Significance: The N -glycan composition of the constant (Fc) domain of IgG can modulate antibody effector functions by affecting the ability of the Fc to bind various Fc receptors (FcγRs). Most therapeutic IgG antibodies carry heterogeneous N -glycans, which might not be optimal for their therapeutic purpose. To understand the contribution of each N -glycan component on antibody effector function, we generated homogeneous IgG1 glycoforms using a chemoenzymatic approach and performed side-by-side in vitro binding, antibody-dependent cell-mediated cytotoxicity (ADCC), and in vivo IgG-mediated cell depletion assays. Our results confirm the dominant positive effect of removing the core fucose on FcγRIIIA binding and ADCC. Our study further reveals that sialylation adversely impacts ADCC in the context of core fucosylation but not in its absence.

IgG molecules exert both pro- and antiinflammatory effector functions based on the composition of the fragment crystallizable (Fc) domain glycan. Sialylated IgG Fc domains have antiinflammatory properties that are attributed to their ability to increase the activation threshold of innate effector cells to immune complexes by stimulating the upregulation of the inhibitory Fcγ receptor IIB (FcγRIIB). Here, we report that IgG Fc sialylation of human monoclonal IgG1 molecules impairs their efficacy to induce complement-mediated cytotoxicity (CDC). Fc sialylation of a CD20-targeting antibody had no impact on antibody-dependent cellular cytotoxicity and did not change the affinity of the antibody for activating Fcγ receptors. In contrast, the presence of sialic acid abrogated the increased binding of C1q to Fc-galactosylated IgG1 and resulted in decreased levels of C3b deposition on the cell surface. Similar to monoclonal antibodies, sialic acid inhibited the increased C1q binding to galactosylated Fc fragments in human polyclonal IgG. In sera derived from patients with chronic inflammatory demyelinating polyneuropathy, an autoimmune disease of the peripheral nervous system in which humoral immune responses mediate tissue damage, induction of IgG Fc sialylation was associated with clinical disease remission. Thus, impairment of CDC represents an FcγR-independent mechanism by which Fc-sialylated glycovariants might limit proinflammatory IgG effector functions.