Background and aims Programmed cell death protein 1 (PD-1/CD279) is a cell surface receptor that belongs to the extended CD28/CTLA-4 family and is expressed on T cells and pro-B cells. PD-1 plays an important role in down regulating the immune system by preventing the activation of T-cells. Soluble PD-1 (sPD-1), which is produced by the alternative splicing, can functionally block the regulatory effect of membrane-bound PD-1 on T cell activation. We aimed to retrospectively evaluate the usefulness of sPD-1 in patients with systemic lupus erythematosus (SLE).

Methods We measured the levels of sPD-1 by enzyme-linked immunosorbent assay (ELISA) kit in sera of patients with SLE (n=59) and systemic sclerosis, and healthy controls, and compared them. We also analysed the association between the levels of sPD-1 and clinical information in patients with SLE.

Results The levels of sPD-1 in SLE patients with SLEDAI-2K≥6 were significantly higher than those in SLE patients with SLEDAI-2K<6, patients with systemic sclerosis, and healthy controls (p<0.05 in all comparisons), whereas there was no significant difference in other comparisons. In SLE patients, the levels of sPD-1 were moderately correlated with the titers of anti-ds DNA antibodies and SLEDAI-2K, and were moderately and inversely correlated with the levels of C3 and C4. In addition, the levels of sPD-1 were significantly higher in SLE patients with arthritis, mucosal ulcers, fever, leucopenia, or anaemia than those without (p<0.05 in all comparisons).

Conclusions The present study suggested that sPD-1 can serve as a new biomarker reflecting disease activity in patients with SLE.