Evidence of variable human Fcγ receptor-Fc affinities across differentially-complexed IgG

Area of Science:

• Immunology
• Molecular Biology
• Biophysics

Background:

• Antibody effector functions are typically explained by an associative model of IgG-Fcγ receptor (FcγR) interactions.
• This model assumes FcγRs bind free and antigen-bound IgG equally, with clustering driving immune responses.
• An alternative, conformational allostery model proposes antigen binding alters IgG, increasing FcγR affinity.

Purpose of the Study:

• To investigate the binding affinities of FcγRs for free versus antigen-bound IgG.
• To determine if antigen binding to IgG enhances FcγR affinity, supporting the antibody allostery model.
• To explore the thermodynamic differences in FcγR binding to free and immune-complexed IgG.

Main Methods:

• Utilized multiplexed, label-free kinetic experiments.
• Characterized FcγR affinities for covalently immobilized, captured, and antigen-bound IgG.
• Employed an orthogonal label-free method to measure thermodynamic signatures of FcγR binding.

Main Results:

• Fcγ receptors demonstrated significantly greater affinity for antigen-bound IgG compared to free IgG across tested methods.
• This enhanced affinity phenomenon was consistent across multiple FcγRs, antigens, antibody specificities, and subclasses.
• Thermodynamic signatures differed between free and immune-complexed IgG, but overall affinity trends were not fully recapitulated.

Conclusions:

• The findings provide strong evidence supporting the antibody allostery model over the traditional associative model.
• Antigen-bound IgG exhibits a distinct conformation recognized with higher affinity by FcγRs.
• Further research is needed to elucidate additional factors influencing FcγR binding thermodynamics and immune complex recognition.