Loss of ST8Sia6 mediated α-2,8-linked di-sialylation enhances T cell-dependent antibody responses and promotes autoimmunity in aged mice

Introduction Sialic acids are terminal nine-carbon sugars that form sialoglycans on the glycocalyx of all vertebrate cell surfaces. In this exposed position, they fulfill various functions in the immune system, including involvement in host-pathogen interactions and control of immune cell activation. The synthesis of sialoglycans is catalyzed by sialyltransferases. One of these is ST8Sia6, which is known to catalyze the synthesis of α-2,8-linked di-sialyl motifs and to be upregulated in plasma cells. Methods We studied the role of ST8Sia6 in the immune system by examining a St8sia6 knockout mouse model. Results The St8sia6 -/- mice showed normal B cell and plasma cell development. However, in St8sia6 -/- B cells, a stronger proliferation was induced by T cell-dependent stimuli. Additionally, an increased IgG1 response was observed upon T cell-dependent immunization. In the study, 60-week-old St8sia6 -/- mice showed enlarged populations of germinal center B cells, age-induced B cells, and plasma cells. Furthermore, aged St8sia6 -/- mice exhibited increased activation markers on their B cells and displayed elevated levels of anti-nuclear IgG autoantibodies, all of which are indicative of autoimmunity. We identified CD45 as a carrier protein of O -acetylated α2,8-linked di-sialic acids on plasma cells. Discussion These results demonstrate that the loss of ST8Sia6-mediated α-2,8-linked di-sialylation enhances T cell-dependent immune responses and promotes an accumulation of strongly activated B cells and an autoimmune-like phenotype upon aging.