Immunoglobulin A (IgA) nephropathy is the most prevalent form of glomerulonephritis worldwide. A renal biopsy is required for an accurate diagnosis, as no convenient biomarker is currently available. We developed a serological test based upon the observation that this nephropathy is characterized by undergalactosylated IgA1 in the circulation and in mesangial immune deposits. In the absence of galactose, the terminal saccharide of O-linked chains in the hinge region of IgA1 is terminal or sialylated N-acetylgalactosamine. A lectin from Helix aspersa, recognizing N-acetylgalactosamine, was used to develop an enzyme-linked immunosorbent assay that measures galactose-deficient IgA1 in serum. The median serum lectin-binding IgA1 level was significantly higher for 153 Caucasian adult patients with IgA nephropathy without progression to end-stage renal disease as compared with that for 150 healthy Caucasian adult controls. As the lectin-binding IgA1 levels for the controls were not normally distributed, the 90th percentile was used for determination of significant elevation. Using a value of 1076 U/ml as the upper limit of normal, 117 of the 153 patients with IgA nephropathy had an elevated serum lectin-binding IgA1 level. The sensitivity as a diagnostic test was 76.5%, with specificity 94%; the positive predictive value was 88.6% and the negative predictive value was 78.9%. We conclude that this lectin-binding assay may have potential as a noninvasive diagnostic test for IgA nephropathy.

Complement activation has a role in the pathogenesis of IgA nephropathy, an autoimmune disease mediated by pathogenic immune complexes consisting of galactose-deficient IgA1 bound by antiglycan antibodies. Of three complement-activation pathways, the alternative and lectin pathways are involved in IgA nephropathy. IgA1 can activate both pathways in vitro, and pathway components are present in the mesangial immunodeposits, including properdin and factor H in the alternative pathway and mannan-binding lectin, mannan-binding lectin-associated serine proteases 1 and 2, and C4d in the lectin pathway. Genome-wide association studies identified deletion of complement factor H-related genes 1 and 3 as protective against the disease. Because the corresponding gene products compete with factor H in the regulation of the alternative pathway, it has been hypothesized that the absence of these genes could lead to more potent inhibition of complement by factor H. Complement activation can take place directly on IgA1-containing immune complexes in circulation and/or after their deposition in the mesangium. Notably, complement factors and their fragments may serve as biomarkers of IgA nephropathy in serum, urine, or renal tissue. A better understanding of the role of complement in IgA nephropathy may provide potential targets and rationale for development of complement-targeting therapy of the disease.

IgA nephropathy (IgAN) is characterized by circulating immune complexes composed of galactose-deficient IgA1 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.

Aberrant glycosylation of IgA1 plays an essential role in the pathogenesis of IgA nephropathy. This abnormality is manifested by a deficiency of galactose in the hinge-region O-linked glycans of IgA1. Biosynthesis of these glycans occurs in a stepwise fashion beginning with the addition of N-acetylgalactosamine by the enzyme N-acetylgalactosaminyltransferase 2 and continuing with the addition of either galactose by β1,3-galactosyltransferase or a terminal sialic acid by a N-acetylgalactosamine–specific α2,6-sialyltransferase. To identify the molecular basis for the aberrant IgA glycosylation, we established EBV-immortalized IgA1-producing cells from peripheral blood cells of patients with IgA nephropathy. The secreted IgA1 was mostly polymeric and had galactose-deficient O-linked glycans, characterized by a terminal or sialylated N-acetylgalactosamine. As controls, we showed that EBV-immortalized cells from patients with lupus nephritis and healthy individuals did not produce IgA with the defective galactosylation pattern. Analysis of the biosynthetic pathways in cloned EBV-immortalized cells from patients with IgA nephropathy indicated a decrease in β1,3-galactosyltransferase activity and an increase in N-acetylgalactosamine–specific α2,6-sialyltransferase activity. Also, expression of β1,3-galactosyltransferase was significantly lower, and that of N-acetylgalactosamine–specific α2,6-sialyltransferase was significantly higher than the expression of these genes in the control cells. Thus, our data suggest that premature sialylation likely contributes to the aberrant IgA1 glycosylation in IgA nephropathy and may represent a new therapeutic target.