L20- the complement system

What does the complement system provide

Protection in early infections

What is the complement system

A major effector system of the humoral branch of the innate and adaptive immune system → is a group of nearly 30 serum and membane proteins

How deos teh complement system act

In concert in an orderly sequence + as a cascade

Goal fo teh implement system

Inflammatory, phagocytosis, memrbane attack → KILL PATHOGEN though hihgly regualted enzymatic cascade

Roles (4)

• some activated proteins bind covalently to bacteria opsonising them → are phagocytosed by the cell with complement receptors

• Some small fragments of complement recruit phagocytes to the site and regulate the inflammatory response

• Some products activate B cells

• Terminal component of system generates membrane attack complex (MAC) → lysis of pathogen

What does C3B do

Coats things with C3B receptors on their surface to improve the phagocytosis process

Pathways involved in complement activation (3)

• classical pathway

• Lectin pathway

• Alternative pathway

Classical pathway

• antibody binds to specific antigen on pathogen surface forming antigen-antibody complexes

Lectin pathway (2)

• Initiated by acute pahse proteins that bind glycoproteins or carbohydrates on microorganisms

• is the process of mannose-binding protein binding to pathogen surface

Alternative pathway

• when the pahtogen surface creates local environment conducive to complement activaiton

• Trigger by some pathogen surfaces

What do all pathways in complement activation produce

Enzymes known as c3 convertase needed for effector activtiy

What does c3 convertase production by activation pathways lead to

Opsonisation or antigen, inflammatory repsonse and production of membrane attack complex

What is the nomenclature of teh classical pathway (2)

• All components are designated by the letter C followed by a numebr

• Products of their cleavage reactions are designated by addition of a lower case letter

What is the nomenclature of the alternative pathway

• components are designated by different upper case letters

• Larger fragment is b and the smaller fragment is a EXCEPT for C2 where C2a is the larger part and C2b is the smaller part

Classical pathway (12)

(C1 → C1Q → C1r → C1s → C4 → C2 ) → C3 → C5→C6→C7→C8→C9

Alteranative pathway (9)

(Factor D → Factor B→ properdin (factor P) ) → C3 → C5→ C6→ C7→ C8→ C9

What do the terminal components (c5-c9) make up

The memrbane attack complex

What si teh first component of the classical pathway

C1

What is C1 a complex of (3)

• C1q

• C1r

• C1s

How do the subunits of C1 fit together

2 moelcuels of C1r and C1s bind to each C1q whih has Fc binding globules that bind to the Fc part of the antibody

Fc reigon of an antibody

Fragment crystallisable region = Straight part

FAB region of antibody

Fragment antigen binding region

Wen is teh classical pathwya activated

Activation is initiated when antibodies bind tot he surface of an antigen or pathogen bind C1q

Strucutre of teh C1q molecule

Has 6 globular heads, echa of which can bind to one Fc domain on an antibody, joined to a common stem

How many globular heads of C1q must bind to Fc domains for C1q to be actiaveted

At least 2

What is required to trigger the conformational chagne in C1Q that reveleas a proteolytic site on C1r

• either binding of C1q to a singel IgM or its binding to 2 or more IgG molecule

What is teh effect of C1qbinding to either 1 IgM or 2 or more IgG moelcuels

triggers the conformational chagne in C1Q that reveleas a proteolytic site on C1r

What does the revelations of the proteolytic site on C1r mean

C1r now has enzymatic activity to cleave C1s, generating a serine protease enzyme C1s

How does IgM and IgG travel round

• IgM → as a pentamer

• IgG → as an individual

What does the activated C1s Act on and awhat happens (1+5)

Acts on next 2 components of classical pathway, c4 and c2:

• cleaves C4 to C4a and C4b

• C4b binds covalently to the pathogen surface

• C2 then binds to the C4b on the antigen or pathogen surface

• C2 is cleaved by the adjacted C1s to C2a and C2b leaving the C4b2a complex = c3 convertase

• C3 convertase breaks down C3 into C3a and C3b

What is teh C4b2a complex

An active enzyme called C3 convertase which cleaves large numbers of C3 moelcuels to C3b and C3a

What happens to C3a and C3b once they’ve been cleaved by c3 convertase

• C3a is the small bit so is disregarded

• C3b joined on to the surface forming C4bC2aC3b

What does some of teh C3b do

Is teh. Opsonin which helps to create the surface sugar + opsonisation of pathogen → phagocytosis of pahtogen → pahtogen death

What does sugar coating opsonisation by C3b do o

Increases capcaity of phagocytosis

Stages of opsonisation (4)

• encapsulated bacteria resist uptake by neutrophils and avoid engulfment

• Binding of anitbody on bacterial surface leads to bidning of complemetn protein C3b

• Uptake of bacteria into neutrophil phagosome mediated by Fc and complement region

• Granule s fuse with phagosome producing toxic oxygen metabolites that kill bacteria

How does a single C3 convertase moelcuels cuase tremendous amplification

Can generate up to 1000 moelcuels of C3b

What type of bond does C3b have

A reactive thioester bond that is exposed after cleavage allwos in C3b to bind covalently tohe pathogen surface

What happens to C3b if it does bind to teh surface of a pathogen

Is almost immediately hydrolysed by water and becomes inactive

What binds to the C4b2a3b complex and what does this form

C5 binds forming C5 convertase

What does C5 convertase do

Cleaves C5 to C5a and C5b

What what does C5b do/ C5 cascade

Binds to the pathogen surface and binds C6 to form C5bC6 whcih binds C7 to form C5bC6C7 wchih triggers conformational chagne and C7 inserts into the lipid bilayer of the pathogen wall leading to a pore formation in the memrbaen

What does C5b67 bind and what does this eventually form

C8→ C8beta binds to C5b allowing binding of C8alpha-gamma which inserts into teh lipid bilayer and inducer the polymerisation of 10-16 molecules of C9 forming a ring strucutre C5b6789= membrane attack complex

C5b6789

Memrbaen attack complex

Wat does teh Memrbaen attack complex do (5)

• displaces cell memrbane phospholipids

• Channels

• Disrupts memrbane

• Causes cell lysis

• Cell death

How is teh alternative pathway different front eh classical pathway

C3 convertase is different but still has the same role

What does C3 undergo

Spontaneous hydrolysis

How is teh alternative pathway c3 convertase formed (4)

• C3b binds to the surface

• Factor B binds to C3b

• Cleaved by Factor D

• Forms C3bBb= c3 convertase = C4b2a

What is factor P and what is its role

Properdin → stabilised C3 convertase

Alternative pathway steps

• C3 convertase C3bBb

• Second C3b is added to form C3b2Bb= C5 convertase

• C5 → C5a and C5b

• Forms MAC: C5bC6C7C8C9

• Inserts into the memrbane forming a pore

What can teh alternative pathway be activated by

Spontaneous hydrolysis of C3

What does the lectin pathway use

Soluble receptors → acute pahse proteins eg mannose proteins made by liver

What does the lectin pathway activate and what does it recoginse

• recognises carbohydrates on microbial surfaces → get complexes of mannose binding lectin and MBL associated serine proteases

• Activates complement cascade

Types of MBL assocated serine proteases (2)

• MASP-1

• MASP-2

What do MASP1 and 2 do

• cleavage and activaiton

• MASP 2 celaves C4 and C2 the same way Cs does in the classical system

Similarities between then lectin and classical pathway (2)

• same C3 convertase

• Same C5 convertase

Lectin pathway steps (4)

• activated MASP 2 associated with MBL or ficolin cleaves C4 to C4a and C4b which binds to the microbial surface

• C4b then binds C2 which is cleaved by MASP 2 to C2a and C2b forming the C4b2a complex

• C4b2a is an active C3 convertase which cleaved C3 to C3a and C3b which can bind to the microbial surface or to the convertase itself

• One molecule of C4b2a can cleave up to 1000 molecules of c3 to C3b and many C3b molecules bind to the microbial surface

How does teh complement system cuase inflammation

Smaller components released cause inflammation, making the vessels leaky → C5 is most potent of the 3 that are relased

What does a C3 deficiency affect and cause

• Affects opsonisation, inflammation + cytolysis

• Causes life threatening infection wiht a rnage of bacteria

What does a deficiency of C6-C9 affect and cause

• affects cytolysis

• May have problems with infection iwth Neisseria

Why must the system be kept in check

Can rapidly amplify repsonse otherwise

Role of C1 inhibitor

Binds to activated C1r and C1s removin them from C1q adn to activated MASP2, removing it from MBL

What does the complement receptor 1 do in regulation of complement activation

Binds C4b displasing C2a, or c3b displacing Bb, is a cofactors fro I

What does factor I (I) do in complement activation regulation

Is a serine protease that cleaves C3b and C4b, aided by H, MCP, C4BP or CR1

How is C1q binding to antigen:antibody complexes to acitivate C1r and C1s regualted

C1 inhibitor dissociates C1r and C1s from teh active C1 compelx

How is C4b2a cleaving c3 to C3a and c3b regualted (2)

• DAF, C4BP and CR1 displace C2a from the C4b2a compelx

• C4b bound by C4BP, MCP or CR1 is cleaved by a soluble protease I to inactive forms C4b and C4

How is c5 convertase cleaving c5 to c5a and C5b regulated

CR1 and H displace C3b and act as cofactors in the cleavage of C3b by I

How is the terminal component of the complemetn forming a memrbane pore and MAC regualted

CD59 prevents final assembly of the MAC at the C8-C9 stage