Neurogenic syphilis is a significant clinical feature of early and late syphilis infections. The disease process triggered by Treponema pallidum is closely related to the activation of the host’s inflammatory response. However, the exact molecular pathways of neuroinflammation caused by Treponema pallidum infection in the central nervous system remain incompletely understood. We utilized human microglial cells (HMC3) as a model system to investigate the effects of the Treponema pallidum lipoprotein Tp0768 on the secretion of inflammatory cytokines. We cocultured HMC3 cells with Tp0768 at concentrations of 0, 5, 10, and 20 μg/mL and measured the mRNA levels of inflammation‐related cytokines (IL‐1β, IL‐6, and TNF‐α) using qRT‐PCR to determine the effective concentration for subsequent experiments. Treatment of HMC3 cells with 20 μg/mL of Tp0768 for 0, 12, 24, and 48 h significantly promoted the secretion of IL‐1β, IL‐6, and TNF‐α at the 48‐hour mark, exhibiting a concentration‐ and time‐dependent manner. Further investigations using western blotting revealed that Tp0768 activates Toll‐like receptors 4 (TLR4) and 9 (TLR9). After downregulating the expression of TLR4 and TLR9 using three specific siRNAs, ELISA assays showed a significant reduction in the secretion levels of IL‐1β, IL‐6, and TNF‐α compared to the control group. Additionally, western blotting demonstrated that Tp0768 significantly increased the phosphorylation levels of ERK, JNK, p38 MAPK, and p65 within 180 min in HMC3 cells compared to the control group. Furthermore, immunofluorescence colocalization confirmed that Tp0768 promotes the translocation of p65 into the nucleus of HMC3 cells. Pretreatment of HMC3 cells with ERK inhibitor (PD98059), p38 inhibitor (SB203580), JNK inhibitor (SP600125), and p65 inhibitor (BAY‐11 7082) followed by coincubation with Tp0768 resulted in reduced secretion levels of IL‐1β, IL‐6, and TNF‐α compared to the control group, as demonstrated by ELISA. Using three siRNAs to downregulate the expression of TLR4 and TLR9, we detected that the activation of the MAPKs/NF‐κB pathway in the coculture group with Tp0768 was significantly lower than that in the Tp0768 group, as assessed by western blotting. These results indicate that Tp0768 promotes the secretion of IL‐1β, IL‐6, and TNF‐α in HMC3 cells via the TLR4/9 receptors, triggering the MAPKs/NF‐κB signaling pathway.
Peng et al. (Thu,) studied this question.