With the recent approval of the human Cell Line Activation Test (h-CLAT) for skin sensitization (allergy), toxicologists now have a battery of methods that allows them to test for sensitization without using animals.
Testing a chemical substance for skin irritation or corrosion is pretty straight-forward: the substance is applied to a skin sample (there are many non-animal in vitro options) and damage will be seen relatively quickly. But to learn if the chemical has the potential to cause skin allergies, testing is more complicated. Skin sensitization is a two-stage process. In the first stage, a chemical exposure “primes” the immune system. Additional exposures then provoke an allergic response (inflammation, redness, itching, etc). Because of the biological complexity of the process, skin sensitization testing is usually conducted on mice or guinea pigs.
But mice and guinea pigs don’t always react the same way as humans would to potential skin allergens. To replace these animal tests with more human-relevant methods, toxicologists have long recommended developing a battery of in vitro tests that could be combined in an Integrated Testing Strategy – where each test method captures a different part of the skin sensitization process. The biological processes that underlie the skin allergy reaction are pretty well understood, and have been described (the Organisation on Economic Co-operation and Development (OECD) has published a description of this process – the Adverse Outcome Pathway leading to skin allergies). Two of the pieces of this strategy are already in place: the OECD approved the use of the Direct Peptide Reactivity Assay (DPRA) and the KeratinoSens test – each measuring a different step in the sensitization process. The DRPA assays measures whether a chemical can react with proteins in a way that causes the protein to become an allergen. The KeratinoSens assay measures activation of genes involved in the allergic reaction in skin cells (keratinocytes). The h-CLAT method completes the battery of tests: it detects biomarkers that indicate activation of immune (dendritic) cells in the skin.
Many countries require that chemicals used in manufacturing, agriculture, medicine, and cosmetics be tested for their potential to cause skin allergies in humans. Without approved in vitro options, REACH regulations in the EU alone would force industry to use hundreds of thousands of animals for skin sensitization testing. The animal-free, 3-test strategy is a “textbook” example of the mechanistic, pathway-based approach to chemical testing promoted by the Human Toxicology Project Consortium. Use of this strategy has the potential to greatly reduce the numbers of animals used for skin sensitization testing, while also reducing the cost and time it takes to produce human-relevant results.