Skin sensitisation, part of the immune response, is an important occupational and environmental health issue, accounting for 10-15% of reported occupational illnesses in Europe and the US. As such, skin sensitisation represents a critical component of the risk assessment of chemicals for many sectors, including pesticides, cosmetics and industrial chemicals. In addition, skin sensitisation is typically required to classify and label chemicals under systems such as EU REACH.
Many hundreds of chemicals have been identified as possible skin sensitisers, and allergic contact dermatitis, although not life-threatening, can be associated with considerable morbidities. As with other forms of allergic disease, allergic contact dermatitis proceeds in two phases: (i) the induction/sensitisation phase during which a person becomes sensitised, a process which is dependent upon the expansion of allergen-reactive T cells in lymph nodes draining the site of exposure, and (ii) the elicitation phase which occurs upon repeat exposure of a sensitised person to the sensitising material resulting in a localised cutaneous inflammatory reaction characteristic of an allergic response.
Traditionally, in vivo models such as the Guinea Pig Maximisation Test (GPMT), Buehler Test, and Local Lymph Node Assay (LLNA) have assessed skin sensitising potential. Human patch testing has also been used in some industries, such as cosmetics, once a preliminary assessment has been conducted. However, there is increasing ethical and political demand to develop new approaches for the assessment of skin sensitisation that do not require the use of animals.
Understanding the biology of skin sensitisation has been the focus of intensive research over many years resulting in general agreement on the key biological events underlying this process. Consequently, the Organisation for Economic Co-operation and Development (OECD) has identified and adopted four key events. They are described in the Adverse Outcome Pathway (AOP) for Skin Sensitisation Initiated by Covalent Binding to Proteins, providing a framework against which several in vitro tests have been developed.
Adverse Outcome Pathway
The AOP for skin sensitisation describes the sequence of events from the molecular initiating event, through intermediate steps, to the adverse outcome of allergic sensitisation and comprises the four Key Events (KE) summarised below.
AOP for skin sensitisation
The first key event (KE1) is based on the premise that for a chemical to be a sensitiser, it must first interact with skin proteins for it to be recognised by cells of the immune system. KE1 represents the molecular initiating event and requires that the chemical binds covalently with protein to form haptens. Sometimes, a chemical may require oxidation (a pre-hapten) or activation by host enzymes (a pro-hapten) to react with skin proteins. The second key event (KE2) is defined as keratinocyte activation and is associated with the ability of a sensitiser to induce a local inflammatory response and alterations in gene expression in keratinocytes, changes that are known to be induced by contact sensitisers. Key event 3 (KE3) relates to dendritic cell (DC) activation and refers to the requirement for a sensitising material to bind to and activate local DC causing migration of activated DC to draining lymph nodes with simultaneous upregulation of membrane expression of maturation markers, including CD54 (an adhesion molecule) and CD86 (a co-stimulatory molecule). Upregulation of these maturation markers on DC enables their interaction with and activation of allergen-reactive T cells in draining lymph nodes, leading to T cell proliferation which defines KE4. These key events describe the cellular and molecular processes required for a susceptible individual to become sensitised.
Together as a testing strategy, these predictive tests can be used instead of animal tests as part of an Integrated Approach to Testing and Assessment (IATA) to assess a potential contact allergen. The use of fully validated in vitro methods helps reduce, refine and replace animal testing. In the event of obtaining positive results in these assays, Gentronix can offer expert advice on moving forward.