Abstract: Xie Zhen, Chen Yuanhan, Wang Siyu, Wan Huiying, Lei Hua, Yang Ge, Lin Zhaochun Institute of Dermatology and Venereology, Sichuan Academy of Medical Sciences and Sichuan Provincial People′s Hospital, Chengdu 610031, China （Xie Z, Wang SY, Wan HY, Lei H, Yang G, Lin ZC）; Department of Nephrology, Guangdong Academy of Medical Sciences, Guangdong General Hospital, Guangzhou 510080, China （Chen YH） Corresponding author: Xie Zhen, Email: xiezhen6289@126.com 【Abstract】 Objective To measure the of indoleamine 2, 3?dioxygenase （IDO） in condy?loma acuminatum （CA） lesions, and to evaluate its ability to locally metabolize tryptophan. Methods Immunohistochemical study was performed to observe the protein of IDO in skin lesions of patients with CA, and count the number of IDO?positive cells. Immunofluorescence assay was conducted to estimate the relationship between IDO?positive cells and dendritic cells. Epidermal cells and keratinocytes were isolated from warts of 30 patients with CA and prepuces of 11 healthy controls respectively, and both in vitro incubated with tryptophan solution for 4 hours. Then, high?performance liquid chromatography （HPLC） was performed to detect the level of tryptophan metabolite, kynurenine, in the culture supernatant of the above cells, which could reflect the ability of epidermal cells to metabolize tryptophan. Results Rare IDO?positive cells were found in the normal skin, but a lot of IDO?positive cells gathered in the epidermis of the wart tissues. The IDO?positive cell/total cell ratio was significantly higher in the wart tissues than in the normal skin （48.3% ± 15.4% vs. 5.2% ± 2.4%, P < 0.05）. The fluorescence signals of IDO?positive cells and CD1a?positive Langerhans cells were not overlapped with each other, suggesting that IDO?positive cells were derived from epidermal cells of the wart tissues. Compared with the keratinocytes from the healthy skin, the epidermal cells from warts had a stronger ability to metabolize tryptophan in vitro. Conclusion A large number of IDO?positive cells exist in CA warts, and may be involved in occurrence of CA.