Glycobiology of rheumatic diseases

Abstract

Glycosylation has a profound influence on protein activity and cell biology through a variety of mechanisms, such as protein stability, receptor interactions and signal transduction. In many rheumatic diseases, a shift in protein glycosylation occurs, and is associated with inflammatory processes and disease progression. For example, the Fc-glycan composition on (auto)antibodies is associated with disease activity, and the presence of additional glycans in the antigen-binding domains of some autoreactive B cell receptors can affect B cell activation. In addition, changes in synovial fibroblast cell-surface glycosylation can alter the synovial microenvironment and are associated with an altered inflammatory state and disease activity in rheumatoid arthritis. The development of our understanding of the role of glycosylation of plasma proteins (particularly (auto)antibodies), cells and tissues in rheumatic pathological conditions suggests that glycosylation-based interventions could be used in the treatment of these diseases.

Fig. 1. Biosynthesis of protein glycans and the IgG-specific glycan signatures in rheumatoid arthritis.

a, N-linked glycans are attached to asparagine (N) residues in N-X-S/T motifs, where ‘X’ is any amino acid except for proline. A precursor N-glycan is co-translationally or post-translationally attached in the endoplasmic reticulum (ER). After the removal of terminal glucose and mannose residues (folding control) by α-glucosidase (α-Glc) and α-mannosidase (α-Man) enzymes, and addition of N-acetylglucosamine (GlcNAc) by GlcNAc transferase (GlcNAcT), the protein enters the Golgi apparatus, where the high-mannose structure is trimmed down and subsequently built up to complex-type di-antennary (A2), tri-antennary (A3) and tetra-antennary (A4) N-glycans that may carry sialyl-Lewis X terminal motifs. O-linked glycans are attached post-translationally in the Golgi apparatus to serine (S) or threonine (T) amino acids, and can be further extended. b, Schematic representation of an IgG molecule. The fragment crystallizable (Fc) domain is 100% N-glycosylated at N297, and the fragment antigen-binding (Fab) domain is 15–25% N-glycosylated at N-X-S/T consensus motifs. Fab glycosylation is increased on anti-citrullinated protein antibody (ACPA) IgG from patients with rheumatoid arthritis (RA), on anti-myeloperoxidase (MPO) and anti-proteinase 3 (PR3) IgG from patients with anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV), on anti-muscle-specific kinase (MuSK) receptor antibodies from patients with myasthenia gravis (MG) and on total IgG from patients with primary Sjögren syndrome (pSS), multiple sclerosis (MS) or systemic lupus erythematosus (SLE). Rheumatic-disease-specific IgG Fc glycans are characterized by the presence of fucose but not galactose (G0F), and G0F is also observed on antigen-specific ACPA and anti-MPO IgG. c, The glycan signatures of ACPA IgG in RA. Fc galactosylation decreases and processed Fab glycosylation (represented by ‘G2FBS2’, where G is galactose, F is fucose, B is bisecting GlcNAc and S is sialic acid) increases towards disease onset and is associated with disease severity. FucT, fucosyltransferase; GalT, galactosyltransferase; PsA, psoriatic arthritis; SialylT, sialyltransferase; SpA, spondyloarthritis.

Fig. 2. Functional effects of disease-specific Fc and Fab glycosylation in humans.

a, Fragment crystallizable (Fc) region N-glycan galactosylation increases Fcγ receptor FcγRIIIa binding and antibody-dependent cellular cytotoxicity (ADCC) in the absence of core fucosylation (represented by ‘G2F0’, where G is galactose and F is fucose). ADCC is diminished by G0F1-modified IgG immune complexes because of repulsion between the IgG and FcγR N-glycan fucose residues (fucose clashing). Galactosylation of Fc glycans enhances hexamerization and subsequent C1q binding and complement-dependent cytotoxicity (CDC). b, Processed disialylated IgG Fab glycans can influence antigen binding via steric or charge-induced repulsion or by competing with the antigen for the binding pocket, as evidenced by dynamic simulations on Fab crystal structures. Fab glycans can enhance B cell receptor (BCR) signalling while downregulating BCR internalization and antigen uptake. B cell activation is potentially influenced by differences in BCR downregulation, clustering (steric or charge-induced repulsion) or interactions with membrane-bound or soluble lectins. These effects on B cells may foster a breach of tolerance by mediating the escape from important tolerance checkpoints. Gal-9, galectin-9; Siglec, sialic acid-binding lectin. Part b is adapted from ref., CC BY-NC 4.0 (https://creativecommons.org/licenses/by-nc/4.0/).

Fig. 3. Disease-specific tissue glycosylation.

a, The N-glycome of the synovial fibroblast is rich in high-mannose and short di-antennary glycans with differential terminal sialylation in both health and disease states. In rheumatoid arthritis (RA), TNF stimulates the downregulation of β-galactoside α-2,6-sialyltransferase 1 (ST6Gal1), resulting in reduction of surface sialylation and a molecular switch to a more pro-inflammatory synovial fibroblast phenotype. Desialylated synovial fibroblast surface glycans may interact with galectin-3 (Gal-3), inducing the secretion of pro-inflammatory cytokines (IL-6, CCL2), which in turn upregulate TNF expression and thus the desialylation of the synovial fibroblast surface (in a vicious cycle). b, Abnormal glomerulus tissue glycosylation in systemic lupus erythematosus. Accumulation of mannose N-glycans is the result of a deficient complex-N-glycosylation pathway characterized by downregulation of expression of α-mannosidase II (α-Man II) and promotion of O-mannosylation by upregulation of expression of protein O-mannosyl-transferase 1 (POMT1). Mannose-enriched N-glycans are typically found on the surface of pathogens and can therefore trigger activation of antigen-presenting cells (APCs) by mannose glycan-binding receptors such as dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), ultimately leading to a loss of self-tolerance. GlcNAcT, N-acetylglucosamine transferase.

Fig. 4. Therapeutic potential of glycosylation.

Glycan-based therapies that intervene in inflammatory processes include the administration of enzymes such as endoglycosidase S, which alters the fragment crystallizable (Fc)-domain glycosylation of antibodies. One cost-effective and non-toxic intervention is the direct administration of glycans to promote alteration of glycan biosynthesis. Other glycosylation-related therapeutic approaches include specific glycoengineering of proteins such as antibodies, alteration of glycosylation by TNF inhibition, lectin-mediated inhibition of autoreactive B cells and targeted autoantibody degradation. a, Hypothetical method to change the pro-inflammatory phenotype of synovial fibroblasts in rheumatic diseases by manipulating glycosylation by treatment with TNF inhibitors. Abrogation of TNF-mediated downregulation of β-galactoside α-2,6-sialyltransferase 1 (ST6Gal1) results in an anti-inflammatory hypersialylated synovial fibroblast glycan coat. Sialylation prevents interaction with galectin-3 (Gal-3), which further breaks the vicious cycle in which TNF promotes desialylation. b, Sialic-acid-binding immunoglobulin-type lectin (Siglec)-engaging tolerance-inducing antigenic liposomes (STALs) carry autoantigens and high-affinity ligands for CD22 (also known as Siglec-2) or Siglec-10, negative regulators of B cell receptor (BCR) signalling, and can thus enforce an association between the BCR and the negative regulators. Inhibited BCR signalling results in pro-apoptotic downstream signalling events and the apoptosis of antigen-specific B cells. c, Targeted autoantibody degradation by MoDE-As (molecular degraders of extracellular proteins through the asialoglycoprotein receptor (ASGPR)), small molecules carrying an N-acetylgalactosamine (GalNAc) ASGPR binding motif and an autoantigen. MoDE-As target autoantibodies and facilitate target-specific internalization and subsequent lysosomal degradation through ASGPR expressed on liver cells. SHP1, SRC homology 2 domain-containing protein tyrosine phosphatase-1.