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.


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.
Salmhofer
W
W
Department of Dermatology, Medical University of Graz, Graz, Austria. wolfgang.salmhofer@meduni-graz.at
Hermann
J
J
Joch
M
M
Kerl
H
H
Graninger
W
W
D000328:Adult; D000368:Aged; D001324:Autoantigens / Q000097:blood / Q000276:immunology; D015415:Biomarkers / Q000097:blood; D003430:Cross-Sectional Studies; D003937:Diagnosis, Differential; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D006801:Humans; D008180:Lupus Erythematosus, Systemic / Q000175:diagnosis* / Q000276:immunology; D008297:Male; D008875:Middle Aged; D008947:Mixed Connective Tissue Disease / Q000175:diagnosis* / Q000276:immunology; D011994:Recombinant Proteins / Q000097:blood* / Q000276:immunology; D017411:Ribonucleoproteins, Small Nuclear / Q000097:blood* / Q000276:immunology; D012680:Sensitivity and Specificity
D001324:Autoantigens; D015415:Biomarkers; D011994:Recombinant Proteins; D017411:Ribonucleoproteins, Small Nuclear