Ito Cells Are Liver-Resident Antigen-Presenting Cells for Activating T Cell Responses

Absence of invariant natural killer T cells deteriorates liver inflammation and fibrosis in mice fed high-fat diet

BACKGROUND

Invariant natural killer T (iNKT) cells have been suggested to play critical roles in a wide range of immune responses by acting in a proinflammatory or anti-inflammatory manner. Nonalcoholic steatohepatitis (NASH) is a chronic liver disease progressing to advanced cirrhosis and hepatocellular carcinoma. Despite the abundance of iNKT cells in the liver, their role in the pathogenesis of NASH remains obscure. Here, we investigated their role in the development of diet-induced steatosis/steatohepatitis.

METHODS

We used BALB/c wild-type mice and Jα18-deficient (KO) mice lacking iNKT cells fed either a normal diet or a high-fat diet (HFD). The liver and blood were collected from these mice to examine liver inflammation, steatosis, and fibrosis at the indicated time points.

RESULTS

KO mice fed the HFD, compared with control mice fed the HFD, exhibited a clearly higher serum alanine aminotransferase level and a greater number of hepatic inflammatory foci, although there was no significant difference in hepatic lipid retention between these groups of mice. The HFD enhanced hepatic messenger RNA expression of inflammatory cytokines and chemokines in KO but not in control mice. The HFD also increased the proportion of hepatic CD4 T cells and CD8 T cells that composed hepatic inflammatory foci in KO mice, but not in the controls. Prolonged feeding with the HFD augmented liver fibrosis in KO but not in control mice.

CONCLUSIONS

These findings indicate that iNKT cells play a protective role against liver inflammation progressing to fibrosis, but not against steatosis, enhanced by dietary excess fat, suggesting a key role of these cells in NASH pathogenesis.

B7-H4 mediates inhibition of T cell responses by activated murine hepatic stellate cells

Liver fibrosis is mediated by the transformation of hepatic stellate cells (HSC) from a quiescent to an activated state. To understand the role of HSC in liver immunity, we investigated the effect of this transition on T cell stimulation in vitro. Unlike quiescent HSC, activated HSC did not induce proliferation of antigen-specific T cells. Phenotypic analysis of quiescent and activated HSC revealed that activated HSC expressed the coinhibitory molecule B7-H4. Silencing B7-H4 by small interfering RNA (siRNA) in activated HSC restored the ability of T cells to proliferate, differentiate, and regain effector recall responses. Furthermore, expression of B7-H4 on HSC inhibits early T cell activation and addition of exogenous interleukin (IL)-2 reversed the T cell anergy induced by activated HSC.

CONCLUSION

These studies reveal a novel role for activated HSC in the attenuation of intrahepatic T cell responses by way of expression of the coinhibitory molecule B7-H4, and may provide fundamental insight into intrahepatic immunity during liver fibrogenesis.

Spontaneous focal activation of invariant natural killer T (iNKT) cells in mouse liver and kidney

BACKGROUND

Invariant natural killer T (iNKT) cells differ from other T cells by their hyperactive effector T-cell status, in addition to the expression of NK lineage receptors and semi-invariant T-cell receptors. It is generally agreed that the immune phenotype of iNKT cells is maintained by repeated activation in peripheral tissues although no explicit evidence for such iNKT cell activity in vivo has so far been reported.

RESULTS

We used an interferon (IFN)-γ-inducible cytoplasmic protein, Irga6, as a histological marker for local IFN-γ production. Irga6 was intensely expressed in small foci of liver parenchymal cells and kidney tubular epithelium. Focal Irga6 expression was unaffected by germ-free status or loss of TLR signalling and was totally dependent on IFN-γ secreted by T cells in the centres of expression foci. These were shown to be iNKT cells by diagnostic T cell receptor usage and their activity was lost in both CD1 d and Jα-deficient mice.

CONCLUSIONS

This is the first report that supplies direct evidence for explicit activation events of NKT cells in vivo and raises issues about the triggering mechanism and consequences for immune functions in liver and kidney.

CXCL10 promotes liver fibrosis by prevention of NK cell mediated hepatic stellate cell inactivation

Chemokines, such as CXCL10, promote hepatic inflammation in chronic or acute liver injury through recruitment of leukocytes to the liver parenchyma. The CXCL10 receptor CXCR3, which is expressed on a subset of leukocytes, plays an important part in Th1-dependent inflammatory responses. Here, we investigated the role of CXCL10 in chemically induced liver fibrosis. We used carbon tetrachloride (CCl(4)) to trigger chronic liver damage in wildtype C57BL/6 and CXCL10-deficient mice. Fibrosis severity was assessed by Sirius Red staining and intrahepatic leukocyte subsets were investigated by immunohistochemistry. We have further analyzed hepatic stellate cell (HSC) distribution and activation and investigated the effect of CXCL10 on HSC motility and proliferation. In order to demonstrate a possible therapeutic intervention strategy, we have examined the anti-fibrotic potential of a neutralizing anti-CXCL10 antibody. Upon CCl(4) administration, CXCL10-deficient mice showed massively reduced liver fibrosis, when compared to wildtype mice. CXCL10-deficient mice had less B- and T lymphocyte and dendritic cell infiltrations within the liver and the number and activity of HSCs was reduced. In contrast, natural killer (NK) cells were more abundant in CXCL10-deficient mice and granzyme B expression was increased in areas with high numbers of NK cells. Further detailed analysis revealed that HSCs express CXCR3, respond to CXCL10 and secrete CXCL10 when stimulated with IFNγ. Blockade of CXCL10 with a neutralizing antibody exhibited a significant anti-fibrotic effect. Our data suggest that CXCL10 is a pro-fibrotic factor, which participates in a crosstalk between hepatocytes, HSCs and immune cells. NK cells seem to play an important role in controlling HSC activity and fibrosis. CXCL10 blockade may constitute a possible therapeutic intervention for hepatic fibrosis.

The NF-kappaB p50:p50:HDAC-1 repressor complex orchestrates transcriptional inhibition of multiple pro-inflammatory genes

BACKGROUND & AIMS

The pro-inflammatory functions of NF-kappaB must be tightly regulated to prevent inappropriate tissue damage and remodelling caused by activated inflammatory and wound-healing cells. The p50 subunit of NF-kappaB is emerging as an important repressor of immune and inflammatory responses, but by mechanisms that are poorly defined. This study aims to delineate p50 target genes in activated hepatic stellate cells and to outline mechanisms utilised in their repression.

METHODS

Hepatic stellate cells were isolated from nfkb1(p50)-deficient or Wt mice and gene expression compared using microarray. Target genes were verified by qRT-PCR and p50-mediated HDAC-1 recruitment to the target genes demonstrated using chromatin immunoprecipitation.

RESULTS

We identify p50 as transcriptional repressor of multiple pro-inflammatory genes including Ccl2, Cxcl10, Gm-csf, and Mmp-13. These genes are over-expressed in nfkb1(p50)-deficient mice suffering from chronic hepatitis and in fibrogenic/inflammatory hepatic stellate cells isolated from nfkb1(-/-) liver. We identify Mmp-13 as a bona-fide target gene for p50 and demonstrate that p50 is required for recruitment of the transcriptional repressor histone deacetylase (HDAC)-1 to kappaB sites in the Mmp-13 promoter. Chromatin immunoprecipitations identified binding of HDAC-1 to specific regulatory regions of the Ccl2, Cxcl10, Gm-csf genes that contain predicted kappaB binding motifs. Recruitment of HDAC-1 to these genes was not observed in nfkb1(-/-) cells suggesting a requirement for p50 in a manner similar to that described for Mmp-13.

CONCLUSIONS

Recruitment of HDAC-1 to inflammatory genes provides a widespread mechanism to explain the immunosuppressive properties of p50.

Effects and regulation of autoreactive CD8+ T cells in a transgenic mouse model of autoimmune hepatitis

BACKGROUND & AIMS

Autoimmune hepatitis (AIH) is a chronic inflammatory liver disease of unknown etiology. Autoreactive T cells are thought to mediate liver injury, but the pathogenesis of AIH is poorly understood because of the lack of suitable animal models. We established a mouse model to investigate liver-specific T-cell responses and assess the effects and regulation of autoreactive CD8(+) T cells in the pathogenesis of AIH.

METHODS

We generated transgenic mice expressing the influenza virus hemagglutinin (HA) autoantigen under control of mouse albumin regulatory elements and alpha-fetoprotein enhancers (Alb) specifically in the liver (Alb-HA mice); they were crossed with mice that express a specific T-cell receptor (TCR) (CL4-TCR). CL4-TCR transgenic CD8(+) T cells were also adoptively transferred into Alb-HA mice. We investigated immunologic mechanisms of CD8(+) T cell-induced liver damage and of counteracting peripheral tolerance.

RESULTS

The double-transgenic mice (Alb-HA/CL4-TCR) spontaneously developed chronic, autoimmune-mediated hepatitis, characterized by necroinflammatory lesions, hepatic fibrosis, and increased levels of aminotransferase; these features resembled those of AIH. Interestingly, most liver-infiltrating, HA-specific CD8(+) T cells had an anergic phenotype; regulatory CD4(+)CD25(+)Foxp3(+) T cells accumulated in the inflamed liver.

CONCLUSIONS

The continuous development of a few, HA-specific CD8(+) effector T cells is sufficient to induce chronic hepatitis. Peripheral tolerance mechanisms such as induction of T-cell anergy and accumulation of regulatory CD4(+)CD25(+)Foxp3(+) T cells protect the liver from severe damage. Our mouse model that spontaneously develops chronic autoimmune-mediated hepatitis provides a new tool to investigate autoantigen-specific T-cell responses and regulatory mechanisms involved in the prevention and pathogenesis of AIH.

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.

Vitamin A deficiency alters splenic dendritic cell subsets and increases CD8(+)Gr-1(+) memory T lymphocytes in C57BL/6J mice

Vitamin A-deficient populations have impaired T cell-dependent antibody responses. Dendritic cells (DCs) are the most proficient antigen-presenting cells to naïve T cells. In the mouse, CD11b(+) myeloid DCs stimulate T helper (Th) 2 antibody immune responses, while CD8α(+) lymphoid DCs stimulate Th1 cell-mediated immune responses. Therefore, we hypothesized that vitamin A-deficient animals would have decreased numbers of myeloid DCs and unaffected numbers of lymphoid DCs. We performed dietary depletion of vitamin A in C57BL/6J male and female mice and used multicolor flow cytometry to quantify immune cell populations of the spleen, with particular focus on DC subpopulations. We show that vitamin A-depleted animals have increased polymorphonuclear neutrophils, lymphoid DCs, and memory CD8(+) T cells and decreased CD4(+) T lymphocytes. Therefore, vitamin A deficiency alters splenic DC subpopulations, which may contribute to skewed immune responses of vitamin A-deficient populations.

Hepatocyte death: a clear and present danger

The hepatocyte is especially vulnerable to injury due to its central role in xenobiotic metabolism including drugs and alcohol, participation in lipid and fatty acid metabolism, its unique role in the enterohepatic circulation of bile acids, the widespread prevalence of hepatotropic viruses, and its existence within a milieu of innate immune responding cells. Apoptosis and necrosis are the most widely recognized forms of hepatocyte cell death. The hepatocyte displays many unique features regarding cell death by apoptosis. It is quite susceptible to death receptor-mediated injury, and its death receptor signaling pathways involve the mitochondrial pathway for efficient cell killing. Also, death receptors can trigger lysosomal disruption in hepatocytes which further promote cell and tissue injury. Interestingly, hepatocytes are protected from cell death by only two anti-apoptotic proteins, Bcl-x(L) and Mcl-1, which have nonredundant functions. Endoplasmic reticulum stress or the unfolded protein response contributes to hepatocyte cell death during alterations of lipid and fatty acid metabolism. Finally, the current information implicating RIP kinases in necrosis provides an approach to more fully address this mode of cell death in hepatocyte injury. All of these processes contributing to hepatocyte injury are discussed in the context of potential therapeutic strategies.

Cotransplantation of hepatic stellate cells attenuates the severity of graft-versus-host disease

Liver allografts seem to be immunologically privileged; they are the only solid transplant that can protect cotransplanted organs from rejection. The mechanisms of this "hepatic tolerance" are poorly understood. The aim of this study was to examine the immunomodulatory effect of hepatic stellate cells (HSCs) in mixed lymphocyte reactions in vitro and in graft-versus-host disease (GVHD) in vivo. Using a carboxyfluorescein diacetate succinimidyl ester-labeling method, we observed that the percentage of proliferative T cells was reduced when HSCs were present in a mixed lymphocyte culture. In addition, the degree of reduction was the same when HSCs were co-cultured in transwell inserts. Thus, soluble factors may participate in the immunomodulatory effect of HSCs. In GVHD experiments, irradiated BALB/c (H-2d) mice simultaneously received an intravenous mixture of bone marrow and splenic T cells from C57BL/6 (H-2b) mice. The HSCs prominently reduced symptoms and pathologic severity of GVHD in target organs. HSC cotransplanted mice survived for 57.5+/-30.6 days whereas the control hosts only survived for 15.3+/-5.2 days (P<.01). We concluded that HSCs may reduce T-cell proliferation against alloantigens and suppress acute GVHD to prolong recipient survival. Our study sheds some light on the immunosuppressive nature of the liver, suggesting a biological manipulation of alloreactivity for transplantation medicine.

Vitamin A as a regulator of antigen presenting cells

Vitamin A has been long associated with immune system competence. Vitamin A deficiency is known to compromise many aspects of both innate and adaptive immune responses. Recent advances in retinol uptake and metabolism have identified the antigen presenting cell (APC) as a central immune cell capable of vitamin A metabolism. APC are now known to express retinaldehyde dehydrogenase and secrete retinoic acid. The retinoic acid produced has both autocrine and paracrine effects. Autocrine effects include upregulation of CD1d nonclassical major histocompatibility class I-like molecule and matrix metalloproteinase-9. Paracrine effects influence multiple lymphocyte lineage cell populations. Specifically, retinoic acid increases IgA isotype class switching by B lymphocytes, enhances regulatory T cell differentiation, and directs homing of lymphocytes to mucosa. CD1d lipid antigen presentation expands natural killer T cell populations. Previously, the focus of vitamin A action in adaptive immunity was on lymphocytes, but these recent advances suggest the APC may be the central player in carrying out the immune system functions of vitamin A.

Human immunodeficiency virus (HIV)-1 infects human hepatic stellate cells and promotes collagen I and monocyte chemoattractant protein-1 expression: implications for the pathogenesis of HIV/hepatitis C virus-induced liver fibrosis

Patients coinfected with human immunodeficiency virus (HIV) and hepatitis C virus (HCV) develop more rapid fibrosis than those infected with HCV only. In HIV/HCV-coinfected patients, fibrosis progression correlates with HIV RNA levels, suggesting a direct role of HIV in liver fibrogenesis. Chemokine (C-C motif) receptor 5 (CCR5) and cysteine-X-cysteine receptor 4 (CXCR4), the two major coreceptors required for HIV entry into cells, are expressed on activated hepatic stellate cells (HSCs), the principle fibrogenic cell type in the liver. We therefore examined whether HIV can infect HSCs, explored the potential mechanisms of viral entry, and assessed the impact of infection as reflected by the ability of HSCs to transfer virus to T lymphocytes and elicit a proinflammatory and profibrogenic response. We report that the laboratory-adapted viruses HIV-IIIB (CXCR4-tropic or X4) and HIV-BaL (CCR5-tropic or R5) and primary HIV isolates can infect both a human stellate cell line, LX-2, and primary human HSCs. HIV entry and gene expression in HSCs was confirmed using HIV-green fluorescent protein (GFP) expression viral constructs in the presence or absence of the reverse-transcriptase inhibitor azidothymidine. CD4 expression on a subset of primary HSCs was demonstrated using fluorescence-activated cell sorting and immunofluorescence staining. Blocking experiments in the presence of anti-CD4, anti-CXCR4, and anti-CCR5 revealed that HIV entry into HSCs is predominantly CD4/chemokine coreceptor-independent. HIV infection promoted HSC collagen I expression and secretion of the proinflammatory cytokine monocyte chemoattractant protein-1. Furthermore, infected LX-2 cells were capable of transferring GFP-expressing virus to T lymphocytes in a coculture system.

CONCLUSION

Taken together, our results suggest a potential role of HIV in liver fibrosis/inflammation mediated through effects on HSCs. The role of early highly active antiretroviral therapy initiation in patients with HIV/HCV coinfection warrants further investigation.

Role of gut-resident dendritic cells in inflammatory bowel disease

The gastrointestinal immune system, innate and adaptive, is continuously exposed to challenges provided by the enteric flora. In most cases, the result of mucosal immune responses is the development of tolerance. Mucosal dendritic cells initiate and regulate local immune responses. Uncontrolled local immune responses are thought to be a major factor in the development of inflammatory bowel disease, such as Crohn's disease and ulcerative colitis. This review will discuss the function of dendritic cells in the recognition of the enteric flora and their role in the development of intestinal inflammation.

Hepatic stellate cells and fibrogenesis in hepatitis C virus infection: an ultrastructural insight

An ultrastructural quantitative assessment of hepatic stellate cells (HSCs) was made in relation to hepatic fibrosis, apoptotic cellular changes, intracellular fat deposition, circulating inflammatory cells in the sinusoids, and the necroinflammatory activity in liver specimens of 33 patients proven to be positive for hepatitis C virus (HCV)-RNA by polymerase chain reaction with the intention that electron microscopy may throw more light on the role of HSCs in the complicated process of fibrogenesis. A detailed review concerning these parameters and observed evidence suggesting the potential properties of HSCs to recycle cellular debris into collagen fibers are reported.

Circulating markers of liver fibrosis progression

INTRODUCTION

Fibrogenesis is a typical reaction of the liver to injury. In the case of overstimulation of fibrogenesis clinically significant fibrosis and, eventually, cirrhosis occur. Treatment of liver cirrhosis is limited, therefore it is important to screen and monitor patients at risk of cirrhosis. Noninvasive parameters are ideal for this purpose due to their risk profile and repeatability.

METHODS

Systematic review of literature.

RESULTS

Among large number of proposed biomarkers, there is a distinct difference between two groups or classes. Class I biomarkers are associated with the process of fibrogenesis, their presence in the serum is the result of the increased turnover of extracellular matrix. Class II biomarkers and their combinations are mostly markers of liver function or structural damage. We have identified 27 Class I and 13 Class II biomarkers that have been proposed in the literature. We have evaluated in detail those which reached limited clinical application.

CONCLUSION

General clinical acceptance of these biomarkers is low because of various drawbacks. Simple and readily available biomarkers have low accuracy in predicting liver fibrosis and more advanced markers have low cost-benefit ratio. Therefore liver biopsy remains the "gold standard" for diagnosis of fibrosis. However potential noninvasive alternatives exist and their implementation could be valuable.

Analysis of antigen-presenting functionality of cultured rat hepatic stellate cells and transdifferentiated myofibroblasts

Here, we demonstrate that hepatic stellate cells (HSC) isolated from Lewis rats have in vitro antigen-presentation cell (APC) functionality and are able to process and present exogenous antigens. We show activation of a major histocompatibility complex II (RT1BI)-restricted T-cell hybridoma specific for guinea pig myelin basic protein (gpMBP) after coculture with HSC. During transdifferentiation of HSC into myofibroblasts (MFB) the APC function was markedly decreased but restorable by addition of interferon-gamma (IFN-gamma). Based on our findings we conclude that HSC play a key role in hepatic immune function and that IFN-gamma treatment might mediate its beneficial therapeutic effects via activation of APC function in MFB.

[Liver fibrosis - pathogenesis and novel therapeutic approaches]

Chronic liver injury, such as viral hepatitis, alcohol, metabolic syndrome or other toxic damages, leads to an inflammatory response including the infiltration and activation of immune cells and to the proliferation and transdifferentiation of mesenchymal cells within the liver, especially of hepatic stellate cells. These cells produce an excess of extracellular matrix proteins that are deposited in the liver. Hepatic fibrosis may progress to liver cirrhosis and liver failure. This review aims at summarizing the current view on the pathogenic sequence during fibrogenesis highlighting the essential role of cytokines and chemokines. Understanding the complex cellular interactions in liver fibrosis may help to develop novel antifibrotic therapies in the near future.

An intravascular immune response to Borrelia burgdorferi involves Kupffer cells and iNKT cells

Here we investigate the dynamics of the hepatic intravascular immune response to a pathogen relevant to invariant natural killer T cells (iNKT cells). Immobilized Kupffer cells with highly ramified extended processes into multiple sinusoids could effectively capture blood-borne, disseminating Borrelia burgdorferi, creating a highly efficient surveillance and filtering system. After ingesting B. burgdorferi, Kupffer cells induced chemokine receptor CXCR3-dependent clustering of iNKT cells. Kupffer cells and iNKT cells formed stable contacts via the antigen-presenting molecule CD1d, which led to iNKT cell activation. An absence of iNKT cells caused B. burgdorferi to leave the blood and enter the joints more effectively. B. burgdorferi that escaped Kupffer cells entered the liver parenchyma and survived despite Ito cell responses. Kupffer cell-iNKT cell interactions induced a key intravascular immune response that diminished the dissemination of B. burgdorferi.

Dynamic imaging of experimental Leishmania donovani-induced hepatic granulomas detects Kupffer cell-restricted antigen presentation to antigen-specific CD8 T cells

Kupffer cells (KCs) represent the major phagocytic population within the liver and provide an intracellular niche for the survival of a number of important human pathogens. Although KCs have been extensively studied in vitro, little is known of their in vivo response to infection and their capacity to directly interact with antigen-specific CD8⁺ T cells. Here, using a combination of approaches including whole mount and thin section confocal microscopy, adoptive cell transfer and intra-vital 2-photon microscopy, we demonstrate that KCs represent the only detectable population of mononuclear phagocytes within granulomas induced by Leishmania donovani infection that are capable of presenting parasite-derived peptide to effector CD8⁺ T cells. This restriction of antigen presentation to KCs within the Leishmania granuloma has important implications for the identification of new candidate vaccine antigens and for the design of novel immuno-therapeutic interventions.

Leishmania donovani is a protozoan parasite that causes severe disease in humans with associated pathology in the spleen and liver. In experimental models of L. donovani infection, the hepatic response to infection is characterised by the presence of a focal mononuclear cell-rich inflammatory response (a granuloma) surrounding cells infected with intracellular amastigotes. Granulomas provide focus to the ensuing immune response, helping to contain parasite dissemination and providing the major effector site responsible for parasites elimination from the liver. Although granulomas are believed to form around infected resident liver macrophages (Kupffer cells), the role of these cells in intra-granuloma antigen presentation is currently unknown. As CD8⁺ T cells have been shown to play an important role in hepatic resistance to L. donovani following natural infection, vaccination and during immunotherapy, we asked which cells within the granuloma microenvironment serve as targets for antigen recognition by effector CD8⁺ T cells. Here we provide evidence that the heavily infected mononuclear cell core of the granuloma is composed almost entirely of Kupffer cells, many having migrated from the surrounding sinusoids. Furthermore, by intra-vital 2-photon microscopy, we show that only Kupffer cells laden with intracellular amastigotes are able to form long-lasting antigen-specific interactions with CD8⁺ T cells within the granuloma microenvironment. These data have important implications for the understanding of how granulomas function to limit infection and may have important implications for the development of vaccines to Leishmania that are designed to induce CD8⁺ T cell responses.

Leishmania donovani-induced expression of signal regulatory protein alpha on Kupffer cells enhances hepatic invariant NKT-cell activation

Signal regulatory protein α (SIRPα) and its cognate ligand CD47 have been documented to have a broad range of cellular functions in development and immunity. Here, we investigated the role of SIRPα–CD47 signalling in invariant NKT (iNKT) cell responses. We found that CD47 was required for the optimal production of IFN-γ from splenic iNKT cells following exposure to the αGalCer analogue PBS-57 and in vivo infection of mice with Leishmania donovani. Surprisingly, although SIRPα was undetectable in the liver of uninfected mice, the hepatic iNKT-cell response to infection was also impaired in CD47^−/− mice. However, we found that SIRPα was rapidly induced on Kupffer cells following L. donovani infection, via a mechanism involving G-protein-coupled receptors. Thus, we describe a novel amplification pathway affecting cytokine production by hepatic iNKT cells, which may facilitate the breakdown of hepatic tolerance after infection.

The immunological functions of saposins

Saposins or sphingolipid activator proteins (SAPs) are small, nonenzymatic glycoproteins that are ubiquitously present in lysosomes. SAPs comprise the five molecules saposins A-D and the GM2 activator protein. Saposins are essential for sphingolipid degradation and membrane digestion. On the one hand, they bind the respective hydrolases required to catabolize sphingolipid molecules; on the other hand, saposins can interact with intralysosomal membrane structures to render lipids accessible to their degrading enzymes. Thus, saposins bridge the physicochemical gap between lipid substrate and hydrophilic hydrolases. Accordingly, defects in saposin function can lead to lysosomal lipid accumulation. In addition to their specific functions in sphingolipid metabolism, saposins have membrane-perturbing properties. At the low pH of lysosomes, saposins get protonated and exhibit a high binding affinity for anionic phospholipids. Based on their universal principle to interact with membrane bilayers, we present the immunological functions of saposins with regard to lipid antigen presentation to CD1-restricted T cells, processing of apoptotic bodies for antigen delivery and cross-priming, as well as their potential antimicrobial impact.

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions.

Signal molecule-mediated hepatic cell communication during liver regeneration

Liver regeneration is a complex and well-orchestrated process, during which hepatic cells are activated to produce large signal molecules in response to liver injury or mass reduction. These signal molecules, in turn, set up the connections and cross-talk among liver cells to promote hepatic recovery. In this review, we endeavor to summarize the network of signal molecules that mediates hepatic cell communication in the regulation of liver regeneration.

Mast cell-mediated antigen presentation regulates CD8+ T cell effector functions

The characteristics, importance, and molecular requirements for interactions between mast cells (MCs) and CD8(+) T cells have not been elucidated. Here, we demonstrated that MCs induced antigen-specific CD8(+) T cell activation and proliferation. This process required direct cell contact and MHC class I-dependent antigen cross-presentation by MCs and induced the secretion of interleukin-2, interferon-gamma, and macrophage inflammatory protein-1alpha by CD8(+) T cells. MCs regulated antigen-specific CD8(+) T cell cytotoxicity by increasing granzyme B expression and by promoting CD8(+) T cell degranulation. Because MCs also upregulated their expression of costimulatory molecules (4-1BB) and released osteopontin upon direct T cell contact, MC-T cell interactions probably are bidirectional. In vivo, adoptive transfer of antigen-pulsed MCs induced MHC class I-dependent, antigen-specific CD8(+) T cell proliferation, and MCs regulated CD8(+) T cell-specific priming in experimental autoimmune encephalomyelitis. Thus, MCs are important players in antigen-specific regulation of CD8(+) T cells.

Role for hedgehog pathway in regulating growth and function of invariant NKT cells

Lymphocyte accumulation is characteristic of chronic hepatitis, but the mechanisms regulating lymphocyte numbers and their roles in liver disease progression are poorly understood. The Hedgehog (Hh) pathway regulates thymic development and lymphopoeisis during embryogenesis, and is activated in fibrosing liver disease in adults. Our objective was to determine if Hh ligands regulate the viability and phenotype of NKT cells, which comprise a substantial sub-population of resident lymphocytes in healthy adult livers and often accumulate during liver fibrosis. The results demonstrate that a mouse invariant NKT cell line (DN32 iNKT cells), mouse primary liver iNKT cells, and human peripheral blood iNKT cells are all responsive to sonic hedgehog (Shh). In cultured iNKT cells, Shh enhances proliferation, inhibits apoptosis, induces activation, and stimulates expression of the pro-fibrogenic cytokine, IL-13. Livers of transgenic mice with an overly active Hh pathway harbor increased numbers of iNKT cells. iNKT cells also express Shh. These results demonstrate that iNKT cells produce and respond to Hh ligands, and that Hh pathway activation regulates the size and cytokine production of liver iNKT cell populations. Therefore, Hh pathway activation may contribute to the local expansion of pro-fibrogenic iNKT cell populations during certain types of fibrosing liver damage.

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.

Gut homing receptors on CD8 T cells are retinoic acid dependent and not maintained by liver dendritic or stellate cells

BACKGROUND & AIMS

Lymphocytes primed by intestinal dendritic cells (DC) express the gut-homing receptors CCR9 and alpha4beta7, which recognize CCL25 and mucosal addressin cell-adhesion molecule-1 in the intestine promoting the development of regional immunity. In mice, imprinting of CCR9 and alpha4beta7 is dependent on retinoic acid during T-cell activation. Tissue specificity is lost in primary sclerosing cholangitis (PSC), an extraintestinal manifestation of inflammatory bowel disease, when ectopic expression of mucosal addressin cell-adhesion molecule-1 and CCL25 in the liver promotes recruitment of CCR9+alpha4beta7+ T cells to the liver. We investigated the processes that control enterohepatic T-cell migration and whether the ability to imprint CCR9 and alpha4beta7 is restricted to intestinal DCs or can under some circumstances be acquired by hepatic DCs in diseases such as PSC.

METHODS

Human and murine DCs from gut, liver, or portal lymph nodes and hepatic stellate cells were used to activate CD8 T cells. Imprinting of CCR9 and alpha4beta7 and functional migration responses were determined. Crossover activation protocols assessed plasticity of gut homing.

RESULTS

Activation by gut DCs imprinted high levels of functional CCR9 and alpha4beta7 on naïve CD8 T cells, whereas hepatic DCs and stellate cells proved inferior. Imprinting was RA dependent and demonstrated plasticity.

CONCLUSIONS

Imprinting and plasticity of gut-homing human CD8 T cells requires primary activation or reactivation by gut DCs and is retinoic acid dependent. The inability of liver DCs to imprint gut tropism implies that alpha4beta7+CCR9+ T cell that infiltrate the liver in PSC are primed in the gut.

Mononuclear cells in liver fibrosis

Fibrosis is a multicellular wound healing process, where myofibroblasts that express extracellular matrix components extensively cross-talk with other cells resident in the liver or recruited from the bloodstream. Macrophages and infiltrating monocytes participate in the development of fibrosis via several mechanisms, including secretion of cytokines and generation of oxidative stress-related products. However, macrophages are also pivotal in the process of fibrosis resolution, where they contribute to matrix degradation. T lymphocytes modulate the fibrogenic process by direct interaction with myofibroblasts and secreting cytokines. In general, Th2 polarized responses promote fibrosis, while Th1 cytokines may be antifibrogenic. NK cells limit the development of fibrosis and favor its resolution, at least in part via killing of fibrogenic cells. The possible role of NKT cells and B cells is emerging in recent studies. Thus, mononuclear cells represent a critical regulatory system during fibrogenesis and may become an appealing target for therapy.

The liver as a lymphoid organ

The liver receives blood from both the systemic circulation and the intestine, and in distinctive, thin-walled sinusoids this mixture passes over a large macrophage population, termed Kupffer cells. The exposure of liver cells to antigens, and to microbial products derived from the intestinal bacteria, has resulted in a distinctive local immune environment. Innate lymphocytes, including both natural killer cells and natural killer T cells, are unusually abundant in the liver. Multiple populations of nonhematopoietic liver cells, including sinusoidal endothelial cells, stellate cells located in the subendothelial space, and liver parenchymal cells, take on the roles of antigen-presenting cells. These cells present antigen in the context of immunosuppressive cytokines and inhibitory cell surface ligands, and immune responses to liver antigens often result in tolerance. Important human pathogens, including hepatitis C virus and the malaria parasite, exploit the liver's environment, subvert immunity, and establish persistent infection.

Systemic antigen cross-presented by liver sinusoidal endothelial cells induces liver-specific CD8 T-cell retention and tolerization

Peripheral CD8 T-cell tolerance can be generated outside lymphatic tissue in the liver, but the course of events leading to tolerogenic interaction of hepatic cell populations with circulating T-cells remain largely undefined. Here we demonstrate that preferential uptake of systemically circulating antigen by murine liver sinusoidal endothelial cells (LSECs), and not by other antigen-presenting cells in the liver or spleen, leads to cross-presentation on major histocompatibility complex (MHC) I molecules, which causes rapid antigen-specific naïve CD8 T-cell retention in the liver but not in other organs. Using bone-marrow chimeras and a novel transgenic mouse model (Tie2-H-2K(b) mice) with endothelial cell-specific MHC I expression, we provide evidence that cross-presentation by organ-resident and radiation-resistant LSECs in vivo was both essential and sufficient to cause antigen-specific retention of naïve CD8 T-cells under noninflammatory conditions. This was followed by sustained CD8 T-cell proliferation and expansion in vivo, but ultimately led to the development of T-cell tolerance.

CONCLUSION

Our results show that cross-presentation of circulating antigens by LSECs caused antigen-specific retention of naïve CD8 T-cells and identify antigen-specific T-cell adhesion as the first step in the induction of T-cell tolerance.

Diverse roles of invariant natural killer T cells in liver injury and fibrosis induced by carbon tetrachloride

Liver fibrosis is a common scarring response to all forms of chronic liver injury and is always associated with inflammation that contributes to fibrogenesis. Although a variety of cell populations infiltrate the liver during inflammation, it is generically clear that CD8 T lymphocytes promote while natural killer (NK) cells inhibit liver fibrosis. However, the role of invariant natural killer T (iNKT) cells, which are abundant in the liver, in hepatic fibrogenesis, remains obscure. Here we show that iNKT-deficient mice are more susceptible to carbon tetrachloride (CCl(4))-induced acute liver injury and inflammation. The protective effect of naturally activated iNKT in this model is likely mediated via suppression of the proinflammatory effect of activated hepatic stellate cells. Interestingly, strong activation of iNKT through injection of iNKT activator alpha-galactosylceramide (alpha-GalCer) accelerates CCl(4)-induced acute liver injury and fibrosis. In contrast, chronic CCl(4) administration induces a similar degree of liver injury in iNKT-deficient and wild-type mice, and only a slightly higher grade of liver fibrosis in iNKT-deficient mice than wild-type mice 2 weeks but not 4 weeks after CCl(4) injection, although iNKT cells are able to kill activated stellate cells. An insignificant role of iNKT in chronic liver injury and fibrosis may be attributable to hepatic iNKT cell depletion. Finally, chronic alpha-GalCer treatment had little effect on liver injury and fibrosis, which is attributable to iNKT tolerance after alpha-GalCer injection.

CONCLUSION

Natural activation of hepatic iNKT cells inhibits, whereas strong activation of iNKT cells by alpha-GalCer accelerates CCl(4)-induced acute liver injury, inflammation, and fibrosis. During chronic liver injury, hepatic iNKT cells are depleted and play a role in inhibiting liver fibrosis in the early stage but not the late stage of fibrosis.

Enrichment of human CD4+ V(alpha)24/Vbeta11 invariant NKT cells in intrahepatic malignant tumors

Invariant NKT cells (iNKT cells) recognize glycolipid Ags via an invariant TCR alpha-chain and play a central role in various immune responses. Although human CD4(+) and CD4(-) iNKT cell subsets both produce Th1 cytokines, the CD4(+) subset displays an enhanced ability to secrete Th2 cytokines and shows regulatory activity. We performed an ex vivo analysis of blood, liver, and tumor iNKT cells from patients with hepatocellular carcinoma and metastases from uveal melanoma or colon carcinoma. Frequencies of Valpha24/Vbeta11 iNKT cells were increased in tumors, especially in patients with hepatocellular carcinoma. The proportions of CD4(+), double negative, and CD8alpha(+) iNKT cell subsets in the blood of patients were similar to those of healthy donors. However, we consistently found that the proportion of CD4(+) iNKT cells increased gradually from blood to liver to tumor. Furthermore, CD4(+) iNKT cell clones generated from healthy donors were functionally distinct from their CD4(-) counterparts, exhibiting higher Th2 cytokine production and lower cytolytic activity. Thus, in the tumor microenvironment the iNKT cell repertoire is modified by the enrichment of CD4(+) iNKT cells, a subset able to generate Th2 cytokines that can inhibit the expansion of tumor Ag-specific CD8(+) T cells. Because CD4(+) iNKT cells appear inefficient in tumor defense and may even favor tumor growth and recurrence, novel iNKT-targeted therapies should restore CD4(-) iNKT cells at the tumor site and specifically induce Th1 cytokine production from all iNKT cell subsets.

Hepatic sinusoidal cells in health and disease: update from the 14th International Symposium

This review aims to give an update of the field of the hepatic sinusoid, supported by references to presentations given at the 14th International Symposium on Cells of the Hepatic Sinusoid (ISCHS2008), which was held in Tromsø, Norway, August 31-September 4, 2008. The subtitle of the symposium, 'Integrating basic and clinical hepatology', signified the inclusion of both basal and applied clinical results of importance in the field of liver sinusoidal physiology and pathophysiology. Of nearly 50 oral presentations, nine were invited tutorial lectures. The authors of the review have avoided writing a 'flat summary' of the presentations given at ISCHS2008, and instead focused on important novel information. The tutorial presentations have served as a particularly important basis in the preparation of this update. In this review, we have also included references to recent literature that may not have been covered by the ISCHS2008 programme. The sections of this review reflect the scientific programme of the symposium (http://www.ub.uit.no/munin/bitstream/10037/1654/1/book.pdf): 1. Liver sinusoidal endothelial cells. 2. Kupffer cells. 3. Hepatic stellate cells. 4. Immunology. 5. Tumor/metastasis. Symposium abstracts are referred to by a number preceded by the letter A.

Hepatic stellate cell: a star cell in the liver

Hepatic stellate cells represent a highly versatile cytotype that plays a significant role in liver development and differentiation, regeneration, xenobiotic response, immunoregulation, control of hepatic blood flow and inflammatory reactions. Because of the wide panel of molecular intermediates they may produce and secrete, particularly after their sustained activation in a disease state, hepatic stellate cells are definitely involved in the pathogenesis of various liver pathologies, besides the well know key role in fibrosis and extracellular matrix remodelling. In particular, they can actively contribute to the progression of hepatitis and steatohepatitis of different aetiology, and of liver carcinogenesis.

Inflammatory pathways in liver homeostasis and liver injury

The liver is a unique organ with respect to its anatomical location, allowing continuous blood flow from the gastrointestinal tract through the sinusoids, and its cellular composition, comprising metabolically active hepatocytes, nonhepatocytic parenchymal cells, and various immune cell populations. Cytokines are key mediators within the complex interplay of intrahepatic immune cells and hepatocytes, as they can activate effector functions of immune cells, as well as hepatocytic intracellular signaling pathways controlling cellular homeostasis. Kupffer cells and liver-infiltrating monocyte-derived macrophages are primary sources of cytokines such as tumor-necrosis factor-alpha (TNF-alpha) and interleukin-6. The liver is also enriched in natural killer (NK) and NK T cells, which fulfill functions in pathogen defense, T cell recruitment, and modulation of liver injury. TNF-alpha can activate specific intracellular pathways in hepatocytes that influence cell fate in different manners, e.g., proapoptotic signals via the caspase cascade, but also survival pathways, namely the nuclear factor (NF)-kappaB pathway. NF-kappaB regulates important functions in liver physiology and pathology. Recent experiments with genetically modified mice demonstrated important and partly controversial functions of this pathway, e.g., in cytokine-mediated hepatocyte apoptosis or ischemia-reperfusion injury. The exact dissection of the contribution of recruited and resident immune cells, their soluble cytokine and chemokine mediators, and the intracellular hepatocytic response in liver homeostasis and injury could potentially identify novel targets for the treatment of acute and chronic liver disease, liver fibrosis, or cirrhosis.

Hepatic stellate cells preferentially expand allogeneic CD4+ CD25+ FoxP3+ regulatory T cells in an IL-2-dependent manner

BACKGROUND

Organ transplantation has been successfully practiced for decades, but the outcome of cell transplantation remains disappointing. This is the case in animal models; liver allografts in mice are spontaneously accepted without requirement of immunosuppression, whereas hepatocyte transplants in the same combination are acutely rejected, apparently resulting from immune attacks because syngeneic hepatocyte transplants survive indefinitely. This suggests that liver nonparenchymal cells play an important role in protecting parenchymal cell from rejection. We have shown that hepatic stellate cells (HpSC), well known to participate in liver repairing and fibrosis, mediate potent immunomodulatory functions through induction of activated T-cell death.

METHODS AND RESULTS

Here, we report that HpSC acquired antigen presenting capacity after activated by interferon-gamma. In contrast to professional antigen-presenting cells dendritic cells that predominantly stimulated CD4+ T cells to generate CD25+ forkhead box P3 (Foxp3)- effector cells, HpSC selectively expanded CD4+ CD25+ Foxp3+ cells in an interleukin-2-dependent manner. These expanded CD4+ CD25+ Foxp3+ cells showed T regulatory cell (Treg) activity in effectively inhibiting T-cell proliferation in responses to anti-CD3 monoclonal antibody or alloantigens in a major histocompatibility complex nonspecific fashion. The Treg cells were expanded from the CD4+ CD25+ population with the help of interleukin-2, independent of B7-H1 and transforming growth factor-beta. Administration of HpSC into allogeneic recipients resulted in expansion of CD4+ CD25+ FoxP3+ cells in vivo.

CONCLUSION

Liver stromal HpSC acted as nonprofessional antigen-presenting cells, and preferentially expanded CD25+FoxP3+ Treg cells, which may contribute to immune regulation in the liver.

Antigen-presenting cells under the influence of alcohol

The negative influence of alcohol (ethanol) and its metabolites on innate and adaptive immunity is well-recognized. Much attention has recently been focused on the impact of acute and chronic alcohol exposure on antigen-presenting cells (APC). In particular, insights have been gained into how the properties of human blood monocytes and rodent macrophages are influenced by alcohol in vitro and in vivo. Here, we review the impact of alcohol on various aspects of APC function and the underlying mechanisms, including its effects on intracellular signaling events. We also discuss new information regarding the influence of alcohol on various APC populations in the liver, a primary site of alcohol metabolism.

Hepatic stellate cell lipid droplets: a specialized lipid droplet for retinoid storage

The majority of retinoid (vitamin A and its metabolites) present in the body of a healthy vertebrate is contained within lipid droplets present in the cytoplasm of hepatic stellate cells (HSCs). Two types of lipid droplets have been identified through histological analysis of HSCs within the liver: smaller droplets bounded by a unit membrane and larger membrane-free droplets. Dietary retinoid intake but not triglyceride intake markedly influences the number and size of HSC lipid droplets. The lipids present in rat HSC lipid droplets include retinyl ester, triglyceride, cholesteryl ester, cholesterol, phospholipids and free fatty acids. Retinyl ester and triglyceride are present at similar concentrations, and together these two classes of lipid account for approximately three-quarters of the total lipid in HSC lipid droplets. Both adipocyte-differentiation related protein and TIP47 have been identified by immunohistochemical analysis to be present in HSC lipid droplets. Lecithin:retinol acyltransferase (LRAT), an enzyme responsible for all retinyl ester synthesis within the liver, is required for HSC lipid droplet formation, since Lrat-deficient mice completely lack HSC lipid droplets. When HSCs become activated in response to hepatic injury, the lipid droplets and their retinoid contents are rapidly lost. Although loss of HSC lipid droplets is a hallmark of developing liver disease, it is not known whether this contributes to disease development or occurs simply as a consequence of disease progression. Collectively, the available information suggests that HSC lipid droplets are specialized organelles for hepatic retinoid storage and that loss of HSC lipid droplets may contribute to the development of hepatic disease.

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.

Immune interactions in hepatic fibrosis

Liver cirrhosis is caused by iterative cycles of tissue injury, inflammation, and repair. Although most causes of acute hepatitis resolve without scarring, chronic hepatitis is associated with persistent inflammation and matrix remodeling, which leads to fibrosis and, eventually, cirrhosis. The mechanisms that govern wound healing involve interactions between the innate and adaptive immune systems and stromal cells within a microenvironment composed of cytokines, growth factors, and modified matricellular proteins. The immune system plays a central role in the regulation of fibrosis, tissue repair, and recovery that is vital for the maintenance of tissue homeostasis. Chronic inflammation and fibrosis are inextricably linked and the cellular interactions between immune effector cells, local fibroblasts, and tissue macrophages at sites of scar formation determine the outcome of liver injury and the development of scarring.

Redox mechanisms in hepatic chronic wound healing and fibrogenesis

Reactive oxygen species (ROS) generated within cells or, more generally, in a tissue environment, may easily turn into a source of cell and tissue injury. Aerobic organisms have developed evolutionarily conserved mechanisms and strategies to carefully control the generation of ROS and other oxidative stress-related radical or non-radical reactive intermediates (that is, to maintain redox homeostasis), as well as to 'make use' of these molecules under physiological conditions as tools to modulate signal transduction, gene expression and cellular functional responses (that is, redox signalling). However, a derangement in redox homeostasis, resulting in sustained levels of oxidative stress and related mediators, can play a significant role in the pathogenesis of major human diseases characterized by chronic inflammation, chronic activation of wound healing and tissue fibrogenesis. This review has been designed to first offer a critical introduction to current knowledge in the field of redox research in order to introduce readers to the complexity of redox signalling and redox homeostasis. This will include ready-to-use key information and concepts on ROS, free radicals and oxidative stress-related reactive intermediates and reactions, sources of ROS in mammalian cells and tissues, antioxidant defences, redox sensors and, more generally, the major principles of redox signalling and redox-dependent transcriptional regulation of mammalian cells. This information will serve as a basis of knowledge to introduce the role of ROS and other oxidative stress-related intermediates in contributing to essential events, such as the induction of cell death, the perpetuation of chronic inflammatory responses, fibrogenesis and much more, with a major focus on hepatic chronic wound healing and liver fibrogenesis.