Download PG2003 Lecutre 14 The Complement Cascade

TR056/PG2003
Lecture 14
The Complement Cascade
Dr Neil Docherty
My Teaching Objectives
• To explain the role of the complement system as a key pathogen
recognition and targeting system in response to infection
• To illustrate how the complement system components are generated
by three main routes all of which rely on cascades of enzyme
activation
• To emphasise the “classical” activation route as a salient example
of the concept that innate and adaptive immune responses are
functionally linked
Complement Cascade
• 
• 
30 plasma and cell-surface complement components mainly liver derived
Originally shown to complement antibody mediated opsonisation and killing of
bacteria (Nobel Prize Physiology or Medicine 1919 Jules Bordet)
3 pathways involved in complement
attack upon pathogens
1.  Classical pathway
2.  Alternative pathway
3.  Mannose-binding lectin
pathway (MBL-MASP)
Complement Effects
1. 
2. 
3. 
4. 
5. 
Initiation of an inflammatory response
Recruitment of phagocytes to site of infection
Direct destruction of gram negative bacteria
Assistance in phagocytosis
Promotion of phagocyte oxidative burst activity and
granule release
6.  Generation of a humoral immune response
7.  Clearance of soluble immune complexes from
circulation
Alternative Pathway
•  C3 convertase formation splits C3 into
C3b (large) and C3a (small)
•  C3bBb convertase requires 3 other
serum proteins: factors B & D,
properdin and C3b
•  Activity is harmless at host cell surface
•  Activity is increased on surface of
bacteria, fungi, protozoa
•  Large amounts of C3b deposited
Lectin Pathway
• 
• 
• 
• 
• 
Mannose-binding lectin (MBL) recognise microbial surfaces to generate C3
convertase
Sugars such as mannose are expressed on microbial cell surfaces that MBLs
can bind to.
Activate proteases
•  MASP-1 & MASP-2 (Major Binding Protein –associated serine protease)
Cleave 2 complement components: C4 & C2
Results in the formation of C3 convertase C4b2a, acts similarly to C3bBb
Mannose Binding Lectins (MBLs)
and C3 Activation
•  MBLs recognize sugar patterns that are unique to pathogens
(hidden by sialic acid on vertebrate cells)
•  Collectins since they have a collagen like domain & globular
lectin domain
•  MASPs are proteases that generate the active C3 convertase
•  Classical pathway has analogous components (next slide)
Classical Pathway
Note that the basic
Structures are similar
• 
• 
• 
Antigen complexed antibodies are bound by complement component C1q
via constant regions of heavy chains
C1q action with other complement proteins, C1r+C1s, induces cleavage of
C4 and C2 to form the C4b2a component (C3 convertase)
N.B. Not just sugar residue restricted response as it occurs via antibody
Classical C3 Convertase Formation
Effector Pathways Downstream of
C3 Conversion
•  Common downstream cascade for 3 complement pathways
•  C3b is the primary mediator of biological effects of complement activation
•  Once C3b binds to a pathogen; it attaches to C3b receptors on phagocytic
cells inducing phagocytosis
•  C3b cleaves C5 to C5b; associates with C6, C7, C8 & C9 components to
form a membrane attack complex (MAC)
•  MAC inserted into membranes of microbes eg gram (-) bacteria; induces
leakage and death
•  Small fragment C3a & C5a promotes inflammation by binding to mast cells
Cascade of events during classical
complement activation
AntigenAntibody
complex
C1
C2a
C4b
C3
C3b
C5
C5a
C3b
C3a
C5b
C6,C7,C8,C9
Centrally Important Product=C3
Processes
-Zymogen activation
-Opsonisation and phagocytosis
-MAC formation
-Pro-inflammatory Signalling
Opsonisation
•  Opsonisation is the process whereby plasma factors act on
bacteria to facilitate phagocytic attraction
•  Opsonins
-Antibody
-Complement proteins (C3b is main opsonin)
•  Coated bacteria bind to receptors on neutropils cell
membrane and trigger lysis of cell and respiratory burst
Phagocytosis and Complement Receptors
Type 1, 3,4 Bind C3b Phagocytosis Which cells?
How might this link with antigen presentation?
Complement Components and Receptors
(More Than Phagocytosis)
Receptor Expressed by Binds to Effect on cell func6on CR1 (CD35) Dendri>c cells, B cells, macrophages C3b, C4b Binds immune complexes, phagocytosis with C5a CR2 (CD21) B cells, dendri>c cells C3d,C3dg,iC3b Binds EBV, B cell ac>va>on (component of B-­‐cell receptor complex) CR3 (CD11b) Macrophages, Neutrophils, NK cells iC3b Phagocytosis, Cell adhesion CR4 (CD11c) Monocytes, Macrophages, Neutrophils iC3b Phagocytosis C3a/4aR Mast cell, basophils C3a, 4a Degranula>on C5aR Mast cells, basophils, C5a monocytes, macrophages Degranula>on, migra>on, adhesion of phagocytes to endothelium Formation of MAC
How would a large pore
potentially disrupt and
Kill the bacteria
i.e. What is the physiological
Consequence?
Regulation of complement
C1 Inhibitor Binds to C1r, C1s and removes it from C1q CD46 Inac>vator of C3b & C4b Factor I Serine protease cleaves C3b & C4b aided by H C4-­‐Binding protein Binds C4b & displaces C2b or Binds C3b displaces Bb Factor H Binds C3b Cofactor for I Decay Accelera>ng Factor (DAF) Displaces Bb from C3b & C2b from C4b Pathogens can escape from complement control
Anti-complement mechanisms:
•  Peptidoglycan of Gram-positive membranes prevent insertion
of the MAC complex
•  Leishmania can expel MACcomplex from its membrane &
reseals
•  Elastase of Pseudomonas destroy C3b and C5a
•  Proteins of viruses, bacteria and others mimic the inhibitors of
complement which are involved in its regulation
Complement defects
Decreased C1s esterase inhibitor Low levels of C1, C2 or C4 Low levels of C2 & C4 Decrease in clearance of an>gen-­‐an>body complexes Inflamma>on & >ssue swelling Hereditary Angiodema Deposi>on of immune complexes damages kidney Lupus erythematosus Low levels of C5-­‐
C9 Absence of C3 Forma>on of ly>c membrane-­‐
aYack complex Survival extremely rare Predisposed to infec6ons such as gonorrhoea & meningococcal meningi6s Your Learning Objectives
Your learning should focus on being able to;
1) Explain how the complement system components are generated
by three main routes all of which rely on cascades of enzyme
activation
2) Describe the “classical” activation route as a salient example
of the concept that innate and adaptive immune responses are
Functionally linked