4. The complement system

Question 1

where are complement proteins produced

Answer 1

the liver

Question 2

what is opsonization

Answer 2

coating of the pathogen with antibodies for recognition by phagocytic cells

Question 3

complement proteins in circulation

Answer 3

circulate in their inactive forms, “activated” in the presence of pathogens or antibodies bound to pathogens

Question 4

3 different proteolytic pathways that lead to complement activation

Answer 4

first to act = alternative pathway
second to act = lectin pathway
third to act = classical pathway

Question 5

many complement proteins are proteases and zymogens, what does this mean

Answer 5

complement proteins are synthesized as their inactive forms (zymogens) and successively cleave each other to become activated (proteases)

Question 6

stages of complement activation

Answer 6

• pattern recognition trigger
• protease cascade amplification/C3 convertase
• 1 of 3 effector pathways (inflammation, phagocytosis or MAC)

Question 7

outcome of complement activation: inflammation

Answer 7

C3a and C5a recruit and activate leukocytes at site of infection

Question 8

outcome of complement activation: opsonization

Answer 8

C3b bound to microbe surface is recognized by C3b receptor on phagocyte which destroys the pathogen

Question 9

outcome of complement activation: membrane attack complex

Answer 9

insertion of complement proteins C5b-C9 into the pathogen membrane forms pores which disrupts the osmotic balance across the membrane and promotes pathogen death

Question 10

exceptions in complement protein nomenclature

Answer 10

• C2 product C2a is a larger fragment than C2b
• C1q, C1r and C1s are not cleavage products but distinct proteins that compose C1

Question 11

extra proteins in the alternative pathway

Answer 11

factor B (Ba and Bb) and factor D

Question 12

complement proteins were named by…

Answer 12

their order of discovery (C1-4-2-3-5…)
* C4 acts before C2 and 3 but it was discovered first

Question 13

which complement proteins are NOT in the alternative pathway

Answer 13

C1, 4 and 2

Question 14

which complement protein is NOT in the lectin pathway

Answer 14

C1

Question 15

lectin pathway initiation

Answer 15

initiated by mannose-binding lectin and ficolins
bind to carbohydrate structures on microbial surfaces
MASPs trigger the cleavage of complement routines (since there is no C1)

Question 16

classical pathway initiation

Answer 16

initiated when C1 either recognizes a microbial surface via CRP or directly binds to antibodies already bound to a pathogen

Question 17

alternative pathway activation

Answer 17

initiated by spontaneous hydrolysis and activation of C3

Question 18

which complement pathway boosts the adaptive immune system

Answer 18

classical pathway (involves antibodies)

Question 19

where do all 3 complement pathways converge

Answer 19

the generation of C3 convertase - C3 is cleaved leaving C3 bound to the microbial surface and releases C3a to trigger inflammation

Question 20

role of C3a and C3b

Answer 20

C3a: an anaphylatoxin - activates inflammatory response by triggering degranulation of cells that cause inflammation
C3b: covalently attaches to the pathogen surface and marks it for destruction (more susceptible to phagocytosis)

Question 21

when C3 convertase creates C3a and C3b it exposes a…

Answer 21

highly reactive thioester bond in C3b

Question 22

composition of C3 convertase in the different complement pathways

Answer 22

lectin and classical: C4b2a
alternative: C3bBb

Question 23

composition of C5 convertase in the different complement pathways

Answer 23

lectin and classical: C4b2a3b
alternative: C3b2Bb

Question 24

how to MBL and ficolins initiate the lectin pathway

Answer 24

• they form complexes with serine protease and recognize carbohydrates on protein surfaces
• their associated MASPs cleave complement proteins to start the pathway
• MBL: binds to mannose and fructose residues
• ficolins: bind oligosaccharides containing acetylated sugars

Question 25

how does C3 convertase result in the binding of many C3b to the pathogen surface

Answer 25

MASP-2 (or C1s) cleaves C4 into C4a and C4b - C4b binds to microbial surface C2 comes and MASP (or C1s) cleaves it - C2a joins and the C4b2a complex is created C4b2a is an active C3 convertase - can cleave up to 1000 C3 proteins

Question 26

what is the role of each proteins that makes up C1

Answer 26

C1q: binds to pathogen surfaces at CRP or indirectly to antibody, allowing auto-activation of C1r C1r: cleaves C1s to activate it C1s: cleaves C2 and C4

Question 27

what are the roles of Factor B and D in the alternative pathway

Answer 27

Factor D: protease that functions to activate factor B Factor B: protease zymogen which when activated functions as a subunit of C3 convertase

Question 28

why do we have the alternative pathway?

Answer 28

it is an amplification loop for C3b formation that is accelerated by properdin in the presence of pathogens more C3b = more opsonization = more phagocytosis

Question 29

what is properdin (factor P)

Answer 29

a plasma protein that enhances the activity of the alternative C3 convertase to aid in complement fixation

Question 30

different complement receptors and their ligands

Answer 30

CR3 and CR4: ligand is C3b and iC3b CR2: ligand is C3dg

Question 31

what is Factor I

Answer 31

a serine protease that inactivates C3b through its cleavage into iC3b - cannot function as a component of C3 convertase

Question 32

which complement receptor do macrophages express

Answer 32

CR1 - binds C3b to enhance recognition and phagocytosis

Question 33

what plasma proteins regulate complement activation

Answer 33

Factor H and Factor I: regulate cleavage of C3b

Question 34

how does factor H regulate complement activation

Answer 34

enhances the cleavage of C3b into iC3b by factor I binds to cell membranes by interacting with sialic acid which inhibits complement activation

Question 35

what membrane proteins regulate complement activation

Answer 35

1. DAF: breakdown the alternative C3 convertase 2. MCP: binds to C3b and enhances its cleavage to inactive iC3b by factor I *these proteins are expressed by human cells to inhibit C3b fixation because it would destruct the cell

Question 36

what is the membrane attack complex

Answer 36

complement proteins C5b-C9 work together to form holes in bacterial and eukaryotic membranes

Question 37

how is the MAC regulated

Answer 37

plasma proteins: S protein, clusterin and factor J inhibit C6 and C7 from binding to the membrane human cell-surface proteins: CD59 and HRF prevent C9 from binding to the membrane