Cutaneous infiltration of plasmacytoid dendritic cells and T regulatory cells in skin lesions of polymorphic light eruption

Background Polymorphic light eruption ( PLE ) is the most common autoimmune photodermatosis. Plasmacytoid dendritic cells ( PDC s) are important mediators of innate antimicrobial immunity involved in the pathogenesis of many inflammatory skin diseases. In addition to PDC s, regulatory T cells (Tregs) are involved in controlling inflammation and adaptive immunity in skin by their immunosuppressive capacity. Objective The aim of this study was to investigate the presence of PDC s and Tregs in photoexposed skin from PLE compared to healthy skin. Methods Patients with PLE diagnosis and healthy controls were recruited and underwent a photoprovocative test. A 4‐mm punch biopsy was taken from the site of positive photoprovocation test reaction, and immunohistochemistry for BDCA 2 as marker for PDC s, CD 4 and FOXP 3 as markers for Tregs was performed. Double immunostain for FOXP 3 and CD 4 was performed as well. Absolute counts for CD 4, BDCA 2 and FOXP 3 were performed in at least 5 High Power Fields ( HPF ). Percentage of CD 4‐, BDCA 2‐ and CD 4 FOXP 3‐positive cells over the total inflammatory infiltrate was assessed for each case. Results We enrolled 23 patients and controls. BDCA 2+ cells were present in 91.3% of PLE skin samples and 100% of healthy volunteer. Both in PLE patients and healthy controls, PDC s distribution was mainly dermic ( P < 0.05). Compared to healthy controls, both epidermic and dermic BDCA 2+ cells count were significantly higher in PLE patients ( P < 0.05). Both in PLE patients and healthy controls, Tregs distribution was mainly dermic ( P < 0.05). The presence of both CD 4+ cells and FOXP 3+ cells was significantly higher in the dermis of PLE patients compared to controls ( P < 0.05). Relative percentages of cellular infiltrations confirmed these results. Conclusions D‐ PDCS and Tregs may play a significant role in the development of PLE , and dermal distribution of PDC s in PLE skin biopsies seems to confirm a possible overlap with cutaneous lupus erythematosus ( CLE ).