Antigen-independent binding of IgG dimers to C1 q as studied by sedimentation equilibrium, complement fixation and electron microscopy

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Abstract
Rabbit anti-SII pneumococcal polysaccharide IgG antibody was cross-linked by dithiobis (succinimidyl propionate). The IgG dimers were visualized by electronmicroscopy and resembled two Y-shaped structures connected in random orientations. The three arms of the Y's were of equal length. The six globular domains of the IgG protomers could be seen, but the Fab arms and Fc stems could not be distinguished from each other. Electron microscopy revealed that the dimers were bound to the globular heads of C1 q. From the dependence of the weight average molecular weight on dimer concentration, an equilibrium constant of about 106 M−1 was derived for the binding of dimers to C1 q. The number of IgG dimers which could be accommodated at a single C1q molecule was derived to be three. The data do not allow a clear distinction between noncooperative and cooperative binding. The binding equilibrium was independent of whether the IgG dimers were liganded with an SII nonasaccharide hapten or not. The results are at variance with an allosteric mechanism of the action of antigen in C1 activation. They lend support to the association hypothesis of complement activation. The data suggest that clusters of about six IgG molecules, connected by a multivalent antigen or arranged at the cell surface, are recognized by C1 q with a binding constant of about 1010 M−1.