Polyethylene glycol enhances the binding of C1q to circulating immune complexes

By radioimmunoassay we measured the amount of endogenous C1q that was precipitated by polyethylene glycol (PEG) under the conditions of the 125I-C1q-binding test (C1q-BT). We found a linear correlation between the percentage endogenous C1q that was precipitated and the 125I-C1q-binding activity (C1q-BA). We concluded that the 125I-C1q behaves like the endogenous C1q. To detect circulating immune complexes (CIC) which had already bound C1q, human sera were added to tubes coated with anti-C1q. Under the conditions used, no C1q-bearing CIC were detected. In addition, 7 sera from patients with high C1q-BA were analyzed by sucrose-gradient ultracentrifugation. No C1q was found in the fast sedimenting fractions, although C1q-BA was detected in these fractions. With IgG-coated tubes we observed that PEG enhanced the binding of 125I-C1q as well as endogenous C1q to aggregated and monomeric IgG. PEG also enhanced the binding of CIC to C1q-coated tubes. The results suggest that CIC detected by the C1q-BT do not bear C1q in significant amounts in the circulation and that these CIC become detectable only in the presence of PEG.