Induction of Immune Tolerance by Periodontal pathogen Lipopolysaccharide

Gram-negative bacteria Lipopolysaccharide (LPS) is a potent inducer of proinflammatory cytokines by mononuclear cells.  Prior exposure of monocytic cells to bacterial LPS, has been reported to reduce responsiveness upon secondary stimulation, known as “endotoxin tolerance”.  Objective: The purpose of this study was to investigate the degree of endotoxin tolerance to LPS of Prevotella denticola (Pd) by monocytes, which were exposed to LPS of E. coli (Ec) LPS. Methods: Human monocytic THP-1 cells were differentiated to monocytes by incubating the cells with PMA for 48 h. The cells (5x10⁵) in a tissue culture dish were exposed to E. coli LPS (10 ng/ml) for 24 h and the culture supernatant was collected and cytokine(IL-1beta, TNF-alpha and IL-6) levels were measured by ELISA kits (R and D Systems). The cells were rested for another 24 h and re-challenged with Pd LPS (10 ng/ml) for 24 h. At the end of incubation, culture supernatant was assayed for cytokines as described above. The study was repeated by treating the cells with Pd LPS first and then challenged with LPS of Ec. Results: Cells exposed to Ec LPS, secreted significantly less cytokines upon secondary challenge with Pd LPS: A reduction of 87% of IL-1beta, 77% of TNF-alpha and 28 % of IL-6, was observed. Only 12-48% inhibition was seen when the monocytes were first exposed to Pd LPS prior to Ec LPS challenge. Conclusion: The data suggest that LPS of Ec LPS is a stronger elicitor of cytokines than Pd. The low degree of stimulation of inflammatory cytokines by P. denticola LPS and its ability to induce immune tolerance may contribute to the chronic nature of periodontal infections.

Supported by College of Dentistry Alumni Endowment Fund.