Antibodies against the 52 kDa Ro(SSA) protein are an important laboratory variable in autoimmune diseases, most notably in the diagnosis of primary Sjogren's syndrome (SS), congenital heart block (CHB), and certain varieties of systemic lupus erythematosus (SLE). However, there is controversy about the differential reactivity of these sera, both with regard to immunogenic regions on the target protein and the respective antibody titers. Therefore, sera from various autoimmune diseases were tested against a broad panel of recombinant 52 kDa Ro(SSA) fusion proteins.
Sera were obtained from 20 patients with SLE, 10 with primary SS, 15 children with CHB, 6 healthy anti-52 kDa Ro(SSA) positive infants born to mothers with SLE and 7 anti-52 kDa Ro(SSA) positive patients with primary biliary cirrhosis/secondary SS. Epitope mapping was performed using different fusion proteins in ELISA and immunoblot.
All sera reacted with whole recombinant antigen as well as with the protein carrying the amino acid sequence (AA) 1-245. The proportion of positive sera against the 52kDa Ro fusion proteins tested was found in descending order in patients with CHB, down to primary SS, the healthy infants group, patients with SLE and finally primary biliary cirrhosis/secondary SS. In general, CHB and primary SS sera exhibited the broadest reactivity against the recombinant protein compared to the limited and lower reactivity of sera from patients with primary biliary cirrhosis. Sera from infants with CHB had significantly higher antibody levels to AA 1-245 compared to SLE sera (p < 0.0005) and to sera from healthy infants born to SLE mothers (p < 0.05), as well as to serum samples from patients with primary biliary cirrhosis/SS (p < 0.005). The strongest antigenicities recognized by anti-52 kDa Ro(SSA) autoantibodies are located within the AA 197-245 region that represents an essential epitope. Further antigenic sites preferentially recognized by SS, CHB, and healthy infant sera are located within AA 153-196.
Thus, the central region AA 153-245 is the major immunogenic region of the 52 kDa Ro(SSA) antigen containing a strong antigenic epitope between AA 197-245. The antibody response is directed to this major antigenic region regardless of the underlying autoimmune disease. However, the strikingly different quantities of antibody levels and the recognition of epitopes on AA 153-196 may be associated with different disease expressions.