Download ACTIVATION OF THE COMPLEMENT SYSTEM

THE COMPLEMENT SYSTEM
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
- Membran proteins wich inhibit the lysis of host cells
Most of the complement proteins and glycoproteins
are produced in the liver in an inactive form.
Activation is induced by proteolitic cleavage.
Enzyme cascades of the plasma: complement and coagulation
Markiewski MM 2007
(i) complement activation, (ii) C3-convertase C3a, C3b, (iii) C3a  platelet activation, (iv) C3b  C5-convertase
C5a, C5b, (v) C5a  tissue factor, PAI-1 expression, (vi) C5b  MAC,…(ix) contact activation (intrinsic pathway)
(prekallikrein, factor XII), (x) extrinsic pathway (TF+factor VII), (xi) factor X, (xii) from protrombin (II) to trombin (IIa),
(xiii) trombin  fibrinpolimerisation (fibrin), (xiv) trombin C3 and C5 breakdown
A plazmacascades
Enzyme
enzim rendszerei
of the plasma
Coagulation cascade
Kallikrein-kinin system
Complement cascade
Alternative pathway
MB-lectin pathway
MAC
Classical pathway
AMPLIFICATION OF THE COMPLEMENT CASCADE
limited
inactive precursors proteolysis
enzyme
activating surface
Activating sutface needed!
ACTIVATION OF THE COMPLEMENT SYSTEM
COMPLEMENT SYSTEM
CLASSICAL PATHWAY
MB-LECTIN PATHWAY
ALTERNATIVE PATHWAY
COMPLEMENT ACTIVATION
RECRUITMENT OF
INFLAMMATORY CELLS
OPSONIZATION OF
PATHOGENS
KILLING OF
PATHOGENS
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
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 C-terminal of antibody - Multiple interaction with immune
complexes
Only in classical pathway!
Immunoglobulin Fragments: Structure/Function
Relationships
antigen
binding
complement binding site
binding to Fc
receptors
C1 component
placental
transfer
Association between native and adaptive immunity
Only the antigen-linked antibodies are able to associate to complement. Why?
The classical pathway of complement activation is initiated
by binding of C1q to antibody on a bacterial surface
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
THE CENTRAL COMPONENT OF THE
COMPLEMENT SYSTEM
C3
CGEQ
CLEAVAGE SITE
One of the proteins present at the highest concentration in serum
1.2mg/ml
Is it a lot???
3 900.000.000.000.000 molecules/ml
ACTIVATION OF C3
C3
Active thioester
CGEQ
CGEQ
CGEQ
C3a C3b
Inflammation Binding
OH
R
OH
R
O
R ROH
Cell
CGEQ
OH
OH
R RR
O
Bacterium
Membrane attack complex (MAC)
• The membrane attack complex affects to the
bacterial cell wall, but complement fragments can
be attached to the body’s cell surface also
• Complement-mediated lysis of the cells is blocked
by cell surface and soluble inhibitory factors
• Certain bacteria can activate the C3 complement
component directly (ALTERNATIVE PATHWAY)
• Complement-mediated lysis of bacteria opsonized
by antibodies takes place in the absence of
alternative pathway also
The membrane-attack complex assembles to generate a pore
in the lipid bilayer membrane
MAC in the cell membrane
live
and
bacteria
dead
Homologue components of classical and alternative
pathways
Complement receptors
Name
Ligand
Expression
CR1
C3b>C4b, iC3b
RBC, Mo/MØ, Gr, B
Act-T, FDC
C3d, C3dg, iC3b
EBV, IFNa, CD23
B, activated T, FDC
iC3b> C3dg, C3d
ICAM-1, LPS, fibrinogen
Mo/MØ, Gr, NK
Mo/MØ, Gr, NK
CD11c/CD18
iC3b, C3dg, C3d
Fibriogen
C3aR
C3a
M, B, Gr, Mo/MØ,
Trombocites, simaizom,
Neur
C5aR
C5a,, des-Arg-C5a
M, B, Mo/MØ, Trombocites,
SMC, Neur
C1qR
C1q collagen part
B, NGr, Mo/MØ, endothel
C1qRp
C1q
Fagocyte
CD35
CR2
CD21, CD21L
CR3
CD11b/CD18
CR4
The role of complement system in in vivo
Lectin and alternative
pathway
lysis
MAC
classical pathway
C3
C3a
C3b
C3b
C3b
C4a
C5a
C3b
C3b
opsonization
phagocytosis
OPSONIZATION
C3b
bacterium
complement receptor
macrophage
Local inflammatory responses can be induced by the small
complement fragments C3a, C4a, and especially C5a
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, it can effect in many
steps)
Factor H: inhibits C3-konvertase of alternative pathway, co-factor of
factor I, cleaves C4b and C3b
Properdin: ballasts convertases of alternative pathway
DAF: Decay Accelerating Factor
MCP: Membrane Cofactor Protein
CD59: inhibits the linking of C9 and C8
Regulation of C3 convertase
f= FACTOR
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
Other functions are on the Figure 33.
Regulatory proteins on
human cells protect them from
complement-mediated attack
CD59 prevents assembly of terminal complement
components into a membrane pore
Problem of xenotransplantation
Cascalho M & Platt JL Nat Rev Immunol 2001
•
•
•
The ABO blood group antigens, cell surface carbohydrate components of endothel
cells and the CD55/DAFand CD59 molecules are genus specific.
Xenotransplantation from minipig – the complement regulators on pig cells can not
protect from the attack of the recipient complement system.
Transgenic (human CD55) animals – lower cytotoxic activity of human serum
xenoreactive antibodies against xenotransplanted cells. (Transplant Proc. 2008
Mar;40(2):551-3 )
Deficiencies of complement system – cascade molecules
Deficiencies of complement system – regulatory molecules,
receptors
Hereditary angioneurotic edema (HANO)
(hereditary C1INH defect)
• 17-year old boy - severe abdominal pain (frequent sharp spasms, vomiting)
• appendectomia – normal appendix
• family history of prior illness
• immunologist’s suspicion: hereditary angioneurotic edema
• level of C1INH: 16% of the normal mean
• daily doses of Winstrol (stanozolol) – marked diminution in the frequency and
severity of symptoms
• purified C1INH intravenously – the infusion relieves the symptoms within 25 minutes
Main symptoms:
• swellings of skin, guts, respiratory tracts
• serious acut abdomenal pain, vomiting
• larynx swelling – may cause death
Treatment:
• IV C1INH
• kallikrein and bradykinin receptor antagonists
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
plasminogen
to generate plasmin
cleveage of C2 to
generate C2a
activation of C1
activating surface because
plasmine is always active
Questions
hereditary angioneurotic edema
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?
- In HANE, C4b and C2b both generated free in plasma because plasmine always
actives the C1
- There are not an activating surface, so C4b are not able to bind to a surface, so it is
rapidly inactivated. The concentration of C4b and C2b are relatively low, no C3/C5
convertase is formed.
 C3 and C5 are not cleaved and C3a and C5a are not generated.
 After the complement activation histamine do not release which is caused by
C3a
 Without C5a there are not cell recruitment
BUT there are C2a-kinin and bradykinin which cause edema.
2. Which complement component levels will be decreased? Why?
C2 and C4, because of the continous cleveage by activated C1.
Questions
hereditary angioneurotic edema
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)
• acqired clonal mutation of PIG-A
gene – no GPI-enchored proteins in
the cell membrane
• CD59 (upper pic) and CD55
complement regulatory proteins
• no CD59 and/or CD55: PNH patients
are highly susceptible to
complement-mediated lysis (lower
pic).
• Eleveted levels of TF derived from
complement-damaged leukocytes
• Treating PNH with a humanized antiC5 antibody
GPI= glycosylphosphatidylinositol
Paroxysmal nocturnal hemoglobinuria (PNH)
symptoms and therapy
• haemolytic anaemia (9%) and
associated symptoms (35%)
• haemoglobin and its products
in the urine (26%)
• thrombosis (6%): in brain
veins, mesentheric veins,
vv. hepaticae (Budd-Chiarisyndrome)
• transformation to acut
lymphoid leukemia (ALL)
• aplastic anaemia (13%)
• eculizumab (anti-C5
monoclonal antibody)
• steroids
• Iron replacement
• transfusion
• bone marrow transplantation