COMPLEMENT SYSTEM
D.VENKATESHWARLUROLL NO;BBM051505
DEPORTMENT OF BIOCHEMISTRY AND MOLECULAR BIOLOGYCENTRAL UNIVERSITY OF KERALA
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 consists of more than 20 proteins present in plasma and on cell surfaces that interact with each other to produce biologically active inflammatory mediators that promote cell and tissue injury
Nomenclature:
a. the first component of complement is named C1 (etc.) other components are designated by capital letters and names: Factor B, Properidin
b. when cleaved: fragments of complement components are designated by small letters (e.g. C3a and C3b)
C3C3a
C3b
Factor B Ba + Bb
Factor H
Factor I
Complement Activationcomplement activation involves 3 pathways which forms separate
homologous varients of C3 convertase
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 mannose- binding lectin (MBL) to mannose residues on glycoproteins or carbohydrates on the surface of microorganisms – Ab-independent
C1 complex
C1 exists in blood serum as a molecular complex containing:•6 molecules of C1q•2 molecules of C1r•2 molecules of C1s
•The constant regions of mu chains (IgM) and some gamma chains (IgG) contain a binding site for C1q.
The first protein in the classical pathway is C1
• IgG The C1 must bind to at least
two IgG molecules that are close enough together so that it can bind to both of them at the same time.
• IgMThe C1 must bind at least 2 CH3 domains of one IgM molecule to be activated.
IgM is the best complement activator because it is a pentamer.
The building of a C3 activation complex
• Once C1 is activated, it activates 2 other complement proteins, C4 and C2 by cutting them in half
• C4 is cleaved into C4a and C4b
• C2 is cleaved into C2a and C2b
• Both C4b and C2b bind together on the surface of the bacteria
• C4a and C2a diffuse away
C3 Activation complex• C4b and C2b bind
together on the surface to form a C3 activation complex
• The function of the C3 activation complex is to activate C3 proteins.– This is done by cleaving C3
into C3a and C3b
C3b• Many C3b molecules are produced by
the C3 activation complex.• The C3b bind to and coat the surface of
the bacteria.
• C3b is an opsonin– Opsonins are molecules that bind both to
bacteria and phagocytes– Opsonization increases phagocytosis by 1,000
fold.
Bacteria
Opsonins
Macrophages and neutrophils have receptors for C3b and can bind the C3b-coated cell or particle preparatory to phagocytosis.
C3a
C3a increases the inflammatory response by binding to mast cells and causing them to release histamine
This small fragment is released into the surrounding fluids. It can bind to receptors on basophils and mast cells triggering them to release their vasoactive contents (e.g., histamine). Because of the role of these materials in anaphylaxis, C3a is called an anaphylatoxin.
the C5 activation complex
• Eventially enough C3b is cleaved that the surface of the bacteria begins to become saturated with it.
• C2b and C4b which make up the C3 activation complex has a slight affinity for C3b and C3b binds to them
• When C3b binds to C2b and C4b it forms a new complex referred to as the C5 activation complex
The C5 activation complex
The C5 activation complex (C2b, C4b, C3b) activates C5 proteins by cleaving them into C5a and C5b
Many C5b proteins are produced by the C5activation complex. These C5b begin to coat the surface of the bacteria.
The function of C5a
C5a disperses away from the bacteria.› Binds to mast cells and increases inflammation.› Most powerful chemotactic factor known for leukocytes
MAC: a lytic complex of the terminal
components of the complement cascade, including C5,6,7,8 and multiple copies of C9, that forms in the membrane of target cells. The MAC causes lethal ionic and osmotic changes in cells.
Membrane Attack complex
C5 convertase cleaves C5 into C5a and C5b.
C5 convertase
C5b
C5a
C5b
C5b binds to the surface and C6 binds to C5b,stabilizing it.
C6
C5b
C6C7
Then C7 binds. C7 inserts into the phospholipid bilayer of the plasma membrane.
C5b
C6
C7
Then C8 binds to the complex and alsoinserts into the bilayer.
C8
C5b
C6
C7
Finally, C9 molecules bind to the complex and polymerize. Twelve to fifteen C9 molecules form a pore in the membrane.
C8
C9
C5b
C6
C7
Twelve to fifteen C9 molecules form a pore in the membrane.
C8
C5b
C6
C7
The membrane attack complex is a pore in the plasma membrane.
C8
C1qC2C4
2b2a4b4a
C3-convertase
C3
C3aC3b
C5-Convertase
C3a binds to receptors on basophils and mast cells triggering them to release there vasoactive compounds (enhances vasodilation and vasopermeability) - ANAPHYLATOXIN
C5
C5aC5b
C5a is a:
1. Potent anaphylatoxin
2. Chemoattractant for neutrophils
C6
C7C8
C9
Classical Pathway
C1C1Activated
C4b2b
C3 CONVERTASE
C5
C9C8C7C6
BacteriumIgGIgM
C4 C2C4a C4bC2b C2a
C3
C3aC3b
C4b2b3b
C5 CONVERTASE
C5a C5bLysis
MEMBRANE ATTACK
COMPLEX
C5-C9 MAC
The Lectin Pathway
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.
MASP-1 & MASP-2: MBL-associated serine protease
MB-lectin forms a complex with two protease : MBL associated serine protease; MASP-1 and MASP-2Closely homologous to C1r and C1s and activated to cleave C4 and C2
MBL binds to mannose on glycoproteinson the surface of microorganisms. Then MASPsbind to it.
MASP-1MASP-1
MASP-2MASP-2
MASP = mannose associated serine protease
- MBL is induced during inflammatory responses.- 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
C4b2b
C3 CONVERTASE
C5
C9C8C7C6
C4 C2C4a C4bC2b C2a
C3
C3aC3b
C4b2b3b
C5 CONVERTASE
C5a C5bLysis
MEMBRANE ATTACK
COMPLEX
C5-C9 MAC
Bacterium
MBL
MASP
The alternative pathway
• The alternative pathway is part of the non-specific defense because it does not need antibodies to initiate the pathway.
- 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 ®).
___analogous to the C4b2a complex in the classical pathway
Ba
(stabilization of C3bBb)
Mg++
©
C3 contains in unstable thioester bond.
This unstable bond makesC3 subject to slow spontaneous hydrolysis to C3b and C3a
The C3b is able to bind to foreign surface antigens.
Mammalian cells contain sialic acid which inactivates C3b
Initiation of The Alternative pathway
Factor B
• C3b on the surface of a foreign cells binds to another plasma protein called factor B
Factor D• The binding of C3b to
factor B allows a protein enzyme called Factor D to cleave Factor B to Ba and Bb.
• Factor Bb remains bound to C3b while Ba and Factor D disperse away.
The C3 activation complex
• Properdin, also called factor P, binds to the C3bBb complex to stabilize it.
• C3bBbP make up the C3 activation complex for the alternative pathway
The C3 activation Complex• The C3 activation
complex causes the production of more C3b.
• This allows the initial steps of this pathway to be repeated and amplified
• 2X106 molecules can be generated in 5 minutes
C5 activation complex• When an additional C3b
binds to the C3 activation complex it converts it into a C5 activation complex.
• The C5 activation complex cleaves C5 into C5a and C5b.
• C5b begins the production of the MAC.
C3C3bC3a
Anaphylatoxin
BD
Bb Ba
C3
C3a C3bC5-ConvertaseC3-Convertase
C5
C5aC5b
Alternative Pathway
C6
C7C8
C9
C3 C3b
C5
C6C7C8C9
Bacterium Factor D
Ba Bb
C3C3a C3b
C3bBb3b
Alternative C5 convertase
C5b
C5a
Lysis
MEMBRANE ATTACK
COMPLEX
C5-C9 MAC
Factor B
C3bB C3bBbAlternative C3
convertaseProperdin
SUMMARY OF COMPLEMENT ACTIVATION
ClassicalPathway Lectin-Binding
PathwayAlternativePathway
MBP C3C1q
[C4b2b] [C3bBbP]
C3b, C3bi(opsonlzation)
C5b-C 9
C5bC5a
(membrane attack complex)
Cell Injury
C3b
anaphylatoxins
C3a
C3 Convertase
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.
Biological Effects Mediated by Complement
• anaphylatoxins: C3a and C5a: mast cell degranulation– smooth muscle contraction– mast cell degranulation mediator release (histamine, leukotrienes)
– vascular changes: dilation, increased permeability (edema)
– C5a also leukocyte adhesion and chemotaxis (recruitment)
• opsonization: C3b, C3bi, C3d: (binding to complement receptors and enhanced phagocytosis by neutrophils and macrophages)
• clearance of circulating immune complexes
- 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.
Viral neutralization
Regulatory components
Summary
• Complement system• Complement activation -Classical pathway -lectin pathway -alternative pathway • Biological Effects Mediated by Complement • 1. Cell lysis 2. Inflammatory response
Regulatory components3. Opsonizatin4.Viral neutralization5.Immune clearance
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