Epidermal photoreceptors

To induce immune suppression the electromagnetic energy of UV radiation must first be absorbed by an epidermal photoreceptor and then converted into a biologically recognizable signal. Three epidermal photoreceptors have been identified DNA, urocanic acid (UCA) and membrane lipids. 

Meisner H, Daga A, Buxton J et al 1997 Interactions of Drosophila Cbl with epidermal growth factor receptors and role of Cbl in R7 photoreceptor cell development. Mol Cell Biol 17 2217-... 

Urocanic acid (2-propanoic acid 3-[lH-imidazol-4-yl] is located superficially in the stratum comeum. Metabolism of epidermal UCA does not occur in situ due to the absence of urocanase, resulting in the accumulation of UCA in the epidermis. Upon UV exposure, naturally occurring trans-UCA converts to the d.s-isomer, in a dose dependent manner, until the photostationary state is reached, when equal quantities of trans- and m-UCA are found in the skin.15 Based on an analysis of the action spectrum for UV-induced immune suppression, and the fact that no immune suppression was observed in mice whose stratum comeum was previously removed by tape stripping, De Fabo and Noonan suggested that urocanic acid was the photoreceptor for UV-induced immune suppression.16 Since the initial experiments many others have documented, the ability of ris-UCA to initiate immune suppression, documented its presence in the serum of UV-irradiated mice, and demonstrated that m-UCA plays a role in UV-induced skin cancer induction. (For a more complete review of the role of m-UCA in immune suppression see two excellent reviews by Norval and colleagues.1718)... 

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