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MIMM214 > adaptive immunity - b cells > Flashcards

adaptive immunity - b cells Flashcards

1
Q

brief description of the 3 mechanisms of humoral immunity

A

Abs promote pathogen neutralization, opsonization, complement activation

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2
Q

what is IgM?

A

Immunoglobulin M, an antibody class that serves as a receptor on naïve B cells

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3
Q

name properties of B cells

A
  • antigen-specific
  • ## clonotypic
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4
Q

what are plasma cells?

A

-Activated and differentiated B cells.
the main antibody-secreting cells

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5
Q

what are plasmablasts?

A

B cells in a lymph node that already show some plasma cells features but don’t secrete (or not a lot of) antibody

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6
Q

what do B cells do once they are activated?

A

secrete BCR (antibody)

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7
Q

what happens during clonal selection?

A

activated B cells undergo proliferation and differentiation until they become plasma cells

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8
Q

what is B cells signal 1? (survival signal)

A

antigen

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9
Q

through what vessel do b cells enter and exit lymph node?

A

enter via HEV
exit via efferent lymphatics

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10
Q

in what cases do naive B cells die?

A

after a few month if they haven’t encounter signal 1 (antigen)

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11
Q

does b cell bind antigen on MHC?

A

No! binds antigen DIRECTLY

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12
Q

what are the 3 ways how antigen end up in lymph node?

A
  1. from pathogens via afferent lymphatics
  2. has been opsonized via complement pathway
  3. transported via activated conventional DCs & transferred to follicular DCs
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13
Q

what are subcapsular sinus macrophages?

A

SCS macrophages: lymph node resident macrophages that express complement receptors on their surface to bind the complement on the opsonized antigen.
retain antigen in the lymph node.

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14
Q

what does it mean that SCS macrophages have low endocytic and degradative activity?

A

they don’t phagocytose antigens, they keep them on their surface in the lymph node

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15
Q

what does B cell bind to specifically?

A

epitope on antigen

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16
Q

what are B cell’s co-receptor for signal 1? what do they bind?

A

CD19 and CD21: bind complement (they are not necessary but enhance signaling)

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17
Q

what happens in BCR when it encounters antigen?

A

Igalpha and Igbeta domains associated to BCR become phosphorylated (well their ITAM motifs get phosphorylated)

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18
Q

what happens to BCR-Ag complex after phosphorylation of ITAMs?

A

it gets endocytosed! (internalized) with the Ag

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19
Q

3 main outcomes of signal 1

A
  1. Transcription factors are activated -> gene transcription
  2. Survival signal
  3. Cytoskeletal reorganization
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20
Q

what happens to the internalized BCR-Ag complex?

A

gets processed and presented on MHC so it can interact with TCR

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21
Q

why is antigen processing by B cell considered exogenous pathway?

A

because it’s the cell that decides when to endocytose.
If it’s the pathogen that wanted to enter the cell, it would be endogenous pathway.

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22
Q

what are TD vs TI antigen?

A

thymus-dependent vs thymus-independent (highly repetitive molecules, ex LPS and cross-link on BCR)

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23
Q

what are the difference between TD and TI?

A

TD-antigen provides signal 2 by an activated CD4+ Tfh cell via BCR.
TI-antigen provides signal 2 by TLR signaling!

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24
Q

can TD or TI lead to generation of memory B cells?

A

only TD antigens are specific Ab and can provide memory

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25
Q

what are the 2 parts of signal 2?

A
  1. Signal from pMHC that has bound to TCR & co-receptor on TFH cell
  2. Signal from CD40 on B cells that has bound to CD40L on TFH cell
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26
Q

what are the outcomes of signal 2?

A
  • signaling & activation of transcription factors
  • Leads to activation, proliferation, differentiation: antibody secretion
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27
Q

what cytokine is always secreted in high amounts by TFH to help with proliferation?

A

IL-12

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28
Q

what is linked recognition?

A

for signal 2. the same antigen (can be different epitope) must be presented to T cell by DC AND by B cell, but via a different route

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29
Q

where are SCS macrophages located?

A

subcapsular sinus. where they encoutner Ag

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30
Q

what is in the T cell zone?

A

T cells: they get activated by DCs there

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31
Q

what’s in the B cell zone?

A

it is where B cells encounter Ag (signal 1) and undergo later stages of proliferation and differentiation.
where follicles and germinal centers are found.

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32
Q

what happens at the T-B border?

A

B cell’s signal 2

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33
Q

what happens in the follicle vs in the germinal center?

A

follicle: activation and development of B cells
germinal center: B cell proliferation and differentiation

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34
Q

what changes happen to B cell after signal 1?

A

increased expression of pMHC II, increased expression of chemokines receptors to target to T-B border

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35
Q

what are activated B cell’s 2 options?

A
  1. form primary focus in subcapsular region
  2. migrate to follicle to form germinal center
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36
Q

what happens in primary focus?

A

B cells go near subcapsular region and become plasmablasts

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37
Q

what antibodies can be secreted by plasmablasts?

A

the basic IgM

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38
Q

difference between naive B cell and plasmablast?

A

naive B cell has surface IgM but can not secrete antibody.
plasmablast has surface Abs and can secrete them, and can proliferate

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39
Q

what other than plasmablast can come out of primary focus?

A

IgM MEMORY B cells that produce IgM

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40
Q

where can primary focus form?

A
  • near subcapsular region
  • in interfollicular regions
  • medullary cords
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41
Q

how long does it take for primary focus to be apparent after a primary infection?

A

5 days

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42
Q

what happens to plasmablasts?

A
  • USUALLY stay in lymph node and die by apoptosis within 5-10 days
  • migrate to bone marrow and become plasma cells and continue antibody production
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43
Q

what are the 2 outcomes of germinal center?

A
  1. plasma cells: secrete large quantities of high affinity antibodies
  2. memory B cells: important for memory response
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44
Q

how do memory B cells maintain their memory response?

A

they maintain the capacity to produce high affinity antibodies

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45
Q

what happens in germinal centers?

A
  • b cells recieves signals 1 and 2 again
  • b cells differentiate
  • somatic hypermutation, affinity maturation, class switching
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46
Q

what’s another name for germinal center

A

secondary lymphoid follicle

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47
Q

how long does it take for germinal center to peak in size?

A

7-12 days

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48
Q

name 3 ways how antibodies can act?

A
  • neutralization
  • opsonization
  • complement activation
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49
Q

what’s an antibody?

A

BCR: Y shaped glycoprotein

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50
Q

what is a regular antibody structure?

A

2 identical disulfide-linked heavy (H) chains and 2 identical light (L) chains

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51
Q

where is the variable region on an antibody? what does it form?

A

on both the heavy and light chain, on the N terminus.
forms the 2 identical antigen-binding sites

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52
Q

what does Vl, Vh, Cl, Ch stand for?

A
  • VL : Variable region in light chain
  • VH : Variable region in heavy chain
  • CL : Constant region in light chain
  • CH : Constant region in heavy chain
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53
Q

what can result from variable region of Ab binding to antigen?

A

neutralization of the antigen

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54
Q

what is the role of antibody’s constant region (Fc)?

A
  • involved in complement activation
  • can bind Fc receptors on phagocytes, eosinophils, mast cells
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55
Q

what are Fab fragments?

A

Fragment Antigen Binding: 2 fragments on an antibody that bind the antigen.
part of constant heavy AND light chain.

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56
Q

what is Fc fragment?

A

one fragment on an antibody on the constant region of one of the heavy chain.
is recognized by antibody receptors.

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57
Q

what are the heavy and light chains held together with?

A

disulfide covalent bonds

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58
Q

what type of 3D structure if found in antibody?

A

Beta strand

59
Q

what are where are CDRs?

A

Complementarity Determining Region: antigen-binding site in direct contact with Ag made of 3 hypervariable loops.
found at the antibody extremities.

60
Q

what type of bonds are involved in
ab-ag binding?

A

non-covalent bonding:
* Hydrogen bonds
* van der Waals
* Hydrophobic
* Ionic

61
Q

where on the antigen is the epitope?

A

can be anywhere

62
Q

what varies in length on the antibody to confer specificity?

A

CDR

63
Q

what differs between the different classes of antibodies?

A
  • length/sequence of the constant region of heavy chain
  • number of Ig-like domain
64
Q

describe IgM

A
  • pentameric: 5 antibodies linked via disulphide bonds
  • heavy chain = one variable region and 4 constant regions
  • expressed on mature B cells BEFORE activation
65
Q

what antibody is the most effective initiator of complement cascade

A

IgM

66
Q

describe IgD

A
  • heavy chain = one variable region, 3 constant regions
  • part of the first wave of secreted antibodies (right after IgM)
67
Q

describe IgG

A
  • heavy chain = one variable region and 3 constant regions
  • most abundant in plasma
  • has 4 subclasses
68
Q

when is IgG produced?

A

following differentiation in Germinal Center

69
Q

describe IgE

A
  • heavy chain = 1 variable region and 4 constant regions
  • produced in response to Helminth infections
70
Q

describe IgA

A
  • heavy chain = one variable region and 3 constant regions
  • monomer in plasma, dimer in mucous
  • dimer linked via J chain
  • 2 subclasses
71
Q

give 2 examples of antibody treatment

A
  • antibody against TNF-a against Rheumatoid arthritis
  • antibody against IgG Fc region
72
Q

where are the heavy and light chain encoded?

A

on different chromosomes

73
Q

how many CDRs are there in 1 Ig?

A

12 (3 per chain)

74
Q

each chain as which region genes?

A
  • Light chain: Variable (V) and Joining (J) constant (C) region gene
  • Heavy chain: Variable (V), diversity (D), joining (J), and constant (C) region gene segments
75
Q

where are CDR1, 2, and 3 encoded?

A

CDR1 and 2 are encoded in the V segments of light and heavy chains.
CDR3 is encoded in the joining of V-J segments of light chain; and in the V, D, J gene segments of the heavy chain

76
Q

what CDR is the most variable?

A

CDR3

77
Q

in what order are the different gene segments found? where?

A

light chain variable region: C, V, J
heavy chain variable region: C, V, D, J

78
Q

what chain has 2 different loci (only one is expressed)? name them

A

light chain.
k chain or lambda chain

79
Q

what do antibody’s different constant regions represent?

A

the different isotypes (IgM, IgD, IgG)

80
Q

is somatic recombination reversible?

A

no it’s irreversible

81
Q

what is combinatorial diversity?

A

Diversity that is the result of different combinations of V, D, and J regions

82
Q

what are RSSs and their role?

A

Recombination Signal Sequences: they flank each antibody gene segment and get recognized by recombinase enzymes.
They direct the pairing of segments

83
Q

what are RSSs structure?

A

nonamer + heptamer sequence in which either a 12 or 23 bp long spacer lies

84
Q

what’s the 12/23 rule?

A

a 12-bp RSS must pair with a 23-bp RSS for recombination to occur. this allows for only 1 segment of each to be chosen

85
Q

in what order does the heavy chain build itself?

A

D-J, then V-DJ

86
Q

how long is the spacer of RSS for VH (variable region of heavy chain)

A

23 bp

87
Q

what is RAG? what are they for?

A

Recombination Activating Gene: RAG-1 and -2 are necessary for recombination. they recognize and cut DNA at the RSS and COVALENTLY close the DNA hairpin ends.

88
Q

what is the signal joint?

A

loop of DNA that gets excised and deleted

89
Q

what is the coding joint?

A

Coding region of selected V and J regions that remains

90
Q

what is junctional diversity?

A

During recombination, nucleotides may be added or removed at the junctions between V & D, and D & J (or the V & J for the light chain)

91
Q

what is artemis?

A

an endonuclease that opens the DNA hairpin once signal joint has been excised

92
Q

what can REMOVE nucleotides on each side of the coding joint?

A

exonuclease

93
Q

what kind of nucleotides can be ADDED at overhangs ends?

A

palindromic nucleotides: TCGA in 5’ -> 3’

94
Q

what can add up to 20 N-nucleotides to the cleaved strands primarily in the heavy chain?

A

TdT

95
Q

what does N-nucleotides addition by TdT cause?

A

variation in length of CDRs

96
Q

in what order do the enzymes work to generate BCR diversity?

A

RAG1//2, Artemis, Exonuclease, TdT

97
Q

what is TCR made of?

A

one alpha chain, one beta chain, each with a variable and a constant region

98
Q

what segments are found on each TCR chain?

A

alpha: V and J segments
beta: V, D, J segments

99
Q

where does TCR somatic recombination take place?

A

in the thymus

100
Q

what is the difference between BCR and TCR recombination?

A

BCR: D segment is surrounded by 2 RSS of 12bp spacing
TCR: D segment is surrounded by a 5’ 12 bp RSS and a 3’ 23 bp RSS

101
Q

how many CDR does TCR have?

A

6 (3 on each chain)

102
Q

what CDR is the most important for diversity in TCR? where is it encoded?

A

CDR3: encoded in D and J segments of TCR chains

103
Q

where are CDR1 and 2 encoded in TCR?

A

they are encoded in the V segment of alpha AND beta chains

104
Q

what is allelic exclusion for?

A

ensure that each B cell synthesizes only one heavy and one light chain, even if we receive 2 of each from parents

105
Q

how does allelic exclusion happen?

A

one chromosome gets methylated and silenced

106
Q

what antibodies are expressed by mature but naive B cells?

A

IgM, IgD

107
Q

how does the production of Ig subtypes happen?

A

Alternative RNA splicing

108
Q

what can alternative splicing also decide?

A

if an mRNA is secreted or transmembrane

109
Q

where does secondary diversification happen?

A

in germinal center in lymph node follicles

110
Q

what does somatic hypermutation cause?

A

increases affinity for a BCR’s antigen, without changing specificity

111
Q

what is class switching?

A

a process that replaces one heavy chain constant region with one of a different isotype

112
Q

where do somatic hypermutations happen in the BCR?

A

in the variable region, mostly in CDR loops

113
Q

how to BCR know what “class” to switch to? (what isotype)

A

from TFH cytokines released in the germinal center

114
Q

can naive B cells undergo somatic hypermutation and class switching?

A

no, only activated B cells

115
Q

what structure in BCRs guide class switching?

A

switch regions upstream of C genes

116
Q

what extra diversity is found in BCRs that isn’t in TCRs?

A
  • H and L chain combination
  • secondary diversification (somatic hypermutation and class switching)
117
Q

what is found in germinal center light zone?

A

TFH (retain antigen in the light zone)

118
Q

can B cells who enter germinal center produce antibodies?

A

yes they can produce IgM/IgD

119
Q

where does secondary diversification happen?

A

germinal center

120
Q

what mostly happens in the light zone of germinal center?

A

plasma and memory cell differentiation

121
Q

what happens in the dark zone of germinal center?

A

somatic hypermutation

122
Q

what are FDCs?

A

Follicular Dendritic Cells: DCs in germinal center that condense Ag

123
Q

where are FDCs?

A

in light zone of germinal centers! because that’s where differentiation happens

124
Q

how does affinity maturation work?

A

higher affinity B cells bind antigen in germinal center light zone and recieve signal 1 again.
lower affinity B cells don’t and die by apoptosis.

125
Q

what are the outcomes of germinal center?

A
  1. plasma cells: secrete IgG, IgA, or IgE antibodies of high affinity
  2. Memory B cells:
126
Q

what is AID?

A

Activation-Induced cytidine Deaminase: responsible for somatic mutations

127
Q

how does AID work?

A

it deaminates cytidine residues in ssDNA: cytidine -> uridine. Uridine then gets removed. produces individual point mutations in the variable chain

128
Q

are all AID changes productive?

A

no

129
Q

when does class switching occur?

A

in the germinal center after the second exposure to antigen (2nd signal 2)

130
Q

what is required for class switching?

A

complete signal 2 (Ag + CD40)

131
Q

between what regions does recombination occur in BCR?

A

between switch regions

132
Q

in class switch recombination: When B cell receives cytokine signal, transcription is activated _______ of constant region

A

upstream

133
Q

after transcription begins, DNA is now accessible to what enzyme?

A

AID

134
Q

how is immunological memory easier to detect/monitor?

A

Abs can be measured in serum
Memory T cells reside in tissues

135
Q

where do most IgM (sometime IgD) producing cells come from in primary response?

A

primary focus

136
Q

what do memory B cells express higher levels of than naive B cells?

A

MHC class II and B7 co-stimulatory ligand

137
Q

why do memory B cells express more MHC class II and B7 molecules?

A
  • makes it easier to present antigen to Tfh
  • increases antibody production
138
Q

where do memory B cells reside?

A

circulation, spleen, lymph nodes

139
Q

what do memory T cells have a large expression of?

A

CD28: don’t require strong co-stimulatory signal or cytokines

140
Q

what distinguishes Tcm (central memory t cells) vs Tem (effector memory t cells)

A

location (Tcm in secondary lymphoid tissue) (Tem in tertiary/peripheral tissue)

141
Q

does passive immunization generate memory response?

A

no. no memory cells are made out of it because we inject preformed antibodies

142
Q

what is the original antigenic sin?

A

when presented to a know infection, memory cells are enlisted instead of activating naive cells that target new unique epitopes

143
Q

what are adjuvants?

A

molecules in vaccines that enhance the immune response

MIMM214 (5 decks)

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