AbstractAntibodies are glycoproteins that carry a conserved N-linked carbohydrate attached to the Fc, whose presence and fine structure profoundly impacts on theirin vivoimmunogenicity, pharmacokinetics and functional attributes. The host cell line used to produce IgG has a major impact on this glycosylation, as different systems express different glycosylation enzymes and transporters that contribute to the specificity and heterogeneity of the final IgG-Fc glycosylation profile. Here we compare two panels of glycan-adapted IgG1-Fc mutants expressed in either the HEK 293-F or CHO-K1 systems. We show that the types of N-linked glycans between matched pairs of Fc mutants vary significantly, and in particular with respect to sialylation. These cell line effects on glycosylation profoundly influence the ability of the engineered Fcs to interact with either human or pathogen receptors. For example, we describe Fc mutants that potently disrupted influenza B-mediated agglutination of human erythrocytes when expressed in CHO-K1 but not in HEK 293-F cells.