Figure 6.

Proposed associative models of IgG hexamerization and complement activation. (A) Structure-function relationship of a triple mutant IgG1 mAb RGY (E345R/E430G/S440Y) in solution: 1) enhanced CDC activity relative to wild-type IgG1–005 and IgG1-E345R; 2) an overview electron tomography (ET) image showing a monomer (small circle) and a hexamer (large circle); 3) a representative hexamer with colored Fab pairs; and 4) ET average of 200 subtomograms at a resolution of 2.9 nm. Figure reproduced from¹⁰² with permission from AAAS. (B) Native mass spectrometry analysis of reconstructed C1, C1:IgG, and C1:IgG:Ag complexes. The assembly C1q:C1r:C1s stoichiometry of 1:2:2 is consistent with the reported composition of natural C1.¹⁰³ The C1 exhibits the same IgG binding stoichiometry as C1q. In the presence of excess soluble antigen, the assembly C1:IgG:Ag stoichiometry of 1:6:12 is the predominant species. (C) Model summarizing the molecular determinants for IgG-mediated activation of the classical component pathway: 1) availability of antigen and epitope distribution; 2) ability of antigen to cluster IgG at the cell surface or in solution; 3) Fc-Fc associative cooperativity required for hexamerization; 4) avidity binding sites for hexavalent C1q; 5) composition of Fc oligosaccharide; 6) Fab-Fab intermolecular cooperativity; and 7) antigen-induced conformational allostery to affect downstream Fc-mediated complement activation. Figures reproduced from¹⁰³ with permission from Elsevier.