Systemic administration of urocanic acid generates suppression of the delayed type hypersensitivity response to herpes simplex virus in a murine model of infection.

Urocanic acid (UCA), the putative photoreceptor for ultraviolet radiation (UV)-induced immunosuppression, undergoes a UV-dependent trans to cis isomerization. When UCA, containing a known proportion of the cis isomer, is administered by the intravenous route and live herpes simplex virus is given subcutaneously shortly afterwards, suppression of the delayed type hypersensitivity response to the virus is induced. Two T suppressor cell subsets are generated, one [Ly2+, L3T4-] and the other [Ly2-, L3T4+]. The results are discussed with regard to the differences indicated between local (epidermal) and systemic immunosuppression.