TLR2 Transmodulates Monocyte Adhesion and Transmigration via Rac1- and PI3K-Mediated Inside-Out Signaling in Response toPorphyromonas gingivalisFimbriae

Abstract

We present evidence for a novel TLR2 function in transmodulating the adhesive activities of human monocytes in response to the fimbriae of Porphyromonas gingivalis, a pathogen implicated in chronic periodontitis and atherosclerosis. Monocyte recruitment into the subendothelium is a crucial step in atherosclerosis, and we investigated the role of P. gingivalis fimbriae in stimulating monocyte adhesion to endothelial cells and transendothelial migration. Fimbriae induced CD11b/CD18-dependent adhesion of human monocytes or mouse macrophages to endothelial receptor ICAM-1; these activities were inhibited by TLR2 blockade or deficiency or by pharmacological inhibitors of PI3K. Moreover, this inducible adhesive activity was sensitive to the action of Clostridium difficile toxin B, but was not affected by Clostridium botulinum C3 exoenzyme, pertussis toxin, or cholera toxin. Accordingly, we subsequently showed through the use of dominant negative signaling mutants of small GTPases, that Rac1 mediates the ability of fimbria-stimulated monocytes to bind ICAM-1. A dominant negative mutant of Rac1 also inhibited the lipid kinase activity of PI3K suggesting that Rac1 acts upstream of PI3K in this proadhesive pathway. Furthermore, fimbriae stimulated monocyte adhesion to HUVEC and transmigration across HUVEC monolayers; both activities required TLR2 and Rac1 signaling and were dependent upon ICAM-1 and the high-affinity state of CD11b/CD18. P. gingivalis-stimulated monocytes displayed enhanced transendothelial migration compared with monocytes stimulated with nonfimbriated isogenic mutants. Thus, P. gingivalis fimbriae activate a novel proadhesive pathway in human monocytes, involving TLR2, Rac1, PI3K, and CD11b/CD18, which may constitute a mechanistic basis linking P. gingivalis to inflammatory atherosclerotic processes.

Subversion of Innate Immunity by Periodontopathic Bacteria via Exploitation of Complement Receptor-3

“…CR3 exploitation by P. gingivalis is initiated at the level of TLR2, since TLR2 inside-out signaling is required for effective interaction of P. gingivalis fimbriae with CR3 (Harokopakis et al, 2006;Harokopakis and Hajishengallis, 2005). Consistent with this, we have now found that the intracellular survival of fimbriated P. gingivalis is dramatically reduced in TLR2-deficient macrophages relative to normal controls (Fig.…”

“…The ability of P. gingivalis fimbriae to activate TLR2 inside-out signaling for CR3 activation suggested that this pathogen may stimulate monocyte transmigration. Indeed, we found that fimbriated P. gingivalis (or purified fimbriae) stimulate CR3-dependent monocyte adhesion to endothelial ICAM-1 and transmigration across endothelial cell monolayers (Harokopakis et al, 2006). This may represent a potentially protective mechanism which can contribute to monocyte recruitment to sites of P. gingivalis infection.…”

“…P. gingivalis exploits TLR2 and CR3 and undermines innate immunity P. gingivalis exploits its fimbrial-mediated interaction with macrophage TLR2 to activate the high-affinity conformation of CR3. This pathway proceeds through Rac1/PI3K-mediated inside-out signaling and requires CD14 for facilitating the fimbria-TLR2 interaction Harokopakis et al, 2006). TLR2 stimulation by P. gingivalis also induces NF-κB activation and cytokine production Hajishengallis et al, 2006a).…”

“…These interactions take place in membrane lipid rafts where CD14 is constitutively found, whereas TLR2 and CR3 are recruited there following cell stimulation with P. gingivalis fimbriae (Hajishengallis et al, 2006a;Hajishengallis et al, 2006b). Unlike wild-type P. gingivalis, the ability of nonfimbriated mutants to activate and interact with CR3 is relatively poor (Hajishengallis et al, 2006b;Harokopakis et al, 2006).…”

Involvement of the TREM-1/DAP12 pathway in the innate immune responses to Porphyromonas gingivalis

“…Apart from its well accepted role as an oral pathogen in the establishment of chronic inflammation at the site of infection, there is increasing evidence of a link between P. gingivalis-associated periodontal disease and systemic inflammatory conditions, such as diabetes, preterm birth, aspiration pneumonia and atherosclerotic cardiovascular disease (Brodala et al, 2005;Liao et al, 2009;Lin et al, 2003;Maekawa et al, 2011;Pussinen and Mattila, 2004). On the cellular level, a convincing body of evidence indicates that P. gingivalis possesses unique signalling properties, which enable it to evade, subvert or manipulate the host immune defences by exploiting the toll-like receptor (TLR) family, in particular TLR2 (Burns et al, 2006;Hajishengallis et al, 2002;Hajishengallis et al, 2008b;Harokopakis et al, 2006;Wang et al, 2010). As an opportunistic pathogen, P. gingivalis can preferentially target innate immunity, which may in turn disable adaptive immunity, hence causing a general deregulation of the immune responses (Hajishengallis, 2009;Hajishengallis et al, 2008a).…”