PDCD1 and rheumatoid arthritis: ScRNAseq of the synovial tissue isolated from patients with either OA or RA revealed three subpopulations of CD4+ [CCR7+ (called SC-T1), forkhead box P3 (FOXP3)+ regulatory (SC-T2), PDCD1+ peripheral helper (TPH) and follicular helper T cells (TFH) (SC-T3)] and three subpopulations of CD8+ T cells [GZMK+ (SC-T4), GNLY+GZMB+ cytotoxic (SC-T5), GZMK+GZMB+ T cells (SC-T6)] [19].