These findings suggest that cytotoxic CD8+ PD-1+ HLA-DRHIGH KLRG1LOW T cells in the SF are different than the same cells from the PB (Fig. 4J) and, accordingly, could mediate some elements of synovial inflammation in RA and also serve as a target for therapy in the context of the PD-1 agonism. Here, PDCD1 is linked to rheumatoid arthritis.