Nickel allergy is one of the most common causes of allergic contact dermatitis and a team led by researchers at the University of Giessen have now shown that the response is linked to activation of a single receptor, toll-like receptor 4 (TLR4).
The family of toll-like receptors normally recognizes structurally similar molecules derived from microbial pathogens and plays a key role in host defense. The team showed that nickel directly activated human, but not mouse, TLR4 and studies with mutant proteins showed that non-conserved histidine residues at positions 456 and 458 were necessary for activation of human TLR4. Wild type mice do not show an allergy to nickel but transgenic mice expressing human, rather than mouse, TLR4 could be efficiently sensitized to the metal. This is the first time that an inorganic substance has been shown to activate this innate immune pathway and, since histidines 456 and 458 are not essential for responses to microbial lipopolysaccharide, the authors suggest that site-specific inhibition of TLR4 could provide a potential strategy for treatment of nickel allergy, which would not compromise normal immune responses. Nickel allergy affects millions of people and is often associated with earrings and jewellery for other body piercings.
The study is published in the journal Nature Immunology.