Sialic Acid and Fucose Residues on the SARS-CoV‑2 Receptor-Binding Domain Modulate IgG Antibody Reactivity

The receptor-binding domain (RBD) of the SARS-CoV-2 spike
protein
is a conserved domain and a target for neutralizing antibodies. We
defined the carbohydrate content of the recombinant RBD produced in
different mammalian cells. We found a higher degree of complex-type
N-linked glycans, with less sialylation and more fucosylation, when
the RBD was produced in human embryonic kidney cells compared to the
same protein produced in Chinese hamster ovary cells. The carbohydrates
on the RBD proteins were enzymatically modulated, and the effect on
antibody reactivity was evaluated with serum samples from SARS-CoV-2
positive patients. Removal of all carbohydrates diminished antibody
reactivity, while removal of only sialic acids or terminal fucoses
improved the reactivity. The RBD produced in Lec3.2.8.1-cells, which
generate carbohydrate structures devoid of sialic acids and with reduced
fucose content, exhibited enhanced antibody reactivity, verifying
the importance of these specific monosaccharides. The results can
be of importance for the design of future vaccine candidates, indicating
that it is possible to enhance the immunogenicity of recombinant viral
proteins.