Cellular senescence in the cholangiopathies: a driver of immunopathology and a novel therapeutic target

ELMO1 regulates macrophage directed migration and attenuates inflammation via NF-κB signaling pathway in primary biliary cholangitis

BACKGROUND & AIMS

Primary biliary cholangitis (PBC), a typical autoimmune liver disease, is characterized by an increased infiltration of immune cells. However, the specific molecular mechanisms regulating immune cell migration in PBC are unknown. Engulfment and cell motility 1 (ELMO1) plays an important function in cellular dynamics. In view of this, the aim of this study was to explore the expression of ELMO1 in PBC, its effects on the proliferation, migration, and secretion of inflammatory factors by the mainly regulated immune cells and the specific molecular mechanisms behind it.

METHODS

To determine the expression of ELMO1 in PBC and its major regulatory immune cells in PBC. The migratory and proliferative capacities of ELMO1-deficient macrophages were measured, and their pro-inflammatory cytokine secretion was also detected and explored mechanistically.

RESULTS

ELMO1 expression was up-regulated in the PBC patients and positively correlated with alkaline phosphatase (ALP). ELMO1 mainly regulated macrophages in the liver of PBC patients. Knockdown of ELMO1 did not affect macrophage proliferation, however,knockdown of ELMO1 significantly inhibited macrophage migration,downstream RAC1 activity was diminished, and reduced F-actin synthesis. Knockdown of ELMO1 reduced macrophage inflammatory factor secretion and NF-κB signaling pathway activity was decreased.

CONCLUSIONS

ELMO1 regulates macrophage directed migration and attenuates inflammation via NF-κB signaling pathway in primary biliary cholangitis.

Immune-mediated cholangiopathies in children: the need to better understand the pathophysiology for finding the future possible treatment targets

Cholangiopathies are defined as focal or extensive damage of the bile ducts. According to the pathogenetic mechanism, it may be immune-mediated or due to genetic, infectious, toxic, vascular, and obstructive causes. Their chronic evolution is characterized by inflammation, obstruction of bile flow, cholangiocyte proliferation, and progression toward fibrosis and cirrhosis. Immune-mediated cholangiopathies comprise primary sclerosing cholangitis (PSC), autoimmune cholangitis and IgG4-associated cholangitis in adults and biliary atresia (BA), neonatal sclerosing cholangitis (NSC) in children. The main purpose of this narrative review was to highlight the similarities and differences among immune-mediated cholangiopathies, especially those frequent in children in which cholangiocyte senescence plays a key role (BA, NSC, and PSC). These three entities have many similarities in terms of clinical and histopathological manifestations, and the distinction between them can be hard to achieve. In BA, bile duct destruction occurs due to aggression of the biliary cells due to viral infections or toxins during the intrauterine period or immediately after birth. The consequence is the activation of the immune system leading to severe inflammation and fibrosis of the extrahepatic biliary tract, lumen stenosis, and impairment of the biliary flow. PSC is characterized by inflammation and fibrosis of intra- and extrahepatic bile ducts, leading to secondary biliary cirrhosis. It is a multifactorial disease that occurs because of genetic predisposition [human leukocyte antigen (HLA) and non-HLA haplotypes], autoimmunity (cellular immune response, autoantibodies, association with inflammatory bowel disease), environmental factors (infections or toxic bile), and host factors (intestinal microbiota). NSC seems to be a distinct subgroup of childhood PSC that appears due to the interaction between genetic predisposition (HLA B8 and DR3) and the disruption of the immune system, validated by elevated IgG levels or specific antibodies [antinuclear antibody (ANA), anti-smooth muscle antibody (ASMA)]. Currently, the exact mechanism of immune cholangiopathy is not fully understood, and further data are required to identify individuals at high risk of developing these conditions. A better understanding of the immune mechanisms and pathophysiology of BA, NSC, and PSC will open new perspectives for future treatments and better methods of preventing severe evolution.

Panic at the Bile Duct: How Intrahepatic Cholangiocytes Respond to Stress and Injury

In the liver, biliary epithelial cells (BECs) line intrahepatic bile ducts (IHBDs) and are primarily responsible for modifying and transporting hepatocyte-produced bile to the digestive tract. BECs comprise only 3% to 5% of the liver by cell number but are critical for maintaining choleresis through homeostasis and disease. To this end, BECs drive an extensive morphologic remodeling of the IHBD network termed ductular reaction (DR) in response to direct injury or injury to the hepatic parenchyma. BECs are also the target of a broad and heterogenous class of diseases termed cholangiopathies, which can present with phenotypes ranging from defective IHBD development in pediatric patients to progressive periductal fibrosis and cancer. DR is observed in many cholangiopathies, highlighting overlapping similarities between cell- and tissue-level responses by BECs across a spectrum of injury and disease. The following core set of cell biological BEC responses to stress and injury may moderate, initiate, or exacerbate liver pathophysiology in a context-dependent manner: cell death, proliferation, transdifferentiation, senescence, and acquisition of neuroendocrine phenotype. By reviewing how IHBDs respond to stress, this review seeks to highlight fundamental processes with potentially adaptive or maladaptive consequences. A deeper understanding of how these common responses contribute to DR and cholangiopathies may identify novel therapeutic targets in liver disease.

Primary biliary cholangitis: primary autoimmune disease or primary secretory defect

INTRODUCTION

Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease characterized by the immune-mediated destruction of small and medium intrahepatic bile ducts, involving predominantly females. PBC has long been described as an autoimmune liver disease, also because it is very often associated with many autoimmune conditions. More recently, another pathogenic mechanism exploring the damage of cholangiocytes has been hypothesized, i.e. a defect in the biliary umbrella which is physiologically responsible for the exchange of the ions Cl- and HCO3- and maintains the integrity of glycocalyx. To provide a state-of-the-art analysis of this topic, a systematic review of literature in PubMed, Scopus, and Science Direct was conducted (inclusive dates: 1986-2023).

AREA COVERED

Although the etiology remains unknown, pathogenesis consists of a complex immune-mediated process resulting from a genetic susceptibility. PBC can be triggered by an immune-mediated response to an autoantigen, which leads to a progressive destruction of bile ducts and eventually to a progressive fibrosis with cirrhosis. The defect in the 'bicarbonate umbrella' acts as a protection against the toxic hydrophobic bile acids, leading to a toxic composition of bile.

EXPERT OPINION

This review offers a summary of the current knowledge about the pathogenesis of PBC, indicating that this is probably based on the mutual relationship between the immune insult and the unbalanced secretory mechanisms.

New Treatment Paradigms in Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is an archetypal autoimmune disease. Chronic lymphocytic cholangitis is associated with interface hepatitis, ductopenia, cholestasis, and progressive biliary fibrosis. People living with PBC are frequently symptomatic, experiencing a quality-of-life burden dominated by fatigue, itch, abdominal pain, and sicca complex. Although the female predominance, specific serum autoantibodies, immune-mediated cellular injury, as well as genetic (HLA and non-HLA) risk factors, identify PBC as autoimmune, to date treatment has focused on cholestatic consequences. Biliary epithelial homeostasis is abnormal and contributes to disease. The impact of cholangiocyte senescence, apoptosis, and impaired bicarbonate secretion enhances chronic inflammation and bile acid retention. First-line therapy is a non-specific anti-cholestatic agent, ursodeoxycholic acid. For those with residual cholestasis biochemically, obeticholic acid is introduced, and this semisynthetic farnesoid X receptor agonist adds choleretic, anti-fibrotic, and anti-inflammatory activity. Future PBC licensed therapy will likely include peroxisome proliferator activated receptor (PPAR) pathway agonists, including specific PPAR-delta agonism (seladelpar), as well as elafibrinor and saroglitazar (both with broader PPAR agonism). These agents dovetail the clinical and trial experience for off-label bezafibrate and fenofibrate use. Symptom management is essential, and encouragingly, PPAR agonists reduce itch; IBAT inhibition (eg, linerixibat) also appears promising for pruritus. For those where liver fibrosis is the target, NOX inhibition is being evaluated. Earlier stage therapies in development include therapy to impact immunoregulation in patients, as well other approaches to treating pruritus (eg, antagonists of MrgprX4). Collectively the PBC therapeutic landscape is exciting. Therapy goals are increasingly proactive and individualized and aspire to rapidly achieve normal serum tests and quality of life with prevention of end-stage liver disease.

Development of liver inflammatory injury in biliary atresia: from basic to clinical research

Biliary atresia (BA) is a severe cholangiopathy in infants. It is characterized by inflammatory fibro-obliteration of the intra- and extrahepatic bile ducts. Although the restoration of bile flow can be successful after Kasai operation, the rapid progression of liver fibrosis can continue, leading to cirrhosis. It is believed that the progression of liver fibrosis in BA is exacerbated by complicated mechanisms other than the consequence of bile duct obstruction. The fibrogenic cascade in BA liver can be divided into three stages, including liver inflammatory injury, myofibroblast activation, and fibrous scar formation. Recent studies have revealed that the activation of an immune response following bile duct injury plays an important role in promoting the inflammatory process, the releasing of inflammatory cytokines, and the development of fibrogenesis in BA liver. In this article, we summarized the evidence regarding liver inflammatory injury and the possible mechanisms that explain the rapid progression of liver fibrosis in BA.

Cholangiokines: undervalued modulators in the hepatic microenvironment

The biliary epithelial cells, also known as cholangiocytes, line the intra- and extrahepatic bile ducts, forming a barrier between intra- and extra-ductal environments. Cholangiocytes are mostly known to modulate bile composition and transportation. In hepatobiliary diseases, bile duct injury leads to drastic alterations in cholangiocyte phenotypes and their release of soluble mediators, which can vary depending on the original insult and cellular states (quiescence, senescence, or proliferation). The cholangiocyte-secreted cytokines (also termed cholangiokines) drive ductular cell proliferation, portal inflammation and fibrosis, and carcinogenesis. Hence, despite the previous consensus that cholangiocytes are bystanders in liver diseases, their diverse secretome plays critical roles in modulating the intrahepatic microenvironment. This review summarizes recent insights into the cholangiokines under both physiological and pathological conditions, especially as they occur during liver injury-regeneration, inflammation, fibrosis and malignant transformation processes.

Bile acid-mediated signaling in cholestatic liver diseases

Chronic cholestatic liver diseases, such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), are associated with bile stasis and gradually progress to fibrosis, cirrhosis, and liver failure, which requires liver transplantation. Although ursodeoxycholic acid is effective in slowing the disease progression of PBC, it has limited efficacy in PSC patients. It is challenging to develop effective therapeutic agents due to the limited understanding of disease pathogenesis. During the last decade, numerous studies have demonstrated that disruption of bile acid (BA) metabolism and intrahepatic circulation promotes the progression of cholestatic liver diseases. BAs not only play an essential role in nutrition absorption as detergents but also play an important role in regulating hepatic metabolism and modulating immune responses as key signaling molecules. Several excellent papers have recently reviewed the role of BAs in metabolic liver diseases. This review focuses on BA-mediated signaling in cholestatic liver disease.

Morphological aspects of small-duct cholangiopathies: A minireview

The biliary system consists of intrahepatic and extrahepatic bile ducts lined by biliary epithelial cells (cholangiocytes). Bile ducts and cholangiocytes are affected by a variety of disorders called cholangiopathies, which differ in aetiology, pathogenesis, and morphology. Classification of cholangiopathies is complex and reflects pathogenic mechanisms (immune-mediated, genetic, drug- and toxin-induced, ischaemic, infectious, neoplastic), predominant morphological patterns of biliary injury (suppurative and non-suppurative cholangitis, cholangiopathy), and specific segments of the biliary tree affected by the disease process. While the involvement of large extrahepatic and intrahepatic bile ducts is typically visualised using radiology imaging, histopathological examination of liver tissue obtained by percutaneous liver biopsy still plays an important role in the diagnosis of cholangiopathies affecting the small intrahepatic bile ducts. To increase the diagnostic yield of a liver biopsy and determine the optimal therapeutic approach, the referring clinician is tasked with interpreting the results of histopathological examination. This requires knowledge and understanding of basic morphological patterns of hepatobiliary injury and an ability to correlate microscopic findings with results obtained by imaging and laboratory methods. This minireview describes the morphological aspects of small-duct cholangiopathies pertaining to the diagnostic process.

Recent Advances in Intrahepatic Biliary Epithelial Heterogeneity

Biliary epithelium (i.e., cholangiocytes) is a heterogeneous population of epithelial cells in the liver, which line small and large bile ducts and have individual responses and functions dependent on size and location in the biliary tract. We discuss the recent findings showing that the intrahepatic biliary tree is heterogeneous regarding (1) morphology and function, (2) hormone expression and signaling (3), response to injury, and (4) roles in liver regeneration. This review overviews the significant characteristics and differences of the small and large cholangiocytes. Briefly, it outlines the in vitro and in vivo models used in the heterogeneity evaluation. In conclusion, future studies addressing biliary heterogeneity's role in the pathogenesis of liver diseases characterized by ductular reaction may reveal novel therapeutic approaches.

Primary sclerosing cholangitis-A long night's journey into day

Content available: Audio Recording.

Primary Biliary Cholangitis and Primary Sclerosing Cholangitis: Current Knowledge of Pathogenesis and Therapeutics

Cholangiopathies encompass various biliary diseases affecting the biliary epithelium, resulting in cholestasis, inflammation, fibrosis, and ultimately liver cirrhosis. Primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are the most important progressive cholangiopathies in adults. Much research has broadened the scope of disease biology to genetic risk, epigenetic changes, dysregulated mucosal immunity, altered biliary epithelial cell function, and dysbiosis, all of which interact and arise in the context of ill-defined environmental triggers. An in-depth understanding of the molecular pathogenesis of these cholestatic diseases will help clinicians better prevent and treat diseases. In this review, we focus on the main underlying mechanisms of disease initiation and progression, and novel targeted therapeutics beyond currently approved treatments.

Role of Immune Cells in Biliary Repair

The biliary system is comprised of cholangiocytes and plays an important role in maintaining liver function. Under normal conditions, cholangiocytes remain in the stationary phase and maintain a very low turnover rate. However, the robust biliary repair is initiated in disease conditions, and different repair mechanisms can be activated depending on the pathological changes. During biliary disease, immune cells including monocytes, lymphocytes, neutrophils, and mast cells are recruited to the liver. The cellular interactions between cholangiocytes and these recruited immune cells as well as hepatic resident immune cells, including Kupffer cells, determine disease outcomes. However, the role of immune cells in the initiation, regulation, and suspension of biliary repair remains elusive. The cellular processes of cholangiocyte proliferation, progenitor cell differentiation, and hepatocyte-cholangiocyte transdifferentiation during biliary diseases are reviewed to manifest the underlying mechanism of biliary repair. Furthermore, the potential role of immune cells in crucial biliary repair mechanisms is highlighted. The mechanisms of biliary repair in immune-mediated cholangiopathies, inherited cholangiopathies, obstructive cholangiopathies, and cholangiocarcinoma are also summarized. Additionally, novel techniques that could clarify the underlying mechanisms of biliary repair are displayed. Collectively, this review aims to deepen the understanding of the mechanisms of biliary repair and contributes potential novel therapeutic methods for treating biliary diseases.