Department of General Dentistry, Goldman School of Dental Medicine, Boston University, Boston, Massachusetts 02118,1 Department of Dental Public Health, Nihon University School of Dentistry at Matsudo, Chiba 271-8587, Japan,2 Department of Medicine, Section of Infectious Diseases, Boston University School of Medicine, Boston, Massachusetts 02118,3 Department of Conservative Dentistry, Tokushima University School of Dentistry, Tokushima 770-8504, Japan,4 Department of Oral Microbiology, Kanagawa Dental College, Yokosuka, Kanagawa 238-8580, Japan,5 Department of Periodontology and Oral Biology, Goldman School of Dental Medicine, Boston University, Boston, Massachusetts 02118,6 Department of Microbiology, Boston University School of Medicine, Boston, Massachusetts 021187
Received 13 June 2005/ Returned for modification 29 August 2005/ Accepted 2 November 2005
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P. gingivalis rapidly accelerated atherosclerosis. Five-week-old male ApoE/ mice (Jackson Laboratories, Bar Harbor, Maine) were cared for in accordance with National Institutes of Health- and Boston University Institutional Animal Care and Use Committee-approved procedures, received standard chow diet and water ad libitum, and were randomly placed into three groups (n = 10 for each group). One group of ApoE/ mice was challenged orally with P. gingivalis strain 381 five times a week for 3 weeks to mimic chronic exposure to P. gingivalis, as described previously (7, 9). A second group of animals was immunized subcutaneously two times a week for 3 weeks with 0.1 ml of heat-killed P. gingivalis 381 in sterile, pyrogen-free saline prior to oral challenge (6, 7). The third group of mice were not treated and served as age-matched controls (Fig. 1). A subset of similar groups of animals (n = 6) was followed for 6 weeks as appropriate controls for plaque accumulation in the established murine model of late events in the atherosclerotic process. All animals were monitored daily until sacrifice (24 h or 6 weeks after the final oral challenge) and appeared healthy throughout the course of this study. By a modification of the method of Paigen et al. (14), we examined cryosections of the aortic sinus for atherosclerotic plaque accumulation by oil red O staining. The average total lesion size and percentage of the lumen occupied by atheroma was determined by two independent observers, who were blinded to the identities of the individual groups. Analysis was completed by utilizing the first 10 µm of cryosection possessing all three leaflets of the aortic sinus and a second 10-µm cryosection 140 µm distal to the first section of each animal (n = 4 for each group) with a microscope coupled to a computer-assisted morphometry system (IPLabs; Scanalytics, Inc., Fairfax, Va.). Our result showed that ApoE/ mice challenged with P. gingivalis demonstrated significantly more atherosclerotic plaque accumulation in the aortic sinus compared to the unchallenged ApoE/ mice (Fig. 2B). As expected, the group of ApoE/ mice which were immunized and challenged orally with P. gingivalis did not exhibit accelerated atheroma development and resembled unchallenged ApoE/ mice (Fig. 2B). In addition, the levels of atherosclerotic plaque accumulation observed at 6 weeks were similar to those observed in our previous studies (data not shown; 3). This demonstrates that as early as 3 weeks after initial pathogen exposure, ApoE/ mice chronically challenged with P. gingivalis present with increased atherosclerosis.
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P. gingivalis accelerates atherosclerosis without an elevated systemic host response. The association of inflammation with the initiation and progression of atherosclerosis suggests that serum markers such as interleukin-6 (IL-6) and C-reactive protein may be useful in predicting an increased risk of coronary heart disease (15). At the time of sacrifice, serum was collected from each animal and examined for levels of IL-6 and serum amyloid A (SAA; the murine equivalent of human C-reactive protein) by enzyme-linked immunosorbent assay (Pierce Endogen, Rockford, Ill.). We observed that the levels of IL-6 and SAA in the sera of mice challenged with P. gingivalis were not significantly different than those of uninfected mice or mice immunized and subsequently challenged with P. gingivalis (Fig. 5). This was observed in samples obtained 1 h, 24 h, and 6 weeks after the final oral challenge. These observations suggest that oral infection with P. gingivalis does not result in a significant increase in the systemic inflammatory response.
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In addition, we have demonstrated that P. gingivalis infection does not result in significant increases in the serum levels of IL-6 and SAA. We conclude from these results that systemic activation of inflammatory mediator expression by the host does not in itself contribute significantly to the observed increase in atheroma development following P. gingivalis infection. Interestingly, in contrast to our results, Lalla et al. (9) reported that oral challenge of ApoE/ mice with P. gingivalis resulted in increased IL-6 in serum compared with unchallenged animals. Likewise, Li et al. (10) reported that intravenous infection with P. gingivalis resulted in increased SAA compared with unchallenged controls. The differences observed in those two studies, compared to our study, may reflect different routes of challenge, animal genotypes, numbers of animals examined, and time points used for assessment of IL-6 and SAA (5, 6). Future studies challenging mice deficient in IL-6 or SAA, in an ApoE/ background, are required to clarify the role of these molecules in P. gingivalis-mediated acceleration of atherosclerosis. Our observations of ICAM-1 and VCAM-1 expression and macrophage infiltration early after P. gingivalis challenge demonstrate that P. gingivalis infection leads to localized activation of the aortic vascular endothelium. This local endothelial activation may lead to macrophage fatty streak formation and accelerated atherosclerosis. Further studies are needed to define the interaction of CAM-expressing vascular endothelial cells and macrophages and the temporal events that occur during these interactions. In summary, with P. gingivalis as a model organism, we have demonstrated that invasive bacterial infection elicits a local innate immune response via TLRs and up-regulation of cell adhesion molecules and that these events specifically accelerate atherosclerosis in hyperlipidemic mice. Importantly, we have shown that innate immune activation and atherosclerosis are detectable shortly after bacterial infection and that this was prevented by immunization. Taken together, these results indicate (i) that early innate immune activation occurs locally in the aortic arch in response to infectious challenge and is associated with pathogen-accelerated atherosclerosis and (ii) that immunization may be sufficient for prevention of pathogen-accelerated atherosclerosis.
| ACKNOWLEDGMENTS |
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This work was supported by National Institutes of Health grant P01-DE-13191 to C.A.G. and National Institute of Dental and Craniofacial Research grant R01-DE-14774 to F.C.G.
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