4) Complement system

Alternative pathway

-Initiates first

-No reliance on antibodies or PAMPS

-Binds to antigens

-Involves recruitment of phagocytes

-Relies on C3b

Role of factor B in alternative pathway?

1) Factor B associates with aqueous C3 but is cleaved by factor D to produce aqueous C3Bb

2) Factor B associates again with aqueous C3b and is cleaved by factor D to form aqueous C3bBb

What is aqueous C3Bb?

Fluid phase C3 convertase

What does aqueous C3Bb do?

Converts C3 into C3a and C3b which enables binding of C3b to pathogen antigens

What is aqueous C3bBb?

Surface bound C3 convertase

What does aqueous C3bBb do?

Converts C3 into C3a and C3b so allows more C3b opsonin for positive feedback loop.

Why is the C3b positive feedback loop important?

Required:

- to produce several opsonins to attach to antigen

-to combine C3b with C3bBb to form C3bBbC3b  

What is the C3bBbC3b ?

The alternative pathway C5 convertase

What does C3bBbC3b do?

Convert C5 to C5a and C5b

What is the importance of C5b?

Required for formation of the MAC (membrane attack complex)

What do C3a,C4a and C5a do?

They are anaphylatoxins which contribute to inflammatory response by:

• Causing degranulation of mast cells and basophils.

• Releasing histamine, heparin, and other mediators.

• Result → vasodilation, increased vascular permeability, local swelling, redness.

• Recruiting immune cells (especially neutrophils, eosinophils, monocytes) to the site of infection or injury.

• Inducing smooth muscle contraction in some tissues (e.g. airway, vasculature).

• Can contribute to systemic inflammation in severe infections (e.g. septic shock).

Lectin pathway

-Requires MBL (mannose binding lectin) and (M/H/L)- ficolin

-Involves cleavage of C4 and C2 fragments

-Helps initiate phagocytosis

What is MBL?

-Collagen and lectin (collectin)

-Soluble PRR

-Binds to close knit arrays of mannose and fucose residues 

-Circulate in plasma bound to MASP1&2

What does MBL first bind to and what happens?

Binds to C4 and cleaves into C4a and C4b then binds to and cleaves C2 into C2a (larger) and C2b.

What does C4b do?

Can bind to antigens and joins with C2a to form C4bC2a.

What is C4bC2a?

Classical C3 convertase

What does C4bC2a do?

Converts C3 into C3a and C3b to contribute to opsonin pool and allows binding to C4bC2a complex to form C4bC2aC3b.

What is C4bC2aC3b?

classical pathway C5 convertase

What does C4bC2aC3b do?

Converts C5 into C5a and C5b which can allow formation of MAC.

What receptor does C3b bind to and what is it’s relevance?

Binds to C3bR which can be found on surface of phagocytes to enable phagocytosis.

Classical pathway

-Requires antibody and C-reactive protein

-Relies on C1 activity

What does c-reactive protein do in the classical pathway?

Activate C1q

What does C1q do?

Binds to multiple Fc regions of antibodies to induce conformational change that activates C1r and C1s subunits of C1.

What does C1S do?

Cleaves C4 into C4b which can then bind to antigen.

What does C2 do?

Associates with C4b where it gets cleaved into C2a and C2b to form C4bC2a.

C3 threshold

Determines whether activation pathways should be initiated or aborted

How does the C3 threshold determine pathway decision?

-Critical mass of C3 activation which depends on quantity and speed of C3 activation.

Function of complement system?

1) Opsonisation -C3b 

2) Inflammation- C3a,C4a,C5a

3) Phagocytosis- CR1,CR3,CR5 ( required to fully induce phagocytosis)

4) Immune complex clearance- CR1 highly expressed on erythrocytes and binds to C3b 

How to generate the MAC?

1) Relies on C5b which binds to C6 and C7 and allows C7 to insert into the membrane

2) Then C8 binds to C5bC6C7 complex and inserts into membrane

3) C9 polymerises complex to form pore.

Why must we regulate complement?

-Complements are unstable with short half lives

-Need to fine tune their activation to prevent damage to host tissue and ensure efficient pathogen clearance

Positive Regulator

Properdin (Factor P)

Properdin (Factor P)

• Stabilises C3bBb to increases activity 5–10 fold.

• Also stabilises the C5 convertase complex, enhancing:

  • C5 cleavage.

  • MAC (membrane attack complex) formation.

Negative Regulator

-Factor I

-Membrane Cofactor Protein (MCP)

-Complement Receptor 1 (CR1)

-Factor H

-Decay Accelerating Factor (DAF)

-Protectin (CD59)

Factor I

• A soluble, constitutively active serine protease.

• Degrades C3b and C4b.

• Requires cofactors (e.g. MCP and Factor H) to function effectively.

-Membrane Cofactor Protein (MCP)

• Expressed on host cell membranes.

• Functions:

  • Induces dissociation of C3bBb.

  • Promotes cleavage of C3b and C4b by Factor I.

• MCP has:

  • Decay-accelerating activity.

  • Cofactor activity for Factor I.

-Complement Receptor 1 (CR1)

• Acts similarly to MCP.

• Promotes:

  • Dissociation of C3 convertases.

  • Cleavage of C3b/C4b by Factor I.

-Factor H

• The main control factor regulating complement activation in solution.

• A soluble cofactor for Factor I.

• Binds to sialic acid on host cell membranes (absent on most bacteria).

• Functions:

  • Dissociates the C3bBb complex.

  • Makes C3b susceptible to cleavage by Factor I.

• Important for self vs non-self discrimination.

-Decay Accelerating Factor (DAF)

• Present on the surface of host cells.

• Dissociates alternative pathway C3 convertases (C3bBb).

• Prevents amplification of complement on host surfaces.

-Protectin (CD59)

• Host cell surface protein.

• Binds to MAC intermediate C5b678, preventing:

  • Its insertion into membranes.

  • Recruitment of C9 subunits.

• Effectively blocks MAC formation on host cells

Defects in C1–C4

• Inefficient immune complex clearance from the blood.

• Development of immune-complex disease (e.g. lupus-like disease).

C3 Deficiency

• Activated C3 (C3b) is a key opsonin for promoting phagocytosis of bacteria.

• Defects in C3 or failure to activate C3 result in:

  • High susceptibility to pyogenic infections (pus-forming bacteria).

  • Common pathogens: Staphylococcus and Streptococcus.

 C5–C9 Deficiency

• C5–C9 form the membrane attack complex (MAC).

• Complement-mediated lysis is the most effective defence against Neisseria.

• Deficiencies in C5–C9 result in:

  • Recurrent Neisseria infections (e.g. N. meningitidis).

Defects Affecting Opsonisation

• Control of encapsulated bacteria depends heavily on:

  • Opsonisation (especially C3b)

  • Subsequent phagocytosis

• Defects in factor D or properdin → reduced generation of C3b → susceptibility to pyogenic infection.

• Depletion of C3 due to uncontrolled alternative pathway activation (factor I or factor H deficiency) causes similar susceptibility.

DAF & Protectin Deficiency

• AF and Protectin (CD59) are GPI-anchored proteins on erythrocytes.

• Reduced GPI synthesis → reduced DAF and Protectin expression →

  • Paroxysmal nocturnal hemoglobinuria (PNH).

  • Caused by complement-mediated lysis of red blood cells.

Complement Evasion Strategies by Pathogens

 

• Interference with antibody–complement interactions.

• Binding and inactivation of complement components.

• Destruction of complement components by proteases.

• Mimicry of host inhibitory regulators or recruitment of host inhibitors.