Flashcards in Lecture 25 - Complement Proteins Deck (55):
When and how were complement discovered?
A heat-labile factor in fresh serum that complements the function of antibodies is discovered
What are complement proteins?
Inactive proteins (often pro-enzymes, zymogens) in serum activated by proteolysis to carry out a range of immune functions
Complement protein functions
1) Bacterial, infected cell, foreign cell lysis
Number of C' proteins
What produce C' proteins?
3) Some epithelial cells
4) Neutrophils (less commonly)
Proportion of globin plasma that is C'
Protein component of plasma
How are C' activated?
Significance of 'a' and 'b' fragments of a C'
'a' is smaller fragment, 'b' is larger fragment.
Exception is C2, where 'a' is larger
Where does activation of C' often occur?
Surface of pathogen
How do host cells minimise self damage by C'?
Self cells have regulatory factors on their surfaces for reducing C' activity.
Pathogens lack these
Activity of soluble/fluid phase C'
Often transiently active, inactive.
Which C' pathway is an effector of humoral immunity?
Alternative pathway origins
Evolutionarily older than the lectin or classical pathways
Steps in C' cascade
1) 3 pathways for activation
2) Different pathways use different, but homologous components
3) Result in formation of different, but homologous C3 convertases
1) Cleavage of C3
2) Formation of C5 convertase
How can C3 be cleaved?
1) C3 convertase (C4b/C2a, C3b/Bb)
2) Spontaneous hydrolysis of C3 (tickover)
Types of C3 convertase
1) Classical/lectin - C4b/C2a
2) Alternative - C3b/Bb
Late steps (effector phase)
1) After C3 cleavage, C5 convertases are formed
2) C5 activation results in pore formation, inflammation, cell lysis
Common steps in complement activation
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)
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
Two types of C5 convertase
1) Classical/lectin - C4b/C2a/C3b
2) Alternative - C3b/Bb/C3b
Alternative pathway initiation
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
What cleaves B into Bb?
What stabilises C3 convertase on microbial surface in alternative pathway?
How is C5 convertase formed in the alternative pathway?
When C3 convertase cleaves C3, C3b joins C3b/Bb on cell surface, forms C5 convertase
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
1) Serine protease
2) Cleaves factor B when bound to C3b
How is alternative initial phase regulated?
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
Factor I and factor H role
Inactivate C3b in fluid phase
What initiates the classical C' pathway?
C1q binding to an antibody or C-reactive protein, bound to a pathogen
A compex of C1q (6x collagen-like tails, 6x globular heads), C1r and C1s (serine proteases)
What does C reactive protein bind?
Phosphocholine residues on bacterial surfaces
How is C1 activated?
C1q binding activates C1r to cleave C1s, which forms an active serine protease
IgGs that best activate classical pathway
Antibody that best activates classical pathway
Minimum number of Ig heavy chains that C1q must bind to activate classical pathway
Can soluble IgM activate classical pathway?
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
Classical complement pathway formation of C3 convertase
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
Classical pathway equivalent to alternative factor B
Classical pathway equivalent to alternative C3b
Classical and alternative formation of C5 convertase
1) C3 convertase cleaves C3 into C3a, C3b
2) C3b binds to C3 convertase, forms C5 convertase
Powerful inflammatory mediator
Lectin pathway formation of C3 convertase
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
What is MBL equivalent to?
Alternative pathway amplification loop
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
Formation of membrane attack complex
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
Bacteria that is controlled with C9 pore formation
Factors involved in forming classical C3 convertase
C1(q, r, s), C2a, C4b
Factors involved in forming lectin C3 convertase
MBL, MASP, C2a, C4b