Complement

• Describe the general operation of the complement system

  Complement proteins are a system of first line humoral innate defence and respond to inflammatory cytokines.

  • Over 50 soluble or membrane bound proteins existing as inactive precursors, which are activated by proteolytic cleavage
  • Activation triggers an amplifying cascade
  • Range of effector functions in adaptive and innate immunity
  • Highly regulated
• Describe the nomenclature of complement proteins

  • C= complement
  • Classical pathway: C1 to C9
  • Alternative pathway: factor B, D and Properdin
  • Cleavage:
  • Smaller fragment: a
  • Larger fragment: b
  • Exception: C2
• Describe the sensor molecule in the classical pathway

  C1q interacts with pathogen surface or pathogen-bound antibodies

  • Binding of multiple globular heads induce conformational change
  • Auto-activation of inactive C1r, a serine protease, which cleaves and activates C1s
  • C1s cleaves C4 to form C4a to C4b
• Describe the formation of the C3 convertase in the classical pathway

  1. Conformational change in C4b exposes reactive thioester, resulting in covalent binding of C4b to local cell surface
  2. Conformational change in C4b allows C2 serine protease to bind, which, in the presence of C1, can be cleaved to release C2b and activates its serine protease activity in C2a fragment.
  3. C2a forms active classical C3 convertase, C4bC2a.
• Describe the sensor molecules in the lectin pathway

  MASP: MBL-associated serine proteases

  • Mannose binding lectins and ficolins recognise and bind to carbohydrates on pathogen surface, act as PRRs

  Then follows classical pathway
• Describe the formation of the C3 convertase in the alternative pathway

  1. Spontaneous hydrolysis of thioester bond in a small amount of circulating C3
  2. Conformational change reveals binding site for Factor B, inactive serine protease
  3. Circulating C3(H2O)B binds to inactive Factor D and, alongside MASP-3, stimulates a conformational change
  4. Activated Factor D cleaves and activates Factor B to produce fluid phase C3(H2O)bBb, C3 convertase, which cleaves C3 to produce and deposit C3b on any nearby surface
• Describe the amplification loop

  • Cleaved C3b has a binding site for Factor B
  • Factor B binds to membrane-bound C3b to reveal a binding site for Factor D
  • C3bB binds inactive Factor D and induces a conformational change
  • Factor D cleaves and activates Factor B
  • Surface-bound C3bBb is an active C3 convertase that generates further C3a and C3b so that Factor B can bind newly deposited C3b
• What is the function of the alternative pathway?

  Tickover: AP provides immune surveillance in the resting state by constant low level tickover activation and deposition of small amounts of C3b on any nearby surface

  Amplification: source of initial C3b deposition may have been triggered by classical or lectin pathway but AP is responsible for 80-90% of C3 activation.
• Describe the terminal pathway

  C3b binds to membrane bound C3 convertase and causes them to change substrate and preferentially cleave C5. C5 convertases:

  • C4bC2aC3b
  • C3bBbC3b

  C5 cleavage is the last enzymatic step of the cascade and initiates assembly and deposition of the membrane attack complex.
• Describe the action of properdin

  Properdin is a positive regulator of the AP pathway. It binds to and stabilises C3bBb, which is a short lived complex. Properdin acts as a PRR as it binds to certain bacteria and apoptotic cell and acts as a platform for the assembly of C3b or C3(H2O) and factor B. This allows generation of the C3 convertase C3bBb. Properdin is stored in neutrophil granules and released during inflammation promoting further localised AP activation.
• Describe progressive C3 cleavage

  Progessive cleavage of C3 limits the duration of complement:

  • factor I cleave C3b to iC3b
  • serum proteases cleave iC3b to C3c and C3dg
  • leaves pathogens and apoptosis cells tagged
• Describe the membrane attack complex

  This complex decreases membrane integrity by causing an imbalance of ions and osmotic lysis of target cells. Sub-lethal targeting of MAC mediates signalling to cells and enhances TLR effects by stimulating inflammasome activation and release of the pro-inflammatory cytokine IL-1beta.
• Describe the action of Cd59

  Prevents formation of the MAC

  • binds to C5b678 complex and prevents recruitment of C9 to form the pore
• Describe the action of the C1 inhibitor

  Binds to activated C1r and C1s, removing them from C1q

  • Binds to MASP-2, removing it from MBL
• Describe the action of Factor H as a negative regulator

  Factor H binds to C3b and displaces Bb

  • co-factor for Factor I, which is a serine protease that cleaves C3b and C4b
  • change properties means that it preferentially associates with ECM and self-cells through interaction with self-associated molecular patterns. Therefore, preferentially regulates at self-surfaces.
• Describe the complement receptor 1

  Binds to C4b and displaces C2a

  • Binds to C3b and displaces Bb
  • Cofactor Factor I
• Describe the action of decay-accelerating factor

  Membrane protein that displaces Bb from C3b and C2a from C4b
• Describe how complement system interacts with opsonisation 


  C3b binds to pathogen surface and acts as an opsonin for phagocytic cells that carry C3 receptors.

  • CR3 and CR4 on macrophages
  • CR1 and CR2 on B cells

  C5a can activate macrophages to engulf C3b coated bacterium via CR1. Apoptotic cells undergo cell modifications to reveal DAMPs and decreases expression of complement regulatory receptors.

  • opsonised by C1q and iC3b
  • triggers production of anti-inflammatory cytokines, impaired maturation of DC and induction of Tregs
  • Immunologically silent clearance of apoptotic cells
• Which complement receptors act as anaphylatoxins?

  C3a receptor: activates G protein on macrophages, endothelial Cole’s and mast cells

  C3b receptor: activates G protein on neutrophils, macrophages, endothelial cells and mast cells

  • C5L2: decoy receptor mops up ligand without signalling, therefore limiting response
  • C5a > C3a in potency, but both regulated by serum carboxypeptidases in humans
• Describe the action of anaphylatoxins

  In crease vascular permeability and alter cell adhesion molecules

  • increase recruitment of macrophages, PMNs and lymphocytes
  • increase activation of mast cells, release other inflammatory mediators
  • cross-talk with TLRs and other receptors and increase phagocytic uptake