Binding of aggregated human gamma globulin by Raji cells: C1q will enhance only if it is dissociated from the C1 macromolecular complex.

The role of complement components in binding of aggregated human gamma globulin (AHG) to Raji cells was examined using the Raji cell radioimmunoassay. Incubation of AHG in normal human serum enhanced up to five-fold the binding of these complexes by Raji cells. This enhanced binding was medicated primarily by C3 receptors, however, as much as 30% of the enhanced binding was due to a heat-labile protein in serum. AHG incubated with serum-EDTA bound to Raji cells up to two-fold more than AHG incubated with unchelated serum. Since purified Clq also enhanced binding, binding of AHG after incubation with serum-EDTA was probably mediated by Clq. The enhancement effected by Clq occurred only if Clq bound first to AHG, not to the Raji cells, and if Clq bound in the absence of Clr and Cls. Speculations on a role for Clq in biological processes must consider whether the Clq in serum is available to participate. The results presented here suggest that whole serum activated by AHG contained only a small amount of Clq available for cross-linking of particles. Thus, the potential involvement of Clq in biological reactions in vivo is probably limited.