Chemical Mediators of Inflammation - Lecture Slides

CHEMICAL MEDIATORS OF INFLAMMATION Dr. Bernard Petershie Introduction These are chemicals responsible for the events that takes place in inflammation Many mediators have been identified and the coordination of these chemicals is still being studied Mediators are derived from plasma or cell...

CHEMICAL MEDIATORS OF INFLAMMATION Dr. Bernard Petershie Introduction These are chemicals responsible for the events that takes place in inflammation Many mediators have been identified and the coordination of these chemicals is still being studied Mediators are derived from plasma or cells Plasma derived Complement proteins Kinins clotting factors Cell derived Preformed- histamine Newly formed- prostaglandins or cytokines Cells of origin include platelets, neutrophils, monocytes/macrophages, mast cells, mesenchymal cells like smooth muscles, fibroblasts and epithelial cells. Characteristic of chemical mediators Active mediators are triggered by microbial products as well as host proteins Mediators bind to receptors on target cells Mediators can stimulate release of other mediators from target cells Mediators can act on 1 or many target cells, have diverse targets, have differing effects Mediators are short lived, either by decay or inactivated by enzymes or scavenged or inhibited Mediators have potential to cause harmful effect VASOACTIVE AMINES Composed of histamine and serotonin Histamine Derived from mast cells, platelets and basophils Released in response to stimuli- physical injury i.e trauma, cold, immune reactions, fragments of complements c5a,c3a, histamine releasing proteins from leukocytes, neuropeptides substance p, cytokines IL1, IL8 Serotonin Also called 5 hydroxytryptamine Present in platelets, enterochromafin cells, mast cells in rodents Serotonin released from platelets when platelets aggregate due to contact with collagen, thrombin, adenosine diphosphate and antigen antibody reaction Platelet aggregation is mediated by PAF derived from mast cells during IgE mediated reactions Function of vasoamines Potent vasodilator of arterioles Increase venular permeability Elaborate pain Constrictor of large arteries Plasma Proteins-Complement system Composed of 20 proteins Function in innate and adaptive immunity Function in vascular permeability, chemotaxis, opsonization Inactive form in plasma labelled c1-c9 which could be activated Activation of complements Classical pathway- Antigen/antibody reaction Mediated by IgM and IgG Alternate pathway- involve no antibody, antigen include endotoxins or LPS, complex polysacharides, cobra venom Lectin pathways – plasma mannose binding lectin binds to carbonhydrates on microbes and directly activates C1 CLASSICAL PATHWAY Activation is by antigen-antibody complex or aggregated immunoglobulins. IgG1, IgG2, IgG3 subclasses and IgM can activate the classical pathway. One molecule of IgM or at least two cross-linked molecules of IgG are capable of activating the classical pathway. IgG4, IgA, IgD and IgE are incapable of activating the classical pathway. Among IgG subclasses, IgG3 is the most active. IgG, however, is a far less efficient compliment activator than IgM. The classical pathway can also be activated non immunologically by DNA, C-reactive protein, Trypsin-like enzymes, etc. BIOCHEMICAL REACTION In this pathway, Ag-Ab complex activates C1. C1 is a trimolecular complex consisting of C1q, C1r and C1s in a molar ratio of 1:2:2 After binding to Ab C1q undergoes a conformational change which results in the activation of C1r. C1r is then activates C1s which cleaves C2 and C4 to form C4b2b Alternate pathway This pathway is important in the early stages of host defence against microbes. Activation occurs more slowly than in the classical pathway. Activation is mainly by non immunological stimuli – bacterial lipopolyssacharide, fungal cell wall, viral envelopes, plasmin, cobra venom, IgA. BIOCHEMICAL REACTION In the alternative pathway, C3 undergoes hydrolysis, which induces a conformational change. C3 now accepts factor B and the complex is cleaved by factor D in the presence of magnesium to form C3bBb All pathways lead to formation of C3 convertase Splits C3 to C3a and C3b C3a is released C3b binds to to previously generated fragment to form C5 convertase C5a is released from C5 and the remaining C5b binds the late components C6-C9 MAC is formed which lyses cells MAC is C5b-C9 Functions of complement system Anaphylatoxins- C5a, C3a and C4a Causes release of histamine from mast cells C5a also stimulate lipooxygenase pathways C5a causes leukocytes adhesion and is a Chemotactic agent C3b and iC3B are opsonins C3 and C5 can be activated by plasmin and lysosomal enzymes Regulation of complement system Regulator proteins are present in normal host cells and absent from microbes and include C1 inhibitor – recognises and binds to C1 to destroy its enzymatic activity, thereby blocking the classical pathway. It also inactivates factor XIIa, Kallikrein and plasmin. Individuals with hereditary angioneurotic oedema (HANE) have low levels of C1 inhibitor. The alternative pathway is regulated by binding of C3b to factors H and I, which cleaves C3b to iC3b. Factor I also inactivates C4b. The alternative pathway C3 convertase is very labile but is stabilised by Properdin (C3NeF has similar action to Properdin).  Decay accelerating Factor (DAF) is a protein that protects human cells from lysis by the MAC. It accelerates the decay of C3 and C5 convertases by removal of C2b from C4b and Bb from C3b. Membrane Cofactor Protein (MCP) has similar action to DAF. It is also a cofactor for factor I in the inactivation of C3b and C4b. Complement receptor type 1 (CRI) is similar to DAF. It also inactivates C3b and C4b. It also functions as a cofactor for factor I that cleaves C3b.  Vitronectin. Vitronectin, also called S-protein is a component of the ECM. It binds to C5b-7 and inhibits the action of MAC CD59 (MAC, protecting, homologous restriction factor-20). It inhibits the incorporation of C9 into C5b-9 complexes. Complement Deficiencies Are associated with abnormal complement activation and various related clinical manifestations Deficiencies involving C1q, C1r, C4, C2 and C3 have been described. C2 deficiency is the most common. In over 50% of those with C2 and C4 deficiencies, an SLE-like disease develops. Deficiencies of factor B, properdin and C3 results in increased susceptibility to pyogenic infections. Individuals with a deficiency of the terminal complement components are predisposed to infection by Neisseria organisms. Deficiencies of Complement regulating proteins These deficiencies are associated with abnormal complement activation and various related clinical abnormalities. Deficiency of C1 inhibitor leads to HANE, with an AD mode of inheritance. Deficiency of DAF causes paroxysmal nocturnal haemoglobinuria (PNH) due to unregulated complement activation on the surface of erythrocytes. Individuals with a deficiency of factor I have increased susceptibility to infection by pyogenic bacteria. Factor H deficiency is rare, and is characterized by excessive activation of the alternative pathway and glomerulonephritis. KININ SYSTEM Factor XII (Hageman factor) is activated by contact with collagen and basement membrane Factor XIIa is formed which activate Prekallikrein to Kallikrein Kallikrein cleave high molecular weight Kininogen to produce Bradykinin High molecular weight kininogen is a cofactor in the activation of Factor XII Kallikrein is a potent activator of Hageman factor Kinin is degraded by Kininase in plasma and angiotensin convertin enzyme in the lungs Clotting Factor Intrinsic – activated by factor XII Chain of reaction which converts prothrombin to thrombin Thrombin converts fibrinogen to insoluble fibrin by binding to protease activated receptors Receptors are also seen on platelets, endothelial cells and smooth muscle cells etc. Binding to thrombin triggers numerous inflammatory reaction Inflammatory response due to thrombin and PAR 1 Mobilization of P Selectin Production of chemokines Expression of endothelial adhesion molecules for leukocyte integrins Induction of Cox 2 and production of prostaglandins Production of PAF and NO Changes in endothelial shape Fibrinolytic pathway Also activated by factor XIIa Plasminogen activator cleaves plasminogen to plasmin Plasmin lyses fibrin clots Plasmin activates C3 to produce C3 fragments Plasmin degrade fibrin to produce fibrin split products which has permeability properties Plasmin can also activate Hageman factor Interrelationships between the four plasma mediator systems triggered by activation of factor XII Arachidonic Acid Metabollites Are generated from membrane lipids of activated cells Are biologically active lipid mediators with intra and extracellular signaling and function in inflammation and hemostasis Are autocoids, formed rapidly, exerts effects locally and either decayed spontaneously or enzymatically  AA are liberated from membrane phospholipids by Phospholipase A2 activated by mechanical, chemical and physical stimuli or by C5a Process involve increase in cytosolic Ca2+ and activation of various kinases AA metabolites are eicosanoids synthesize by cyclooxygenase pathway(prostaglandins and thromboxanes) or lipooxygenase (lipoxins and leukotrienes) Cyclooxygenase pathway Initiated by COX-1 OR COX-2 enzymes Produce prostaglandins PGD,PGE,PGF,PGG, PGH Important is PGE2,PGD2, PGF2alpha, and PGI2(prostacyclins), TXA2 (thromboxane) PGI2 produce by endothelium is a potent vasodilator(enzyme is prostaglandin synthetase) Thromboxane produce from platelet is a potent vasoconstrictor(enzyme is thromboxane synthetase) Note opposing effect of the above in thrombosis  COX-1 is present in most tissues and function in hemostasis and inflammation COX-2 is present in few tissue and function in inflammation only There is preference of COX-2 inhibitors because of less side effect, aspirin and other NSAID is a COX-1 inhibitor Lipoxygenase present in few cells, 5-HETE is produce from 5-LO and is chemotactic for neutrophils. Converted into family of compound called leukotrienes  Leukotrienes LTB4 is a potent chemotactic agent and activator of neutrophil functional response LTC4, LTD4, LTE4 causes vasoconstriction, bronchospasm and increase vascular permeability. Vascular leakage is restricted to venules like histamine Lipoxygenase inhibitor used in management of asthma  Lipoxins Leukocytes produce intermediates in lipoxin synthesis and converted to lipoxins by interaction with platelets LXA4 and LXB4 are generated from neutrophil derive LTA4 by effect of platelet 12-lipoxygenase Lipoxins inhibit leukocyte recruitment, chemotaxis and adhesion to endothelium PAF Platelet activating factor Is a phospholipid derived mediator Chemically AGEPC- acetyl glyceryl ether phosphocholine PAF causes vasoconstriction and bronchospasm At low concentration it causes vasodilatation Elaborated by platelets, basophils, mast cells, neutrophils ,macrophages and endothelial cells It is degraded by PAF acetylhydrolases PAF can elicit most of the cardinal signs of inflammation CYTOKINES Proteins produced by many cell types that modullate the function of other cell types Cells include lymphocytes, macrophages, endothelium, epithelium and connective tissues etc Example is IL1, TNF Induces acute phase response associated with infection or injury which include fever, loss of appetite,slow wave sleep, release of neutrophils into circulation, haemodynamic effect of septic shock, etc Effect of IL1 & TNF Chemokines Cytokines that acts as chemo-attractants Has 4 major groups C-X-C CC C CX3C NITRIC OXIDE Causes vasodilatation by relaxing vascular smooth muscle Derived from endothelial cells NO reduces platelet aggregation and is an endogenous response to reduce inflammation NO is a mediator of host defense in inflammation OTHERS Lysosomal constituents Oxygen derived free radicals mainly superoxide anions, hydroxyl radicals,H2O2 Neuropeptides substance P and neurokinase A Effect of hypoxia- hypoxia induced factor 1 alpha Effect of necrosis- uric acid suspected  Thank You