1
Q
What is complement good for (big picture)?
A
- Lyse bacteria
2. Clearing immune complexes
2
Q
What is the broad definition and function of the complement system?
A
- A group of plasma proteins that acts as an auxiliary system in immunity, both on its own and in conjunction with humoral immunity.
- –A primitive surveillance system for microbes, independent of T cells and antibodies
- –A major effector system for humoral immunity
3
Q
What are the specific functions of complement?
A
- Lysis of many microorganisms, viruses, and nucleated cells
- Opsonization of antigen - uptake of particulate antigen by phagocytes
- –Opsonin - molecule that binds to antigen and phagocyte to enhance phagocytosis - Source of mediators of the inflammatory response
- Solubilization and clearance of immune complexes
- Clearance of apoptotic cells
- Augments stimulation of the B cell through the CR2 receptor to increase the humoral immune response.
4
Q
What are the four basic roles of complement?
A
- Lysis
- Opsonization
- Activation of Inflammatory Response
- Clearance of Immune Complexes
5
Q
What is the order of the basic components of complement?
A
142356789
6
Q
You don’t have to have lysis to. . .
A
. . .gain something from complement.
7
Q
What diseases are associated with deficiency in C1q, C1r, C1s, C4 and C2?
A
- SLE
- Glomerulonephritis
- Encapsulated bacterial infections or pyogenic infections
8
Q
What diseases are associated with deficiency in MBL?
A
-Increased susceptibility to bacterial infection
9
Q
What disease is associated with deficiency in C1-INH?
A
Hereditary angioedema
10
Q
What disease is associated with C3 deficiency?
A
Glomeronephritis, SLE, Pyogenic infection, neisseria infection (recurrent)
11
Q
What diseases is associated with B deficiency?
A
Meningococcal infection
12
Q
What diseases are associated with D deficiency?
A
Meningococcal and encapsulated bacterial infection, Neisseria infection
13
Q
What diseases are associated with Properdin deficiency?
A
Meningococcal infection
14
Q
What is Factor I associated with?
A
Encapsulated bacterial infection, Recurrent infections
15
Q
What becomes a problem if you don’t have complement?
A
Immune Complexes!
16
Q
What is Factor H deficiency associated with?
A
Atypic hemolytic uremic syndrome, age-related macular degeneration
17
Q
What is deficiency in MAC (C5, C6, C7, C8 and C9) associated with?
A
Meningococcal infection, Recurrent neiserrial infections (otherwise usually healthy)
18
Q
What pathways are most important in getting rid of Neisseria?
A
- MAC complex (C5b, C6, C7, C8, C9)
- Start of alternative pathway
19
Q
What do complement component deficiencies usually present as?
A
Infections!
- Pyogenic infections and infection with encapsulated bacteria (classical and alternative)
- –Opsonization and phagocytosis are a primary host defense
20
Q
What types of infections are most common with complement deficiency?
A
- Pyogenic infections and infection with encapsulated bacteria (classical and alternative)
- –Opsonization and phagocytosis are a primary host defense - Neisseria infections (C3, alternative pathway and terminal lytic pathway)
- Serious pyogenic infections with MBL deficiency
21
Q
What types of infections do you get with MBL deficiency?
A
Serious pyogenic infections
22
Q
What deficiencies present as immune complex disease or autoimmune disease?
A
Classical pathway or C3 deficiencies
23
Q
Where is complement?
A
- Primarily in plasma, but also interstitially and in secretions (bronchoalveolar lavage fluid, etc.).
- Present at portals of entry
- Concentration of different proteins in the plasma ranges:
- –1-2 ug/ml - MBL and Factor D
- –300 ug/ml - C4
- –1,200 ug/ml - C3 - LOTS!!
24
Q
Where is complement synthesized?
A
-Primarily by liver hepatocytes & by tissue macrophages, but also by epithelial cells, fibroblasts and monocytes
25
What are the three pathways of complement activation and what are their start signals?
1. Classical - antigen antibody complexes
2. Mannose binding lectin - mannose
3. Alternative - LPS, carbohydrates, etc.
26
Where do proteolytic cleavages of complement components operate?
C1-C5
| non-proteolytic events for C6789
27
What does MBL and MASP stand for?
MBL - mannose-binding lectin
| MASP - MBL-associated serine protease
28
During the proteolytic cleavage steps through C5, what can smaller fragments do? What do larger fragments do?
Smaller - have biologic effects
| Larger - remain bound in a complex required for next step
29
How do you know if a protein is big or small?
- Smaller proteins are denoted with 'a'
- Larger proteins are denoted with 'b'
- Notable exception is C2 (C2a is the larger, active fragment)
30
What is the order of the classical pathway? What triggers it?
- C142356789
| - It's triggered by antigen binding to IgG (2) or IgM
31
What is the cascade that occurs in the classical pathway? What attaches via thioester bonds?
- Cascade of proteolytic steps performed by serine esterases
| - Covalent attachment of C4b and C3b to surfaces via thioester bonds
32
How is C1 activated?
- C1q binds antigen-bound antibody
- C1r activates auto-catalytically and activates the second C1r; both activate C1s
- C1q is C1 esterase (enzyme - short half life) that cleaves C4 into C4a and C4b
33
What controls C1 esterase's cleavage of C4?
C1 inhibitor (C1INH)
34
What odes absence or mutation of C1 inhibitor cause?
Hereditary angioedema (life threatening swelling)
35
How is C4 activated?
- C1 esterase cleaves C4
- C4b has a thioester regions which forms covalent bonds with activating surface
- C4b can at as an opsonin and interacts with complement receptors on white cells (CR1)
36
How is C2 activated?
- C2 interacts with C4b and is cleaved by C1s, forming a C4b2a complex on the surface
- C4b2a is the classical pathway C3 convertase
37
How is C3 activated?
- C4b2a cleaves C3, activating a labile thioester bond on C3b
- This thioester can bind COVALENTLY to free hydroxyl or amino groups, resulting in C3b covalently binding to surfaces
38
What are key points to remember for the classical pathway?
- Activation in conjunction with specific antibody
- C3b and C4b covalently bind via thioester bonds (ON EXAM!!)
- Enzymatic cleavage of proteins with amplification
39
What is important about the MLB pathway?
- Does not require specific antibody
- Primary constituent is the plasma protein mannose binding lectin (also called the mannan binding lectin)
- Pathway triggered by polysaccharide structure of microbes (Salmonella, Listeria, Neisseria, Candida, etc. bind MBL)
- Binds to sugar residues like N-acetyl glucosamine or mannose
- Leads to the C1 independent formation of the classical pathway C3 convertase (C4b2a)
40
What is important about the alternative pathway?
- It's promiscuous
- Triggered by recognition of forging surface structures (zymosan, endotoxin (LPS), etc.) by complement itself
- Does not require specific antibody
41
What is true about all of these things:
- Gram - and + bacteria
- LPS from gram negative bacteria
- Teichoic acid from gram positive cell walls
- Fungal and yeast cell walls (zymosan)
- Some viruses and virus infected cells
- Some tumor cells
- Some parasites
- Human IgA, IgG, and IgE in complexes
- Anionic polymers (dextran sulfate)
- Pure carbohydrates (agarose, insulin)
All are initiators or activators of the Alternative Pathway
42
What surface is deficient in sialic acid? What surface has lots of sialic acid?
Deficient - Activator surface (bacteria has a deficient surface)
LOTS - Non-activator surface
43
Formation of the alternative pathway C3 convertase (C3bBb):
- C3 tickover - spontanteous conformational change of a few C3 molecules, leading to water hydrolyzing the thirster bond of C3 to form C3(H2O) = C3b.
- Factor B binds to C3b or C3(H2O) and then Factor D will cleave the bound Factor B. This results in the formation of C3bBb (C3 convertase)
- Properdin acts to stabilize the alternative pathway C3 convertase (C3bBb(
- C3b is deposited continuously in a random and nonspecific way on the surfaces of cells and pathogenic organisms alike. Whether activation occurs is determined by surface properties.
- Surfaces rich in carbohydrates and deficient in sialic acid tend to be the best activators
44
What makes C5 convertase different than C3 convertase?
Added a molecule, C3b to the end of both
| -Trimolecular complexes!
45
What are the two C3 convertase?
C4b2a - classical
| C3bBb - alternative
46
How does amplification occur?
- Occurs in all three pathways
1. Single C3 convertase can cleave many C3 molecules, resulting in the deposition of multiple C3b molecules on the surface
2. A single C5 convertase can cleave many C5 molecules, increasing the probabilty that you will form a complete MAC!
47
Activation of the complement system by antibody coated Strep, pneumoniae leads to the formation of a C3 convertase enzyme. What is the subunit composition for this enzyme?
C4bC2a
48
What complement protein covalently binds to surfaces after enzymatic cleavage and exposure of an internal thioester bond?
C4b
49
Excessive complement activation in immune complex disease would most likely result in depletion of what?
C4
50
What types of interactions occur after C5 is cleaved?
Protein-protein interactions
51
Activation of C5 and the Terminal Complement Pathway:
- C5 is cleaved by either the C5 convertase C3bBbC3b or C4bC2aC3b into two fragments
- C5b then interacts with C6, C7, C8 and C9 to form the MAC in the lipid membrane
- The MAC is a transmembrane channel that allows passage of ions and lysis of the cell
- Cleavage of C5 is the last enzymatic step
52
How does the Membrane Attack Complex (MAC) form?
- If the interaction with C5b-9 occurs in proximity to a membrane, then the MAC assembly occurs in the membrane and lysis occurs
- Alternatively C5b-9 can float away
- C5b-7 can bind to S protein in the fluid phase, preventing membrane insertion and MAC formation
53
What are some key points in the formation of the MAC complex?
- C5b-9 formation involves conformational changes, but not enzymatic reactions.
- Lysis can occur in the absence of C9, but it is slower
- Complement system activation can have important biological consequences even if C5b-9 does not deposit in the membrane and lead to lysis
- C3b and C4b are still covalently attached to the membrane and can result in other biological effects by interaction with complement receptors 1-4 (CR1-4)
54
What things limit complement activation?
1. Short half life of the enzymes formed
2. Properties of non-activator surfaces
3. Inhibitors
- -Fluid phase inhibitors: so active fragments don't go too far
- -Membrane bound inhibitors: on our own membranes, so C3b and C4b don't attach or don't lead to lysis of our own cells
- -Presence of sialic acid
55
How does Factor H serve as a cofactor for cleavage of C3b by Factor I?
1. If Factor H sees C3bBb on membrane with sialic acid (like humans), it will bind to sialic acid residue and C3b, displacing Bb from eh convertase and inactivating C3bBb
2. Factor I can then degrade the C3b with Factor H as a cofactor
3. In an activator surface (bacteria) Factor H cannot bind and replace Bb (no sialic acid present). In this case, Factor H does not inhibit the C3 convertase activity
56
If Factor H can bind...
...then Factor I will come along and chew up the C3b.
57
What if you lack control of complement?
- Deficiencies in complement control proteins can lead to uncontrolled activation of the complement system
- --Consequences of activation - lysis, etc.
- --Consumption of the complement components leading to the consequences of secondary complement deficiency (IC disease and infections)
58
What is C1 Inh Deficiency?
- HAE (hereditary angioedema)
- Recurrent episodes of localized edema in skin, GI tract, or larynx
- C1 inhibitor inhibits C1 esterase, MASP, kallikrein, plasmin, Factor XIa and Factor XIIa
- Uncontrolled complement activation leads to consumption of C4 and C2
- --Possibility of immune complex disease as well
59
How do you treat C1 inh deficiency?
- Anabolic steroids to increase synthesis of C1 Inh
- Purified C1 Inh
- Kallikrein inhibitors and B2 receptor inhibitors
- -Evidence for excess kinin formation responsible for episodes of edema
60
What happens with a deficiency in DAF (CD55) and CD59?
- Increased susceptibility of erythrocytes to MAC-mediated lysis
- Complement-mediated intravascular hemolysis in paroxysmal nocturnal hemoglobinuria (PNH)
- Defect in post-translational modification of the peptide anchors that bind the proteins to the cell membrane
- Recent studies suggest that treatment with an antibody to C5 (eculizumab) reduces hemolysis
61
What does MAC do in the complement cascade?
Lyses cells
62
What does C3a and C5a do in the complement cascade?
Inflammatory mediators (T-cell regulation)
63
What does the CR1 receptor do in the complement cascade?
Solubilization and clearance of immune complexes
64
What does the CR2 receptor (CD21) do in the complement cascade?
Augmentation of humoral immunity
65
Where are the anaphylatoxin receptors?
Neutrophils, endothelial cells, many different cells
66
How do anaphylatoxins work?
They are small chunks of complement proteins.
- C3a and C5a can mimic the symptoms of inflammation and anaphylaxis
- Chemotaxis, smooth muscle contraction, increased vascular permeability, degranulation of mast cells, etc.
- Distinct receptors on many cell types
- Implicated in pathology in many inflammatory diseases
67
What three things can C4b and/or C3b on surfaces do?
1. Participate in continued pathway activation leading to MAC ---> Lysis
2. Interact with CR1 --> Opsonization, Clearance of IC
3. Degraded to fragments --> Interact with CR2 and CR3 --> Opsonization, Clearance of IC, Augmentation of humoral immunity - C2
68
What is CR1 and what is its function?
(CD35)
- Major ligands - C3b, C4b
- Monocytes, macrophages, PMN, Eosinphil, RBC, B and T cells
- Transport of immune complexes by RBC
- Promotes immune adherance (binding of opsonized microbes to primate RBCs)
- Promotes phagocytosis in cooperation with Fc receptors
- Blocks formation of C3 convertase
69
What is CR2 and what is its function?
(CD21)
- Major ligands = C3d, C3dg, iC3b
- B cells, activated T cells, epithelial cells
- CR2 forms an additional signal with antibody to augment stimulation of the B cell to increase the humoral immune response (CR2/CD19/CD81)
- CR2 has high affinity for an envelope protein of EBV, allowing the virus to enter the B cell!
IHO Week 2
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