The Possible Clinical Significance of a Decreased Serum Level of Soluble PD-L1 in Discoid Lupus Erythematosus, but Not in Subacute Cutaneous Lupus Erythematosus—A Pilot Study

Cutaneous lupus erythematosus (CLE) is an autoimmune skin disease with various clinical forms, including the subtypes of discoid lupus erythematosus (DLE) and subacute cutaneous lupus erythematosus (SCLE). The altered function of the programmed cell death 1/programmed cell death ligand 1 (PD-1/PD-L1) axis in CLE pathogenesis has been suggested. Here, the soluble forms of PD-1 (sPD-1) and PD-L1 (sPD-L1) were explored in untreated DLE and SCLE. Levels of sPD-1 and sPD-L1 were determined by enzyme-linked immunosorbent assay in serums of 21 DLE, 18 SCLE, 13 systemic lupus erythematosus (SLE) patients and 20 healthy controls (HCs). Differences between patient groups and HCs, and the association between clinical activity of skin symptoms and sPD-1/sPD-L1 levels were analyzed with Mann–Whitney U-test and Spearmann’s correlation. Regarding sPD-1 levels, no statistically significant differences were found between DLE and SCLE groups, nor compared to HCs. As for sPD-L1, a significantly lower level was found in the DLE group compared to the SCLE and HC groups (p = 0.027 and p = 0.009, respectively). In SLE, significantly higher sPD-1 was found compared to HCs (p = 0.002). No association between skin symptom activity and sPD-1/sPD-L1 levels was found in CLE. Alterations of the inhibitory effect of sPD-L1 on T-cell activity might elucidate the differences between DLE and SCLE.