Antibody class differences are detected in anti-nRNP and anti-Sm antibodies directed against distinct antigen subunits.

Antibodies to nRNP, Sm and La were detected and characterised by immunoblot analysis. A comparison was made between IgG and IgM autoantibodies in 77 patients with systemic lupus erythematosus (SLE) and 50 normal subjects. No antibodies were detected in the normal subjects. In all 3 antigen specificity groups, a heterogeneity of antibody class was observed between patients. Antibodies to the 2 nRNP-specific polypeptides (33 and 67 kD) were approximately equally frequent. Although IgG antibodies to the 67 kD polypeptide were detected in 88% of patients with antibodies to this polypeptide, IgG antibodies to the 33 kD polypeptide were only detected in 43% of patients with antibodies to this polypeptide. This suggested either that anti-33 kD antibody is produced by a B cell which cannot mature to an IgG-secretor, or that anti-33 kD antibody production succeeds an initial immune response producing anti-67 kD. Reactivity with the 29 kD, Sm-specific polypeptide appeared to be the most frequent in anti-Sm sera compared with the 16 kD polypeptide suggesting that this polypeptide may be the primary immunogenic component of Sm.