A murine model of type 2 autoimmune hepatitis: Xenoimmunization with human antigens

Autoimmune hepatitis. Part A: pathogenesis

Autoimmune hepatitis is a consequence of a triggering antigen and genetic factors that favor the presentation of autoantigens, polymorphisms that affect immunocyte activation and durability, cytokine alterations that promote proliferation of liver-infiltrating cytotoxic T cells, and perturbations in the number and function of immune-regulatory cell populations, including T regulatory cells and natural killer T cells. The triggering epitope is probably a short sequence peptide that is common in multiple infectious or toxic agents. Homologies between this epitope and self-antigens (molecular mimicry) may stimulate humoral and cellular responses that are cross-reactive. Sensitized immunocytes extend and perpetuate the inflammation through imprecise targeting of self-antigens that resemble foreign antigens (promiscuous behavior). The occurrence and clinical phenotype of the disease may relate to genetic susceptibility factors that favor protracted exposure to indigenous etiological agents, and these genetic factors can vary in different geographical regions and ethnic groups. The clinical phenotype within a population can be modified further by genetic polymorphisms that are not disease specific and that affect immunocyte activation, differentiation, proliferation and programmed death (apoptosis). Autoimmune hepatitis is a model of autoreactivity that reflects multiple disturbances in the counter-regulatory mechanisms essential for immune homeostasis.

Autoimmune diseases of the liver, autoimmune hepatitis and primary biliary cirrhosis: Unfinished business

Autoimmune liver diseases (AILD) including autoimmune hepatitis (AIH) and primary biliary cirrhosis (PBC) have attracted much attention since their discovery 50 years ago, but there remain items of unfinished business. These relate to disease susceptibility including genetic influences (HLA and non-HLA genes, genes associated with female predisposition, and others) and environmental influences (infections, chemicals, xenobiotics and medications). Also needed is better characterization of autoantigenic molecules, particularly the anti-F-actin specificity characteristic of AIH, shown here to have functional effects in vitro. Deeper analysis of T-lymphocyte function in AILD should reveal relative contributions of eachof the multiple subsets of T cells now being defined in studies on laboratory animals, CD4(+), CD8(+), Th1, Th2, Th17, memory subsets and regulatory subsets. Diagnostic immunology providers now offer high-performance assay formats that call for systematic clinical assessments to achieve standardization, calibration and optimal information.

Detection of auto-antibodies against cytochrome P4502E1 (CYP2E1) in chronic hepatitis C

BACKGROUND/AIMS

Chronic hepatitis C (CHC) is often associated with auto-immune reactions. In the light of the role of alcohol in promoting CHC progression, we have investigated the possible presence of auto-reactivity against the ethanol-inducible cytochrome P4502E1 (CYP2E1) in CHC patients with and without alcohol consumption.

METHODS

The IgG reactivity against recombinant human CYP2E1 was evaluated by solid-phase immunoassays in 102 CHC patients with different alcohol consumption and 59 HCV-free controls.

RESULTS

Auto-antibodies against CYP2E1 were significantly (p<0.0001) increased in CHC patients as compared to controls. Anti-CYP2E1 IgG above the 97th percentile in the controls were evident in 41 (40%) CHC patients. Competition experiments revealed that CYP2E1 recognition was not due to the cross-reactivity with CYP2D6. The detection of anti-CYP2E1 IgG was unrelated to alcohol consumption and no difference in gender, age, aminotransferase levels and virus genotype was evident among the patients with or without anti-CYP2E1 auto-antibodies. However, anti-CYP2E1 auto-reactivity was significantly (p=0.025) associated with the severity of periportal/periseptal interface hepatitis. Moreover, confocal microscopy demonstrated that anti-CYP2E1 IgG associated with CHC recognized CYP2E1 exposed on the outer side of hepatocyte plasma membranes.

CONCLUSIONS

HCV infection favours the breaking of self-tolerance against CYP2E1 that might contribute to hepatocyte injury.

Animal models for autoimmune hepatitis

The liver is the target of adverse immune reactions in three putative autoimmune diseases: autoimmune hepatitis (AIH), primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC). These three diseases can be distinguished by clinical, histological, and immunological features. However, especially on the level of specific antibody formation an overlap can occur, which sometimes complicates diagnosis. In this review, we will concentrate on autoimmune hepatitis and the current state of animal models for this severe disease. AIH is characterized by the presence of interface hepatitis and portal plasma cell infiltration, hypergammaglobulinemia, and autoantibodies. The hallmark of type 2 AIH is the generation of autoantibodies of the LKM-1 type. The major target of these antibodies is the cytochrome P450 isoform 2D6 (CYP2D6). In the past several attempts have been made to develop a reliable animal model that reflects the persistent hepatic destruction that occurs in human AIH. However, most models were only successful in causing a transient form of hepatic damage and often used rather complex ways of disease induction.

Pathogenesis of autoimmune hepatitis: from break of tolerance to immune-mediated hepatocyte apoptosis

Understanding the pathogenesis and progression of autoimmune hepatitis (AIH) at the molecular level could prove essential in developing new preventive and therapeutic strategies. Recently developed murine models have enabled the identification of various mechanisms involved in the development and perpetuation of this autoimmune disorder. Studies on these models have shown that a peripheral break of tolerance against liver-expressed antigens is sufficient to induce an autoimmune liver disease, which can occur without prior liver damage. Recent data have also shown that the liver selectively recruits and induces the apoptosis of activated CD8+ T cells after an immune response. This process of T-cell trapping involves the expression of specific chemokines and adhesion molecules, and these molecules are believed to play an important role in the initiation and perpetuation of autoimmune hepatitis. Hepatocyte apoptosis, induced by autoreactive T cells, follows specific pathways that could be targeted by new therapeutic agents. Basic research on the break of immune tolerance against liver antigens would be beneficial for patients with autoimmune hepatitis, as well as those suffering from other chronic inflammatory liver diseases, such as primary biliary cirrhosis and graft-versus-host diseases.

Autoimmune hepatitis and primary sclerosing cholangitis

Autoimmune liver disease in children presents predominantly as autoimmune hepatitis (AIH) and primary sclerosing cholangitis (PSC). These diagnoses must be considered in patients who have acute and chronic hepatitis, particularly when an extrahepatic autoimmune disorder is present. In AIH, the timely and sustained control of liver inflammation is critical to improve the short- and long-term outcomes. No effective treatment for PSC has been identified to date, but supportive care, careful attention to complications and associated nonhepatic diseases, and liver transplantation significantly improve the long-term outcome.

Autoimmune hepatitis, from mechanisms to therapy

In 1950, Waldenström was the first to describe a chronic form of hepatitis in young women. Subsequently, the disease was found to be associated with other autoimmune syndromes and was later termed "lupoid hepatitis" because of the presence of antinuclear antibodies. In 1965, it became designated by Mackay et al. as "autoimmune hepatitis" at an international meeting, at which the general concept of autoimmunity was endorsed by the scientific community. In the early 1960s and 1970s, the value of immunosuppressive therapy with glucocorticoids and/or azathioprine was well documented in several studies. The original association of autoimmune hepatitis (AIH) and HLA alleles, which has remarkably stood the test of time, was published in 1972. In the 1970s and 1980s, several autoantibodies were identified in patients with autoimmune hepatitis directed against proteins of the endoplasmatic reticulum expressed in liver and kidney and against soluble liver antigens. Subsequently, the molecular targets of these antibodies were identified and more precisely characterized. In the last two decades many additional pieces of the AIH puzzle have been collected leading to the identification of additional antibodies and genes associated with AIH and to the emergence of new therapeutic agents. Meanwhile, the immunoserological and genetic heterogeneity of AIH is well established and it has become obvious that clinical manifestations, disease behavior, and treatment outcome may vary by racial groups, geographical regions and genetic predisposition. Currently, the International Autoimmune hepatitis group is endorsing multi-center collaborative studies to more precisely define the features at disease presentation and to define prognostic indices and appropriate treatment algorithms. Given the importance of serological testing, the IAHG is also working on guidelines and procedures for more reliable and standardized testing of autoantibodies.

Type 2 autoimmune hepatitis murine model: the influence of genetic background in disease development

Genetic predisposition is recognized as an important factor for the development of autoimmune hepatitis (AIH). To assess the potential contribution of MHC and non-MHC genes, type 2 AIH was reproduced in three mice strains, taking advantage of their different genetic makeup with regard to MHC and non-MHC genes. Mice (C57BL/6, 129/Sv and BALB/c) were DNA vaccinated with a pCMV-CTLA4-CYP2D6-FTCD plasmid coding for the extracellular region of CTLA-4 and for the antigenic region of the CYP2D6 and FTCD, and with pCMV-IL12. ALT and total IgG levels, liver histology, FACS analysis and liver T-cell cytotoxicity assays were monitored up to 8 months post-injection. C57BL/6 mice showed elevated serum ALT levels, autoantibodies, antigen-specific cytotoxic T-cells and lobular and periportal inflammatory infiltrate. The 129/Sv mice showed slightly elevated ALT levels, sparse liver lobular infiltrate and cytotoxic T-cells. The BALB/c mice showed no liver inflammation. All mice had elevated total serum IgG levels. This murine model of type 2 AIH shows that MHC and non-MHC genes contribute to the susceptibility to autoimmune hepatitis. The understanding of the genetic determinants implicated in AIH development will be a major advance in the study of its pathogenesis and could lead to a better diagnostic approach and preventive strategies.

Autoimmune liver disease

PURPOSE OF REVIEW

This review aims to demonstrate how recent insights into disease behavior and mechanisms and the availability of new drugs can be assimilated into evolving concepts of diagnosis and treatment.

RECENT FINDINGS

Autoimmune hepatitis has a global distribution, and its clinical manifestations are similar in different regions. Concurrent immune diseases are common, and immunoglobulin A nephropathy may explain ascites in some patients. Subclinical celiac disease can cause cryptic liver dysfunction or be associated with autoimmune hepatitis. A fulminant presentation that is associated with de novo rather than exacerbated pre-existent disease is possible, and these patients may have centrilobular zone 3 necrosis. Bile duct injury as a background histologic finding should not change the diagnosis or therapy, and the clinical significance of autoantibodies can be determined only by examination of liver biopsy tissue. Molecular mimicry may be important in breaking self-tolerance, and a murine model based on DNA immunization with self-antigens supports this hypothesis. Corticosteroid therapy reduces or prevents hepatic fibrosis, and noninvasive techniques promise to facilitate the development of treatments that enhance this effect. Mycophenolate mofetil is a possible salvage therapy that requires clinical trial, and liver transplantation has a 5-year patient survival of 78% but is frequently followed by recurrent disease and acute rejection.

SUMMARY

Autoimmune hepatitis should be considered in all patients with acute or chronic liver disease. Therapy can reduce or prevent fibrosis. The new immunosuppressive drugs should undergo clinical trial before empiric use. Animal models promise to identify key pathogenic sites that can be targeted by therapies.