ABSTRACT
Connective tissue diseases often have overlapping clinical features and laboratory abnormalities. The distinctiveness of mixed connective tissue disease (MCTD) as an entity is of scientific interest and practical importance. In order to discriminate between MCTD and SLE patients we used a newly developed, commercially not available ELISA with recombinant antigen expressed in Baculovirus infected cells. This ELISA detects antibodies against RNP and Sm in complex as well as the subsets U1-snRNP 68 kDa, RNP-A, RNP-C (RNP), Sm-BB' and SS-D. We analyzed 66 RNP-positive consecutive patients prediagnosed as SLE or MCTD/overlap-syndrome. 45/66 patients were found to be U1-snRNP-68 kDa positive (27 SLE, 18 MCTD), 51/66 RNP-A [36,15] and 44/66 RNP-C [31,13]. 35/66 had antibodies against Sm-BB' (30 SLE, 5 MCTD), 10/66 against Sm-D (all SLE). 28/66 were found to be U1-snRNP-68 kDa and Sm-BB' positive (23 SLE, 5 MCTD), while 8/66 where U1-snRNP-68 kDa and Sm-D positive (all SLE). The combination of antibodies against 68 kDa, Aand C was exclusively observed in 6 MCTD patients, while the combination against 68 kDa, A, C, Sm-BB' and Sm-D was restricted to 8 patients with SLE. The antibody combination to 68 kDa, A, C and Sm-BB' was also found in 11/20 SLE patients with major organ involvement. In SLE and MCTD, determination of subsets of antibodies against Ul-snRNP-68 kDa and Sm-complex allows a differentiation of patient subgroups with more definite diagnoses and potential prognostic impact.