Inflammatory monocytes recruited to the liver within 24 hours after virus-induced inflammation resemble Kupffer cells but are functionally distinct

Cellular uptake and in vivo distribution of mesenchymal-stem-cell-derived extracellular vesicles are protein corona dependent

Extracellular vesicles (EVs) derived from mesenchymal stem cells are promising nanotherapeutics in liver diseases due to their regenerative and immunomodulatory properties. Nevertheless, a concern has been raised regarding the rapid clearance of exogenous EVs by phagocytic cells. Here we explore the impact of protein corona on EVs derived from two culturing conditions in which specific proteins acquired from media were simultaneously adsorbed on the EV surface. Additionally, by incubating EVs with serum, simulating protein corona formation upon systemic delivery, further resolved protein corona-EV complex patterns were investigated. Our findings reveal the potential influences of corona composition on EVs under in vitro conditions and their in vivo kinetics. Our data suggest that bound albumin creates an EV signature that can retarget EVs from hepatic macrophages. This results in markedly improved cellular uptake by hepatocytes, liver sinusoidal endothelial cells and hepatic stellate cells. This phenomenon can be applied as a camouflage strategy by precoating EVs with albumin to fabricate the albumin-enriched protein corona-EV complex, enhancing non-phagocytic uptake in the liver. This work addresses a critical challenge facing intravenously administered EVs for liver therapy by tailoring the protein corona-EV complex for liver cell targeting and immune evasion.

TLR8 agonist selgantolimod regulates Kupffer cell differentiation status and impairs HBV entry into hepatocytes via an IL-6-dependent mechanism

OBJECTIVE

Achieving HBV cure will require novel combination therapies of direct-acting antivirals and immunomodulatory agents. In this context, the toll-like receptor 8 (TLR8) agonist selgantolimod (SLGN) has been investigated in preclinical models and clinical trials for chronic hepatitis B (CHB). However, little is known regarding its action on immune effectors within the liver. Our aim was to characterise the transcriptomic changes and intercellular communication events induced by SLGN in the hepatic microenvironment.

DESIGN

We identified TLR8-expressing cell types in the human liver using publicly available single-cell RNA-seq data and established a method to isolate Kupffer cells (KCs). We characterised transcriptomic and cytokine KC profiles in response to SLGN. SLGN's indirect effect was evaluated by RNA-seq in hepatocytes treated with SLGN-conditioned media (CM) and quantification of HBV parameters following infection. Pathways mediating SLGN's effect were validated using transcriptomic data from HBV-infected patients.

RESULTS

Hepatic TLR8 expression takes place in the myeloid compartment. SLGN treatment of KCs upregulated monocyte markers (eg, S100A12) and downregulated genes associated with the KC identity (eg, SPIC). Treatment of hepatocytes with SLGN-CM downregulated NTCP and impaired HBV entry. Cotreatment with an interleukin 6-neutralising antibody reverted the HBV entry inhibition.

CONCLUSION

Our transcriptomic characterisation of SLGN sheds light into the programmes regulating KC activation. Furthermore, in addition to its previously described effect on established HBV infection and adaptive immunity, we show that SLGN impairs HBV entry. Altogether, SLGN may contribute through KCs to remodelling the intrahepatic immune microenvironment and may thus represent an important component of future combinations to cure HBV infection.

LPS differentially affects expression of CD14 and CCR2 in monocyte subsets of Post-STEMI patients with hyperglycemia

AIMS

Following ST-segment elevation myocardial infarction (STEMI), recruitment and activation of monocytes [classical (CD14⁺⁺CD16^-CCR2⁺⁺), intermediate (CD14⁺⁺CD16⁺CCR2⁺), non-classical (CD14^LowCD16⁺⁺CCR2^Low)] are needed for myocardial wound healing. Monocyte surface receptor CC chemokine receptor type 2 (CCR2) is responsible for monocyte chemotaxis to sites of inflammation and the lipopolysaccharide (LPS)-binding protein co-receptor, CD14, is involved in pro-inflammatory monocyte activation. The purpose of this investigation was to determine the effects of ex-vivo LPS activation on monocyte subset CD14 and CCR2 expression in post-STEMI individuals with normal and elevated random blood glucose.

METHODS

Post-STEMI subjects were identified as normal random glucose (NG, <98 mg/dL, n = 13) or impaired random glucose (IG, ≥98 mg/dL, n = 26) and monocytes were analyzed for non-activated and LPS-activated (1 µg/mL for 4 h) CCR2 and CD14 expression.

RESULTS

Non-activated intermediate monocytes from IG showed decreased CD14 expression when compared to NG, which was maintained following LPS-activation. The NG group showed a larger absolute reduction in classical CCR2 expression, leading to a significant difference between NG and IG following LPS-activation.

CONCLUSION

Results suggest a heightened response to pro-inflammatory activation in IG following STEMI, which may impair or delay post-STEMI myocardial healing, and thus increase the incidence of chronic heart failure. NIH 1R34HL121402.

Molecular design of a pathogen activated, self-assembling mechanopharmaceutical device

Weakly basic small molecule drugs like clofazimine can be used as building blocks for endowing cells with unnatural structural and functional elements. Here, we describe how clofazimine represents a first-in-class mechanopharmaceutical device, serving to construct inert, inactive and stimulus responsive drug depots within the endophagolysosomal compartment of cells of living organisms. Upon oral administration, clofazimine molecules self-assemble into stable, membrane-bound, crystal-like drug inclusions (CLDI) that accumulate within macrophages to form a "smart" biocompatible, pathogen activatable mechanopharmaceutical device. Upon perturbation of the mechanism maintaining pH and ion homeostasis of these CLDIs, the inert encapsulated drug precipitates are destabilized, releasing bioactive drug molecules into the cell and its surrounding. The resulting increase in clofazimine solubility activates this broad-spectrum antimicrobial, antiparasitic, antiviral or cytotoxic agent within the infected macrophage. We present a general, molecular design strategy for using clofazimine and other small molecule building blocks for the cytoplasmic construction of mechanopharmaceutical devices, aimed at rapid deployment during infectious disease outbreaks, for the purpose of pandemic prevention.

Innate immune surveillance of the circulation: A review on the removal of circulating virions from the bloodstream

Many viruses utilize the lymphohematogenous route for dissemination; however, they may not freely use this highway unchecked. The reticuloendothelial system (RES) is an innate defense system that surveys circulating blood, recognizing and capturing viral particles. Examination of the literature shows that the bulk of viral clearance is mediated by the liver; however, the precise mechanism(s) mediating viral vascular clearance vary between viruses and, in many cases, remains poorly defined. Herein, we summarize what is known regarding the recognition and capture of virions from the circulation prior to the generation of a specific antibody response. We also discuss the consequences of viral capture on viral pathogenesis and the fate of the captor cell. Finally, this understudied topic has implications beyond viral pathogenesis, including effects on arbovirus ecology and the application of virus-vectored gene 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.

The Gut-Liver Axis in Chronic Liver Disease: A Macrophage Perspective

Chronic liver disease (CLD) is a growing health concern which accounts for two million deaths per year. Obesity, alcohol overconsumption, and progressive cholestasis are commonly characterized by persistent low-grade inflammation and advancing fibrosis, which form the basis for development of end-stage liver disease complications, including hepatocellular carcinoma. CLD pathophysiology extends to the intestinal tract and is characterized by intestinal dysbiosis, bile acid dysregulation, and gut barrier disruption. In addition, macrophages are key players in CLD progression and intestinal barrier breakdown. Emerging studies are unveiling macrophage heterogeneity and driving factors of their plasticity in health and disease. To date, in-depth investigation of how gut-liver axis disruption impacts the hepatic and intestinal macrophage pool in CLD pathogenesis is scarce. In this review, we give an overview of the role of intestinal and hepatic macrophages in homeostasis and gut-liver axis disruption in progressive stages of CLD.

Neutrophils are important for the development of pro-reparative macrophages after irreversible electroporation of the liver in mice

Irreversible electroporation (IRE) is a non-thermal tissue ablative technology that has emerging applications in surgical oncology and regenerative surgery. To advance its therapeutic usefulness, it is important to understand the mechanisms through which IRE induces cell death and the role of the innate immune system in mediating subsequent regenerative repair. Through intravital imaging of the liver in mice, we show that IRE produces distinctive tissue injury features, including delayed yet robust recruitment of neutrophils, consistent with programmed necrosis. IRE treatment converts the monocyte/macrophage balance from pro-inflammatory to pro-reparative populations, and depletion of neutrophils inhibits this conversion. Reduced generation of pro-reparative Ly6C^loF4/80^hi macrophages correlates with lower numbers of SOX9⁺ hepatic progenitor cells in areas of macrophage clusters within the IRE injury zone. Our findings suggest that neutrophils play an important role in promoting the development of pro-reparative Ly6C^lo monocytes/macrophages at the site of IRE injury, thus establishing conditions of regenerative repair.

Liver Injury and the Macrophage Issue: Molecular and Mechanistic Facts and Their Clinical Relevance

The liver is an essential immunological organ due to its gatekeeper position to bypassing antigens from the intestinal blood flow and microbial products from the intestinal commensals. The tissue-resident liver macrophages, termed Kupffer cells, represent key phagocytes that closely interact with local parenchymal, interstitial and other immunological cells in the liver to maintain homeostasis and tolerance against harmless antigens. Upon liver injury, the pool of hepatic macrophages expands dramatically by infiltrating bone marrow-/monocyte-derived macrophages. The interplay of the injured microenvironment and altered macrophage pool skews the subsequent course of liver injuries. It may range from complete recovery to chronic inflammation, fibrosis, cirrhosis and eventually hepatocellular cancer. This review summarizes current knowledge on the classification and role of hepatic macrophages in the healthy and injured liver.

Glucocorticoids as Regulators of Macrophage-Mediated Tissue Homeostasis

Our immune system has evolved as a complex network of cells and tissues tasked with maintaining host homeostasis. This is evident during the inflammatory responses elicited during a microbial infection or traumatic tissue damage. These responses seek to eliminate foreign material or restore tissue integrity. Even during periods without explicit disturbances, the immune system plays prominent roles in tissue homeostasis. Perhaps one of the most studied cells in this regard is the macrophage. Tissue-resident macrophages are a heterogenous group of sensory cells that respond to a variety of environmental cues and are essential for organ function. Endogenously produced glucocorticoid hormones connect external environmental stress signals with the function of many cell types, producing profound changes in immune cells, including macrophages. Here, we review the current literature which demonstrates specific effects of glucocorticoids in several organ systems. We propose that tissue-resident macrophages, through glucocorticoid signaling, may play an underappreciated role as regulators of organ homeostasis.

Type I Interferon-Mediated Regulation of Antiviral Capabilities of Neutrophils

Interferons (IFNs) are induced by viruses and are the main regulators of the host antiviral response. They balance tissue tolerance and immune resistance against viral challenges. Like all cells in the human body, neutrophils possess the receptors for IFNs and contribute to antiviral host defense. To combat viruses, neutrophils utilize various mechanisms, such as viral sensing, neutrophil extracellular trap formation, and antigen presentation. These mechanisms have also been linked to tissue damage during viral infection and inflammation. In this review, we presented evidence that a complex cross-regulatory talk between IFNs and neutrophils initiates appropriate antiviral immune responses and regulates them to minimize tissue damage. We also explored recent exciting research elucidating the interactions between IFNs, neutrophils, and severe acute respiratory syndrome-coronavirus-2, as an example of neutrophil and IFN cross-regulatory talk. Dissecting the IFN-neutrophil paradigm is needed for well-balanced antiviral therapeutics and development of novel treatments against many major epidemic or pandemic viral infections, including the ongoing pandemic of the coronavirus disease that emerged in 2019.

Monocyte dysregulation: consequences for hepatic infections

Liver disorders due to infections are a substantial health concern in underdeveloped and industrialized countries. This includes not only hepatotropic viruses (e.g., hepatitis B, hepatitis C) but also bacterial and parasitic infections such as amebiasis, leishmaniasis, schistosomiasis, or echinococcosis. Recent studies of the immune mechanisms underlying liver disease show that monocytes play an essential role in determining patient outcomes. Monocytes are derived from the mononuclear phagocyte lineage in the bone marrow and are present in nearly all tissues of the body; these cells function as part of the early innate immune response that reacts to challenge by external pathogens. Due to their special ability to develop into tissue macrophages and dendritic cells and to change from an inflammatory to an anti-inflammatory phenotype, monocytes play a pivotal role in infectious and non-infectious liver diseases: they can maintain inflammation and support resolution of inflammation. Therefore, tight regulation of monocyte recruitment and termination of monocyte-driven immune responses in the liver is prerequisite to appropriate healing of organ damage. In this review, we discuss monocyte-dependent immune mechanisms underlying hepatic infectious disorders. Better understanding of these immune mechanisms may lead to development of new interventions to treat acute liver disease and prevent progression to organ failure.

Reparative System Arising from CCR2(+) Monocyte Conversion Attenuates Neuroinflammation Following Ischemic Stroke

Monocytes recruitment from the blood to inflamed tissues following ischemic stroke is an important immune response to wound healing and tissue repair. Mouse monocytes can be endogenously divided into two distinct populations: pro-inflammatory or classical monocytes that express CCR2^highCX3CR1^low and circulate in blood, and anti-inflammatory or non-classical monocytes that express CCR2^lowCX3CR1^high and patrol locally. In this study of transgenic mice with functional CX3CR1^GFP/+ or CX3CR1^GFP/+-CCR2^RFP/+, we found that CCR2^highCX3CR1^low monocytes recruited to the injured brain were cytokine-dependently converted into CCR2^lowCX3CR1^high macrophages, especially under the influence of IL-4 and IL-13, thereby attenuating the neuroinflammation following sterile ischemic stroke. The overall data suggest that (1) the regulation of monocyte-switching is one of the ultimate reparative strategies in ischemic stroke, and (2) the adaptation of monocytes in a locally inflamed milieu is vital to alleviating the effects of ischemic stroke through innate immunity.

Recent advances targeting C-C chemokine receptor type 2 for liver diseases in monocyte/macrophage

Liver plays a critical role in metabolism, nutrient storage and detoxification. Emergency signals or appropriate immune response leads to pathological inflammation and breaks the steady state when liver dysfunction appears, which makes body more susceptible to chronic liver infection, autoimmune diseases and tumour. Compelling proof has illustrated the non-redundant importance of C-C chemokine receptor type 2 (CCR2), one of G-protein-coupled receptors, in different diseases. Selectively expressed on the surface of cells, CCR2 is involved in various signalling pathways and regulates the migration of cells. Especially, a peculiar role of CCR2 has been identified within decades in the onset and progression of hepatic diseases, which led to particular focusing on CCR2 as a new therapeutic and diagnostic target for non-alcoholic fatty liver disease and hepatocellular carcinoma. In this review, we discuss the effect of CCR2 in monocytes/macrophages on liver diseases. The application and translation of the decades of discoveries into therapies promise novel approaches in the treatment of liver disease.

Irradiated and CCl4 -treated bone marrow-derived liver macrophages exhibit different gene expression patterns and phenotypes

Myeloid cells infiltrate into the liver and differentiate into macrophages in different liver injury mouse models. However, the heterogeneity of bone marrow (BM)-derived LMs populations remains to be understood. To investigate this and understand the impact of the macrophage niche on the properties of recruited BM-derived macrophages, we used a non-myeloablation BM transplantation model to label and trace BM-derived LMs. Subsequently, we quantified the number of embryonic-derived liver-resident macrophages, BM-derived LMs and total LMs in CCl₄ and irradiated acute liver injury mouse models, respectively. Finally, we compared the cell fate, gene expression patterns, chemokine signals, and surface markers of irradiated and CCl₄ -treated BM-derived LMs. We observed that, as compared to CCl_4, radiation generated a macrophage niche by depleting embryonic-derived liver-resident macrophages and induced the recruitment of BM-derived LMs that further settled in the liver. Irradiated and CCl₄ -treated BM-derived LMs are different with respect to their cell fates, gene expression patterns, and chemokine expression and recruitment. They also have different surface markers shortly after differentiating from their progenitors. Our findings suggest that irradiated and CCl₄ -treated LM populations derived from the bone marrow display different patterns of gene expression and phenotypes; these differences may be due to the availability of macrophage niche.

HBV X protein-based therapeutic vaccine accelerates viral antigen clearance by mobilizing monocyte infiltration into the liver in HBV carrier mice

Background

Hepatitis B virus (HBV) persistently infected about 250 million people worldwide, and a curative treatment remains an unmet medical need. Among many approaches to treat chronic hepatitis B (CHB), therapeutic vaccines have been developed for two decades, but none have yielded promising results in clinical trials. Therefore, dissection of HBV clearance mechanisms during therapeutic vaccination in appropriate models, which could give rise to new curative therapies, is urgently needed. Growing evidence indicates that prolonged and intensive exposure of antigen-specific T cells to viral antigens is a major cause of T cell exhaustion, and decreases anti-HBV immunity efficacy of therapeutic vaccination. HBV X protein (HBx) is expressed at low levels, and the understanding of its immunogenicity and potential in therapeutic CHB vaccines is limited.

Methods

HBV genome sequences from CHB patients were cloned into a pAAV plasmid backbone and transfected into immunocompetent mouse hepatocytes through hydrodynamic injection. Mice carrying > 500 IU/mL serum HBV surface antigen (HBs) for more than 4 weeks were considered HBV carriers mimicking human CHB and received 3 doses of weekly HBx vaccine by subcutaneous immunization. Serum HBV clearance was evaluated by monitoring serum HBs and HBV-DNA titers. Residual HBV in the liver was evaluated by western blotting for HBV core antigen. The splenic antigen-specific T cell response was quantified by a 15-mer overlapping peptide-stimulated interferon-γ enzyme-linked immunospot assay. Blood and hepatic immune cells were quantified by flow cytometric analysis.

Results

Our HBx-based vaccine induced systemic HBx-specific CD4⁺ and CD8⁺ T cell responses in HBV carrier mice and demonstrated significant HBs and HBV-DNA elimination. The protective effect persisted for at least 30 days without additional booster immunization. Different infiltrating myeloid cell subsets, each with distinctive roles during immune-mediated HBV clearance, were found in the liver of vaccinated mice. During vaccine therapy, inflammatory monocyte depletion resulted in sustained HBV clearance inhibition, whereas phagocytic monocyte-derived macrophage and Kupffer cell elimination resulted in only transient inhibition of vaccine-induced HBV clearance.

Conclusions

We report the potential role of HBx as a major immunogen in an HBV therapeutic vaccine and the significance of a liver-infiltrating monocyte subset during hepatic viral clearance.

Extracellular vesicles participate in macrophage-involved immune responses under liver diseases

The liver serves as a central participant in immune system owing to its particular blood supply and large amounts of immune cells, in which macrophages play a significant role in liver homeostasis and disorders. Extracellular vesicles (EVs), membrane-defined nanometer-sized vesicles released by cells in a tightly controlled manner, have attracted intensive research attention as a critical vehicle for cell-cell communication in the pathophysiology of liver. Accumulating evidence has proved that extracellular vesicles are frequently involved in macrophage-mediated biological behaviors. Not only can macrophages produce and secrete EVs containing multifarious cargo themselves to exert immunomodulatory functions, but also macrophages may serve as target cells of EVs from other cells eliciting the alteration of their phenotype and function. Since both macrophage as well as EVs show pleiotropic and central effects in the progression of liver diseases, their roles in adjusting innate immunity of liver often present a crossover. In this review we are dedicated to deciphering the complex immunological network constituted by macrophages and EVs in several common liver diseases, including acute liver injury or failure and a set of chronic liver diseases such as viral hepatitis B and C, metabolic and alcoholic liver diseases, as well as hepatocellular carcinoma (HCC). From the aspect of immunology, we integrate the mechanism of EVs and hepatic macrophages in the setting of liver diseases and show a promising significance of utilizing this association into clinical immunotherapy.

Synergy of therapeutic heterologous prime-boost hepatitis B vaccination with CpG-application to improve immune control of persistent HBV infection

Therapeutic vaccination against chronic hepatitis B must overcome high viral antigen load and local regulatory mechanisms that promote immune-tolerance in the liver and curtail hepatitis B virus (HBV)-specific CD8 T cell immunity. Here, we report that therapeutic heterologous HBcore-protein-prime/Modified-Vaccinia-Virus-Ankara (MVA-HBcore) boost vaccination followed by CpG-application augmented vaccine-induced HBcAg-specific CD8 T cell-function in the liver. In HBV-transgenic as well as AAV-HBV-transduced mice with persistent high-level HBV-replication, the combination of therapeutic vaccination with subsequent CpG-application was synergistic to generate more potent HBV-specific CD8 T cell immunity that improved control of hepatocytes replicating HBV.

The Role of Myeloid-Derived Cells in the Progression of Liver Disease

Control of homeostasis and rapid response to tissue damage in the liver is orchestrated by crosstalk between resident and infiltrating inflammatory cells. A crucial role for myeloid cells during hepatic injury and repair has emerged where resident Kupffer cells, circulating monocytes, macrophages, dendritic cells and neutrophils control local tissue inflammation and regenerative function to maintain tissue architecture. Studies in humans and rodents have revealed a heterogeneous population of myeloid cells that respond to the local environment by either promoting regeneration or driving the inflammatory processes that can lead to hepatitis, fibrogenesis, and the development of cirrhosis and malignancy. Such plasticity of myeloid cell responses presents unique challenges for therapeutic intervention strategies and a greater understanding of the underlying mechanisms is needed. Here we review the role of myeloid cells in the establishment and progression of liver disease and highlight key pathways that have become the focus for current and future therapeutic strategies.

Liver macrophages: Friend or foe during hepatitis B infection?

The Hepatitis B virus chronically infects the liver of 250 million people worldwide. Over the past decades, major advances have been made in the understanding of Hepatitis B virus life cycle in hepatocytes. Beside these parenchymal cells, the liver also contains resident and infiltrating myeloid cells involved in immune responses to pathogens and much less is known about their interplay with Hepatitis B virus. In this review, we summarized and discussed the current knowledge of the role of liver macrophages (including Kupffer cells and liver monocyte-derived macrophages), in HBV infection. While it is still unclear if liver macrophages play a role in the establishment and persistence of HBV infection, several studies disclosed data suggesting that HBV would favour liver macrophage anti-inflammatory phenotypes and thereby increase liver tolerance. In addition, alternatively activated liver macrophages might also play in the long term a key role in hepatitis B-associated pathogenesis, especially through the activation of hepatic stellate cells. Therapies aiming at a transient activation of pro-inflammatory liver macrophages should therefore be considered for the treatment of chronic HBV infection.

Type I interferon receptor signaling delays Kupffer cell replenishment during acute fulminant viral hepatitis

BACKGROUND & AIM

Virus-induced fulminant hepatitis is a major cause of acute liver failure. During acute viral hepatitis the impact of type I interferon (IFN-I) on myeloid cells, including liver-resident Kupffer cells (KC), is only partially understood. Herein, we dissected the impact of locally induced IFN-I responses on myeloid cell function and hepatocytes during acute liver inflammation.

METHODS

Two different DNA-encoded viruses, vaccinia virus (VACV) and murine cytomegalovirus (MCMV), were studied. In vivo imaging was applied to visualize local IFN-β induction and IFN-I receptor (IFNAR) triggering in VACV-infected reporter mice. Furthermore, mice with a cell type-selective IFNAR ablation were analyzed to dissect the role of IFNAR signaling in myeloid cells and hepatocytes. Experiments with Cx3cr1^+/gfp mice revealed the origin of reconstituted KC. Finally, mixed bone marrow chimeric mice were studied to specifically analyze the effect of IFNAR triggering on liver infiltrating monocytes.

RESULTS

VACV infection induced local IFN-β responses, which lead to IFNAR signaling primarily within the liver. IFNAR triggering was needed to control the infection and prevent fulminant hepatitis. The severity of liver inflammation was independent of IFNAR triggering of hepatocytes, whereas IFNAR triggering of myeloid cells protected from excessive inflammation. Upon VACV or MCMV infection KC disappeared, whereas infiltrating monocytes differentiated to KC afterwards. During IFNAR triggering such replenished monocyte-derived KC comprised more IFNAR-deficient than -competent cells in mixed bone marrow chimeric mice, whereas after the decline of IFNAR triggering both subsets showed an even distribution.

CONCLUSION

Upon VACV infection IFNAR triggering of myeloid cells, but not of hepatocytes, critically modulates acute viral hepatitis. During infection with DNA-encoded viruses IFNAR triggering of liver-infiltrating blood monocytes delays the development of monocyte-derived KC, pointing towards new therapeutic strategies for acute viral hepatitis.

LAY SUMMARY

Viral infection can cause fulminant hepatitis, which in turn is a major cause of acute liver failure. Herein, we aimed to study the role of type 1 interferon responses in acute viral hepatitis. We identified that during infection with DNA-encoded viruses, type 1 interferon receptor triggering of blood monocytes delays the development of monocyte-derived Kupffer cells. This points to new therapeutic strategies for acute viral hepatitis.

The diversity of myeloid immune cells shaping wound repair and fibrosis in the lung

In healthy circumstances the immune system coordinates tissue repair responses in a tight balance that entails efficient inflammation for removal of potential threats, proper wound closure, and regeneration to regain tissue function. Pathological conditions, continuous exposure to noxious agents, and even ageing can dysregulate immune responses after injury. This dysregulation can lead to a chronic repair mechanism known as fibrosis. Alterations in wound healing can occur in many organs, but our focus lies with the lung as it requires highly regulated immune and repair responses with its continuous exposure to airborne threats. Dysregulated repair responses can lead to pulmonary fibrosis but the exact reason for its development is often not known. Here, we review the diversity of innate immune cells of myeloid origin that are involved in tissue repair and we illustrate how these cell types can contribute to the development of pulmonary fibrosis. Moreover, we briefly discuss the effect of age on innate immune responses and therefore on wound healing and we conclude with the implications of current knowledge on the avenues for future research.

Interferon-alpha treatment rapidly clears Hepatitis E virus infection in humanized mice

Antiviral treatment options for chronic Hepatitis E Virus (HEV) infections are limited and immunological determinants of viral persistence remain largely unexplored. We studied the antiviral potency of pegylated interferon-α (pegIFNα) against HEV infections in humanized mice and modelled intrahepatic interferon stimulated gene (ISG) responses. Human gene expression levels in humanized mouse livers were analyzed by qPCR and Nanostring. Human CXCL10 was measured in mouse serum. HEV genotype 3 (gt3) infections were cleared from liver and feces within 8 pegIFNα doses in all mice and relapsed after a single pegIFNα injection in only half of treated animals. Rapid viral clearance by pegIFNα was confirmed in HEV gt1, but not in Hepatitis B Virus infected animals. No ISG induction was observed in untreated HEV gt3 and gt1 infected humanized livers compared to control chimeric mice, irrespective of the human hepatocyte donor, viral isolate or HEV infection duration. Human specific ISG transcript levels in mouse liver increased significantly after pegIFNα treatment and induced high circulating human CXCL10 in mouse serum. In conclusion, HEV gt1 and gt3 infections do not elicit innate intrahepatic immune responses and remain highly sensitive to pegIFNα in immunocompromised humanized mice.

Unique intrahepatic transcriptomics profiles discriminate the clinical phases of a chronic HBV infection

Chronic hepatitis B is a highly heterogeneous liver disease characterized by phases with fluctuations in viral replication and progressive liver damage in some, but not all infected individuals. Despite four decades of research, insight into host determinants underlying these distinct clinical phases-immunotolerant, immune active, inactive carrier, and HBeAg-negative hepatitis-remains elusive. We performed an in-depth transcriptome analysis of archived FFPE liver biopsies of each clinical phase to address host determinants associated with the natural history. Therefore, we determined, for the first time, intrahepatic global expression profiles of well-characterized chronic HBV patients at different clinical phases. Our data, obtained by microarray, demonstrate that B cells and NK/cytotoxic-related genes in the liver, including CD19, TNFRSF13C, GZMH, and KIR2DS3, were differentially expressed across the clinical HBV phases, which was confirmed by modular analysis and also Nanostring arrays in an independent cohort. Compared to the immunotolerant phase, 92 genes were differentially expressed in the liver during the immune active phase, 46 in the inactive carrier phase, and 71 in the HBeAg-negative phase. Furthermore, our study also revealed distinctive transcription of genes associated with cell cycle activity, NF-κB signaling, cytotoxic function and mitochondrial respiration between clinical phases. Our data define for the first time using microarray unique transcriptomes in the HBV-infected liver during consecutive clinical phases. We demonstrate that fluctuations of viral loads and liver damage coincide with fluctuations in the liver transcriptome and point to functional- immune and non-immune- components contributing to the clinical phenotype in patients.

Targeting hepatic macrophages to treat liver diseases

Our view on liver macrophages in the context of health and disease has been reformed by the recognition of a remarkable heterogeneity of phagocytes in the liver. Liver macrophages consist of ontogenically distinct populations termed Kupffer cells and monocyte-derived macrophages. Kupffer cells are self-renewing, resident and principally non-migratory phagocytes, serving as sentinels for liver homeostasis. Liver injury triggers Kupffer cell activation, leading to inflammatory cytokine and chemokine release. This fosters the infiltration of monocytes into the liver, which give rise to large numbers of inflammatory monocyte-derived macrophages. Liver macrophages are very plastic and adapt their phenotype according to signals derived from the hepatic microenvironment (e.g. danger signals, fatty acids, phagocytosis of cellular debris), which explains their manifold and even opposing functions during disease. These central functions include the perpetuation of inflammation and hepatocyte injury, activation of hepatic stellate cells with subsequent fibrogenesis, and support of tumor development by angiogenesis and T cell suppression. If liver injury ceases, specific molecular signals trigger hepatic macrophages to switch their phenotype towards reparative phagocytes that promote tissue repair and regression of fibrosis. Novel strategies to treat liver disease aim at targeting macrophages. These interventions modulate Kupffer cell activation (e.g. via gut-liver axis or inflammasome formation), monocyte recruitment (e.g. via inhibiting chemokine pathways like CCR2 or CCL2) or macrophage polarization and differentiation (e.g. by nanoparticles). Evidence from mouse models and early clinical studies in patients with non-alcoholic steatohepatitis and fibrosis support the notion that pathogenic macrophage subsets can be successfully translated into novel treatment options for patients with liver disease.

LAY SUMMARY

Macrophages (Greek for "big eaters") are a frequent non-parenchymal cell type of the liver that ensures homeostasis, antimicrobial defense and proper metabolism. However, liver macrophages consist of different subtypes regarding their ontogeny (developmental origin), differentiation and function. Understanding this heterogeneity and the critical regulation of inflammation, fibrosis and cancer by macrophage subsets opens promising new options for treating liver diseases.

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.

Immunoregulatory Role of NK Cells in Tissue Inflammation and Regeneration

NK cells represent an important first line of defense against viral infection and cancer and are also involved in tissue homeostasis. Studies of NK cell activation in the last decade have revealed that they are able to respond to the inflammatory stimuli evoked by tissue damage and contribute to both progression and resolution of diseases. Exacerbation of the inflammatory response through interactions between immune effector cells facilitates the progression of non-alcoholic fatty liver disease (NAFLD) into steatosis, cirrhosis, and hepatocellular carcinoma (HCC). When hepatic damage is incurred, macrophage activation is crucial for initiating cross talk with neighboring cells present in the liver, including hepatocytes and NK cells, and the importance of this interaction in shaping the immune response in liver disease is increasingly recognized. Inflicted structural damage can be in part regenerated via the process of self-limiting fibrosis, though persistent hepatic damage will lead to chronic fibrosis and loss of tissue organization and function. The cytotoxic activity of NK cells plays an important role in inducing hepatic stellate cell apoptosis and thus curtailing the progression of fibrosis. Alternatively, in some diseases, such as HCC, NK cells may become dysregulated, promoting an immunosuppressive state where tumors are able to escape immune surveillance. This review describes the current understanding of the contributions of NK cells to tissue inflammation and metabolic liver diseases and the ongoing effort to develop therapeutics that target the immunoregulatory function of NK cells.

Liver Monocytes and Kupffer Cells Remain Transcriptionally Distinct during Chronic Viral Infection

Due to the scarcity of immunocompetent animal models for chronic viral hepatitis, little is known about the role of the innate intrahepatic immune system during viral replication in the liver. These insights are however fundamental for the understanding of the inappropriate adaptive immune responses during the chronic phase of the infection. We apply the Lymphocytic Choriomenigitis Virus (LCMV) clone 13 mouse model to examine chronic virus-host interactions of Kupffer cells (KC) and infiltrating monocytes (IM) in an infected liver. LCMV infection induced overt clinical hepatitis, with rise in ALT and serum cytokines, and increased intrahepatic F4/80 expression. Despite ongoing viral replication, whole liver transcriptome showed baseline expression levels of inflammatory cytokines, interferons, and interferon induced genes during the chronic infection phase. Transcriptome analyses of sorted KC and IMs using NanoString technology revealed two unique phenotypes with only minimal overlap. At the chronic viral infection phase, KC showed no increased transcription of activation markers Cd80 and Cd86, but an increased expression of genes related to antigen presentation, whereas monocytes were more activated and expressed higher levels of Tnf transcripts. Although both KCs and intrahepatic IM share the surface markers F4/80 and CD11b, their transcriptomes point towards distinctive roles during virus-induced chronic hepatitis.

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.

Kupffer Cells Undergo Fundamental Changes during the Development of Experimental NASH and Are Critical in Initiating Liver Damage and Inflammation

Non-alcoholic fatty liver disease has become the leading liver disease in North America and is associated with the progressive inflammatory liver disease non-alcoholic steatohepatitis (NASH). Considerable effort has been made to understand the role of resident and recruited macrophage populations in NASH however numerous questions remain. Our goal was to characterize the dynamic changes in liver macrophages during the initiation of NASH in a murine model. Using the methionine-choline deficient diet we found that liver-resident macrophages, Kupffer cells were lost early in disease onset followed by a robust infiltration of Ly-6C⁺ monocyte-derived macrophages that retained a dynamic phenotype. Genetic profiling revealed distinct patterns of inflammatory gene expression between macrophage subsets. Only early depletion of liver macrophages using liposomal clodronate prevented the development of NASH in mice suggesting that Kupffer cells are critical for the orchestration of inflammation during experimental NASH. Increased understanding of these dynamics may allow us to target potentially harmful populations whilst promoting anti-inflammatory or restorative populations to ultimately guide the development of effective treatment strategies.

Hepatic macrophages in homeostasis and liver diseases: from pathogenesis to novel therapeutic strategies

Macrophages represent a major cell type of innate immunity and have emerged as a critical player and therapeutic target in many chronic inflammatory diseases. Hepatic macrophages consist of Kupffer cells, which are originated from the fetal yolk-sack, and infiltrated bone marrow-derived monocytes/macrophages. Hepatic macrophages play a central role in maintaining homeostasis of the liver and in the pathogenesis of liver injury, making them an attractive therapeutic target for liver diseases. However, the various populations of hepatic macrophages display different phenotypes and exert distinct functions. Thus, more research is required to better understand these cells to guide the development of macrophage-based therapeutic interventions. This review article will summarize the current knowledge on the origins and composition of hepatic macrophages, their functions in maintaining hepatic homeostasis, and their involvement in both promoting and resolving liver inflammation, injury, and fibrosis. Finally, the current strategies being developed to target hepatic macrophages for the treatment of liver diseases will be reviewed.

The role of innate immunity in the immunopathology and treatment of HBV infection

In this review we give a brief update on sensors recently determined to be capable of detecting HBV, and examine how the virus represses the induction of pro-inflammatory cytokines like type I interferons. We overview cellular components of innate immunity that are present at high frequencies in the liver, and discuss their roles in HBV control and/or pathogenesis. We argue that many innate effectors have adaptive-like features or can exert specific effects on HBV through immunoregulation of T cells. Finally we consider current and possible future strategies to manipulate innate immunity as novel approaches towards a functional cure for HBV.

Immunology in the liver--from homeostasis to disease

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