Antibodies to double-stranded DNA: purification and characterization of binding specificities

Antibodies to double-stranded DNA are found almost exclusively in serum of patients with the autoimmune disease systemic lupus erythematosus (SLE). Very little is known about the nature of the antibody binding site(s) on DNA. We report here the affinity purification from SLE serum of a group of IgG and IgM antibodies that bind in nitrocellulose filter binding assays to entirely double-stranded radiolabeled DNA devoid of single-stranded regions. We have studied the specificities of these purified antibodies by competition with various unlabeled polynucleotides and have visualized their binding to double-stranded DNA by electron microscopy. From the competition studies we could identify a type of purified antibody that binds only to entirely double-stranded DNA and other types that bind to both double- and single-stranded DNA. We report also that purified antibodies that bind to double-stranded poly d(AT) bind well to single-stranded DNA. They appear to be different from those purified antibodies that bind to native double-stranded DNA; antibodies to poly d(AT) can be shown to bind independently of antibodies to native double-stranded DNA. Our results suggest that a heterogeneous group of antibodies to double-stranded DNA can exist in a single SLE serum.