⚔️ Section D · The Conflict
Act 3 · The Conflict

Pathogenesis

The villain does not destroy the periodontium directly.
The host's own immune response does.
🦠
MICROBIAL CHALLENGE
Dysbiotic biofilm · Keystone pathogens · PAMPs · LPS
⚔️
🔥
HOST RESPONSE
Cytokines · MMPs · Complement · Immune cells
→
💀
TISSUE DESTRUCTION
Collagen loss · Bone resorption · Attachment loss
Page & Kornman Non-Linear Model, 1997
Bacterial products regulate host immune-inflammatory systems. The clinical expression of disease is then modified by environmental and genetic risk factors. Destruction is host-mediated — bacteria provide the trigger, not the bullet.
The Molecular Weapons

The Cytokine Storm

Click each mediator card to reveal its mechanism and destructive role
IL-1β
Interleukin-1 Beta
  • Source: Macrophages, monocytes, neutrophils
  • MMP induction: Activates MMP-1, MMP-3, MMP-13
  • Bone: Stimulates RANKL on osteoblasts → osteoclast activation
  • PGE2: Induces prostaglandin E2 → vasodilation, pain, bone loss
  • Most studied cytokine in periodontitis
TNF-α
Tumour Necrosis Factor-Alpha
  • Source: Macrophages, T-cells, mast cells
  • Synergises with IL-1β — amplifies all downstream effects
  • NF-κB: Major activator → sustained inflammatory gene expression
  • Bone: Direct osteoclastogenic effect via RANKL pathway
  • Elevated in GCF of periodontitis patients
IL-6
Interleukin-6
  • Source: Fibroblasts, macrophages, endothelium
  • Acute phase: Stimulates CRP, fibrinogen — systemic inflammation
  • B-cell differentiation: → plasma cell production → IgG antibodies
  • Osteoclastogenesis: Independent pathway alongside RANKL
  • Systemic link: elevated serum IL-6 in severe periodontitis
IL-17
Interleukin-17 (Th17)
  • Source: Th17 T-cells (key adaptive response driver)
  • RANKL: Stimulates RANKL on stromal cells → bone loss
  • MMP induction: Activates MMP-1, MMP-3 in fibroblasts
  • Neutrophil recruitment: IL-17 → IL-8 → neutrophil influx
  • Critical bone destruction driver (Gaffen, 2009)
IL-8 / CXCL8
Chemokine · Neutrophil Attractant
  • Source: Epithelial cells, macrophages, fibroblasts
  • Primary function: Neutrophil chemotaxis into sulcus
  • Degraded by: P. gingivalis gingipains → chemokine paralysis
  • Dual role: Protective (defence) and destructive (excess neutrophil activity)
  • Key molecule in leukopenia of the sulcus
PGE₂
Prostaglandin E₂
  • Source: Macrophages, fibroblasts via COX-2 pathway
  • Vasodilation: Increased blood flow → redness, BOP, oedema
  • Bone: Directly stimulates osteoclast activity — major bone loss driver
  • Pain: Sensitises nociceptors — periodontal pain
  • COX-2 inhibitors (NSAIDs) reduce bone loss — therapeutic target
Key principle: These cytokines do not act in isolation — they form a self-amplifying network. IL-1β induces TNF-α; both induce IL-6; IL-17 amplifies all three. The result is a cytokine cascade that becomes largely host-driven and independent of continued bacterial presence.
Tissue Destruction Pathway

Matrix Metalloproteinases

The molecular scissors dismantling the periodontium

MMPs are zinc-dependent endopeptidases secreted by neutrophils, macrophages, fibroblasts and epithelial cells in response to pro-inflammatory cytokines (IL-1β, TNF-α, IL-17).

They are secreted as inactive pro-MMPs, activated extracellularly by plasmin or other MMPs (cascade activation), and regulated by TIMPs (Tissue Inhibitors of Metalloproteinases).

MMP-8 (Collagenase-2) — Primary marker
Produced by neutrophils · Cleaves collagen I, II, III · Most abundant MMP in GCF · Used as a point-of-care diagnostic biomarker for periodontitis activity
MMP-1 (Collagenase-1) — Fibroblast origin
Produced by gingival fibroblasts · Activated by IL-1β and TNF-α · Degrades collagen I, II, III in PDL · Elevated in diseased gingival tissue
MMP-13 (Collagenase-3) — Bone matrix
Produced by osteoblasts and chondrocytes · Degrades collagen I, II, III and aggrecan · Critical in alveolar bone matrix breakdown · Activated by Th17/IL-17 signalling
TIMP-1 & TIMP-2 — The regulators
Natural endogenous inhibitors of all MMPs · In periodontitis: TIMP levels overwhelmed by MMP overproduction · MMP/TIMP ratio shifts toward destruction · Therapeutic target: doxycycline (sub-antimicrobial) inhibits MMPs
MMP ACTIVATION CASCADE Cytokines (IL-1β, TNF-α) activate fibroblasts, macrophages Pro-MMP Secreted (inactive precursor — latent form) plasmin / MT-MMPs Active MMP MMP-1 · MMP-8 · MMP-13 Collagen I/III PDL · Gingival fibres Bone Matrix Osteoid · Collagen II degradation Clinical Outcome Attachment loss · Pocket deepening Bone loss · Tooth mobility TIMP inhibits
Clinical application: Sub-antimicrobial doxycycline (SDD — 20mg twice daily) inhibits MMP activity without antibiotic effect. FDA-approved for adjunctive periodontitis treatment.
Bone Destruction Mechanism

The RANKL / OPG Axis

The molecular switch controlling alveolar bone fate

Bone remodelling requires a precise balance between osteoclastic resorption and osteoblastic formation. In periodontitis, pro-inflammatory cytokines overwhelmingly tip this balance toward resorption.

🔺 RANKL (Receptor Activator of NF-κB Ligand)
  • Expressed on: activated T-cells, osteoblasts, stromal fibroblasts
  • Upregulated by: IL-1β, TNF-α, IL-17, PGE₂, PTH
  • Binds RANK receptor on osteoclast precursors → osteoclast differentiation and activation
  • Result: Alveolar bone resorption
🔻 OPG (Osteoprotegerin) — The Decoy Receptor
  • Expressed by: osteoblasts, stromal cells
  • Functions as soluble decoy receptor — binds RANKL, prevents it reaching RANK
  • Downregulated by: IL-1β, TNF-α — inflammatory cytokines suppress OPG
  • Result: Without OPG, RANKL acts unopposed → bone resorption
Therapeutic implication: Denosumab (anti-RANKL monoclonal antibody) reduces bone loss — in trials for periodontitis. OPG-Fc fusion proteins under investigation.
HEALTHY BALANCE RANKL osteoclast ↑ OPG inhibits RANKL = Bone Homeostasis PERIODONTITIS — IMBALANCE RANKL ↑↑ IL-1β · TNF-α · IL-17 OPG ↓↓ suppressed → Alveolar Bone Resorption
Evolution of Understanding

Models of Disease Progression

A century of shifting paradigms — click each model to explore
Early 20th Century
Continuous Model
time → bone loss
Slow, steady, linear progression. All sites lose bone at the same rate. Disproved — didn't explain remission or rapid bursts.
Socransky et al., 1984
Random Burst Model
time → bursts
Episodic bursts of destruction followed by remission. Different sites active at different times. Explains clinical variance across patients.
Kornman & Page, 1997
Non-Linear Model
Microbe + Host Modified by Risk Factors
Host immune response as primary driver. Genetic & environmental risk factors modify clinical expression. Landmark in periodontal pathogenesis thinking.
Hajishengallis, 2014
PSD Model
Pg
Polymicrobial Synergy & Dysbiosis — keystone pathogen orchestrates community-level disease. Current gold standard conceptual model.
Clinical implication of burst models: Disease does not progress uniformly — active periods of destruction alternate with remission. This explains why some patients with chronic periodontitis show minimal change over years, then sudden aggressive bone loss. Treatment timing and maintenance intervals must account for this episodic nature.
Why The Conflict Persists

Resolution of Inflammation — Why It Fails

Inflammation is designed to terminate. In periodontitis, it does not.
✅ Normal Acute Inflammation
🦠 PAMPs
→
🔥 Inflammation
→
💊 SPMs
→
✓ Resolved
Lipoxins, resolvins and protectins actively terminate neutrophil recruitment, clear apoptotic cells and restore tissue homeostasis.
❌ Periodontitis — Unresolved
🦠 Dysbiosis
→
🔥 Inflammation
→
⛔ SPMs blocked
→
✗ Chronic
P. gingivalis cleaves lipoxin receptors · Sustained NF-κB activation overrides resolution signals · Smoking, diabetes, omega-3 deficiency suppress SPM production.
Specialised Pro-resolving Mediators (SPMs)
Lipoxins
Arachidonic acid-derived · Stop neutrophil influx · Promote efferocytosis
Resolvins
Omega-3 (EPA/DHA) · Reduce IL-1β, TNF-α · Promote tissue repair
Protectins
DHA-derived · Neuroprotectin D1 · Suppress pro-inflammatory gene expression
Host Modulation Therapy (HMT)
Targeting the host response to tip the balance toward resolution
💊
Sub-antimicrobial Doxycycline (SDD)
20mg twice daily · MMP inhibition · FDA-approved · No antibiotic effect · Reduces collagen loss
🐟
Omega-3 Supplementation
EPA/DHA → resolvin & protectin precursors · Boosts SPM levels · Reduces PGE₂ · Anti-inflammatory
🧬
NSAIDs (Topical)
COX-2 inhibition → reduces PGE₂-driven bone loss · Topical ketorolac / flurbiprofen studied in trials
🦴
Bisphosphonates / Anti-RANKL
Inhibit osteoclast activity directly · Denosumab (anti-RANKL mAb) in clinical trials for periodontitis
Key insight: HMT targets the host, not the bacteria — complementing mechanical debridement to restore the balance between inflammation and resolution.