Immunology - 3 - Complement

Question 1

What is the Complement System?

Answer 1

• Soluble proteins made by the liver
  
  • present in blood/lymph/ECF
• cascade of sequential proteolytic cleavages
  
  • multiple effector functions
• Part of BOTH innate & adaptive immunity
  
  • ​Innate:
    
    • ​opsonization
    • lysis of pathogens
    • chemotaxis
    • inflammation
    • cell activation
  • Adaptive:
    
    • augmentation of Ab response
    • promotion of t-cell response
    • elimination of self-reactive B cells
    • enhancement of immunologic memory

Question 2

Explain creation of the C3 convertase for each of the complement pathways

Answer 2

Classical Pathway

• C1s cleaves C4 –> C4a & C4b
  
  • bind to microbial surface
• C4b binds C2
  
  • cleaved by C1s –> C2a & C2b –> C4b2a
    
    • = active C3 convertase

Alternative Pathway

• C3 undergoes spontaneous hydrolysis –> binds to factor B
  
  • allows it to be cleaved by factor D –> Ba & Bb
• C3(H2O)Bb = C3 convertase
  
  • binding of factor P = stabilizes complex on pathogen

Question 3

Outline the late stages of complement activation

Answer 3

C5 Convertase

• C3b binds both to C4b2a & C3bBb –> active C5 convertase
  
  • C5 binds to C3b
• C5 is cleaved by C2a or Bb –> C5b & C5a

Anaphylatoxins

• small complement-cleavage products act on blood vessels
  
  • increase permeability & cell-adhesion molecules
    
    • C3a, C5a, C4a
• Increased permeability
  
  • allows increased fluid leakage from blood vessels & extravasation of Ig & complement molecules
• migration of macrophages, polymorphnuclear leukocytes (PMN), and lymphocytes = increased
  
  • microbicidal activity of macrophages & PMNs = increased

Question 4

Compare and contrast the 3 complement pathways.

Answer 4

1. Classical pathway
   
   • Ag/Ab complex on pathogen surface
     
     • C1q, C1r, C1s, C4, C2
2. Lectin Pathway
   
   • Mannose-binding lectin or
   • ficolin binds polysaccahrides on pathogen surfaces
     
     • C4, C2, MASP-2
3. Alternative Pathway (1st!)
   
   • Pathogen surfaces
     
     • C3, B, D

—> all converge & become common pathway<—

• C3 convertase

1. C3a, C5a
   
   • peptide mediators of inflammation & phagocyte recruitment
2. C3b
   
   • binds to complement receptors on phagocytes
   • opsonization of pathogens
   • removal of immune complexes
     
     • —> terminal complement components
       
       • C5b, C6,7,8,9
         
         • membrane attack complex (MAC)
         • lysis of certain pathogens & cells

Question 5

Describe the 3 main functions of complement.

Answer 5

1. Cell lysis
   
   • C5b6789 –>membrane attack complex (MAC)
   • ruptures bacterial cell wall via osmotic lysis
2. Opsonization & enhanced phagocytosis
   
   • C3b –> activates macrophages/neutrophils
   • phagocytosis of pathogen
3. Chemotaxis
   
   • C5a –> neutrophil & macrophage migration to location of Ag
   • recruitment of phagositic cells & inflammatory progress

Notes:

• C3a, C4a, C5a = activation of Mast cells + Basophils
  
  • mediators of inflammation

Question 6

steps of complement activation that are regulated.

Answer 6

Stages at which complement activity is regulated:

Part 1:

• C1q binding to Ag:Ab complex
  
  • activates C1r & C1s
• C1 inhibitor (C1INH)
  
  • dissociates C1r & C1s from active C1 complex

Part 2:

• C4b2a = active C3 convertase
  
  • cleaves C3 –> C3a & C3b
• DAF, C4BP, CR1 displaces C2a from C4b2a complex
  
  • C4b is cleaved by soluble protease I –>
    
    • inactive forms of C4d & C4c
• **W/o this = NO ACTIVATION OF PATHWAY

Part 3:

• C5 convertases cleaves C5 –> C5a & C5b
  
  • allows for MAC activation
• CR1 & H displaces C3b
  
  • CR1 & H act as cofactors in cleavage of C3b by I

Part 4:

• Terminal components of complement form a membrane pore
  
  • MAC (membrane attack complex)
• CD59 prevents final assembly of MAC
  
  • at stages C8 & C9