Testing for contact allergy in experimental animals

Immune sensitization to methylene diphenyl diisocyanate (MDI) resulting from skin exposure: albumin as a carrier protein connecting skin exposure to subsequent respiratory responses

Background

Methylene diphenyl diisocyanate (MDI), a reactive chemical used for commercial polyurethane production, is a well-recognized cause of occupational asthma. The major focus of disease prevention efforts to date has been respiratory tract exposure; however, skin exposure may also be an important route for inducing immune sensitization, which may promote subsequent airway inflammatory responses. We developed a murine model to investigate pathogenic mechanisms by which MDI skin exposure might promote subsequent immune responses, including respiratory tract inflammation.

Methods

Mice exposed via the skin to varying doses (0.1-10% w/v) of MDI diluted in acetone/olive oil were subsequently evaluated for MDI immune sensitization. Serum levels of MDI-specific IgG and IgE were measured by enzyme-linked immunosorbant assay (ELISA), while respiratory tract inflammation, induced by intranasal delivery of MDI-mouse albumin conjugates, was evaluated based on bronchoalveolar lavage (BAL). Autologous serum IgG from "skin only" exposed mice was used to detect and guide the purification/identification of skin proteins antigenically modified by MDI exposure in vivo.

Results

Skin exposure to MDI resulted in specific antibody production and promoted subsequent respiratory tract inflammation in animals challenged intranasally with MDI-mouse albumin conjugates. The degree of (secondary) respiratory tract inflammation and eosinophilia depended upon the (primary) skin exposure dose, and was maximal in mice exposed to 1% MDI, but paradoxically limited in mice receiving 10-fold higher doses (e.g. 10% MDI). The major antigenically-modified protein at the local MDI skin exposure site was identified as albumin, and demonstrated biophysical changes consistent with MDI conjugation.

Conclusions

MDI skin exposure can induce MDI-specific immune sensitivity and promote subsequent respiratory tract inflammatory responses and thus, may play an important role in MDI asthma pathogenesis. MDI conjugation and antigenic modification of albumin at local (skin/respiratory tract) exposure sites may represent the common antigenic link connecting skin exposure to subsequent respiratory tract inflammation.

Guinea-pig contact sensitization assays

After reviewing general aspects and the historical development of predictive animal skin sensitization tests, the recommended tests of the OECD and EC are discussed. Background information is given on the changes which occurred in the last 2 years. From the many factors known to influence induction and challenge of contact allergy in animals, the discussion is focused on the choice of induction concentration and the vehicle. The difference of general screening strategies in Europe and the USA is presented and discussed. The paper closes with a discussion on the importance of inhouse validation of the methods used for proper prediction of the sensitizing potential of chemicals and with remarks on predictive in vitro tests.

Optimization of the mouse ear swelling test for in vivo and in vitro studies of weak contact sensitizers

Murine models for the assessment of the contact sensitizing properties of chemicals rely on mouse ear swelling tests (Mest), which are not sensitive enough to detect weak sensitizers. The aim of the present study was to develop in mice an adjuvant-free Mest appropriate for in vivo detection of any type of sensitizer (weak to strong), and useful for in vitro assessment of contact sensitivity (CS). 3 haptens were tested: dinitrochlorobenzene (DNCB), para-phenylenediamine (pPD) and isoeugenol. We compared various protocols for induction of the CS reaction, differing by the site of induction, the number of applications and the concentrations of the 3 haptens. Comparison of the induction site for optimal CS reaction showed that, in Balb/c mice, the back was a better site of induction than the abdomen. Detection of the sensitizing properties of weak sensitizers (pPD, isoeugenol) was possible using an adjuvant-free protocol, provided that the induction phase comprised hapten applications on 3 consecutive days on the backs of animals. For DNCB, one application was sufficient to obtain optimal CS reaction. For all 3 haptens, a secondary response in vitro was obtained using semi-purified lymph node T cells from animals sensitized 5 days before with the optimized Mest. These results demonstrate that the Mest could be a useful experimental model for the study of all types of contact sensitizers.

Allergenic potential of two orthodontic bonding materials

The sensitization potential of two orthodontic bonding materials, one conventional premix product and one nonmix product, was evaluated by a modified guinea pig maximization test. The nonmix product, consisting of a liquid to be applied to the tooth surface and a paste which was placed on the bracket base, caused reaction in 50% of the tested animals. This material should be considered to be a possible cause of allergy in patients or professionals handling it.

Skin sensitization--a critical review of predictive test methods in animals and man

With the exception of the Draize Test, the guinea-pig test methods currently accepted by regulatory authorities worldwide are well able to predict the potential of a material to cause skin sensitization. Nevertheless, (a) some methods are more sensitive than others (e.g. adjuvant tests are generally more sensitive than non-adjuvant tests); (b) methods cannot be sufficiently standardized to give full reproducibility of results between laboratories; and (c) most methods are based on subjective visual grading of skin reactions--difficulties thus arise when testing coloured or irritant materials. Laboratories must be able to show the sensitivity of the method(s) they use by demonstrating that positive reactions occur with mild/moderate contact allergens rather than the strong/extreme sensitizers currently recommended in certain guidelines, specifically in the EEC Test Method. The sensitivity of the adjuvant tests is such that it is possible to halve the minimum number of animals required by present regulatory guidelines without compromising the capacity of the tests to detect weak/mild sensitizers. A similar review has not yet been made for non-adjuvant tests. Alternative test methods, including some recently developed mouse models, offer several advantages, including more objective endpoints. These tests have not been extensively validated and this precludes their use at present for regulatory purposes other than to confirm the sensitization potential of a material. Two new test methods using mice, the Mouse Ear-swelling Test and the Local Lymph Node Assay, appear promising. They should undergo rigorous interlaboratory testing to determine their sensitivity and specificity. In vitro methods do not represent a viable alternative in the foreseeable future. An approach using quantitative structure-activity relationships is the most likely route to a non-animal model, but this will require considerable research, development and validation. Human sensitization tests have generally not been used for the classification of substances as non-sensitizers. This is because of an absence of internationally agreed test protocols, the lack of positive controls and because the methods for establishing the sensitivity of human tests are less developed than for animal tests. Nevertheless, for products for which direct human contact is intended, predictive tests in human volunteers can be considered. The EEC Directive for the Classification, Packaging and Labelling of Dangerous Substances provides a reasonable approach to the evaluation of skin sensitizers.(ABSTRACT TRUNCATED AT 400 WORDS)

Oral induction of immunological tolerance to chromium in the guinea pig

Metal alloys used in dentistry may elicit adverse side-effects. Contact allergic reactions to metals released from such alloys are among the most frequently encountered problems. In an earlier study, we observed that oral contacts with nickel or chromium salts did not sensitize, but rather decreased the risk of subsequent sensitization to these metals. In the present study, we focused on chromium allergy and extended our earlier observations by further dose-response studies. In addition, we compared different chromium valencies as to their potential oral tolerogenic effects. Development of immunological tolerance in chromium-fed guinea pigs was demonstrated by their inability to develop chromium hypersensitivity after a subsequent immunization attempt. For these studies, the techniques of immunization and skin testing were first improved. One feeding with a high dose of K2Cr2O7 (containing hexavalent chromium) was effective in full tolerance induction. In contrast, trivalent chromium (CrCl3) induced a distinctly lower degree of tolerance, whereas metallic chromium powder was not detectably tolerogenic after a limited number of feedings. Dose-frequency-response studies with K2Cr2O7 showed that full tolerance could also be induced by an increase in the number of feedings with sub-optimal tolerogenic doses. The present results therefore support our hypothesis that long-lasting oral contact with chromium-releasing metal alloys may ultimately result in strong immune tolerance to this metal in subjects without previous skin contact with it. This view is further supported by recent insights into the unique tolerogenicity of oral, as compared with gastro-intestinal, allergenic contacts.

The maximization test for skin sensitization potential--updating the standard protocol and validation of a modified protocol

Since Magnusson and Kligman (1969) first described the maximization test, for assessing skin sensitization potential, the test has been substantially validated and data have been obtained on dose-activity relationships. The original method remains acceptable for qualified risk assessment but several modified protocols have been developed to overcome problems with particular test materials. A modified maximization test is described for compounds that are not soluble in the standard vehicles and, therefore, not injectable, and for formulations, to simulate user exposure conditions. The test compounds 2,4-dinitrochlorobenzene, p-phenylenediamine, 1-(3-chlorophenyl)-3- (4-chlorophenyl)-2-pyrazoline, formaldehyde, benzocain, penicillin G, benzylsalicylate, potassium dichromate and isoeugenol, all gave positive reactions. The incidences of positive reactions varied with the sensitization potentials of the compounds. The results for the first six compounds listed above were compared with those obtained by the standard procedure and were found to be similar. We propose that testing guidelines for skin sensitization should accommodate validated modifications of existing standard procedures.