POS0335 DECODING THE T-CELL RECEPTOR REPERTOIRE IN CHRONIC ARTHRITIS ACROSS DIFFERENT AGES, DISEASE GROUPS, AND T-CELL SUBSETS
Authors: M. Ha, V. Van Deuren, J. Dehoorne, T. Renson, E. Geens, N. Aerts, R. Wittoek, K. Heusdens, E. De Wachter, A. Andrzejewski, M. Kuznetsova, E. Bartholomeus, N. De Vrij, I. Maes, A. Suls, E. Lion, S. Vanhee, W. Adriaensen, K. Mullan, R. Joos, K. Laukens, P. Meysman, B. Ogunjimi
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> One of the key players in autoimmune arthritis is T-cells. Both CD4<sup>+</sup> and CD8<sup>+</sup> T-cells have been implicated in mediating many aspects of synovial inflammation [1,2]. Oligoclonal populations of CD8<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> T-cells [7]. <h3>Objectives:</h3> 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. <h3>Methods:</h3> 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<sup>+</sup>, CD8<sup>+</sup>, and Treg fractions. <h3>Results:</h3> 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<sup>–</sup> patients, B27<sup>+</sup> patients had upregulated HLA-DP, HLA-DQ, HLA-DR, and GZMA genes in central memory CD8<sup>+</sup> 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<sup>+</sup> patients. From the bulk TCRβ sequencing data (Figure 2), we observed that HLA-B27<sup>+</sup> 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. <h3>Conclusion:</h3> 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. <h3>REFERENCES:</h3> [1] Black, A.P.B. et al. <i>Arthritis Res Ther</i><b>2002</b>, <i>4</i>, 177. [2] Skapenko, A. et al. <i>Arthritis Res Ther</i><b>2005</b>, <i>7</i>, S4. [3] Fazou, C. et al. <i>Arthritis and Rheumatology</i><b>2001</b>, <i>44</i>, 2038–2045. [4] Cañete, J.D. et al. <i>Ann Rheum Dis</i><b>2000</b>, <i>59</i>, 263–268. [5] Davis, L.S. et al. <i>Arthritis Res Ther</i><b>2000</b>, <i>3</i>, 54. [6] Möller, B. et al. <i>Rheumatology</i><b>2001</b>, <i>40</i>, 302–309. [7] Mijnheer, G. et al. <i>Elife</i><b>2023</b>, <i>12</i>, e79016. <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>