Differential Induction of Endotoxin Tolerance by Lipopolysaccharides Derived from Porphyromonas gingivalis and Escherichia coli

Abstract

Exposure of mononuclear phagocytes to enterobacterial LPS induces a state of transient hyporesponsiveness to subsequent LPS exposure, termed endotoxin tolerance. In the present study, LPS derived from the oral periodontal pathogen, Porphyromonas gingivalis, was compared with that derived from the enterobacterium, Escherichia coli, for the ability to induce endotoxin tolerance. Pretreatment of the human macrophage cell line, THP-1, with E. coli LPS resulted in a severe reduction in the levels of IL-1β, IL-6, and TNF-α upon secondary stimulation. In contrast, pretreatment of THP-1 cells with P. gingivalis LPS resulted in a mitigation of IL-1β, but not IL-6 and TNF-α production upon subsequent exposure to P. gingivalis LPS: primary or secondary stimulation with ≤100 ng/ml P. gingivalis LPS resulted in comparable levels of IL-6 and TNF-α, while stimulation of THP-1 cells with ≥1 μg/ml P. gingivalis LPS induced a significant enhancement in IL-6 and TNF-α levels upon secondary exposure. To identify possible mechanisms for these differences, changes in the expression of molecules involved in the LPS-signaling pathway were assessed. Pretreatment of THP-1 cells with E. coli LPS resulted in a significant reduction in surface Toll-like receptor 4 (TLR4) expression and an inability to degrade Ι-κB-α or Ι-κB-β proteins upon secondary stimulation. In contrast, pretreatment of THP-1 cells with P. gingivalis LPS resulted in a significant enhancement of both CD14 and TLR2, while maintaining the ability to degrade Ι-κB-β only upon secondary stimulation. Thus, E. coli and P. gingivalis LPS differentially affect CD14 and TLR expression as well as secondary LPS-associated responses.