High serum levels of antibodies against the recombinant 70 kDa ribonucleoprotein are useful for diagnosing mixed connective tissue disease

BACKGROUND: Anti-Sm antibodies and anti-RNP antibodies are considered to be diagnostic markers of systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD). However, cross-reactivity between the antibodies diminishes their discriminating specificity between these diagnoses. OBJECTIVE: We examined whether we could achieve better differentiation between these two disease entities using recombinant antigens to RNP70 and SmD and quantitative immunoassays. PATIENTS/METHODS: Sera from 51 patients with SLE and 10 patients with MCTD and from a control group of 59 patients were used in a cross-sectional setting. Semiquantitative ELISAs for the detection of antibodies to RNP-70, RNP-A, RNP-C, SmBB' and SmD were used and the results were compared to conventional ELISA tests using U(1)-snRNP and a mixture of SmBB' and SmD as antigenic substrates. RESULTS: Sera from MCTD patients showed higher levels of anti-RNP-70 antibodies than sera from SLE patients. Levels of anti-SmBB' or anti-SmD antibodies were not significantly different between SLE and MCTD sera. However, the presence of antibodies directed against SmD was more frequent in SLE. CONCLUSIONS: Our results suggest that the use of RNP-70 and SmD antigens may increase the practical value of immunoassays used to confirm a diagnosis of SLE or MCTD in patients with connective tissue disease.