EPIDERMAL UROCANIC ACID AND SUPPRESSION OF CONTACT HYPERSENSITIVITY BY ULTRAVIOLET-RADIATION IN MONODELPHIS-DOMESTICA

A single specific epidermal photoreceptor for the immunosuppressive ac tion of UV radiation has not been defined, although separate evidence is accruing in favour of each of two candidates, trans-urocanic acid a nd DNA. In Monodelphis domestica, specific photoreactivation repair of UV radiation-induced pyrimidine dimers has been shown to abrogate the suppression of contact hypersensitivity (CHS), thus suggesting that D NA is the target for this impairment. However, in both haired and hair less mice, immunosuppressive effects of UV radiation have been reprodu ced by the exogenous administration of the UV photoproduct of urocanic acid, cis-urocanic acid. We show here that the epidermis of M. domest ica contains urocanic acid, that UV irradiation of the shaved dorsal s kin has resulted in an increase in epidermal cis-urocanic acid and tha t the topical application of a cis-urocanic acid-containing lotion sig nificantly depressed the capacity of Monodelphis to respond to contact sensitisers, in a manner analogous to these responses in the hairless mouse. Therefore in Monodelphis, suppression of CHS by UV irradiation appears to involve both urocanic acid photo-isomerisation and epiderm al DNA damage.