O-glycosylation of IgA1 and the pathogenesis of an autoimmune disease IgA nephropathy

Abstract

IgA nephropathy is a kidney disease characterized by deposition of immune complexes containing abnormally O-glycosylated IgA1 in the glomeruli. Specifically, some O-glycans are missing galactose that is normally β1,3-linked to N-acetylgalactosamine of the core 1 glycans. These galactose-deficient IgA1 glycoforms are produced by IgA1-secreting cells due to a dysregulated expression and activity of several glycosyltransferases. Galactose-deficient IgA1 in the circulation of patients with IgA nephropathy is bound by IgG autoantibodies and the resultant immune complexes can contain additional proteins, such as complement C3. These complexes, if not removed from the circulation, can enter the glomerular mesangium, activate the resident mesangial cells, and induce glomerular injury. In this review, we briefly summarize clinical and pathological features of IgA nephropathy, review normal and aberrant IgA1 O-glycosylation pathways, and discuss the origins and potential significance of natural anti-glycan antibodies, namely those recognizing N-acetylgalactosamine. We also discuss the features of autoantibodies specific for galactose-deficient IgA1 and the characteristics of pathogenic immune complexes containing IgA1 and IgG. In IgA nephropathy, kidneys are injured by IgA1-containing immune complexes as innocent bystanders. Most patients with IgA nephropathy progress to kidney failure and require dialysis or transplantation. Moreover, most patients after transplantation experience a recurrent disease. Thus, a better understanding of the pathogenetic mechanisms is needed to develop new disease-specific treatments.

Keywords: IgA nephropathy; IgA1 O-glycans; autoimmune disease; immune complexes.

Fig. 1

Hinge-region amino-acid sequence and O-glycosylation of circulatory IgA1. A) IgA1 usually has three to six sites with attached O-glycans. The six commonly used sites include T225, T228, S230, S232, T233, and T236 (marked by stars in the amino-acid sequence). B) Biosynthesis of O-glycans of circulatory IgA1. Glycosylation of IgA1 begins in the Golgi apparatus with the addition of N-acetylgalactosamine (GalNAc) to S and T residues by GalNAc-transferase (GalNAc-T). GalNAc can be then modified by core 1 synthase (C1GalT1) with a β1,3-linked galactose (Gal). This enzyme requires expression of C1GalT1-specific chaperone 1 (C1GalT1C1). In the follow-up reaction, one or both sugars can be modified by sialic acid: Galactose by α2,3-linked sialic acid and GalNAc by α2,6-linked sialic acid. These reactions are catalyzed by ST3Gal and ST6GalNAc2 enzymes, respectively. Alternatively, GalNAc can be modified before galactosylation by α2,6-linked sialic acid; this modification blocks any other glycosylation and thus yields a Gal-deficient site, also termed sialyl-Tn antigen. If GalNAc remains unmodified, it is called Tn antigen.

Fig. 2

Jacalin and HPA lectin structures and glycan-binding specificity. Panels (A–D) show jacalin, a lectin from Artocarpus integer, with preference for binding to O-linked Gal(β1-3) α-GalNAc. Panel (A) shows a cartoon representation of the hetero-octameric structure of jacalin composed of four chains each of the α- and β-chains [PDB ID: 1M26; (Jeyaprakash et al. 2002)] with Gal(β1-3) α-GalNAc shown in space-filling model. In (A–C), each chain is shown in alternating colors with α- and β-chains shaded in lighter and darker shades, respectively. In (B), a monomeric version of the jacalin α/β heterodimer is shown with glycan in stick model. Panel (C) shows a closeup of the glycan-binding site with amino-acid residues within five angstroms of the glycan shown as sticks and labeled. Panel (D) shows the surface rendering of jacalin with bound glycan. The surface is colored by electrostatic potential. The surface of jacalin provides an extensive pocket to accommodate the disaccharide with high specificity. Panels (E–H) show the lectin from Helix pomatia, which has a preference to bind terminal GalNAc. Panel E shows the hexameric H. pomatia complex [PDB ID: 2CVV; (Sanchez et al. 2006)], where two trimeric assemblies are covalently linked together through disulfide bonds. Each monomer harbors an N-glycan (shown as sticks) and bound GalNAc (shown in space filling model). Panel F shows the monomeric model with the GalNAc shown in stick model. Panel (G) shows a closeup of the GalNAc-binding site with amino-acid residues and water molecules within five angstroms of the glycan shown as sticks and labeled. The GalNAc binding site is generated by two adjacent lectin monomers in the trimeric assembly. Panel H features a surface rendering of the GalNAc-binding pocket colored according to electrostatic potential. For both jacalin and H. pomatia lectins, hydrogen bonding to the O4 atom of the GalNAc moiety (labeled in D and H) provides specificity to accommodate GalNAc. An (*) denotes the point of attachment of GalNAc to serine or threonine residues of a glycopeptide form of Tn antigen. Figures 2A–C and E–G were generated with PyMol (DeLano 2002). Figures 2D and H were generated with ChimeraX (Meng et al. 2023).

Fig. 3

Pathogenesis model of IgA nephropathy. Pathogenesis of the autoimmune disease IgA nephropathy (IgAN) has been described as a multi-step process of IgA1 and IgG antibody generation, immune-complex formation, and deposition in the glomeruli, leading to kidney injury (Suzuki et al. 2011). Polymeric IgA1 (dimeric in this figure) harboring O-glycans in the hinge region, with an incomplete repertoire of galactose moieties [galactose (Gal)-deficient IgA1 (Gd-IgA1)], is found elevated in the circulation of most IgAN patients and some of these glycoforms are recognized as an autoantigen. IgG autoantibodies with a specificity for the glycosylated hinge region of Gd-IgA1 bind Gd-IgA1 (antigen-binding sites of IgG marked by *), and pathogenic immune complexes are formed. Larger complexes are formed as additional proteins are added, including complement C3. C3 or activated C3b covalently binds to IgA1 or IgG through the thioester bond found in the thioester-containing domain. Some of these immune complexes pass through the fenestration of glomerular endothelial cells, enter glomerular mesangial space, and activate mesangial cells. A cascade of effects is associated with mesangial-cell activation, including cellular proliferation and overproduction of extracellular matrix (ECM), cytokines, chemokines, some of which and can activate podocytes. These events can lead to glomerular injury. Glycan content at up to six Ser/Thr residues (225, 228, 230, 232, 233, 236; shaded red) and the IgA1 hinge-region conformation distinguishes whether the IgA1 molecule is viewed as normal IgA1 or is identified as an autoantigen. Space filling models of IgA1, IgG, and complement C3 were generated from coordinates published in (Person et al. 2022), (PDB ID: 1IGT; (Harris et al. 1997)), and (PDB ID: 2I07; (Janssen et al. 2006)), respectively. Component chains of Gd-IgA1 and IgG are shown in different colors. Complement C3b is colored by domain as in (Rajasekaran et al. 2023).