Autoimmune hepatitis (AIH) is an a priori chronic disease of the hepatic parenchyma occurring predominantly in women, characterized by hypergammaglobulinemia, circulating autoantibodies and the morphological changes of interface hepatitis with a marked lymphoplasmacytic inflammatory infiltrate and piecemeal necrosis of mainly periportal hepatocytes.1 Although recognized in one form or another for more than 50 years and more recently successfully codified,1 it remains something of a conundrum, reflecting the dichotomy of organ-specific autoimmunity in the context of circulating non-organ-specific autoantibodies. Considerable progress has been made in the diagnosis and management of the disease, resulting in successful treatment of more than 85% of sufferers by standard therapy with corticosteroids and/or azathioprine.2 However, in a sense, hepatologists and hepatopathologists seeking greater insight into its etiopathogenesis have perhaps been victims of their own success, for the pressure to undertake basic research into the mechanisms involved has consequently been somewhat less than for other autoimmune conditions which cannot be so successfully treated.
Nonetheless, knowledge has advanced. Recognition that there is an underlying genetic predisposition to the disease is suggested by its marked female (70%–80%) preponderance, its well-established association with inheritance of the HLA A1-B8-DR3 haplotype, and the DR3, DR4 and other allotypes, as well as with polymorphisms in other immunoregulatory genes, and the high frequency of other autoimmune disorders in sufferers or their families.3, 4 Current thinking is that there must be (probably both specific and non-specific) defects in the control mechanisms that normally regulate self-tolerance and that some initiating factor (e.g., a virus or toxin) is required to trigger the disease in susceptible individuals.3, 4 Thereafter, the pathway to tissue injury is unclear. It is presumed that the target(s) of damaging autoreactions must be self antigen(s) expressed on the surfaces of hepatocytes and, to account for the organ-specificity of the disorder, that such target(s) must be liver-specific.
Initiation of an autoimmune response requires presentation of a self-peptide by an HLA class II molecule to an uncommitted helper T cell (TH0) by a professional antigen presenting cell (APC), leading to activation of the TH0 cell and its differentiation into functional phenotypes (TH1 or TH2) depending on the nature of the autopeptide, the affinity of binding between the T cell receptor and the peptide/HLA complex, and the cytokine milieu in the microenvironment (Fig. 1). It is generally assumed that hepatocytes cannot usually act as conventional APCs because they do not normally express HLA class II molecules, express class I only weakly, and lack the co-stimulatory molecules required for strong interactions with the T cell receptor. However, triggering of an inflammatory response in the liver can lead to upregulation of expression of both class I and class II on hepatocytes — raising the possibility that these cells may be able to present self-peptides to TH0 cells.3–5 Which pathway supervenes in AIH is unclear, because there is evidence that both T cell (via the TH1 pathway) and antibody-dependent (ADCC, via TH2) mechanisms may be involved and a definitive target antigen has not yet been identified.3, 4
The wide range of circulating autoantibodies in AIH has been a pivotal focus of much research to identify potential targets of tissue damage in the disease. Although aimed primarily at elucidating the pathogenic mechanisms, these studies have also provided a serological method for classifying the disease. However, the antinuclear (ANA) and smooth muscle antibodies (SMA) characteristic of type 1 AIH fulfill none of the above criteria for potential mediators of hepatocyte-specific damaging immune reactions, in that they are directed at intracellular, non-organ-specific antigens and, furthermore, these autoantibodies are not disease-specific and titres do not correlate closely with disease severity or progression. The type 1 anti-liver/kidney microsomal antibodies (anti-LKM1) associated with type 2 AIH are directed at the cytochrome isoform P4502D6. Although this is also not a liver-specific antigen, there is good evidence that it can be expressed on hepatocyte surfaces.6, 7 Most commonly detected in patients with type 2 AIH, anti-LKM1 antibodies have been described in sera from a proportion of patients with hepatitis C virus (HCV) infection. Indeed, anti-cytochrome P4502D6 antibody titers in patients with HCV infection and type 2 AIH are similar, yet, epitope mapping has shown that significant differences exist regarding the epitopes recognized by the anti-LKM1 antibodies in these two situations.8–10 Anti-LC1, is an autoantibody which reacts with the liver-specific cytosolic enzyme formiminotransferase cyclodeaminase,11 at titres that correlate with disease activity in type 2 AIH, occurring in up to 50% of patients with LKM1 positive sera.12 This antibody has also been described in 10% of patients with type 2 AIH where it represents the only detectable marker of disease, and in some patients with HCV infection.13 Antibodies reacting with a soluble liver/pancreas antigen (anti-SLA/LP) have been reported in up to 60% of patients with AIH,14 seem to be specific for the disease, and correlate with severity.14, 15 Their target has been identified as the UGA-suppressor tRNA-associated protein (tRNP(Ser)Sec) involved in co-translational selenocysteine incorporation in human cells, but this is not a liver-specific component.16, 17
Of all of the targets of autoreactions in AIH that have been studied, so far only one, the hepatocyte-specific asialoglycoprotein receptor (ASGPR), fulfills all of the previously mentioned criteria for an antigen that might be directly involved in pathogenesis of the disease. There is evidence to suggest that this receptor is expressed at high density on the surfaces of periportal hepatocytes and that anti-ASGPR antibodies can bind to cells in this area of the liver lobule.18 Previous studies have shown that AIH patients have circulating and liver-infiltrating T cells that recognize the ASGPR and can induce autologous B-lymphocytes to produce anti-ASGPR antibodies in vitro.19–23 Furthermore, there is also some evidence that AIH patients have a specific defect in controlling autoreactions to the ASGPR.19, 20 Almost all AIH patients with active disease have high titres of circulating anti-ASGPR which correlate with severity of interface hepatitis.24 However, these autoantibodies are also found in patients with other liver disorders in which similar hepatic morphological changes occur. They therefore appear to be a marker of interface hepatitis rather than of AIH per se. This, together with a number of technical problems, has led to further investigation of this interesting potential target antigen being sadly neglected.
Undoubtedly, progress in understanding the pathogenesis of AIH has been seriously hampered by the lack of a reliable animal model. Over the past 30 years there have been numerous attempts to develop the disease in rodents, including both transgenic and nontransgenic models, but a spontaneously relapsing chronic hepatitis closely resembling type 1 AIH has yet to be achieved in animals.25–27 In contrast, a plausible model of type 2 AIH has been generated in mice by DNA immunization against type 2 AIH self-antigens.28
In this issue of HEPATOLOGY, Yamauchi et al. describe yet another putative animal model of AIH.29 The work is an extension of their recent studies on the generation of a human IgM (λ-type) monoclonal antibody (MoAb) by Epstein-Barr virus (EBV) transformation of peripheral blood mononuclear cells from a Japanese patient with type 1 AIH.30 The MoAb recognises a liver-specific 190 kDa molecule expressed on the surfaces of hepatocytes and could induce complement-dependent lysis of a hepatocyte-derived (HuH2) cell line.30 In that report, they also documented the detection of circulating IgM (but not IgG) antibodies reacting with the 190 kDa antigen (affinity-purified using the MoAb) in patients with AIH but not in healthy subjects or patients with chronic hepatitis C. Furthermore, titres of the anti-190 kDa IgM antibodies correlated with serum aminotransferase levels in the AIH patients. In the present study, the authors show that the MoAb also reacts with a 190 kDa molecule on mouse hepatocytes, indicating that this target antigen is both liver-specific and species cross-reactive.29 Importantly, they further show that injection of the MoAb into mice rapidly induced massive, predominantly perivenular, complement-mediated hepatocellular necrosis. The authors conclude that the data from these two studies suggest that the IgM anti-190 kDa antibody detected in patients may have an important role in the immunopathogenesis of AIH.
Is this a valid model of AIH ? Probably not. On the one hand, as with all animal models so far developed, the mice presumably do not have the underlying genetic defect(s) in regulation of self-tolerance postulated for AIH. Furthermore, the short duration of the in vivo experiments does not provide information about possible progression to chronicity. More importantly, massive perivenular necrosis is not considered to be a feature of AIH. Indeed, if found, it would normally raise questions about the diagnosis. The authors counter this argument by suggesting that it might represent a very early stage in the evolution of AIH and cite two case series of a total of 6 patients with presumed AIH in which perivenular necrosis was documented.31, 32 There is some merit in this suggestion, because it is now generally accepted that in AIH there is often a prolonged presymptomatic phase before the disease becomes clinically manifest. Due to this latency of clinical onset, initiating events crucial to understanding pathogenesis are usually missed. However, a further problem with the model is that the hepatocellular necrosis is complement-mediated. The authors point to two previous reports of complement-mediated liver damage,33, 34 but these relate to studies in other experimental models and there has been no evidence that this occurs in AIH patients. On the contrary, the available evidence indicates that complement-mediated cytolysis is not involved in AIH, due to the frequent finding in patients of null alleles at either the C4A or C5B loci (often in linkage disequilibrium with HLA A1-B8-DR3), resulting in low complement levels which are positively associated with severity of disease.35–37 Yamauchi et al. point out, however, that this evidence comes from studies in caucasoids and that such complement gene deletions are not common in Japanese AIH patients.29 Again, this has some merit, because there is no reason to suppose that what we see as the clinical expression of AIH necessarily arises through the same pathogenetic mechanisms in all individuals with the disease.
The above caveats notwithstanding, the findings of Yamauchi et al. are intriguing.29, 30 It will be interesting to know the identity of this 190 kDa molecule, why it is preferentially expressed on perivenular hepatocytes, why there does not appear to be a class switch from an IgM to an IgG response to it in AIH patients, and whether mice injected with their MoAb develop chronic hepatitis over a longer period of observation.