Expression in systemic lupus erythematosus of an idiotype common to DNA-binding and nonbinding monoclonal antibodies produced by normal human lymphoid cells.

Rabbit antiserum raised against a normal-derived monoclonal anti-DNA antibody KIM 4.6.3 (IgM lambda) was used for idiotype analyses. This anti-serum (anti-4.6.3 ID) was rendered specific for KIM 4.6.3 idiotype (4.6.3 ID) by absorption with normal human IgM and IgG. The specificity of anti-4.6.3 was shown by its ability to bind to KIM 4.6.3 antibody but not to normal human IgM and IgG, by inhibition of anti-4.6.3 ID reactivity with KIM 4.6.3 antibody by the homologous monoclonal antibody and by the ability of anti-4.6.3 ID to inhibit the binding of single stranded DNA with KIM 4.6.3 antibody. The 4.6.3 ID was found to be commonly expressed since it was detected among 33% (10/30) DNA and 32% (23/72) non-DNA-reactive monoclonal antibodies that were obtained from five different unrelated normal individuals. The binding to ssDNA of the majority of idiotype positive anti-DNA antibodies however was not blocked by anti-4.6.3 ID suggesting that among these other monoclonal antibodies its expression is outside of the antigen binding site. The 4.6.3 ID, which was present among some normal-derived monoclonal IgM molecules was also found at a high frequency (90%) in the sera of patients with systemic lupus erythematosus (SLE) but only at a low frequency (24%) and concentration in normal sera. The level of 4.6.3 ID in SLE did not correlate with serum IgM and IgG nor with anti-DNA antibody concentrations. Idiotypic relatedness between SLE serum antibodies and monoclonal anti-DNA antibodies of normals implies the existence of a cross-reactive idiotype family and implies that a conserved common gene or closely related genes exist in the germ line encoding these 4.6.3 ID positive antibodies some of which are not exclusively associated with nucleic acid reactivity. The expression of these germ line genes in vivo thus distinguishes SLE from normals.