Targets of Antibodies to Soluble Liver Antigen in Patients with Autoimmune Hepatitis

The immunopathogenetic role of autoantibodies in canine autoimmune hepatitis: lessons to learn from human autoimmune hepatitis

Autoimmune hepatitis (AIH) is not a disease entity restricted to man, but it can be found in other animals including canines. An increasing number of studies have focused on the immunopathogenesis of human autoimmune hepatitis (hAIH), but little is known of what triggers canine autoimmune hepatitis (cAIH). Several drugs, toxins, microbial and viral agents are able to induce autoantibodies and indeed immune-mediated chronic canine hepatitis with immunological and serological features similar of those seen in the human disease. We discuss the features of cAIH paying attention to the autoantibody profile of the disease in comparison to that seen in hAIH. We also discuss the immunomodulatory role of specific molecular signaling pathways such as those mediated by tumor growth factor and p38 mitogen-activated kinase in the induction of AIH, and the potential of these molecules to act as targets of specialized immunotherapeutic interventions. Review of the literature indicates that we have more to learn for the delineation of autoantibody profile and the antigen-specific immunoregulatory mechanisms involved in the pathogenesis of cAIH from the human disease, rather than the other way around.

Immunopathogenesis of primary biliary cirrhosis: an old wives' tale

Primary biliary cirrhosis (PBC) is a cholestatic liver disease characterised by the autoimmune destruction of the small intrahepatic bile ducts. The disease has an unpredictable clinical course, but may progress to fibrosis and cirrhosis. Although medical treatment with urseodeoxycholic acid is largely successful, some patients may progress to liver failure requiring liver transplantation. PBC is characterised by the presence of disease specific anti-mitochondrial (AMA) antibodies, which are pathognomonic for PBC development. The disease demonstrates an overwhelming female preponderance and virtually all women with PBC present in middle age. The reasons for this are unknown; however several environmental and immunological factors may be involved. As the immune systems ages, it become less self tolerant, and mounts a weaker response to pathogens, possibly leading to cross reactivity or molecular mimicry. Some individuals display immunological changes which encourage the development of autoimmune disease. Risk factors implicated in PBC include recurrent urinary tract infection in females, as well as an increased prevalence of reproductive complications. These risk factors may work in concert with and possibly even accelerate, immune system ageing, contributing to PBC development. This review will examine the changes that occur in the immune system with ageing, paying particular attention to those changes which contribute to the development of autoimmune disease with increasing age. The review also discusses risk factors which may account for the increased female predominance of PBC, such as recurrent UTI and oestrogens.

Peculiar antibody reactivity to human connexin 37 and its microbial mimics in patients with Crohn's disease

BACKGROUND/AIMS

We found that pooled Crohn's disease (CD) sera strongly react with a human gap-junction connexin 37 (Cx37) peptide and tested for anti-Cx37 antibody reactivity in sera from CD patients and controls. We also investigated whether peptide-recognition is due to Cx37/microbial molecular mimicry.

METHODS

The PSI-BLAST program was used for Cx37(121-135)/microbial alignment. Reactivity to biotinylated human Cx37(121-135) and its microbial mimics was determined by ELISA using sera from 44 CD, 30 ulcerative colitis and 28 healthy individuals.

RESULTS

Anti-Cx37(121-135) reactivity (1/200 dilution) was present in 30/44 (68%) CD cases and persisted at 1/1000 dilution. Database search shows that Cx37(121-135) contains the -ALTAV- motif which is cross-recognized by diabetes-specific phogrin and enteroviral immunity. Testing of 9 Cx37(121-135)-microbial mimics revealed 57-68% reactivity against human enterovirus C, Lactococcus lactis, coxsackie virus A24 and B4. Anti-Cx37(121-135) was inhibited by itself or the microbial mimics. No reactivity was found against the poliovirus, rubella, and Mycobacterium tuberculosis mimics, or the beta cell phogrin autoantigen. Microbial/Cx37 reactivity was not able to differentiate CD patients from UC or healthy controls, in terms of overall prevalence and antibody titres, but microbial mimics were unable to inhibit reactivity to human Cx37 in the majority of the controls.

CONCLUSIONS

Sera from CD patients react with connexin 37 and cross-react with specific Cx37-mimicking enteroviral peptides. Microbial/self reactivity can be seen in UC and healthy controls. The lack of responses to other Cx37(121-135) microbial mimics and the inability of the reactive microbes to inhibit reactivity to self is intriguing and warrants further investigation.

The role of autoantibodies as diagnostic markers of autoimmune hepatitis

Autoantibody testing is the first step towards the diagnosis of autoimmune hepatitis, and it is essential in the evaluation of acute and chronic hepatitis of undetermined cause and allograft dysfunction following liver transplantation. A standard diagnostic repertoire has been promulgated, and other autoantibodies are emerging that may have prognostic value. Supplemental autoantibodies may prove useful in assessing patients who lack the standard markers or who are distinctive among those with conventional markers. Serologic testing will improve as assays are standardized by serum exchange workshops, core diagnostic batteries are codified and promulgated, and markers emerge that are tightly associated with pathogenic mechanisms, and closely reflect disease activity and outcome.

SLA/LP/tRNP((Ser)Sec) antigen in autoimmune hepatitis: identification of the native protein in human hepatic cell extract

A diagnostic subgroup of AIH type 1 is characterized by specific serum antibodies against soluble liver protein. The respective autoantigen was named SLA/LP/tRNP((Ser)Sec), after three homologous recombinant polypeptides were isolated from expression gene libraries. We analyzed human cultured liver cells for the human homologue of recombinant SLA/LP/tRNP((Ser)Sec) by antigen purification. In addition, a monoclonal antibody was generated against recombinant SLA-p35, a truncated recombinant SLA-reactive polypeptide. With a positive patient serum, immune affinity chromatography was performed on the 52 kD-SLA main antigenic determinant pre-enriched by ion exchange chromatography. By mass spectrometry, the 52 kD-SLA/LP/tRNP ((Ser)Sec) autoantigen was unambiguously identified in the purification product. The identity of the recombinant SLA-p35 and its human homologue was further confirmed by a specific signal of the anti SLA-p35 monoclonal antibody with purified human SLA/LP/tRNP((Ser)Sec). The 48 kD-SLA species frequently comigrating in SLA-immunoblotting however was not identified by either approach. We conclude that the native counterpart of recombinant tRNP((Ser)(Sec)) indeed is detectable with a molecular weight of 52 kD in soluble liver extract of human cells as the major antigenic component of SLA/LP/tRNP((Ser)Sec).

Autoimmune liver serology: Current diagnostic and clinical challenges

Liver-related autoantibodies are crucial for the correct diagnosis and classification of autoimmune liver diseases (AiLD), namely autoimmune hepatitis types 1 and 2 (AIH-1 and 2), primary biliary cirrhosis (PBC), and the sclerosing cholangitis variants in adults and children. AIH-1 is specified by anti-nuclear antibody (ANA) and smooth muscle antibody (SMA). AIH-2 is specified by antibody to liver kidney microsomal antigen type-1 (anti-LKM1) and anti-liver cytosol type 1 (anti-LC1). SMA, ANA and anti-LKM antibodies can be present in de-novo AIH following liver transplantation. PBC is specified by antimitochondrial antibodies (AMA) reacting with enzymes of the 2-oxo-acid dehydrogenase complexes (chiefly pyruvate dehydrogenase complex E2 subunit) and disease-specific ANA mainly reacting with nuclear pore gp210 and nuclear body sp100. Sclerosing cholangitis presents as at least two variants, first the classical primary sclerosing cholangitis (PSC) mostly affecting adult men wherein the only (and non-specific) reactivity is an atypical perinuclear antineutrophil cytoplasmic antibody (p-ANCA), also termed perinuclear anti-neutrophil nuclear antibodies (p-ANNA) and second the childhood disease called autoimmune sclerosing cholangitis (ASC) with serological features resembling those of type 1 AIH. Liver diagnostic serology is a fast-expanding area of investigation as new purified and recombinant autoantigens, and automated technologies such as ELISAs and bead assays, become available to complement (or even compete with) traditional immunofluorescence procedures. We survey for the first time global trends in quality assurance impacting as it does on (1) manufacturers/purveyors of kits and reagents, (2) diagnostic service laboratories that fulfill clinicians’ requirements, and (3) the end-user, the physician providing patient care, who must properly interpret test results in the overall clinical context.

Autoimmune hepatitis-specific antibodies against soluble liver antigen and liver cytosol type 1 in patients with chronic viral hepatitis

BACKGROUND

Non-organ specific autoantibodies are highly prevalent in patients with chronic hepatitis C (HCV). Among them, anti-liver kidney microsomal type 1 (LKM1) antibody--the serological marker of type 2 autoimmune hepatitis (AIH-2)--is detected in up to 11% of the HCV-infected subjects. On the other hand, anti-liver cytosol type 1 antibodies (anti-LC1)--either in association with anti-LKM1, or in isolation--and anti-soluble liver antigen antibodies (anti-SLA) have been considered as useful and specific diagnostic markers for AIH. However, their specificity for AIH has been questioned by some recent studies, which have shown the detection of anti-LC1 and anti-SLA by immunoprecipitation assays in HCV patients irrespective of their anti-LKM1 status. The aim of the present study was to test the anti-LC1 and anti-SLA presence by specific enzyme linked immunosorbent assays (ELISAs), in a large group of Greek HCV-infected patients with or without anti-LKM1 reactivity as firstly, immunoprecipitation assays are limited to few specialized laboratories worldwide and cannot be used routinely and secondly, to assess whether application of such tests has any relevance in the context of patients with viral hepatitis since antibody detection based on such ELISAs has not been described in detail in large groups of HCV patients.

METHODS

One hundred and thirty eight consecutive HCV patients (120 anti-LKM1 negative and 18 anti-LKM1 positive) were investigated for the presence of anti-LC1 and anti-SLA by commercial ELISAs. A similar number (120) of chronic hepatitis B virus (HBV) infected patients seronegative for anti-LKM1 was also tested as pathological controls.

RESULTS

Six out of 18 (33%) anti-LKM(pos)/HCV(pos) patients tested positive for anti-LC1 compared to 1/120 (0.83%) anti-LKM(neg)/HCV(pos) patients and 0/120 (0%) of the anti-LKM1(neg)/HBV(pos) patients (p < 0.001 for both comparisons). Anti-SLA antibodies were not present in any of the HCV (with or without anti-LKM1) or HBV-infected patients.

CONCLUSION

We showed that anti-LC1 and anti-SLA autoantibodies are not detected by conventional assays in a large group of anti-LKM1 negative patients with chronic hepatitis B and C infections. Based on these results we cannot find any justification for the application of anti-LC1 and anti-SLA tests in the routine laboratory testing of viral hepatitis-related autoantibody serology with the only potential exception being the anti-LC1 screening in anti-LKM1(pos)/HCV(pos) patients.

Autoantibodies in autoimmune liver disease

Autoantibodies indicate an immune reactive state, but in liver disease they lack pathogenicity and disease specificity. Antinuclear antibodies, smooth muscle antibodies, antibodies to liver/kidney microsome type 1, antimitochondrial antibodies, and perinuclear antineutrophil cytoplasmic antibodies constitute the standard serological repertoire that should be assessed in all liver diseases of undetermined cause. Antibodies to soluble liver antigen/liver pancreas, asialoglycoprotein receptor, actin, liver cytosol type 1, nuclear antigens specific to primary biliary cirrhosis, and pore complex antigens constitute an investigational repertoire that promises to have prognostic and diagnostic value. These autoantibodies may emerge as predictors of treatment response and outcome. Antibodies to histones, doubled-stranded DNA, chromatin, and lactoferrin constitute a supplemental repertoire, and they support the immune nature of the liver disease. Final diagnoses and treatment strategies do not depend solely on serological markers. Autoantibodies are floating variables, and their behavior does not correlate closely with disease activity. There are no minimum levels of significant seropositivity, especially in children. Over-interpretation is the major pitfall in the clinical application of the serological results. New autoantibodies will emerge as the search for target antigens and key pathogenic pathways continues.

Anti-tRNP(ser)sec/SLA/LP autoantibodies. Comparative study using in-house ELISA with a recombinant 48.8 kDa protein, immunoblot, and analysis of immunoprecipitated RNAs

BACKGROUND

Antibodies against tRNP((ser)sec) (ribonucleoproteins, RNP) have been described in our laboratory as markers of poor outcome in type 1 autoimmune hepatitis (AIH). The antigenic protein has been sequenced and cloned as a 48.8 kDa protein and identified with soluble liver antigen (SLA) and liver-pancreas (LP) antigen. The aim of this paper was to determine the best assay by which to detect these antibodies in type 1 AIH.

METHODS

A simple and reliable enzyme linked immunoassay based on prokaryotically expressed protein was compared with an immunoblot assay using prokaryotically- and eukaryotically-expressed proteins and an assay based on immunoprecipitated RNAs from HeLa cell extracts. Eighty-one sera from 58 patients with type 1 AIH, 168 sera from patients with autoimmune diseases or chronic hepatitis C, and 60 sera from healthy subjects were similarly tested.

RESULTS

The specificity of the assays was 100%, but the frequency of seropositivity was higher in the assay based on immunoprecipitated RNAs (44.4%) than in the enzyme-linked immunosorbent assay (ELISA) (16%) and the immunoblot assay with prokaryotically (12.34%) and eukaryotically (14.8%)-expressed protein. There were no clinical differences between the patients positive by ELISA, immunoblot assay, or immunoprecipitated RNAs.

CONCLUSIONS

These results suggest that the analysis of the immunoprecipitated RNAs is the most useful, sensitive and specific method to detect anti-tRNP((ser)sec)/SLA/LP autoantibodies.

Antibodies to soluble liver antigen and alpha-enolase in patients with autoimmune hepatitis

BACKGROUND: Antibodies to a cytosolic soluble liver antigen (SLA) are specifically detected in patients with autoimmune hepatitis (AIH). The target of anti-SLA has been identified as a ~50 kDa UGA serine tRNA-associated protein complex (tRNP(Ser)Sec), through the screening of cDNA libraries. A recent report questioned the identity of tRNP(Ser)Sec as the real SLA antigen. The latter study identified alpha-enolase as a major anti-SLA target, through proteomic analysis. METHODS: In an attempt to explain the observed discrepancy we have investigated reactivity of SLA positive sera against alpha-enolase and tRNP(Ser)Sec using rat and primate liver homogenate and the recombinant antigens. Thirty-three serum samples, 11 from SLA-positive patients and 22 from SLA negative controls were investigated. SLA antibodies were detected by an inhibition ELISA and confirmed by immunoblot using human liver homogenate. Autoantibody reactivity was further evaluated using preparations of primate and rat liver homogenates. Anti-alpha-enolase antibody reactivity has been tested by immunoblot using recombinant alpha-enolase. An affinity purified goat polyclonal anti-alpha-enolase IgG antibody was used as reference serum sample. Anti-tRNP(Ser)Sec antibody reactivity was detected by ELISA or dot blot using recombinant tRNP(Ser)Sec antigen. RESULTS AND DISCUSSION: The affinity purified IgG antibody directed to human alpha-enolase gave a band of approximately 48 kDa in both human and rat liver homogenates. A high titre anti-tRNP(Ser)Sec antibody serum gave a single band of ~50 kDa in both liver preparations. All but one anti-SLA antibody positive sera reacted with a ~50 kDa but none immunofixed a 48 kDa band. All anti-SLA antibody positive sera reacted strongly with the recombinant full length tRNP(Ser)Sec protein. None of the anti-SLA negative sera reacted with tRNP(Ser)Sec. Anti-SLA positive, and anti-SLA negative sera reacted equally against recombinant alpha-enolase by immunoblot. Pre-incubation of anti-SLA positive sera with tRNP(Ser)Sec completely abolished the 50 kDa band. The findings of the present study indicate that alpha-enolase and tRNP(Ser)Sec are both expressed in primate and rat liver and have a respective MW of 48 and 50 kDa. They also show that anti-tRNP(Ser)Sec - but not anti-alpha-enolase - correlates with anti-SLA antibody reactivity. CONCLUSION: Our findings indicate that tRNP(Ser)Sec is the most likely target of anti-SLA.