The role of urocanic acid in UVB-induced suppression of immunity to Trichinella spiralis infection in the rat

The naturally occurring trans-isomer of urocanic acid (trans-UCA), found in the stratum corneum, absorbs ultraviolet light (UV) and isomerizes to the cis-form. Cis-UCA has been shown to impair some cellular immune responses, and has been proposed as an initiator of the suppression that follows UV ir radiation. UVB exposure leads to an increase in cis-UCA in the skin of rats from about 10%;, to 40% of the total UCA. Previously it has been demonstra ted that UVB lowers immune responses to Trichinella spiralis after oral inf ection of rats with the parasitic worm. Tn the present study we investigate d the role of cis-UCA in the control of this parasitic infection. Rats were infected orally with T. spiralis and injected with different doses of cis- or trans-UCA subcutaneously. Mitogenic responses and the mixed lymphocyte reaction were not affected by either isomer. In contrast, the number of T. spiralis larvae in muscle tissue of infected rats was increased significant ly in the cis-UCA-treated animals compared with the trans-UCA-treated anima ls. In addition, delayed-type hypersensitivity (DTH) to T, antigen in infec ted rats was significantly impaired by cis-UCA but not by trans-UCA. If rat s were injected with a monoclonal antibody with specificity for cis-UCA 2 h r prior to UVB exposure, the UVB-induced suppression in DTH to T. spiralis and the increase in larvae counts were significantly inhibited compared wit h rats that were similarly injected with a control antibody. Thus cis-UCA c an inhibit the specific resistance to parasitic infections and acts as an i mportant mediator of UVB-induced suppression of immunity to. spiralis in th e rat.