Endotoxin Tolerance Induced by Lipopolysaccharides Derived from Porphyromonas gingivalis and Escherichia coli: Alternations in Toll-Like Receptor 2 and 4 Signaling Pathway

Abstract

Periodontitis is a chronic inflammatory disease induced by bacteria. Exposure of the host to periodontal pathogens and their virulence factors induces a hyporesponsive state to subsequent challenge, which is termed endotoxin tolerance. In this experiment, we studied the cytokine production in THP-1 cells upon single or repeated Porphyromonas gingivalis (P. gingivalis) lipopolysaccharide (LPS) or Escherichia coli (E. coli) LPS stimulation by ELISA. In addition, the protein expression profiles of Toll-like receptor 2 (TLR2), TLR4, IL-1 receptor-associated kinase 4 (IRAK4) and IRAK-M and the gene expression changes of Toll-interacting protein (Tollip) and suppressor of cytokine-signaling-1 (SOCS1) were explored to identify possible mechanisms for changes in cytokine secretion. After repeated stimulation with P. gingivalis LPS or E. coli LPS, secretions of TNF-α and IL-1β were decreased significantly compared with those following single challenge, while the levels of IL-10 were increased (p < 0.05). Only comparable levels of IL-8 were confirmed in P. gingivalis LPS-tolerized cells (p > 0.05). In addition, severe downregulation of TLR2 was detected in THP-1 cells retreated with P. gingivalis LPS, and the reduction of TLR4 expression was observed in cells restimulated with E. coli LPS (p < 0.05). Precondition with P. gingivalis LPS or E. coli LPS also led to an enhancement of IRAK-M and SOCS1, while maintaining the expressions of IRAK4 and Tollip. This pattern of cytokine production indicates the different effects of endotoxin tolerance triggered by P. gingivalis LPS and E. coli LPS, which might contribute to limiting inflammatory damage. Moreover, TLR2, TLR4, IRAK-M, and SOCS1 might play important roles in developing tolerance.