Antibodies to soluble liver antigen and α-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 α-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 α-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-α-enolase antibody reactivity has been tested by immunoblot using recombinant α-enolase. An affinity purified goat polyclonal anti-α-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 α-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 α-enolase by immunoblot. Preincubation of anti-SLA positive sera with tRNP^(Ser)Sec completely abolished the 50 kDa band. The findings of the present study indicate that α-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-α-enolase - correlates with anti-SLA antibody reactivity. Conclusion: Our findings indicate that tRNP^(Ser)Sec is the most likely target of anti-SLA.