PDCD1 and rheumatoid arthritis: However, despite the high expression of PD‐1, T cells in RA are not exhausted and are actively involved in its pathogenesis.[5b] The PD‐1/PD‐ligand 1 (PD‐L1) axis is essential in maintaining immune homeostasis by limiting activated T cells.[6] However, why PD‐1+ T cells in RA remain persistently activated instead of receiving sufficient inhibitory signal from PD‐L1 to limit their function is unclear.