Detection of <i>Treponema pallidum</i> in Skin Lesions of Secondary Syphilis and Characterization of the Inflammatory Infiltrate

<i>Background:</i> Syphilis is an ancient sexually transmitted disease. However, the pathogenesis of mucocutaneous lesions of secondary syphilis is not completely understood. <i>Methods:</i> We analyzed the presence of <i>Treponema pallidum</i> in formalin-fixed, paraffin-embedded biopsy specimens from mucocutaneous lesions of secondary syphilis using highly sensitive nested polymerase chain reaction (PCR). The inflammatory infiltrates from the same specimens are also characterized using immunohistochemical methods. <i>Results and Conclusions:</i> Ten out of 24 (41.7%) specimens are <i>T. pallidum</i> positive using nested PCR, whereas none of them is <i>T. pallidum</i> positive using traditional silver staining. The presence of <i>T. pallidum </i>in the mucocutaneous lesions indicates that mucocutaneous lesions of secondary syphilis might be caused by direct <i>T. pallidum</i> invasion rather than by an allergic reaction. Furthermore, the majority of inflammatory infiltrating cells are CD45RO-positive T cells and CD68-positive macrophages, suggesting that cellular immunity plays an important role in the host reaction against <i>T. pallidum</i> infection in secondary syphilis.