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

dheeravathvenkateswarlu070@gmail.com

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