Abnormal IgA glycosylation in Henoch-Schönlein purpura restricted to patients with clinical nephritis.

Alice C. Allen 0 1 Frank R. Willis 0 1 T. James Beattie 0 1 John Feehally 0 1 0 of Nephrology, Leicester General Hospital , Gwendolen Road, Leicester LE5 4PW , UK 1 Department of Nephrology, Leicester General Hospital , Gwendolen Road, Leicester LE5 4PW , and 1Renal Unit, Royal Hospital for Sick Children , Yorkhill, Glasgow G3 8SJ , UK Background. Glomerular deposition of IgA1 is a common feature of Henoch-Scho nlein purpura, and is indistinguishable from that seen in IgA nephropathy. Serum IgA1 is abnormally O-glycosylated in IgA nephropathy, and this may contribute to mesangial IgA1 deposition and the development of glomerular injury. This altered O-glycosylation of IgA1 can be detected by its increased binding to the lectin Vicia villosa. Methods. To investigate whether IgA1 is abnormally glycosylated in Henoch-Scho nlein purpura, the binding of Vicia villosa lectin to serum IgA1 was studied in the following subject groups: IgA nephropathy; adults and children with Henoch-Scho nlein purpura and nephritis; children with clinically diagnosed Henoch-Scho nlein purpura but no renal involvement; adults and children with non-IgA associated glomerulonephritis; and matched controls. Results. The abnormality of lectin binding seen in IgA nephropathy was also found in both adults and children with Henoch-Scho nlein purpura with nephritis. However, the lectin binding of serum IgA1 from children with Henoch-Scho nlein purpura lacking renal involvement did not diVer from controls, and similarly, no abnormality of lectin binding was seen in patients with non-IgA associated glomerulonephritis. Conclusions. These data indicate that the abnormality of IgA1 O-glycosylation seen in IgA nephropathy is also found in Henoch-Scho nlein purpura, but only in those subjects with renal involvement, while IgA1 O-glycosylation is normal in patients with other forms of renal disease. These findings lend strong support to a role for altered IgA1 O-glycosylation in the pathogenesis of IgA-associated glomerular disease. - Henoch-Scho nlein purpura ( HSP) is a form of systemic vasculitis characterized by IgA deposition in aVected blood vessels. HSP can occur at any age, but is most common in children. Renal involvement is common and in the glomerular mesangium there is IgA1 deposition which is indistinguishable from that found in IgA nephropathy ( IgAN ). The mechanisms of mesangial IgA1 deposition and subsequent glomerular injury in IgAN and HSP are unknown. Altered IgA1 glycosylation has recently received attention as a candidate physicochemical abnormality in the pathogenesis of IgAN [ 1], but has not been studied in HSP. IgA1 is an unusual serum protein in possessing a series of O-linked glycans in the hinge region of the molecule ( Figure 1A), a feature which distinguishes it from IgA2 and other serum immunoglobulins [2]. Each of the O-linked glycans is based on N-acetylgalactosamine (GalNAc ) O-linked to serine residues; O-glycosylation of threonine residues, though theoretically possible, has not been described in IgA1. The glycan chains may be elongated with the further addition of galactose (Gal ) in b1,3 linkage with GalNAc, and a variable degree of sialylation ( Figure 1B) [3,4]. Altered O-glycosylation of serum IgA1 in IgAN has been described in a number of studies [510], but the abnormality has yet to be defined precisely. Most of the evidence for this abnormality has come from reports of altered binding of IgA1 to various lectins. Though lectins cannot provide exact structural information about the abnormality, they are useful and convenient tools with which to detect altered glycosylation. We have used the lectin from Vicia villosa ( VV ) to demonstrate abnormal O-glycosylation of serum IgA1 in IgAN [8] and the Wiskott-Aldrich syndrome [11 ]. VV lectin binds to terminal O-linked GalNAc, and shows higher binding to IgA1 from patients with IgAN than controls. This finding suggests that the abnormality may take the form of a reduction in the degree of galactosylation of GalNAc moieties in IgAN, resulting in increased exposure of GalNAc Fig. 1. (A) IgA1 molecule, showing the position of the hinge region O-glycans. The hinge region lies between the CH1 and CH2 domains. On the right is the amino acid sequence of the hinge region, with the serine-linked O-glycosylation sites marked by arrows. O-glycosylation of threonine residues is also possible, but has not been described in IgA1. (B) Structure of the O-linked glycan moiety of IgA1. GalNAc is O-linked to serine or threonine, and is frequently further substituted by galactose in the b1,3 configuration. Mono- or di-sialylation (N-acetyl neuraminic acid; NeuNAc) may also occur. Sugars not consistently present are shown in italics: Gal and NeuNAc. to lectin binding. This interpretation of our data is consistent with the findings of other investigators [57,9], though more detailed structural information is still required to confirm precisely the glycan abnormality of IgA1. Altered O-glycosylation of IgA1 has the potential to aVect the interactions of the molecule in various ways. It may compromise the eYciency of hepatic clearance of circulating IgA1 by the asialoglycoprotein receptor which displays high aYnity for O-linked glycans [12 ]. Furthermore, altered IgA1 O-glycosylation may favour mesangial deposition or the initiation of glomerular inflammatory processes by promoting self-aggregation [13], or interaction of IgA1 with extracellular matrix components, Fca receptors expressed by leucocytes and mesangial cells, and with complement. In this study, we report that the O-glycosylation of serum IgA1 is abnormal in adults and children with HSP nephritis, but not in subjects with other glomerular diseases. Furthermore in children with HSP but without apparent renal involvement IgA1 glycosylation is normal, an observation which lends support to a role for altered IgA1 glycosylation in the pathogenesis of IgA-associated glomerulonephritis. Subjects and methods All chemicals were purchased from Sigma Chemical Co, Poole, UK, except where otherwise specified. Antibodies and buVered 1,2-phenylenediamine dihydrochloride (OPD) tablets were purchased from Dako, Amersham, UK, and biotinylated Vicia villosa lectin and peroxidase-conjugated avidin D from Vector Laboratories, Peterborough, UK. Subjects and samples Details of the patient and control groups are given in Table 1. HSP: Of 24 children with HSP nephritis (HSPN ), half had undergone renal biopsy, and the rest had unequivocal clinical evidence of renal disease: persistent proteinuria and/or haematuria with or without impairment of renal function. The 22 children with HSP but no nephritis (HSPN0) all had a clinical diagnosis of HSP, but no proteinuria, haematuria, hypertension or renal impairment. PSGN: seven children with acute nephritic syndrome due to post-streptococcal glomerulonephritis were also studied. Control: 22 age and sex-matched children were under investigation for endocrine (...truncated)