Dimeric, trimeric and tetrameric complexes of immunoglobulin G fix complement

Abstract

The binding of pure dimers, trimers and tetramers of randomly cross-linked non-immune rabbit immunoglobulin G to the first component and subcomponent of the complement system, C1 and C1q respectively, was studied. These oligomers possessed open linear structures. All three oligomers fixed complement with decreasing affinity in the order: tetramer, trimer, dimer. Complement fixation by dimeric immunoglobulin exhibited the strongest concentration-dependence. No clear distinction between a non-co-operative and a co-operative binding mechanism could be achieved, although the steepness of the complement-fixation curves for dimers and trimers was better reflected by the co-operative mechanism. Intrinsic binding constants were about 10(6)M-1 for dimers, 10(7)M-1 for trimers and 3 X 10(9)M-1 for tetramers, assuming non-co-operative binding. The data are consistent with a maximum valency of complement component C1 for immunoglobulin G protomers in the range 6-18. The binding of dimers to purified complement subcomponent C1q was demonstrated by sedimentation-velocity ultracentrifugation. Mild reduction of the complexes by dithioerythritol caused the immunoglobulin to revert to the monomeric state (S20,w = 6.2-6.5S) with concomitant loss of complement-fixing ability.

Structural Analysis of the Protein/Lipid Complexes Associated with Pore Formation by the Bacterial Toxin Pneumolysin

“…Such a phenomenon may also occur with pneumolysin, because the domain through which pneumolysin activates complement (the Cterminal domain) has a fold very similar to IgG-Fc, with which it interacts during complement activation (8,2,17). IgG itself has a hyperactivating effect on complement when oligomerized (27), which suggests the possibility that the lipids associated with free oligomers may themselves elicit an immune response because nonbilayer (nonplanar) lipid structures have been shown to be highly immunogenic (28). The immunogenicity of such structures is thought to result from their high curvature (28).…”

Assembly of subcomponents C1r and C1s of first component of complement: electron microscopic and ultracentrifugal studies.

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ABSTRACTMonomeric Cis (Mr, 85,000; sso,,,4.3 S), a subcomponent of first component of complement (C),the dimer (Mr, 170,000; s20,w, 6.7 (sow, 16.3 S) reconstituted from Clq, Cir, and Cis dissociates under the conditions used for electron microscopy. Some features of the C1 complex are revealed in the dissociation products.

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Conformational changes in C1q upon binding to IgG oligomers

“…Samples containing predominantly IgG trimers and tetramers were pooled and stored at 4°C prior to use. Such oligomers have been shown to be stable for up to 2 months at -70°C [9]. Samples containing monomeric IgG were treated similarly.…”

“…Antiovalbumin rabbit IgG and immune complexes (prepared at the equivalence IgG : ovalbumin ratio) were obtained as in [71. Human IgG was prepared as in [8]; 250 mg of lgG was chemically crosslinked by dithiobis(succinimidylpropionate), with a crosslinker:lgG ratio of 1.8:1 (tool/tool), as in method B of [9]. The IgG oligomers were fractionated by gel filtration over Sephacryl S-300, equilibrated with 0.01 M phosphate, 0.15 M NaCI, pH 7.2.…”