Bloque 8: The complement system

Question 1

Functions of the complement system

Answer 1

Lysis of cells, bacteria and viruses by the formation of the membrane attack complex

Opsonisation by binding the surface of the pathogens (b subunits) and the CR1 receptors of macrophages

Tigger the activation of immune response and enhance inflammation

Immune clearance by removing immune complexes from circulation

Question 2

Functions of the complement system

Answer 2

• OPSONISATION (C3b and C4b): Proteins cote the outer surface of a pathogen and are recognised by CR1 in the macrophage
• Complement-mediated CYTOLYSIS (C5b-C9): Building of membrane attack complex (MAC)
• ENHANCE INFLAMMATION (C5a, C4a and C3a): Activation of mast cells that produce histamine, neutrophils and endothelial cells.
• IMMUNE CLEARANCE: By binding to antigen-antibody complexes, complement proteins make them more soluble and easily to be removed.

Question 3

Alternative pathway

Answer 3

This pathway starts when cleaved C3 (coming from other pathways or present in blood due to C3 tickover) binds to the surface of a microbe in the absence of an antibody.
C3b binds the surface of the microbe through the thioester domain.
When C3b undergoes its post-cleavage conformational change, a binding site for Factor B (plasma protein) is exposed. Factor B will then bind C3b and will be cleaved by factor D generating two fragments: Bb (larger and remains attached to C3b) and Ba (smaller and released).

C3bBb is the C3 convertase of the alternative pathway and will keep cleaving C3 and amplifying the cascade. C3 convertase can be stabilised by properdin, an enzyme released by neutrophils and the only known positive regulator of the complement system.

C3bBb may join another C3b forming C3bBbC3b, which is the C5 convertase of the alternative pathway.

This pathway is the phylogenetically oldest pathway.

Question 4

Classical pathway

Answer 4

This pathway is initiates by binding of C1 complex, specifically of C1q subunit, to the constant region of either IgG (specifically subunits 1 and 3) or IgM bound to antigens. C1 complex is divided in 3 subunits: C1q that binds antibodies, C1r and C1s. The two last ones are proteases.
Once the C1q region is bound to the antibody, C1r and C1s are activated. These two proteases will cleave and activate C4 to generate C4b and C4a. C4b covalently binds to the surface of the microbe to which antibodies are attached. C1s will then generate a C2a fragment that will bind to C4b forming C4bC2a, the classical pathway C3 convertase. The subunit C4b of C4bC2a binds to C3, while C2a subunit activates C3. C3b binds to the surface of the microbe or forms C4bC2aC3b complex or C5 convertase.

Question 5

Late steps in the complement system

Answer 5

These steps are common in the three pathways as all of them end in the generation of C5 convertase, C4bC2aC3b in the case of lectin and classical pathways and C3bBbC3b in the case of the alternative pathway.
C5 convertase will cleave C5. C5b will initiate a cascade in which different complement proteins join. This cascade results in the generation of C5b-C6-C7-C8-C9 complex which will form the MAC or membrane attack complex. This leads to a formation of a pore in the microbial cell and finally, to its lysis.

Question 6

Regulation of the complement system (proteins)

Answer 6

• C1 inhibitor: Affects the classical pathway
• C4-b binding protein: Affects classical and lectin protein
• H factor: Affects the alternative pathway by binding C3b and blocking the formation of C3 convertase
• Complement receptor type 1 and membrane co-factor protein: Blocks the whole complement by binding C3b
• Decay accelerating factor: Blocks the whole complement by accelerating the dissociation of C3 convertase in the different pathways
• Factor 1: Blocks the whole complement
• Homologous restriction factor and membrane inhibitor reactive lysis

Question 7

Lectin pathway

Answer 7

In order for this pathway to be activated, MBL must bind a mannose residue from the microbial surface or a ficolin must bind to a polysaccharide. Both ficolin and MBL have mannose-associated serine proteases or MASPs that cleave C2 and C4 forming C4bC2a complex or C3 convertase that cleave C3 into C3b and C3a.
C3b can join C4bC2a and form C4bC2aC3b or C5 convertase.