Suppression of contact hypersensitivity by short-term ultraviolet irradiation: II. The role of urocanic acid.

Cis-urocanic acid (cis-UCA), produced from trans-UCA (a normal component of epidermis) by UV irradiation, suppressed cell-mediated immunological reactions in vivo and in vitro. It suppressed the development of contact hypersensitivity (CHS) when injected into mice, and it suppressed leucocyte adherence inhibition (LAI) reactions of previously sensitized lymphocytes exposed to antigen. Serum from mice injected with cis-UCA was also immunosuppressive in vitro. Normal murine spleen cells cultured with cis-UCA produced a non-dialysable factor which suppressed LAI reactivity. Trans-UCA was ineffectual in all of these systems. Both the ability of cis-UCA to induce an immunosuppressive serum factor and its ability to suppress CHS were abrogated by prior administration of cyclophosphamide, indicating that cis-UCA (normally from irradiated epidermis) stimulates T suppressor cells to produce the previously described suppressor factor in serum and the immunosuppression associated with short-term irradiation.