***Inflammatory mediators

Question 1

Where do inflammatory mediators come from?

Which cells produce them?

Answer 1

• Inflammatory mediators are produced by cells at the site of inflammation. Examples are prostaglandins, leukotrienes and cytokines
• They can also be plasma- derived, i.e, they roam the blood in an inactive state and are activated at the site of inflammation
• Macrophages, dendritic and mast cells are the major producers of mediators during acute inflammation

Question 2

What are amines?

Which cells produce amines?

Answer 2

• Amines are cell derived mediators. They are rapidly released intracellular granules
• Mast cells are the richest source of histamine, and to a lesser extent basophils and platelets. These cells are located adjacent to blood vessels in connective tissue

Question 3

How is histamine activated?

What is histamines mechanism of action?

Answer 3

• Histamine works by binding to H1 (and H2/3/4) receptors on endothelial cells. Anti-histamines antagonise H1 receptors
• It causes dilation of arterioles and increases permeability of venules (producing interendothelial gaps in postcapillary venules – oedema), and contraction of bronchial smooth muscle cells

Question 4

What causes histamine to be released?

Answer 4

Histamine is released because:
○ Physical injury – trauma (mechanosensitive), cold/heat, drugs & foreign chemicals

○ Cell surface receptors are engaged
§ Antibody binding to mast cells– underlines immediate hypersensitivity (allergic) reactions
§ Complement products (anaphylatoxin) binding to mast cells – C3a and C5a

Question 5

What is arachidonic acid?

Answer 5

• It is a polyunsaturated fatty acid found in the plasma membrane
• It is important in metabolism, especially in the synthesis of prostaglandins and leukotrienes, and is an essential constituent of the diet

Question 6

How is arachidonic acid formed?

Answer 6

• Mechanical, chemical or inflammatory stimuli activate cellular phospholipases which stimulates the release of arachidonic acid from the membrane

• Once freed, arachidonic acid is converted to eicosanoids by two major classes of enzymes:
○ Cyclooxygenases (COX) – generates prostaglandins
○ Lipoxygenases – generates leukotrienes and lipoxins

Question 7

List eicosanoids

Answer 7

Prostaglandins:
PGI2, PGE1, PGE2, PGD2

Thromboxane A2, Leukotrienes

Leukotrienes

Leukotriene B4

Prostaglandins:
PGC4, PGD4, PGE4

Question 8

What are the actions of:
Prostaglandins:
PGI2, PGE1, PGE2, PGD2

Answer 8

Vasodilation

Question 9

What are the actions of:

Thromboxane A2, Leukotrienes

Answer 9

Vasoconstriction

Question 10

What are the actions of:

Leukotrienes

Answer 10

Increased vascular permeability

Question 11

What are the actions of:

Leukotriene B4

Answer 11

Chemotaxis, leukocyte adhesion

Question 12

What are the actions of:
Prostaglandins:
PGC4, PGD4, PGE4

Answer 12

Smooth muscle contraction

Question 13

Describe prostaglandins in terms of:
○ Cells that produce them
○ The pathway that generates then
○ Their actions

Answer 13

• A group of mediators produced by mast cells, macrophages, endothelial cells and other cell types
• Mediate fever and pain
• Generated by COX-1 & COX-2

Question 14

Describe leukotrienes in terms of:
○ Cells that produce them
○ The roles of LTB4 and LTC4
○ Their actions

Answer 14

• Produced by leukocytes and mast cells via lipoxygenase

• LTB4: produced by neutrophils and macrophages and is a potent chemotactic. It also generates ROS and related lysosomal enzymes for neutrophils
• LTC4: produced mainly in mast cells and causes intense baited constriction, bronchospasm (important in asthma) and increased permeability of venules

• Involved in vascular and smooth muscle reactions and leukocyte recruitment

Question 15

Describe lipoxins in terms of:
○ The pathway that generates them
○ Their actions

Answer 15

• Also generated by the lipoxygenase pathway, however has contradictory effects – suppresses inflammation by inhibiting leukocyte recruitment

• Inhibits neutrophil chemotaxis and adhesion to the endothelium
• Regulatory mechanism

Question 16

Describe TNF and IL-1 as mediators of inflammation and their role in cell migration to site of inflammation

Answer 16

TNF:
• Critical in leukocyte recruitment; they promote adhesion to endothelium and thus migration. In addition, the prolong neutrophil life and make macrophages more biocidal
• Produced mainly by activated macrophages and dendritic cells (T cells, mast cells).

IL-1
• Activates fibroblasts for collagen synthesis and stimulates proliferation of synovial cells and other mesenchymal cells.
• IL-1 in combination with IL-6 stimulates the differentiation T cells to the proinflammatory subset TH17

Question 17

What are chemokines?

Answer 17

Family of a small proteins that act as leukocyte attractants and also activate cells such as neutrophils (i.e., IL-8).They bind to G- protein coupled receptors

Question 18

What are the four groups of chemokines?

Answer 18

Classified into 4 groups
  • C-X-C
  • C-C
  • C
  • CX3C

Question 19

What is the role of chemokines in acute inflammation?

Answer 19

• Guide leukocytes along a chemoattractant gradient to the site of infection/inflammation
• Increase affinity of integrins on leukocytes to enhance attachment to the endothelium

Question 20

What is the complement system?

Answer 20

They are a collection of soluble proteins and their membrane receptors that function in host defence against microbes

Question 21

What are the three ways that activate the complement system?

Answer 21

Complement proteins are present in inactive forms in the plasma. Cleavage products of complement proteins are required to activate them

• In complement activation, proteolysis of the complement protein C3 is required. C3 can be cleaved by one of 3 pathways
• Classical pathway – occurs when an antigen binds to an antibody. This results in the activation of the C1 protein which eventually sets off a cascade that cleaves C3
• Alternative pathway - triggered by bacterial endotoxin, LPS
• Lectin pathway - Mannose binding lectin produced by the liver, opsonises carbohydrates on microbes, directly activating C1

Question 22

What are the functions of the complement system in:

Inflammation
Opsonisation/ phagocytosis
Cell lysis

Answer 22

Inflammation
• C5a stimulates histamine release from mast cells
• Activates the lipoxygenase pathway in neutrophils and monocytes

Opsonization and phagocytosis
• C3b fixes to microbial cell wall (opsonization) and promotes phagocytosis

Cell lysis
• Membrane attack complex proteins ‘drills’ holes in the microbial membrane

Question 23

Which two inflammatory mediators induce

• Vasodilation

Answer 23

· Histamine

· Prostaglandins

Question 24

Which three inflammatory mediators induce

• Vascular permeability

Answer 24

· Histamine
· C3a and C5a
· Leukotrienes C4, D4, E4

Question 25

Which five inflammatory mediators induce

• Chemotaxis/ leukocyte recruitment and activation

Answer 25

· TNF
· IL-1
· Chemokines
· C3a, C5a
· Leukotriene B4

Question 26

Which three inflammatory mediators induce

• Fever

Answer 26

· IL-1
· TNF
· Prostaglandins

Question 27

Which two inflammatory mediators induce

• Pain

Answer 27

· Prostaglandins

· Bradykinin

Question 28

Which two inflammatory mediators induce

• Tissue damage

Answer 28

· Lysosomal enzymes of leukocytes

· Reactive oxygen species