Lecture 2: Inflammation

5 steps of Acute Inflammation

Step 1: Recognition

Cellular receptors for microbes

Toll like receptors (TLR)

Expressed on epithelial cells, macrophages, dendritic cells, lymphocytes

Sensors of cell damage

Respond to disruption of cellular homeostasis and activate production of IL-1

IL-1 activates the inflammatory cascade

DAMPS and PAMPS

Damage Associated Molecular Pattern (DAMP):

Molecules associated with cellular damage that trigger inflammatory response (heat shock proteins, mitochondrial DNA)

Often lead to production of IL-1 via inflammasomes

Pathogen Associated Molecular Pattern (PAMP):

Highly conserved pathogen molecular structure recognized by the immune system (lipopolysaccharide, some bacterial toxins)

Step 2: Recruitments of Leukocytes and Plasma Proteins

Vascular Changes

Changes in vascular flow

Increased vascular permeability

Lymphatic flow increases to drain extracellular fluid, cells and debris

Leukocyte recruitment

Leukocytes migrate through epithelium from blood into damaged region

Leukocytes follow chemokines to site of injury (chemotaxis)

Vascular Changes

Decreased flow velocity

Histamine causes vasodilation

Increased diameter reduces flow speed allowing leukocytes to slow down and stick to endothelial walls

Increased permeability (leaky capillaries)

Histamine causes endothelial contraction (gaps between cells)

Allows fluid, proteins, and cells to enter the extravascular space

Increased lymphatic flow to drain extracellular space

Leukocyte Recruitment

Flow characteristics allow leukocytes to accumulate at the endothelial cell surface moving at very low speed

Selectins allow for weak reversible attraction that produces a rolling motion

Integrins allow for a stronger bond that stops leukocytes and facilitates migration through the endothelium

Endothelial Selectin and Integrin ligand expression is promoted by cytokines (IL1, TNF)

Integrins are activated (higher affinity) by chemokines on the epithelial cell surface

Transmigration between adjacent endothelial cells is facilitated by PECAM1 (CD31) on the endothelial cells and the leukocytes

Chemotaxis

Locomotion along a chemical gradient

Injured cells and activated immune cells release chemokines

Leukocytes move toward high gradients of chemokines to the site of injury

Step 3: Removal of Stimulus

Phagocytosis and clearance of the offending agent

Neutrophils and Macrophages are the major phagocytic cells

Recognition by Phagocytic receptors

May detect microbial cell wall elements or opsonized microbes (IgG, C3b)

Engulfment of microbe

Endocytosis

Killing of infectious agent

Fusion of endosome with lysosome

May include release of granule content extracellularly (degranulation)

Neutrophils

Rapid, short lives response

Major response: degranulation

Macrophages

Slower response

Cytokine production is major functional activity

Lysosome Enzymes

Neutrophils and macrophages contain lysosomal granules with enzymes for microbial killing and tissue damage

Neutrophils

Specific (secondary) granules and Azurophil (primary) granule

Different contents but can both fuse with endosomes or release extracellularly

Neutral proteases digest host proteins like collagen, basement membrane, fibrin, elastin

Cause tissue destruction associated with acute inflammation

Neutrophil Extracellular Traps (NETS)

Fibrillar networks produced by neutrophils

Concentrate antimicrobial substances at infection sites

Trap microbes to prevent their spread

Meshwork of nuclear chromatin that binds granule proteins (antimicrobial peptide and enzymes)

Step 4: Regulation of Inflammatory Response

Anti-inflammatory mediators from macrophages and other cells: TGF beta, IL-10

Removal of stimuli via step 3

Neutrophils are self limited

Purulent (Suppurative) Inflammation

Pulmonary abscesses – contain neutrophils and cellular debris

Purulent (Suppurative) Inflammation

Purulent (Suppurative) Inflammation

Acute inflammation dominated by pus – a thick exudate containing neutrophils, necrotic debris, and edema fluid

Pyogenic bacteria stimulate intense neutrophil recruitment

Liquefactive necrosis of tissue → pus formation

Gross: Yellow-white creamy fluid, localized or diffuse

Microscopic: Dense neutrophil infiltrate, tissue necrosis, protein-rich fluid

Abscess – localized collection of pus in a cavity (may have fibrous capsule if chronic)

Cellulitis – diffuse spread of purulent inflammation in tissue

Serous inflammation

Blistered skin following thermal burn

Serous inflammation

Acute inflammation with cell-poor, protein-poor fluid (serous exudate) in a body cavity or tissue space

Mild injury to vasculature → increased vascular permeability

Plasma or serous fluid leaks into confined space

Gross: Clear or pale yellow fluid accumulation

Microscopic: Scant inflammatory cells, few proteins, mostly transudate-like fluid (but inflammatory in origin)

Skin: Burns, viral infections → blister formation

Serosal cavities: Pleura, pericardium, peritoneum (e.g., early inflammation, autoimmune disease)

Fibrinous pericarditis (inflammation)

Deposits of fibrin on the pericardium

Fibrinous pericarditis (inflammation)

A pink meshwork of fibrin exudate (F) overlies the pericardial surface (P)

Fibrinous inflammation

Acute inflammation with fibrin-rich exudate on serosal or mucosal surfaces due to severe vascular injury

Marked endothelial damage → large plasma proteins (including fibrinogen) leak into tissue

Fibrinogen polymerizes into fibrin in the extracellular space

Gross: “Bread-and-butter” appearance — shaggy strands of fibrin between inflamed surfaces

Microscopic: Eosinophilic fibrin mesh overlying tissue; may contain inflammatory cells

Pericardium – post-MI fibrinous pericarditis

Pleura – pneumonia, uremia

Peritoneum – post-surgical inflammation

Ulcer

Ulcer

Ulcer

Local defect in the epithelial surface caused by sloughing of inflamed necrotic tissue

Persistent or severe injury → necrosis of surface epithelium

Acute inflammation (neutrophils) often overlies chronic inflammation (macrophages, lymphocytes)

Morphology:

Gross: Depressed lesion with irregular borders; base may contain granulation tissue or fibrinopurulent exudate

Microscopic: Necrotic debris on surface; fibrin layer; underlying inflammatory infiltrate

GI tract – peptic ulcer disease, inflammatory bowel disease

Skin – ischemic ulcers, pressure sores

GU tract – chronic infections, malignancy

Ulceration =

1) Loss of epithelium

2) Exposure of underlying connective tissue

Histamine

Source: Mast cells, basophils, platelets

Action: Vasodilation, Increased vascular permeability, endothelial activation

Prostaglandins

Source: Mast cells, leukocytes

Action: Vasodilation, pain, fever

Leukotrienes

Source: Mast cells, leukocytes

Action: Increased vascular permeability, chemotaxis, leukocyte adhesion and activation

Cytokines (TNF, IL-1, IL-6)

Source: Macrophages, endothelial cells, mast cells

Local: endothelial activation (expression of adhesion molecules)

Systemic: fever, metabolic abnormalities, hypotension (shock)

Complement

Source: Plasma (produced in liver)

Action: Leukocyte chemotaxis and activation, direct target killing (membrane attack complex), vasodilation (mast cell stimulation)

Arachidonic Acid

Precursor to many inflammatory mediators

Metabolism pathway is the target of multiple drugs

PRINCIPAL ACTIONS OF ARACHIDONIC ACID METABOLITES

Vasodilation: Prostaglandins PGI2 (prostacyclin), PGE1, PGE2, PGD2

Vasoconstriction: Thromboxane A2, leukotrienes C4, D4, E4

Increased vascular permeability: Leukotrienes C4, D4, E4

Chemotaxis, leukocyte adhesion: Leukotrienes B4, HETE

Systemic Response to Inflammation

Cytokines enter the circulation and cause a systemic response

Fever

TNF, IL-1 act on hypothalamus to raise temp

Acute-phase proteins – markers of inflammation

Produced in liver to help with immune response

C-reactive protein, SAA (serum amyloid A) bind to microbes and opsonize

Fibrinogen increases and can cause red cell adhesion

High serum ferritin (binds free iron, depriving microbes)

Leukocytosis: Colony Stimulating Factors (CSFs) stimulate production of leukocytes from precursors in the bone marrow

Neutrophils in bacterial infection, lymphocytes in viral infection

Complement System

Group of over 20 plasma proteins (including C1-C9)

Play a crucial role in host defense against microbes and inflammation

Functions in both innate and adaptive immunity

Activation leads to:

Production of cleavage products that cause increased vascular permeability, chemotaxis, and opsonization

Direct cytotoxic effect due to formation of membrane attack complex (MAC)

Vasodilation

Histamine

Prostaglandins

Increased vascular permeability

Histamine

Chemotaxis, leukocyte recruitment, and activation

TNF, IL-1

Fever

IL-1, TNF

Prostaglandins

Pain

Prostaglandins

Substance P

Acute Inflammation

Chronic Inflammation

Onset: Slow; days

Cellular infiltrate: Monocytes/macrophages and lymphocytes

Tissue injury, fibrosis: Often severe and progressive

Less systemic and local signs

Angiogenesis (new blood vessels) and fibrosis - Healing

Acute inflammation mediators

TNF, IL-1, IL-6, Chemokines, IL-17

Chronic inflammation mediators

IL-12, IFN-y, IL-17

Chronic Inflammation Cells and Mediators

Macrophages

Lymphocytes

T Cells (CD8+ T cells – cytotoxic T cells; CD4+ T cells)

Th1 - IFN-γ - which activates macrophages (classical pathway)

Th2 - IL-4, IL-5, and IL-13:

Recruit and activate eosinophils

Alternative pathway of macrophage activation

Th17 - IL-17 - induce the secretion of chemokines

B Cells→ activated B cell → plasma cell

Source of macrophages

Activated macrophages – bone marrow (monocytes)

Resident tissue macrophages – Yolk Sac and fetal liver

Macrophage-Lymphocyte Interaction

Activated T cells produce cytokines that Recruit macrophages (TNF, IL-17, chemokines)

Activate macrophages (IFN-γ)

Activated macrophages stimulate T cells by Presenting antigens

Cytokines (IL-12)

Granulomatous Inflammation

Chronic inflammation characterized by focal aggregates of activated macrophages (epithelioid cells), often with lymphocytes and sometimes multinucleated giant cells

Cellular attempt to contain and isolate an offending agent that is difficult or impossible to eradicate

Gross: Firm, nodular lesions (may be caseating or non-caseating)

Microscopic: Central area may have necrosis (caseating) or remain viable (non-caseating); Rim of epithelioid histiocytes, multinucleated giant cells, lymphocytes

Foreign Body Granulomas – Inert material, minimal T-cell response (e.g., sutures, talc)

Immune Granulomas – Persistent T-cell–mediated immune response (e.g., TB, sarcoidosis, certain fungi).

Common Causes: Mycobacteria (e.g., Mycobacterium tuberculosis); Fungi; Foreign bodies, chronic irritants; Immune-mediated diseases (sarcoidosis, Crohn disease)

Foreign Body Granuloma

Macrophages engulfing foreign body

Commonly seen with sutures or biopsy site

Necrotizing Granuloma

Rim of epithelioid histiocytes

Central necrotic region

Multinucleated giant cells

Rim of lymphocytes and plasma cells

Non-caseating Granuloma

Well-formed granuloma = sarcoidosis

No central necrosis

Caused by T cell activation of macrophages