Session 2: Immunotoxicity and AllergyAlternative approaches to the identification and characterization of chemical allergens
Introduction
Allergic disease resulting from exposure of susceptible individuals to sensitizing chemicals is an important health issue. There is clearly a need to identify accurately chemicals that have the potential to induce allergic sensitization and to assess the risks they pose to human health. In the context of occupational disease two forms of allergic sensitization are of greatest importance; skin sensitization resulting in allergic contact dermatitis and sensitization of the respiratory tract associated with rhinitis, asthma and other symptoms. In both instances methods for hazard identification focused originally on the use of guinea pig tests in which activity is measured usually as a function of challenge-induced allergic reactions (dermal or pulmonary) in previously sensitized animals (for reviews see Manome, Aiba, & Tagami, 1999, Sarlo & Ritz, 1997). However, during the last two decades there have been significant advances in allergy research and in the definition of the important immune mechanisms that influence the acquisition and regulation of allergic sensitization to chemicals. Associated with this progress there have been opportunities to design alternative approaches to hazard identification based on analysis of immune responses induced in mice (and rats) by chemical allergens. Among these new methods is one which is already well established, the local lymph node assay, a predictive test for skin sensitizing potential (Kimber, Cumberbatch, Dearman, Bhushan, & Griffiths, 2000, Dearman, Basketter, & Kimber, 1999, Gerberick et al., 2000), and others that show promise but which have not yet been formally validated. Among the latter are included the mouse (and rat) IgE test and cytokine fingerprinting, both of which methods are designed to identify chemicals that have the ability to cause sensitization of the respiratory tract (Kimber and Dearman, 1999a).
All of the approaches summarized above have a requirement for animals, and it is legitimate to consider whether there are emerging opportunities to develop alternative strategies for predictive testing that are based solely on in vitro methods. It is timely to address this now since a more sophisticated appreciation of the relevant molecular mechanisms facilitates, in principle at least, the design of cell or tissue culture systems that reflect what is known of the induction phase of sensitization to chemicals. Although some of these opportunities are explored below, it is important to recognize that the development of realistic in vitro methods is not a trivial exercise and poses experimental toxicologists with some substantial challenges.
Section snippets
Contact and respiratory allergens
Chemicals can cause various forms of allergic disease. There are many chemicals that are known to cause skin sensitization and allergic contact dermatitis, but which are believed not to induce sensitization of the respiratory tract. In contrast, there are other chemicals (fewer in number) that have been shown to induce respiratory allergy among exposed subjects, but which only rarely, if ever, induce contact sensitization in humans. There is now a substantial body of evidence that the
Approaches to skin sensitization testing in vitro
Proposed in vitro test methods are in the main based on an understanding of the cell and molecular processes that characterize the induction phase of contact sensitization. Briefly, the key events are as follows. Epidermal Langerhans cells (LC) recognize, internalize and process chemical allergen in the form of a hapten–protein conjugate. Antigen is transported from the skin to draining lymph nodes by LC. While in transit from the epidermis, LC are subject to a functional maturation such that
The way forward
Currently the approach we (and others) are taking is to identify candidate genes that are up- or down-regulated selectively by contact allergens using microarray transcript profiling. For this purpose we are using custom-designed microarrays. The first of these, ToxBlot I, comprises approximately 600 genes selected on the basis of their relevance for immune and inflammatory reactions and other biological processes, including the regulation of cell division and differentiation, apoptosis,
Concluding comments
In recent years there has been very substantial progress in our understanding of the cellular and molecular mechanisms that serve to initiate and regulate skin sensitization. Such advances, combined with the application of new technology platforms, is now providing realistic opportunities to design novel approaches to hazard identification. The goal of a robust in vitro method that can serve as a realistic alternative to current predictive tests has yet to be achieved. In practice, realising
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