Lecture 25 - Complement Proteins Flashcards Preview

Immunology > Lecture 25 - Complement Proteins > Flashcards

Flashcards in Lecture 25 - Complement Proteins Deck (55):
1

When and how were complement discovered?

1900.
A heat-labile factor in fresh serum that complements the function of antibodies is discovered

2

What are complement proteins?

Inactive proteins (often pro-enzymes, zymogens) in serum activated by proteolysis to carry out a range of immune functions

3

Complement protein functions
1)
2)
3)

1) Bacterial, infected cell, foreign cell lysis
2) Chemotaxis
3) Inflammation

4

Number of C' proteins

Over 30

5

What produce C' proteins?
1)
2)
3)
4)

1) Hepatocytes
2) Macrophages/monocytes
3) Some epithelial cells
4) Neutrophils (less commonly)

6

Proportion of globin plasma that is C'

10%

7

Globin plasma

Protein component of plasma

8

How are C' activated?

Enzymatic cascade

9

Significance of 'a' and 'b' fragments of a C'

'a' is smaller fragment, 'b' is larger fragment.

Exception is C2, where 'a' is larger

10

Where does activation of C' often occur?

Surface of pathogen

11

How do host cells minimise self damage by C'?

Self cells have regulatory factors on their surfaces for reducing C' activity.
Pathogens lack these

12

Activity of soluble/fluid phase C'

Often transiently active, inactive.

13

Which C' pathway is an effector of humoral immunity?

Classical pathway

14

Alternative pathway origins

Evolutionarily older than the lectin or classical pathways

15

Antibody-independent pathways

Lectin, alternative

16

Steps in C' cascade
1)
2)
3)

1) Initiation
2) Early
3) Late

17

Initiation
1)
2)
3)

1) 3 pathways for activation
2) Different pathways use different, but homologous components
3) Result in formation of different, but homologous C3 convertases

18

Early stages
1)
2)

1) Cleavage of C3
2) Formation of C5 convertase

19

How can C3 be cleaved?
1)
2)

1) C3 convertase (C4b/C2a, C3b/Bb)
2) Spontaneous hydrolysis of C3 (tickover)

20

Types of C3 convertase
1)
2)

1) Classical/lectin - C4b/C2a
2) Alternative - C3b/Bb

21

Late steps (effector phase)
1)
2)

1) After C3 cleavage, C5 convertases are formed
2) C5 activation results in pore formation, inflammation, cell lysis

22

Common steps in complement activation
1)
2)
3)

1) C3 convertase cleaves C3 (C4b/C2a or C3b/Bb)
2) C3 is cleaved, C3a is an inflammatory mediator, C3b binds to the surface of microbe (acts as an opsonin)
3) C3 convertases form the C5 convertases (C4b/C2a/C3b or C3b/Bb/C3b)

23

How does C3b bind to microbial surface?

Cleavage exposes reactive thioester groups on C3b
Reactive thioester groups bind amino and hydroxyl groups on microbial surface

24

Two types of C5 convertase

1) Classical/lectin - C4b/C2a/C3b
2) Alternative - C3b/Bb/C3b

25

Alternative pathway initiation
1)
2)
3)
4)
5)
6)

1) Low-levels of C3 hydrolysis initiate formation of active intermediates
2) Intermediates cleave C3 to C3a and C3b. In fluid, C3b is short-lived
3) C3b thioester group is revealed, C3b binds to microbial surface
4) B cleaved by factor D to Bb
5) C3b and Bb form C3 convertase on microbial surface
6) Properdin binds and stabilises C3 convertase on microbial surface

26

What cleaves B into Bb?

Factor D

27

What stabilises C3 convertase on microbial surface in alternative pathway?

Properdin

28

How is C5 convertase formed in the alternative pathway?

When C3 convertase cleaves C3, C3b joins C3b/Bb on cell surface, forms C5 convertase

29

Factor Bb
1)
2)
3)

1) Active form of factor B
2) Cleaved by factor D
3) Forms alternative C3 convertase with C3b, and alternative C5 convertase with two C3b's

30

Factor D
1)
2)

1) Serine protease
2) Cleaves factor B when bound to C3b

31

How is alternative initial phase regulated?
1)
2)

1) Factors I and H rapidly degrade C3b in fluid phase
2) C3b can bind host-cell surfaces, but is rapidly inactivated by complement regulatory proteins on these cells

32

Factor I and factor H role

Inactivate C3b in fluid phase

33

What initiates the classical C' pathway?

C1q binding to an antibody or C-reactive protein, bound to a pathogen

34

C1 structure

A compex of C1q (6x collagen-like tails, 6x globular heads), C1r and C1s (serine proteases)

35

What does C reactive protein bind?

Phosphocholine residues on bacterial surfaces

36

How is C1 activated?

C1q binding activates C1r to cleave C1s, which forms an active serine protease

37

IgGs that best activate classical pathway

IgG1, IgG3

38

Antibody that best activates classical pathway

IgM

39

Minimum number of Ig heavy chains that C1q must bind to activate classical pathway

2

40

Can soluble IgM activate classical pathway?

No

41

How does IgM/IgG activate classical pathway?

Binds to pathogen, this results in a conformational change.
IgM Fc region involved in C1q binding is exposed

42

Classical complement pathway formation of C3 convertase
1)
2)
3)
4)
5)
6)
7)
8)

1) C1q binds antibody
2) C1r2, C1s2 are activated
3) C4 binds activated C1q
4) Activated C1s cleaves C4 into C4a, C4b
5) C4b binds covalently to cell surface
6) C4b binds C2
7) C2 is cleaved by C1s
8) C4b/C2a C3 convertase formed

43

Classical pathway equivalent to alternative factor B

C2

44

Classical pathway equivalent to alternative C3b

C4b

45

Classical and alternative formation of C5 convertase
1)
2)

1) C3 convertase cleaves C3 into C3a, C3b
2) C3b binds to C3 convertase, forms C5 convertase

46

C5a

Powerful inflammatory mediator

47

C3a

Inflammatory mediator

48

Lectin pathway formation of C3 convertase
1)
2)
3)
4)
5)
6)
7)

1) MBL binds to sugars on microbial surface
2) MBL-associated serine protease (MASP) binds to collagen-like region of MBL
3) MASP cleaves C4 into C4a, C4b (from here on, the same as classical pathway)
4) C4b covalently binds to cell surface
5) C4b binds C2
6) C2 cleaved by MASP into C2a, C2b
7) C4b and C2a form C3 convertase

49

What is MBL equivalent to?

C1q

50

Alternative pathway amplification loop
1)
2)
3)
4)

1) C3b produced by any pathway can interact with factors B and D
2) C3b binds to cell surface with thioester region
3) Factor B is cleaved by factor D
4) Bb binds to C3b, forms C3 convertase

51

Formation of membrane attack complex
1)
2)
3)
4)
5)
6)
7)

1) C5b bound to cell membrane
2) C5b recruits C6, C7 (hydrophobic)
3) C5b/C6/C7 insert into cell membrane, recruit C8
4) C8 is a trimer. 1 unit inserts into cell
5) C5b/C6/C7/C8 capable of transiently lysing cells
6) C5b/C6/C7/C8 casuses polymerisation of C9
7) Polymerised C9 forms a 100 Angstrom hole

52

Bacteria that is controlled with C9 pore formation

Neisseria spp

53

Factors involved in forming classical C3 convertase

C1(q, r, s), C2a, C4b

54

Factors involved in forming lectin C3 convertase

MBL, MASP, C2a, C4b

55

Factors involved in forming alternative C3 convertase

C3b, Bb, factor D (cleaves B)