Chemical mediators of inflammation

chemical mediators of inflammation

1. Chemical mediators of inflammation presented by karuna sharma pg STUDENT

2. Mediators are derived from plasma or by local production of cells . plasma derived mediators (complement kinins,coagulation factors) are present as circulating precursors that must be activated,usually by proteolytic cleavage,to acquire their biological properties. cell-derived mediators are normally sequestered in intracellular granules ( e.g.,histamine in mast cells) that are subsequently secreted,or are synthesized de novo( e.g.,prostaglandins )in response to a stimulus.

3. Most mediators perform their biologic activity by initially binding to specific receptors on target cells. However some have direct enzymatic or toxic activities(e.g., lysosomal proteases or reactive oxygen species) Mediators may stimulate target cells to release secondary effector molecules.These secondary mediators may have activities similar to the initial effector molecule,in which case they may amplify a particular response. On the other hand they may have opposing activities, and thereby function to counter-regulate the initial stimulus.

4. Mediators may act on only one or a very few targets,or may have widespread activity,, and may have widely differing outcomes depending on which cell type they affect. Mediator function is generally tightly regulated.once activated and released from the cell,most mediators quickly decay( e.g.,AA metabolites),are inactivated by enzymes( e.g.,kininase inactivates bradykinin ),or are eliminated( e.g.,antioxidants scavenge toxic oxygen metabolites). A major reason for the checks and balances is that most mediators have the potential to cause harmful effects.

5. VASOACTIVE AMINES The two amines, Histamine & Serotonin are the first mediators to be released during inflammation. Histamine is widely distributed in tissues, particularly in mast cells adjacent to vessels,as well as in circulating basophils and platelets. Preformed histamine is present in mast cell granules that are released in response to a variety of stimuli:

6. 1. Physical injury such as trauma and heat 2. Immune reactions involving binding of IgE antibodies to Fc receptors on mast cells 3. C3a and C5a fragments of complement,the so called anaphylotoxins 4. Leukocyte-derived histamine-releasing proteins

7. 5.Neuropeptides ( e.g.,substance p)and 6.Certain cytokines(e.g.,IL-1andIL-8) In humans , histamine causes arteriolar dilation & is the principal mediator of the immediate phase of increased vascular permeability, causing venular endothelial contraction and widening of the interendothelial cell junctions. soon after its release, histamine is inactivated by histaminase .

8. Serotonin (5-hydroxytryptamine) - is also a preformed vasoactive mediator,with effects similar to histamine . it is found primarily within platelet-dense body granules (along with histamine,adenosine diphosphate and calcium) and release is stimulated by platelet aggregation.

9. PLASMA PROTEASES Many of the effects of inflammation are mediated by three interrelated plasma derived factors- The clotting system, Complement,and The kinins , all linked by the initial activation of hageman factor.

10. THE CLOTTING SYSTEM Is a cascade of plasma proteases,which can be triggered by proteoytic action of activated Hageman factor. Hageman factor (factor XII of the intrinsic coagulation cascade) is a protein synthesized by liver that circulates in an inactive form untill it encounters collagen,basement membrane,or activated platelets(as at a site of endothelial injury). Factor XII then undergoes a conformational change(becoming factor XIIa )

11. In the clotting system,the resultant proteolytic cascade results in activation of thrombin (factor IIa ) from precursor prothrombin (factor II) which in turn cleaves circulating soluble fibrinogen to generate an insoluble fibrin clot. Thrombin also enhances leukocyte adhesion to endothelium,and fibrinopeptides resulting from fibrinogen cleavage increase vascular permeability and are chemotactic for leukocytes.

12. At the same time,when it is inducing clotting , it can activate the fibrinolytic system. This mechanism is to counterregulate clotting by causing further cleavage of the fibrin molecule to fibrin split products.

13. Without fibrinolysis , initiation of the coagulation cascade ,even by trivial injury, would result in continuous and irrevocable clotting of the entire vasculature In addition it contributes to the vascular phenomenon of inflammation Plasminogen activator (released from endothelium, leukocytes, and other tissues) cleaves plasminogen , a plasma protein bound up in the evolving fibrin clot.

14. This generates plasmin , a protease that degrades fibrin to fibrin split products and is therefore important in lysing fibrin clots. Fibrin split products also increase vascular permeability,while plasmin cleaves the complement C3 component to C3a,resulting in vasodilation and increased vascular permeability.

15. Kinin system activation leads ultimately to the formation of bradykinin from its circulating precursor,high molecular weight kininogen (HMWK). Like histamine , bradykinin causes increased vascular permeability,arteriolar dilation, and extravascular smooth muscle contraction( e.g.,in bronchial smooth muscle ) It also causes pain when injected into the skin. It is short lived because it is rapidly inactivated by kininases .

16. COMPLEMENT SYSTEM Consists of a cascade of plasma proteins that play an important role in both immunity and inflammation . They function in immunity by ultimately generating a porelike membrane attack complex(MAC). In the process of generating the MAC ,a number of complement fragments are produced e.g. C3b opsonins .

17. Complement components (C1 to C9) are present in plasma. The most critical step in the elaboration of the biologic functions of complement is the activation of the third component,C3. C3 cleavage can occur via the classic pathway,triggered by fixation of C1 to antigen-antibody complexes. Through the alternative pathway,triggered by bacterial polysaccharides, or aggregated IgA.The alternative pathway involves a distinct set of serum components, including properdin and factors B and D.

18. 3.C3 convertase cleaves C3 to C3a and C3b. 4.C3b then binds to the C3 convertase complex to form C5 convertase . 5.This complex cleaves C5 to generate C5a and initiate the final stages of assembly of the C5a to C9MAC

19. Th e various effects of the complement derived mediators in acute inflammation : Vascular effects: C3a and C5a ( anphylotoxins ) increase vascular permeability and cause vasodilation by inducing mast cells to release their histamine.C5a also activates the lipoxygenase pathway of AA metabolism in neutrophils and monocytes

20. Leukocyte activation,adhesion and chemotaxis:C5a activates leukocytes and increases the avidity of their integrins ,thereby increasing adhesion to endothelium Phagocytosis when fixed to to a microbial surface,C3b and C3bi act as opsonins,augmenting phagocytosis by cell bearing C3b receptors. The significance of C3 and C5 is further increased by the fact that they can also be activated by the fact that they can also be activated by proteolytic enzymes present within the inflammatory exudate .

21. ARACHIDONIC ACID METABOLITES (EICOSANOIDS):PROSTAGLANDINS AND LEUKOTRIENES Arachidonic acid is a 20 C polyunsaturated fatty acid derived primarily from linoleic acid present in the body in the cell membrane phospholipids. It is released from these phospholipids via cellular phospholipases that have been activated by mechanical,chemical or physical stimuli or by inflammatory mediators such as C5a.

22. AA metabolism proceeds along one of two major pathways Are named for the enzymes that initiate the reactions Cycloxygenase pathway Lipoxygenase pathway

23. CYCLOXYGENASE PATHWAY Include PGE 2 ,PGD 2 ,PGF 2 α ,PGI 2 (PROSTACYCLIN ) each of which is derived by the action of specific enzyme Some of these enzymes have a restricted tissue distribution. E.g. platelets contain the enzyme thromboxane synthetase , and hence TXA 2 ,a potent platelet aggregating agent and vasoconstrictor, is the major prostaglandin product in these cells.

24. Endothelium,on the other hand, lacks thromboxane synthetase but possess prostacyclin synthetase,which leads to the formation of PGI 2 ,a vasodilator and a potent inhibitor of platelet aggregation. PGD 2 is the major metabolite of the cycloxygenase pathway in mast cells;along with PGE 2 and PGF 2 α (which are most widely distributed), it causes vasodilatation and causes edema formation. Recent studies revealed that there are two forms of cycloxygenases (COX), called COX1 and COX2 .

25. COX1 is expressed in gastric mucosa. At this site the mucosal prostaglandins generated by the actions of COX1 are protective because they prevent the acid induced damage. While inhibition of cycloxygenases by aspirin and NSAID reduces inflammation by blocking prostaglandin synthesis,these drugs also predispose to gastric ulceration. To preserve the anti-inflammatory effects and prevent the harmful effects on the gastric mucosa,several highly selective COX2 inhibitors are being developed.

26. LIPOXYGENASE PATHWAY 5 Lipoxygenase is the predominant AA metabolizing enzyme in neutrophils , and the products of its actions are the best characterized. The 5-hydroxyperoxy derivative of AA,5-HPETE, is quite unstable and is either reduced to 5-HETE or converted to leukotrienes . The first leukotriene generated from 5-HPETE is called leukotriene A 4 (LTA 4 ),which in turn gives rise to LTB 4 or LTC 4

27. LTB 4 is a chemotactic agent and causes aggregation of neutrophils . LTC 4 and its subsequent metabolites LTD 4 and LTE 4 ,cause vasoconstriction, bronchospasm , and increased vascular permeability.

28. CLINICAL UTILITY The fact that eicosanoids hold a central role in inflammatory processes is borne out by the clinical anti-inflammatory utility of agents that suppress cycloxygenase activity ( e.g.,aspirin and NSAIDs). Glucocorticoids , which are powerful anti-inflammatory agents,may act in part by inhibiting the activity of phospholipase A 2 .

29. PLATELET ACTIVATING FACTOR Originally named for its ability to aggregate platelets and cause degranulation . PAF is phospholipid derived mediator. Formally PAF is acetyl glycerol ether phosphocholine . It is derived from the membrane phospholipids of neutrophils , monocytes , basophils , endothelium, and platelets by the action of phospholipids A 2 .

30. Besides platelet stimulation PAF causes vasoconstriction and bronchoconstriction . It is 100-10,000 times more potent than histamine in inducing vasodilation and increased vascular permeability. PAF also causes enhanced leukocyte adhesion (via integrin conformational changes), chemotaxis,leukocyte degranulation , and the oxidative burst.

31. PAF also stimulates the synthesis of other mediators,particularly eicosanoids .

32. CYTOKINES Cytokines are polypeptide products of many cell types(but principally activated lymphocytes and macrophages) that modulate the function of other cell types. Cytokines can act On the same cell ( autocrine effect), On other cells in the immediate vicinity( paracrine effect), or Systemically (endocrine effect)

33. H istorically associated with cellular immune responses,various cytokines, in particular (IL-1), (TNF- α and – β ), (INF- ᵧ ), and the chemokines,have additional effects that are important in the inflammatory response These cytokines can exert local effects on endothelium, leukocytes, and fibroblasts, as well as induce systemic acute-phase reactions.

34. NITRIC OXIDE AND OXYGEN DERIVED FREE RADICALS NO is a short acting, soluble free radical gas produced by a variety of cells. In the CNS, it regulates neurotransmitter release, as well as blood flow. Macrophages use it as a cytotoxic metabolite for microbes and tumor cells. When produced by endothelium(EDRF or endothelium-derived relaxation factor),it activates guanylyl cyclase in vascular smooth muscle,resulting in increased cGMP and ultimately smooth muscle relaxation( vasodilation ).

35. Since the half life of NO is measured in seconds, it can affect only those cells in near proximity to the source where it is generated. Moreover, the short half life of NO dictates that its effects are regulated primarily by the rate of synthesis.

36. NO plays multiple roles inflammation,including Vascular smooth muscle relaxation( vasodialation ) Antagonism of all stages of platelet activation( adhesion,aggregation,and degranulation ), and Acting as a microbicidal agent(with or without superoxide radicals) in activated macrophages

37. OXYGEN DERIVED FREE RADICALS are synthesized via the NADPH oxidase pathway and are released from neutrophils and macrophages after stimulation by chemotactic agents, immune complexes, or phagocytic activity Superoxide is subsequently converted to H2O2, OH - And toxic NO derivatives. These shortlived mediators have been implicated in a variety of tissue injuries,including

38. Endothelial damage, with thrombosis and increased permeability; Protease activation and antiprotease inactivation, with a net increase in breakdown of the extracellular matrix;and Direct injury to other cell types( e.g.,tumor cells,erythrocytes,parenchymal cells). Fortunately, a variety of antioxidant protective mechanisms ( e.g , catalase , superoxide dismutase, and glutathione) are present in tissues and serun to minimize the toxicity of the oxygen metabolites.

39. LYSOSOMAL CONSTITUENTS The lysosomal granules of neutrophils and monocytes contain a number of molecules that can potentially act as mediators of acute inflammation. They may be released after cell death , by leakage during the formation of the phagocytic vacuole, or by frustrated phagocytosis against large surfaces. While acid proteases have acidic pH optima and are generally active only within phagolysosomes , neutral proteases, including enzymes such as elastase,collegenase , and cathepsin,are active in the extracellular matrix and cause destructive,deforming tissue injury by degrading elastin,collagen,basement membrane, and other matrix proteins.

40. Neutral proteases can also cleave C3 and C5 directly to generate the C3a and C5a anaphylotoxins , and can promote the generation of bradykinin like peptides from kininogen . Thus if the initial leukocyte infiltration is left unrestrined,substantial vascular permeability and tissue damage may result . These effects are checked, however,by a series of antiproteases present in the serum and extracellular matrix

41. MOST LIKELY MEDIATORS IN INFLAMMATION Vasodilation Prostaglandins Nitric oxide Increased vascular permeability Vasoactive amines C3a and C5a Bradykinin Leukotrienes C 4 ,D 4 ,E 4 PAF

42. Chemotaxis and leukocyte activation C5a Leukotriene B 4 Bacterial products Chemokines (e.g:IL8) Fever IL-1,IL-6,TNF- α PG Pain PG’s bradykinin

43. Tissue damage Neutrophil and macrophage lysosomal enzymes Oxygen metabolites Nitric oxide

44. OUTCOMES OF ACUTE INFLAMMATION Complete resolution Scarring or fibrosis Abscess formation Progression to chronic inflammation

45. CHRONIC INFLAMMATION Chronic inflammation is characterized by: Infiltration with mononuclear cells including macrophages, lymphocytes, and plasma cells. Tissue destruction Repair involving new vessel proliferation(angiogenesis) and fibrosis .

46. Chronic inflammation can be considered to be inflammation of prolonged duration(weeks to months to years) in which active inflammation ,tissue injury , and healing proceed simultaneously Chronic inflammation may follow acute inflammation. This transition occurs when the acute response cannot be resolved,either because of the persistence of the injurious agent or because of interference in the normal process of healing

47. For e.g., a peptic ulcer of the duodenum initially shows acute inflammation followed by the beginning stages of resolution. However recurrent bouts of duodenal epithelial injury interrupt this process and result in a lesion characterized by both acute and chronic inflammation. Alternatively, some forms of injury ( e.g.,viral infections) engender a response that involves a chronic inflammation esentially from the beginning. Although the injurious agents mediating chronic inflammation may be less noxious than those that cause acute inflammation, the overall failure to resolve the process may lead to substantially more long term injury.

48. Fibrosis –the proliferation of fibroblasts and accumulation of excess extracellular matrix-is also a common feature of many chronic inflammatory diseases and is an important cause of organ dysfunction. Chronic inflammation arises in the following settings. Persistent infections,most characteristically by a set of microorganisms including mycobacteria,treponema pallidum and certain fungi. These organisms are of low direct pathogenicity , but typically evoke an immune response called DELAYED HYPERSENSITIVITY that may culminate in a granulomatous reaction.

49. 2. Prolonged exposure to potentially toxic agents.e.g . nondegradable exogenous material such as inhaled particulate silica, which can induce a chronic inflammatory response in the lungs(silicosis) 3.Autoimmune diseases, in which individual develops an immune response to self antigens and tissues.

50. CHRONIC INFLAMMATORY CELLS Macrophages, are but one component of the mononuclear phagocyte system, consisting of closely related cells of bone marrow origin, including the circulating blood monocytes , and tissue macrophages. Macrophages are diffusely scattered in connective tissues, or clustered in organs such as the liver( kupffer cells),spleen and lymph nodes (sinus histiocytes ),central nervous system(microglia), and lungs (alveolar macrophages).

51. The half life of circulating monocytes is about 1 day Under the influence of adhesion molecules and chemotactic factors, they begin to emigrate at a site of injury within the first 24 to 48 hours after onset of acute inflammation. When monocytes reach the extravascular tissue, they undergo transformation into larger phagocytic cells called macrophages. Macrophages may also become activated, a process resulting in increased cell size, increased content of lysosomal enzymes, more active metabolism, and greater ability to kill ingested microorganisms.

52. Activation signals include cytokines secreted by sensitized T lymphocytes, bacterial endotoxins , various mediators produced during acute inflammation, and extracellular matrix proteins such as fibronectin .

53. After activation, macrophages produce a wide variety of biologically active products- Acid and neutral proteases Complement components and coagulation factors Reactive oxygen species and NO Eicosanoids Cytokines such as IL-1 and TNF

54. At the site of acute inflammation, where the irritant is removed and the process is resolved, macrophages eventually die or wander off into lympatics . In chronic inflammatory sites, however macrophage accumulation persists. Other types of cells present in chronic inflammation are lymphocytes, plasma cells, and eosinophils .

55. Both T and B lymphocytes migrate into inflammatory sites via adhesion molecules. T lymphocytes are activated by macrophages. The activated lymphocytes produce cytokines like IFN- ᵧ , a stimulant for monocytes and macrophages. Activated macrophages release cytokines, including IL-1 and TNF. The end result is an inflammatory focus where macrophages and T cells can persistently stimulate one another until the triggering antigen is removed. Plasma cells are the end product of B-cell activation. They can produce antibodies directed against antigens in the inflammatory site.

56. GRANULOMATOUS INFLAMMATION Is a distinctive pattern of chronic inflammation characterized by aggregations of activated macrophages, that have acquired an enlarged, squamous cell-like (called epitheloid ) appearance. Examples Bacterial Tuberculosis (Mycobacterium tuberculosis) Leprosy (M. Leprae ) Parasitic schistosomiasis

57. Fungal Histoplasma Capsulatum Blastomycosis Inorganic metals or dusts Silicosis Berylliosis Foreign bodies Suture, vascular graft Unknown sarcoidosis

58. Morphologic patterns in acute and chronic inflammation SEROUS INFLAMMATION Is characterized by the outpouring of a watery , relatively protein poor fluid that ,depending on the site of injury,derives either from the serum or from the secretions of mesothelial cells lining the peritoneal, pleural and pericardial cavities. The skin blister resulting from a burn or a viral infection is a good example.

59. FIBRINOUS INFLAMMATION This occurs as a consequence of more severe injuries,with the resultant greater vascular permeability allowing larger molecules(fibrinogen) to pass the endothelial barrier. Fibrinous exudates may be degraded by fibrinolysis and the accumulated debris may be removed by macrophages

60. resulting in restoration of the normal tissue structure(resolution).however ,failure to completely remove the fibrin results in the ingrowth of fibroblasts and blood vessels, leading ultimately to scarring(organization)

61. example:organization of a fibrinous pericardial exudate to form scar tissue can lead to the development of fibrous strands that bridge the pericardial space and restrict myocardial function .

62. SUPPURATIVE INFLAMMATION (PURULENT) This is manifested by the presence of large amounts of purulent exudate (pus) consisting of neutrophils,necrotic cells, and edema fluid. Staphylococci are more likely to induce this localized suppuration and are therefore referred to as pyogenic . Abscesses are focal collections of pus that may be caused by deep seeding of pyogenic organisms into a tissue or by secondary infections of necrotic foci.

63. Abscesses typically have a central, largely necrotic region rimmed by a layer of preserved neutrophils,with a surrounding zone of dilated vessels and fibroblastic proliferation . In time ,the abscess may become completely walled off and eventually replaced by connective tissue.

64. ULCERATION This refers to a site of inflammation where an epithelial surface has become necrotic and eroded,often with associated subepithelial acute and chronic inflammation. Can occur as a consequence of toxic or traumatic injury to the epithelial surface E.g. peptic ulcers or May be due to vascular compromise (as in the foot ulcers associated with the vasculopathy of diabetes)

65. There is usually an early intense neutrophilic infiltrate with associated vascular dilation. In chronic lesions where there has been repeated insult ,the area surrounding the ulcer develops fibroblastic proliferation, scarring, and the accumulation of chronic inflammatory cells.

66. SYSTEMIC EFFECTS OF INFLAMMATION Fever Higher erythrocyte sedimentation rate Leukocytosis Neutrophilia Lymphocytosis leukopenia

67. Fever is one of the most obvious of the systemic effects of inflammation,collectively identified as acute phase reactions. Acute phase reactions includes Increased slow wave sleep Anorexia Accelerated degradation of skeletal muscle proteins Hypotension

68. Hepatic synthesis of a variety of acute phase proteins Alterations in the circulating white blood cell pool.

69. The cytokines IL-1,IL-6 and TNF are the most important mediators of the acute phase reactions. These cytokines are released by leukocytes in response to infection, or to immune and toxic injury, and are released systemically, frequently in a sort of cytokine cascade.

70. TNF can induce the production of IL-1,which inturn induces the production of IL-6. IL-6 stimulates the hepatic synthesis of several plasma proteins,most notably fibrinogen, Elevated fibrinogen levels cause erythrocytes to agglutinate more readily,which explains for the higher erythrocyte sedimentation rate.

71. Although IL-1 and TNF induce other consequences ,their effects are similar to one another. For example they both act on the thermoregulatory centre of the hypothalamus, most likely via local prostaglandin E production, to induce fever (hence the efficacy of aspirin and NSAIDs in reducing fever)

72. Leukocytosis (increased white blood cell count) is a common feature of inflammatory reactions, especially those induced by bacterial infection. The leukocyte count usually increases to 15000 or 20000 cells/µl. It may exceptionally may climb as high as 40000 to 100000 cells/µl , a so called leukemoid reaction.

73. Leukocytosis initially results from the release of cells from the bone marrow(caused by IL-1 and TNF) and is associated with an increased number of relatively immature neutrophils in the blood Most bacterial infections induce a relatively selective increase in polymorphonuclear cells ( neutrophilia )

74. Parasitic infections as well as allergic responses characteristically induce eosinophilia . Certain viruses, such as infectious mononucleosis,mumps , rubella, engender selective increase in lymphocytes ( lymphocytosis ) However most viral infections as well as rickettsial , protozoal and certain types of bacterial infections (typhoid fever) are associated with a decreased number of circulating white cells. ( leukopenia )

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