Repeated low-dose ultraviolet (UV) B exposures of humans induce limited photoprotection against the immune effects of erythemal UVB radiation

BACKGROUND: Exposure of human subjects to ultraviolet (UV) B radiation causes immunosuppression. Most experiments to date have not tested the effects of low daily doses of UVB radiation. OBJECTIVES: To ascertain whether photoprotection against several UV-induced immune effects might develop following repeated exposure. METHODS: Groups of approximately 30 healthy individuals were given whole-body UVB irradiation on each of 10 consecutive days with 0.7 minimal erythema dose, or whole-body irradiation as before followed by a single erythemal UVB dose on a small body area, or irradiated only with a single erythemal UVB dose on a small body area, or were not irradiated. They were sensitized with diphenylcyclopropenone (DPCP) 24 h after the final dose, and skin biopsies collected to assess cytokine mRNA expression and the number of cells with thymine dimers and expression cyclooxygenase (COX)-1 and COX-2. RESULTS: The contact hypersensitivity (CHS) response to DPCP was significantly lower in the three irradiated groups compared with the unirradiated controls, while cutaneous interleukin (IL)-1beta, IL-6, IL-10 and tumour necrosis factor-alpha mRNAs, COX-1 and COX-2 and thymine dimers were all significantly higher. When the single erythemal UVB dose was given following the repeated low exposures, a slight downregulation in cytokine expression and thymine dimer formation was indicated. CONCLUSIONS: The repeated low doses of UVB protected to a limited extent against the effects of an erythemal UVB dose on cytokine expression and thymine dimer formation, but not on CHS or COX enzymes.