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Complement 1,Introduction
2,The Complement Components
3,Complement Activation
4,Regulation of the Complement System
5,Biological Consequences of Complement Activation
Jules Bordet (1870-1961), discoverer of complement
Belgian bacteriologist and immunologist who received the Nobel Prize for Physiology or Medicine in 1919 for his discovery of immunity factors in blood serum; this was a development vital to the diagnosis and treatment of many dangerous contagious diseases.
1,Introduction
Sheep antiserum + vibrio cholerae
Lysis of bacteria
Heated sheep antiserum + vibrio cholerae
Lysis of bacteria
destroyed
Fresh serumwithout antibody+
restored
Lysis of bacteriaunable
Paul Ehrlich in Berlin independently carried out similar experiments and coined the term complement, defining it as “the activity of blood serum that completes the action of antibody.”
Paul Ehrlich ( 1854~1915 )
A group of sequentially reacting proteins, which upon
activation, mediate a number of biological reactions that are
important to host defense.
Complement
OpsonizationImmune clearanceB cell activation
Lysis of cells, bacteria and viruses
ChemotaxisInflammation
2, The Complement Components
I
Ia Ib II
IIb IIa III
IIIa IIIb
biological effects
Sequential reaction
name molecule weight kDa con. in serum mg/ml
460 8083 5083 50200 600102 20
Classical pathway
C1qC1rC1sC4C2C3 185 1300
2490
1210
Alternative pathway
D factorB factor
MBL pathwayMBL 30 x 3 1
Terminal pathwayC5 204 70C6 120 65C7 120 55C8 160 55C9 70 60
Regulatory factorsC1-INH 105 200C4-bp 550 250
H 150 480I 88 35P 4 x 56 20
(1)The complement components in serum
(2)Nomenclature
①“C” - designation for 11 of the complement proteins (C1, C2, etc.)
②Factor - designation for many alternative pathway components (factor B)
③Overbar - indicates an enzymatically active protein or complex(C3bBb)
④Lower case letters - indicates a proteolytic cleavage fragment (C3a or C5a)
⑤“R” - designation for receptors in the complement system (CR1 or C5aR)
(3)physical and chemical properties of complement
① The concentration of complement in serum is stable( 10% of
serum proteins) , C3 is highest one in all of complement
components:1.20-1.60g/L
② Heat –labile feature
56 C 30 min—inactivation
③ Synthesized sites: liver cells, macrophages
④ The concentration of complement is highest in the serum
of guinea pig
(1)Classical pathway from C1 activated by Ag-Ab complex
(2)Alternative pathwayfrom C3 by the surface of microbe
(3)MBL pathway: from C4 and C2 by the binding of MBL to mannose surface of microbes
3,Complement Activation
CLASSICAL MBL ALTERNATIVE pathway pathway pathway
C3a C3 C3b
C5a C5 C5b + C6-C9TERMINAL
pathway
①Phase of recognition (initiation ): recognizing unit (C1qrs) :activated C1
②Phase of activation: activating unit( C4,C2,C3):C3 convertase and C5 convertase
③Phase of attack( effector): membrane-attack complex (MAC) ,C5-9
(1)Classical Pathway The binding of antibodyantibody to its antigen often triggers the complement system through the so-called classical pathway. It can occur in solution or when the antibodies have bound to antigens on a cell surface.
VH
CH1
CH2
CH3
VL
CL
Hinge region
COO–
NH3+
Complement Binding Site ( Fc )
AntigenBinding Site
( Fab )
The changes of structure of IgG molecule by binding antigen
Before binding antigen
Fab
Fc
C1q binding site covered
CH1
CH2
IgM CH3 region , IgG CH2 region
After binding antigen
C1q binding site exposed
C1sC1q C1r
C1qr2s2
Ab
<40nm Ag Ag
The structure and function of C1
C1: C1q C1r×2 C1s×2
Each C1q molecule must bind to at least two Fc sites for a stable C1-antibody interaction to occur. Then C1r and C1s are activated and complement activation is initiated.
C1
① Phase of recognition (initiation )
C1s Cleavage of C4
antibody
C1qr2s2
<40nmantigen antigen
C4
C4b
C4a
C1s
②Phase of activation
C3 Convertase Formation
C4b
C2b
C2C1s
C4bC2a
C1s Cleavage of C2
C3 Cleavage
C3 C3a
C3b
C3 Convertase
C4b C2a
C5 Convertase formation (Classical Pathway)
C5
C5b
C5a
C4b
C2a
C3b
1) Formation of the Membrane Attack Complex (MAC)
2) Effect of MAC 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 . Since ions and small molecules can diffuse freely through the central channel of the MAC, the cell cannot maintain its osmotic stability and is killed by an influx of water and loss of electrolytes.
③Phase of attack( effector) (Common terminal pathway)
C5bC9
polymer
C5b+C6+C7+C8+C9 = MACs
The structure of MACs
C7C6
C8
10-17
MACs-induced lesion on the membrane
Figure 2-35
(1)Classical Pathway
(2)MBL(The lectin) pathway
The pathway is activated by the binding of mannose-binding lectin, MBL( also called mannose-binding protein, MBP ) to mannose residues on glycoproteins or carbonhydrates on the surface of the microorganisms.
*Independent on the antibody*The mechanism is more like that of the classical pathway
MBLC1q
The function and structure of MBL are similar to that of C1q in the complement pathway.
mannose
MBL
C4 C4a + C4b
C2 C2a + C2b
C4b2b( C3 convertase )
+ MASP
MASP
MBL
mannose
MASP:MBL-associated serine protease (similar to C1r and C1s).
There is a spontaneous conversion of C3 to C3b. However,
ordinarily C3b is quickly inactivated: if C3b binds to inhibitor
y proteins and sialic acid present on the surface of body's own c
ells, the process is aborted. After C3b binds to the surface of ba
cteria, the alternative pathway will be initiated.
(3)The Alternative Pathway
Antibody-independent
C3C3 is the most abundant protein of the complement system (~1.3 mg/ml).
Because of its abundance and its ability to activate itself, it greatly magnifiesmagnifies the response.
Factor B
LPS
C3b C3b Bb
Ba
C3 Convertase Formation
Factor D
C3 Cleavage
LPS
C3b Bb
C3a
C3 convertase C3
C3b
(Alternative pathway)
LPS
C3b Bb
C3 convertase
C3b
C5
C5b
C5a
C3
C5 Convertase Formation
Contains a labile thioester bond
C3b can bind to cell surfacesInactivated on host cells by surface sialic acids
Schematic diagram of intermediates in the formation of bound C5b by the alternative pathway of complement activation. The C3bBb complex is stabilized by binding of properdin. Conversion of bound C5b to the membrane-attack complex occurs by the same sequence of reactions as in the classical pathway.
classical MBL Alternative
Components C1~C9 C2~C9 C3,C5~C9, Factor D,B, P
C3 convertase C4b2a C4b2b C3bBb
FunctionSpecific immune response
Activator Ag-Ab complex MBL-mannose Bacterium, LPS
C5 convertase C4b2a3b C4b2b3b C3bBb3b
Activated enzyme C1s MASP Factor D
Non-specific immune response in the early infection
Non-specific immune response in the early infection
Comparison of three pathways
CLASSCAL PATHWAY
LECTIN PATHWAY
ALTERNATIVE PATHWAY
Antigen-antibody
C1qr2s2 C1qr2s2
C4 C4b
C4a
C4b2a C4b2a3b
C2 C2b
Microbial surfaces
C3 C3b
C3a
Factor D
B Ba
C3bBb C3bBb3b
C5 C5b
C5a
(C3 convertase) (C5 convertase)
C6C7C8C9
MACMBL
MASP
Microbial cell wall
Activated C1-like complex
C3 C3b
C3a
C3a
(C5 convertase)(C3 convertase)
4,Regulation of the Complement System
The explosive potential of the complement system requires to be kept under tight control.
At least 12 proteins are known to contribute to regulation of the complement system.
(1)The Membrane-Attack Complex Can Lyse a Broad Spectrum of Cells
The complement system lyses target microbe cells through alternative pathway and MBL pathway of complement-activating in absence of antibody and through classical pathway in presence of antibodies.
5,Biological Consequences of Complement Activation
form
(2)Cleavage Products of ComplementComponents Mediate Inflammation
Anaphylatoxins : C3a,C4a, C5a
Kinin-like action: C2a, C4a
Chemokine-like action: C3a, C5a
(3)C3b and C4b Binding Facilitates Opsonization
The process that phagocytosis of phagocytes are enhanced by antibody or complement binding to microbial surface is call opsonization.
The bound C3b ,C4b to CR1 on macrophage andneutrophils enhances their actions.
(4)The Complement System AlsoNeutralizes Viral Infectivity
virus
C3bC3b C3b
C3b
C3bC3b C3b
C3b
C3b
C3bC3b
C3b+
(5)The Complement System Clears Immune Complexes from Circulation
The coating of soluble immune complexes with C3b is thought to facilitate their binding to CR1 on erythrocytes.Erythrocytes may carry these complexes to the liver and spleen where immune complexes are removed and phagocytosed.
1,Master the concept of complement
2,Understand basic components of complement system and their nomenclature
3,Master three pathways of complement activation and their comparison
4,Master biological Consequences of Complement activation
Learning Objectives