Quantitative determination of anti-dsDNA antibodies and antibody/dsDNA stoichiometries in prepared,soluble complement-fixing antibody/dsDNA immune complexes

We have investigated quantitatively the complement-mediated binding of prepared, soluble ¹²⁵I-7S IgG antibody/³H-dsDNA immune complexes to human red blood cells (RBCs). We have performed these studies by using a detailed modification of the RBC-CF assay [Pedersen et al., J. Immun. Meth. 38, 2692–2280 (1980)] which now allows for the simultaneous measurement of both ³H-DNA and ¹²⁵I-binding to the cells. Our results indicate that, in the case of three SLE patients, their anti-dsDNA antibody titers are sufficiently high that a small fraction of their ¹²⁵I-7S IgG antibodies (ca 0.1–0.2%) can be identified as specifically anti-dsDNA. We have also used an indirect method (with ¹²⁵I-labelled rabbit anti-human IgG) for the determination of IgG anti-dsDNA antibodies in complement-fixing antibody/dsDNA immune complexes that bind to RBCs, and the results of these measurements are in reasonable agreement with the direct binding experiments. These studies have also allowed us to estimate the antibody/DNA stoichiometries in complement-fixing immune complexes. The results of these experiments may provide a useful standard for the analysis of monoclonal anti-dsDNA antibodies.