Comparison of the local lymph node assay with the guinea-pig maximization test for the detection of a range of contact allergens

Skin sensitization, false positives and false negatives: experience with guinea pig assays

The advent of the local lymph node assay (LLNA), and efforts to develop in vitro alternatives for the identification of skin sensitizing chemicals has focused attention on the issue of false positive and false negative results. In essence, the question becomes 'what is the gold standard?' In this context, attention has focused primarily on the LLNA as this is now the preferred assay for skin sensitization testing. However, for many years prior to introduction of the LLNA, the guinea pig maximization test and the occluded patch test of Buehler were the methods of choice. In order to encourage a more informed dialogue about the relative performance, accuracy and applicability of the LLNA and guinea pig tests, we have here considered the extent to which guinea pig methods were themselves subject to false positives and negative results. We describe and discuss here well-characterized examples of instances where both false negatives (including abietic acid and eugenol) or false positives (including vanillin and sulfanilic acid) have been recorded in guinea pig tests. These and other examples are discussed with particular reference to the fabrication of a gold standard dataset that is required for the validation of in vitro alternatives.

Evaluation of Allergenicity of Constituents of Myoga Using the Murine Local Lymph Node Assay

Myoga ( Zingiber Myoga Roscoe) is a perennial plant with a pungent smell from its flower buds. It is native to East Asia and has been reported to cause allergic contact dermatitis. The purpose of this study is to assess the allergenicity of myoga related to its major chemical components, α-pinene, β-pinene, limonene, limonene oxide and β-phellandrene, which are supposed to be the causative agents of contact dermatitis among myoga cultivators. We performed a toxicity study of the volatile constituents of myoga using the local lymph node assay (LLNA), in which limonene, limonene oxide and β-phellandrene had positive responses and the EC3 was 35.8%, 8.22%, and 0.54%, respectively. EC3 for both α-pinene and β-pinene was over 100%. Both chemicals failed to induce positive responses in the LLNA. While the maximization rating of limonene, limonene oxide and phellandrene were evaluated as moderate, extreme, and extreme respectively, α-pinene and β-pinene were evaluated as weak in the previously reported GPMT. The usage of LLNA was also confirmed by comparing with previously reported GPMT results to detect the allergenicity of myoga constituents. The actual risk of humans developing an allergy to myoga constituents depends on many factors. The concentration of the compounds, the frequency and duration of exposure and the condition of the skin are supposed to be important factors.

In conclusion, limonene, limonene oxide and β-phellandrene are constituents of myoga which revealed their allergenicity by the LLNA. The usefulness of the LLNA to detect the allergenicity of myoga constituents was also confirmed when considering the differences between the LLNA in the present study and the previously reported GPMT. The actual risk of humans developing an allergy to myoga constituents depends on many factors. The concentration of the compounds, the frequency and duration of exposure and condition of the skin are all important factors.

The local lymph node assay compared with the human maximization test as an indicator of allergic potency in humans using patch test clinic populations

The human maximization test (HMT) is a method to evaluate potency in humans, while the local lymph node assay (LLNA) is a test method that allows for the measuring of the allergic potency of a substance in a rodent. It has been proposed that an EC3 value (the value obtained by the LLNA test, ie, the concentration of an allergen leading to a 3-fold increase of baseline proliferation rate) would be a reliable indicator for a compound's allergic potency in humans. This paper compares the correlation between the EC3 value of a compound and its allergic occurrence in the general population with the correlation between the HMT of the compound and its allergic occurrence in the general population, to determine the relationship to potency. The correlation values when outliers were removed from the sample were -0.56 and -0.71 for LLNA and HMT, respectively, suggesting that there is a possible 20% error margin in LLNA's ability to predict potency. The data also suggest that other factors (such as exposure) could play up to a 30% role in the determination of allergic occurrence in the general population. The potency assays might be made more clinically relevant for predicting allergic frequencies by including a frequency factor and other factors in its dermatotoxicological interpretation.

Nothing is perfect, not even the local lymph node assay: a commentary and the implications for REACH

For many regulatory authorities, the local lymph node assay (LLNA) is the preferred assay for the predictive identification of skin-sensitizing chemicals. It is the initial requirement for sensitization testing within the new REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances) regulations in the European Union. The primary reasons for the preferment of the LLNA are the animal welfare benefits it provides compared with traditional guinea-pig methods (refinement and reduction of animal usage) and the general performance characteristics of the assay with regard to overall reliability, accuracy, and interpretation. Moreover, a substantial published literature on the LLNA is available making it appropriate for use as a benchmark against which new approaches, including in vitro alternatives, can be evaluated and validated. There is, therefore, a view that the LLNA represents the 'gold standard' for skin sensitization testing. However, although this is probably correct, it is important to recognize and acknowledge that in common with all other predictive tests (whether they be validated or not), the LLNA has limitations, in addition to strengths, some of which were mentioned above. Arguably, it is the limitations (e.g., the occurrence of false positive and false negative results) of test methods that are most important to understand. With respect to the LLNA, these limitations are similar to those associated with guinea-pig skin sensitization methods. Among these are the occurrence of false positive and false negative results, susceptibility of results to changes in vehicle, and the possibility that interspecies differences may confound interpretation. In this commentary, these issues are reviewed and their impact on the utility of the LLNA for identification, classification, and potency assessment of skin sensitizers are considered. In addition, their relevance for the future development and validation of novel in vitro and in silico alternatives is explored.

Dose-Response Relationships and Threshold Levels in Skin and Respiratory Allergy

A literature study was performed to evaluate dose-response relationships and no-effect levels for sensitization and elicitation in skin- and respiratory allergy. With respect to the skin, dose-response relationships and no-effect levels were found for both intradermal and topical induction, as well as for intradermal and topical elicitation of allergenic responses in epidemiological, clinical, and animal studies. Skin damage or irritation may result in a significant reduction of the no-effect level for a specific compound. With respect to the respiratory tract, dose-response relationships and no-effect levels for induction were found in several human as well as animal studies. Although dose-response relationships for elicitation were found in some epidemiological studies, concentration-response relationships were present only in a limited number of animal studies. Reported results suggest that especially relatively high peak concentrations can induce sensitization, and that prevention of such concentrations will prevent workers from developing respiratory allergy. Moreover, induction of skin sensitization may result in subsequent heightened respiratory responsiveness following inhalation exposure. The threshold concentration for the elicitation of allergic airway reactions in sensitized subjects is generally lower than the threshold to induce sensitization. Therefore, it is important to consider the low threshold levels for elicitation for recommendation of health-based occupational exposure limits, and to avoid high peak concentrations. Notwithstanding the observation of dose-response relationships and no-effect levels, due to a number of uncertainties, no definite conclusions can be drawn about absolute threshold values for allergens with respect to sensitization of and elicitation reactions in the skin and respiratory tract. Most predictive tests are generally meant to detect the potential of a chemical to induce skin and/or respiratory allergy at relatively high doses. Consequently, these tests do not provide information of dose-response relationships at lower doses such as found in, for example, occupational situations. In addition, the observed dose-response relationships and threshold values have been obtained by a wide variety of test methods using different techniques, such as intradermal exposure versus topical or inhalation exposure at the workplace, or using different endpoints, which all appear important for the outcome of the test. Therefore, especially with regard to respiratory allergy, standardized and validated dose-response test methods are urgently required in order to be able to recommend safe exposure levels for allergens at the workplace.

Gene expression changes induced by type IV allergy-inducible chemicals in dendritic cells

In the present study, the changes of gene expression profile in dendritic cell (DC)-derived DC2.4 cells sensitized with two allergenic chemicals were analyzed by microarray analysis to develop a basis for an in vitro assessment system of type IV allergenic chemicals. Consequently, 26 genes were significantly up-regulated, and 53 were down-regulated in both groups. Interestingly, some of up-regulated genes were associated with the maturation process of DCs. A set of genes was further evaluated by real-time reverse transcription-polymerase chain reaction to identify the gene expression changes specifically induced by type IV allergy-inducible chemicals in DC2.4 cells, and 2 possible candidates, syndecan-1 (Sdc1) and smoothened (SMO) genes were identified. Thus, up-regulation of Sdc1 gene and down-regulation of SMO gene in DC2.4 cells may be diagnostic markers for the screening of type IV-allergenic chemicals.

Interlaboratory validation of the modified murine local lymph node assay based on adenosine triphosphate measurement

INTRODUCTION

The murine local lymph node assay (LLNA) is a well-established alternative to the guinea pig maximization test (GPMT) or Buehler test (BT) for the assessment of the skin sensitizing ability of drugs and chemicals. Daicel Chemical Industries Ltd. has developed a modified LLNA based on the adenosine triphosphate (ATP) content (LLNA-DA). We conducted 2 interlaboratory validation studies to evaluate the reliability and relevance of LLNA-DA.

METHODS

The experiment involved 17 laboratories, wherein 14 chemicals were examined under blinded conditions. In the first study, 3 chemicals were examined in 10 laboratories and the remaining 9 were examined in 3 laboratories. In the second study, 1 chemical was examined in 7 laboratories and the remaining 4 chemicals were examined in 4 laboratories. The data were expressed as the ATP content for each chemical-treated group, and the stimulation index (SI) for each chemical-treated group was determined as the increase in the ATP content relative to the concurrent vehicle control group. An SI of 3 was set as the cut-off value for exhibiting skin sensitization activity.

RESULTS

The results of the first study obtained in the experiments conducted for the 3 chemicals that were examined in all the 10 laboratories and for 5 of the remaining 9 chemicals were sufficiently consistent with small variations in their SI values. The sensitivity, specificity, and accuracy of LLNA-DA against those of GPMT/BT were 7/8 (87.5%), 3/3 (100%), and 10/11 (90.9%), respectively. In the second study, all the 5 chemicals studied demonstrated acceptably small interlaboratory variations.

DISCUSSION

In the first study, a large variation was observed for 2 chemicals; in the second study, this variation was small. It was attributed to the application of dimethylsulfoxide as the solvent for the metallic salts. In conclusion, these 2 studies provide good evidence for the reliability of the LLNA-DA.

Characterization and evaluation of a modified local lymph node assay using ATP content as a non-radio isotopic endpoint

INTRODUCTION

The murine local lymph node assay (LLNA) is an accepted and widely used method for assessing the skin-sensitizing potential of chemicals. Here, we describe a non-radio isotopic modified LLNA in which adenosine triphosphate (ATP) content is used as an endpoint instead of radioisotope (RI); the method is termed LLNA modified by Daicel based on ATP content (LLNA-DA).

METHODS

Groups of female CBA/JNCrlj mice were treated topically on the dorsum of both ears with test chemicals or a vehicle control on days 1, 2, and 3; an additional fourth application was conducted on day 7. Pretreatment with 1% sodium lauryl sulfate solution was performed 1 h before each application. On day 8, the amount of ATP in the draining auricular lymph nodes was measured as an alternative endpoint by the luciferin-luciferase assay in terms of bioluminescence (relative light units, RLU). A stimulation index (SI) relative to the concurrent vehicle control was derived based on the RLU value, and an SI of 3 was set as the cut-off value.

RESULTS

Using the LLNA-DA method, 31 chemicals were tested and the results were compared with those of other test methods. The accuracy of LLNA-DA vs LLNA, guinea pig tests, and human tests was 93% (28/30), 80% (20/25), and 79% (15/19), respectively. The estimated concentration (EC) 3 value was calculated and compared with that of the original LLNA. It was found that the EC3 values obtained by LLNA-DA were almost equal to those obtained by the original LLNA.

DISCUSSION

The SI value based on ATP content is similar to that of the original LLNA as a result of the modifications in the chemical treatment procedure, which contribute to improving the SI value. It is concluded that LLNA-DA is a promising non-RI alternative method for evaluating the skin-sensitizing potential of chemicals.

The contact allergen dinitrochlorobenzene (DNCB) and respiratory allergy in the Th2-prone Brown Norway rat

All LMW respiratory allergens known to date can also induce skin allergy in test animals. The question here was if in turn skin allergens can induce allergy in the respiratory tract. Respiratory allergy was tested in Th2-prone Brown Norway (BN) rats by dermal sensitization with the contact allergen dinitrochlorobenzene (DNCB; 1%, day 0; 0.5%, day 7) and a head/nose-only inhalation challenge of 27mg/m3 of DNCB (15 min, day 21), using a protocol that successfully identified chemical respiratory allergens. Skin allergy to DNCB was examined in BN rats and Th1-prone Wistar rats in a local lymph node assay followed by a topical patch challenge of 0.1% DNCB. Sensitization of BN rats via the skin induced DNCB-specific IgG in serum, but not in all animals, and an increased number of CD4+ cells in the lung parenchyma. Subsequent inhalation challenge with DNCB did not provoke apneas or allergic inflammation (signs of respiratory allergy) in the BN rats. However, microarray analysis of mRNA isolated from the lung revealed upregulation of the genes for Ccl2 (MCP-1), Ccl4 (MIP-1beta), Ccl7 and Ccl17. Skin challenge induced considerably less skin irritation and allergic dermatitis in the BN rat than in the Wistar rat. In conclusion, the Th2-prone BN rat appeared less sensitive to DNCB than the Wistar rat; nevertheless, DNCB induced allergic inflammation in the skin of BN rats but even a relatively high challenge concentration did not induce allergy in the respiratory tract, although genes associated with allergy were upregulated in lung tissue.

Human skin in organ culture and human skin cells (keratinocytes and fibroblasts) in monolayer culture for assessment of chemically induced skin damage

Human skin cells (epidermal keratinocytes and dermal fibroblasts) in monolayer culture and human skin in organ culture were exposed to agents that are known to produce irritation (redness, dryness, edema and scaly crusts) when applied topically to skin. Among the agents used were three well accepted contact irritants (i.e., all-trans retinoic acid [RA], sodium lauryl sulfate [SLS] and benzalkonium chloride) as well as the corrosive organic mercury compound, aminophenyl mercuric acetate (APMA), and 5 contact sensitizers (oxazolone, nickel sulfate, eugenol, isoeugenol and ethylene glycol dimethacrylate [EGDM]). As a group, the contact irritants (including the corrosive mercuric compound) were cytotoxic for keratinocytes and fibroblasts and suppressed growth at lower concentrations than the contact sensitizers. The contact irritants also produced histological changes (hyperplasia, incomplete keratinization, loss of the granular layer, acantholysis and necrosis) in organ-cultured skin at dose levels at which the contact sensitizers appeared to be inert. Finally, the profile of secreted molecules from organ-cultured skin was different in the presence of contact irritants versus contact sensitizers. Taken together, these data suggest that the use of organ-cultured skin in conjunction with cells derived from the skin in monolayer culture may provide an initial approach to screening agents for deleterious changes in skin.

Fragrance material review on methyl salicylate

A toxicologic and dermatologic review of methyl salicylate when used as a fragrance ingredient is presented.

Assessment of sensitization potential of monoterpenes using the rat popliteal lymph node assay

The popliteal lymph node assay (PLNA) has been proposed as a screening test for detecting chemicals with potential of inducing allergic and auto-immune-like reactions in humans. In the present study, we used the rat PLNA to evaluate the immuno-sensitizing potential of 10 monoterpenes found in the essential oils of a variety of aromatic, edible and medicinal plants. The primary or direct PLNA was performed with the monoterpenes, and chlorpromazine (CPZ) and barbital were used as positive and negative controls, respectively. Female, 7-8 week-old Wistar rats were injected subcutaneously (50 microL) with the test substance (0.5, 2.5 or 5mg) into the right hind footpad while the contralateral footpad was injected with the vehicle (DMSO) alone. Weight (WI) and cellularity (CI) indices for draining PLNs were determined 7 days after treatment. PLNA was positive (WI >or= 2 and CI >or= 5) for CPZ, citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene, and negative for barbital, DMSO, (-)-menthol, 1,8-cineole, (+/-) citronellal, (+)-limonene, (+/-) camphor and terpineol. A secondary PLNA, a T-cell priming test, was carried out with the four substances that had been positive in the primary assay. Six weeks after being locally primed with 5 mg/paw, rats were sc injected into the same footpad with a dose (0.5 mg/paw) of the substance that had been previously found to be insufficient to cause a positive response. WI and CI were then calculated 4 and 7 days after the second injection. CPZ was also positive in the secondary assay thereby confirming that it is a sensitizing agent. Citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene, however, were negative in the secondary assay. In summary, citral, alpha-terpinene, beta-myrcene and (-)-alpha-pinene induced a clear immuno-stimulatory response due to their irritant properties but no monoterpene proved to be a sensitizing agent in the PLNA.

Quantitative structure activity relationship model for predicting the depletion percentage of skin allergic chemical substances of glutathione

A quantitative model was developed to predict the depletion percentage of glutathione (DPG) compounds by gene expression programming (GEP). Each kind of compound was represented by several calculated structural descriptors involving constitutional, topological, geometrical, electrostatic and quantum-chemical features of compounds. The GEP method produced a nonlinear and five-descriptor quantitative model with a mean error and a correlation coefficient of 10.52 and 0.94 for the training set, 22.80 and 0.85 for the test set, respectively. It is shown that the GEP predicted results are in good agreement with experimental ones, better than those of the heuristic method.

Identification and classification of skin sensitizers: identifying false positives and false negatives

The first step in regulatory evaluation of substances involves the identification of their intrinsic hazards, including the potential for skin sensitization. This is, quite properly, entirely different from assessment of the risks to human health, which might arise from incorporation of substances in products. EU guidance on regulations concerning the classification of skin sensitizers suggests a range of sources of information be deployed in the hazard identification process. These include chemical structure, predictive animal tests, and various types of human data. Where the information is clear-cut, then uncertainties rarely arise. However, for some materials, discordant information arises, perhaps because the substance is on the borderline of test sensitivity and classification (sensitizing materials of insufficient potency do not classified according to the EU scheme), due to conflicting results in predictive tests or for other reasons. In this study, we review data on a number of substances where a classification decision is complicated by such discordances and seek to use these examples to demonstrate how best to make a weight of evidence decision on whether a substance should, or should not, be classified as a skin sensitizer.

Predictive identification of human skin sensitization thresholds

For years, methods have been available for the predictive identification of chemicals that possess the intrinsic potential to cause skin sensitization. However, many have proven less suitable for the determination of relative sensitizing potency. In this respect, the local lymph node assay (LLNA) has been shown to have a number of important advantages. Through interpolation of LLNA dose-response data, the concentration of a chemical required to produce a threshold positive response (a 3-fold increase in activity compared with concurrent vehicle controls, the EC3 value) can be measured. The robustness of this parameter has been demonstrated rigorously in terms of inter- and intralaboratory reproducibility. Additionally, the relationship between potency estimates from the LLNA and an appreciation of human potency based on clinical experience has been reported previously. In the present investigations, we have sought to consolidate further our understanding of the association between EC3 values and human skin-sensitization potency by undertaking a thorough and extensive analysis of existing human predictive assays, particularly where dose-response information is available, from historical human repeated insult patch tests (HRIPTs). From these human data, information on the approximate threshold for the induction of skin sensitization in the HRIPT was determined for 26 skin-sensitizing chemicals. These data were then compared with LLNA-derived EC3 values. The results from each assay, expressed as dose per unit area (microg/cm(2)), revealed a clear linear relationship between the 2 values, thereby substantiating further the utility of LLNA EC3 values for prediction of the relative human sensitizing potency of newly identified skin sensitizers.

An European inter-laboratory validation of alternative endpoints of the murine local lymph node assay

The original local lymph node assay (LLNA) is based on the use of radioactive labelling to measure cell proliferation. Other endpoints for the assessment of proliferation are also authorized by the OECD Guideline 429 provided there is appropriate scientific support, including full citations and description of the methodology (OECD, 2002. OECD Guideline for the Testing of Chemicals; Skin Sensitization: Local Lymph Node Assay, Guideline 429. Paris, adopted 24th April 2002.). Here, we describe the outcome of the second round of an inter-laboratory validation of alternative endpoints in the LLNA conducted in nine laboratories in Europe. The validation study was managed and supervised by the Swiss Agency for Therapeutic Products (Swissmedic) in Bern. Ear-draining lymph node (LN) weight and cell counts were used to assess LN cell proliferation instead of [3H]TdR incorporation. In addition, the acute inflammatory skin reaction was measured by ear weight determination of circular biopsies of the ears to identify skin irritation properties of the test items. The statistical analysis was performed in the department of statistics at the university of Bern. Similar to the EC(3) values defined for the radioactive method, threshold values were calculated for the endpoints measured in this modification of the LLNA. It was concluded that all parameters measured have to be taken into consideration for the categorisation of compounds due to their sensitising potencies. Therefore, an assessment scheme has been developed which turned out to be of great importance to consistently assess sensitisation versus irritancy based on the data of the different parameters. In contrast to the radioactive method, irritants have been picked up by all the laboratories applying this assessment scheme.

Fragrance material review on cinnamaldehyde

A toxicologic and dermatologic review of cinnamaldehyde when used as a fragrance ingredient is presented.

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.

Advantage of using CBA/N strain mice in a non-radioisotopic modification of the local lymph node assay

The murine local lymph node assay (LLNA) is currently recognized as a stand-alone test method for determining the skin sensitizing potential of chemicals. It has been incorporated into the official test guidelines published by some authorities, including the OECD. To avoid the use of radioisotopes, efforts have been made recently to develop non-radioisotopic modifications of the LLNA. A non-radioisotopic modification of the LLNA was developed previously using 5-bromo-2'-deoxyuridine (BrdU) incorporation (non-RI LLNA). However, the non-RI LLNA was found to be somewhat less sensitive than the standard assay. This study reports the advantage of using mice of the CBA/N strain in the non-RI LLNA to improve the sensitivity of this method. The non-RI LLNA was performed using CBA/JN and CBA/N mice exposed to one of four confirmed skin sensitizers, 2,4-dinitrochlorobenzene (DNCB), eugenol (EG), isoeugenol (IEG) or alpha-hexylcinnamic aldehyde (HCA), and to one non-sensitizer, propylene glycol (PG). The EC3 values for DNCB, IEG, EG, HCA and PG were calculated to be 0.1%, 9.6%, 40.6%, 45.5% and >50% in CBA/JN mice and 0.08%, 1.9%, 10.7%, 20.3% and >50% in CBA/N mice, respectively. The EC3 values for DNCB, IEG, EG, HCA and PG in the standard LLNA using CBA/Ca mice and radioisotopes were reported elsewhere as being 0.08%, 1.3%, 13.0%, 8.0% and >50%, respectively. The EC3 values derived from the CBA/N mice in the non-RI LLNA were nearly equivalent to the EC3 values obtained using the standard radioisotopic LLNA with CBA/Ca mice. These data suggest that the use of CBA/N mice may provide a realistic opportunity to develop a version of the LLNA that does not have a requirement for the use of radioisotopes, but which nevertheless has sensitivity approaching, or comparable to, the standard method.

Evaluation of the skin sensitizing potency of chemicals by using the existing methods and considerations of relevance for elicitation

The Technical Committee of Classification and Labelling dealing with harmonized classification of substances and classification criteria under Directive 67/548/EEC on behalf of the European Commission nominated an expert group on skin sensitization in order to investigate further the possibility for potency consideration of skin sensitizers for future development of the classification criteria. All substances and preparations should be classified on the basis of their intrinsic properties and should be labelled accordingly with the rules set up in the Directive 67/548/EEC. The classification should be the same under their full life cycle and in the case that there is no harmonized classification the substance or preparation should be self-classified by the manufacturer in accordance with the same criteria. The Directive does not apply to certain preparations in the finished state, such as medical products, cosmetics, food and feeding stuffs, which are subject to specific community legislation. The main questions that are answered in this report are whether it would be possible to give detailed guidance on how to grade allergen potency based on the existing methods, whether such grading could be translated into practical thresholds and whether these could be set for both induction and elicitation. Examples are given for substances falling into various potency groups for skin sensitization relating to results from the local lymph node assay, the guinea pig maximization test, the Buehler method and human experience.

Novel approach for classifying chemicals according to skin sensitizing potency by non-radioisotopic modification of the local lymph node assay

The murine local lymph node assay (LLNA) is currently recognized as a stand-alone sensitization test for determining the sensitizing potential of chemicals, and it has the advantage of yielding a quantitative endpoint that can be used to predict the sensitization potency of chemicals. The EC3 has been proposed as a parameter for classifying chemicals according to the sensitization potency. We previously developed a non-radioisotopic endpoint for the LLNA based on 5-bromo-2'-deoxyuridine (BrdU) incorporation (non-RI LLNA), and we are proposing a new procedure to predict the sensitization potency of chemicals based on comparisons with known human contact allergens. Nine chemicals (i.e. diphencyclopropenone, p-phenylenediamine, glutaraldehyde, cinnamicaldehyde, citral, eugenol, isopropyl myristate, propyleneglycol and hexane) categorized as human contact allergen classes 1-5 were tested by the non-RI LLNA with the following reference allergens: 2,4-dinitrochlorobenzene (DNCB) as a class 1 human contact allergen, isoeugenol as a class 2 human contact allergen and alpha-hexylcinnamic aldehyde (HCA) as a class 3 human contact allergen. Consequently, nine test chemicals were almost assigned to their correct allergen class. The results suggested that the new procedure for non-RI LLNA can provide correct sensitization potency data. Sensitization potency data are useful for evaluating the sensitization risk to humans of exposure to new chemical products. Accordingly, this approach would be an effective modification of LLNA with regard to its experimental design. Moreover, this procedure can be applied also to the standard LLNA with radioisotopes and to other modifications of the LLNA.

Ranking of hair dye substances according to predicted sensitization potency: quantitative structure-activity relationships

Allergic contact dermatitis following the use of hair dyes is well known. Many chemicals are used in hair dyes and it is unlikely that all cases of hair dye allergy can be diagnosed by means of patch testing with p-phenylenediamine (PPD). The objectives of this study are to identify all hair dye substances registered in Europe and to provide their tonnage data. The sensitization potential of each substance was then estimated by using a quantitative structure-activity relationship (QSAR) model and the substances were ranked according to their predicted potency. A cluster analysis was performed in order to help select a number of chemically diverse hair dye substances that could be used in subsequent clinical work. Various information sources, including the Inventory of Cosmetics Ingredients, new regulations on cosmetics, data on total use and ChemId (the Chemical Search Input website provided by the National Library of Medicine), were used in order to identify the names and structures of the hair dyes. A QSAR model, developed with the help of experimental local lymph node assay data and topological sub-structural molecular descriptors (TOPS-MODE), was used in order to predict the likely sensitization potential. Predictions for sensitization potential were made for the 229 substances that could be identified by means of a chemical structure, the majority of these hair dyes (75%) being predicted to be strong/moderate sensitizers. Only 22% were predicted to be weak sensitizers and 3% were predicted to be extremely weak or non-sensitizing. Eight of the most widely used hair dye substances were predicted to be strong/moderate sensitizers, including PPD - which is the most commonly used hair dye allergy marker in patch testing. A cluster analysis by using TOPS-MODE descriptors as inputs helped us group the hair dye substances according to their chemical similarity. This would facilitate the selection of potential substances for clinical patch testing. A patch-test series with potent, frequently used, substances representing various chemical clusters is suggested. This may prove useful in diagnosing PPD-negative patients with symptoms of hair dye allergy and would provide some clinical validation of the QSAR predictions.

A chemical dataset for evaluation of alternative approaches to skin-sensitization testing

Allergic contact dermatitis resulting from skin sensitization is a common occupational and environmental health problem. In recent years, the local lymph node assay (LLNA) has emerged as a practical option for assessing the skin-sensitization potential of chemicals. In addition to accurate identification of skin sensitizers, the LLNA can also provide a reliable measure of relative sensitization potency, information that is pivotal in successful management of human health risks. However, even with the significant animal welfare benefits provided by the LLNA, there is interest still in the development of non-animal test methods for skin sensitization. Here, we provide a dataset of chemicals that have been tested in the LLNA and the activity of which correspond with what is known of their potential to cause skin sensitization in humans. It is anticipated that this will be of value to other investigators in the evaluation and calibration of novel approaches to skin-sensitization testing. The materials that comprise this dataset encompass both the chemical and biological diversity of known chemical allergens and provide also examples of negative controls. It is hoped that this dataset will accelerate the development, evaluation and eventual validation of new approaches to skin-sensitization testing.

Evaluation of lymphocyte subpopulations in draining lymph node cells following allergen and irritant

The murine local lymph node assay (LLNA) has been developed as an alternative to guinea pig models for the assessment of the contact sensitization potential. However, there is a need to develop a non-radioisotopic endpoint for the LLNA, because of the radioisotopic method's requiring the use of special facilities. In this study, we investigated to evaluate the lymphocyte subpopulations in the lymph node cells following allergen and irritant treatment. Female Balb/c mice were treated by the topical application on the dorsum of both ears with sensitizers, 2,4-dinitrochlorobenzene (DNCB), toluene diisocyanate (TDI), and α-hexylcinnamaldehyde (HCA), and an irritant, sodium lauryl sulfate (SLS), once daily for three consecutive days. The lymph node (LN) cells were harvested 72h after the final treatment. Phenotypic analysis of lymphocytes subsets was performed with a flow cytometry. The allergens DNCB, TDI, and HCA and an irritant, SLS increased cell number compared to the vehicle. Mice were treated with DNCB, HCA, and TDI showed a preferential increase in the percentage of B220+CD40+ cells compared with vehicle and irritant-treated mice. There was an increase in B220+CD86+ cells of mice treated with DNCB, TDI, and HCA, but no significant increases were observed in mice treated with SLS. Mice were treated with DNCB and TDI showed an increase in the percentage of B220+CD23+ cells compared with vehicle and irritant-treated mice. These results suggest that analysis of B cell activation marker, CD40 on B cells may be useful in differentiating allergen and irritant responses in the draining lymph nodes of chemically treated mice.

Quantitative relationship between the local lymph node assay and human skin sensitization assays

The local lymph node assay (LLNA) is a new test method which allows for the quantitative assessment of sensitizing potency in the mouse. Here, we investigate the quantitative correlation between results from the LLNA and two human sensitization tests--specifically, human repeat insult patch tests (HRIPTs) and human maximization tests (HMTs). Data for 57 substances were evaluated, of which 46 showed skin sensitizing properties in human tests, whereas 11 yielded negative results in humans. For better comparability data from mouse and human tests were transformed to applied doses per skin area, which ranged over four orders of magnitude for the substances considered. Regression analysis for the 46 human sensitizing substances revealed a significant positive correlation between the LLNA and human tests. The correlation was better between LLNA and HRIPT data (n=23; r=0.77) than between LLNA and HMT data (n=38; r=0.65). The observed scattering of data points is related to various uncertainties, in part associated with insufficiencies of data from older HMT studies. Predominantly negative results in the LLNA for another 11 substances which showed no skin sensitizing activity in human maximization tests further corroborate the correspondence between LLNA and human tests. Based on this analysis, the LLNA can be considered a reliable basis for relative potency assessments for skin sensitizers. Proposals are made for the regulatory exploitation of the LLNA: four potency groups can be established, and assignment of substances to these groups according to the outcome of the LLNA can be used to characterize skin sensitizing potency in substance-specific assessments. Moreover, based on these potency groups, a more adequate consideration of sensitizing substances in preparations becomes possible. It is proposed to replace the current single concentration limit for skin sensitizers in preparations, which leads to an all or nothing classification of a preparation as sensitizing to skin ("R43") in the European Union, by differentiated concentration limits derived from the limits for the four potency groups.

Proposal for a risk assessment methodology for skin sensitization based on sensitization potency data

In this paper, we propose a quantitative risk assessment methodology for skin sensitization aiming at the derivation of 'safe' exposure levels for sensitizing chemicals, used e.g., as ingredients in consumer products. Given the limited number of sensitizers tested in human sensitization tests, such as the human repeat-insult patch test (HRIPT) or the human maximization test (HMT), we used EC3 values from the local lymph node assay (LLNA) in mice because they provide the best quantitative measure of the skin sensitizing potency of a chemical. A comparison of LLNA EC3 values with HRIPT and HMT LOEL, and NOEL values was carried out and revealed that the EC3, expressed as area dose, can be used as a surrogate value for the human NOEL in risk assessment. The uncertainty/extrapolation factor approach was used to derive (a) an 'acceptable non-sensitizing area dose' (ANSAD) to protect non-allergic individuals against skin sensitization and (b) an 'acceptable non-eliciting area dose' (ANEAD) to protect allergic individuals against elicitation of allergic contact dermatitis. For ANSAD derivation, interspecies, intraspecies and time extrapolation factors are applied to the LLNA EC3. For ANEAD derivation, additional application of a variable sensitization-elicitation extrapolation factor is proposed. Values for extrapolation factors are derived and discussed, the proposed methodology is applied to the sensitizers methylchloroisothiazolinone/methylisothiazolinone, cinnamic aldehyde and nickel and results are compared to published risk assessments.

Use of the local lymph node assay in the evaluation of the sensitizing potential of pharmaceutical process intermediates

The murine local lymph node assay (LLNA) has recently been developed to determine the contact sensitization potential of chemicals. Since its original development, the LLNA results have been the subject of extensive comparisons with guinea pig and human data. The investigations described here were designed to explore the ability of the LLNA to identify accurately, pharmaceutical process intermediates (PIs) known to cause contact allergy in humans. To that end, 16 PIs previously tested in the guinea-pig maximization test (GPMT) were tested in the LLNA. Another PI known to be a contact sensitizer in humans was tested only in the LLNA. Cases of contact sensitization in humans were reported only for PIs that were extreme sensitizers in the GPMT and had low EC3 values (concentration of the test substance required to generate a threefold increase in lymph node cell proliferation) in the LLNA. These data provide additional evidence that the LLNA is able to discriminate skin sensitizers from chemicals that do not possess a significant skin sensitization potential and is thus a useful method for hazard identification. In addition, this method also offers important animal welfare benefits and may also be useful for risk assessment purposes.

Citral a fragrance allergen and irritant

Citral is a well known contact allergen and a contact irritant. Routine patch testing in the past may have been restricted because of possible irritant (IR) patch test responses. 586 consecutive patients, with hand eczema, were patch tested with a selection of fragrances including citral 2% petrolatum and the European standard series. 28 of the patients showed a positive patch test reaction (+ to +++) to citral and 82 at least 1 IR patch test reaction and no positive patch test reaction to citral. A statistically significant association between a positive patch test reaction to citral and positive patch test reactions to other fragrances compared with IR reactions (n = 82) was established. The difference regarding fragrance history found between those with IR and positive reactions to citral was not significant. Citral could be an allergen and/or irritant, worthy of further more extensive studies.

Fragrance allergy in patients with hand eczema - a clinical study

Fragrance allergy and hand eczema are both common among dermatological patients. Fragrance mix (FM) and its constituents have a recognized relevance to exposure to fine fragrances and cosmetic products. Based on extensive chemical analysis and database search, a new selection of fragrances was established, including 14 known fragrance allergens present in products to which hand exposure would occur. A non-irritating patch-test concentration for some fragrances was established in 212 consecutive patients. 658 consecutive patients presenting with hand eczema were patch tested with the European standard series and the developed selection of fragrances. 67 (10.2%) of the 658 patients had a positive reaction to 1 or more of our selection of fragrance chemicals present in the new selection. The most common reactions to fragrances not included in the FM were to citral, Lyral (hydroxyisohexyl-3-cyclohexene carboxaldehyde) and oxidized l-limonene. A concomitant reaction to the FM identified potential fragrance allergy in less than (1/2) of these patients. Exposure assessment and a statistically significant association between a positive patch test to our selected fragrances and patients' history support the relevance of this selection of fragrances. Those with a positive reaction to our selected fragrances were significantly more likely to have 1 or more positive patch tests in the standard series. This observation is the basis for the hypothesis concerning cross-reactivity and the effect of simultaneous exposure. The study found that fragrance allergy could be a common problem in patients with eczema on the hands.

Effect of prolonged exposure to low antigen concentration for sensitization

The local lymph node assay (LLNA) is an assay in mice to identify potential allergens. Compounds that do not induce a stimulation index (SI)>or=3 are not considered sensitizers. Of the chemicals that do, the SI of 3 is used as a benchmark, and indicates the sensitizing potency of a chemical. Compared to the exposure duration of the LLNA (3 days), real life exposure often lasts for months or years. We therefore investigated whether prolonged exposure to sensitizers at concentrations that do not induce a SI>or=3 in the LLNA, were able to surpass this threshold. Mice were treated for 2 months at 7-day intervals with a range of concentrations of the known allergens ethyl-p-aminobenzoate (benzocaine, BENZ), 2,4-dinitrochlorobenzene (DNCB), and tetramethyl thiuram disulfide (TMTD). Both proliferative activity and cytokine production were established at day 60. Neither BENZ nor TMTD showed a significant increase in the proliferation rate compared to vehicle controls. Only DNCB at concentrations originally above the EC(3) a significant increase in proliferation was seen after prolonged exposure. No significant effect on IFN-gamma and IL-4 production was observed for all three compounds compared. These findings indicate that for classification of sensitizers the shorter exposure period employed in the standard LLNA is sufficient, and longer periods of exposure have no bearing on this classification.

Peptide-binding assessment using mass spectrometry as a new screening method for skin sensitization

Skin sensitization potential of low molecular weight chemicals was assessed by analyzing peptide-conjugate formation. Chemicals were incubated with a peptide, glutathione, and resultant mixtures were analyzed by mass spectrometry. Eighteen chemicals were assessed, and new peaks corresponding to chemical-peptide conjugates were detected for 13 of 14 known sensitizers. Conjugates were not detected for 4 negative chemicals. The method has advantages as a simple screening assay for assessing the sensitization potential of chemicals.

Application of the risk assessment paradigm to the induction of allergic contact dermatitis

The National Academy of Science (NAS) risk assessment paradigm has been widely accepted as a framework for estimating risk from exposure to environmental chemicals (NAS, 1983). Within this framework, quantitative risk assessments (QRAs) serve as the cornerstone of health-based exposure limits, and have been used routinely for both cancer and noncancer endpoints. These methods have focused primarily on the extrapolation of data from laboratory animals to establish acceptable levels of exposure for humans. For health effects associated with a threshold, uncertainty and variability inherent in the extrapolation process is generally dealt with by the application of "uncertainty factors (UFs)." The adaptation of QRA methods to address skin sensitization is a natural and desirable extension of current practices. Based on our chemical, cellular and molecular understanding of the induction of allergic contact dermatitis, one can conduct a QRA using established methods of identifying a NOAEL (No Observed Adverse Effect Level) or other point of departure, and applying appropriate UFs. This paper describes the application of the NAS paradigm to characterize risks from human exposure to skin sensitizers; consequently, this method can also be used to establish an exposure level for skin allergens that does not present an appreciable risk of sensitization.

Evaluation of CD86 expression and MHC class II molecule internalization in THP-1 human monocyte cells as predictive endpoints for contact sensitizers

The aim of this study was to explore the usefulness of a human monocyte cell line in the development of in vitro models for predictive testing of contact sensitizers. Several studies have shown that contact sensitizers induce CD86 expression and enhanced internalization of MHC class II molecules in dendritic cells (DCs). We used THP-1, a human monocyte cell line, as a replacement for DCs for evaluation of these phenotypical alterations as predictive endpoints for contact sensitizers. Known sensitizers and irritants were evaluated. After 24-h exposure to samples, the expression of CD86 on THP-1 cells was measured by flow cytometry. Sensitizers such as dinitrochlorobenzene (DNCB), 2-mercaptobenzothiazole (MBT), eugenol, p-phenylenediamine (PPDA) and ammonium tetrachloroplatinate (Pt) enhanced CD86 expression on THP-1 cells, while nickel sulfate, cobalt sulfate and irritants such as methylsalicylate (MS), sodium dodecyl sulfate (SDS) and dimethyl sulfoxide (DMSO) did not augment CD86 expression. A synergistic effect was observed when DNCB and IFN-alpha were added simultaneously to a culture of THP-1 cells. Furthermore, internalization of MHC class II molecules was observed when the cells were treated with some of sensitizers for 2 h. The inducing effects of chemicals on the two phenotypical alterations were the same. These results suggest that these test systems can be used to predict contact-sensitizing ability of chemicals as an in vitro sensitization assay.

The local lymph node assay: past, present and future

The local lymph node assay (LLNA) was developed originally as a method for the identification of chemicals that have the potential to cause skin sensitization and allergic contact dermatitis. The assay is based on an understanding that the acquisition of contact sensitization is associated with, and dependent upon, the stimulation by chemical allergens of lymphocyte proliferative responses in skin-draining lymph nodes. Those chemicals that provoke a defined level of lymph node cell (LNC) proliferation (a 3-fold or greater increase compared with concurrent vehicle controls) are classified as skin sensitizers. Following its original inception and development, the LLNA was the subject of both national and international interlaboratory collaborative trials, and of very detailed comparisons with other test methods and with human skin sensitization data. The assay has now been validated fully as a stand-alone test for the purposes of hazard identification. In recent years, there has been a growing interest also in the use of the LLNA to assess the potency of contact allergens and in risk assessment. There is reason to believe that the extent of skin sensitization achieved is associated with the vigour of LNC proliferation induced in draining nodes. Given this relationship, the relative potency of skin sensitizing chemicals is measured in the LLNA by derivation of an EC3 value, this being the concentration of chemical required to provoke a 3-fold increase in the proliferation of LNC compared with controls. Experience to date indicates that relative potency as determined using this approach correlates closely with what is known of the activity of skin sensitizing chemicals in humans. In this article, we review the development, evaluation and validation of the LLNA for the purposes of hazard identification, and the more recent application of the method for evaluation of potency in the context of risk assessment. In addition, we consider what new applications and modifications are currently being investigated.

Quantitative aspects of contact allergy to chromium and exposure to chrome-tanned leather

The potential of trivalent and hexavalent chromium to induce and elicit allergic contact dermatitis and the degree of chromium exposure from leather products are reviewed. Chromium dermatitis is often due to exposure in the occupational environment, with cement being one of the most common chromium sources. However, consumer products such as chromium(III)-tanned leather products are also an important source of chromium exposure. Apart from Cr(III), which is used for tanning, leather often also contains trace amounts of Cr(VI), which is formed by oxidation of Cr(III) during the tanning process. In a recent study of the Cr(VI) content of leather products bought on the Danish market, 35% of such articles had a Cr(VI) content above the detection limit of 3 p.p.m., ranging from 3.6 p.p.m. to 14.7 p.p.m. Leachable Cr(III) was detected at levels of 430-980 p.p.m. An examination of available dose-response studies showed that exposure to occluded patch test concentrations of 7-45 p.p.m. Cr(VI) elicits a reaction in 10% of the chromium-sensitive patients. When reviewing repeated open exposure studies, it is seen that either exposure to 5 p.p.m. Cr(VI) in the presence of 1% sodium lauryl sulfate (SLS) or exposure to 10 p.p.m. Cr(VI) alone both elicit eczema in chromium-sensitive patients. The eliciting capacity of Cr(III) has not been systematically investigated but, compared to Cr(VI), much higher concentrations are needed to elicit eczema.