Complement Flashcards
Why is complement so named
It was identified as a heat-labile component of the serum that complemented the antibacterial properties of Ab
Are complement molecules always free
Some are soluble and some are membrane associated
In general, is fragment b bigger or smaller
Bigger
What are the 3 major functions of complement
Name some additional roles
Activation of inflammation
Opsonisation
Lysis of target cells
Clearance of immune complexes and apoptotic cells, stimulating adaptive responses
Which unusual bond in C3 is v important
Internal thioester bond
What happens when C3 is cleaved into C3a and C3b
What does this allow
The thioester bond in C3b is exposed and can react with hydroxyl or amino groups
Covalent linking of C3b to pathogen surface, opsonising the pathogen for phagocytosis
Also stimulates generation of more C3b
What does C3a do
It is an anaphylotoxin that stimulates local inflammation
Probably the most important function of complement is to facilitate the uptake and killing of pathogen is by phagocytic cells. How is this controlled?
Complement Receptors (CRs) that recognise proteolytic derivatives of C3
Which CRs are particularly important for the phagocytosis of complement tagged bacteria?
CR1 and CR3
CR1 binds directly to C3b
In the absence of an antibody, macrophages require an additional signal to C3b. What is this signal?
C5a and C5a receptor
Which blood cell highly expresses CR1
What does this mean
erythrocytes
It can bind C3b attached to the antibody/antigen immune complexes so they can be removed from the blood for clearance in the liver
What causes immune-complex disease
Low CR1 levels
What are the 3 major pathways that lead to activation of complement of
What order do they act in
1st - Alternative pathway
2nd - Lectin pathway
3rd- Classical pathway
What does the alternative pathway in complement rely on
Spontaneous conformational changes in C3 that expose the internal thioester group
What does the spontaneous exposure of the internal Thioester group in C3 lead to an aqueous environment?
What does this allow
Generation of C3H2O
Association with factor B
What can happen when C3(H2O) has associated with factor B?
What happens next?
It is susceptible to cleavage by the factor D
The large fragment (Bb) remains associated with C3(H2O) —-> C3(H2O)Bb
What type of enzyme is a factor D
Serine protease
What does C3(H2O)Bb do
What is another name for this molecule
What is its role
It is a protease that can cleave C3 into C3a and C3b
Aqueous/ fluid phase C3 convertase
To produce enough C3b so that some will attach to the activating surface of the pathogen
True or false
Microbial services are by default “activating”
True
They generally lack regulatory mechanisms to deactivate complement
What happens to C3B when attached to a surface in the alternative pathway
What is this called
It combined factor B and generate surface bound C3bBb
Alternative pathway C3 convertase
What is Alternative pathway C3 convertase able to do
What do these molecules do
Generate more membrane-bound C3b and soluble C3a
C3b acts as an opsonin and provides a positive feedback loop to form more C3bBb
What is alternative C5 convertase?
Instead of binding to a service, cleaved C3b can remain attached to C3bBb to form C3b2Bb
What does C3b2Bb do?
What is the point?
Cleaves C5 into C5a and C5b (C3b2Bb = Alternative C5 convertase)
To form the membrane attack complex
What happens in the alternative pathway when C5b is formed
C6 and C7 bind to C5b, allowing C7 to insert into the lipid membrane
C8 binds to C5b67 - this allows insertion into the membrane and subsequent polymerisation of C9 subunits to form a membrane pore
Did alternative pathway is effective against which pathogens
Gram negative bacteria and enveloped viruses
How is the Lectin pathway activated
Mannose binding lectin (MBL) and M-,H-, or L-ficolin
What kind of molecule is MBL
What is it comprised of
A ‘collectin’ and a PRR
Collagen and Lectin domains, hence COLL- LECTIN
MBL is a PRR. what does it bind
Close knit a raise of mannose and fructose residues that are found on microbial services of bacteria, yeast, fungi and viruses but not the host
Are MBL molecules membrane bound
What are they associated with
No
They circulate in plasma in association with 2 serine proteases
What are the 2 MBL associated serine proteases
When do they activate
What do they do
MBL-associated Serine Protease (MASP) 1 and 2
Upon binding the pathogen surface
Cleave C4 and C2
Why is C4 similar to C3
Both have an internal thioester bond and C4b can attach to the pathogen surface
What happens when C4b binds to the pathogen surface
It is joined by C2a to form C4bC2a
What is another name for C4bC2a
The Classical C3 Convertase
True or false
Complement components usually act sequentially
This is mostly true of C1-9 with the exception of C4
What is unusual about C2
C2a > C2b
When can the Classical Pathway be activated
If the surface of the pathogen can be recognised by antibody or C-reactive protein
How does the binding of antibody to a surface help the classical pathway
It exposes a binding site for C1q present in the Fc region of some antibody isotypes
What happens when a molecule of C1q binds to several Fc regions?
The conformational change activates the C1r and C1s subunits
What does C1s do
What happens to C1 next
It is able to cleave C4 to expose the thioester group which can covalently attach C4b to pathogen surface
Activated C1 associates with and cleaves C2, allowing C4bC2a
What is C4bC2a
What does it do
The Classical C3 Convertase
Cleaves C3 to allow C3b to bind to microbial surface
This then uses the alternative pathway amplification loop by binding to factor B to generate C3bBb
What is a classical pathway C5 convertase
C4bC2aC3b
Generated when C4bC2a (classical C3 Convertase) associates with C3b
How long is the half life of C3bBb
Is this representative for the life span of complement components
~90 secs
Yes they are relatively unstable and quickly lose activity
Name a positive regulator of the alternative pathway
How many positive regulators are known
Properdin (Factor P)
Only 1
What does Factor P do
Stabilises C3bBb and increases activity 5-10 fold
Stabilises 5 convertase complex assisting in MAC complex formation
What is Factor I
A soluble constitutively active serine protease
What does Factor I degrade
Does it do this alone?
C3b and C4b
It requires cofactors such as MCP and Factor H
What is MCP
Where is it expressed
What does it do
Membrane Cofactors Protein
Host cell membranes
Induces dissociation of C4bC2a and C3bBb, and cleavage of C3b and C4b by Factor I
True or false
MCP has decay accelerating capacity but not factor I cofactor activity
False
It has both decay accelerating capacity and factor I cofactor activity
Which Complement Receptor acts in a similar way to MCP
CR1
What is the main control factor responsible for regulating complement activation in solution
Factor H
What is factor H
A soluble cofactor for Factor I that attaches sialic acid present on host membranes
What does Factor H attach to
Where is this found
Sialic acid
Host membranes but is absent in most bacteria
What does Factor H induce
Dissociation of C3bBb complexes and makes C3b susceptible to cleavage by Factor I
What is DAF
Where is it found
What does it do
Decay accelerating factor
On the surface of host cells
Dissociates the classical (C4bC2a) and alternative (C3bBb) C3 convertases
What is protectin
A host cell surface protein that binds the MAC intermediate C5b678 and prevents its insertion into membranes
Also interacts with C9 and prevents its recruitment to the MAC complex
How do early components of complement pathways facilitate clearance of soluble immune complexes?
Components of Which pathway is responsible for this
Coating them with C4b and C3b
Classical pathway
Who suffers from immune complex disease
Individuals with defects in C1-4 as they cannot clear immune complexes from the blood
What does defects in C3 result in
Why is this
What else can lead to the same result
Susceptibility to pyogenic infections (ie pus forming infections such as staphylococcus and streptococcus)
It is an important opsonin assisting in phagocytosis of bacteria
Defects in activating C3
What is the most effective defence against Neisseria
Who would suffer from recurrent Neisseria infections
Complement mediated lysis
Individuals with deficiencies in membrane attack components C5-9
Control of which kind of bacteria relies heavily on opsonisation
Encapsulated bacteria
Defects in generation of which complement components result in susceptibility pyogenic infection
C3b
Factor D and properdin
What can lead to C3 depletion
What does this result in
Uncontrolled alternative pathway activation
Factor I and Factor H deficiency
And leads to susceptibility to pyogenic infection
Why does a reduced GPI synthesis lead to reduced DAF and protectin on RBCs
What does this cause
DAF and protectin are GPI-anchored proteins
Paroxysmal Nocturnal Hemoglobinuria
What is PHN?
Paroxysmal Nocturnal Hemoglobinuria
Complement mediated lysis of RBCs because of reduced expression of DAF and protectin on erythrocytes
Name 4 complement evasion strategies
Interference of antibody-complement interactions
Binding and inactivation of complement components
Destruction of complement components by proteases
Mimicry of inhibitory regulators or recruitment of host inhibitors
Which pathogen is a good example of complement evasion
Staphylococcus aureus
How does staphylococcus aureus avoid complement (6)
The cell wall is protected by a polysaccharide capsule to prevent opsonisation
Secretes proteins that bind or degrade C3
Staphylococcal Protein A binds the Fc region of IgG blocking complement recruitment
Clumping factor A recruits Factor I to the bacterial surface
Chemotaxis Inhibitory Protein blocks C5a chemotactic receptor on neutrophils
Neutrophil extravasation is interfered with
What happens if C3 Convertase does manage to assemble at the site of staphylococcus aureus infection
C3 Convertase is bound and inactivated by Staphylococcus Complement Inhibitor (SCIN) preventing generation of opsonins (eg C3b)
