A solid-phase radioimmunoassay for anti-SNP antibodies.

A highly sensitive and discriminatory solid-phase radioimmunoassay has been developed to detect anti-SNP antibodies in sera. Polystyrene tubes are coated with SNP and incubated with the test sera. The fixed antibodies are detected by a double layer technique using rabbit anti-human γ-globulin antiserum followed by incubation with a ¹²⁵I-labelled sheep anti-rabbit γ-globulin antiserum. Results are expressed in ng of the ¹²⁵I reagent fixed by 30 ωl of serum. The mean binding of 47 normal human sera was 22 ng ± 12 ng: 22/50 SLE sera gave over 34 ng binding. The specificity of the assay was studied in 3 different types of experiment: inhibition of the binding of positive sera either by pre-incubation with NDNA, SNP or RNA, DNAse I digestion of the SNP coated tubes, and incubation of SNP coated tubes with sera of known reactivity. It was shown that the solid-phase assay detects mainly antibodies directed against NDNA; however antibodies directed against the DNA-protein complex or the protein alone can easily be detected. Our results obtained with the solid-phase assay correlated well with those of the Farr assay. However, this new assay presents major improvements: it is simple, highly reproducible, and avoids the need for labelled antigen. A single labelled antiglobulin reagent allows identification of the class or subclass of reactive antibodies in a given species. Quantitation is more precise, particularly for sera containing high amounts of antibodies.