The Complement System

Question 1

___________ components leave the host extremely vulnerable to both infectious and autoimmune diseases.

Answer 1

Early

Question 2

Biological Consequence of Complement Activation

Answer 2

Lysis of Ab- coated cells

Promoting Ab formation

Cell lysis and viral neutralization

Question 3

Which complement components are deficient in recurrent Neisseria infections?

Answer 3

C5, C6, C7, C8, properin, Factor D

Question 4

Deficiencies of the Alternative Pathway
Factors D and B

Answer 4

Factor D deficiency is

• very rare and has only been described in two families.
• Both of these families had multiple members with a history of serious infections.

Factor B is an acute phase protein and increases during inflammation.

•There is only one unconfirmed report of this deficiency in humans.

Question 5

The tickover pathway is rapid and, once the ALPW has been initiated, more than_______________ molecules of C3b can be deposited on a microbial surface in less than 5 minutes.

Answer 5

2X 10⁶

Question 6

MAC is a doughnut-shaped structure with a ____________ exterior that allows association with the pathogen membrane. Its internal ___________ channel acts as a pore in the lipid bilayer and results in loss of ______________ and eventual destruction of the pathogen.

Answer 6

hydrophobic; hydrophilic; membrane integrity

Question 7

Deficiencies of the Lectin Pathway Components
MBL, M-ficolin, L-ficolin, H-ficolin, CL-11, MASPs

Answer 7

• Serious pyrogenic (fever-inducing) infections in babies and children.
• found w/ a frequency 2 to 3 times higher in SLE patients than in normal subjects.

MBL is not a primary immunodeficiency

Question 8

Three pathways of the complement system

Answer 8

Classical: antibody-antigen complex

Lectin: Lectin binding to pathogen surfaces

Alternative pathway: Pathogen surfaces

Question 9

Deficiencies of the Alternative Pathway
Factor Properdin

Answer 9

• Properdin is the only CM protein that is X-linked.
• The protein is synthesized by monocytes, granulocytic cells and T-cells.
• Several mutant forms of the protein have been identified that result in decreased AP function.
• Properdin deficiency increases the susceptibility to bacterial infections of the Neisseria family of organisms.
• The most prominent in the group is N. meningitis, the cause of a serious form of meningitis.
• Typical family histories include male relatives who have had or died from Neisserial infections.

Question 10

What role does the complement system play in triggering infection?

Answer 10

• C3a, C4a, and C5a act to stimulate smooth muscle contraction and increase vascular permeability.
• C5a and C3a bind to receptors on mast cells and basophils, and induce the release of pro-inflammatory mediators, such as histamine.
• When present at a high level, these complement fragments are involved in a generalized circulatory collapse termed anaphylactic (anaphylatoxins).
  
  • Treated with epinephrine (adrenaline) shot ASAP

Question 11

C3 is cleaved into C3a and C3b. What does C3a do?

Answer 11

binds to receptors on basophils & mast cells triggering the release of histamine (anaphylatoxin)

Question 12

What role does CR1 play?

Answer 12

• Increases uptake of Ag by antigen-presenting cells
• Can help provide secondary signals to B cells for activation and increase their uptake of antigen to present to helper T cells
• Can help to stimulate T cells to release cytokines, further potentiating innate inflammatory and adaptive immune responses

Question 13

•Patients with a deficiency in properdin production are uniquely susceptible to ______________induced by the _____________ bacterium. These findings suggest that properdin has the capacity to act as a ______________, specifically directing the activation of the alternative pathway onto the surface of Neisseria and other microbial cells.

Answer 13

meningococcal disease; Neisseria gonorrhoeae; pattern recognition receptor (PRR)

Question 14

What bond is formed when C4 is cleaved? What effect does this have?

Answer 14

A thioester bond is exposed.

• This bond is reactive, binding to amino groups on the surface of a target
• If no target is present, the bond is quickly hydrolyzed and rendered inert

Question 15

Which complement components are deficient in asymptomatic?

Answer 15

C9

Question 16

What are the major components of the classical pathway?

Answer 16

C1 through C9, which are inactive until they are cleaved

Question 17

the first lectin demonstrated to be capable of initiating CM activation

Answer 17

Mannose-binding Lectin

Question 18

The Alternative Protease-Activated Pathway

Answer 18

1. Thrombin cleaves C3 and C5 in vitro with the release of the active anaphyltoxins C3a and C5a
2. Plasmin is capable of generatin both C3 a and C5a
3. Platelet activation releases ATP, Ca2+ and serine/ threonine kinases that could stabilize C3b in fluid space

This indicates that strong inflammatory reactions could potentially activate CM systems

Question 19

Which complement components are deficient in Immune complex disease?

Answer 19

C1q, C1s, or C1r2s2, C2, C4

Question 20

Which complement components are deficient in recurrent pyogenic infections?

Answer 20

C3, factor H, factor I

Question 21

How does the complement system mediate the interface between innate and adaptive immunities?

Answer 21

• Increases uptake of Ag by antigen-presenting cells
• Can help provide secondary signals to B cells for activation and increase their uptake of antigen to present to helper T cells
• Can help to stimulate T cells to release cytokines, further potentiating innate inflammatory and adaptive immune responses

Question 22

How is the Complement System Regulated?

Answer 22

1. Dissociation of C1 complex, prevents cleaving of C4 and C2
2. Decay-accelerating activity for C3 Convertases (Accelerates the decay of C3 convertase and Factor H binds C3Bb to make it accesible to be cleave by Factor I
3. Regulation by catalytic activity (Factor H binds C3Bb and serves as a cofactor for the cleavage of surface bound C3b by Factor I. CD46 binds either C3b or C4 promoting inactivation by Factor I. Cd46 functions to limit C3 and C5 Convertase activity.
4. Inhibition of lysis by MAC via protectin (CD59), vironection, and S protein
5. Cleavage of anaphylatoxins (removes arginine residues from the carboxy termini of C3a and C5a via carboxylpeptidase

Question 23

Contraction phase is also known as ____________

Answer 23

terminating phase

Question 24

Classical Complement Pathway (CCPW)

• Part of the __________immune response
• It is initiated by ___________
• C1 becomes activated when it binds to an ____________
• Only complexes formed by ___________or certain subclasses of __________Abs are capable of activating the CCPW
• potent activators:_______ and ________
• poor activators: ________ and __________

Answer 24

adaptive

Ab binding

Ag-Ab Complex

IgM; IgG

IgG1 & IgG3

IgG2 & IgG4

Question 25

Steps of the Classical Pathway

Answer 25

1. C1q binds antigen-bound antibody, and induces a conformational change in one C1r molecule, activating it. This C1r then activates the other C1r and the two C1s molecules. 2. C1s cleaves C4 and C4b binds to the membrane close to the C1. C4b binds C2 and exposes it to the action of C1s. C1s cleaves C2, creating the C3 convertase, C4b2a. 3. C3 convertase hydrolyzes many C3 molecules. Some combine with C3 convertase to form C5 convertase. 4. The C3b component of the C5 convertase binds C5, permitting C4b2a to cleave C5. 5. C5b initiates the generation of the MAC

Question 26

The alternative pathway is considered to be a part of the \_\_\_\_\_\_\_\_\_- immune system.

Answer 26

Innate

Question 27

Less than _________ molecules of IgM bound to a RBC can activate the CCPW and induce lysis, whereas some \_\_\_\_\_\_\_\_\_\_molecules of cell-bound IgG are required to ensure the same result.

Answer 27

10; 1000

Question 28

CM components constitute ~ \_\_\_\_\_\_\_\_\_\_% of the globulin protein fraction in plasma, and their combined concentration can be as high as \_\_\_\_\_\_\_\_\_mg/ml

Answer 28

15;3

Question 29

4 main functions of the Complement system

Answer 29

1. Triggering inflammation- C3a, C4a, and C5a trigger inflammation 2. Attraction of immune cells 3. Opsonization 4. Pathogen lysis

Question 30

Characteristics of MBL

Answer 30

Binds close-knit arrays of mannose residues that are found on microbial surfaces such as: * Bacteria strains: Salmonella, Listeria, Neisseria * Fungi strains: Cryptococcus neoformans, C. albicans * Viruses: HIV-1 & RSV (respiratory syncytial virus).

Question 31

Components of the C1 macromolecular complex. How does a stable interaction occur?

Answer 31

1. one molecule of C1q 2. two molecules each of the serine proteases: C1r and C1s * held together in a Ca2+ stabilized complex (C1qr2s2) C1q globular heads must be bound to two Fc sites

Question 32

Additional initiators of the LPW of CM activation

Answer 32

L-ficolin, H-ficolin, and M-ficolin * each bind specific types of carbohydrates on microbial surfaces.

Question 33

Which complement components are deficient in andioderma?

Answer 33

C1 inhibitor

Question 34

•Paroxysmal Nocturnal Hemoglobinuria (PNH)

Answer 34

* "PNH Thrombosis" * Pancytopenia * with Nocturnal build up of dark urine * Coombs negative Hemolytic anemia (complement-mediated) * venous Thrombosis * rom pro-coagulant and pro-inflammatory state formed by complement activation * May cause * aplastic anemia (pancytopenia) * myelodysplasia * acute leukemia * Defect in glycosylphosphatidylinositol (GPI) anchors DAF (CD55) and Protectin (CD59) on RBC membrane * these protect RBC from complement * DAF inhibits C3 convertase * CM-mediated destruction of RBCs * activation of platelets and WBCs * Clonal stem cell defect from mutation in hematopoietic stem cell * Somatic mutations in the X-chromosomal gene PIGA

Question 35

The ALPW is initiated in three ways:

Answer 35

## Footnote A.The Alternative Tickover Pathway B.The Alternative Properdin-Activated Pathway C.The Alternative Protease-Activated Pathway

Question 36

The Alternative Properdin-Activated Pathway

Answer 36

1. Properdin binds first and initiates the Complement on the membrane 2. C3b and Factor B are then recruited 3. Factor D is recruited to cleave Factor B 4. C3bPBb complex acted as an effective C3 convertase

Question 37

* Breakdown of C3b generates a fragment (\_\_\_\_\_\_\_\_\_\_) that binds to Ags * enhancing their uptake by dendritic cells and B cells. * The C3d-Ag complex binds to the same receptor on B cells that the \_\_\_\_\_\_\_\_\_\_\_\_\_uses to gain entry into B cells * where it may cause mononucleosis and, * sometimes, \_\_\_\_\_\_\_\_\_\_\_\_.

Answer 37

C3d; Epstein-Barr virus (EBV);Burkitt's lymphoma

Question 38

•Most CM components are synthesized in the \_\_\_\_\_\_\_\_\_\_\_\_\_.Some are produced by other cell types: \_\_\_\_\_\_\_\_\_\_\_\_

Answer 38

liver by hepatocytes blood monocytes, tissue macrophages, fibroblasts, and epithelial cells of the GIT and GUT.

Question 39

CM proteins

Answer 39

a set of serum proteins that cooperates with both the innate and the adaptive immune systems to eliminate blood and tissue pathogens.

Question 40

C1 esterase inhibitor deficiency

Answer 40

* leads to uncontrolled CM activation * •a complex disorder: hereditary angioedema (HAE): * excessive production of vasoactive mediators (molecules that control blood vessel diameter and integrity), * leads to tissue swelling and extracellular fluid accumulation * suffocation can occur if the airways are obstructed, and * the acute swelling of the abdominal region produces intense pain often resulting in exploratory surgery. Acute treatments * C1 inhibitor, * a replacement therapy (both plasma derived and recombinant products are available); * Prophylactic treatments include attenuated androgens and C1 inhibitor.

Question 41

What role the complement system play in promotion of opsonization?

Answer 41

* Phagocytic leukocytes, including neutrophils & macrophages, carry receptors for C3b (CR1). * When an antigenic particle is coated with C3b, C3b (also C4b) assists in the adherence and ultimate ingestion of the particle by the phagocytic cell. * Opsonized microbes easier to ingest/destroy * Opsonized immune complexes easier to clear * The C5a fragment also enhances phagocytosis by stimulating phagocytic cells to ingest C3b coated Ags

Question 42

What role does complement system play in clearance of immune complexes?

Answer 42

Clearance of Immmune complexes from the circulation depends upon C3b Via C3b, Ag-Ab complexes bind to complement receptors on circulating RBCs As the RBCs pass through the spleen and liver, the coated complexes are stripped off of the RBCs by resident phagocytes Deficiency in the process can lead to renal damage due to accumulation of immune complexes

Question 43

C5 is cleaved to C5a and C5b. What does each component do?

Answer 43

•C5a is a potent anaphylatoxin and a chemotactic attractant for neutrophils C5b serves as the anchor for the assembly of a single molecule each of C6, C7 & C8 and activates MAC.

Question 44

Role CR2 on B cells

Answer 44

•Binds to C3b on opsonized bacteria/antigens •Helps provide secondary signals to B cells through BCR complex for more efficient activation

Question 45

All three pathways of the complement system function to:

Answer 45

1. Recruit inflammatory cells 2. Opsonize pathogens 3. Kill pathogens

Question 46

The Initiation of the Alternative Tickover Pathway of CM

Answer 46

1. C3 undergoes spontaneous hydrolysis at its internal thioester bond, yielding the molecule C3 (H2O) 2. Factor B binds to C3 (H2O) 3. Factor B becomes susceptible to cleavage by Factor D. It cleaved to Ba and Bb. 4. Ba is diffused away and Bb binds to C3 (H2O) in plasma, forming fluid phase C3 convertase 5. Fluid phase C3 convertase cleaves C3 to C3a and C3b 6. C3b binds to factor B on the microbial cell surface 7. Factor B is cleaved by Factor D 8. Membrane bound C3 Convertase is formed (C3bBb) 9. The unstable membrane bound C3 Convertase is stabilized by Properdin 10. Addition of another C3b molecule to the C3bBb complex forms the C5 convertase, which is also stabilized by Properdin

Question 47

Lectin Pathway Steps

Answer 47

1. Lectin recognizes microbial antigens and bind to the surface via lectin rectors 2. MASP-2 on the surface of the microbial antigens cleaves C4 and C2 to form the C3 Convertase (C4b2a) 3. C3 convertase hydrolyzes many C3 molecules. Some combine with C3 convertase to form C5 convertase 4. The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5. 5. C5b initiates the generation of the MAC

Question 48

Alternative Pathway Control Proteins Deficiencies of ADF & H Factors

Answer 48

* Deficiencies of factor H Decay-accelerating (DAF) activity for C3 convertase are linked with a wide variety of symptoms: * leads to uncontrolled activation of the AP, and depletion of C3 occurs. * Bacterial infections * Deficiency or dysfunction of factor H and the resulting dysregulation of the AP is associated with various forms of * kidney disease including atypical Hemolytic Uremic Syndrome (aHUS), * as well as age-related macular degeneration (AMD).