immunology g9

8/2/2019 Immunology g9

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P G UPADHYAYA, KIMSMICRO,BANGALORE


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ComplementDefinition : series of heat-labile serum proteins

Site : serum and all tissue fluids except urine and CSF

Synthesis : in liver appear in fetal circulation during 1st 13 W

Function : Responsible for certain aspects of

immune response and inflammatory response

Activation : antigen-antibody complex or endotoxin, capsule

series of proteins activated sequentially

Inactivation: inhibitors in plasma (short lived)

Biological effects: either beneficial or harmful to hostP G UPADHYAYA, KIMSMICRO,BANGALORE


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Complement: History

Discovered in 1894 byBordet

It represents lytic activityof fresh serum

Its lytic activity is

destroyed whenheated at 56C for 30min



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Properties

Approximately 30 circulatory & membranebound proteins

complement refers to ability of these

proteins to complement, i.e., augment, theeffects of other components of immunesystem

Important component of innate hostdefenses

Synthesized in the liver and by cells

involved in the inflammatory response.P G UPADHYAYA, KIMSMICRO,BANGALORE


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Effects

Lysis : of cells such as bacteria,allografts, and tumor cells.

Generation of mediators that participatein inflammation and attract neutrophils

Opsonization, ie, enhancement of

phagocytosis



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Complement Functions

Host

detriment:Inflammation

Anaphylaxis

Figure-1. Complement has a central role in inflammationas it causes chemotaxis of phagocytes, opsonization

and lysis of pathogens and clearence of immunecomplexes.P G UPADHYAYA, KIMSMICRO,BANGALORE


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Proteins of the complementsystem (nomenclature)

C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9

factors B, D, H and I, properdin (P)

mannose binding lectin (MBL), MBL associatedserine proteases (MASP-1 MASP-2)

C1 inhibitor (C1-INH, serpin), C4-binding

protein (C4-BP), decay accelerating factor(DAF), Complement receptor 1 (CR1), protein-S (vitronectin)



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C-activation: alteration of C proteins such that theyinteract with the next component

C-fixation: utilization of C by Ag-Ab complexes

Hemolytic units (CH50): dilution of serum whichlyses 50% of a standardized suspension of Ab-coatedr.b.c.

C-inactivation: denaturation (usually by heat) of anearly C-component resulting in loss of hemolytic activity

Convertase/esterase: altered C-protein which acts

as a proteolytic enzyme for another C-component

Definitions



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Activation

several complements are pro-enzymes, whichmust be cleaved to form active enzymes.

Initiated either by antigen-antibody complexesor by a variety of non-immunologic molecules,

eg, endotoxin



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Activation product of complementproteins (nomenclature)

When enzymatically cleaved, the larger moiety,

binds to the activation complex or membraneand the smaller peptide is released in themicroenvironment

Letter b is usuallyadded to the larger,

membrane-binding, peptide and a to thesmaller peptide (e.g., C3b/C3a, C4b/C4a,C5b/C5a), EXCEPTC2 (the larger, membrane-binding moiety is C2a; the smaller one is C2b)

Activated component are usually over-lined: e.g.C1qrs



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Pathways of complementactivation

CLASSICALPATHWAY

ALTERNATIVEPATHWAY

activationof C5

LYTIC ATTACKPATHWAY

antibody

dependent

LECTINPATHWAY

antibodyindependent

Activation of C3 andgeneration of C5 convertase



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Importance

Alternative pathway is more important the firsttime we are infected by a microorganisms,because the antibody required to trigger theclassic pathway is not present.

Both pathways lead to the production of C3b,the central molecule of the complement

cascade. C3b has 2 important functions: (1) It combines

with other complement components to generateC5



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Importance of C3b

(1) It combines with other complement

components to generate C5 convertase, the

enzyme that leads to production of MAC.

(2) Opsonizes bacteria because phagocytes have

receptors for C3b on surfaces



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Components of the ClassicalPathway

C4C3

C1 complex

C1s is an enzyme and cleaves C4 and C2



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Classical PathwayGeneration of C3-convertase

Cleavage of C4 by C1s

produces C4a and C4b



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C4b

C4b binds C2 andC4b2 is cleaved by

C1s. C2b is releasedbut C2a remainsbound to C4b on thesurface. C4b2a is C3Convertase



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C4bC3b

________

C4b2a3b is C5 convertase;it leads into the MembraneAttack Pathway



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Biological Activities of ClassicalPathway Components

Component Biological Activity

C2b Prokinin; cleaved by plasmin to yield kinin, whichresults in edema

C3a Anaphylotoxin; can activate basophils and mastcells to degranulate resulting in increased vascularpermeability and contraction of smooth muscle cells,which may lead to anaphylaxis

C3b OpsoninActivation of phagocytic cells

C4a Anaphylotoxin

C4b OpsoninP G UPADHYAYA, KIMSMICRO,BANGALORE


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Control of Classical PathwayComponents

Component Regulation

All C1-inhibitor (C1-INH); dissociates C1r and C1s fromC1q

C3a C3a-inactivator (C3a-INA; Carboxypeptidase B)

C3b Factors H and I; Factor H facilitates the degradationof C3b by Factor I

C4a C3a-INHC4b C4 binding protein (C4-BP) and Factor I; C4-BP

facilitates degradation of C4b by Factor I; C4-BPalso prevents the association of C2a with C4b thusblocking formation of C3 convertase



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C1-inhibitor deficiency:hereditary angioedema



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Components of mannose-bindinglectin pathway

MBL MASP1

Pathogen



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Mannose-binding lectin pathway

MBL

_____

C4b2a is C3 convertase; itwill lead to the generation of

C5 convertaseMASP1

Binding to lectins cause autocatalytic activation

of MASPs which then cleave C4 & C2P G UPADHYAYA, KIMSMICRO,BANGALORE


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Components of thealternative pathway

C3



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Spontaneous C3 activation

C3

Generation of C3 convertase

fB activate fD which then cut fB releasing

Ba, while Bb becomes an active protease

C3Bb complex has a very short half life

b C3b



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bC3b

If spontaneously-generatedC3b is not degraded

C3-activationthe amplification loop

C3b



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C3b

C3b


C3bb



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C3b


C3bC3b



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Control of spontaneousC3 activation via DAF

C3bDAF prevents

the binding of

factor B to C3bAutologous cell membrane

CR1



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Control of spontaneousC3 activation via DAF

DAF dislodges

C3b-bound

factor Bb

bb C3b

Autologous cell membrane

CR1



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Autologous cell membrane

C3bC3b

iC3b

Control of spontaneousC3 activation via CR1

CR1CR1



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Degradation of spontaneouslyproduced C3b

C3bC3b

iC3biC3b C3dgC3dg

C3c C3c



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C3b stabilization andC5 activation

C3b

C3b finds an activator(protector) membrane

C3bb

This is stable C5 convertaseof the alternative pathway



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C3b regulation on self andactivator surfaces

C3b



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C5-convertase of the twopathways

C3b C3b

C5-convertase of theAlternative Pathway

C4bC3b

C5-convertase of theClassical and lectin

Pathways



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Generation of C5 convertaseleads to the activation of the

Lytic pathway

Lytic pathway



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Components of the lytic pathway

C6

C9

C7



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Lytic pathwayC5-activation

C3bC4b

b



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Lytic pathwayassembly of the lytic complex

b

C6

C7

C5b first binds C6 and then C7

from the plasma. Membrane bound

C5b67 recruits C8 and C9 to form

the Membrane Attack Complex (MAC)



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Lytic pathway:insertion of lytic complex into cell membrane

b

C6

C7

C9

C9

C9

C9C9

C9 C

9C9

C9



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Biological effects of C5a



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Product Biological Effects Regulation

Biological properties of C-activationproducts

C2b(prokinin) edema C1-INH

C3a(anaphylatoxin)

mast cell degranulation;enhanced vascular

permeability;anaphylaxis

carboxy-peptidase- B

(C3-INA)



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as C3, but lesspotent

(C3-INA)C4a(anaphylatoxin)

opsonization;phagocytosis

C4b(opsonin)

C4-BP,factor I

C3b

(opsonin)

opsonization;

phagocyte activation

factors H & I



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anaphylactic as C3, butmuch more potent;attracts & activates PMNcauses neutrophilaggregation, stimulationof oxidative metabolism

and leukotriene release

C5a(chemotacticfactor)

carboxy-peptidase-B(C3-INA)

C5b67 protein-Schemotaxis, attachesto other membranes



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Complement Deficiencies and DiseaseClassical Pathway

Pathway Component Disease Mechanism

C1INH HereditaryAngioedema

Overproduction of C2b(prokinin)

C1, C2, C4 Predispositionto SLE

Opsonization of immunecomplexes help keepthem soluble, deficiency

results in increasedprecipitation in tissuesand inflammation



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Complement Deficiencies and DiseaseLectin Pathway


MBL Susceptibility tobacterial infectionsin infants orimmunosuppressed

Inability to initiatelectin pathway



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Complement Deficiencies and DiseaseAlternative Pathway

Pathway/Component Disease Mechanism

Factors B or D Susceptibilityto pyogenic

(pus-forming)bacterialinfections

Lack of sufficientopsonization of bacteria

C3 Susceptibilityto bacterial

infections

Lack of opsonization andinability to utilize the

membrane attack pathwayC5, C6, C7 C8, orC9

Susceptibilityto Gram-negativeinfections

Inability to attack the outermembrane of Gram-negative bacteria



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Complement Deficiencies and DiseaseAlternative Pathway cont.


Properdin (X-linked) Susceptibility

meningococcalmeningitis

Lack of opsonization of

bacteria

Factors H or I C3 deficiency

andsusceptibility tobacterialinfections

Uncontrolled activation of

C3 via alternativepathway resulting indepletion of C3

immunology g9

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