Molecular Defence, Complement

Question 1

What is complement?

Answer 1

Complement describes a group of plasma protein, normally inactive when triggered lead to activation of the complement pathway.

Question 2

What makes up the complement pathway.

Answer 2

There are more than 30 plasma and cell surface proteins and 9 central components C1 - C9.

Question 3

Is the complement pathway are part innate it hum oral immunity?

Answer 3

The complement pathway are part of both innate and hum oral immunity.

Question 4

What does the complement activation involve?

Answer 4

Complement activation involves a cascade of enzymatic cleavage of complement proteins. So activation lead to proteolytic cascade and a production of multiple products down the line.

Question 5

What happens to the products of complement?

Answer 5

Products of complement proteolytic attach covalent it to microbial surfaces (or absorbed or bound) stabilisation is when complement proteins are inactivate/transiently active when in fluid form but is stably activated when on microbes.

Question 6

What does regulatory proteins do to complement action?

Answer 6

Regulatory proteins inhibit complement actions on the healthy host cells to minimise tissue damage

Question 7

What is the complement roles?

Answer 7

The complement role is to eliminate microbes, this is done by opsonisation of pathogens, inflammation (to recruit and activate leukocyte a) and lysis of microbes which is done via membrane attack complex.

They also eliminate apoptotic cells and debris, clearance of AgAb complexes and to promote B cell activation. Complement is also involved in disease pathogenesis.

Question 8

What does the membrane attack complex do?

Answer 8

The membrane attack complex stack the membrane of the microbe and disrupt it causing it to burst by filling with water.

Question 9

What are the three complement pathways?

Answer 9

Classical
Alternative
Lectin
Pathways

Question 10

What does the classical pathway describe?

Answer 10

This pathway describe when C1 interacts with antibodies (IgM, IgG) bound to microbes. It is an effective mechanism of hum oral immunity.

Question 11

The classical pathway involves C1 and antibodies, how is the alternative pathway different?

Answer 11

The alternative pathway is part of the innate immunity. It involves the direct pathway of microbial structures

Question 12

When we are first exposed to the virus, the first pathway we use is the alternative pathway. Within the alternative pathway describe how a complementary protein attach to the microb

Answer 12

It starts with the spontaneous cleavage of C3 in plasma result in the exposure of thioester bond. The exposed thioester bond reacts with amino or hydroxyl groups on surface of microbes to form ester bond which cause a covalent attachment of C3b to surface of microbes.

Question 13

Describe the thioester bond in C3

Answer 13

C3 cleavage induces conformational changes in C3b, this expose the thioester bond. This bond react with amino or hydroxyl groups on the surface of the microbes to form an eater bonds.

Question 14

What happens if C3b never attaches to a microbe?

Answer 14

In the absence of covalent attachment of C3b remain in the fluid phase and is rapidly inactivated by hydrolysis. So further complement activation is stopped.

Question 15

What happens within the alternative pathway after C3b is attached to the microbe membrane?

Answer 15

When C3b bonds to the membrane it can react with proteins on the surface and bond to it it is stabilised. C3b hen bind to factor B. Factor D cause factor B to dissociate so only factor Bb remains with C3b. The complex on the membrane is C3bBb which is anther name for C3 convertase. This complex is stabilised by Properdin.

Question 16

What is the role of C3 convertase?

Answer 16

C3 convertase’s main function is to cleave more C3 molecules which lead to the amplification of complement action. More C3b is generated.

Question 17

How is C5 convertase formed?

Answer 17

If a C3b deposition forms a bond with C3 convertase then it forms C5 convertase which is C3bBbC3b. This is the alternative pathway C5 convertase.

Question 18

What is the role of alternative pathway c5 convertase.

Answer 18

C5 convertase cleaves C5 and initiates late steps of complement activation. c5 convertase cleaves C5 and initiate the late steps of complement action.

Question 19

What happens in the late stage of complement activation in the alternative pathway after cleave of C5.

Answer 19

C5 convertase cleave C5 so only C5b remains, this complex will recruit c6, then C7, then C8 and C9. After C9 is recruited that is when the membrane attack complex is formed. This complex allows water to enter through C9 into the cell to undergo microbe lysis.

C7 is hydrophobic so it inserts itself into lipid membranes. C8 have three chains, one of which is inserted into the membrane. C9 polymerase lead to the creation of pores.

Question 20

How is the classical pathway of complement initiated?

Answer 20

The classical pathway is j twisted by binding of C1 to antigen bound IgG or IgM. C1 is a multimeric protein complex of C1q, C1r and C1s. C1q binds to the antibodies and C1r and C1s are proteases. C1 is not able to bind to soluble antibody molecules. C1 only binds to antibodies that are bound to Antigens.

Question 21

How does C1 bind to antibodies?

Answer 21

C1q part of the C1 bind between two antibodies on their Fc regions. They can only bind to it when the distance are close enough. The head can only move marginally so C1 can only bind to a microbe with lots of antibodies attached to it.

Question 22

How is IgM different to IgG for C1 activation?

Answer 22

Free IgM is pentameric and in planar configuration so C1q can access Fc region of free IgM. When IgM bind with Antigen it exposes the binding site of C1q on the Fc region.

Question 23

What happens in the classical pathway after C1 is activated.

Answer 23

Binding of two or more globular head of C1q to Ag bound antibodies lead to activation of C1r which cleaves and activate C1s. C1s cleaves C4 which produce C4a and C4b. C3a is small and soluble, C4b is not. Both C4 and C3 have a thioester bond.

C4b binds covalently to the surface of the antigen.

Question 24

What happens in the classical pathway after C4b is membrane bound.

Answer 24

C4b recruits C2, C2 is then cleaves by C1s into C2a (larger) and C2b (smaller and have no biological function) C4bC2a complex is classical pathway convertase.

Classic C3 convertase cleaves more C3 molecules and C3b is recruited to form C5 convertase of classical pathway.

Everything else are the same as alternative pathway.

Question 25

How is the lectin pathway initiated?

Answer 25

The lectin pathway is initiated in absence of antibody. It is triggered by microbial carbohydrate recognition by pattern recognition receptors (PRR).

Question 26

What are examples of pattern recognition receptors?

Answer 26

PRR involved are the mannan binding lectin (MBL) and ficolins. They have similar structures to C1q.

Mannan binding lectin bind to mannose which is a type of sugar that residues on microorganisms.

Mannan binding lectin is in a complex with MBL associated serine proteases 1 and 2 (MASP1 and 2) Masps cleave C4 then C2 which precede similarly to classical pathway.

Question 27

What are complement functions other than cell lysis?

Answer 27

C3 and C5 cleavage lead to B cell activation,

C3 and C5 cleavage lead to neutrophil via inflammation and chemotaxis and microbe destruction by leukocyte a.

C3 and C5 cleavage of the microbe lead to opsonisation which promote phagocytosis.

C3a -C5a trigger acute inflammation. They bind to mast cell which lead to degranulation which release histamine. This lead to phagocyte activation.

C5a also act on neutrophils which lead to chemotaxis plus reactive oxygen species. (It is a signalling molecule for neutrophil.

C5a increase permeability of endothelial cells and neutrophil adhesion to the endothelium.

Question 28

What is the mechanism of complement regulation?

Answer 28

Complement activation is tightly regulated to prevent complement activation on healthy cells and limited duration of complement activation on microbes or AgAb complexes.

Mechanism of complement regulation inhibit formation of C3 convertase, breakdown and inactivate C3, C5 convertase. Which in turn inhibit MAC formation.

Question 29

What is the fluid regulator of complement?

Answer 29

Fluid phase regulator presents in plasma, body fluids.

Factor H is a fluid phase regulator in alternative pathway where as Properdin is a activator in the alternative pathway.

C1 inhibitor and C4 binding protein both are classical and lectin pathway fluid phase regulator

Question 30

What are membrane bound regulators?

Answer 30

Membrane bound regulators are CD46 (MCP, membrane cofactor protein)

CD55 (DAF compliment decay accelerating factor),

CD59 (protectin) and complement receptor 1 are all membrane bound regulators.

Question 31

What does C1 inhibitor do?

Answer 31

C1INH inhibit the formation of C3 convertase, t dissociates C1r and C1s from C1q so it blocks proteolytic activity of C1r and C1s.

Question 32

What does CD59 do?

Answer 32

Protectin is present in surface of healthy cells, it binds to C5-C8 and inhibits the recruitment and polymerisation of C9 therefore inhibit MAC formation.