9.2 Acute & Chronic Inflammation, Granulomas, Systemic Effects & Tissue Repair

Why this area matters

The TOS basic pathology area lists "inflammations; cell and tissue regeneration". Every infectious, autoimmune and post-surgical item in Pathology, Medicine and Surgery uses these concepts. Common question types are:

• a child with recurrent infections and an unusual leukocyte defect;
• a granuloma to classify;
• a laboratory picture of the acute-phase response;
• a wound that heals badly.

Acute Inflammation: The Leukocyte Recruitment Cascade

Acute inflammation develops rapidly (minutes to hours), featuring exudation of fluid and plasma proteins (edema) and emigration of leukocytes, predominantly neutrophils (polymorphonuclear leukocytes, PMNs).

Leukocyte exit from the microvascular lumen to the interstitial site of injury follows an orderly sequence of cellular events:

1. Margination and Rolling:

   • Increased vascular permeability and slowing of post-capillary venular blood flow (stasis) pushes heavier leukocytes outward from the central axial stream toward the vascular endothelium (margination).
   • Leukocytes roll along the endothelial surface via low-affinity, transient binding mediated by the Selectin family:
     • P-selectin: Pre-formed and stored inside endothelial Weibel-Palade bodies and platelet alpha-granules; rapidly translocated to the cell surface by histamine and thrombin.
     • E-selectin: Synthesized de novo on endothelial cells in response to tumor necrosis factor (TNF) and interleukin-1 (IL-1).
     • L-selectin: Constitutively expressed on leukocyte microvilli.
     • Selectins bind to sialylated oligosaccharides (such as Sialyl Lewis X) displayed on leukocyte or endothelial glycoprotein backbones.

2. Firm Adhesion:

   • Rolling leukocytes are arrested and firmly adhere to the endothelial surface via Integrins.
   • Endothelial chemokines bind to leukocyte G-protein coupled receptors, triggering an "inside-out" conformational switch that converts low-affinity leukocyte integrins into high-affinity active conformations.
   • LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) on leukocytes bind firmly to ICAM-1 (Intercellular Adhesion Molecule-1) on the endothelium.
   • VLA-4 (alpha-4/beta-1 integrin) on leukocytes binds VCAM-1 (Vascular Cell Adhesion Molecule-1).
   • Clinical Correlation: Leukocyte Adhesion Deficiency Type 1 (LAD-1) is an autosomal recessive deficiency of the CD18 subunit (beta-2 integrin). Hallmark findings include delayed umbilical cord separation ($> 30$ days), absent pus formation at infection sites, severe recurrent bacterial infections, and marked peripheral blood leukocytosis.

3. Diapedesis (Transmigration):

   • Leukocytes extend pseudopodia and squeeze between endothelial intercellular junctions to breach the basement membrane.
   • This process is mediated primarily by homophilic interactions of PECAM-1 (Platelet Endothelial Cell Adhesion Molecule-1, CD31) present on both leukocytes and endothelial cell borders.

4. Chemotaxis:

   • Once across the vessel wall, leukocytes migrate directionally along a chemical concentration gradient toward the focus of tissue injury.
   • Major exogenous chemoattractants include bacterial products (N-formylmethionyl peptides).
   • Major endogenous chemoattractants include:
     • Complement component C5a
     • Leukotriene B4 ($LTB_4$)
     • Interleukin-8 (IL-8 / CXCL8)
     • Platelet-activating factor (PAF)

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Phagocytosis & Intracellular Killing: The Respiratory Burst

Upon arrival at the extravascular site, neutrophils and macrophages engulf and destroy targets through three sequential steps:

1. Recognition and Opsonization: Targets are coated by host serum proteins called opsonins. The principal opsonins are the Fc portion of IgG (recognized by leukocyte Fc$\gamma$R), complement component C3b (recognized by complement receptor CR1/CR3), and plasma mannose-binding lectin (MBL).
2. Engulfment: Binding triggers pseudopod extension around the microbe, enclosing it within a cytoplasmic vesicle called a phagosome. The phagosome fuses with a primary lysosome to form a phagolysosome.
3. Intracellular Microbial Killing: Microbicidal killing occurs predominantly through oxygen-dependent mechanisms mediated by the respiratory (oxidative) burst:

$\text{NADPH} + 2O_2 \xrightarrow{\text{NADPH Oxidase}} \text{NADP}^+ + H^+ + 2O_2^{\bullet-}$
$2O_2^{\bullet-} + 2H^+ \xrightarrow{\text{Superoxide Dismutase (SOD)}} H_2O_2 + O_2$
$H_2O_2 + Cl^- \xrightarrow{\text{Myeloperoxidase (MPO)}} HOCl + H_2O$

• Superoxide radical ($O_2^{\bullet-}$) is generated by the membrane-bound enzyme complex NADPH oxidase (phagocyte oxidase).
• Superoxide dismutase converts superoxide to hydrogen peroxide ($H_2O_2$).
• Myeloperoxidase (MPO), located in the azurophilic granules of neutrophils, utilizes chloride ions to convert $H_2O_2$ into hypochlorous acid ($HOCl$, bleach), the most potent bactericidal oxidant in phagocytes.

Clinical Deficiencies in Microbial Killing

• Chronic Granulomatous Disease (CGD): An inherited defect in NADPH oxidase (most commonly X-linked mutation in the $gp91^{phox}$ subunit). Phagocytes ingest microbes normally but cannot produce superoxide or hydrogen peroxide. Catalase-negative organisms generate their own $H_2O_2$, which the deficient neutrophil can exploit to kill them. However, catalase-positive organisms (Staphylococcus aureus, Burkholderia cepacia, Serratia marcescens, Nocardia, Aspergillus) degrade their own endogenous $H_2O_2$, leading to recurrent life-threatening catalase-positive infections and granuloma formation. Diagnostic tests include the abnormal Dihydrorhodamine (DHR) flow cytometry test (absent green fluorescence) and the Nitroblue Tetrazolium (NBT) reduction test (fails to turn blue).
• Myeloperoxidase (MPO) Deficiency: Absence of MPO. Respiratory burst is completely intact (normal/positive NBT and DHR tests). Most patients are clinically asymptomatic, though they display increased susceptibility to Candida infections.

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Chemical Mediators of Inflammation

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Chronic Inflammation & Granulomatous Diseases

Chronic inflammation is inflammation of prolonged duration (weeks to months to years) in which active inflammation, tissue destruction, and attempts at repair (fibrosis) proceed simultaneously.

Cellular Players and Macrophage Polarization

Macrophages are the dominant cellular effectors in chronic inflammation, capable of differentiating into two distinct functional programs:

• Classically Activated (M1) Macrophages: Induced by microbial endotoxins (TLR ligands) and Interferon-gamma (IFN-$\gamma$) secreted by Th1 lymphocytes. M1 cells produce microbicidal substances (ROS, NO) and pro-inflammatory cytokines (IL-1, TNF, IL-12), promoting host defense and tissue destruction.
• Alternatively Activated (M2) Macrophages: Induced by cytokines IL-4 and IL-13 produced by Th2 lymphocytes. M2 cells secrete growth factors (TGF-$\beta$, VEGF, PDGF) and anti-inflammatory cytokines (IL-10), functioning primarily to terminate inflammation, promote tissue repair, and stimulate fibrosis.

Granulomatous Inflammation

Granulomatous inflammation is a distinctive pattern of chronic inflammation characterized by focal collections of activated epithelioid histiocytes—macrophages with abundant pink granular cytoplasm and elongated slipper-shaped vesicular nuclei resembling epithelial cells—surrounded by a collar of mononuclear leukocytes (principally CD4+ T lymphocytes and plasma cells).

• Multinucleated Giant Cells: Formed by the fusion of multiple epithelioid macrophages. Langhans giant cells exhibit nuclei arranged peripherally in a horseshoe pattern (classic for mycobacterial infection). Foreign body giant cells exhibit nuclei scattered haphazardly throughout the cytoplasm.
• Caseating Granulomas: Contain a central core of structureless, acellular, eosinophilic necrotic debris; hallmark of Mycobacterium tuberculosis and endemic fungi (Histoplasma, Coccidioides).
• Non-Caseating Granulomas: Lack central necrotic debris; classic for Sarcoidosis (associated with elevated ACE, hypercalcemia, Asteroid and Schaumann bodies), Crohn disease, and Berylliosis.

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Tissue Repair, Angiogenesis & Scar Formation

Tissue repair occurs by two processes: regeneration by proliferation of uninjured cells (in tissues with high proliferative capacity like liver and gastrointestinal epithelium) and maturation of scar tissue by connective tissue deposition (when the extracellular matrix framework is destroyed).

Stages of Wound Healing

1. Hemostasis and Acute Inflammation (0 to 48 hours): Platelet plug formation, fibrin clot stabilization, and neutrophil infiltration, followed by macrophage dominance by day 2.
2. Granulation Tissue Formation (Days 3 to 5): The microscopic hallmark of early wound healing, characterized by a pink, soft, granular tissue bed composed of proliferating fibroblasts, loose extracellular matrix, and newly formed, delicate capillary loops (angiogenesis). Angiogenesis is driven by Vascular Endothelial Growth Factor (VEGF) and basic Fibroblast Growth Factor (bFGF).
3. Matrix Remodeling and Scar Deposition (Weeks to Months): Synthesis of extracellular matrix. Type III collagen is laid down initially during early repair and is subsequently degraded and replaced by strong, mature Type I collagen by zinc-dependent Matrix Metalloproteinases (MMPs). Specialized myofibroblasts containing alpha-smooth muscle actin mediate wound contraction.
4. Tensile Strength: Reaches approximately 10% of original uninjured skin strength at 1 week, accelerating to approximately 70% to 80% at 3 months, which represents the plateau (scarred tissue never regains 100% of original strength).

Pathologic Wound Repair

Morphologic patterns and outcomes of acute inflammation

Outcomes of acute inflammation:

• Complete resolution: injury is short-lived and tissue can regenerate, as in lobar pneumonia.
• Healing by fibrosis: substantial tissue destruction, or tissue that cannot regenerate.
• Progression to chronic inflammation: the agent persists.

Systemic effects of inflammation

• Fever: IL-1, TNF and IL-6 induce cyclooxygenase in the hypothalamus. Prostaglandin E2 resets the thermostat, which is why NSAIDs reduce fever.
• Acute-phase reactants are made by the liver, driven mainly by IL-6:
  • C-reactive protein;
  • fibrinogen, which raises the erythrocyte sedimentation rate by promoting rouleaux;
  • serum amyloid A, which causes AA amyloidosis in long-standing inflammation;
  • hepcidin, which traps iron in macrophages and causes the anemia of chronic disease.
• Leukocytosis:
  • Neutrophilia with a left shift occurs in bacterial infection.
  • Lymphocytosis occurs in viral infections and pertussis.
  • Eosinophilia occurs in allergy and helminth infection.
  • Typhoid fever and some viral and rickettsial infections cause leukopenia.
• Sepsis: massive cytokine release leads to vasodilation, increased vascular permeability, disseminated intravascular coagulation and septic shock.

Regeneration and the cell cycle

Tissues heal according to the ability of their cells to proliferate:

• Labile tissues regenerate readily: surface epithelia and bone marrow.
• Stable tissues can regenerate if the framework is intact: liver, kidney tubules, pancreas, endothelium and fibroblasts. Liver regeneration after partial hepatectomy is the classic example.
• Permanent tissues heal by scar: neurons and cardiac muscle. A myocardial infarct always heals with a fibrous scar.

Healing by first and second intention

• First intention (primary union): a clean, sutured surgical incision with apposed edges. There is little granulation tissue and minimal scar. Epithelial bridging occurs within about 24 to 48 hours, and sutures are usually removed at about 1 week, when strength is only about 10%.
• Second intention (secondary union): large wounds, abscess cavities and ulcers with separated edges. There is abundant granulation tissue, prominent wound contraction by myofibroblasts, and a larger scar.

Factors that impair healing:

• Local: infection, the most important local factor; foreign bodies; poor blood supply; movement and mechanical stress; a large wound gap.
• Systemic:
  • diabetes mellitus;
  • malnutrition: protein deficiency; vitamin C deficiency, which impairs collagen hydroxylation (scurvy); zinc deficiency;
  • glucocorticoids, which reduce inflammation and collagen synthesis;
  • advanced age;
  • poor perfusion.

Abnormal repair:

• Wound dehiscence, from abdominal strain or infection.
• Hypertrophic scars and keloids, compared in the table above. Keloids are more common in people of Asian and African ancestry.
• Exuberant granulation ("proud flesh").
• Contractures, especially after burns over joints.