LMP299Y1 Lecture Notes - C5-Convertase, Fibrin Degradation Product, Complement Component 3

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Inflammatory Mediators
Virtually anything could be an inflammatory mediator
If given substance X, would characteristics/tests would tell us it is an inflammatory mediator?
Inject substance X, does it induce inflammatory response?
Should be able to see it at the site of inflammation
Found at sites of tissue damage
Injection of purified substance induces inflammatory response
Inflammatory effects of this substance inhibited by antagonist or receptor antagonist
Biological effects of these mediators
Prolonged inflammation can damage tissue thus short-lived
Where they are often found? In blood if not rapidly inactivated, will not have local inflammation to
keep inflammation local
Stimulate target cells to produce secondary mediators amplification of the response
Vasodilation to increase blood flow (increase delivery of mediators and leukocytes to site of tissue injury
or infection)
Increased vessel permeability (allow mediators into tissue)
Emigration of leukocytes into tissues
These called vascular events of inflammation
Smooth muscle cells line the smooth muscle walls
Exogenous inflammatory mediators
Chemnotaxin recruitment of leukocytes
Endogenous mediators products of host released in response to the pathogen plasma derived,
meaning they are inactive until cleaved
Cell derived in response to pathogen
Plasma derived mediators largely produced by liver, circulate in plasma as INACTIVE precursor
Factor 12 important activator of all four inflammatory cascades that form the
Inactive until in contact with collagen, BM, and activated platelets this happens when there is damge
to vessel wall, pathogen, some tissue injurty that exposes BM
Co-factor as well, together cleave so 12a is active inflammatory mediator BOUND TO VESSEL WALL,
LOCALIZED TO SITE OF TISSUE INJURY can activate all four arms of plasma derived mediators
First system
Activated factor 12a cleaves prekallikrein into kallikrein
Kallikrein cleaves HMWK to form bradykinin, the active inflammatory mediator of kinin system
Bradykinine major component of bee venome, wasp venom that’s why they hurt
Activate fibinoloytic pathway activates other plasma derived mediators
Plasma-derived inflammatory mediators
The coagulation system
Clotting cascade forms protective protein mesh over sites of injurty mediated by the protein
Variety of factors can activate thrombin cascade
Extrinsic pathway that involves tissue factor
Intrinsic pathway involves factor 12 a activate factor 10a 10a cleaves prothrombin to produce
thrombin cleaves soluble plasma fibrinogen to produce fibrin, fibrin aggregates and forms blood clot
can also act as inflammatory mediator by par1 receptor induce other cells to produce inflammatory
Other inflammatory mediators production can also be induced
Plasma derived 4
Fibrinolytic system acts in opposition to coagulation system
Plasmin protein derived from plasminogen breaks down fibrin clots thrombin makes thrombin clots
fibrin degradation products can have plasma permeability effects
Another effect is can cleave complement c3
Amplifying the process
Complement cascade
Fourth system
Circulate as pro-enzymes in plasma
C3 cleaved by enzyme to form peptides C3 A and B
This is the case for all complement proteins A is soluble, has inflammatory effects
B is the active fragments, cleaves the next zymogen in the cascade
Activation occurs on pathogen surface active enzyme localize complement response inflammatory
effect of the complement cascade include c3a and c5a these are soluble mediators they are often
called anaphylatoxins
Act on blood vessels to increase vascular permeability c5a is also chemoattractant for neutrophils to
site of infection c3b is opsonin is a molecule that binds to pathogen and targets it for phagocytosis
and then c5b to 9 forms MAC
Complement cascade three arms on the left three different pathways for activation of complement
Microbe has C3b deposited
Formation of c3 convertase is important for effector production
C3 convertatse critical for effector production
Generation of C3 converstase on the left
Generation of effectors on the right
C3b opsonin binds to the pathogen, targets it for phagocytosis and then also downstream is MAC
Complement cascade
Lead to generation of C3 convertase
Triggered enzyme cascades
Classical pathway C1 activated when antibody binds to antigen on pathogen; C1 is soluble, binds to
this complex, undergoes conformation change that activates it; now activated C1 C4 can now bind to
C1; makes C4a soluble; C4b binds to the pathogen; when C4b is bound to pathogen, now C2 can bind;
activating C1 can cleave C2, get C2a; C2b remains bound to pathogen; C4b2b bound to surface (this is
the classical C3 convertase covalently linked to pathogen surface can cleave many C3, over a
C3 when cleaved, forms C3a which is soluble C3b can bind to pathogen as opsonin or it can bind to
C3 convertase to make C4b2b3b (this is C5 convertase)
C5b initiates formation of MAC
This General pathway is true for all three pathways just the initiating aspect that is different
For example, lectin pathway it is the mannose-binding lectin binds to mannose residue; human
mannose are often covered by other residues, not available for lectin to bind;
Alternate pathway in this case, there is no involvement of the C1 spontaneous hydrolysis of these
proteins in the blood C3 binds to microbial surfaces polysachcardies and becomes spontaneously
hydrolyzed to form C3a and C3b once bound to pathogen, other factors in plasma, like factor B that is
cleaved by D leading to C3bBb
This only happens on pathogens, not host cells, because there are host regulatory protines that can
stabilize the complex
Properdin will stabilize C3b complex if bound to pathogen, NOT IF BOUND TO HOST CELL
CL1 proteins will dissociated this from a host cell; decay-accelerating factor wil dissociate this from a
host cell
Membrane Attack Complex
After the C5 convertase was formed cleaved C5 into C5a and b
C5b associates with C6 and C7 this whole complex binds to pathogen membrane via C7 C8 then
binds, inserting itself into cell membrane; then C9 binds many molecules of C9, forming pore in the
Disrupt bacterial membrane lead to destruction of bacterial membrane
This cannot happen to host cells regulatory proteins that prevent (CD59) that inhibits binding of C9
Interactions between kinin, fibrin, coagulation, and complement cascades
Factor 12 binding to collagen can activate kinin and clotting cascade
Kini cascade can activate fibrinolyit csystem act in opposition to each other to induce formation and
breakdown of clots
Plasmin can activate C3 to produce C3a all four of these pathways
There is feedback amplifccation; can initiate production of inflammatory mediators
PLASMA to FIBRIN arrow should be inhibitory BREAKING DOWN FIBRIN
Vasoactive amines histamine, serotonin
Histamine is a vasoactive amine formed by decarboxylation of L-histidine
Mast cells
Stain of WBC smear; with basophil, mast cells stains dark purple filled with granules for release
Black circles granules full of histamine
Histamine relase from cultured mast cells stimulated with complement proteins and bradykinin
Had cultured mast cells look for substances that will cause release of these granules
C5a , C3a, and braykinin causes releases of histamine