Aetiological links between oral pathogens and dementia.

Abstract

Introduction: Several observational studies support an association between periodontal disease and Alzheimer’s disease (AD). Poorly managed oral hygiene together with the immunosuppressed status of demented patients appears central to this hypothesis as together they contribute not only to an increased incidence of oral infections but also to recurrent bacteraemia that can seed oral bacteria into systemic circulation. The aim of this study was to establish a link between periodontal disease and AD with a view to identifying the red complex periodontal disease bacteria (Treponema denticola, Tannerella forsythia and Porphyromonas gingivalis) and/or bacterial components in human AD and non-AD brain tissue and explore the proof of concept of the red complex bacteria accessing the brain of ApoEnull mice during experimental periodontitis, and assess how they may contribute to the development of AD pathology.
Methods: Molecular techniques (PCR, cloning and sequencing) were employed for investigating the presence of bacterial DNA within the specimens (human or mouse) using two different approaches (universal and species specific primers). The presence of specific virulence factors were determined using anti-bacterial antibodies. The innate immune responses were detected using antibodies against complement activation, alongside inflammatory assessment using specific antibodies for activated microglia and astrocytes. Further, histology staining was used to assess tissue preservation and the presence of pathological hallmarks of AD.
Results: Human AD and non-AD controls failed to demonstrate the presence of red complex pathogens when analysed using molecular methodologies. However, immunofluorescence labelling of a virulence factor (LPS) was positive for P. gingivalis in 4 out of 10 AD cases. Immunoblotting demonstrated bands corresponding to P. ginigivalis LPS in the same AD brain specimens (p = 0.029). Analysis of brain tissue from ApoEnull mice induced with periodontal disease using molecular methods demonstrated 6 out of 12 ApoEnull mice brains contained P. gingivalis genomic DNA at 12 weeks (P = 0.006), and increased to 9 out of 12 at 24 weeks (P = 0.0001). In addition, tissue sections of infected mice demonstrated periodic acid-Schiff (PAS)-positive, argyrophilic inclusions in the hippocampus at both time points, which also labelled positive with the bacterial-specific anti-peptidoglycan antibody. Also, it was noted that microglia in both infected and control groups demonstrated strong intracellular labelling with C3 and C9, presumed on-going biosynthesis, however, the pyramidal neurons of the hippocampus in 4 out of 12 P. gingivalis infected mice brains were clearly opsonised with C3 activation fragments (P = 0.032) suggesting they were under attack from complement mediated lysis.
Conclusion: These results show P. gingivalis was able to access the brain of humans and ApoEnull mice, supporting the concept of the focal infection theory. Together these results suggest a potential link with AD via the periodontal pathogen translocating from its original oral niche to the brain. ApoEnull mice induced with periodontal disease demonstrated the intracerebaral innate immune responses were initiated by local CNS cells, which not only contributed to a higher inflammatory burden but also bystander damage of functional neurons in the hippocampus area of the brain which is associated with memory. Although further research is needed to establish clinical measures that demonstrate a cause and effect relationship between oral infections and AD, this study does provide initial support to the role of periodontal pathogens in the development of dementia. Early treatment of periodontal disease in addition to greater awareness of the importance of maintaining good oral health may halt or slow down the progression of this debilitating disease.