PDCD1LG2 and rheumatoid arthritis: Herein, we have demonstrated that the synovial tissue expression of PDCD1 was upregulated in early and established RA, and that of its ligands (PD-L1 and PD-L2) was significantly induced in arthralgia, IA/UA, early and established RA in comparison to their OA and healthy control counterparts (Fig 1A–1C).