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INFLAMMATION - Complete Topic

Cellular and Molecular Immunology - Abbas, Lichtman & Pillai

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1. DEFINITION AND OVERVIEW

• Leukocytes and plasma proteins normally circulate in blood and must be recruited to extravascular sites of infection/injury
• They perform effector functions: kill microbes, clear dead cells, and begin tissue repair
• Acute inflammatory responses begin with recognition of microbial PAMPs (pathogen-associated molecular patterns) or DAMPs (damage-associated molecular patterns) by tissue sentinel cells

• Increased blood flow
• Delivery of plasma proteins
• Migration of leukocytes into tissues

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2. INITIATION: SENTINEL CELLS AND PATTERN RECOGNITION

• Toll-like receptors (TLRs)
• Cytosolic innate pattern recognition receptors (NLRs, cGAS, etc.)

• Histamine - causes vasodilation and increased capillary permeability
• Prostaglandins - also cause vasodilation and increased permeability

1. Injury, barrier break, or microbe entry
2. Microbes/injury activate sentinel cells (macrophages, DCs, mast cells)
3. Sentinel cells secrete inflammatory mediators (cytokines, histamine, prostaglandins)
4. Vasodilation and increased vascular permeability → fluid and proteins enter tissues
5. Complement, antibodies, and antimicrobial proteins kill microbes
6. Adhesion molecules and chemokines cause leukocyte migration into tissue
7. Phagocytosis and killing of microbes

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3. THE MAJOR PROINFLAMMATORY CYTOKINES OF INNATE IMMUNITY

• Produced mainly by tissue macrophages and DCs (also mast cells, endothelial cells, some epithelial cells)
• Usually act paracrine (on nearby cells); in severe infections, may enter circulation and act endocrine (at a distance)
• May have overlapping or unique actions; one cytokine may stimulate production of others (cytokine cascades)
• Serve multiple roles: induce inflammation, inhibit viral replication, promote T cell responses, and limit innate immune responses

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TABLE 4.5 - Key Cytokines of Innate Immunity

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3A. TUMOR NECROSIS FACTOR (TNF)

• Also called TNF-α (to distinguish from TNF-β/lymphotoxin)
• Produced mainly by macrophages; also DCs and mast cells
• Originally named for causing necrosis of tumors (inflammation and thrombosis of tumor blood vessels)
• Structure: synthesized as a non-glycosylated type II membrane homotrimeric protein
• Cleaved by a membrane metalloproteinase → released polypeptide fragment
• Three polypeptide chains polymerize into a triangular pyramid-shaped circulating protein
• Receptor-binding sites are at the base of the pyramid → can simultaneously bind three receptor molecules
• Member of the TNF superfamily (all form homotrimers)
• Primary signal: TLR and NLR activation → NF-κB and AP-1 transcription factors

• Activates endothelial cells → upregulates adhesion molecules (E-selectin, ICAM-1, VCAM-1) → promotes leukocyte recruitment
• Stimulates neutrophil and monocyte activation
• Works with IL-1 to stimulate chemokine secretion (CXCL8, CCL2)
• Induces acute-phase protein synthesis in liver
• Acts on hypothalamus → fever (endogenous pyrogen)

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3B. INTERLEUKIN-1 (IL-1)

• Two forms: IL-1α and IL-1β (less than 30% homologous, but same receptors and same biologic activities)
• Main active secreted form: IL-1β
• Sources: activated macrophages (major), neutrophils, DCs, epithelial cells (keratinocytes), endothelial cells
• Actions similar to TNF

1. Signal 1: TLR, NLR, or KLR signaling → activates NF-κB → transcription and production of 33-kD pro-IL-1β
2. Signal 2: Inflammasome activation → caspase-1 is activated → proteolytically cleaves pro-IL-1β to generate 17-kD mature IL-1β

• Binds type I IL-1 receptor (expressed on endothelial cells, epithelial cells, leukocytes)
• Receptor has extracellular Ig domain + cytosolic TIR domain (same as TLRs)
• Signaling: activates NF-κB and AP-1
• Type II IL-1 receptor: decoy receptor - cannot activate signals, limits IL-1 responses

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3C. INTERLEUKIN-6 (IL-6)

• Produced by macrophages and endothelial cells (stimulated by TNF and IL-1)
• Stimulates liver to produce acute-phase proteins (CRP, SAP, fibrinogen)
• Promotes B cell growth and differentiation
• Promotes Th17 T cell differentiation
• Part of the cytokine cascade: TNF → IL-1 → IL-6 → acute-phase response

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4. SEQUENCE OF EVENTS IN INFLAMMATION: VASCULAR CHANGES AND LEUKOCYTE RECRUITMENT

4A. Vascular Changes

• Vasodilation → increased blood flow → more leukocytes delivered to affected area
• Increased capillary permeability → plasma proteins (complement, pentraxins, collectins, antibodies) enter tissues
• Changes blood flow characteristics → enhance physical interactions of circulating leukocytes with vessel walls

4B. Leukocyte Recruitment

• Postcapillary venule endothelial cells increase surface expression of adhesion molecules:
  • E-selectin (induced by TNF and IL-1 via NF-κB)
  • ICAM-1 (intercellular adhesion molecule 1) - ligand for integrins
  • VCAM-1 (vascular cell adhesion molecule 1) - ligand for integrins
  • P-selectin - stored in cytoplasmic granules, mobilized to surface by thrombin from coagulation cascade

• TNF and IL-1 stimulate cells to secrete chemokines:
  • CXCL8 (IL-8) → binds receptors on neutrophils
  • CCL2 (MCP-1) → binds receptors on monocytes
• Chemokines increase affinity of leukocyte integrins for their ligands
• Chemokines stimulate directional movement (chemotaxis) of leukocytes
• Result: increased neutrophil and monocyte adhesion to endothelium → transmigration through vessel wall

• Leukocytes accumulate in tissues → form the inflammatory infiltrate
• Neutrophils arrive first (most abundant leukocyte in blood, most rapid responder to chemotactic signals)
• Monocytes (become macrophages in tissue) become increasingly prominent over time; may be dominant in some reactions
• If TNF is absent or blocked (e.g., in patients treated with anti-TNF drugs, or TNF knockout mice) → failure to contain infections

• TNF, IL-1, and IL-6 from inflammatory sites enter the blood → delivered to bone marrow
• Work with colony-stimulating factors → enhance production of neutrophils from progenitors
• Promote release of mature neutrophils into blood
• Increases supply of cells that can be recruited and replace consumed leukocytes

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5. PHAGOCYTOSIS AND KILLING OF MICROBES

5A. Phagocytosis - Definition

• Active, energy-dependent process of engulfment of large particles (> 0.5 μm diameter) into vesicles
• Phagocytic vesicles fuse with lysosomes → ingested particles are destroyed
• Killing mechanisms are isolated from the rest of the cell inside the phagolysosome

5B. Receptors for Phagocytosis

1. Pattern recognition receptors (C-type lectins, scavenger receptors): bind specific molecular patterns (e.g., mannose receptor binds mannose residues on microbes)
2. Fc receptors (FcγRI): high-affinity receptor for IgG → phagocytosis of antibody-opsonized microbes
3. Complement receptors (C3b receptor): bind complement-opsonized microbes
4. Plasma lectin receptors

5C. Mechanism of Phagocytosis

1. Microbe binds to receptor on phagocyte
2. Plasma membrane invaginates and extends a cup-shaped projection around the microbe
3. Membrane cup extends beyond the diameter of the particle, then closes and pinches off
4. Forms a phagosome (inside-out intracellular vesicle containing the foreign particle)
5. Receptor signals activate microbicidal activities of the phagocyte
6. Phagosome fuses with lysosome → phagolysosome

5D. Microbicidal Mechanisms (inside the phagolysosome)

• Generated by phagocyte oxidase (NADPH oxidase) in the phagolysosome membrane
• O₂ is reduced to superoxide (O₂⁻), which is converted to H₂O₂ and then to hypochlorous acid (HOCl)
• HOCl (the principal microbicidal reactive oxygen intermediate) kills microbes
• Patients with Chronic Granulomatous Disease (CGD): phagocyte oxidase mutations → cannot generate ROS → susceptible to recurrent bacterial/fungal infections

• Generated by inducible nitric oxide synthase (iNOS)
• Uses arginine as substrate → produces NO + citrulline
• NO combines with ROS to form peroxynitrite and other reactive nitrogen species → toxic to microbes
• iNOS is especially important in macrophages (induced by IFN-γ from T cells and NK cells)

• Acid proteases, phospholipases, and other hydrolases in lysosomes
• Digest killed microbes
• Also contribute to killing in the phagolysosome environment (low pH)

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6. ROLE OF MACROPHAGES IN TISSUE REPAIR

• Clear dead cells (efferocytosis)
• Secrete growth factors that promote regeneration and angiogenesis
• Secrete TGF-β and other cytokines → stimulate collagen synthesis by fibroblasts → scar tissue formation

• Classically activated macrophages (M1): microbicidal, promote inflammation - dominant early in the reaction
• Alternatively activated macrophages (M2): promote tissue repair - dominant later in the reaction

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7. SYSTEMIC AND PATHOLOGIC CONSEQUENCES OF INFLAMMATION

7A. Protective Systemic Effects

• CRP (C-reactive protein) and SAP (serum amyloid P) - pentraxins that play protective roles in infection (opsonization, complement activation)
• Fibrinogen - precursor of fibrin; contributes to hemostasis and tissue repair
• Elevated plasma levels of these proteins are clinically used as markers of infection/inflammation (ESR elevated by fibrinogen; CRP is a direct acute-phase marker)

7B. Pathologic Effects of Excess TNF (Sepsis)

7C. Sepsis Syndrome

• Clinically: fever, fast heart rate, fast respiratory rate, metabolic abnormalities, mental disturbances
• Most often initiated by LPS (lipopolysaccharide/endotoxin) from gram-negative bacteria activating TLR4
• LPS triggers massive TNF production → septic shock
• The cytokine storm (TNF + IL-1 + IL-6 + other mediators) causes the organ dysfunction seen in sepsis

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8. FUNCTIONAL RESPONSES OF INNATE IMMUNITY: THREE PILLARS

8A. Inflammation

8B. Cell Death (Programmed Death Pathways)

• Apoptosis of infected cells helps eliminate viral replication sites
• Virally infected cells are hypersensitive to TNF-induced apoptosis
  • TNF receptor engages both proinflammatory AND pro-apoptotic pathways; viral infection shifts balance toward apoptosis
• Misfolded viral proteins → unfolded protein response → may culminate in apoptosis
• Pyroptosis: inflammatory cell death mediated by gasdermin D pores (formed downstream of inflammasome → caspase-1 → gasdermin D cleavage)

8C. Antiviral Response (Type I Interferons)

• IFN-α: mainly from macrophages and plasmacytoid DCs (the specialist IFN-α-producing cell)
• IFN-β: mainly from fibroblasts and other cell types

1. Induce antiviral resistance in uninfected cells (block viral replication pathways)
2. Activate NK cells (which kill virally infected cells)
3. Enhance adaptive immune responses

• Patients with severe COVID-19 often have defects in type I IFN:
  • ~10% produce autoantibodies against their own type I IFN (may predate infection)
  • 3.5-4% have inherited mutations affecting type I IFN production or signaling
• IFN-α is in clinical use as antiviral in viral hepatitis; also used in some tumors (boosts CTL activity or inhibits proliferation)
• IFN-β is therapy for multiple sclerosis (mechanism not fully known)

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9. STIMULATION OF ADAPTIVE IMMUNITY BY INFLAMMATION

Two-Signal Hypothesis for Lymphocyte Activation

1. Signal 1 (Antigen): Ensures the response is specific
2. Signal 2 (Innate-derived molecules): Ensures adaptive responses are triggered only when there is a dangerous infection (not when lymphocytes recognize harmless/self antigens)

• Costimulators (e.g., B7 molecules on APCs) → for T cells
• Cytokines (IL-1, IL-2, IL-6, IL-12, etc.) → for both T and B cells
• Complement breakdown products (C3d) → for B cells

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10. SUMMARY: KEY MEDIATORS AND THEIR ROLES

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Key Points to Remember (Abbas perspective)

1. Inflammation is fundamentally an innate immune response - the principal defensive action of the innate system
2. The key cellular sentinels initiating inflammation are macrophages, DCs, and mast cells
3. The three master proinflammatory cytokines are TNF, IL-1, and IL-6 - they act locally AND systemically
4. Neutrophils arrive first; monocytes/macrophages follow and dominate later
5. Phagocytes kill microbes using ROS (phagocyte oxidase), NO (iNOS), and lysosomal enzymes inside phagolysosomes
6. Systemic inflammation causes fever, leukocytosis, and the acute-phase response - all protective
7. Excess TNF in sepsis causes shock, DIC, and cachexia - pathologic consequences
8. Inflammation connects innate to adaptive immunity via the two-signal hypothesis (Signal 2 comes from innate reactions)
9. M1 macrophages (classically activated) fight infection early; M2 macrophages (alternatively activated) repair tissue later
10. Type I IFNs are the antiviral arm of innate immunity, distinct from the pro-inflammatory cytokines

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