C1q solid-phase radioimmunoassay: evidence for detection of antibody directed against the collagen-like region of C1q in sera from patients with systemic lupus erythematosus.

In earlier studies we showed that the C1q-binding IgG in the sera from patients with systemic lupus erythematosus (SLE) tested by C1q solid-phase radioimmunoassay is cofractionated with monomeric IgG on gel filtration and mostly binds to C1q via the F(ab')2 region. In this study, we found that C1q, even when stripped of its immune complex-binding globular regions by pepsin digestion, retained a substantial part of its ability to bind IgG from SLE sera, suggesting that the collagen-like region of C1q is involved in binding to the SLE IgG. Heat-inactivation of C1q also failed to abolish its ability to bind IgG from SLE sera. In contrast, the binding of C1q to heat-aggregated IgG was completely abrogated by these treatments. In addition, the reaction of heat-aggregated IgG with the solid-phase C1q was markedly dependent on ionic strength whereas the binding of IgG from SLE sera with the solid-phase C1q persisted at high concentrations of salt. These findings suggest that the Clq-binding IgG in SLE sera is, at least in part, antibody directed against the collagen-like region of C1q.