Establishment of an in vitro photoassay using THP-1 cells and IL-8 to discriminate photoirritants from photoallergens

•No in vitro methods have been adopted to assess photosensitization.•We explore the use of IL-8 release by THP-1 cells to identify photosensitizers.•Photoallergens induced a dose-related increase of IL-8 after irradiation.•A tentative cut-off is proposed to discriminate photoallergens from photoirritants.•The assay may become a useful cell-based in vitro test to assess photosensitization.

At present, there are no in vivo or in vitro methods developed which has been adopted by regulatory authorities to assess photosensitization induced by chemicals. Recently, we have proposed the use of THP-1 cells and IL-8 release to identify the potential of chemicals to induce skin sensitization. Based on the assumption that sensitization and photosensitization share common mechanisms, the aim of this work was to explore the THP-1 model as an in vitro model to identify photoallergenic chemicals.THP-1 cells were exposed to 7 photoallergens and 3 photoirritants and irradiated with UVA light or kept in dark. Non phototoxic allergens or irritants were also included as negative compounds. Following 24 h of incubation, cytotoxicity and IL-8 release were measured. At subtoxic concentrations, photoallergens produced a dose-related increase in IL-8 release after irradiation. Some photoirritants also produced a slight increase in IL-8 release. However, when the overall stimulation indexes of IL-8 were calculated for each chemical, 6 out of 7 photoallergens tested reached a stimulation index above 2, while the entire set of negative compounds had stimulation indexes below 2. Our data suggest that this assay may become a useful cell-based in vitro test for evaluating the photosensitizing potential of chemicals.