Sensitization to 2,4-dinitrochlorobenzene: influence of vehicle on absorption and lymph node activation

Ethanol and Diethyl Phthalate: Vehicle Effects in the Local Lymph Node Assay

The vehicle in which an allergen is presented to the skin has been recognized to have an effect on the skin-sensitizing potency of the allergen. Typical vehicles used to evaluate the skin sensitization potential of fragrance materials include ethanol, diethyl phthalate, or a combination of the two. The authors conducted a series of studies to evaluate each of these vehicles for their utility in the murine local lymph node assay and to investigate the potential differences in skin sensitization resulting from their use. Four fragrance materials were tested in four different vehicles. The test materials were p-t-butyl- α-methylhydrocinnamic aldehyde, geraniol, eugenol, and hydroxycitronellal. The vehicles were diethyl phthalate, 1:3 ethanol:diethyl phthalate, 3:1 ethanol:diethyl phthalate, and ethanol. Each of the fragrance materials was tested at five dose levels ranging from 0.3% to 50% w/v. In all four vehicles, each material tested elicited positive responses, exhibiting weak to moderate skin sensitization potential. Overall, p-t-butyl- α-methylhydrocinnamic aldehyde exhibited the most potency, followed by eugenol, geraniol, and hydroxycitronellal. The sensitization potential of both p-t-butyl- α-methylhydrocinnamic aldehyde and geraniol was greatest when the vehicle was ethanol. The sensitization potential of eugenol was greatest in 3:1 ethanol:diethyl phthalate, but the sensitization potential for hydroxycitronellal was greatest in 1:3 ethanol:diethyl phthalate. The strength of the sensitization response was observed to vary with the vehicle; however, the results did not show any clear pattern of one vehicle over another regarding skin sensitization.

Vehicle effects on skin sensitizing potency of four chemicals: assessment using the local lymph node assay

The murine local lymph node assay (LLNA) can be used to determine the relative skin sensitizing potency of chemicals via interpolation of the quantitative dose response data generated. Using this approach we have demonstrated previously that the vehicle matrix in which a chemical allergen is encountered on the skin can have a significant influence on sensitizing potency. Estimates of relative potency are calculated from LLNA dose responses as a function of the mathematically derived EC3 value, this being the concentration estimated to induce a stimulation index (SI) of 3. To investigate further the influence of application vehicle on sensitizing potency, the LLNA has been used to examine the activity of four recognized human contact allergens: isoeugenol and cinnamic aldehyde, two fragrance chemicals; 3-dimethylaminopropylamine (a sensitizing impurity of cocamidopropyl betaine, a surfactant used in shower gel) and dibromodicyanobutane (the sensitizing component of Euxyl K 400, a preservative used in cosmetics). The four chemicals were applied in each of seven different vehicles (acetone: olive oil [4 : 1]; dimethylsulphoxide; methylethylketone; dimethyl formamide; propylene glycol; and both 50 : 50 and 90 : 10 mixtures of ethanol and water). It was found that the vehicle in which a chemical is presented to the epidermis can have a marked effect on sensitizing activity. EC3 values ranged from 0.9 to 4.9% for isoeugenol, from 0.5 to 1.7% for cinnamic aldehyde, from 1.7 to > 10% for dimethylaminopropylamine and from 0.4 to 6.4% for dibromodicyanobutane. These data confirm that the vehicle in which a chemical is encountered on the skin has an important influence on the relative skin sensitizing potency of chemicals and may have a significant impact on the acquisition of allergic contact dermatitis. The data also demonstrate the utility of the LLNA as a method for the prediction of these effects and thus for the development of more accurate risk assessments.

The LLNA: A Brief Review of Recent Advances and Limitations

Allergic contact dermatitis is the second most commonly reported occupational illness, accounting for 10% to 15% of all occupational diseases. This highlights the importance of developing rapid and sensitive methods for hazard identification of chemical sensitizers. The murine local lymph node assay (LLNA) was developed and validated for the identification of low molecular weight sensitizing chemicals. It provides several benefits over other tests for sensitization because it provides a quantitative endpoint, dose-responsive data, and allows for prediction of potency. However, there are also several concerns with this assay including: levels of false positive responses, variability due to vehicle, and predictivity. This report serves as a concise review which briefly summarizes the progress, advances and limitations of the assay over the last decade.

Multi-species assessment of electrical resistance as a skin integrity marker for in vitro percutaneous absorption studies

Assessment of percutaneous absorption in vitro provides key information when predicting dermal absorption in vivo. Confirmation of skin membrane integrity is an essential component of the in vitro method, as described in test guideline OECD 428. Historically, assessment of the membrane's permeability to tritiated water (T2O) and the generation of a permeability coefficient (Kp) were used to confirm that the skin membrane was intact prior to application of the test penetrant. Measuring electrical resistance (ER) across the membrane is a simpler, quicker, safer and more cost effective method. To investigate the robustness of the ER integrity measure, the Kp values for T2O for a range of human and animal skin membranes were compared with corresponding ER data. Overall, for human, rat, pig, mouse, rabbit and guinea pig skin, the ER data gave a good inverse association with the corresponding Kp values; the higher the Kp the lower the ER values. In addition, the distribution across a large dataset for individual skin samples was similar for Kp and ER, allowing a cut-off value for ER to be established for each skin type. Based on CTL's (Syngenta Central Toxicology Laboratory) standard static diffusion cells and databridge, we propose that intact skin should have an ER equal to or above (in kOmega): human (10), mouse (5) guinea pig (5), pig (4) rat (3), and rabbit (0.8). We conclude that measurement of ER across in vitro skin membranes provides a robust measurement of skin barrier integrity and is an appropriate alternative to Kp for T2O in order to identify intact membranes that have acceptable permeability characteristics for in vitro percutaneous absorption studies.

Classification of contact allergens according to potency: proposals

It is clear that contact allergens vary substantially with regard to the relative potency with which they are able to induce skin sensitisation. Considerations of potency will in the future become a significant factor in the classification of skin sensitising chemicals. It is therefore appropriate to establish what is known of potency and thresholds in the induction of skin sensitisation and the elicitation of allergic contact dermatitis, and to identify approaches that might be available for assessment of relative potency for the purposes of categorising chemical allergens. This paper was prepared by an ECETOC (European Centre for Ecotoxicology and Toxicology) Task Force that had the objective of recommending approaches for the measurement of potency and definition of thresholds for both the induction and elicitation of contact sensitisation. The deliberations recorded here build upon recommendations made previously by an ECETOC Task Force that considered the conduct of standard skin sensitisation test methods for the purposes of hazard identification and risk assessment (ECETOC, Monograph No. 29, Brussels, 2000). The emphasis in this present paper is also on standard and accepted methods for the assessment of skin sensitisation, and for which OECD guidelines are available: the local lymph node assay (LLNA), the guinea pig maximisation test and the occluded patch test of Buehler. For various reasons, discussed in detail herein, attention focused primarily upon consideration of categorisation of chemical allergens and the identification of thresholds with respect to the induction of skin sensitisation, rather than the elicitation of allergic contact dermatitis. It is concluded that although the LLNA is the method of choice for the determination of skin sensitisation potency for the purposes of categorisation, if data are already available from appropriate guinea pig tests then their judicious interpretation may provide information of value in determinations of potency and categorisation. Included here are detailed and specific recommendations for how best the results of the three test methods considered can be used for the categorisation of chemical allergens as a function of skin sensitisation potency.

What makes a chemical an allergen?

OBJECTIVE

To consider the factors that confer on chemicals the ability to cause allergic sensitization, with particular emphasis on the induction of skin sensitization.

DATA SOURCES

Original and review articles available in the scientific literature.

STUDY SELECTION

The expert opinion of the authors was used to select studies for inclusion in this review.

RESULTS

A number of requirements must be met if a chemical is to induce skin sensitization. The most important requirements are access to the viable epidermis, the formation of stable conjugates with proteins, elicitation of cytokine production by skin cells, and the initiation of T-lymphocyte responses. In addition, qualitative aspects of induced immune responses influence the form of allergic sensitization.

CONCLUSIONS

An increasingly sophisticated understanding of the factors required for the development of skin sensitization and other forms of chemical-induced allergy provides new opportunities for toxicological investigation and clinical management.

A prevalidation study on the in vitro skin irritation function test (SIFT) for prediction of acute skin irritation in vivo: results and evaluation of ECVAM Phase III

A prevalidation study sponsored by the European Centre for the Validation of Alternative Methods (ECVAM) on in vitro tests for acute skin irritation is aimed at identifying non-animal tests capable of discriminating irritants (I) from non-irritants (NI), as defined according to European Union and OECD. This paper reports on Phase III for one of the methods, the skin integrity function test (SIFT), assessing the protocol performance of the SIFT, in terms of reproducibility and predictive ability, in three laboratories. The barrier function properties of excised mouse skin were determined using a set of 20 coded chemicals (10 I, 10 NI), using the endpoints of trans-epidermal water loss (TEWL) and electrical resistance (ER). The basis of the SIFT prediction model is if the ratios of the pre- and post-application values for either TEWL or ER are greater than five-fold, then the test chemical is deemed irritant (I). If the ratio of both parameters is less than five-fold then the chemical is deemed non-irritant (NI). Analysis of variance (ANOVA) indicated that the intra-lab reproducibility was acceptable but that the inter-lab reproducibility was not. Overall, the SIFT test under-predicted the irritancy of the test chemicals chosen for Phase III with an overall accuracy of only 55%. The sensitivity value (ability to correctly predict I) was only 30%. The specificity (ability to predict NI) of the test was better at 80%. A retrospective examination of the SIFT results was undertaken using Student's t-test and a significance level of P<0.05 to predict an irritant based on changes in the TEWL ratio values. This improved the predictivity of the SIFT test, giving a specificity of 60%, a sensitivity of 80% and an overall accuracy of 70%. Appropriate modifications to the prediction model have now been made and the SIFT will be re-examined in a new validation exercise to investigate the potential of this non-animal method to predict acute skin irritation potential.

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.

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

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

Insights into the quantitative relationship between sensitization and challenge for allergic contact dermatitis reactions

The ability of chemical or pharmaceutical agents to induce allergic contact dermatitis (ACD) is of major health and regulatory concern. As such, tests to identify their sensitizing capacity, such as the guinea pig maximization test and the more recently developed local lymph node assay, are broadly used. Ideally, for risk assessment it is useful to translate results from animal data into establishing safe or no-effect levels for occupational or environmental agents. This, of course, would require consideration of the quantitative relationships between sensitizing and challenge doses as well as other exposure conditions. In the present studies, we modeled two sensitizers, 2,4-dinitrochlorobenzene and squaric acid dibutyl ester, over a large range of concentrations using the LLNA and more traditional tests that measure both sensitization and elicitation responses. Both the sensitization and challenge phases provided similar dose-response curves, demonstrating a threshold followed by a shallow linear increase and eventual plateau at increasing doses. Extending earlier studies by P. S. Friedmann (1994, Immunotoxicology and Immunopharmacology, pp. 589-616, Raven Press, New York) in humans, we observed that the minimum dose required to elicit sensitization or challenge was not static, but rather reflected a "sliding-scale." That is, as the sensitization dose was increased, the concentration required to elicit a challenge response was decreased. Correspondingly, as the challenge dose was increased, the dose required for sensitization was lessened. Taken together, these findings indicate that there is a need to consider dose-response relationships for sensitization and challenge in establishing minimum exposure levels for chemicals that cause ACD.

Determination of the sensitising activity of the rubber contact sensitisers TMTD, ZDMC, MBT and DEA in a modified local lymph node assay and the effect of sodium dodecyl sulfate pretreatment on local lymph node responses

A modified local lymph node assay (LLNA) was used to determine the sensitising activity of four chemicals used for the production of natural rubber latex products. Tetramethylthiuramdisulfide (TMTD), 2-mercaptobenzothiazole (MBT) and zincdimethyldithiocarbamate (ZDMC), three moderate human sensitisers, and diethylamine (DEA) a known human sensitiser, were epicutaneously administered on the ear and the proliferating activity in the draining (auricular) lymph node (LN) was determined by ex vivo (3)H-thymidine incorporation. Consistent results were obtained for TMTD and ZDMC with stimulation indices (SI) above 3, identifying these compounds as sensitiser, while for DEA and MBT inconsistent results were obtained. For all parameters determined such as LN weight, LN cell number, cell proliferation per 2 x 10(6) cells, and cell proliferation per LN statistical significant increases were observed. The SI, expressed as cellular proliferation per LN or per animal (left and right LN combined), was the most sensitive parameter with an optimum at day 5 after start of treatment.Furthermore, we investigated whether the use of sodium dodecyl sulfate (SDS) was able to enhance weak responses in the LLNA. SDS treatment with dosages of 10% and higher resulted in a SI above 3, while a dosage of 1% SDS showed no activity. Pretreatment with 1% SDS 1 h before application of the rubber chemicals enhanced the responses to these chemicals consistently, identifying also DEA and MBT as sensitisers. Our results indicate that SDS had synergistic activity on the LN responses of the administered rubber chemicals in the LLNA. For the moderately responding sensitisers TMTD and ZDMC both IFN-gamma and IL-4 production was observed. For the weakly responding sensitisers DEA and MBT both IFN-gamma and IL-4 cytokine production was only observed after pretreatment of the animals with 10% SDS. For 10% and 20% SDS, inducing approximately a SI of 20 in the LLNA, no induction of cytokines was observed.

Joint positive control testing in guinea pig skin sensitization tests. A harmonized approach

A scheme for the performance of positive control studies within a coordinated group of laboratories was proposed (joint positive control testing). The procedure has been described, as well as the first results of the validation phase of this joint positive control testing project. Adoption of this proposal within the participating six laboratories would lead to a reduction in the number of guinea pigs required for reliability and sensitivity checks from current approximate 12 studies per year down to 2 studies, i.e., 150-300 fewer animals per year. Another benefit would be the use of a harmonized, and therefore more comparable, method to perform guinea pig tests and interpret the data. In the validation phase of joint reading of the positive control studies, the congruency of reading could clearly be demonstrated. From the experience gained up to now, it was possible to draw the conclusion that a coordinated interlaboratory approach for positive control testing was fully acceptable and an improvement with regard to animal welfare.

Understanding fragrance allergy using an exposure-based risk assessment approach

Conducting a sound skin sensitization risk assessment prior to the introduction of new ingredients and products into the market place is essential. The process by which low-molecular-weight chemicals induce and elicit skin sensitization is dependent on many factors, including the ability of the chemical to penetrate the skin, react with protein, and trigger a cell-mediated immune response. Based on our chemical, cellular and molecular understanding of allergic contact dermatitis, it is possible to carry out a quantitative risk assessment. Specifically, by estimating the exposure to the allergen and its allergenic potency, it is feasible to assess quantitatively the sensitization risk of an ingredient in a particular product type. This paper focuses on applying exposure-based risk assessment tools to understanding fragrance allergy for 2 hypothetical products containing the fragrance allergen cinnamic aldehyde. The risk assessment process predicts that an eau de toilette leave-on product containing 1000 ppm or more cinnamic aldehyde would pose an unacceptable risk of induction of skin sensitization, while a shampoo, containing the same level of cinnamic aldehyde, would pose an acceptable risk of induction of skin sensitization, based on limited exposure to the ingredient from a rinse-off product application.

Comparison of tissue sources for the skin integrity function test (SIFT)

One of the in vitro models involved in an ECVAM-sponsored prevalidation study for acute skin irritation is the skin integrity function test (SIFT), which utilises full-thickness mouse skin. We have evaluated nine different skin types in order to identify the most useful model for assessing skin barrier function using transepidermal water loss (TEWL), electrical resistance (ER) and tritiated water flux (TWF) and sodium lauryl sulphate (SLS) as a standard skin irritant. Tissues were: human skin (epidermis and whole), reconstituted human epidermis (RHE), pig (dermatomed and whole), rabbit (whole), rat (epidermis and whole) and mouse (whole). Barrier function was measured following sodium lauryl sulphate (SLS) exposure and expressed as a damage ratio. Human epidermis gave good responses at high doses of SLS only. RHE had abnormally high permeability to water and therefore had little or no response to SLS. Pig skin gave low TEWL ratios and rabbit skin was a poor responder to SLS. Mouse whole skin performed best in this study, giving consistent high damage ratios to TEWL, ER and TWF following SLS treatment. Rat whole skin also performed well but was generally less responsive. We conclude that mouse skin is the best and most practical in vitro model for the SIFT approach for skin irritation prediction.

Skin sensitisation, vehicle effects and the local lymph node assay

Accurate risk assessment in allergic contact dermatitis is dependent on the successful prospective identification of chemicals which possess the ability to behave as skin sensitisers, followed by appropriate measurement of the relative ability to cause sensitisation; their potency. Tools for hazard identification have been available for many years; more recently, a novel approach to the quantitative assessment of potency--the derivation of EC3 values in the local lymph node assay (LLNA)--has been described. It must be recognised, however, that these evaluations of chemical sensitisers also may be affected by the vehicle matrix in which skin exposure occurs. In this article, our knowledge of this area is reviewed and potential mechanisms through which vehicle effects may occur are detailed. Using the LLNA as an example, it is demonstrated that the vehicle may have little impact on the accuracy of basic hazard identification; the data also therefore support the view that testing ingredients in specific product formulations is not warranted for hazard identification purposes. However, the effect on potency estimations is of greater significance. Although not all chemical allergens are affected similarly, for certain substances a greater than 10-fold vehicle-dependent change in potency is observed. Such data are vital for accurate risk assessment. Unfortunately, it does not at present appear possible to predict notionally the effect of the vehicle matrix on skin sensitising potency without recourse to direct testing, for example by estimation of LLNA EC3 data, which provides a valuable tool for this purpose.

A prevalidation study on in vitro tests for acute skin irritation. results and evaluation by the Management Team

A prevalidation study on in vitro tests for acute skin irritation was conducted during 1999 and 2000. The overall objective of validation in this area, of which this prevalidation study is an initial stage, is to identify tests capable of discriminating irritants (I) from non-irritants (NI), as defined according to European Union (EU) risk phrases ("R38"; no classification) and the harmonised OECD criteria ("Irritant"; no label). This prevalidation study specifically addressed aspects of: protocol refinement (phase I), protocol transfer (phase II), and protocol performance (phase III), in accordance with the prevalidation scheme defined by the European Centre for the Validation of Alternative Methods (ECVAM). The tests evaluated were: EpiDerm (phases I, II and III), EPISKIN (phases I, II and III), PREDISKIN (phases I and II, and additional protocol refinement), the non-perfused pig ear method (phases I and II, and additional protocol refinement), and the mouse skin integrity function test (SIFT; phases I and II). Modified, standardised test protocols and well-defined prediction models were available for each of the tests at the end of phase I. The results of phase I (intralaboratory reproducibility) were sufficiently promising for all of the tests to progress to phase II. Protocol transfer between the Lead Laboratory and Laboratory 2 was undertaken for all five tests during phase II, and additional refinements were made to the test protocols. For EpiDerm, EPISKIN and the SIFT, the intralaboratory and interlaboratory reproducibilities were acceptable; however, better standardisation of certain aspects of the test protocols was needed prior to commencing phase III. Neither PREDISKIN nor the pig ear test performed sufficiently well in phase II to progress to phase III. The PREDISKIN protocol was overly sensitive, resulting in the prediction of all the NI chemicals as I. The variability in the pig ear test results was too great, indicating that the test would show limited predictive ability. In additional studies (a repeat of phase I), further modification of the PREDISKIN protocol and a change in the prediction model considerably improved the ability of the test to distinguish I from NI chemicals. However, attempts to improve the intralaboratory reproducibility of the pig ear test were unsuccessful. In phase III an initial assessment of the reproducibility and predictive ability, in three independent laboratories per test, was undertaken for the EpiDerm and EPISKIN tests (the SIFT was a late inclusion in the prevalidation study, and is being evaluated in a separate phase III study). A set of 20 coded chemicals (10 I, 10 NI) were tested with the final, refined, test protocols. The intralaboratory reproducibility was acceptable for both EpiDerm and EPISKIN. The interlaboratory reproducibility was considered to be acceptable for EPISKIN; however, for EpiDerm, analysis of variance (ANOVA) indicated that there was a statistically significant laboratory effect on the overall variability, suggesting that the interlaboratory transferability of the test needs to be improved. The EpiDerm test had an overall accuracy of 58%, with an over-prediction rate of 37% and an under-prediction rate of 47%. The EPISKIN test had an overall accuracy of 58%, showing an under-prediction rate of 23% and an over-prediction rate of 60%. It is concluded that, as yet, none of the tests evaluated in this prevalidation study are ready for inclusion in a formal validation study on in vitro tests for acute skin irritation. Overall protocol performance of the SIFT is currently being evaluated in a phase III study. Further studies are also in progress to improve the test protocols and prediction models for EpiDerm and EPISKIN.

Distribution of MHC-II and CD1 molecules in the skin of lambs and changes during experimentally-induced contact hypersensitivity

The presentation of antigen to specific T-cell populations is a crucial event during the elicitation phase of contact hypersensitivity (CHS). Significant changes in CD4(+) T-cell and gammadelta T-cell populations occur in the skin of sheep 48h after re-exposure to dinitrochlorobenzene but the expression of antigen presentation molecules such as MHC-II and CD1 at this stage of the hypersensitivity response has not been investigated. In the present study, a panel of monoclonal antibodies recognising CD1 and MHC-II subtypes was used in combination with computer assisted morphometric analysis to estimate the distribution of antigen presentation molecules in the superficial and deep dermis of the ears of lambs during the elicitation phase of CHS. The MHC-II molecules showed predominantly a perivascular and peri-appendageal distribution in the dermis and there were scattered MHC-II(+) cells in the basal and suprabasal layers of the epidermis. The CD1w2(+) (CD1b-like) molecules were present on distinct cells that were scattered evenly through the dermis, whereas CD1w3(+) (CD1c-like) molecules were almost exclusively detected on or in close association with the vascular endothelium. There was a significant increase in the presence of MHC-DQ(+) cells in the superficial dermis of dinitrochlorobenzene-treated animals compared with both an untreated control group and a vehicle-treated control group. However, MHC-DQ/DR(+) and CD1w3(+) cells only showed a significant increase compared with the vehicle-treated control group. The present study shows that the distribution of molecules involved in antigen presentation to CD4(+) T-cells and gammadelta T-cells changes during the elicitation phase of CHS in sheep, and suggests a role for MHC-DQ molecules on antigen presenting cells. However, the changes in distribution and expression of MHC-II and CD1 subtypes argue against a prominent role for a CD1-dependent pathway for T-cell recognition in the clinical cutaneous hypersensitivity response in sheep. Based on the expression of MHC-II molecules and CD1c molecules, we also suggest a potential role for endothelial cells in antigen presentation during the clinical dermatitis reaction.

The diagnostic evaluation, treatment, and prevention of allergic contact dermatitis in the new millennium

Identifying the etiology of allergic contact dermatitis is a rewarding yet challenging endeavor. Not all allergic contact reactions are eczematous in appearance. The most reliable clinical clue to the allergic nature of the dermatitis is its geographic distribution. Once a list of culprit allergens has been identified by patch testing, the practitioner must identify the relevant allergen(s) and counsel the patient in avoidance. For most individuals, allergen avoidance results in resolution of the dermatitis; however, some patients will require continuing symptomatic therapy despite avoidance. For those patients unable to avoid known allergens, immunosuppressant therapies (including phototherapy) or barriers can be beneficial. Currently, hyposensitization is not a viable alternative for the treatment of allergic contact dermatitis.

Influence of application vehicle on skin sensitization to methylchloroisothiazolinone/methylisothiazolinone: an analysis using the local lymph node assay

The murine local lymph node assay (LLNA) is a method for the identification of skin sensitizing chemicals in which activity is measured as a function of proliferative responses induced in draining lymph nodes following topical exposure of mice to the test material. More recently, the LLNA has also been used for the determination of relative skin sensitizing potency based upon the mathematical derivation of an EC3 value, this being the estimated concentration of test chemical necessary to provoke a 3-fold increase in lymph-node cell-proliferative activity compared with concurrent vehicle-treated controls. Here we describe the use of the LLNA to determine the influence of vehicle on the skin-sensitizing potency of methylchloroisothiazolinone/methylisothiazolinone (MCI/MI), the active ingredient of preservatives such as Kathon CG. To this end, LLNA responses to MCI/ MI were measured using the vehicles 4:1 acetone:olive oil (AOO), methyl ethyl ketone, dimethylsulfoxide, dimethylformamide, propylene glycol (PG) and acetone. It was found that the vehicle in which MCI/MI was applied had a substantial impact on activity, with derived EC3 values varying from 0.0049% with AOO to 0.048% with PG. With the other vehicles, EC3 values ranged from 0.0068 to 0.0076%. The skin sensitizing potency of MCI/MI as judged from LLNA responses is consistent with what is known of the requirements for sensitization in humans. It is proposed that the LLNA not only provides a method for determination of relative skin sensitizing potency, but is also appropriate for assessing the influence of vehicle matrix on sensitizing activity.

Local lymph node assay: use in hazard and risk assessment

The murine local lymph node assay (LLNA) was developed as an alternative method for the identification of chemicals that have the ability to cause skin sensitization and allergic contact dermatitis. The assay now has been evaluated extensively in the context of both national and international inter-laboratory collaborative trials and has been the subject of detailed comparisons with guinea pig test methods and human skin sensitization data. On the basis of these evaluations the LLNA has been endorsed recently by the US Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) as a stand-alone method for skin sensitization testing. The assay offers a number of important benefits compared with conventional guinea pig test methods, among these being provision of an objective and quantitative endpoint. Moreover, the LLNA provides advantages in the context of animal welfare; compared with guinea pig tests, fewer animals are required and these animals are subject to less trauma. It is important now that the validation status of the LLNA is recognized and the method applied widely so that these advantages may be realized. Hazard identification represents only the first step in the risk assessment process. A full toxicological evaluation of skin sensitization activity requires an understanding of relative potency. Guinea pig methods do not lend themselves readily to assessment of potency, and interest recently has focused on the utility of the LLNA for this purpose. Contained within this review article are brief descriptions of the history of the LLNA and the immunobiological basis for the method, together with detailed accounts of the conduct and interpretation of the assay. Procedural modifications to, and alternative endpoints for, the LLNA are considered also. Finally, the current regulatory status of the LLNA is summarized and the application of the method for the purposes of defining relative potency and developing risk assessments is reviewed.

A comparison of statistical approaches to the derivation of EC3 values from local lymph node assay dose responses

Effective risk assessment and management of allergic contact dermatitis require three key factors: adequate hazard identification, measurement of the relative potency of identified hazards and an understanding of the nature, extent and duration of exposure. Suitable methods for hazard identification, such as the murine local lymph node assay (LLNA) and the guinea-pig maximization test, are well established and conditions of human exposure normally can be well anticipated. Thus, the need is for a robust and quantitative method for the estimation of relative skin sensitizing potency. One possible approach is via the analysis of LLNA dose-response data. In the LLNA, contact allergens are defined currently as those chemicals that cause a threefold or greater increase in lymph node cell proliferative activity compared with concurrent vehicle-treated controls. It is possible to estimate the concentration of a sensitizer required to generate a threefold stimulation of proliferation in draining lymph nodes; such a concentration is known as the EC3 value. Using a variety of statistical approaches to derive EC3 values from LLNA dose-response data for 10 chemicals, it has been demonstrated that simple linear interpolation between the values either side of the threefold stimulation index provides a robust assessment of the EC3 value without the need for recourse to more sophisticated statistical techniques. Provided that the appropriate concentrations of test chemical have been selected, EC3 values obtained in this way are reproducible both within and between laboratories and form the basis for examination of the utility of this approach for the estimation of relative skin sensitizing potency.

Thresholds in contact sensitization: theoretical and practical considerations

The existence of thresholds for both the induction and the elicitation of contact sensitization is an important issue for hazard assessment in this area of toxicology. In this paper, the evidence for such thresholds has been reviewed from both theoretical and practical perspectives. When the mechanisms of skin sensitization are considered, the existence of thresholds can be shown to occur at several stages. They are both quantitative, for example in terms of the degree of protein haptenation and in the sufficiency of the signals for Langerhans cell migration and maturation, as well as qualitative, in terms of the type of immune response that is engaged. Such considerations are evidenced by a substantial body of practical observation. In humans and in animal models of skin sensitization there is abundant evidence for the existence of thresholds for both the induction and the elicitation of reactions. In addition to, and in distinction from, the experimental situation, in the general human population there is extensive evidence for threshold effects. This evidence arises directly from the observation that only a proportion of those who are exposed become sensitized (i.e. are patch test positive), and of that latter group only a proportion develop allergic contact dermatitis.

Aliphatic and alicyclic diols induce melanogenesis in cultured cells and guinea pig skin

We have found that several aliphatic and alicyclic diols induce melanogenesis in cultured S91 mouse melanoma cells and normal human epidermal melanocytes (NHEM). In addition, these compounds induce melanogenesis when applied to guinea pig skin, with transfer of melanin to keratinocytes and formation of "supranuclear caps," as occurs in naturally pigmented skin. The relative order of potency of some of these diols in NHEM is 5-norbornene-2,2-dimethanol > 3,3-dimethyl-1,2-butanediol > cis-1,2-cyclopentanediol > 2,3-dimethyl-2,3-butanediol > 1,2-propanediol. Following treatment with these diols or 3-isobutyl-1-methylxanthine, melanin and tyrosinase activity are increased within S91 cells and NHEM; however, for cultured NHEM, the largest increases of melanin and tyrosinase occur in an extracellular particulate fraction, shown by electron microscopy to consist almost entirely of stage III and IV melanosomes. These results indicate that cultured NHEM treated with diols export melanosomes in a fashion that is commensurate with natural melanogenic processes. In contrast, S91 mouse melanoma cells exhibit aberrant melanosomal trafficking, in accordance with the known defect in myosin-V mediated melanosomal transport. Both S91 cells and NHEM exhibit morphologic changes and growth arrest indicative of differentiation following treatment with diols. The diols described in this report are candidates for use as cosmeceutical tanning agents.

Skin sensitization thresholds: determination in predictive models

For many years, test methods for the prospective identification of skin sensitizing chemicals have been widely available. However, although these techniques have permitted the identification of the great majority of skin sensitizers, their use in assessing the relative potency of a particular chemical as a human contact allergen has not been well described. A primary reason for this is the inherent difficulty of such an exercise. A complex phenomenon involving interactions between the vehicle, the allergen, the skin and its inflammatory responses takes place during the induction and elicitation of sensitization. All these factors can have a profound effect on the threshold values determined for a skin sensitizer. Consequently, whether the assessment is conducted in humans or in animal models, a threshold concentration is always a function of the method of measurement as much as the potency of the allergen. Although an exhaustive review has not been carried out, this paper considers the attempts that have been made to assess relative potency by the measurement of dose-response relationships and the determination of induction and elicitation thresholds in both animal models and in humans. The latter has special relevance for regulatory toxicology and this matter is given particular attention in this article. Finally, recommendations are made: (a) that threshold concentrations for skin sensitizers should be determined on a case by case basis in relation to the likely mode of skin contact; (b) where the data are used in comparisons of skin sensitization potency, then there should be standardization of the method used for the determinations.