Lecture 4

Question 1

What is opsonization?

Answer 1

Coating of a pathogen with antibodies and/or complement proteins so that it can be more readily taken and destroyed by phagocytic cells

Question 2

What is the complement system and which form of immunity is it involved in?

Answer 2

• system of soluble plasma proteins (complement proteins)
• act to opsonize and lyse pathogens
• innate immunity

Question 3

Where are complement proteins produced? Do they circulate in their active or inactive forms?

Answer 3

• In the liver
• circulate in inactive form, activated in the presence of pathogens or antibodies bound to pathogens

Question 4

What are the three different proteolytic pathways that lead to complement activation

Answer 4

• alternative
• lectin
• classical

Question 5

What is the significance of the many complement proteins being proteases?

Answer 5

They successively cleave and activate e/o

Question 6

What are the three general stages of complement action?

Answer 6

1. Pattern-recognition trigger: complement pathways triggered by proteins acting as PRRs
2. Protease cascafe amplification/C3 convertase: detection activates initial zymogen, triggering a cascade of proteolysis and amplifying the signal as the cascade progresses
3. Inflammation, Phagocytosis, Membrane Attack Complex

Question 7

What are the names of the complement proteins in order of their discovery?

Answer 7

C1, C4, C2, C3, C5, C6, C7, C8, C9

Question 8

What are the cleavage products of a complement protein like C3? Which cleavage product is bigger and which is smaller?

Answer 8

• C3a and C3b
• C3b is the larger fragment
• C3a is the smaller fragment

Question 9

What are the two exceptions to the general cleavage product rule of complement proteins?

Answer 9

1. C2 produces C2a and C2b, but C2a is bigger than C2b
2. C1q, C1r and C1s are not cleavage products of C1, but distinct proteins that compose C1

Question 10

What are two complement proteins that are exclusive to the alternative pathway?

Answer 10

• factor b
• factor d

Question 11

What does factor b cleave into?

Answer 11

• Ba (smaller)
• Bb (bigger)

Question 12

What are the three exceptional complement proteins that are not involved in all three complement pathways? Which pathway(s) are these proteins involved in?

Answer 12

• C1 - only Classical
• C2 - Lectin and Classical
• C4 - Lectin and Classical

Question 13

What initiate the lectin pathway?

Answer 13

• mannose-binding lectin (MBL)
• ficolins

Question 14

What initiates the classical pathway?

Answer 14

C1

Question 15

What initiates the alternative pathway?

Answer 15

spontaneous hydrolysis and activation of C3

Question 16

What step do the three pathways of complement activation converge on?

Answer 16

C3 cleavage

Question 17

What enzyme cleaves C3

Answer 17

C3 convertase

Question 18

What effector activities of the complement system does C3 directly or indirectly lead to?

Answer 18

• C3a and C5 recruit phagocytic cells to infection site to promote inflammation
• phagocytes with receptors for C3b engulf and destroy the pathogen
• all pathogens generate a C5 convertase that leads to formation of a membrane attack complex which disrupts cell membranes

Question 19

What type of protein is C3a and what are its functions?

Answer 19

an anaphylatoxin; activates inflammatory response by triggering degranulation of cells capable of inducing inflammation

Question 20

What type of protein is C3b and what are its functions?

Answer 20

an opsonin; covalently attaches (complement fixation) to the pathogen surface to mark the pathogen for destruction via phagocytosis

Question 21

What is the characteristic of C3b that allows it to act as an opsonin?

Answer 21

A highly reactive thioester bond that is revealed when C3 convertase cleaves C3

Question 22

What is the C3 convertase of the lectin pathway?

Answer 22

C4b2a

Question 23

What is the C3 convertase of the classical pathway?

Answer 23

C4b2a

Question 24

What is the C3 convertase of the alternative pathway?

Answer 24

C3bBb

Question 25

What is the C5 convertase for the lectin and classical pathways?

Answer 25

C4b2a3b

Question 26

What is the C5 convertase for the alternative pathway?

Answer 26

C3b2Bb

Question 27

In what order do the complement activation pathways act?

Answer 27

Alternative -> Lectin -> Classical

Question 28

What are the steps of the lectin pathway?

Answer 28

1. MBL and ficolin form complexes with MASPs and recognize particular carbs on pathogen sufaces 2. Activated MASP associated with MBL or ficolin cleaves C4 to C4a and C4b which binds to microbial surface 3. C4b binds C2 4. C2 cleaved by MASP to C2a and C2b 5. C2a forms a complex with C4 on the microbial surface (C4b2a complex) 6. C4b2a is an active C3 convertase 7. C4b2a cleaves C3 to C3b and C3a 8. C3b coats the microbial surface or binds to the convertase itself 9. One molecule of C4b2a can cleave many molecules of C3 and C3b will bind to the microbial surface to induce destruction of the pathogen

Question 29

What is the only difference between the Lectin pathway and the Classical pathway?

Answer 29

- instead of MBL and ficolins, the classical pathway uses C1 - C1q of the C1 protein binds directly to pathogen surfaces or indirectly to antibody bound to pathogens which allows the autoactivation of C1r - C1r cleaves C1s to active protease - C1s cleaves C4 and C2

Question 30

What are the steps of the alternative pathway?

Answer 30

1. C3 is spontaneously hydrolysed into iC3 (iC3 production is accelerated in presence of pathogen) 2. Factor B binds to iC3 3. Factor D cleaves factor B into Bb (which stays bound to iC3) and Ba 4. iC3Bb is a C3 convertase (but not the convertase of the alternative pathway) that can help amplify the C3 cleavage amplification loop 5. factor B binds to C3b bound to the pathogen surface 6. factor B is cleaved by factor D 7. C3bBb is formed - the activated alternative pathway C3 convertase 8. C3bBb carries out C3 cleavage activities (C3b acting as an opsonin and C3a as an anaphylatoxin

Question 31

The alternative pathway is an amplification loop for C3b formation that is accelerated by ____ in the presence of pathogens

Answer 31

properdin

Question 32

What is properdin?

Answer 32

- a plasma protein - enhances activity of alternative C3 convertase to aid in complement activation and fixation

Question 33

The alternative pathway of complement activation can ____ the classical or the lectin pathway by _____

Answer 33

amplify; forming an alternative C3 convertase (iC3Bb) and depositing more C3b molecules on the pathogen

Question 34

iC3b is a ligand for ____

Answer 34

CR3 and CR4

Question 35

C3dg is a ligand for ___

Answer 35

CR2

Question 36

What is factor I and what are its mechanisms?

Answer 36

- serine protease - inactivates C3b by cleaving it into a smaller fragment (iC3b) which cannot function as a component of C3 convertase

Question 37

Macrophages express ____ which binds to C3b allowing for enhanced pathogen recognition and phagocytosis

Answer 37

CR1

Question 38

What is Factor H and what are its mechanisms?

Answer 38

- plasma protein - enchances cleavage of C3b into iC3b by Factor I - binds to cell membranes by interacting with sialic acid (common carb on eukaryotic cells)

Question 39

What is one way some bacteria (such as S. aureus and S. pyogenes) have evolved to avoid complement activation

Answer 39

They incorporate sialic acid on their surfaces, a common carbohydrate on eukaryotic cell surfaces

Question 40

To ensure that C3b fixation on host cell surfaces dows not result in targeting these cells for destruction, human cells express ____ and ____ membrane proteins that inhibit complement.

Answer 40

- DAF (decay-accelerating factor) - MCP (membrane cofactor protein)

Question 41

What is the mechanism of DAF (decay-accelerating factor)?

Answer 41

Breakdown of the alternative C3 convertase to ensure host cells are not targeted for destruction

Question 42

What is the mechanism of MCP (membrane cofactor protein)?

Answer 42

Binds to C3b and enhances its cleavage into inactive iC3b by factor I

Question 43

What 5 complement proteins form the membrane attack complex?

Answer 43

C5, C6, C7, C8, C9

Question 44

What is the initiating factor in the formation of the membrane attack complex?

Answer 44

C5b

Question 45

How does the membrane attack complex work?

Answer 45

complement proteins that work in concert to form holes in bacterial and eukaryotic membranes

Question 46

What regulates the membrane attack complex?

Answer 46

S protein, clusterin, Factor J and human cell-surface proteins (CD59, HRF)

Question 47

What are the functions of C3a and C5a

Answer 47

- anaphylatoxins - act on blood vessels to increase permeability and cell-adhesion molecules - allows fluid leakage from blood vessels and extravasation of Igs and complement molecules - migration of macrophages, neutrophils also increased

Question 48

What happens when complement system malfunctions?

Answer 48

- abnormal clearance of bacteria and fungi - hypersensitivity responses and autoimmune disorders (lupus) due to improper clearance of soluble immune complexes - increased infection rate due to malfunction of MAC