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Why does inflammation occur?

- injurious stimuli cause a protective vascular connective tissue reaction called "inflammation"
-- dilute
-- destroy
-- isolate
-- initiate repair


What forms of inflammation are there?

- acute and chronic forms


What are the key components of inflammation?

- cells: endothelial cells, migratory cells such as neutrophils
- mediators: many chemicals released
- immune system: innate and acquired


What is the chemical mediator theory?

- chemical substances called mediators, released from injured or activated cells coordinate the development of the inflammatory response


What are chemical mediators?

- generated either from cells or from plasma proteins
- activated in response to various stimuli (inactive form or more produced)
- one mediator can stimulate the release of other mediators. Such cascades provide mechanisms for amplifying - or, in certain instances, counteracting - the initial action of a mediator
- mediators vary in their range of cellular targets
- once activated and released from the cell, most of these mediators are short-lived (don't want to keep promoting acute response inappropriately)
- mediators constitute a self regulated system of checks and balances that regulate their actions


Why does acute inflammation occur?

- prevent and limit injury, infection: protection
- interact with components of the adaptive immune system
- prepare the area of injury for healing


What are the immediate and early responses to tissue injury?

1. vasodilation (leads to redness/heat)
2. vascular leakage and edema
3. leukocyte migrations (mostly PMNs = neutrophils)


What is vasodilation?

Brief arteriolar vasoconstriction followed by vasodilation
- expansion of capillary bed
- increased blood flow
- accounts for warmth and redness
- opens microvascular 'beds'
- increased intravascular pressure causes an early transudate (protein-poor filtrate of plasma) into interstitium (vascular permeability still not increased yet)


What is vascular permeability?

- transudate gives way to exudate (protein-rich)
- increases interstitial osmotic pressure contributing to edema (water and ions)


What mechanisms cause vascular permeability?

- many mechanisms known to cause vascular leakiness/endothelial retraction or damage
-- histamines**, bradykinins, leukotriens cause an early, brief (15-30min) immediate transient response in the form of endothelial cell contraction that widens intercellular gaps of venules
-- cytokine mediators (TNF, IL-1) induce endothelial cell junction retraction through cytoskeleton reorganisation (4-6 hrs post injury, lasting 24 hours or more)


What might severe injuries cause in regards to vascular permeability?

severe injuries may cause:
- immediate direct endothelial cell damage (necrosis, detachment) making them leaky until they are repaired (immediate sustained response)
- or may cause delayed as in thermal or UV injury or some bacterial toxins (delayed prolonged leakage)


What is increased transcytosis?

- certain mediators (VEGF) may cause increased transcytosis via intracellular vesicles which travel from the luminal to basement membrane surface of the endothelial cell


What is immune cell mediated endothelial damage?

- marginating and endothelial cell-adherent leukocytes may pile-up and damage the endothelium through activation and release of toxic oxygen radicals and proteolytic enzymes (destroy pathogens but can destroy endothelial cells)


Through what sequence of events do leukocytes leave the vasculature?

1. margination and rolling
2. adhesion and transmigration
3. chemotaxis and activation


What are leukocytes free to do once they leave the vasculature?

- phagocytosis and degranulation
- collateral damage: leukocyte-induced tissue injury


What are the kinetics of oedema and cellular infiltration?

- oedema first, rapid
- neutrophils rapid but later
- macrophages slower, peak later


What is the mechanism of margination and rolling?

-with increased vascular permeability, fluid leaves the vessel causing leukocytes to settle-out of the central flow and "marginate" along the endothelial surface
- P-selectin and E-selectin on the endothelial cell interact with the Integrins and Sialyl-Lewis X-modified glycoproteins on the neutrophil (receptor-ligand interaction)
- P-selectins and E selectins in high numbers on the inner endothelial surface only around the site of injury (retracted in normal endothelial cells)
- once neutrophils see these receptors they begin to slow down and begin this rolling behaviour until they interact with more and more receptors and once there is a 'threshold of triggering'
- cells change to migrate through using another type of receptor (PECAM-1/CD31): pushes the cells through the gaps between the endothelial cells
- some release of chemokines trigger change in cell behaviour response


What is early rolling adhesion mediated by?

- the selectin family
- e-selectin (endothelium), p-selectin (platelets, endothelium), bind other surface molecules that are upregulated on endothelium by cytokines (TNF, IL-1) at injury sites
- p-selectin in cell in Weibel-Palade bodies but not on cell surface - stimulated to surface by histamine and thrombin


What is adhesion?

- rolling comes to a stop and adhesion results
- other sets of adhesion molecules participate:
-- endothelial: ICAM-1, VCAM-1
-- Leukocyte: LFA-1, Mac-1, VLA-4
(ICAM-1 binds LFA-1/Mac-1, VCAM-1 binds VLA-4)
- ordinarily down-regulated or in an inactive conformation, but inflammation alters this (main triggers are TNF, IL-1) - very specific receptor-ligand interaction occurring
- cells that do not have correct receptors/ligands will not bind


What is transmigration (diapedesis)?

- occurs after firm adhesion within the systemic venules and pulmonary capillaries via PECAM-1 (CD31) (present on neutrophils and endothelial cells)
- must then cross basement membrane
-- collagenases
-- integrins


How do cells know where to go?

- leukocytes follow chemical gradient to site of injury (chemotaxis)
-- soluble bacterial products
-- complement components (C5a)
-- cytokines (chemokine subfamily e.g. IL-8)
-- Leukotrienes
-chemotactic agents bind surface receptors inducing calcium mobilisation and assembly of cytoskeletal contractile elements

- extend pseudopods with overlying surface adhesion molecules (integrins) that bind ECM during chemotaxis

When undergoing chemotaxis the neutrophil develops a front and a back - becomes polarised


What happens to macrophages (and to some extent neutrophils) once they come in contact with chemotaxins?

undergo activation:
- prepare AA metabolites from phospholipids
- prepare for degranulation and release of lysosomal enzymes (oxidative burst)
- regulate leukocyte adhesion molecule affinity as needed


What are chemical mediators?

- histamine: mast cells, basophils, platelets
- serotonin: platelets
Newly synthesised:
- prostaglandins: all leukocytes, mast cells
- leukotrienes: ","
- platelet-activating factor: ", EC
- Reactive oxygen species: "
- Nitric oxide: macrophages, EC
- Cytokines: macrophages, lymphocytes, EC, mast cells
- Neuropeptides: leukocytes, nerve fibres

Plasma derived:
-C3a/5a (anaphylotoxins),
-C3b, C5b-9 (membrane attack complex)
Factor XII (Hageman factor) activation
- kinin system (bradykinin)
- coagulation/fibrinolysis
many in "pro-form" requiring activation (enzymatic cleavage)


What are the major chemical mediator systems in inflammation?

Plasma proteins
i) complement
ii) kinin system
iii) clotting system
iv) fibrinolytic system (system used to get rid of blood clot/return system to normal)

Vasoactive amines

Arachidonic Acid Metabolites

Cytokines and Chemokines


What are the major features of chemical mediators?

- may or may not utilise a specific cell surface receptor for activity
- may also signal target cells to release other effector molecules that either amplify or inhibit initial response (regulation)
- are tightly regulated
-- quickly decay (AA metabolites), are inactivated enzymatically (kininase), or are scavenged (antioxidants)


What are the plasma mediator systems initiated by coagulation factor XII?

Factor XII = Hageman factor
- involved in blood clot mechanism
- kinin cascade that ultimately produces bradykinin that is an important mediator
- clotting cascade: most important factor here in terms of inflammation is thrombin
- fibrinolytic system
- complement cascade: C3 and C5 are the only complement proteins to have a direct influence on the inflammatory response
- integration of all these pathways --> feed into one another/influence each other


What is the clotting cascade?

Cascade of plasma proteases
- factor XII (Hageman factor)
- activated platelets converts XII to XIIa (active form)
- ultimately XIIa activates thrombin which in turn converts soluble fibrinogen to insoluble fibrin clot
- factor XIIa simultaneously activates the "brakes" through the fibrinolytic system to prevent continuous clot propagation - you want the clot to be in a defined region (opposing mechanism)
- components role in inflammation: thrombin regulates leukocyte adhesion and fibroblast proliferation


What is the kinin system?

leads to formation of bradykinin from cleavage of precursor (HMWK = high MW kininogen)
- vascular permeability
- arteriolar dilation
- non-vascular smooth muscle contraction (e.g. bronchial smooth muscle)
- causes pain
- rapidly inactivated (kininases)
- intimate association with clotting pathway


What is the complement system?

- extremely complex
- normally used to attack a pathogen
- in this context the important part for the inflammatory response is C5a and C3a
- recruitment and activation of white blood cells
- involved in the destruction of microbes


What are vasoactive amines?

- vasodilation and venular endothelial cell contraction
- junctional widening
- released by mast cells
- basophils
- platelets in response to injury (trauma, heat, cold)
- preformed in mast cell granules - connective tissue adjacent to blood vessels, also basophils and platelets


What is the role of nitric oxide?

- sort-acting soluble free-radical gas with many functions
- produced by endothelial cells, macrophages
- causes:
-- vascular smooth muscle relaxation and vasodilation
-- kills microbes in activated macrophages
-- counteracts platelet adhesion, aggregation, and degranulation


What is the role of arachidonic acid metabolites (eicosanoids)?

- prostacyclin and thromboxane have opposing effects: vasodilation vs vasoconstriction
- COS is the target of aspirin and other NSAIDs (non steroidal anti-inflammatory drug)
- Leukotrienes: via lipoxygenase pathway; are chemotaxins, vasoconstrictors, cause increased vascular permeability, and bronchospasm
- PAF (platelet activating factor): derived also from cell membrane, phospholipid, causes vasodilation, increased vascular permeability, increases leukocyte adhesion (integrin conformation)


What do steroids inhibit in the arachidonic acid pathway?

- steroids are inhibitors of the whole downstream pathway - more potent


What do aspirin and other NSAIDs inhibit in the AA pathway?

- COX-1 and COX-2 (cyclooxygenase)
- still have the ability of the other pathways


What are cytokines?

- proteins that act via specific cell surface receptors, as a message to other cells modulating cell function
- IL-1, TNF-alpha and -beta, IFN-gamma are especially important in inflammation
- increase endothelial cell adhesion molecule expression, activation and aggregation of PMNs and many other immune roles


What are strategies for inhibiting cytokines?

- reduce cytokine producing cells (e.g. with cytostatics)
- inhibitory cytokines (e.g. IL-10)
- inhibitors of signal transduction (e.g. cyclosporin)
- regulation of gene expression (e.g. glucocorticoids) (glucocorticoids have widespread effects, often in negative ways, so important to try and find other drugs to deal with inflammation)
- reduction in circulating cytokines (e.g. monoclonals, soluble receptors)
- receptor blockade (e.g. antagonists or monoclonals)


What are lysosomal components in relation to mediators of inflammation?

- leak from PMNs and macrophages after cell death
- acid proteases (only active within lysosomes)
- neutral proteases such as elastase and collagenase are destructive in ECM
- counteracted by serum and ECM anti-proteases


What are some of the systemic effects of acute inflammation?

- one of the easily recognised cytokine-mediated (esp. IL-1, IL-6, TNF) acute-phase reactions including
-- anorexia
-- skeletal muscle protein degradation
-- hypotension

- elevated white blood cell count


What are natural mediators which suppress inflammation?

- ACTH, GCs - products of the HPA axis
- Cytokines such as IL-10
- Lipocortin-1 (Annexin-1)