Objective:To investigate the phagocytosis of Treponema pallidum (Tp) by macrophages and the polarization direction of macrophages after Tp stimulation. Methods:Human THP-1 monocyte-derived M0 macrophages were stimulated with the Tp Nichols strain, and the phagocytosis of Tp by macrophages and changes in the intracellular structure of macrophages were observed by transmission electron microscopy and immunofluorescence staining. After 12-hour stimulation by Tp, Tp was removed, the M0 macrophages continued to be cultured for 24, 48, 72 hours and 6 days. Western blot analysis and immunofluorescence staining were performed to determine the expression of the M1 macrophage marker CD86 and M2 macrophage marker CD163, and enzyme-linked immunosorbent assay was conducted to detect levels of M1-type cytokines interleukin (IL) -12 p70, interferon (IFN) -γ, chemokine ligand 10 (CXCL10) , IL-6, tumor necrosis factor (TNF) -α and IL-1β, as well as the M2-type cytokine transforming growth factor (TGF) -β1 in the culture supernatant of macrophages. Dunnett- t test was used for multiple comparisons. Results:As transmission electron microscopy showed, after the stimulation by Tp, the macrophages extended pseudopodia and engulfed Tp, leading to swelling and obviously irregular hyperplasia of endoplasmic reticulum as well as enlargement of mitochondria. Moreover, after additional culture for 24, 48, 72 hours and 6 days, CD86 was highly expressed, but CD163 was lowly expressed in the Tp-treated macrophages; at 24 hours, the supernatant levels of IL-12 p70, IFN-γ, CXCL10, IL-6, TNF-α and IL-1β were significantly higher in the Tp-treated group than in the control group (all P<0.001) , but there was no significant difference in the TGF-β1 supernatant level between the 2 groups ( P>0.05) . Conclusions:After engulfment of Tp, the structures of endoplasmic reticulum and mitochondria in THP-1-derived macrophages markedly changed. Tp could induce the polarization of M0 macrophages into M1 macrophages, and phenotypic switch from M1 to M2 macrophage polarization was not observed within 6 days after Tp stimulation.