Background It is well known that tissue immune response plays an important role in the pathogenetic mechanisms of several bacterial and viral infections. Several studies focused on the cutaneous host immune-response during infections by mycobacteria, HIV, HHV8, HCV and HSV, while few data are known about cell-mediated immunity in skin syphilitic lesions.

Methods By an immunohistochemistry technique, we characterised the cutaneous inflammatory infiltrate in 5 cases of secondary syphilis, using a large panel of monoclonal antibodies (MAbs) and a specific polyclonal antibody against Treponema pallidum (TP) antigen.

Results TP bacteria were mainly localised in the epithelial sheets, even if some TP were also detected in perivascular areas of the dermis. The infiltrate was characterised, as expected, by large number of plasmocytes, detected using MAbs anti- CD138, CD79a and MUM-1. Plasmocytes showed active in situ proliferation, as demonstrated by MIB-1/Ki-67 proliferation marker. An abundant epidermotropic T-lymphocytes infiltrate consisting of the CD8+ subset was easily demonstrated in the superficial dermis, where several macrophages (M1 and M2) were also detected. Unexpectedly the staining of dendritic cells by using anti CD1a MAb evidenced the quite complete absence of Langerhans cells, both in the epidermis and in the dermis.

Conclusions Our results, showing that syphilis infiltrate is mainly composed by abundant plasmocytes and CD8+ lymphocytes in absence of dendritic Langerhans cells, suggest the presence of a characteristic immune-disregulation in the skin affected from TP infections. Further studies on a larger number of patients are needed in order to better clarify the exact immunological mechanisms in skin syphilitic lesions.