Differential effects of a monoclonal antibody to cis-urocanic acid on the suppression of delayed and contact hypersensitivity following ultraviolet irradiation.

Urocanic acid (UCA) occurs naturally in the stratum corneum of the skin as the trans-isomer and, upon exposure to UVB radiation, converts to cis-UCA. It has been proposed that trans-UCA is the photoreceptor for and, following its isomerization to cis-UCA, a mediator of the suppressive effects of UVB irradiation on systemic T cell-mediated immune responses, such as contact hypersensitivity (CH) and delayed-type hypersensitivity (DTH). To address this question directly, we studied the consequence of deleting the in vivo function of cis-UCA on systemic suppression of CH and DTH, by injecting mice with a anti-cis-UCA mAb several hours before exposure to UVB radiation. We found that while DTH responses were completely restored, the anti-cis-UCA Ab had no effect on UV-induced immunosuppression of the CH response, even though suppressor cell formation was inhibited in both cases. Further, the kinetics of IL-10 expression in the skin of irradiated mice injected with the anti-cis-UCA mAb was altered and the diminished APC function of spleen-adherent cells from UVB-irradiated mice was totally reversed by the Ab. These findings suggest that cis-UCA acts as a mediator for some but not all of the systemic suppressive effects of UVB irradiation. They also suggest that cis-UCA may act indirectly via IL-10 to modulate immune function.