A solid-phase radioimmunoassay for C1q-binding immune complexes. I. Delta IgG as indicator molecule.

A solid-phase radioimmunoassay for detection of circulating C1q-binding immune complexes (IC) or heat-aggregated IgG (delta IgG) is described. Purified human C1q protein was adsorbed to a fixed area of polystyrene tube surface for 1 hr at 22 degrees C, pH 7.3, mu 0.15. Binding of delta IgG or IC to solid-phase C1q at 22 degrees C progressed over several hours and was enhanced at low mu (0.05). Heating of C1q (56 degrees C for 30 min) reduced the binding by 85%-90%. Binding of IC or delta IgG was retained after several weeks' storage of solid-phase C1q at 4 degrees C. Detection of 2-5 ng delta IgG and less than 50 ng IC (in antibody excess) was achieved in competitive binding or inhibition tests with [125I]delta IgG. Preliminary testing of 48 human sera indicated the usefulness of the assay for detection of IC in patient sera.