Increased sialylation of polymeric lambda-IgA1 in patients with IgA nephropathy

The mechanism of mesangial IgA deposition is poorly understood in IgA nephropathy (IgAN). Abnormal glycosylation of carbohydrate moieties in the hinge region of the IgA molecule has recently attracted much attention. In this report, we studied galactosylation and sialylation profiles in kappa- and lambda-IgA1 from patients with IgAN. Total serum IgA1 was isolated from patients with IgAN or healthy controls by jacalin-affinity chromatography. Six fractions of molecular weight (MW) 50-1,000 kDa were separated by fast protein liquid chromatography (FPLC). Four lectin-binding assays were used to study the sialylation and the presence of terminal galactose or N-acetylgalactosamine (GalNAc) in the O-linked carbohydrate moieties of kappa- or lambda-IgA1. Maackia amurensis agglutinin (MAA) and Sambucus nigra agglutinin (SNA) lectin recognize alpha(2,3)- and alpha(2,6)-linked sialic acid, respectively. Peanut agglutinin (PNA) and Helix aspersa (HA) lectin recognize terminal galactose and GalNAc, respectively. Reduced HA was demonstrated in macromolecular kappa or lambda-IgA1 (300-825 kDa) isolated from patients with IgAN (P < 0.05 compared with healthy controls). Lambda- but not kappa-IgA1 from patients with IgAN bound less to PNA (P < 0.05). The alpha(2,3)-linked sialic acid content in lambda- but not kappa-IgA1 of MW 150-610 kDa from patients was higher than that of controls (P < 0.005). The alpha(2,6)-linked sialic acid content in lambda-IgA1 (300-825 kDa) and kappa-IgA1 (150-610 kDa) from patients was also higher than that of controls. This unusual glycosylation and sialylation pattern of the lambda-IgA1 may have important implications for the pathogenesis of IgAN, as both the masking effect of sialic acid on galactose and the reduced galactosylation will hinder the clearance of macromolecular lambda-IgA1 by asialoglycoprotein receptor of hepatocytes. The negative charge from sialic acid may also favor mesangial deposition of macromolecular lambda-IgA1 in IgAN.