In vivo exposure to Porphyromonas gingivalis up-regulates nitric oxide but suppresses tumour necrosis factor-alpha production by cultured macrophages.

The present study was designed to test whether the functional response of mouse macrophages elicited by chronic exposure to bacteria will be different from that of cells elicited by a non-bacterial irritant. Macrophage elicitation was conducted by Porphyromonas gingivalis, a major periodontal pathogen, in comparison to a standard elicitation by thioglycollate (TG). We measured lipopolysaccharide (LPS)-induced nitric oxide (NO) and tumour necrosis factor-alpha (TNF-alpha) secretion by the elicited macrophages, and the expression of inflammatory cytokines in the whole elicited cell population. In addition, we tested the response of TG-elicited macrophages to pretreatment with P. gingivalis LPS in vitro. Mouse peritoneal macrophages were harvested 4 days after intraperitoneal injection of TG or heat-killed P. gingivalis. TG-elicited macrophages produced undetectable levels of TNF-alpha and approximately 0.5 microM of NO. The stimulation of the macrophages with LPS resulted in the secretion of NO and TNF-alpha in a dose-dependent manner. The P. gingivalis-elicited macrophages produced basal levels of approximately 5 microM NO, but TNF-alpha was not detectable. LPS stimulation of these cells further increased the secretion of NO eightfold while TNF-alpha remained undetectable. The NO secretion by P. gingivalis-elicited cells was significantly higher than that by TG-elicited cells. Examination of cytokine expression in the whole elicited cell population revealed that both P. gingivalis-elicited cells and TG-elicited cells expressed messenger RNA for interleukin-2 (IL-2), TNF-alpha and interferon-gamma (IFN-gamma), but not for IL-4. IL-6 was expressed in P. gingivalis-elicited cells only. Pretreatment of TG-elicited macrophages with P. gingivalis LPS for 24 hr prior to a second LPS challenge resulted in down-regulation of TNF-alpha secretion and up-regulation of NO secretion, a response similar to that seen in P. gingivalis-elicited peritoneal macrophages. The results suggest that the in vivo exposure of resident macrophages to P. gingivalis induces functional changes in peritoneal macrophages. These changes might be due to the effect of P. gingivalis LPS.