The structural basis for the anti-inflammatory activity of sialylated antibodies (THER5P.822)

Abstract

Abstract Antibodies (Ab) mediate pro-inflammatory effector functions through the engagement of Fc receptors (FcγRs) expressed on leukocytes. A complex, N-linked glycan attached to the Fc domain of IgG regulates the interaction to FcγRs by maintaining the Fc in a biologically active conformation. However, attaching sialic acid sugar residues to this Fc-associated glycan reduces affinity for FcγRs and confers binding to an alternate class of receptors, the C-type lectins (DC-SIGN). This switch in receptor specificity coincides with a switch in effector function in vivo as sialylated IgG (sFc) suppresses inflammation in mouse models of autoimmunity. The structural mechanism that regulates the sialic acid-dependent change in Ab effector function remains poorly understood. Biophysical analysis reveals that sialylation destabilizes the Fc fragment by increasing the hydrophobic surface area exposed to solvent. Upon solving a crystal structure of sFc, we observe that this loss of stability correlates with inter-domain flexibility such that the sFc in the crystal is found in both “open” and “closed” states. Mutagenic analysis of sFc further identifies key contact sites between the Fc-glycan and the Fc-backbone necessary to induce this conformational change and attain anti-inflammatory activity in vivo. These studies demonstrate that the conformational diversity of the Fc fragment serves as a general strategy to shift receptor specificity in order to effect different immunological outcomes.