Treponema denticolaSuppresses Expression of Human β-Defensin-3 in Gingival Epithelial Cells through Inhibition of the Toll-Like Receptor 2 Axis

Abstract

We reported previously that Treponema denticola, one of the periodontal pathogens, suppresses the expression of human ␤-defensins (HBDs) in human gingival epithelial cells. To identify the mechanisms involved in this suppression, immortalized and normal human gingival epithelial cells were infected with live or heat-killed T. denticola for 24 h, and then the expression of HBDs was examined by real-time RT-PCR. Live T. denticola suppressed the expression of HBD-3 substantially and also suppressed the expression of HBD-1 and HBD-2. However, heat-killed bacteria did not produce a suppressive effect but instead slightly upregulated the levels of HBD-2 and HBD-3. In contrast to live T. denticola, which reduced the activation of mitogen-activated protein kinase (MAPK) and NF-B within an hour of infection, heat-killed bacteria did not show any inhibitory effect on the MAPK and NF-B signaling pathways. Knockdown of Toll-like receptor 2 (TLR2) via RNA interference abolished the suppressive effect of T. denticola on the expression of HBD-3. Heat-killed T. denticola but not live bacteria could activate TLR2 in CHO/CD14/TLR2 reporter cells, suggesting that T. denticola contains a heat-labile inhibitor(s) of TLR2 in addition to ligands recognized by TLR2. Indeed, live T. denticola was able to inhibit TLR2 activation by Pam 3 CSK. In conclusion, T. denticola suppressed the expression of HBD-3 by inhibiting the TLR2 axis in gingival epithelial cells. These results may provide new insight into the pathogenesis of periodontitis caused by T. denticola.Periodontitis is the inflammation of periodontal tissue that results from the interaction of plaque-associated bacteria with the host immune system. During the transition from health to periodontitis, the amount of total bacteria and the proportion of periodontopathic bacteria in the plaque microflora increase (22). Among the periodontitis-associated bacteria, the members of the so-called red complex triad (Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola) are the most well-studied periodontal pathogens, and these bacteria have been characterized as the primary causal agents of periodontal disease (10,22). The gingival epithelium provides the first line of defense against these plaque bacteria by forming a physical and chemical barrier. The barrier function of epithelia is attributed to the unique architectural integrity of the cells and to the production of antimicrobial peptides, such as human ␤-defensins (HBDs) and the cathelicidin LL-37 (9). Patients with Kostmann syndrome lack LL-37 in their saliva and develop severe periodontitis in young adulthood (21), suggesting the importance of antimicrobial peptides in the maintenance of periodontal health.The regulation of HBD expression has been studied extensively. HBD-1 is expressed constitutively, whereas the expression of HBD-2 and HBD-3 is under the control of the NF-B and/or mitogen-activated protein kinase (MAPK) signaling pathway (11). At the receptor level, induction of HBD-2 expression is mediated via p...

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Virulence Factors of the Oral Spirochete Treponema denticola

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Periodontal Disease and Gingival Innate Immunity – Who Has the Upper Hand?

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