Stereospecific inhibition of human epidermal cell interleukin-1 secretion and HLA-DR expression by cis-urocanic acid.

UV radiation is known to photoisomerize trans-urocanic acid (trans-UCA) into a stable isomer, cis-urocanic acid (cis-UCA). To study the possible immunomodulatory effects of cis-UCA, the two isomers were added separately to different in vitro assays employing human epidermal cell suspensions or purified human peripheral T lymphocytes, supplemented with epidermal cells. Cis-UCA but not trans-UCA suppressed interleukin-1 (IL-1) production by epidermal cell suspensions in a dose-dependent fashion and diminished the number of HLA-DR positive epidermal cells to 61% of control values. An inhibitory effect on epidermal cell accessory function could be demonstrated with both isomers of UCA, but only if UVB-irradiated epidermis was used as a source for the epidermal cells. Taken together, the findings of our study lend indirect support to the concept of cis-UCA as a possible mediator of UV-radiation-induced immunosuppression.