Periodontal Disease Pathogenesis and Mechanisms

Dr. Enas Razzoqi B.D.S., M.Sc., Ph.D in Periodontics

Mechanisms of pathogenicity:- pathogenesis the origination and development of a disease. Essentially, this refers to the step-by step processes that lead to the development of a disease and that result in a series of changes in the structure and function of the periodontium. In broad terms, the pathogenesis of a disease is the mechanism by which a causative factor (or factors) causes the disease. . .

Periodontal disease results from a complex interplay between the subgingival biofilm and the host immune inflammatory events that develop in the gingival and periodontal tissues in response to the challenge presented by the bacteria. These defensive processes, which are protective by intent (i.e., to prevent the ingress of the bacteria and their products into the tissues), result in the majority of tissue damage that leads to the clinical manifestations of disease. The poor oral hygiene results in increased plaque accumulation, and resulted in periodontal disease. However, there are many individuals with poor oral hygiene who do not develop advanced periodontal disease, and conversely, some individuals, despite good oral hygiene and compliance with periodontal treatment protocols, present with advanced and progressing periodontitis

In periodontal disease, many species of organisms are identifiable in the periodontal pocket, and it is impossible to conclude that a single species or even a group of species causes periodontal disease. Many of the species that are considered important in periodontal predominate in deep pockets because the pocket is a favorable environment in which they can survive (i.e., it is warm, moist, and anaerobic, with a ready supply of pathogenesis may nutrients).

Overt clinical signs of gingivitis (i.e., redness, swelling, and bleeding on probing) do not develop because of several innate and structural defense mechanisms, including the following: 1- The maintenance of an intact epithelial barrier(the junctional and sulcular epithelium). 2- The outflow of GCF from the sulcus (dilution effect and flushing action). 3- The sloughing of surface epithelial cells of the junctional and sulcular epithelium. 4- The presence of neutrophils and macrophages in the sulcus that phagocytose bacteria 5- The presence of antibodies in the GCF.

The dentogingival junction is a unique anatomic feature that functions to attach the gingiva to the tooth. It has an epithelial portion and a connective tissue portion.

The widened intercellular spaces in the junctional epithelium permit the migration of neutrophils; they also allow macrophages from the gingival connective tissues to enter the sulcus to phagocytose bacteria, and the ingress of bacterial products and antigens occurs as well. The connective tissue component of the dentogingival unit contains densely packed collagen fiber bundles (a mixture of type I and III collagen fibers) that are arranged in distinct patterns that maintain the functional integrity of the tissues and the tight adaptation of the soft tissues to the teeth.

Histopathology of periodontal disease (gingivitis and periodontitis) In 1976, Page and Schroeder classified the progression of gingival and periodontal inflammation into four phases: initial, early, established and advanced stages or lesions. 1- Initial lesion: (Corresponds With Clinically Healthy Gingival Tissues) Pink, are not swollen or inflamed, and are firmly attached to the underlying tooth and bone, with minimal bleeding on probing Develop within 2 to 4 days of the accumulation of plaque Low-grade inflammation occurs in response to the continued presence of bacteria and their products in the gingival crevice.

Slightly elevated vascular permeability and vasodilation Permitting the migration of leukocytes,(neutrophils and monocytes,) Gingival connective tissue contains at least some inflammatory cells, particularly neutrophils accumulate). (Lymphocytes and macrophages also Across the junctional epithelium Sulcus toward the source of the chemotactic stimulus: the bacterial products in the gingival sulcus as well as from the chemoattractant factors produced by the host

There is a continuous exudate of fluid from the gingival tissues that enters the crevice and flows out as gingival crevicular fluid (GCF). . . Increased GCF flow has the effect of diluting bacterial products, and it also potentially has a flushing action to remove bacteria and their products from the crevice. A decrease in perivascular collagen occurs in this area

after about 1 week of continued plaque accumulation and corresponds to the early clinical signs of gingivitis. 2- Early lesion:- Early gingivitis Increased vascular permeability and vasodilation The proliferation of capillaries, the opening up of microvascular beds The gingiva are erythematous in appearance Increased gingival crevicular fluid (GCF) flow Large numbers of infiltrating leukocytes. The predominant infiltrating cell types are neutrophils and lymphocytes(primarily thymic lymphocytes [T cells]),

Fibroblasts degenerate, primarily by apoptosis (programmed cell death), which increases the space available for infiltrating leukocytes. Collagen destruction occurs, that results in collagen-depleted areas of the connective tissue. Proliferation of the junctional and sulcular epithelium into collagen- depleted areas of the connective tissues to maintain an intact barrier against the bacteria and their products As a result of edema of the gingival tissues, the gingiva may appear slightly swollen, and, accordingly, the gingival sulcus becomes deeper. slightly

3- Established lesion:- (Chronic Gingivitis) after 2-3 weeks of plaque accumulation. Dense inflammatory cell infiltrate (i.e., plasma cells, lymphocytes, and neutrophils) Dominated by plasma cells Neutrophils accumulate in the tissues and release their lysosomal contents extracellularly (in an attempt to kill bacteria that are not phagocytosed), thereby resulting in further tissue destruction Elevated release of matrix metalloproteinases (MMP-8 and MMP-9) from neutrophils

Collagen depletion continues significantly, with further proliferation of the epithelium into the connective tissue spaces .The junctional epithelium and sulcular epithelium form a pocket epithelium that is not firmly attached to the tooth surface, that contains large numbers of neutrophils, and that is more permeable to the passage of substances into or out of the underlying connective tissue. The pocket epithelium may be ulcerated and less able to resist the passage of the periodontal probe, so bleeding on probing is a common feature of chronic gingivitis. .

4- Advanced lesion Transition from gingivitis to periodontitis. Continued collagen breakdown that results in large areas of collagen-depleted connective tissue that extends into the periodontal ligament and the alveolar bone. Predominance of neutrophils in the pocket epithelium and the periodontal pocket. Dense inflammatory cell infiltrate in the connective tissues(primarily plasma cells) Apical migration of junctional epithelium along the root surface into the collagen-depleted areas to maintain (preserve) an intact epithelial barrier (pocket deepens) Osteoclastic bone resorption commences and progresses.

The progression from the early lesion to the established lesion and transition from gingivitis to periodontitis depends on many factors, including: The plaque (bacterial) challenge (the composition and quantity of the biofilm), Host inflammatory response Host susceptibility factors, and risk factors including environmental and genetic risk factors

Inflammatory Responses in the Periodontium The molecules that play a role in the pathogenesis of periodontitis divided into two main groups: derived from the subgingival microbiota microbial virulence factors derived from the host immune inflammatory response. The subgingival bacteria also contribute directly to tissue damage by the release of noxious substances, but their primary importance in periodontal pathogenesis is that of activating immune inflammatory responses that, in turn, result in tissue damage, which may well be beneficial to the bacteria located within the periodontal pocket by providing nutrient sources.

Lipopolysaccharide {LPS} Endotoxins large molecules composed of a lipid component (lipid A) and a polysaccharide component a constituent of the cell wall of G-ve bacteria LPS is of key importance for initiating and sustaining inflammatory responses in the gingival and periodontal tissues. LPSs are highly conserved in gram-negative bacterial species, a finding that reflects their importance in maintaining the structural integrity of the bacterial cells.

Interacts with the CD14/TLR-4/MD-2 receptor complex on immune cells such as macrophages, monocytes, dendritic cells, and B cells. This interaction triggers a series of intracellular events: The increased production of proinflammatory mediators such as cytokines from these cells The differentiation of immune cells (e.g., dendritic cells) for the development of effective immune responses against the pathogens.

Lipoteichoic acid, a component of gram-positive cell walls, also stimulates immune responses. Lipoteichoic acid signals through TLR-2. LPS and lipoteichoic acid (released from the bacteria ) Stimulate inflammatory responses in the tissues Increased vasodilation and vascular permeability Recruitment of inflammatory cells by chemotaxis Release of proinflammatory mediators by the leukocytes that are recruited to the area.

Bacterial Enzymes and Noxious Products Noxious Products several metabolic waste products contribute directly to tissue damage. These include Ammonia (NH3) Hydrogen sulfide (H2S) Short-chain carboxylic acids(fatty acids )such as butyric acid and propionic acid. These acids are detectable in GCF and are found in increasing concentrations as the severity of periodontal disease increases. Butyric acid induces apoptosis in T cells, B cells, fibroblasts, and gingival epithelial cells) Create a nutrient supply for the organism by increasing bleeding into the periodontal pocket. Influence cytokine secretion by immune cells,

Bacterial Enzymes Proteases Breaking down structural proteins of the periodontium such as collagen, elastin, and fibronectin and thereby provide peptides for bacterial nutrition. Bacterial proteases disrupt host responses, compromise tissue integrity, and facilitate the microbial invasion of the tissues. P. gingivalis produces two classes of cysteine proteases known asgingipains arginine-specific gingipains RgpAand RgpB. lysine-specific gingipain Kgp

1-Disrupt immune inflammatory responses, thus potentially leading to increased tissue breakdown. 2-Reduce the concentrations of cytokines in cell culture systems, and they digest and . inactivate TNF- 3-The gingipains can also stimulate cytokine secretion such as IL-6 and IL-8 secretion by monocytes through the activation of protease- activated receptors (PARs).

Bacterial invasion P. gingivalis Aggregatibacter actinomycetemcomitans invade the gingival tissues, including the connective tissues Fusobacterium nucleatum Can invade epithelial cells and persist intracellularly bacteria that routinely invade host cells may facilitate the entry of noninvasive bacteria by coaggregating with them. species such as P. gingivalis can be found located within the tissues, they are mainly within the epithelium, and it is unusual for the bacteria to reach the connective tissue until extensive tissue destruction has occurred, and even then it is as a result of inflammation rather than invading bacteria the use of antibiotics for treatment of periodontitis as a means to try to eliminate those organisms that are located in the tissues and that are therefore protected from mechanical disruption by root surface debridement..

Fimbriae P. gingivalis fimbriae Stimulate nuclear factor (NF)- B and IL-8 in a gingival epithelial cell line Stimulate monocytes secreting IL-6, IL-8, and TNF- . Interact with complement receptor-3 (CR-3) Activate intracellular signaling pathways Inhibit IL-12 production mediated by TLR-2 signaling IL-12 is important in the activation of naturalkiller (NK) cells and CD8+ cytotoxic T cells, which themselves may be important in killing P. gingivalis infected host cells, such as epithelial cells.

Bacterial Deoxyribonucleic Acid and Extracellular Deoxyribonucleic Acid Bacterial (DNA) stimulates immune cells through TLR-9, which recognizes hypomethylated CpG regions of the DNA. Extracellular DNA (eDNA) Derived from the chromosomal DNA of bacteria in biofilms, The majority of eDNA is released after bacterial cell lysis or by another mechanisms. eDNA plays a number of important roles in biofilm integrity, in adhesion and formation on hard and soft tissues in the oral cavity, protection against antimicrobial agents, and nutrient storage, as well as genetic exchange, and it is possible that eDNA may ultimately prove to be an important target for biofilm control

Host-Derived Inflammatory Mediators Cytokines They are soluble proteins, and they act as messengers to transmit signals from one cell to another. They bind to specific receptors on target cells and initiate intracellular signaling cascades that result in phenotypic changes in the cell by altered gene regulation.). Play a fundamental role in inflammation and at all stages of the immune response, and they are key inflammatory mediators in periodontal disease Proinflammatory and antiinflammatory cytokines A key proinflammatory cytokine is interleukin-1 , which up- regulates inflammatory responses

produced by a large number of cell types infiltrating inflammatory resident cells in the periodontium fibroblasts, epithelial cells neutrophils, macrophages, lymphocytes Cytokines are able to induce their own expression in either an autocrine or paracrine fashion

The prolonged and excessive production of cytokines and other inflammatory mediators in the periodontium tissue damage For example, cytokines mediate connective tissue and alveolar bone destruction through the induction of fibroblasts and osteoclasts to produce proteolytic enzymes (i.e., MMPs) that break down structural components of these connective tissues.

Prostaglandins lipid compounds Arachidonic acid a polyunsaturated fatty acid found in the plasma membrane of most cells. cyclooxygenase-2 (COX-2) cyclooxygenase-1 (COX-1) prostanoids include the PGs, the thromboxanes, and the prostacyclins PGs are important mediators of inflammation, particularly prostaglandin E2 (PGE2), which results in vasodilation and induces cytokine production by a variety of cell types.

PGE2 macrophages and fibroblasts Results in the induction of MMPs and osteoclastic bone resorption It has a major role in contributing to the tissue damage that characterizes periodontitis and in periodontitis progression COX-2 is up-regulated by IL-1 , TNF- , and bacterial LPS, thus resulting in increased production of PGE2 in inflamed tissues.

Matrix Metalloproteinases Family of proteolytic enzymes that degrade extracellular matrix molecules (proteins) such as collagen, gelatin, and elastin. Group Collagenases Enzyme MMP-1 Name Collagenase 1, fibroblast collagenase Collagenase 2, neutrophil collagenase Collagenase 3 Gelatinase A Gelatinase B Stromelysin 1 Stromelysin 2 Stromelysin 3 Matrilysin 1, pump-1 Matrilysin 2 MT1-MMP MT2-MMP MT3-MMP MT4-MMP MT5-MMP MT6-MMP Macrophage elastase Enamelysin MMP-8 MMP-13 MMP-2 MMP-9 MMP-3 MMP-10 MMP-11 MMP-7 MMP-26 MMP-14 MMP-15 MMP-16 MMP-17 MMP-24 MMP-25 MMP-12 MMP-19 MMP-20 Gelatinases Stromelysins Matrilysins Membrane-type MMPs Others

MMPs are secreted in a latent form (inactive) and are activated by the proteolytic cleavage of a portion of the latent enzyme. This is achieved by proteases, such as cathepsin G, produced by neutrophils. Inhibitors of MMPs found in the serum found in the tissues glycoprotein 1-antitrypsin 2- macroglobulin the tissue metalloproteinases (TIMPs) inhibitors of TIMP-1. MMPs are also inhibited by the tetracycline. Doxycycline, like all the tetracyclines, possesses the ability to down-regulate MMPs, The subantimicrobial formulation of doxycycline as an adjunctive systemic drug treatment for periodontitis has been shown to inhibit collagenase activity in the gingival tissues and GCF periodontitis of patients with chronic