Complement

Complement: History

Discovered in 1894 by Bordet

It was used to refer to a heat-labile serum component

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

However it is now known that complement contributes to many other functions

Complement Functions

• Host benefit:– opsonization to enhance phagocytosis– phagocytes 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

Proteins of the complementsystem

Complement comprises over 20 different serum proteins

These proteins are produced by a variety of cells:

Hepatocytes, macrophages, gut epithelial cells

Some components can bind to Immunoglobulins

While others are proenzymes that when activated can cleave and activate other components

Proteins of the complementsystem (nomenclature)

5

C1(q r s), 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)

• 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.Cs

• 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

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 into 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)

EXCEPTION: 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

Components of the Classical Pathway

C4C2 C3

C1 complex

Ca++

C1r C1s

C1q

Ca++

C1r C1s

C1q

C4

C4a

b

Classical Pathway Generation of C3-convertase


C4b

Mg++

C4a

Ca++

C1r C1s

C1q

C2

C2ba

C2a

_____

C4b2a is C3 convertase


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

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Mannose Binding Lectin (MBL),

which binds to bacterial surface with mannose-containing polysaccharides

MBL associated serine proteases (MASP-1 MASP-2)

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

Components of thealternative pathway

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C3

factors B & D

properdin (P)

Control and regulation of the alternative pathway

Factor I & H DAF Complement receptor 1

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The alternative pathway can be activated by:

Many gram-negative bacteria:

Neisseria meningitidis and N. gonorrhoea

Some gram-positive bacteriaCertain viruses and parasitesAggregated immunoglobulina(particularly IgA)

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the result is lysis of these

organisms

Activation of the alternative pathway

Spontaneous hydrolysis of C3 to produce C3i C3i cleaves Factor B into Bb The C3iBb complex acts as C3 convertase(has

very short half life) Once C3b is formed Factor B binds to it and

becomes susceptible to cleavage by Factor D C3bBb is a more stable C3 convertase which

continues to generate more C3b(amplfication loop)

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Components of thealternative pathway

C3 B

D

P

Degradation of spontaneously produced C3b

C3b

C3b

iC3biC3b

II

C3dgC3dg

C3c C3c

C3b stabilization andC5 activation

When C3b finds the appropriate surface it binds to factor B, which is cleaved by Factor D to produce C3 convertase(C3bBb) (which is more stable)

C3 convertase(C3bBb) is further stabilized by Poperdin

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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

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

Generation of C5 convertase leads to the activation of the

Lytic pathway

Lytic pathway

Components of the lytic pathway

C6

C9

C8

C7C5

Lytic pathway

C5-convertase (of the Classical and lectin

Pathways or the Alternative Pathway) cleaves C5 into C5a & C5b

C5b associates with C6 &C7 and insrets into the cell membrane

Then C8 binds, followed by several molecules of C9

C9 molecules form a pore in the membrane C5bC6C7C8C9 is MAC

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Biological effects of C5a

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)



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



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

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