Complement: History Discovered in 1894 by Bordet It represents lytic activity of fresh serum Its...

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Complement: History

Discovered in 1894 by Bordet

It represents lytic activity of fresh serum

Its lytic activity destroyed when heated at 56C for 30 min

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Complement functions

• Host benefit:– opsonization to enhance phagocytosis– phagocyte attraction and activation– lysis of bacteria and infected cells– regulation of antibody responses– clearance of immune complexes– clearance of apoptotic cells

• Host detriment:– Inflammation, anaphylaxis

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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 associated serine 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 they interact with the next component

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

• Hemolytic units (CH50): dilution of serum which lyses 50% of a standardized suspension of Ab-coated r.b.c

• C-inactivation: denaturation (usually by heat) of an early 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 product of complement proteins (nomenclature)

When enzymatically cleaved, the larger moiety, binds to the activation complex or membrane and the smaller peptide is released in the microenvironment

Letter “b” is usually added to the larger, membrane-binding, peptide and “a” to the smaller peptide (e.g., C3b/C3a, C4b/C4a, C5b/C5a), EXCEPT C2 (the larger, membrane-binding moiety is C2a; the smaller one is C2b)

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

Pathways of complement activation

CLASSICALPATHWAY

ALTERNATIVEPATHWAY

activationof C5

LYTIC ATTACKPATHWAY

antibodydependent

LECTINPATHWAY

antibodyindependent

Activation of C3 andgeneration of C5 convertase

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Components of the Classical Pathway

C4C2 C3

C1 complex

Ca++

C1r C1s

C1q

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Ca++

C1r C1s

C1q

C4

C4a

b

Classical Pathway Generation of C3-convertase

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C4b

Mg++

C4a

Ca++

C1r C1s

C1q

C2

C2ba

C2a

_____

C4b2a is C3 convertase

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C4b

Mg++

C4a

Ca++

C1r C1s

C1q

C2b

C2a

C3

C3a

b

________

C4b2a3b is C5 convertase; it leads into the Membrane

Attack Pathway

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Biological Activities of Classical Pathway Components

Component Biological Activity

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

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

C3b Opsonin

Activation of phagocytic cells

C4a Anaphylaotoxin

C4b Opsonin

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Control of Classical Pathway Components

Component Regulation

All C1-inhibitor (C1-INH); dissociates C1r and C1s from C1q

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

C3b Factors H and I; Factor H facilitates the degradation of C3b by Factor I

C4a C3a-INH

C4b C4 binding protein (C4-BP) and Factor I; C4-BP facilitates degradation of C4b by Factor I; C4-BP also prevents the association of C2a with C4b thus blocking formation of C3 convertase

Components of mannose-binding lectin pathway

C4

MBL C2 MASP1

MASP2

Mannose-binding lectin pathway

C4

MBL

C4b

C4a

C4b

C2

C2b

C2a

C2a

_____

C4b2a is C3 convertase; it will lead to the generation of

C5 convertaseMASP1

MASP2

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Components of thealternative pathway

C3 B

D

P

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Spontaneous C3 activation

C3

H2O

iB

D

Generation of C3 convertase

C3iBb complex has a very short half life

b C3

C3a

b

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B

D

bC3b

If spontaneously-generated C3b is not degraded

C3-activationthe amplification loop

C3C3a b

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C3a

B

D

BbC3b

C3 b


C3b

C3a

b

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C3a

C3a BbC3b

C3bC3 BbB

D

bb

C3a


C3b

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C3a

C3a BbC3b

BbBbC3b

C3a


C3bC3b

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C3a

C3a BbC3b

BbBb

C3a


C3bC3b

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Control of spontaneousC3 activation via DAF

C3b

DAF prevents

the binding of

factor B to C3b

B

Autologous cell membrane

DA

F

CR1

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Control of spontaneousC3 activation via DAF

DAF dislodges

C3b-bound

factor Bb

B bb C3b


DA

F

CR1

B b

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C3b C3b

Bb

H

I

iC3b

Control of spontaneousC3 activation via CR1

B b

I

iC3b DA

F

CR1DA

F

CR1

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Degradation of spontaneously produced C3b

C3b

C3b

iC3biC3b

II

C3dgC3dg

C3c C3c

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C3b stabilization andC5 activation

C3b

C3b finds an activator (protector) membrane

C3

C3a

bB

D

b

PThis is stable C5 convertase

of the alternative pathway

C3b regulation on self and activator surfaces

C3b

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C5-convertase of the two pathways

C3b Bb C3b

C5-convertase of the Alternative Pathway

C4b C2a C3b

C5-convertase of the Classical and lectin

Pathways

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Generation of C5 convertase leads to the activation of the

Lytic pathway

Lytic pathway

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Components of the lytic pathway

C6

C9

C8

C7C5

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Lytic pathwayC5-activation

C3b C2 aC4b

C5 b

C5a

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Lytic pathwayassembly of the lytic complex

C5 b

C6

C7

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Lytic pathway:insertion of lytic complex into cell membrane

C5 b

C6

C7C8

C9

C9

C9

C9C9

C9 C

9C9

C9

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Neutrophil

Neutrophil

Activation

AdhesionNeutrophil

Neutrophil

Chemotaxis

Respiratory burst

Monocyte

Activation

Cytokineproduction

Mastcell

Degranulation

Vascular permeability

Vascular wall transmigration

Biological effects of C5a

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Product Biological Effects Regulation

Biological properties of C-activation products

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 less potent

(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, but much more potent;attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release

C5a (chemotactic factor)

carboxy-peptidase-B (C3-INA)

C5b67 protein-Schemotaxis, attaches to other membranes

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Complement Deficiencies and DiseaseClassical Pathway

Pathway Component Disease Mechanism

C1INH HereditaryAngioedema

Overproduction of C2b (prokinin)

C1, C2, C4 Predisposition to SLE

Opsonization of immunecomplexes help keepthem soluble, deficiencyresults in increasedprecipitation in tissuesand inflammation

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Complement Deficiencies and DiseaseLectin Pathway


MBL Susceptibility to bacterial infections in infants orimmunosuppressed

Inability to initiate lectin pathway

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Complement Deficiencies and DiseaseAlternative Pathway

Pathway/Component Disease Mechanism

Factors B or D Susceptibility to pyogenic (pus-forming) bacterial infections

Lack of sufficient opsonization of bacteria

C3 Susceptibility to bacterial infections

Lack of opsonization and inability to utilize the membrane attack pathway

C5, C6, C7 C8, or C9

Susceptibility to Gram-negative infections

Inability to attack the outer membrane of Gram-negative bacteria

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Complement Deficiencies and DiseaseAlternative Pathway cont.


Properdin (X-linked) Susceptibility meningococcal meningitis

Lack of opsonization of bacteria

Factors H or I C3 deficiency and susceptibility to bacterial infections

Uncontrolled activation of C3 via alternative pathway resulting in depletion of C3

Complement: History Discovered in 1894 by Bordet It represents lytic activity of fresh serum Its...

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