Antigen-Induced Allosteric Changes in a Human IgG1 Fc Increase Low-Affinity Fcγ Receptor Binding

Abstract

Antibody structure couples adaptive and innate immunity via Fab (antigen binding) and Fc (effector) domains that are connected by unique hinge regions. Because antibodies harbor two or more Fab domains, they are capable of crosslinking multi-determinant antigens, which is required for Fc-dependent functions through associative interactions with effector ligands, including C1q and cell surface Fc receptors. The modular nature of antibodies, with distal ligand binding sites for antigen and Fc-ligands, is reminiscent of allosteric proteins, suggesting that allosteric interactions might contribute to Fc-mediated effector functions. This hypothesis has been pursued for over 40 years and remains unresolved. Here, we provide evidence that allosteric interactions between Fab and Fc triggered by antigen binding modulate binding of Fc to low-affinity Fc receptors (Fc?R) for a human IgG1. This work opens the path to further dissection of the relative roles of allosteric and associative interactions in Fc-mediated effector functions. Orlandi et al. show that antigen binding to Fab increases binding of IgG1 Fc to low-affinity Fc?R by conformational allostery. Leucine to alanine mutations at positions 234 and 235 in the lower hinge of IgG1 globally alter Fc structure and biological activity by configurational allostery.