Chronic<i>Porphyromonas gingivalis</i>infection accelerates the occurrence of age-related granules in ApoE^–^/^–mice brains

Abstract

This study explored the origin of age-related granules in the apolipoprotein E gene knockout (ApoE^-/-) B6 background mice brains following chronic gingival infection with <i>Porphyromonas gingivalis</i> for 24 weeks. Intracerebral localization of <i>P. gingivalis</i> was detected by fluorescence <i>in situ</i> hybridization (FISH) and its protease by immunohistochemistry. The age-related granules were observed by periodic acid-Schiff (PAS), silver impregnation, and immunostaining. FISH showed intracerebral dissemination of <i>P. gingivalis</i> cells (<i>p</i> = 0.001). PAS and silver impregnation demonstrated the presence of larger inclusions restricted to the CA1, CA2, and dentate gyrus sectors of the hippocampus. A specific monoclonal antibody to bacterial peptidoglycan detected clusters of granules with variable sizes in mice brains infected with <i>P. gingivalis</i> (<i>p</i> = 0.004), and also highlighted areas of diffuse punctate staining equating to physical tissue damage. Mouse immunoglobulin G was observed in the capillaries of the cerebral parenchyma of all <i>P. gingivalis</i>-infected brains (<i>p</i> = 0.001), and on pyramidal neurons in some severely affected mice, compared with the sham-infected mice. Gingipains was also observed in microvessels of the hippocampus in the infected mice. This study supports the possibility of early appearance of age-related granules in ApoE^-/- mice following inflammation-mediated tissue injury, accompanied by loss of cerebral blood-brain barrier integrity.