Lecture 4 Pathology

• The lecture is about the mediators of inflammation

Learning Objectives

• Define and describe the general properties of principal inflammation mediators.
• Know the properties of vasoactive Amines such as "Histamine and Serotonin".
• Comprehend the properties of Arachidonic acid metabolites like Prostaglandins, thromboxane, Leukotrienes, and Lipoxins, including their pharmacologic inhibitors.
• Understand the role of Cytokines, tumor necrosis factor(TNF), Interleukin-1, and Chemokines.
• Be aware of the activation and function of the complement system in inflammation, opsonization, and phagocytosis
• Know other inflammation mediators like Hageman Factor, Products of coagulation, Kinins, and neuropeptides

Mediators of the Inflammatory Response

• Tissue injury can result from trauma, ischemia, neoplasms, infectious agents, and foreign particles
• Tissue injury leads to production of inflammatory mediators
• Inflammatory mediators contribute to:
• Vasodilation
• Increased vascular permeability (leads to edema)
• Recruitment and stimulation of inflammatory cells (Acute and Chronic Inflammation)
• Chemotactic factors
• Vasoactive mediators such as
• Histamine
• Serotonin
• Bradykinin
• Leukotrienes
• Prostaglandins
• Platelet-activating factor (PAF)
• Anaphylatoxins
• Nitric oxide
• Acute inflammation involves neutrophils, platelets, and mast cells
• Chronic inflammation involves macrophages, lymphocytes, and plasma cells

Review Question 1: Recognition

• Question: A 4-year-old child has high-volume diarrhea, is dehydrated, tests positive for rotavirus, and recovers with intravenous fluids. Which component on intestinal cells recognizes double-stranded RNA of this virus to signal transcription factors that upregulate interferon production for elimination? A) Caspase-1, B) Complement receptor, C) Lectin, D) T cell receptor, E) Toll-like receptor
• Answer: E) Toll-like receptor
• Toll-like receptors (TLRs) recognize nonhuman microbial substances like viral double-stranded RNA, bacterial DNA, and endotoxin, serving as an innate defense mechanism.
• TLRs are located on cells of the innate immune system (macrophages, dendritic cells) and adaptive immunity (lymphocytes)
• TLRs are activated by pathogens-associated molecular patterns (PAMPs).
• TLR activation upregulates NF-kB, a transcription factor that activates immune response genes for the synthesis of immune mediators.
• Caspase-1 is activated by an inflammasome complex responding to bacterial organisms, producing biologically active interleukin-1 (IL-1).
• Complement receptors on inflammatory cells recognize complement components, aiding in costimulatory signals for immune responses.
• Lectins on cell surfaces bind various substances, like fungal polysaccharides, triggering cellular defenses.
• T cell receptors respond to peptide antigens, initiating a cell-mediated immune response.

Review Question 2: Arachidonic Acid Metabolites

• Question: A 19-year-old woman develops a sore throat and fever with pharyngeal erythema and swelling. She is given naproxen. Which feature of the acute inflammatory response is most affected by this drug? A) Chemotaxis, B) Emigration, C) Leukocytosis, D) Phagocytosis, E) Vasodilation
• Naproxen, is a nonsteroidal anti-inflammatory drug which targets the cyclooxygenase pathway of arachidonic acid metabolism.
• Reduced Prostaglandin generation can reduce vasodilation
• Correct answer: E) Vasodilation
• Prostaglandins promote vasodilation at sites of inflammation.
• Chemotaxis is driven by chemokines, and complement C3b may promote phagocytosis; aspirin does not affect either.
• Leukocyte emigration is aided by adhesion molecules.
• Leukocyte release is driven by interleukin-1 (IL-1) and tumor necrosis factor (TNF).
• Arachidonic acid (AA) is released from the phospholipid cell membrane by phospholipase A2 and then acted upon by cyclooxygenase or 5-lipoxygenase.
• Cyclooxygenase produces prostaglandins (PG):
  • PGI2 , PGD2 , and PGE2 mediate vasodilation and increase vascular permeability
  • PGE2 also mediates pain and fever.
• 5-lipoxygenase produces leukotrienes (LT):
  • LTB4 attracts activated neutrophils.
  • LTC4 , LTD4 , and LTE4 mediate vasoconstriction, bronchospasm, and increase vascular permeability.

Arachidonic Acid Metabolites

• Steroids inhibit Phospholipases from activating Cell membrane phospholipids
• Arachidonic acid is converted by Cyclooxygenase into Prostaglandin G2 (PGG2) and Prostaglandin H2 (PGH2).
• Prostacyclin (PGI2) Causes vasodilation and inhibits platelet aggregation.
• Thromboxane A2 (TXA2) Causes vasoconstriction and promotes platelet aggregation.
• Prostaglandin D2 (PGD2) and Prostaglandin E2 (PGE2) cause vasodilation, increased vascular permeability, and leukocyte chemotaxis

Review Question 3: Mediators of Inflammation - Mast Cells

• Question: A woman allergic to cats develops nasal congestion with abundant nasal secretion after inhaling cat dander. Which substance is most likely responsible? A) Bradykinin, B) Complement C5a, C) Histamine, D) Interleukin-1 (IL-1), E) Phospholipase C, F) Tumor necrosis factor (TNF)
• Answer: C) Histamine
• Histamine is abundant in mast cells, located near blood vessels beneath mucosal surfaces in airways.
• Antigen (allergen) binding to IgE antibodies (attached to mast cells via the Fc receptor) triggers mast cell degranulation, releasing histamine, increasing vascular permeability, and mucous secretions.
• Bradykinin, generated on surface contact of Hageman factor with collagen and basement membrane due to vascular injury, promotes vascular permeability, smooth muscle contraction, and pain.
• Complement C5a is a potent chemotactic factor for neutrophils
• Interleukin-1 (IL-1) and tumor necrosis factor (TNF), produced by activated macrophages, mediate systemic effects, including fever, metabolic wasting, and hypotension.
• Phospholipase C, catalyzing arachidonic acid release, is generated from platelet activation.
• Histamine and serotonin are major vasoactive amines with actions on blood vessels.

Mediators of Inflammation: Mast Cells

• Mast cells degranulation releases histamine
• Mast cells are widely distributed throughout connective tissue.
• Histamine is stored in mast cell granules and released in response to:
  • Tissue trauma
  • Complement proteins C3a and C5a
  • Binding of antigen to IgE antibodies on mast cells (immediate hypersensitivity reactions)
  • Neuropeptides (e.g., substance P)
  • The response causes immediate and delayed effects

Histamine Function:

• Immediate response:
  • Mediate vasodilation of arterioles.
  • Increases vascular permeability (interendothelial gaps in venules)
  • Antihistamines treat some inflammatory reactions (allergies)
  • Antihistamines are histamine receptor antagonists that bind and block the receptor.
• Delayed response:
  • Production of Arachidonic acid metabolites like leukotrienes

Serotonin

• Serotonin (5-hydroxytryptamine) is a preformed vasoactive mediator present in platelets and certain neuroendocrine (GI) cells.
• Its primary function is as a neurotransmitter in the GI and the CNS.
• Serotonin is a vasoconstrictor; its importance is unclear.

Review Question 4: Mediators of Inflammation - Complement

Question # 7: In an experiment, bacteria are inoculated into aliquots of normal human blood that have been treated with an anticoagulant. The bacteria are either phagocytized by neutrophils or undergo lysis. Which blood plasma component is most likely to facilitate these effects? A) Complement, B) Fibrin, C) Kallikrein, D) Plasmin, E) Thrombin

• Answer: A) Complement
• Complement activation may occur via:
  • Microbial cell wall components such as polysaccharides (alternative pathway)
  • Mannose (lectin pathway)
  • Antibody attached to surface antigens (classic pathway)

Activation in the complement pathways generates:

• Complement C5a is a neutrophil chemoattractant.
• Complement C3b is an opsonin
• Complements C5-9 is the membrane attack complex.
• The remaining options are closely associated with coagulation.
  • Fibrin is generated, without anticoagulation.
  • Kallikrein may aid in bradykinin and plasmin generation, and is involved in complement C5a generation
  • Plasmin is generated from plasminogen and helps lyse clots
  • Thrombin is generated by the coagulation cascade

Mediators of Inflammation: Complement

• Complement consists of Pro-inflammatory serum proteins that "complement" inflammation, circulating as inactive precursors
• Activation of complement can occur via:
  • Classical pathway: triggered by antigen-bound IgG or IgM.
  • Alternative pathway: directly activated by microbial products.
  • Mannose-binding lectin (MBL) pathway: MBL binding to mannose on microorganisms
• All pathways produce. C3 convertase
• Mediates C3->C3a and C3b, which produces C5 convertase
• Mediates C5->C5a and C5b
• C5b complexes with C6 and C9 to form the membrane attack complex (MAC)
• Anaphylatoxins (C3a, C4a, C5a) trigger mast cell degranulation
• C5a is chemotactic for neutrophils.
• Vascular permeability increased through histamine mediated vasodilation
• C3b is an opsonin for phagocytosis.
• MAC lyses microbes by creating a hole in the cell membrane.

Biologic activity of the anaphylatoxins

• Anaphylatoxins promote:
  • Neutrophil chemotaxis and stimulate immune responses.
  • Suppress Immune responses
  • Mast cell Stimulation, smooth muscle, and vascular constriction
• Tissue Activation leads to COX,LOX products Release
• Release histamine to synthesize leukotrienes and increase permeability
• Edema results

Mediators of Inflammation: Hageman factor "Factor XII

• Hageman factor is an inactive proinflammatory protein produced in the liver
• It is activated upon exposure to subendothelial or tissue collagen, which in turn then activates:
• Coagulation and fibrinolytic systems
• Complement
• Kinin system: Kinin cleaves high-molecular-weight kininogen (HMWK) to bradykinin, which mediates vasodilation and increases vascular permeability, and pain

Hageman factor activation and inflammatory mediator production

• Agents Associated with Injury:
  • Negatively Charged Surfaces: Basement membrane, collagen, elastin, glycosaminoglycans
  • Bacterial Lipopolysaccharide, Sodium Urate Crystals, Enzymes like Trypsin, Plasmin stimulate the activation of Hageman Factor (XII)
• Hageman Factors activates:
  • Fibrinolysis to generate Fibrin Degradation Products
  • Complement Activation to trigger Anaphylatoxin Chemataxis
  • Activates Kallikrein Kinin, which leads to Kinin Generation
  • The Coagulation System, resulting in Clot Formation

Cytokines as Cell-Derived Protein Mediators

• Cytokines are low, molecular-weight proteins which are secreted by activated cells
• Cytokines include:
  • Interleukins
  • Growth factors
  • Colony-stimulating Factors
  • Interferons
  • Chemokines
• Cytokines act in three ways
  • Autocrine: Cytokine acts of secreting cell
  • Paracrine: Cytokine acts on neighboring cells
  • Endocrine: Cytokine acts on cells in distant tissues

Class of Cytokine-Species-Related Actions

• Inflammatory cell activation occurs from Interleukins such as IL-1, IL-6, IL-8, IL-10, and IL-13
• Growth factors such as GM-CSF and M-CSF: Stimulate macrophage population, providing Bactericidal activity while supporting NK and dendritic cell function
• Chemokines (CC, CXC, XC, CX3C): Leukocyte Chemotaxis and Activation
• Interferons (IFNα, IFNβ, IFNγ): Antiviral and Leukocyte Activation.
• Proinflammatory Cytokines (TNFα): Cause fever, Anorexia and Shock, and inducing Cytotoxicity and Cytokine induction. It can cause activation of endothelial and tissue cells

Central Role of Interleukin Il-1 and TNF-a in Inflammation

• Gram-negative bacteria, T-cells, LPS, and IFNy cause TNFα, IL-1 to be created to produce responses in
  • Endothelial cells using Adhesion molecules, Cytokines, Eiconosoids
  • Neutrophils causes Aggregation and Priming
  • Acute phase response causes Fever Anorexia, Hypotension, Increased heart rate and Corticosteroid and ACTH Release