Identification of target antigen for SLA/LP autoantibodies in autoimmune hepatitis

doi:10.1016/S0140-6736(00)02166-8

The Lancet

Volume 355, Issue 9214, 29 April 2000, Pages 1510-1515

https://doi.org/10.1016/S0140-6736(00)02166-8 Get rights and content

Summary

Background

Autoantibodies are a hallmark of autoimmune hepatitis, but most are not disease specific. Autoantibodies to soluble liver antigen (SLA) and to liver and pancreas antigen (LP) have been described as disease specific, occurring in about 30% of all patients with autoimmune hepatitis, but no standardised assays are available.

Methods

We tested 2000 serum samples from patients with various liver diseases and controls for SLA autoantibodies by inhibition ELISA. Serum samples positive for SLA antibodies were used for immunoscreening of cDNA expression libraries. Identified clones were tested against a panel of serum samples positive for SLA and LP autoantibodies and control serum samples, and the epitope mapped by deletion mutants and exonuclease digestion.

Findings

SLA and LP autoantibodies were identical. Of 2000 serum samples screened, 35 were positive for SLA autoantibodies. These positive samples came from patients with autoimmune hepatitis; three from patients with an overlap syndrome (primary biliary cirrhosis and secondary autoimmune hepatitis). Expression cloning and absorption experiments identified a 422 aminoacid protein present in two splice variants as the sole target antigen. Aminoacids 371–409 were critical for immune recognition.

Interpretation

The identified cDNA encodes the primary target antigen of SLA/LP autoantibodies. The SLA/LP antigen has a previously unknown aminoacid sequence, and presumably codes for an unindentified enzyme, suggested to be UGA-suppressor tRNA-associated protein. SLA/LP autoantibodies are disease specific and recognise a dominant epitope, suggesting a specific antigen-driven immune response. Identification of the SLA/LP target antigen will allow establishment of a reliable, widely available diagnostic assay. Furthermore, its role in the pathogenesis of autoimmune hepatitis can now be studied.

Introduction

Autoimmune hepatitis is a chronic progressive liver disease that responds well to immunosuppressive therapy, but has a poor prognosis if untreated.1, 2, 3 Early and accurate diagnosis is therefore of great importance. There is no single diagnostic test for autoimmune hepatitis.2, 4 Autoantibodies are an important diagnostic criterion,⁴ but only about 70% of all patients with autoimmune hepatitis have significant titres of autoantibodies detected by routine immunofluorescence testing (antibodies to nuclear antigens [ANA], smooth-muscle autoantibodies [SMA], or liver-kidney microsomal autoantibodies [LKM]).1, 2 These antibodies, in addition, are not specific for autoimmune hepatitis since they also occur in 10–15% of patients with viral hepatitis and other immune-mediated diseases.⁵ Antibodies to soluble liver antigen (SLA)⁶ and antibodies to a liver and pancreas antigen (LP)7, 8, 9 have been described as autoantibodies characteristic for autoimmune hepatitis, and are present in about 30% of all patients, many of whom are negative for other autoantibodies. There is some debate as to whether these patients represent a distinct subgroup; however, the clinical picture of patients with autoimmune hepatitis who have antibodies to SLA is very similar to that of patients with autoimmune hepatitis who have other autoantibodies.¹⁰

Use of these autoantibodies in the diagnosis of autoimmune hepatitis has been severely limited by the lack of well-defined test systems, because their target antigens are not known.² Liver cytokeratins 8 and 18,¹¹ and, later, glutathione-S-transferase (GST)¹² have been suggested as possible target antigens, but have not been confirmed.¹³ It has been suggested that SLA and LP autoantibodies are identical.13, 14 In this study we follow up previous suggestions that SLA autoantibodies are highly specific for autoimmune hepatitis, thus suggesting a pathogenetic role for their target antigen. We also describe identification by molecular cloning of the target antigen of SLA autoantibodies.

Section snippets

Serum samples

2000 serum samples from consecutive patients seen at our outpatient clinic were tested for the presence of SLA autoantibodies by inhibition immunoassay, as described by Manns and colleagues.⁶ These samples included those from patients with autoimmune hepatitis (98), an overlap syndrome of primary biliary cirrhosis with secondary autoimmune hepatitis (20),¹⁵ primary biliary cirrhosis (84), primary sclerosing cholangitis (23), hepatitis C (350), hepatitis B (215), various rheumatic autoimmune

Results

Of 2000 serum samples screened by inhibition ELISA for the presence of SLA autoantibodies, 35 were found to be positive (table 1). These SLA-positive serum samples all came from patients fulfilling standard diagnostic criteria for autoimmune hepatitis.⁴ Three of these patients in addition fulfilled diagnostic criteria for primary biliary cirrhosis, thus representing patients with primary biliary cirrhosis with secondary autoimmune hepatitis (overlap syndrome).¹⁵ 15 of 35 SLA-positive patients

Discussion

In this study we have shown the exquisite degree of disease specificity of SLA autoantibodies, and show that SLA and LP autoantibodies are indeed the same autoantibodies, hence our suggestion of the term SLA/LP autoantibody. Their specificity for the diagnosis of autoimmune hepatitis make them important for diagnostic purposes. In addition, the disease specificity suggests that these autoantibodies or their antigen are somehow closely related to the immunopathogenesis of autoimmune hepatitis.

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Citation Excerpt :
Patients with AIH-2 have detectable anti–liver kidney microsomal type-1 (anti-LKM1) or rarely anti–liver kidney microsomal type-3 (anti-LKM3), and/or anti–liver cytosol type-1 (anti-LC1) antibodies [1–3,6,18]. Antibodies against soluble liver antigens/liver pancreas autoantigen (anti-SLA/LP) can also be detected, mainly in AIH-1 patients [18–20]. The clinical significance of the subclassification of AIH is shown in Table 1.
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Early ReportIdentification of target antigen for SLA/LP autoantibodies in autoimmune hepatitis

Summary

Background

Methods

Findings

Interpretation

Introduction

Section snippets

Serum samples

Results

Discussion

J Hepatol

Lancet

J Hepatol

Gastroenterology

Gastroenterology

Lancet

J Autoimmunity

J Hepatol

Immunol Today

Autoimmune Hepatitis

N Engl J Med

Autoimmune Hepatitis

N Engl J Med

Treatment of autoimmune hepatitis

Early Report
Identification of target antigen for SLA/LP autoantibodies in autoimmune hepatitis