Complement

Question 1

Functions of Complement (4)

Answer 1

1. Lysis (enveloped viruses, RBCs, gram neg bacteria)
2. Opsonization (parasites, bacteria)
3. Activation of inflammatory response
4. Clearance of immune complexes

Question 2

Complement proteins: most to least abundant

Answer 2

C3 > C4 > C5 > C2

Question 3

Outline the Classical complement pathway

Answer 3

• antibody (IgM or two IgG) bind target
• C1q binds
• 2x: C1r and C1s bind + Ca2+
• C4 + C2 = C4bC2a (C3 convertase)
• C3 cleaved and C3b binds = C4bC2aC3b (C5 convertase)
• C5 cleaved and C5b binds + C678 + C9 = MAC

Question 4

What is a challenge with the classical complement cascade ?

Answer 4

• needs an antibody to a foreign antigen to be activated
• foreign antigen must have been seen before (memory)

Question 5

Outline the Alternative complement pathway

Answer 5

Fluid Phase:
- no antibody involved; spontaneous hydrolysis of C3
- Factor B attaches to hydrolyzed C3 = C3B
- Factor D activates Factor B = C3Bb (C3 convertase)
- C3 cleaved to C3b = rapidly degraded UNLESS microbe is nearby to bind

Cell-bound Phase: (organism)
- C3b binds to microbe (no sialic acid)
- Factor B attaches to bound C3b
- Factor D activates Factor B = C3bBb
- Properdin stabilizes complex
- C3bBb binds another C3b = C3bBbC3b (C5 convertase)

• C5b binds + C678 + C9 = MAC

NOTE: sialic acid on host cell inactivates C3b

Question 6

Pathogen activators of Alternative pathway

Answer 6

• gram neg bacteria (LPS)
• gram pos bacteria (Teichoic acid)
• fungi and yeast cell wall (zymosan)
• viruses

Question 7

Non-pathogen activators of Alternative pathway

Answer 7

• IgG, IgA, IgE complexes
• cobra venom
• carbohydrates (insulin, agarose)

Question 8

Outline the Lectin complement pathway

Answer 8

• Mannose-binding Lectin (MBL) binds CHO residue on microbes
• MASP binds = MBL activated

Same as classical pathway:
- C4 and C2 cleaved = C4bC2a (C3 convertase)
- C3 cleaved = C4bC2aC3b (C5 convertase)
- C5 cleaved = C5b + C678 + C9 = MAC

MASP = MBL-associated serine proteases

Question 9

Pathogens with D-mannose/ L-fructose for Lectin Pathway

Answer 9

• gram neg bacteria (Salmonella)
• gram pos bacteria (Streptococci)
• yeast
• viruses (HIV, Influenza A)
• parasites (Leishmania)

Question 10

Describe CR1

Answer 10

• facilitates phagocytosis by monocytes, macrophages,
  neutrophils
• complement receptor; binds C3b, C4b
• immune complex clearance; delivers RBCs to spleen
• Required for Factor I

NOTE: Plasmodium falciparum uses CR1 to enter RBCs

Question 11

Describe CR2

Answer 11

• Co-receptor on B cell surface
• Participates in amplification signalling with associated
  CD19 and CD81 (TAPA-1)

NOTE: EBV imitates CR2 receptor

Question 12

Describe CR3 and CR4 Integrins

Answer 12

• facilitates phagocytosis by monocytes, macrophages, neuts
• Help neutrophils and monocytes with extravasation
• complement receptor on NK cells; if bacteria is tagged and has no MHC I = direct killing

Question 13

How is complement regulated ?

Answer 13

• inhibitors localize complement activation to protect host
• soluble plasma proteins
• membrane bound proteins

Question 14

Describe C1-Inhibitor

Answer 14

• plasma protein
• affects classical pathway
• removes C1r and C1s from C1q = cannot activate C4 and C2

Question 15

Describe Factor H

Answer 15

• plasma protein
• affects alternative pathway
• prevents binding of B to fluid phase C3 and bound C3b
• co-factor for Factor I

Question 16

Describe Factor I

Answer 16

• plasma protein
• requires cofactors (CR1, factor H, CD46)
• affects classical, alternative, and lectin pathway:
• cleaves C3b into C3d + C3c (all pathways)
• cleaves C4b into C4d + C4c (classical and lectin)

Question 17

Describe DAF

Answer 17

Decay-accelerating Factor (CD55):
- membrane-bound inhibitor
- affects classical, alternative, and lectin pathway

• inactivates C3 convertase (C4b2a and C3bBb)

Question 18

Describe MIRL

Answer 18

Membrane Inhibitor of Reactive Lysis (protectin/ CD59):
- affects classical, alternative, and lectin pathway
- binds C5b678 = prevents C9 from binding

Question 19

Early Complement Deficiencies

Answer 19

C1q, C1r, C1s, C4, C2
- susceptible to gram pos infections (Staph and Strep)

Question 20

Middle Complement Deficiencies

Answer 20

C3
- suffer from wide range of infections

Question 21

Late Complement Deficiencies

Answer 21

C5, C6, C7, C8, C9
- susceptible to gram neg infections (E. coli, N. meningitidis)

Question 22

Describe SLE

Answer 22

Systemic Lupus Erythematosus:
- C1, C2, C4-deficient people at higher risk
- immune complexes non-specifically deposit onto tissues = immune system (ie. neutrophil) removes own cells = tissue damage

Question 23

Describe C1-Inhibitor deficiency

Answer 23

• hereditary angioedema (autosomal dominant) = localized edema in subcutaneous tissue, bowel, respiratory tract
• leads to excess bradykinin formation due to excess kallikrein
• treated with C1-Inh or frozen plasma

Question 24

Describe Factor I and H deficiency

Answer 24

• increased complement activation = low levels of complement proteins (C3 and C4) = recurrent infections

NOTE: Factor I and H regulators prevent complement activation

Question 25

Describe DAF and MIRL deficiencies

Answer 25

Paroxysmal Nocturnal Hemoglobinuria: - no anchoring protein (PIG-A) = absent CD55 and CD59 - anemia, cytopenia, thrombosis, DVT (10-15 year survival)

Question 26

Gram-positive evasion of complement

Answer 26

- thick peptidoglycan layer prevents insertion of MAC - capsule provides physical barrier between C3b and CR1

Question 27

Gram-negative evasion of complement

Answer 27

- long polysaccharide chains prevent MAC insertion - outer membrane prevents MAC insertion (N. gonorrhoeae) - elastase inhibits C3a and C5a (P. aeruginosa)

Question 28

Protein Mimicry used in Microbial Evasion of Complement. Provide examples.

Answer 28

- microbial membrane-bound proteins similar to inhibitor proteins Ie. Herpes, EBV, T. cruzi, C. albicans

Question 29

HIV evasion of complement

Answer 29

- gp-41 and gp-120 on **lipid envelope binds Factor H** (alternative pathway inhibitor) - HIV **incorporates host DAF and MIRL into own membrane = protects HIV from complement** - **complement-coated virus** gets **phagocytosed due to CR1, CR3, and CR4 = RBC transports virus**