Inflammation and Repair: Mechanisms, Types, and Clinical Relevance

Inflammation and Repair

Learning Objectives

• Define inflammation and explain its protective role in host defense against infections and tissue injury.

• Identify and describe the major causes and triggers of inflammation, including infections, tissue necrosis, foreign bodies, and immune reactions.

• Explain the mechanisms and key cellular events involved in acute inflammation, including recognition of the offending agent, vascular changes, leukocyte recruitment, and mediator release.

• Compare and contrast acute and chronic inflammation, highlighting their causes, cellular participants, and outcomes.

• List the major chemical mediators of inflammation, such as cytokines and chemokines, and describe their functions in regulating the inflammatory response.

• Describe the process of tissue repair following inflammation, including regeneration, scarring, angiogenesis, and connective tissue remodeling.

• Discuss the potential harmful consequences of inflammation, including tissue damage, chronic disease development, and systemic effects like fever and leukocytosis.

Inflammation: General Properties

Definition and Protective Role

• Inflammation is the response of vascularized tissues to infections and damaged tissues, bringing cells and molecules of host defense from the circulation to the site of injury to eliminate the offending agents.

• It is a protective response (defense mechanism) essential for survival, serving to rid the host of both the initial cause of cell injury (e.g., microbes, toxins) and the consequences of such injury (e.g., necrotic cells and tissues).

• Major mediators: phagocytic leukocytes, antibodies, and complement proteins act as first responders.

Causes of Inflammation

• Infections: Bacterial, viral, fungal, and parasitic infections, as well as microbial toxins (e.g., lipopolysaccharide).

• Tissue necrosis: Regardless of the cause, necrotic tissue triggers inflammation.

• Foreign bodies: Both exogenous (dirt, sutures) and endogenous substances (e.g., urate crystals).

• Immune reactions (hypersensitivity): Normally protective, but can damage the individual's own tissues if misdirected.

• If the immune response is directed against self-antigens, it leads to autoimmune diseases.

• If the response is inappropriate against environmental substances/microbes, it results in allergies.

Recognition of Offending Agents

Mechanisms of Recognition

• The first step in all inflammatory reactions is the recognition of offending agents by cellular receptors and circulating proteins.

• Cellular receptors sense the presence of foreign bodies in different cellular compartments:

  • Plasma membrane: Recognizes extracellular microbes (e.g., Toll-like receptors (TLRs)).

  • Endosomes: Detect ingested microbes.

  • Cytosol: Detects intracellular microbes (e.g., inflammasome).

  • Fc receptors: Located on the membrane, bind antibodies.

  • Complement proteins: Present in plasma, recognize pathogens and damaged cells.

• These receptors are expressed on epithelial cells, macrophages, and dendritic cells, which are the first line of tissue exposed to foreign bodies.

Acute vs. Chronic Inflammation

Comparison of Types

• Acute inflammation is an initial, rapid response to infections and tissue damage, representing a type of innate immunity.

• It develops within minutes or hours and is of short duration (hours to a few days).

• Key characteristics: exudation of fluid and plasma proteins (edema), and emigration of leukocytes (mainly neutrophils).

• If the acute reaction fails to clear the stimulus, the reaction progresses to chronic inflammation.

• Chronic inflammation recruits macrophages and lymphocytes, induces angiogenesis, and connective tissue growth (adaptive immunity).

Acute Inflammation

Major Components

• Dilation of small vessels and an increase in blood flow.

• Increased permeability of the microvasculature to allow plasma proteins and leukocytes to leave the circulation.

• Migration of leukocytes from the microcirculation to the site of injury, and their activation to eliminate the offending agent.

Sequence of Acute Inflammation

1. Causes of inflammation

2. Recognition of the offending agent

3. Acute inflammation

4. Phagocytosis and clearance of offending agent

Changes in Vascular Flow and Permeability

• Vasodilation (mediated by histamine) is the earliest manifestation, leading to increased blood flow, heat, and redness (erythema).

• Increased permeability of the microvasculature allows plasma proteins and leukocytes to exit the circulation.

• Loss of fluid increases blood viscosity, slowing down blood cells and causing stasis (engorgement of microvessels).

• Leukocytes (mainly neutrophils) adhere to the endothelium and migrate through the vessel wall to the site of injury.

• Endothelial cell contraction (induced by histamine, bradykinin, and leukotrienes) increases interendothelial spaces, resulting in vascular leakage (exudate).

Leukocyte Recruitment and Emigration

• Following changes in vascular flow and permeability, leukocytes are recruited to the site of injury.

• Leukocytes roll along the endothelium (rolling), adhere (adhesion), and migrate (transmigration) to the injury site.

• Once at the site, leukocytes are activated to eliminate the offending agent.

Soluble Mediators of Inflammation

• Substances that initiate and regulate inflammation are called mediators.

• Mediators are often short-lived and act in a tightly regulated, interdependent manner.

• Examples include histamine, bradykinin, leukotrienes, cytokines, and chemokines.

Cytokines and Chemokines

• Cytokines are produced by activated leukocytes and endothelial cells, mediating local and systemic inflammation.

• Chemokines are small proteins (8-10 kDa) that act primarily as chemoattractants for leukocytes.

• Key cytokines: TNF-α, IL-1, IL-6 (systemic effects), IL-8 (neutrophil recruitment), IL-17 (neutrophil activation).

• Key chemokines: CCL2, CXCL8 (IL-8).

Resolution of Inflammation and Tissue Repair

• When the offending agent is eliminated, inflammatory mediators dissipate and leukocytes die by apoptosis.

• Inflammation resolves and anti-inflammatory mechanisms are activated.

• Tissue repair begins, involving regeneration of surviving cells (or stem cells) and filling of residual defects with connective tissue (scarring).

Harmful Consequences of Inflammation

• Inflammation is often accompanied by focal tissue damage, pain, and functional impairment.

• If self-limited, these effects resolve as inflammation abates.

• Mis-directed inflammation can damage tissues not involved in the initial insult (e.g., allergies, autoimmune diseases).

• Inadequately controlled inflammation can cause chronic diseases (e.g., rheumatoid arthritis, atherosclerosis) or life-threatening hypersensitivity reactions.

Chronic Inflammation

Features and Causes

• Chronic inflammation is a response of prolonged duration (weeks or months) in which inflammation, tissue injury, and attempts at repair coexist.

• Causes include persistent infections (e.g., mycobacteria), hypersensitivity diseases (autoimmune or allergies), and prolonged exposure to toxic agents (exogenous or endogenous).

• Pathological features: infiltration with mononuclear cells (macrophages, lymphocytes), tissue destruction, and attempts at healing by connective tissue replacement (scarring) and angiogenesis.

Cells of Chronic Inflammation

• Macrophages: Dominant cells, derived from monocytes, responsible for phagocytosis and cytokine production.

• Lymphocytes: T and B cells amplify and propagate chronic inflammation, especially in autoimmune diseases.

• Other cells: Eosinophils, mast cells, and sometimes neutrophils.

Systemic Effects of Chronic Inflammation

• Fever: Due to changes in hypothalamic temperature set-point, mediated by prostaglandins (PGs).

• Acute-phase proteins: Increased levels of C-reactive protein (CRP), TNF, and IL-6 in plasma.

• Leukocytosis: Elevated white blood cell count (>15,000 leukocytes/ml).

• Increased pulse and blood pressure.

Tissue Repair

Mechanisms of Repair

• Tissue repair is the restoration of tissue architecture and function after injury.

• Occurs by two main mechanisms:

  • Regeneration: Proliferation of residual (uninjured) cells and maturation of tissue stem cells.

  • Scar formation: Deposition of connective (fibrous) tissue when supporting structures are severely damaged.

Regeneration and Scarring

• Regeneration depends on the intrinsic proliferative capacity of the tissue:

  • Labile tissues: Continuously dividing (e.g., bone marrow stem cells, epithelial cells).

  • Stable tissues: Quiescent but capable of division upon injury (e.g., vascular smooth muscle cells, kidney).

  • Permanent tissues: Terminally differentiated, little to no regenerative capacity (e.g., heart, neurons).

• Angiogenesis: Formation of new blood vessels to supply nutrients and oxygen (requires VEGF).

• Granulation tissue formation: Migration and proliferation of fibroblasts and deposition of loose connective tissue and new vessels (requires TGF-beta and MMPs).

• Remodeling: Maturation and organization of connective tissue to produce a stable fibrous scar.

Summary Table: Acute vs. Chronic Inflammation

Feature	Acute Inflammation	Chronic Inflammation
Onset	Rapid (minutes to hours)	Slow (days to weeks)
Duration	Short (hours to days)	Long (weeks to months or years)
Cellular infiltrate	Mainly neutrophils	Mononuclear cells (macrophages, lymphocytes)
Tissue injury/fibrosis	Usually mild and self-limited	Often severe and progressive
Local and systemic signs	Prominent	Less prominent; may be subtle

Key Equations and Concepts

• Starling's Law of Capillary Exchange (relevant to edema formation):

$J_v = K_f [(P_c - P_i) - (_c - _i)]$

• Where $J_v$ = net fluid movement, $K_f$ = filtration coefficient, $P_c$ = capillary hydrostatic pressure, $P_i$ = interstitial hydrostatic pressure, $_c$ = capillary oncotic pressure, $_i$ = interstitial oncotic pressure.

Summary

• Inflammation is a complex, protective response aimed at eliminating harmful agents and initiating tissue repair.

• Acute inflammation is rapid and self-limited, while chronic inflammation is prolonged and associated with tissue destruction and repair.

• Tissue repair occurs via regeneration or scarring, depending on the tissue's capacity for renewal and the extent of injury.