Hirahara S, et al. - This study was performed to assess whether and how serum soluble PD-1 (sPD-1) and related molecules were associated with SLE and their usefulness as biomarkers were explored. The serum levels of sPD-1, soluble PD-ligand 1 (sPD-L1), soluble PD-ligand 2 (sPD-L2) and interleukin (IL)-21 were retrospectively measured by ELISA in SLE patients, systemic sclerosis patients and healthy controls. Researchers collected repeat sera samples (post treatment). The study reveals higher serum levels of sPD-1 and sPD-L2 in SLE patients with high disease activity than those in SLE patients with low disease activity, systemic sclerosis patients and healthy controls. The results exhibit that serum sPD-1 and sPD-L2 correlated with SLE and implies their utility as disease activity biomarkers for SLE.