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.

Acetylated bacterial proteins as potent antigens inducing an anti-modified protein antibody response.

OBJECTIVE: Gut-residing bacteria, such as Escherichia coli, can acetylate their proteome under conditions of amine starvation. It is postulated that the (gut) microbiome is involved in the breach of immune tolerance to modified self-proteins leading to the anti-modified protein antibodies (AMPAs), hallmarking seropositive rheumatoid arthritis (RA). Our aim was to determine whether acetylated bacterial proteins can induce AMPA responses cross-reactive to modified self-proteins and be recognised by human AMPA (hAMPA). METHODS: E. coli bacteria were grown under amine starvation to generate endogenously acetylated bacterial proteins. Furthermore, E. coli proteins were acetylated chemically. Recognition of these proteins by hAMPA was analysed by western blotting and ELISA; recognition by B cells carrying a modified protein-reactive B cell receptor (BCR) was analysed by pSyk (Syk phosphorylation) activation assay. C57BL/6 mice were immunised with (modified) bacterial protein fractions, and sera were analysed by ELISA. RESULTS: Chemically modified bacterial protein fractions contained high levels of acetylated proteins and were readily recognised by hAMPA and able to activate B cells carrying modified protein-reactive BCRs. Likely due to substantially lower levels of acetylation, endogenously acetylated protein fractions were not recognised by hAMPA or hAMPA-expressing B cells. Immunising mice with chemically modified protein fractions induced a strong cross-reactive AMPA response, targeting various modified antigens including citrullinated proteins. CONCLUSIONS: Acetylated bacterial proteins are recognisable by hAMPA and are capable of inducing cross-reactive AMPA in mice. These observations provide the first conceptual evidence for a novel mechanism involving the (endogenous) acetylation of the bacterial proteome, allowing a breach of tolerance to modified proteins and the formation of cross-reactive AMPA.

Type 17-specific immune pathways are active in early spondyloarthritis.

OBJECTIVE: Undifferentiated, early inflammatory arthritis (EIA) can differentiate into seropositive or seronegative rheumatoid arthritis (RA), peripheral spondyloarthritis (SpA) or remain as seronegative undifferentiated inflammatory arthritis (UIA). Little is known about immune pathways active in the early stages of SpA and seronegative UIA, in contrast to detailed knowledge of seropositive RA. The aim of this study was to examine if specific immune pathways were active in synovial CD4+ and CD8+ T cells in EIA. METHODS: Synovial fluid (SF) samples from 30 patients with EIA were analysed for expression of IL-17A, IFNγ and TNFα in CD8+ or CD4+ T cells. Final clinical diagnoses were made at least 12 months after sample collection, by two independent clinicians blind to the study data. RESULTS: Flow cytometry analysis of all EIA samples indicated considerable variation in synovial IL-17A+CD8+ T cells (Tc17) cell frequencies between patients. The group with a final diagnosis of SpA (psoriatic arthritis or peripheral SpA, n=14) showed a significant enrichment in the percentage of synovial Tc17 cells compared with the group later diagnosed with seronegative UIA (n=10). The small number of patients later diagnosed with seropositive RA (n=6) patients had few Tc17 cells, similar to our previous findings in established disease. In contrast, RA SF contained a significantly higher percentage of CD8+IFNγ+ T cells compared with SpA or seronegative UIA. CONCLUSION: These results suggest that adaptive T cell cytokine pathways differ not only between RA and SpA but also seronegative UIA early in the disease process, with a particular activation of Tc17 pathways in early SpA.

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.