Download Complement

Complement
Objectives
 Discuss the role of complement in the immune
system.
 Discuss complement regarding its:



Components
Activation pathways
Biological activities
Complement
 System comprised of more than 25
glycoproteins

Make up about 10% of total serum proteins
 Components formed mainly in the liver


C1 forms in intestinal epithelial cells
Factor D forms in adipose tissues
 Components form a cascade, with each step
triggering (and often amplifying) the next step
Complement
 In plasma – the components are in inactive form
 Once activated, each component splits
 The smaller “a” fragment serves to stimulate the
immune system
 The larger “b” fragment further activates the cascade
 The exception to this is C2… C2a is the larger
molecule that promotes the cascade
 Activated components are written with a line over the
letter or number. For example:

____
C4b2a
3 Functions
1. Cell lysis – Cell swells and bursts
2. Opsonization - Neutrophils and
macrophages have receptors for C4b or
C3b, which promotes phagocytosis
3 Functions
3. Regulate immune and inflammatory response
 Immune adherence – enhanced response to an antigen.
Receptors for complement found on Red Blood Cells
(RBCs), platelets, B lymphocytes, endothelial and epithelial
cells
 Anaphylatoxins - chemicals that increase vascular
permeability, contract smooth muscle, and cause the release
of histamine from basophils and mast cells
 C3a, C4a, and C5a are anaphylatoxins
 Chemotaxins – signal leukocytes to migrate to an affected
area
 C5a is also a chemotaxin
 C5b67 promotes monocyte and neutrophil adherence
to blood vessel endothelium, and extravasation
3 Activation Pathways
 Classical
 Alternative
 Lectin
Classical Pathway
 Begins with antibody
sensitization of antigen on cell
 2 Fc pieces in close proximity

IgM better than IgG (1 IgM vs.
800 IgG)
IgG1 & IgG3 better at
activation than IgG2 or IgG4
 C1 = Recognition unit
 C1q
 C1r
 C1s
C1
Y

Recognition Unit
 C1q combines with the Fc piece
6 globular heads attached to
collagen-like tails.
 C1r that acts as a protease on
C1s
 C1r and C1s intertwine with
the tails.
 2 C1q heads must interact with
Fc pieces for complement to be
activated.

Y
Classical - Activation Unit
C4a
C4
C4b2a
C1
C1s
C2
C2b
 C4, C2, and C3 components participate
 Amplification of cascade
 1 molecule of C1s activates approximately 30 C4
molecules.
Activation Unit
 C2 is active only if it binds to C4b before
being cleaved by C1s

This reaction is enhanced if C4b binds to the
antigen rather than being free in serum
 Once C4b2a is formed, antibody is no longer
necessary to ensure cell lysis
Activation Unit
C4a
C3a
C4
C4b2a
C1
C1s
C2
C3
C3b
C4b2a3b
C2b
 C4b2a = C3 convertase
 One C4b2a complex converts about 200 C3
molecules
 Requires Mg+
Activation Unit
 C3 most abundant complement component
 C3 common to all pathways
 C3b on a cell enhances opsonization
 C3b combines with C4b2a to form C4b2a3b =
C5 convertase
Classical – MAC
C4a
C4b2a
C1s
C8
C6
C5a
C4
C9
C7
C3a
C1
C2
C2b
C3
C3b
C4b2a3b
C5
C5b
 Membrane Attack Complex = C5, C6, C7, C8, C9
MAC
 C5b binds to C6 and C7 in serum
 This complex will bind to any nearby cell membrane,
not just the cell that originally triggered the cascade.
(The “innocent by-stander” effect)
 May also form a micelle – free floating sphere- which
has antiviral properties
 C5b678 form a surface on the cell membrane for
polymerization of C9
 A pore forms in the cell, allowing an influx of water
 The cell swells and lyses
Hemolysin

An antibody that can activate complement,
resulting in lysis of an RBC is termed a
hemolysin.
C1
Alternative (Properdin) Pathway
Properdin
C3b
C9
C8
C7
C6
Factor B
C3bBb
Factor D
C5a
C3bBbP
C5
C5b
 Activated by bacteria, fungus, yeast, viruses,
parasites, and tumor cells
 Oldest pathway
 Relies on the natural splitting of C3 into C3a
and C3b (exact process unclear)
Alternative
 C3b and Factor B need Mg+ to combine
 Factor D splits Factor B that has been bound
to C3b
 Properdin stabilizes the C3bBb complex
 C3bBbP is a C5 convertase
 Once C5b formed, cascade continues as in
the classical pathway
Lectin Pathway
C4a
C3a
C4
MBL
C4b2a
C3
C5a
C2
C2b
C9
C8
C7
C6
C3b
C4b2a3b
C5
C5b
 Mannose is a sugar commonly found in
bacterial cell walls
 This pathway can be activated by bacteria,
yeast, viruses and protozoa
Lectin Pathway
 MBL= Mannose Binding Lectin equivalent to
C1q of classical pathway

MBL also increases opsonization
 MASP-1 and MASP-2 function similarly to C1r
and C1s respectively

MASP = Mannose Associated Serine
Proteases
 Once MBL activates the cascade, the
cascade continues as in the classical
pathway
Inhibitors of Complement
 Activated enzymes decay quickly.
 Half life of activated components ranges from
a fraction of a second to approximately 30
minutes.
 If Ca+ and Mg+ are not present, the cascade
can NOT be activated.
 Carboxypeptidase N inactivates
anaphylatoxins.
 Heating serum at 56oC for 30 minutes
destroys complement components.
Specific Inhibitors
 C1-INH irreversibly binds to the active sites of
C1r and C1s.
 Factor I degrades C4b and C3b
 Membrane cofactor protein –cofactor for
Factor I
 C4-binding protein – prevents C4 and C2 from
joining (cofactor for Factor I)
Specific Inhibitors
 Complement Receptor 1 (CR1)
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
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Found as a surface protein on most WBCs
and follicular dendritic cells
Binds C3b allowing degradation by Factor I
Increases clearance of C3b coated cells via
macrophages of liver and spleen
Also increases the length of time antigen
remains near germinal centers of lymph nodes
(May assist with B cell differentiation)
Specific Inhibitors
 Sialic acid of cell membrane inactivates C3b
bound to cells
 Decay Accelerating Factor (DAF) –
dissociates C2 from C3
 S protein – prevents C5b67 complex from
binding to cell
 MIRL – Binds to C8

MIRL = Membrane Inhibitor of Reactive Lysis
 Protectin – Prevents binding of C5b678 to
cell, so no polymerization of C9 is allowed
Inhibitors - Alternative
Properdin
C9
C8
C7
C6
C3b
Factor B
C3bBb
C5a
C3bBbP
C5
C5b
 DAF & Factor H – compete with
Factor B for binding with C3b
Complement Deficiencies
 Complement deficiencies rare
 C2 deficiency most common (1 in 10,000)
 C3 deficiency most problematic as it
participates in all pathways
 Deficiencies in complement components can
lead to:



Increased susceptibility to infection
Accumulation of immune complexes
Autoimmune disease
Pathologic Conditions
 Complement is harmful if:
 Activated systemically
 Activated by tissue necrosis
 Allows build up of immune complexes

Immune complexes play a role in
Goodpasture’s syndrome, SLE, MS, GuillainBarre’ syndrome and other autoimmune
diseases
Pathologic Conditions
 Paroxysmal Nocturnal Hemoglobinuria (PNH)


Abnormal DAF on RBCs
Cells more susceptible to lysis
 C1-INH deficiency – Hereditary angioedema

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C2b accumulates, but does not participate in
the complement cascade
Increases vascular permeability
The End
Can you list the complement components
in order of activation?