Pos0335 Decoding the T-Cell Receptor Repertoire in Chronic Arthritis Across Different Ages, Disease Groups, and T-Cell Subsets

Background: One of the key players in autoimmune arthritis is T-cells. Both CD4⁺ and CD8⁺ T-cells have been implicated in mediating many aspects of synovial inflammation [1,2]. Oligoclonal populations of CD8⁺ T-cells specific for epitopes from Epstein-Barr virus, cytomegalovirus, and influenza virus were found enriched in the synovial fluid of patients with inflammatory arthritis [3]. There are also data suggesting that CD4⁺ T-cells from the inflamed synovium in rheumatoid arthritis (RA) represent activated Th1 cells that secrete IFNγ, which in turn orchestrates synovial inflammation [4–6]. Among CD4⁺ T-cells, regulatory T-cells (Treg), which are crucial in preserving immune homeostasis, are believed to be dysfunctional in autoimmunity. In the inflamed joints of juvenile idiopathic arthritis (JIA) patients, a higher frequency of clonally expanded Tregs was observed than conventional CD4⁺ T-cells [7]. Objectives: Mapping the immune phenotypes in synovial fluid of juvenile and adult patients with chronic autoimmune arthritis. Decoding the synovial fluid T-cell receptor repertoire across different arthritis groups and T-cell subsets. Identifying T-cell receptors specific to HLA-B27 associated spondyloarthropathies. Methods: 27 patients were recruited: 6 had oligoarticular JIA (oJIA), 4 had enthesitis-related JIA (JIA-ERA), 3 had juvenile psoriatic arthritis (JIA-PsA), 5 had adult psoriatic arthritis (PsA), 5 had RA, 3 had Lyme arthritis (borrelia), and 1 had spondyloarthritis (SpA). Their synovial fluid was collected and processed into mononuclear cells (SFMC). Single-cell RNA-seq and TCR-seq were performed on the SFMC. Bulk TCR-seq was done on the CD4⁺, CD8⁺, and Treg fractions. Results: Our single-cell sequencing data indicate that among the 28 identified cell phenotypes, Th1/Th17 cells were the dominant population among all patient groups, having the highest proportion out of the total T-cell count in JIA-ERA (Figure 1A). Furthermore, compared to B27– patients, B27⁺ patients had upregulated HLA-DP, HLA-DQ, HLA-DR, and GZMA genes in central memory CD8⁺ T-cells, implying an increase in T-cell activation and cytotoxic activity (Figure 1B). Gene set enrichment analysis also suggests pro-inflammatory pathways (e.g., TNFα, IFNα, and IFNγ signalling), and T-cell activation in response to viruses in B27⁺ patients. From the bulk TCRβ sequencing data (Figure 2), we observed that HLA-B27⁺ patients shared broad TCRBV20 clusters specific for various viral epitopes, indicating a possible cross-reactive response. Furthermore, this method highlighted TCR clusters with sequence similarity to those known to be associated with HLA-B27 pathologies, as well as TCRs with unknown specificity, but a conserved GRGR motiflet. Conclusion: The findings in this study provide insights into how TCR repertoires differ between arthritis groups, patients' ages, T-cell subsets, and whether viral/bacterial infection contributes to arthritis onset and flares. Such knowledge will greatly contribute to the understanding of T-cells' and TCRs' participation in the underlying pathophysiology of arthritis. REFERENCES: [1] Black, A.P.B. et al. Arthritis Res Ther2002, 4, 177. [2] Skapenko, A. et al. Arthritis Res Ther2005, 7, S4. [3] Fazou, C. et al. Arthritis and Rheumatology2001, 44, 2038–2045. [4] Cañete, J.D. et al. Ann Rheum Dis2000, 59, 263–268. [5] Davis, L.S. et al. Arthritis Res Ther2000, 3, 54. [6] Möller, B. et al. Rheumatology2001, 40, 302–309. [7] Mijnheer, G. et al. Elife2023, 12, e79016. Acknowledgements: NIL. Disclosure of Interests: None declared.