Abstract: Objective To study the biological characteristics of dendritic cells （DCs） derived from peripheral blood mononuclear cells （PBMCs） of patients diagnosed with syphilis. Methods PBMCs were isolated from 16 patients clinically and serologically diagnosed with syphilis, and from 16 healthy human controls, then cultured with GM-CSF and IL-4. On day 10, the monocyte-derived dendritic cells （MoDCs） of the patients and controls were collected and subjected to the detection of surface molecules by flow cytometry; TpN17 was used to stimulate MoDCs from the controls, the expression of phosphorylated ERK was detected by Western blotting 20 minutes following the stimulation. Results The positivity rate of CD80 was significantly increased in the patients with syphilis than that in the controls （51.90% vs 33.67, P < 0.05）, while no significant difference was observed in the expressions of CD83, CD86 or HLA-DR between the two groups （16.53% vs 15.99%, 66.13% vs 59.32%, 91.29% vs 90.51%, all P > 0.05）. The expressions of CD80 and CD83 on the surface of MoDCs were enhanced in a dose-dependent manner after exposure to TpN17. The expression of cytoplasmic phosphorylated ERK was observed in MoDC stimulated by TpN17, but not in those without the treatment. Conclusions Antigenic stimulation with Treponema pallidum may be a reason for phenotypic abnormality of MoDCs derived from patients with syphilis. TpN17 may stimulate the maturation of DCs through the ERK signal transduction pathway.