Skin Sensitisation

Skin sensitisation is the regulatory endpoint aiming at the identification of chemicals able to elicit an allergic response in susceptible individuals.

Following repeated exposure to a sensitising agent, the adverse health effect of allergic contact dermatitis (ACD) may be provoked.

Thus the development of ACD is characterised by two distinct phases:

a) the induction of specialised immunological memory following the initial exposure to an allergen, called sensitisation and

b) elicitation of the clinical allergic response following subsequent exposure to the allergen.

The evaluation of the skin sensitisation potential represents an important component of the safety assessment of substances, relevant in the context of several EU regulations aiming at the protection of human health and the environment. Until recently, skin sensitisation assessment has relied on animal assays (see Regulatory acceptance and legislation).

The key biological events underpinning the skin sensitisation process are well established and have been summarised in the OECD report on "The Adverse Outcome Pathway (AOP) for Skin Sensitisation Initiated by Covalent Binding to Proteins". These key events include:

1. KE1 - the covalent binding of the chemical to skin proteins (haptenation)
2. KE2 - the release of pro-inflammatory cytokines and the induction of cyto-protective pathways in keratinocytes
3. KE3 - the maturation and mobilisation of dendritic cells (DC), immuno-competent cells in the skin
4. KE4 - the antigen presentation to naïve T-cells and proliferation of memory T-cells.

In 2013 EURL ECVAM made an explicit commitment to invest in the formal evaluation of non-animal methods for skin sensitisation (Casati et al., 2013) and to take a leading role at international level for the development of guidance on the reporting of defined approaches (OECD Guidance Documents 255, 256), i.e. approaches based on the use of various type of data, including in silico, in chemico and in vitro data.

Regulatory acceptance and legislation

Regulatory adopted animal-based tests, which are part of Council Regulation No 440/2008, include:

• Local lymph node assay, LLA (OECD TG 429), and its non-radioactive modifications, LLNA-DA (OECD TG 442A) and LLNA-BrdU Elisa (OECD TG 442B)
• Guineau pig maximisation test, GPMT, by Magnusson & Kligman (OECD TG 406)
• Buehler occluded patch test in the guinea pig (OECD TG 406)

EURL ECVAM validated test methods

Today a number of non-animal methods addressing each a specific key mechanism of the induction phase of skin sensitisation have been adopted by the OECD and can be employed for skin sensitisation hazard assessment and to inform potency subcategorization (i.e. Globally Harmonised System to Classification and Labelling potency categories 1A and 1B) when used in conjunction with other relevant information.

Test methods under validation by EURL ECVAM

Other methods are under formal validation and/or evaluation, for example the LuSens, a similar method to the KeratinoSens, was peer reviewed by the EURL ECVAM Scientific Advisory Committee (ESAC) in 2016.

Development and optimisation of alternative methods for skin sensitisation

EURL ECVAM continues to partner with other organisations to continue the development of alternative methods for skin sensitisation.