THE COMPLEMENT SYSTEM
THE COMPLEMENT SYSTEM
• The complement system is a set of plasma proteins that
act in a cascade to attack and kill extracellular
pathogens.
• Approximately 30 components:
–
–
–
–
activating molecules
regulator factors
complement receptors
membrane proteins wich inhibit the lysis of host cells
• Most of the complement proteins and glycoproteins are
produced in the liver in an inactive form (zymogen).
Activation is induced by proteolitic cleavage.
AMPLIFICATION OF THE COMPLEMENT CASCADE
inactive
precursors
limited
proteolysis
enzyme
activating surface
Activating surface needed!
ACTIVATION OF THE COMPLEMENT SYSTEM
COMPLEMENT SYSTEM
CLASSICAL PATHWAY
MB-LECTIN PATHWAY
ALTERNATIVE PATHWAY
COMPLEMENT ACTIVATION
RECRUITMENT OF
INFLAMMATORY CELLS
OPSONIZATION OF
PATHOGENS
FACILITATING
PHAGOCYTOSIS
DIRECT KILLING OF
PATHOGENS
ACTIVATION VIA THE
CLASSICAL PATHWAY
THE C1 COMPLEX
Collagen „legs”
Gobular „heads”
C1 is always present in serum but it can operate on an activating surface in normal case
Low affinity binding to the Fc region of antibody  conformational change  activation
Multiple interaction with immune complexes
Different isotypes of antibodies activate the
complement system differently
The classical pathway: Fixation of complement,
generation of C3b by the classical C3 convertase
ACTIVATION VIA THE
MANNAN-BINDING LECTIN
PATHWAY
GLYCOSYLATION OF PROTEINS
IS DIFFERENT IN VARIOUS SPECIES
Prokariotic cells
Eukariotic cells
Mannose
Glucoseamine
Mannose
Galactose
Neuraminic acid
(sialic acid)
MANNAN-BINDING LEKTIN ACTIVATES THE
COMPLEMENT SYSTEM
MASP = MBL associated serin protease
ACTIVATION VIA THE
ALTERNATIVE PATHWAY
Alternative pathway is
instantly inactivated on
eukariotic cell surfaces
(in the presence of sialic
acid molecules)
C3b can derive
from classical or the
lectin pathway too
THE CENTRAL COMPONENT OF THE
COMPLEMENT SYSTEM
Complement fixation
Strong covalent binding
(3 900 000 000 000 000 molecules/ml)
C5-CONVERTASE
(C4bC2bC3b)
C3-convertase + C3b=
C5-convertase
The classical and alternative C3-convertase is different in
structure but common in function
MEMBRANE ATTACK COMPLEX (MAC)
MAC in the cell membrane
live
and
bacteria
dead
COMPLEMENT ACTIVATION
SUMMARY
COMPLEMENT SYSTEM
CLASSICAL PATHWAY
Antigen-antibody
complex
C1q, C1r, C1s
Serin protease
C4, C2
C4a*
MB-LECTIN PATHWAY
ALTERNATIVE PATHWAY
Mannose
Pathogen surface
MBL
MASP-1/MASP-2
C3
B, D
Serin protease
C4, C2
C3 CONVERTASE
C3a, C5a
C3b
Inflammatory peptid
mediators
Phagocyte recruitment
Opsonization
Binding to phagocyte CR
Immune complex removal
Terminal C5b – C9
MAC
Pathogen/cell
lysis
The role of complement system in in vivo
Alternative, lectin & classical pathway
lysis
MAC
C3
C3a
C3b
C3b
C3b
C3b
C3b
C4a
C5a
inflammation
opsonization
phagocytosis
Local inflammatory responses can be induced by the small
complement fragments C3a, C4a, and especially C5a
OPSONIZATION
C3b
bacterium
complement receptor
macrophage
Complement receptors
Name
Ligand
Expression
CR1
C3b>C4b, iC3b
rbc, Mo/MF, Gr, B
activated T, follicular DC
C3d, C3dg, iC3b
EBV, IFNa, CD23
B, act. T, foll. DC
iC3b> C3dg, C3d
ICAM-1, LPS, fibrinogen
Mo/MF, Gr, NK
Mo/MF, Gr, NK
CD11c/CD18
iC3b, C3dg, C3d
fibrinogen
C3aR
C3a
M, B, Gr, Mo/MF, platelet,
SMC, neuron
C5aR
C5a,, des-Arg-C5a
M, B, Mo/MF, platelet, SMC,
neuron
C1qR
C1q collagen part
B, NGr, Mo/MF, EC
C1qRp
C1q
phagocyte
CD35
CR2
CD21, CD21L
CR3
CD11b/CD18
CR4
ACTIVATION VIA THE CLASSICAL PATHWAY
REGULATION OF THE
COMPLEMENT SYSTEM
Regulatory proteins on
human cells protect them from
complement-mediated attack
CD59 prevents assembly of terminal complement
components into a membrane pore
Regulation of complement system
Factor I
a-2macrogl
C1Inh
DAF C4bp CR1 MCP
LECTIN PATHWAY
HRF
C-pept.ase N
CD59
Properdin
S-protein
DAF
positive feedback
Fact-H CR1 MCP
Factor I
membrane protein
soluble molecule
MAJOR REGULATING FACTORS OF
COMPLEMENT SYSTEM
C1Inh: C1-inhibitor (serine-protease inhibitor)
Factor I: inhibits both C3 convertases in the presence of co-factors
(C4bp – classical pw., factor H – alternative pw., MCP – both)
DAF(CD55): Decay Accelerating Factor
MCP: Membrane Cofactor Protein
MIRL(CD59): Membrane Inhibitor of Reactive Lysis
Properdin: stabilize convertases of alternative pathway
Deficiencies of complement system – cascade molecules
Not the lysis of cells is the most important
function of the complement system
Deficiencies of regulatory molecules, receptors
One of the major function of C1 INHIBITOR
C1q binds to IgM on
bacterial surface
C1q binds to at least two IgG
molecules on bacterial surface
Binding of C1q to Ig activates C1r, which cleaves
and activates the serine protease C1s
C1INH dissociates C1r and C1s from the active C1 complex
HEREDITARY ANGIONEUROTIC EDEMA (HANE)
(HEREDITARY C1INH DEFECT)
• 17-year old boy - severe abdominal pain (frequent sharp spasms, vomiting)
• appendectomia  normal appendix
• similar symptoms occured repeatedly earlier in his life with watery diarrhea
• family history of prior illness
• immunologist’s suspicion: hereditary angioneurotic edema
• level of C1INH: 16% of the normal mean
• daily doses of Winstrol (steroid) – marked diminution in the frequency and
severity of symptoms
• intravenous purified C1INH became avaible by the time
Main symptoms:
• swellings of skin, guts, respiratory tracts
• serious acute abdominal pain, vomiting
• larynx swelling – suffocation, may cause death
Treatment:
• iv C1INH, FFP, steroid
• kallikrein and bradykinin receptor antagonists
Child with symptoms
of HANE
Pathogenesis of hereditary angioneurotic edema
activation of XII factor
Inhibition by C1INH in many steps
• bradykinin and C2-kinin:
enhance the permeability of
postcapillar venules
activation of
kallikrein
activation of
proactivator
by contraction of endothel
• holes in the venule walls
• edema formation
cleveage of kininogen
to generate bradykinin,
vasoactive peptide
• C1 is always active without
cleveage of C2a to
generate C2-kinin,
vasoactive peptide
cleveage of C2 to
generate C2a
cleveage of
plasminogen to
generate plasmin
activation of C1
activating surface because
plasmine is always active
Q&A
HANE
1. Activation of complement system results in the release of histamine
and chemokines, which normally produce pain, heat and itching. Why
is the edema fluid in HANE free of cellular components, and why does
the swelling not itch?
Histamine release on complement activation and recruiting of leukocytes
is caused by C3a and C5a, both generated by the C3/C5 convertases. In
HANE C1 constantly activate C2 and C4 in the plasma but C4b is rapidly
inactivated because it does not bind to activating surface; for that reason,
and because the concentrations of C2 and C4 are relatively low, no C3/C5
convertase is formed.
Edema is caused by C2-kinin and bradykinin.
2. Which complement component levels will be decreased? Why?
C2 and C4, because of the continous cleavage by activated C1.
Q&A
HANE
3. Would you expect the alternative pathway components to be low, normal or
elevated?
C1 plays no part in the alternative pathway. This pathway is not affected.
4. What about the levels of the terminal components?
The unregulated activation of the early components does not lead to the formation
of the C3/C5 convertase, so the terminal components are not abnormally activated.
5. Despite the complement deficiency in patients with HANE, they are not
unduly susceptible to infection. Why not?
The alternative pathway of complement activation is intact and these are
compensated for by the potent amplification step from the alternative pathway.
6. How might you decide the background of the laryngeal edema
(HANO or anaphylactic reaction)?
If the laryngeal edema is anaphylactic, it will respond to epinephrine.
If it is due to HANO, it will not, C1INH needed.
PAROXYSMAL NOCTURNAL HEMOGLOBINURIA (PNH)
•
Acquired clonal mutation of PIG-A
gene in myeloid progenitors – no GPIenchored proteins in the cell membrane
of affected cells (rbc, plt, wbc)
•
CD59 and CD55 complement regulatory
proteins are GPI-enchored proteins
•
No CD59 and/or CD55  PNH
patients are highly susceptible to
complement-mediated lysis
•
The lysis of red blood cells leads to high
levels of hemoglobins in the blood that
appears in the urine (hemoglobinuria)
•
Elevated levels of TF derived from
complement-damaged leukocytes cause
thromboses
Change in the colour of urine samples
taken from PNH patient during the day
Paroxysmal nocturnal hemoglobinuria (PNH)
symptoms and therapy
• Haemolytic anaemia and
associated symptoms
• Specific th.: eculizumab (Soliris)
- anti-C5 monoclonal antibody
• Haemoglobin and its products
in the urine
• Curative th.: bone marrow
transplantation
• Thrombosis:
• Alternative th.: steroids (general
immunosuppression)
– brain veins,
– mesentheric veins,
– hepatic veins (Budd-Chiarisyndrome)
• Chance of transformation to
acut myelogenous leukemia
(AML), aplastic anaemia,
myelodisplastic syndrome
(MDS)
• Anticoagulants: sc. heparin 
p.o. kumarin
• Iron replacement
• Transfusion (filtered-irradiated
blood)
Abbreviations
C1Inh: inhibitor of C1 and MBL (serin protease inhibitor – multiple effects)
α2-macroglobulin: inhibitor of MBL
C4bp: C4 binding protein - inhibitor of the classical C3 convertase
Factor H: inhibitor of the alternative C3 convertase
Factor I: cleaves C4b and C3b
Properdin: stabilizes the convertases of the alternative pathway
DAF (CD55): Decay Accelerating Factor (of C3 convertases)
MCP (CD46): Membrane Cofactor Protein, cleavage of C3 convertases with factor I
CR1: complement receptor 1, inhibitor of C3 convertases
CD59 (MIRL): Membrane Inhibitor of Reactive Lysis – inhibits binding of C9 to C8
HRF: Homologous Restriction Factor (inhibits binding of C8 and C9)