Comparison of a monoclonal and polyclonal anti-idiotype against a human IgG anti-DNA antibody.

Over the last decade a number of idiotypes (Id) have been identified on monoclonal IgM anti-DNA antibodies which represent germ-line genes. This study describes two new idiotypes present on a monoclonal human IgG anti-DNA antibody, D5 derived from a patient with active SLE. The two idiotypes, designated D5-RId and D5-MId, are defined by a polyclonal and monoclonal anti-Id respectively. Both anti-Ids inhibited each other's binding to D5 in an ELISA and did not bind to human IgG. Using western blotting the D5-RId was located on the light chain whereas the D5-MId bound to conformational determinants; both idiotypes were close to or at the binding site for DNA. The anti-Id reagents were used in ELISAs to screen human sera and tissue samples for the presence of the D5-MId and the D5-RId. The upper limit of normal for sera in the D5-MId ELISA was much lower than the D5-RId ELISA indicating a greater degree of specificity in the former. The idiotypes were found only in the IgG fraction of the sera. About 30% of SLE patients had either the D5-MId or the D5-RId and 20% had elevated levels of both, showing a considerable overlap in the expression of the two idiotypes. This overlap was also observed in the other disease groups including patients with other autoimmune diseases, though the numbers of patients expressing the idiotypes were significantly lower than in the SLE group. The idiotypes were present on both DNA binding and non-DNA binding fractions of lupus sera. D5-MId was present in 6/10 renal lupus biopsies and only in 2/15 disease control renal biopsies in which immunoglobulin was deposited. D5-RId did not stain any sections. There is a close correlation between the presence of the D5-MId and D5-RId in SLE sera and the level of expression. It is evident that both idiotypes are associated with SLE and are markers of a population of IgG anti-DNA antibodies, the isotype associated with active disease. Since the idiotypes are not found on IgM antibodies they are likely to be generated by somatic mutation.