Clonally expanded PD‐1‐expressing T cells are enriched in synovial fluid of juvenile idiopathic arthritis patients

Juvenile idiopathic arthritis (JIA) is the most common chronic rheumatic condition in childhood. Disease etiology remains largely unknown, however a key role in JIA pathogenesis is surely mediated by T cells. T lymphocytes activity is controlled via signals, known as immune-checkpoints (IC). Delivering an inhibitory signal or blocking a stimulatory signal to achieve immune suppression is critical in autoimmune diseases. However, the role of IC in chronic inflammation and autoimmunity must still be deciphered. In this study, we investigated at single cell level the feature of T cells in JIA chronic inflammation both at transcriptome level via single-cell RNA sequencing and at protein level by flow cytometry. We found that despite the heterogeneity in the composition of synovial CD4+ and CD8+ T cells, those characterized by PD-1 expression were clonally expanded Trm-like cells and displayed the highest pro-inflammatory capacity, suggesting their active contribution in sustaining chronic inflammation in situ. Our data support the concept that novel therapeutic strategies targeting PD-1 may be effective in the treatment of JIA. With this approach, it may become possible to target overactive T regardless of their cytokine production profile.