Association between periodontal pathogens and systemic disease.

A growing body of literature suggests that there is a link between periodontitis and systemic diseases. These diseases include cardiovascular disease, gastrointestinal and colorectal cancer, diabetes and insulin resistance, and Alzheimer's disease, as well as respiratory tract infection and adverse pregnancy outcomes. The presence of periodontal pathogens and their metabolic by-products in the mouth may in fact modulate the immune response beyond the oral cavity, thus promoting the development of systemic conditions. A cause-and-effect relationship has not been established yet for most of the diseases, and the mediators of the association are still being identified. A better understanding of the systemic effects of oral microorganisms will contribute to the goal of using the oral cavity to diagnose and possibly treat non-oral systemic disease.

Fusobacterium nucleatum infection of gingival epithelial cells leads to NLRP3 inflammasome-dependent secretion of IL-1ß and the danger signals ASC and HMGB1.

Fusobacterium nucleatum is an invasive anaerobic bacterium that is associated with periodontal disease. Previous studies have focused on virulence factors produced by F. nucleatum, but early recognition of the pathogen by the immune system remains poorly understood. Although an inflammasome in gingival epithelial cells (GECs) can be stimulated by danger-associated molecular patterns (DAMPs) (also known as danger signals) such as ATP, inflammasome activation by this periodontal pathogen has yet to be described in these cells. This study therefore examines the effects of F. nucleatum infection on pro-inflammatory cytokine expression and inflammasome activation in GECs. Our results indicate that infection induces translocation of NF-κB into the nucleus, resulting in cytokine gene expression. In addition, infection activates the NLRP3 inflammasome, which in turn activates caspase-1 and stimulates secretion of mature IL-1ß. Unlike other pathogens studied until now, F. nucleatum activates the inflammasome in GECs in the absence of exogenous DAMPs such as ATP. Finally, infection promotes release of other DAMPs that mediate inflammation, such as high-mobility group box 1 protein and apoptosis-associated speck-like protein, with a similar time-course as caspase-1 activation. Thus, F. nucleatum expresses the pathogen-associated molecular patterns necessary to activate NF-κB and also provides an endogenous DAMP to stimulate the inflammasome and further amplify inflammation through secretion of secondary DAMPs.