Role of host response and mechanisms of periodontal tissue destruction in periodontitis

Host Responses:

• are essential to prevent serious progressive infection

• may also result in local tissue damage

• also stimulate attempts at tissue repair

While some of the damage to the local tissue is caused by local host responses -the host response is protective and without an adequate host response, periodntitis would result in severe progressive infection which could potentially be life threatening

gingivitis can be a protective host response

bacterial accumulation along the gingival margins - host response limits the ingress of bacteria and confining it to the gingival tissues

stages of gingivitis development

increasing inflammatory infiltrate within the gingival tissues due to the accumulation of plaque/biofilm at the gingival crevice

yet no loss od pdl attachment - so no apical migration of the junctional epithlium

therefore, this can be regarded as a sucessful host response as the infection is limited to the tissues

initial lesion

Early lesion

Established lesion

An inadequate host response

• periodontal tissue loss - necrotising periodontitis - loss of interproximally and cratering of the periodontal tissue loss resulting in recession

• painful

• NPD can have some systemic effects as there is invasion of periodontal bacteria into deeper tissues

• still mostly localised

periodontitis - formation of the advanced lesion

Established lesion

Advanced lesion

deepening of the inflammation causes loss of the pdl attachemnt and andveolar bone resorption

apical migration of the junctional epithelium - causing deepening of the pocket

microbial changes

• in periodontal health - symbiotic bacterial community and helps regulate host immune repsonse - homeostatsis

• gingivitis - increase in number of immflamic pathobionts and commensal bacteria → accesory pathogens. there are also keystone pathogens becuase although they are low in abundance they can orchestrate a biofilm formation and can manipulate the host resposneto their favour in order to aquire nutrients - enabling their survival but also causes tissue damage

• which may lead to periodontal tissue destruction

terminology - pathobionts and pathogens

not infectious as such - also present in low levels in health but the balance tips and their numbers increase as inflammation proceeds

interaction between bacteria and response in susceptible host

• co-operative interactions between the symbiotic micriobiota (which includes inflammophillic pathobionts, keystone pathogens, commensals) - results in a situation where with time and accumulation you can get colonisation and immune subversion - which leads to impaired immunity inflammation

• this favours a dysbiotic environment in a susceotible host

• as dysbiosis proceeds you get more overt inflammation with inflammatory cells (neutrophils, T,B cells, macrophages)

• this results in local tissue damage

• tissue breakdown products provide further nutrients and positive signalling for the pathogenic biofilm such that dysbiosis is further enhanced and positive feedback loop is perpetuated

symbiotic bacteria and keystone pathogens are found in very low levels in health - which show that the host immune response plays a key role in the development of the development of perio

periodontitis is a classic chronic inflammation - what is that?

Chronic Inflammation - Persisting inflammation + attempts at repair

Attempts at repair

• reparative responses include granulation tissue formation (vascular immature connective tissues) and fibrosis (scarring)

• regulated by cytokines such as IL-1, TNF-α and many others

• In gingivitis, tissue damage is typically in balance with tissue repair, in perio the balance is favoured at damage (but repair still happens)

attempts at repair as regulated by…cytokines

• regulated by cytokines such as IL-1, TNF-α and many others

• also important in repair

Histological features of the periodontitis inflammatory lesion 3

1. Epithelial proliferation from the junctional and sulcular aspects to increase the widths - to try to limit any ingress of bacteria / wall them off from gaining acees to deeper tissues

2. Extensive collagen loss - loss of the connective tissue

3. Bone loss

Cellular Inflammatory infiltrate: 4 in both gingival and periodontal connective tissue

1. Neutrophils - acute but also perpetuated in chronic

2. Macrophages - link innate with adaptive immune system

3. T lymphocytes

4. Plasma cells - antibodies

overview of pathogenesis

• bacteria initiates the inflammatory response - which results in further bacterial colonisation

• results in tissue daamge by bacteria (minor) and by the host immune response (major)

• tissue damage exaccerbated tissue response

• concurrent attempts at tissue repaie

Molecular mediators of disease activity 2

1. Bacterial virulence factors - direct damage, subvert host response so it can be perpetuated

2. Signalling molecules controlling host response - excessive - bystander damage to host tissue

Bacterial virulence factors

1. Secreted by the bacteria, • eg. enzymes - collagenases and leukotoxin

2. Metabolites that create favourable conditions - ROS

3. Structural components of the bacteria released during growth or death of the bacteria, • eg. endotoxins, fimbriae etc.

Bacterial virulence factors can:

1. Breakdown host tissues directly (enzymes)

2. Damage / kill host cells (toxins)

3. Stimulate host - mediated tissue breakdown (e.g. bone resorption)

4. Stimulate inflammatory / immune responses

Bacterial Virulence Factors - case study - LPS and PAMPs

• PAMPs - “Pathogen Associated Molecular Patterns” eg lipopolysaccharide (LPS) on Gram negative bacteria such as P. gingivalis

• Bind to cell surface receptors on macrophages, - eg TLRs ( Toll-like receptors)

• Results in cytokine stimulation and immune activation

• TLRs expressed by many cell types – junctional and sulcular epithelium

TLRs expressed by many cell types

macrophages junctional and sulcular epithelium

So, if host responses are protective, why do they cause local tissue damage?

Local tissue damage provides access for the inflammatory response (bystander damage?)

Why do some individuals with significant biofilm accumulation maintain attachment levels, while others with reasonable plaque control become edentulous?

• This remains one of the most significant unanswered questions in periodontology.

• What is evident, however, is the need for a paradigm shift.

• While bacteria initiate the inflammatory process in all individuals, it is the host response that ultimately determines the progression to periodontitis.

Molecular Mediators of Host Responses include:

1. Vasoactive inflammatory mediators (eg. histamine)

2. Cytokines (inter-cellular soluble protein messengers) , Interleukin-1, IL-6, IL-8, IL-10, IL-17, IL-23, TNF, Interferons

3. Prostaglandins, Leukotrienes.

Overall make a complex interacting network of pro- and anti-inflammatory signals to allow an appropriate inflammatory response

Connective Tissue Damage

Direct Bacterial Actions?:

• Enzymes. eg Collagenase

Host Mediated Actions: - MAIN

• Cells - Fibroblasts - mmps and cathepthins and Macrophages

• Enzymes : MMPs and Cathepsins

• Regulated by Inflammatory Cytokines - via macrophages

Connective Tissue Damage

• Bacterial enzymes can damage tissues directly

• Most damage is mediated by host enzymes such as MMPs and cathepsins

• These enzymes are produced by host cells in response to stimulation by interleukins,TNFs and other inflammatory mediators, and to some extent by bacterial factors

net loss of connective tissues - increased utilisation of space by the inflammatory immune cells

Matrix Metalloproteinases

• Series of metal ion (Zn2+) containing enzymes regulating connective tissue turnover and breakdown

• Produced by fibroblasts and macrophages

• Stimulated by inflammatory cytokines, LPS, prostaglandins

• Important in chronic inflammatory diseases, cancer metastases, etc

• Can be inhibited by chelating agents, including tetracyclines

Matrix Metalloproteinases - examples

• Collagenases (MMP-1, -8, -13) - MMP8 - biomarker to characterise perio - prognostic factor - if their perio is active/progressive

• Gelatinases ( MMP-2, -9)

• Stromelysins (MMP-3, -10, -11)

• Elastases

Cells and mediators involved in periodontal host response

• both innate and adaptive immune system

• proinflammatory - red arrows

• attempt to control - blue arrows and blue pills

• tooth is on the left side

pale pink at the top = neutrophils - attempting to stop bacterial ingress - secreting pro-inflammatory cytokines

m1 - type 1 pro-inflammatory macrophages - mmp production via fibroblasts

TIMMPs - attempts from fibroblasts to control this - they get exhausted ☹ and become quiescent - lack of new collagen - net loss of tissue

macrophages also tey to link innate and adaptive immune system Th cells

Th 1 and 2 - classic, Th1 - more direct pro-inflammatory - IL1B interferon gamma etc - eccentuates macrophages

Th2 - more involved in producing specific response - activating B cells to plasma cells - antibody production against bacterial ingress

TH17 - IL17 - highly pro-inflammatory - perpetuates host immune reponse and tissue damage

Treg - immune tolerance and try to limit - IL10 and TGFB which stimulates repair

SPMs - specialised pro-resolving mediators - fatty acid derived- enhance repair and tone down inflammation - eg resolvins, hypoxins, protectins

Cells and mediators involved in periodontal bone resorption

cytokines may oromote bone resorption

small influence of bacterial products having a direct effect on bone resorption - though host response is pretty good at walling off the bacterial products from binding to osteoblasts

major influence - host immune response

Th17 , neutrophils and macrophages - directly signalling to osteoblasts - reduction

increase in osteoclast activity

cytokines such as IL17 can bind onto pre-osteoclasts which can cause them to mature

The RANK/RANKL/OPG system

• RANKL expression is essential for osteoclasts to form, and therefore is essential for bone resorption

• RANKL is either membrane bound or secreted by cell types including lymphocytes and osteoblasts

• It binds to its receptor, RANK, present on pre-monocytes to induce osteoclasts

• RANKL is blocked by its inhibitor, OPG, therefore the ratio of RANKL to OPG determines whether bone resorption occurs.

RANKL is either

membrane bound or secreted by cell types including lymphocytes and osteoblasts

RANKL in periodontitis

• High levels of OPG expressed in health

• RANKL expression is induced in periodontitis by inflammatory cytokines, prostaglandins, bacterial virulence factors

• OPG expression reduced in periodontitis

• High RANKL/OPG ratio associated with bone resorption in progressive periodontitis

RANKL expression is induced in periodontitis by

• inflammatory cytokines, prostaglandins, bacterial virulence factors

cellular infiltrate is slightly differnt in gingivitis than perio

can’t be oral hygiene alone -

risk factors having an influence

Major risk factors for progressive periodontitis

• Smoking • Diabetes • Genetics • Specific bacteria - non modifiable

Pathological factors associated with progressive periodontitis 3

1. Microbial variation

2. Neutrophil function

3. Hyper-inflammatory responses

Microbial variation

Species associated with destructive periodontitis - direct tissue damage, though for some patients, the amount of bacterial loas is not significant, though for patients with an average to low susceptibility - biofilm managemnet is very important

• P. gingivalis, T. forsythia, F. alocis, P. stomatis

• A.a (incl JP-2 clone)

Specific virulence factors

• P.g. proteolytic enzymes - gingipains

• Aa Leukotoxin

Amount of bacteria / bacterial load?

Neutrophil function

• Where the real action takes place -

• Defective neutrophil function is consistently associated with severe periodontal breakdown

• Neutrophil migration may be impaired in smoking, diabetes, and other systemic conditions

Neutrophil function 3

mediate beenficial effects in 3 ways

pahgoctytosis, degranulation to destroy pathogens, NETosis (neutrophil extracellular trap) - contents are ejected to encase bacteria so they can be phagocytosed

Examples of pathobionts subverting neutrophil function

• neutrophil mechanisms may be manipulated/subverted by pathobionts so the tissue can be further invaded and for inflammation to be perpetuated - so we get an increase in pathobionts and breakdown of tissues - leading to nutrients and more space for colonisation

• read slide

Too many or too few neutrophils lead to periodontal tissue loss

too little - profound perio and loss of teeth at a very yound age

Trained immunity resulting from bacterial stimulation

in chronic inflammation eg perio, then this leads to increased capacity for tissue destruction

delicate balance to treat

Neutrophils play a central role in perpetuating periodontal tissue loss

by complement activation or complement activation, direct effects on local tissue environment by causing tissue damage - they also signal to Th17 cells and adaptive immune cells that include B cells - plasma cells - antibodies - increase in RANKL - osteoclast progenitors - osteoclast form - bone resorption

IL-17 – key mediator involved in periodontal tissue loss

pro-inflammatory stimulus - tissue breakdown, bone resorption, and further production of pro-inflammatory mediators and pro-inflammatory macrophages, promote sneutrophil production from bone marrow, migration and recruitment

Potential therapeutic implications

Adjunctive therapies for specific susceptible patients ?

• Antimicrobial

• Specific inflammatory inhibitors (anti-cytokine therapies, NSAIDS)

• Anti-tissue damage - MMP inhibitors

• Anti-bone resorption - OPG, etc

Therapeutic options targeting host response

note therapies

Some Protective Factors in Host Response

• Saliva • Gingival Crevicular Fluid • Neutrophils • Antibodies • Complement • Attempts at repair

Antibacterial actions of saliva include:

A vehicle for swallowing bacteria • Inhibition of bacterial attachment • Aggregation of bacteria (agglutinins) • Killing of bacteria by the peroxidase system • Killing of bacteria by antimicrobial peptides (defensins) • Killing of bacteria by lysozyme, lactoferrin etc

Protective effects of gingival crevicular fluid/ inflammatory exudate

Mechanical washing in crevice • Complement • Antibodies • Other soluble proteins such as • Inflammatory mediators • Protease Inhibitors

Inflammatory Cells

• Neutrophils (PMNs): –Present in tissues and in large numbers within the pocket; –Kill bacteria by phagocytosis and extra-cellular mechanisms; • Macrophages: –Phagocytosis and removal of damaged tissue; –Modulation of inflammation by cytokine production; –Antigen presentation and control of immune mechanisms;

Immune Mechanisms

• Humoral immunity • B-cell (plasma cell) production of antibodies. • Cell Mediated Immunity • T-lymphocyte mediated immune mechanisms (T helper cells - Th1, Th2, Th17, Tregs - produce characteristic specific cytokine profiles)

Humoral immunity

Antibodies have three possible basic actions:

• Neutralisation:

–Neutralising toxins and enzymes by direct blocking action

–Blocking bacterial binding sites to prevent colonisation

• Opsonisation:

–Specific binding to bacteria to promote phagocytosis

• Complement activation:

–Activates complement by the Classical Pathway

Humoral immunity in periodontitis