Common epitopes on the SS-B/La proteins of 41 and 29 kD recognized by human autoantibodies

The calf thymus SS-B/La antigen precipitated by specific antibodies or purified by affinity chromatography has two major antigenic proteins. The proteins of 41 and 29 kD were found whether whole calf thymus extract (CTE) or the 60-80% saturated ammonium sulfate fraction of CTE were used as a source of SS-B/La. Both proteins were shown to react with human autoantibodies to SS-B/La by immunoblotting. Purified anti-41 and 29 kD SS-B/La antibodies reacted with both 29 and 41 kD proteins in immunoblotting. In agar diffusion, purified anti-41 and 29 kD SS-B/La antibodies and anti-SS-B/La antibodies in the original serum showed a reaction of identity. Both anti-41 and 29 kD protein antibodies showed three immunofluorescent staining patterns on rat liver cells: speckled; nucleolar and speckled associated with disperse cytoplasmic. These results indicate that the common antigenic determinants present on the 41 and 29 kD SS-B/La proteins are recognized by human autoantibodies to the SS-B/La antigen produced in connective tissue diseases.