Serum levels of soluble programmed cell death protein 1 and soluble programmed cell death protein ligand 2 are increased in systemic lupus erythematosus and associated with the disease activity

Objective Programmed cell death protein 1 (PD-1) pathway plays important roles in systemic lupus erythematosus (SLE). We aimed to elucidate the association of serum soluble PD-1 (sPD-1) and related molecules with SLE and to explore their usefulness as biomarkers. Methods We retrospectively measured the serum levels of sPD-1, soluble PD-ligand 1 (sPD-L1), soluble PD-ligand 2 (sPD-L2) and interleukin (IL)-21 by ELISA in SLE patients, systemic sclerosis patients and healthy controls. Repeat sera samples were also obtained post treatment. Results The serum levels of sPD-1 and sPD-L2 in SLE patients with high disease activity were significantly higher than those in SLE patients with low disease activity, systemic sclerosis patients and healthy controls (n = 58, 15, 20 and 21, respectively; p