WEEK FOUR Flashcards
(22 cards)
L1. List the stages of cellular extravasation and migration.
◦ How does this relate to the vascular changes previously discussed?
LEUKOCYTE EXTRAVASATION
Adhesion molecules on endothelial cell surface and leukocyte bind together. Surface expression of adhesion molecules are mediated by chemical mediators, chemo-attractants and cytokines.
Leukocytes extravasation by:
1. Margination: Slowed blood flow pushes leukocytes towards vessel walls.
2. Rolling: Tumbling of leukocyte, subsequent adhesion to endothelium.
3. Pavementing: Endothelium accumulates white cells
4. Leukocyte pseudopods: Leukocytes insert into junctions between endothelial cells and traverse the basement membrane.
This relates to vascular changes as the vasculature as increased blood flow (from increased BP) and increased permeability must occur to permit leukocyte extravasation and migration. Leukocytes will subsequently use chemotaxis to locate the relevant site.
L1. Discuss adhesion molecules:
Selectins
Integrins
Mucin-like glycoproteins
Immunoglobulin family
L1. How do adhesion molecules facilitate the process of extravasation and migration?
IL-I and TNF are secreted by macrophages and endothelial cells to induce the expression of adhesion molecules. Endothelial cells begin to express E-selectin.
Leukocytes express L-selectins that weakly bind to E-selectins to permit tumbling of leukocytes along endothelium.
During pavementing, integrins with stronger affinity bind to leukocytes to stop rolling.
Chemokines act on leukocytes to stimulate cells to migrate through inter-endothelial spaces.
Leukocytes pierce the membrane and traverse via chemotactic gradient.
Leukocytes adhere via integrins to glycoproteins in the ECM to retain leukocytes in ECM.
L1. Describe the role of opsonins in phagocytosis. Give an example of an opsonin.
Opsinons: Antibodies (IgG), complement proteins (C3b) and lectins. Leukocytes have receptors for opsonins.
Opsinons coat particles which targets them for phagocytosis. The most efficient system for opsonisation is coating with IgG Ab.
L1. How are phagocytosed microbes killed and degraded?
Phagocytosis occurs in three steps.
- *1. Recognition and attachment of foreign particles**
- Phagocytes can engulf bacteria without specific receptor attachment.
- BUT phagocytosis still requires recognition of leukocyte receptors, which is then enhanced by opsinons.
- *2. Engulfment of particle**
- Binding of phagocyte to particle initiates active phagocytosis.
- Cytoplasm flows around targeted particle, forming a complete enclosure in phagosome.
- Phagosome fuses with lysosome to form phagolysosome.
- Discharge of lysosome contents into phagolysosome.
3. Killing and degradation of ingested particle.
Oxygen dependent and independent mechanisms.
- O2 dependent: Phagocytosis stimulates O2 consumption and glucose oxidation which produces ROS (hydrogen peroxide).
- O2 independent: Enzymes and lysosomes in phagocytic granules kill bacteria and degrade within phagolysosomes.
L1. What mechanisms are in place to control the acute inflammatory response?
- Short half-lives of inflammation mediators, only produced in quick bursts.
- Stop signals triggered by inflammation.
- Production of pro-inflammatory leukotrienes switched to anti-inflammatory leukotrienes.
- Neural impulses limit macrophage TNF production.
L1.
A: Chronic -> Lymphocytes (Chronic, cell mediated immunity). Macrophages (clean up cells). Distortion of cellular structures. B: Acute -> Many neutrophils (quick responders, mins-hours). RBC?
L3. Where are inflammatory mediators derived from and what is their overall function (in general terms)?
- Chemical inflammatory mediators are derived directly from cells or from plasma proteins.
- They function to coordinate inflammatory response by activating inflammation and coagulation, termination of response and initiating of repair.
L3. Name two types inflammatory mediators involved in the following: EXAM Q
◦ Vasodilation: Histamine (preformed). Prostaglandins (newly synthesised).
◦ Fever: Prostaglandins (newly synthesised), TNF.
◦ Pain: Prostaglandins (newly synthesised), bradykinin (of kinin system via Hageman factor).
◦ Chemotaxis: Chemokines, TNF. ◦ Increased vascular permeability: Histamine (preformed), C3a/C5a.
L3. In which organ are plasma-derived mediators produced primarily?
Plasma-derived mediators are primarily formed in the liver, via Hageman activation or complement activation.
Plasma-derived mediators circulate in inactive form, requiring cleavage for activation.
L3. Which three inter-related systems do plasma proteins belong to?
Plasma-derived mediators belong to 3 inter-related systems:
- *1. Complement:** Leukocyte chemotaxis/activation. Direct killing (MAC). Vasodilation.
- *2. Kinin:** Vascular permeability, vasodilation, smooth muscle contraction, pain.
- *3. Clotting systems:** Proteases activated during coagulation – Endothelial activation and leukocyte recruitment.
These systems therefore have cascade effects (amplification) post-cleavage.
L3. What is complement? Describe the three pathways that result in activation, including some of the enzymes involved.
The complement system ‘complements’ the ability of antibodies and phagocytic cells to remove pathogens from an organism.
Complement proteins become proteolytic upon activation, resulting in a powerful enzymatic amplification.
Complement activation results in increased vascular permeability, chemotaxis and opsonisation.
Complement activation occurs by Ab-Ag complexes or contact with microbial surface molecules.
COMPLEMENT PATHWAYS
There are three complement pathways, each requiring the activation of C3, the most abundant complement protein.
Each pathway generates variants of the protease C3 convertase.
C3 convertase Splits C3 into C3a + C3b.
C3b forms C5 convertase Splits C5 into C5a + C5b.
C5b forms MAC
Classical pathway – binding of C1 to Ab-Ag complexes. C1 binds with C4/C2 and is cleaved, forming C3 convertase, which acts on C3. Acts on Ab.
Alternate pathway – Binding of exotoxin or LPS. Activates other pathways in complement system – C3 is cleaved directly, and products bind to factors B and D, forming an alternative form of C3 convertase that acts on C3.
Lectin pathway – Plasma mannose-binding lectin binds to microbes, activating C1, binds with C4/C2 and is cleaved, forming C3 convertase, which acts on C3. Acts on mannose-binding lectin.
L3. What are some of the effects produced by complement when it becomes activated?
The activation of complement occurs by the critical activation of C3, the most abundant complement protein. Activation of complement permits the formation of anaphylotoxins (Complement fragments C3a, C4a, C5a).
Following this, C3 is cleaved into C3a/C3b, which eventually generates C5 that is cleaved into C5a/C5b.
C3a + C5a have roles in inflammation, C3b is involved in phagocytosis, and C5b is involved in the formation of the Membrane Attack Complex (MAC).
L3. Describe the roles of the following:
- C3a
- C3b
- C5a
- C5b
◦ C3a: Increases vascular permeability via histamine release from mast cells. Rolls in chemotaxis.
◦ C3b: Important opsinon that enhances neutrophil phagocytosis.
◦ C5a: Powerful chemotactic agent. Behaves as an anaphylotoxin (releases histamine/other chemical mediators from mast cells/basophils).
◦ C5b: Aids in MAC formation.
L3. How is the membrane attack complex (MAC) formed and what is its function?
The MAC complex is formed by C5b acting as an anchor point for C6-C8 assembly forming a single molecule.
This facilitates polymerisation of C9, with up to 18 C9 molecules.
Ultimately, the MAC complex forms a tube that is inserted into the cell membrane to permit cellular lysis by action of ion influx and H2O osmosis.
L3. Why is formation of C3 so crucial in protecting the body from micro-organisms?
MAC plays a critical role in immune defence. If there is no MAC complex (C6-C9) formed, the body is not severely immunodeficient but is more susceptible to some bacterial infections. The body is still able to undergo C3b opsoninisation, and can form anaphylotoxins (via C3a/C5b), which act to target and destroy bacterial cells.
If C3 cannot be formed or activation, the body is severely immunodeficient. This is due the inability of C3 to undergo proteolytic cleavage to form C3b (opsonininisation), which prevents the subsequent formation of anaphylotoxins C3a and C5a.
L3. What is the role of the kinin system? Which major product is produced when the kinin system is activated?
The kinin system functions to alter blood pressure and inflammation, via the formation of short-lived bradykinins; the most important kinin with functions in pain response and increased vasculature permeability.
Inflammation and coagulation are intertwined process, concurrently promoting each other.
- *Kinins** are vasoactive peptides, derived from plasma proteins Kininogens.
- *Kininogens** are activated by the action of Kallikreins to produce bradykinin.
Kallikrein is activated by Hageman Factor XII within the clotting cascade.
Upon exposure of collagen and basement membranes of damaged endothelium, the kinin system is activated.
This activates Hageman Factor XII and subsequent kinin cascades, producing Kallikreins, which cleaves a kininogen HMKW (High Molecular Weight Kininogen), resulting in the formation of bradykinins. Bradykinins subsequently function as a vasoactive amine to permit vasodilation and pain. The short-lived action of bradykinins function to increase vascular permeability, contraction of smooth muscle, and dilation of blood vessels.
Activation of the kinin system (inflammation) occurs concurrently with activation of the clotting cascade.
L3. What are the two pathways of the clotting cascade called? How is each activated and which tests are used to evaluate each pathway?
There are two pathways within the clotting cascade, the intrinsic (thrombin) and extrinsic (fibrin) pathways. Both pathways are activated via Hageman factor XII.
In real life, these pathways are not separate and occur concurrently.
Thrombin is derived from prothrombin, fibrin is derived from fibrinogen. Prothrombin and fibrinogen are both produced by the liver.
1. INTRINSIC CLOTTING PATHWAY
- Intrinsic clotting pathway is activated, not measured via Partial Thromboplastin Time (PTT)
Hageman Factor XII activates plasma proteins that result in clotting, leading to formation of thrombin (factor IIa).
Activated thrombin cleaves circulating soluble fibrinogen to form an insoluble fibrin clot.
2. EXTRINSIC CLOTTING PATHWAY
- Measured via Prothrombin time (PT)
Tissue factor acts on Hageman factor XII to form XIIa, that progresses into a common pathway with instrinsic pathway, to form a fibrin clot as in the intrinsic pathway.
L3. Formation of which products are the ultimate goal of the clotting cascade?
The clotting cascades ultimate goal is the formation of a fibrin clot. The fibrin clot functions to aid in tissue repair.
Histologically, the plasma proteins caught up in the fibrin mesh are eosinophilic, so will stain pink indicating tissue damage.
L3. What is Hageman Factor? What is its function?
HAGEMAN FACTOR XII
Produced in liver, circulates in inactive form until encounter with negatively charged surfaces collagen, basement membrane, activated platelets (endothelial injury) or endotoxins.
Factor CII transforms into XIIa, inducing the clotting cascade and activating the kinin cascade and fibrinolytic system.
L3. What is Thrombin and what is its role in the body?
Thrombin is an enzyme activated by the coagulation cascade.
Activated thrombin cleaves circulation soluble fibrinogen to generate an insoluble fibrin clot.
Thrombin is a major coagulation protease (cleaving fibrinogen into fibrin), triggering other inflammatory responses by feedback mechanism in coagulation cascades.
L3. What is the role of the fibrinolytic system and describe the role of plasmin?
The fibrinolytic system functions to remove the fibrin clot formed by the coagulation system in order to maintain homeostasis.
Hageman Factor XII concurrently activates the clotting cascade and fibrinolytic system.
The fibrinolytic system solubilises the fibrin clot by the action of plasmin. Plasmin also functions to activate/cleave C3 of the complement cascade. Plasmin also activates Hageman Factor XII.
Plasmin is involved in the production of fibrin split productions, which result in increased vascular permeability (inflammation).
