Complement

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Complement
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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 56C
  for 30 min
• However it is now known that complement
  contributes to many other functions

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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

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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

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Proteins of the complementsystem (nomenclature)

• 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)

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Definitions

• 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

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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 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)
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Pathways of complement activation
LECTIN PATHWAY
CLASSICAL PATHWAY
ALTERNATIVE PATHWAY
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Components of the Classical Pathway

C4
C2
C3
C1 complex
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Classical Pathway Generation of C3-convertase
C4

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Classical Pathway Generation of C3-convertase
C2
C4a

_____ C4b2a is C3 convertase
Mg
C4b
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Classical Pathway Generation of C5-convertase
C2b
C4a

________ C4b2a3b is C5 convertase it leads into
the Membrane Attack Pathway
Mg
C3
C4b
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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
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Components of mannose-binding (lectin) pathway

• Mannose Binding Lectin (MBL),
• which binds to bacterial surface with
  mannose-containing polysaccharides
• MBL associated serine proteases (MASP-1 MASP-2)

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Components of mannose-binding lectin pathway
C4
MASP2
C2
MASP1
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Mannose-binding lectin pathway
_____ C4b2a is C3 convertase it will lead to
the generation of C5 convertase
C4
C2
MASP2
MASP1
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Components of thealternative pathway

• C3
• factors B D
• properdin (P)

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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 bacteria
• Certain viruses and parasites
• Aggregated immunoglobulina(particularly IgA)

the result is lysis of these organisms
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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
D
C3
B
P
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Degradation of spontaneously produced C3b
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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
This is stable C5 convertase of the alternative
pathway
D
P
B
C3
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C3b regulation on self and activator surfaces
C3b
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C5-convertase of the two pathways
C5-convertase of the Classical and lectin Pathways
C5-convertase of the Alternative Pathway
Bb
C3b
C3b
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Lytic pathway
Generation of C5 convertase leads to the
activation of the Lytic pathway
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Components of the lytic pathway
C6
C5
C 9
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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
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Biological properties of C-activation products
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Biological properties of C-activation products
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Biological properties of C-activation products
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Complement Deficiencies and DiseaseClassical
Pathway
Pathway Component Disease Mechanism
C1INH Hereditary Angioedema Overproduction of C2b (prokinin)
C1, C2, C4 Predisposition to SLE Opsonization of immune complexes help keep them soluble, deficiency results in increased precipitation in tissues and inflammation
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Complement Deficiencies and DiseaseLectin Pathway
Pathway Component Disease Mechanism
MBL Susceptibility to bacterial infections in infants or immunosuppressed 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.
Pathway Component Disease Mechanism
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