Studies on poison ivy. In vitro lymphocyte transformation by urushiol-protein conjugates

Integrating chemistry and immunology in allergic contact dermatitis: more questions than answers?

In this issue, Simonsson and colleagues shed light on the chemical mechanisms determining hapten formation in the skin, which precede the elicitation of an antigen-specific immune response in allergic contact dermatitis. Combining fluorescence microscopy, proteomics, and mass spectrometry, the investigators identified keratins K5 and K14, particularly cysteine 54 of K5, in the human basal epidermal layer as the major molecular targets of caged thiol-reactive fluorescent haptens (i.e., bromobimanes). Anti-keratin antibody responses in mice exposed to bromobimanes suggest the generation of immunogenic epitopes by cysteine-reactive haptens. Although many issues await further investigation, Simonsson and co-workers' observations advance our understanding of the molecular basis of hapten-protein complex formation in skin.

Dermal toxicity and environmental contamination: electron transfer, reactive oxygen species, oxidative stress, cell signaling, and protection by antioxidants

Large numbers of chemicals are known to produce diverse types of skin injury, and these substances fit into a wide variety of both organic and inorganic chemical classes. Skin contact with toxins is difficult to avoid, because they are widely distributed, e.g., in industrial substances, agricultural chemicals, household products, and plants. Although various hypotheses have been advanced, there is no universal agreement as to how dermal toxins act to produce their effects. In this review, we provide evidence and numerous literature citations to support the view that oxidative stress (OS) and electron transfer (ET) comprise a portion of a key mechanism, and perhaps unifying theme that underlie the action of dermatotoxins. We apply the concept that ET and OS are key elements in the induction of dermatotoxic effects to all of the main classes of toxins, and to other toxins, as well. We believe it is not coincidental that the vast majority of dermatotoxic substances incorporate recurrent ET chemical functionalities (i.e., quinone, metal complexes, ArNO2, or conjugated iminium), either per se or as metabolites; such entities potentially give rise to reactive oxygen species (ROS) by redox cycling. However, in some categories, wherein agents cause dermal damage, e.g., peroxides and radiation, it appears that ROS are generated by non-ET routes. As expected, if ET and oxidative process do constitute the mechanistic framework by which most dermal toxins act, then antioxidants (AOs), if present, should prevent or mitigate effects. This is exactly what has been discovered to occur. Because ET and OS either cause or contribute to dermal toxicity, and AOs may offer protection therefrom, policy makers and researchers may be better positioned to prevent human dermatotoxicity.

Dermal sensitization quantitative risk assessment (QRA) for fragrance ingredients

Based on chemical, cellular, and molecular understanding of dermal sensitization, an exposure-based quantitative risk assessment (QRA) can be conducted to determine safe use levels of fragrance ingredients in different consumer product types. The key steps are: (1) determination of benchmarks (no expected sensitization induction level (NESIL)); (2) application of sensitization assessment factors (SAF); and (3) consumer exposure (CEL) calculation through product use. Using these parameters, an acceptable exposure level (AEL) can be calculated and compared with the CEL. The ratio of AEL to CEL must be favorable to support safe use of the potential skin sensitizer. This ratio must be calculated for the fragrance ingredient in each product type. Based on the Research Institute for Fragrance Materials, Inc. (RIFM) Expert Panel's recommendation, RIFM and the International Fragrance Association (IFRA) have adopted the dermal sensitization QRA approach described in this review for fragrance ingredients identified as potential dermal sensitizers. This now forms the fragrance industry's core strategy for primary prevention of dermal sensitization to these materials in consumer products. This methodology is used to determine global fragrance industry product management practices (IFRA Standards) for fragrance ingredients that are potential dermal sensitizers. This paper describes the principles of the recommended approach, provides detailed review of all the information used in the dermal sensitization QRA approach for fragrance ingredients and presents key conclusions for its use now and refinement in the future.

A review of the scientific basis for uncertainty factors for use in quantitative risk assessment for the induction of allergic contact dermatitis

Safety evaluations for chemicals which possess the ability to cause sensitization by skin contact have traditionally been done using an ad hoc comparative risk assessment technique. Recently, several papers have been published supporting the use of an alternative, and potentially better, quantitative risk assessment approach. While they represent a relatively new approach to risk assessment for sensitizers, quantitative methods have been used for decades to support risk assessments for systemic toxicity. Historically, these methods have involved the extrapolation of toxicity data - generally from studies in laboratory animals at relatively high doses to human exposures at lower doses. For toxicity endpoints with a threshold, this process has traditionally involved the use of uncertainty factors. For example, uncertainty factors are commonly used to extrapolate from laboratory animals to humans, and from 'average' humans to sensitive subpopulations. In the absence of data to support a different value, a default factor of 10 is widely accepted for each of these areas. Recent papers have advocated the use of a similar approach to characterize the risk of the induction of skin sensitization by allergens of varying potency and potential for skin contact. As with other forms of toxicity, a quantitative assessment of risk for allergic skin reactions can be approached by identifying a NOAEL (no observed adverse effect level) and applying appropriate uncertainty factors. Three major areas of data extrapolation have been identified: inter-individual susceptibility, the influence of vehicle or product matrix, and exposure considerations. This paper provides an overview of each of these areas with an evaluation of the available scientific database to support an uncertainty factor in the range of 1-10 for each area.

The chemistry of contact allergy: why is a molecule allergenic?

This review concentrates on some specific aspects of the chemistry of allergic contact dermatitis. The way low molecular weight chemicals react with skin proteins to form complete antigens will be discussed and the development of molecular modelling techniques to analyse molecular recognition presented. Subsequently, how knowledge of the chemical structure can be used to estimate the allergenic activity of a molecule will be considered. This aspect includes work with qualitative and quantitative structure-activity relationships (SAR) in the field of contact allergy.

Are free radicals and not quinones the haptenic species derived from urushiols and other contact allergenic mono- and dihydric alkylbenzenes? The significance of NADH, glutathione, and redox cycling in the skin

The induction of allergic contact dermatitis to urushiols from poison ivy and related plants is generally believed to involve an initial oxidation event by which a protein-reactive quinone is formed. However, this does not readily account for the contact allergenicity of closely related mono- and dihydric alkylbenzenes such as the alkylphenols and alkylresorcinols which are not so easily oxidised to quinones in vitro. When the redox processes known to occur in living tissues are taken into consideration, a more plausible unifying mechanism involving the formation of protein-reactive radical species becomes apparent. Experiments described here examine the autoxidation of p-benzoquinone and various mono- and dihydric benzenes and alkylbenzenes, and their reactions with the diphenylpicrylhydrazyl radical, cysteine, glutathione, and NADH. We have also demonstrated that administration to mice of 2-oxo-4-thiazolidine carboxylate, a compound known to elevate intracellular glutathione levels, inhibits the irritancy and sensitising activity of 3-pentadecylphenol. This work suggests that redox cycling in the skin following penetration of allergenic mono- and dihydric alkylbenzenes initially depletes local levels of endogenous reducing equivalents such as glutathione and NADH; once depleted, further cycling results in the uncontrolled generation of radical species which may reasonably be expected to exhibit protein reactivity.