Functions Of Chemical Mediators Of Inflammation Notes

Chemical Mediators Of Inflammation

These are a large and increasing number of endogenous chemical substances which mediate the process of acute and chronic inflammation.

Mediators of inflammation have some common properties as under:

• They are released from the cells or derived from plasma proteins:
  • Cell-derived mediators are released either from their storage in the cell granules or are synthesised in the cells.

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• • Most common site of synthesis of plasma-derived mediators is the liver. After their release from the liver, these mediators require activation.
• All mediators are released in response to certain stimuli:
  • These stimuli may be a variety of injurious agents, dead and damaged tissues, or a mediator itself.
  • When one mediator stimulates the release of another, it is called a secondary mediator, which may perform the function of the initial mediator or may have opposing action.
• Mediators act on different targets:
  • They may have a similar action on different target cells or differ in their action on different target cells. They may act on cells which formed them or on other body cells.
  • In general, the spectrum of actions of different mediators are increased vascular permeability, vasodilatation, chemotaxis, fever, pain and tissue damage.
• Mediators have a short lifespan after their release:
  • After release, they are rapidly removed from the body by various mechanisms for example, By enzymatic inactivation, antioxidants, regulatory proteins or may even decay spontaneously.

Two main groups of substances acting as chemical mediators of inflammation—released from the cells and those from the plasma proteins, and members in each group are listed in given in the table and are discussed below. Their range of actions in acute inflammation is schematically illustrated in

Cell-Derived Mediators:

This group includes chemical substances which are either preformed or are newly synthesised in different cells of the body; on stimulation of the cell they are released and act as potent mediators of inflammation.

1. Vasoactive Amines:

Two important pharmacologically active amines that have role in the early inflammatory response (first one hour) are histamine and 5-hydroxytryptamine (5-HT) or serotonin; another addition to this group is neuropeptides.

Mediators of inflammation:

1. Cell-Derived Mediators:
   • Vasoactive amines (Histamine, 5-hydroxytryptamine, neuropeptides)
   • Arachidonic acid metabolites (Eicosanoids)
     • Metabolites via cyclo-oxygenase pathway (prostaglandins, thromboxane A2, prostacyclin, resolvins)
     • Metabolites via lipo-oxygenase pathway (5-HETE, leukotrienes, lipoxins)
   • Cytokines: interleukins (IL-1, IL-6, IL-8, IL-12, IIL-17) TNF -α, TNF-β; IFN-γ; other chemokines)
   • Platelet-activating factor
   • Free radicals (Oxygen intermediates, nitric oxide)
2. Plasma Protein-Derived Mediators (Plasma Proteases) Products of:
   • The kinin system (kallikrein, bradykinin)
   • The clotting system (fibrin, fibrinopeptide’s)
   • The fibrinolytic system (plasmin)
   • The complement system (C3a, C3b, C5a, MAC)

(IL, interleukin; TNF, tumour necrosis factor; IFN, interferon; 5-HETE, 5-hydroxyeicosatetranoic acid; MAC, membrane attack complex).

•  Histamine: It is stored as preformed granules in the mast cells, basophils and platelets.
  • Histamine is released from these cells in response to various agents:
  • Stimuli or substances inducing acute inflammation for example, Heat, cold, irradiation, trauma, irritant chemicals, type I hypersensitivity reaction etc
  • Anaphylatoxins such as activated products of complement, C3a, and C5a
  • Histamine-releasing chemical factors from neutrophils, monocytes and platelets
  • The main actions of histamine are:
  • Vasodilatation of arterioles
  • Increased venular permeability
  • Itching, and
  • Pain.

Stimulation of mast cells and basophils also releases products of arachidonic acid metabolism including the release of slow-reacting substances of anaphylaxis (SRS-As). The SRSAs consist of various leukotrienes (LTC4, LTD4 and LTE4).

• 5-Hydroxytryptamine (5-HT or serotonin): 
  • It is present as a preformed mediator in platelets and in enterochromaffin cells of GIT.
  • The actions of 5-HT are similar to histamine but it is a less potent mediator of increased vascular permeability and vasodilatation than histamine.
  • It may be mentioned here that a carcinoid tumour is a serotonin-secreting tumour.
• Neuropeptides:
  • Another class of vasoactive amines is tachykinin neuropeptides such as substance P, neurokinin A, vasoactive intestinal polypeptide (VIP) and somatostatin.
  • These small peptides are produced in the central and peripheral nervous systems.

The major proinflammatory actions of these neuropeptides are:

• Increased vascular permeability
• Transmission of pain stimuli, and
• Mast cell degranulation.

2. Arachidonic Acid Metabolites (Eicosanoids):

Arachidonic acid metabolites or eicosanoids are the most potent mediators of inflammation, much more than reactive oxygen species.

Arachidonic acid is a fatty acid, eicosatetraenoic acid; the Greek word ‘eikosa’ means ‘twenty’ because of 20 carbon atom composition of this fatty acid.

Arachidonic acid is a constituent of the phospholipid cell membrane, besides its presence in some constituents of the diet. Arachidonic acid is released from the cell membrane by phospholipases.

It is then activated to form arachidonic acid metabolites or eicosanoids by one of the following 2 pathways: via cyclo-oxygenase pathway or via lipo-oxygenase pathway:

Metabolites Via Cyclo-Oxygenase Pathway: Prostaglandins, Thromboxane A2, Prostacyclin, Resolvin:

The name ‘prostaglandin’ was first given to a substance found in human seminal fluid but now the same substance has been isolated from a number of other body cells for example, All leucocytes, endothelial cells, platelets etc.

Prostaglandins and related compounds are also called autocoids because these substances are mainly autocrine or paracrine agents.

The terminology used for prostaglandins is abbreviation as PG followed by a suffix of an alphabet and a serial number for example, PGD₂, PGE₂, and PGF₂. PGG₂ etc.

Cyclo-oxygenase (COX), a fatty acid enzyme present as COX-1 and COX-2 acts on activated arachidonic acid to form prostaglandin endoperoxide (PGG₂). PGG₂ is enzymatically transformed into PGH₂  with generation of free radicals of oxygen.

PGH₂ is further acted upon by enzymes and results in formation of the following 3 metabolites:

• Prostaglandins (PGD₂, PGE₂ and PGF₂α): PGD₂ and PGE₂ act on blood vessels and cause increased venular permeability, vasodilatation and bronchodilatation and inhibit inflammatory cell function. PGF₂α is a potent stimulant of uterine and bronchial smooth muscle contraction, and vasoconstriction.
• Prostacyclin (PGI₂) and PGF₁α: Endothelial cells contain enzyme prostacyclin synthase that forms PGI2 which induces vasodilatation, bronchodilatation and inhibits platelet aggregation. PGI₂  is converted to its stable product, PGF₁α
• Thromboxane A₂ and B₂ (TXA₂, TXB₂): Platelets contain the enzyme thromboxane synthase and hence the metabolite, thromboxane A₂
• , is formed which is active in platelet aggregation, besides its role as a vasoconstrictor and bronchoconstrictor. However, TXA₂ is unstable and is rapidly converted to its inactive form TXB₂.
• Resolvins: Are another derivative of COX pathway which act by inhibiting the production of proinflammatory cytokines. Thus, resolvins are actually helpful—drugs such as aspirin act by inhibiting COX activity and stimulate the production of resolvins.

It may be mentioned here that some of the major anti-inflammatory drugs act by inhibiting the activity of the enzyme COX; for example, Non-steroidal anti-inflammatory drugs (NSAIDs), COX-2 inhibitors.

Metabolites Via Lipo-Oxygenase Pathway: 5-Hete, Leukotrienes, Lipoxins:

The enzyme, lipo-oxygenase, a predominant enzyme in neutrophils, acts on activated arachidonic acid to form hydroperoxy eicosatetraenoic acid (5-HPETE) which on further peroxidation forms following 2 metabolites:

1. 5-HETE (hydroxy compound): An intermediate product, which is a potent chemoattractant for neutrophils.
2. Leukotrienes (LT):  So named because they were first isolated from leucocytes. On activation, initially unstable leukotriene A4 (LTA4) is formed which is acted upon by enzymes to form other LTs:
   • LTB 4 chemotactic for phagocytic cells and stimulates phagocytic cell adherence), and LTC 4, LTD4 and
   • LTE4, all of which cause smooth muscle contraction, which in turn, induces vasoconstriction, bronchoconstriction and increased vascular permeability.Hence, these LTs they are also called as slow-reacting substances of anaphylaxis (SRS-As).
3. Lipoxins (LX): A ct to regulate and counterbalance actions of leukotrienes i.e. they cause vasodilatation. Lipooxygenase-12 present in platelets acts on LTA4 derived from neutrophils and forms LXA 4 and LXB4.

3. Cytokines And Chemokines:

Cytokines are polypeptide substances produced by activated lymphocytes (lymphokines) and activated monocytes (monokines).

The term chemokine is used for a family of substances which act as chemoattractants for specific types of inflammatory cells.

All these agents may act on ‘self’ cells which produced them or on other cells. Although over 200 cytokines have been described, major cytokines and their role in inflammation are as under

Interleukins (IL-1, IL-6, IL-8, IL-12, IL-17):

While IL-1 and IL-6 are active in mediating acute inflammation, IL-12 and IL-17 play a potent role in chronic inflammation. IL-8 is a chemokine for acute inflammatory cells:

Major Cytokines In Inflammation:

 

IL-1:   IL-1 is elaborated by several body cells-monocytes and macrophages, B lymphocytes, fibroblasts, endothelial and some epithelial cells.

• Similarly, it can target all body cells. Its major actions are:
  • Expression of adhesion molecules.
  • Emigration of neutrophils and macrophages.
  • Role in fever and shock and.
  • Hepatic production of acute phase protein.

IL-6: IL-6 is similar in its sources and target cells of action.

• Its major role are:
  • Hepatic production of acute phase protein  and
  • Differentiation and growth of T and B cells.

IL-8: IL-8 is also elaborated by the same cells as for IL-2 and IL-6 except that it is secreted by T cells instead of B lymphocytes.  Its target cells are neutrophils, basophils, T cells, monocytes/macrophages, endothelial cells.

• IL-8 is hemokine and its major actions are:
  • Induces migration of neutrophils
  • Macrophages and T cells
  • Stimulates release of histamine from basophils and
  • Stimulates angiogenesis.

IL-12:  IL-12 is synthesised by macrophages, dendritic cells and neutrophils while it targets T cells and NK cells.

• Its major actions in chronic inflammation are as under:
  • Induces the formation of T helper cells and killer cells
  • Promotes CTL cytolytic activity; increases production of IFN-γ and
  • Decreases production of IL-17.

IL-17:  IL-17 is formed by CD4+T cells while it targets fibroblasts, endothelial cells and epithelial cells.

• Its action in chronic inflammation are:
  • Increased secretion of other cytokines and
  • Migration of neutrophils and monocytes.

Tumour Necrosis Factor (TNF-α AND β):

TNF-α is a mediator of acute inflammation while TNF-β is involved in cellular cytotoxicity and in development of spleen and lymph nodes.

TNF-α is formed by various cells (Monocytes/macrophages, mast cells/basophils, eosinophils, B cells, T cells, NK cells) while TNF-β is formed by B and T lymphocytes only.

Both can target all body cells except erythrocytes. Major actions of TNF-α are:

• Hepatic production of acute phase proteins
• Systemic features (fever, shock, anorexia)
• Expression of endothelial adhesion molecules;
• Enhanced leucocyte cytotoxicity  and
• Induction of pro-inflammatory cytokines.

Interferon (IFN)-γ:

It is produced by T cells and NK cells and may act on all body cells.

• It acts as mediator of acute inflammation as under:
  • Activation of macrophages and NK cells
  • Stimulates secretion of immunoglobulins by B cells and
  • Role in differentiation of T helper cells.

Chemokines:

Chemokines are a form of cytokines which act as chemoattractants for the specific type of inflammatory cells.

Structurally, chemokines are small proteins (molecular weight 8-10 kD) containing cysteine (C) residues. Based on the structural organisation of C, chemokines and

Their receptors are divided into following 4 groups, each having distinct examples:

1. C-X-C chemokine (α-chemokine):

Contains two conserved C residues separated by one amino acid residue (X). A typical example is IL-8 (discussed above).

2. C-C chemokine (β-chemokine):

Contains two conserved C residues adjacent to each other. Examples are as under:

• MCP-1 (monocyte chemoattractant proteins) is elaborated by fibroblasts, smooth muscle cells, and peripheral blood mononuclear cells. Its actions are:
  • Chemoattractant for monocytes, T cells and NK cells.
  • Stimulates release of histamine from basophils
• MIP-1α (macrophage inflammatory protein) is sourced from monocyte-macrophages. Its actions are:
  • Chemoattractant for monocytes, dendritic cells, T cells, NK cells.
• Eotaxin is formed by the alveolar cells of the lung and in the heart. Its actions are:
  • Chemoattractant for eosinophils and basophils.
  • Induces allergic pulmonary disease
• Rates (regulated and normal T cell expressed and secreted) It is produced by monocytes macrophages, T cells, fibroblasts and eosinophils. Its main actions are:
  • Chemoattractant for monocytes-macrophages, T cells and NK cells

3. C-chemokine (γ-chemkine):

Lacks two conserved C residues out of four. An example of this group is:

• Lymphotactin is produced by NK cells, T cells, and mast cells. Its actions are:
  • Chemoattractant is specific for lymphocytes.

4. CX3-C chemokine:

Contains two C residues separated by three amino acid residues. Example is:

• Fractalkine is activated and released from endothelial cells. Its actions are:
  • As cell-surface chemoattractant for monocytes and T cells
  • As cell adhesion molecule for monocytes and T lymphocytes

4. Platelet Activating Factor (PAF):

It is released from IgE-sensitised basophils or mast cells, other leucocytes, endothelium and platelets. Apart from its action on platelet aggregation and release reaction, the actions of PAF as mediator of inflammation are:

• Increased vascular permeability
• Vasodilatation in low concentration and vasoconstriction otherwise;
• Bronchoconstriction
• Adhesion of leucocytes to endothelium; and
• Cemotaxis.

5. Free Radicals: Oxygen Intermediates And Nitric Oxide:

Free radicals act as potent mediators of inflammation:

• Oxygen-derived intermediates (or reactive oxygen species): Are released from activated neutrophils and macrophages and include superoxide oxygen (O^–₂), H₂O₂, and OH^–.
• These oxygen-derived free radicals have the following actions in inflammation:
  • Endothelial cell damage and thereby increased vascular permeability.
  • Activation of protease and inactivation of antiprotease causing tissue matrix damage.
  • Damage to other cells.

The actions of free radicals are counteracted by antioxidants present in tissues and serum which play a protective role.

• Nitric oxide (NO): Was originally described as vascular relaxation factor produced by endothelial cells. Now it is known that NO is formed by activated macrophages during the oxidation of arginine by the action of enzyme, NO synthase.
• NO plays the following roles in mediating inflammation:
  • Vasodilatation
  • Anti-platelet activating agent
  • Possibly microbicidal action.

Plasma-Derived Mediators (Plasma Proteases):

These include various products derived from activation and interaction of 4 interlinked systems: kinin, clotting, fibrinolytic and complement. Each of these systems has its inhibitors and accelerators in plasma with negative and positive feedback mechanisms respectively.

Hageman factor (factor XII) of the clotting system plays a key role in interactions of the four systems.

Activation of factor XII in vivo by contact with basement membrane and bacterial endotoxins, and in vitro with glass or kaolin, leads to activation of clotting, fibrinolytic and kinin systems.

In inflammation, activation of factor XII is brought about by contact of the factor leaking through the endothelial gaps. The end-products of the activated clotting, fibrinolytic and kinin systems activate the complement system that generates permeability factors.

These permeability factors, in turn, further activate the clotting system:

Thus, these four pathways are interlinked as illustrated in given in below

1. The Kinin System:

This system on activation by factor Xlla generates bradykinin, so named because of the slow contraction of smooth muscle induced by it.

First, kallikrein is formed from plasma prekallikrein by the action of the prekallikrein activator which is a fragment of factor Xlla.

Kallikrein then acts on high molecular weight kininogen to form bradykinin.

Bradykinin acts in the early stage of inflammation and its effects include:

• Smooth muscle contraction
• Vasodilatation
• Increased vascular permeability; and
• Pain.

2. The Clotting System;

Factor Xlla initiates the cascade of the clotting system resulting in the formation of fibrinogen which is acted upon by thrombin to form fibrin and fibrinopeptides.

The actions of fibrinopeptide’s in inflammation are:

• Increased vascular permeability
• Chemotaxis for leucocyte; and
• Anticoagulant activity

3. The Fibrinolytic System:

This system is activated by a plasminogen activator, the sources of which include kallikrein of the kinin system, endothelial cells and leucocytes. Plasminogen activator acts on plasminogen present as a component of plasma proteins to form plasmin.

Further breakdown of fibrin by The actions of plasmin in inflammation is as follows:

• Activation of factor XII to form a prekallikrein activator that stimulates the kinin system to generate bradykinin
• Splits off complement C3 to form C3a which is a permeability factor; and
• Degrades fibrin to form fibrin split products which increase vascular permeability and are chemotactic to leucocytes.

4. The Complement System:

Complement system is a group of approximately 30 widely distributed proteins that includes normal serum plasma proteins, cell surface proteins and receptors, which function to add or augment the host defenses against bacterial infection and play a role in pathologic inflammatory reactions.

In effect, complement system acts as a ‘complement’ to antibody-mediated bacterial killing. The system is an important part of innate and adaptive immune responses.

Activation Pathways:

On activation, the complement system yields important cleavage products; most important is enzyme C3 convertase, which splits C3 into C3a and C3b both of which play significant role in inflammation. Activation of the complement system can occur by one of the following three pathways

• Classical pathway: It is triggered by antigen-IgG or IgM class antibody complex that causes activation of C1 to form C3 convertase, which in turn, breaks down C3 to produce active cleavage products C3a and C3b.
• Alternative pathway:  It can be triggered via non-immunologic agents such as bacterial toxins or lipopolysaccharides, and cobra venoms. In this, C3 is hydrolysed directly to C3b, and then by alternate pathway converted to C3 convertase and C5 convertase, which in turn complexes with other components of complement C6, 7, 8, 9. Thus, the end-product formed by this pathway is C5b, 6, 7, 8, 9 or membrane attack complex (MAC).
• Mannose-biding lectin (MBL) pathway: In this, MBL binds to mannose residues on the microbial surface and activates MBL-associated serine proteases. These, in turn, split C4 into C4a and C4b, and C2 into C2a and C2b. C4b and C2a together form C3 convertase as in the classic pathway.

Control And Regulation:

Complement activation is controlled and regulated by body proteins present in circulation as well as on the cells so that activation products of complement do not damage normal host tissues.

Important regulatory proteins are as follows:

• C1 inhibitor It blocks the activation of C1 in the classic pathway.
• Decay accelerating factor (DAF) It is linked to plasma membrane glycophosphatadyl and prevents the formation of C3 convertase.
• CD59 It prevents the formation of MAC (C5b, 6, 7, 8, 9).

Actions: Activation products of the complement system by the above pathways mediate the following actions in inflammation:

• C3a, C4a, and C5a (anaphylatoxins) activate mast cells and basophils to release of histamine, cause increased vascular permeability causing oedema in tissues, and augments phagocytosis.
• C3b acts as an opsonin in phagocytosis. C5a is chemotactic for leucocytes.
• MAC (C5b, 6, 7, 8, 9) is a lipid-dissolving agent that causes holes in the phospholipid membrane of the cell and causes cell lysis.

Chemical Mediators of Inflammation:

These are endogenous chemical substances which mediate the process of inflammation. They have some common properties:

They are released either from the cells or are derived from plasma proteins:

• They are released in response to certain stimuli
• They act on different targets
• They have a short lifespan after their release
• They have several actions, the most important being increased vascular permeability.

Cell-derived mediators of inflammation are vasoactive amines (histamine, 5HT, neuropeptides), arachidonic acid metabolites (prostaglandins, 5-HETE, leukotrienes, lipoxins), platelet-activating factor, cytokines (ILs, TNF, IFN, chemokines), and free radicals (reactive oxygen species and nitric oxide).

Plasma protein-derived mediators are products of kinin (kallikrein, bradykinin), clotting (fibrin, fibrinopeptides), fibrinolytic (plasmin) and complement (C3a, C5a, C4a, C5a, MAC) system.