Evaluation of the C1q solid-phase binding assay for immune complexes. A clinical and laboratory study

The solid-phase C1q binding assay for circulating immune complexes has been evaluated. The assay provides a rapid, sensitive (detecting as little as 1 microgram of aggregated IgG) and reproducible procedure for the detection of immune complexes in biological fluids. Using artificially prepared immune complexes, the assay detects complexes at four-times antigen-excess. Gel filtration over Sepharose 6B showed that these complexes were distributed over a range of molecular weights from greater than 4 x 10(6) to 300,000 daltons. Using radiolabelled anti-BSA, antigen (BSA) could be detected in these complexes. Screening of gel-filtered SLE showed that the assay detects complexes of both high and low molecular weight, but does not detect all complexes in the SLE sera. Clinical studies showed that immune complexes are frequently found in the sera of patients with SLE and measurement of the concentrations of complexes provides a more sensitive index of disease activity than either serum C3 or C4 concentrations or DNA binding capacity. In patients with RA concentrations of immune complexes were generally higher in synovial fluid than serum, although a patient with systemic rheumatoid disease with hypocomplementaemia had an extremely high level of circulating immune complexes. The assay only infrequently detects circulating immune complexes in glomerulonephritis and in renal transplant recipients. It is concluded that the assay provides a useful clinical tool, but detects only a limited species of immune complexes. It can be used in the detection of antigens in complexes.