The regulatory power of glycans and their binding partners in immunity

Abstract

Glycans and glycan-binding proteins are central to a properly functioning immune system. Perhaps the best known example of this is the selectin family of surface proteins that are primarily found on leukocytes, and which bind to endothelial glycans near sites of infection or inflammation and enable extravasation into tissues. In the past decade, however, several other immune pathways that are dependent on or sensitive to changes in glycan-mediated mechanisms have been revealed. These include antibody function, apoptosis, T helper (Th)1 versus Th2 skewing, T cell receptor signaling, and MHC class II antigen presentation. Here, we highlight how regulated changes in protein glycosylation both at the cell surface and on secreted glycoproteins can positively and negatively modulate the immune response.

Galectin structural motifs

The galectin family is categorized by the structural arrangement of the carbohydrate recognition domains (CRDs). Prototypical galectins exist as monomers in the cytoplasm and are secreted as homodimers containing two CRDs (one per subunit). Tandem-repeat galectins are monomeric, yet contain two CRDs connected by a linker domain. Chimeric galectins exist as homopolymers with a variable number of subunits, each containing a single CRD.

Figure 1. Protein Glycosylation and Important Glycan Epitopes

Schematic representations of N-linked, O-linked (O-GalNAc), and O-GlcNAc protein glycosylation pathways. For the N-linked glycosylation pathway, the structures falling into ER-localized, high mannose, hybrid, and complex groups are indicated to illustrate the general steps of synthesis and structural diversification that occurs in the Golgi apparatus. Differences in branching patterns are also illustrated, together with details about the enzymes (Mgat1–5) responsible for the addition of GlcNAc residues that seed each branch. Branching is a modulated structural attribute of N-glycans during disease and inflammation, which can be regulated through differential expression of the Mgat genes. For the O-linked glycosylation pathway, each of the “Core” structures are shown to illustrate the structural relationships between each glycan type. All of the “Core” structures can serve as a platform for further diversification, as shown for Core 1 and Core 3. Within both N- and O-linked glycans, a number of key glycan epitopes and ligands that are relevant for the immune system are possible and indicated in red boxes. For example, sialyl-Lewis^x is a ligand for selectin molecules, whereas LacNAc and poly-LacNAc represent minimal glycans for many of the galectins. We also show both Type-1 and Type-2 A/B/O(H) blood group antigens which can also be found in both N- and O-linked glycans as examples relevant to transplantation and blood transfusion.

Figure 2. Pathways for immune regulation by glycans and glycan-binding proteins

Glycan binding proteins, such as the galectin and C-type lectin families, play both positive and negative roles in the induction of an immune response. For example, galectin-1 (Gal-1) and galectin-3 (Gal-3) hold CD45 at the cell surface, such that T cell receptor signaling is muted by preventing Lck phosphorylation, while Gal-1 can also impact the differentiation of the FoxP3⁻ Tr1 subset of inducible T regulatory cells through an unknown mechanism. The C-type lectin DC-SIGN functions not only to recognize foreign carbohydrates from H. pylori and M. tuberculosis, but it also binds to sialylated IgG molecules (sIgG) as part of an anti-inflammatory cascade. Galactosylated IgG (gIgG) crosslinks the inhibitory FcγRIIB receptor with Dectin-1, which leads to inhibition of C5aR signaling, whereas galectin-9 associates with glycans on Tim-3 leading to apoptosis. The glycans themselves can regulate these interactions through programmed changes in glycan structure and diversification, which could be mediated by changes in expression of glycosylation-specific Golgi enzymes or their carbohydrate substrate concentrations.