Quantitation of antibodies to nucleoribonucleoprotein by ELISA: relation between antibody levels and disease activity in patients with connective tissue disease.

We describe a solid-phase enzyme-linked immunosorbent assay (ELISA) for quantitation of antibodies to nucleoribonucleoprotein (nRNP/Sm). nRNP/Sm was purified from rabbit thymus acetone powder by immunoaffinity chromatography and characterized by counterimmunoelectrophoresis (CIE) and immunoblotting using sera with well-known specificities. The purified antigen was used in ELISA. Positive results in ELISA were obtained only in sera with anti-nRNP or anti-Sm specificity as determined in CIE. Levels of anti-nRNP/Sm as quantitated by ELISA were higher in the sera of patients with active connective tissue disease (n = 7) than in those with inactive disease (n = 6) (P less than 0.01). Differences in anti-nRNP/Sm levels were also found between patients with mildly active disease (n = 19) and those with active disease (P less than 0.01). Fluctuations of anti-nRNP/Sm levels related to disease activity were seen in longitudinal observation. Although anti-nRNP/Sm levels as quantitated by ELISA correlated with titres of antinuclear antibodies as determined by immunofluorescence (r = 0.46, P less than 0.05), quantitation of anti-nRNP/Sm by ELISA is superior since the assay is antigen-specific and its quantitation independent of titration-related inaccuracies.