Fulminant hepatic failure in murine hepatitis virus strain 3 infection: tissue-specific expression of a novel fgl2 prothrombinase

FGL1 and FGL2: emerging regulators of liver health and disease

Liver disease is a complex group of diseases with high morbidity and mortality rates, emerging as a major global health concern. Recent studies have highlighted the involvement of fibrinogen-like proteins, specifically fibrinogen-like protein 1 (FGL1) and fibrinogen-like protein 2 (FGL2), in the regulation of various liver diseases. FGL1 plays a crucial role in promoting hepatocyte growth, regulating lipid metabolism, and influencing the tumor microenvironment (TME), contributing significantly to liver repair, non-alcoholic fatty liver disease (NAFLD), and liver cancer. On the other hand, FGL2 is a multifunctional protein known for its role in modulating prothrombin activity and inducing immune tolerance, impacting viral hepatitis, liver fibrosis, hepatocellular carcinoma (HCC), and liver transplantation. Understanding the functions and mechanisms of fibrinogen-like proteins is essential for the development of effective therapeutic approaches for liver diseases. Additionally, FGL1 has demonstrated potential as a disease biomarker in radiation and drug-induced liver injury as well as HCC, while FGL2 shows promise as a biomarker in viral hepatitis and liver transplantation. The expression levels of these molecules offer exciting prospects for disease assessment. This review provides an overview of the structure and roles of FGL1 and FGL2 in different liver conditions, emphasizing the intricate molecular regulatory processes and advancements in targeted therapies. Furthermore, it explores the potential benefits and challenges of targeting FGL1 and FGL2 for liver disease treatment and the prospects of fibrinogen-like proteins as biomarkers for liver disease, offering insights for future research in this field.

Fibrinogen-like protein 2 in inflammatory diseases: A future therapeutic target

Fibrinogen-like protein 2 (FGL2), a member of the fibrinogen family, exists as a membrane-bound protein with immune-associated coagulation activity and a soluble form possessing immunosuppressive functions. The immunomodulatory role of FGL2 is evident in fibrin deposition-associated inflammatory diseases and cancer, suggesting that FGL2 expression could be exploited as a disease biomarker and a therapeutic target. Recently, in vitro studies and knockout and transgenic animal FGL2 models have been used by us and others to reveal the involvement of FGL2 in the pathogenesis of various inflammatory diseases. This review summarizes our current knowledge of the immunomodulatory role of FGL2 in inflammatory diseases and examines the role of FGL2 as a potential therapeutic target.

The distinctive role of membrane fibrinogen-like protein 2 in the liver stage of rodent malaria infections

Viral infection often induce the expression of murine fibrinogen-like protein 2 (mFGL2) triggering immune coagulation, which causes severe liver pathogenesis via increased fibrin deposition and thrombosis in the microvasculature. We aimed to investigate the role of mFGL2 in the liver stage of malaria infections. We reveal that infection with malaria sporozoites also induces increased expression of mFGL2 and that this expression is primarily located within the liver Kupffer and endothelial cells. In addition, we report that inhibition of FGL2 has no significant effect on immune coagulation but increases the expression of inflammatory cytokines in the livers of infected mice. Interestingly, FGL2 deficiency had no significant impact on the development of liver stage malaria parasites or the pathogenesis of the infected liver. In contrast to viral infections, we conclude that mFGL2 does not contribute to either parasite development or liver pathology during these infections, revealing the unique features of this protein in liver-stage malaria infections.

Fibrinogen-like protein 2 deficiency inhibits virus-induced fulminant hepatitis through abrogating inflammatory macrophage activation

BACKGROUND

Heterogeneous macrophages play an important role in multiple liver diseases, including viral fulminant hepatitis (VFH). Fibrinogen-like protein 2 (FGL2) is expressed on macrophages and regulates VFH pathogenesis; however, the underlying mechanism remains unclear.

AIM

To explore how FGL2 regulates macrophage function and subsequent liver injury during VFH.

METHODS

Murine hepatitis virus strain 3 (MHV-3) was used to induce VFH in FGL2-deficient (Fgl2-/-) and wild-type (WT) mice. The dynamic constitution of hepatic macrophages was examined. Adoptive transfer of Fgl2-/- or WT bone marrow-derived macrophages (BMDMs) into WT recipients with macrophages depleted prior to infection was carried out and the consequent degree of liver damage was compared. The signaling cascades that may be regulated by FGL2 were detected in macrophages.

RESULTS

Following MHV-3 infection, hepatic macrophages were largely replenished by proinflammatory monocyte-derived macrophages (MoMFs), which expressed high levels of FGL2. In Fgl2-/- mice, the number of infiltrating inflammatory MoMFs was reduced compared with that in WT mice after viral infection. Macrophage depletion ameliorated liver damage in WT mice and further alleviated liver damage in Fgl2-/- mice. Adoptive transfer of Fgl2-/- BMDMs into macrophage-removed recipients significantly reduced the degree of liver damage. Inhibition of monocyte infiltration also significantly ameliorated liver damage. Functionally, Fgl2 deletion impaired macrophage phagocytosis and the antigen presentation potential and attenuated the proinflammatory phenotype. At the molecular level, FGL2 deficiency impaired IRF3, IRF7, and p38 phosphorylation, along with NF-κB activation in BMDMs in response to viral infection.

CONCLUSION

Infiltrated MoMFs represent a major source of hepatic inflammation during VFH progression, and FGL2 expression on MoMFs maintains the proinflammatory phenotype via p38-dependent positive feedback, contributing to VFH pathogenesis.

Fibrinogen-related protein, FGL2, of hamster cauda epididymal fluid: Purification, kinetic analysis of its prothrombinase activity, and its role in segregation of nonviable spermatozoa

Although the epididymal environment promotes the maturation and survival of spermatozoa, not all spermatozoa remain viable during passage through the epididymis. Does the epididymis has a protective mechanism(s) to segregate the viable sperm from defective spermatozoa? Previously, we identified 260/280 kDa oligomers (termed eFGL-Epididymal Fibrinogen-Like oligomer) are composed of two disulfide-linked subunits: a 64 kDa polypeptide identified as fibrinogen-like protein-2 (FGL2) and a 33 kDa polypeptide identified as fibrinogen-like protein-1 (FGL1). Our morphological studies demonstrated that the eFGL, secreted from the principal cells of the cauda epididymis, is polymerized into a death cocoon-like complex (DCF), masking defective luminal spermatozoa but, not the viable sperm population. In the present study, we purified FGL2 from hamster cauda epididymal fluid toward homogeneity and its prothrombinase catalytic activity was examined. Time-course conversion studies revealed that all prothrombin was converted to thrombin by purified hamster FGL2. Our biochemical studies demonstrate that FGL2 is a lipid-activated serine protease and functions as a lectin by binding specific carbohydrate residues. Co-immunoprecipitation analysis demonstrated that FGL2 of cauda epididymal fluid is ubiquitinated but not the FGL1. We propose that FGL2/FGL1 oligomers represent a novel and unique mechanism to shield the viable sperm population from degenerating spermatozoa contained within the tubule lumen.

Of Mice and Men: The Coronavirus MHV and Mouse Models as a Translational Approach to Understand SARS-CoV-2

The fatal acute respiratory coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Since COVID-19 was declared a pandemic by the World Health Organization in March 2020, infection and mortality rates have been rising steadily worldwide. The lack of a vaccine, as well as preventive and therapeutic strategies, emphasize the need to develop new strategies to mitigate SARS-CoV-2 transmission and pathogenesis. Since mouse hepatitis virus (MHV), severe acute respiratory syndrome coronavirus (SARS-CoV), and SARS-CoV-2 share a common genus, lessons learnt from MHV and SARS-CoV could offer mechanistic insights into SARS-CoV-2. This review provides a comprehensive review of MHV in mice and SARS-CoV-2 in humans, thereby highlighting further translational avenues in the development of innovative strategies in controlling the detrimental course of SARS-CoV-2. Specifically, we have focused on various aspects, including host species, organotropism, transmission, clinical disease, pathogenesis, control and therapy, MHV as a model for SARS-CoV and SARS-CoV-2 as well as mouse models for infection with SARS-CoV and SARS-CoV-2. While MHV in mice and SARS-CoV-2 in humans share various similarities, there are also differences that need to be addressed when studying murine models. Translational approaches, such as humanized mouse models are pivotal in studying the clinical course and pathology observed in COVID-19 patients. Lessons from prior murine studies on coronavirus, coupled with novel murine models could offer new promising avenues for treatment of COVID-19.

Fibrinogen-like protein 2 deficiency aggravates renal fibrosis by facilitating macrophage polarization

Renal fibrosis has no effective target for its prevention or reversal. Fibinogen-like protein 2 (Fgl2) is a novel prothrombinase exhibiting coagulation activity and immunomodulatory effects. Although Fgl2 is known to play a vital role in the development of liver and interstitial fibrosis, its function in renal fibrosis remains unclear. In this study, Fgl2 expression was found to be markedly increased in kidney tissues from mice with unilateral ureteral obstruction (UUO)-induced renal fibrosis and patients with chronic kidney disease. However, Fgl2 deficiency aggravated UUO-induced renal fibrosis, as evidenced by the significantly increasing collagen I, fibronectin, and α-SMA expression, extracellular matrix deposition, and profibrotic factor (TGF-β1) secretion. Administration of rmFgl2 (recombinant mouse Fgl2) significantly alleviated UUO-induced renal fibrosis in mice, suggesting that the increased fibrosis can be reversed by supplementing rmFgl2. Although there was no difference in the percentages of total macrophages between Fgl2^+/+ and Fgl2^-/- mice, Fgl2 deficiency remarkably facilitated M2 macrophage polarization and accelerated M1 macrophage polarization to a low degree, during UUO-induced renal fibrosis development in mice. Similar results were observed when Fgl2^+/+ and Fgl2^-/- mice bone marrow-derived macrophages were treated for M1 or M2 polarization. Moreover, Fgl2 deficiency significantly increased the phosphorylation of STAT6, a critical mediator of M2 polarization, in both UUO-induced fibrotic kidney tissues and bone marrow-derived M2 macrophages. In conclusion, the aggravation of renal fibrosis by Fgl2 deficiency is facilitated by the p-STAT6-dependent upregulation of macrophage polarization, especially of M2.

Fibrinogen-like protein 2 aggravates nonalcoholic steatohepatitis via interaction with TLR4, eliciting inflammation in macrophages and inducing hepatic lipid metabolism disorder

Rationale: The functions of fibrinogen-like protein 2 (fgl2) have been studied in many inflammatory and neoplastic diseases, but the role of fgl2 in nonalcoholic fatty liver disease has not yet been elucidated. In this study, we sought to investigate the role of fgl2 in the pathogenesis of nonalcoholic steatohepatitis (NASH). Methods: Hepatic fgl2 expression was tested in patients with nonalcoholic fatty liver (NAFL) or NASH and controls. Wild-type and fgl2-/- C57BL/6 mice were subjected to a methionine/choline-deficient (MCD) diet or a high-fat diet (HFD) to establish NASH models. Bone marrow-derived macrophages (BMDMs) stimulated with LPS or free fatty acids were used for the in vitro study. Results: In both humans and mice with NASH, macrophage accumulation was concomitant with significantly increased fgl2 expression in the liver. Fgl2 deficiency attenuated liver steatosis and inflammation in diet-induced murine models of NASH. In both liver tissues and BMDMs from NASH mice, fgl2 deficiency resulted in reduced levels of proinflammatory cytokines and reactive oxygen species (ROS) compared with levels in wild-type controls. Activation of NF-κB, p38-MAPK and NLRP3 inflammasomes was also suppressed upon fgl2 disruption. Moreover, lipogenic genes (Fasn and SREBP-2) were downregulated while lipolytic genes (PPAR and CPT1A) were upregulated in the livers of fgl2-/- NASH mice. Primary hepatocytes incubated with the medium collected from fgl2-/- BMDMs showed less fat deposition than those incubated with WT BMDMs. Furthermore, we discovered that fgl2 combined with TLR4 mediates the activation of the Myd88-dependent signaling pathway, which may contribute to inflammation and lipid metabolism disorders. Conclusions: These data suggest that fgl2 aggravates the progression of NASH through activation of NF-κB, p38-MAPK and NLRP3 inflammasomes in macrophages, which consequently induces overproduction of proinflammatory cytokines and lipid metabolism disorders. An interaction of fgl2 and TLR4 may in part contribute to the activation of inflammatory signaling pathways in macrophages.

The Role of Fibrinogen-Like Protein 2 on Immunosuppression and Malignant Progression in Glioma

BACKGROUND

Virtually all low-grade gliomas (LGGs) will progress to high-grade gliomas (HGGs), including glioblastoma, the most common malignant primary brain tumor in adults. A key regulator of immunosuppression, fibrinogen-like protein 2 (FGL2), may play an important role in the malignant transformation of LGG to HGG. We sought to determine the mechanism of FGL2 on tumor progression and to show that inhibiting FGL2 expression had a therapeutic effect.

METHODS

We analyzed human gliomas that had progressed from low- to high-grade for FGL2 expression. We modeled FGL2 overexpression in an immunocompetent genetically engineered mouse model to determine its effect on tumor progression. Tumors and their associated microenvironments were analyzed for their immune cell infiltration. Mice were treated with an FGL2 antibody to determine a therapeutic effect. Statistical tests were two-sided.

RESULTS

We identified increased expression of FGL2 in surgically resected tumors that progressed from low to high grade (n = 10). The Cancer Genome Atlas data showed that LGG cases with overexpression of FGL2 (n = 195) had statistically significantly shorter survival (median = 62.9 months) compared with cases with low expression (n = 325, median = 94.4 months, P < .001). In a murine glioma model, HGGs induced with FGL2 exhibited a mesenchymal phenotype and increased CD4+ forkhead box P3 (FoxP3)+ Treg cells, implicating immunosuppression as a mechanism for tumor progression. Macrophages in these tumors were skewed toward the immunosuppressive M2 phenotype. Depletion of Treg cells with anti-FGL2 statistically significantly prolonged survival in mice compared with controls (n = 11 per group, median survival = 90 days vs 62 days, P = .004), shifted the phenotype from mesenchymal HGG to proneural LGG, and decreased M2 macrophage skewing.

CONCLUSIONS

FGL2 facilitates glioma progression from low to high grade. Suppressing FGL2 expression holds therapeutic promise for halting malignant transformation in glioma.

Blood-stage malaria parasites manipulate host innate immune responses through the induction of sFGL2

Malaria parasites suppress host immune responses to facilitate their survival, but the underlying mechanism remains elusive. Here, we found that blood-stage malaria parasites predominantly induced CD4⁺Foxp3⁺CD25⁺ regulatory T cells to release soluble fibrinogen-like protein 2 (sFGL2), which substantially enhanced the infection. This was attributed to the capacity of sFGL2 to inhibit macrophages from releasing monocyte chemoattractant protein-1 (MCP-1) and to sequentially reduce the recruitment of natural killer/natural killer T cells to the spleen and the production of interferon-γ. sFGL2 inhibited c-Jun N-terminal kinase phosphorylation in the Toll-like receptor 2 signaling pathway of macrophages dependent on FcγRIIB receptor to release MCP-1. Notably, sFGL2 were markedly elevated in the sera of patients with malaria, and recombinant FGL2 substantially suppressed Plasmodium falciparum from inducing macrophages to release MCP-1. Therefore, we highlight a previously unrecognized immune suppression strategy of malaria parasites and uncover the fundamental mechanism of sFGL2 to suppress host innate immune responses.

Interferon Control of Neurotropic Viral Infections

Interferons (IFNs) comprise a pleiotropic family of signaling molecules that are often the first line of defense against viral infection. Inflammatory responses induced by IFN are generally well tolerated during peripheral infections; yet, the same degree of inflammation during infection of the central nervous system (CNS) could be catastrophic. Thus, IFN responses must be modified within the CNS to ensure host survival. In this review, we discuss emerging principles highlighting unique aspects of antiviral effects of IFN protection following neurotropic viral infection, chiefly using new techniques in rodent models. Evaluation of these unique responses provides insights into how the immune system eradicates or controls pathogens, while avoiding host damage. Defining these principles may have direct impact on the development of novel clinical approaches.

Clara Cell 10 kDa Protein Alleviates Murine Hepatitis Virus Strain 3-Induced Fulminant Hepatitis by Inhibiting Fibrinogen-Like Protein 2 Expression

Background: Fulminant hepatitis (FH) is a serious threat to human life, accompanied by massive and rapid necroinflammation. Kupffer cells, the major immune cell population involved in innate immune responses, are considered to be central for FH. Fibrinogen-like protein 2 (Fgl2) is a pro-coagulant protein that is substantially induced in macrophages upon viral infection, and Fgl2 depletion represses murine hepatitis virus strain 3 (MHV-3) infection. Clara cell 10 kDa (CC10) protein is a secretory protein with anti-inflammatory properties in allergic rhinitis and asthma. However, its mechanisms of action and pathogenic roles in other disease are still unclear. In this study, we aimed to determine the role of CC10 in FH and the regulation of Fgl2 by CC10.

Methods: A mouse FH model was established by peritoneal injection of MHV-3. The mice received CC10 protein through tail vein injection before viral infection. Survival rate, liver function, liver histology, fibrin deposition, and necrosis were examined. The regulatory effect of CC10 on Fgl2 expression was investigated using THP-1 cells and mouse peritoneal macrophages in vitro.

Results: In the mouse FH model induced by MHV-3, the survival rate increased from 0 to 12.5% in the CC10 group compared to that in the saline-only control group. Meanwhile, the levels of ALT and AST in serum were significantly decreased and liver damage was reduced. Furthermore, hepatic Fgl2, TNF-α, and IL-1β expression was obviously downregulated together with fibrin deposition, and hepatocyte apoptosis was reduced after administration of CC10 protein. In vitro, CC10 was found to significantly inhibit the expression of Fgl2 in IFN-γ-treated THP-1 cells and MHV-3-infected mouse peritoneal macrophages by western blot and real-time PCR. However, there was no direct interaction between CC10 and Fgl2 as shown by co-immunoprecipitation. Microarray investigations suggested that HMG-box transcription factor 1 (HBP1) was significantly low in CC10-treated and IFN-γ-primed THP-1 cells. HBP1-siRNA treatment abrogated the inhibitory effect of CC10 on Fgl2 expression in Human Umbilical Vein Endothelial cells (HUVECs).

Conclusion:CC10 protects against MHV-3-induced FH via suppression of Fgl2 expression in macrophages. Such effects may be mediated by the transcription factor HBP1.

Gene expression profiles of rat MMECs with different glucose levels and fgl2 gene silencing

BACKGROUND

Cardiac microvascular endothelial cells (MMECs) is one of the key factors in the process of diabetic cardiomyopathy, a common chronic complication of diabetes. Fibrinogen-like protein 2 (FGL2) is linked to apoptosis, angiogenesis, and inflammatory response, all of which also occur in diabetes. Thus, we investigate the role of FGL 2 and other genes in the pathology of diabetic cardiomyopathy.

METHODS

In the present study, we used high-throughput microarray to profile gene expression in rat myocardial MMECs with or without silencing the fgl2 gene and in different glucose environments. We use volcanic maps to isolate genes with significantly different expression levels between conditions, using the standard statistical criteria of fold changes ≥1.5 and P-values ≤0.05. From this list, we identified genes with the most signicant changes in RNA levels and confirmed their protein-level changes with Western blot. Furthermore, bioinformatic analysis predicts possible pathophysiology and clinical relevance of these proteins in diabetic cardiomyopathy.

RESULTS

We identified 17 upregulated and 15 downregulated genes caused by silencing fgl2 gene. Most of them are involved in metabolism, ion transport, cell membrane surface recognition signal modification, inflammatory response, and immune response. Using Western blot, we were able to confirm protein-level expression changes of three genes. Specifically, in both normal and high glucose conditions, silencing fgl2 significantly decreased the expression levels of CCL3 and PLAGL1 while increasing the expression level of CTSC. Significantly, bioinformatic analyses show that CCL3 is related to type 1 diabetes, PLAGL1 to cardiomyocytes, and CTSC to albuminuria in type 2 diabetes.

CONCLUSIONS

Our study provides clues for further studies on the mechanism of diabetic cardiomyopathy as well as function of FGL2 in this process, potentially offering new therapeutic strategies for treating diabetic cardiomyopathy.

Involvement of Pro-Inflammatory Macrophages in Liver Pathology of Pirital Virus-Infected Syrian Hamsters

New World arenaviruses cause fatal hemorrhagic disease in South America. Pirital virus (PIRV), a mammarenavirus hosted by Alston’s cotton rat (Sigmodon alstoni), causes a disease in Syrian golden hamsters (Mesocricetus auratus) (biosafety level-3, BSL-3) that has many pathologic similarities to the South American hemorrhagic fevers (BSL-4) and, thus, is considered among the best small-animal models for human arenavirus disease. Here, we extend in greater detail previously described clinical and pathological findings in Syrian hamsters and provide evidence for a pro-inflammatory macrophage response during PIRV infection. The liver was the principal target organ of the disease, and signs of Kupffer cell involvement were identified in mortally infected hamster histopathology data. Differential expression analysis of liver mRNA revealed signatures of the pro-inflammatory response, hematologic dysregulation, interferon pathway and other host response pathways, including 17 key transcripts that were also reported in two non-human primate (NHP) arenavirus liver-infection models, representing both Old and New World mammarenavirus infections. Although antigen presentation may differ among rodent and NHP species, key hemostatic and innate immune-response components showed expression parallels. Signatures of pro-inflammatory macrophage involvement in PIRV-infected livers included enrichment of Ifng, Nfkb2, Stat1, Irf1, Klf6, Il1b, Cxcl10, and Cxcl11 transcripts. Together, these data indicate that pro-inflammatory macrophage M1 responses likely contribute to the pathogenesis of acute PIRV infection.

IL-33 protects murine viral fulminant hepatitis by targeting coagulation hallmark protein FGL2/fibroleukin expression

Fulminant hepatitis (FH) is characterized by rapid liver failure and high mortality. The pathogenesis of viral FH includes virus-induced immune activation, inflammation, and subsequent hepatic apoptosis and necrosis. However, the mechanisms that underlie FH progression are unclear. IL-33 is a member of the IL-1-related cytokines, considered to be an "alarmin" that participates in various diseases, but its precise role in the coagulation of FH is not very clear. In our study, we found that IL-33 is significantly elevated in mice infected with murine hepatitis virus strain 3 (MHV-3). This is accompanied by an increase in pro-coagulant fibrinogen-like protein 2 (FGL2) in the liver. Previous studies have suggested that an increase in FGL2 is diagnostic of FH and liver necrosis, and animals with no FGL2 had better survivorship during FH. Our studies showed that IL-33 administration in a MHV-3 infection promoted survival during FH, with a significant reduction in FGL2 expression and liver inflammation. In vitro IL-33 treatment abrogated MHV-3 and IFN-γ induced FGL2 expression in RAW264.7 and THP-1 cells, respectively. In conclusion, our research suggests that IL-33 protects against viral fulminant hepatitis in mice by antagonizing expression of the pro-coagulant protein FGL2.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

Two fibrinogen-like proteins, FGL1 and FGL2 are disulfide-linked subunits of oligomers that specifically bind nonviable spermatozoa

Nevertheless, a nonviable sperm population is present in the cauda epididymidis of many species. Degenerating spermatozoa release enzymes that could have detrimental effects on the viability of neighboring cells, and they are source of autoantigens that induce an autoimmune response if they escape the blood-epididymis barrier. Does the epididymis have specialized protective mechanism(s) to segregate the viable sperm population from defective spermatozoa? Previously, we identified a fibrinogen-like protein-2 (fgl2) that specifically binds to and polymerizes into a cocoon-like complex coating defective spermatozoa and sperm fragments. The objective of the present study is to identify the subunit composition of the fgl2-containing oligomers both in the soluble and cocoon-like complex. Our proteomic studies indicate that the 260/280kDa oligomers (termed eFGL) contain two distinct disulfide-linked subunits; 64kDa fgl2 and 33kDa fgl1. Utilizing a PCR-based cloning strategy, the 33kDa polypeptide has been identified as fibrinogen-like protein-1 (fgl1). Immunocytochemical studies revealed that fgl1 selectively binds to defective spermatozoa in the cauda epididymidis. Northern blot analysis and in situ hybridization demonstrated the high expression of fgl1 in the principal cells of the proximal cauda epididymidis. Co-immunoprecipitation analyses of cauda epididymal fluid, using anti-fgl2, demonstrate that both fgl1 and fgl2 are present in the soluble eFGL. Our study is the first to show an association of fgl1 and fgl2 both in the soluble and in the sperm-associated eFGL. We conclude that our results provide new insights into the mechanisms by which the potentially unique epididymal protein functions in the recognition and elimination of defective spermatozoa.

Toll-like receptor-2 exacerbates murine acute viral hepatitis

Viral replication in the liver is generally detected by cellular endosomal Toll‐like receptors (TLRs) and cytosolic helicase sensors that trigger antiviral inflammatory responses. Recent evidence suggests that surface TLR2 may also contribute to viral detection through recognition of viral coat proteins but its role in the outcome of acute viral infection remains elusive. In this study, we examined in vivo the role of TLR2 in acute infections induced by the highly hepatotrophic mouse hepatitis virus (MHV) type 3 and weakly hepatotrophic MHV‐A59 serotype. To address this, C57BL/6 (wild‐type; WT) and TLR2 knockout (KO) groups of mice were intraperitoneally infected with MHV3 or MHV‐A59. MHV3 infection provoked a fulminant hepatitis in WT mice, characterized by early mortality and high alanine and aspartate transaminase levels, histopathological lesions and viral replication whereas infection of TLR2 KO mice was markedly less severe. MHV‐A59 provoked a comparable mild and subclinical hepatitis in WT and TLR2 KO mice. MHV3‐induced fulminant hepatitis in WT mice correlated with higher hepatic expression of interferon‐β, interleukin‐6, tumour necrosis factor‐α, CXCL1, CCL2, CXCL10 and alarmin (interleukin‐33) than in MHV‐A59‐infected WT mice and in MHV3‐infected TLR2 KO mice. Intrahepatic recruited neutrophils, natural killer cells, natural killer T cells or macrophages rapidly decreased in MHV3‐infected WT mice whereas they were sustained in MHV‐A59‐infected WT mice and MHV3‐infected TLR2 KO. MHV3 in vitro infection of macrophagic cells induced rapid and higher viral replication and/or interleukin‐6 induction in comparison to MHV‐A59, and depended on viral activation of TLR2 and p38 mitogen‐activated protein kinase. Taken together, these results support a new aggravating inflammatory role for TLR2 in MHV3‐induced acute fulminant hepatitis.

Fibrinogen-like protein 2 gene silencing inhibits cardiomyocytes apoptosis, improves heart function of streptozotocin-induced diabetes rats and the molecular mechanism involved

We assumed that Fibrinogen-like protein 2 (Fgl2) plays an important role during the process of diabetic cardiomyopathy (DCM). In the present study, we test that the feasibility of Fgl2 as a therapeutic target for the treatment of DCM and its possible molecular mechanism involved.

Fibrinogen-like protein 2 (Fgl2) is involved in apoptosis, angiogenesis and inflammatory response. Diabetes is closely associated with apoptosis, angiogenesis and coagulation. So it allowed us to assume that Fgl2 plays an important role during the process of diabetic cardiomyopathy (DCM). In the present study, we test that the feasibility of Fgl2 as a therapeutic target for the treatment of DCM and its possible molecular mechanism involved. We found that Fgl2 gene silencing inhibits apoptosis and improves heart function of streptozotocin (STZ)-induced diabetes rats, the possible mechanism maybe that Fgl2 gene silencing reduces the tumour necrosis factor (TNF)±levels, decreases the expression of B-cell lymphoma-2 (bcl2), bcl-2-associated X (bax), toll-like receptors 4 (TLR4) and p38 mitogen-activated protein kinase (MAPK). In conclusion, Fgl2 is a potent target to treat DCM.

C5aR, TNF-α, and FGL2 contribute to coagulation and complement activation in virus-induced fulminant hepatitis

BACKGROUND & AIMS

Viral fulminant hepatitis (FH) is a disease with a high mortality rate. Activation of the complement system correlates with the development of FH. However, the key factors mediating complement activation in FH remain elusive.

METHODS

Liver tissues were isolated from FH patients infected by hepatitis B virus (HBV) and from mice infected with murine hepatitis virus strain 3 (MHV-3). Wild type mice were treated with or without antagonists of C5aR or TNF-α, and mice deficient for C5aR (C5aR(-/-)), Fgl2 (Fgl2(-/-)), and Tnfα (Tnfα(-/-)) mice were not treated with the antagonists. C5b-9, C5aR, FGL2, CD31, CD11b, fibrin, TNF-α, and complement C3 cleavage products were detected by immunohistochemistry, immunofluorescence, or ELISA. Sorted liver sinusoidal endothelial cells (LSECs) or myeloid-derived (CD11b(+)) cells were stimulated with C5a, TNF-α or MHV-3 in vitro. The mRNA expressions levels of Fgl2 and Tnfα were determined by qRT-PCR analyses.

RESULTS

We observed that complement activation, coagulation and pro-inflammatory cytokine production were upregulated in the HBV(+) patients with FH. Similar observations were made in the murine FH models. Complement activation and coagulation were significantly reduced in MHV-3 infected mice in the absence of C5aR, Tnfα or Fgl2. The MHV-3 infected C5aR(-/-) mice exhibited reduced numbers of infiltrated inflammatory CD11b(+) cells and a reduced expression of TNF-α and FGL2. Moreover, C5a administration stimulated TNF-α production by CD11b(+) cells, which in turn promoted the expression of FGL2 in CD31(+) LSEC-like cells in vitro. Administration of antagonists against C5aR or TNF-α ameliorated MHV-3-induced FH.

CONCLUSIONS

Our results demonstrate that C5aR, TNF-α, and FGL2 form an integral network that contributes to coagulation and complement activation, and suggest that those are potential therapeutic targets in viral FH intervention.

The cell-membrane prothrombinase, fibrinogen-like protein 2, promotes angiogenesis and tumor development

The aim of the study was to further investigate the role of fibrinogen-like protein 2 (FGL-2), a transmembrane prothrombinase that directly cleaves prothrombin to thrombin, in angiogenesis and tumor development and the mechanism(s) underlying these processes. To study angiogenesis HUVEC clones with decreased fgl-2 mRNA were generated by specific siRNA. To study tumorigenesis SCID mice were implanted with intact (wild type) and fgl-2-silenced PC-3 clones. IFN-γ treated HUVEC expressing increased fgl-2 mRNA exhibited significant capillary sprouting that was not inhibited by hirudin, whereas fgl-2 silencing completely inhibited blood-vessel formation. Tumors (poorly differentiated carcinoma) developed in all 12 mice injected with wild type PC-3 compared with 8/12 mice injected with the fgl-2-silenced PC-3 clone. The tumors developed by fgl-2-silenced PC-3 clones were smaller and less aggressive and contained significantly fewer blood vessels (p<0.05). All tumors' sections were negative for thrombin staining, indicating that FGL-2-induced tumorigenesis was not mediated by thrombin. In fgl-2-silenced tumors there was a decrease in fgl-2 mRNA (p=0.02) and ERK1/2 phosphorylation (p<0.05) by 80% and a 20%, respectively. The mechanism underlying these processes, studied in PC-3 clones, revealed that fgl-2 silencing was associated with a 65% decrease in FGF-2 mRNA (p<0.01) and a 30% down regulation of ERK1/2 phosphorylation (p<0.05). Together, these results suggest that FGL-2 mediates angiogenesis and tumorigenesis not by thrombin-mediated mechanism but rather through FGF-2/ERK signaling pathway. FGL-2 may serve as a valuable therapeutic target in the future.

The Duality of Fgl2 - Secreted Immune Checkpoint Regulator Versus Membrane-Associated Procoagulant: Therapeutic Potential and Implications

Fibrinogen-like protein 2 (Fgl2), a member of the fibrinogen family, can be expressed as a membrane-associated protein with coagulation activity or in a secreted form possessing unique immune suppressive functions. The biological importance of Fgl2 is evident within viral-induced fibrin depositing inflammatory diseases and malignancies and provides a compelling rationale for Fgl2 expression to not only be considered as a disease biomarker but also as a therapeutic target. This article will provide a comprehensive review of the currently known biological properties of Fgl2 and clarifies future scientific directives.

C5a/C5aR pathway is essential for the pathogenesis of murine viral fulminant hepatitis by way of potentiating Fgl2/fibroleukin expression

Viral fulminant hepatitis (FH) remains a serious clinical problem with very high mortality. Lacking understanding of FH pathogenesis has in essence hindered efficient clinical treatment. Inferring from a correlation observed between the genetic differences in the complement component 5 (C5) and the susceptibility of mouse strains to murine hepatitis virus strain-3 (MHV-3) infections, we propose that excessive complement activation plays a critical role in the development of FH. We show that MHV-3 infection causes massive complement activation, along with a rapid increase in serum C5a levels and quick development of FH in susceptible strains. Mice deficient in the C5a receptor (C5aR) or the susceptible strains treated with C5aR antagonists (C5aRa) exhibit significant attenuation of the disease, accompanied by a remarkable reduction of hepatic fibrinogen-like protein 2 (Fgl2), a hallmark protein that causes necrosis of infected livers. In accordance, biopsy of FH patients shows a dramatic increase of Fgl2 expression, which correlates with C5aR up-regulation in the liver. In vitro C5a administration accelerates MHV-3-induced Fgl2 secretion by macrophages. Furthermore, inhibiting ERK1/2 and p38 efficiently blocks C5a-mediated Fgl2 production during viral infections.

CONCLUSION

These data provide evidence that mouse susceptibility to MHV-3-induced FH may rely on C5a/C5aR interactions, for which ERK1/2 and p38 pathways participate in up-regulating Fgl2 expression. Inhibition of C5a/C5aR interactions is expected to be beneficial in the clinical treatment of FH patients.

Intrahepatic microcirculatory disorder, parenchymal hypoxia and NOX4 upregulation result in zonal differences in hepatocyte apoptosis following lipopolysaccharide- and D-galactosamine-induced acute liver failure in rats

Although the mechanisms responsible for acute liver failure (ALF) have not yet been fully elucidated, studies have indicated that intrahepatic macrophage activation plays an important role in the pathogenesis of ALF through intrahepatic microcirculatory disorder and consequent parenchymal cell death. Intrahepatic microcirculatory disorder has been demonstrated in animal models using intravital microscopy; however, the limitations of this method include simultaneously evaluating blood flow and the surrounding pathological changes. Therefore, in this study, we devised a novel method involving tetramethylrhodamine isothiocyanate (TRITC)-dextran administration for the pathological assessment of hepatic microcirculation. In addition, we aimed to elucidate the mechanisms through which intrahepatic microcirculatory disorder progresses with relation to activated macrophages. ALF was induced in Wistar rats by exposure to lipopolysaccharide and D-galactosamine. Intrahepatic microcirculation and microcirculatory disorder in zone 3 (pericentral zone) of the livers of rats with ALF was observed. Immunohistochemical examinations in conjunction with TRITC-dextran images revealed that the macrophages were mainly distributed in zone 2 (intermediate zone), while cleaved caspase-3-positive hepatocytes, pimonidazole and hypoxia-inducible factor 1-α were abundant in zone 3. We also found that 4-hydroxy-2-nonenal and nicotinamide adenine dinucleotide phosphate oxidase (NOX)4-positive cells were predominantly located in the zone 3 parenchyma. The majority of apoptotic hepatocytes in zone 3 were co-localized with NOX4. Our results revealed that the apoptotic cells in zone 3 were a result of hypoxic conditions induced by intrahepatic microcirculatory disorder, and were not induced by activated macrophages. The increased levels of oxidative stress in zone 3 may contribute to the progression of hepatocyte apoptosis.

Tumor necrosis factor α (TNF-α) receptor-I is required for TNF-α-mediated fulminant virus hepatitis caused by murine hepatitis virus strain-3 infection

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TNFR1^−/− mice displayed a dramatic decrease in tissue necrosis and cell apoptosis in the MHV-3 infected spleens and livers.

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TNFR1 deficiency directly impeded FGL2 secretion as well as reduced Fas and FasL expression in MHV-3 infected livers.

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TNFR1 deficiency impeded neutrophils, which produce proinflammatory factors and FGL2 directly, infiltration into liver and spleen.

TNF-α plays an essential role in the pathogenesis of fulminant virus hepatitis (FH) caused by infection with murine hepatitis virus strain-3 (MHV-3). However, the specific TNF-α receptors (TNFR) involved in this disease and how they mediate this effect are uncertain. Here, we showed that the expression of TNFR1 and TNFR2 in the liver and spleen was triggered by MHV-3. However, only TNFR1^−/− mice were resistant to MHV-3 mediated FH, as displayed by a dramatic decrease in tissue necrosis and cell apoptosis in the infected spleens and livers from TNFR1^−/− mice, as well as prolonged survival in these mice compared to wild type littermate controls. Mechanistically, TNFR1 deficiency directly impeded the serum and tissue levels of fibrinogen-like protein 2 (FGL2), a virus-induced procoagulant molecule that promotes cell apoptosis. Additionally, the expression of apoptosis-associated molecules, Fas and Fas ligand (FasL) in the infected organs from TNFR1^−/− mice were also decreased. Moreover, the infiltration of neutrophils rather than Foxp3⁺ regulatory T cells, which produce proinflammatory factors and FGL2 directly, into the infected liver and spleen tissues was also decreased in TNFR1^−/− mice. These combined results indicate that signaling through TNFR1 plays an essential role in the pathogenesis of FH caused by MHV-3 infection, and interruption of this signaling pathway could be useful for clinical therapy.

Expression of B and T lymphocyte attenuator (BTLA) in macrophages contributes to the fulminant hepatitis caused by murine hepatitis virus strain-3

OBJECTIVES

Fulminant viral hepatitis (FH) remains a serious clinical problem for which the underlying pathogenesis remains unclear. The B and T lymphocyte attenuator (BTLA) is an immunoglobulin-domain-containing protein that has the capacity to maintain peripheral tolerance and limit immunopathological damage during immune responses. However, its precise role in FH has yet to be investigated.

DESIGN

BTLA-deficient (BTLA-/-) mice and their wild-type littermates were infected with murine hepatitis virus strain-3 (MHV-3), and the levels of tissue damage, cell apoptosis, serum liver enzymes, fibrinogen-like protein 2 (FGL2) and cytokine production were measured and compared. Survival rate was studied after MHV-3 infection with or without adoptive transferring macrophages.

RESULTS

FGL2 production, liver and spleen damage, and mortality were significantly reduced in BTLA-/- mice infected with MHV-3. This effect is due to rapid, TRAIL (TNF-related apoptosis-inducing ligand)-dependent apoptosis of MHV-3-infected macrophages in BTLA-/- mice. The early loss of macrophages resulted in reduced pathogenic tumour necrosis factor α (TNFα) and FGL2 levels and lower viral titres. The importance of TNFα in MHV-3-induced pathology was demonstrated by increased mortality in TNFα-treated MHV-3-infected BTLA-/- mice, whereas TNFα-/- mice were resistant to the infection. Moreover, adoptively transferring macrophages to BTLA-/- mice caused sensitisation, whereas blocking BTLA protected wild-type mice from virus-induced FH mortality.

CONCLUSIONS

BTLA promotes the pathogenesis of virus-induced FH by enhancing macrophage viability and function. Targeting BTLA may be a novel strategy for the treatment of FH.

Intrahepatic expression of programmed death-1 and its ligands in patients with HBV-related acute-on-chronic liver failure

Hepatitis B virus (HBV) infection is a major public health problem, and HBV-related acute-on-chronic liver failure (HBV-ACLF) has an extremely poor prognosis due to a lack of understanding of pathogenesis as well as a lack of effective treatments. Signals from the inhibitory receptor programmed death-1 (PD-1) have been demonstrated to be involved in regulating the pathogenesis of infectious diseases. However, the expression of PD-1 and its ligands in HBV-ACLF patients has yet to be evaluated. In this study, the expression of PD-1 and its ligands, PD-L1 and PD-L2, in liver biopsies from HBV-ACLF as well as chronic hepatitis B (CHB) patients were analyzed by immunohistochemistry. The results showed that all three molecules were observed in the HBV-ACLF samples and their levels were significantly higher than they were in CHB. Immunofluorescence double-staining showed that PD-1 was found on CD3(+), CD8(+) T cells, CD56(+) NK cells, CD68(+) macrophages, CK-18(+) epithelial cells, and CD16(+) monocytes. The PD-L1 expression was observed on all cell types detected and the PD-L2 was chiefly on CK-18(+) epithelial cells and CD31(+) endothelial cells. Interestingly, high levels of virus-induced procoagulant molecule fibrinogen-like protein 2 (FGL2) were observed in liver sections from HBV-ACLF, and PD-L1 and PD-L2 expression was also observed on FGL2(+) cells in these patients. Our combined results suggest that the expression of PD-L1 and PD-L2 may be biomarkers to identify and diagnose ACLF, and a clear understanding of their functional roles should further elucidate the pathogenesis of this disease.

Soluble FGL2 induced by tumor necrosis factor-α and interferon-γ in CD4+ T cells through MAPK pathway in human renal allograft acute rejection

BACKGROUND

Acute rejection (AR), initiated by alloreactive CD4(+) T cells, hampers allograft survival. Soluble fibrinogen-like protein 2 (sFGL2) is a novel effector of CD4(+) T cells. We previously found that serum sFGL2 significantly increased in renal allograft recipients with AR. In this study, sFGL2 secretion by CD4(+) T cells and its mechanism were further explored both in vivo and in vitro.

MATERIALS AND METHODS

Forty cases of living-related renal transplant recipients with biopsy-proven AR or stable renal function were collected and detected serum sFGL2, tumor necrosis factor (TNF)-α and interferon (IFN)-γ, and peripheral CD4(+) T cells. In vitro, the isolated human CD4(+) T cells were stimulated by TNF-α or IFN-γ. sFGL2 in the supernatant and mitogen-activated protein kinase (MAPK) proteins in the CD4(+) T cells were investigated. Approval for this study was obtained from the Ethics Committee of Fudan University.

RESULTS

sFGL2, TNF-α, IFN-γ, and CD4(+) T cells were significantly increased in the peripheral blood of renal allograft recipients with AR. Stimulation with 1000 U/mL TNF-α or 62.5 U/mL IFN-γ for 48 h provided an optimal condition for CD4(+) T cells to secrete sFGL2 in vitro. Phosphorylated (p-) c-Jun N-terminal kinase was remarkably upregulated in the activated CD4(+) T cells, whereas no significant changes were found in p-p38 MAPK or p-ERK1/2 expression. Furthermore, inhibition of c-Jun N-terminal kinase significantly reduced sFGL2 secretion by CD4(+) T cells.

CONCLUSIONS

sFGL2 secretion by CD4(+) T cells can be induced with TNF-α and IFN-γ stimulation through MAPK signaling in renal allograft AR. Our study suggests that sFGL2 is a potential mediator in the pathogenesis of allograft rejection.

CD4-CD8-T cells contribute to the persistence of viral hepatitis by striking a delicate balance in immune modulation

Viral hepatitis remains the most common cause of liver disease and a major public health problem. Here, we focus on the role of CD4 CD8 double negative T (DN T) cells involved in the mechanisms of viral persistence in hepatitis. C3H/HeJ mice infected with murine hepatitis virus strain 3 (MHV-3) were used to display chronic viral hepatitis. DN T cells dramatically increased in MHV-3 infected mice. Adoptive transfer of DN T cells from MHV-3 infected mice led to a significant increase in mice survival. The DN T cells with production of IFN-γ and IL-2 are able to kill virus-specific CD8⁺ T cells via the Fas/FasL dependent pathway. The delicate balance of multiple effects of DN T cells may lead to viral persistence in MHV-3 induced hepatitis. In short, our study identified DN T cells contributing to viral persistence in MHV-3 induced hepatitis in C3H/HeJ mice, which provides a rationale for modulating DN T cells for the management of viral hepatitis.

Characterization of fibrinogen-like protein 2 (FGL2): monomeric FGL2 has enhanced immunosuppressive activity in comparison to oligomeric FGL2

Fibrinogen-like protein 2 (FGL2), a novel effector molecule of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg), mediates its suppressive activity through binding to low affinity Fcγ receptors expressed on antigen presenting cells (APCs). FGL2 has been implicated in the pathogenesis of viral hepatitis, xeno- and allotransplant rejection, and rheumatoid arthritis. Here we fully analyzed the structure-function relationships of recombinant murine FGL2 generated in COS-7 cells and identified the receptor binding domains. Native FGL2 exists as an oligomer with a molecular weight of approximately 260 kDa, while under reducing conditions, FGL2 has a molecular weight of 65 kDa suggesting that native FGL2 is composed of four monomers. By site-directed mutation, cysteines at positions 94, 97, 184 and 187, found in the coiled-coil domain were shown to be crucial for FGL2 oligomerization. Monomeric FGL2 had a lower affinity binding to APCs, but increased immunosuppressive activity compared to oligomeric FGL2. Deglycosylation demonstrated that sugar moieties are critical for maintaining solubility of FGL2. SWISS-MODEL analysis suggested that FGL2 has a similar tertiary structure with other members of the fibrinogen family such as fibrinogen and tachylectin. Mutational analysis of cysteine residues and Western blots suggested an asymmetric bouquet-shaped quaternary structure for oligomeric FGL2, resembling many pattern-recognition molecules in the lectin pathway of innate immunity. The functional motifs of FGL2 were mapped to the C terminal globular domain, using a peptide blockade assay. These results collectively define the biochemical and immunological determinants of FGL2, an important immunosuppressive molecule of Treg providing important insights for designing FGL2-related therapeutics.

Inhibitory function of Tregs via soluble FGL2 in chronic hepatitis B

CD4(+)CD25(+)CD127(dim/-) regulatory T cells (Tregs) have been implicated in suppressing T cell immune responses to hepatitis B virus (HBV), but the inhibition mechanism has not being clear yet. This study investigated the effects of soluble FGL2 (sFGL2) secreted by Tregs on immune suppression in chronic HBV-infected patients. We verified that sFGL2 protein and mRNA were highly expressed in Tregs. The separated Tregs by using magnetic beads from peripheral blood mononuclear cells (PBMCs) in 20 patients with chronic hepatitis B were co-cultured with PBMCs at a ratio of 1:3 with anti-CD3 stimulating antibody or FGL2 blocking antibody. The proliferation index of CD8(+)T cells after blocking FGL2 was higher than that in blank group (3.58±0.18 vs. 3.28±0.17, P=0.034) in 18 of 20 samples, and lower than that in CD3 stimulation group (3.82±0.19, P=0.026) in 16 of 20 samples. The IFN-γ secreted in the mixed culture in the absence of Tregs was higher than that in the culture in the presence of Tregs, but it could be abolished by FGL2 blocking antibody. These results suggest that sFGL2 protein secreted by Tregs suppresses the proliferation and function of CD8(+) T cells in chronic hepatitis B.

The evolving story of macrophages in acute liver failure

Acute liver failure (ALF) remains a worldwide problem. The innate immune system acts as an important regulator of ALF. Kupffer cells (KCs), the resident macrophages in liver, play a key role in liver innate immune response. Recent researches have shown that macrophages display a remarkable plasticity and can differentiate into functionally diverse subsets. However, the dynamic polarized phenotypes and functional status of macrophages at different stage of ALF are not clear. In this paper, we present a review of evidence that KCs play a significant role in the pathogenesis of ALF, including the phenotype and functions of macrophages, signaling pathways involved in macrophage functional status and cell-crosstalks of KCs with other immune cells. More information on macrophages will promote a better understanding of the cellular molecular mechanisms of ALF and provide new insights for the development of therapeutic targets for ALF.

FGL2/fibroleukin mediates hepatic reperfusion injury by induction of sinusoidal endothelial cell and hepatocyte apoptosis in mice

BACKGROUND & AIMS

Sinusoidal endothelial cell (SEC) and hepatocyte death are early, TNF-α mediated events in ischemia and reperfusion of the liver (I/Rp). We previously reported that TNF-α induced liver injury is dependent on Fibrinogen like protein 2 (FGL2/Fibroleukin) and showed that FGL2 binding to its receptor, FcγRIIB, results in lymphocyte apoptosis. In this study we examine whether I/Rp is induced by specific binding of FGL2 to FcγRIIB expressed on SEC.

METHODS

Hepatic ischemia and reperfusion was induced in wild type (WT) mice and in mice with deletion or inhibition of FGL2 and FcRIIB. Liver injury was determined by AST release, necrosis and animal death. Apoptosis was evaluated with caspase 3 and TUNEL staining.

RESULTS

FGL2 deletion or inhibition resulted in decreased liver injury as determined by a marked reduction in both levels of AST and ALT and hepatocyte necrosis. Caspase 3 staining of SEC (12% vs. 75%) and hepatocytes (12% vs. 45%) as well as TUNEL staining of SEC (13% vs. 60%, p=0.02) and hepatocytes (18% vs. 70%, p=0.03), markers of apoptosis, were lower in Fgl2(-/-) compared to WT mice. In vitro incubation of SEC with FGL2 induced apoptosis of SEC from WT mice, but not FcγRIIB(-/-) mice. Deletion of FcγRIIB fully protected mice against SEC and hepatocyte death in vivo. Survival of mice deficient in either Fgl2(-/-) (80%) or FcγRIIB(-/-) (100%) was markedly increased compared to WT mice (10%) which were subjected to 75min of total hepatic ischemia (p=0.001).

CONCLUSIONS

FGL2 binding to the FcγRIIB receptor expressed on SEC is a critical event in the initiation of the hepatic reperfusion injury cascade through induction of SEC and hepatocyte death.

Antiviral resistance mutations potentiate HBV surface antigen-induced transcription of hfgl2 prothrombinase gene

Antiviral resistance mutations in the hepatitis B virus (HBV) polymerase (pol) gene have been demonstrated to play an important role in the progression of liver disease and the development of hepatocellular carcinoma. The HBV pol gene overlaps the S gene encoding surface antigen (HBsAg). Previous studies from our laboratory have shown that HBV core protein (HBc) and X protein (HBx), but not HBV S protein (HBs), promote hfgl2 prothrombinase transcription. To investigate whether the nucleotide (nucleoside)-induced resistant mutations of HBs potentiate transcription of hfgl2 prothrombinase gene, we generated two mutant HB expression constructs harboring rtM204V/sI195M or rtM204I/sW196L mutations. Two mutant expression plasmids were co-transfected with hfgl2 promoter luciferase-reporter plasmids and β-galactosidase plasmid in CHO cells and HepG2 cells, respectively. Luciferase assay showed that the rtM204I/V mutant HBs could activate the transcription of hfgl2 promoter compared with the wild type HBs. Site-directed mutagenesis and further experiment (co-transfection) demonstrated that transcription factor Ets translocated to its cognate cis-element in the hfgl2 promoter. The results show that mutated HBs caused by antiviral drug resistance induce transcription of the hfgl2 gene dependent on the transcription factor Ets.

Programmed death (PD)-1-deficient mice are extremely sensitive to murine hepatitis virus strain-3 (MHV-3) infection

The inhibitory receptor programmed death-1 (PD-1) has the capacity to maintain peripheral tolerance and limit immunopathological damage; however, its precise role in fulminant viral hepatitis (FH) has yet to be described. Here, we investigated the functional mechanisms of PD-1 as related to FH pathogenesis induced by the murine hepatitis virus strain-3 (MHV-3). High levels of PD-1-positive CD4⁺, CD8⁺ T cells, NK cells and macrophages were observed in liver, spleen, lymph node and thymus tissues following MHV-3 infection. PD-1-deficient mice exhibited significantly higher expression of the effector molecule which initiates fibrinogen deposition, fibrinogen-like protein 2 (FGL2), than did their wild-type (WT) littermates. As a result, more severe tissue damage was produced and mortality rates were higher. Fluorescence double-staining revealed that FGL2 and PD-1 were not co-expressed on the same cells, while quantitative RT-PCR demonstrated that higher levels of IFN-γ and TNF-α mRNA transcription occurred in PD-1-deficient mice in response to MHV-3 infection. Conversely, in vivo blockade of IFN-γ and TNF-α led to efficient inhibition of FGL2 expression, greatly attenuated the development of tissue lesions, and ultimately reduced mortality. Thus, the up-regulation of FGL2 in PD-1-deficient mice was determined to be mediated by IFN-γ and TNF-α. Taken together, our results suggest that PD-1 signaling plays an essential role in decreasing the immunopathological damage induced by MHV-3 and that manipulation of this signal might be a useful strategy for FH immunotherapy.

The principal characteristic of fulminant viral hepatitis (FH) induced by the murine hepatitis virus strain-3 (MHV-3) is severe hepatocellular necrosis, which is mediated by the fibrinogen-like protein 2 (FGL2), a molecule that has the capacity to promote fibrinogen deposition and activate the coagulation cascades. Here, we report that MHV-3 infection of program death-1 (PD-1)-deficient mice results in tissue damage throughout multiple organs, including the liver, spleen, thymus and lymph nodes. The liver damage, in particular, occurred earlier and was more severe in PD-1-deficient mice than in their wild type (WT) littermates. Further investigation determined that MHV-3 infection was associated with high levels of IFN-γ and TNF-α in the damaged organs of PD-1-deficient mice. Conversely, intraperitoneal injection of a combination of anti-IFN-γ and anti-TNF-α blocking mAbs led to inhibition of FGL2 expression, greatly attenuated tissue lesions and reduced mortality. Our results demonstrate that PD-1 signaling controls immunopathological damage following MHV-3 infection, indicating that manipulation of the PD-1 signal might represent a useful strategy for FH immunotherapy.

Dual interference with novel genes mfgl2 and mTNFR1 ameliorates murine hepatitis virus type 3-induced fulminant hepatitis in BALB/cJ mice

Our studies and those of many others have implicated hepatocyte necrosis and apoptosis mediated by fibrinogen-like protein-2 (fgl2) prothrombinase and tumor necrosis factor receptor (TNFR) in the development of fulminant viral hepatitis, a disease with a mortality rate greater than 80% in cases lacking immediate organ transplantation. This study was designed to explore the efficacy of dual short hairpin RNA (shRNA) interference with fgl2 and TNFR1 in the treatment of murine hepatitis virus strain 3 (MHV-3)-induced fulminant hepatitis in mice. Plasmids p-mfgl2shRNA and p-mTNFR1shRNA, complementary to the sequences for mfgl2 and mTNFR1, were constructed. Plasmids pEGFP-mfgl2 and pEGFP-mTNFR1 expressing mfgl2-EGFP (enhanced green fluorescent protein) and mTNFR1-EGFP fusion proteins were also constructed to screen the inhibitory effect of p-mfgl2shRNA and p-mTNFR1shRNA on mfgl2 and mTNFR1 expression. Cotransfection of individual shRNA plasmids and pcDNA3.0-mfgl2 and pcDNA3.0-mTNFR1 expression constructs into Chinese hamster ovary (CHO) cells significantly inhibited mfgl2 and mTNFR1 gene expression, as evidenced by fluorescence microscopy, reverse transcription-polymerase chain reaction, and Western blotting. In vivo hydrodynamic delivery of dual-interference shRNA plasmids for mfgl2 and mTNFR1 significantly decreased mfgl2 and mTNFR1 expression; markedly ameliorated fibrin deposition, hepatocyte necrosis, and apoptosis; and prolonged survival against fulminant viral hepatitis induced by MHV-3 in BALB/cJ mice compared with mfgl2 or TNFR1 single-gene interference. These results indicate that in vivo interference with genes for more than one key target provides superior treatment efficacy compared with single-gene interference.

Expression and significance of fgl2 prothrombinase in cardiac microvascular endothelial cells of rats with type 2 diabetes

Microthrombosis may be involved in the pathogenesis of cardiac microangiopathy due to diabetes. Recent studies have shown that fibrinogen-like protein 2 (fgl2) plays a pivotal role in microthrombosis in viral hepatitis, acute vascular xenograft rejection and cytokine-induced fetal loss syndrome. The current study was designed to examine the expression of fgl2 in microvascular endothelial cells and investigate the effects of microthrombi due to fgl2 on cardiac function and structure in rats with type 2 diabetes. Following induction of type 2 diabetes, 24 rats were observed dynamically. Fgl2 expression and related cardiac microthrombosis were examined. Local or circulating TNF-α was measured. Coronary flow (CF) per min was calculated as an index of cardiac microcirculation. Cardiac function and morphology were evaluated. It was found that Fgl2 was highly expressed in cardiac microvascular endothelial cells of rats with type 2 diabetes, which was promoted by local or circulating TNF-α. The Fgl2 expression was associated with cardiac hyaline microthrombosis. In parallel with the fgl2 expression, CF per min, cardiac diastolic or systolic function and cardiac morphology were aggravated to some extent. It was concluded that in rats with type 2 diabetes, microthrombosis due to fgl2 contributes to the impairment of cardiac diastolic or systolic function and morphological changes.

Melatonin prevents the decreased activity of antioxidant enzymes and activates nuclear erythroid 2-related factor 2 signaling in an animal model of fulminant hepatic failure of viral origin

This work was undertaken to investigate whether treatment with melatonin prevents oxidative stress and changes in the expression and activity of factor erythroid 2-related factor 2 (Nrf2)-mediated antioxidant enzymes in an animal model of fulminant hepatic failure of viral origin. Rabbits were experimentally infected with 2 x 10(4) hemagglutination units of a rabbit hemorrhagic disease virus (RHDV) isolate and received melatonin at two concentrations of 10 mg/kg and 20 mg/kg at 0, 12 and 24 hr postinfection. Blood transaminases, blood lactate dehydrogenase, liver concentration of thiobarbituric reactive acid substances and the liver oxidized to reduced glutathione ratio significantly increased at 36 hr postinfection in infected animals. Significant decreases were found in the mRNA levels and in the liver activities of Mn-superoxide dismutase, glutathione peroxidase and glutathione-S-transferase in infected rabbits. These effects were prevented by melatonin administration in a concentration-dependent manner. Melatonin treatment was not accompanied by changes in protein levels of Kelch-like ECH-associating protein 1 (Keap1) but resulted in an increased protein expression of Nrf2 in the cytoplasm and the nucleus, which was confirmed by the results of Nrf2 immunostaining. Nuclear extracts from livers of melatonin-treated rats displayed an enhanced antioxidant responsive element (ARE)-binding activity of Nrf2. Our results suggest a potential hepatoprotective role of melatonin in fulminant hepatic failure, partially mediated through the abrogation of oxidative stress and the prevention of the decreased activity of antioxidant enzymes via the Nrf2 pathways.

Telbivudine preserves T-helper 1 cytokine production and downregulates programmed death ligand 1 in a mouse model of viral hepatitis

Telbivudine is an orally bioavailable L-nucleoside with potent and specific anti-hepatitis B virus activity. The higher rate of hepatitis B e antigen (HBeAg) seroconversion during telbivudine treatment than other potent anti-HBV agents suggests a potential immunomodulatory effect. We sought to determine the effects of telbivudine on the immune system, particularly on cytokine production and T-cell response, using an animal model with mouse hepatitis virus strain 3 (MHV-3)-induced hepatitis. The effects of telbivudine on virus replication and cytokine production were investigated in vitro using MHV-3-infected macrophages, and the effects on T-cell response were investigated in vivo in an MHV-3-induced viral hepatitis model. Telbivudine had no effect on MHV-3 replication in macrophages. However, the production of tumour necrosis factor-alpha and interleukin-12 was increased significantly in MHV-3-induced macrophages treated with telbivudine. In vivo survival was enhanced in telbivudine-treated mice, with marked normalization in clinical conditions and histological lesions. Serum levels of interferon-gamma were elevated significantly after telbivudine treatment in MHV-3-infected C3H mice. In contrast, serum interleukin-4 levels were decreased significantly. Furthermore, telbivudine treatment enhanced the ability of T cells to undergo proliferation and secrete cytokines but did not affect cytotoxicity of infected hepatocytes. Of note, we found that telbivudine treatment suppressed programmed death ligand 1 expression on T cells. The results demonstrate the immunomodulatory properties of telbivudine, independent of its antiviral activity, in a mouse model of MHV-3-induced hepatitis.

An overview of animal models for investigating the pathogenesis and therapeutic strategies in acute hepatic failure

Acute hepatic failure (AHF) is a severe liver injury accompanied by hepatic encephalopathy which causes multiorgan failure with an extremely high mortality rate, even if intensive care is provided. Management of severe AHF continues to be one of the most challenging problems in clinical medicine. Liver transplantation has been shown to be the most effective therapy, but the procedure is limited by shortage of donor organs. Although a number of clinical trials testing different liver assist devices are under way, these systems alone have no significant effect on patient survival and are only regarded as a useful approach to bridge patients with AHF to liver transplantation. As a result, reproducible experimental animal models resembling the clinical conditions are still needed. The three main approaches used to create an animal model for AHF are: surgical procedures, toxic liver injury and infective procedures. Most common models are based on surgical techniques (total/partial hepatectomy, complete/transient devascularization) or the use of hepatotoxic drugs (acetaminophen, galactosamine, thioacetamide, and others), and very few satisfactory viral models are available. We have recently developed a viral model of AHF by means of the inoculation of rabbits with the virus of rabbit hemorrhagic disease. This model displays biochemical and histological characteristics, and clinical features that resemble those in human AHF. In the present article an overview is given of the most widely used animal models of AHF, and their main advantages and disadvantages are reviewed.

Targeted deletion of fgl2 leads to impaired regulatory T cell activity and development of autoimmune glomerulonephritis

Mice with targeted deletion of fibrinogen-like protein 2 (fgl2) spontaneously developed autoimmune glomerulonephritis with increasing age, as did wild-type recipients reconstituted with fgl2-/- bone marrow. These data implicate FGL2 as an important immunoregulatory molecule and led us to identify the underlying mechanisms. Deficiency of FGL2, produced by CD4+CD25+ regulatory T cells (Treg), resulted in increased T cell proliferation to lectins and alloantigens, Th 1 polarization, and increased numbers of Ab-producing B cells following immunization with T-independent Ags. Dendritic cells were more abundant in fgl2-/- mice and had increased expression of CD80 and MHCII following LPS stimulation. Treg cells were also more abundant in fgl2-/- mice, but their suppressive activity was significantly impaired. Ab to FGL2 completely inhibited Treg cell activity in vitro. FGL2 inhibited dendritic cell maturation and induced apoptosis of B cells through binding to the low-affinity FcgammaRIIB receptor. Collectively, these data suggest that FGL2 contributes to Treg cell activity and inhibits the development of autoimmune disease.

Expression of prothrombinase/fibroleukin gene fg12 in lung impairment in a murine severe acute respiratory syndrome model

To evaluate the role of murine fibrinogen like protein 2 (mfgl2) /fibroleukin in lung impairment in Severe acute respiratory syndrome (SARS), a murine SARS model induced by Murine hepatitis virus strain 3 (MHV-3) through trachea was established. Impressively, all the animals developed interstitial pneumonia with extensive hyaline membranes formation within alveoli, and presence of micro-vascular thrombosis in the pulmonary vessels. MHV-3 nucleocapsid gene transcripts were identified in multiple organs including lungs, spleen etc. As a representative proinflammatory gene, mfgl2 prothrombinase expression was evident in terminal and respiratory bronchioles, alveolar epithelia and infiltrated cells in the lungs associated with fibrin deposition and micro-vascular thrombosis. In summary, the established murine SARS model could mimic the pathologic characteristics of lungs in patients with SARS. Besides the physical damages due to virus replication in organs, the up-regulation of novel gene mfgl2 in lungs may play a vital role in the development of SARS associated lung damage.