Predominance of IgM anti-U1RNP antibodies in patients with systemic lupus erythematosus.

Anti-U1RNP antibodies occur in patients with mixed connective tissue disease (MCTD), systemic sclerosis (SSc), systemic lupus erythematosus (SLE) and other ill-defined connective tissue diseases. To associate the isotypes of anti-U1RNP antibodies with the diagnosis of the disease, namely SLE or MCTD, sequential sera of patients positive for anti-U1RNP antibodies by counterimmunoelectrophoresis (CIE) (32 with SLE, 35 with MCTD) were tested for IgG and IgM anti-U1RNP antibodies by enzyme-linked immunosorbent assay (ELISA) using affinity-purified U1snRNP complexes. Results from ELISA were confirmed by RNA precipitation. IgG RNA precipitation of HeLa cellular extracts was performed using the bulk of the IgG fraction removed from each serum after binding to protein A-Sepharose beads. IgM RNA precipitation was carried out on the IgM fraction of the serum bound to protein A-Sepharose-rabbit anti-human IgM immune complexes. RNAs were electrophoresed in 10.5% acrylamide-7 M urea gels and detected with the silver stain. ELISA showed that all sera were positive to IgG anti-U1RNP, while 12 of the 35 MCTD and 21 of the 32 SLE patients possessed IgM anti-U1RNP (P < 0.025). IgM anti-U1RNP reactivity was found during the follow-up in 20% of 44 sera from 17 MCTD patients and 68% of 112 sera from 23 SLE patients (P < 0.0001). IgG from all the sera precipitated U1RNPs. Eight of the MCTD sera also precipitated U2RNPs and 14 of the SLE sera U2 and/or U4/U6, U5 RNPs. IgM from MCTD sera did not precipitate URNPs, while IgM from SLE sera precipitated predominantly U1RNPs. These data suggest that IgM anti-U1RNP antibodies occur predominantly in patients with SLE. The occurrence of IgG anti-U1RNP without IgM is more frequent in MCTD.