Complement Immune System

Question 1

What are the general “5 steps” of alternative complement system?

Answer 1

1. initiation of complement activation
2. C3 cleaved: C3a inflammation
3. C3b deposited on microbe: opsonization and phagocytosis
4. C5 cleaved: C5a inflammation
5. membrane attack complex (MAC): lysis of microbe

Question 2

What other medically important proteolytic cascades does complement system cooperate with? (2)

Answer 2

• blood coagulation pathways
• kinin-kallikrein system: regulates vascular permeability

Question 3

What does Factor B do in alternative complement system?

Answer 3

• Bb is a serine protease
• active enzyme of C3 and C5 convertases

Question 4

What does Factor D do in alternative complement system?

Answer 4

• plasma serine protease
• cleaves factor B when it is bound to C3b

Question 5

What does properdin (Factor P) do in alternative complement system?

Answer 5

• stabilizes C3 convertases (C3bBb) on microbial surfaces

Question 6

What does C1 do in classical pathway?

Answer 6

initiates the pathway

Question 7

What does C1q do in the classical pathway?

Answer 7

binds to Fc portion of antibody that has bound to microbe antigen, to apoptotic cells, or to cationic surfaces

Question 8

What does C1r do in classical pathway?

Answer 8

• serine protease
• cleaves C1s to make it an active protease

Question 9

What does C1s do in classical pathway?

Answer 9

• serine protease
• cleaves C4 and C2

Question 10

What does C4 do in classical pathway?

Answer 10

• C4b covalently binds to microbe, complement is activated
• C4b binds C2 for cleavage by C1s
• C4a: unknown

Question 11

What does C2 do in classical pathway?

Answer 11

• C2a is a serine protease
• active enzyme of C3 and C5 convertases to cleave C3 and C5

Question 12

What composes the classical C3 convertase?

Answer 12

C4bC2a

Question 13

What composes the classical C5 convertase?

Answer 13

C4bC2aC3b

Question 14

What does MBL do in lectin pathway?

Answer 14

agglutinin, opsonin, complement fixing

Question 15

What does MASP1 do in lectin pathway?

Answer 15

forms complex with MASP2 and collectins for ficolins and activates MASP3

Question 16

What does MASP2 do in lectin pathway?

Answer 16

forms complex with lectins, especially ficolin-3

Question 17

What does MASP3 do in lectin pathway?

Answer 17

associates with collectins or ficolins and MASP1, cleaves C4

Question 18

What is the activation requirement for classical pathway?

Answer 18

• antibodies: 1 IgM or 2 IgG
• C1

Question 19

What is the activation requirement for lectin pathway?

Answer 19

• does not use C1
• bacteria/pathogen needs to have mannose antigen
• MBP binds to bacteria, MASPs are activated

Question 20

What is the activation requirement for alternative pathway?

Answer 20

• C3 undergoes spontaneous hydrolysis
• FB and FD are needed

Question 21

What does C1-inhibitor do in the regulation of complement pathways?

Answer 21

• inactivates C1r, C1s (classical) and MASPs (lectin) (all irreversible)
• also inhibits kallikrein-kinin and coagulation cascades (complement activation)

Question 22

What does Factor 1 (F1) do in the regulation of complement system?

Answer 22

• inactivates/degrades C3b and C4b (inhibits formation of convertases)
• requires cofactor activity by one of the following: MCP, CR1, Factor H, C4BP

Question 23

Which regulatory proteins destabilize CP C3 convertase? AP C3 convertase?

Answer 23

• CP C3: DAF, CR1, C4BP
• AP C3: DAF, CR1, Factor H

Question 24

Which regulatory proteins inhibit MAC assembly?

Answer 24

CD59, S protein, vitronectin

Question 25

- disease caused by complement deficiency - immune-complex disease

Answer 25

C1, C2, C4

Question 26

- disease caused by complement deficiency - susceptibility to capsulated bacteria

Answer 26

C3

Question 27

- disease caused by complement deficiency - susceptibility to *Neisseria*

Answer 27

C5-C9

Question 28

- disease caused by complement deficiency - susceptibility to capsulated bacteria *Neisseria* but no immune-complex disease

Answer 28

Factor D, properdin (factor P)

Question 29

- disease caused by complement deficiency - similar affects to deficiency of C3 (suspectibility to capsulated bacteria)

Answer 29

Factor I deficiency

Question 30

- disease caused by complement deficiency - autoimmune-like conditions including paroxysmal nocturnal hemoglobinuria

Answer 30

DAF, CD59

Question 31

- disease caused by complement deficiency - hereditary angioneurotic edema (HAE) - the edema in HAE is caused by this deficiency, as the enzyme inactivates cleaving of kininogen to bradykinin; build up of bradykinin is causing of edema

Answer 31

C1-inhibitor (C1-INH) deficiency

Question 32

What are the diagnostic tests for meningitis?

Answer 32

**blood cultures** (bacterial growth), **imaging** (CT or MRI of the head), **lumbar puncture**

Question 33

What would the CSF from a lumbar puncture show in the case of meningitis infection?

Answer 33

- low glucose level - increased WBC count - increased protein - **PCR** would be needed to determine presence of pathogen, testing needed for presence of **anti-viral antibodies**

Question 34

What are the most common causes of meningitis?

Answer 34

- **viral infections** - bacterial infections: pnemococcus (most common for bacteria) and meningococcus (highly contageous, teenagers and YAs, the one that causes local epidemics)

Question 35

How should you (the physician) proceed with treatment in a suspected meningitis case?

Answer 35

start **empiric antimicrobial therapy** aka IV antibiotics Ceftriaxone and Vancomycin (until more specific results can be obtained, bacterial infections tend to worsen without tx)

Question 36

What are the warning signs of immunodeficiency disorder? (8)

Answer 36

- eight+ ear infections in 1 year - 2+ serious sinus infections in 1 year - 2+ bouts of pneumonia in 1 year - 2+ deep-seated infections, or infections in unusual areas - recurrent deep skin or organ abscesses - **need for IV antibiotic therapy to clear infection** - infections with unusual or opportunistic organisms - family history of primary immunodeficiency

Question 37

How do you clinically test **classical**, **alternative**, and **terminal** (**MAC** complex) pathways of the complement system?

Answer 37

- **CH50 test**: amount of patient's serum that will lyse 50% of sheep RBCs coated WITH antibody (tests **classical** and **terminal** components) - **AH50**: same as CH50 except sheep RBCs are WITHOUT antibody (tests **alternative** and **terminal** components)

Question 38

- deficiency in complement system - rare, hereditary genetic condition that is life threatening - sx: edema in various body parts, hands, feet, face, airway; abd pain, nausea, vomiting (caused by intestinal wall swelling) - airway edema can cause death by asphyxiation - **defect in gene** that encodes for protein **C1-inhibitor (C1-INH)**

Answer 38

hereditary angioedema (HAE) or C1 inhibitor deficiency

Question 39

What is the cause of swelling in hereditary angioedema (HAE)?

Answer 39

- **C1-INH inactivates plasma kallifrein**, which is an enzyme that cleaves plasma kininogen to produce **bradykinin** - in HAE, patients have **deficiency** in **C1-INH**, thus there is **more bradykinin** production than normal - the **bradykinin** causes **swelling** seen in HEA

Question 40

How do you confirm diagnosis of a patient suspected of paroxysmal nocturnal hemoglobinuria (PNH)?

Answer 40

test the expression of **DAF** (CD55) and **CD59** on the surface of WBCs by a **flow cytometry**

Question 41

- deficiency in complement system - failure to regulate formation of MAC - **somatic mutation** causes **deficiency** of glycosylphosphatidylinositol **(GPI)** - cells **lack surface proteins** (~40) linked through GPI anchor of cell membranes, most important being **DAF** (CD55) and **CD59** that **protect RBCs from complement action** - sx: **intravascular hemolysis**, thrombosis, anemia

Answer 41

paroxysmal nocturnal hemoglobinuria (PNH)

Question 42

What type of therapeutic antibody would be used for: - autoimmune diseases - sepsis - age-related degenerative diseases - transplant rejection, I/R injury

Answer 42

complement inhibiting antibodies (inhibition targets key components of complement cascade: C1, C3, C5, F1, and FD)

Question 43

What type of therapeutic antibody would be used for: - viral infections - bacterial infections - cancer: hematological tumors - cancer: solid tumors

Answer 43

complement stimulating antibodies (activation via Fc-induced complement activation (e.g. anti-CD20 for B-cell lymphomas))

Question 44

- deficiency in compliment system - half of all patients have renal disease (most important prognostic factor for poor outcome) - renal involvement manifested clinically by **hypertension** and **edema** - sx: malar erythema (butterfly rash), fatigue, headaches, edema, joint pain - lab findings: proteinuria, pyuria, hematuria

Answer 44

Systemic lupus erythematosus (SLE)

Question 45

How does the complement system play a role in pathogenesis of glomerular injury?

Answer 45

- production of **reactive oxygen intermediates** and **inflammatory cytokines** (**C5a**) by attracted inflammatory cells - production of **proteases** (when **MAC complex** forms, it is believed to activate mesangial and epithelial cells to release proteases)

Question 46

What are the 2 mechanisms in immune-mediated vasculitis?

Answer 46

- immune-complex deposition - direct antibody interaction