Activation of the Alternative Pathway

1
Complement
S. Barbour
Suggested Reading Janeway, Chapter 2, pp.
54-75 Chapter 9, pp. 395-397
Office hours by arrangement Please contact me by
email (sbarbour_at_hsc.vcu.edu) to schedule an
appointment.
2
I. Overview of the Complement System
3
Hallmarks of Complement

• Sequential Activation
• Amplification
• Regulation

4
Overview of Complement
Membrane Attack Pathway Cytolysis of some
pathogens
5
Complement Nomenclature
1. CX (X 1-9) example C5
2. Factor X (alternative pathway) example
Factor B (B)
3. complement fragments (usually a and b)
example C3 ? C3a C3b
Note the b fragment remains cell associated
a fragment is soluble

• Note i denotes inactive fragments that do
  not support complement activation. However,
  these fragments can have other biological
  functions. (eg. iC3b or C3bi)

6
II. Activation of the Complement System
7
Activation of the Classical Pathway
IgG (IgG1, IgG2, IgG3)
IgM
Y
Y
Y
Y
Y
Y
Y
Antigen (bacterial or yeast cell surface)
-Note only antigen-antibodies complexes can
activate the classical pathway! Antibody alone
cannot -Classical pathway results in formation of
a C3 convertase (C4b2b) that generates C3b -C3b
is covalently attached to pathogen triggers
complement activation and biological activity
8
Activation of the Alternative Pathway(Sources of
C3b)
C3 tickover
Y
Y
classical pathway
activating surface (bacterial or yeast cell
surface)
-the alternative pathway is initiated by C3b
binding to a bacterial or yeast cell
surface -this C3b can come from the classical
pathway or C3 tickover -C3b preferentially
interacts with bacterial or yeast cells host
cells are spared -this is a primitive distinction
of self versus non-self
9
Activation of the Alternative Pathway(C3
Convertase)
C3 tickover
activating surface (bacterial or yeast cell
surface)
-alternative pathway results in the formation of
a C3 convertase (C3bBb) that generates C3b -this
is the same C3b that is produced by the classical
pathway two C3b molecules in alternative
pathway -C3 tickover occurs constantly
regulatory proteins prevent C3b from binding host
cells
10
Activation of the Lectin Pathway
mannan binding lectin (MBL)
mannose sugars
MBL
activating surface (bacterial or yeast cell
surface)
-mannan binding lectin (MBL) recognizes mannose
sugars on microbial cells -host mannose is hidden
and is not accessible to MBL -lectin pathway
results in the formation of a C3 convertase
(C4b2b) that generates C3b
11
C3 Convertase and C5 Convertase

• C3 convertases generate C3b
• C3b forms covalent attachment with pathogen
  surface
• Together with other components, attached C3b
  forms C5 convertase
• C5 convertase proteolyzes (activates) C5 last
  step in complement activation

12
II. Regulation of Complement Activation
13
Importance of C3

• Activation of Classical, Lectin, and Alternative
  pathways (alternative pathway is constantly
  activated in serum!)
• Fragments have biological activity
• Opsonization / Phagocytosis
• B cell activation
• Inflammation
• Most abundant complement protein in serum
• ( gt 1 mg/ml)

14
C3 Convertases
C3 convertase
Classical / Lectin Pathway
Alternative Pathway
receptor subunit
receptor subunit
catalytic subunit
catalytic subunit
P
covalent association with pathogen surface
-C3 convertases consist of two subunits one with
catalytic activity, one that binds C3
-subunits must be associated to have active C3
convertase
-regulatory mechanisms are designed to disrupt
this association
15
Regulation of C3 Convertases
Dissociation
Proteolysis (Factor I cofactor)
Protein Classical/Lectin Alternative C4b C3b
Factor H - -
C4bp - -
DAF - -
MCP - -
CR1
In general, regulatory proteins are expressed on
host cells, but not on bacteria. Therefore,
host cells are spared from complement attack.
16
III. Effecter Functions of Complement
17
The Membrane Attack Pathway
MAC
The MAC is especially important for the immune
response against Neisseria spp. Membrane
proteins (CD59, HRF) prevent MAC formation on
host cells.
18
Opsonization / Phagocytosis
C3b or iC3b
opsonized bacteria
CR1, CR3, or CR4
19
Inflammation
anaphylotoxins
C5achemoattraction C3a, C4a----activation
C3a, C4a---increased vascular permeability
20
Complement Proteins and Inflammation

• C4a, C3a, and C5a are pro-inflammatory
  anaphylotoxins

• C5a binds a G protein coupled receptor
• -neutrophils express high levels of the C5a
  receptor
• -C5a binding induces chemotaxis and activation /
  
• increased cytocidal activity

• C3a, C4a bind another G protein coupled receptor
• -C3a/4a receptor is expressed by phagocytes,
  endothelial
• cells
• -C3a/ C4a binding induces cell activation,
  increased
• cytocidal activity, increased vascular
  permeability

21
Clearance of Immune Complexes
CR1
22
Activation of B Cells
or C3dg
-microbial cell expresses antigens recognized by
B cell receptor -microbial cell is coated with
C3d or C3dg, recognized by CR2 -Simultaneous
binding to B cell receptor and CR2 results in
efficient B cell activation
23
F. Clearance of Apoptotic Cells

• Phagocyte recognizes C1q deposited on the surface
  of apoptotic cell

• Apoptotic cell is ingested and destroyed by
  phagocyte

• This may be an important mechanism for clearing
  self antigens

- Alternatively, uptake of apoptotic host cells
by C1q on immature DC may induce self
tolerance
- prevalence of autoimmune disease is very high
in subjects lacking C1q
24
Overview of Complement
25
Clearance of apoptotic cells
B cell activation