Mass cytometry and single-cell sequencing technologies have previously been used to compare synovial tissue from patents with RA versus osteoarthritis, with enrichment of pro-inflammatory cells observed in RA synovia across a broad spectrum of cell types including CD4+PDCD1+ T peripheral helper cells, CD8+granzyme+ T cells, ITGAX+TBX21+ B cells, IL1B+ monocytes, and THY1 + HLA-DRAhi sublining fibroblasts23. Here, PDCD1 is linked to rheumatoid arthritis.