Complement

Question 1

What is complement?

Answer 1

• group of proteins whose functions complement the antigen binding function of antibodies
• essentially a proteolytic cascade
• leads to the disposition or fixing of complement components to the pathogen surface

Question 2

What does the complement system consist of? function?

Answer 2

serum and cell surface proteins that interact with one another and with other molecules of the immune system in a highly regulated manner to generate products that function to eliminate microbes

Question 3

Where are complement components made? Where do they circulate? What are they?

Answer 3

• made in liver
• circulate in plasma
• many are enzymes (proteases) that circulate in an inactive form (zymogen) and activate complement (fixation)

Question 4

What are the major functions of complement?

Answer 4

• opsonization
• direct lysis of target cells
• chemoattraction and activation of inflammatory cells
• can move in the direction towards high complement concentration

Question 5

What are the three complement pathways?

Answer 5

1. classical- antibody binds to specific antigen on pathogen surface
2. lectin- mannose binding lectin binds to pathogen surface
3. alternate- pathogen surface creates local environment conducive to complement activation

Question 6

What happens after complement is activated?

Answer 6

C3b covalently is bound to surface components of pathogen

Question 7

After C3b is bound, what are the results that lead to the death of the pathogen?

Answer 7

1. recruitment of inflammatory cells
2. opsonization of pathogens, facilitating uptake and killing by phagocytes
3. perforation of pathogen cell membrane

Question 8

Difference of proteins bound in the classical vs the alternate pathways of complement? lectin?

Answer 8

classical- C1, C2, C4

alternate- Bb

lectin- MASP 1, MASP 2, no C1

Question 9

In the classical path, what does C1 consist of? function of each?

Answer 9

Three proteins:

• C1q- stable association with IgG or IgM (have receptors for C1q)
• C1r- activate C1s
• C1s- proteolytic activity, cleaves complement components

Question 10

How is C1 activated in the classical path?

Answer 10

the receptors of C1q become available following conformational changes that take place on at least 2 antibody molecules each of which binding two epitopes on a multivalent antigen

Question 11

Where specifically does C1q bind?

Answer 11

binds to the Fc portion of antibody

Question 12

How does C1r activate C1s?

Answer 12

cleaves C1s to make it an active protease -serine protease

Question 13

What does C1s do?

Answer 13

cleaves C2 and C4 -serine protease

Question 14

What are the basic steps to the classical path of activating complement?

Answer 14

1. activation of C1
2. activation of C4
3. activation of C2
4. activation of C3
5. activation of C5
6. membrane attack complex
7. binding of C8
8. polymerization of C9
9. target cell lysis

Question 15

Explain step by step process of how C1 is activated?

Answer 15

• C1q binds the Fc portion of antibody and is activated
• C1q being bound activates C1r
• C1r cleaves C1s and activates it
• activated C1s has proteolytic activity on C4

Question 16

Explain step by step process of how C4 is activated? what does C4 bind to?

Answer 16

• activated C1s cleaves C4 into C4b and C4a
• C4b binds to the cell surface near the Ag-Ab complex
• C4b also attaches C2

Question 17

Explain step by step process of how C2 is activated? what does it associate with? what is the complex it forms?

Answer 17

• cleaved by the action of C1s and C4b
• remains associated with C4b
• active complex of C4b-C2a is called C3 convertase

Question 18

Explain step by step process of how C3 is activated? what is important about this step? where does C3b go?

Answer 18

• C3 is activated by C3 converts and is split into C3a and C3b -important amplification step
• C3b becomes attached to the target cell membrane
• C3 most abundant form in serum

Question 19

Explain step by step process of how C5 is activated? what is C5 convertase composed of?

Answer 19

• C5 convertase splits C5 into C5a and C5b
• C5 convertase = C4b-C2a-C3b-C5

Question 20

Explain step by step process of how the membrane attack complex is formed? function?

Answer 20

• C5b binds to the target cell membrane and binds stoichiometrically to C6 and C7
• C5b, C6, C7 through conformational changes, inserts into the lipid bilayer
• structural- no enzymatic activity left

Question 21

Binding of C8 step? results?

Answer 21

• C5b, C6, C7 also focuses the activities of C8 and C9
• association of C8 results in stable membrane spanning complex formation and facilitates the formation of a pore by C9 polymerization
• exposes hydrophobic region that inserts into cell membrane

Question 22

polymerization of C9 step? results?

Answer 22

• the final hit that leads to lysis is caused by the polymerization of C9, a perforin like molecule, around the C5b, C6, C7 channel
• polymerization results in the formation of transmembrane channels
• disrupts cell integrity and results in cell death

Question 23

What is the final step in the classic path?

Answer 23

target cell lysis

Question 24

How is the alternate path initiated? C3b?

Answer 24

• C3 is spontaneously hydrolyzed in plasma at a low level, products are unstable and rapidly degraded
• when C3b produced by any method binds to microbial surfaces, degradation is prevented
• more primitive process

Question 25

Describe the process of alternative path?

Answer 25

1. spontaneous cleavage of C3 to C3b 2. C3b is prevented from binding normal hosts by regulatory proteins, so it binds the microbe 3. on the microbe, C3b combines with serum factor B to form a complex which is further activated by factor D 4. factor D cleaves factor B while it is attached to C3b to form C3b-Bb complex 5. C3b-Bb acts as C3 convertase (can also form by classical path, then interact with B, alternative) 6. additional C3 molecules are cleaved by C3 convertase 7. C3b binds to the C3b-Bb (convertase) and C5 to make C5 convertase 8. Membrane attack complex 9. bind C8 10. polymerize C9 11. cell lysis

Question 26

What are three other biologic functions of complement?

Answer 26

1. anaphylatoxins 2. chemotaxins 3. opsonization

Question 27

What are anaphylatoxins?

Answer 27

- substance that induces degranulation of mast cells and basophils release of histamine - they are certain complement fragments that promote an inflammatory response via binding to complement receptors on mast cells and triggering release of histamine, which increases blood vessel permeability and smooth muscle contraction - C3a, C5a, C4a

Question 28

What does C5a stimulate?

Answer 28

inflammation

Question 29

What are chemotaxins?

Answer 29

- substances that attract phagocytic cells and cause their migration from areas of lower concentration to areas of higher concentration - C5a potent chemoattractant for neutrophils- many critical steps happen early - C3a and C5a generated no matter what path is taken

Question 30

Opsonization in complement?

Answer 30

- coating of a particular antigen by antibody or complement components renders the cell more attractive to phagocytic cells - attachment is facilitated by specific receptors on the phagocytic cell surface - CR1 (complement receptor) binds to the opsonin fragments C3b and C4b and promotes phagocytosis (direct)

Question 31

Role of Types 1, 3, and 4 complement receptors?

Answer 31

phagocytosis

Question 32

Role of Type 2 complement receptor?

Answer 32

- trap antigens in germinal centers - CR2, CD21