Sialic acid (Sia) occupies the outermost terminus of glycans. Owing to their position, negative charge, and structural diversity, sialoglycans are key determinants in various molecular recognition events. Recently, we revealed that a monoclonal antibody, GL7, recognizes glycans with a terminal α2, 6 linked Sia and that this binding is specific to Sia species. Although its epitope is unclear, GL7 has been used as a marker for activated B cells and germinal centers. In a germinal center, immunoglobulins undergo affinity maturation and class switch recombination, making it an important microenvironment for acquired immune response mediated by B cells. Our results indicated that GL7 probes the activation-dependent conversion of major Sia species, from N-glycolylneuraminic acid (Neu5Gc) to N-acetylneuraminic acid (Neu5Ac), in mouse germinal center B cells. This change in Sia species occurred with the concomitant loss of the CD22 ligand on B cells. Furthermore, the phenotypes of Neu5Gc-deficient mice suggested that Neu5Gc negatively modulates B cell proliferation.
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