Abstract: 【Abstract】 Objective To investigate distribution characteristics of Treponema pallidum （Tp） in secondary syphilis lesions, and to analyze its correlation with histopathological findings. Methods Totally, 41 patients were collected from Department of Dermatology and Venereology, Peking Union Medical College Hospital from January 2008 to December 2018, who were confirmedly diagnosed with secondary syphilis according to clinical manifestations and serological examinations, and had undergone histopathological examinations. Immunohistochemical results of skin tissue sections were analyzed, and differences in clinical and histopathological characteristics were analyzed between immunohistochemically Tp-positive and Tp-negative sections. Continuous data were compared by using t test or Kruskal-Wallis test, and categorical data were compared by using Chi-square test or Fisher′s exact test. Results Immunohistochemical examination showed that Tp was detected in 68.3% of the 42 secondary syphilis tissue sections, and Tp was mainly distributed in the lower epidermis and superficial and middle dermis. The positive rate of Tp was significantly higher in secondary syphilis lesions mainly manifesting as maculae （80% ［16/20］） than in those mainly manifesting as papules （50% ［11/22］, P < 0.05）. Among 10 pathological characteristics, extended rete ridges, basal cell liquefaction degeneration, neutrophil infiltration in the stratum corneum, lichenoid pattern of infiltration and punctate keratinocyte necrosis were observed more frequently in immunohistochemically Tp-positive sections than in Tp-negative sections（all P < 0.05）. Immunohistochemical study revealed that tissue sections with a larger number of Tp showed more pathological features （H = 17.914, P < 0.001）. Immunohistochemical staining showed that there were 8, 7 and 6 syphilis-specific histopathological characteristics on average in 8 tissue sections with a larger number of Tp, 14 with a medium number of Tp and 5 with a small number of Tp, respectively; while only 4 syphilis-specific histopathological characteristics were observed on average in 15 immunohistochemically Tp-negative tissue sections. Conclusion Immunohistochemical staining could show the distribution of Tp in secondary syphilis lesions, and it seems that tissue sections with a larger number of Tp present with more syphilis-specific histopathological characteristics.

Key words: Syphilis, Diagnosis, Treponema pallidum, Immunohistochemistry, Pathologic processes