Soluble liver antigen: isolation of a 35-kd recombinant protein (SLA-p35) specifically recognizing sera from patients with autoimmune hepatitis.

Autoantibodies to soluble liver antigen (SLA) are considered a specific marker of autoimmune hepatitis. We have performed immunoscreening of a human liver gene expression library with an anti-SLA-positive serum. A reactive clone with a 35-kd open reading frame (ORF) and a 563 base pair (bp) 3' untranslated region (UTR) was isolated (soluble liver antigen [SLA]-p35), showing strong homology to an independently isolated putative SLA/liver-pancreas antigen (LP) sequence (Acc. No. AF146396), and a UGA serine tRNA-protein complex (tRNP)((Ser) Sec) related protein (AJ238617), as well as different expression sequence tag (EST)-clones from lymphatic and oncofetal tissues. Expressed in Escherichia coli, SLA-p35 showed dose-dependent and complete blocking of reactivity to native SLA antigen after preabsorption with the 35-kd recombinant protein. It recognized 67/85 (78.8%) precharacterized anti-SLA-positive sera in dilutions up to 1:40,000 in immunoblot, without detectable cross reactivity in the controls. The commercially available SLA/LP enzymelinked immunosorbent assay (ELISA), by comparison, recognized 63/85 samples (74.1%). Of the negative samples, 18% showed strong inhibition rates (80% and above) in the polyclonal inhibition ELISA. We conlude that the complementary DNA now isolated by 3 independent approaches encodes for the major but not sole antigenic component of soluble liver antigen. Although its truncated form presented here may serve to improve diagnostics based on the new recombinant polypeptide, it currently cannot fully replace the polyclonal inhibition ELISA.

Immunoreactivity to cytokeratin 8/18 in patients with soluble liver antigen (SLA) positive autoimmune hepatitis (AIH type 3) is not sufficient for diagnostic use.

"Soluble liver antigen" (SLA) has been reported to be an infrequent but in certain cases a unique marker of autoimmune hepatitis, with cytokeratins 8 and 18 as major antigenic components. Using precharacterized sera, we could confirm trypsin sensitivity and a molecular weight of approximately 50 kD of the reactive protein. However, the reaction differed from that of cytokeratins 8 and 18 by molecular weight and a pI of 7.5. A significant reactivity to cytokeratin 8 and 18 preparations was seen in only 1/12 patients. Immunoscreening of a human liver expression gene bank yielded no clones with sequence homology to cytokeratins. We conclude that reactivity in SLA positive sera is not mainly directed against cytokeratins 8/18 and recommend native antigen preparations for diagnostic use until the exact molecular nature of the 52 kD SLA antigen has been elucidated.