Notes on Inflammation (NDMU PAS504)

Innate vs Adaptive Immunity

  • Adaptive immunity
    • Slower acting but highly specific
    • Provides long-term protection via memory
    • 3rd line of defense
  • Innate immunity
    • Rapid, nonspecific response
    • Short-lived; no memory
    • 1st line of defense: epithelial barriers and their secretions + microbiota
    • 2nd line of defense: inflammatory response

Physical and Mechanical Barriers

  • Physical barriers involve tightly-associated epithelial cells forming surfaces exposed to the outside world
    • Locations: skin, GI tract, respiratory tract, genitourinary tract
    • Key features: high turnover rate; quick replacement after damage
    • Cleansing mechanisms: mucus production, vomiting, urination
  • Purpose: prevent pathogen entry and reduce infection risk

Biochemical Barriers

  • Epithelial cells release antimicrobial proteins
    • Examples: defensins, cathelicidin, collectin
    • Modes: kill directly or tag pathogens for destruction
  • Microbiota
    • Complex community that benefits host
    • Locations: skin, mouth, gut, vagina
    • Functions: prevent attachment, compete for nutrients, release antimicrobial factors

Inflammatory Response: Overview

  • Triggers include: infection, nutrient deprivation, mechanical damage, extreme temperatures, ischemia, radiation
  • Non-specific response; same mechanism regardless of stimulus
  • Rapidly initiated; no memory cells involved
  • Not always present; can be activated when needed
  • Purposes
    • Limit and control infection
    • Interact with components of adaptive immunity
    • Remove debris and prepare injured area for healing
  • First responders include dead organelles and debris

Components of the Inflammatory Response

  • Three highly integrated levels:
    • Vascular response
    • Plasma protein response
    • Cell mediators
  • Receptors and signals involved include: TLRs, scavenger receptors, cytokines (IFNs, ILs, TNF), chemokines
  • Effects are overlapping and synergistic

Vascular Response

  • Vasodilation: increased vessel diameter → increased blood flow
  • Increased capillary permeability: tight junctions between endothelial cells become permeable
  • Leukocyte margination: WBCs adhere to inner vessel walls near injury
  • Diapedesis: leukocytes migrate from capillaries into tissue
  • Mast cells, damaged tissue, and circulating inflammatory mediators contribute signals that drive these changes

Plasma Protein Response (Overview)

  • Plasma proteins provide biochemical barriers against pathogens
  • Key systems: Complement, Clotting, Kinin
  • These proteins exist as inactive proenzymes that are sequentially activated in cascades
  • Cascade principles ensure amplification and control

Complement System

  • 30+ globular proteins participate in innate and adaptive immunity
  • Activation pathways (depending on pathogen type):
    • Classical: antibody-antigen complexes
    • Lectin: mannose-containing bacterial carbohydrates
    • Alternative: direct recognition on Gram-negative bacteria, yeasts, tumor cells, viruses
  • Main functions
    • Opsonization: tagging microbes for destruction (e.g., C3b)
    • Leukocyte chemotaxis: recruitment of immune cells (e.g., C5a)
    • Anaphylatoxic activity: mast cell degranulation (C3a, C5a)
    • Cell lysis: form membrane attack complexes
  • Important intermediates and concepts (LaTeX):
    ext{Classical/ Lectin pathway: } C4b2a ext{ (C3 convertase)}
    ext{Alternative pathway: } C3bBb ext{ (C3 convertase)}
    C3
    ightarrow C3a + C3b,\ C5
    ightarrow C5a + C5b
    ext{Membrane attack complex: } C5b ext{–}C6 ext{–}C7 ext{–}C8 ext{–}C9

Clotting System

  • Purpose: form a fibrinous mesh at injury to prevent spread of infection, confine microbes, and provide a scaffold for repair
  • Main component: fibrin
  • Activation converges to Factor X (Xa) in the common pathway
  • Common pathway (high level)
    • Xa, Va, Ca^{2+}, and phospholipid surface generate a Prothrombin activator
    • Prothrombin (II) → Thrombin (IIa)
    • Thrombin converts Fibrinogen (I) → Fibrin (Ia)
    • Fibrin mesh is cross-linked by Factor XIIIa
  • Phases of coagulation (schematic)
    • Phase 1 (Intrinsic): vessel endothelium rupture exposes collagen; platelets adhere; contact activation starts with XII → XIIa, XI → XIa, IX → IXa, with VIII and others forming the IXa–VIIIa complex
    • Phase 1 (Extrinsic): tissue trauma exposes tissue factor (TF) with VIIa to form TF–VIIa complex
    • Phase 2: generation of Prothrombin Activator and Thrombin
    • Phase 3: Fibrin formation and clot stabilization
  • Crosslinks and interplay ensure rapid formation of a hemostatic plug at sites of injury

Kinin System

  • Bradykinin is the primary mediator
  • Functions
    • Vasodilation
    • Pain induction
    • Increased vascular permeability
    • Leukocyte chemotaxis
    • Contraction of non-vascular smooth muscle
  • Activated by the intrinsic pathway of the clotting cascade

Interaction Between Clotting, Complement, and Kinins

  • These systems are highly interactive with overlapping effects
  • They reinforce each other to ensure efficient activation at the site of injury
  • Tight spatial control is essential to prevent damage to healthy tissue

Inhibition of Inflammatory Cascades

  • Natural inhibitors help limit tissue injury and resolve inflammation:
    • Carboxypeptidase: inactivates C3a and C5a
    • Kininases: degrade kinins
    • C1 esterase inhibitor: inhibits the complement system
    • Histaminase: degrades histamine

Cell Mediators of Inflammation

  • Composed of cells and substances produced by cells
  • Localized in blood and perivascular tissue
  • Functions
    • Drive the vascular response
    • Modulate localization and activity of other inflammatory cells/molecules

Key Cellular Players in Inflammation

  • Endothelium
    • Single-cell layer lining all blood vessels
    • Produces nitric oxide (NO) from arginine and prostacyclin (PGI_{2}) from arachidonic acid
    • NO and PGI_{2} maintain blood flow and inhibit platelet activation
    • Damage promotes clotting; during inflammation, endothelium upregulates receptors to attract WBCs and increases permeability
  • Platelets (thrombocytes)
    • Fragments from megakaryocytes
    • Activation leads to platelet aggregation and plug formation (thromboxane A2 promotes this)
    • Secrete procoagulants and vasoconstrictors; release growth factors for healing
  • White blood cells (WBCs)
    • Include neutrophils, monocytes, NK cells, eosinophils, basophils, mast cells
    • Involved in both innate and adaptive immunity
    • WBCs can patrol for damage, migrate to sites of tissue injury, and participate in healing

White Blood Cells in Inflammation

  • Neutrophils
    • First responders
    • Ingest bacteria, dead cells, debris
    • Short-lived; contribute to purulent exudate (pus)
  • Monocytes
    • Circulate in blood; become macrophages (mφ) in tissue
    • Wandering macrophages actively seek pathogens; fixed macrophages reside in particular tissues (e.g., microglia,Kupffer cells, dermal macrophages, alveolar macrophages)
  • Natural killer (NK) cells
    • Lymphocytes that kill virally infected and neoplastic cells
  • Eosinophils
    • Defense against parasites; also modulate allergic responses
  • Basophils
    • Similar to mast cells; participate in allergic responses
  • Mast cells
    • Located in tissues near surfaces (skin, GI, respiratory, GU tracts)
    • Contain granules that release contents upon activation
    • Activation stimuli: physical injury, toxins/venoms, immunologic triggers (anaphylatoxins), TLR activation
    • Two response types:
    • Degranulation: immediate release of granule contents
    • Synthesis: production of lipid mediators and other inflammatory products
    • Degranulation products
    • Histamine: causes rapid vasodilation and increased vascular permeability; raises flow to microcirculation
    • Heparin, TNF-α, IL-3, IL-8
    • Synthesis products (lipid mediators):
    • Leukotrienes (LAT): increase permeability and vasodilation; promote leukocyte chemotaxis
    • Prostaglandins (PGE series): similar effects; contribute to pain
    • Platelet-activating factor (PAF): effects similar to leukotrienes

Cytokines and Receptors

  • Cell surface/secreted molecules that regulate inflammation and immune responses
  • Cytokines can be proinflammatory or anti-inflammatory and are pleiotropic (one cytokine can have multiple effects depending on target cells)
  • Major families include:
    • Interleukins (ILs)
    • Interferons (IFNs)
    • Tumor necrosis factor (TNF)
    • Chemokines

Cytokines: Highlights

  • Interleukins (ILs)
    • Produced by macrophages and lymphocytes; >30 identified with varying effects
    • Proinflammatory examples:
    • IL-1: endogenous pyrogen; activates phagocytes and lymphocytes
    • IL-6: stimulates liver synthesis of acute-phase reactants
    • Anti-inflammatory examples:
    • IL-10: down-regulates production of proinflammatory cytokines
    • Transforming growth factor-beta (TGF-β)
  • Interferons (IFNs)
    • IFN-α: produced by macrophages; induces antiviral proteins
    • IFN-β: produced by virus-infected cells; prevents others from becoming infected
    • IFN-γ: released by T cells; enhances macrophage microbicidal activity
  • Tumor necrosis factor (TNF)
    • Two receptors: TNFR1 (proinflammatory) and TNFR2 (anti-inflammatory)
    • TNFR1 roles: enhances leukocyte adhesion, induces fever, upregulates acute-phase proteins
    • TNFR2 roles: contributes to wound healing and anti-inflammatory processes
  • Chemokines
    • Small peptides that induce leukocyte chemotaxis; >40 identified
    • Classified by cysteine motifs into subfamilies: CXC, CC, CX3C, XC
    • Produced by macrophages, fibroblasts, endothelial cells in response to proinflammatory signals

Local vs Systemic Manifestations

  • Local manifestations (from vascular changes):
    • Heat (vasodilation)
    • Redness (vasodilation)
    • Swelling (exudate and increased permeability)
    • Pain (pressure/exudate and prostaglandins/bradykinin)
    • Purpose: dilute toxins, deliver plasma proteins and WBCs, remove toxins and debris
  • Exudates (types)
    • Serous: early or mild inflammation; watery fluid with few cells
    • Fibrinous: thick and clotted; can be seen in pneumonia or strep throat
    • Purulent: rich in pus; suggests persistent bacterial infection
    • Hemorrhagic: contains blood
    • Catarrhal: mucus-rich exudate seen in nose/throat (e.g., common cold)

Systemic Manifestations of Acute Inflammation

  • Fever: triggered by pyrogens; can be endogenous (e.g., IL-1) or exogenous (pathogens)
  • Leukocytosis: increased circulating WBCs
  • Acute-phase protein synthesis: liver-produced proteins (e.g., fibrinogen, plasminogen, α1-antitrypsin, complements); can be pro- or anti-inflammatory

Acute vs Chronic Inflammation

  • Major difference: duration
  • Acute inflammation: protective, typically short-term
  • Chronic inflammation: destructive; may follow an unsuccessful acute response or occur with pathogens that resist clearance

Chronic Inflammation Characteristics

  • Dense infiltration by lymphocytes and macrophages
  • Destruction of surrounding healthy tissue
  • Chronic inflammation is involved in many disease processes
  • Formation of granulomas as the body isolates the area

Healing after Inflammation

  • Healing typically begins 3–4 days after injury and can continue for ~2 weeks
  • Key drivers in healing during inflammation include:
    • Clotting system activity
    • Macrophage activity
    • TNF → TNFR2 and anti-inflammatory mediators (e.g., IL-10)
    • Transforming growth factor-beta (TGF-β)
    • Down-regulation of proinflammatory activity
  • Mechanisms of healing
    • Destruction/deactivation of proinflammatory molecules and cells (e.g., IL-10, TGF-β, TNF → TNFR2)
    • Connective tissue and epithelial growth into damaged area; formation of a scaffold for tissue regeneration
    • Growth factors promote cell proliferation; matrix metalloproteinases (MMPs) remodel extracellular matrix
    • Debridement: removal of clots, microorganisms, dead cells; debris cleared via blood vessels and lymphatics
    • Reversal of vasodilation and permeability as healing progresses
  • Types of healing
    • Regeneration: restoration to original structure and function (best outcome but not always possible, especially in adults)
    • Resolution (primary intention): return to near-original structure and function (e.g., surgical wound closure)
    • Repair (secondary intention): significant tissue remodeling with scar formation; may involve abscess, granuloma, or persistent fibrin

Review and Practice Questions (Key Concepts)

  • Why are epithelial cells a good physical barrier? They form continuous, tightly joined layers with rapid turnover and mucociliary/cleansing mechanisms.
  • What is the vascular response to inflammation, and how does it benefit the response? Vasodilation and increased permeability raise blood flow, allow plasma proteins and leukocytes to reach tissue, and facilitate leukocyte extravasation.
  • Where do plasma proteins originate, and what are their general functions? They are liver-derived acute-phase reactants and components of the clotting, complement, and kinin systems that mediate inflammation, coagulation, and defense.
  • Name 3 different white blood cells and their protective roles. Neutrophils (phagocytosis of bacteria), monocytes/macrophages (phagocytosis and antigen presentation), mast cells (granule release and mediator production).
  • Which cytokines support healing by diminishing inflammation? IL-10 and TGF-β are anti-inflammatory mediators that help resolve inflammation; TNF and IL-1 are proinflammatory during the early phase but can be downregulated later.
  • Difference between interferons and interleukins; cytokines vs chemokines. Interferons (IFNs) primarily antiviral and immunomodulatory; interleukins (ILs) are a broad family with varied pro- and anti-inflammatory roles. Chemokines are a subset that primarily induce leukocyte chemotaxis.
  • Main differences between acute and chronic inflammation? Duration, cellular composition, tissue damage, and outcome (acute is often protective; chronic is destructive and may involve granuloma formation).

Practice Questions (with answers)

  • Practice Question 1: The characteristic vascular changes at the site of an injury produce:
    • Increased permeability and leakage (correct)
  • Practice Question 2: Which statement about mast cells is correct?
    • They degranulate in response to chemical agents (correct) [also can be activated by various stimuli, but the option about degranulation best fits the mast cell immediate response]
  • Practice Question 3: How do Toll-like receptors (TLRs) detect an invading pathogen?
    • They recognize pathogen-associated molecular patterns (PAMPs) (correct)
  • Practice Question 4: Ibuprofen and other NSAIDs help relieve inflammation and pain by their action at what enzyme?
    • Cyclooxygenase (correct)
  • Practice Question 5: Match the item with the associated mast cell activity:
    • I. Leukotrienes and prostaglandins → B (synthesis)
    • II. Anaphylaktotoxic activity → A (degranulation)
    • III. Vasodilation and increased permeability → A (degranulation)
    • IV. A long-term response → B (synthesis)
    • V. Histamine → A (degranulation)
    • VI. An immediate response → A (degranulation)
  • Practice Question 6: Determine whether the statements are TRUE or FALSE:
    • The clotting system has only one method of activation. FALSE
    • Purulent exudate is watery and contains few cells. FALSE
    • Complement proteins can tag pathogens for destruction (opsonization) but do not kill them directly. FALSE
    • Two effects of bradykinin are vasodilation and pain. TRUE
    • Interferons kill virus-infected cells. FALSE

Notation and Formulas (LaTeX)

  • Common coagulation pathway (high level):
    ext{Prothrombin activator} = Xa \, Va \, Ca^{2+} \, PL
    ext{Prothrombin (II)}
    ightarrow ext{Thrombin (IIa)}
    ext{Fibrinogen (I)}
    ightarrow ext{Fibrin (Ia)} ext{ (via Thrombin)}
    ext{Cross-linking: XIIIa} ext{ stabilizes fibrin mesh}

  • Complement pathways (summary):
    ext{Classical/ Lectin: } C4b2a ext{ (C3 convertase)}
    ext{Alternative: } C3bBb ext{ (C3 convertase)}
    C3
    ightarrow C3a + C3b, ext{ } C5
    ightarrow C5a + C5b
    ext{MAC: } C5b-C6-C7-C8-C9

  • Key mediators and terms to remember

    • PAMPs: pathogen-associated molecular patterns
    • DAMPs: damage-associated molecular patterns
    • PRRs: pattern recognition receptors
    • TLRs: toll-like receptors
    • NO: nitric oxide; PGI2: prostacyclin
    • IL-1, IL-6, IL-10, TNF-α, IFN-γ, C3a, C5a, C3b, VEGF, TGF-β

Practical Takeaways for Studying

  • Inflammation is an integrated system; multiple pathways can be activated and intercommunicate to ensure a rapid and effective response, while also containing damage to surrounding tissue.
  • Therapeutic interventions (e.g., NSAIDs) target specific steps (like COX enzymes) and can alter healing dynamics; balance between reducing harm from inflammation and preserving host defense is critical.
  • Healing involves both suppression of proinflammatory signals and active tissue repair processes (growth factors, ECM remodeling, debridement).