Complement - I&I

Function of the complement system:

Enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen's cell membrane.

What does the complement system produce when it is activated?

Release of small bioactive fragments (they are peptides called anaphylatoxins) - attract immune cells to site of infection

3 different pathways of the complement system:

• Alternative pathway (1st to act) - Triggered by pathogen surfaces - it accelerates other pathways - most important as it leads to production of an enzyme that cleave C3

• MBLectin pathway (2nd to act) - Involves the interaction of sugars on the surface of pathogens - you can live without this pathway

• Classical pathway (3rd to act) - antibody binds to specific antigen

First and most important step in complement activation:

C3 gets cleaved into:

C3a - the anaphylotoxin

C3b - binds to surface of pathogen - opsonin

Classic complement pathway:

• An antibody binds to an antigen on the pathogens surface

• The C1 complex binds to the antibody - activating C1

• C4 is cleaved by C1 into 4a and 4b

• C2 is cleaved by C1 into 2a and 2b

• 4b and 2a form an enzyme called C3 convertase

• C3 convertase cleaves C3 into 3a and 3b

• 3b joins the complex to produce C5 convertase

• C5 gets cleaved by C5 convertase into 5a and 5b

• This leads to the initiation of the terminal pathway - by 5b

• 5b binds C6 and C7 in solution then fixes to the membrane

• To cause the drilling of a hole in the microbial membrane

What can anaphylatoxins do?

They can chemotechnically attract immune cells to come to the site of infection

They do this by increasing vascular permeability - acting on blood vessels

Where are complement components made?

Many tissues make their own source of complement components

What does C1 (complement factor 1) consist of?

Consist of collagen structure with 6 heads

C1r and C1s are interlocked into C1q - integral to the structure

What is it exactly that cleaves C2 and C4?

C1r and C1s cleave C2 and C4

How is C1q able to bind to a pathogens surface?

2 different ways:

• Pentameric IgM binds to antigens and adopts a staple form

• C1q binds to single IgM

OR

• IgG molecules bind to antigens on bacterial surface

• C1q binds to at least two IgG molecules

THIS ACTIVES C1r and C1s

What do C4a and C4b do?

We think C4a is an anaphylatoxin

C4b - opsonises the surface of the pathogen

What do C2a and C2b do?

C2a is important for the formation of C3 convertase

We are unsure of what C2b does

Most abundant and important of the complement proteins?

C3

Structure of C3?

A large, multi-domained protein

How is C3 convertase formed in the alternative pathway?

C3 in your bloodstream can spontaneously become cleaved when it meets a pathogen

• 3b will become attached to the surface

• It recruits 2 other components factor B and factor D

• Factor D cleaves factor B into Ba and Bb

• 3b + Bb forms an alternative pathway convertase, which can cleave a lot of C3

• This will allow for complete coverage of the surface of the pathogen by opsonins (3b) - this can be easily detected by the immune cells

How does C5 convertase form via the alternative pathway?

Another 3b molecule can bind to the complex (C3b-Bb), and this forms C5 convertase

how many ways of activating C3 are there?

3

The classical pathway: C3 convertase consisting of C4b and C2a

The alternative pathway: C3 convertase consisting of C3b and Bb (product of cleaved factor B)

It can also be activated by the Lectin pathway

Why does fluid-phase C3 convertase exist?

To spontaneously "tick over" and initiate the alternative complement pathway in the blood.

Also allows for constant surveillance against pathogens without needing initial antibodies

What is fluid-phase C3 convertase?

When C3 convertase forms in solution - without the need of a pathogen surface

How does the fluid C3 convertase form?

C3 undergoes spontaneous hydrolysis

Which binds factor B allowing it to be cleaved by factor D into Ba and Bb

C3(H20)Bb complex is a C3 convertase > cleaving C3 into C3a and C3b

• C3b is rapidly inactivated unless binds to cell surface

If C3b binds to a pathogen surface factor B comes along and binds to it. Then, factor D cuts factor B into 2 pieces: Ba and Bb

This creates the C3bBb complex on the cell surface

Difference in outcome if the C3 convertase complex forms on a host cell vs. a pathogen cell surface?

H: complement control proteins bind to C3b and displace Bb

C3b bound to complement control proteins is cleaved by factor I making it inactive - NO ACTIVATION OF COMPLEMENT

P: binding of factor P may stabilize the C3b,Bb complex

The C3 convertase deposits many molecules of C3b on the pathogen surface - OPSONISATION - ACTIVATION OF COMPLEMENT

How is complement activation prevented on host cells?

By the breakdown of the deposited C3b molecules on the surface

 

Similarity between C3 and C4?

They both have a thioester bond

What is the importance of the thioester bond in C3 and C4?

It allows the protein to bind to the surface of the microbe

After C5 gets cleaved what happens?

C5b binds to C6 and C7

This complex to able to bind to the membrane and the binding of C8 inserts it into the membrane

This primes the formation of the membrane attack complex that purely exists of C9

What does the MAC cause?

Lysis of the cell - caused by extra-cellular fluid flooding into the cell

When does C3 hydrolysis happen?

it is happening all the time spontaneously

What is the function of DAF

It is a protein that regulates complement activation to ensure it doesn’t happen on host cells by accelerating the decay of C3b

What are factor I and factor H?

They are both proteins that regulate complement activation to ensure it doesn’t happen on host cells:

Factor I - C3b/C4b inactivator

Factor H - co-factor for factor I

6 types of complement activation regulation proteins:

DAF - decays accelerating factor

Factor I - C3b/C4b inactivator

Factor H - co-factor for factor I

MCP - membrane cofactor protein

CR1 - complement receptor 1

Structure of C3 convertase in each of the 3 pathways:

Classical + lectin: C4b + C2a

Alternative: C3b + Bb

Structure of all of the complement activation regulation proteins

All involve multiple domains

What are DAF, MCP and CR1 known as now?

They are named using CD nomenclature

DAF: CD55

MCP: CD46

CR1: CD35

Generally how do regulators of complement activation regulate complement activation?

Either by disturbing the convertase structure or preventing the convertase from forming

How exactly do complement activator regulators regulate alternative pathway complement activation?

Complement Receptor 1 (CR1/CD35) and DAF/CD55 (decay accelerating factor) compete with Factor B in binding with C3b on the cell surface. They can even remove Bb from an already formed C3bBb complex.

CR1 and DAF/CD55 come into action when the alternative pathway C3 convertase has already formed

The formation of a C3 convertase (C3bBb) can be prevented when Factor I cleaves C3b into its inactive form, iC3b. Factor I requires a C3b-binding protein cofactor such as complement factor H, CR1/CD35 and Membrane Cofactor of Proteolysis (MCP/CD46).

Function of factor H in the regulation of complement activation:

Factor H can inhibit the formation of the C3 convertase by competing with factor B for binding to C3b; accelerate the decay of the C3 convertase; and act as a cofactor for Factor I-mediated cleavage of C3b.

How does factor H allow preferential protection of the host?

Factor H preferentially binds to vertebrate cells (because of affinity for certain sugars on the surface of host cells)

How have some bacteria evolved to recruit factor H to prevent them being killed by the complement system?

They have evolved to express these certain sugars that the host cell express on their surface to mimic the host cells and prevents C3 cleavage

Structure of factor H?

made up of 20 domains - very big

The first 4 domains are involved in complement regulation (binding to C3b)

The last 2 domains are used for attachment to the surface of the host cell (particularly the sugars on the cell)

It tends to sit on the surface of the host cell

What is the general function of factor I and factor H?

Critical fluid-phase regulators of the alternative complement pathway (AP) that prevent excessive immune system activation and self-tissue damage.

What do factor H, MCP and CR1 all have in common in terms of their function in cleaving C3b?

They all require the help of factor I

What is the classical pathway version of activation regulation?

C4 binding protein - can bind to C3 convertase and helps recruit factor I and cleave activated complement components into pieces

2 other key functions of the complement system?

Phagocytosis and clearance

Role of the complement system in phagocytosis:

CR2 can bind to opsinized antigens and facilitate their engulfment

FC receptors can also take up the opsinized antigens

Role of the complement system in clearance:

Red blood cells have complement receptor 1 (CR1) on their surface

Opsinized immune complexes are removed in this way and delivered to macrophages

2 types of G-protein complement receptors:

C3a and C5a receptors

Complement receptor deficiencies can cause serious diseases, which CR is less severe if you lack it?

CR2 - often means infectious disease can easily infect you

What do factor B and D deficien