Single cell RNA and TCR sequencing reveals substantive similarities between synovial fluid and synovial tissue T-cells in inflammatory arthritis.
Objective: Synovial fluid (SF) derived T-cells are frequently studied as a proxy for investigating the synovial tissue (ST) T-cell infiltrate in inflammatory arthritis. However, since ST is the primary site of inflammatory activity, there is debate as to whether SF provides a true reflection of the ST T-cell population.  Methods: In this study, we used single cell RNA sequencing paired with single cell TCR sequencing to directly compare memory T-cells from paired samples of SF and ST from 6 patients with inflammatory arthritis to investigate their similarity in terms of T-cell receptor (TCR) repertoire and T-cell subset composition. Results: The TCR repertoires of SF and ST T-cells were strikingly similar, particularly for CD8+ T-cells. A median of 49% of the total CD8+ TCR repertoire in SF was shared with ST, compared to 20% shared with blood and 47% of the ST CD8+ TCR repertoire was shared with SF compared to 25% with blood. Furthermore, once the effect of collagenase digestion on gene expression by ST T-cells had been accounted for, the frequencies of specific CD8+ and CD4+ T-cell subsets were, in general, very similar in SF and ST and were distinct from blood. Conclusion: Our results suggest that T-cells migrate and equilibrate between SF and ST and maintain similar phenotypes in both sites. We conclude that SF is an appropriate proxy for investigating the T-cell infiltrate in inflamed synovium using single cell RNA sequencing, particularly in terms of investigating the TCR repertoire. Overall design: Single cell RNA and TCR sequencing analysis of memory T cells  from blood, synovial fluid and synovial tissue from patients with psoriatic arthritis