Lec 5 - Innate II - Complement

Question 1

what are complements? general

Answer 1

soluble proteins
that cooperate w/ BOTH innate + adaptive immune systems (to eliminate pathogens, dying cells + immune complexes from the body (ex. Abs bound to Ag)

Question 2

what are complements? specific (w/ naming patterns)

Answer 2

1. proteases in blood & other fluids

naming:
2. either called ‘C’ (most), followed by number (and lowercase letter) ex. C3a
3. or called ‘factor’, followed by capital letter. ex. factor B

Question 3

what is a protease?

Answer 3

enzyme that performs proteolysis (breaks down proteins)

Question 4

how are protease numbered?

Answer 4

based on when discovered

Question 5

where are complement proteins produced?

Answer 5

liver

Question 6

what is the purpose of complement?

Answer 6

set off chain reaction (initiate signalling cascade) that helps clear pathogens

Question 7

what are the 3 key mechanisms of action of complement?

Answer 7

1. increase recognition of pathogens and facilitate phagocytosis via opsonisation
2. ↑ vascular permeability + chemotaxis (inflammation)
3. destroy pathogen cell membranes

Question 8

what is opsonisation?

Answer 8

coating of pathogen surface by antibody and/or complement making the pathogen more easily ingested by phagocytes

Question 9

what is phagocytosis?

Answer 9

internalization of particular matter by cells by a process of engulfment, in which cell membrane surrounds the material, forming an intracellular vesicle (phagosome) containing ingested material

Question 10

how does complement get activated? general + ways

Answer 10

after being cleaved by proteases
3 ways/pathways (CAL):
classical, alternative, lectin

Question 11

what is an inactive complement component called?

Answer 11

pro-protease

Question 12

how does complement activation act?

Answer 12

as a (signalling) cascade

Question 13

what does proteolytic cleavage generate?

Answer 13

2 fragments:
1. small - letter ‘a’, w/ special function
2. large - letter ‘b’, w/ proteolytic activity on NEW substrate

both fragments are proteases (pro-protease -> undergoes proteolytic cleavage -> 2x protease)

Question 14

what are the two C3 convertase and how do they arise?

Answer 14

C4b2a and C3bBb
C4b2a: C4b covalently bound to C2a
C3bBb: C3b covalently bound to Bb

Question 15

what is the role of C3 convertase?

Answer 15

cleave C3 pro-proteases, activate them
C3 -> C3a and C3b via C4b2a/C3bBb

Question 16

how and where are pro-proteases found?

Answer 16

circulate in groups in blood

Question 17

what are functions of different complement?

Answer 17

1. initiators - initiate respective complement reactions + cascade
2. convertase activators - cleave & activate next member of complement reaction sequence
3. opsonins - for opsonisation (facilitate phagocytosis)
4. anaphylatoxins - increase inflammation
5. MAC (membrane attack complex) - create pores on pathogen membrane
6. receptors - on immune cells
7. regulators - degrade complement components

Question 18

3 important general characteristics of complement activation

Answer 18

1. 3 pathways w/ 3 different initiators
2. all pathways generate C3 convertase (either C4b2a or C3bBb to cleave C3 into C3a and C3b)
3. 3 different main outcomes (↑ opsonisation, ↑ inflammation, MAC)

Question 19

what triggers the lectin pathway?

Answer 19

lectins (initiator) binds to pathogen surface. lectins are soluble proteins and PPRs (pattern recognition receptors)

Question 20

what are the types of lectins and where do they circulate?

Answer 20

Mannose-binding lectin (MBL) and ficolins
blood

Question 21

what happens to lectins during infection?

Answer 21

expression of lectins increases

Question 22

what happens when the initiator binds to pathogen surface/PAMPs during lectin pathway?

Answer 22

MBL/Ficolin (PPR) binds causing MASPs (MBL associated serine protease) activation triggering signalling cascade. C4b2a generated. C3 -> C3a and C3b

Question 23

what triggers the classical pathway? how?

Answer 23

C1q (initiator) binds either directly onto pathogen surface OR to antibodies on pathogen surface

Question 24

what is unique to the classical pathway? 2-ish points

Answer 24

can bind to antibodies on pathogen surface (present because of adaptive immunity - classical pathway links adaptive + innate immunity together (they coexist simultaneously)

Question 25

what happens after the initiator binds during classical pathway?

Answer 25

C1q binds causing confimation change of serine proteases C1r and C1s (think QRS in alphabet) triggering a signaling cascade on pathogen surface. C4b2a is generated. C3 cleaved -> C3a and C3b

Question 26

what are 3 steps common to both the classical and lectin pathway?

Answer 26

C4b2a generation (C3 convertase) trigger of signaling cascade on pathogen surface initiator binds to pathogen surface

Question 27

what are the products of C3 cleavage involved in?

Answer 27

C3a = enhancing inflammation C3b = opsonization AND C5 convertase (C5 cleaved -> C5a and C5b)

Question 28

what triggers the alternative pathway?

Answer 28

C3b C3b formation is enhanced via amplification loop of C3bBb such that more C3b deposited on pathogen surface

Question 29

what is special about the alternative pathway?

Answer 29

requires lectin and/or classical pathway to generate C3b

Question 30

what is required for C3b amplification loop?

Answer 30

factor B and protease factor D. factor B is cleaved by factor D

Question 31

what is another way C3b is generated without the amplification loop?

Answer 31

when high concentration of C3, C3 undergoes SPONTANEOUS hydrolysis to C3h2o. C3h2o binds to factor B and is cleaved by factor D into C3bBb. C3bBb is a C3 convertase.

Question 32

what is the issue w/ the alternative pathway and how is it fixed?

Answer 32

C3bBb is very unstable. requires properdin (factor P) to stabilise C3bBb by binding to microbial surfaces

Question 33

what secretes properdin. what is another name for properdin?

Answer 33

neutrophils. factor P

Question 34

what are the downstream effects of C3 convertase cleavage general?

Answer 34

inflammation, opsonisation, pathogen lysis

Question 35

expand on inflammation downstream effect

Answer 35

additional signaling results in other complement molecules cleaving (ex. C5). C3a + C5a recruite phagocytes + promote inflammation complement receptors C3aR and C5aR on granulocytes connect complement-tagged pathogens to immune cells by releasing proinflammatory cytokines + granule components from basophils, eosinophils, neutrophils, mast cells + anaphylatoxins

Question 36

what is the danger of downstream inflammation effect?

Answer 36

it large amounts, can lead to anaphylatic shock

Question 37

expand on increased phagocytosis downstream effect

Answer 37

phagocytes have receptors for C3b (an opsonin) and antibodies. once C3b or antibody (on pathogen surface) attaches to phagocyte receptor, phagocyte more readily takes pathogen

Question 38

expand on pathogen lysis downstream effect

Answer 38

additional complement factors create membrane-attack complex (MAC) causing cell lysis. involves C5 and C3 activation. C3b is C5 convertase. C5b starts creating MAC and will recruit other complements. signaling cascade leads to recruitment of MAC components and MAC formation. eventually creates pore in surface of pathogen that lyses pathogen.

Question 39

how is C activation regulated?

Answer 39

negatively via complement-regulatory proteins in plasma or cell surfaces under normal/basal conditions. minimize C3 convertase presence by preventing generation and promoting disappearance. each pathway and process has its own regulatory component.

Question 40

what is an example of a regulatory component in an immune pathway?

Answer 40

CD59 (protectin) inhibits MAC formation by preventing C9 binding

Question 41

what is associated to problems w/ complement ?

Answer 41

many clinical diseases

Question 42

what is C5a and C5b involved in?

Answer 42

C5a - inflammation C5b - MAC

Question 43

under what category does C3b fall?

Answer 43

opsonin