Detection of Immune Complexes in Unheated Sera by a Modified 125I-C1q Binding Test

Abstract The 125I-C1q binding test was modified in order to allow for the detection of immune complexes in native unheated human serum. Indeed, heat-inactivation (56°, 30 min) was found to reduce the C1q-binding activity of immune complexes mixed with native serum. This effect was not observed when EDTA was added to the native serum before mixing with immune complexes. The modified 125I-C1q binding test was performed in two steps: first, the tested native serum sample was incubated for 30 min at 37°C with 0.13 M EDTA in order to prevent the integration of 125I-C1q into the intrinsic C1qrs complex, second, 125I-C1q and polyethylene glycol (final concentration 2.5%) were added to this mixture, and further incubated for 1 hr at 4°C. Under these conditions, free C1q remained soluble whereas C1q bound to macromolecular complexes was precipitated. The competitive effect of intrinsic C1q and the interference of other substances such as DNA or bacterial LPS were very limited. The modified C1q binding test was applied to the clinical investigation of 44 patients with systemic lupus erythematosus; and increased C1q binding activity (C1q-BA) was observed in 91% of the samples. The level of C1q-BA was found to be significantly correlated to the DNA-binding capacity and to the decrease of the level of some complement components.