Charge Distribution of IgA-Lambda in IgA Nephropathy

Abstract

The finding that eluted mesangial IgA and serum IgA from patients with IgA nephropathy had a restricted anionic charge contrasting with normal serum IgA prompted us to study the charge of the kappa and lambda subclasses of IgA. Serum IgA from 11 patients with IgA nephropathy and 11 controls was purified by affinity chromatography by binding to jacalin. The charges of IgA were studied by a novel method. The spectrotype of total IgA was first studied by isoelectric focusing and immunoblotting. IgAλ and IgAκ was further analyzed by reacting with specific monoclonal antibodies. The amount of IgA with different pis was analyzed by computerized densitometry. The anionic :cationic (A: C) ratio of IgA using pi 5.6 as the dividing point was greater in patients (at clinical quiescence and during exacerbation) than in controls (1.67 ± 0.31 versus 1.36 ± 0.27, p < 0.025). IgAλ in both groups was anionic (A:C ratio 2.22 ± 0.77 versus 2.36 ± 0.36) and IgAκ was cationic (A:C ratio 1.15 ± 0.36 versus 1.04 ± 0.39) but no difference in the A:C ratio was demonstrated. The increased A:C ratio in total IgA in patients was due to raised serum IgAλ (κ/λ ratio 1.11 ± 0.14 in patients and 1.51 ± 0.16 in controls, p < 0.01). We had previously shown a predominant mesangial deposition of IgAλ in IgA nephropathy. Animal experiments have revealed the preferential mesangial deposition of immune complexes is related to their anionic charges. Our data of raised anionic IgAλ in IgA nephropathy may be important in determining its selective mesangial binding that could contribute to the immunopathogenesis.

Sera of IgA Nephropathy Patients Contain a Heterogeneous Population of Relatively Cationic Alpha-Heavy Chains

“…Our results demonstrate that champedak lectin-C was a better and more sensitive detecting reagent for Western blotting of the α-heavy chain spots as compared to the α-heavy chain-specific anti-human IgA polyclonal antiserum that was earlier used. The pI of the α-heavy chain from IgAN patients and healthy controls resolved by this method spanned from 4.4 to 6.8, a result which is compatible with several previously reported studies on IgA [10, 11, 12]. …”

“…2). The α-heavy chain band detected from the Western blot analysis was divided into the anionic (pI <5.6) and cationic (pI >5.6) regions according to previously adopted methods [8, 9, 11]. When compared with the Coomassie-stained 2-D gels that were subjected to densitometric scanning and analysis, it was obvious that the Coomassie-stained 2-D separation was only capable of resolving the anionic region of the α-heavy chain band as the cationic region of the band was superimposed with several other protein spots.…”

“…In addition, the λ-containing IgA1 of IgAN patients were shown to be more negatively charged [8, 9, 10, 11], and higher association of λ than ĸ light chains with IgA1 of IgAN patients has been observed [12, 13, 14]. The unusually glycosylated IgA from IgAN patients and IgA1 with modified O -glycan structures were also demonstrated to have increased binding to mesangial cells [15, 16].…”

IgA Structure Variations Associate with Immune Stimulations and IgA Mesangial Deposition

“…1 Various inconsistent abnormalities of deposited IgA have been reported: antigen coprecipitation, 2,3 negative electrostatic charge, [4][5][6] and altered patterns of IgA glycosylation with hinge hypogalactosylation 7 and increased exposition of N-acetylgalactosamine (GalNac). 8 However, hypogalactosylation also occurs without IgAN.…”

Polymeric IgA1 from Patients with IgA Nephropathy Upregulates Transforming Growth Factor-β Synthesis and Signal Transduction in Human Mesangial Cells via the Renin-Angiotensin System

“…The disease is characterized by mesangial deposition of polymeric IgA1 (pIgA1), proliferation of mesangial cells, increased synthesis of extracellular matrix, and infiltration by macrophages, monocytes, and T cells (2). Immunoregulatory abnormalities involving IgA1 synthesis in IgAN include overproduction of IgA1 by B lymphocytes in vitro (3,4), increased serum levels of IgA, IgA-containing immune complexes (5,6), and anionic IgA (7), and abnormal glycosylation of the carbohydrate moieties in IgA1 from these patients (8). Five different IgA receptors have been identified in human subjects, including Fc␣R1 or CD89, the polymeric Ig receptor, the asialoglycoprotein receptor, Fc␣/ R, and the transferrin receptor or CD71.…”