Mixed IgG Fc immune complexes exhibit blended binding profiles and refine FcR affinity estimates
Immunoglobulin (Ig)G antibodies coordinate immune effector responses by selectively binding to target antigens and then interacting with various effector cells via the Fc{gamma} receptors. The Fc domain of IgG can promote or inhibit distinct effector responses across several different immune cell types through variation based on subclass and Fc domain glycosylation. Extensive characterization of these interactions has revealed how the inclusion of certain Fc subclasses or glycans results in distinct immune responses. During an immune response, however, IgG is produced with mixtures of Fc domain properties, so antigen-IgG immune complexes are likely to almost always be comprised of a combination of Fc forms. Whether and how this mixed composition influences immune effector responses has not been examined. Here, we measured Fc{gamma} receptor binding to immune complexes of mixed Fc domain composition. We found that the binding properties of the mixed-composition immune complexes fell along a continuum between those of the corresponding pure cases. Binding quantitatively matched a mechanistic binding model, except for several low-affinity interactions mostly involving IgG2. We found that the affinities of these interactions are different than previously reported, and that the binding model could be used to provide refined estimates of these affinities. Finally, we demonstrated that the binding model can predict effector-cell elicited platelet depletion in humanized mice, with the model inferring the relevant effector cell populations. Contrary to the previous view in which IgG2 poorly engages with effector populations, we observe appreciable binding through avidity, but insufficient amounts to observe immune effector responses. Overall, this work demonstrates a quantitative framework for reasoning about effector response regulation arising from IgG of mixed Fc composition. Summary pointsO_LIThe binding behavior of mixed Fc immune complexes is a blend of the binding properties for each constituent IgG species. C_LIO_LIAn equilibrium, multivalent binding model can be generalized to incorporate immune complexes of mixed Fc composition. C_LIO_LIParticularly for low-affinity IgG-Fc{gamma} receptor interactions, immune complexes provide better estimates of affinities. C_LIO_LIThe Fc{gamma}R binding model predicts effector-elicited cell clearance in humanized mice. C_LI