LECTURE 8

Question 1

At what stage in infection is complement activated?

Answer 1

early on in infection

Question 2

how was complement discovered?

Answer 2

found heat labile substance in serum that had non-specific antimicrobial activity

Question 3

describe complement proteins in general

Answer 3

ZYMOGENS: released from liver as inactive proteins, then activated via enzymes at infection site

Question 4

Describe classical complement pathway up to C3 convertase

Answer 4

1. C1q/C1s/C1r complex cleave C4
2. C4b binds pathogen
3. C4b binds C2, allowing cleavage by C1s
4. C2a+C4b = C3 convertase

Question 5

Describe MBL pathway up to C3 convertase

Answer 5

1. MBL binds mannose on pathogen
2. MASP1/MASP2 activate
3. C2 and C4 cleaved
4. C2a+C4b = C3 convertase

Question 6

describe classical/MBL complement pathway after C3 convertase

Answer 6

1. C3a leaves as inflammatory mediator
2. C3b coats pathogen as serine protease and binds C4b and C2b
3. C3b + C4b + C2b = C5 convertase

Question 7

describe alternative complement pathway up to C5 convertase

Answer 7

1. C3 spontaneously hydrolyzed
2. C3(H2O) binds factor B
3. Factor D cleaves B
4. Bb cleaves C3(H2O)
5. Bb+C3b = C3 convertase
6. C3b+C3b+Bb = C5 convertase

Question 8

where do the complement pathways converge?

Answer 8

classical and MBL converge at C3 convertase, and then converge with alternative once C5 convertase has cleaved C5

Question 9

complement pathway after C5 is cleaved

Answer 9

1. C5b binds C6 and C7
2. C7 undergoes conformational change and inserts into bacterial membrane
3. C8 and C9 bind and polymerize
4. form MAC
5. disrupt homeostasis, lose H+ gradient, pathogen dies

Question 10

which complement proteins induce inflammation? how?

Answer 10

C3a, C4a, and C5a

induce smooth muscle contraction which causes vasodilation and vascular permeability

Question 11

complement-induced inflammation by mast cells

Answer 11

C3a and C5a trigger histamine + TNFa

Question 12

complement-induced inflammation by endothelial cells

Answer 12

C3a and C5a induce expression of adhesion molecules

Question 13

complement-induced migration of immune cells

Answer 13

C5a attracts neutrophils and monocytes to site of infection

Question 14

which complement molecule is involved in opsonization?

how?

Answer 14

C3b

binds CR1, CR3, CR4 on macrophages
binds CR2 on B cells

Question 15

what is iC3b?

Answer 15

inhibits C3 convertase BUT stimulates phagocytosis

Question 16

why do complement proteins need to be regulated?

Answer 16

prevent spont. activation on host cells

Question 17

6 methods of negative regulation

Answer 17

1. C3b and C4b that don’t bind will react with water to be inactivated
2. C1 inhibitor removes C1q from C1 complex
3. CR1 and DAF/CD55 displace Bb in C3 convertase
4. Factor I cleaves C3b to iC3b
5. Factor H competes with Bb for C3b binding
6. CD59/protectin prevents C9 binding

Question 18

1 method of positive regulation

Answer 18

Factor P/properdin stabilizes C3 convertase on pathogen surface

Question 19

intracellular sensing of C3 for viruses and bacteria

Answer 19

virus targeted to proteasome

virus and bacteria targeted to MAVS-dependent signaling to induce IRF3/5/7, NFkB, and AP1 (pro-inflammatory)

Question 20

complement system and B cells

Answer 20

opsonized Ag and immune complexes taken up by CR2 on B cells so they can migrate to T/B cell boundary to activate and proliferate

binding to CR1 inhibits TLR9-mediated activation of B cells

Question 21

complement system and T cells

Answer 21

complement is necessary for T cell function

TCR induces:
- C3
- C5
- factor B
- factor D
- C3aR –> mTOR, ROS, cell survival

C3b binding to CD46 = Treg

Question 22

2 types of diseases

Answer 22

1. deficiency in component –> recurrent infection
2. excessive activation –> inflammatory, autoimmune, etc. diseases

Question 23

2 tests for complement diseases

Answer 23

1. CH50 assay –> ability of patient serum to lyse sheep RBCs with rabbit anti-sheep Ab
2. ELISA –> measure quantities of components

Question 24

hereditary angioneurotic edema

Answer 24

C1 inhibitor deficiency
- spont. C1 activation
- makes C2 kinin –> vasoactive peptide
- causes edema

• treat with C1 inhibitor from human plasma (Cinryze)

Question 25

factor I deficiency

Answer 25

*cannot make iC3b* - genetic - uncontrolled activation - causes cerebral inflammation and complement depletion --> infection

Question 26

factor H deficiency

Answer 26

*cannot regulate alternative pathway* - defective causes Macular Degeneration --> deposits in retina, treat with Compstatin - deficiency causes Atypical Hemolytic Uremic syndrome, schizophrenia, stroke

Question 27

paroxysmal nocturnal hemoglobinuria

Answer 27

*PIGA deficiency --> DAF/CD55 and CD59 not active* - RBCs and leukocytes vulnerable to complement-mediated lysis - causes reduced hematopoietis, venus thrombosis, intravascular hemolysis, Hb in urine - treat with BM transplant or anti-C5 Ab (eculizumab/Soliris or ravulizumab)

Question 28

complement deficiency

Answer 28

mostly undiagnosed bc redundancy - recurring infection - autoimmunity - glomerulonephritis - joint and lung issues - angioedema - dermatomyositis - vasculitis

Question 29

heart attack and stroke

Answer 29

inflammation seen in stroke and heart attack is due to complement

Question 30

transplant rejection

Answer 30

C5a triggers inflammatory response to make MAC on all cells of transplant organ

Question 31

AD

Answer 31

C3 and C1q bind amyloid-B to cause neuron lysis

Question 32

schizophrenia

Answer 32

C4 binds neurons

Question 33

asthma

Answer 33

environmental factors, like smoke, induce C3a and C5a in bronchoalveolar lavage fluid to cause inflammation

Question 34

SLE --> double edged sword

Answer 34

complement deficiency = can't clear immune complexes complement activation = inflammation

Question 35

COVID

Answer 35

a) Env binds and activates MBL and C1q (complement) b) Nucleocapsid activates MASP1/MASP2 (complement, coagulation) c) blocks factor H d) infected epithelial cells make C5

Question 36

cancer

Answer 36

tumour cells normally removed by ADCC or complement-dependent cytotoxicity BUT they express the complement regulatory proteins normally expressed by host cells to evade complement

Question 37

anti-complement immunotherapy

Answer 37

1. eculizumab (soliris) against C5 2. compstatin prevents C3 cleavage 3. PMX-53 is C5aR antagonist for SLE and sepsis

Question 38

4 methods of complement evasion by bacteria

Answer 38

1. complement inhibitor --> SSL7 inhibits C5 2. inhibitor of complement inhibitor --> Borrelia makes CRASP that binds factor H 3. regulatory proteins --> Borrelia makes CD59-like protein that binds C3 to block C3 convertase 4. bacterial protease --> pseudomonas elastase cleaves C3

Question 39

3 methods of complement evasion by viruses

Answer 39

1. variola/small pox makes inhibitor SPICE 2. vaccinia makes inhibitor VCP 3. HCV makes C3 and C9 inhibitor