 stimulation or combined with anti-inflammatory cytokines in cirrhotic and immortalised LSECs. Th17 and regulatory T cells were progressively increased in the hepatic tissue from compensated to decompensated patients. A significant inverse correlation was present between gene expression levels of CLEC4G/LSECtin and RORγT and FOXP3 in liver tissues.

CONCLUSION

LSECtin restrains TLR proinflammatory secretome induced on LSECs by interfering immune response control, survival and MAPKs signalling pathways. The cytokine-dependent induction of LSECtin and the association between LSECtin loss and Th17 cell subset expansion in the liver, provides a solid background for exploring LSECtin retrieval as a mechanism to reprogram LSEC homeostatic function hampered during cirrhosis.

Exploring the Relationship between Liver Disease, Bacterial Translocation, and Dysbiosis: Unveiling the Gut-Liver Axis

Background

The global burden of liver disease and cirrhosis has been progressively increasing in the last decade. The interplay between gut microbiota and immune system and the bidirectional relationship with the liver, known as the gut-liver axis, has arisen as a fundamental aspect of liver disease.

Summary

Alterations of the gut microbiome have been described and include both dysbiotic microbial signatures and intestinal bacterial overgrowth. The integrity of the intestinal epithelial barrier is essential for preventing the access of harmful substances and bacterial products into the host. Bacterial translocation due to altered host-microbiota interactions triggers local immune cell activation and facilitates a chronic inflammatory state that can ultimately lead to immune exhaustion, characteristic of cirrhosis. In cirrhosis, breakdown of the gut vascular barrier allows access of bacterial products to portal blood circulation and facilitates their influx into the liver, further contributing to disease progression.

Key Messages

A better understanding of the contributing factors to pathological bacterial translocation and the impact of dysbiosis in liver disease will lead to achieve innovative therapeutic strategies in cirrhosis.

The progress of autoimmune hepatitis research and future challenges

Autoimmune hepatitis (AIH) is a chronic liver inflammatory disease with various immune system manifestations, showing a global trend of increased prevalence. AIH is diagnosed through histological abnormalities, clinical manifestations, and biochemical indicators. The biochemical markers involve interfacial hepatitis, transaminase abnormalities, positive autoantibodies, etc. Although AIH pathogenesis is unclear, gene mutations and immunological factors could be the leading factors. AIH usually presents as a chronic liver disease and sometimes as acute hepatitis, making it challenging to distinguish it from drug-related hepatitis due to similar clinical symptoms. Normalizing transaminases and serum IgG levels is essential in assessing the remission status of AIH treatment. Glucocorticoids and azathioprine are the first-line AIH treatment, with lifelong maintenance therapy in some patients. The quality of life and survival can be improved after appropriate treatment. However, certain limitations jeopardize the quality of treatment, including long treatment cycles, side effects, poor patient compliance, and inability to inhibit liver fibrosis and cirrhosis. Accurate AIH animal models will help us understand the pathophysiology of the disease while providing fresh perspectives for avoiding and treating AIH. This review will help us understand AIH better, from the cellular and molecular causes to the clinical features, and will provide insight into new therapy techniques with fewer side effects.

Lipid oxidation dysregulation: an emerging player in the pathophysiology of sepsis

Sepsis is a life-threatening organ dysfunction caused by abnormal host response to infection. Millions of people are affected annually worldwide. Derangement of the inflammatory response is crucial in sepsis pathogenesis. However, metabolic, coagulation, and thermoregulatory alterations also occur in patients with sepsis. Fatty acid mobilization and oxidation changes may assume the role of a protagonist in sepsis pathogenesis. Lipid oxidation and free fatty acids (FFAs) are potentially valuable markers for sepsis diagnosis and prognosis. Herein, we discuss inflammatory and metabolic dysfunction during sepsis, focusing on fatty acid oxidation (FAO) alterations in the liver and muscle (skeletal and cardiac) and their implications in sepsis development.

The evolving role of liver sinusoidal endothelial cells in liver health and disease

LSECs are a unique population of endothelial cells within the liver and are recognized as key regulators of liver homeostasis. LSECs also play a key role in liver disease, as dysregulation of their quiescent phenotype promotes pathological processes within the liver including inflammation, microvascular thrombosis, fibrosis, and portal hypertension. Recent technical advances in single-cell analysis have characterized distinct subpopulations of the LSECs themselves with a high resolution and defined their gene expression profile and phenotype, broadening our understanding of their mechanistic role in liver biology. This article will review 4 broad advances in our understanding of LSEC biology in general: (1) LSEC heterogeneity, (2) LSEC aging and senescence, (3) LSEC role in liver regeneration, and (4) LSEC role in liver inflammation and will then review the role of LSECs in various liver pathologies including fibrosis, DILI, alcohol-associated liver disease, NASH, viral hepatitis, liver transplant rejection, and ischemia reperfusion injury. The review will conclude with a discussion of gaps in knowledge and areas for future research.

Accelerated clearance by antibodies against methoxy PEG depends on pegylation architecture

Methoxy polyethylene glycol (mPEG) is attached to many proteins, peptides, nucleic acids and nanomedicines to improve their biocompatibility. Antibodies that bind PEG are present in many individuals and can be generated upon administration of pegylated therapeutics. Anti-PEG antibodies that bind to the PEG "backbone" can accelerate drug clearance and detrimentally affect drug activity and safety, but no studies have examined how anti-methoxy PEG (mPEG) antibodies, which selectively bind the terminus of mPEG, affect pegylated drugs. Here, we investigated how defined IgG and IgM monoclonal antibodies specific to the PEG backbone (anti-PEG) or terminal methoxy group (anti-mPEG) affect pegylated liposomes or proteins with a single PEG chain, a single branched PEG chain, or multiple PEG chains. Large immune complexes can be formed between all pegylated compounds and anti-PEG antibodies but only pegylated liposomes formed large immune complexes with anti-mPEG antibodies. Both anti-PEG IgG and IgM antibodies accelerated the clearance of all pegylated compounds but anti-mPEG antibodies did not accelerate clearance of proteins with a single or branched PEG molecule. Pegylated liposomes were primarily taken up by Kupffer cells in the liver, but both anti-PEG and anti-mPEG antibodies directed uptake of a heavily pegylated protein to liver sinusoidal endothelial cells. Our results demonstrate that in contrast to anti-PEG antibodies, immune complex formation and drug clearance induced by anti-mPEG antibodies depends on pegylation architecture; compounds with a single or branched PEG molecule are unaffected by anti-mPEG antibodies but are increasingly affected as the number of PEG chain in a structure increases.

The liver in sepsis: molecular mechanism of liver failure and their potential for clinical translation

Liver failure is a life-threatening complication of infections restricting the host's response to infection. The pivotal role of the liver in metabolic, synthetic, and immunological pathways enforces limits the host's ability to control the immune response appropriately, making it vulnerable to ineffective pathogen resistance and tissue damage. Deregulated networks of liver diseases are gradually uncovered by high-throughput, single-cell resolved OMICS technologies visualizing an astonishing diversity of cell types and regulatory interaction driving tolerogenic signaling in health and inflammation in disease. Therefore, this review elucidates the effects of the dysregulated host response on the liver, consequences for the immune response, and possible avenues for personalized therapeutics.

Why is endothelial resilience key to maintain cardiac health?

Myocardial injury as induced by myocardial infarction results in tissue ischemia, which critically incepts cardiomyocyte death. Endothelial cells play a crucial role in restoring oxygen and nutrient supply to the heart. Latest advances in single-cell multi-omics, together with genetic lineage tracing, reveal a transcriptional and phenotypical adaptation to the injured microenvironment, which includes alterations in metabolic, mesenchymal, hematopoietic and pro-inflammatory signatures. The extent of transition in mesenchymal or hematopoietic cell lineages is still debated, but it is clear that several of the adaptive phenotypical changes are transient and endothelial cells revert back to a naïve cell state after resolution of injury responses. This resilience of endothelial cells to acute stress responses is important for preventing chronic dysfunction. Here, we summarize how endothelial cells adjust to injury and how this dynamic response contributes to repair and regeneration. We will highlight intrinsic and microenvironmental factors that contribute to endothelial cell resilience and may be targetable to maintain a functionally active, healthy microcirculation.

The Hepatic Sinusoid in Chronic Liver Disease: The Optimal Milieu for Cancer

Simple Summary

During the development of chronic liver disease, the hepatic sinusoid undergoes major changes that further compromise the hepatic function, inducing persistent inflammation and the formation of scar tissue, together with alterations in liver hemodynamics. This diseased background may induce the formation and development of hepatocellular carcinoma (HCC), which is the most common form of primary liver cancer and a major cause of mortality. In this review, we describe the ways in which the dysregulation of hepatic sinusoidal cells—including liver sinusoidal cells, Kupffer cells, and hepatic stellate cells—may have an important role in the development of HCC. Our review summarizes all of the known sinusoidal processes in both health and disease, and possible treatments focusing on the dysregulation of the sinusoid; finally, we discuss how some of these alterations occurring during chronic injury are shared with the pathology of HCC and may contribute to its development.

Abstract

The liver sinusoids are a unique type of microvascular beds. The specialized phenotype of sinusoidal cells is essential for their communication, and for the function of all hepatic cell types, including hepatocytes. Liver sinusoidal endothelial cells (LSECs) conform the inner layer of the sinusoids, which is permeable due to the fenestrae across the cytoplasm; hepatic stellate cells (HSCs) surround LSECs, regulate the vascular tone, and synthetize the extracellular matrix, and Kupffer cells (KCs) are the liver-resident macrophages. Upon injury, the harmonic equilibrium in sinusoidal communication is disrupted, leading to phenotypic alterations that may affect the function of the whole liver if the damage persists. Understanding how the specialized sinusoidal cells work in coordination with each other in healthy livers and chronic liver disease is of the utmost importance for the discovery of new therapeutic targets and the design of novel pharmacological strategies. In this manuscript, we summarize the current knowledge on the role of sinusoidal cells and their communication both in health and chronic liver diseases, and their potential pharmacologic modulation. Finally, we discuss how alterations occurring during chronic injury may contribute to the development of hepatocellular carcinoma, which is usually developed in the background of chronic liver disease.

Gene Signatures Detect Damaged Liver Sinusoidal Endothelial Cells in Chronic Liver Diseases

Liver sinusoidal endothelial cells have a gatekeeper function in liver homeostasis by permitting substrates from the bloodstream into the space of Disse and regulating hepatic stellate cell activation status. Maintenance of LSEC's highly specialized phenotype is crucial for liver homeostasis. During liver fibrosis and cirrhosis, LSEC phenotype and functions are lost by processes known as capillarization and LSEC dysfunction. LSEC capillarization can be demonstrated by the loss of fenestrae (cytoplasmic pores) and the manifestation of a basement membrane. Currently, no protein or genetic markers can clearly distinguish healthy from damaged LSECs in acute or chronic liver disease. Single cell (sc)RNA sequencing efforts have identified several LSEC populations in mouse models for liver disease and in human cirrhotic livers. Still, there are no clearly defined genesets that can identify LSECs or dysfunctional LSEC populations in transcriptome data. Here, we developed genesets that are enriched in healthy and damaged LSECs which correlated very strongly with healthy and early stage- vs. advanced human liver diseases. A damaged LSEC signature comprised of Fabp4/5 and Vwf/a1 was established which could efficiently identify damaged endothelial cells in single cell RNAseq data sets. In LSECs from an acute CCl₄ liver injury mouse model, Fabp4/5 and Vwf/a1 expression is induced within 1-3 days while in cirrhotic human livers these 4 genes are highly enriched in damaged LSECs. In conclusion, our newly developed gene signature of damaged LSECs can be applicable to a wide range of liver disease etiologies, implicating a common transcriptional alteration mechanism in LSEC damage.

Role of liver sinusoidal endothelial cells in liver diseases

Liver sinusoidal endothelial cells (LSECs) form the wall of the hepatic sinusoids. Unlike other capillaries, they lack an organized basement membrane and have cytoplasm that is penetrated by open fenestrae, making the hepatic microvascular endothelium discontinuous. LSECs have essential roles in the maintenance of hepatic homeostasis, including regulation of the vascular tone, inflammation and thrombosis, and they are essential for control of the hepatic immune response. On a background of acute or chronic liver injury, LSECs modify their phenotype and negatively affect neighbouring cells and liver disease pathophysiology. This Review describes the main functions and phenotypic dysregulations of LSECs in liver diseases, specifically in the context of acute injury (ischaemia-reperfusion injury, drug-induced liver injury and bacterial and viral infection), chronic liver disease (metabolism-associated liver disease, alcoholic steatohepatitis and chronic hepatotoxic injury) and hepatocellular carcinoma, and provides a comprehensive update of the role of LSECs as therapeutic targets for liver disease. Finally, we discuss the open questions in the field of LSEC pathobiology and future avenues of research.