No abstract

f Porphyromonas gingivalis is an important member of the anaerobic oral flora. Its presence fosters growth of periodontal biofilm and development of periodontitis. In this study, we demonstrated that lipophilic outer membrane vesicles (OMV) shed from P. gingivalis promote monocyte unresponsiveness to live P. gingivalis but retain reactivity to stimulation with bacterial DNA isolated from P. gingivalis or AIM2 ligand poly(dA·dT). OMV-mediated tolerance of P. gingivalis is characterized by selective abrogation of tumor necrosis factor (TNF). Neutralization of interleukin-10 (IL-10) during OMV challenge partially restores monocyte responsiveness to P. gingivalis; full reactivity to P. gingivalis can be restored by inhibition of mTOR signaling, which we previously identified as the major signaling pathway promoting Toll-like receptor 2 and Toll-like receptor 4 (TLR2/4)-mediated tolerance in monocytes. However, despite previous reports emphasizing a central role of TLR2 in innate immune recognition of P. gingivalis, our current findings highlight a selective role of TLR4 in the promotion of OMV-mediated TNF tolerance: only blockade of TLR4 -and not of TLR2-restores responsiveness to P. gingivalis. Of further note, OMV-mediated tolerance is preserved in the presence of cytochalasin B and chloroquine, indicating that triggering of surface TLR4 is sufficient for this effect. Taking the results together, we propose that P. gingivalis OMV contribute to local immune evasion of P. gingivalis by hampering the host response. P eriodontitis (PD) is probably the most frequent chronic inflammatory disorder associated with an alteration of the local microbiota. Clinically, release of inflammatory mediators induces collagen degradation and bone resorption, which ultimately result in tooth loss. The carbohydrate-deficient subgingival environment fosters the growth of Porphyromonas gingivalis (1). Tissue degradation caused by enzymes released from bacteria and neutrophils provokes an inflammatory response and supplies bacteria with additional nutrients. This specific milieu permits growth of P. gingivalis, whose sophisticated mechanisms for immune evasion enhance growth of the periodontal biofilm, thus leading to bacterial overgrowth and excessive immune stimulation (2, 3).P. gingivalis releases outer membrane lipophilic microvesicles, which contain lipopolysaccharide (LPS) as a major structural component (4-6). These outer membrane vesicles (OMV) overcome the epithelial barrier, thus transporting LPS and other virulence factors into the host tissue (7). Subsequently, OMV elicit a robust mucosal immune response (8), an effect exploited in OMV-based vaccines (8, 9) and mainly attributed to their LPS content (6). However, unlike LPS from Escherichia coli or Salmonella spp., which are Toll-like receptor 4 (TLR4) agonists, P. gingivalis LPS contains a mixture of chemically diverse lipid A species with distinct immune stimulatory properties (10). Albeit P. gingivalis LPS was formerly thought to act as a TLR2 ligand (11, 12), recent studi...