Lecture 16-17 - Complement

Question 1

Fixation of complement involves forming ___ and ____ from ____

Answer 1

C3 –> C3a + C3b

Question 2

cleavage of C3 exposes ______ which undergoes _____ attack by ______

Answer 2

exposes thioester bond

undergoes nucleophilic attack by H₂0, R-OH or R-NH2

Question 3

Attack by H20 gives _______

Attack by R-OH or R-NH2 gives _________

Answer 3

• Soluble C3b
• C3b bound to pathogen surface

Question 4

Features of the complement pathways (3)

Answer 4

1. Amplification
2. Factors produced by liver (as zymogens)
3. Converge onto C3 (the factor with the highest concentration)

Question 5

What are the three complement pathways?

(and their relative orders, and general schemes)

Answer 5

1. Alternative
   
   1. Pathogen surface creates a local environment conduction to complement activation
2. Lectin
   
   1. Mannose-binding lectin binds to pathogen surface
3. Classical
   
   1. CRP (acute phase protein) or antibody binds to specific antigen on pathogen surface

Question 6

In the alternative pathway, you need to form…

Answer 6

C3bBb

= Alternative C3 convertase

Question 7

Alternative pathway steps (6)

Answer 7

1. H20 exposes C3’s thioester bond (becomes iC3b)
2. Factor B binds to C3b
3. Factor D cleaves Factor B —–> Bb + Ba
4. forms iC3bBb (unstable)
5. binds another plain C3, which is cleaved into C3a + C3b (starts over…Amplification!)
6. New C3 leaves, thioester is bound by another ROH/RNH₂ = C3bBb = Alt. C3 Convertase

Question 8

Positive and Negative Regulators of Alt. Pathway

1. Stabilizer of C3Bb?
2. Inactivator of C3b on both Pathogen and human cell surface?

Answer 8

1. Stabilizer = Properdin stabilizes C3 convertase C3bBb (active) on a pathogen surface
2. Pathogen = C3bBb is bound by factor H and then bound to factor I. When this all breaks up, it takes a part of the C3bBb, leaving iC3b (which is inactive)
3. Human = DAF and MCP inhibit surface bound C3bBb

Question 9

What are DAF and MCP (function and name)

Answer 9

Inhibitors of Human cell surface bound C3bBb

• DAF = Decay Accelerating Factor
• CD55 = Membrane cofactor protein

Question 10

Consequences of Factor I deficiency in Alt pathway

Answer 10

• Unchecked C3 activation
• C3 depletion (too low to attack when pathogens infect)
• Chronic *encapsulated *bacterial infection

Question 11

1. Factor I deficiency inheritance?
2. Symptoms?
3. What will the blood counts look like (lymphocyte, IG)

Answer 11

1. Autosomal Recessive
2. Recurring infections of URT, ears, skin, UG (PNA, Meningitis, sepsis). Glomerulonephritis with isolated C3 deposits, **RA **or SLE.
3. Normal lymphocyte ct, nml immunoglobulin concentration

Question 12

Human cells are covered in ____, which is negative, and it reduces affinity for _____ and increases affinity for ___

Answer 12

Sialic acid

C3b

Factor H

Question 13

C3b binding causes _____ by cells

Answer 13

opsonization

Question 14

C3b on the pathogen binds to ____ on the MQ?

What happens next? (2)

Answer 14

C3b binds CR1 on MQ

1. Endocytosis
2. vesicle fusion –> phagocytosis

Question 15

Ligands for CR1, CR2, and CR3/4

Answer 15

CR1 and CR2 = C3b

CR3/4 = iC3b

Question 16

Associated cells for

CR1, CR2, CR3/4

Answer 16

CR1 = MQ, PMN, RBC

CR2 = FDC, B cell

CR3/4 = MQ, PMN

Question 17

Function of CR1, CR2, and CR3/4

Answer 17

1 = phagocytosis, clearance of immune complexes

2= Antigen trapping, B cell activation

3/4 = phagocytosis

Question 18

CR1 reduces the ___________

What happens when RBC’s CR1 recepor binds?

Answer 18

amount of Ag-AB complexes in the blood (some are coated with C3b)

–RBC will take the immune complex to the liver/spleen where it is eaten by a MQ

Question 19

Terminal pathway of the Alternative path involves activation of

Answer 19

activation of C5

Question 20

What does C5 bind to in the terminal pathway?

What happens next?

Answer 20

Binds to C3b₍₂₎Bb

(= the *active *version of C3bBb that had two C3b’s bound to it)

• The C5 is split into C5b and C5a
• –> C5b sets off the terminal pathway

Question 21

What is recruited by C5b? What does this result in?

Answer 21

combines with C6, 7, 8, 9 to make the Membrane attack complex

*The pore is lined with C9, so many are recruited per C5b

Question 22

Functions of C7 and C8 in MAC formation?

Answer 22

Both expose hydrophobic regions

C7 allows attachment to membrane

C8 allows insertion into membrane

Question 23

Why doesnt the MAC get formed in all cells that have C3b2Bb?

Answer 23

CD9 can’t bind in human cells because C5b678 is bound by** CD59**

(CD59 = CI factor 5)

Question 24

PNH?

1. Main feature/ pathophysiology
2. Defective expression of ____ and ____
3. Somatic mutation in _____ gene

Answer 24

Paroxysomal Nocturnal hemoglobinuria

1. Intermittent intravascular hemolysis
2. No CD55 (DAF) or CD59
3. Mutation of PIGA gene (X-linked)

Question 25

PNH path: DAF defect causes \_\_\_\_\_\_ CD59 defect causes \_\_\_\_\_\_

Answer 25

_DAF_ = extravascular hemolysis _CD59_ = intravascular hemolysis --\> Free Hb --\> NO scavenging = *fatigue*, *esophageal* *spasms*, *thrombosis* \*\*both result in anemia

Question 26

PNH treatments

Answer 26

1. Bone marrow transplantation 2. Eculizumab 1. (*Soliris*, a humanized anti-C5 antibody) 2. prevents C5 activation by C5 convertase (**C3b(2)Bb**)

Question 27

Anaphylatoxins * potencies * act on ____ to increase\_\_\_\_\_ * also causes migration of...

Answer 27

C5a \> C3a \> C4a act on vessels to increase vascular permeability MQ and PMN into tissue from bloodstream

Question 28

Two types of C3 convertase

Answer 28

ALt = C3bBb Classical = C2aC4b

Question 29

Classical C3 pathway steps (4)

Answer 29

1. Immune complex's Fc receptor recruits **C1q,r,s** 2. C4 and C2 are cleaved, giving **C4b2a** (C4a and C2b leave...mediate inflammation) 3. C4b2a (= C3 convertase) cleaves C3 into C3b (C3a leaves, also mediates inflamm.) 4. Forms **C4b2aC3b** = C5 convertase (classical)

Question 30

Classical pathway can be activated by... How?

Answer 30

antibodies and CRP * If CRP binds to phosphocholine on the pathogen surface, it can recruit C1 in the same way as IgM

Question 31

Initiation of the classical pathway involves binding of \_\_\_\_\_

Answer 31

Binding of **IgM** to antigen on a pathogen surface

Question 32

IgM binds in ___ conformation to bacterial cell surface

Answer 32

staple | (soluble form is planar)

Question 33

Can IgG activate complement classical pathway?

Answer 33

Yes. _Multiple IgG_ are required for activation * IgG binds to surface antigen --\> C1q binds to *two or more IgG* and initiates complement OR * IgG binds to soluble *multivalent* antigen --\> C1q binds to soluble immune complex and initiates complement activation

Question 34

Amplification by \_\_\_\_\_\_\_ Steps?

Answer 34

Alt. C3 convertase 1. C1 binds to immune complex 2. Deposition of **C4b** (by C1) 3. Deposition of **C3b** (by C4b2a *and* C3bBb)

Question 35

Ability of isotypes to activate complement (rank)

Answer 35

1. IgM best 2. then IgG 3 \> 1 \> 2 3. Then IgA 1+2 "So *M31212*"

Question 36

C4 haplotype - people can have ______ genes (and what are the combos) Absence of C4A = Absence of C4B =

Answer 36

People can have 1, 2, or 3 genes. (_A/B_, _AA/BB/AB_, _AAB/ABB_) _C4A_ = SLE susceptibility _C4B_ = increased infections

Question 37

**C1 r + s** (in classical pathway) are \_\_\_\_

Answer 37

serine proteases

Question 38

Lectin pathway steps

Answer 38

1. MBP binds bacterial surface 2. MASP1 and 2 are recruited 3. C4 activated 4. C2 activated 5. C4b2a is formed (as in the classical pathway

Question 39

What are MASP1/2?

Answer 39

MBP-associated *serine proteases*

Question 40

Hereditary angioedema lab tests

Answer 40

C1 inhibitor deficiency Low serum C4 Normal C3

Question 41

* Hereditary angioneurotic edema = deficiency of \_\_\_\_\_ * Genetics * Increase of \_\_\_\_, decrease of \_\_\_ and \_\_\_

Answer 41

* Low functional **C1-INH** * Autosomal dominant * increaed **bradykinin,** decreased C4 + C2

Question 42

HAE Sx Why do these patients NOT have increased susceptibility to infectious disease?

Answer 42

Recurrent edema of _Skin, GI, UG, larynx_ Abdomal/pelvic pain, suffocation \*Because C3 is still active = _Alternative pathway still intact_ and you still have antibodies doing their thing

Question 43

C1 inhibitor inhibits activation of ___ and ___ (among severeal other steps)

Answer 43

kallikrein and plasminogen

Question 44

C2 Kinin =

Answer 44

a vasoactive peptide generated by C2b cleavage from plasmin \*results in edema