Abstract
The chlamydial plasmid, an essential virulence factor, encodes plasmid proteins that play important roles in chlamydial infection and the corresponding immune response. However, the virulence factors and the molecular mechanisms of Chlamydia psittaci are not well understood. In the present study, we investigated the roles and mechanisms of the plasmid-encoded protein CPSIT_P7 of C. psittaci in regulating the inflammatory response in THP-1 cells (human monocytic leukemia cell line). Based on cytokine arrays, CPSIT_P7 induces the expression of interleukin-6 (IL-6), interleukin-8 (IL-8), and monocyte chemoattractant protein-1 (MCP-1) in THP-1 cells. Moreover, the expression levels of IL-6, IL-8, and MCP-1 stimulated by CPSIT_P7 declined after silencing of the Toll-like receptor 4 (TLR4) gene using small interfering RNA and transfection of a dominant negative plasmid encoding TLR4 (pZERO-hTLR4). We further demonstrated that transfection with the dominant negative plasmid encoding MyD88 (pDeNy-hMyD88) and the dominant negative plasmid encoding Mal (pDeNy-hMal) could also abrogate the expression of the corresponding proteins. Western blot and immunofluorescence assay results showed that CPSIT_P7 could activate nuclear factor κB (NF-κB) signaling pathways in THP-1 cells. Altogether, our results indicate that the CPSIT_P7 induces the TLR4/Mal/MyD88/NF-κB signaling axis and therefore contributes to the inflammatory cytokine response.
Highlights
Chlamydia psittaci is an intracellular pathogen of zoonotic pathogens responsible for atypical pneumonia
Among the 42 cytokines tested, the four cytokines mainly secreted by THP-1 cells were granulocytemonocyte colony-stimulating factor (GM-CSF), IL-6, IL-8, and monocyte chemoattractant protein-1 (MCP-1) (Figure 1B), which indicated that the increased levels of inflammatory molecules may contribute to inflammation in vitro
The results showed that silencing of Toll-like receptor 4 (TLR4) by dominant negative plasmids could significantly downregulate the secretion levels of IL-6, IL-8, and MCP-1 in cells
Summary
Chlamydia psittaci is an intracellular pathogen of zoonotic pathogens responsible for atypical pneumonia. TLRs play a crucial role in activating innate immune cells including monocytes, macrophages, and dendritic cells. They can recognize related structures in pathogens, called pathogen-associated molecular patterns (PAMPs), and initiate signaling cascades (Xie et al, 2017). The main proinflammatory cytokines, IL-6, IL-8, and MCP1(CCL2) are the key chemokines produced by different types of cells, such as monocytes, macrophages, and endothelial and epithelial cells (Deshmane et al, 2009; Klapproth and Sasaki, 2010; Yang et al, 2018; Kanmani et al, 2019). Given the significance of plasmid-encoded proteins in the innate immune response and the inflammatory response during chlamydial infections, we mainly focused on investigating the role and molecular mechanism of CPSIT_P7 in the inflammatory response in human monocytic cells
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