Antibody and effector B cell functions (Humoral Immunity) Flashcards
1
Q
Involvement of complement in periodontitis
A
- the perio pocket is >4 mm deep
- severe inflammation due to bacteria leads to alveolar bone loss
- the sub gingival biofilm in the pocket is dysbiotic
- the subverted immune response, including complement, is the culprit
2
Q
What mediates humoral immunity?
A
antibodies: perform their effector function at sites distant from their production and can be transported across epithelial barrires. They provide defense against extracellular microbes, microbial toxins, and viruses or intracellular bacteria before they infect cells or when they are released from infected cells
3
Q
describe the role of antibodies in protective immunity
A
- protective immunity is mediated by antibodies derived from short-lived and long-lived antibody-producing plasma cells and by activation of memory B cells
- antibodies can be harmful and mediate tissue injury
4
Q
the most effective vaccines induce protection by…
A
stimulating antibody production
5
Q
what are the 7 NEED-to-know effector functions of antibodies (immunoglobulins)?
A
- neutralization of microbes and toxins by inhibiting binding of microbes to receptor so they cannot infect cells
- opsonization and phagocytosis of microbes (Fc receptor binding)
- antibody-dependent cellular cytotoxicity (Fc binding to NK cells)
- lysis of microbes
- phagocytosis of microbes opsonized with complement fragments (eg. C3b)
- inflammation
- complement activation
6
Q
which region of the Ig mediates neutralization of pathogen?
A
Variable region (Fab)
7
Q
which region of the Ig mediates elimination of pathogen?
A
Constant region (Fc)
*binding of FcR from phagocytes, mast cells, eosinophils, and NK cells
*complement activation
8
Q
Both neutralization and elimination are triggered by binding of…
A
antigens to the variable region of Ig
9
Q
Istotype responsible for activation of the classical pathway of complement
A
IgM
10
Q
Istotype responsible for eosinophil and mast cell-mediated defense against helminths
A
IgE
11
Q
Istotype responsible for mucosal immunity (secretion into lumens of GI and respiratory tracts, neutralization microbes and toxins)
A
IgA
12
Q
What contributes to the increased half-life of IgG?
A
Neonatal Fc receptor (FcRn)
- circulating proteins half-life: hours to days
- IgG half-life: 3 weeks
- IgG are sequestered by endosomal FcRn in endothelial cells and phagocytes, where recycling of the endosome allows IgG released from FcRn and re-expressed on cell surface
- concept exploited for therapy (TNF receptor-Etanercept)
13
Q
How does neutralization of extracellular microbes by antibodies occur?
A
Antibodies bind molecules on the surface of the microbes and block their entrance into the cells/tissues (binding molecules on the microbes required for infecting cells/tissues)
14
Q
Examples of binding molecules on the microbes required for infecting cells/tissues
A
- hemagglutinin (influenza virus)
- pilli (gram- bacteria)
15
Q
IgG is found in ___, IgA is found in ___
A
blood, mucosa
16
Q
Antibodies with ___ affinity for antigens are the most effective at neutralization of microbes
A
high affinity
17
Q
describe the neutralization of a virus
A
- virus binds to receptors on cell surface
- receptor-mediated endocytosis of virus
- acidification of endosome after endocytosis triggers fusion of virus with cell and entry of virus DNA
*an antibody would block binding of virus to receptor and also can block the fusion event
18
Q
What is the polio vaccine?
A
Attenuated polio virus - mucosal IgA
19
Q
What is the Hep A or B vaccine?
A
recombinant viral envelope proteins - systemic IgG
20
Q
describe the neutralization of microbial toxins
A
Antibody blocks binding of toxin to cellular receptor
- anti-toxin Ig
- Toxin examples: tetanus toxin induces paralysis, diphtheria toxin is a protein synthesis inhibitor. Vaccines use toxoid to stimulate immune response
- steric hindrance: prevention of molecular interactions as a result of the spatial structure
21
Q
How do Ig mediate phagocytosis of microbes? (overview)
A
Binding of Ig-coated microbes by Fc receptors
22
Q
FcgammaRIIIA (CD16)
A
- distrusted on NK cells
- functions in antibody-dependent cellular cytotoxicity (ADCC)
23
Q
Fc receptors
A
Receptors on the phagocyte’s surface that specifically bind to antibodies
- bind constant region of Ig and FcR clustering results in cellular activation except for Fc[gamma]RIIB
- clustering of FcR requires recognition of multivalent antigen by Ig
24
Q
FcgammaRI (CD64)
A
- has high affinity for Ig
- distrusted on macrophages, neutrophils, mast cells, eosinophils
- functions in phagocytosis and activation of phagocytes
25
describe antibody-mediated opsonization and phagocytosis
- IgG coat microbes and promote their phagocytosis by binding to Fc receptors on phagocytes (macrophages and neutrophils)
- coating process = opsonization. The substances that perform this function are the opsonins.
- Fc receptors deliver signals via FcR chain that promote phagocytosis and stimulate the microbial activities
26
step-by-step antibody-mediated opsonization and phagocytosis
1. opsonization of microbe by IgG (multivalent antibody-coated microbe)
2. binding of opsonized microbes to phagocyte Fc receptors (FcR clustering required)
3. Fc receptor signals activate phagocyte
4. phagocytosis and killing of ingested microbe
27
How do IgG induce NK cell cytotoxicity of infected cells? (overview)
Binding of IgG-coated cells by Fc receptors
28
describe antibody-dependent cell-mediated cytotoxicity (ADCC)
- NK cells bind IgG-coated cells via FcgRIII (CD16) and discharge granules resulting in killing of antibody-coated cells
- FcgRIII binds aggregated IgG not circulating monomeric IgG
- for infected cells displaying infectious agent antigen on surface
29
How do IgE induce mast cells/eosinophil degranulation?
Binding of IgE-helminth or IgE-allergens by Fc receptors
30
Describe IgE and eosinophil-mediated killing of helminth
- FceRI binds Fc portion of IgE-coated helminth
- triggers degranulation of toxic mediators that kill helminth
31
Describe Fce-mediated allergic disease
- mast cells/basophils are coated with IgE specific for allergens via their FceRI
- clustering of FceRI bound to IgE by multivalent allergen triggers degranulation
32
How do vaccines work? (overview)
Microbe-specific Ig are being produced by memory B cells
33
What was the first vaccine?
- Edward Jenner 1796: inoculation of cowpox virus (obtained from pus from skin blister) protected individual against smallpox
- terminology derives from vaccus: cow
- first prescribed vaccine which eradicated the disease
- cowpox and smallpox viruses share some surface antigens: cowpox antibodies bind to neutralize the smallpox virus
34
What makes the most effective vaccine?
Production of high-affinity neutralizing antibodies and memory cells
35
which form of vaccines are more potent?
Live attenuated or killed vaccine are more potent. Allows for better immune response with TLRs
Ex: BCG, cholera
36
What do subunit vaccines contain?
Non-living vaccine antigens and Alum (adjuvant - means "to help" to try and stimulate better immune response)
Ex: tetanus toxoid, diphtheria toxoid
37
describe the possibility of a caries vaccine
- Sub-cutaneous administration of Streptococcus mutans (Glucan-binding protein subunit) was used successfully in monkeys and elicited predominantly IgA, IgG, and IgM
- the antibodies find their way into the oral cavity via gingival crevicular fluid and are protective against dental caries
- passive administration of antibody to functional epitopes of S. mutans virulence antigens has provided a degree of protection in preclinical studies and small-scale human investigations
38
Which part of the immune system is complement a part of?
Innate immunity (but can also bind to some immunoglobulin)
39
Jules Bordet demonstrated the presence of a ___ component in antiserum (alexin) that mediate bacteriolysis. Redefined by Paul Ehrlich as its complements the lytic function of antibodies.
heat sensitive
*at the time he didn't realize there were two things in the serum involved: immunoglobulin not sensitive to heat and complement (heat sensitive)
40
Complement complements...
the ability of antibodies to bind to bacteria to lyse the bacteria
41
complement system are proteins synthesized by...
hepatocytes and are inactive
(C1 produced by macrophage but is only exception)
42
activation of complement involves...
the sequential proteolysis of those proteins produced by the liver to generate enzymes (zymogens) with proteolytic activity
43
Products of complement activation covalently bind...
Microbial cell surface
Ab bound to microbes
Ab bound to tissues
44
what are the three major pathways of complement activation?
Classical pathway
Alternative pathway
Lectin pathway
45
classical pathway of complement activation
Activated by the Fc portion of an immunoglobulin in an antigen-antibody complex. It can also be activated by enzymes and a variety of substances which include endotoxins, cell membranes, and viruses
Summary: binding of antibody on surface of microbe
46
alternative pathway of complement activation
Does not depend on antigen-antibody reaction in order to become active. Biological activators of this pathway include bacterial endotoxins, yeast cell walls, aggregated immunoglobulins and snake venom.
*binding of C3b on surface of microbe
47
lectin pathway of complement activation
Activation is dependent on the binding of a lectin to mannose on the surface of a pathogen
48
What differs in the major pathways of complement activation?
- early steps differ (triggering events and convertases)
- all result in phagocytosis, inflammation, and destruction of microbes
49
C4b2a
C3 convertase of classical pathway and lectin pathway
50
C3bBb
C3 convertase of the alternative pathway
51
Mammalian cells express several plasma and membrane proteins that block...
complement fixation and subsequent activation
52
C3bBbC3b
C5 convertase of alternative pathway
53
C4b2aC3b
C5 convertase of classical pathway
54
membrane attack complex (MAC)
complement system components assembled to form pores in membranes of invading cells; leads to movement of ions and water across the membrane and ultimately lysis
55
C3 convertase
cleaves C3 into C3a and C3b
56
In the early steps of complement activation, several ___ proteases are involved in proteolytic cleavage of the complement proteins
serine
57
most important step in complement activation
Cleavage of C3 by C3 convertase
58
net result of complement activation
Coating of microbes with C3b
59
C3b
opsonin: C3b receptor: CR1
*binds to surface of microbes
60
C3a
anaphylatoxin (Leukocyte recruitment and activation)
61
oldest complement pathway, phylogenetically
alternative pathway
62
step-by-step alternative pathway
1. spontaneous cleavage of C3
2. hydrolysis and inactivation of C3b in fluid phase
3. C3b binds covalently to microbial surfaces, binds Factor B
4. cleavage of Factor B by Factor D; stabilization by properdin
5. cleavage of additional C3 molecules by cell-associated C3 convertase
6. C3b covalently binds to cell surface, binds to C3bBb to form C5 convertase
7. cleavage of C5; initiation of late steps of complement activation
63
what triggers the alternative pathway?
Binding of C3b to microbial surfaces (there is always some spontaneous cleavage of C3). Thioester group becomes accessible in C3b and forms ester or amide bond.
*If there is no microbe surface to bind to, spontaneous lysis of C3 by hydrolysis into C3H2O occurs, creating inactive C3b
64
What happens after C3b binds to the surface of the microbe in the alternative pathway?
Cleavage of Factor B by Factor D: B is bound to C3b and when cleaved, turns into Bb and Ba.
65
a vs. b in complement pathways
B is usually big, a is usually small
*exceptions exist
66
Factor D
enzyme that cleaves bound Factor B; plasma serine protease
67
Bb
serine protease and active enzyme of C3 and C5 convertase: cleaves even more C3 into C3a and C3b
68
step-by-step classical pathway
1. immunoglobulin binding on surface of the microbe: more than one Ig needs to be binding and proximal
2. C1 complex binds to multiple immunoglobulins
3. C1 binding activates C1r, which cleaves C1s
4. C1s is activated - now C4 can bind
5. C4 binding causes it to be cleaved by C1s
6. C2 binds and is cleaved by C1s into C2a and C2b
*this is the exception: C2a is the bigger molecule
7. C3 convertase formed by C4b and C2a (C2a is the enzyme that cleaves into C3a and C3b)
69
What triggers the classical pathway?
binding of C1 to microbe-bound antibodies (IgG, IgM)
70
C1r
serine protease which cleaves C1s to activate it
71
C1s
Serine protease that cleaves C4 and C2
72
Lectin pathway step-by-step
1. mannose binding lectin produced by the liver (acute phase reactant) can recognize mannose.
2. Once mannose binds to mannose-binding lectin, it sends a signal that activates MASP1
3. MASP1 cleaves MASP2 into an active protease
4. MASP2 cleaves C4 and C2
73
what triggers the lectin pathway?
Binding of circulating lectins to microbe polysaccharides
74
MASP1
Serine protease that cleaves MASP2
75
MASP2
serine protease that cleaves C4 and C2
76
MASP
MBL-associated serine protease
77
How have most microbes evolved to be resistant to MAC?
- Gram+ bacteria because of thick cell wall
- protozoan parasites, gram- bacteria and viruses because of inhibition mechanisms
- Neisseria susceptible due to thin cell wall
78
membrane attack complex (MAC) structure
Made up of C6, C5b, C7, C8 (C8 inserted into membrane)
C9 polymerizes and forms a pore
79
what does the MAC induce?
osmotic swelling
80
functions of complement system
- opsonization: phagocytosis and killing of microbe
- complement-mediated cytolysis: osmotic lysis of microbe
- stimulation of inflammatory reactions: destruction of microbes by leukocytes
81
opsonization of microbes by ___ induces their phagocytosis
C3b
82
Rank the anaphylatoxins in potency
C5a > C3a > C4a
- increase vascular permeability
- increase extravasation of plasma proteins and monocytes/neutrophils
- increase microbicidal activities
83
describe the clearance of immune complexes by C3b
- complement proteins promote the solubilization of antigen-antibody complexes by binding them
- Immune complexes with attached C3b are bound to CR1 on RBC and cleared by phagocytes in the liver
84
deficiency in C2/C4 is associated with...
lupus
85
describe the coactivation of B cells
- C3b is cleaved by factor I into C3d
- C3d binds to surface of microbe
- C3d binds to CR2 on B cells
- when BCR recognizes microbe at the same time as C3d is bound to CR2, sends signal for B cell activation
- there is impairment in antibody production in mice which lack C3, C4, or CR2
86
Why is regulation of complement activation necessary?
- low level complement activation need to be regulated to avoid damage to normal tissues
- degradation products of compliment can diffuse to adjacent cells
*regulation may be overcome by increasing amounts of complement activation
87
describe the regulation of complement activation
Plasma and membrane proteins prevent complement fixation at the surface of mammalian cells at various stages of complement activation
88
C1 inhibitor
dissociates C1r and C1s from C1q: thus inactivates enzyme responsible for activating pathway
*plasma
89
Factor I
- Inactivates C3b/C4b attached to cell surface
*plasma
90
How is the late-step membrane-attack complex inhibited?
1. S protein inhibits insertion of the complex into the membrane
2. CD59 inhibits C9 polymerization
91
How is complement involved with periodontitis?
- P. gingivalis is a keystone bacteria
- Gingipain enzymes generates C5a and degrades C5b
- crosstalk between TLR2 and C5aR
92
explain the crosstalk between TLR2 and C5aR
- blocks neutrophil phagocytosis
- triggers inflammatory cytokines
- leads to altered growth of microbial community
