Abstract: Objective To establish a method of murine local lymph node assay (LLNA) combined with measurement of ear swelling for the identification of photosensitive agents. Methods The female BALB/c mice received test chemicals or vehicle alone on the back skin of both ears and then were exposed to UVA irradiation on three consecutive days. The post-auricular lymph nodes draining the ear tissue were excised 24 h following the last irradiation. Lymph node cell counts and lymphocyte proliferative rates were determined, which indicated lymph node hyperplasia. While ear swelling was also measured, to reveal skin irritating reactions. Some cytokines (IFN-γ, IL-2 and IL-4) released by the draining lymph node were also analyzed by ELISA. Results Tetrachlorosalicylanilide, a strong photoallergen, caused lymph node hyperplasia without ear swelling. The mild photoallergen (6-methylcoumarin) showed neither lymph node proliferation nor ear swelling reactions. But 8-methoxypsoralen, a phototoxic substance, induced both lymph node hyperplasia and ear swelling. The level of IL-4 was significantly increased when the mice were previously sensitized with tetrachlorosalicylanilide (P<0.05). Conclusions LLNA combined with the measurement of ear swelling makes it possible to differentiate strong photoallergens from phototoxic substance, but is not sensitive enough to detect mild photoallergens. Cytokine analysis of the draining lymph node may provide an additional method for the differentiation of photoallergic agents from phototoxic substances.

Key words: Allergens, Phototoxic agents, Local lymph node assay