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MIIM30002 - Principles of Immunology > Lecture 25 - Complement Activation 1 > Flashcards

Lecture 25 - Complement Activation 1 Flashcards

1
Q

Where is complement located?

A
  • In the serum

* In the inactive form (pro-enzyme)

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2
Q

In general, how is C’ activated?

A

Proteolysis

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3
Q

What are the generalised effector functions of C’?

A
  1. Lysis of cells (MAC formation)
  2. Opsonisation (Enhanced phagocytosis)
  3. Inflammation (recruitment of lymphocytes)
  4. B cell activation
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4
Q

Which cells can by lysed by C’?

A

• Bacteria
• Infected cells
• Transplanted cells
etc.

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5
Q

How many different C’ proteins are there?

A

More than 30

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6
Q

Where are C’ proteins produced?

A
  • Hepatocytes
  • Monocytes & Macrophages
  • Some epithelial cells
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7
Q

Is C’ a major component of the blood?

A

Yes

They make up 10% of the ‘globulin’ fraction (i.e. the protein component) of the blood

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8
Q

Describe the generalised process of the complement cascade

A

→ Trigger
S → T

Active T converts:
U → V

Active V converts:
W → X

Active X converts:
Y → Z

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9
Q

What is the nomenclature of C’ protein?

A

C1, C2 etc.

Products of a cleaved protein:
 • 'a': smaller fragment
 • 'b': larger fragment
e.g. 
C3 → C3a + C3b
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10
Q

Physically, where does activation of the C’ system occur?

A

On the surface of cells (pathogens or others)

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11
Q

How are host cells protected from C’?

A

Cell surface proteins
• Present on healthy host cells
• Absent on microbes

These proteins regulate C’ activation to minimise host damage

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12
Q

Soluble C’ components are often…

A

Inactive

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13
Q

Describe the mechanisms of C’ activation

A
  1. Classical pathway
    • Ag-Ab complexes
    • IgM & IgG
  2. Alternative pathway
    • Ab independent
    • C’ directly binds to bacterial surfaces
  3. Lectin pathway
    • Mannan binding lectin (MBL)
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14
Q

Which pathway of C’ activation is evolutionarily older?

A

Alternative pathway

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15
Q

What are the steps in C’ activation?

A
  1. Initiation
    • 3 different pathways of activation
    • Formation of a C3 convertase complex
  2. Early steps
    • Cleavage of C3
    • Formation of the C5 a convertase complex
3. Late steps
 • Cleavage of C5
 • Effector mechanisms:
- lysis
- MAC formation
- inflammation
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16
Q

What is the outcome of initiation of C’ activation?

A

Formation of C3 convertase

The different pathways create different convertases, but they are homologous

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17
Q

What are the common steps in C’ activation?

A

These steps are the same, regardless of which pathways has been initiated

  1. C3 cleavage
    C3 → C3a + C3b
  2. C5 cleavage
    C5 → C5a + C5b
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18
Q

What type of molecule is C3a?

Describe its function

A

Inflammatory mediator

Recruitment of inflammatory cells

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19
Q

What type of molecule is C3b?

Describe its function

A

Opsonin

  • Binds to the microbe surface
  • Phagocytes bind C’ receptors, and engulf the pathogen
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20
Q

What are the different mechanisms of C3 cleavage?

A

Specific molecules act as convertases

  1. Classical / lectin pathway
    • C4bC2a
  2. Alternative pathway
    • C3bBb
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21
Q

Describe the different mechanisms of C5 cleavage

A

The C3 convertases join with C3b to form the C5 convertases

  1. Classical / lectin pathway
    • C4b2aC3b
  2. Alternative pathway
    • C3bBbC3b
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22
Q

Describe the following steps of the alternative pathway:
• Initiation
• Early steps
• Late steps

A

“Tickover”

– Initiation –

  1. Natural low-level hydrolysis of C3
    C3 → C3a + C3b

If there are microbes present:
2. C3b binds to microbial surfaces

– Early steps –

  1. ‘B’ binds C3b on microbial surface
  2. B cleaved by Factor D
    B → Bb + Ba
  3. C3b + Bb come together to form a C3 convertase
  4. Properdin binds and stabilises C3bBb (the C3 convertase) on the microbial surface
  5. C3 convertase cleaves C3:
    C3 → C3a + C3b

– Late steps –

  1. C3b binds C3 convertase to form C5 convertase
    C3bBb + C3b → C3bBbC3b
  2. Convertase cleaves C5
    C5 → C5a + C5b
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23
Q

How does C3b bind to microbes?

A

C3b: Thioester bonds

Microbial surface: amino and hydroxyl groups

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24
Q

What cleaves ‘B’?

A

Factor D

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25
Q

What is the structure of the C3 convertase in the following pathways:
• Classical
• Lectin
• Alternative

A

Classical
• C4bC2a

Lectin
• C4bC2a

Alternative:
• C3bBb

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26
Q

What is the structure of the C5 convertase in the following pathways:
• Classical
• Lectin
• Alternative

A

Classical:
• C4bC2aC3b

Lectin
• C4bC2aC3b

Alternative:
• C3bBbC3b

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27
Q

What is the function of Factor D?

A

Cleaves B:
B → Bb + Ba

When B is bound to C3b

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28
Q

What is the function of Factor B?

A

Inactive form

Factor B binds to C3b on the microbe surface in the alternative pathway

Factor B is then cleaved by Factor D:
B → Bb + Ba

29
Q

What is Factor Bb?

A

Active form of Factor B

Formed by cleavage of Factor B by Factor D

Forms part of the C3 convertase

30
Q

What is the function of Properdin?

A

Binds and stabilises C3 convertase on the microbial surface in the alternate pathway

→ stabilisation of the C3 convertase

31
Q

How is the alternative pathway regulated?

A
  1. Factor I & Factor H
    • C3b is rapidly degraded by these factors
  2. Complement regulatory proteins
    • When C3b binds healthy host cells it is rapidly inactivated by these proteins
32
Q

What degrades C3b?

A

Factor I and Factor H

33
Q

What is the “Pathogen sensing complex”?

A

C1 complex

Involved in initiation of the Classical pathway

34
Q

Describe how C1q is activated

A 
Activated by binding:
1. IgM or IgG 
 • binds heavy domain
 • Ig has bound pathogens already
 • Must bind at least two Ig heavy chains
  1. CRP
    • CRP binds bacterial surfaces at phosphocholine residues
35
Q

Describe the classical pathway
• Initiation
• Early steps
• Late steps

A

– Initiation –

  1. C1q binds CRP or Ig on bacterial surface
  2. C1r cleaves C1s to form an active serene protease (C1s)

– Early steps –

  1. C4 binds Ig-bound C1q
  2. Activated C1s cleaves C4
    C4 → C4b + C4a
  3. C4b binds covalently to the surface of pathogen
  4. C4b binds C2
  5. C2 cleaved by C1s
    C2 → C2a + C2b
  6. C2a associates with C4a to form C3 convertase:
    C4bC2a
  7. Cleavage of C3 by C4bC2a
    C3 → C3a + C3b

– Late steps –

  1. C3b that has just been cleaved binds to C4bC2a to form C5 convertase:
    C4bC2aC3b
  2. Cleavage of C5 by C4bC2aC3b
    C5 → C5a + C5b
36
Q

Which Ig best activates C1q?

A

In order of efficacy:
IgM
IgG1, IgG3

37
Q

What is the structure of the C1 complex?

A 
Composed of:
 • C1q (6 strands)
- recognises foreign stuff
- collagen-like tails
- globular heads

• C1r(2)
• C1s(2)
- serine proteases involved in activation of the complex

38
Q

What are the Ig requirements for C1 complex activation?

A
  • C1 complex must bind at least two Igs

* Ig must be bound to the bacterial surface

39
Q

Why must Ig be bound to the surface of the pathogen to activate the C1 complex?

A 
Soluble IgM (pentamer):
 • Inaccessible Fc

Soluble IgG:
• Fc portions not adjacent

Antigen bound IgM:
• There was a conformational change
• Fc exposed

Antigen-bound IgG:
• Fc portions adjacent

40
Q

What allows C1 to bind IgM when bound to the surface?

A

Binding stimulates a conformational change in IgM

Fc portion is now exposed

41
Q

Which part of the C1 complex actually binds Ig?

A

C1q globular heads

42
Q

What is active C1s?

How does it become activated?

A

Active serine protease

Activation:

  1. Binding of C1q to Ig
  2. C1r cleaves C1s
  3. C1s now active
43
Q 
What are the homologous comounds for:
 • Factor B
 • Factor Bb
 • C4bC2a
 • C4bC2aC3b
A

Factor B = C2

Factor Bb = C2a

C3 convertase:
C3bBb = C4bC2a

C5 convertase:
C3bBbC3b = C4bC2aC3b

44
Q

What type of molecule is C5a?

Describe its function

A

Powerful inflammatory mediator

Recruits inflammatory cells

45
Q

Describe the lectin pathway

A

– Initiation –

  1. MBL binds sugars on the surface of microbes with lectin domains
  2. MASP associated with MBL cleaves C4
    C4 → C4a + C4b
  3. C4b binds covalently to surface of pathogen
  4. C2 binds C4b and is cleaved:
    C2 → C2a + C2b
  5. C2a associates with C4b to form the C3 convertase
  6. C3 cleaved by C4bC2a
    C3 → C3a + C3b

– Late steps –

  1. C3b associates with C4bC2a to form C5 convertase:
    C4bC2aC3b
  2. Cleavage of C5 by C4bC2aC3b
    C5 → C5a + C5b
46
Q

Where is MBL normally found?

What is it similar to?

A

In the serum

Similar to C1q

47
Q

What is MASP?

A

MBL-associated serine protease

48
Q

What is the homologous factor to C1s in the lectin pathway?

A

MASP (MBL-associated serine protease)

49
Q

Describe how C3b can feedback positively on itself

A

C3b produced by any of the three pathways at the cleavage stage can interact with Factors B and D

→ more C3b produced through the alternative pathway

50
Q

Is C’ heat stable or heat labile?

A

Heat labile

Only active at 37°

51
Q

How is amplification achieved in the C’ cascade?

A

A signal molecule will activate many of the next molecule in the cascade, and so on and so forth

52
Q

Which C’ pathway is formally an effector mechanism of humoral immunity?

A

Classical pathway

Because it involves Ag-Ab immune complexes

53
Q

Describe the structure of C1q

A

6 collagen-like tails

6 globular heads
• bind Fc on Ab that have bound a pathogen

54
Q

What can C1q bind?

A
  • Ab that have bound pathogens

* CRP

55
Q

How does CRP bind pathogens?

A

CRP binds phosphocholine residues on bacterial surfaces

56
Q

What is the structure of MBL?

A

(Mannose binding lectin)

MASP1 and 2
• Serine proteases

Collagen like domain

Lectin domains:
• Globular heads

57
Q

Where is MBL found normally?

A

In the serum

58
Q

What does MBL bind?

Which bit of MBL binds?

A

Sugars found on the surface of microbial cells (and not human cells)

The Lectin domains (globular heads) bind the sugars

59
Q

Compare the molecule that performs cleavage of C4 in the following pathways:
• Classical
• Lectin pathway

A

Classical: C1s

Lectin: MASP

60
Q

Is there any cross over between the Classical/lectin pathways and the Alternative pathway?

A

Yes

C3b produced through the Classical / Lectin pathways can go back and feed the Alternative pathway

61
Q

What happens to the cleavage products of C5?

A

C5b: Formation of MAC

C5a: Inflammation

62
Q

Describe the process of formation of the MAC

A
  1. Cell bound C5b from common cleavage step
  2. C5b binds C6 and C7
  3. C5bC6C7 inserts into the cell membrane
  4. C8 binds, one of its subunit inserts into membrane

→ transient lysis of cells

  1. C5bC6C7C8 causes polymerisation of C9

→ formation of MAC

63
Q

What are the physical properties of C6 and C7?

A

Hydrophobic

64
Q

What is the structure of C8?

A

Trimer

65
Q

How big is the pore of the MAC?

A

100 Angstroms

Made up of poly-C9

66
Q

How does the MAC bring about cell death?

A
  • Osmotic rupture

* Triggers apoptosis

67
Q

What is the function of C5bC6C7C8?

A

Transiently able to lyse cells (to some degree)

More importantly, leads to the polymerisation of C9 to form the MAC

68
Q

Discuss the role of CRP in the C’ cascade

A

CRP bound to microbial surfaces can trigger the classical pathway (like IgM and IgG)

  1. CRP bound to phosphocholine residues on microbial surfaces
  2. C1q binds CRP and is activated
  3. C1(r2s2) activated to become active serine protease
  4. C4 binds C1q bound to CRP
  5. C1s cleaves C4
  6. C4b binds covalently to microbe surface
  7. C2 binds C4b
  8. C2 cleaved by C1s to form C2a
    – C3 convertase formed –
  9. C4bC2a cleaves C3 to form C3a and C3b
  10. C3b binds C4bC2a to form C5 convertase
  11. C5 cleaved to form C5a and C5b
    → effector functions
69
Q 
Describe how the following bind to the surface of the pathogen:
 • MBL
 • C3b
 • C4b
 • CRP
A

MBL:
• Lectin binding domains bind to mannose (sugars) on the pathogen surface

C3b:
• Thioester on C3b binds hydroxyl and amino groups on pathogen surface

C4b:
• Binds covalently to pathogen surface

CRP:
• Phosphocholine on pathogen

MIIM30002 - Principles of Immunology (30 decks)

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