General Veterinary Pathology

Chemical mediators of inflammation

CHEMICAL MEDIATRS OF INFLAMMATION

I.Cellular

• Preformed mediator in secretary granules

Mediators	Source
Histamine	Mast cell, basophils, platelets
Serotonin	Platelets
Lysosomal enzymes	Neutrophils

• Newly synthesised
  • Mediators Source
    • Prostaglandin All leukocytes, platelets, Endothelial cell
    • Leukotrienes All leucocytes
    • Platelet activating factor All leucocytes, endothelial cell
    • Activated oxygen species All leucocytes
    • Nitric oxide Macrophages
    • Cytokines Lymphocytes, macrophages

II. Liver – plasma

• Factor XII (Hageman factor) activation – kinin system (Bradykinin), Coagulation system
• Complement activation – C3a, C5a – Anaphylatoxins, C3b – phagocytosis of bacteria, C5b-9 – Membrane attack complex

Biologic activity

• Specific receptors on target cells
• Direct-enzymes
• Mediate oxidative damage

Chemical mediators

• Stimulate release and mediation of target cells themselves. The secondary mediators have similar or opposite effect.
• Chemical action – one or many target cells with different effects.
• Chemical mediators are short lived and scavenge oxygen species.
• Histamine and serotonin cause tissue damage.

Preformed mediators in secretary granules

Histamine

• Histamine is found in the granules of mast cells, basophils and platelets. It increases the vascular permeability of venules and dilates arterioles and induces endothelial junctional gap early response to inflammation.

Serotonin

• Serotonin (5-hydroxy tryptamine) - It is present in mast cell and platelets of rodents. It increases vascular permeability and involved in early inflammatory response.

Lysosomal components

• Lysosomal components leak during phagocytosis or regurgitation. Small granules contain lysosomes, collagen, alkaline phosphatase, histaminases and plasminogen activator. Large granules (azurophils) myeloperoxidases, bactericidal factor, acid hydrolases and neutral proteases are responsible for vascular permeability, chemotaxis and tissue damage.

Lysozyme

• Lysozyme (neuraminidase) is found in granulocytes, monocytes, macrophages and produced by epithelial cells of mucosa and glands of intestinal tract and secreted in milk, tear and saliva. Lysozyme catalyses the hydrolysis of peptidoglycans in bacterial cell wall.

Arachidonic acid metabolites

• The cell membrane phospholipids of neutrophils are acted upon by phospholipases. When arachidonic acid enters 5-lipoxygenase pathway, leukotrienes are produced e.g. LTc4, LTD4, LTE4. When acted through platelets lipoxins which are potent chemoattractants are produced.
• If acted by cyclooxgenase pathway, prostaglandins are produced.

Newly synthesised mediators

Eicosanoids

• These are derived from arachidonic acid from injures cell membrane phospholipids. These are prostaglandins and leukotrienes. Prostaglandin derivatives PGI2, PGE2, PGD2 cause vasodilatation while thromboxaneA2, LTC4, LTD4, LTE4 cause vasoconstriction. LTC4, LTD4, LTE4 are also responsible for increased vascular permeability. LTD4 and HETE can induce chemotaxis and leucocytic adhesion

ARACHIDONIC ACID METABOLITES

(Prostaglandins, Leukotrienes, Lipoxins)

Note: Leukotrienes are 1000 times more potent than histamine

Cytokines

• These are derived from activated macrophages and lymphocytes and are proteins e.g. TNFα, IL-1 are produced by macrophages.
• γ-Interferon also induce acute phase response whereas interleukin 10 is a potent anti inflammatory cytokine.

Platelet activating factor (PAF)

• It is derived from degeneration of membrane phospholipids (platelets, neutrophils, endothelium). PAF causes increased vascular permeability and 100 to 10,000 times more potent than histamine higher concentration of platelet activating factor can stimulate platelets, enhance leucocyte adhesion to endothelium and stimulate vasoconstriction.
• IL-2 and TNF can produce endothelial activation adhesion of leucocytes production of arachidonic acid metabolite and nitric oxide.

Chemokines

• Chemokines are responsible for activation and migration of leucocytes in acute inflammation. alpha chemokines (C-H-C)- attract neutrophils, beta chemokines (C-C) attract monocytes, lymphocytes, eosinophils and basophils, gamma chemokines (C) attracts lymphocytes and CX3C causes attraction and adhesion of monocytes and T-cells.
• Acute phase proteins - They are not normally present in plasma but markedly increase after injury. Hence of diagnostic value in inflammation. These are synthesized in liver in response to cytokines released by inflammation leucocytes (IL-1, TNF alpha)

Exmples function of acute phase protein

• Fibrinogen, fibrin Coagulation forming coagulant polymers
• C3 Backbone of complement cascade responsible for destruction of bacteria
• C-reactive protein Initiating complement dependent opsonisation
• Haptoglobulin Antioxidant by binding haemoglobin and saving iron

Interferons

• Interferons are produced by host cells in response to stimulation by virus, intracellular bacteria, foreign material and soluble protein. It is considered as first host defence against viral infection. Fibronectin, an alpha 2 glycoprotein on fibroblast surface and basement membrane. when in plasma helps in opsonisation of bacteria and promotes phagocytosis.

Oxygen derived free radicals

• Oxygen derived free radicals (superoxide, hydrogen peroxide and hydroxide radicals) derived from membrane damage like neutrophils can cause tissue injury and endothelial damage.

Nitric oxide

• Nitric oxide (NO) - Microbicidal agent in activated macrophages causes vascular dilatation. It is soluble and short lived free radical gas.
• Activation of Hageman factor results in cascade of reactions. The fragment which enhances inflammatory process and this stimulates complement system, kinin system, clotting system and fibrinolytic system.

PLASMA PROTEIN SYSTEM

Plasma proteases

(Kinin, Clothing, Fibrinolytic systems)

Kinin, clotting and fibrinolytic system

• The Hageman factor (factor XII) is activated on contact with collagen, basement membrane and platelets to produce prekallikrein. The prekallikrein is converted to kallikrein which will be converted to bradykinin that induces vascular permeability, pain and smooth muscle contraction.
• Kallikrein also mediates plasminogen, vascular permeability and vascular dilatation
• Activated Hageman factor is also involved in conversion of prothrombin to thrombin which in turn aids in conversion of fibrinogrn to fibrin. The fibrinolytic peptides and split products of fibrin can induces vascular permeability and chemotaxis.

Complement system

• Complement represented as C consists of 20 proteins in an inactive form in plasma and body fluids. Complement is mainly synthesised by liver. Complement system may be activated in one of the two ways, classic or alternate pathway. But both the pathways converge to produce a membrane attack complex (MAC) which is responsible for lysis of bacterial cell membrane and also results in mediating inflammation. e.g. chemotaxis, histamine release from mast cells (C3a) and procoagulant from platelets. C3b is a major opsonin protein which adhere to bacteria (opsonisation). It is recognised, phagocytosed and destroyed by neutrophils and monocytes.

Kinin system

• The vasoactive polypeptide (kinins) are derived from kininogen (plasma globulins). The kinins are potent mediator of vasodilatation, pain, increased capillary permeability. The bradykinin induces vascular leakage from post capillary venules. It is 10 times more active than histamine but short lived.

Clotting system

• Coagulation is seen following damage of endothelium in inflammation through fibrinolytic system the initiated by activated Hageman factor.

Actions of mediators in acute inflammation

Action	Mediators
Vasodilation	Histamine Prostaglandins Nitric oxide
Vascular permeability	Vasoactive amines C3a and C5a Bradykinin Leukotrienes C4, d4, E4 Platelet activating factor Substance P
Leukocyte chemotaxis and activation	C5a Leukotriene B4 Chemokines TNF and IL-1
Fever	TNF and IL-1 prostaglandins
Pain	Prostaglandins Bradykinin