Antibody binding geometry and affinity control inhibitory hFcγRIIB receptor signaling

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• The inhibitory human Fc gamma receptor IIB (hFcγRIIB) plays a crucial role in humoral immunity and regulates antibody-mediated effector functions.
• Antibodies targeting hFcγRIIB can modulate its function, but the mechanistic differences between agonistic and antagonistic effects are not fully understood.

Purpose of the Study:

• To elucidate the molecular mechanisms differentiating agonistic and antagonistic anti-hFcγRIIB antibody binding.
• To investigate how antibody binding influences hFcγRIIB mobility, clustering, and interaction with lipid rafts.

Main Methods:

• Crystallographic structure determination and alanine-scanning mutagenesis to identify antibody epitopes.
• Small-angle X-ray scattering (SAXS) and molecular dynamics simulations to analyze receptor complex formation.
• Measurements of receptor mobility, clustering, and lipid raft redistribution in the plasma membrane.

Main Results:

• Agonistic antibodies, characterized by lower affinity and higher off-rates, reduce hFcγRIIB mobility, induce receptor clustering, and promote redistribution into lipid rafts.
• Antagonistic antibodies exhibit distinct binding geometries and lower off-rates, hindering effective receptor clustering.
• Epitopes targeted by agonistic and antagonistic antibodies are overlapping yet distinct, influencing receptor complex formation and dynamics.

Conclusions:

• Agonists likely employ a 'catch-and-release' mechanism via high off-rates to facilitate hFcγRIIB clustering and activation.
• Antagonists' binding geometry and low off-rates prevent efficient clustering, thereby inhibiting receptor function.
• These findings provide fundamental principles for understanding agonism versus antagonism in immunomodulatory receptor signaling.