ICL 3.1 & 3.2: Complement

Question 1

which part of the immune system is the complement system part of?

Answer 1

innate immune system

Question 2

what does the complement system do?

Answer 2

1. protects the host from pathogenic microorganisms
2. contributes to immune complex regulation, and
3. represents an important link between the innate and specific/adaptive immune system

Question 3

how does the complement system act as a link between the innate and adaptive immune system?

Answer 3

1. major effector system for antibody-mediated killing

2. affects cell-mediated immunity

Question 4

through what three pathways can the complement system be activated?

Answer 4

1. classical pathway
2. lectin pathway
3. alternative pathway

these three pathways all converge to activate C3 which then goes on to cause inflammation, pathogen killing and membrane disruption

Question 5

what are the three consequences of the complement system?

Answer 5

1. recruitment of inflammatory and immunocompetent cells
2. opsonization of pathogens
3. killing of pathogens

Question 6

how does complement system opsonization of pathogens work in general?

Answer 6

binding of C3b to microbe = opsinozation

phagocyte C3b receptor recognizes C3b bound to the surface of the microbe

this leads to phagocytosis of microbe

Question 7

how does complement system stimulation of inflammatory reactions work in general?

Answer 7

C3b binds to microbe and releases C3a and C5a fragment

C3a and C5a are chemotactic agents that recruit and activate leukocytes

this leads to destruction of microbes by leukocytes

Question 8

how does complement-mediated cytolysis of pathogens work in general?

Answer 8

C3b binds to microbe which activates the terminal components of complement

MAC complex is formed which leads to osmotic lysis of bacteria

Question 9

what happens overall in the classical pathway?

Answer 9

there is both antibody-dependent and independent recognition of pathogens or apoptotic self cells

C1 binds to antibody bound to specific antigens on pathogen surface

Question 10

what happens overall in the lectin pathway?

Answer 10

antibody independent recognition of danger via sugar residue patterns that are recognized by MBL and ficolins on pathogens

MBL or ficolin binds carbohydrate on pathogen surfaces

Question 11

what happens overall in the alternative pathway?

Answer 11

recognition of possible danger by default; it will activate on any surface that “allows” it to

it’s always active!! the only reason it would be inactive is if the surface of the cell can control it; pathogens don’t have the ability to control it!

pathogen surface creates environment conductive to complement activation

Question 12

where are complement proteins produced?

Answer 12

liver

complement production increases during the acute-phase response

complement components are largely secreted as inactive stable zymogens (enzyme precursors) and circulate widely through body fluids and tissues

Question 13

what does activation of the complement system do too zymogens?

Answer 13

zymogens are complement precursors that circulate throughout the body in an inactive form

activation causes a precursor zymogen to be cleaved by serine proteases and become enzymatically active

this activated enzyme then activates a different zymogen and it’s a whole cascade –? each successive enzymatic cleavage causes amplification

Question 14

what does completion of the complement cascade lead to?

Answer 14

1. membrane attack complex (MAC): lysis of microbes
2. opsonization of microbes: phagocytic uptake
3. solubilization and removal of immune complexes
4. proteolytic events produce inflammatory mediators

Question 15

what is the big list of all the consequences of the complement system?

Answer 15

1. opsonization of bacteria
2. lysis of bacteria
3. kills parasites
4. blocks viruses
5. chemotaxis of inflammatory cells to site of attack
6. mast cell degranulation
7. increased vascular permeability
8. intern regulation
9. smooth muscle contraction
10. DC cell maturation
11. B cell memory enhancement
12. B cell activation
13. influence regulatory T cell development
14. influence t cell lineage commitment
15. clearance of immune complexes
16. recognition of apoptotic cells
17. clearance of cellular debris

Question 16

what is the recognition molecule of the classical pathway?

Answer 16

C1

Question 17

what is the structure of C1?

Answer 17

it’s a C1 (q,r,s) complex

C1q is the stalk that’s made of 2 chains linked together to form a triple helix structure repeated 6 times

there’s also 2r and 2s serine proteases

looks like a tulip bouquet

Question 18

what part of an antibody does C1 recognize?

Answer 18

Fc

Question 19

how does C1 bind to an antibody?

Answer 19

C1 binds to the Fc portions of IgG and IgM

the catch is that when IgM is in the blood it’s a pentamer in a planar shape but when the Fb region binds to the antigen, the Fc regions become exposed

if there’s enough antibodies next to each other then C1 can bind to the Fc region

this is why C1 doesn’t bind to IgM and IgG just floating in the blood

Question 20

which antibodies activate C1?

Answer 20

IgM > IgG3 > IgG1 > IgG2

IgD, IgG4 and IgE DONT activate C1!

Question 21

what can C1q bind to?

Answer 21

1. antibodies
2. cell wall component and membrane proteins of all kinds of microorganisms.
3. fragments of cellular and sub cellular membranes and modified host proteins and phospholipids
4. C-reactive protein, and serum amyloid P protein

Question 22

what are the steps in the classical pathway?

Answer 22

1. C1q binds to an antibody which activates C1R when then activates C1s
2. C1s cleaves C4 into C4a and C4b
3. if C4b is close enough to a pathogen surface, it will covalently bind to the surface
4. C1s cleaves C2 into C2a and C2b
5. C2b binds to C4b to form C3 convertase (C4b,2b)
6. C3 convertase cleaves C3 into C3a and C3b
7. if C3b is close enough to the pathogen cell surface, it will covalently bind to it = opsonization
   or. ..C3b can binds to C4b2b to form C5 convertase = C4b,2b,3b

Question 23

how does C4b bind to a pathogen surface?

Answer 23

covalent binding of C4b to the cell surface via hydroxyl and amino groups present on carbohydrates and proteins, respectively

however, most C4b is not close enough to any surface and is quickly inactivated in the fluid phase of blood

it’s the same thing for C3b!

Question 24

what does C3a do?

Answer 24

chemotaxis

it recruits inflammatory cells!

Question 25

what are the subunits of C3 convertase?

Answer 25

C4b C2b

Question 26

what are the subunits of C5 convertase?

Answer 26

C4b C2b C3b

Question 27

what does C1q deficiency cause?

Answer 27

susceptible to bacterial infection systemic lupus erythematosus (SLE)

Question 28

what does CRP stand for?

Answer 28

C-reactive protein

Question 29

what does CRP do?

Answer 29

CRP can initiate the classical pathway of complement CRP recognizes phosphocholine on bacteria and on disrupted cell membranes (e.g. apoptotic cells) C1 can bind to CRP on the pathogen surface which activates the classical pathway of complement fixation this is antibody-independent activation of the common pathway!!!

Question 30

what is antibody-independent activation of the common pathway?

Answer 30

C1 binding to CRP on pathogen surface!

Question 31

what is the structure of MBL?

Answer 31

MBL binds specifically to mannose and certain sugar patterns on microbial surfaces MBL forms a complex with two serine proteases, MASP-1 and MASP-2 (homologous to C1r and C1s)

Question 32

what are the steps in the lectin pathway?

Answer 32

1. MBL complex binds to carbohydrates on pathogen surface 2. MASP-2 cleaves C4 into C4a and C4b 3. C4b covalently binds to pathogen surface if its close enough 4. MASP-2 cleaves C2 into C2a and C2b 5. C2b binds to C4b on the pathogen surface to form C3 convertase 6. C3 convertase cleaves C3 into C3a and C3b 7. C3b binds to pathogen surface if it's close enough = opsonization or. ..C3b binds to C4b2b complex to form C5 convertase

Question 33

what does an MBL deficiency cause?

Answer 33

Increased susceptibility to bacterial infections especially infants and immunocompromised

Question 34

are C3 and C5 convertase of the classical and lectin pathways the same?

Answer 34

YES C3 convertase = C4b,2b C5 convertase = C4b,2b,3b

Question 35

what is tickover?

Answer 35

spontaneous cleavage of C3 that is constantly occurring so there is a constant pool of active C3

Question 36

what is the recognition molecule in the lectin pathway?

Answer 36

MBL

Question 37

what is the recognition molecule in the alternative pathway?

Answer 37

trick question, there isn't one! AP is not dependent on a recognition molecule for initiation

Question 38

what are the steps in the alternative pathway?

Answer 38

1. a small portion of C3 in the plasma can undergo spontaneous hydrolysis into C3H2O = iC3 2. factor B binds to C3H2O 3. factor D cleaves factor B to form Ba and Bb -- Bb stays bound to C3H2O = soluble C3 convertase 4. Bb-C3H2O cleaves C3 into C3b and C3a 5. C3b binds to pathogen surface if it's close enough 6. factor B now binds to C3b because it's structurally similar to C3 7. factor D cleaves factor B into Ba and Bb -- Bb stays bound to C3b 8. properdin binds to Bb-C3b complex = membrane bound C3 convertase 9. membrane bound C3 convertase cleaves C3 into C3a and C3b 10. C3b binds to pathogen surface or. ..C3b binds to properdin-Bb-C3b complex to form C5 convertase

Question 39

why is it important that C3b only binds to a cell surface if it's close?

Answer 39

because C3b is being randomly generated by spontaneous hydrolysis you don't want it to just be floating around forever so unless it's close enough to a cell surface, it'll get destroyed

Question 40

what are the C3 convertases in the alternative pathway?

Answer 40

soluble C3 convertase = Bb-C3H2O membrane bound C3 convertase = Bb-C3b

Question 41

what is the alternative pathway C3 convertase amplification loop?

Answer 41

1. C3b binds to pathogen surface if it's close enough 2. factor B now binds to C3b because it's structurally similar to C3 3. factor D cleaves factor B into Ba and Bb -- Bb stays bound to C3b 4. properdin binds to Bb-C3b complex = membrane bound C3 convertase 5. membrane bound C3 convertase cleaves C3 into C3a and C3b 6. C3b binds to pathogen surface

Question 42

what is the function of properdin?

Answer 42

properdin binds to C3b-Bb complex in the alternative pathway properdin stabilizes the alternative pathway C3 and C5 convertases, increasing the half-life of the enzyme and making possible the efficient amplification of C3b deposition (amplification loop)

Question 43

what subunits make up C5 convertase in the alternative pathway?

Answer 43

properdin, Bb, C3b, C3b = C3bn,Bb

Question 44

what is the alternative pathway of complement amplification loop important for?

Answer 44

amplifying the classical and lectin pathways C3b gets deposited on pathogen cell surfaces from the classical or lectin pathways this C3b can bind factor B which is then cleaved by factor D into Ba and Bb this forms C3 convertase which can cleave even more C3 molecules into C3a and C3b

Question 45

how does the alternative pathway know when to activate?

Answer 45

AP is always active; it just depends if that particular cell type can regulate the AP and shut it down by inactivating C3b pathogens and altered-self cells can't regulate the alternative pathway so they get killed C3b is constantly deposited in at low levels on all our cells and tissues and any pathogen that happens to be in our bodies…. activation will only proceed if the surface on which the C3b has deposited: 1. does NOT have sufficient or any membrane-bound negative complement regulatory proteins (DAF, MCP, CD59, CR1). 2. does NOT efficiently bind soluble complement regulator factor H (fH). 3. most pathogens have these disadvantages and are cleared by complement-mediated lysis and/or phagocytosis. BUT, many pathogens make their own complement regulatory proteins or sequester host regulatory proteins as a complement EVASION strategy

Question 46

what is factor H?

Answer 46

it regulates the alternative pathway by accelerating the decay of the C3b-Bb complex factor H is in our blood and tissues at VERY high concentrations it only binds to surfaces where there's a deposited C3b AND polyanions; good news is our tissues have lots of polyanion markers factor H binds to C3b-Bb on the surface of cells and makes Bb detach from C3b = inactive

Question 47

what does facto I do?

Answer 47

it's a factor H cofactor that inactivates C3b that is bound to factor H by cleaving C3b into C3f and iC3b C3f floats away iC3b is bound to the surface of the cell but it's inactive and can't bind to factor B at all

Question 48

what are the membrane bound regulatory proteins that turn off the alternative pathway?

Answer 48

factor H DAF MCP CR1

Question 49

what do the membrane bound regulatory proteins of the alternative pathway do?

Answer 49

1. decay acceleration: they bind to C3b and displace Bb ex. DAF, CR1 2. cofactor for I activity = they help factor I cleave C3b to form inactive iC3b which can no longer bind to factor B ex. MCP, CR1 **there is an insignificant amount of iC3b on the surface

Question 50

what happens if cells can't bind factor H or if they have no membrane-bound regulators?

Answer 50

if cells (i.e. pathogens) cannot bind factor H efficiently/quickly and if they have little to no membrane-bound regulators, they become ACTIVATORS of the alternative pathway tons of C3b will deposit on the cell surface and without factor H or membrane-bound regulators around, C3b will build up and activate factor I thousands of C3b is converted into thousands of iC3b and C3d c3d is important for generating B cell responses and iC3b is important for phagocytosis

Question 51

how are the classical and lectin pathways regulation?

Answer 51

soluble C4 binding protein (C4bp) carries out the decay acceleration (displacement of C2b) and cofactor for factor I activities (inactivation of C4, forming iC4b)

Question 52

what does C1INH do?

Answer 52

C1INH = C1 inhibitor binds to activates C1r and C2s which removes them from C1q binds to activated MASP-2 to remove it from MBL

Question 53

what does factor H do?

Answer 53

binds to C3b and displaces Bb it's also a cofactor for I

Question 54

what does DAF do?

Answer 54

DAF = decay-accelerating factor it displaces Bb from C3b and C2b from C4b

Question 55

what does CD59 do?

Answer 55

CD59 = protectin prevents formation of membrane-attack complex on autologous or allogenic cells

Question 56

what does CR1 do?

Answer 56

binds C4b, displacing C2b binds C3b, displacing Bb it's a cofactor for factor I

Question 57

what is the central component of the 3 complement pathways?

Answer 57

C3 it's super abundant in our blood

Question 58

what happens when C3 is cleaved during tickover?

Answer 58

cleavage exposes a highly reactive thioester bond in C3b, which allows it to bind covalently to ANY surface that has available –NH2 (proteins) or -OH (carbohydrates) groups, if it is close enough to the surface… cleavage into C3b exposes internal thioester bonds which are susceptible to nucleophilic attack by oxygen (as shown) or nitrogen atoms C4 is structurally homologous to C3 and has an identical thioester group if not, the thioester bond is spontaneously hydrolyzed and the C3b is inactivated --> ~90% of the C3b or C4b binds a water molecule, and becomes inactive (~60 microseconds).

Question 59

what is the structure of C5 convertase in the common pathway, leptin pathway and alternative pathway? what do they do?

Answer 59

CP & LP = C4b2b3b AP = C3b2Bb C5 binds to the C3b components of the C5 convertase enzyme C5 is cleaved by C2b or Bb to form C5b and C5a

Question 60

what are the terminal complement components?

Answer 60

C5b C6 C7 C8 C9 these come together to form the MAC complex!

Question 61

how is the MAC complex formed?

Answer 61

1. C5 convertase cleaves 5 into C5b and C5a 2. C5b binds C6 and C7, exposing a hydrophobic site on C7 (binding) 3. C5b67 complex binds to membrane via C7 4. binding of C8 to this complex exposes a hydrophobic site; C8 inserts into the cell membrane 4. this complex promotes polymerization of C9 to form membrane pore all reactions after C5b production are nonenzymatic (assembly) only C9 completely penetrates the bilipid cell membrane

Question 62

what does C5, C6, C7, C8, or C9 deficiency cause?

Answer 62

increased susceptibility to neisserial infections

Question 63

what regulates the formation of MAC complex?

Answer 63

CD59 prevents final assembly of the MAC complex at the C8 to C9 stage

Question 64

what are the three consequences of the complement pathway?

Answer 64

1. peptide mediators of inflammation and recruitment of phagocytes 2. opsonization of pathogens 3. MAC complex formation

Question 65

what are the 4 types of things the complement does?

Answer 65

1. host defense 2. pro inflammatory mediators 3. adaptive humoral and cellular immune response 4. housekeeping

Question 66

what are the three things the complement system does related to host defense?

Answer 66

1. direct killing of pathogens 2. neutralization of viruses 3. opsonization of pathogens --> phagocytosis and elimination

Question 67

how does the complement system do direct killing of pathogens?

Answer 67

- binding and activation of complement on pathogen surface (C1q, C3, lectin) - formation of MAC and lysis of pathogens direct killing of pathogens is initiated or enhances by antibodies (IgG or IgM)

Question 68

how does the complement system do neutralization of viruses?

Answer 68

binding of C1q, C4b, C3b on viral surface interferes with the ability of the virus to bind and penetrate host cells

Question 69

how does opsonization via C3b for phagocytosis work?

Answer 69

1. complement activation leads to deposition of C3b on the bacterial cell surface 2. CR1 on macrophage binds C3b on bacteria 3. endocytosis of the bacterium by the macrophage 4. macrophage membranes fuse, creating a membrane-bound vesicle, the phagosome 5. lysosomes fuse with the phagosomes forming the phagolysosome

Question 70

what is the function and ligands of CR1 receptor?

Answer 70

aka CD35 ligands = C3b > C4b > iC3b phagocytosis

Question 71

what is the function and ligands of CR3 receptor?

Answer 71

aka Mac-1, CD11b/CD18 ligands = iC3b phagocytosis!

Question 72

what is the function and ligands of CR4 receptor?

Answer 72

aka gp150,95 or CD11c/CD18 ligand = iC3b phagocytosis!

Question 73

what are the C3b fragments that are bound to a pathogen surface eventually converted to?

Answer 73

iC3b

Question 74

what happens on cells on which complement activates and amplifies??

Answer 74

AMPLIFIED C3b deposition on ACTIVATING SURFACE (many THOUSANDS of C3b) C3b is slowly converted to thousands of iC3b and C3d by fluid phase factor I this allows for phagocytosis because iC3b is recognized by CR1, CR3, CR4 receptors on phagocytes the quantity of iC3b and how quickly the obsinization occurs is the difference between our cells and a pathogen cell

Question 75

which receptors on phagocytes are the most important in complement activation?

Answer 75

CR3 and CR4 they bind to iC3b!

Question 76

in what ways have pathogens found ways to evade the complement system?

Answer 76

1. non-activating capture of complement initiators (such as immunoglobulins) 2. inactivation or depletion of complement components by secreted proteases 3. recruitment of complement regulators to the pathogen surface or secretion of regulator mimics 4. interference with MAC formation 5. some pathogens enter cells (escape) by binding cell-bound complement receptors and regulators via pathogen-expressed surface proteins or via C3b fragments deposited on their surface

Question 77

which functions of the complement system are related to the generation of pro inflammatory mediators?

Answer 77

1. chemotaxis of inflammatory cells to site of attack 2. mast cell degranulation 3. increased vascular permeability 4. intern regulation 5. smooth muscle

Question 78

how is the complement system related to the generation of pro inflammatory mediators?

Answer 78

small soluble cleavage fragments of some complement proteins can initiate local inflammatory responses C3, C4 and C5 are cleaved during the course of complement activation fragments C3a, C4a, and C5a have specific receptors on different cell types like phagocytes, endothelial cells, mast cells when present at low levels, binding of these components induce localized inflammation when present at high levels, they can induce general circulatory collapse (anaphylactic shock) --> anaphylatoxins

Question 79

which parts of the complement system are considered anaphylatoxins?

Answer 79

C5a > C3a >> C4a anaphylatoxins = can induce general circulatory collapse

Question 80

what does C5a do when it comes to phagocytosis of pathogens?

Answer 80

it promotes phagocytosis it increases ability to ingest particles and increases expression of CR1 and CR3 when a bacteria is coated with complement by the alternative pathway and MBL pathogens, C3b can bind to CR1 on the macrophage but the bacteria isn't phagocytosed C5a can activate macrophages to phagocytose via CR1

Question 81

C5a is a chemotactic factor for what?

Answer 81

it is an extremely potent chemotactic factor for neutrophils, monocytes, macrophages and other inflammatory cells C3a is also chemotactic but it's not as potent

Question 82

what are the functions of C5a?

Answer 82

1. anaphylatoxin 2. chemotactic factor 3. increases phagocytosis of pathogens 4. induces acute inflammation if activated in tissue by soluble antigen-antibody complexes

Question 83

how are C3a and C5a broken down?

Answer 83

in tissues, C3a and C5a are rapidly broken down to C5a des-Arg by cleavage of the N-terminal arginine (carboxipeptidase N) they're chemotactic factors that cause migration of cells to the sites of complement activation so they need to be degraded

Question 84

which two complement fragments have chemotactic activity?

Answer 84

C5a and C3a

Question 85

which functions of the complement system are related to the adaptive humoral and cellular immune response?

Answer 85

1. influence T cell lineage commitment 2. influence regulatory T cell development 3. B cell activation 4. B cell memory enhancement 5. DC cell maturation

Question 86

what is the function and ligand for CR2 receptors?

Answer 86

aka CD21 ligands = C3d > C3dg > iC3b its a coreceptor for B cell action it's found on B lymphocytes and follicular dendritic cells

Question 87

what is the role of complement in B cell activation?

Answer 87

C3d is essential for lowering the threshold of B cell activation aka the amount of antigen that is need for the activation an antigen binds to IgM on the surface of a B cell AND C3d is bound to the antigen which binds to the CR2 receptor on the B cell it's the combination of these two signals that allows for B cell action -- if there was no C3d or IgM hasn't bound to the epitope, then there isn't a strong activated B cell response

Question 88

what are the functions of the complement system related to housekeeping?

Answer 88

1. clearance of cellular debris 2. recognition of apoptotic cells 3. clearance of immune complexes

Question 89

how does complement regulate immune complex formation?

Answer 89

immune complexes are good for removing unwanted antigens, BUT... immune complexes need to be removed in order to not cause tissue damage, inflammation, other harmful effects complement activation prevents the formation of very large, insoluble immune complexes (solubilization of immune complexes) and opsonizes immune complexes for elimination (phagocytosis of immune complexes) if complexes are not cleared the immune complexes can precipitate at the basement membrane of small blood vessels

Question 90

which phagocyte receptor is involved with RBC transport of immune complexes?

Answer 90

CR1 aka CD35 on RBCs

Question 91

how do RBCs help clear immune complexes from circulation?

Answer 91

RBC CR1 helps to clear immune complexes from circulation RBCs bind C3b on immune complexes via CR1 on their surface 1. small antigen; antibody complex forms in circulation and activates complement 2. this immune complex is coated with covalently bound C3b 3. CR1 on RBC surface binds C3b-tagged immune complex 4. RBC carries immune complex to the liver or spleen where it is detached and taken up by a macrophage 5. resident macrophages bind immune complexes via complement receptors like CR3, CR4, CR1 or Fc receptors

Question 92

how does the complement system participate in waste disposal?

Answer 92

C1q, C3b, C4b or other fragments bind to damaged and dying cells (apoptotic/necrotic), cellular fragments subcellular membranes, mitochondria, microsomes C5a induces chemotactic response of inflammatory cells and phagocytosis to remove tagged particles and cells C5a induces chemotactic response of inflammatory cells and phagocytosis to remove tagged particles and cells

Question 93

what does failure of the complement system to do waste disposal cause?

Answer 93

autoimmune disease due to increase of dangerous autoantigens from dead cells that have not been cleared and immune complex disease like SLE