Complement C1q binding affects spin-labeled heterosaccharides of rabbit antibodies in immune but not artificial immunoglobulin G aggregates.

IgG anti-hapten antibodies were purified from the sera of rabbits homozygous for allotypic determinants d11 and d12 in the constant region of the heavy chain. Correlative with this determinant is the absence (d11) or presence (d12) of an oligosaccharide chain just below the hinge region of the IgG molecule. Both d11 and d12 molecules contain a complex heterosaccharide chain located near the carboxyl terminus of the second constant region domain. The two populations of IgG antibodies were thus selectively labeled with the spin probe Tempamine in their second constant region domains by reductive amination primarily of terminal N-acetylneuraminic acid residues. Chemical and enzymatic cleavages showed about 80% of the attached spin labels were N-acetylneuraminic acid-associated. Analysis of probe adducts by ESR spectrometry showed the presence of slower and faster moving subcomponents. Formation of immune complexes by antigen induces slight but significant restrictions of spin label mobility for both d11 and d12 IgG molecules. This restriction is qualitatively different from that seen in glutaraldehyde-, carbodiimide-, or ethanol-induced aggregates of the same IgG antibodies. The addition of purified complement C1 subcomponent C1q to immune aggregates resulted in marked immobilization of spin labels, the rotational correlation time of which was 30-40 mu s for both d11 and d12 molecules (evaluated by saturation transfer spectroscopy). A similar spin probe immobilizing effect is not seen when C1q binds to chemically aggregated IgG antibodies (which also do not activate C1). A novel model is proposed in which C1q is hypothesized to juxtapose Fc moieties in a discrete fashion required for subsequent C1 activation processes mediated by immune complexes.