Glycan Structures of Avian IgG: The Presence of Both Conserved and Species-Specific Glycans

Abstract

Avian IgGs, also called IgYs, are a major immunoglobulin in serum and egg yolk. Unlike mammalian IgGs, avian IgGs lack a highly flexible hinge between the Fab and Fc regions, but possess one additional immunoglobulin domain (CH2 domain) on the constant region of a heavy chain. By contrast, the CH3 domain of avian IgGs resembles the CH2 domain of mammalian IgGs, and one N-glycosylation site is located at a corresponding position in these two domains. N-Glycans at this position are located inside a cavity of two heavy chains, and are considered important for stabilization of the Fc region. However, only high mannose-type N-glycans including monoglucosylated forms (Glc₁Man_8–9GlcNAc₂), are present at the conserved N-glycosylation site in the CH3 domain of avian IgGs, whereas biantennary complex-type N-glycans are generally present on mammalian IgGs. On the other hand, the structures of complex-type N-glycans of avian IgGs, which are most likely located on the CH2 domain and variable regions, are highly heterogeneous due to alteration of glycan sequences at non-reducing termini. N-Glycan structures from chicken, quail, pigeon, gull, turkey, guineafowl, and peafowl IgGs, revealed that some contain Galα1-4Gal and/or Galβ1-4Gal sequences in a species-specific manner. The data suggested that the glycan structures of avian IgGs are very useful indicators to explore species-specific glycan differentiations among avian species.