Chapter 7 The Complement System
Complement-induced lesions on the membrane of a RBC
Nov 7 & 9, 2006
Discovery of Complement
1890s Jules Bordet (Institut Pasteur in Paris) observed:
1. Sheep antiserum to the bacterium Vibrio cholerae
caused lysis of the bacteria.
2. Heating the antiserum destroyed its bacteriolytic
activity.
3. Addition of fresh normal serum, that contained no
Abs against the bacterium and was unable to kill
the bacterium by itself, restored the ability to lyse
the bacteria by the heated antiserum.
Q1: If you were Bordet, what would
you hypothesize?
A1: Bacteriolytic activity requires 2 different
substances.
a. the specific antibacterial Abs, which
are resistant to the heating process
b. heat-sensitive component responsible
for the lytic activity
c. 56˚C, 30 min will inactivate complement
Paul Ehrlich in Berlin carried out similar
experiments and named the substance
complement defining it as “the activity of
serum that completes the action of Ab.”
Bordet won the Nobel Prize in 1919 –
Complement-mediated bacteriolysis
Complement Plays a Key Role in Both
Innate and Adaptive Immunity
- The complement system is the major effector
of the humoral branch of the immune system.
- Although the discovery of complement and
most early studies were linked to the activity
of complement following Ab binding, a major
role for this system is the recognition and
destruction of pathogens based on recognition
of pathogen-associated molecular patterns, or
PAMPs, rather than on Ab specificity.
The Complement Components
- More than 30 soluble and cell-bound proteins
- Participate in both innate and adaptive immunity
- Produced by hepatocytes (mainly), monocytes and
epithelial cells of the gastrointestinal and genitourinary
tracts
- Constitute 5% (by weight) of the serum globulin
fraction
- Many components are proenzymes (zymogens), which
are functionally inactive until proteolytic cleavage,
which removes an inhibitory fragment and exposes the
active site.
- Reaction starts with an enzyme cascade.
The Functions of Complement
1. Lysis of cells, bacteria, and viruses – the major effector
of the humoral branch of the immune system
2. Opsonization, which promotes phagocytosis of
particulate Ags
3. Binding to specific complement receptors on cells of
the immune system, triggering specific cell functions,
inflammation, and secretion of immunoregulatory
molecules
4. Immune clearance, which removes immune complexes
from the circulation and deposits them in the spleen
and liver
Complement Activation
1. Classical Pathway – begins with the formation of Ag-Ab
complex
2. Alternative Pathway – is initiated by cell-surface
constituents that are foreign to
the host
– Ab-independent
3. Lectin Pathway – is activated by the binding of mannosebinding lectin (MBL) to mannose
residues on glycoproteins or
carbohydrates on the surface of
microorganisms
– Ab-independent
Classical Pathway Begins with Ag-Ab Binding
C1 molecule
soluble Ag-Ab* or bacteria-Ab*
↓
conformational changes in the
Fc portion of Ig
↓
expose a binding site on the
CH2 domain of the Fc portion
for the C1 component of
the complement system
* complement-activating Abs:
IgM, IgG1, IgG2, IgG3 (human)
C1qr2s2 stabilized by Ca++
C1q binds to Ag-bound Ab
↓
Binding of C1q to Fc induces a conformational change in C1r
↓
__
C1r converts to an active serine protease enzyme, C1r, __
which cleaves C1s to a similar active enzyme, C1s
CH2 domain
of the Ab
bacterium
__
C1s has two substrates, C4 and C2
__
↓
C1s hydrolyzes C4 into C4a and C4b,
and hydrolyze C2 into C2b and C2a
↓
___
C4b and C2a form a C4b2a complex, also called C3 convertase,
referring to its role in converting the C3 into an active form.
an anaphylatoxin*,
or
a mediator of inflammation
* Anaphylaxis
C2a
C4b
(
)
____
C4b2a (C3 convertase) hydrolyzes C3 into C3b and C3a ☺
↓
____
______
C3b binds to C4b2a and form C4b2a3b (C5 convertase)
_______
↓
C4b2a3b cleaves C5 into C5b and C5a
opsonization
inflammatory
responses
inflammatory
responses
©
_____
Hydrolysis of C3 by C3 Convertase C4b2a
formation of a labile
internal thioester
bound in C3
bind to free – OH
or – NH2 groups on
a cell membrane
generates > 200 C3b
☺®
Bound C3b exhibits various
biological activities, e.g., binding
of C5 and binding to C3b
receptors on phagocytes.
The Alternative Pathway Is
Ab-independent
- The activation of alternative pathway doesn’t
need Ab; thus, it is a component of the innate
immune system.
- It is initiated by cell-surface constituents that
are foreign to the host, e.g., bacterial cell wall.
- C1, C4 and C2 are not involved in the alternative
pathway.
- Four serum proteins, C3, factor B, factor D, and
properdin, are involved in this pathway.
plasma C3, with an unstable thioester
bond, can be hydrolyzed
spontaneously into C3a and C3b.
C3b attaches to the surface of bacteria,
yeasts, viruses (or even host’s own
cells ®).
Mg++
Ba
(stabilization
of C3bBb)
___
analogous to the C4b2a complex in the
classical pathway
©
The Lectin Pathway Originates with Host
Proteins Binding Microbial Surfaces
Lectin: proteins that bind to a carbohydrate
MBL (mannose-binding lectin):
- an acute phase protein which binds to mannose
residues on glycoproteins or carbohydrates on
the surface of microorganisms (structurally
similar to C1q)
MASP-1 & MASP-2: MBL-associated serine protease
(structurally similar to C1r and C1s)
- MBL is induced during inflammatory responses.
- After MBL binds to the surface of a microbe,
MBL-associated serine proteases, MASP-1 and
MASP-2, bind to MBL.
- The MBL-MASP-1/2 complex mimics the
activity of C1r and C1s, and causes cleavage and
activation of C4 and C2.
- Thus, the lectin pathway is Ab-independent. It is
an important innate defense mechanism comparable
to the alternative pathway, but utilizing the elements
of the classical pathway, except for the C1 proteins.
Q2: Why doesn’t mannose-binding lectin (MBL)
bind to host carbohydrates?
A2: Mammalian cells normally have sialic
acid residues covering the sugar groups
recognized by MBL and are not a target
for binding.
The Three Complement Pathways Converge at the
Membrane-attack Complex
The three complement
pathways converge at
the production of an
active C5 convertase
↓
C5b6789
membrane-attack
complex (MAC)
Formation of C5b6789,
Membrane-attack Complex
C5b attaches to C6, then to C7, and the
C5b67 complex inserts into the
membrane.
↓
binding of C8 to membrane-bound
C5b67 induces a 10 Å pore.
↓
binding and polymerization of C9, a
perforin-like molecule, to C5b678
↓
The completed membrane-attack
complex (MAC) has a tubular form and
functional pore size of 70 – 100 Å
Lysis of an E. coli by Complement
Strict Regulation of the Complement System
- Discrimination between microorganisms and self
- Passive mechanisms of regulation :
highly labile components that undergo
spontaneous inactivation if they are not
stabilized by reaction with other components
- A series of specific regulatory proteins:
inactivate various components
a
b
c
d
e
f
g
h
i
j
(B)
____
C4b2a
____
C3bBb
C3 convertase
b
c
f
(C)
h
i
j
Biological Effects Mediated by Complement
1. Cell lysis
The membrane-attack complex can lyse a broad spectrum
of cells:
G(-) bacteria
parasites
viruses
erythrocyte
nucleated cells (tumor cells)
Because the activation of alternative and lectin pathways is
Ab-independent, these pathways serve as important innate
immune defenses against infectious microorganisms.
2. Inflammatory response
- Various peptides generated during activation of
complement play a decisive role in the development
of an effective inflammatory response.
- C3a, C4a, C5a (called anaphylatoxin) bind to
complement receptors on mast cells and basophils
and induce degranulation with release of histamine
and other mediators.
- The anaphylatoxins also induce smooth-muscle
contraction, increased vascular permeability,
extravasation, and chemoattraction (induced by
C5a, C3a, and C5b67)
3. Opsonization
- C3b is the major opsonin of the complement system,
although C4b and iC3b also have opsonizing activity.
binds to the surface of microbes
C3 convertase
Opsonization by Ab and complement
4. Viral neutralization
- Formation of larger viral aggregates reduces the
net number of infectious viral particles
- The deposits of Ab and complement on viral
particle neutralizes viral infectivity by blocking
attachment to susceptible host cells and facilitates
binding of the viral particle to cells possessing
FcR or CR1.
5. Clearance of
immune complexes