W7L10 - Complement Systems Part 1

Question 1

Introduction to the Complement System

Answer 1

Comprises a large group of roughly 30 circulating membrane expressed proteins making up roughly 10% of serum protein
Complement components synthesised in the liver and by inflammatory cells
It is a major pathway acting to protect against infection and inflammation

Question 2

Zymogens

Answer 2

Inactive proteases that need to undergo enzyme cleavage to become activated

Question 3

Functions of the Complement System

Answer 3

1. Production of Opsonins
2. Production of Anaphylatoxins
3. Direct killing of pathogens
4. Other functions

Question 4

Functions of the Complement System - Production of Opsonins

Answer 4

Opsonin is a molecule that enhances the phagocytosis of particles by macrophages and neutrophils
They coat bacteria and it is these molecules that are recognised by receptors on phagocytes
Fragments from the cleavage of particular complement components coat bacteria
Specific receptors and macrophages or neutrophils bind the complement components and this triggers phagocytosis

Question 5

Functions of the Complement System - Production of Anaphylatoxins

Answer 5

Anaphylatoxins are peptides/fragments from cleavage of complement proteins that induce local and systemic inflammatory responses
They bind receptors on many cell types
- e.g. they bind neutrophils stimulating directional migration - neutrophils migrate from an area of lesser concentration to an area of higher concentration

Question 6

Functions of the Complement System - Direct killing of pathogens

Answer 6

Terminal complement components make pores in bacterial membranes

Question 7

Functions of the Complement System - Other functions

Answer 7

Maintenance and homeostasis by removing dead and dying cells and immune complexes
Enhancing antigen specific immune responses

Question 8

Complement System Pathways

Answer 8

Classical - initiated by binding of C1q to pathogen surface
Mannose-binding Lectin - initiated by binding of mannose-binding lectin to carbs on bacteria and viruses
Alternative - initiated by binding of spontaneously activated C3 in plasma to pathogen surfaces

Question 9

Complement System Nomenclature

Answer 9

Components of classical pathway - C and number e.g. C3
Products of cleavage reactions - lower case letters, the larger fragment being b and the smaller a (except for C e.g. C1b smaller than C1a)
Components of alternative pathway - different letters e.g. factor D and B
MB-Lectin pathway - first enzymes activated are the mannose-binding lectin associated serine proteases MASP-1 and 2
Convertase - name for a complement enzyme that converts an inactive complement protein into an active one

Question 10

What is the first component of the classical pathway?

Answer 10

C1 complex
Consists of C1q which is made of 6 subunits with globular heads and collagen-like tails
These bind to 2 molecules of C1r and C1s make up the complex
Antibody-antigen complexes are the predominant activators
- globular parts of C1q bind to receptors on Fc region of either one IgM or two closely spaced IgG molecules that are bound to pathogen
When C1 binds to complex C1s becomes enzymatically active making it C1s esterase

Question 11

Classical Pathway Steps

Answer 11

• C1s esterase cleaves C4 -> C4a and C4b which attaches to pathogen surface
• pathogen attached C4b binds C2 making C2 susceptible to cleavage by C1s esterase
• C2b in fluid phase and C2a forms complex with C4b to make C4b2a
• C4b2a remains linked to surface of pathogen and is the C3 convertase of pathway
• C2a component is the active protease and it cleaves many C3 molecules to produce C3b which binds to pathogen surface

Question 12

Mannose Binding-Lectin Pathway

Answer 12

Mannose binding lectin specifically binds to mannose residues and certain other sugars arranged in a pattern that allows binding
- common on bacteria but not on mammalian cells
- means of discriminating self from non self
MBL has two associated proteases MASP-1 and 2

Question 13

Mannose Binding Lectin Pathway Steps

Answer 13

• MBL binds to a pathogen surface MASP-2 is activated to cleave C4 and C2
• like the classical pathway the cleavage of C4 by MAST-2 produces C4a (in fluid phase) and C4b which attaches to surface of bacteria
• MAST-2 then cleaves C2 producing C2b which leaves in the fluid phase and C2a which combines with C4b to form the C3 convertase C4b2a
• C3 convertase cleaves many C3 molecules to produce C3b which binds to the pathogen surface

Question 14

Alternative Pathway

Answer 14

Occurs on microbial surfaces without antibody and without a pathogen binding protein
Initiated through spontaneous hydrolysis of C3
C3b binds to hydroxyl groups on proteins and carbs on cell surfaces
Serum factor B combines with C3b on cell surface (C3bB)
Factor D cleaves B in C3bB complexes releasing Ba and leaving Bb attached to C3b -> C3bBb which is the alternative pathway C3 convertase

Question 15

Properdin and Properdin stabilised C3bBb

Answer 15

C3bBb rapidly dissociates unless stabilised by properdin
Properdin stabilised C3bBb rapidly cleaves high levels of C3 to C3b
Alternative pathway must be regulated to avoid injury of self cells/tissue

Question 16

C5 convertase - Classical Pathway

Answer 16

Classical pathway C3 convertase cleaves C3 to give C3a (fluid phase) and C3b (binds to bacterial surface)
On the bacterial surface C3b may act as an opsonin or ti can combine with C3 convertase for form a C5 convertase
C5 convertase formed when C3b binds C4b2a giving C4b2a3b

Question 17

C5a and C5b

Answer 17

C5 convertase cleaves C5 -> C5a and C5b
C5a - potent fluid phase anaphylatoxin
C5b - binds to the pathogen surface to form the nucleus for the binding of the terminal complement components

Question 18

C5 convertase - Alternative Pathway

Answer 18

Same as classical pathway except C5 formed when C3b binds to C3bBb giving C3bBb3b which is the alternative pathway C5 convertase