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

Local lymph node assay with non-radioisotope alternative endpoints

The local lymph node assay has recently been accepted by regulatory agencies as a stand-alone alternate method for predicting allergic contact dermatitis. To compare the sensitivity of non-radioisotope methods with that of the standard assay, we determined if these modified methods would affect evaluation of sensitization potency. For this reason, we used 2,4-dinitrochlorobenzene (DNCB) and benzocaine for different sensitizing criteria. Female CBA mice were treated for 3 days with a test compound or vehicle applied to each side of both ears. Bilateral auricular lymph node proliferative activity was assessed by the following endpoints with incorporation of 3H-methyl thymidine (3H-TdR), bromodeoxyuridine (BrdU) in vivo, and BrdU ex vivo, IL-2 production, and proliferating cell nuclear antigen (PCNA) expression. Ear thickness was also tested. The strong sensitizer DNCB was detectable by any of the non-radioisotope endpoints as well as by radioisotope-dependent standard assay. On the other hand, when evaluating the weak sensitizer benzocaine, significant changes were evident in BrdU incorporation ex vivo and in vivo, and IL-2 production. We believe that these non-radioisotope methods can assess allergic contact dermatitis caused by chemicals even in the laboratory, where it can be difficult to handle radioisotopes.

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.

A nonradioisotopic endpoint for measurement of lymph node cell proliferation in a murine allergic contact dermatitis model, using bromodeoxyuridine immunohistochemistry

INTRODUCTION

The murine local lymph node assay (LLNA) was developed as an alternative to guinea pig models for the assessment of the xenobiotic contact sensitization potential. However, it would be advantageous to have an alternative endpoint to the usual radioisotopic-dependent measures. In the present study, we investigated the development of a nonradioisotopic endpoint for LLNA using immunohistochemistry.

METHODS

Female Balb/c mice were treated by the topical application of strong sensitizers, 2,4-dinitrochlorobenzene (DNCB) and toluene diisocyanate (TDI), and a strong irritant, sodium lauryl sulfate (SLS), on the dorsum of both ears once daily for three consecutive days. The proliferation of cells in the auricular lymph node and ears was analyzed by means of the labeling index (LI) of bromodeoxyuridine (BrdU) incorporation into cells.

RESULTS

Skin reactions, consisting of increased ear thickness and the presence of inflammatory cell infiltrates, were observed in mice treated with DNCB and TDI. The cell number and the weight of the lymph nodes in the mice treated with the allergens, DNCB and TDI, were increased compared to vehicle control. We observed an increase in the areas of the B220(+) cells in the lymph nodes of mice treated with allergens, as determined by immunohistochemistry. There was an increase in the percentage of B220(+) cells in mice treated with DNCB and TDI compared to the vehicle control, but not in those treated with SLS. Because we observed an increase in the percentage of B cells in the allergen-treated group, we measured the stimulation index (SI) in the cortex and medulla (C+M) of the lymph node. The SI values of the C+M in the lymph nodes of the mice treated with DNCB and TDI were increased more than threefold compared with that of the control. However, the SI of the C+M in the lymph nodes of the mice exposed to 25% SLS was not significantly increased compared to the vehicle control, although the lymph node weight of the SLS group was significantly increased.

DISCUSSION

In Balb/c mice, BrdU immunohistochemistry showed its potential use for the identification and differentiation of chemicals with the capacity to induce irritation and sensitization. The results suggest that the measurement of the SI in the cortex and medulla of the lymph node using BrdU immunohistochemistry could provide a useful method to screen irritants and allergens.

Local lymph node assay - validation, conduct and use in practice

The validation of alternative methods is a relatively new activity in toxicology. The local lymph node assay (LLNA), a novel method for the identification of chemicals that have the potential to cause skin sensitization, was the first test to pass through the formal regulatory validation process established in the USA under the auspices of ICCVAM, the Interagency Coordinating Committee on the Validation of Alternative Methods. ICCVAM approved the LLNA as an alternative to guinea pig tests for the identification of skin sensitisation hazards. In this report, we explore the nine recommendations made by ICCVAM and discuss their interpretation in relation to the new OECD Guideline 429, which describes the LLNA. In particular, the value and limitations of the use of statistical evaluation of data and of the inclusion of routine positive controls is examined. It is concluded that the OECD Guideline as currently written embodies the necessary flexibility to permit conduct of the LLNA in a manner necessary to meet the varying needs of regulatory agencies and toxicologists around the world.

ICCVAM evaluation of the murine local lymph node assay. The ICCVAM review process

New test methods are being developed to improve the prediction of human and environmental risks and to benefit animal welfare by reducing, refining, and replacing animal use. Regulatory adoption of new test methods is often a complex and protracted process, requiring test method validation, regulatory acceptance, and implementation. Assessments of new test methods have not always been uniform within or among regulatory agencies. Thus, there have been increased pressures for a harmonized approach to test method evaluation and acceptance. In 1997, in response to these pressures and to U.S. Public Law 103-43, the National Institute of Environmental Health Sciences (NIEHS) established the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) to coordinate interagency consideration of new and revised test methods. This article describes the validation and acceptance criteria and process used for the first test method evaluated by ICCVAM, the murine local lymph node assay (LLNA). Based on ICCVAM's conclusions and recommendations, the LLNA has been accepted by U.S. regulatory agencies as a stand-alone assay for allergic contact dermatitis. Two related articles in this series of three present the results of the independent peer review evaluation of the LLNA and summarize the performance characteristics of the database substantiating the validity of the LLNA.

ICCVAM evaluation of the murine local lymph node assay. Conclusions and recommendations of an independent scientific peer review panel

The validation status of the murine local lymph node assay (LLNA), a method for assessing the allergic contact dermatitis potential of chemicals, was evaluated by an independent peer review panel (Panel) convened by the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). The LLNA measures lymphocyte proliferation using incorporation of radioactive thymidine or iododeoxyuridine into cells of the draining lymph nodes of mice topically exposed to a test article. The Panel concluded that the assay performed as well as currently accepted guinea pig methods [guinea pig maximization test (GPMT)/Buehler assay (BA)] for the hazard identification of strong to moderate chemical sensitizing agents, but that it might not correctly identify all weak sensitizers or metals (potential false negative response) or all strong irritants (potential false positive response). The Panel concluded also that the LLNA involves less pain and distress than conventional guinea pig methods. The Panel unanimously recommended the LLNA as a stand-alone alternative for contact sensitization hazard assessment, provided that certain protocol modifications were made. These included collection of individual, rather than pooled, animal response data; the inclusion of a concurrent positive control; and consideration of dose-response information and statistical analyses. A standardized LLNA protocol is provided.

Development of non-radio isotopic endpoint of murine local lymph node assay based on 5-bromo-2'-deoxyuridine (BrdU) incorporation

Allergic contact dermatitis is a serious health problem. Over the last decade, the murine local lymph node assay (LLNA) has been developed to detect chemical allergens, and international validation studies have been conducted. We have tried to establish an alternative non-radioisotopic endpoint for the LLNA by using 5-bromo-2'-deoxyuridine (BrdU) incorporation in place of radioisotopes, such as [3H]thymidine, employed in the standard method. BrdU was given as a single administration at 5 mg/animal 2 days following three consecutive daily applications of a test chemical. BrdU incorporation into draining lymph node cells was measured using an enzyme immunosorbent assay technique. In this study, p-benzoquinone(PBQ), trimellitic anhydride (TMA), citral(CT) and dextran (DEX) were used as pilot chemicals. PBQ, TMA and CT, which are classified as moderate to strong sensitizers in the guinea pig maximization test and were positive in the original LLNA, were also found to elicit positive responses in the alternative LLNA using BrdU incorporation. In contrast, DEX tested negative in the modified assay consistent with previous guinea pig and LLNA data. Consequently, the modified LLNA endpoint using BrdU incorporation may represent a useful alternative to the standard assay in situations, where there is a need to avoid the use of radioisotopes.

Analyses of cutaneous fluoroquinolones photoreactivity using the integrated model for the differentiation of skin reactions

Currently available test models for the differentiation of photoallergic and photoirritant reactions are extremely time consuming and the protocols are very heterogeneous. In vitro tests are of proven value in predicting irritant or toxic effects, but these tests fail to predict chemical-induced allergic side effects. We developed test systems for this endpoint which is not easily detected by existing assays. In a previous publication we were able to discriminate between a contact sensitizer and a skin irritant with a combination of primary ear swelling analysis and cell counting of the ear-draining lymph nodes [Toxicol. Appl. Pharm. 153 (1998) 83; Arch. Toxicol. 73 (2000) 501]. This combination of tests was called the Integrated Model for the Differentiation of chemical-induced allergic and irritant Skin reactions (IMDS). In addition, it had been shown before that inclusion of UV irradiation in the local lymph node assay enables discrimination of photoallergic from photoirritant reactions after dermal application [Photodermatol. Photoimmunol. Photomed. 10 (1994) 57]. Because of the fact that fluoroquinolones are known to induce photoreactions after oral but not dermal treatment, the aim of the present study was to apply the IMDS for the fast and reliable differentiation of photoreactions due to fluoroquinolones after oral treatment. Enoxacin, lomefloxacin, ofloxacin, sparfloxacin and BAY y 3118 were tested in this system. We found a good correlation between the results of UV light-irradiated IMDS and a guinea pig model with the quinolones as far as photoirritancy was concerned. This holds true also for the photoallergic standard olaquindox and the photoirritant standard 8-methoxypsoralen. However, in contrast to the guinea pig assays the IMDS is fast and extremely predictive for the risk of both photosensitization and photoirritancy depending on the route of exposure. Thus, the UV light-irradiated IMDS turned out to be a good tool for the preclinical risk assessment procedure in terms of discriminating photoreactions. In addition, flow cytometric analyses were used to underline the fact that antigen-independent activation occurred after the induction of photoirritant reactions.

A quantitative method for assessing the sensitizing potency of low molecular weight chemicals using a local lymph node assay: employment of a regression method that includes determination of the uncertainty margins

Risk assessment of sensitizing chemicals requires, besides hazard identification, the assessment of potency. To examine the sensitizing capacity of low molecular weight chemicals, a murine local lymph node assay (LLNA) was used. The sensitizing capacity of known allergens was quantified by dose-response modeling. At a stimulatory index (SI) of 3, the corresponding estimated concentration was calculated (EC(3)), together with a confidence interval to take account of the quality of the particular data set. We tested ten allergens (ethyl-p-aminobenzoate (benzocaine), diethylamine (DEA), 2,4-dinitrochlorobenzene (DNCB), 2-mercaptobenzothiazole (MBT), 4-ethoxymethylene 2-phenyl oxazol-5-one (oxazolone), phthalic anhydride (PA), toluene diisocyanate (TDI), trimellitic anhydride (TMA), tetramethylthiuramdisulfide (TMTD) and zincdimethyldithiocarbamate (ZDMC)). Oxazolone showed the strongest sensitizing potency followed in this order by DNCB, TDI, TMA, PA, TMTD, ZDMC, MBT, benzocaine and DEA. The approach performed in this study is a way to accurately assess the potency of sensitizing chemicals and thus a possibility for classification.

In vitro evaluation of the sensitization potential of weak contact allergens using langerhans-like dendritic cells and autologous T cells

Contact hypersensitivity is a major public health concern in most industrial countries, which is why predictive tests which could identify potential allergens are needed. We have established an in vitro approach for the detection of primary immune response. This model uses Langerhans-like dendritic cells (LLDC) derived from cord blood progenitors and autologous T lymphocytes, isolated from the same blood sample. Treatment of day 12-14 LLDC, with strong haptens trinitrobenzene sulfonic acid (TNP), fluorescein isothiocyanate (FITC) or Bandrowski's base (BB), results in the proliferation of T lymphocytes, whereas weak allergens and irritants, such as sodium dodecyl sulfate (SDS) are ineffective. The use of immature (day 8) LLDC and the addition of a 48 h stage of incubation after hapten contact, result in phenotypic maturation of LLDC in addition to lymphocyte activation in all the cultures with strong haptens. The 48 h stage of incubation, results in sensitization and in some cases the induction of T cell proliferation to citronellal (1/8), coumarine (1/8) and to a prohapten p-phenylenediamine (pPDA; 2/8). The phenotype of DC after 48 h of contact with a strong hapten, becomes that of mature DC (CD83(+), CD86(+) and HLA-DR(++)). With fragrance molecules, weak haptens and prohaptens, a comparable phenotype is observed only when T lymphocytes are activated. These data suggest that the unresponsiveness observed with weak haptens, may be the consequence on an incomplete maturation of LLDC.

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.

Cytokine fingerprinting: characterization of chemical allergens

Chemical allergy is a common and important occupational health issue. Allergic sensitization induced by chemicals may take a variety of forms, including allergic contact dermatitis (skin sensitization) and allergic asthma and rhinitis (sensitization of the respiratory tract). There is a need to identify and characterize chemicals that have the potential to cause such sensitization reactions. Although a number of methods are available for the prospective analysis of skin sensitizing activity, there are currently no widely accepted tests for the identification of chemical respiratory allergens. We here describe a novel approach, cytokine fingerprinting, that has the potential to distinguish between chemical contact and respiratory allergens. The pattern of cytokine production by draining lymph node cells (LNCs) is evaluated following repeated topical exposure of mice to test chemicals. Experience to date reveals that contact allergens stimulate the selective development of type 1 immune responses associated with the secretion by draining LNCs of interferon gamma (IFN-gamma), but little interleukin-4 (IL-4) or interleukin-10 (IL-10). In contrast, chemical respiratory allergens are found to induce the appearance of preferential type 2 immune responses characterized by IL-4 and IL-10 production, but comparatively low levels of IFN-gamma. It is proposed that cytokine fingerprinting may permit the simultaneous identification and characterization of those chemicals that have the potential to cause allergic sensitization.

Cytokine endpoints for the local lymph node assay: consideration of interferon-gamma and interleukin 12

The murine local lymph node assay (LLNA) is a method for the prospective identification of contact allergens. Skin sensitization potential is assessed as a function of induced proliferative responses in lymph nodes draining the site of topical exposure measured in situ by incorporation of radiolabelled thymidine ([3H]thymidine). The results of previous investigations have demonstrated that the analysis of [3H]thymidine incorporation represents a robust and reliable endpoint for the LLNA for the assessment of skin sensitizing activity for strong and moderate allergens and, in addition, many weaker sensitizers. The aim of the current experiments was to explore the utility of the production of the cytokines interferon-gamma (IFN-gamma) and interleukin 12 (IL-12) by draining lymph node cells (LNC) as alternative readouts for the LLNA. Animals were exposed to a range of skin sensitizers at two application concentrations. The first of these was chosen on the basis of results from previous investigations to stimulate a strong proliferative response (tenfold or greater increase in proliferation compared with concurrent vehicle controls). The second concentration of test material in each case was the amount of chemical estimated to be necessary mathematically for elicitation of a stimulation index of 3 (EC3 value); the induction of a threefold or greater increase in proliferation is the current criterion for a positive response in the LLNA. In addition, analyses were conducted with para-aminobenzoic acid (PABA), a non-sensitizing chemical shown previously not to induce LLNA responses. Secretion of IFN-gamma and the p40 subunit of IL-12 by draining LNC was measured by cytokine-specific enzyme-linked immunosorbent assay. In parallel experiments, LNC activity was assessed as a function of [3H]thymidine incorporation in situ. All the chemical allergens tested provoked robust proliferative responses, with the stimulation indices recorded at both test concentrations reflecting only small changes in activity compared with previously recorded data. Exposure to vehicle (4:1 acetone:olive oil, AOO) alone resulted in detectable, although variable, expression of both IFN-gamma and IL-12. Treatment with chemical allergen in each case caused a marked increase in IFN-gamma secretion, with particularly vigorous production of cytokine being stimulated following exposure to oxazolone or hexyl cinnamic aldehyde. In contrast, application of chemical allergens was not generally associated with elevated IL-12 p40 secretion. Exposure of mice to PABA did not result in increased IFN-gamma or IL-12 production compared with vehicle-treated controls. In general, however, cytokine secretion did not correlate closely with the induction of LNC proliferation. These data indicate that expression by allergen-activated LNC of IFN-gamma or IL-12 does not provide a reliable or sufficiently sensitive endpoint for the LLNA compared with [3H]thymidine incorporation in situ.

Development of a non-radioactive endpoint in a modified local lymph node assay

A murine local lymph node assay (LLNA) has been developed as an alternative to guinea pig models for contact sensitization testing. Although the LLNA appears to be a little less sensitive than the most stringent of guinea pig assays, it provides a rapid, objective, quantitative and cost-effective method for screening strong contact sensitizers and has advantages with respect to animal welfare. However, a potential disadvantage is the need for the use of radioactive material. We have reported previously that an ex vivo assay based on similar principles to the original in vivo LLNA, but using a non-radioactive endopoint, was valid for the prediction of strong sensitizers. This ex vivo assay was not sensitive enough to allow prediction of moderately potent ones. In this study, we propose a new parameter, Corrected IL-2 Index (CII), for the prediction of moderate sensitizers. To obtain CII the IL-2 release in the supernatant of the cell culture is corrected for lymph node weight ratio and ratio of CD4-positive subset. We found that CII predicted the allergenicity of moderate sensitizers, including the ones recommended by the OECD in guideline 406, such as mercaptobenzothiazole and hexyl cinnamic aldehyde. The allergenicity of metal salts, such as potassium dichromate, ammonium tetrachloroplatinate and cobalt chloride, was also predicted by the CII. We conclude that the use of CII as an index significantly increases the sensitivity of the ex vivo method so that moderate sensitizers may also be detected.

Value of animal models for predicting hypersensitivity reactions to medicinal products

Although hypersensitivity reactions induced by medicinal products and chemicals are relatively common, few predictive models are available. A major difficulty is our currently limited understanding of the mechanisms involved, and efforts should be paid to better defining drug immunogenicity, hapten formation and immune effector mechanisms. A second difficulty is the multiplicity of clinical manifestations presumably due to varying mechanisms. Available models can only predict a few of these reactions. Anaphylaxis models in guinea-pigs can be only used for the safety assessment of macromolecules which are neither humanized or of human origin, whereas guinea-pig or mouse models can detect the majority of human contact sensitizers. In addition to the extensive validation of existing models, promising avenues of research are expected to be found in the use of novel animal models, particularly those using genetically modified animals, such as transgenic and knock-out mice.

Strategies for identifying false positive responses in predictive skin sensitization tests

It is important that predictive toxicological test methods are selective for their intended endpoint and that their limitations are understood and acknowledged. The local lymph node assay (LLNA) is a relatively new predictive test for skin sensitization potential that can replace traditional guinea pig tests and offers significant scientific and animal welfare advantages. However, there has been some concern that certain irritant materials may yield false positive results, although it must be emphasized that false positives also occur in guinea pig methods. Consequently, we have examined the performance in the LLNA of a range of skin irritants, from varying chemical classes and covering a range of irritation potency. The results presented here demonstrate clearly that the majority of skin irritants are negative in the LLNA. These results are reviewed in the context of the occurrence of false positive reactions in the guinea pig maximization test and the strategies for dealing with such results are discussed. The need for careful scientific evaluation of the results in all predictive tests for sensitization is thus emphasized. In terms of specificity, the LLNA has been more fully evaluated than other predictive test methods and is at least as accurate. In terms of animal welfare, objectivity, reproducibility and reliability it is superior to other methods. In summary, all predictive skin sensitization test results should be evaluated in a scientifically rigorous manner and the additional data provided herein further support the adoption of the LLNA as a complete replacement for the traditional guinea pig methods.

Evaluation of the capacity of dendritic cells derived from cord blood CD34+ precursors to present haptens to unsensitized autologous T cells in vitro

Langerhans cells play a key role in contact hypersensitivity reactions. The application of haptens on the skin leads to many modifications of these cells, including the increase of major histocompatibility complex II expression, allogeneic stimulation potency, and migration towards lymph nodes to activate T cells. Moreover, it has been shown that Langerhans cells cultured in vitro are able to prime naive T cells in response to hapten contact. From CD34+ progenitors present in cord blood, we generated dendritic cells of which some presented the phenotypic markers of Langerhans cells. We show that these cells are able to sensitize syngeneic naive (CD45RA+) T cells to haptens such as trinitrophenyl conjugate of trinitrobenzene sulfonic acid (TNP) and fluoroscein isothiocyanate. The response to TNP is higher than to fluoroscein isothiocyanate, whereas sodium dodecyl sulfate, an irritant molecule used as a control, never caused this effect. Phenotypic analysis of cellular suspensions and experiments of cell sorting lead to the conclusion that only CD1a+ cells are able to induce a primary response of syngeneic T cells to TNP or fluoroscein isothiocyanate. Furthermore, we have shown a close relationship between the differentiation state of dendritic cells and their ability to prime T lymphocytes. Dendritic cells are able to present haptens in an efficient manner between day 10 and 14 of culturing CD34+ progenitors, whereas they were efficient in presenting alloantigens from day 6 until after day 20. This dissociation suggests the need of an active metabolic process for hapten presentation in the direct treatment of dendritic cells with haptens. This model of hapten presentation was used for a panel of fragrance molecules and other molecules considered as weaker haptens than TNP and fluoroscein isothiocyanate.

Assessment of the sensitization potential of five metal salts in the murine local lymph node assay

The murine local lymph node assay (LLNA) has been proposed as a predictive test for the identification of sensitizing agents. We used this test to compare the sensitization potential of NiSO4, K2Cr2O7, CoCl2, Na2PtCl6 and BeSO4, salts of metals which have all been associated with allergic contact dermatitis and either bronchial asthma orinterstitial lung disease, by either humoral or cell-mediated allergic mechanisms. BALB/c mice (n = 3 per concentration studied, three concentrations studied per metal) received three daily applications of the metal salt (in DMSO) on the dorsum of both ears. On the fourth day the draining auricular lymph nodes were removed and the incorporation of [3H]-thymidine in the lymphocytes in culture was compared to that of concurrent vehicle-treated control mice, thus enabling to derive a stimulation index (SI), indicative of immunological sensitization potential. Each experiment was performed three times. Oxazolone and toluene diisocyanate, chosen as positive controls, yielded strongly positive SI values (> 20 and > 30 respectively). Na2PtCl6 (SI 2.6 +/- 1.0 at 2.5%), CoCl2 (SI 2.8 +/- 0.5 at 5%) and possibly also K2Cr2O7 (SI 2.1 +/- 1.2 at 0.5%) were positive in the LLNA, whereas NiSO4 (SI 0.9 +/- 0.2 at 5%) and BeSO4 (SI 1.3 +/- 0.6 at 4%) were negative. Although our results are still limited by the fact that only one mice strain was tested, they indicate that there is no strict relationship between the sensitization potential of metal salts, as evaluated in the murine LLNA, and their potential to cause either respiratory or dermal allergic disease. Consequently, caution should be exercised before proposing the murine LLNA as a valid test to predict the sensitization potential of low molecular weight chemicals.

A re-appraisal of the skin-sensitizing activity of 2,4-dinitrothiocyanobenzene

A debate continues regarding the immunological properties of 2,4-dinitrothiocyanobenzene (DNTB). In some investigations this chemical was shown not to cause skin sensitization when applied topically but to induce instead hyporesponsiveness or immunological tolerance. In other studies DNTB was found to cause skin sensitization, but not tolerance. However, this chemical continues to be used to discriminate between the properties of skin sensitizing and non-sensitizing chemicals. This study demonstrates that topical exposure of mice to DNTB induces skin sensitization in mice and that this is associated with the accumulation of dendritic cells in draining lymph nodes and the stimulation of lymph node cell proliferation; the latter responses being of equivalent magnitude to those stimulated by 2,4-dinitrochlorobenzene (DNCB), a chemical known to cause contact sensitization. Moreover, exposure of mice to DNTB, as with exposure to DNCB, resulted in the development of a cytokine secretion pattern by draining lymph node cells (LNC) characteristic of contact allergens. Thus, DNTB and DNCB each induced the production by LNC of high levels of interferon-gamma, but little or no interleukin 4 or interleukin 10. Finally, DNTB was shown in the guinea pig maximization test to behave as an extreme skin sensitizer. These results confirm that DNTB should not be regarded as a universal tolerogen and that it possesses a significant potential to induce contact sensitization. The use of this chemical as a presumptive non-sensitizer and/or tolerogen for the evaluation of the selectivity of new predictive test methods for the identification of contact allergens is therefore considered to be inappropriate.

Evaluation of skin sensitization potential of nickel, chromium, titanium and zirconium salts using guinea-pigs and mice

Contact sensitization capacity of four metal salts, nickel sulphate (NiSO4), potassium dichromate (K2Cr2O7), titanium chloride (TiCl4) and zirconium chloride (ZrCl4), was evaluated using guinea-pigs and mice. In the guinea-pig sensitization tests, we set up an injection concentration to 1% for all chemicals, and changed the challenge concentration. Guinea-pigs were sensitized with NiSO4, K2Cr2O7 and TiCl4. Among the test metal salts, K2Cr2O7 showed the highest sensitization rate and strongest skin reactions. ZrCl4 did not cause any sensitization responses under our experimental conditions. Minimum challenge concentration to cause a skin response was < 0.25% for K2Cr2O7, 0.5% for NiSO4 and 2% for TiCl4, respectively. A sensitive mouse lymph node assay (SLNA) also determined NiSO4 and K2Cr2O7 as a sensitizer. In the SLNA, TiCl4 caused mild lymph node responses, but was classified as a non-sensitizer as well as ZrCl4. Considering these results, the order of sensitization potential was K2Cr2O7 > NiSO4 > ZrCl4. NiSO4- and K2CrO7-sensitized animals did not show any reactions to ZrCl4 and TiCl4. No cross-reaction among these metal salts was found.

The local lymph node assay: a viable alternative to currently accepted skin sensitization tests

The prospective identification of skin sensitizing chemicals is a vital prerequisite for their proper risk management. Traditionally this has been achieved largely by the conduct of guinea pig assays such as the maximization and Buehler tests. These methods are recommended by the Organisation for Economic Cooperation and Development (OECD) and are required by the European Union (EU) for the evaluation of new substances. However, a novel mechanistically based method, the local lymph node assay (LLNA), has been the focus of substantial validation activity in recent years. This material is reviewed in this paper. It is shown that the LLNA has been validated successfully by five interlaboratory assessments as well as by comparisons with guinea pig tests and human data. The method also offers clear advantages to the user in terms of objectivity, time and cost, and delivers important animal welfare benefits. In consequence, it is recommended that the LLNA be formally adopted by the OECD in Guideline 406 and accepted by the EU and US EPA as a method suitable for the classification of the skin sensitizing potential of chemicals.

Experimental assessment of the sensitizing properties of formaldehyde

Formaldehyde causes upper respiratory tract irritation and has been reported in some investigations to be a cause of occupational allergic asthma. The data are equivocal, however, and it has proved difficult to confirm that exposure to formaldehyde induces respiratory sensitization or provokes the production of specific immunoglobulin E (IgE) antibody. In this study the sensitizing properties of formaldehyde were examined experimentally. This chemical elicited strong positive responses in three independent methods for the prospective identification of contact sensitizing chemicals-the guinea pig maximization test, the occluded patch test of Buehler and the murine local lymph node assay. In contrast, in a novel predictive test method for assessment of respiratory sensitization potential-the mouse IgE test-formaldehyde at the same test concentrations was negative. Furthermore, formaldehyde induced in mice a pattern of cytokine secretion by draining lymph node cells inconsistent with the stimulation of IgE antibody responses or respiratory sensitization. These data indicate that, although formaldehyde is a potent contact allergen, it lacks a significant potential to cause sensitization of the respiratory tract.

Further evaluation of the local lymph node assay in the final phase of an international collaborative trial

The local lymph node assay (LLNA) is a method used for the prospective identification in mice of chemicals that have the potential to cause skin sensitization. We report here the results of the second and final phase of an international trial in which the performance of the assay has been evaluated using seven test materials in five independent laboratories. The additional chemicals examined here included compounds which are considered less potent allergens than some of those tested in the first phase of the investigation, and includes hexylcinnamic aldehyde (HCA), a chemical recommended by the Organization for Economic Cooperation and Development (OECD) as a positive control for skin sensitization studies. In each laboratory all skin sensitizing chemicals examined (2,4-dinitrochlorobenzene {DNCB}, HCA, oxazolone, isoeugenal and eugenol) elicited positive responses of comparable magnitude as judged by the derived lowest concentration of test chemical required to elicit a 3-fold or greater increase in the proliferative activity of draining lymph node cells compared with vehicle-treated controls. We observed that sodium lauryl sulphate, considered to be a non-sensitizing skin irritant, also induced a positive response in the assay. Para-aminobenzoic acid (pABA), a nonsensitizing chemical, was negative at all test concentrations in each laboratory. Some laboratories incorporated minor modifications into the standard assay procedure, including the evaluation of lymph nodes pooled from individual mice rather than treatment groups and the use of statistical analyses. The use of statistics did not markedly change the determination of the lowest concentration yielding a positive response. These data confirm that the local lymph node assay is robust and yields equivalent results when performed independently.

Dichloronitrobenzene: a reappraisal of its skin sensitization potential

Unlike the closely related chemical dinitrochlorobenzene (DNCB), which is a very strong contact allergen, dichloronitrobenzene (DCNB) has been widely regarded as a non-allergen and, as such, a useful control for its strongly sensitizing counterpart. Nevertheless, it is still an organic chemical species readily capable of penetrating skin and, rather than being regarded as completely inert, it has even been suggested to react with the immune system in such a way that it induces specific tolerance to its chemical structure. We investigated whether DCNB was in reality a non-allergen, or rather merely a weak contact sensitizer. In both a rigorously conducted guinea pig maximization test and in a modified murine local lymph node assay, DCNB was demonstrated to possess weak sensitizing activity. On this basis, DCNB cannot be regarded as inert with respect to contact allergic potential, and is therefore inappropriate as a negative control in studies of skin sensitization.

Local lymph node activation in rats after dermal application of the sensitizers 2,4-dinitrochlorobenzene and trimellitic anhydride

Five rat strains were compared for their performance in the local lymph node assay (LLNA), a promising test system for the identification of the skin-sensitizing potential of chemicals in the mouse. The contract sensitizer 2,4-dinitrochlorobenzene (DNCB) and the contact and respiratory sensitizer trimellitic anhydride (TMA) were used as model chemicals and responses in rats were compared with those in BALB/c mice. The chemicals were applied to the dorsum of both ears, once daily for three consecutive days; 2 days (mice) or 3 days (rats) thereafter, proliferating cells were labelled by i.p. injection of BrdU 2 hr before the animals were killed. Systemic effects were subsequently assessed by determination of spleen, liver and kidney weights, skin effects by determination of swelling and inflammatory cell infiltration of the ears, and immune effects by determination of weight and proliferative activity of the local lymph nodes (LLN). Following application (x 3) of DNCB or TMA, minor systemic effects were observed, as indicated by slightly elevated spleen and liver weights in a few rat strains and the mice. Skin effects, consisting of increased ear thickness and presence of mononuclear inflammatory cell infiltrates, were observed in all rat strains treated with DNCB or TMA, LLN weights had increased, as had the proliferative activity in these nodes. It was concluded that effects induced by DNCB and TMA in all five rat strains were comparable with those in mice.

An alternative strategy to the use of guinea pigs for the identification of skin sensitization hazard

For over half a century, guinea pig methods have dominated the field of toxicology concerned with the identification of skin sensitizers. Specific protocols, for example the guinea pig maximization test (GPMT), have been pre-eminent in the identification of skin sensitization hazard for regulatory purposes. However, there are increasingly several forces driving change, not least animal use/welfare considerations. In response to this and to address the need for a rapid screen for chemical allergens, an alternative strategy has been developed. In the first instance, a chemical is assessed by a computer-based expert system. This system is constructed from some 50 rules describing the key chemically reactive substructures of known skin sensitizers. The output from the expert system is also evaluated in the light of the understanding of the skin penetration characteristics of the chemical. In this way, and without use of animals, the likelihood that a chemical represents a skin sensitization hazard is assessed based on the two key characteristics of a skin sensitizer: (1) its direct or indirect ability to react with skin protein (i.e. does it contain a structural alert?); and (2) the ability of the chemical to partition into the appropriate epidermal compartment. When the chemical does possess a structural alert and has the capacity to penetrate skin sufficiently, then it may be regarded as a potential skin sensitizer. Subsequent to this screening phase, if necessary the chemical may be assessed in the murine local lymph node assay. This assay is quicker and cheaper than traditional guinea pig assays and importantly is less stressful to the fewer animals that it requires. The assay is well validated and produces objective results which are equivalent to the GPMT in terms of identifying significant skin sensitization hazard. In this paper, the above strategy is described in more detail, focusing on its relevance to hazard identification and its value in animal welfare terms. It is concluded that the strategy provides an important opportunity for both substantial reduction and refinement of animal use in a manner which will not compromise the existing standard of classification and labelling of skin sensitization hazard in the European Union.

An international evaluation of the murine local lymph node assay and comparison of modified procedures

The murine local lymph node assay is a predictive test for the identification of skin-sensitizing chemicals. The method has been the subject both of national inter-laboratory studies and of extensive comparisons with guinea pig tests. In the investigations reported here, the local lymph node assay has been evaluated further in the context of an international study comprising five independent laboratories. In addition, the influence of minor modifications to the standard assay procedure on the performance of the test has been examined. The modified procedures investigated were exposure of mice for 4 rather than 3 consecutive days, excision of lymph nodes 4 rather than 5 days after the initiation of exposure and the use of an alternative isotope. All five laboratories, irrespective of whether the standard or a modified protocol was used, were able to identify accurately, and with comparable sensitivity, potassium dichromate and 2,4-dinitrochlorobenzene as skin sensitizers. Using standard criteria, none of the laboratories recorded positive responses with methyl salicylate, a non-sensitizer. In the standard protocol, lymph nodes are pooled for each experimental group and the vigor of responses measured as a stimulation index relative to vehicle controls. A stimulation index of 3 or greater is considered to indicate skin-sensitizing potential. One further modification adopted by three of the laboratories was to analyze nodes from individual animals and, thereby, permit statistical evaluation. This allowed a direct comparison of statistical significance with the conventional stimulation index as criteria for a positive response. The data indicate that, while statistical evaluation may provide, in some instances, for small increases in sensitivity, this may be at the expense of some loss of selectivity. There are, however, insufficient data presently to draw firm conclusions regarding the relative value of statistical analysis. These studies demonstrate that the local lymph node assay is sufficiently robust to accommodate minor procedural and technical modifications without material changes in test performance.

An evaluation of the suitability of benzocaine as a positive control skin sensitizer

Although they are subject to some limitations, sensitization tests such as the guinea pig maximization test (GPMT) have for many years provided a valuable basis for the identification of skin sensitization potential. Thus they have been used widely by regulatory authorities, such as those in Europe, as a means to identify significant sensitization hazards associated with new chemicals. However, the standard of performance of guinea pig sensitization assays has been demonstrated to be widely variable. Consequently, the OECD sensitization test guideline (the de facto world standard) has been updated to incorporate recommendations for action whose aim is to achieve a minimum standard of test conduct. The principle is that a test laboratory should be able to demonstrate an acceptable level of response using a moderately sensitizing chemical. A list of 3 such chemicals is provided, hexyl cinnamic aldehyde, mercaptobenzothiazole and benzocaine. It is our experience that whilst good results can readily be obtained with the first 2 of these, benzocaine is much more difficult. Using both the GPMT and the local lymph node assay (LLNA), an OECD-recommended screening test, benzocaine has given highly variable results. A range of from 0% to 60% positive in the GPMT was found and, in most tests, benzocaine would not classify as a skin sensitizer according to EU criteria. In the LLNA, from a series of 12 tests conducted in 2 laboratories, only occasional positive results were obtained. Furthermore, these positive results were not reproducible. Reasons for this variability are discussed. However, the main conclusion must be that benzocaine does not represent a useful moderately sensitizing positive control.

A critical commentary and updating of the guinea pig maximization test

The guinea pig maximization test (GPMT) of Magnusson and Kligman was published in 1969. Since then, a vast body of practical experience with the test has been accumulated. New information requires that certain aspects of the procedure be re-evaluated, especially with regard to the interpretation of challenge results. In particular, awareness of the phenomenon of hyperirritable skin (the 'angry back' phenomenon) suggests that presently used controls are not always adequate and may overstate allergenicity owing to false-positive reactions. The control group should be exposed to a chemical insult at induction which provokes an inflammatory reaction comparable to the test substance. We present strategies to distinguish irritant from allergic responses. Allergic reactions should persist on rechallenge weeks later, while nonspecific irritant reactions generally fade and are irreproducible in particular animals. Finally, when a chemical is identified as a contact sensitizer in the GPMT, that result is simply a categorical statement of a theoretical hazard. An assessment of risk is necessary to estimate the relevance of the test result to usage in the real world.

Predictive immunotoxicological test systems: suitability of the popliteal lymph node assay in mice and rats

This article reviews results obtained with popliteal lymph node assays (PLNAs) in rodents and discusses their ability to detect and analyze immunotoxic effects of drugs and other low molecular weight (LMW) chemicals. In its basic form, the PLNA measures activation of the draining lymph node of the hind paw (i.e., the PLN) after injection of a test chemical into the hind foot pad. The assay appears to be appropriate to recognize sensitizing, that is, allergenic and autoimmunogenic, chemicals, as well as nonsensitizing immunostimulatory chemicals. With modifications, PLNAs can detect immunosuppressive chemicals and distinguish sensitizing from nonsensitizing chemicals. Furthermore, modified PLNAs enable detection of known as well as unknown sensitizing metabolites, and may assist in the identification of the self-molecules that act as carriers for chemical sensitization or as targets of chemical-induced autoimmune disease. Experience with PLNAs shows that they are rapid, reproducible, and objective tests for recognition of sensitizing or otherwise immunomodulating chemicals. Because current protocols of toxicity testing are insensitive in predicting a chemical's potential to result in immunomodulation, PLNAs, when further validated, may provide welcome supplements to routine toxicity screening of chemicals, thus enhancing chemical safety.

Assessment of contact sensitivity of four thiourea rubber accelerators: comparison of two mouse lymph node assays with the guinea pig maximization test

Contact sensitivity of four thiourea rubber accelerators, diphenylthiourea (DPTU), dilaurylthiourea (DLTU), dibutylthiourea (DBTU) and diethylthiourea (DETU), was evaluated by a new sensitive mouse lymph node assay (SLNA) and the murine local lymph node assay (LLNA). The results of the SLNA and LLNA were compared with the data of the guinea pig maximization test (GPMT). In the LLNA and SLNA, the sensitizing activity was measured as a function of draining lymph node activation following application of the test chemicals. Of these four thioureas, three (DETU, DBTU and DPTU) were not classified as skin sensitizers in the LLNA. The SLNA successfully detected the sensitivity of all thioureas tested. This result indicated that the SLNA was, in these cases, more sensitive than the LLNA for identification of contact allergens. The order of sensitization potential observed from the SLNA was DPTU (greatest), DLTU, DBTU and then DETU (least). The predictions of sensitizing potential and the order of the sensitizing capacity of four thioureas by the SLNA and the GPMT are very similar.

The local lymph node assay: developments and applications

The murine local lymph node assay is a predictive test method for the identification of contact allergens in which sensitizing activity is measured as a function of induced proliferative responses in lymph nodes draining the site of application. In this article the development and validation of the assay are described and comparisons with guinea pig predictive test methods discussed. In addition we examine the advantages and limitations of the method and consider new opportunities and applications of the assay in the context of the toxicological evaluation of sensitizing potential.

Contact sensitization assays in guinea-pigs: are they predictive of the potential for systemic allergic reactions?

Guinea-pig assays have been used extensively to detect contact sensitizers. In contrast, almost no reliable assays are available to detect the potential for low-molecular-weight drugs and chemicals to induce systemic allergic reactions in humans. Based on clinical data, and, to some extent, on recent immunological findings, it is proposed that guinea-pig assays can predict the hazard for systemic allergic reactions in man. Seventy drugs and chemicals were compared from published results in guinea-pig assays and in the clinic. A close correlation was found with 43 substances and a relatively good one with 16 substances. Conflicting results were found with 11 substances only. However, substances known to induce systemic allergic reactions in man were all detected as weak sensitizers, at least in guinea-pigs. Guinea-pig contact sensitization assays may therefore prove useful until more suitable and specific assays are available to predict the risk for systemic allergic reactions.

The local lymph node assay: an interlaboratory evaluation of interleukin 6 (IL-6) production by draining lymph node cells

The murine local lymph node assay is a predictive method for the identification of skin-sensitizing chemicals in which activity is measured as a function of proliferative activity induced in lymph nodes draining the site of exposure. In the present study, the induction by topically applied chemicals of draining lymph node cell (LNC) production of the cytokine interleukin 6 (IL-6) has been evaluated as an alternative endpoint for the local lymph node assay. In addition, results derived from studies of IL-6 production by LNC performed independently in two separate collaborating laboratories have been compared. Of the nine skin sensitizing chemicals examined, six provoked detectable levels (> 150 pg ml-1) of IL-6 production by draining LNCs (as measured by enzyme-linked immunosorbent assay) following exposure of mice to at least one test concentration of the material in both of the laboratories. Three other sensitizing chemicals failed to induce measurable IL-6 production at any test concentration in either one or both of the participating laboratories. Both of the non-sensitizing chemicals evaluated (sodium lauryl sulphate and methyl salicylate) also failed to result in detectable IL-6 synthesis. There was a high level of agreement between the two laboratories. The rank order of chemicals with respect to IL-6 production by LNCs was identical in both cases, as was the dose-response relationship observed with each test material. These data reveal that, although inducible IL-6 production by draining LNCs provides a robust approach to the measurement of strong sensitizing activity, as performed here the method is of insufficient sensitivity for the routine identification of skin allergens.

The performance of the local lymph node assay with chemicals identified as contact allergens in the human maximization test

For many years, tests in the guinea pig have been the favoured option for the identification of the skin sensitization potential of chemicals. However, the mouse has been used widely in immunology research and can represent a viable alternative. A variety of murine assays have been described, including several methods based on ear swelling as an endpoint. Another option is to assess induced lymph node cell proliferation and it is this which forms the basis of the murine local lymph node assay (LLNA). The LLNA has undergone several successful interlaboratory validations and compares well with standard guinea pig assays. In the present study, the performance of the LLNA was examined with chemicals tested previously in the human maximization test (HMT). 30 chemicals, 23 of which proved positive in the HMT and seven of which were negative, have been tested. All but four of the materials found positive in the HMT also tested positive in the LLNA. Of these four, sulfanilamide and paraben esters would not classify as skin sensitizers in the guinea pig maximization test and nickel has been found to yield variable results in a number of predictive animal tests. Of the seven substances which proved negative in the HMT, six were also negative in the LLNA, the exception being sodium dodecyl sulfate. These data demonstrate that the LLNA is able to identify accurately chemicals which have the potential to cause significant allergic contact dermatitis in humans.

Contact sensitivity to Tinuvin P in mice

The contact sensitizing potential of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (Tinuvin P) was investigated using the murine local lymph node assay (LLNA) and the mouse ear swelling test with some modification in the induction procedure (modified MEST). Topical exposure of the ears to Tinuvin P in acetone:olive oil (AOO) for 3 consecutive days induced no increase in auricular lymph node weight and lymph node cell proliferation. In the LLNA, Tinuvin P was negative, and its sensitizing potential was determined as not strong. In the modified MEST, mice were sensitized with a combination of an intradermal injection of 0.2% Tinuvin P in Freund's complete adjuvant into the shaved abdomen and topical application of 1% Tinuvin P in AOO on both ears for 3 consecutive days. The ear thickness in mice previously treated with Tinuvin P increased significantly on challenge with Tinuvin P. A cross-reaction test between Tinuvin P and various substituted benzotriazole ultraviolet light (UV) absorbers was performed. Challenge with these benzotriazole compounds on the ears of the mice previously sensitized to Tinuvin P showed no increase in ear thickness. These data indicated that Tinuvin P had a contact sensitizing ability, while Tinuvin P-sensitized mice did not cross-react with the other benzotriazole UV absorbers.

The identification of chemicals with sensitizing or immunosuppressive properties in routine toxicology

In the context of this paper, immunotoxicity is taken to encompass immunosuppression/immunopotentiation and allergy. Over the last 10 to 15 years, well characterized methods for the assessment of altered immune competence have been reported. This has led to proposals for tiered testing schemes. This review examines the suitability of immunotoxicity parameters for inclusion in routine 28-day studies and comments on methods that have been proposed for incorporation within the guidelines issued by the US FDA and US EPA and OECD. It is recommended that the existing OECD Guideline 407 is modified to incorporate total and differential blood cell counts, spleen and thymus weight and histopathology, and draining and distal lymph node histopathology for Tier I level testing. Data so generated will provide a reliable and accurate means of identifying at an early stage potential immunotoxic effects. Tier II testing should be carried out on a case by case basis and only assuming positive results are obtained at Tier I. An increasingly sophisticated understanding of the nature of immune responses to chemical allergens has facilitated the design of novel predictive methods for the identification of sensitizing activity. Opportunities which arise from these new developments in allergy testing such as the local lymph node assay, mouse ear swelling test, and the mouse IgE test should be monitored closely.

A sensitive mouse lymph node assay with two application phases for detection of contact allergens

A predictive test using mice for the identification of contact sensitizing chemicals was developed. Contact sensitizing activity is measured as a function of draining lymph node activation following application of test chemical. Experimental conditions for assessment of induced lymph node cell (LNC) responses have been optimized. BALB/c mice were initially treated with intradermal injections of test chemical in Freund's complete adjuvant emulsion. Five days after intradermal injection, mice were exposed topically to chemical in vehicle on the ears daily for 3 consecutive days. Next day following the final exposure, changes in lymph node weight, total cell number in the draining lymph nodes and LNC proliferation for 24 h culture were assessed. The performance of the method was evaluated with ten sensitizing chemicals and a non-sensitizing irritant, sodium lauryl sulfate (SLS). The LNC proliferation induced by combination of intradermal injection and topical application of sensitizing chemicals was more clearly increased than that following only topical application. With the single exception of sulfanilic acid, the method developed was able to detect the sensitizing capacity of chemicals that failed to induce sensitization in the local lymph node assay. Under the conditions used, SLS did not induce measurable lymph node responses. These results suggest that the mouse lymph node assay can provide a sensitive screening test for weak to moderate sensitizers. In addition, the assay offers the advantages of objective and quantitative endpoints, and is suitable for the evaluation of colored or irritant chemicals.

Approaches to the identification and classification of chemical allergens in mice

During the last 10 years understanding of the immunobiological mechanisms whereby chemicals induce allergy has increased significantly. In parallel there have emerged opportunities to develop alternative methods for the prospective identification and classification of chemical contact and respiratory allergens. Attention has focused largely on the mouse, and several novel approaches to the identification of contact allergens have been described, among them the local lymph node assay, the mouse ear swelling test and, most recently, the noninvasive mouse ear swelling assay (MESA). Progress has been made also in defining methods for measuring respiratory sensitization potential in mice. Recent evidence indicates that respiratory and contact allergens provoke qualitatively different immune responses characteristic of selective T helper (Th) cell activation. Evaluation of the nature of immune responses induced in mice by chemicals may permit not only assessment of allergenic potential, but also prediction of the form allergic reactions will take.

Results with OECD recommended positive control sensitizers in the maximization, Buehler and local lymph node assays

The guinea pig maximization test and the Buehler occluded patch test are used widely to identify the sensitization potential of new chemicals. This information enables toxicologists and/or regulatory authorities to determine whether a chemical should be classified formally as a skin sensitizer. Both to improve and to harmonize these assessments internationally, the OECD has recommended recently that moderate rather than strong contact sensitizers are used as positive control substances. The purpose is to ensure an adequate level of sensitivity in sensitization assays performed at specific testing establishments. Results from two laboratories reported here show that the minimum acceptable standard laid down by the OECD can be achieved and indeed commonly exceeded by a substantial margin. Furthermore, results with these positive controls in a new method, the local lymph node assay, also appear to satisfy similar criteria, suggesting results from this assay, including negative data, should be acceptable for classification purposes. However, a review of the way in which results with new chemicals will be interpreted for regulatory purposes, in the context of positive control data, reveals that considerable inadequacies still exist. It is recommended that ultimately, sensitization data can only be interpreted meaningfully (i.e. to protect humans from sensitization hazards) by considering the potency of the contact allergen in the context of the sensitivity of the assay performed at the particular testing institution.

Differences of draining lymph node cell proliferation among mice, rats and guinea pigs following exposure to metal allergens

Contact sensitivities of three well known metal allergens (nickel sulfate, potassium dichromate and cobalt chloride) were examined using the local lymph node assay in CBA/N mice, F344 rats and Hartley guinea pigs. The effect of various species sera on lymph node cell (LNC) proliferation was also investigated. Exposure to potassium dichromate and cobalt chloride induced significant LNC proliferative responses in the three species. The LNC responses to potassium dichromate in the rats were higher than those in the mice and guinea pigs. Mice exhibited the highest response to cobalt chloride among the three species, whereas, exposure to nickel sulfate failed to induce a marked LNC proliferation. Increased draining lymph node weights and LNC numbers were also observed following exposure to the metal salts. However, these parameters were less sensitive compared with the LNC proliferative response. There was a large difference in the lymph node weight between individual guinea pigs. The [methyl-3H]thymidine incorporation into LNC of each species cultured in the presence of the homologous serum in vitro was lower than in the presence or absence of fetal calf serum. However, there was no significant difference in stimulation indices among the different culture conditions. The local lymph node assay may be performed in rats as well as in mice for the detection of metal allergens.

Classification of chemicals as sensitisers based on new test methods

For the last decade, classification schemes worldwide have recognised that certain chemicals may need to be categorised as skin or respiratory sensitisers. Although differing in detail, the schemes use similar criteria for designating materials as sensitisers, based on either direct evidence from exposed humans or the results of predictive guinea pig tests. In the case of respiratory sensitisation, however, there are currently no acceptable animal test methods. With an increasing understanding of cellular immunology in general, and of immune responses in skin and respiratory sensitisation in particular, several laboratories have recently been developing more objective, immunologically-based tests. For skin sensitisation, the two most promising methods are the murine local lymph node assay (LLNA) and the mouse ear swelling test (MEST). Both assays have undergone inter-laboratory validation and it has been shown that they are able to detect reliably moderate to strong sensitisers. The 1992 update of the OECD test guideline for skin sensitisation suggests the use of the LLNA or MEST as a first stage of testing; if a positive result is seen in either assay, a chemical may then be designated (and classified) as a potential sensitiser and it may not be necessary to conduct a guinea pig test. However, if a negative result is obtained, a guinea pig test must be performed. For respiratory sensitisation, although certain guinea pig models of asthma appear to be predictive of the known human response to sensitisers such as diisocyanates and acid anhydrides, the measurement of changes in serum IgE antibodies in mice treated topically with chemicals may represent a simpler and more accurate method of designating chemicals as respiratory sensitisers.

Sulphanilic acid: divergent results in the guinea pig maximization test and the local lymph node assay

The guinea pig maximization test (GPMT) has proven to be a valuable tool for the identification of the skin sensitization potential of chemicals. The method identifies a hazard which can lead in the EC to compulsory labelling of that chemical. In the present study, data on sulphanilic acid derived from the GPMT has been compared with results from a second guinea pig assay (the cumulative contact enhancement test) and the murine local lymph node assay, both of which require only topical application of chemical. Except for the GPMT, no test identified any sensitizing activity associated with exposure to sulphanilic acid. These latter results are consistent with the experience gained from substantial human exposure in an occupational setting and from which no cases of allergic contact dermatitis to sulphanilic acid have arisen over a 20-year period. In consequence, it is questioned which test protocol in practice has given the more accurate identification of sensitization hazard relevant to man.