Lack of activation of C1, despite circulating immune complexes detected by two C1q methods, in patients with rheumatoid arthritis

Abstract The activation of C1 by circulating immune complexes in patients with rheumatoid arthritis was investigated. C\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm{1r}}} $\end{document} C\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm{1s}}} $\end{document} 1s(C\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{\bar 1}} $\end{document} ‐In)2 complexes in EDTA‐plasma, reflecting C1 activation in vivo, were slightly raised in 35 of 57 patients with rheumatoid arthritis, though most patients had elevated levels of circulating immune complexes as measured with either the 125I‐C1q binding test or the C1q solid phase assay. The activation of C1 by circulating immune complexes in vitro was investigated by measuring the generation of C\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm{1r}}} $\end{document} C\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm{1s}}} $\end{document} (C\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{\bar 1}} $\end{document} ‐In)2 complexes during 60 minutes at 37oC in diluted recalcified EDTA‐plasma. In 16 of the 57 patients, a slightly increased C1 activation in vitro was observed. These patients tended to have high levels of circulating immune complexes. However, the majority of the patients with high levels of circulating immune complexes showed a normal C1 activation in vitro. Therefore, it was concluded that measurement of circulating immune complexes by either of the two C1q methods in patients with rheumatoid arthritis does not imply that these circulating immune complexes are able to activate C1.