Defective T helper response of hepatocyte-stimulated CD4 T cells impairs antiviral CD8 response and viral clearance

The lack of HBsAg secretion does neither facilitate induction of antiviral T cell responses nor Hepatitis B Virus clearance in mice

Immune tolerance to the hepatitis B virus (HBV) is crucial for developing chronic hepatitis B, and the HBV surface antigen (HBsAg) produced and secreted in high amounts is regarded as a key contributor. HBsAg is expressed in HBV-infected hepatocytes and those carrying an HBV integration. Whether either HBsAg secretion or the high antigen amount expressed in the liver determines its immunomodulatory properties, however, remains unclear. We, therefore, developed a novel HBV animal model that allowed us to study the role of secreted HBsAg. We introduced a previously described HBs mutation, C65S, abolishing HBsAg secretion into a replication-competent 1.3-overlength HBV genome and used adeno-associated virus vectors to deliver it to the mouse liver. The AAV-HBV established a carrier state of wildtype and C65S mutant HBV, respectively. We investigated antiviral B- and T-cell immunity in the HBV-carrier mice after therapeutic vaccination. Moreover, we compared the effect of a lacking HBsAg secretion with that of an antiviral siRNA. While missing HBsAg secretion allowed for higher levels of detectable anti-HBs antibodies after therapeutic vaccination, it did neither affect antiviral T-cell responses nor intrahepatic HBV gene expression, irrespective of the starting level. A treatment with HBV siRNA restricting viral antigen expression within hepatocytes, however, improved the antiviral efficacy of therapeutic vaccination, irrespective of the ability of HBV to secrete HBsAg. Our data indicate that clearing HBsAg from blood cannot significantly impact HBV persistence or T-cell immunity. This indicates that a restriction of hepatic viral antigen expression will be required to break HBV immunotolerance.

Lipid metabolism in the immune niche of tumor-prone liver microenvironment

The liver is a common primary site not only for tumorigenesis, but also for cancer metastasis. Advanced cancer patients with liver metastases also show reduced response rates and survival benefits when treated with immune checkpoint inhibitors. Accumulating evidence has highlighted the importance of the liver immune microenvironment in determining tumorigenesis, metastasis-organotropism, and immunotherapy resistance. Various immune cells such as T cells, natural killer and natural killer T cells, macrophages and dendritic cells, and stromal cells including liver sinusoidal endothelial cells, Kupffer cells, hepatic stellate cells, and hepatocytes are implicated in contributing to the immune niche of tumor-prone liver microenvironment. In parallel, as the major organ for lipid metabolism, the increased abundance of lipids and their metabolites is linked to processes crucial for nonalcoholic fatty liver disease and related liver cancer development. Furthermore, the proliferation, differentiation, and functions of hepatic immune and stromal cells are also reported to be regulated by lipid metabolism. Therefore, targeting lipid metabolism may hold great potential to reprogram the immunosuppressive liver microenvironment and synergistically enhance the immunotherapy efficacy in the circumstance of liver metastasis. In this review, we describe how the hepatic microenvironment adapts to the lipid metabolic alterations in pathologic conditions like nonalcoholic fatty liver disease. We also illustrate how these immunometabolic alterations promote the development of liver cancers and immunotherapy resistance. Finally, we discuss the current therapeutic options and hypothetic combination immunotherapies for the treatment of advanced liver cancers.

Hepatocytes: A key role in liver inflammation

Hepatocytes, the major parenchymal cells in the liver, are responsible for a variety of cellular functions including carbohydrate, lipid and protein metabolism, detoxification and immune cell activation to maintain liver homeotasis. Recent studies show hepatocytes play a pivotal role in liver inflammation. After receiving liver insults and inflammatory signals, hepatocytes may undergo organelle damage, and further respond by releasing mediators and expressing molecules that can act in the microenvironment as well as initiate a robust inflammatory response. In this review, we summarize how the hepatic organelle damage link to liver inflammation and introduce numerous hepatocyte-derived pro-inflammatory factors in response to chronic liver injury.

Harnessing the liver to induce antigen-specific immune tolerance

Autoimmune diseases develop when the adaptive immune system attacks the body’s own antigens leading to tissue damage. At least 80 different conditions are believed to have an autoimmune aetiology, including rheumatoid arthritis, type I diabetes, multiple sclerosis or systemic lupus erythematosus. Collectively, autoimmune diseases are a leading cause of severe health impairment along with substantial socioeconomic costs. Current treatments are mostly symptomatic and non-specific, and it is typically not possible to cure these diseases. Thus, the development of more causative treatments that suppress only the pathogenic immune responses, but spare general immunity is of great biomedical interest. The liver offers considerable potential for development of such antigen-specific immunotherapies, as it has a distinct physiological capacity to induce immune tolerance. Indeed, the liver has been shown to specifically suppress autoimmune responses to organ allografts co-transplanted with the liver or to autoantigens that were transferred to the liver. Liver tolerance is established by a unique microenvironment that facilitates interactions between liver-resident antigen-presenting cells and lymphocytes passing by in the low blood flow within the hepatic sinusoids. Here, we summarise current concepts and mechanisms of liver immune tolerance, and review present approaches to harness liver tolerance for antigen-specific immunotherapy.

Functional heterogeneity of CD4+ T cells in liver inflammation

CD4⁺ T cells play an essential role in orchestrating adequate immunity, but their overactivity has been associated with the development of immune-mediated inflammatory diseases, including liver inflammatory diseases. These cells can be subclassified according to their maturation stage, cytokine profile, and pro or anti-inflammatory functions, i.e., functional heterogeneity. In this review, we summarize what has been discovered so far regarding the role of the different CD4⁺ T cell polarization states in the progression of two prominent and still different liver inflammatory diseases: non-alcoholic steatohepatitis (NASH) and autoimmune hepatitis (AIH). Finally, the potential of CD4⁺ T cells as a therapeutic target in both NASH and AIH is discussed.

In Situ Vaccination as a Strategy to Modulate the Immune Microenvironment of Hepatocellular Carcinoma

Hepatocellular Carcinoma (HCC) is a highly prevalent malignancy that develops in patients with chronic liver diseases and dysregulated systemic and hepatic immunity. The tumor microenvironment (TME) contains tumor-associated macrophages (TAM), cancer-associated fibroblasts (CAF), regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSC) and is central to mediating immune evasion and resistance to therapy. The interplay between these cells types often leads to insufficient antigen presentation, preventing effective anti-tumor immune responses. In situ vaccines harness the tumor as the source of antigens and implement sequential immunomodulation to generate systemic and lasting antitumor immunity. Thus, in situ vaccines hold the promise to induce a switch from an immunosuppressive environment where HCC cells evade antigen presentation and suppress T cell responses towards an immunostimulatory environment enriched for activated cytotoxic cells. Pivotal steps of in situ vaccination include the induction of immunogenic cell death of tumor cells, a recruitment of antigen-presenting cells with a focus on dendritic cells, their loading and maturation and a subsequent cross-priming of CD8+ T cells to ensure cytotoxic activity against tumor cells. Several in situ vaccine approaches have been suggested, with vaccine regimens including oncolytic viruses, Flt3L, GM-CSF and TLR agonists. Moreover, combinations with checkpoint inhibitors have been suggested in HCC and other tumor entities. This review will give an overview of various in situ vaccine strategies for HCC, highlighting the potentials and pitfalls of in situ vaccines to treat liver cancer.

The Role of Diverse Liver Cells in Liver Transplantation Tolerance

Liver transplantation is the ideal treatment approach for a variety of end-stage liver diseases. However, life-long, systemic immunosuppressive treatment after transplantation is required to prevent rejection and graft loss, which is associated with severe side effects, although liver allograft is considered more tolerogenic. Therefore, understanding the mechanism underlying the unique immunologically privileged liver organ is valuable for transplantation management and autoimmune disease treatment. The unique hepatic acinus anatomy and a complex cellular network constitute the immunosuppressive hepatic microenvironment, which are responsible for the tolerogenic properties of the liver. The hepatic microenvironment contains a variety of hepatic-resident immobile non-professional antigen-presenting cells, including hepatocytes, liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, that are insufficient to optimally prime T cells locally and lead to the removal of alloreactive T cells due to the low expression of major histocompatibility complex (MHC) molecules, costimulatory molecules and proinflammatory cytokines but a rather high expression of coinhibitory molecules and anti-inflammatory cytokines. Hepatic dendritic cells (DCs) are generally immature and less immunogenic than splenic DCs and are also ineffective in priming naïve allogeneic T cells via the direct recognition pathway in recipient secondary lymphoid organs. Although natural killer cells and natural killer T cells are reportedly associated with liver tolerance, their roles in liver transplantation are multifaceted and need to be further clarified. Under these circumstances, T cells are prone to clonal deletion, clonal anergy and exhaustion, eventually leading to tolerance. Other proposed liver tolerance mechanisms, such as soluble donor MHC class I molecules, passenger leukocytes theory and a high-load antigen effect, have also been addressed. We herein comprehensively review the current evidence implicating the tolerogenic properties of diverse liver cells in liver transplantation tolerance.

Hepatocytes Delete Regulatory T Cells by Enclysis, a CD4+ T Cell Engulfment Process

CD4+ T cells play critical roles in directing immunity, both as T helper and as regulatory T (Treg) cells. Here, we demonstrate that hepatocytes can modulate T cell populations through engulfment of live CD4+ lymphocytes. We term this phenomenon enclysis to reflect the specific enclosure of CD4+ T cells in hepatocytes. Enclysis is selective for CD4+ but not CD8+ cells, independent of antigen-specific activation, and occurs in human hepatocytes in vitro, ex vivo, and in vivo. Intercellular adhesion molecule 1 (ICAM-1) facilitates T cell early adhesion and internalization, whereas hepatocytes form membrane lamellipodia or blebs to mediate engulfment. T cell internalization is unaffected by wortmannin and Rho kinase inhibition. Hepatocytes engulf Treg cells more efficiently than non-Treg cells, but Treg cell-containing vesicles preferentially acidify overnight. Thus, enclysis is a biological process with potential effects on immunomodulation and opens a new field for research to fully understand CD4+ T cell dynamics in liver inflammation.

Liver-Mediated Adaptive Immune Tolerance

The liver is an immunologically tolerant organ that is uniquely equipped to limit hypersensitivity to food-derived antigens and bacterial products through the portal vein and can feasibly accept liver allografts. The adaptive immune response is a major branch of the immune system that induces organ/tissue-localized and systematic responses against pathogens and tumors while promoting self-tolerance. Persistent infection of the liver with a virus or other pathogen typically results in tolerance, which is a key feature of the liver. The liver's immunosuppressive microenvironment means that hepatic adaptive immune cells become readily tolerogenic, promoting the death of effector cells and the “education” of regulatory cells. The above mechanisms may result in the clonal deletion, exhaustion, or inhibition of peripheral T cells, which are key players in the adaptive immune response. These tolerance mechanisms are believed to be responsible for almost all liver diseases. However, optimal protective adaptive immune responses may be achieved through checkpoint immunotherapy and the modulation of hepatic innate immune cells in the host. In this review, we focus on the mechanisms involved in hepatic adaptive immune tolerance, the liver diseases caused thereby, and the therapeutic strategies needed to overcome this tolerance.

Liver infiltrating T cells regulate bile acid metabolism in experimental cholangitis

BACKGROUND & AIMS

T cells are central mediators of liver inflammation and represent potential treatment targets in cholestatic liver disease. Whereas emerging evidence shows that bile acids (BAs) affect T cell function, the role of T cells for the regulation of BA metabolism is unknown. In order to understand this interplay, we investigated the influence of T cells on BA metabolism in a novel mouse model of cholangitis.

METHODS

Mdr2-/- mice were crossed with transgenic K14-OVAp mice, which express an MHC class I restricted ovalbumin peptide on biliary epithelial cells (Mdr2-/-xK14-OVAp). T cell-mediated cholangitis was induced by the adoptive transfer of antigen-specific CD8+ T cells. BA levels were quantified using a targeted liquid chromatography-mass spectrometry-based approach.

RESULTS

T cell-induced cholangitis resulted in reduced levels of unconjugated BAs in the liver and significantly increased serum and hepatic levels of conjugated BAs. Genes responsible for BA synthesis and uptake were downregulated and expression of the bile salt export pump was increased. The transferred antigen-specific CD8+ T cells alone were able to induce these changes, as demonstrated using Mdr2-/-xK14-OVAp recipient mice on the Rag1-/- background. Mechanistically, we showed by depletion experiments that alterations in BA metabolism were partly mediated by the proinflammatory cytokines TNF and IFN-γ in an FXR-dependent manner, a process that in vitro required cell contact between T cells and hepatocytes.

CONCLUSION

Whereas it is known that BA metabolism is dysregulated in sepsis and related conditions, we have shown that T cells are able to control the synthesis and metabolism of BAs, a process which depends on TNF and IFN-γ. Understanding the effect of lymphocytes on BA metabolism will help in the design of combined treatment strategies for cholestatic liver diseases.

LAY SUMMARY

Dysregulation of bile acid metabolism and T cells can contribute to the development of cholangiopathies. Before targeting T cells for the treatment of cholangiopathies, it should be determined whether they exert protective effects on bile acid metabolism. Herein, we demonstrate that T cell-induced cholangitis resulted in decreased levels of harmful unconjugated bile acids. T cells were able to directly control synthesis and metabolism of bile acids, a process which was dependent on the proinflammatory cytokines TNF and IFN-γ. Understanding the effect of lymphocytes on bile acid metabolism will help in the design of combined treatment strategies for cholestatic liver diseases.

Chemotactic effects in reaction-diffusion equations for inflammation

Predator-prey systems are used to model time-dependent virus and lymphocyte population during a liver infection and to discuss the influence of chemotactic behavior on the chronification tendency of such infections. Therefore, a model family of reaction-diffusion equations is presented, and the long-term behavior of the solutions is estimated by a critical value containing the reaction strength, the diffusion rate, and the extension of the liver domain. Fourier techniques are applied to evaluate the influence of chemotactic behavior of the immune response to the long-term behavior of locally linearized models. It turns out that the chemotaxis is a subordinated influence with respect to the chronification of liver infections.

Intrahepatic regulation of antiviral T cell responses at initial stages of viral infection

It is generally accepted that the appropriate boost of early immune response will control viral replications and limit the immune-mediated pathology in viral hepatitis. However, poor immunity results in viral persistence, chronic inflammation and finally liver cirrhosis and carcinoma. As a peripheral non-lymphoid organ of immune surveillance, the liver continually encounters hundreds of molecules from the blood, including nutrients, toxins and pathogens. In this way, the liver maintains immune tolerance under healthy conditions, but responds quickly to the hepatotropic pathogens during the early stages of an infection. Although our knowledge of liver cell compositions and functions has been improved significantly in recent years, the intrahepatic immune regulation of antiviral T cells at the initial stage is complex and not well elucidated. Here, we summarize the role of liver cell subpopulations in regulating antiviral T cell response at the initial stages of viral infection. A better understanding of early hepatic immune regulation will pave the way for the development of novel therapies and vaccine design for human viral hepatitis.

Attenuated viral hepatitis in Trem1-/- mice is associated with reduced inflammatory activity of neutrophils

TREM1 (Triggering Receptor Expressed on Myeloid Cells 1) is a pro-inflammatory receptor expressed by phagocytes, which can also be released as a soluble molecule (sTREM1). The roles of TREM1 and sTREM1 in liver infection and inflammation are not clear. Here we show that patients with hepatitis B virus (HBV) or hepatitis C virus (HCV) infection manifest elevated serum levels of sTREM1. In mice, experimental viral hepatitis induced by infection with Lymphocytic Choriomeningitis Virus (LCMV)-WE was likewise associated with increased sTREM1 in serum and urine, and with increased TREM1 and its associated adapter molecule DAP12 in the liver. Trem1−/− mice showed accelerated clearance of LCMV-WE and manifested attenuated liver inflammation and injury. TREM1 expression in the liver of wild-type mice was mostly confined to infiltrating neutrophils, which responded to LCMV by secretion of CCL2 and TNF-α, and release of sTREM1. Accordingly, the production of CCL2 and TNF-α was decreased in the livers of LCMV-infected Trem1−/− mice, as compared to LCMV-infected wildtype mice. These findings indicate that TREM1 plays a role in viral hepatitis, in which it seems to aggravate the immunopathology associated with viral clearance, mainly by increasing the inflammatory activity of neutrophils.

Hepatocytes induce Foxp3⁺ regulatory T cells by Notch signaling

The liver plays a pivotal role in maintaining immunological tolerance, although the exact molecular mechanism is still largely unknown. The induction of systemic tolerance by liver resident APCs has been attributed to peripheral deletion and to the induction of Tregs. HCs, the parenchymal cells in the liver, could function as nonprofessional APCs and interact and establish cell-cell contact with T lymphocytes. We hypothesized that HCs from healthy or regenerated livers may contribute to induction of functional Tregs. Here, we show that murine HCs induced Foxp3(+) Tregs within CD4(+) T cells in vitro, which increased in the presence of TGF-β. Interestingly, a further Foxp3(+) Treg expansion was observed if HCs were isolated from regenerated livers. Additionally, the induction of Foxp3(+) Tregs was associated with the Notch signaling pathway, as the ability of HCs to enhance Foxp3 was abolished by γ-secretase inhibition. Furthermore, HC-iTregs showed ability to suppress the proliferative response of CD4(+) T cells to anti-CD3 stimulation in vitro. Thus, HCs may play a pivotal role in the induction of tolerance via Notch-mediated conversion of CD4(+) T cells into Foxp3(+) Tregs upon TCR stimulation.

Role of major histocompatibility complex class II expression by non-hematopoietic cells in autoimmune and inflammatory disorders: facts and fiction

It is well established that interactions between CD4(+) T cells and major histocompatibility complex class II (MHCII) positive antigen-presenting cells (APCs) of hematopoietic origin play key roles in both the maintenance of tolerance and the initiation and development of autoimmune and inflammatory disorders. In sharp contrast, despite nearly three decades of intensive research, the functional relevance of MHCII expression by non-hematopoietic tissue-resident cells has remained obscure. The widespread assumption that MHCII expression by non-hematopoietic APCs has an impact on autoimmune and inflammatory diseases has in most instances neither been confirmed nor excluded by indisputable in vivo data. Here we review and put into perspective conflicting in vitro and in vivo results on the putative impact of MHCII expression by non-hematopoietic APCs--in both target organs and secondary lymphoid tissues--on the initiation and development of representative autoimmune and inflammatory disorders. Emphasis will be placed on the lacunar status of our knowledge in this field. We also discuss new mouse models--developed on the basis of our understanding of the molecular mechanisms that regulate MHCII expression--that constitute valuable tools for filling the severe gaps in our knowledge on the functions of non-hematopoietic APCs in inflammatory conditions.

The liver works as a school to educate regulatory immune cells

Because of its unique blood supply, the liver maintains a special local immune tolerogenic microenvironment. Moreover, the liver can impart this immune tolerogenic effect on other organs, thus inducing systemic immune tolerance. The network of hepatic regulatory cells is an important mechanism underlying liver tolerance. Many types of liver-resident antigen-presenting cells (APCs) have immune regulatory function, and more importantly, they can also induce the differentiation of circulating immune cells into regulatory cells to further extend systemic tolerance. Thus, the liver can be seen as a type of 'school', where liver APCs function as 'teachers' and circulating immune cells function as 'students.'

Role of T cell death in maintaining immune tolerance during persistent viral hepatitis

Virus-specific T cells play an important role in the resolution of hepatic infection. However, during chronic hepatitis infection these cells lack their effector functions and fail to control the virus. Hepatitis B virus and hepatitis C virus have developed several mechanisms to generate immune tolerance. One of these strategies is the depletion of virus-specific T cells by apoptosis. The immunotolerogenic liver has unique property to retain and activate naïve T cell to avoid the over reactivation of immune response against antigens which is exploited by hepatotropic viruses to persist. The deletion of the virus-specific T cells occurs by intrinsic (passive) apoptotic mechanism. The pro-apoptotic molecule Bcl-2 interacting mediator (Bim) has attracted increasing attention as a pivotal involvement in apoptosis, as a regulator of tissue homeostasis and an enhancer for the viral persistence. Here, we reviewed our current knowledge on the evidence showing critical role of Bim in viral-specific T cell death by apoptotic pathways and helps in the immune tolerance.

Inhibition of inflammatory CD4 T cell activity by murine liver sinusoidal endothelial cells

BACKGROUND & AIMS

The liver can mitigate the inflammatory activity of infiltrating T cells by mechanisms that are not entirely clear. Here we investigated the role of liver sinusoidal endothelial cells (LSECs) in regulating the activity of inflammatory CD4 T cells.

METHODS

Interactions between T helper (Th) 1 or Th17 cells and LSEC were studied by intravital microscopy and by in vitro stimulation assays.

RESULTS

Circulating CD4 T cells established lasting and repeated interactions with liver endothelium in vivo. Stimulation of Th1 and Th17 cells by LSEC greatly inhibited their capacity to secrete interferon-γ or interleukin-17 in vitro; in contrast, stimulation by dendritic cells (DCs) resulted in considerable secretion of both cytokines. Cytokine release by Th1 or Th17 cells seemed to be actively suppressed by LSEC, as indicated by the inhibition of cytokine secretion even in the presence of Th1- and Th17-promoting DC. This inhibition of CD4 T cell effector function seemed to depend on the dominance of inhibitory over activating co-stimulatory signals on LSEC, since (1) cytokine secretion could be restored by increased CD28 co-activation; (2) LSEC from interleukin-10(-/-) mice, which manifest increased activating signals, such as MHC II, and decreased inhibitory signals, such as PD-L1, failed to suppress cytokine secretion; and (3) cytokine secretion by Th1 or Th17 cells that lacked PD-1, the ligand for inhibitory PD-L1, could not be suppressed by LSEC.

CONCLUSIONS

LSEC inhibit inflammatory cytokine secretion of Th1 and Th17 effector CD4 T cells in dependence of interleukin-10 and PD-1.

Hepatitis C virus-mediated inhibition of cathepsin S increases invariant-chain expression on hepatocyte surface

Hepatocytes are the main source of hepatitis C virus (HCV) replication and contain the maximum viral load in an infected person. Chronic HCV infection is characterized by weak cellular immune responses to viral proteins. Cathepsin S is a lysosomal cysteine protease and controls HLA-DR-antigen complex presentation through the degradation of the invariant chain. In this study, we examined the effect of HCV proteins on cathepsin S expression and found it to be markedly decreased in dendritic cells (DCs) exposed to HCV or in hepatocytes expressing HCV proteins. The downregulation of cathepsin S was mediated by HCV core and NS5A proteins involving inhibition of the transcription factors interferon regulatory factor 1 (IRF-1) and upstream stimulatory factor 1 (USF-1) in gamma interferon (IFN-γ)-treated hepatocytes. Inhibition of cathepsin S by HCV proteins increased cell surface expression of the invariant chain. In addition, hepatocytes stably transfected with HCV core or NS5A inhibited HLA-DR expression. Together, these results suggested that HCV has an inhibitory role on cathepsin S-mediated major histocompatibility complex (MHC) class II maturation, which may contribute to weak immunogenicity of viral antigens in chronically infected humans.

Different sites of xenoantigen delivery lead to a virally induced late-onset hepatitis in mice through molecular mimicry

BACKGROUND

Epidemiological and laboratory evidences led to the hypothesis that molecular mimicry between viruses and self-proteins could be linked to the onset of autoimmune hepatitis (AIH). Hepatotropic viruses could be good candidates, as a pro-inflammatory environment may facilitate the development of AIH.

AIMS

The aims of this study were to test a virus ability to induce an AIH through molecular mimicry and the influence of hepatic inflammation in this process.

METHODS

C57BL/6 mice were injected i.v. or i.m. with recombinant adenoviral vectors (RecAdV) encoding for human type 2 AIH antigens to target xenoantigens expression in the liver and to create a transient hepatitis (i.v.) or for 'peripheral' xenoantigens expression (i.m.). Liver injury and B-cell response were evaluated.

RESULTS

Late-onset hepatitis was observed 8 months after i.v. or i.m. RecAdV injections, despite presence or absence of an initial transient hepatitis. Intensity of B-cell response was similar for both type of injections, but the Ig isotypes produced were different. B-cell autoimmune response spread to several liver proteins.

CONCLUSIONS

Liver autoimmune response can be initiated using molecular mimicry over a long period of time, validating the hit-and-run hypothesis. Initial liver inflammatory injury is neither necessary, nor detrimental to the development of AIH. These results highlight the significance of initial events on the pathogenesis of autoimmune liver injury.

Failure of CD4 T-cells to respond to liver-derived antigen and to provide help to CD8 T-cells

CD4 T-cell help is required for the induction of efficient CD8 T-cells responses and the generation of memory cells. Lack of CD4 T-cell help may contribute to an exhausted CD8 phenotype and viral persistence. Little is known about priming of CD4 T-cells by liver-derived antigen. We used TF-OVA mice expressing ovalbumin in hepatocytes to investigate CD4 T-cell priming by liver-derived antigen and the impact of CD4 T-cell help on CD8 T-cell function. Naïve and effector CD4 T-cells specific for ovalbumin were transferred into TF-OVA mice alone or together with naïve ovalbumin-specific CD8 T-cells. T-cell activation and function were analyzed. CD4 T-cells ignored antigen presented by liver antigen-presenting cells (APCs) in vitro and in vivo but were primed in the liver-draining lymph node and the spleen. No priming occurred in the absence of bone-marrow derived APCs capable of presenting ovalbumin in vivo. CD4 T-cells primed in TF-OVA mice displayed defective Th1-effector function and caused no liver damage. CD4 T-cells were not required for the induction of hepatitis by CD8 T-cells. Th1-effector but not naïve CD4 T-cells augmented the severity of liver injury caused by CD8 T-cells. Our data demonstrate that CD4 T-cells fail to respond to liver-derived antigen presented by liver APCs and develop defective effector function after priming in lymph nodes and spleen. The lack of CD4 T-cell help may be responsible for insufficient CD8 T-cell function against hepatic antigens.

Chronic inflammatory IFN-γ signaling suppresses hepatocarcinogenesis in mice by sensitizing hepatocytes for apoptosis

Chronic liver inflammation is a critical component of hepatocarcinogenesis. Indeed, inflammatory mediators are believed to promote liver cancer by upholding compensatory proliferation of hepatocytes in response to tissue damage. However, inflammation can also mediate the depletion of malignant cells, but the difference between tumor-suppressive and tumor-promoting inflammation is not defined at the molecular level. Here, we analyzed the role of the major inflammatory mediator IFN-γ in chemical hepatocarcinogenesis of transgenic mice that overexpress IFN-γ in the liver; these mice manifest severe chronic inflammatory liver damage and lasting compensatory regeneration. We found that chronic exposure to IFN-γ suppressed chemical hepatocarcinogenesis, despite overt liver injury. Indeed, IFN-γ-transgenic mice had significantly fewer and significantly less advanced malignant lesions than nontransgenic mice. This tumor-suppressive effect of IFN-γ seemed to be mediated in part by its known immune activating function, indicated by infiltration of IFN-γ-transgenic livers with CD8 T cells, natural killer T cells, and natural killer cells. However, IFN-γ seemed to prevent carcinogenesis also by activating the cell-intrinsic p53 tumor suppressor pathway. Indeed, exposure to IFN-γ in vivo or in vitro was associated with accumulation of p53 in hepatocytes and the sensitization of hepatocytes to apoptosis induced by genotoxic stress. The IFN-γ-induced increase in apoptosis of hepatocytes seemed to be p53 dependent. Thus, chronic inflammation dominated by IFN-γ may prevent hepatocarcinogenesis, despite continued inflammatory liver injury and regeneration. Therefore, the carcinogenic potential of inflammation seems to be determined by type and composition of its mediators and manipulating the type of chronic inflammation may serve the prevention of cancer.

A role for intestinal alkaline phosphatase in the maintenance of local gut immunity

BACKGROUND AND AIMS

Intestinal alkaline phosphatase (IAP) is a gut mucosal defense factor known to dephosphorylate lipopolysaccharide (LPS); however, the role of IAP in the gut response to luminal bacteria remains poorly defined. We investigated immune responses of wild-type (WT) and IAP-knockout (IAP-KO) mice to LPS and Salmonella typhimurium challenges.

METHODS

Cryostat sectioning and standard indirect immunohistochemical staining for major histocompatibility complex (MHC) class II molecules were performed on liver tissue from WT and IAP-KO mice. WT and IAP-KO mice were orally gavaged with S. typhimurium; bacterial translocation to mesenteric nodes, liver, and spleen was determined by tissue homogenization and plating. In other experiments, WT and IAP-KO mice received intraperitoneal injections of LPS, with subsequent quantification of complete blood counts and serum interleukin (IL)-6 by enzyme-linked immunosorbent assay (ELISA). WT and IAP-KO whole blood were plated and stimulated with LPS and Pam-3-Cys, followed by cytokine assays.

RESULTS

Immunohistologic liver examinations showed increased expression of MHC class II molecules in IAP-KO mice. Following S. typhimurium challenge, WT mice appeared moribund compared with IAP-KO mice, with increased bacterial translocation. WT mice had >50% decrease (P<.005) in platelets and 1.8-fold (P<.05) increased serum IL-6 compared with IAP-KO mice in response to LPS injections. IAP-KO whole-blood stimulation with LPS and Pam-3-Cys resulted in increased IL-6 and tumor necrosis factor (TNF)-alpha secretion compared with WT.

CONCLUSIONS

IAP-KO mice exhibit characteristics consistent with local LPS tolerance. Whole-blood response of IAP-KO mice did not reflect systemic tolerance. These data suggest that IAP is a local immunomodulating factor, perhaps regulating LPS-toll-like receptor 4 (TLR4) interaction between commensal microflora and intestinal epithelium.

Antigen-presenting cell function in the tolerogenic liver environment

The demands that are imposed on the liver as a result of its function as a metabolic organ that extracts nutrients and clears gut-derived microbial products from the blood are met by a unique microanatomical and immunological environment. The inherent tolerogenicity of the liver and its role in the regulation of innate and adaptive immunity are mediated by parenchymal and non-parenchymal antigen-presenting cells (APCs), cell-autonomous molecular pathways and locally produced factors. Here, we review the central role of liver APCs in the regulation of hepatic immune function and also consider how recent insights may be applied in strategies to target liver tolerance for disease therapy.

Direct, help-independent priming of CD8+ T cells by adeno-associated virus-transduced hepatocytes

Both hepatitis B and C viruses frequently establish chronic infection, raising the question whether T cells are poorly primed in the liver. To determine the role of different cell types in the activation of CD8+ T cells against hepatocellular antigens, we used an Adeno-associated virus to deliver ovalbumin to hepatocytes. In contrast to CD8+ T cells, CD4+ T cells were not activated. The CD8+ T cells were activated even in the absence of endogenous CD4+ T cells; however, in the liver, these cells were high in the programmed death-1 protein and low in CD127. Chimera experiments revealed that these CD8+ T cells were activated on a solid tissue cell.

CONCLUSION

Priming of CD8+ T cells directly on nonhematopoietic cells, in the absence of CD4+ T cell help, results in suboptimal T cell activation. This could explain the impaired function of CD8+ T cells seen in chronic liver infection.

CD4 T cells in hepatic immune tolerance

The liver features a unique immune microenvironment, which seems to favour immune tolerance, both locally and systemically. The hepatic microenvironment is formed by the unique anatomical structure of the liver sinusoids, a peculiar composition of antigen presenting cells and the relative abundance of anti-inflammatory cytokines. The outcome of T cell stimulation within the hepatic microenvironment is often tolerance. This is illustrated by the observations that antigen delivered to the portal vein, or allografts co-transplanted with allogeneic liver are not attacked by the immune system. Moreover, the tolerogenic properties of the liver seem to be part of the cause for the frequent persistence of hepatitis virus infections. This review summarizes some of the mechanisms of tolerance induction in the liver with a focus on CD4 T cells. Hepatic CD4 T cell tolerance seems to emerge from various tolerogenic mechanisms, including immune deviation from inflammatory to non-inflammatory effector function, a relative preponderance of negative co-stimulation notably through PD-1, generation and expansion of regulatory T cells, or the relative abundance of immunoinhibitory cytokines, such as inteleukin-10 and TGF-beta. Understanding the mechanisms of hepatic tolerance induction may teach us how to develop or improve therapies for inflammatory diseases of the liver and other organs. Indeed, novel therapeutic options that utilize hepatic tolerance mechanisms are beginning to emerge, such as the generation of Treg in the liver for therapy of autoimmune disease or the blockade of PD-1 for the therapy of chronic viral hepatitis.

Immune tolerance: what is unique about the liver

The 'liver tolerance effect' mediates local and systemic tolerance to self and foreign antigens and has been attributed to specialized resident cells expressing anti-inflammatory mediators and inhibitory cell surface ligands for T cell activation. Non-parenchymal liver cells responsible for the tolerogenic properties of the liver are the resident dendritic cells (DCs), which comprise myeloid as well as plasmacytoid DCs, liver sinusoidal endothelial cells (LSECs), Kupffer cells (KCs) as well as hepatic stellate cells (HSCs), also known as Ito cells. These cells mediate immunosuppression by production of anti-inflammatory cytokines such as IL-10 and TGFbeta as well as by expression of the negative co-stimulator for T cell activation programmed cell death ligand-1 (PD-L1). An interesting observation in this context is that knockout of IL-10 or PD-L1 (or the receptor PD-1) does not necessarily result in inflammatory liver damage whereas transgenic inhibition of TGFbeta signaling induces liver disease in mice resembling chronic cholangitis. However, depending on the mouse model and on the type of injury, e.g. autoimmune disease, allograft rejection or viral infection, IL-10 or TGFbeta and/or PD-1 as well as cytotoxic T lymphocyte antigen-4 (CTLA-4) contribute to the immunosuppressive mechanisms of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs), which seem to be converted in the liver from infiltrating conventional naïve CD4(+) T cells and/or effector CD4(+) T cells to control the disease. Finally, hepatocytes also contribute to the 'liver tolerance effect' by expression of MHC class II molecules, probably low levels of co-stimulatory molecules and high levels of the co-inhibitory molecule PD-L1.

Liver architecture, cell function, and disease

The liver is an organ consisting of the largest reticulo-endothelial cell network in the body and playing an important role in host defense against invading microorganisms. The organ is comprised of parenchymal cells and many different types of non-parenchymal cells, all of which play a significant role. Even biliary epithelial cells are not only the target in autoimmune liver diseases but also have central role in orchestrating several immune cells involved in both innate and acquired immunity. Tissue damage caused by various agents results in inflammation, necrosis, fibrosis, and, eventually, distortion of normal hepatic architecture, cirrhosis, and functional deterioration.

Influence of genes, sex, age and environment on the onset of autoimmune hepatitis

The pathogenesis of autoimmune hepatitis (AIH) is complex. However, it is believed that a susceptible individual, owing to his genetic background, sex and age, can develop the disease following exposure to an environmental trigger. Autoimmune hepatitis does not follow a Mendelian pattern of inheritance; hence no single causative genetic locus has been identified. However, several genes, inside and outside the HLA locus, have been linked to an increased susceptibility to AIH. Epidemiological evidence also suggests that the sex and age of the patient plays a role in AIH pathogenesis as the disease onset occurs mainly in the two first decades of life and a higher disease incidence is observed in females. No environmental trigger has been identified, but several have been proposed, mainly viruses and xenobiotics. This article aims at reviewing the current knowledge on susceptibility factors leading to AIH and putative triggers, emphasizing fundamental mechanisms responsible for the break of liver immunological tolerance.

Ectopic expression of neural autoantigen in mouse liver suppresses experimental autoimmune neuroinflammation by inducing antigen-specific Tregs

Tregs are important mediators of immune tolerance to self antigens, and it has been suggested that Treg inactivation may cause autoimmune disease. Therefore, immunotherapy approaches that aim to restore or expand autoantigen-specific Treg activity might be beneficial for the treatment of autoimmune disease. Here we report that Treg-mediated suppression of autoimmune disease can be achieved in vivo by taking advantage of the ability of the liver to promote immune tolerance. Expression of the neural autoantigen myelin basic protein (MBP) in the liver was accomplished stably in liver-specific MBP transgenic mice and transiently using gene transfer to liver cells in vivo. Such ectopic MBP expression induced protection from autoimmune neuroinflammation in a mouse model of multiple sclerosis. Protection from autoimmunity was mediated by MBP-specific CD4+CD25+Foxp3+ Tregs, as demonstrated by the ability of these cells to prevent disease when adoptively transferred into nontransgenic mice and to suppress conventional CD4+CD25- T cell proliferation after antigen-specific stimulation with MBP in vitro. The generation of MBP-specific CD4+CD25+Foxp3+ Tregs in vivo depended on expression of MBP in the liver, but not in skin, and occurred by TGF-beta-dependent peripheral conversion from conventional non-Tregs. Our findings indicate that autoantigen expression in the liver may generate autoantigen-specific Tregs. Thus, targeting of autoantigens to hepatocytes may be a novel approach to prevention or treatment of autoimmune diseases.