Kupffer cell-monocyte communication is essential for initiating murine liver progenitor cell-mediated liver regeneration

Kupffer cells abrogate homing and repopulation of allogeneic hepatic progenitors in injured liver site

BACKGROUND

Allogeneic hepatocyte transplantation is an emerging approach to treat acute liver defects. However, durable engraftment of the transplanted cells remains a daunting task, as they are actively cleared by the recipient's immune system. Therefore, a detailed understanding of the innate or adaptive immune cells-derived responses against allogeneic transplanted hepatic cells is the key to rationalize cell-based therapies.

METHODS

Here, we induced an acute inflammatory regenerative niche (3-96 h) on the surface of the liver by the application of cryo-injury (CI) to systematically evaluate the innate immune response against transplanted allogeneic hepatic progenitors in a sustained micro-inflammatory environment.

RESULTS

The resulting data highlighted that the injured site was significantly repopulated by alternating numbers of innate immune cells, including neutrophils, monocytes and Kupffer cells (KCs), from 3 to 96 h. The transplanted allo-HPs, engrafted 6 h post-injury, were collectively eliminated by the innate immune response within 24 h of transplantation. Selective depletion of the KCs demonstrated a delayed recruitment of monocytes from day 2 to day 6. In addition, the intrasplenic engraftment of the hepatic progenitors 54 h post-transplantation was dismantled by KCs, while a time-dependent better survival and translocation of the transplanted cells into the injured site could be observed in samples devoid of KCs.

CONCLUSION

Overall, this study provides evidence that KCs ablation enables a better survival and integration of allo-HPs in a sustained liver inflammatory environment, having implications for rationalizing the cell-based therapeutic interventions against liver defects.

Cell transplantation-based regenerative medicine in liver diseases

This review aims to evaluate the current preclinical state of liver bioengineering, the clinical context for liver cell therapies, the cell sources, the delivery routes, and the results of clinical trials for end-stage liver disease. Different clinical settings, such as inborn errors of metabolism, acute liver failure, chronic liver disease, liver cirrhosis, and acute-on-chronic liver failure, as well as multiple cellular sources were analyzed; namely, hepatocytes, hepatic progenitor cells, biliary tree stem/progenitor cells, mesenchymal stromal cells, and macrophages. The highly heterogeneous clinical scenario of liver disease and the availability of multiple cellular sources endowed with different biological properties make this a multidisciplinary translational research challenge. Data on each individual liver disease and more accurate endpoints are urgently needed, together with a characterization of the regenerative pathways leading to potential therapeutic benefit. Here, we critically review these topics and identify related research needs and perspectives in preclinical and clinical settings.

The role of gut microbiota in liver regeneration

The liver has unique regeneration potential, which ensures the continuous dependence of the human body on hepatic functions. As the composition and function of gut microbiota has been gradually elucidated, the vital role of gut microbiota in liver regeneration through gut-liver axis has recently been accepted. In the process of liver regeneration, gut microbiota composition is changed. Moreover, gut microbiota can contribute to the regulation of the liver immune microenvironment, thereby modulating the release of inflammatory factors including IL-6, TNF-α, HGF, IFN-γ and TGF-β, which involve in different phases of liver regeneration. And previous research have demonstrated that through enterohepatic circulation, bile acids (BAs), lipopolysaccharide, short-chain fatty acids and other metabolites of gut microbiota associate with liver and may promote liver regeneration through various pathways. In this perspective, by summarizing gut microbiota-derived signaling pathways that promote liver regeneration, we unveil the role of gut microbiota in liver regeneration and provide feasible strategies to promote liver regeneration by altering gut microbiota composition.

The liver-resident immune cell repertoire - A boon or a bane during machine perfusion?

The liver has been proposed as an important “immune organ” of the body, as it is critically involved in a variety of specific and unique immune tasks. It contains a huge resident immune cell repertoire, which determines the balance between tolerance and inflammation in the hepatic microenvironment. Liver-resident immune cells, populating the sinusoids and the space of Disse, include professional antigen-presenting cells, myeloid cells, as well as innate and adaptive lymphoid cell populations. Machine perfusion (MP) has emerged as an innovative technology to preserve organs ex vivo while testing for organ quality and function prior to transplantation. As for the liver, hypothermic and normothermic MP techniques have successfully been implemented in clinically routine, especially for the use of marginal donor livers. Although there is evidence that ischemia reperfusion injury-associated inflammation is reduced in machine-perfused livers, little is known whether MP impacts the quantity, activation state and function of the hepatic immune-cell repertoire, and how this affects the inflammatory milieu during MP. At this point, it remains even speculative if liver-resident immune cells primarily exert a pro-inflammatory and hence destructive effect on machine-perfused organs, or in part may be essential to induce liver regeneration and counteract liver damage. This review discusses the role of hepatic immune cell subtypes during inflammatory conditions and ischemia reperfusion injury in the context of liver transplantation. We further highlight the possible impact of MP on the modification of the immune cell repertoire and its potential for future applications and immune modulation of the liver.

Inhibition of NLRP3-mediated crosstalk between hepatocytes and liver macrophages by geniposidic acid alleviates cholestatic liver inflammatory injury

The excessive accumulation of bile acids (BA) in hepatocytes can trigger inflammatory response and recruit macrophages, thereby accelerating cholestatic liver injury. The crosstalk between hepatocytes and macrophages has been recently implicated in the pathogenesis of cholestasis; however, the underlying mechanisms remain unclear. Here, we demonstrated that BA initiate NLRP3 inflammasome activation in hepatocytes to release proinflammatory cytokines and promote the communication between hepatocytes and macrophages, thus enhancing liver inflammation in an NLRP3-dependent manner. NLRP3-inhibition by geniposidic acid (GPA), a novel NLRP3-specific covalent inhibitor that directly interacts with NLRP3, in hepatocytes and macrophages abated BA-induced inflammation. Moreover, NLRP3-deletion or its inhibition mitigated ANIT-induced cholestatic inflammation, whereas disrupting the crosstalk between hepatic macrophages and hepatocytes attenuated the hepatoprotective effect of GPA against ANIT-induced cholestatic inflammation. Therefore, blocking this crosstalk by suppressing NLRP3 inflammasome activation may represent a novel therapeutic strategy for cholestasis.

Liver Regeneration in Chronic Liver Injuries: Basic and Clinical Applications Focusing on Macrophages and Natural Killer Cells

BACKGROUND & AIMS

Liver regeneration is a necessary but complex process involving multiple cell types besides hepatocytes. Mechanisms underlying liver regeneration after partial hepatectomy and acute liver injury have been well-described. However, in patients with chronic and severe liver injury, the remnant liver cannot completely restore the liver mass and function, thereby involving liver progenitor-like cells (LPLCs) and various immune cells.

RESULTS

Macrophages are beneficial to LPLCs proliferation and the differentiation of LPLCs to hepatocytes. Also, cells expressing natural killer (NK) cell markers have been studied in promoting both liver injury and liver regeneration. NK cells can promote LPLC-induced liver regeneration, but the excessive activation of hepatic NK cells may lead to high serum levels of interferon-γ, thus inhibiting liver regeneration.

CONCLUSIONS

This review summarizes the recent research on 2 important innate immune cells, macrophages and NK cells, in LPLC-induced liver regeneration and the mechanisms of liver regeneration during chronic liver injury, as well as the latest macrophage- and NK cell-based therapies for chronic liver injury. These novel findings can further help identify new treatments for chronic liver injury, saving patients from the pain of liver transplantations.

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.

GITR/GITRL reverse signalling modulates the proliferation of hepatic progenitor cells by recruiting ANXA2 to phosphorylate ERK1/2 and Akt

Hepatic stem/progenitor cells are the major cell compartment for tissue repair when hepatocyte proliferation is compromised in chronic liver diseases, but the expansion of these cells increases the risk of carcinogenesis. Therefore, it is essential to explore the pathways restricting their expansion and abnormal transformation. The ligand of glucocorticoid-induced tumour necrosis factor receptor (GITRL) showed the most highly increased expression in hepatic progenitor cells treated with transforming growth factor (TGF)-β1. If overexpressed by hepatic progenitor cells, GITRL stimulated cell proliferation by activating the epithelial–mesenchymal transition pathway and enhancing ERK1/2 and Akt phosphorylation via GITRL binding to ANXA2. However, GITR, the specific GITRL receptor, suppressed the epithelial–mesenchymal transition pathway of GITRL-expressing cells and decreased their growth by dissociating ANXA2 from GITRL and reducing downstream ERK1/2 and Akt phosphorylation. This study identifies GITR/GITRL reverse signalling as a cross-interaction pathway between immune cells and hepatic stem/progenitor cells that restricts the expansion of hepatic stem/progenitor cells and reduces the possibility of carcinogenesis.

OUP accepted manuscript

Numerous components of the immune system, including inflammatory mediators, immune cells and cytokines, have a profound modulatory effect on the homeostatic regulation and regenerative activity of endogenous stem cells and progenitor cells. Thus, understanding how the immune system interacts with stem/progenitor cells could build the foundation to design novel and more effective regenerative therapies. Indeed, utilizing and controlling immune system components may be one of the most effective approaches to promote tissue regeneration. In this review, we first summarize the effects of various immune cell types on endogenous stem/progenitor cells, focusing on the tissue healing context. Then, we present interesting regenerative strategies that control or mimic the effect of immune components on stem/progenitor cells, in order to enhance the regenerative capacity of endogenous and transplanted stem cells. We highlight the potential clinical translation of such approaches for multiple tissues and organ systems, as these novel regenerative strategies could considerably improve or eventually substitute stem cell-based therapies. Overall, harnessing the power of the cross-talk between the immune system and stem/progenitor cells holds great potential for the development of novel and effective regenerative therapies.

Macrophages as key regulators of liver health and disease

Macrophages are a heterogeneous population of innate immune cells and key cellular components of the liver. Hepatic macrophages consist of embryologically-derived resident Kupffer cells (KC), recruited monocyte-derived macrophages (MDM) and capsular macrophages. Both the diversity and plasticity of hepatic macrophage subsets explain their different functions in the maintenance of hepatic homeostasis and in injury processes in acute and chronic liver diseases. In this review, we assess the evidence for macrophage involvement in regulating both liver health and injury responses in liver diseases including acute liver injury (ALI), chronic liver disease (CLD) (including liver fibrosis) and hepatocellular carcinoma (HCC). In healthy livers, KC display critical functions such as phagocytosis, danger signal recognition, cytokine release, antigen processing and the ability to orchestrate immune responses and maintain immunological tolerance. However, in most liver diseases there is a striking hepatic MDM expansion, which orchestrate both disease progression and regression. Single-cell approaches have transformed our understanding of liver macrophage heterogeneity, dynamics, and functions in both human samples and preclinical models. We will further discuss the new insights provided by these approaches and how they are enabling high-fidelity work to specifically identify pathogenic macrophage subpopulations. Given the important role of macrophages in regulating injury responses in a broad range of settings, there is now a huge interest in developing new therapeutic strategies aimed at targeting macrophages. Therefore, we also review the current approaches being used to modulate macrophage function in liver diseases and discuss the therapeutic potential of targeting macrophage subpopulations as a novel treatment strategy for patients with liver disorders.

Maraviroc Prevents HCC Development by Suppressing Macrophages and the Liver Progenitor Cell Response in a Murine Chronic Liver Disease Model

Simple Summary

Liver stem cells and activated macrophages have been implicated as contributors to liver cancer; hence, reducing their abundance is a potential avenue for therapy. In this article, we demonstrate that Maraviroc, a drug approved for human use, reduces the liver stem cell response and macrophage activation in a mouse model of liver cancer. These findings underline the preventive potential of this drug in liver cancer, a deadly disease for which there are few effective treatments.

Abstract

Maraviroc (MVC), a CCR5 antagonist, reduces liver fibrosis, injury and tumour burden in mice fed a hepatocarcinogenic diet, suggesting it has potential as a cancer therapeutic. We investigated the effect of MVC on liver progenitor cells (LPCs) and macrophages as both have a role in hepatocarcinogenesis. Mice were fed the hepatocarcinogenic choline-deficient, ethionine-supplemented diet (CDE) ± MVC, and immunohistochemistry, RNA and protein expression were used to determine LPC and macrophage abundance, migration and related molecular mechanisms. MVC reduced LPC numbers in CDE mice by 54%, with a smaller reduction seen in macrophages. Transcript and protein abundance of LPC-associated markers correlated with this reduction. The CDE diet activated phosphorylation of AKT and STAT3 and was inhibited by MVC. LPCs did not express Ccr5 in our model; in contrast, macrophages expressed high levels of this receptor, suggesting the effect of MVC is mediated by targeting macrophages. MVC reduced CD45+ cells and macrophage migration in liver and blocked the CDE-induced transition of liver macrophages from an M1- to M2-tumour-associated macrophage (TAM) phenotype. These findings suggest MVC has potential as a re-purposed therapeutic agent for treating chronic liver diseases where M2-TAM and LPC numbers are increased, and the incidence of HCC is enhanced.

Liver regeneration and inflammation: from fundamental science to clinical applications

Liver regeneration is a complex process involving the crosstalk of multiple cell types, including hepatocytes, hepatic stellate cells, endothelial cells and inflammatory cells. The healthy liver is mitotically quiescent, but following toxic damage or resection the cells can rapidly enter the cell cycle to restore liver mass and function. During this process of regeneration, epithelial and non-parenchymal cells respond in a tightly coordinated fashion. Recent studies have described the interaction between inflammatory cells and a number of other cell types in the liver. In particular, macrophages can support biliary regeneration, contribute to fibrosis remodelling by repressing hepatic stellate cell activation and improve liver regeneration by scavenging dead or dying cells in situ. In this Review, we describe the mechanisms of tissue repair following damage, highlighting the close relationship between inflammation and liver regeneration, and discuss how recent findings can help design novel therapeutic approaches.

Novel Mechanisms for Resolution of Liver Inflammation: Therapeutic Implications

Traditional concepts have classically viewed resolution of inflammation as a passive process yet insight into the pathways by which inflammation is resolved has challenged this idea. Resolution has been revealed as a highly dynamic and active event that is essential to counteract the dysregulated inflammatory response that drives diverse disease states. Abrogation of the hepatic inflammatory response through the stimulation of proresolving mechanisms represents a new paradigm in the setting of chronic inflammatory-driven liver diseases. Elucidation of the role of different cells of the innate and adaptive immune system has highlighted the interplay between them as an important orchestrator of liver repair. A finely tuned interaction between neutrophils and macrophages has risen as revolutionary mechanism that drives the restoration of hepatic function and architecture. Specialized proresolving mediators have also been shown to act as stop signals of the inflammatory response and promote resolution as well as tissue regeneration. In this review, we discuss the discovery and understanding of the mechanisms by which inflammation is resolved and highlight novel proresolving pathways that represent promising therapeutic strategies.

Stem cell therapy and tissue engineering strategies using cell aggregates and decellularized scaffolds for the rescue of liver failure

Liver failure is a lethal condition with hepatocellular dysfunction, and liver transplantation is presently the only effective treatment. However, due to the limited availability of donors and the potential immune rejection, novel therapeutic strategies are actively sought to restore the normal hepatic architectures and functions, especially for livers with inherited metabolic dysfunctions or chronic diseases. Although the conventional cell therapy has shown promising results, the direct infusion of hepatocytes is hampered by limited hepatocyte sources, poor cell viability, and engraftment. Hence, this review mainly highlights the role of stem cells and progenitors as the alternative cell source and summarizes the potential approaches based on tissue engineering to improve the delivery efficiency of cells. Particularly, the underlying mechanisms for cell therapy using stem cells and progenitors are discussed in two main aspects: paracrine effect and cell differentiation. Moreover, tissue-engineering approaches using cell aggregates and decellularized liver scaffolds for bioengineering of functional hepatic constructs are discussed and compared in terms of the potential to replicate liver physiological structures. In the end, a potentially effective strategy combining the premium advantages of stem cell aggregates and decellularized liver scaffolds is proposed as the future direction of liver tissue engineering and regeneration.

Fuzheng Huayu Recipe Prevented and Treated CCl4-Induced Mice Liver Fibrosis through Regulating Polarization and Chemotaxis of Intrahepatic Macrophages via CCL2 and CX3CL1

Background

Fuzheng Huayu recipe (FZHY) is an original Chinese patent medicine which was developed and marketed by our institute. It could markedly improve liver tissue inflammation and ameliorate hepatic fibrosis in the clinical study. The intrahepatic macrophages recruitment and polarization play an important role in the progress of liver inflammation and fibrosis. Whether FZHY exerted its antiliver fibrosis effects through regulating intrahepatic macrophages phenotypic ratios is still unknown. This study aims to explore the antifibrosis mechanism of FZHY on regulating the recruitment and polarization of intrahepatic macrophages.

Methods

C57/B6 mice were used for the establishment of the CCl4-induced mice liver fibrosis model. Liver inflammation and fibrosis were evaluated by HE and Sirius red staining, hydroxyproline assays, and biochemical tests. The levels of chemokines and inflammatory cytokines in liver tissue were measured by RNA-Seq transcriptome analysis, western blot assay, RT-qPCR, and immunofluorescence assay. The macrophages recruitment and phenotypic polarization were observed by flow cytometry.

Results

FZHY significantly improved liver inflammation and reduced liver fibrosis degree. TNF signaling pathway, involved in macrophages recruitment and phenotypic polarization, was discovered by RNA-Seq transcriptome analysis. In TNF signaling pathway, CCL2 expression was significantly decreased and CX3CL1 expression was significantly upregulated by FZHY in liver tissue and primary intrahepatic macrophages. The ratio of proinflammatory hepatic resident macrophage-Kupffer cells (F4/80⁺CD11b⁻CD86⁺) was downregulated by FZHY, while the proportion of anti-inflammatory Kupffer cells (F4/80⁺CD11b⁻CD206⁺) was upregulated. Meanwhile, the ratio of proinflammatory Ly6C^high macrophages (F4/80⁺CD11b⁺Ly6C^high) which were recruited from blood circulation by CCL2 was reduced by FZHY, while the ratio of restorative Ly6C^low macrophages (F4/80⁺CD11b⁺Ly6C^low) which were recruited from blood circulation or induced from Ly6C^high macrophages polarization by CX3CL1 was significantly increased.

Conclusions

FZHY could regulate the recruitment and polarization of intrahepatic macrophages via CCL2 and CX3CL1, so as to play its anti-inflammation and antifibrosis pharmacological effects in the liver.

Hepatic macrophages in liver homeostasis and diseases-diversity, plasticity and therapeutic opportunities

Macrophages, which are key cellular components of the liver, have emerged as essential players in the maintenance of hepatic homeostasis and in injury and repair processes in acute and chronic liver diseases. Upon liver injury, resident Kupffer cells (KCs) sense disturbances in homeostasis, interact with hepatic cell populations and release chemokines to recruit circulating leukocytes, including monocytes, which subsequently differentiate into monocyte-derived macrophages (MoMϕs) in the liver. Both KCs and MoMϕs contribute to both the progression and resolution of tissue inflammation and injury in various liver diseases. The diversity of hepatic macrophage subsets and their plasticity explain their different functional responses in distinct liver diseases. In this review, we highlight novel findings regarding the origins and functions of hepatic macrophages and discuss the potential of targeting macrophages as a therapeutic strategy for liver disease.

Regeneration and activation of liver progenitor cells in liver cirrhosis

Cirrhosis is characterized as the progress of regenerative nodules surrounded by fibrous bands in response to chronic hepatic injury and causes portal hypertension and end-stage hepatic disease. Following liver injury, liver progenitor cells (LPCs) can be activated and differentiate into hepatocytes in order to awaken liver regeneration and reach homeostasis. Recent research has uncovered some new sources of LPCs. Here, we update the mechanisms of LPCs-mediated liver regeneration in cirrhosis by introducing the origin of LPCs and LPCs’ niche with a discussion of the influence of LPC-related cells. This article analyzes the mechanism of regeneration and activation of LPCs in cirrhosis in recent years aiming to provide help for clinical application.

Hepatic Macrophages in Liver Injury

Ample evidence suggests that hepatic macrophages play key roles in the injury and repair mechanisms during liver disease progression. There are two major populations of hepatic macrophages: the liver resident Kupffer cells and the monocyte-derived macrophages, which rapidly infiltrate the liver during injury. Under different disease conditions, the tissue microenvironmental cues of the liver critically influence the phenotypes and functions of hepatic macrophages. Furthermore, hepatic macrophages interact with multiple cells types in the liver, such as hepatocytes, neutrophils, endothelial cells, and platelets. These crosstalk interactions are of paramount importance in regulating the extents of liver injury, repair, and ultimately liver disease progression. In this review, we summarize the novel findings highlighting the impact of injury-induced microenvironmental signals that determine the phenotype and function of hepatic macrophages. Moreover, we discuss the role of hepatic macrophages in homeostasis and pathological conditions through crosstalk interactions with other cells of the liver.

Suberoylanilide hydroxamic acid alleviates orthotopic liver transplantation‑induced hepatic ischemia‑reperfusion injury by regulating the AKT/GSK3β/NF‑κB and AKT/mTOR pathways in rat Kupffer cells

Multiple mechanisms are involved in regulating hepatic ischemia-reperfusion injury (IRI), in which Kupffer cells (KCs), which are liver-resident macrophages, play critical roles by regulating inflammation and the immune response. Suberoylanilide hydroxamic acid (SAHA), a pan-histone deacetylase inhibitor, has anti-inflammatory effects and induces autophagy. To investigate whether SAHA ameliorates IRI and the mechanisms by which SAHA exerts its effects, an orthotopic liver transplantation (OLT) rat model was established after treatment with SAHA. The results showed that SAHA effectively ameliorated OLT-induced IRI by reducing M1 polarization of KCs through inhibition of the AKT/glycogen synthase kinase (GSK)3β/NF-κB signaling pathway. Furthermore, the present study found that SAHA upregulates autophagy 5 protein (ATG5)/LC3B in KCs through the AKT/mTOR signaling pathway and inhibition of autophagy by knockdown of ATG5 in KCs partly impaired the protective effect of SAHA on IR-injured liver. Therefore, the current study demonstrated that SAHA reduces M1 polarization of KCs by inhibiting the AKT/GSK3β/NF-κB pathway and upregulates autophagy in KCs through the AKT/mTOR signaling pathway, which both alleviate OLT-induced IRI. The present study revealed that SAHA may be a novel treatment for the amelioration of OLT-induced IRI.

Endogenous Interleukin 18 Suppresses Liver Regeneration After Hepatectomy in Mice

The comprehensive role of interleukin (IL) 18 during liver regeneration is barely studied. Our aim is to evaluate the role of IL18 in liver regeneration after partial hepatectomy (PH) in mice. The expression profile of IL18 in the liver and the gut after 70% PH was measured. Liver samples after 70% and 85% PH from IL18 knockout (IL18^-/- ) mice and wild type (WT) mice were collected for comparison of liver regeneration. The effect of recombinant IL18 on liver regeneration was tested in IL18^-/- mice, and the utility of IL18 binding protein (BP) was also evaluated following 70% PH in WT mice. Expression levels of IL18 in the liver and the gut elevated after 70% PH. The liver weight/body weight ratios (LBWRs) after PH were significantly higher in IL18^-/- mice than those in WT mice. Recombinant IL18 injection significantly decreased LBWR at 7 days after 70% PH in IL18^-/- mice. The expression of cyclin D1, EdU labeling index, and Ki-67 proliferation index were much higher in IL18^-/- mice than those in WT mice after 70% PH. The expression level of glypican 3 (GPC3) in WT mice significantly elevated during liver regeneration. In contrast, the expression level of GPC3 in IL18^-/- mice remained roughly unchanged during liver regeneration. IL18BP injection significantly increased the LBWR at 7 days after 70% PH in WT mice. In conclusion, endogenous IL18 inhibited liver regeneration after PH in mice, possibly through up-regulating GPC3. IL18BP may be an effective agent to promote liver regeneration after PH.

Macrophage Phenotype and Function in Liver Disorder

Hepatic macrophages are a remarkably heterogeneous population consisting of self-renewing tissue-resident phagocytes, termed Kupffer cells (KCs), and recruited macrophages derived from peritoneal cavity as well as the bone marrow. KCs are located in the liver sinusoid where they scavenge the microbe from the portal vein to maintain liver homeostasis. Liver injury may trigger hepatic recruitment of peritoneal macrophages and monocyte-derived macrophages. Studies describing macrophage accumulation have shown that hepatic macrophages are involved in the initiation and progression of various liver diseases. They act as tolerogenic antigen-presenting cells to inhibit T-cell activation by producing distinct sets of cytokines, chemokines, and mediators to maintain or resolve inflammation. Furthermore, by releasing regenerative growth factors, matrix metalloproteinase arginase, they promote tissue repair. Recent experiments found that KCs and recruited macrophages may play different roles in the development of liver disease. Given that hepatic macrophages are considerably plastic populations, their phenotypes and functions are likely switching along disease progression. In this review, we summarize current knowledge about the role of tissue-resident macrophages and recruited macrophages in pathogenesis of alcoholic liver disease (ALD), non-alcoholic steatohepatitis (NASH), viral hepatitis, and hepatocellular carcinoma (HCC).

Carcinogenesis and Metastasis in Liver: Cell Physiological Basis

Hepatocellular carcinoma (HCC) incidence is rising. This paper summarises the current state of knowledge and recent discoveries in the cellular and physiological mechanisms leading to the development of liver cancer, especially HCC, and liver metastases. After reviewing normal hepatic cytoarchitecture and immunological characteristics, the paper addresses the pathophysiological factors that cause liver damage and predispose to neoplasia. Particular attention is given to chronic liver diseases, metabolic syndrome and the impact of altered gut microbiota, disrupted circadian rhythm and psychological stress. Improved knowledge of the multifactorial aetiology of HCC has important implications for the prevention and treatment of this cancer and of liver metastases in general.

Liver Macrophages: Old Dogmas and New Insights

Inflammation is a hallmark of virtually all liver diseases, such as liver cancer, fibrosis, nonalcoholic steatohepatitis, alcoholic liver disease, and cholangiopathies. Liver macrophages have been thoroughly studied in human disease and mouse models, unravelling that the hepatic mononuclear phagocyte system is more versatile and complex than previously believed. Liver macrophages mainly consist of liver‐resident phagocytes, or Kupffer cells (KCs), and bone marrow‐derived recruited monocytes. Although both cell populations in the liver demonstrate principal functions of macrophages, such as phagocytosis, danger signal recognition, cytokine release, antigen processing, and the ability to orchestrate immune responses, KCs and recruited monocytes retain characteristic ontogeny markers and remain remarkably distinct on several functional aspects. While KCs dominate the hepatic macrophage pool in homeostasis (“sentinel function”), monocyte‐derived macrophages prevail in acute or chronic injury (“emergency response team”), making them an interesting target for novel therapeutic approaches in liver disease. In addition, recent data acquired by unbiased large‐scale techniques, such as single‐cell RNA sequencing, unraveled a previously unrecognized complexity of human and murine macrophage polarization abilities, far beyond the old dogma of inflammatory (M1) and anti‐inflammatory (M2) macrophages. Despite tremendous progress, numerous challenges remain in deciphering the full spectrum of macrophage activation and its implication in either promoting liver disease progression or repairing injured liver tissue. Being aware of such heterogeneity in cell origin and function is of crucial importance when studying liver diseases, developing novel therapeutic interventions, defining macrophage‐based prognostic biomarkers, or designing clinical trials. Growing knowledge in gene expression modulation and emerging technologies in drug delivery may soon allow shaping macrophage populations toward orchestrating beneficial rather than detrimental inflammatory responses.

Accumulation of free cholesterol and oxidized low-density lipoprotein is associated with portal inflammation and fibrosis in nonalcoholic fatty liver disease

Background

Macrophages engulf oxidized-LDL (oxLDL) leading to accumulation of cellular cholesterol and formation of foam cells, which is a hallmark of atherosclerosis. Moreover, recent studies showed that accumulation of free cholesterol in macrophages leading to activation of NLRP3 inflammasome and production of interleukin-1β (IL-1β) has been linked to atherosclerosis-associated inflammation. However, it is not clear if cholesterol accumulation is associated with hepatic inflammation and fibrosis in the liver. In this study, we investigated the association of free cholesterol and oxLDL accumulation in portal vein with the inflammation, atherosclerosis, and fibrosis in human nonalcoholic fatty liver disease (NAFLD).

Methods

Serial sections derived from surgical specimens of NAFLD were stained with filipin and antibodies against IL-1β, CD68, α-smooth muscle actin (α-SMA), oxLDL and lectin-like oxLDL receptor-1 (LOX-1).

Results

We show that free cholesterol was colocalized with oxLDL in the wall of portal vein, and which was associated with lumen narrowing, plaque formation, endothelium deformation, and portal venous inflammation. The inflammation was evidenced by the colocalization of Kupffer cells and IL-1β and the expression of LOX-1. Notably, ruptured plaque was closely associated with portal venous inflammation. Moreover, free cholesterol and oxLDL accumulation in periportal and sinusoidal fibrosis, which was associated with regional stellate cell activation and chicken-wire fibrosis.

Conclusion

These findings reveal a direct association between cholesterol accumulation, portal venous inflammation and fibrosis in NAFLD.

Electronic supplementary material

The online version of this article (10.1186/s12950-019-0211-5) contains supplementary material, which is available to authorized users.

Regulation of Monocyte-Macrophage Responses in Cirrhosis-Role of Innate Immune Programming and Checkpoint Receptors

Many aspects of the innate immune system have been studied in cirrhosis, and abnormalities have been described supporting both a pro-inflammatory and anti-inflammatory phenotype of myeloid cells. However, the findings of these studies vary by stage of disease and methodology. The recent description of the syndrome of acute-on-chronic liver failure (ACLF) has refined our understanding of the natural history of cirrhosis. In this context, we review the regulatory mechanisms at play that contribute to the immune abnormalities described in advanced liver disease. Specifically, we review the evidence for epigenetic mechanisms regulating monocyte phenotype, and the role of checkpoint receptors on regulating innate and adaptive immune cell function.

Cytokine profiles in acute liver injury-Results from the US Drug-Induced Liver Injury Network (DILIN) and the Acute Liver Failure Study Group

Changes in levels of cytokines and chemokines have been proposed as possible biomarkers of tissue injury, including liver injury due to drugs. Recently, in acute drug-induced liver injury (DILI), we showed that 19 of 27 immune analytes were differentially expressed and that disparate patterns of immune responses were evident. Lower values of serum albumin (< 2.8 g/dL) and lower levels of only four analytes, namely, IL-9, IL-17, PDGF-bb, and RANTES, were highly predictive of early death [accuracy = 96%]. The goals of this study were to assess levels of the same 27 immune analytes in larger numbers of subjects to learn whether the earlier findings would be confirmed in new and larger cohorts of subjects, compared with a new cohort of healthy controls. We studied 127 subjects with acute DILI enrolled into the US DILIN. We also studied 118 subjects with severe acute liver injury of diverse etiologies, enrolled into the ALF SG registry of subjects. Controls comprised 63 de-identified subjects with no history of liver disease and normal liver tests. Analytes associated with poor outcomes [death before 6 months, n = 32 of the total of 232 non-acetaminophen (Apap) subjects], were lower serum albumin [2.6 vs 3.0 g/dL] and RANTES [6,458 vs 8,999 pg/mL] but higher levels of IL-6 [41 vs 18], IL-8 [78 vs 48], and MELD scores [30 vs 24]. Similar patterns were observed for outcome of death/liver transplant within 6 months. A model that included only serum albumin < 2.8 g/dL and RANTES below its median value of 11,349 had 83% (or 81%) accuracy for predicting early death (or early death/liver transplant) in 127 subjects from DILIN. No patterns of serum immune analytes were reflective of the etiologies of acute liver failure, but there were cytokine patterns that predicted prognosis in both acute DILI and ALF.

Myeloid Cells and Chronic Liver Disease: a Comprehensive Review

Myeloid cells play a major role in the sensitization to liver injury, particularly in chronic inflammatory liver diseases with a biliary or hepatocellular origin, and the interplay between myeloid cells and the liver may explain the increased incidence of hepatic osteodystrophy. The myeloid cell-liver axis involves several mature myeloid cells as well as immature or progenitor cells with the complexity of the liver immune microenvironment aggravating the mist of cell differentiation. The unique positioning of the liver at the junction of the peripheral and portal circulation systems underlines the interaction of myeloid cells and hepatic cells and leads to immune tolerance breakdown. We herein discuss the scenarios of different chronic liver diseases closely modulated by myeloid cells and illustrate the numerous potential targets, the understanding of which will ultimately steer the development of solid immunotherapeutic regimens. Ultimately, we are convinced that an adequate modulation of the liver microenvironment to modify the functional and quantitative characteristics of myeloid cells will be a successful approach to treating chronic liver diseases of different etiologies.

Curcumin reduces Ly6Chi monocyte infiltration to protect against liver fibrosis by inhibiting Kupffer cells activation to reduce chemokines secretion

Curcumin has been reported to have anti-fibrotic effect. However, the anti-fibrotic mechanism of curcumin for liver fibrosis remains obscure. In the presenting study, we aimed to investigate whether curcumin reduce chemokines secretion by inhibiting kupffer cells (KCs) activation to decrease Ly6C^hi monocyte infiltration in the treatment of liver fibrosis. Liver fibrosis was induced by intraperitoneal carbon tetrachloride (CCl₄)-injection in mice. Mice in curcumin group received curcumin treatment by gavage. Pretreatment with curcumin significantly protected mice from liver inflammation and fibrosis. Compared to CCl₄ group, mice in the curcumin group showed significantly less intrahepatic infiltration of Ly6C^hi monocytes, but no difference of other leucocyte subtypes. Moreover, curcumin significantly reduced Ly6C^hi monocytes associated pro-inflammatory and pro-fibrogenic cytokines, which was in line with the decreased numbers of intrahepatic Ly6C^hi monocytes. Further study found that curcumin is able to decrease KCs activation and monocyte chemokines, which explains why curcumin can reduce Ly6C^hi monocytes infiltration during liver fibrosis. In vitro, we discovered that curcumin prevents the polarization of macrophages toward M1 and reduces monocyte chemokines secretion, which is involved with ERK1/2 and p38 pathways. Taken together, for the first time, we verified that curcumin can reduce chemokines secretion by inhibiting KCs activation to decrease Ly6C^hi monocyte infiltration in the treatment of liver fibrosis. These results suggested that curcumin may be considered a promising candidate in the prevention and treatment of liver fibrosis.

Interleukins-17 and 27 promote liver regeneration by sequentially inducing progenitor cell expansion and differentiation

Liver progenitor cells (LPCs)/ductular reactions (DRs) are associated with inflammation and implicated in the pathogenesis of chronic liver diseases. However, how inflammation regulates LPCs/DRs remains largely unknown. Identification of inflammatory processes that involve LPC activation and expansion represent a key step in understanding the pathogenesis of liver diseases. In the current study, we found that diverse types of chronic liver diseases are associated with elevation of infiltrated interleukin (IL)-17-positive (+) cells and cytokeratin 19 (CK19)⁺ LPCs, and both cell types colocalized and their numbers positively correlated with each other. The role of IL-17 in the induction of LPCs was examined in a mouse model fed a choline-deficient and ethionine-supplemented (CDE) diet. Feeding of wild-type mice with the CDE diet markedly elevated CK19⁺Ki67⁺ proliferating LPCs and hepatic inflammation. Disruption of the IL-17 gene or IL-27 receptor, alpha subunit (WSX-1) gene abolished CDE diet-induced LPC expansion and inflammation. In vitro treatment with IL-17 promoted proliferation of bipotential murine oval liver cells (a liver progenitor cell line) and markedly up-regulated IL-27 expression in macrophages. Treatment with IL-27 favored the differentiation of bipotential murine oval liver cells and freshly isolated LPCs into hepatocytes. Conclusion: The current data provide evidence for a collaborative role between IL-17 and IL-27 in promoting LPC expansion and differentiation, respectively, thereby contributing to liver regeneration. (Hepatology Communications 2018;2:329-343).

Mesenchymal stem cells rescue acute hepatic failure by polarizing M2 macrophages

AIM

To investigate whether M1 or M2 polarization contributes to the therapeutic effects of mesenchymal stem cells (MSCs) in acute hepatic failure (AHF).

METHODS

MSCs were transfused into rats with AHF induced by D-galactosamine (DGalN). The therapeutic effects of MSCs were evaluated based on survival rate and hepatocyte proliferation and apoptosis. Hepatocyte regeneration capacity was evaluated by the expression of the hepatic progenitor surface marker epithelial cell adhesion molecule (EpCAM). Macrophage polarization was analyzed by M1 markers [CD68, tumor necrosis factor alpha (TNF-α), interferon-γ (IFN-γ), inducible nitric oxide synthase (INOS)] and M2 markers [CD163, interleukin (IL)-4, IL-10, arginase-1 (Arg-1)] in the survival and death groups after MSC transplantation.

RESULTS

The survival rate in the MSC-treated group was increased compared with the DPBS-treated control group (37.5% vs 10%). MSC treatment protected rats with AHF by reducing apoptotic hepatocytes and promoting hepatocyte regeneration. Immunohistochemical analysis showed that MSC treatment significantly increased the expression of EpCAM compared with the control groups (P < 0.001). Expression of EpCAM in the survival group was significantly up-regulated compared with the death group after MSC transplantation (P = 0.003). Transplantation of MSCs significantly improved the expression of CD163 and increased the gene expression of IL-10 and Arg-1 in the survival group. IL-4 concentrations were significantly increased compared to the death group after MSC transplantation (88.51 ± 24.51 pg/mL vs 34.61 ± 6.6 pg/mL, P < 0.001). In contrast, macrophages showed strong expression of CD68, TNF-α, and INOS in the death group. The concentration of IFN-γ was significantly increased compared to the survival group after MSC transplantation (542.11 ± 51.59 pg/mL vs 104.07 ± 42.80 pg/mL, P < 0.001).

CONCLUSION

M2 polarization contributes to the therapeutic effects of MSCs in AHF by altering levels of anti-inflammatory and pro-inflammatory factors.

Differential Location and Distribution of Hepatic Immune Cells

The liver is one of the main organs in the body, performing several metabolic and immunological functions that are indispensable to the organism. The liver is strategically positioned in the abdominal cavity between the intestine and the systemic circulation. Due to its location, the liver is continually exposed to nutritional insults, microbiota products from the intestinal tract, and to toxic substances. Hepatocytes are the major functional constituents of the hepatic lobes, and perform most of the liver’s secretory and synthesizing functions, although another important cell population sustains the vitality of the organ: the hepatic immune cells. Liver immune cells play a fundamental role in host immune responses and exquisite mechanisms are necessary to govern the density and the location of the different hepatic leukocytes. Here we discuss the location of these pivotal cells within the different liver compartments, and how their frequency and tissular location can dictate the fate of liver immune responses.

Cardamom powder supplementation prevents obesity, improves glucose intolerance, inflammation and oxidative stress in liver of high carbohydrate high fat diet induced obese rats

Background

Cardamom is a well-known spice in Indian subcontinent, used in culinary and traditional medicine practices since ancient times. The current investigation was untaken to evaluate the potential benefit of cardamom powder supplementation in high carbohydrate high fat (HCHF) diet induced obese rats.

Method

Male Wistar rats (28 rats) were divided into four different groups such as Control, Control + cardamom, HCHF, HCHF + cardamom. High carbohydrate and high fat (HCHF) diet was prepared in our laboratory. Oral glucose tolerance test, organs wet weight measurements and oxidative stress parameters analysis as well as liver marker enzymes such as alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) activities were assayed on the tissues collected from the rats. Plasma lipids profiles were also measured in all groups of animals. Moreover, histological staining was also performed to evaluate inflammatory cells infiltration and fibrosis in liver.

Results

The current investigation showed that, HCHF diet feeding in rats developed glucose intolerance and increased peritoneal fat deposition compared to control rats. Cardamom powder supplementation improved the glucose intolerance significantly (p > 0.05) and prevented the abdominal fat deposition in HCHF diet fed rats. HCHF diet feeding in rats also developed dyslipidemia, increased fat deposition and inflammation in liver compared to control rats. Cardamom powder supplementation significantly prevented the rise of lipid parameters (p > 0.05) in HCHF diet fed rats. Histological assessments confirmed that HCHF diet increased the fat deposition and inflammatory cells infiltration in liver which was normalized by cardamom powder supplementation in HCHF diet fed rats. Furthermore, HCHF diet increased lipid peroxidation, decreased antioxidant enzymes activities and increased advanced protein oxidation product level significantly (p > 0.05) both in plasma and liver tissue which were modulated by cardamom powder supplementation in HCHF diet fed rats. HCHF diet feeding in rats also increased the ALT, AST and ALP enzyme activities in plasma which were also normalized by cardamom powder supplementation in HCHF diet fed rats. Moreover, cardamom powder supplementation ameliorated the fibrosis in liver of HCHF diet fed rats.

Conclusion

This study suggests that, cardamom powder supplementation can prevent dyslipidemia, oxidative stress and hepatic damage in HCHF diet fed rats.

Treatment of severe alcoholic hepatitis: past, present and future

Alcoholic hepatitis (AH) manifests as a clinical syndrome characterized by recent jaundice and liver function deterioration in an actively drinking patient. The principal cause of AH is alcoholic steatohepatitis (ASH) defined histologically by the coexistence of steatosis, hepatocyte ballooning and satellitosis. While nonsevere AH usually responds to alcohol abstinence, severe AH, identified by Maddrey scoring ≥ 32, has a bad prognosis and is traditionally treated by a 28-day course of prednisone therapy. A recent trial, which showed no improvement of long-term survival but significant reduced mortality after 28 days of corticoid therapy compared to placebo, opens a debate on its efficacy. N-acetyl-cysteine supplementation combined with steroid therapy is also able to reduce the 28-day mortality compared to steroid alone. While guidelines recommend high-calorie intake and protein supplementation in decompensated liver diseases, intensive enteral nutrition together with corticoid treatment does not reduce mortality compared to corticoid alone in a recent study with ASH patients. Stimulation of liver regeneration through interleukin-22, granulocyte colony-stimulating factor or farnesoid X receptor agonists, inhibition of apoptosis, early liver transplantation and modulation of gut microbiota through antibiotic or faecal transplantation approaches constitute new therapeutic perspectives that are investigated in current clinical trials. Inhibition of oxidative stress, modulation of gut fungal populations and stimulation of progenitor cell proliferation and pro-regenerative inflammatory pathways constitute prospects for future human trials. For long-term survival, strategies for persistent alcohol abstinence remain the key of success, opening another large research field.

Hepatic Progenitor Cells: An Update

Liver regeneration is a fascinating and complex process with many medical implications. An important component of this regenerative process is the hepatic progenitor cell (HPC). These appealing cells are able to participate in the renewal of hepatocytes and cholangiocytes when the normal homeostatic regeneration is exhausted. Moreover, the HPC niche is of vital importance toward the activation, differentiation, and proliferation of the HPC. This niche provides a rich microenvironment for the regulation of the HPC, thanks to the intercellular secretion of molecules. New findings indicate that the regenerative possibilities in the liver could provide a diverse basis for therapeutic targets.

Ly6Chi Monocytes and Their Macrophage Descendants Regulate Neutrophil Function and Clearance in Acetaminophen-Induced Liver Injury

Monocyte-derived macrophages (MoMF) play a pivotal role in the resolution of acetaminophen-induced liver injury (AILI). Timely termination of neutrophil activity and their clearance are essential for liver regeneration following injury. Here, we show that infiltrating Ly6C^hi monocytes, their macrophage descendants, and neutrophils spatially and temporally overlap in the centrilobular necrotic areas during the necroinflammatory and resolution phases of AILI. At the necroinflammatory phase, inducible ablation of circulating Ly6C^hi monocytes resulted in reduced numbers and fractions of reactive oxygen species (ROS)-producing neutrophils. In alignment with this, neutrophils sorted from monocyte-deficient livers exhibited reduced expression of NADPH oxidase 2. Moreover, human CD14⁺ monocytes stimulated with lipopolysaccharide or hepatocyte apoptotic bodies directly induced ROS production by cocultured neutrophils. RNA-seq-based transcriptome profiling of neutrophils from Ly6C^hi monocyte-deficient versus normal livers revealed 449 genes that were differentially expressed with at least twofold change (p ≤ 0.05). In the absence of Ly6C^hi monocytes, neutrophils displayed gene expression alterations associated with decreased innate immune activity and increased cell survival. At the early resolution phase, Ly6C^hi monocytes differentiated into ephemeral Ly6C^lo MoMF and their absence resulted in significant accumulation of late apoptotic neutrophils. Further gene expression analysis revealed the induced expression of a specific repertoire of bridging molecules and receptors involved with apoptotic cell clearance during the transition from Ly6C^hi monocytes to MoMF. Collectively, our findings establish a phase-dependent task division between liver-infiltrating Ly6C^hi monocytes and their MoMF descendants with the former regulating innate immune functions and cell survival of neutrophils and the later neutrophil clearance.

Modulation of liver regeneration via myeloid PTEN deficiency

Molecular mechanisms that modulate liver regeneration are of critical importance for a number of hepatic disorders. Kupffer cells and natural killer (NK) cells are two cell subsets indispensable for liver regeneration. We have focused on these two populations and, in particular, the interplay between them. Importantly, we demonstrate that deletion of the myeloid phosphatase and tensin homolog on chromosome 10 (PTEN) leading to an M2-like polarization of Kupffer cells, which results in decreased activation of NK cells. In addition, PTEN-deficient Kupffer cells secrete additional factors that facilitate the proliferation of hepatocytes. In conclusion, PTEN is critical for inhibiting M2-like polarization of Kupffer cells after partial hepatectomy, resulting in NK cell activation and thus the inhibition of liver regeneration. Furthermore, PTEN reduces growth factor secretion by Kupffer cells. Our results suggest that targeting PTEN on Kupffer cells may be useful in altering liver regeneration in patients undergoing liver resection.

The spleen in liver cirrhosis: revisiting an old enemy with novel targets

The spleen is a secondary lymphoid organ which can influence the progression of multiple diseases, notably liver cirrhosis. In chronic liver diseases, splenomegaly and hypersplenism can manifest following the development of portal hypertension. These splenic abnormalities correlate with and have been postulated to facilitate the progression of liver fibrosis to cirrhosis, although precise mechanisms remain poorly understood. In this review, we summarize the literature to highlight the mechanistic contributions of splenomegaly and hypersplenism to the development of liver cirrhosis, focusing on three key aspects: hepatic fibrogenesis, hepatic immune microenvironment dysregulation and liver regeneration. We conclude with a discussion of the possible therapeutic strategies for modulating splenic abnormalities, including the novel potential usage of nanomedicine in non-surgically targetting splenic disorders for the treatment of liver cirrhosis.

Autologous bone marrow-derived cell transplantation in decompensated alcoholic liver disease: what is the impact on liver histology and gene expression patterns?

Background

Liver stem cell therapy (SCT) has been suggested as a promising means to improve liver regeneration in advanced liver disease. However, data from trials are heterogeneous, with no systematic histological evaluation. The aim of this study is to specifically analyze the effect of autologous SCT on liver regeneration and on gene expression changes.

Methods

Individuals in the randomized controlled trial of SCT in alcoholic hepatitis with paired liver biopsies were included (n = 58). Immunohistochemistry (Ki67, K7, and CD68), in situ hybridization (SPINK1), and global gene expression analysis were performed on liver biopsies (30 control patients and 28 patients with transarterial administration of bone marrow-derived stem cells) both at baseline and after 4 weeks of follow-up.

Results

No difference between the two groups could be observed regarding the proliferative hepatocyte number, proliferative K7-positive cells, or total K7-positive cells at the 4-week follow-up liver biopsy. However, patients who received SCT showed a more important liver macrophagic expansion as compared to standard treatment. Transcriptome data revealed changes in genes linked with inflammation (CD68 and SAA), regeneration (SPINK1 and HGF), fibrosis (COL1A1), and stem cells (CD45). No changes in gene pathways involved in liver growth and cell cycle proteins were evident. SPINK1 mRNA was present by in situ hybridization at week 4 in SCT patients in the liver parenchyma areas adjacent to macrophage recruitment and liver cell proliferation.

Conclusions

The analysis of liver tissue after SCT demonstrated an expansion of macrophages concurrent with an upregulated expression of genes involved in inflammatory and regenerative pathways. With the negative results from the clinical trial, the impact of the SCT has to be interpreted as weak, and it is not able to modify the clinical course of this severe liver disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0541-2) contains supplementary material, which is available to authorized users.

Liver macrophages in tissue homeostasis and disease

Macrophages represent a key cellular component of the liver, and are essential for maintaining tissue homeostasis and ensuring rapid responses to hepatic injury. Our understanding of liver macrophages has been revolutionized by the delineation of heterogeneous subsets of these cells. Kupffer cells are a self-sustaining, liver-resident population of macrophages and can be distinguished from the monocyte-derived macrophages that rapidly accumulate in the injured liver. Specific environmental signals further determine the polarization and function of hepatic macrophages. These cells promote the restoration of tissue integrity following liver injury or infection, but they can also contribute to the progression of liver diseases, including hepatitis, fibrosis and cancer. In this Review, we highlight novel findings regarding the origin, classification and function of hepatic macrophages, and we discuss their divergent roles in the healthy and diseased liver.

The progenitor cell dilemma: Cellular and functional heterogeneity in assistance or escalation of liver injury

Liver progenitor cells (LPCs) are quiescent cells that are activated during liver injury and thought to give rise to hepatocytes and cholangiocytes in order to support liver regeneration and tissue restitution. While hepatocytes are capable of self-renewal, during most chronic injuries the proliferative capacity of hepatocytes is inhibited, thus LPCs provide main source for regeneration. Despite extensive lineage tracing studies, their role and involvement in these processes are often controversial. Additionally, increasing evidence suggests that the LPC compartment consists of heterogeneous cell populations that are actively involved in cellular interactions with myeloid and lymphoid cells during regeneration. On the other hand, LPC expansion has been associated with an increased fibrogenic response, raising concerns about the therapeutic use of these cells. This review aims to summarize the current understanding of the identity, the cellular interactions and the key pathways affecting the biology of LPCs. Understanding the regulatory circuits and the specific role of LPCs is especially important as it could provide novel therapeutic platforms for the treatment of liver inflammation, fibrosis and regeneration.

Immune cells in liver regeneration

After partial hepatectomy, hepatocytes proliferate to restore mass and function of the liver. Macrophages, natural killer (NK) cells, natural killer T (NKT) cells, dendritic cells (DC), eosinophils, gamma delta T (γδT) cells, and conventional T cells, as well as other subsets of the immune cells residing in the liver control liver regeneration, either through direct interactions with hepatocytes or indirectly by releasing inflammatory cytokines. Here, we review recent progress regarding the immune cells in the liver and their functions during liver regeneration.

Possible Involvement of Liver Resident Macrophages (Kupffer Cells) in the Pathogenesis of Both Intrahepatic and Extrahepatic Inflammation

Liver resident macrophages designated Kupffer cells (KCs) form the largest subpopulation of tissue macrophages. KCs are involved in the pathogenesis of liver inflammation. However, the role of KCs in the systemic inflammation is still elusive. In this study, we examined whether KCs are involved in not only intrahepatic inflammation but also extrahepatic systemic inflammation. Administration of clodronate liposomes resulted in the KC deletion and in the suppression of liver injury in T cell-mediated hepatitis by ConA as a local acute inflammation model, while the treatment did not influence dextran sulfate sodium- (DSS-) induced colitis featured by weight loss, intestinal shrink, and pathological observation as an ectopic local acute inflammation model. In contrast, KC deletion inhibited collagen-induced arthritis as a model of extrahepatic, systemic chronical inflammation. KC deleted mice showed weaker arthritic scores, less joint swelling, and more joint space compared to arthritis-induced control mice. These results strongly suggest that KCs are involved in not only intrahepatic inflammatory response but also systemic (especially) chronic inflammation.

Infliximab and Dexamethasone Attenuate the Ductular Reaction in Mice

Chronic hepatic injury is accompanied by a ductular response that is strongly correlated with disease severity and progression of fibrosis. To investigate whether anti-inflammatory drugs can modulate the ductular response, we treated mice suffering from a steatotic or cholestatic injury with anti-TNF-α antibodies (Infliximab) or glucocorticoids (Dexamethasone). We discovered that Dexamethasone and Infliximab can both modulate the adaptive remodeling of the biliary architecture that occurs upon liver injury and limit extracellular matrix deposition. Infliximab treatment, at least in these steatotic and cholestatic mouse models, is the safer approach since it does not increase liver injury, allows inflammation to take place but inhibits efficiently the ductular response and extracellular matrix deposition. Infliximab-based therapy could, thus, still be of importance in multiple chronic liver disorders that display a ductular response such as alcoholic liver disease or sclerosing cholangitis.

Infiltrating monocytes in liver injury and repair

Noninfectious liver injury causes many acute and chronic liver diseases around the globe, and particularly in developed nations. Bone marrow-derived monocytes infiltrate the damaged liver tissue and are a critical component of the innate immune response that may drive injury resolution or host death in the short term or chronic inflammation, fibrosis and hepatocellular carcinoma in the long term. Monocytes often play dual roles in liver injury—both perpetuating inflammation and promoting resolution of inflammation and fibrosis. Thus, we will address the role that monocytes play in different experimental forms of noninfectious liver injury; considering in particular the importance of the transition from inflammatory Ly6C^hi monocytes to pro-resolution Ly6C^lo monocyte-derived macrophages and the consequences of this transition for disease progression and resolution.

Monocyte Conversion During Inflammation and Injury

Monocytes are circulating leukocytes important in both innate and adaptive immunity, primarily functioning in immune defense, inflammation, and tissue remodeling. There are 2 subsets of monocytes in mice (3 subsets in humans) that are mobilized from the bone marrow and recruited to sites of inflammation, where they carry out their respective functions in promoting inflammation or facilitating tissue repair. Our understanding of the fate of these monocyte subsets at the site of inflammation is constantly evolving. This brief review highlights the plasticity of monocyte subsets and their conversion during inflammation and injury.

Macrophage colony-stimulating factor (CSF1) controls monocyte production and maturation and the steady-state size of the liver in pigs

Macrophage colony-stimulating factor (CSF1) is an essential growth and differentiation factor for cells of the macrophage lineage. To explore the role of CSF1 in steady-state control of monocyte production and differentiation and tissue repair, we previously developed a bioactive protein with a longer half-life in circulation by fusing pig CSF1 with the Fc region of pig IgG1a. CSF1-Fc administration to pigs expanded progenitor pools in the marrow and selectively increased monocyte numbers and their expression of the maturation marker CD163. There was a rapid increase in the size of the liver, and extensive proliferation of hepatocytes associated with increased macrophage infiltration. Despite the large influx of macrophages, there was no evidence of liver injury and no increase in circulating liver enzymes. Microarray expression profiling of livers identified increased expression of macrophage markers, i.e., cytokines such as TNF, IL1, and IL6 known to influence hepatocyte proliferation, alongside cell cycle genes. The analysis also revealed selective enrichment of genes associated with portal, as opposed to centrilobular regions, as seen in hepatic regeneration. Combined with earlier data from the mouse, this study supports the existence of a CSF1-dependent feedback loop, linking macrophages of the liver with bone marrow and blood monocytes, to mediate homeostatic control of the size of the liver. The results also provide evidence of safety and efficacy for possible clinical applications of CSF1-Fc.

Model-Based Characterization of Inflammatory Gene Expression Patterns of Activated Macrophages

Macrophages are cells with remarkable plasticity. They integrate signals from their microenvironment leading to context-dependent polarization into classically (M1) or alternatively (M2) activated macrophages, representing two extremes of a broad spectrum of divergent phenotypes. Thereby, macrophages deliver protective and pro-regenerative signals towards injured tissue but, depending on the eliciting damage, may also be responsible for the generation and aggravation of tissue injury. Although incompletely understood, there is emerging evidence that macrophage polarization is critical for these antagonistic roles. To identify activation-specific expression patterns of chemokines and cytokines that may confer these distinct effects a systems biology approach was applied. A comprehensive literature-based Boolean model was developed to describe the M1 (LPS-activated) and M2 (IL-4/13-activated) polarization types. The model was validated using high-throughput transcript expression data from murine bone marrow derived macrophages. By dynamic modeling of gene expression, the chronology of pathway activation and autocrine signaling was estimated. Our results provide a deepened understanding of the physiological balance leading to M1/M2 activation, indicating the relevance of co-regulatory signals at the level of Akt1 or Akt2 that may be important for directing macrophage polarization.

Macrophages are essential cells of the immune system and indispensable for a defense against bacterial infection. They reside as resting, immune modulatory cells in several tissues of the human body where they continuously sense inputs from their local environment. They react to stimuli such as toxins, injury or bacterial products in a process termed macrophage activation or polarization. For example, the bacterial component lipopolysaccharide induces so-called classical activation of macrophages into the M1 phenotype that secretes a number of inflammatory cytokines and chemokines leading to killing of bacteria and resolution of inflammation. Another prominent phenotype of macrophages is the M2 polarization state that is associated with wound healing and tissue regeneration. Unbalanced activation of macrophages is implicated in a number of diseases. An improved knowledge and extensive characterization of these macrophages as well as the factors determining their phenotypes will improve the understanding of the role of macrophages in disease progression.

Pathogenesis of Kupffer Cells in Cholestatic Liver Injury

Kupffer cells are the resident macrophages in the liver. They are located in hepatic sinusoid, which allows them to remove foreign materials, pathogens, and apoptotic cells efficiently. Activated Kupffer cells secrete various mediators, including cytokines and chemokines, to initiate immune responses, inflammation, or recruitment of other liver cells. Bile duct ligation (BDL) surgery in rodents is often studied as an animal model of cholestatic liver disease, characterized by obstruction of bile flow. BDL mice show altered functional activities of Kupffer cells compared with sham-operated mice, including elevated cytokine secretion and impaired bacterial clearance. Various mediators produced by other liver cells can regulate Kupffer cell activation, which suggest that Kupffer cells orchestrate with other liver cells to relay inflammatory signals and to maintain liver homeostasis during BDL-induced liver injury. Blocking or depletion of Kupffer cells, an approach for the treatment of liver diseases, has shown controversial implications. Procedures in Kupffer cell research have limitations and may produce various results in Kupffer cell research. It is important, however, to reveal underlying mechanisms of activation and functions of Kupffer cells, followed by hepatic inflammation and fibrosis. This review summarizes present Kupffer cell studies in cholestatic liver injury.