Compared with the control group, the levels of sPD-1 and sPD-L1 were significantly increased in the RA group (RA group: sPD-1: 6.17 ± 0.81, sPD-L1: 2.33 ± 0.56 Figures 2(a) and 2(b)). Here, SPDL1 is linked to rheumatoid arthritis.