Resolution of Inflammation

Resolution of Inflammation - Calm After Storm

• Ideal outcome: Return to normal tissue architecture & function post-acute inflammation.
• Core Processes:
  • Removal of injurious stimuli.
  • Cessation of pro-inflammatory signals.
  • Switch to anti-inflammatory & pro-resolving mediators.
  • Neutrophil apoptosis & macrophage-mediated clearance (efferocytosis).
  • Drainage of edema fluid & debris.
• Key Pro-Resolving Mediators:
  • Anti-inflammatory cytokines: IL-10, TGF-β.
  • Specialized Pro-resolving Mediators (SPMs):
    • Lipoxins (e.g., LXA₄).
    • Resolvins (from EPA/DHA).
    • Protectins (from DHA).
    • Maresins (from DHA).
• Failure leads to: Chronic inflammation, scarring, or abscess.

⭐ SPMs like Lipoxins and Resolvins actively drive resolution, not just passive decay of inflammation.

Resolution of Inflammation - The Peacekeepers

• Goal: Complete restoration of tissue structure & function. Active, programmed process.
• Triggers: Removal of offending agent.
• Key Events:
  • Cessation of pro-inflammatory signals (e.g., leukotrienes switch to lipoxins).
  • Neutrophil apoptosis & subsequent efferocytosis by macrophages.
  • Clearance of edema fluid & proteins via lymphatics.
  • Macrophage phenotype switch: M1 (pro-inflammatory) → M2 (anti-inflammatory, pro-repair).
• Specialized Pro-resolving Mediators (SPMs): 📌 "LRPM" (Lipoxins, Resolvins, Protectins, Maresins)
  • Lipoxins (LXA4, LXB4): Inhibit neutrophil chemotaxis, stimulate monocyte recruitment.
  • Resolvins (E-series from EPA, D-series from DHA): Reduce PMN infiltration, enhance efferocytosis.
  • Protectins (e.g., Neuroprotectin D1 from DHA): Potent anti-inflammatory.
  • Maresins (MaR1 from DHA, by macrophages): Stimulate tissue regeneration.
• Anti-inflammatory Cytokines: IL-10, TGF-β.

⭐ Lipoxins are unique as they are generated through transcellular biosynthesis, often involving neutrophil-platelet or neutrophil-epithelial cell interactions, signifying a "stop signal" for inflammation.

Resolution of Inflammation - Clean Sweep

Active, programmed return to homeostasis; not passive decay. Switches off pro-inflammatory signals, actively clears cells & debris.

• Hallmarks & Mechanisms:
  • Cessation of stimulus: Removal of offending agent.
  • Mediator Switch: From pro-inflammatory (e.g., Leukotrienes, Prostaglandins) to anti-inflammatory.
    • Specialized Pro-resolving Mediators (SPMs): Lipoxins, Resolvins (e.g., RvE1, RvD1), Protectins, Maresins. 📌 (LRPM: "Let Resolution Promote Mend")
    • Anti-inflammatory cytokines: IL-10, TGF-β.
  • Stop signals: Inhibit leukocyte recruitment.
  • Cellular Cleanup:
    • Neutrophil apoptosis & subsequent efferocytosis (engulfment by macrophages).
    • Macrophage phenotype switch: M1 (pro-inflammatory) → M2 (anti-inflammatory, pro-repair).
  • Debris Clearance: Edema, proteins, necrotic cells via lymphatics & phagocytosis.
  • Outcome: Tissue regeneration or repair (scarring).

⭐ Lipoxins (e.g., LXA4, LXB4) are key SPMs that inhibit neutrophil chemotaxis and adhesion, stimulate monocyte recruitment, and promote efferocytosis, acting as "braking signals" for acute inflammation.

Resolution of Inflammation - Healing Hiccups

• Ideal Outcome: Complete restoration of normal tissue structure & function.
• "Hiccups" - Factors Impeding Resolution:
  • Persistent injurious agent (e.g., M. tuberculosis)
  • Extensive necrosis or non-regenerating tissue
  • Inadequate drainage (e.g., empyema)
  • Foreign bodies
  • Impaired host immunity
• Alternative Outcomes if Resolution Fails:
  • Chronic Inflammation: Ongoing inflammation, angiogenesis, fibrosis.

    ⭐ Persistent macrophage activation (e.g., epithelioid, giant cells) is key in chronic inflammation, often driving fibrosis.

  • Fibrosis/Scarring: Replacement by connective tissue; loss of function.
  • Abscess Formation: Localized pus (neutrophils, necrotic cells, edema fluid) walled off by fibrous tissue.

High‑Yield Points - ⚡ Biggest Takeaways

• Resolution is the ideal outcome, restoring normal tissue structure and function post-inflammation.
• Driven by Specialized Pro-resolving Mediators (SPMs) like lipoxins, resolvins, and protectins.
• Involves neutrophil apoptosis, efferocytosis by M2 macrophages, and cessation of leukocyte influx.
• M2 macrophages clear debris and release anti-inflammatory cytokines (e.g., IL-10, TGF-β).
• Failure leads to chronic inflammation, fibrosis, or abscess formation.
• Favored by minimal tissue damage and rapid removal of the injurious stimulus.