An enzyme-linked immunoassay for detection of IgG- and C3-containing circulating immune complexes: comparison with a radioimmunoassay and quantitation in patients with rheumatic diseases.

We developed a simplified, relatively rapid, inexpensive, antigen-nonspecific, enzyme-linked immunosorbent assay (ELISA) for immunoglobulin G (IgG)- and C3-containing circulating immune complexes (CICs), adapted from a solid-phase anti-C3 radioimmunoassay (RIA). Standards (containing purified, heat-aggregated IgG and fresh human serum) or samples were allowed to react with goat F(ab')2 antihuman C3 bound to the matrix of microtiter plates. Then alkaline phosphatase conjugated to goat IgG fraction antihuman IgG was added, followed by p-nitrophenylphosphate, optical densities determined, and concentrations of CICs calculated. We found excellent correlations between serum and plasma CIC levels by either ELISA (r = 0.95, p less than 0.01) or RIA (r = 0.89, p less than 0.01). Furthermore, ELISA quantitation of CICs correlated well with RIA (serum, n = 75, r = 0.64, p less than 0.01; plasma, n = 101, r = 0.56, p less than 0.01). By ELISA we found 32 normal subjects had 38 +/- 12 micrograms CIC/ml in serum and 34 +/- 10 micrograms CIC/ml in plasma. Patients with systemic lupus erythematosus (39% of 27 patients, p less than 0.05) had significantly elevated CIC levels compared with normal (serum, 157 +/- 50 micrograms/ml, p less than 0.01; plasma, 89 +/- 23 micrograms/ml, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)