IgM anti-A and D SnRNP proteins and IgM anti-dsDNA are closely associated in SLE sera.

U1RNP and Sm precipitins occur in systemic lupus erythematosus (SLE) and in overlap syndromes, while precipitins to Ro/SSA and La/SSB occur in SLE and its variants as well as Sjögren's syndrome. In studying IgM and IgG antibodies to these polypeptides by Western blot, we found the expected frequency of antibodies to the A protein of the U1RNP specificity and the D protein of the Sm specificity in sera with precipitins to U1RNP and Sm, but also found the frequent occurrence of both IgM and IgG anti-A and D in SLE sera with no precipitins, and in sera with anti-Ro/SSA and/or anti-La/SSB precipitins, but not in sera from patients with rheumatoid arthritis, polymyositis, scleroderma, or normals. We have studied the sera which bind the A and D polypeptides on Western blot to test their ability to bind native U1RNP (which contain the A and D proteins) in ELISA. The results indicate that most anti-Ro/SSA alone, anti-Ro/SSA, and anti-La/SSB sera as well as SLE sera with no precipitins react preferentially with the denatured A and D proteins. Further study of sera with IgM anti-A and D in Western blot reveals that these sera also frequently contain anti-double-stranded DNA antibodies. These results show that the anti-A and D responses, especially IgM anti-A and D, occur not only in patients with precipitating antibodies to SnRNPs, but in almost half of patients across the lupus spectrum. The extended prevalence of antibodies to the denatured A and D proteins in Western blot is associated with the cross-reaction of antibodies to dsDNA.