Defensins.

Antimicrobial peptides are components of the innate immune response in eukaryotes, providing defense against a wide spectrum of gram-positive and gram-negative bacteria, viruses, and fungi.142,372,391 In the oral cavity, at least 45 different antimicrobial peptides belonging to different biochemical classes are found in the saliva and the gingival crevicular fluid.¹¹⁹ Defensins and the cathelicidin LL-37 are the most studied antimicrobial peptides (for comprehensive reviews of various antimicrobial peptides, the reader is referred to References 119 and 177). Defensins and LL-37 are cationic peptides that bind to negatively charged molecules on the microbial cell surface (e.g., LPS in gram-negative bacteria and lipoteichoic acid in gram-positive bacteria), thereby depolarizing and permeabilizing the cell membrane and leading to bacterial cell death.^199,322

Defensins are the predominant type of antimicrobial peptides in humans, and, on the basis of the spacing and pairing of cysteine residues, they can be subdivided into α- and β-defensins. α-Defensins can be further subdivided into those produced by polymorphonuclear neutrophils (also called human neutrophil peptides 1 through 4), which are most predominant in infection sites, and those produced by the Paneth cells of the small intestine (HD5 and 6). β-Defensins were initially identified in upper respiratory mucosal epithelial cells 20 years ago,⁸⁵ and they are now known to be produced by a variety of epithelial cells, including cells from the skin, the upper respiratory tract mucosa, the kidneys, the pancreas, the lung, the uterus, the eye, and the urinary tract.⁷⁵

The presence of defensins in the oral cavity of humans was first reported in 1995,³²⁰ and they have since been confirmed by various studies describing the abundant production of human β-defensins by the tissues in the oral cavity and by salivary glands. These defensins—particularly the β-defensins 1, 2, and 3—can be found in the saliva and the gingival crevicular fluid.^{75,91,197,277,306}

On the periodontium, β-defensins are located in different regions of the epithelium: β-defensins 1 and 2 are observed in the upper layers of the gingival and sulcular epithelium, adjacent to the microbial biofilm and external environment, which is consistent with the innate immune “barrier” function of the epithelium. Interestingly, neither β-defensin 1 nor β-defensin 2 are found in the junctional epithelium. Protection in the junctional epithelium may be provided by the higher concentration of α-defensins and LL-37 produced by granulocytes migrating toward the gingival sulcus.75,74,233

The expression of defensins induced by whole periodontopathogenic bacteria such as F. nucleatum, P. gingivalis, A. actinomycetemcomitans, and T. denticola is largely dependent on TLR signaling, as various studies have demonstrated by silencing TLR expression in epithelial cell lines.174,278,328,364 For further information about the induction of defensins by periodontopathic bacteria, the reader is referred to Reference 118.

In addition to their primary antimicrobial function, defensins are modulated by immune response mediators and also present immunomodulatory functions of their own, which suggests that these peptides may actually function as an enhancer of innate immune response and even as a bridge between innate and adaptive immunity. Evidence that β-defensins 2, 3, and 4 induce the production of IL-6, IL-10, IL-18, IP-10, MCP1, MIP3, and RANTES by epithelial cells supports the idea of this immunoregulatory function of defensins.248,247 On the other hand, defensins are also modulated by soluble mediators secreted by immune cells, such as proinflammatory cytokines IL-1β, tumor necrosis factor-α (TNF-α), interferon-γ, and IL-17, which induce the expression of β-defensins 1, 2, and 3 by epithelial cells^182,186,364; anti-inflammatory IL-4 and IL-10 suppress their production.¹⁸⁶ Defensins also have a chemotactic effect on immune cells. The mechanisms are not fully understood and may involve direct interaction with unknown receptors in the target cells as well as indirectly inducing the production of chemokines. β-Defensins exert a chemotactic effect on macrophages, immature dendritic cells, memory T cells, and mastocytes via CCR6.³⁸⁴ Neutrophil-derived α-defensins are also involved in the chemokine receptor 6 of naive T cells and immature dendritic cells as well as the degranulation of mastocytes and complement activation.^382,383

In the periodontium, the expression of β-defensins 1, 2, and 3 is observed at the mRNA level both in clinically healthy and diseased tissues; the expression of these epithelium-derived peptides appears to be correlated with periodontal health, thereby suggesting a protective role.38,46,365 On the other hand, the expression of neutrophil-derived defensins (α-defensins 1, 2, and 3) and LL-37 was significantly elevated in the gingival crevicular fluid of patients with chronic periodontitis^282,357; however, the role of defensins and LL-37 in periodontal disease is currently not clear.

These antimicrobial and immunomodulatory roles of defensins have obvious attractiveness for therapeutic applications. However, the biochemical purification process is cost-inefficient and the synthesis process is complicated by the size and tridimensional structure of the peptides. Recently, novel analogs of defensins have shown even higher antibacterial activity than the endogenous β-defensins 1 and 3, without any cytotoxic effects on host cells,³²¹ thus indicating the promise of this approach. The issues with the direct use of antimicrobial peptides with therapeutic purposes may also be dealt with by stimulating the endogenous production of these peptides.

Activation of the innate immune system is critical for lymphocyte activation and other immune cells to help clear the infectious microorganisms. However, the exuberant production of proinflammatory cytokines leads to severe pathology, including periodontal bone loss.18,81,82,122,123 To help prevent the deleterious effects of TLR activation, a number of signaling mechanisms have evolved. These mechanisms include the downregulation of surface TLR receptor expression; the transcriptional induction of negative regulators such as IL-1 receptor-associated kinase; the suppression of cytokine signaling-1 and SH2-containing inositol phosphatase; and the production of anti-inflammatory cytokines, including IL-10 and transforming growth factor-β (TGF-β).²¹⁶ In contrast with the release of proinflammatory cytokines and mediators, the production of these negative immune regulators occurs over a much longer time frame, thereby providing a vital role for controlling the extent of pro- and anti-inflammatory mediators in a proper temporal sequence.²¹⁶

Cell Signaling Pathways and the Expression of Biologically Active Mediators in the Innate Immune Response

After MAMPs are recognized by the appropriate pattern-recognition receptors, a signal is initiated within the cell. This signal is transduced through the cytoplasm and the nucleus, more commonly by the sequential post-translational modification (e.g., phosphorylation) of a cascade of kinases that are constitutively expressed and that ultimately will determine the cell’s response to the MAMPs that are detected.

TLRs are single-pass transmembrane proteins with N-terminals that present leucine-rich repeats that are responsible for the recognition of their ligands and with C-terminal cytoplasmic domains that are very similar to the cytoplasmic region of the interleukin-1 receptor,⁴ which are called Toll/IL-1 receptor domains (TIR domains). Thus, subsequent to the recognition of a ligand by TLRs, the signal generated makes use of pathways similar to those used by the IL-1 receptor. TLR signaling was originally described in the context of the activation of interferon regulatory factors (IRF) family of transcription factors and NF-κβ, thereby leading to the expression of interferon-γ and early-response inflammatory genes, respectively.

Interestingly, the participation of at least four adaptor proteins (MyD88, TRIF, Mal/TIRAP, and TRAM) that contain TIR domains and that can be recruited by activated TLRs results in important branching of the signal transduction and yields a significant flexibility to TLR signaling by allowing for cross-talk with other pathways, most notably the mitogen-activated protein kinase (MAPK) pathway. These adaptor proteins are recruited to TLRs by homophilic interactions between their TIR domains, and they are used differently by the TLRs. TLR5, TLR7, and TLR9 were shown to depend on the recruitment of MyD88 to signal,152,159 whereas TLR3 is the only TLR that does not use MyD88.³⁷⁹ TLR4, on the other hand, can use all four adaptor proteins: MyD88, TRIF, Mal/TIRAP, and TRAM.^378–380 Although the activation of the canonical NF-κβ pathway is usually affected by all TLRs, the timing of NF-κβ activation^188,190 as well as the additional signaling pathways that are activated by the branching of the signal varies among TLR receptors and with the participation of different adaptor proteins (Figure 9-2). These variations will ultimately affect the biological result in terms of gene expression, as demonstrated by the finding that, even though NF-κβ activation is observed after TLR4 stimulation by LPS, this may or may not result in inflammatory gene expression, depending on the adaptor protein used. In wild-type cells, LPS stimulation results in inflammatory cytokine expression, whereas, in MyD88-deficient cells, LPS fails to induce cytokine expression. In the absence of MyD88, the activation of NF-κβ occurs with delayed kinetics as compared with wild-type cells.¹⁸⁸

The shift of the microbial population present in the oral biofilm from predominantly gram-positive to gram-negative bacteria that is associated with the onset of periodontal disease may lead to different patterns of immune response as a result of the type of TLR predominantly activated. Gram-positive bacteria were shown to activate TLR2, which induced increased expression of IL-8, whereas gram-negative bacteria activated predominantly TLR4, thereby resulting in the increased expression of TNF-α.³⁵¹

However, some gram-negative microorganisms that are present in the oral biofilm and associated with periodontal disease are rather unique in their capacity to activate NF-κβ via the preferential use of TLR2.⁴⁹ It has been reported that most gram-negative bacteria associated with periodontal disease—including Porphyromonas gingivalis, Tannerella forsythensis, Prevotella intermedia, Prevotella nigrescens, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, and Veillonella parvula—are all capable of activating TLR2, whereas the latter two microorganisms also activate TLR4.¹⁹³ Even though all of these disease-associated microorganisms activate TLR2 signaling, this pathway can also be activated in vitro by microorganisms that are present in an oral biofilm composed primarily of gram-positive bacteria, which are common colonizers of the oral biofilm and not associated with clinical signs of periodontal disease.³⁸⁸ The fact that TLR2 is activated by both pathogenic and non-pathogenic microorganisms is an interesting finding and suggests differences in the use of adaptor proteins as well as the concomitant activation of other TLRs by different MAMPs expressed by the various bacterial species that are present in an oral biofilm associated with disease. These differences can lead to the activation of different signaling pathways and the subsequent modulation of the host response.

NOD-like receptors include 23 genes in humans, even though some of these genes are primarily expressed in macrophages and polymorphonuclear leukocytes; the best studied and best characterized members, Nod1 and Nod2, are expressed by a wide variety of cells, including epithelial cells,358,360 monocytes/macrophages,²⁶⁰ and dendritic cells³⁴⁵ (see Table 9-1). The exact mechanism by which these proteins recognize their ligands is still unknown, and there is no evidence for direct interaction between ligands and NOD proteins. According to the proposed mechanism of activation of NOD proteins, they are kept in an inactive state via intramolecular interactions among the C-terminal leucine-rich repeat domain, the N-terminal fragment caspase-activating recruitment domain, and the NOD domains. Ligand recognition results in conformational changes that relieve the autoinhibitory intramolecular interactions and that allow for NOD-domain dependent nucleotide binding and oligomerization.¹⁶² The stimulation of Nod1 and Nod2 will result in the activation of NF-κβ and MAPKs, which is similar to the activation of TLRs (see Figure 9-2); however, the signal transduction pathways require different signaling intermediates. The activation of Nod2 leads to the recruitment of a kinase called receptor-interacting protein 2 (Rip2, also known as RICK), which will bind directly to the inhibitor of NF-κβ kinase-γ (IKKγ; also known as NF-κβ essential modulator or NEMO) and promote its ubiquitination and the activation of the catalytic regulatory subunits of the inhibitor of the IKK complex. Once activated, IKK phosphorylates the inhibitor Iκβ, thereby leading to its degradation by the proteasome, releasing NF-κβ, and allowing for its translocation to the nucleus, where it will affect the expression of various inflammatory genes.^150,265 In this pathway, Rip2 has been shown to be required for Nod1 and Nod2 activation of NF-κβ,²⁶⁵ but, interestingly, its kinase activity is not essential.¹⁴⁷ The same signaling intermediate Rip2 has also been shown to be involved in Nod1- and Nod2-induced MAPK activation by mediating the recruitment of the upstream activator of MAPK, TGF-β–activated kinase 1 (TAK1).¹⁴⁷ However, although Rip2 and TAK1 are required for Nod1- and Nod2-mediated activation of MAPKs, the intermediate steps in the activation of this pathway are not well known.^265,315

Because TLRs and NOD proteins are PRRs involved in the recognition of bacteria and considering that the signals generated by their activation converge to the same signaling pathways, there may be a synergistic or “amplifying” effect for simultaneous NOD and TLR activation by MAMPs. This cooperation between different PRRs in the activation of NF-κβ and MAPK signaling pathways may increase the sensitivity and potency of the host response to the presence of bacteria.98,99,363 Indeed, there is evidence that demonstrates that the activation of Nod1 and Nod2 has synergistic effects with TLR signaling on the production of proinflammatory cytokines, including IL-1, IL-4, IL-6, IL-10, IL-12, granulocyte macrophage colony-stimulating factor, and TNF.^{99,265,345,363} Moreover, it has been suggested that Nod1 and Nod2 signaling augments Th1 polarization induced by TLR signaling, except for TLR2 signals, which inhibit Th1-type cytokines as a result of IL-10 production.³⁴⁵

In addition to their role as PRRs and their synergism with TLR signaling, other members of the NOD family of proteins can also affect innate immune response via their role in the formation of the inflammasome, which is a multi-protein complex that activates caspase-1. Activated caspase-1 will process the inactive pro-IL-1β and pro-IL-18 forms that are produced and convert them into the biologically active forms that will be secreted.²⁰⁷

Inflammasomes.

Inflammasomes are multi-protein complexes that include a NOD leucine-rich repeat-containing receptor (NLR), caspase-1, and an adaptor protein. These complexes participate in the sensing of microbial-derived (MAMPs) and tissue-damage–derived (DAMPs) molecular patterns by the innate immunity. Caspase-1 is the central effector molecule in these protein complexes, and it participates in the processing of inflammatory cytokines and apoptosis.

The NLR proteins represent the “core” of the multi-protein complex, and they are reflected in the name of the inflammasome. NLRP1, NLRP3, and NLRC4 are the only inflammasomes with known physiological functions (Figure 9-3).²⁷³ In addition to the core NLR protein, inflammasomes also include caspase-1 and the apoptosis-associated speck-like protein, which contain caspase-activating recruitment domains as central proteins. The relevance of inflammasomes to the immune response is supported by the associations between mutations in the genes that encoding the proteins of the inflammasome as well as inflammatory autoimmune conditions, including vitiligo, Muckle-Wells syndrome, cryopyrin-associated periodic syndrome, and type 2 diabetes.^179,156,210

The different inflammasomes are “assembled” and activated in response to various exogenous and endogenous signals. MAMPs and DAMPs may activate NLRP3, whereas NLRC4 is preferentially activated by MAMPs, including double-stranded DNA and specific bacterial proteins. The protease caspase-1 is the effector protein of the inflammasomes; it plays an important role in cell death (specifically by pyroptosis) and also in the final processing and activation of the inflammatory cytokines IL-1β, IL-18, and IL-33. Thus, the gain of function caused by constitutive or prolonged activation of the inflammasomes and consequently of caspase-1 may lead to an exacerbation of the inflammatory response by increasing the bioavailability of active IL-1β. On the other hand, loss or reduced function of the inflammasomes can dampen inflammation and attenuate the immune response, thereby reducing the ability of the host to detect and respond to various MAMPs. Various microorganisms (e.g., Salmonella typhimurium) present evasion mechanisms to prevent the activation of the inflammasomes and to facilitate their invasion, thereby indicating the relevance of inflammasomes for the immune response and host–microbial interactions.²⁴¹

With periodontal diseases, there is scarce evidence of the importance of inflammasomes. There is an increase in the gene expression of NLRP3 and its endogenous antagonist NLRP2 in human gingival tissues with various forms periodontal disease as compared with gingival biopsies of periodontally healthy sites. Moreover, the expression levels were positively correlated with the levels of IL-1 and IL-18 in the tissues,⁴¹ thereby suggesting a role for NLRP3 inflammasome in inflammatory periodontal diseases. In vitro evidence indicates that microorganisms from the dental biofilm may modulate the expression of NLRP3 inflammasome and that this modulation was correlated with the production of IL-1 and IL-18.⁴² Interestingly, some periodontopathogens from the subgingival biofilm may evade immune surveillance by avoiding the activation of inflammasomes, as was demonstrated in vitro for P. gingivalis that attenuated NLRP3 and IL-1 gene expression by gingival fibroblasts.³¹

The production of cytokines is crucial for the establishment of a competent innate immune response and also for the subsequent activation of adaptive immunity. Cytokines such as IL-1, IL-18, and IL-33 are processed by the inflammasomes, and they play an important role in the polarization of T-cell response toward Th17, Th1, and Th2, respectively.60,209 Thus, inflammasome activation may bridge innate and adaptive immunity. An indication of the role played by inflammasomes on adaptive immunity comes from the experimental models of respiratory allergy and encephalitis. In these models, mice that are deficient in NLRP3, caspase-activating recruitment domains, or caspase-1 failed to develop an allergic reaction^97,200 and showed reduced progression of encephalitis^125,326 as a result of a marked shift in the adaptive response. Further evidence for the role of inflammasomes in the modulation of adaptive immunity includes models of host–microbial interactions that indicate a higher susceptibility for disseminated fungal infection in caspase-1–deficient animals; this is associated with the reduced Th1 and Th17 polarization of T cells.³⁶²

Complement Systems.

In cases of periodontal disease, the host defense also is dependent on a functional complement system, which coordinates the recruitment and activation of inflammatory cells, bacterial opsonization, phagocytosis, and lysis.128,131 In addition, complement can amplify the TLR4-mediated inflammatory response toward bacterial LPS challenge (this was reviewed in Reference 291). The complement system is critical to the linking of innate and adaptive immunity by regulating the activation of both B cells and T cells, either directly or through effects on antigen-presenting cells.

The activation of the complement cascade involves the sequential activation and proteolytic cleavage of a series of serum proteins via three distinct mechanisms, namely the classical, lectin, and alternative (Figure 9-4).^128,291 Classical pathway activation occurs in response to antigen–antibody complexes that are recognized by the C1q subunit of C1. However, the lectin pathway is triggered through the interaction of a secreted pattern-recognition receptor (mannose-binding lectin) with specific carbohydrate groups on the surface of a variety of microorganisms. Both the classical and the lectin pathways then proceed through C4 and C2 cleavage for the generation of the classical/lectin C3 convertase (C4bC2b) (see Figure 9-4). The alternative pathway is initiated by the hydrolysis of C3 to C3(H₂O), which is a C3b analog that forms the initial alternative pathway for C3 convertase, or through bacterial lipopolysaccharide and lipooligosaccharide molecules in concert with microbial-derived properdin.¹²⁸ The alternative pathway also serves as a positive feedback loop for the classical and lectin pathways. All three pathways converge at the third component of complement (C3), which upon activation by pathway-specific C3 convertases leads to the generation of key effector molecules. These include the C3a and C5a anaphylatoxins, which activate specific G-protein–coupled receptors and which mediate the mobilization and activation of leukocytes. Also important are the C3b opsonins, which promote phagocytosis through complement receptors, and the C5b-9 membrane attack complex, which can lyse targeted pathogens (see Figure 9-4).¹²⁸

The complement systems have also been shown to provide a barrier against the spread of bacterial infections; to facilitate clotting mechanisms²⁹¹; to mobilize hematopoietic stem cells and progenitor cells from the bone marrow; to help replenish new leukocytes²¹¹; and to activate the differentiation of specific T-cell subsets.²²⁰ The dysregulation of complement activities may lead to a failure to protect the host against pathogens and amplify inflammatory tissue damage (this was reviewed in Reference 128). In the context of periodontal inflammation, complement subversion appears to play a major role in periodontal pathogenesis.¹³²

Activated complement components are found in the gingival crevicular fluid of patients with chronic periodontitis as compared with healthy sites. Virtually all of the complement components have been detected in chronically inflamed gingiva or in the gingival crevicular fluid of patients as compared with healthy control samples. Finally, local complement activation may promote periodontal inflammation predominantly via C5a-induced vasodilation, increased vascular permeability and flow of inflammatory exudate, and chemotactic recruitment of inflammatory cells, especially neutrophils.¹²⁸

Relative to periodontitis, therapeutic strategies are evolving that target either CR3 or CR5 (see Figure 9-4). Because C3 is a central component of all three activation pathways, blockade at this level is reasonable approach to treating complement-associated diseases, including periodontal diseases. CR3 antagonism through topical small molecule inhibitors has been shown to reduce P. gingivalis–induced alveolar bone loss.^130,135 The complement-generated fragment C5a functions as a potent mediator of complement signaling and neutrophil recruitment that may protect but also mediate excessive neutrophil activation, and it has the potential to augment tissue damage during periodontal disease progression. As such, C5aR inhibitors have been recently used in preclinical models to indicate that the potential to inhibit C5aR may be a viable option for the treatment and management of periodontal diseases.¹

Activation of Adaptive Immunity in Periodontal Diseases

In addition to its role in the immediate detection of infectious agents and toxic/degradation (“danger”) products to elicit an inflammatory response and to activate effector processes aimed at the elimination or confinement of microbial invaders, innate immunity also has the pivotal role of initiating and modulating adaptive immune responses. Thus, adaptive immunity requires the innate “branch” of immunity to be activated; however, in vivo, this didactic distinction does not exist, and the immune response should be understood as a continuum and as a constantly adjusting host reaction to the presence of microbes and their MAMPs. In the context of this chapter and to avoid excessive overlap of information with other chapters in this book, only the signaling and molecular aspects of the host–microbe interactions related to the adaptive immune response will be emphasized.

Innate immune mechanisms and signaling are not “turned off” when the adaptive immune response is activated; rather, they are “supplemented” by the latter in their defense against microbes. On the other hand, cells from adaptive immune response also express PRRs and respond to MAMPs, which further stresses the dynamic interaction between the two “arms” of the immune response.

In T lymphocytes, TLR agonists have been shown to modulate the expression of co-stimulatory receptors (e.g., CD28, CD69, and CD152) and to increase their proliferation and interferon-γ production, thereby suggesting that microbes and their MAMPs may also have a direct functional role in the regulation of adaptive immunity.51,331 This is especially important to realize in the periodontal disease context, which has been classically described as a chronic inflammatory condition and, as such, a condition that involves a dense lymphocytic infiltrate. Ever since the classical studies of the host response in periodontal disease—including the recognition of immunoglobulin-producing plasma cells in the gingival tissues of patients with periodontal disease⁴⁷; the classical descriptions of initial, established, and advanced lesions²⁶⁴; and the characterization of the inflammatory infiltrate in the dog model²¹⁷—the nature of the inflammatory infiltrate and its association with a certain type of inflammatory response and ultimately with the quiescence or progression of disease, as characterized by bone resorption, has been debated.

The adaptive immune response is characterized by the activities of pathogen-specific B and T lymphocytes; however, the cell type that is primarily responsible for translating innate signals into adaptive immunity is the dendritic cell (DC). In fact, the generation of adaptive immunity initiates with DCs recognizing MAMPs at the site of infection and subsequently migrating into the regional draining lymph nodes to present the processed antigen peptides in the context of MHC molecules to naïve T lymphocytes. Once inside the lymph nodes, DCs migrate to the T-cell areas, seek out antigen-specific T cells, and induce their activation and differentiation into effector cells (i.e., helper or cytotoxic cells).

The maturation process induced by MAMPs that leads to the migration of DCs to the lymph nodes involves the modulation of the expression of various chemokine receptors that renders DCs less responsive to inflammatory signals and more responsive to lymphocyte-derived signals. Notably, the LPS-induced maturation of DCs has been shown to result in the downregulation of “inflammatory” CCR1 and CCR5 (which responds to macrophage inflammatory protein-1α, for example) and the upregulation of CXCR4 and CCR7 (which respond to CXCL12 and CCL19, respectively).³⁰⁹ These changes ultimately control DC trafficking and migration to the lymph nodes, and they represent one mechanism whereby the modulation of cell-signaling pathways (i.e., MAMPs activating PRR signaling versus inflammatory cytokine signaling) can influence the host response. The profound phenotypic changes that DC cells undergo during the process of activation by MAMPs and conversion into antigen-presenting cells is a multi-step process that includes not only trafficking and migration into lymph nodes but also the expression of high levels of major histocompatibility complex (MHC) bearing the processed microbial-derived peptides that will engage T-cell receptors in naïve T cells; the expression of co-stimulatory membrane-bound molecules (e.g., CD80, CD86), which are important for T-cell survival and proliferation; and finally the production of mediators (e.g., IL-12) that act on T cells and that induce their terminal differentiation into an effector cell type and their profile of cytokine expression. Ultimately, the integration of these signals (i.e., MHC antigens, co-stimulatory molecules, cytokines) will determine the fate and nature of the adaptive immune response.¹⁸¹

Interestingly, many of the features of DC activation can be induced by inflammatory cytokines in the absence of MAMPs and PRR signaling. This fact is interpreted by some as being supportive of the “danger model” of immunity, in which endogenous danger signals (e.g., toxic products from necrotic, apoptotic, or infected cells; inflammatory cytokines) can also trigger an immune response, similarly to exogenous MAMPs.30,231 There are two important arguments against this concept:

The signaling aspects involved in DC maturation and activation are especially interesting, because most PRR and inflammatory receptor signals converge into similar pathways (e.g., NF-κβ, MAPKs). Despite similar signaling pathways activated by MAMPs and inflammatory cytokines, MyD88-defficient DCs stimulated with inflammatory cytokines fail to produce IL-12 and other inflammatory cytokines (e.g., IL-1β, IL-6, TNF-α, IFN-β),¹⁶⁰ which will have important consequences for the differentiation and activation of naïve CD4⁺ and CD8⁺ T cells⁷¹ and also indirectly affect the activation of B cells and the humoral response.³³⁶ Among the possible reasons for this differential role of MAMPs and inflammatory cytokines as external signals, despite the activation of similar signaling pathways, are the kinetics of the activation of the signaling pathways and also the other signaling pathways that are simultaneously activated by each external signal. The integration of multiple signal pathways may be required for a given profile of cytokine expression by DCs. Specifically, in patients with periodontal diseases, both MAMPs and inflammatory cytokines are usually present to fully activate the DCs, which suggests that there is no impairment to a competent activation of adaptive immunity.

Host–Microbe Interactions in Adaptive Immunity

Considering that humans are exposed to bacteria in various mucosal sites, including the oral mucosa, shortly after birth and that this exposure continues throughout our lives, it is reasonable to expect the chronic presence of adaptive immune cells as part of a vigilance mechanism in the sites that are chronically exposed to microbes and their MAMPs. The crucial role of adaptive immunity to periodontal disease progression is/>