Complementary Tolls in the periodontium: how periodontal bacteria modify complement and Toll-like receptor responses to prevail in the host

Abstract

The complement and the Toll-like receptors are rapidly activatable systems which, in concert, provide first-line innate defense against infection and act as mediators between the innate and the adaptive immune response. The ability of periodontal bacteria to persist and establish chronic infections in the periodontium suggests that they may have evolved strategies to evade, disarm, or subvert these defense systems to their own advantage. Indeed, accumulating evidence indicates that at least some of the major periodontal pathogens utilize ingenious mechanisms to not only undermine each system separately, but also exploit crosstalk points between the complement and the Toll-like receptor pathways. It is conceivable that immune subversive activities by certain keynote periodontal pathogens, such as those comprising the so-called “red complex”, may be critical for the persistence of the entire mixed-species biofilm community in the diseased periodontium. This review summarizes and synthesizes recent discoveries in this field, which offers important insights into the pathology associated with the complex periodontal host-microbe interplay.

Fig.1. Activation pathways of the complement system

All three pathways converge at a central step, involving activation of the third component of complement (C3) by pathway-specific C3 convertases. The classical pathway is initiated by antigen-antibody (Ag-Ab) complexes and requires the participation of C1, C2, and C4. The lectin pathway is triggered through interaction of the mannose-binding lectin (MBL) with specific carbohydrate groups on the surface of microorganisms. The alternative pathway is initiated by spontaneously hydrolyzed C3 [C3(H₂O)] which can thereby form a complex with factor B, followed by factor B cleavage by factor D and formation of the initial alternative pathway C3 convertase (104). Morerover, the alternative pathway can be induced by bacterial lipopolysacharide (LPS) and lipooligosacharide (LOS) in a properdin-dependent way (86). Proteolytic cleavage of a series of proteins downstream of C3 leads to the generation of potent effector molecules. These include the anaphylatoxins C3a and C5a, which activate specific receptors (C3aR and C5aR, respectively), although C5a also interacts with the so-called C5a receptor-like 2 (C5L2), which is only modestly characterized (91). Additional effectors generated downstream of C3 are the opsonins C3b and iC3b, the latter of which coats microbes and promotes their phagocytosis by complement receptor-3 (CR3). In the terminal pathway, C5b initiates the assembly of the C5b-9 membrane attack complex (MAC), which in turn induces microbial cell lysis (104). Complement activation can also occur through cross-talk with other physiological pathways, such as the coagulation system, in which thrombin acts as a C5 convertase (71).

Fig. 2. Microbial ligand specificities of human TLRs

Those TLRs which recognize extracellular microbial structures (i.e., TLRs 1, 2, 4, 5, and 6) are expressed on the host cell surface. TLR2 in cooperation with its signaling partners, TLR1 or TLR6, detect mostly microbial cell wall components, such as lipoproteins, lipoteichoic acid (LTA), firmbriae, or yeast zymosan (1, 12). TLR4 and TLR5 recognize lipopolysaccharide (LPS) and bacterial flagellin, respectively, whereas no ligand has been identified for TLR10. Those TLRs specializing in detecting viral or bacterial nucleic acids (i.e., TLRs 3, 7, 8 and 9) are expressed intracellularly on endocytic vesicles. TLR3 recognizes double-stranded viral RNA, TLR7 and TLR8 single-stranded viral RNA, and TLR9 detects microbial CpG DNA.

Fig. 3. Evasion or subversion of TLR activation by P. gingivalis

P. gingivalis uses an elaborate system of lipid A phosphatase and deacylase activities that modify the lipid A structure of its lipopolysaccharide (21, 22). These modifications result in lipopolysaccharide molecules that can either evade or actively antagonize TLR4 activation (depicted as a homodimer; TLR4/4) (21, 22). Although the activation of the TLR2/TLR1 heterodimer (TLR2/1) is not antagonized at the TLR receptor level, P. gingivalis instigates a molecular cross-talk between the CXC-chemokine receptor 4 and TLR2/1. Unlike CD14 which facilitates TLR2/1 activation by the pathogen (57), CXCR4 suppresses TLR2 signaling (62). Mechanistically, P. gingivalis uses its fimbriae to bind CXCR4 and induce cAMP-dependent PKA signaling, which in turn inhibits the activation of nuclear factor-κB (NF-κB) activation (62).

Fig. 4. Cross-talk pathways between TLRs and complement in P. gingivalis-activated macrophages

TLR recognition of P. gingivalis is predominantly mediated by the TLR2/TLR1 heterodimer (TLR2/1), aided by the CD14 co-receptor (57). This interaction induces phosphatidylinositol 3-kinase (PI3K)-dependent inside-out signaling, which transactivates the high-affinity state of complement receptor-3 (CR3) (54, 64) (CR3). Interestingly, P. gingivalis interacts with activated CR3 and induces extracellular signal-related kinase 1/2 (ERK1/2) signaling, which in turn downregulates mRNA expression of cytokines of the interleukin-12 family (56). Moreover, P. gingivalis uses its gingipains to attack C5 and release biologically C5a (128, 176). Through its receptor (C5aR), C5a can activate PI3K and ERK1/2, which in turn suppress critical transcription factors (the interferon regulatory factors 1 and 8; IRF-1, -8), required for expression of cytokines of the interleukin-12 family (67). Intriguingly, inhibition of bioactive interleukin-12 though these mechanisms results in impaired immune clearance of P. gingivalis in vivo (56), suggesting that the pathogen exploits TLR/complement cross-talk signaling to promote its virulence.

Fig. 5. Biphasic virulence effects of P. gingivalis and P. intermedia cysteine proteases

(A) Effects of low concentrations of the proteases, i.e., early in the clonization process, which activate the classical pathway. (B) Effects of high concentrations of the proteases, i.e., in a developed biofilm, which suppress the physiological activation of the complement cascade, but selectively generate anaphylatoxins via limited degradation of C3 and C5. P. gingivalis and P. intermedia are exceptionally resistant to the bactericidal activity of complement and released proteases, gingipains and interpain, respectively, contribute to the resistance. At low concentrations of the proteases, likely to occur at the early stages of infection or at a long distance from the dental plaque, the released proteases activate the C1 complex leading to deposition of C1q on the bacterial surface (128, 131). Complement activation may eliminate complement-sensitive commensal bacteria which could otherwise compete with pathogens for space and nutrients. At high concentrations, the proteases synergistically inhibit the bactericidal activity of complement by degrading C3 and C5, thus protecting complement-sensitive bacteria in their proximity and promoting biofilm development. At the same time, released anaphylatoxins (C3a and C5a; although only the latter was confirmed to retain its biological activity) fuel inflammation resulting in tissue damage and nutrient generation, and are exploited for immune evasion (see also Fig. 4).