Effect of coexistent serum DNA on qualitative properties of anti-DNA antibodies.

We have explored the possibility that inapparent DNA in serum from patients with systemic lupus erythematosus can occupy anti-DNA combining sites and alter the apparent qualitative properties of such an antibody. DNAse digestion of such sera altered both the association rate of anti-DNA with 125I-DNA and the slope of binding isotherms in Scathchard analysis, although no immunoprecipitable DNA was detected in these sera. The association rate of serum after DNAse digestion was a better correlate of nephritis and disease activity. These findings suggest that DNA not detectable by counterimmunoelectrophoresis may affect assessment of qualitative properties of anti-DNA without affecting overall antibody titer, and that the association rate after DNAse digestion of serum remains one of the best correlates of disease activity. Our data further suggest that future qualitative studies of anti-DNA behavior utilize plasma rather than serum to avoid the artifact reported here.