The influence of nuclear antigen form on complement activation by systemic lupus erythematosus antinuclear antibody immune complexes determined by a novel immunocapture assay.

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease in which immune complexes (ICs) consisting of antinuclear autoantibodies (ANAs) and nuclear antigens induce complement activation and contribute to disease pathogenesis. The impact of antigen form on ICs formation and subsequent complement activation is not well understood. To elucidate the formation of these ICs, we used an immunocapture assay (immune complex complement assay [ICCA]) to determine the influence of antigen form on complement activation. In the ICCA, ICs are formed in vitro with SLE patient plasmas and different nuclear antigen preparations (ApoS, a supernatant of apoptotic cells; calf thymus DNA; nucleosomes; or purified Smith [Sm]/ribonucleoprotein [RNP]); healthy donor whole blood or plasma served as a complement source. Complement activation was detected by capturing ICs with either a monoclonal anti-DNA or anti-Sm antibody and assaying for bound C3 fragments. Results show that ICs formed with ApoS produced 4- to 8-fold higher complement activation than those with soluble DNA or nucleosomes. Patient samples with antibodies to RNA binding proteins (anti-Sm/RNP) induced significant complement activation with an Sm/RNP antigen preparation but not with ApoS. Despite a general correlation with anti-DNA antibody titers in patient samples, anti-DNA antibody levels did not fully predict complement activation. These findings highlight the influence of nuclear antigen form on complement activation by ANA ICs and support ICCA as a potential biomarker platform to elucidate disease mechanisms and evaluate new therapies.