Lecture 7 - The Complement System Flashcards
1
Q
What are barrier epithelia?
A
- Skin
- Lungs
- GIT
- Genito-urinary tract
2
Q
Through what barrier epithelia do most pathogens enter? Why?
A
Mucosal barriers (esp. respiratory tree and then GIT) because they are easier to breach than the skin
3
Q
What 2 immune cells are the first to come in contact with a pathogen that has breached the barrier epithelia?
A
Resident MOs and DCs
4
Q
What is the complement cascade?
A
Pattern recognition system that helps antibodies with their antimicrobial activity (what it was first discovered doing) but also compliments both innate and adaptive immune systems
5
Q
What is the first antimicrobial system activated when microorganisms breach barrier epithelia?
A
Complement system
6
Q
What is the complement system?
A
A system of plasma and tissue proteins some of which are inactive (serine) proteases activated by cleavage =
a triggered enzyme cascade with downstream amplification cascade that terminates in the formation of a membrane attack complex (MAC) which creates a ring or pore in the envelope of microorganisms or the membrane of cells
7
Q
What are components of the complement system cleaved into?
A
Cleaved into 2 pieces:
- ‘b’: binding - BIG
- ‘a’: soluble mediator (sometimes chemoattractant) – small
8
Q
Purpose of complement system? 6
A
Augments or complements:
- Phagocytosis via opsonization
- Inflammation via chemotactic factors, etc.
- Kills microorganisms and host cells (pathological)
- B cells activation and maturation
- Immune complex removal
- T cell modulation
9
Q
What is generated along the route of the complement cascade? Purpose?
A
Chemotactic factors, ‘handles’ (opsonins) that allow phagocytes to grasp pathogens => disposal system for antigen-antibody complexes and stimulation of T and B lymphocytes
10
Q
What other cascade is an amplified one in the body?
A
Coagulation cascade
11
Q
What is opsonization?
A
Uptake of antigens
12
Q
What 2 molecules are a powerful opsonin? Why?
A
IgG and C3b (in the complement cascade)
Because phagocytes have receptors for both
13
Q
What immune cells are recruited by the chemotactic factors of the complement cascade?
A
Mainly polymorphonuclear leukocytes
14
Q
How does the complement cascade allow for immune complex removal? 3 mechanisms.
A
- Enabling immune complexes to be picked up by RBCs (with CR1 receptors) to be plucked off in the spleen and liver by resident MOs to be destroyed
- Can solubilize large immune complexes to prevent them from falling out of solution (particularly the Alternative pathway)
- Classical pathway, particularly, C1qrs and C3 but also the Alternative pathway, can inhibit precipitation of antigen-antibody immune complexes
15
Q
What would happen to immune complexes without the complement system?
A
They would get lodged in capillary beds in lungs, kidneys (glomerulus to cause tissue damage), or the choroid plexus causing inflammation
16
Q
3 types of complement cascade pathways? How do they differ from each other?
A
- Classical pathway
- Alternative pathway
- Mannose-binding lectin pathway
Differ from each other in the way the cascade is initiated
17
Q
Describe the nomenclature for the components of the complement cascade.
A
- Classical pathway: components are named C1-C9, except that there’s a glitch in the order of the components such that C4 comes before C2.
- Alternative pathway: C3, factors B, and D and properdin
- Mannose-Binding Lectin pathway: 2 serine proteases associated with it termed MASP-1 and MASP-2 (mannose-binding lectin serine proteases)
All components can be cleaved into a and b fragments, EXCEPT for C2: C2bC2a (switched)
18
Q
Describe the binding between the big piece and the pathogen.
A
Covalent
19
Q
What are MASP-1 and 2?
A
Soluble PPRs
20
Q
How many amplification steps are there in the complement cascade? Describe them.
A
- Convertase C3 (different in alternative pathway)
2. Convertase C5
21
Q
Which 2 complement system pathways are innate? What does this mean?
A
The Mannose-Binding Lectin pathway and the Alternative pathway.
Meaning that they do not require the participation of antibody
22
Q
Which complement system pathway is acquired? What does this mean?
A
The Classical pathway is initiated by either IgM or IgG (IgG3, 1, and 2) antibody binding to the pathogen
23
Q
At what point do all 3 complement pathways converge?
A
C3 convertase
24
Q
What does each complement system pathway handle?
A
- Classical pathway: antigen-antibody complexes on pathogen surfaces
- Alternative pathway: pathogen surfaces
- Mannose-binding lectin pathway: specific arrays of mannose and fucose on pathogen surfaces
25
Which complement has the highest concentration in our blood?
C3
26
Describe the 9 steps of the classical pathway cascade.
1. Binding of IgM or IgG to the pathogen surface
2. Change in conformation of the bound antibody exposes the site for C1q binding (subunit of C1)
3. C1q bound = C1 catalytically active
4. Conformational change to C1r
5. C1s becomes a serine protease
6. C1s cleaves C4 and C2
7a. C4b covalently binds the pathogen and opsonizes it and then binds C2 for cleavage by C1s
7b. C4a acts as a peptide mediator of inflammation (weak)
8a. C4bC2a (AKA C3 CONVERTASE) cleaves C3 and C5
8b. C2b is a precursor of vasoactive C2 kinin
9a. C3b binds to pathogen surface (C4bC2aC3b = C3/C5 CONVERTASE) and acts as an opsonin + initiates amplification via the alternative pathway + binds C5 for cleavage by C3 CONVERTASE
9b. C3a acts as a peptide mediator of inflammation (intermediate)
10a. C5b initiates formation of MAC
10b. C5a is a chemotactic factor
27
Are IgM and IgG the only 2 elements that can initiate the classical pathway by binding the antigen?
NOPE
C1q can bind bacteria-bound C-reactive protein and some bacterial surfaces
28
First step of the alternative pathway? What is this called? Explain what happens after this.
Spontaneous cleavage (hydrolysis) of C3 = C3 tick-over => exposed, labile thioester bond can immediately bind to a pathogen surface => C3b binds and changes the shape of B (C3b + B) => D can cleave B => Ba + Bb => C3bBb cleaves C3 and C5
29
What happens to the alternative pathway when there are no pathogen surfaces?
Binding capacity of C3 is eliminated aka C3b decays
30
Form of MBL?
Heterotrimer
31
What happens once the mannose-binding lectin (MBL) binds a pathogen?
Change in conformation of the bound MBL renders MASP catalytically active => MASP cleaves C4 and C2 to make C3 convertase (just like in classical pathway)
32
Form of C1?
Heterotrimer
33
Why can't other Igs activate the classical complement pathway?
1. IgD: no function known and very low plasma levels
2. IgE: cytophilic antibody that is active when bound to high affinity IgE Fc receptors on the surface of mast cells and basophils, which mask the Fc region hindering complement activation
3. IgA-1 and 2: designed NOT to activate the complement pathway
34
What can ABs do and what can't they do?
Good at agglutinating and clumping antigens but cannot destroy antigens without the complement cascade
35
Can complement function in all compartments of the body, and if not, why not?
Can only function in the body, so cannot function on barrier epithelia (mechanisms to inhibit it in respiratory secretions, saliva, etc.)
Reason: protect the integrity of the barrier epithelia and the activation of complement is strongly proinflammatory and tissue destructive
36
Main Ig at the barrier epithelia?
IgA
37
How many IgM molecules necessary to bind the pathogen to activate the complement cascade?
1
38
How many IgGs molecules necessary to bind the pathogen to activate the complement cascade?
2 and must be sufficiently close one to another to be bridged by the globular heads of C1q
39
What is special about IgG3 and 1 activating the complement pathway?
They can be transported across the placenta
40
Geometry of IgM in solution? Implication? Purpose?
Planar => in this conformation the complement binding site on the Fc region is masked and IgM cannot fix complement
Purpose: safety mechanism to prevent unwanted complement activation by Igs in solution
41
What happens to IgM once it binds to the pathogen surface? Purpose?
It assumes a crab-like, "staple" conformation that exposes the C1q binding site on the CH3 domain of the Fc region
42
What happens to IgG once it binds to the pathogen surface? Purpose?
C1q binding site is exposed on the CH2 domain of the Fc region
43
Main types of epitopes that IgM binds?
Carbs
44
Why does soluble IgM have high affinity for epitopes?
10 binding sites since it is a pentamer in solution
45
What are C3a, C5a, and C4a called? Why?
Anaphylatoxins => inflammation and phagocyte recruitment
46
How do the 3 chemotactic factors generated by C3 convertase promote inflammation?
1. Direct action on endothelium to cause vasodilation
2. Activate mast cells to degranulate and release inflammatory mediators that act on endothelium
3. Recruit neutrophils
47
What are the 6 complement receptors? Many names for each.
1. CR1 = CD35
2. CR2 = CD21
3. CR3
4. CR4
5. C5a receptor
6. C3a receptor
48
What 3 complements does CR1 bind?
1. C3b
2. C4b
3. iC3b
49
What 4 complements does CR2 bind?
1. C3d
2. iC3b
3. C3dg
4. Epstein-Barr virus
50
What 1 complement does CR3 bind?
iC3b
51
What 1 complement does CR4 bind?
iC3b
52
Main function of CR1?
1. Erythrocyte transport of immune complexes
| 2. Phagocytosis
53
Main function of CR2?
Part of B-cell co-receptor also including CD19 and CD81 that binds C3d
54
Function of CR3 and CR4?
Stimulates phagocytosis
55
Function of C5a and C3a?
Binding of ligand activates G protein
56
On what 6 cell types is CR1 found?
1. Erythrocytes
2. MOs
3. Monocytes
4. Polymorphonuclear leukocytes
5. B cells
6. FDCs
57
On what 2 cell types is CR2 found?
1. B cells
| 2. FDCs
58
On what 3 cell types are C5a and C3a receptors found?
1. Endothelial cells
2. Mast cells
3. Phagocytes
59
What is iC3b?
Proteolytically inactive product of the complement cleavage fragment C3b that still opsonizes microbes, but cannot associate with Factor B, thus, preventing amplification of the complement cascade or activation through the alternative pathway
60
What is C3d?
Plays a role in enhancing B cell responses because it is recognized by CD21, which is expressed in B2 cells
61
Breakdown pathway of C3b?
C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d
62
How is C3b an opsonin? What does it require to function?
C3b covalently binds to pathogen surfaces providing ‘handles’ that can be grasped by Complement receptors on the cell membrane of professional phagocytic cells such as the neutrophil
Uptake of C3b-coated pathogens requires a second signal which is provided by the binding of C5a to its receptor on the phagocyte surface => phagocyte opsonization via CR1
63
How is IgG an opsonin? What does it require to function?
Provide the phagocyte with ‘handles’ with which to grasp the pathogen, accomplished by IgG Fc receptors on the cell membrane of the phagocyte ligating the Fc region of the pathogen-bound IgG
Fc receptors of the phagocyte must be cross-linked => safety mechanism to prevent unwanted activation of phagocytic cells
64
What is synergistic opsonization?
Deposition of both C3b and IgG on the pathogen surface provide the most important trigger for uptake by phagocytes
65
What is the membrane attack complex (MAC) composed of?
1. 4 complement components (C5b, C6, C7, and C8) that bind to the outer surface of the cell membrane,
2. Many copies of a 5th component (C9) that hook up to one another, forming a ring in the membrane
66
Describe the formation of the membrane attack complex (MAC) at the end of the complement cascade. 6 steps
1. C5b binds C6 and C7
2. C5b67 complexes bind to membrane via C7
3. C8 binds to the complex and inserts into the cell membrane
4. C9 molecules bind to the complex and polymerize
5. 1-16 molecules of C9 bind to form a pore in the membrane
6. Pore allows free diffusion of molecules in and out of the cell => if enough pores form, the cell is no longer able to survive
67
What other 2 cells are able to perform a mechanism similar to the MAC assembly to destroy cells?
1. Cytotoxic T cells
| 2. Bacterial toxins
68
In what disease is the formation of MACs on RBCs the pathology?
Hemolytic anemia
69
For what immune complexes is solubilization by the complement cascade more efficient?
Solubilization occurs more effectively for complexes where antigen is in excess
70
For what immune complexes is inhibition of precipitation by the complement cascade more efficient?
More effective in complexes formed in antibody excess
71
How do deficiencies in complement components manifest themselves? Why?
Increased susceptibility to extracellular bacterial pathogens and the pathological formation of immune complexes
Reason: because the Complement cascade defends against extracellular pathogens and these are largely bacteria and removes immune complexes from the circulation
72
How do deficiencies in early complement components of the classical pathway manifest themselves? Example?
Inability to remove immune complexes (because C3b cannot be formed) and the patient presents with immune-complex disease (lupus-like disease)
73
Example of deficiencies in early complement components?
Increased susceptibility to staphylococci
74
Example of deficiencies in late complement components?
Associated with recurrent neisseria infections
75
How do deficiencies in early complement components of the alternative and MBL pathways manifest themselves? Example?
Severe, recurrent bacterial infections
76
What should a physician check if a patient has recurrent infections?
1. Check complement component levels
| 2. Check neutrophil levels and function
77
What can deficiencies in complement inhibitor proteins result in? What is an example of this?
Immunopathology
Deficiency in C1 inhibitor, which is involved in control of the kinin (the kinin system plays a role in inflammation, blood pressure control, coagulation and pain) and coagulation system as well as the complement system
Deficiency in C1 inhibitor + minor trauma to skin or mucosa => rapid angioedema = edema of the dermis, subcutaneous tissues, mucosa and sub-mucosal tissues
78
How to treat angioedema?
Cases where angioedema progresses rapidly should be treated as a medical emergency, as airway obstruction and suffocation can occur
79
Cause of hereditary angioedema?
Often no direct identifiable cause, although mild trauma, including dental work and other stimuli, can cause attacks
80
How can pathogens block the complement cascade?
Pathogens can acquire complement inhibitors to block complement activation
81
Class of protein in the complement cascade that binds antigen-antibody complexes and pathogen surfaces?
C1q
82
3 classes of protein in the complement cascade that binds to carb structures such as mannose or GlcNAc on microbial surfaces?
1. MBL
2. Ficolins
3. Properdin
83
Other name for properdin?
Factor P
84
7 activating enzymes of the complement cascade?
1. C1r
2. C1s
3. C2a
4. Bb
5. D
6. MASP-1
7. MASP-2
85
2 surface binding proteins and opsonins of the complement cascade?
1. C4b
| 2. C3b
86
3 peptide mediators of inflammation of the complement cascade?
1. C5a
2. C3a
3. C4a
87
5 membrane-attack proteins of the complement cascade?
1. C5b
2. C6
3. C7
4. C8
5. C9
88
9 complement-regulatory proteins of the complement cascade?
1. C1INH
2. C4BP
3. CR1
4. MCP/CD46
5. DAF/CD55
6. H
7. I
8. P
9. CD59
89
Effect of MAC?
Osmolytic lysis
