OECD GUIDELINE FOR THE TESTING OF CHEMICALS DRAFT PROPOSAL FOR A NEW TEST GUIDELINE In Vitro Skin Sensitization: Keratinocyte-based ARE-Nrf2 Luciferase Reporter Gene Test Method

1. A skin sensitizer refers to a substance that will lead to an allergic response following skin contact (1). There is general agreement regarding the key biological events underlying skin sensitization summarized as an Adverse Outcome Pathway (AOP) which includes four events recognized as key ones, going from the molecular initiating event to the adverse health effect, i.e., allergic contact dermatitis in humans or contact hypersensitivity in rodents (2)(3). The first key event is the molecular interaction with skin proteins, and in particular the covalent binding to cysteine and/or lysine residues. The second key event takes place in the keratinocyte and includes inflammatory responses as well as gene expression associated with specific cell signaling pathways such as the antioxidant/electrophile response element (ARE)-dependent pathways. The third key event is the activation of dendritic cells, typically assessed by expression of specific cell surface markers, chemokines and cytokines. The fourth key event is T-cell proliferation, which is indirectly measured in the murine Local Lymph Node Assay (4).