Increased N-linked glycosylation leading to oversialylation of monomeric immunoglobulin A1 from patients with Sjogren's syndrome

Increased serum immunoglobulin A (IgA) level is a common finding in primary Sjogren's syndrome (pSS). IgA might not be properly eliminated because of an abnormal glycosylation. We reported previously that IgA(1) from patients with pSS was oversialylated. We extend this finding by showing that monomeric IgA(1) contains more sialic acid (SA) in patients than in controls, as determined by enzyme-linked immunosorbent assay (ELISA) and Western blot with Sambucus nigra agglutinin (SNA), a lectin specific for SA. To localize this excess of SA on the N- and/or O-linked oligosaccharides, we analysed them separately, using N- and O-linked oligosaccharide profiling kits based on fluorophore-assisted carbohydrate electophoresis. N-linked, but not O-linked, oligosaccharides of patients' IgA(1) were oversialylated, and this seemed to be linked to an excess of SA on the same number of polysaccharides as normal IgA(1). To localize the abnormality to the Fab and/or Fc fragments, monomeric IgA(1) was digested with protease, separated and transferred to nitrocellulose, where SA was identified by SNA. Both Fab and Fc fragments appeared to be oversialylated. Oversialylation of N-linked oligosaccharides of IgA(1) from patients with pSS might prevent the recognition of IgA by receptors that are responsible for their clearance, resulting in an excess of serum IgA and related immune complexes.