gingivalis Nutlin3 . Notably, P. gingivalis does not rely on immunological mechanisms for C5aR activation, since it can activate this complement receptor through C5a generated locally by its Arg-specific gingipains (HRgpA, RgpB) that have C5 convertase-like activity [64, 65]. Porphyromonas gingivalis also expresses a number of potent TLR2 ligands including serine lipids and lipoproteins [66, 67]. At the molecular level, the P. gingivalis-induced C5aR-TLR2 cross-talk in macrophages leads to synergistic activation of cAMP-dependent protein kinase A for inhibition of glycogen synthase kinase-3β and of iNOS-dependent
intracellular bacterial killing  (Fig. 3). In the murine periodontal tissue, C5aR signaling synergizes with TLR2 to induce secretion of cytokines that promote periodontal inflammation and bone loss (TNF, IL-1β, IL-6, and IL-17A). This is likely to enhance the fitness of P. gingivalis and other periodontitis-associated bacteria that require an inflammatory environment to secure critical nutrients, i.e. tissue breakdown products including peptides and hemin-derived iron. In stark contrast to the upregulation of bone-resorptive inflammatory cytokines, P. gingivalis-induced C5aR signaling in macrophages downregulates TLR2-induced MG-132 purchase IL-12 and hence inhibits IFN-γ production and cell-mediated immunity against the bacteria [63, 65]. The selective inhibition
bioactive IL-12 (IL-12p35/IL-12p40) associated with C5aR-TLR2 cross-talk involves ERK1/2 signaling-dependent suppression of the IFN regulatory factor-1 (IRF-1), a transcription many factor that is crucial for the regulation of IL-12 p35 and p40 mRNA expression [65, 68]. Importantly, genetic ablation of C5aR or TLR2 promotes the killing of P. gingivalis in vivo [64, 69]. The inhibitory ERK1/2 pathway that regulates TLR2-induced IL-12 is also activated when P. gingivalis binds complement receptor 3 (CR3) on macrophages [70, 71] (Fig. 3). CR3 is a β2 integrin (CD11b/CD18) that can bind ligands when its high-affinity conformation is transactivated via inside-out signaling by other receptors such as chemokine receptors. Porphyromonas gingivalis induces TLR2-mediated transactivation of CR3 through an inside-out pathway that involves RAC1, PI3K, and cytohesin-1 [72, 73] (see Fig. 3). Upon binding CR3, P. gingivalis not only downregulates IL-12 but also enters macrophages in a relatively safe way , perhaps because CR3 is not linked to strong microbicidal mechanisms such as those activated by FcγR-mediated phagocytosis . Indeed, P. gingivalis can persist intracellularly in WT macrophages for longer times than in CR3-deficient macrophages . As alluded to above, P. gingivalis can activate C5aR signaling independently of the canonical activation of complement [64, 65]. In fact, P.