Serum from 53 patients with systemic lupus erythematosus (SLE) and 23 patients with primary Sjögren's syndrome (SS) were studied for anti-52-kDa SS-A/Ro, anti-60-kDa SS-A/Ro, and anti-SS-B/La antibodies by immunoblotting and enzyme-linked immunosorbent assay (ELISA). By immunoblotting, anti-SS-A/Ro was detected in 16 (30%) patients with SLE and 17 (74%) patients with SS. Anti-SS-B/La was detected in 22 (41%) patients with SLE and 15 (65%) patients with SS. Serum from 14 of the 16 SLE patients with anti-SS-A/Ro reacted with the 60-kDa protein and 15 serum samples from these patients recognized the 52-kDa protein. Serum with anti-60-kDa SS-A/Ro alone was not found. Serum from all of the 17 SS patients with anti-SS-A/Ro reacted with the 52-kDa protein, whereas serum from only two of these patients recognized the 60-kDa protein. By ELISA, the frequency of anti-SS-A/Ro (antibodies to the 60-kDa and/or 52-kDa of SS-A/Ro proteins) in patients with SLE and SS was 43/53 (81%) and 15/23 (65%), respectively. Anti-48-kDa SS-B/La was found in 28% and 48% of SLE and SS patients, respectively. Serum from 77% of SLE patients and 48% of SS patients reacted with the 60-kDa SS-A/Ro protein. Serum from 45% of SLE patients and 52% of SS patients reacted with the 52-kDa SS-A/Ro protein. Patients with SLE had significantly higher titers of antibodies to 60-kDa SS-A/Ro compared with patients with SS. Anti-SS-A/Ro and anti-SS-B/La are common in both SLE and SS. The different reactivities of anti-52-kDa and anti-60-kDa antibodies in serum from patients with SLE and SS may represent differences in conformation-dependent epitopes of SS-A/Ro autoantigens.
