Reporter cell lines for skin sensitization testing

Skin sensitization has been described as an adverse outcome pathway (AOP), comprising a number of molecular events leading to the final adverse effect. In a new paradigm of toxicology, attempts are made to collect information using single mechanistic tests addressing different targets along such an AOP and to then integrate this information to arrive at a final toxicological prediction. This proposal is strongly influenced by the availability of methods for high-throughput screening of cellular events. Reporter cell lines are a particularly useful tool in such screening paradigms, as they can deliver highly reproducible and easily measureable results, and they can be designed to quantify induction or suppression at the transcription level of very specific molecular targets within cells. The first cell-based assay for skin sensitization, which has recently received ECVAM and OECD endorsement, is the reporter cell assay KeratinoSens™, reflecting activation of the Nrf2 pathway, and other assays measuring the Nrf2 pathway are under development or validation. An alternative approach (THP-G8) was recently developed based on activation of the Interleukin-8 gene. Here, we review these assays, their role in the AOP, their mechanistic interrelationships, their use for hazard and risk assessment, and their application in integrated testing strategies. At the same time, this study reviews (1) other cellular markers for sensitizers, and the potential to develop new reporter gene assays providing additional, non-redundant information, and (2) it presents approaches and new experimental data on attempts to further improve the predictivity of the existing assay.