Host Defense Peptides in the Oral Cavity. Advances in Applied Microbiology Volume 63. Elsevier; 2008. pp. 281-322. [DOI: 10.1016/s0065-2164(07)00008-1]

Current concepts and an alternative perspective on periodontal disease

BACKGROUND

Epidemiological data from countries worldwide show a consistent pattern implying that a fraction of around 10% of those over 40-50 years in all populations will exhibit severe periodontitis with the potential risk of losing teeth during their life-time. The subgingival microbiota shows striking similarities between populations irrespective of disease severity and can only marginally explain the clinical pattern. It is also difficult to explain this pattern by genetic and acquired risk factors such as systemic disease (e.g. diabetes) or habits (e.g. smoking) even if they may have a confounding effect on the disease.

MAIN TEXT

Inflammation of the gingiva appears to be a normal and physiological response to the presence of commensal bacteria along the gingival crevice and in the dental biofilm. Over many years of exposure to the dental biofilm, the chronic inflammation in the gingiva gradually results in a loss of attachment and bone loss. Numerous laboratory and clinical studies have provided insight into the potential role of determinants that are associated with periodontitis. However, it has been difficult to relate the findings to the pattern of the distribution of the disease observed in epidemiological studies. We propose a simple and parsimonious model that considers all the multitude of potential determinants as creating effectively random noise within the dental biofilm to which the tissues react by accumulating the effects of this noise.

CONCLUSIONS

We suggest that such a model can explain many of the epidemiological features of periodontal breakdown over time, and we discuss its clinical implications.

Importance of Virulence Factors for the Persistence of Oral Bacteria in the Inflamed Gingival Crevice and in the Pathogenesis of Periodontal Disease

Periodontitis is a chronic inflammation that develops due to a destructive tissue response to prolonged inflammation and a disturbed homeostasis (dysbiosis) in the interplay between the microorganisms of the dental biofilm and the host. The infectious nature of the microbes associated with periodontitis is unclear, as is the role of specific bacterial species and virulence factors that interfere with the host defense and tissue repair. This review highlights the impact of classical virulence factors, such as exotoxins, endotoxins, fimbriae and capsule, but also aims to emphasize the often-neglected cascade of metabolic products (e.g., those generated by anaerobic and proteolytic metabolism) that are produced by the bacterial phenotypes that survive and thrive in deep, inflamed periodontal pockets. This metabolic activity of the microbes aggravates the inflammatory response from a low-grade physiologic (homeostatic) inflammation (i.e., gingivitis) into more destructive or tissue remodeling processes in periodontitis. That bacteria associated with periodontitis are linked with a number of systemic diseases of importance in clinical medicine is highlighted and exemplified with rheumatoid arthritis, The unclear significance of a number of potential "virulence factors" that contribute to the pathogenicity of specific bacterial species in the complex biofilm-host interaction clinically is discussed in this review.

The oral microbiome - friend or foe?

The microbiome and the human body constitute an integrated superorganism, which is the result of millions of years of coevolution with mutual adaptation and functional integration, and confers significant benefits for both parties. This evolutionary process has resulted in a highly diverse oral microbiome, which covers the full spectrum of acidogenic, aciduric, inflammatory, and anti-inflammatory properties. The relative proportions of members of the microbiome are affected by factors associated with modern life, such as general diet patterns, sugar consumption, tobacco smoking, oral hygiene, use of antibiotics and other antimicrobials, and vaccines. A perturbed balance in the oral microbiome may result in caries, periodontal disease, or candidiasis, and oral bacteria passively transferred to normally sterile parts of the body may cause extra-oral infections. Nevertheless, it should never be our goal to eliminate the oral microbiome, but rather we have to develop ways to re-establish a harmonious coexistence that is lost because of the modern lifestyle. With regard to oral diseases, this goal can normally be achieved by optimal oral hygiene, exposure to fluoride, reduction of sucrose consumption, stimulation of our innate immune defense, smoking cessation, and control of diabetes.

Influence of saliva on the oral microbiota

Saliva plays a major role in determining the composition and activity of the oral microbiota, via a variety of mechanisms. Molecules, mainly from saliva, form a conditioning film on oral surfaces, thus providing receptors for bacterial attachment. The attached cells use saliva components, such as glycoproteins, as their main source of nutrients for growth. Oral bacteria work sequentially and in a concerted manner to catabolize these structurally complex molecules. Saliva also buffers the pH in the biofilm to around neutrality, creating an environment which is conducive to the growth of many oral bacteria that provide important benefits to the host. Components of the adaptive and innate host defences are delivered by saliva, and these often function synergistically, and at sublethal concentrations, so a complex relationship develops between the host and the resident microbiota. Dysbiosis can occur rapidly if the flow of saliva is perturbed.

Potential applications of antimicrobial peptides and their mimics in combating caries and pulpal infections

Antimicrobial peptides (AMPs) are short cationic host-defense molecules that provide the early stage of protection against invading microbes. They also have important modulatory roles and act as a bridge between innate and acquired immunity. The types and functions of oral AMPs were reviewed and experimental reports on the use of natural AMPs and their synthetic mimics in caries and pulpal infections were discussed. Natural AMPs in the oral cavity, predominantly defensins, cathelicidins and histatins, possess antimicrobial activities against oral pathogens and biofilms. Incomplete debridement of microorganisms in root canal space may precipitate an exacerbated immune response that results in periradicular bone resorption. Because of their immunomodulatory and wound healing potentials, AMPs stimulate pro-inflammatory cytokine production, recruit host defense cells and regulate immuno-inflammatory responses in the vicinity of the pulp and periapex. Recent rapid advances in the development of synthetic AMP mimics offer exciting opportunities for new therapeutic initiatives in root canal treatment and regenerative endodontics.

STATEMENT OF SIGNIFICANCE

Identification of new therapeutic strategies to combat antibiotic-resistant pathogens and biofilm-associated infections continues to be one of the major challenges in modern medicine. Despite the presence of commercialization hurdles and scientific challenges, interests in using antimicrobial peptides as therapeutic alternatives and adjuvants to combat pathogenic biofilms have never been foreshortened. Not only do these cationic peptides possess rapid killing ability, their multi-modal mechanisms of action render them advantageous in targeting different biofilm sub-populations. These factors, together with adjunctive bioactive functions such as immunomodulation and wound healing enhancement, render AMPs or their synthetic mimics exciting candidates to be considered as adjuncts in the treatment of caries, infected pulps and root canals.

Peptide KSL-W-Loaded Mucoadhesive Liquid Crystalline Vehicle as an Alternative Treatment for Multispecies Oral Biofilm

Decapeptide KSL-W shows antibacterial activities and can be used in the oral cavity, however, it is easily degraded in aqueous solution and eliminated. Therefore, we aimed to develop liquid crystalline systems (F1 and F2) for KSL-W buccal administration to treat multispecies oral biofilms. The systems were prepared with oleic acid, polyoxypropylene (5) polyoxyethylene (20) cetyl alcohol (PPG-5-CETETH-20), and a 1% poloxamer 407 dispersion as the oil phase (OP), surfactant (S), and aqueous phase (AP), respectively. We characterized them using polarized light microscopy (PLM), small-angle X-ray scattering (SAXS), rheology, and in vitro bioadhesion, and performed in vitro biological analysis. PLM showed isotropy (F1) or anisotropy with lamellar mesophases (F2), confirmed by peak ratio quantification using SAXS. Rheological tests demonstrated that F1 exhibited Newtonian behavior but not F2, which showed a structured AP concentration-dependent system. Bioadhesion studies revealed an AP concentration-dependent increase in the system’s bioadhesiveness (F2 = 15.50 ± 1.00 mN·s) to bovine teeth blocks. Antimicrobial testing revealed 100% inhibition of multispecies oral biofilm growth after KSL-W administration, which was incorporated in the F2 aqueous phase at a concentration of 1 mg/mL. Our results suggest that this system could serve as a potential vehicle for buccal administration of antibiofilm peptides.

Immunoreactive pattern of Staphylococcus epidermidis biofilm against human whole saliva

Saliva is essential to interact with microorganisms in the oral cavity. Therefore, the interest in saliva antimicrobial properties is on the rise. Here, we used an immunoproteomic approach, based on protein separation of Staphylococcus epidermidis biofilms by 2DE, followed by Western-blotting, to compare human serum and saliva reactivity profile. A total of 17 proteins were identified by MALDI-TOF/TOF. Serum and saliva presented a distinct pattern of immunoreactive proteins. Our results suggest that saliva seems to have higher propensity to react against S. epidermidis proteins with oxidoreductase activity and proteins involved with L-serine metabolic processes. We show that saliva was a powerful tool for the identification of potential S. epidermidis biofilms proteins.

LL-37 opsonizes and inhibits biofilm formation of Aggregatibacter actinomycetemcomitans at subbactericidal concentrations

Host defense peptides are immediate responders of the innate immunity that express antimicrobial, immunoregulatory, and wound-healing activities. Neutrophils are a major source for oral host defense peptides, and phagocytosis by neutrophils is a major mechanism for bacterial clearance in the gingival tissue. Dysfunction of or reduction in the numbers of neutrophils or deficiency in the LL-37 host defense peptide was each previously linked with proliferation of oral Aggregatibacter actinomycetemcomitans which resulted in an aggressive periodontal disease. Surprisingly, A. actinomycetemcomitans shows resistance to high concentrations of LL-37. In this study, we demonstrated that submicrocidal concentrations of LL-37 inhibit biofilm formation by A. actinomycetemcomitans and act as opsonins and agglutinins that greatly enhance its clearance by neutrophils and macrophages. Improved uptake of A. actinomycetemcomitans by neutrophils was mediated by their opsonization with LL-37. Enhanced phagocytosis and killing of A. actinomycetemcomitans by murine macrophage-like RAW 264.7 cells were dependent on their preagglutination by LL-37. Although A. actinomycetemcomitans is resistant to the bactericidal effect of LL-37, our results offer a rationale for the epidemiological association between LL-37 deficiency and the expansion of oral A. actinomycetemcomitans and indicate a possible therapeutic use of cationic peptides for host defense.

Streptococcus pyogenes infection of tonsil explants is associated with a human β-defensin 1 response from control but not recurrent acute tonsillitis patients

Host defence peptides (HDP), including the defensins and hCAP-18, function as part of the innate immune defences, protecting the host epithelia from microbial attachment and invasion. Recurrent acute tonsillitis (RAT), in which patients suffer repeated symptomatic tonsil infections, is linked to Streptococcus pyogenes, a group A streptococcus, and may reflect the impaired expression of such peptides. To address this, the defensin and hCAP-18 messenger RNA expression profiles of 54 tonsils excised from control and RAT patients undergoing tonsillectomy were quantified and compared. Marked variation in expression was observed between individuals from the two groups, but statistically no significant differences were identified, suggesting that at the time of surgery the tonsil epithelial HDP barrier was not compromised in RAT subjects. Surgical removal of the tonsils occurs in a quiescent phase of disease, and so to assess the effects of an active bacterial infection, HaCaT cells an in vitro model of the tonsil epithelium, and explants of patient tonsils maintained in vitro were challenged with S. pyogenes. The HaCaT data supported the reduced expression of hCAP-18/LL-37, human β-defensin 1 (HBD1;P < 0.01) and HBD2 (P < 0.05), consistent with decreased protection of the epithelial barrier. The tonsil explant data, although not as definitive, showed similar trends apart from HBD1 expression, which in the control tonsils but not the RAT patient tonsils was characterized by increased expression (P < 0.01). These data suggest that in vivo HBD1 may play a critical role in protecting the tonsil epithelia from S. pyogenes.

The antibacterial activity of LL-37 against Treponema denticola is dentilisin protease independent and facilitated by the major outer sheath protein virulence factor

Host defense peptides are innate immune effectors that possess both bactericidal activities and immunomodulatory functions. Deficiency in the human host defense peptide LL-37 has previously been correlated with severe periodontal disease. Treponema denticola is an oral anaerobic spirochete closely associated with the pathogenesis of periodontal disease. The T. denticola major surface protein (MSP), involved in adhesion and cytotoxicity, and the dentilisin serine protease are key virulence factors of this organism. In this study, we examined the interactions between LL-37 and T. denticola. The three T. denticola strains tested were susceptible to LL-37. Dentilisin was found to inactivate LL-37 by cleaving it at the Lys, Phe, Gln, and Val residues. However, dentilisin deletion did not increase the susceptibility of T. denticola to LL-37. Furthermore, dentilisin activity was found to be inhibited by human saliva. In contrast, a deficiency of the T. denticola MSP increased resistance to LL-37. The MSP-deficient mutant bound less fluorescently labeled LL-37 than the wild-type strain. MSP demonstrated specific, dose-dependent LL-37 binding. In conclusion, though capable of LL-37 inactivation, dentilisin does not protect T. denticola from LL-37. Rather, the rapid, MSP-mediated binding of LL-37 to the treponemal outer sheath precedes cleavage by dentilisin. Moreover, in vivo, saliva inhibits dentilisin, thus preventing LL-37 restriction and ensuring its bactericidal and immunoregulatory activities.

Gingipains produced by Porphyromonas gingivalis ATCC49417 degrade human-β-defensin 3 and affect peptide's antibacterial activity in vitro

Porphyromonas gingivalis, one of the major pathogen associated with periodontitis, is a highly proteolytic bacterial species. Production of proteases is a common microbial virulence factor that enables the destruction of host tissues and evasion from host defense mechanisms. Antimicrobial peptides are important effector molecules of the innate immune system with a broad range of antimicrobial and immunoregulatory activities. We and others have previously demonstrated that P. gingivalis is relatively resistant to the bactericidal activity of the human β-defensin 3 (hBD3). In this study, ability of proteases released by the pathogenic strain of P. gingivalis ATCC 49417 to degrade hBD3 and to affect the antibacterial properties of the peptide was assessed. P. gingivalis culture supernatants (CS) were found to degrade hBD3 in a concentration- and time-dependent manner. Such degradation was mainly due to the activity of Arg and Lys-gingipains, as pretreatment of CS with inhibitors selective for this class of proteases abolished CS ability to degrade hBD3. Importantly, preincubation of hBD3 with CS reduced peptide's antibacterial activity against a susceptible strain of Staphylococcus aureus, while the presence of gingipain inhibitors in the bactericidal assay increased P. gingivalis susceptibility to hBD3. Altogether these results suggest that gingipains may have a role in the resistance of P. gingivalis ATCC 49417 to hBD3.

Reduced human beta defensin 3 in individuals with periodontal disease

The human beta defensin 3 (hBD3) is widely expressed in the oral cavity and exerts strong antibacterial and immunomodulatory activities. Hence, we hypothesized that hBD3 could play a protective role in the maintenance of periodontal homeostasis, and that it could be found in gingival crevicular fluid (GCF) of healthy individuals and those with periodontitis at levels correlating with the degree of periodontal health. By using an ELISA assay to quantify hBD3 in GCF, we demonstrated that the peptide is present at levels easily detectable in the majority of healthy individuals, but it is drastically reduced in GCF from those with periodontitis. Furthermore, hBD3 levels inversely correlate with the severity of the disease and the degree of colonization by combinations of bacterial species with elevated periodontopathogenic potential. Both genetic factors and host/bacterial proteases released in diseased sites may be responsible for the observed low/null hBD3 levels in GCF from individuals with periodontitis.

Saliva enables the antimicrobial activity of LL-37 in the presence of proteases of Porphyromonas gingivalis

Proteolysis is a common microbial virulence mechanism that enables the destruction of host tissue and evasion from host defense mechanisms. Antimicrobial peptides, also known as host defense peptides, are effector molecules of the innate immunity that demonstrate a broad range of antimicrobial and immunoregulatory activities. Deficiency of the human LL-37 antimicrobial peptide was previously correlated with severe periodontal disease. Porphyromonas gingivalis, the major pathogen associated with periodontitis, is highly proteolytic. In this study, P. gingivalis was found capable of degrading LL-37 by utilizing its arginine-specific gingipains. Saliva collected from volunteers with a healthy periodontium protected LL-37 from proteolysis by P. gingivalis. Salivary protection of LL-37 was heat resistant and specific and enabled LL-37 to inhibit growth of Escherichia coli in the presence of the P. gingivalis proteases. Previously, saliva and other body fluids have been shown to inhibit the antimicrobial activity of LL-37. Here we demonstrate that at a cost of a small reduction in the bactericidal activity of LL-37, saliva enables the antibacterial activity of LL-37 despite the presence of proteases secreted by the main periodontopathogen.