Photochemistry and Photobiology, 2003, 78(3): 228–234 Viability of the Antigen Determines Whether DNA or Urocanic Acid Act as Initiator Molecules for UV-induced Suppression of Delayed-type Hypersensitivity { Tae-Hueng Kim 1 , Angus M. Moodycliffe 2 , Daniel B. Yarosh 3 , Mary Norval 4 , Margaret L. Kripke 5 and Stephen E. Ullrich* 5 1 Department of Dermatology, College of Medicine, Gyeongsang National University, Kyungnam, Korea; 2 Department of Nutrition, Nestle Research Center, Lausanne, Switzerland; 3 Applied Genetics Inc., Freeport, NY; 4 Medical Microbiology, University of Edinburgh Medical School, Edinburgh, Scotland and 5 The Department of Immunology, The University of Texas, M.D. Anderson Cancer Center, Houston, TX Received 6 May 2003; accepted 11 June 2003 ABSTRACT UV radiation suppresses the immune response, and UV- induced immune suppression contributes to UV-induced photocarcinogenesis. For UV-induced immune suppression to occur, electromagnetic energy (i.e. UV radiation) must be converted to a biological signal. Two photoreceptors have been identified in the skin that serves this purpose, epidermal DNA and trans-urocanic acid (UCA). Although compelling evidence exists to support a role for each pathway (UV-induced DNA damage or photoisomerization of UCA) in UV-induced immune suppression, it is not clear what determines which photoreceptor pathway is activated. To address this question, we injected UV-irradiated mice with a monoclonal antibody with specificity for cis-UCA or applied liposomes containing DNA repair enzymes to the skin of UV-irradiated mice. The effect that each had on UV-induced suppression of delayed- type hypersensitivity was measured. We asked whether the light source used (FS-40 sunlamps vs solar-simulated UV radiation) altered whichever pathway of immune suppression was activated. Different doses of UV radiation and the viability of the antigen were also considered. Neither the dose of UV nor the light source had any influence on determining which pathway was activated. Rather, we found that the viability of the antigen was the critical determinant. When live antigens were used, UV-induced immune suppression was blocked with monoclonal anti–cis-UCA but not with T4 endonuclease V–containing liposomes. The reverse was observed when formalin-fixed or killed antigens were used. Our findings indicate that antigen viability dictates which photoreceptor pathway predominates after UV exposure. INTRODUCTION Sunlight-induced skin cancer is the most prevalent type of cancer diagnosed in the industrialized world (www.cancer.org/statistics). In addition to being a complete carcinogen, the UV radiation pres- ent in sunlight is immunosuppressive, and the immune suppression induced by UV radiation is a contributing factor for skin cancer induction (reviewed by Ullrich [1]). Suppressor T cells are found in the lymphoid organs of UV-irradiated mice, and these cells control the development of the primary tumor in the UV-irradiated host (2). Recent results indicate that the UV-induced suppressor T cell belongs to a unique class of immunoregulatory T cells known as natural killer T cells (3). Compelling data from biopsy-proven skin cancer patients and cancer-prone immunosuppressed trans- plant recipients suggest that UV-induced immune suppression is an important risk factor for skin cancer induction in humans (4,5). Moreover, UV exposure of both laboratory animals and human volunteers suppresses cell-mediated immune reactions, such as contact and delayed-type hypersensitivity (DTH) (6–11). The initial photobiologic reaction responsible for triggering the cascade of events leading to the activation of the suppressor pathway remains controversial. Because of its limited ability to penetrate beyond the dermal–epidermal junction, it is thought that UV radiation initiates systemic immune suppression by stimulat- ing epidermal photoreceptors. Candidate molecules that convert the physical energy of UV radiation into a biologic signal include urocanic acid (UCA), a deamination product of histidine present in the stratum corneum (12) and epidermal DNA (13). When UV penetrates the skin, most of its energy is absorbed by the DNA of epidermal keratinocytes. Applying liposomes containing the bacteriophage T4 endonuclease V (T4N5) to the skin of UV-irradiated mice restores immune function (14). Because T4N5 is specific for UV-induced pyrimidine dimers, these data support a role for DNA damage as an initiating event in UV- induced immune suppression. UV-induced DNA damage also appears to be an initiating event when other immunological end points were examined, such as UV-induced cytokine production in {Posted on the website on 8 July 2003 *To whom correspondence should be addressed at: The University of Texas, MD Anderson Cancer Center, Department of Immunology, P.O. Box 301402, Houston, TX 77030-1903, USA. Fax: 713-563-3357; e-mail: sullrich@mdanderson.org Abbreviations: BCG, Bacillus Calmette–Gue ´rin; DTH, delayed-type hypersensitivity; HI-T4N5, heat-inactivated T4N5; IL, interleukin; ISD 50 , 50% immunosuppressive dose; MHC, major histocompatibility complex; PBS, phosphate-buffered saline; SSR, solar-simulated UV radiation; T4N5, T4 endonuclease V; UCA, urocanic acid. Ó 2003 American Society for Photobiology 0031-8655/03 $5.00 þ0.00 228