cis-urocanic acid failed to affect in vitro human Langerhans cell allostimulatory function.

Urocanic acid (UCA) represents the major ultraviolet B (UVB, 290-320 nm)-absorbing component of the skin. Trans-UCA is naturally produced in the stratum corneum and converts to the cis isomer upon UVB irradiation. In this study, we examined the effect of purified cis-UCA (about 99% of cis isomer) on the human Langerhans cell (LC) allostimulatory function by using the mixed epidermal cell-lymphocyte reaction (MELR). We found that addition of increasing amounts (6.5-400 micrograms/mL) of purified cis-UCA or trans-UCA did not modify the T-cell response supported by enriched LC (eLC: 8-25% LC) as well as purified LC (pLC: 70-90% LC) suspensions. Because cis-UCA had no effect on the allostimulatory function of untreated LC, we investigated whether this compound could modify T-cell proliferation induced by UVB-irradiated LC. The UVB exposure of eLC or pLC to 100 J/m2 significantly inhibited the capacity of both suspensions to mount a T-cell response. However, addition of cis-UCA did not potentiate this UVB-induced immunosuppression. The eLC or pLC were then incubated with cis-UCA for 18 h at 37 degrees C and washed before adding to allogeneic T cells. The obtained proliferative response was similar to that induced by control LC incubated in medium alone, demonstrating that pretreatment with cis-UCA did not alter human LC function. In conclusion, these results strongly suggest that cis-UCA has no direct effect on human LC antigen-presenting function.