By Ian Kimber, University of Manchester
G. Frank Gerberick, Procter & Gamble
Skin sensitization resulting in allergic contact dermatitis is a common occupational and environmental health issue. Many hundreds of chemicals have been implicated as skin sensitizers, and allergic contact dermatitis is without doubt the most common manifestation of immunotoxicity in humans.
In common with all forms of allergic disease, contact allergy develops in two phases and is by definition dependent upon the stimulation of an immune response. The first phase – or induction phase - is initiated when an inherently susceptible individual is exposed (usually at the skin surface) to an amount of contact allergen sufficient to trigger a cutaneous immune response resulting in immunological priming. A subject that has acquired skin sensitization now has the ability to mount an accelerated and more aggressive secondary immune response if contact is made with the same allergen again at the same or a distant skin site. This second phase - or elicitation phase - is associated with a localised cutaneous inflammatory reaction at the site of skin exposure that is characterized clinically as allergic contact dermatitis.
Allergic contact dermatitis is not life-threatening, but can be associated with considerable morbidity. Sensitisation is life-long, or at least long-lasting, and can result in the need for redeployment if acquired to chemicals in the workplace.
There is, therefore, a continuing and important need for the accurate identification and characterization of chemicals that have the potential to cause skin sensitization. Initially guinea pig methods were favoured for the purposes of hazard identification, the most widely applied being the Guinea Pig Maximization Test (1) and the Occluded Patch Test (2). More recently, the murine local lymph node assay (LLNA) has been introduced and validated as an alternative to traditional guinea pig test methods, and has been assigned an OECD test guideline (No 429) (3).
The LLNA identifies skin sensitizing chemicals as a function of their ability to provoke T lymphocyte proliferative responses in lymph nodes draining the site of topical exposure. Chemicals are classified as being contact allergens if, at one or more test concentrations, a 3-fold or greater proliferative response is induced in draining lymph nodes compared with concurrent control values (3). The approach employed by the LLNA, wherein activity is measured with respect to events during the induction phase of sensitisation, contrasts with guinea pig tests where sensitisation potential is determined as a function of the ability of chemicals to induce and elicit dermal hypersensitivity reactions (1, 2).
Although it can be argued that there are now available animal models that provide for the needs of skin sensitization hazard identification, there is a real requirement for the purposes of risk assessment to have some understanding of relative potency. Although a case can be made that an appreciation of potency is relevant for all forms of safety assessment, it is particularly the case with respect to skin sensitizers. This is because there is reason to believe that skin sensitising chemicals vary by up to 5 orders of magnitude with regard to their relative potency. Effective risk assessment, and appropriate risk management strategies, rely therefore on information about relative sensitising activity, and the preferred approach currently for providing this is derivation of an EC3 value from analysis of LLNA dose responses (4).
This approach is based on an appreciation that not only does lymph node cell proliferation provide a marker for sensitizing activity, it also correlates causally and quantitatively with the extent to which sensitization is acquired. It is possible therefore to determine relative sensitizing potency as a function of the vigour of induced proliferative responses and for this purpose an EC3 value is derived – this being the amount of chemical required to stimulate a 3-fold increase in lymph node cell proliferation compared with controls (4).
Hazard and risk assessment for skin sensitization is therefore relatively well-served by animal test methods. However, there is interest now in defining new approaches that will allow a reduction or refinement of animal use, or that will allow the replacement of animals altogether. Although it has been proposed recently that a cut-down or reduced version of the LLNA might in some circumstances allow for further refinement and reduction of animal use for hazard identification (but not potency assessment) (5), the real focus is on the development of non-animal methods.
Recent progress has been summarised elsewhere and readers are directed to the following articles for information on recent and current progress (6-11). It is against this background that we hope that debate can be encouraged in the AltTox.org Forum.
©2007 Ian Kimber & G. Frank Gerberick