The influence of endogenous serum C1q on the ¹²⁵I-C1q binding test

Although the ¹²⁵I-C1q binding test is accepted as a reliable method for the detection of immune complexes, a high percent of nonspecific ¹²⁵I-C1q precipitation occurs with some specimens. This has been attributed to precipitation of ¹²⁵I-C1q in samples having low protein concentration. We found no relation between protein concentration and the amount of ¹²⁵I-C1q which precipitated in fractions of normal serum obtained from gel-filtration chromatography. Because less ¹²⁵I-C1q precipitated in those fractions containing C1, the effect of added C1q on the nonspecific ¹²⁵I-C1q precipitation was studied. C1q diminished ¹²⁵I-C1q precipitation in buffer. Heating normal serum to inactivate C1q or removing C1q by euglobulin precipitation increased ¹²⁵I-C1q precipitation, an effect which was reversed by the addition of C1q. Protamine similarly prevented nonspecific ¹²⁵I-C1q precipitation in buffer and heated serum. These data suggest that both protamine and unheated C1q act by dispersing ¹²⁵I-C1q, thus preventing its aggregation and subsequent precipitation. Endogenous C1q also was shown to compete with ¹²⁵I-C1q for binding sites on IgG aggregates added to normal serum, thereby substantially diminishing their reactivity in this assay. These observations indicate that (1) false-positive results for immune complexes may occur with this assay when fluids which contain diminished concentrations of C1q are tested and (2) results may vary inversely with the concentration of endogenous C1q when samples containing immune complexes or immunoglobulin aggregates are studied.