Antibodies to Sm, RNP and SSB detected by solid‐phase ELISAs using recombinant antigens: A comparison study with counter immunoeloctrophoresis and immunoblotting

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Abstract
Sera from 64 patients with systemic lupus erythematosus (SLE) were tested for the presence of anti‐Sm, anti‐RNP, and anti‐SSB antibodies using commercially available solidphase enzyme‐linked immunosorbent assays (ELISAs) and recombinant nuclear proteins as substrates. The results were compared to those obtained with counterimmunoelectrophoresis (CIE) and immunoblotting (IBT) using a rabbit thymus extract (RTE) as the substrate.The ELSAs detected antibodies to Sm, RNP, and SSB in, respectively, 25%, 36%, and 15% of the SLE sera. Neither IBT‐positive/ ELISA‐negative nor CIE‐positive/ELISA‐negative sera were found, regardless of the specificity considered, suggesting that ELISAs using recombinant nuclear antigens are highly sensitive.Discrepancies were observed between the results obtained with these different techniques. In addition to sera positive by both IBT and ELISA but negative by CIE, a substantial number of sera had ELISA‐detectable anti‐Sm, anti‐RNP, and anti‐SSB antibodies which failed to react with the corresponding polypeptides by IBT. The reasons for ELISA/IBT discrepancies were explored; however, no single explanation was found. Instead, a higher sensitivity of the ELISA to detect antibodies directed against certain polypeptides, the possible inability of IBT using RTE as the substrate to detect antibodies reacting with conformational antigenic determinants, and false‐positive reactions in the ELISAs were suggested.Thus, it is still advisable to perform both IB and ELISAs simultaneously in well‐defined autoimmune diseases to further analyze the potential advantages of ELISAs using recombinant antigens.