Mediators of Inflammation	<b>MEDIATORS OF INFLAMMATION - Introduction</b><br><br>Inflammatory mediators are substances that initiate and regulate inflammatory reactions. They may be: (1) Cell-derived - rapidly released from intracellular granules (e.g., histamine) or synthesized de novo (e.g., prostaglandins, leukotrienes, cytokines) in response to a stimulus. Major producing cells: tissue macrophages, dendritic cells, mast cells, platelets, neutrophils, endothelial cells, and epithelia. (2) Plasma-derived - produced mainly in the liver, circulate as inactive precursors, activated by proteolytic cleavage (e.g., complement, kinins). Active mediators are produced only in response to stimuli (microbial products, necrotic cell substances) - ensuring inflammation is triggered only when needed. Most are short-lived: quickly decay, inactivated by enzymes, or scavenged. Key principle: redundancy and synergy ensure robust protective response.<br><br><b>CELL-DERIVED MEDIATORS</b><br><br>(Blank - to be filled)<br><br><b>NEWLY SYNTHESIZED MEDIATORS</b><br><br>These are mediators synthesized de novo upon cell activation, not preformed.<br><br><b>1. PLATELET-ACTIVATING FACTOR (PAF)</b><br><br>Nature: Phospholipid-derived mediator (acetyl-glyceryl-ether-phosphorylcholine). Sources: Platelets, basophils, mast cells, neutrophils, macrophages, endothelial cells - produced in both secreted and cell-bound forms. Functions: (a) Platelet aggregation - the original discovery function, (b) Vasoconstriction and bronchoconstriction at high concentrations, (c) Vasodilation and increased venular permeability at low concentrations, (d) Leukocyte adhesion, (e) Chemotaxis, (f) Degranulation and oxidative burst. Note: PAF antagonists in clinical trials have not shown significant benefit in inflammatory diseases.<br><br><b>2. NITRIC OXIDE (NO)</b><br><br>Physical characteristics: Soluble gas, short half-life, produced from L-arginine by nitric oxide synthase (NOS). Three isoforms of NOS: (i) eNOS - endothelial, constitutive, maintains vascular tone, (ii) nNOS - neuronal, constitutive, acts as neurotransmitter, (iii) iNOS - inducible, induced in macrophages and neutrophils by cytokines (IFN-gamma) and microbial products. Sources: Macrophages (main source in inflammation via iNOS), endothelial cells (eNOS), neurons (nNOS). Functions: (a) Microbicidal killing - NO reacts with superoxide (O2-) to form peroxynitrite (ONOO-), a highly reactive free radical that damages microbial lipids, proteins, and nucleic acids, (b) Vasodilation - relaxes vascular smooth muscle (role in inflammatory vascular response unclear), (c) Neurotransmission (nNOS). Clinical note: Mice lacking both phagocyte oxidase and iNOS succumb rapidly to normally harmless commensal bacteria, showing overlapping microbicidal roles.<br><br><b>3. CYTOKINES</b><br><br>Definition: Proteins produced by many cell types that mediate and regulate immune and inflammatory reactions. Sources: Principally activated lymphocytes, macrophages, and dendritic cells - also endothelial, epithelial, and connective tissue cells. Types and functions in acute inflammation: (a) TNF - from macrophages, dendritic cells, mast cells, T lymphocytes - stimulates endothelial adhesion molecule expression (E-selectin, P-selectin, integrin ligands), induces other cytokines and chemokines, increases procoagulant activity, augments neutrophil responses, systemic effects (fever, SIRS, cachexia - via lipid/protein mobilization and appetite suppression), vasodilation and reduced myocardial contractility at high concentrations. Therapeutic: TNF antagonists used in rheumatoid arthritis, psoriasis, IBD - risk of mycobacterial infection. (b) IL-1 - from macrophages, dendritic cells, some epithelial cells - similar to TNF, greater role in fever, activates fibroblasts to synthesize collagen, stimulates Th17 responses, promotes neutrophil recruitment. IL-1 biologically active form requires inflammasome activation. (c) IL-6 - from macrophages and other cells - systemic effects (acute-phase response). (d) IL-17 - from T lymphocytes - recruits neutrophils and monocytes. Antagonists (anti-IL-6R, anti-IL-17) effective in juvenile arthritis and psoriasis. Chronic inflammation cytokines: IL-12 (from dendritic cells, macrophages - increases IFN-gamma production), IFN-gamma (from T lymphocytes, NK cells - activates macrophages for microbial killing).<br><br><b>CHEMOKINES</b><br><br>(Blank - to be filled)<br><br><b>ARACHIDONIC ACID METABOLITES (Eicosanoids)</b><br><br>Source: Arachidonic acid (AA) - a 20-carbon polyunsaturated fatty acid (5,8,11,14-eicosatetraenoic acid) present in membrane phospholipids, derived from dietary sources or from linoleic acid. Released by phospholipase A2 (activated by mechanical, chemical, physical stimuli, or mediators like C5a). Term: Eicosanoids (Greek: eicosa = 20). Two major enzyme pathways: (A) CYCLOOXYGENASE pathway (COX-1 and COX-2) - generates PROSTAGLANDINS. COX-1: constitutively expressed, homeostatic functions (renal fluid/electrolyte balance, GI cytoprotection). COX-2: induced by inflammatory stimuli. Prostaglandins: PGE2 and PGD2 - vasodilation, increased permeability of postcapillary venules, potentiate edema - PGD2 also chemoattractant for neutrophils. PGI2 (Prostacyclin) - produced by vascular endothelium (prostacyclin synthase) - vasodilator, potent inhibitor of platelet aggregation - prevents thrombus on normal endothelium. TxA2 (Thromboxane A2) - produced by platelets (thromboxane synthase) - potent platelet aggregating agent, vasoconstrictor. Imbalance of TxA2/PGI2 implicated in early thrombosis of coronary/cerebral arteries. PGE2, PGD2, PGF2a, PGI2, TxA2 - named by structural features. Role in pain and fever. NSAIDs: inhibit COX-1 and COX-2 - reduce prostaglandins. Selective COX-2 inhibitors: antiinflammatory, spare GI cytoprotection, but increase cardiovascular/cerebrovascular risk (impair endothelial PGI2 production while leaving platelet TxA2 intact). (B) LIPOXYGENASE pathway (5-lipoxygenase) - generates LEUKOTRIENES and LIPOXINS. Leukotrienes: produced by leukocytes and mast cells. LTB4: potent chemotactic for neutrophils, activates neutrophils (adhesion, ROS generation). Cysteinyl leukotrienes LTC4, LTD4, LTE4: intense vasoconstriction, bronchoconstriction, increased vascular permeability. 5-Lipoxygenase not inhibited by NSAIDs - leukotriene inhibitors (e.g., zileuton) and receptor antagonists (zafirlukast) used in asthma. Lipoxins: generated by cell-cell interactions, INHIBIT inflammation (anti-inflammatory counterpart) - inhibit neutrophil recruitment. Corticosteroids: inhibit COX-2, phospholipase A2, iNOS, and proinflammatory cytokines.<br><br><b>PLASMA-DERIVED (CLASS-DERIVED) MEDIATORS</b><br><br><b>The Complement System</b><br><br>Definition: Collection of more than 20 plasma proteins (numbered C1-C9 and others), produced mainly in the liver. Function: Host defense against microbes and pathologic inflammatory reactions. Participate in innate and adaptive immunity. Exist as inactive proforms activated by proteolytic cascades with tremendous amplification. Critical step: Proteolytic cleavage of C3 (third, most abundant component).<br><br><b>The Complement Cascade - Pathways</b><br><br>Three pathways leading to C3 convertase formation: (1) Classical pathway - triggered by C1 fixation to antibody (IgM or IgG) combined with antigen - sequential binding of C2 and C4 - forms C3 convertase - REQUIRES ANTIBODY. (2) Alternative pathway - triggered by microbial surface molecules (endotoxin/LPS, complex polysaccharides, cobra venom) IN THE ABSENCE OF ANTIBODY - spontaneous C3 cleavage amplified and stabilized on microbes. (3) Lectin pathway - plasma mannose-binding lectin binds carbohydrates on microbes - activates protease homologous to C1 - subsequent steps as in classical pathway - no antibody required. All three pathways form C3 convertase, which splits C3 into C3a (released) and C3b (covalently attached to target cell). C3b then binds with C3 convertase to form C5 convertase, which cleaves C5 into C5a (released) and C5b (cell-bound). C5b binds late components C6, C7, C8, C9 to form the Membrane Attack Complex (MAC) - composed of multiple C9 molecules - creates holes in cell membrane - water and ions leak - cell lysis (especially thin-walled bacteria like Neisseria). Functions of complement: (a) INFLAMMATION - C5a (most potent), C4a, C3a stimulate leukocyte recruitment, induce histamine release from mast cells (anaphylatoxins) - increase vascular permeability and vasodilation. Called anaphylatoxins (effects similar to mast cell mediators in anaphylaxis). (b) OPSONIZATION and PHAGOCYTOSIS - C3b and iC3b (inactive C3b) act as opsonins on microbial walls - recognized by neutrophil and macrophage receptors - promote phagocytosis. (c) CELL LYSIS - MAC on target cells causes osmotic lysis. Complement regulatory proteins: C1 inhibitor (C1 INH) - blocks C1 activation - deficiency causes hereditary angioedema. Decay-accelerating factor (DAF) and CD59 - GPI-anchored membrane proteins - DAF prevents C3 convertase formation, CD59 inhibits MAC formation - deficiency (loss of GPI anchor enzyme) causes paroxysmal nocturnal hemoglobinuria (PNH). Factor H - promotes inactivation of C3 convertase - mutations associated with hemolytic uremic syndrome (HUS) and wet macular degeneration.<br><br><b>The Kinin System</b><br><br>Kinins are vasoactive peptides derived from plasma proteins called kininogens by the action of specific proteases called kallikreins. Mechanism: Kallikrein cleaves high-molecular-weight kininogen (a plasma glycoprotein) to produce BRADYKININ. Functions of bradykinin: (a) Increases vascular permeability, (b) Contraction of smooth muscle, (c) Dilation of blood vessels, (d) Pain when injected into the skin. Effects similar to histamine. Action is short-lived - rapidly inactivated by kininase. Clinical significance: Implicated in some allergic reactions (anaphylaxis). Connection to coagulation: Factor XII (Hageman factor) activates both the coagulation cascade and the kinin system - links inflammation, coagulation, and fibrinolysis. Neuropeptides (substance P, neurokinin A): secreted by sensory nerves and leukocytes - transmit pain signals, increase vascular permeability - provide neural-immune cross-talk.