Cis-urocanic acid (UCA), formed in the stratum corneum by UV irradiation of trans-UCA has been proposed as a mediator of UV-induced immunosuppression in the skin. In this study, we examined the in vitro effect of cis-UCA (6-100 micrograms/mL) on the human mixed lymphocyte reaction (MLR) and the mixed epidermal cell lymphocyte reaction (MECLR). Addition of cis-UCA (purified or in a mixture with trans-UCA) did not affect the MLR but was able to induce a 20% suppression of the MECLR responses. Because this effect of cis-UCA on the MECLR was not as strong as could be expected from previous in vivo results, we designed a set of experiments in order to enhance the in vitro immunosuppressive capacity of cis-UCA. Firstly, we preincubated epidermal cells with UCA (50 micrograms/mL). for 3 or 6 days before culture in the MECLR because in vivo repeated UV exposure can lead to a photostationary state, where cis-UCA may be present for several weeks. This pretreatment with cis-UCA resulted in a maximal decrease of the MECLR response of 27%, whereas trans-UCA had no effect. Secondly, we investigated whether UVB irradiation of epidermal cells could make cells more sensitive to cis-UCA. However, addition of trans- or cis-UCA did not potentiate the reduced alloactivating capacity of UVB-irradiated cells. Finally, we examined the possibility of a synergistic effect of cis-UCA with histamine. Addition of histamine suppressed the MLR and MECLR responses, but neither cis- nor trans-UCA were able to modulate this decrease. We conclude that cis-UCA can partly downregulate the human MECLR but not the MLR. The mechanism involved in this differential downregulation is not known. In this respect it is striking that cis-UCA dose not potentiate the UVB- or histamine-induced suppression of the MECLR.