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CD4+ T cells are implicated in rheumatoid arthritis (RA) pathology from the strong association between RA and certain HLA-class II gene variants. Here, we studied the synovial T cell receptor (TCR) repertoire, T cell phenotypes and T cell specificities in small joints of RA patients at time of diagnosis before therapeutic intervention.Sixteen patients (ACPA-positive n=11, ACPA-negative n=5) underwent synovial biopsy of a small (ultrasound guided, n=13) or large joint (arthroscopic, n=3), followed by direct single T cell sorting for paired sequencing of the αβ-TCR combined with flow cytometry analysis. TCRs from CD4+ T cell clones of four HLA-DRB1*04:01 patients were artificially re-expressed for studies of antigen specificity.T cell analysis demonstrated a CD4+ dominance and the presence of T peripheral helper-like cells in both patient groups. We identified4000 unique TCR-sequences, as well as 225 clonal expansions. Additionally, T cells with double alpha chains were a recurring feature. We identified a biased gene usage of the Vbeta-chain segment TRBV20-1 in CD4+ cells from ACPA-positive patients. In vitro stimulation of T cell lines expressing selected TCRs with an extensive panel of citrullinated and viral peptides identified several different virus-specific TCRs e.g. HCMV and HHV-2. Still, the majority of clones remained orphans, i.e. with unknown specificity.Minimally invasive biopsies of the RA synovium allow for single cell TCR-sequencing and phenotyping. Clonally expanded viral-reactive T cells account for part of the diverse CD4+ T cell repertoire. TRBV20-1 bias in ACPA-positive patients suggests recognition of common antigens. This article is protected by copyright. All rights reserved.

Arthritis is a common clinical feature of systemic lupus erythematosus (SLE) and is usually non-erosive as opposed to rheumatoid arthritis (RA). While RA synovial pathology has been extensively studied, little is known about the pathophysiology of lupus arthritis. Here, we aimed to explore the cytokine and cellular compartments in synovial fluids of SLE patients with arthritic manifestations. Acellular synovial fluid and paired serum samples from SLE patients (n=17) were analyzed with cytokine bead array for T helper associated cytokines. From two SLE patients, synovial fluid mononuclear cells (SFMC) were analyzed by multiparameter flow cytometry to dissect T cell, B cell, monocyte and dendritic cell phenotypes. SLE-derived SFMC were further stimulated in vitro to measure their capacity for producing IFN and IL-17A. All patients fulfilled the ACR 1982 classification criteria for SLE. Clinical records were reviewed to exclude the presence of comorbidities such as osteoarthritis or overlap with RA. IL-17A and IL-6 levels were high in SLE synovial fluid. A clear subset of the synovial CD4+ T cells expressed CCR6+, a marker associated with Th17 cells. IL-17-production was validated amongst CD4+CCR6+ T cells following in vitro stimulation. Furthermore, a strong IFN production was observed in both CD4+ and CD8+ cells. Our study shows high IL-17A and IL-6 levels in synovial fluids of patients with lupus arthritis. The Th17 pathway have been implicated in several aspects of SLE disease pathogenesis and our data points to Th17 involvement also for lupus arthritis.