Clinical manifestations and anti-(U1)snRNP antibodies: a prospective study of 29 anti-RNP antibody positive patients.

Twenty-nine anti-RNP positive patients were followed prospectively with a mean observation time of 65 months (29-120 months). The clinical course was correlated to the presence of IgM and IgG anti-(U1)snRNP antibodies as revealed by immunoblotting from sequentially obtained sera. There was a striking dissociation between the fluctuating course, with the appearance of new manifestations followed by remissions, and the stability of the anti-snRNP antibody specificities where an appearance or a disappearance of anti-snRNP specificities was a rare phenomenon. The main epitope recognized by the IgG antibodies was the 70 kDa protein and of the IgM antibodies the B/B' proteins. No shift from the IgM to the IgG isotype was observed. The presence of IgG anti-70 kDa and IgM anti-B/B' antibodies was highly associated with presence of arthralgias, Raynaud's phenomenon and arthritis. Further, an association was noted between the combined presence of IgG anti-70 kDa, anti-A and anti-C antibodies and IgM anti-B/B' and puffy hands, myositis, pulmonary fibrosis and sclerodactyly, i.e. all manifestations of mixed connective tissue disease (MCTD). On the contrary, serositis as often seen in SLE was correlated to the presence of IgG anti-B/B' antibodies. Thus the longitudinal analysis of the correlation between anti-snRNP antibody specificities and clinical manifestations support the concept of MCTD as a distinct entity.