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 mar ker of autoimmune hepatitis. We have performed immunoscreening of a human l iver gene expression library with an anti-SLA-positive serum. A reactive cl one with a 35-kd open reading frame (ORF) and a 563 base pair (bp) 3' untra nslated region (UTR) was isolated (soluble liver antigen [SLA]-p35), showin g strong homology to an independently isolated putative SLA/liver-pancreas antigen (LP) sequence (Acc, No. AF146396), and a UGA serine tRNA-protein co mplex (tRNP)((Ser)SeC) related protein (AJ238617), as well as different exp ression sequence tag (EST)-clones from lymphatic and oncofetal tissues. Exp ressed in Escherichia coli, SLA-p35 showed dose-dependent and complete bloc king of reactivity to native SLA antigen after preabsorption with the 35-kd recombinant protein. It recognized 67/85 (78.8%) precharacterized anti-SEA -positive sera in dilutions up to 1:40,000 in immunoblot, without detectabl e cross reactivity in the controls. The commercially available SLA/LP enzym e-linked immunosorbent assay (ELISA), by comparison, recognized 63/85 sampl es (74.1%). Of the negative samples, 18% showed strong inhibition rates (80 % and above) in the polyclonal inhibition ELISA. We conclude that the compl ementary DNA now isolated by 3 independent approaches encodes for the major but not sole antigenic component of soluble Liver antigen. Although its tr uncated form presented here may serve to improve diagnostics based on the n ew recombinant polypeptide, it currently cannot fully replace the polyclona l inhibition ELISA.