Aberrantly glycosylated IgA1 in IgA nephropathy patients is recognized by IgG antibodies with restricted heterogeneity

Abstract

IgA nephropathy (IgAN) is characterized by circulating immune complexes composed of galactose-deficientIgA1 and a glycan-specific IgG antibody. These immune complexes deposit in the glomerular mesangium and induce the mesangioproliferative glomerulonephritis characteristic of IgAN. To define the precise specificities and molecular properties of the IgG antibodies, we generated EBV-immortalized IgG-secreting lymphocytes from patients with IgAN and found that the secreted IgG formed complexes with galactose-deficient IgA1 in a glycan-dependent manner. We cloned and sequenced the heavy-and light-chain antigen-binding domains of IgG specific for galactose-deficient IgA1 and identified an A to S substitution in the complementarity-determining region 3 of the variable region of the gene encoding the IgG heavy chain in IgAN patients. Furthermore, site-directed mutagenesis that reverted the residue to alanine reduced the binding of recombinant IgG to galactose-deficient IgA1. Finally, we developed a dot-blot assay for the glycan-specific IgG antibody that differentiated patients with IgAN from healthy and disease controls with 88% specificity and 95% sensitivity and found that elevated levels of this antibody in the sera of patients with IgAN correlated with proteinuria. Collectively, these findings indicate that glycan-specific antibodies are associated with the development of IgAN and may represent a disease-specific marker and potential therapeutic target.

Clustered O-Glycans of IgA1

“…In this work, we demonstrate the complete analysis of O-glycoform microheterogeneity and site localization of the glycoforms in a naturally Gal-deficient IgA1 (Ale) myeloma protein that mimics the nephritogenic IgA1 in patients with IgAN (8,9). Reversed phase (RP) LC FT-ICR MS successfully identified 10 distinct IgA1 HR fragments representing Ͼ99% of total IgA1.…”

“…This distinctive IgA1 is in circulating immune complexes (8,10,15) and in the glomerular deposits of IgAN patients (16,29). The absence of Gal apparently leads to the exposure of neoepitopes, including terminal and sialylated N-acetylgalactosamine (GalNAc) residues (9,10). These epitopes are recognized by naturally occurring antiglycan IgG or IgA1 antibodies and, consequently, circulating immune complexes are formed (9,10,15) that can deposit in the glomerular mesangia.…”

“…The absence of Gal apparently leads to the exposure of neoepitopes, including terminal and sialylated N-acetylgalactosamine (GalNAc) residues (9,10). These epitopes are recognized by naturally occurring antiglycan IgG or IgA1 antibodies and, consequently, circulating immune complexes are formed (9,10,15) that can deposit in the glomerular mesangia. To identify the pathogenic forms of IgA1, a thorough analysis of O-glycan microheterogeneity, including identification of the attachment sites, will be required.…”

Spleen Tyrosine Kinase Is Important in the Production of Proinflammatory Cytokines and Cell Proliferation in Human Mesangial Cells following Stimulation with IgA1 Isolated from IgA Nephropathy Patients

“…IgA1 is unusual in that it has a heavily O-glycosylated hinge region between the CH1 and CH2 domains of the a H chain (1,2). The presence of poorly galactosylated IgA1 O-glycoforms in the serum of patients with IgAN favors formation of circulating immune complexes (IC), which are prone to deposition in the glomerular mesangium (3)(4)(5)(6). Once deposited, these IgA IC trigger mesangial cell activation.…”

Dysfunctions of the Iga system: a common link between intestinal and renal diseases

“…[66][67][68] Interestingly, it has been shown that dimers and polymers of IgA aggregate more readily than monomers, 69 which might explain the propensity of this form to aggregate and deposit in tissues. Our group has also shown that aberrant glycosylation of IgA1 and IC formation constitute essential factors favoring mesangial IgA1-TfR interactions as initial steps in IgAN pathogenesis.…”