Porphyromonas gingivalisFimbriae Proactively Modulate β₂Integrin Adhesive Activity and Promote Binding to and Internalization by Macrophages

Abstract

In monocytes, the fimbriae of the oral pathogen Porphyromonas gingivalis activate cross talk signaling from Toll-like receptor 2 (TLR2) to the ␤ 2 integrin CD11b/CD18, leading to the induction of the high-affinity state of the latter receptor. CD14 plays an important role in this "inside-out" proadhesive pathway by binding fimbriae and facilitating the activation of TLR2 and phosphatidylinositol 3-kinase signaling. In its high-affinity state, CD11b/CD18 mediates monocyte adhesion to endothelial cells and transmigration to sites of infection. We have now shown that P. gingivalis fimbriae function as both an activator and a ligand of CD11b/CD18; thus, fimbriae proactively promote their own binding to monocytes. Indeed, treatments that interfered with fimbriainduced activation of CD11b/CD18 (i.e., blockade of CD14, TLR2, or phosphatidylinositol 3-kinase signaling) also suppressed the cell binding activity of fimbriae, which was largely inducible and CD11b/CD18 dependent. Development of a recombinant inside-out signaling system in Chinese hamster ovary cells confirmed the ability of fimbriae to activate CD14/TLR2 signaling and induce their own CD11b/CD18-dependent binding. Induction of this proadhesive pathway by P. gingivalis fimbriae appeared to take place in lipid rafts. Indeed, methyl-␤-cyclodextrin, a cholesterol-sequestering agent that disrupts lipid raft organization, was found to inhibit the fimbria-induced assembly of CD14/TLR2 signaling complexes and the activation of the high-affinity state of CD11b/CD18. Experiments using macrophages from mice deficient in various pattern recognition receptors indicated that the receptors involved in the inside-out proadhesive pathway (CD14, TLR2, and CD11b/CD18) are important for mediating P. gingivalis internalization within macrophages. It therefore appears that P. gingivalis proactively modulates ␤ 2 integrin adhesive activity for intracellular uptake.

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

“…Asterisks indicate statistically significant (p < 0.05) differences between PRR deficiencies and wild-type controls. Reproduced from Hajishengallis et al 2006b. Copyright 2006 American Society for Microbiology.…”

“…2). In contrast, no significant differences were observed regarding the uptake of nonfimbriated (FimA-deficient) mutants, suggesting that CR3 preferentially takes up fimbriated P. gingivalis (Hajishengallis et al, 2006b). Although TLR2 is not a phagocytic receptor, TLR2 deficiency similarly inhibits the uptake of P. gingivalis, consistent with its role in inside-out signaling for CR3 activation (Hajishengallis et al, 2006b) (Fig.…”

“…Rather, CR3 engages in cooperative interactions with the CD14/TLR2 signaling complex in response to P. gingivalis fimbriae. Hajishengallis et al, 2006a;Hajishengallis et al, 2006b;Harokopakis and Hajishengallis, 2005). According to this model, P. gingivalis fimbriae bind CD14 and activate TLR2-and phosphatidylinositol 3-kinase (PI3K)-mediated inside-out signaling leading to activation of the ligand-binding capacity of CR3 (Fig.…”

“…1). 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).…”

“…We first determined whether CR3 mediates P. gingivalis internalization by macrophages and found that CR3-deficient (CD11b−/−) mouse macrophages display significantly reduced capacity in the uptake of fimbriated P. gingivalis, compared to normal macrophages (Hajishengallis et al, 2006b) (Fig. 2).…”