A photoaffinity glycan-labeling approach to investigate immunoglobulin glycan-binding partners.

Glycans play a pivotal role in biology. However, because of the low-affinity of glycan-protein interactions, many interaction pairs remain unknown. Two important glycoproteins involved in B-cell biology are the B-cell receptor and its secreted counterpart, antibodies. It has been indicated that glycans expressed by these B-cell-specific molecules can modulate immune activation via glycan-binding proteins. In several autoimmune diseases, an increased prevalence of variable domain glycosylation of IgG autoantibodies has been observed. Especially, the hallmarking autoantibodies in rheumatoid arthritis, anti-citrullinated protein antibodies, carry a substantial amount of variable domain glycans. The variable domain glycans expressed by these autoantibodies are N-linked, complex-type, and α2-6 sialylated, and B-cell receptors carrying variable domain glycans have been hypothesized to promote selection of autoreactive B cells via interactions with glycan-binding proteins. Here, we use the anti-citrullinated protein antibody response as a prototype to study potential in solution and in situ B-cell receptor-variable domain glycan interactors. We employed SiaDAz, a UV-activatable sialic acid analog carrying a diazirine moiety that can form covalent bonds with proximal glycan-binding proteins. We show, using oligosaccharide engineering, that SiaDAz can be readily incorporated into variable domain glycans of both antibodies and B-cell receptors. Our data show that antibody variable domain glycans are able to interact with inhibitory receptor, CD22. Interestingly, although we did not detect this interaction on the cell surface, we captured CD79 ß glycan-B-cell receptor interactions. These results show the utility of combining photoaffinity labeling and oligosaccharide engineering for identifying antibody and B-cell receptor interactions and indicate that variable domain glycans appear not to be lectin cis ligands in our tested conditions.

Addressing the key issue: Antigen-specific targeting of B cells in autoimmune diseases.

Autoimmune diseases are heterogeneous pathologies characterized by a breakdown of immunological tolerance to self, resulting in a chronic and aberrant immune response to self-antigens. The scope and extent of affected tissues can vary greatly per autoimmune disease and can involve multiple organs and tissue types. The pathogenesis of most autoimmune diseases remains unknown but it is widely accepted that a complex interplay between (autoreactive) B and T cells in the context of breached immunological tolerance drives autoimmune pathology. The importance of B cells in autoimmune disease is exemplified by the successful use of B cell targeting therapies in the clinic. For example, Rituximab, a depleting anti-CD20 antibody, has shown favorable results in reducing the signs and symptoms of multiple autoimmune diseases, including Rheumatoid Arthritis, Anti-Neutrophil Cytoplasmic Antibody associated vasculitis and Multiple Sclerosis. However, Rituximab depletes the entire B cell repertoire, leaving patients susceptible to (latent) infections. Therefore, multiple ways to target autoreactive cells in an antigen-specific manner are currently under investigation. In this review, we will lay out the current state of antigen-specific B cell inhibiting or depleting therapies in the context of autoimmune diseases.

Multifunctional, Multivalent PIC Polymer Scaffolds for Targeting Antigen-Specific, Autoreactive B Cells.

Multivalent scaffolds that carry multiple molecules with immunophenotyping or immunomodulatory properties are invaluable tools for studying and modulating specific functions of human immune responses. So far, streptavidin-biotin-based tetramers have been widely used for B-cell immunophenotyping purposes. However, the utility of these tetramers is limited by their tetravalency, the inherent immunogenicity of streptavidin (a bacterial protein that can potentially be recognized by B cells), and the limited feasibility to functionalize these reagents. This has rendered tetramers suboptimal for studying rare, in particular, antigen-specific B-cell populations in the context of clinical applications. Here, we used polyisocyanopeptides (PICs), multivalent polymeric scaffolds functionalized with around 50 peptide antigens, to detect autoreactive B cells in the peripheral blood of patients with rheumatoid arthritis. To explore the potential immunomodulatory functionalities, we functionalized PICs with autoantigenic peptides and a trisaccharide CD22 ligand to inhibit autoreactive B-cell activation through interference with the B-cell receptor activation pathway, as evidenced by reduced phospho-Syk expression upon PIC binding. Given the possibilities to functionalize PICs, our data demonstrate that the modular and versatile character of PIC scaffolds makes them promising candidates for future clinical applications in B-cell-mediated diseases.