Lecture 8: Compliment and phagocytosis Flashcards
What is a complement?
An essential component of out innate immune system
- A collection of plasma protein zymogens that are rapidly activated on contact with microbial surfaces resulting in opsonisation and phagocytosis
What are the possible complement activation pathways?
- Classical pathway (activation is mediated by antibody)
- Lectin pathway (activation is mediated by carbohydrate recognition)
- Alternative pathway (Activated by direct contact with surface)
Where do all three compliment activation pathways converge?
All three pathways converge on the action of complement C3.
What does complement activation result in?
- Opsonisation and phagocytosis
- Direct Killing
- Leukocyte recruitment
- Inflammation
What are some key features of complement cascades;
- They’re enzymatic (esterases; zymogens, i.e pro enzymes that are activated by cleavage of the proceeding enzyme in the pathway)
- Positive feedback loops (C3)
Describe in broad terms the regulation of the complement cascade;
- Extensive control mechanisms that result in the rapid decay of activated compliment proteins
- Cells are protected from lethal effects of compliments by numerous inhibitors and decay accelerators
What mediates the three compliment pathways?
- Classical ; Antibody Fc mediated
- Alternative ; Pathogen surfaces
- Lectin ; Pathogen carbohydrates
Describe steps 1-3 of the classical pathway;
- Antibodies (IgG and IgM) bind to surface epitopes of pathogen
- C1q binds to exposed Fc region
- C1r and C1s bind via C1q
Describe steps 4-6 of the classical pathway;
- Serine proteases cleaves (C1S) C4 into C4b opsonin (C4a, soluble, diffuses away)
- C2 binds to bound C4b and is cleaved by activation C1s (C2a and C2b- soluble diffuses away)
- Formation of the C4b2a, soluble version of C3 convertase
Describe the structure of C1q;
C1q has six globular heads attached to the central body by collagen like strands
How is C1q activated?
Two C1qs heads must bind AB for activation, so need two IgG.
IgM is a much better compliment activator than igG
Describe the first three steps of the mannose binding lectin pathway compliment activation;
- Bound and stabilised MBL interacts with two serine esterases (MASP-1 and MAPS-2)
- This activates C4 akin to the classical pathway
i.e
- Cleaves C4 into C4b opsonin (C4a, soluble, diffuses away)
- C2 binds to bound C4b and is cleaved by activation C1s (C2a and C2b- soluble diffuses away)
- Formation of the C4b2a, soluble version of C3 convertase
Describe the MBL;
MBL ; six headed protein similar to C1q with mannose binding lectins at the end
Can MBL target host cells?
Mannose is not a terminal sugar on eukaryotic cells so host cells are not normally targets
How else can MBL exist?
Analogous to C1q, but can also exist as dimers, trimers and tetramers
What are MASP analogous to?
MBL associated serine esterases (MASP) is analogous to C1r-C1s
Describe the first step in the alternative pathway;
C3 is a major plasma protein that is spontaneously activated by SURFACE CONTACT.
This is called tick over
What is c3?
A highly unstable thioester.
Half life in miliseconds
What does C3b bind and how?
C3b bindsi covalently to surface opsonin in the absence of inhibitors such as
Cr1
MCP
DAF
Describe the steps in the alternative pathway;
- Tickover (C3 -> C3b)
- C3b binds Factor B
- Factor B is split by factor D to form Bb
- C3bBb generates more C3b
How can the compliment pathways be amplified?
1 C3 convertase molecule can generate 1000 C3b opsonin molecules
Some bacteria also bind plasma protein (factor P) that accelerates the alternative pathway by stabilising C3bBb complex.
How does opsonisation occur (via the compliment system)?
Covalently
C3b and C4b, are able to form these unique bonds after cleavage, these can react with hydroxyl groups on on the surfaces of bacteria to form thioester linkages.
ms
What is the effector end of the complement?
Surface bound C4b2a and C3bBb;
- Cleave more C3
- Cleave soluble C5 -> C5b
- C5b initiates the Membrane Attack Complex (MAC) and C5a
What is C5a that C5b has activated?
C5a is a potent anaphylotoxin with multiple activities
C3a and C4a are both anaphylotixins too
What is the membrane attack complex MAC?
- C5b binds C6, then joined by C7
- Conformation change
- Hydrophobic domain of C7 exposed , inserts to target membrane.
- Followed by C8, causes the polymerisation of many C9 molecules
What does the polymerisation of C9 molecules in MAC lead to?
C9 polymers forms a membrane pore, ions out, water in, osmotic lysis
Whats the important role of opsonisation?
The activation of myeloid cells to take up and kill the opsonized pathogen
- Neutrophils and macrophages regularly do this
Describe the process of phagocytosis of an opsonised pathogen;
- Rapidly taken up into the phagosome
- Phagosome fuses with lysozome to form pahgolysozome
- Lytic enzymes and oxidation bi-products and other highly toxic compounds produced, cause terminal damage to the organism
Describe the anaphylotoxins receptors and roles;
- 7 TM gprotein coupled receptor
- C5a is most important receptor
- Regulates inflammation and anaphylaxis
- C4a and C3a can kill bacteria directly
List three examples of C5a function;
3/8
- Binds to C5a receptor on mast cell and endothelial cells
- Induces smooth muscle contraction
- Increases adhesion molecules such as E-selectin to capture neutrophils
List three examples of C5a function;
4-6/8
- Act on neutrophils to increase adherence to endothelium
- Increases vasodilation, blood flow and vascular permeability- fluid accumulation increases lymphatic drainage
- Cause Mast cells to release histamine and TNFa
List three examples of C5a function;
7-8/8
- Local pain, swelling and tenderness
- Activate phagocytes and essential trigger for phagocytosis
What are the function of compliment receptors?
Mostly for C3b and its inactivated products. Stimulates phagocyotsis
Removing immune complexes from circulation
Examples of receptors;
- CR1 and CR2
What is the function of CR1?
CR1, probably most important, on RBC hence numerous
What is the function of CR2?
On B cells, 100x increases in sensitivity of B cells to stimulation by specific Antigen
Main receptor for Epstien barr virus to get into b cells
Why does the compliment system attack normal cells?
There are multiple regulators of complement acitvity and each has a different role to play. Deficiencies in several of these lead to susceptibility to infections
What are some examples of complement regulators?
DAF Compliment Receptor C1 inhibitor C4BP Factor H Factor 1 Properdin
What is the function of DAF?
DAF = Decay Accelerating Factor
- Widely expressed on cells
- Binds any C4b, prevents C2 binding.
- Promotes cleavage of C4b by factor 1 protease thus accelerating its decay. Destabilizes the C4b2a complex.
- Stops opsonisation of host cells by preventing formation of C3 convertase
What is the function of complement receptors in regulation?
Cr1 also inhibits C2 binding - same effects as DAF
What is the function of C1 inhibitor (serpin) in regulation?
Serine protease inhibitor (serpin) which inhibits C1s activity. Regulates classical pathway activation of C3 convertase
What is the function of C4BP in regulation?
Binds C4b and displaces C2
What is the function of factor H in regulation?
Binds C3b and displaces Bb. Cofactor for protease Factor 1 which cleaves C3b
What is the function of factor 1 in regulation?
Cleaves C3b to inactive form (still bound by C1R) and is activated by bound Factor H
What is the function of Properdin in regulation?
Binds to certain bacterial surfaces and stabilizes C3 convertase.
Now believed to be a cofactor for C3 convertase
What are the regulators of C3 convertase?
- Presence of DAF or CR1 dramatically inhibit stability of C4b on surfaces
- C4C2b or C4bBb not formed but degraded by factor 1
What does Protectin or CD59 do?
Widely expressed on cells
Prevents formation of MAC inside cells
What are the two last regulatory factors?
- Natural decay (unstable split products rapidly deactivated by water unless bind solid surface)
- Need for co-localisation of crucial components
