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Factor I co-factor activity of CR1 overcomes the protective effect of IgG on covalently bound C3b residues
Insights
Covalently bound C3b (C3b-IgG) interacts similarly with erythrocyte CR1 as free C3b, but its inactivation is slower. This suggests immune complex-bound C3b may rely more on factor H for clearance in the body.
Area of Science:
- Immunology
- Complement System
- Protein Interactions
Background:
- C3b bound to IgG (C3b-IgG) has reduced affinity for factor H, increasing its survival.
- Erythrocyte CR1 is a key co-factor for factor I-mediated inactivation of C3b on immune complexes.
- The interaction of C3b-IgG with CR1 is not well understood.
Purpose of the Study:
- To investigate the effect of covalently bound IgG on the interaction between C3b and erythrocyte CR1.
- To compare the cofactor activity of CR1 for the inactivation of C3b and C3b-IgG by factor I.
Main Methods:
- Binding assays of monomeric C3b and C3b-IgG to human erythrocyte CR1.
- Factor I-mediated cleavage assays using CR1 as a cofactor.
- Analysis of C3b and C3b-IgG inactivation rates at varying ionic strengths.
Main Results:
- C3b and C3b-IgG exhibit identical binding to CR1 regarding ionic strength dependence, binding sites, and affinity.
- CR1 supports similar cleavage rates for C3b and C3b-IgG by factor I.
- CR1-mediated inactivation of C3b-IgG is significantly slower than factor H-mediated inactivation at physiologic ionic strength.
Conclusions:
- CR1 does not recognize C3b-IgG as being in a protected site.
- Efficient CR1 cofactor function for C3b-IgG likely requires multivalent interactions.
- Inactivation of C3b-IgG in vivo may predominantly depend on factor H, leading to increased C3b survival.
Abstract:
We have shown previously that C3b resides in a protected site when it is covalently bound to IgG (C3b-IgG). Such C3b displays a reduced affinity for factor H, with consequent enhanced survival in the presence of factors H and I and increased capacity for promoting alternative pathway consumption of C3. Because erythrocyte CR1 may be a major co-factor for factor I-mediated inactivation of immune complex-borne C3b in blood, we have examined the effect of covalently bound IgG on the C3b-CR1 interaction. Binding of monomeric C3b and C3b-IgG to human erythrocyte CR1 demonstrates identical ionic strength dependence for both species. Identical numbers of binding sites with indistinguishable affinities are detected by both ligands. Cleavage of the alpha'-chain of C3b and the alpha'-heavy chain of C3b-IgG proceeds at the same rate when erythrocyte CR1 serves as co-factor for factor I. Unlike factor H, CR1 supports a second cleavage of fluid-phase iC3b alpha'1 chain (free or bound to IgG) that generates C3c and a 33,000 m.w. fragment, which bears antigenic markers characteristic of C3g. Inactivation of C3b and C3b-IgG by CR1 and factor I also occurs at physiologic ionic strength, but proceeds very slowly relative to rates attainable with sub-physiologic inputs of factor H. CR1 does not recognize IgG-bound C3b as being in a protected site but, because of low binding affinity at physiologic ionic strength, is probably highly dependent on multivalent ligand-receptor interactions to efficiently exert its co-factor functions. Thus, inactivation of C3b-IgG heterodimers or small immune complexes bearing limited numbers of C3b residues may remain largely factor H-dependent in vivo, with resultant enhanced C3b survival.