cis-Urocanic acid does not inhibit mitogen induced lymphocyte transformation in man.

Urocanic acid (UCA) is a photoreceptor in the epidermis which absorbs UVB. To elucidate the role of UCA as an immunomodulator in the mechanism of immune suppression which occurs after UV irradiation in human, we studied the effect trans-UCA as well as that of its UV induced cis-isomer (cis-UCA) on mitogen induced lymphocyte transformation and on delayed type responses elicited by intradermal testing of a standardized battery of microbial antigens. No inhibitory effect was seen either in vitro or in vivo. Our negative results, both in vivo and in vitro, suggest that if UCA and UCA photoproducts exert an immunomodulatory effect, their effect should be analysed by a more subtle evaluation of the immune system.