Rheumatoid Synovial Subsets of CD3+ T cells defined by PD-1+ and PD-1- Cells Drive Fibroblasts into Distinct Endotypes

Introduction: The joint destructive process in rheumatoid arthritis (RA) is highly individual and dominated by complex cellular interactions. In damping the immune activation, the PD-1 pathway is central and highly upregulated in the arthritic joint. Here, we examine how T cells with or without PD-1 influence the function of arthritic mesenchymal cells Methods: We investigated samples from RA synovial fluid and characterized PD-1+ and PD-1- T cells by flow cytometry, ELISA and RNA sequencing (RNA seq). We performed co-culture experiments to investigate the interaction between PD-1+/- cells, osteoclasts, osteoblasts and fibroblasts. We used flow cytometry, ELISA and RNA seq to characterize the RA and HC fibroblasts after co-culture with PD-1+ and PD-1- T cells. Results: We found PD-1+ T cells to express other checkpoint receptors and to have a diminished cytokine production upon CD3/CD28 stimulation. The PD-1+ and PD-1- T cells from the RA synovial fluid clustered differently after RNA seq, and the gene signature of the PD-1+ T shared traits with peripheral T helper cells. Fibroblasts co-cultured with PD-1+ T cells expressed genes associated with collagen synthesis, TGFbeta production and immune regulation. In contrast, the signature in fibroblasts co-cultured with PD-1- cells was associated with immune activation. Conclusion: We demonstrate RA synovial fluid CD3+ T cell subsets can drive healthy fibroblasts into distinct pathological endotypes. When evaluating all PD-1+ T cells, these support fibroblast subsets related to the production of collagen and immune regulation.