The aryl hydrocarbon receptor (AHR) is a cytosolic receptor for low molecular weight molecules, of which the most widely recognized ligand is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and the most widely recognized effect, chloracne. Adverse effects of manipulation were most recently and graphically demonstrated by the poisoning of Viktor Yushchenko during the Ukrainian presidential elections of 2004. However, recent research has revealed a receptor with wide-ranging, and at times, paradoxical actions. It was arguably among the first biological receptors to be utilized by dermatologists, dating from the time of topical tar preparations as a therapeutic agent. I provide a review outlining the role AHR plays in the development, cellular oxidation/antioxidation, responses to ultraviolet light, melanogenesis, epidermal barrier function, and immune regulation and its relationship to tryptophan metabolism. Finally, I will review the role of AHR in diseases of the integument.

The eosinophilia-myalgia syndrome is accompanied by alterations in L-tryptophan metabolism with elevated levels of L-kynurenine and quinolinic acid having been recorded. It has been suggested that this is due to activation of indoleamine 2,3-dioxygenase by interferon-gamma. It is unknown whether these products of tryptophan metabolism play a role in the pathogenesis of this syndrome and the closely related condition of eosinophilic fasciitis. To explore this possibility, the principal author (RN) received a series of subcutaneous injections of quinolinic acid. A total of 1200 mg was administered over a 1-month period. Peripheral blood eosinophil counts were monitored and biopsies taken for H&E and immunohistochemical stains. Over the 1-month period the eosinophil count rose from 0.3x10(9)/l to 0.8x10(9)/l before falling to 0.4x10(9)/l approximately 5 weeks later. H&E sections showed a mixed infiltrate of eosinophils and neutrophils extending through the reticular dermis and septa of the panniculus. No deep fascia was obtained on biopsy. The immunohistochemical stain for transforming growth factor beta 1 showed staining of endothelial cells and dendritic cells. The interleukin-5 stain was negative. Our results suggest that quinolinic acid may play a role in cutaneous eosinophilic disorders.

Sun protection has been recommended by public health authorities to reduce the risk of keratinocyte and melanocyte tumors, yet some sun exposure is required for vitamin D synthesis. Frontal fibrosing alopecia (FFA) is a cicatricial alopecia that has been reported to have an association with facial photoprotection. The brief review proposes the hypothesis that FFA arises as a result of excessive facial photo-protection with a resultant disturbance in immunological homeostasis mediated via the aryl hydrocarbon receptor-kynurenine pathway axis (AHR/KP) leading to the collapse of immune privilege at the hair bulge.

A 60-year-old man with common variable immunodeficiency presented with a 7-year history of violaceous plaques and papules on the thighs, arms and trunk. In the preceding 2 years he had developed new lesions on both hands. He had been previously diagnosed with sarcoidosis on the basis of skin and visceral histology, but subsequent opinion was that these were sarcoid-like granulomas rather than being representative of true sarcoidosis. Biopsy of the hand lesions showed necrotizing granulomas, and a single acid-fast bacillus (AFB) was identified on Wade-Fite stain. Subsequent repeat tissue biopsies for histology, culture and polymerase chain reaction testing failed to confirm the presence of mycobacterial organisms and it was felt that the organism was a contaminant introduced during tissue processing. The hand lesions responded well to intralesional injections of triamcinolone acetonide 10 mg/mL and oral tetracycline 500 mg b.d. was later introduced with a good clinical response. The diagnostic dilemma of finding granulomatous inflammation in a patient with common variable immunodeficiency, and the significance of a single AFB on histology are discussed. The treatment of sarcoid-like granulomas with tetracycline therapy is also commented on.