The inter-laboratory validation study of EpiSensA for predicting skin sensitization potential

The Epidermal Sensitization Assay (EpiSensA) is a reconstructed human epidermis (RhE)-based gene expression assay for predicting the skin sensitization potential of chemicals. Since the RhE model is covered by a stratified stratum corneum, various kinds of test chemicals, including lipophilic ones and pre-/pro-haptens, can be tested with a route of exposure akin to an in vivo assay and human exposure. This article presents the results of a formally managed validation study of the EpiSensA that was carried out by three participating laboratories. The purpose of this validation study was to assess transferability of the EpiSensA to new laboratories along with its within- (WLR) and between-laboratory reproducibility (BLR). The validation study was organized into two independent stages. As demonstrated during the first stage, where three sensitizers and one non-sensitizer were correctly predicted by all participating laboratories, the EpiSensA was successfully transferred to all three participating laboratories. For Phase I of the second stage, each participating laboratory performed three experiments with an identical set of 15 coded test chemicals resulting in WLR of 93.3%, 93.3%, and 86.7%, respectively. Furthermore, when the results from the 15 test chemicals were combined with those of the additional 12 chemicals tested in Phase II of the second stage, the BLR for 27 test chemicals was 88.9%. Moreover, the predictive capacity among the three laboratories showed 92.6% sensitivity, 63.0% specificity, 82.7% accuracy, and 77.8% balanced accuracy based on murine local lymph node assay (LLNA) results. Overall, this validation study concluded that EpiSensA is easily transferable and sufficiently robust for assessing the skin sensitization potential of chemicals.

Skin sensitization data for methyl esters of sulphonic acids-Corrections to published databases required

BACKGROUND

Several methyl esters of sulphonic acids are listed in murine local lymph node assay (LLNA) databases, with dose-response data and EC3 values. However, some of these entries are questionable-in one case the chemical tested is not the chemical named in the databases and in others the EC3 value has been derived by extrapolation from data that do not meet the applicability criteria for the approved extrapolation method.

OBJECTIVES

To consider how LLNA data came to be attributed to the wrong chemical and to address the inappropriate extrapolated EC3 values.

METHODS

Dose-response data for methyl hexadec-3-enesulphonate (wrongly named as methyl hexadec-1-enesulphonate), two other methyl sulphonates and hexadec-1-ene-1,3-sultone are re-evaluated using the single dose probit extrapolation method (SDPEM). The different reaction chemistry profiles of methyl hexadec-3-enesulphonate and methyl hexadec-1-enesulphonate are discussed.

RESULTS

Extrapolated EC3 values for hexadec-1-ene-1,3-sultone are the same by both methods but for the methyl sulphonates the differences are substantial.

CONCLUSIONS

Current databases should be corrected and further analysed to identify other cases where EC3 values are likely to be unreliable due to inappropriate estimation by extrapolation.

Development of a 96-Well Electrophilic Allergen Screening Assay for Skin Sensitization Using a Measurement Science Approach

The Electrophilic Allergen Screening Assay (EASA) has emerged as a promising in chemico method to detect the first key event in the adverse outcome pathway (AOP) for skin sensitization. This assay functions by assessing the depletion of one of two probe molecules (4-nitrobenzenethiol (NBT) and pyridoxylamine (PDA)) in the presence of a test compound (TC). The initial development of EASA utilized a cuvette format resulting in multiple measurement challenges such as low throughput and the inability to include adequate control measurements. In this study, we describe the redesign of EASA into a 96-well plate format that incorporates in-process control measurements to quantify key sources of variability each time the assay is run. The data from the analysis of 67 TCs using the 96-well format had 77% concordance with animal data from the local lymph node assay (LLNA), a result consistent with that for the direct peptide reactivity assay (DPRA), an OECD test guideline (442C) protein binding assay. Overall, the measurement science approach described here provides steps during assay development that can be taken to increase confidence of in chemico assays by attempting to fully characterize the sources of variability and potential biases and incorporate in-process control measurements into the assay.

Validation of Quantitative Structure-Activity Relationship (QSAR) and Quantitative Structure-Property Relationship (QSPR) approaches as alternatives to skin sensitization risk assessment

The aim of this study was conducted to validate the physicochemical properties of a total of 362 chemicals [305 skin sensitizers (212 in the previous study + 93 additional new chemicals), 57 non-skin sensitizers (38 in the previous study + 19 additional new chemicals)] for skin sensitization risk assessment using quantitative structure-activity relationship (QSAR)/quantitative structure-property relationship (QSPR) approaches. The average melting point (MP), surface tension (ST), and density (DS) of the 305 skin sensitizers and 57 non-sensitizers were used to determine the cutoff values distinguishing positive and negative sensitization, and correlation coefficients were employed to derive effective 3-fold concentration (EC3 (%)) values. QSAR models were also utilized to assess skin sensitization. The sensitivity, specificity, and accuracy were 80, 15, and 70%, respectively, for the Toxtree QSAR model; 88, 46, and 81%, respectively, for Vega; and 56, 61, and 56%, respectively, for Danish EPA QSAR. Surprisingly, the sensitivity, specificity, and accuracy were 60, 80, and 64%, respectively, when MP, ST, and DS (MP+ST+DS) were used in this study. Further, MP+ST+DS exhibited a sensitivity of 77%, specificity 57%, and accuracy 73% when the derived EC3 values were classified into local lymph node assay (LLNA) skin sensitizer and non-sensitizer categories. Thus, MP, ST, and DS may prove useful in predicting EC3 values as not only an alternative approach to animal testing but also for skin sensitization risk assessment.

In silico Prediction of Skin Sensitization: Quo vadis?

Skin direct contact with chemical or physical substances is predisposed to allergic contact dermatitis (ACD), producing various allergic reactions, namely rash, blister, or itchy, in the contacted skin area. ACD can be triggered by various extremely complicated adverse outcome pathways (AOPs) remains to be causal for biosafety warrant. As such, commercial products such as ointments or cosmetics can fulfill the topically safe requirements in animal and non-animal models including allergy. Europe, nevertheless, has banned animal tests for the safety evaluations of cosmetic ingredients since 2013, followed by other countries. A variety of non-animal in vitro tests addressing different key events of the AOP, the direct peptide reactivity assay (DPRA), KeratinoSens™, LuSens and human cell line activation test h-CLAT and U-SENS™ have been developed and were adopted in OECD test guideline to identify the skin sensitizers. Other methods, such as the SENS-IS are not yet fully validated and regulatorily accepted. A broad spectrum of in silico models, alternatively, to predict skin sensitization have emerged based on various animal and non-animal data using assorted modeling schemes. In this article, we extensively summarize a number of skin sensitization predictive models that can be used in the biopharmaceutics and cosmeceuticals industries as well as their future perspectives, and the underlined challenges are also discussed.

Identification of gene expression markers and development of evaluation method using cell-based and RT-PCR-based assay for skin sensitising potential of chemicals

Recently, alternatives to animal testing have been used to evaluate skin sensitisers in cosmetic products. However, testing is still complicated and expensive. To develop a simpler, cost-effective and more accurate evaluation method for the skin sensitising chemicals, we employed cell-based and RT-PCR-based assay. Representative sensitiser specific gene expression in THP-1 cells was analysed by microarray. Gene ontology (GO) analysis revealed that 26 genes induced by the sensitisers were associated with immune function. First, seven of the 26 genes were chosen arbitrarily as candidate markers for our sensitisation assay. Then, THP-1 cells were exposed to 13 reference chemicals with known sensitising potential, and real-time RT-PCR assays targeting the candidate marker genes were performed. Among them, six markers were able to properly evaluate the sensitisation potential by classifying the gene induction rates with appropriate criteria. Especially, the results of the assay using TREM1 and TNFRSF12A gene markers showed 100% sensitivity and specificity. An existing test method, h-CLAT, requires a flow cytometer and is complicated to operate. In contrast, our method is relatively simpler and more cost-effective. Therefore, our method is a promising one to evaluate sensitising chemicals.

Application of Spectro-DPRA, KeratinoSens™ and h-CLAT to estimation of the skin sensitization potential of cosmetics ingredients

Ethical issues in animal toxicity testing have led to the search for alternative methods to determine the skin sensitization potential of cosmetic products. The emergence of ethical testing issues has led to the development of many alternative methods that can reliably estimate skin sensitization potentials. However, a single alternative method may not be able to achieve high predictivity due to the complexity of the skin sensitization mechanism. Therefore, several prediction assays, including both in chemico and in vitro test methods, were investigated and integrated based on the skin sensitization adverse outcome pathway. In this study, we evaluated three different integrated approaches to predict a human skin sensitization hazard using data from in vitro assays (KeratinoSens™ and human cell line activation test [h-CLAT]), and a newly developed in chemico assay (spectrophotometric direct peptide reactivity assay [Spectro-DPRA]). When the results of the in chemico and in vitro assays were combined, the predictivity of human data increased compared with that of a single assay. The highest predictivity was obtained for the approach in which sensitization potential was determined by Spectro-DPRA followed by final determination using the result of KeratinoSens™ and h-CLAT assays (96.3% sensitivity, 87.1% specificity, 86.7% positive predictive value, 96.4% negative predictive value and 91.4% accuracy compared with human data). While further optimization is needed, we believe this integrated approach may provide useful predictive data when determining the human skin sensitization potential of chemicals.

Quantitative structure-activity and quantitative structure-property relationship approaches as alternative skin sensitization risk assessment methods

This study aimed to predict skin sensitization potency of selected chemicals by quantitatively analyzing their physicochemical properties by employing quantitative structure-activity relationship (QSAR) and quantitative structure-property relationship (QSPR) approaches as alternative risk assessment methods to animal testing. Correlations between effective concentration for a stimulation index of 3 (EC3) (%), the amount of a chemical required to elicit a threefold increase in lymph node cell proliferative activity (stimulation index, ≥3), were calculated using local lymph node assay (LLNA) and physicochemical properties of 212 skin sensitizers and 38 non-sensitizers were investigated. The correlation coefficients between melting point (MP) and EC3 and between surface tension (ST) and EC3 were 0.65 and 0.69, respectively. The correlation coefficient for MP + ST and EC3 was estimated to be 0.72. Thus, correlation coefficients between EC3 and MP, ST, and MP + ST reliably predicted the skin sensitization potential of the chemicals with sensitivities of 72% (126/175), 70% (122/174), and 73% (116/158); specificities of 77% (27/35), 69% (22/32), and 81% (26/32); and accuracies of 73% (153/210), 70% (144/206), and 75% (142/190), respectively. Our findings suggest that the EC3 value may be more accurately predicted using the ST values of chemicals as opposed to MP values. Thus, information on MP and ST parameters of chemicals might be useful for predicting the EC3 values as not only an alternative approach to animal testing, but as a risk assessment method for skin sensitization.

Alternative skin sensitization prediction and risk assessment using proinflammatory biomarkers, interleukin-1 beta (IL-1β) and inducible nitric oxide synthase (iNOS)

As an alternative to animal tests for skin sensitization potency and risk assessment, cell viability and biomarkers related to skin sensitization were analyzed in THP-1 human monocytic leukemia cells. Cell viabilities of 90% (CV90) and 75% (CV75) were determined for 24 selected test chemicals. Further biomarkers related to skin sensitization were also determined under equivalent comparative conditions. In cell viability analyses, potent skin sensitizers exhibited high cytotoxicity, but non-sensitizers did not display this tendency. In biomarker analyses, interleukin-I beta (IL-1β), inducible nitric oxide synthase (iNOS), IL-1β+iNOS, and THP-1 IL-1β+Raw 264.7 IL-1β were found to be suitable for prediction of skin sensitization potency following classification as either skin sensitizers or non-sensitizers (accuracies of 91.7%, 87.5%, 83.3%, and 82.6%, respectively). A significant positive correlation was found between biomarkers and skin sensitization potency, with a correlation coefficient (R) of 0.7 or more (correlation coefficients of 0.77, 0.72, 0.7, and 0.84, respectively). Finally, the skin sensitization potency effective threefold concentration (EC) 3% was predicted using a biomarker equation, with resulting prediction rates (match rate with actual data) of 58.3%, 54.2%, 62.5%, and 60.9%, respectively. The prediction accuracy for the EC3 value obtained from animal data was calculated as 83.3%, 79.2%, 79.2%, and 73.9%, respectively. Thus, these biomarkers, IL-1β and iNOS, may be alternatively used to predict skin sensitization potency and risk assessment.

Evaluation and improvement of QSAR predictions of skin sensitization for pesticides

In vivo skin sensitization assays have to be provided by applicants to the competent authorities in the European Union for the approval of active substances (AS) in pesticides. This study aimed to test the practicability of in silico predictions for AS by freely available (Q)SAR tools to evaluate their use as a time- and cost-effective alternative to animal testing in the context of the 3R concept. Predictions of skin sensitization for 48 selected sensitizing and non-sensitizing AS by the software programs CAESAR, Toxtree, OECD (Q)SAR Toolbox, CASE Ultra, Leadscope and SciQSAR were collected and compared. Different data evaluation methodologies (score definition, mean, weighted mean, threshold score definition) were applied to optimize the predictions. The calculation methods were internally cross-validated and further validated with an additional validation set of 80 AS. Although the presented calculation methodologies are not suitable as a stand-alone method, this study has shown weaknesses and strengths of some prominent (Q)SAR programs and diverse combinatorial options in the prediction of skin sensitization by pesticidal AS. The present study will help to foster discussions on in silico alternatives to animal testing in the pesticide area.

An evaluation of selected (Q)SARs/expert systems for predicting skin sensitisation potential

Predictive testing to characterise substances for their skin sensitisation potential has historically been based on animal models such as the Local Lymph Node Assay (LLNA) and the Guinea Pig Maximisation Test (GPMT). In recent years, EU regulations, have provided a strong incentive to develop non-animal alternatives, such as expert systems software. Here we selected three different types of expert systems: VEGA (statistical), Derek Nexus (knowledge-based) and TIMES-SS (hybrid), and evaluated their performance using two large sets of animal data: one set of 1249 substances from eChemportal and a second set of 515 substances from NICEATM. A model was considered successful at predicting skin sensitisation potential if it had at least the same balanced accuracy as the LLNA and the GPMT had in predicting the other outcomes, which ranged from 79% to 86%. We found that the highest balanced accuracy of any of the expert systems evaluated was 65% when making global predictions. For substances within the domain of TIMES-SS, however, balanced accuracies for the two datasets were found to be 79% and 82%. In those cases where a chemical was within the TIMES-SS domain, the TIMES-SS skin sensitisation hazard prediction had the same confidence as the result from LLNA or GPMT.

Predicting skin sensitization potential of organic compounds based on toxicity enhancement to Tetrahymena pyriformis, fathead minnow, and Daphnia magna

Skin sensitization is an important occupational health problem and immunotoxicity endpoint. Considering animal welfare and time and cost savings, many alternative approaches, such as those conducted in vitro, in silico, and in chemo, have been proposed and applied to predict skin sensitization of compounds. Toxicologically, sensitizers can elicit excess toxicity at greater levels than non-sensitizers due to their capacity to react with proteins/peptides. Based on this understanding, calculated toxicity enhancements (T_e) of 65 organic compounds from three in vitro bioassays, i.e. 48-hr ciliate (Tetrahymena pyriformis) growth inhibition, and both 96-hr fathead minnow and 48-hr Daphnia magna acute lethal toxicities, were employed to qualitatively and quantitatively predict skin sensitization potencies of the test agents. The sensitivity, specificity, and accuracy reaching 80% strongly suggested toxicity enhancement was an excellent parameter for predicting skin sensitization. Linear regressions of skin sensitization against toxicity enhancement were fitted for each bioassay, and they were improved after the sensitizers were categorized into different reaction mechanistic domains, which, in decreasing order of contribution from T_e to sensitization, were S_NAr > S_N1 > MA. These results indicated that toxicity bioassays are useful tools and that T_e could be a useful parameter that might be applied to predict skin sensitization.

Evaluation of in silico tools to predict the skin sensitization potential of chemicals

Public domain and commercial in silico tools were compared for their performance in predicting the skin sensitization potential of chemicals. The packages were either statistical based (Vega, CASE Ultra) or rule based (OECD Toolbox, Toxtree, Derek Nexus). In practice, several of these in silico tools are used in gap filling and read-across, but here their use was limited to make predictions based on presence/absence of structural features associated to sensitization. The top 400 ranking substances of the ATSDR 2011 Priority List of Hazardous Substances were selected as a starting point. Experimental information was identified for 160 chemically diverse substances (82 positive and 78 negative). The prediction for skin sensitization potential was compared with the experimental data. Rule-based tools perform slightly better, with accuracies ranging from 0.6 (OECD Toolbox) to 0.78 (Derek Nexus), compared with statistical tools that had accuracies ranging from 0.48 (Vega) to 0.73 (CASE Ultra - LLNA weak model). Combining models increased the performance, with positive and negative predictive values up to 80% and 84%, respectively. However, the number of substances that were predicted positive or negative for skin sensitization in both models was low. Adding more substances to the dataset will increase the confidence in the conclusions reached. The insights obtained in this evaluation are incorporated in a web database www.asopus.weebly.com that provides a potential end user context for the scope and performance of different in silico tools with respect to a common dataset of curated skin sensitization data.

Can the Direct Peptide Reactivity Assay Be Used for the Identification of Respiratory Sensitization Potential of Chemicals?

Prospective identification of low molecular weight respiratory sensitizers is difficult due to the current lack of adequate test methods. The direct peptide reactivity assay (DPRA) seems to be a promising method to determine the sensitization potential of chemicals because it determines the intrinsic characteristic of sensitizers to bind to proteins. It is already applied in the field of skin sensitization, and adaptation to respiratory sensitization has started recently. This article further evaluates the ability of the DPRA to predict the respiratory sensitization potential of chemicals. In addition, the added value of applying High Performance Liquid Chromatography (HPLC)-MS and measurements after 20 minutes and 24 hours of incubation was evaluated. Eighteen respiratory sensitizers (10 haptens, 3 prehaptens, and 5 prohaptens) and 14 nonsensitizers were tested with 2-model peptides. Based on peptide depletion, a prediction model was proposed for the identification of (respiratory) sensitizers. Application of mass spectrometry and measurements at 2 time-points increased prediction accuracy of the assay by resolving discordant results. The prediction model correctly identified all haptens and prehaptens as sensitizers. The 5 prohaptens were not identified as sensitizers, most likely due to lack of metabolic activity in the DPRA. All but 1 nonsensitizer was correctly predicted. The model, therefore, shows an accuracy of 78% for the tested dataset. Unfortunately, this assay cannot be used to distinguish respiratory from skin sensitizers. To make this distinction, the DPRA needs to be combined with other test methods that are able to identify respiratory sensitizers.

Evaluation of a High-Throughput Peptide Reactivity Format Assay for Assessment of the Skin Sensitization Potential of Chemicals

The direct peptide reactivity assay (DPRA) is a validated method for in vitro assessment of the skin sensitization potential of chemicals. In the present work, we describe a peptide reactivity assay using 96-well plate format and systematically identified the optimal assay conditions for accurate and reproducible classification of chemicals with known sensitizing capacity. The aim of the research is to ensure that the analytical component of the peptide reactivity assay is robust, accurate, and reproducible in accordance with criteria that are used for the validation of bioanalytical methods. Analytical performance was evaluated using quality control samples (QCs; heptapeptides at low, medium, and high concentrations) and incubation of control chemicals (chemicals with known sensitization capacity, weak, moderate, strong, extreme, and non-sensitizers) with each of three synthetic heptapeptides, viz Cor1-C420 (Ac-NKKCDLF), cysteine- (Ac-RFAACAA), and lysine- (Ac-RFAAKAA) containing heptapeptides. The optimal incubation temperature for all three heptapeptides was 25°C. Apparent heptapeptide depletion was affected by vial material composition. Incubation of test chemicals with Cor1-C420, showed that peptide depletion was unchanged in polypropylene vials over 3-days storage in an autosampler but this was not the case for borosilicate glass vials. For cysteine-containing heptapeptide, the concentration was not stable by day 3 post-incubation in borosilicate glass vials. Although the lysine-containing heptapeptide concentration was unchanged in both polypropylene and borosilicate glass vials, the apparent extent of lysine-containing heptapeptide depletion by ethyl acrylate, differed between polypropylene (24.7%) and glass (47.3%) vials. Additionally, the peptide-chemical complexes for Cor1-C420-cinnamaldehyde and cysteine-containing heptapeptide-2, 4-dinitrochlorobenzene were partially reversible during 3-days of autosampler storage. These observations further highlight the difficulty in adapting in vitro methods to high-throughput format for screening the skin sensitization potential of large numbers of chemicals whilst ensuring that the data produced are both accurate and reproducible.

Skin sensitizers differentially regulate signaling pathways in MUTZ-3 cells in relation to their individual potency

Background

Due to the recent European legislations posing a ban of animal tests for safety assessment within the cosmetic industry, development of in vitro alternatives for assessment of skin sensitization is highly prioritized. To date, proposed in vitro assays are mainly based on single biomarkers, which so far have not been able to classify and stratify chemicals into subgroups, related to risk or potency.

Methods

Recently, we presented the Genomic Allergen Rapid Detection (GARD) assay for assessment of chemical sensitizers. In this paper, we show how the genome wide readout of GARD can be expanded and used to identify differentially regulated pathways relating to individual chemical sensitizers. In this study, we investigated the mechanisms of action of a range of skin sensitizers through pathway identification, pathway classification and transcription factor analysis and related this to the reactive mechanisms and potency of the sensitizing agents.

Results

By transcriptional profiling of chemically stimulated MUTZ-3 cells, 33 canonical pathways intimately involved in sensitization to chemical substances were identified. The results showed that metabolic processes, cell cycling and oxidative stress responses are the key events activated during skin sensitization, and that these functions are engaged differently depending on the reactivity mechanisms of the sensitizing agent. Furthermore, the results indicate that the chemical reactivity groups seem to gradually engage more pathways and more molecules in each pathway with increasing sensitizing potency of the chemical used for stimulation. Also, a switch in gene regulation from up to down regulation, with increasing potency, was seen both in genes involved in metabolic functions and cell cycling. These observed pathway patterns were clearly reflected in the regulatory elements identified to drive these processes, where 33 regulatory elements have been proposed for further analysis.

Conclusions

This study demonstrates that functional analysis of biomarkers identified from our genomics study of human MUTZ-3 cells can be used to assess sensitizing potency of chemicals in vitro, by the identification of key cellular events, such as metabolic and cell cycling pathways.

A dataset on 145 chemicals tested in alternative assays for skin sensitization undergoing prevalidation

Skin sensitization is a key endpoint for cosmetic ingredients, with a forthcoming ban for animal testing in Europe. Four alternative tests have so far been submitted to ECVAM prevalidation: (i) MUSST and (ii) h-Clat assess surface markers on dendritic cell lines, (iii) the direct peptide reactivity assay (DPRA) measures reactivity with model peptides and (iv) the KeratinoSens(TM) assay which is based on detection of Nrf2-induced luciferase. It is anticipated that only an integrated testing strategy (ITS) based on a battery of tests might give a full replacement providing also a sensitization potency assessment, but this concept should be tested with a data-driven analysis. Here we report a database on 145 chemicals reporting the quantitative endpoints measured in a U937- test, the DPRA and KeratinoSens(TM) . It can serve to develop data-driven ITS approaches as we show in a parallel paper and provides a view as to the current ability to predict with in vitro tests as we are entering 2013. It may also serve as reference database when benchmarking new molecules with in vitro based read-across and find use as a reference database when evaluating new tests. The tests and combinations thereof were evaluated for predictivity, and overall a similar predictivity was found as before on three-fold smaller datasets. Analysis of the dose-response parameters of the individual tests indicates a correlation to sensitization potency. Detailed analysis of chemicals false-negative and false-positive in two tests helped to define limitations in the tests but also in the database derived from animal studies.

The definition of the applicability domain relevant to skin sensitization for the aromatic nucleophilic substitution mechanism

This study outlines how a glutathione reactivity assay (so-called in chemico data) can be used to define the applicability domain for the nucleophilic aromatic substitution (S(N)Ar) reaction for benzenes. This reaction is one of the six mechanistic domains that have been shown to be important in toxicological endpoints in which the ability to bind covalently to a protein is a key molecular initiating event. This study has analysed the experimental data, allowing a clear and interpretable structure-activity relationship to be developed for the S(N)Ar domain. The applicability domain has resulted in a series of structural alerts. The definition of the applicability domain for the S(N)Ar reaction and the resulting structural alerts are likely to be beneficial in the development of computational tools for category formation and read-across. The study concludes with how this information can be used in the development of adverse outcome pathways.

Effects of sub-acute exposure to rhodium (as Rh (III) chloride hydrate) on cytokines in female Wistar rats

Quantitative changes in different cytokines were determined in serum of female Wistar rats exposed to Rhodium (III) chloride hydrate to evaluate its early effects on the immune system. Findings revealed an inhibitory effect of Rh salt since each cytokine, with the exceptions of IL-1α and IL-2 levels observed at the highest doses of exposure, was reduced compared to the controls and interestingly, the lowest doses induced the greatest inhibition. This generalized decrease of cytokine levels was not related to a specific cytokine pathway, and may suggest an anti-inflammatory role of Rh salt.

Development and validation of a new in vitro assay designed to measure contact allergen-triggered oxidative stress in dendritic cells

BACKGROUND

Selected contact allergens are known to induce phenotypic and functional maturation of dendritic cells (DCs). Such changes occurring in DCs have been employed as assay readouts to predict skin-sensitizing potentials of small chemicals.

OBJECTIVE

To respond to the urgent needs for reliable in vitro tests to identify contact allergens, we sought to develop a DC-based assay designed to detect early change(s) induced by sensitizers.

METHODS

Signature gene expression profiles of skin sensitization were determined by GeneChip and quantitative RT-PCR analyses of RNA samples harvested from mouse skin and XS106 DC line after exposure to dinitrofluorobenzene (DNFB). Production of reactive oxygen species (ROS) was examined indirectly by measuring the level of oxidative stress-XS106 DCs were labeled with a fluorescent dye, CM-H(2)DCFDA, exposed to test chemicals, and then examined for fluorescence signals by flow cytometer.

RESULTS

DNFB induced abundant mRNA expression of several redox regulatory genes in both mouse skin and XS106DCs. Expression of these genes was inducible by hydrogen peroxide and blocked by a ROS inhibitor, diphenyleneiodonium. Rapid and significant ROS production was induced by 25 of the 28 tested skin sensitizers, but only by 3 of the 21 tested skin irritants.

CONCLUSIONS

Our small-scale validation study demonstrates the practical utility of our DC-based ROS production assay to detect structurally diverse contact allergens with varying sensitizing potencies. It is tempting to speculate that ROS production in DCs may represent an early event during the sensitization phase.

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.

Assessing toxicological data quality: basic principles, existing schemes and current limitations

Existing toxicological data may be used for a variety of purposes such as hazard and risk assessment or toxicity prediction. The potential use of such data is, in part, dependent upon their quality. Consideration of data quality is of key importance with respect to the application of chemicals legislation such as REACH. Whether data are being used to make regulatory decisions or build computational models, the quality of the output is reflected by the quality of the data employed. Therefore, the need to assess data quality is an important requirement for making a decision or prediction with an appropriate level of confidence. This study considers the biological and chemical factors that may impact upon toxicological data quality and discusses the assessment of data quality. Four general quality criteria are introduced and existing data quality assessment schemes are discussed. Two case study datasets of skin sensitization data are assessed for quality providing a comparison of existing assessment methods. This study also discusses the limitations and difficulties encountered during quality assessment, including the use of differing quality schemes and the global versus chemical-specific assessments of quality. Finally, a number of recommendations are made to aid future data quality assessments.

Inter-relationships between different classes of chemical allergens

Although allergic sensitization of the respiratory tract induced by chemicals is not as common as skin sensitization, it is nevertheless an important occupational health issue. Respiratory allergy to chemicals, characterized typically by rhinitis and asthma, is associated with considerable morbidity and with related socioeconomic costs. Several experimental approaches have been proposed for the prospective identification of chemical respiratory allergens, but none of these has yet been validated formally. In the absence of a widely accepted method for respiratory allergen identification, it is appropriate and relevant to explore their relationship with skin-sensitizing chemicals. A series of chemicals known to cause immune-mediated respiratory allergy in humans has been examined. The majority of the respiratory allergens tested were found to elicit positive responses in one or more standard tests used for the identification of skin-sensitizing potential (guinea pig maximization test, the Buehler test and/or the local lymph node assay). We suggest that this observation might form the basis of a potentially useful paradigm for initial characterization of the respiratory-sensitizing potential of chemicals. Specifically, chemicals that fail to elicit positive responses in accepted skin-sensitization test methods might also be regarded as lacking the inherent potential to cause allergic sensitization of the respiratory tract.

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.

A review of the electrophilic reaction chemistry involved in covalent DNA binding

The need to assess the ability of a chemical to act as a mutagen or a genotoxic carcinogen (collectively termed genotoxicity) is one of the primary requirements in regulatory toxicology. Several pieces of legislation have led to an increased interest in the use of in silico methods, specifically the formation of chemical categories for the assessment of toxicological endpoints. A key step in the development of chemical categories for genotoxicity is defining the organic chemistry associated with the formation of a covalent bond between DNA and an exogenous chemical. This organic chemistry is typically defined as structural alerts. To this end, this article has reviewed the literature defining the structural alerts associated with covalent DNA binding. Importantly, this review article also details the mechanistic organic chemistry associated with each of the structural alerts. This information is extremely important in terms of meeting regulatory requirements for the acceptance of the chemical category approach. The structural alerts and associated mechanistic chemistry have been incorporated into the Organisation for Economic Co-operation and Development (OECD) (Q)SAR Application Toolbox.

Chemical allergy: translating biology into hazard characterization

The induction by chemicals of allergic sensitization and allergic disease is an important and challenging branch of toxicology. Skin sensitization resulting in allergic contact dermatitis represents the most common manifestation of immunotoxicity in humans, and many hundreds of chemicals have been implicated as skin sensitizers. There are far fewer chemicals that have been shown to cause sensitization of the respiratory tract and asthma, but the issue is no less important because hazard identification remains a significant challenge, and occupational asthma can be fatal. In all areas of chemical allergy, there have been, and remain still, intriguing challenges where progress has required a close and productive alignment between immunology, toxicology, and clinical medicine. What the authors have sought to do here is to exemplify, within the framework of chemical allergy, how an investment in fundamental research and an improved understanding of relevant biological and biochemical mechanisms can pay important dividends in driving new innovations in hazard identification, hazard characterization, and risk assessment. Here we will consider in turn three specific areas of research in chemical allergy: (1) the role of epidermal Langerhans cells in the development of skin sensitization, (2) T lymphocytes and skin sensitization, and (3) sensitization of the respiratory tract. In each area, the aim is to identify what has been achieved and how that progress has impacted on the development of new approaches to toxicological evaluation. Success has been patchy, and there is still much to be achieved, but the journey has been fascinating and there have been some very important developments. The conclusion drawn is that continued investment in research, if coupled with an appetite for translating the fruits of that research into imaginative new tools for toxicology, should continue to better equip us for tackling the important challenges that remain to be addressed.

A further characteristic of susceptibility to contact allergy: sensitization to a weak contact allergen is associated with polysensitization. Results of the IVDK

Although genetic factors probably account for differences in susceptibility to contact allergy, they have not yet been identified, partly due to an insufficient understanding of 'susceptibility'. Regarding polysensitization (PS) as a sign of increased susceptibility, we studied the relationship between PS and sensitization to weak versus strong allergens. Patch test data from 66,835 patients registered by the multicentre project, Information Network of Departments of Dermatology (IVDK) between 1 January 1997 and 31 December 2004, were analysed. The association between the number of sensitization to standard series allergens, and contact allergy to a strong allergen methyldibromoglutaronitrile (MDBGN) and to a weak allergen (paraben mix), was analysed with adjusted logistic regression analysis. In paraben-positive (++/+++) patients, the risk of > or = 2, > or = 3 or > or = 4 additional reactions were significantly increased by a factor of 2.1-4.6 compared to MDBGN-sensitized (++/+++) patients. Varying the basic model, a higher risk of additional positive reactions associated with paraben sensitization was consistently identified. The association between PS and sensitization to weak allergens adds a further characteristic of susceptibility to former findings of the IVDK, where PS was related to an increased risk of induction, elicitation, and cytokine polymorphisms. PS can be regarded as a phenotype to be considered in genetic studies.

The local lymph node assay and the assessment of relative potency: status of validation

For the prediction of skin sensitization potential, the local lymph node assay (LLNA) is a fully validated alternative to guinea-pig tests. More recently, information from LLNA dose-response analyses has been used to assess the relative potency of skin sensitizing chemicals. These data are then deployed for risk assessment and risk management. In this commentary, the utility and validity of these relative potency measurements are reviewed. It is concluded that the LLNA does provide a valuable assessment of relative sensitizing potency in the form of the estimated concentration of a chemical required to produce a threefold stimulation of draining lymph node cell proliferation compared with concurrent controls (EC3 value) and that all reasonable validation requirements have been addressed successfully. EC3 measurements are reproducible in both intra- and interlaboratory evaluations and are stable over time. It has been shown also, by several independent groups, that EC3 values correlate closely with data on relative human skin sensitization potency. Consequently, the recommendation made here is that LLNA EC3 measurements should now be regarded as a validated method for the determination of the relative potency of skin sensitizing chemicals, a conclusion that has already been reached by a number of independent expert groups.

The skin sensitization potential of resorcinol: experience with the local lymph node assay

Resorcinol is a simple aromatic chemical (1,3-benzenediol) that has found widespread use, particularly as a coupler in hair dyes. Clinical experience clearly shows that resorcinol is a (albeit uncommon) skin sensitizer. By contrast, predictive methods, both animal and human, have previously failed to identify resorcinol as such. Here, we describe the outcome of a recent local lymph node assay performed in accordance with Organisation for Economic Co-operation and Development guideline 429, which correctly identified resorcinol as a skin sensitizer. Clear evidence of a dose response was apparent, and an EC3 value of approximately 6% was calculated. This suggests that the skin-sensitizing potency of resorcinol is approximately 2 orders of magnitude lower than that of p-phenylenediamine but similar to that of hexyl cinnamic aldehyde. These data show the importance of adherence to test guidelines and aligns the clinical experience with resorcinol with that obtained in predictive animal methods.

Platinum Group Elements Enhance the Allergic Immune Response by Acting on Dendritic Cells

BACKGROUND

Atmospheric pollution may play a role in the immune response to allergens either directly or by entering the food chain. While particulate platinum group elements (PLGE) emitted by catalytic converters can be considered biologically inert, approximately 10% of these species accumulate in the environment as bioavailable soluble forms.

METHODS

We challenged in vitro human immature and mature monocyte-derived dendritic cells with subtoxic concentrations of soluble species of PLGE. Dendritic cells were studied both at baseline and following treatment with Na(2)PtCl(6), Na(2)PdCl(6) or Na(3)RhCl(6). (NH(4))(6)Mo(7)O(24) was included as control. The following end-points were considered: expression of differentiation markers, effectiveness of allergen presentation and Th2 cytokine production by cocultured T lymphocytes, expression of IgE-type I receptor and efficiency of IgE-dependent endocytosis.

RESULTS

We found that treatment with PLGE (but not with the control metal) increased costimulatory molecule expression and antigen presentation, amplified IL-5 production by cocultured T lymphocytes, upregulated IgE-type I receptor membrane expression, and augmented IgE-type I receptor-mediated endocytosis.

CONCLUSIONS

We conclude that PLGE have an adjuvant-like effect on dendritic cells that can favor and amplify the immune response to allergens.

Information derived from sensitization test methods: test sensitivity, false positives and false negatives

Predictive toxicology tests for the prospective identification of skin-sensitizing chemicals are well known and have been used for many years. However, of these, only the local lymph node assay (LLNA) has actually undergone formal independent assessment to determine the accuracy of the predictions, particularly with respect to the likelihood of false positives and false negatives. Often, efforts to increase the sensitivity of a test (reducing false negatives) tend to increase the number of false positives. In this short review, these issues are discussed in particular relation to the 3 predictive tests available in regulatory toxicology, the guinea-pig maximization test, the occluded patch test of Buehler and the LLNA. A key perspective is that no predictive test is without limitations; having a good appreciation of these limitations is necessary for making the best use of the information derived from these methods.

Elicitation response characteristics to permanent hair dye in paraphenylenediamine-allergic volunteers

BACKGROUND

Elicitation response characteristics to complete permanent hair dye products in paraphenylenediamine (PPD)-allergic volunteers have not previously been explored in detail.

OBJECTIVES

To assess the elicitation response characteristics observed in PPD-allergic volunteers upon patch testing with complete hair dyes.

METHODS

PPD-allergic volunteers were assigned to 1 of 3 groups depending upon whether they elicited + (group 1), ++ (group 2) or +++ (group 3) reactions following the standard diagnostic procedure. Each group was subsequently patch tested with 2 complete hair dyes (A and B) for 30 min, 1 hr and 24 hr. Patch sites were examined 1 day, 2 days and 3 days after patch removal.

RESULTS

Exposure to either hair dye for 30 min or 1 hr was insufficient to yield positive patch test reactions in all of the PPD-allergic patients in groups 1 or 2. Application of either hair dye for 24 hr was sufficient to yield positive reactions in all of the individuals within groups 2 and 3.

CONCLUSIONS

The frequency of positive patch test reactions observed following 24-hr exposure to complete permanent hair dyes is comparable to that observed following 48-hr exposure to 1% PPD/petrolatum in those individuals whose degree of sensitization is such that they typically present ++ or +++ reactions diagnostically.

The local lymph node assay and skin sensitization: a cut-down screen to reduce animal requirements?

The local lymph node assay (LLNA), an alternative approach to skin-sensitizing testing, has made a significant contribution to animal welfare by permitting a reduction and refinement of animal use. Although there is clearly an aspiration to eliminate the use of animals in such tests, it is appropriate also to consider other opportunities for refinement and reduction of animal use. We have therefore explored the use of a modified version of the LLNA for screening purposes when there is a need to evaluate the sensitizing activity of a large number of chemicals, as will be the case under the auspices of registration, evaluation and authorization of chemicals (REACH). Using an existing LLNA database of 211 chemicals, we have examined whether a cut-down assay comprising a single high-dose group and a concurrent vehicle control would provide a realistic approach for screening chemicals for sensitizing potential. The analyses reported here suggest this is the case. We speculate that the animal welfare benefits may be enhanced further by reducing the number of animals per experimental group. However, a detailed evaluation will be necessary to provide reassurance that a reduction in group size would provide adequate sensitivity across a range of skin sensitization potencies.