5 complement

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


Factor D

C3 Cleavage

LPS

C3b Bb

C3a

C3 convertase C3

C3b

(Alternative pathway)

LPS

C3b Bb

C3 convertase

C3b

C5

C5b

C5a

C3


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

(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

5 complement

Technology