Yukako Ohyama

BACKGROUND: IgA nephropathy (IgAN) is the most common type of primary glomerulonephritis. Elevation in the blood levels of aberrantly glycosylated IgA1 is a crucial initial step in IgAN pathogenesis. Here, we aimed to determine the longitudinal changes in the serum levels of IgA1 O- and N-glycoforms in patients with IgAN receiving different treatments. METHODS: We enrolled Japanese patients diagnosed with primary IgAN: 10 patients who underwent tonsillectomy and corticosteroid therapy (T-CST), 7 who received corticosteroid therapy (CST), 8 who received conservative therapy (CO), and 5 with other renal diseases who received corticosteroid therapy (ORD) as disease controls. IgA was purified from patient sera collected at diagnosis and post-treatment. After sample preparation, O-glycoforms of the hinge region (HR) and N-glycoforms of the fragment crystallizable region were analyzed using high-resolution mass spectrometry (MS). RESULTS: The MS analysis of O-glycoforms of IgA1 showed that the relative abundance of IgA1 with 3GalNAc3Gal, which we previously identified as a characteristic IgA1 O-glycoform in IgAN, decreased post-treatment only in the T-CST group (P = 0.0195). Regarding N-glycoforms, the relative abundance of fucosylated N-glycan at asparagine (Asn)340 increased in the IgAN group compared with that in the ORD group (P = 0.0189) and decreased post-treatment only in the T-CST group (P = 0.0195). CONCLUSION: The MS analysis of O- and N-glycoforms of IgA1 revealed substantial changes in their abundance in the T-CST group but not in the CST, CO, and ORD groups. Our study provides new insights into how specific treatments alter the IgA1 glycoform abundance.

Galactose (Gal)-deficient IgA1 (Gd-IgA1) is involved in IgA nephropathy (IgAN) pathogenesis. To reflect racial differences in clinical characteristics, we assessed disease- and race-specific heterogeneity in the O-glycosylation of the IgA1 hinge region (HR). We determined serum Gd-IgA1 levels in Caucasians (healthy controls [HCs], n = 31; IgAN patients, n = 63) and Asians (HCs, n = 20; IgAN patients, n = 60) and analyzed profiles of serum IgA1 HR O-glycoforms. Elevated serum Gd-IgA1 levels and reduced number of Gal residues per HR were observed in Caucasians. Reduced number of N-acetylgalactosamine (GalNAc) residues per HR and elevated relative abundance of IgA1 with three HR O-glycans were common features in IgAN patients; these features were associated with elevated blood pressure and reduced renal function. We speculate that the mechanisms underlying the reduced GalNAc content in IgA1 HR may be relevant to IgAN pathogenesis.

BACKGROUND: IgA nephropathy (IgAN) primary glomerulonephritis is characterized by the deposition of circulating immune complexes composed of polymeric IgA1 molecules with altered O-glycans (Gd-IgA1) and anti-glycan antibodies in the kidney mesangium. The mesangial IgA deposits and serum IgA1 contain predominantly λ light (L) chains, but the nature and origin of such IgA remains enigmatic. METHODS: We analyzed λ L chain expression in peripheral blood B cells of 30 IgAN patients, 30 healthy controls (HCs), and 18 membranous nephropathy patients selected as disease controls (non-IgAN). RESULTS: In comparison to HCs and non-IgAN patients, peripheral blood surface/membrane bound (mb)-Gd-IgA1+ cells from IgAN patients express predominantly λ L chains. In contrast, total mb-IgA+, mb-IgG+, and mb-IgM+ cells were preferentially positive for kappa (κ) L chains, in all analyzed groups. Although minor in comparison to κ L chains, λ L chain subsets of mb-IgG+, mb-IgM+, and mb-IgA+ cells were significantly enriched in IgAN patients in comparison to non-IgAN patients and/or HCs. In contrast to HCs, the peripheral blood of IgAN patients was enriched with λ+ mb-Gd-IgA1+, CCR10+, and CCR9+ cells, which preferentially home to the upper respiratory and digestive tracts. Furthermore, we observed that mb-Gd-IgA1+ cell populations comprise more CD138+ cells and plasmablasts (CD38+) in comparison to total mb-IgA+ cells. CONCLUSIONS: Peripheral blood of IgAN patients is enriched with migratory λ+ mb-Gd-IgA1+ B cells, with the potential to home to mucosal sites where Gd-IgA1 could be produced during local respiratory or digestive tract infections.