Regulation of alpha 2,6-sialylation in B-cells and the role of sialylation in the pathogenesis of Rheumatoid Arthritis

Sialylation is a common post-translational modification involving the addition of sialic acid to glycoprotein chains. Sialic acid within the Fc fragment of IgG molecules can influence binding to Fc receptors. In rheumatoid arthritis (RA) and other autoimmune conditions, in disease specific auto-antibodies, Fc fragment sialylation is reduced compared to total IgG. Furthermore, plasmablasts from patients with RA display reduced cell surface sialylation compared to cells from healthy donors. Factors which determine B-cell surface sialylation and consequences of altered sialylation are not well understood. α2,6-sialylation was measured in B-cells isolated from healthy donors (HD), patients with pre-RA (PRA) or early RA (ERA) using SNA lectin flow cytometry. Sialylation and markers of activation were measured at baseline or following stimulation with TLR ligands or anti-IgM/G ± CD40L; treatment with neuraminidase (Neu) to digest sialic acid; or culture with serum from HD or patients with ERA. B-cells were differentiated to plasma cells in vitro and sialylation measured at each stage of differentiation. Furthermore, Neu activity in serum was measured by fluorescent assay. Sialylation was confirmed to be decreased in patients with ERA and PRA at baseline compared with HD B-cells. Upon stimulation with TLR ligands, sialylation was increased in HD cells but not cells from patients with ERA or PRA. Differentiated cells showed an initial increase in sialylation before decreasing in terminally differentiated cells. Exposure to serum in culture led to reduced B-cell sialylation and Neu activity was highest in serum from patients with ERA. Exposure to serum in culture as well as direct treatment with Neu led to reduced B-cell activation potential. These results suggest that B-cell sialylation influences activation and function, and control of surface sialylation may be disrupted in RA.