Dose-response study of ultraviolet radiation for induction of photoallergy to tetrachlorosalicylanilide in guinea pigs.

: The dose of ultraviolet light (UV) required to induce photoallergy to tetrachlorosalicylanilide (T4CS) in guinea pigs was studied. A simple induction technique consisting of only one treatment with adjuvant, chemical and UV was used. The concentration of T4CS, which was administered either topically or intradermally, was low to reduce self-screening of UV by the chemical. Doses of UVA from fluorescent blacklamps ranged from 0.4 to 300 kJ/m2. All animals were challenged with topically applied T4CS and 40 kJ/m2 UV. The results showed that incidence of photoallergy increased with increasing dose of UV and reached a plateau between 4.5-45 kJ/m2 following intradermal T4CS and 4-120 kJ/m2 following topical T4CS. The studies demonstrated for the first time decreased response following larger induction doses of UV; photoallergy was not induced by the largest induction dose of 300 kJ/m2. Attenuation of UVB during induction did not affect the decrease. The lack of response may be attributable to further photochemical reactions of a T4CS-protein conjugate with loss of antigenic properties, or to immune suppression initiated by either UVA or an interaction of T4CS and UVA.