Contact allergy: the role of skin chemistry and metabolism

Acute and chronic inflammation alter immunometabolism in a cutaneous delayed-type hypersensitivity reaction (DTHR) mouse model

T-cell-driven immune responses are responsible for several autoimmune disorders, such as psoriasis vulgaris and rheumatoid arthritis. Identification of metabolic signatures in inflamed tissues is needed to facilitate novel and individualised therapeutic developments. Here we show the temporal metabolic dynamics of T-cell-driven inflammation characterised by nuclear magnetic resonance spectroscopy-based metabolomics, histopathology and immunohistochemistry in acute and chronic cutaneous delayed-type hypersensitivity reaction (DTHR). During acute DTHR, an increase in glutathione and glutathione disulfide is consistent with the ear swelling response and degree of neutrophilic infiltration, while taurine and ascorbate dominate the chronic phase, suggesting a switch in redox metabolism. Lowered amino acids, an increase in cell membrane repair-related metabolites and infiltration of T cells and macrophages further characterise chronic DTHR. Acute and chronic cutaneous DTHR can be distinguished by characteristic metabolic patterns associated with individual inflammatory pathways providing knowledge that will aid target discovery of specialised therapeutics.

Nuclear magnetic resonance spectroscopy-based tissue metabolomics is used to define detailed temporal signatures of acute and chronic inflammation in cutaneous delayed-type hypersensitivity reaction.

Dermatokinetics: Advances and Experimental Models, Focus on Skin Metabolism

Numerous dermal contact products, such as drugs or cosmetics, are applied on the skin, the first protective barrier to their entrance into the organism. These products contain various xenobiotic molecules that can penetrate the viable epidermis. Many studies have shown that keratinocyte metabolism could affect their behavior by biotransformation. While aiming for detoxification, toxic metabolites can be produced. These metabolites may react with biological macromolecules often leading to sensitization reactions. After passing through the epidermis, xenobiotics can reach the vascularized dermis and therefore be bioavailable and distributed into the entire organism. To highlight these mechanisms, dermatokinetics, based on the concept of pharmacokinetics, has been developed recently. It provides information on the action of xenobiotics that penetrate the organism through the dermal route. The purpose of this review is first to describe and synthesize the dermatokinetics mechanisms to consider when assessing the absorption of a xenobiotic through the skin. We focus on skin absorption and specifically on skin metabolism, the two main processes involved in dermatokinetics. In addition, experimental models and methods to assess dermatokinetics are described and discussed to select the most relevant method when evaluating, in a specific context, dermatokinetics parameters of a xenobiotic. We also discuss the limits of this approach as it is notably used for risk assessment in the industry where scenario studies generally focus only on one xenobiotic and do not consider interactions with the rest of the exposome. The hypothesis of adverse effects due to the combination of chemical substances in contact with individuals and not to a single molecule are being increasingly studied and embraced in the scientific community.

Strategies to Develop a Suitable Formulation for Inflammatory Skin Disease Treatment

Skin barrier functions, environmental insults, and genetic backgrounds are intricately linked and form the basis of common inflammatory skin disorders, such as atopic dermatitis, psoriasis, and seborrheic dermatitis, which may seriously affect one’s quality of life. Topical therapy is usually the first line of management. It is believed that successful topical treatment requires pharmaceutical formulation from a sufficient dosage to exert therapeutic effects by penetrating the stratum corneum and then diffusing to the target area. However, many factors can affect this process including the physicochemical properties of the active compound, the composition of the formulation base, and the limitations and conditions of the skin barrier, especially in inflammatory skin. This article briefly reviews the available data on these issues and provides opinions on strategies to develop a suitable formulation for inflammatory skin disease treatment.

Temporal transcriptomic analysis of human primary keratinocytes exposed to β-naphthoflavone highlights the protective efficacy of skin to environmental pollutants

The skin covers almost the entire body and plays an important role in detoxification and elimination of xenobiotics. These processes are initiated following the binding of xenobiotics to the aryl hydrocarbon receptor (AhR), which leads to the expression of several detoxification enzymes. To gain some insights on their impacts on skin cells over time, a temporal transcriptional analysis using gene expression arrays was performed in human primary epidermal keratinocyte (HEK) cells exposed for 6, 24 and 48 h to β-naphthoflavone (βNF), a potent agonist of AhR. Our results demonstrated that expression of genes related to xenobiotic, inflammation, and extracellular matrix remodeling was increased upon βNF treatment from 6 h onwards. In contrast, the anti-oxidative response was seen mainly starting at 24 h. While some of the genes controlled by the epidermal differentiation complex was induced as soon as 6 h, expression of most of the S100 related genes located within the same chromosomal locus and keratin genes was increased at later times (24 and 48 h). Altogether our transcriptomic data highlight that following βNF exposure, HEK cells elicited a protective xenobiotic response together with the activation of inflammation and keratinocyte regeneration. Later on these processes were followed by the stimulation of anti-oxidant activity and terminal differentiation.

Utilization of a mouse/human chimeric model for long term metabolic testing of human skin

Until now, ex vivo human skin explant utilization in tissue culture has consisted of limited short-term studies (less than a week). This short timeframe does not allow for the investigation of metabolic responses of complex tissues to specific molecules or compounds. Here, we aim to develop an improved mouse transplantation model that maintains the viability, structure and functionality of the human skin explants for prolonged periods of time. Healthy human skin explants derived from biopsies were grafted onto nude mice and used to perform a toxicological study of the reactivity and functionality of grafted skin explants after one month. Histological observations suggest that the tissue properties and phenotype of the human skin graft are conserved as a result of re-vascularization upon tissue integration. The toxicological test performed shows that the human skin graft reacts to systemic exposure of a xenobiotic metabolic inducer when applied to this mouse model. This mouse/human chimeric model can be effective for the long-term study of human skin reactivity to chemicals as well to study in vivo responses to complex co-exposures.

Skin sensitization: Uncertainties, challenges, and opportunities for improved risk assessment

At the ESCD congress held in Manchester in 2016, a session was organized to encourage more dialogue between clinicians with expertise in skin sensitization and toxicologists seeking to provide effective risk assessment to prevent human health issues. That session focused on the remaining uncertainties regarding the induction and regulation of skin sensitization in humans, and the opportunities and challenges associated with the refinement and improvement of risk assessment methodologies. This short article, prompted by those discussions, debates what the authors regard as being among the most important and most intriguing uncertainties about skin sensitization and allergic contact dermatitis in humans, and the most significant opportunities for improving risk assessment. The aim has been to provide a basis for mapping out the areas that might benefit from a closer alignment between the relevant clinical community and toxicologists charged with the responsibility of ensuring that skin sensitization risks are understood and managed.

The performance of an in vitro skin sensitisation test, IL-8 Luc assay (OECD442E), and the integrated approach with direct peptide reactive assay (DPRA)

In all current in vitro skin sensitisation assays, DMSO is used to dissolve water-insoluble chemicals. However, our previous study suggested the superiority of the modified IL-8 Luc assay (mIL-8 Luc), in which X-VIVO^TM 15 is used to dissolve chemicals, over the original assay using DMSO (oIL-8 Luc). In this study, to confirm the superiority of the mIL-8 Luc, we first increased the number of chemicals examined and demonstrated the superiority of the mIL-8 Luc, in which the mIL-8 Luc provided 87.6% of sensitivity, 74.2% of specificity, and 84.6% of accuracy. Next, to clarify the cause of false negative judgment by the mIL-8 Luc, we examined the effects of physical properties of chemicals on judgment. The results demonstrated that high molecular weight, high LogKo/w, or poor water solubility, did not cause false negative judgment. When it was accepted as an OECD test guideline, the criteria of the mIL-8 Luc to determine sensitisers were modified to further decrease false negative judgment by poor solubility. By applying the new criteria, the test guideline IL-8 Luc assay (tgIL-8 Luc) improved sensitivity but decreased specificity and increased the number of chemicals that cannot be judged. To overcome this problem, we examined a simple combination of the tgIL-8 Luc with direct peptide reactive assay (DPRA), which could improve specificity and decrease the number of the chemicals that cannot be judged. These data suggest that the tgIL-8 Luc is a promising in vitro skin sensitisation assay in combination with other in vitro or in chemico methods.

Skin Sensitization: Modeling Based on Skin Metabolism Simulation and Formation of Protein Conjugates

A quantitative structure-activity relationship (QSAR) system for estimating skin sensitization potency has been developed that incorporates skin metabolism and considers the potential of parent chemicals and/or their activated metabolites to react with skin proteins. A training set of diverse chemicals was compiled and their skin sensitization potency assigned to one of three classes. These three classes were, significant, weak, or nonsensitizing. Because skin sensitization potential depends upon the ability of chemicals to react with skin proteins either directly or after appropriate metabolism, a metabolic simulator was constructed to mimic the enzyme activation of chemicals in the skin. This simulator contains 203 hierarchically ordered spontaneous and enzyme controlled reactions. Phase I and phase II metabolism were simulated by using 102 and 9 principal transformations, respectively. The covalent interactions of chemicals and their metabolites with skin proteins were described by 83 reactions that fall within 39 alerting groups. The SAR/QSAR system developed was able to correctly classify about 80% of the chemicals with significant sensitizing effect and 72% of nonsensitizing chemicals. For some alerting groups, three-dimensional (3D)-QSARs were developed to describe the multiplicity of physicochemical, steric, and electronic parameters. These 3D-QSARs, so-called pattern recognition-type models, were applied each time a latent alerting group was identified in a parent chemical or its generated metabolite(s). The concept of the mutual influence amongst atoms in a molecule was used to define the structural domain of the skin sensitization model. The utility of the structural model domain and the predictability of the model were evaluated using sensitization potency data for 96 chemicals not used in the model building. The TIssue MEtabolism Simulator (TIMES) software was used to integrate a skin metabolism simulator and 3D-QSARs to evaluate the reactivity of chemicals thus predicting their likely skin sensitization potency.

A joint allergist/cardiologist classification for thienopyridines hypersensitivity reactions based on their symptomatic patterns and its impact on the management strategies

The role and importance of thienopyridines such as ticlopidine, clopidogrel, and prasugrel is well-established for several indications, ranging from prevention of acute coronary syndromes to percutaneous coronary interventions, where the dual antiplatelet therapy represents the gold standard to avoid denovo coronary stenosis. However, there is a significant cohort of patients with coronary artery disease who may manifest hypersensitivity reactions to thienopyridines. The examination of the various case reports from medical literature leads to identify mainly four clinical patterns of hypersensitivity to thienopyridines which involves more frequently cutaneous, hematologic, and articular tissues, therefore the kind and predominance of clinical symptoms may determine a different clinical approach to overcome or neutralize thienopyridines hypersensitivity.

In Situ Metabolism of Cinnamyl Alcohol in Reconstructed Human Epidermis: New Insights into the Activation of This Fragrance Skin Sensitizer

Chemical modification of epidermal proteins by skin sensitizers is the molecular event which initiates the induction of contact allergy. However, not all chemical skin allergens react directly as haptens with epidermal proteins but need either a chemical (prehaptens) or metabolic (prohaptens) activation step to become reactive. Cinnamyl alcohol has been considered a model prohapten, as this skin sensitizer has no intrinsic reactivity. Therefore, the prevailing theory is that cinnamyl alcohol is enzymatically oxidized into the protein-reactive cinnamaldehyde, which is the sensitizing agent. Knowing that reconstructed human epidermis (RHE) models have been demonstrated to be quite similar to the normal human epidermis in terms of metabolic enzymes, use of RHE may be useful to investigate the in situ metabolism/activation of cinnamyl alcohol, particularly when coupled with high-resolution magic angle spinning nuclear magnetic resonance. Incubation of carbon-13 substituted cinnamyl derivatives with RHE did not result in the formation of cinnamaldehyde. The metabolites formed suggest the formation of an epoxy-alcohol and an allylic sulfate as potential electrophiles. These data suggest that cinnamyl alcohol is inducing skin sensitization through a route independent of the one involving cinnamaldehyde and should therefore be considered as a skin sensitizer on its own.

Electrophysiological and behavioral characterization of bioactive compounds of the Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum and Cinnamomum zeylanicum essential oils against Anopheles gambiae and prospects for their use as bednet treatments

BACKGROUND

Laboratory and field studies showed that repellent, irritant and toxic actions of common public health insecticides reduce human-vector contact and thereby interrupt disease transmission. One of the more effective strategies to reduce disease risk involves the use of long-lasting treated bednets. However, development of insecticide resistance in mosquito populations makes it imperative to find alternatives to these insecticides. Our previous study identified four essential oils as alternatives to pyrethroids: Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum, Cinnamomum zeylanicum. The objectives of this study were to identify active compounds of these essential oils, to characterize their biological activity, and to examine their potential as a treatment for bednets.

METHODS

We evaluated the electrophysiological, behavioural (repellency, irritancy) and toxic effects of the major compounds of these oils against Anopheles gambiae strain 'Kisumu'.

RESULTS

Aldehydes elicited the strongest responses and monoterpenes the weakest responses in electroantennogram (EAG) trials. However, EAG responses did not correlate consistently with results of behavioral assays. In behavioral and toxicity studies, several of the single compounds did exhibit repellency, irritancy or toxicity in An. gambiae; however, the activity of essential oils did not always correlate with activity expected from the major components. On the contrary, the biological activity of essential oils appeared complex, suggesting interactions between individual compounds and the insect under study. Data also indicated that the three effects appeared independent, suggesting that repellency mechanism(s) may differ from mechanisms of irritancy and toxicity.

CONCLUSIONS

Based on the bioassays reported here, some of the compounds merit consideration as alternative bednet treatments.

The Myeloid U937 Skin Sensitization Test (U-SENS) addresses the activation of dendritic cell event in the adverse outcome pathway for skin sensitization

The U-SENS™ assay, formerly known as MUSST (Myeloid U937 Skin Sensitization Test), is an in vitro method to assess skin sensitization. Dendritic cell activation following exposure to sensitizers was modelled in the U937 human myeloid cell line by measuring the induction of the expression of CD86 by flow cytometry. The predictive performance of U-SENS™ was assessed via a comprehensive comparison analysis with the available human and LLNA data of 175 substances. U-SENS™ showed 79% specificity, 90% sensitivity and 88% accuracy. A four laboratory ring study demonstrated the transferability, reliability and reproducibility of U-SENS™, with a reproducibility of 95% within laboratories and 79% between-laboratories, showing that the U-SENS™ assay is a promising tool in a skin sensitization risk assessment testing strategy.

Applying the skin sensitisation adverse outcome pathway (AOP) to quantitative risk assessment

As documented in the recent OECD report 'the adverse outcome pathway for skin sensitisation initiated by covalent binding to proteins' (OECD, 2012), the chemical and biological events driving the induction of human skin sensitisation have been investigated for many years and are now well understood. Several non-animal test methods have been developed to predict sensitiser potential by measuring the impact of chemical sensitisers on these key events (Adler et al., 2011; Maxwell et al., 2011); however our ability to use these non-animal datasets for risk assessment decision-making (i.e. to establish a safe level of human exposure for a sensitising chemical) remains limited and a more mechanistic approach to data integration is required to address this challenge. Informed by our previous efforts to model the induction of skin sensitisation (Maxwell and MacKay, 2008) we are now developing two mathematical models ('total haptenated protein' model and 'CD8(+) T cell response' model) that will be linked to provide predictions of the human CD8(+) T cell response for a defined skin exposure to a sensitising chemical. Mathematical model development is underpinned by focussed clinical or human-relevant research activities designed to inform/challenge model predictions whilst also increasing our fundamental understanding of human skin sensitisation. With this approach, we aim to quantify the relationship between the dose of sensitiser applied to the skin and the extent of the hapten-specific T cell response that would result. Furthermore, by benchmarking our mathematical model predictions against clinical datasets (e.g. human diagnostic patch test data), instead of animal test data, we propose that this approach could represent a new paradigm for mechanistic toxicology.

What determines skin sensitization potency-myths, maybes and realities. Part 1. The 500 molecular weight cut-off

BACKGROUND

It is widely accepted that there is a molecular weight (MW) cut-off of 500, such that single chemicals with MWs higher than 500 cannot be skin sensitizers. If true, this could serve as a useful principle for designing non-sensitizing chemicals.

OBJECTIVES

To assess whether the 500 MW cut-off is a myth or a reality.

METHODS

A database of 699 chemicals tested for skin sensitization in guinea pigs or mice was analysed to establish the number of tested chemicals with MW > 500, and to establish whether any of these were sensitizers.

RESULTS

Only 13 (2%) of the 699 chemicals in the database have MW > 500. Of the 13 tested compounds with MW > 500 in the database, five are sensitizers and eight are non-sensitizers.

CONCLUSIONS

The 500 MW cut-off for skin sensitization is a myth, probably derived from the widespread misconception that ability to efficiently penetrate the stratum corneum is a key determinant of sensitization potency. The scarcity of sensitizers with MW > 500 simply reflects the general scarcity of chemicals with MW > 500.

Fragrance chemicals lyral and lilial decrease viability of HaCat cells' by increasing free radical production and lowering intracellular ATP level: protection by antioxidants

We investigate in this study the biochemical effects on cells in culture of two commonly used fragrance chemicals: lyral and lilial. Whereas both chemicals exerted a significant effect on primary keratinocyte(s), HaCat cells, no effect was obtained with any of HepG2, Hek293, Caco2, NIH3T3, and MCF7 cells. Lyral and lilial: (a) decreased the viability of HaCat cells with a 50% cell death at 100 and 60 nM respectively; (b) decreased significantly in a dose dependant manner the intracellular ATP level following 12-h of treatment; (c) inhibited complexes I and II of electron transport chain in liver sub-mitochondrial particles; and (d) increased reactive oxygen species generation that was reversed by N-acetyl cysteine and trolox and the natural antioxidant lipoic acid, without influencing the level of free and/or oxidized glutathione. Lipoic acid protected HaCat cells against the decrease in viability induced by either compound. Dehydrogenation of lyral and lilial produce α,β-unsaturated aldehydes, that reacts with lipoic acid requiring proteins resulting in their inhibition. We propose lyral and lilial as toxic to mitochondria that have a direct effect on electron transport chain, increase ROS production, derange mitochondrial membrane potential, and decrease cellular ATP level, leading thus to cell death.

Electrochemistry/liquid chromatography/mass spectrometry to demonstrate irreversible binding of the skin allergen p-phenylenediamine to proteins

RATIONALE

para-Phenylenediamine (PPD) is a potent and well-known allergen, which is commonly used in hair or fur dyes and can cause severe allergic contact dermatitis. In this work, the skin-sensitizing potential of PPD with respect to the conjugation of proteins was evaluated using an approach without animal testing.

METHODS

Electrochemistry (EC) coupled offline to liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS) was employed to convert the pre-hapten PPD into its reactive hapten analogs. A previous study had already shown that this purely instrumental method is suitable for accelerating and simulating the various oxidation processes, which PPD may undergo, and that the emerging products are prone to react with soft thiol groups of small nucleophiles like glutathione and cysteine.

RESULTS

This investigation was extended by successfully demonstrating adduct formation between EC-generated PPD oxidation products and the three proteins β-lactoglobulin A (β-LGA), human serum albumin and human hemoglobin. A tryptic digest of modified β-LGA provided evidence for irreversible protein binding of monomeric PPD, a PPD dimer and the PPD trimer known as Bandrowski's Base. It was shown that the main oxidation product p-phenylene quinone diimine, and the reactive oligomerized species, primarily attack the free thiol function of proteins rather than other nucleophilic amino acid residues.

CONCLUSIONS

The pre-hapten PPD was efficiently activated upon EC oxidation and the resulting species were further reacted with different proteins leading to diverse hapten-protein complexes. Thereby, problems related to the complex matrix present in conventional in vitro or in vivo methods could effectively be avoided.

Chemical sensitization and allergotoxicology

Chemical sensitization remains an important environmental and occupational health issue. A wide range of substances have been shown to possess the ability to induce skin sensitization or respiratory sensitization. As a consequence, there is a need to have appropriate methods to identify sensitizing agents. Although a considerable investment has been made in exploring opportunities to develop methods for hazard identification and characterization, there are, as yet, no validated nonanimal methods available. A state of the art of the different in vitro approaches to identify contact and respiratory capacity of chemicals is covered in this chapter.

Haptenation: chemical reactivity and protein binding

Low molecular weight chemical (LMW) allergens are commonly referred to as haptens. Haptens must complex with proteins to be recognized by the immune system. The majority of occupationally related haptens are reactive, electrophilic chemicals, or are metabolized to reactive metabolites that form covalent bonds with nucleophilic centers on proteins. Nonelectrophilic protein binding may occur through disulfide exchange, coordinate covalent binding onto metal ions on metalloproteins or of metal allergens, themselves, to the major histocompatibility complex. Recent chemical reactivity kinetic studies suggest that the rate of protein binding is a major determinant of allergenic potency; however, electrophilic strength does not seem to predict the ability of a hapten to skew the response between Th1 and Th2. Modern proteomic mass spectrometry methods that allow detailed delineation of potential differences in protein binding sites may be valuable in predicting if a chemical will stimulate an immediate or delayed hypersensitivity. Chemical aspects related to both reactivity and protein-specific binding are discussed.

Assessment of chemical skin-sensitizing potency by an in vitro assay based on human dendritic cells

The skin-sensitizing potential of chemicals is an important concern for public health and thus a significant end point in the hazard identification process. To determine skin-sensitizing capacity, large research efforts focus on the development of assays, which do not require animals. As such, an in vitro test has previously been developed based on the differential expression of CREM and CCR2 transcripts in CD34(+) progenitor-derived dendritic cells (CD34-DC), which allows to classify chemicals as skin (non-)sensitizing. However, skin sensitization is not an all-or-none phenomenon, and up to now, the assessment of relative potency can only be derived using the in vivo local lymph node assay (LLNA). In our study, we analyzed the feasibility to predict the sensitizing potency, i.e., the LLNA EC3 values, of 15 skin sensitizers using in vitro data from the CD34-DC-based assay. Hereto, we extended the in vitro-generated gene expression data set by an additional source of information, the concentration of the compound that causes 20% cell damage (IC20) in CD34-DC. We statistically confirmed that this IC20 is linearly independent from the gene expression changes but that it does correlate with LLNA EC3 values. In a further analysis, we applied a robust linear regression with both IC20 and expression changes of CREM and CCR2 as explanatory variables. For 13 out of 15 compounds, a high linear correlation was established between the in vitro model and the LLNA EC3 values over a range of four orders of magnitude, i.e., from weak to extreme sensitizers.

A QSAR investigation of dermal and respiratory chemical sensitizers based on computational chemistry properties

A wide range of physicochemical properties based on molecular topology, size and shape, and semi-empirical molecular orbital theory were calculated for a variety of dermal and respiratory sensitizers, as well as some non-active substances. Compounds were randomly selected to belong to a training set of substances (approximately 90%) for development of quantitative structure-activity relationship (QSAR) models or to a test set (approximately 10%) for testing the models. A choice was made of those descriptors which were related to sensitization using standard statistics. Pattern recognition methods were then utilized to identify the combination of properties that provided the greatest contribution to the observed biological effect. Principal components (PC) analysis was then performed on the most important properties. The models derived were then applied to a test set of known sensitizers to predict their class. For dermal and respiratory sensitizers respectively, the PC model classified five (100%) of the R-43 active and two (100%) of the R42-active test set compounds correctly. Analysis of the PC loadings showed that the most useful properties distinguishing respiratory and/or dermal sensitizers from inactive substances were the molecular orbital-based terms.

Application of a Systems Biology Approach to Skin Allergy Risk Assessment

We have developed an in silico model of the induction of skin sensitisation, in order to characterise and quantify the contribution of each pathway to the overall biological process. This analysis has been used to guide our research on skin sensitisation and in vitro test development programmes, and provides a theoretical rationale for the interpretation and integration of non-animal predictive data for risk assessment (RA) purposes. The in vivo mouse Local Lymph Node Assay (LLNA) is now in widespread use for the evaluation of skin sensitisation potential and potency. Recent changes in European Union (EU) legislation (i.e. the 7th Amendment to the EU Cosmetics Directive) have made the development of non-animal approaches to provide the data for skin sensitisation RA a key business need. Several in vitro predictive assays have already been developed for the prediction of skin sensitisation. However, these are based on the determination of a small number of pathways within the overall biological process, and our understanding of the relative contribution of these individual pathways to skin sensitisation induction is limited. To address this knowledge gap, a “systems biology” approach has been used to construct a computer-based mathematical model of the induction of skin sensitisation, in collaboration with Entelos, Inc. The biological mechanisms underlying the induction phase of skin sensitisation are represented by nonlinear ordinary differential equations and defined by using information from over 500 published papers. By using the model, we have identified knowledge gaps for future investigative research, and key factors that have a major influence on the induction of skin sensitisation (e.g. TNF-α production in the epidermis). The relative contribution of each of these key pathways has been assessed by determining their contributions to the overall process (e.g. sensitiser-specific T-cell proliferation in the draining lymph node). This information provides a biologically-relevant rationale for the interpretation and potential integration of diverse types of non-animal predictive data. Consequently, the Skin Sensitisation Physiolab® (SSP) platform represents one approach to integration that is likely to prove an invaluable tool for hazard evaluation in a new framework for consumer safety RA.

Skin sensitization: strategies for the assessment and management of risk

Allergic contact dermatitis (ACD) is to a considerable extent a preventable disease. Limitation can be achieved by correct identification of skin sensitizers, characterization of their potency, understanding human skin exposure and application of good risk assessment/management strategies. Various methods exist which are accurate for the predictive identification of chemicals that possess skin-sensitizing properties. These are enshrined in regulations that aim to provide a harmonized approach to hazard identification. One of the methods, the local lymph node assay, also delivers information on the relative potency of sensitizers. Efforts are continuing in the European Union and at the Organization for Economic Cooperation and Development to use elements of this information for regulatory categorization of skin sensitizers. However, greater use can be made of this potency information in the application of quantitative risk assessments. Such assessments depend also on the availability of accurate data on human skin exposure, one aspect where legislation has little role to play. Management of risks by restriction of skin exposure is, in contrast, a key point where legislation can play an important role, helping to establish a level playing field for industry and setting good standards based on the legislator's ability to access all data. Ultimately, the combination of accurate hazard identification, potency measurement, risk assessment and management, underpinned by enabling legislation, will lead to reduction of ACD. For individuals who do still develop contact allergy, avoidance of ACD should continue to be a goal, based on raising awareness of skin protection, allergen labelling and other skincare strategies.

Oxidation of 2-Mercaptobenzothiazole in Latex Gloves and Its Possible Haptenation Pathway

The rubber accelerator, 2-mercaptobenzothiazole (MBT), has been reported to cause allergic contact dermatitis from gloves and other rubber products, but its chemical fate when exposed to occupational oxidants and the mechanism of its pathogenesis are not known. It was hypothesized that the thiol group is critical to MBT's (its oxidation products or metabolites) covalent binding and/or haptenation to nucleophilic protein residues. Oxidative transformation of MBT to the disulfide 2,2'-dithiobis(benzothiazole) (MBTS) was observed within the glove matrix when hypochlorous acid, iodine, and hydrogen peroxide were used as oxidants. Cysteine reduced MBTS to MBT with subsequent formation of the mixed disulfide 2-amino-3-(benzothiazol-2-yl disulfanyl)propionic acid which was identified and characterized. Spectrophotometry and mass spectrometry experiments demonstrated the simultaneous reduction of MBTS and disulfide formation with Cys34 on bovine serum albumin, suggesting a potential route of protein haptenation through covalent bonding between protein cysteinyl residues and the MBT/MBTS thiol moiety. Metabolism of MBT using isoniazid and dexamethasone-induced rat liver microsomes, to give a protein reactive epoxide intermediate and provide an alternative protein haptenation mechanism, was not observed. The data suggest that the critical functional group on MBT is the thiol, and haptenation is via the formation of mixed disulfides between the thiol group on MBT and a protein sulfhydryl group.

TIMES-SS—A promising tool for the assessment of skin sensitization hazard. A characterization with respect to the OECD validation principles for (Q)SARs and an external evaluation for predictivity

The TImes MEtabolism Simulator platform used for predicting Skin Sensitization (TIMES-SS) is a hybrid expert system that was developed at Bourgas University using funding and data from a Consortium comprising industry and regulators. The model was developed with the aim of minimizing animal testing and to be scientifically valid in accordance with the OECD principles for (Q)SAR validation. TIMES-SS encodes structure-toxicity and structure-skin metabolism relationships through a number of transformations, some of which are underpinned by mechanistic 3D QSARs. Here, we describe the extent to which the five OECD principles are met and in particular the results from an external evaluation exercise that was recently carried out. As part of this exercise, data were generated for 40 new chemicals in the murine local lymph node assay (LLNA) and then compared with predictions made by TIMES-SS. The results were promising with an overall good concordance (83%) between experimental and predicted values. Further evaluation of these results highlighted certain inconsistencies which were rationalized by a consideration of reaction chemistry principles for sensitization. Improvements for TIMES-SS were proposed where appropriate. TIMES-SS is a promising tool to aid in the evaluation of skin sensitization hazard under legislative programs such as REACH.

Skin contact irritation conditions the development and severity of allergic contact dermatitis

Irritant contact dermatitis (ICD) is a frequent inflammatory skin disease induced by skin contact with low molecular weight chemicals such as haptens endowed with proinflammatory properties. Allergic contact dermatitis (ACD) is a frequent complication of ICD and is mediated by hapten-specific T cells primed in lymph nodes by skin emigrating dendritic cells. The aim of this study was to analyze the relationship between ICD and ACD to 2,4-dinitrofluorobenezene (DNFB) in C57BL/6 and BALB/C mice, which develop a severe and a moderate skin inflammation, respectively. Upon a single skin painting with DNFB, C57BL/6 developed within hours a more severe dose-dependent ICD response as compared to BALB/C mice, which was associated with enhanced upregulation of IL-1beta, IL-6, and IL-10. Skin exposure to a low dose of DNFB resulted, in both strains, in a low ICD that resolved in a few hours. Alternatively, skin painting with either an intermediate or a high DNFB concentration induced an ICD that subsequently gave rise to an ACD reaction whose intensity was proportional to the magnitude of the ICD response and was more severe in C57BL/6 mice than in BALB/C mice. In conclusion, the hapten-induced skin contact irritation conditions the development and the severity of ACD.

Hapten-protein binding: from theory to practical application in the in vitro prediction of skin sensitization

In view of the forthcoming European Union ban on in vivo testing of cosmetic and toiletry ingredients, following the publication of the 7th amendment to the Cosmetics Directive, the search for practical, alternative, non-animal approaches is gathering pace. For the end-point of skin sensitization, the ultimate goal, i.e. the development and validation of alternative in vitro/in silico assays by 2013, may be achieved through a better understanding of the skin sensitization process on the cellular and molecular levels. One of the key molecular events in skin sensitization is protein haptenation, i.e. the chemical modification of self-skin protein(s) thus forming macromolecular immunogens. This concept is widely accepted and in theory can be used to explain the sensitizing capacity of many known skin sensitizers. Thus, the principle of protein or peptide haptenation could be used in in vitro assays to predict the sensitization potential of a new chemical entity. In this review, we consider some of the theoretical aspects of protein haptenation, how mechanisms of protein haptenation can be investigated experimentally and how we can use such knowledge in the development of novel, alternative approaches for predicting skin sensitization potential in the future.

Quantum Mechanical Structure−Activity Relationship Analyses for Skin Sensitization

Allergic contact dermatitis (ACD) results in inflammation of the skin due to sensitization of the immunologic system to a particular substance. The sensitization process is limited by the compound's ability to both permeate and react with proteins in the integumentary system. Currently, only in vivo animal tests such as the local lymph node assay (LLNA) are recognized by regulatory authorities for risk assessment of ACD. A quantitative structure-activity relationship has been developed to predict relative potency, which allows for the prediction of relative sensitization potentials. The experimental values used in this study include EC3 values (the concentration at which the stimulation index equals 3) from LLNA tests. The predictions in this model enable categorization of the compounds into three groups on the basis of risk of sensitization and enable screening of candidate molecules using rapid SAM1 semiempirical calculations prior to animal testing. The model may also be used to reduce the number of animals subjected to testing by providing estimated concentrations required for useful data of risk assessment. The effect of averaging available literature values on predictive ability is also investigated. The model includes halogenated compounds, aromatic compounds, alcohols, aldehydes, and ketones. The computational investigation resulted in a two-descriptor model that is consistent with the assumed mechanism for sensitization.