Experimental nickel sensitization in the guinea pig: comparison of different protocols

Nickel: Intrinsic Skin Sensitization Potency and Relation to Prevalence of Contact Allergy

Nickel remains the most commonly identified contact allergen. However, it has proven difficult to demonstrate significant skin-sensitizing activity for nickel in toxicology tests, which typically have indicated a weak skin sensitization potential. Information indicates that in vivo assays are not predictive of dermal sensitization hazard or potency for nickel due to a human-specific mechanistic route for nickel sensitization that animals lack. A similar rationale will apply to in vitro alternatives-although these currently have limited ability to determine intrinsic potency. Generally, in silico methods are not designed for metal allergens and cannot contribute to the analysis. For ethical reasons, human experimental work has been limited, with a single study suggesting moderate potency. Accordingly, it seems reasonable to conclude that the high frequency of contact allergy to nickel in humans is a function of both its intermediate potency coupled with a high level of dermal exposure, particularly to damaged/inflamed skin.

[Evaluation of nickel allergy in mice]

This study was performed to establish a convenient mouse model for the evaluation of nickel allergy. For sensitization, 0.2 g of nickel sulfate in petrolatum was applied 4 times every other day to the shaved dorsal skin. Seven days after the first application of the nickel sulfate salt sample, the antigen-specific metal allergy reaction was estimated based on the swelling response of the footpad injected with 20 microliters of nickel sulfate salt in saline. Percutaneously applied nickel powder in petrolatum, as well as the original nickel salt sample, induced a significant nickel allergy reaction. Therefore, it is suggested that this system is applicable for the evaluation of antimetal allergic substances.

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.

Prevention of nickel-induced allergic contact reactions with pentoxifylline

In this study, we investigated the effect of pentoxifylline, an inhibitor of TNF-alpha, on the contact sensitivity response induced by nickel. For induction, open epicutaneous sensitization by NiSO4. 6 H2O (25% aq.) solution was applied on the backs of 38 albino guinea pigs 5 days a week for 4 weeks. NaCl (0.9%) solution was applied epicutaneously to 10 albino guinea pigs as a control group. 19 were sensitized by nickel and developed positive patch test reactions. Patch tests were repeated after 10 of the sensitized pigs were given pentoxifylline 20 mg/kg/day orally. At the end of this study, only 2 positive patch test reactions were observed in the pentoxifylline-treated group, while 7/9 of the untreated guinea pigs developed positive reactions. These results suggest that pentoxifylline inhibits the contact sensitivity response induced by nickel only during drug administration.

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.

Detection of contact sensitivity of metal salts using the murine local lymph node assay

The local lymph node assay (LLNA) is a predictive test for the detection of contact allergens. Nickel and chromium sensitization are common cases in man. However, in a previous study topical application of nickel sulfate and potassium dichromate in aqueous solution failed to induce activation in the draining lymph node. This study describes the application of LLNA to evaluate the contact sensitivity of metal salts. The metal salts were applied in dimethylsulfoxide or aqueous ethanol solution. In some experiments, the skin of the ears was gently abraded using a needle prior to application of metal salts. The ability of seven metal salts to induce lymph node cell (LNC) proliferation was compared. Nickel, cobalt, chromium and copper salts increased LNC proliferation, whereas zinc, manganese and iron salts failed to induce LNC proliferation in this assay.

Nickel-sulphate-induced contact dermatitis in the guinea pig maximization test: a dose-response study

Nickel sulphate is a sensitizer in guinea pigs, but the frequency of sensitization varies from study to study. The dose-response relationship for NiSO4.6H2O was evaluated in the guinea pig maximization test in this study. 6 intradermal (0.01%-3.0% aq.) and 6 topical (0.25%-10.0% pet.) concentrations were chosen for induction and NiSO4.6H2O 1% pet. was used for challenge, based on the absence of skin irritation in a pilot study. Blind reading was performed. A logistic dose-response model was applied to the challenge results. At 48 h, a linear relationship was obtained between the intradermal induction dose (but not topical dose) and the response, resulting in a maximum sensitization rate of 40% after 3% i.d. The reactivity disappeared at re-challenge 1 week later. Following a booster closed patch on day 35, using NiSO4 10% pet., the animals were challenged with NiSO4 2% pet. and statistical analyses of 72-h readings revealed a non-linear dose-response relationship, giving a maximum response frequency of 40% after initial induction with NiSO4 3% i.d. and 2% topical.

Contact sensitization of mice to nickel sulphate and potassium dichromate

The ability of nickel sulphate and potassium dichromate to induce contact sensitization in mice has been investigated. Topical exposure of mice to 0.5% potassium dichromate in dimethylsulphoxide (DMSO) under occlusion resulted in specific contact sensitization in each of 7 experiments. Although sensitization to nickel sulphate under the same conditions was weak and somewhat variable, evidence for significant levels of contact allergy was obtained in 4 of 7 experiments. In addition, repeated topical application of both nickel sulphate and potassium dichromate in DMSO was found to result in the induction of lymphocyte proliferative responses in lymph nodes draining the site of exposure. These data indicate that epicutaneous exposure to both metal salts may cause specific contact sensitization in mice.

Use of an optimized in vitro lymphocyte blastogenesis assay to detect contact sensitivity to nickel sulfate in mice

This study was designed to determine whether an optimized in vitro lymphocyte blastogenesis assay for identification of strong contact sensitizers would also detect sensitization to the weaker, clinically relevant allergen nickel sulfate (NiSO4). Though NiSO4 is effective in eliciting allergic skin reactions in patients sensitized to nickel-containing metals, it has been difficult to assess its potential to induce sensitization using standard or developmental in vivo animal tests. In vitro lymphocyte blastogenesis has been useful in the diagnosis of nickel allergy in humans, but has not been applied to predictive testing in animals. We used a previously optimized lymphocyte blastogenesis assay to determine whether lymphocytes from NiSO4-treated mice would exhibit NiSO4-specific proliferation and whether this would correlate with an in vivo ear swelling response. BALB/c mice given repeated open induction applications of NiSO4 were ear challenged, then lymphocytes from the draining nodes were cultured with Langerhans cell-enriched epidermal cells (EC), EC + soluble NiSO4, or NiSO4-modified EC (modified by preincubation with NiSO4). The NiSO4-modified EC stimulated significant NiSO4-specific proliferation. EC + soluble NiSO4 stimulated a nonspecific blastogenic response in lymphocytes from both NiSO4-treated and naive mice. There was no ear swelling response to NiSO4 using standard challenge procedures. However, exaggeration of the challenge procedure by gently abrading the ears just prior to NiSO4 application resulted in significant ear swelling, thereby supporting the conclusion that the in vitro results were indicative of in vivo sensitization.

A review of the Buehler guinea pig skin sensitization test and its use in a risk assessment process for human skin sensitization

The Buehler test is a valuable procedure for screening the sensitization potential of chemicals prior to human exposure. Our experience of over 20 years has shown it to be effective in detecting strong, moderate, and most weak sensitizers. The topical exposure inherent in the Buehler test allows it to be utilized to investigate dose responses, cross reactivity between structurally related chemicals, and the sensitization potential of contaminants in raw material mixtures. For safety assessment purposes, Buehler test results provide an initial indication of the sensitization potential of the material in question under relevant, but exaggerated, exposure conditions. These results can be compared to results on benchmark chemicals to assess sensitization risk for subsequent human exposure. Optimizing the sensitivity of the Buehler test requires adherence to the published methodology and proper interpretation of the challenge and rechallenge data obtained. Adjuvant-type test methods are generally considered to be more sensitive than topical methods. However, when done properly, topical test procedures such as the Buehler test or the open epicutaneous test can accurately detect most chemicals with any realistic potential for sensitizing humans by the topical route. Moreover, from a risk assessment perspective, these topical tests avoid the problems of overestimating the weak sensitization potential of many topically applied materials or underestimating the sensitization potential of very strong sensitizers; both are potential concerns with invasive adjuvant-type test methods. The Buehler test or other topical test methods are particularly valuable for comparative sensitization risk assessment since human sensitization data on benchmark materials are all derived from topical exposure. The risk assessment is developed by comparing the guinea pig data on the new material versus relevant benchmark chemicals or formulations and also by evaluating the existing human sensitization data on the benchmark material. These data are then utilized to predict human sensitization risk from topical exposure to the new material. Confirmation of human safety can be derived from human repeat insult patch testing (HRIPT) and other clinical tests such as the product use test and the diagnostic patch test. Utilized in this manner, the Buehler test is an integral component of an overall skin sensitization safety assessment program for a new chemical or product formulation.

A risk assessment process for allergic contact sensitization

This review describes an approach that has been used to assess the skin sensitization risk of new chemicals and product formulations prior to launching the new chemical or product on the market. The risk assessment process utilizes a comparative toxicological approach, in which data on the inherent toxicity of a material, and the exposure to it through manufacturing or consumer use or foreseeable misuse, are integrated and compared with data generated by 'benchmark' materials of similar chemistry or product application, or both. This approach has been valuable in providing an accurate assessment of the skin sensitization potential for a wide range of consumer products and pharmaceuticals, ranging from products with a very transient skin exposure (e.g. some paper products), to cosmetics, to products whose ingredients may be deposited on fabrics and thus result in chronic skin exposure. The risk assessment process described includes both guinea-pig (Buehler) and human skin sensitization test methodologies to evaluate inherent toxicity under relevant epicutaneous exposure conditions. Alternative guinea-pig test methods have been reported to be more sensitive than the Buehler method, particularly those employing intradermal injection of the test material in Freund's complete adjuvant (e.g. maximization test). However, by bypassing the skin barrier at induction, these methods can overstate the sensitization risk of epicutaneous exposure to weak sensitizers (Andersen and Hamann, 1983 and 1984; Matsushita et al., 1975a,b), and can understate the risk to very strong sensitizers possibly through tolerance induction (Buehler, 1985). In addition, materials are tested and classified at concentrations that may not be relevant to anticipated human exposure. The Buehler guinea-pig test data are important in assessing skin sensitization risk in the early phases of product development, where human exposure can be limited, controlled and monitored (e.g. manufacturing employees). The Buehler test can often define consumer skin sensitization risk; however, the ultimate consumer skin safety assessment should in general be developed through a series of controlled human tests; the human repeat insult patch test and, when necessary, the provocative or extended product use tests. Post-market monitoring of skin-related consumer comments is the final phase of the data gathering process. These results can be used to assess further each product and to provide valuable feedback to confirm the validity of the overall risk assessment process. Risk assessment for skin sensitization potential is seldom a simple process.(ABSTRACT TRUNCATED AT 400 WORDS)

Toxicity and carcinogenicity of nickel compounds

The toxicity and carcinogenicity of nickel compounds are considered in three broad categories: (1) systemic toxicology, (2) molecular toxicology, and (3) carcinogenicity. The systemic toxicity of nickel compounds is examined based upon human and animal studies. The major organs affected are discussed in three categories: (1) kidney, (2) immune system, and (3) other organs. The second area of concentration is molecular toxicology, which will include a discussion of the chemistry of nickel, its binding to small and large molecular weight ligands, and, finally, its cellular effects. The third major area involves a discussion of the carcinogenicity and genotoxicity of nickel compounds. This section focuses on mechanisms, using studies conducted in vivo and in vitro. It also includes a discussion of the assessment of the carcinogenicity of nickel compounds.

Experimental sensitization of guinea-pigs to nickel and patch testing with metal samples

Two groups each of 30 guinea-pigs were painted on 5 days/wk for 4 wk with 1% nickel sulphate in lanolin. The dose was applied to the shaved dorsal skin, which was prepared daily before treatment with sodium lauryl sulphate and injected intradermally each week during this period with 0.1 ml 1% potassium alum in distilled water, close to the site of NiSO4 application. Of the two groups of guinea-pigs submitted to this epicutaneous procedure, 63 and 80% developed skin allergy in response to challenge with 2% NiSO4 after a rest period of 2 wk, whereas no sensitization response was elicited by 1% NiSO4 in the guinea-pig maximization test. When two further groups were sensitized by the epicutaneous procedure, the first group, with a sensitization rate of 52% at the first challenge, showed no decline in response with five successive monthly challenges. Addition of NiCl2 to the drinking-water of the second group did not increase the sensitization rate induced by the monthly rechallenges with NiSO4. In guinea-pigs allergic to nickel, plating of Ni-coated brass discs with chromium as a special surface treatment prevented the occurrence of the contact allergy elicited by brass discs coated with Ni alone, whereas plating with gold/copper/cadmium did not.