Enzyme immunoassay detection of circulating immune complexes by monoclonal antibodies to C3 neoepitopes with special reference to IgG concentration and to interfering anti-immunoglobulin antibodies

A sensitive solid-phase anti-C3 enzyme immunoassay for detection of circulating immune complexes (CIC) is described. A mixture of the monoclonal antibodies (MoAbs) bH6 and Clone 9 specific for neoepitopes on C3 activation products was used as capture reagent. MoAb bH6 recognized C3b, iC3b and C3c, and Clone 9 recognized iC3b and C3dg. Detection antibody was a polyclonal peroxidase-conjugated rabbit anti-human Ig antiserum. A quantitative assay was constructed using serum incubated with heat aggregated IgG (HAG) as standard. The lower detection limit was 5 micrograms/ml of HAG. Interassay and intra-assay coefficient of variation was 15% and 5%, respectively. Anti-animal immunoglobulin antibodies were detected both in normal and pathological sera. This activity was efficiently absorbed by nonimmune immunoglobulins added to the samples. The present assay was compared with a polyethylene glycol precipitation assay for CIC determination. The latter assay was strongly influenced by the IgG concentration (rs = 0.78; P = 0.006), whereas no such correlation was seen for the anti-C3 immune complex assay (rs = -0.30; P = 0.20).