Characterization of skin sensitizing chemicals: a lesson learnt from nickel allergy.

dermatitis. At first glance it would appear self evident that any worthwhile predictive test for the ability of chemicals to cause skin sensitization should, without difficulty, register nickel salts as clear positives. This has not been the experience however. Although nickel is in fact a comparatively weak allergen (the high prevalence of skin sensitization resulting from ubiquitous exposure, rather than potent allergenicity), the failure to engineer positive responses to this material in tests for sensitizing activity has rankled investigators. Certainly, it was our experience with the mouse local lymph node assay (LLNA) that nickel salts (and usually nickel sulfate) either failed to elicit a positive response or, less commonly, registered only very modest activity (Basketter et al., 1994). Even the adoption of different vehicles and exposure regimen failed to have a material impact on the behavior of nickel sulfate in the assay (Kimber et al., 1990; Ikarashi et al., 1992; Ryan et al., 2002). On the back of that experience, and the investigations of others, nickel has been regarded (correctly) as a “false negative” in the LLNA, and in other animal tests for measurement of skin sensitization potential. The mouse is not completely unresponsive to nickel though and it has proven possible to elicit responses if exposure is performed together with an adjuvant, or if there is some other source of local trauma or inflammation (Sato et al., 2007). It had been proposed also that, at least in part, the unresponsiveness of mice to nickel salts might be attributable to oral tolerance (resulting from oral ingestion of metallic nickel from cage material and drinking nipples). However, even minimization of oral tolerance (and reduced activity of regulatory T-lymphocytes) failed to transform nickel into a really effective skin sensitizer in mice (Van Hoogstraten et al., 1993; Wu et al., 2007). The mystery is now solved, the solution being that mice are different from humans. In a very elegant paper published recently by Schmidt et al. (2010), the immunobiological bases for the observed species differences in responses to nickel have been described very convincingly. The important observation has been that nickel ions can directly trigger activation of human Toll-like receptor 4 (TLR4). This activation requires the presence of non-conserved histidine residues at positions 456 and 458 of the receptor; these residues are found on human TLR4, but not on the same receptor in mice. The significance of this observation is that in humans, but not in mice, Ni can induce skin sensitization because it is able, through activation of TLR4, to provoke the inflammatory signals that are required to support and sustain the initiation of an adaptive immune response. The term “danger signal” has been coined to describe the requirement that encounter with a foreign antigen is accompanied by a certain level of local trauma or cellular damage; the purpose being to prevent unnecessary and inappropriate immune responses being launched when there is no threat as signaled by the damage associated with an important antigen incursion. Such signs of damage or trauma are described collectively as pathogen-associated molecular patterns and these are recognized by cellular vectors of the immune system via pathogen recognition receptors (PRRs) (Vance et al., 2009). Of these, the most thoroughly described are the TLRs of which there are many, including TLR4 that is expressed on the plasma membrane of a variety of cell types. We and others have drawn attention to the fact that the effective acquisition of skin sensitization is dependent upon a number of biological “hurdles” being cleared after encounter with a chemical allergen on the skin (Dearman and Kimber, 2003; Jowsey et al., 2006). Thus, a chemical must gain access to the viable epidermis so that interaction with relevant cells