Lymphocyte genetics and clonal expansion of B cells Flashcards
(137 cards)
1
Q
What is the role of B cells in the immune system?
A
produce antibodies that can bind to any potential antigen.
2
Q
How does the immune system achieve diversity in antibody production?
A
V(D)J recombination where variable (V), diversity (D), and joining (J) gene segments are randomly mixed in developing B cells, leading to a vast array of unique antibodies.
3
Q
Do B cells require antigens for their initial development?
A
No, B cell development is antigen-independent. B cells undergo V(D)J recombination and develop in the bone marrow before they have encountered an antigen
4
Q
How is the specificity of B cells determined?
A
defined by genetic rearrangement during its development. Each B cell produces a unique antibody that defines its specificity
5
Q
What is clonal selection in the context of B cells?
A
natural process by which a B cell with an antibody that successfully binds to an antigen is selected for. This B cell then proliferates, producing a clone of cells that all produce the same antibody specific to the antigen.
6
Q
What is the “one cell-one antibody” rule?
A
Each B cell produces only one type of antibody. Each B cell clone will target a specific antigen.
7
Q
What determines the specificity of B cell antigen receptors?
A
unique genetic mechanism during lymphocyte development in the bone marrow, creating millions of different receptor gene variants.
8
Q
clonal selection theory
A
B cell progenitor gives rise to many pre-B cells, each with a unique specificity. Self-reactive pre-B cells are removed by clonal deletion, while specific immature B cells proliferate and differentiate when activated.
9
Q
How does the immune system deal with self-reactive B cells?
A
Self-reactive B cells are altered or eliminated through a process called clonal deletion to prevent autoimmunity.
10
Q
What are the two stages of B cell development?
A
maturation phase, where stem cells become mature naïve B cells, and a differentiation phase, where mature B cells become plasma cells or memory B cells upon antigen activation
11
Q
What are the three main goals of B cell development?
A
- Generate a diverse array of antigen-binding receptors. 2. Alter or eliminate self-reactive B cells. 3. Promote the maturation of foreign-reactive B cells.
12
Q
What happens to B cells after an antigen is eliminated?
A
specific B cells that were activated and proliferated form memory B cells, which remain in the body to provide a rapid response to future encounters with the same antigen.
13
Q
How do B cells achieve diversity in their antigen receptors?
A
genetic rearrangement during their development in the bone marrow, a key part of clonal selection
14
Q
What are the antigen-independent in B cell development?
A
occurs in the bone marrow during maturation.
15
Q
What are the antigen-dependent phases
A
Occurs in peripheral lymphoid organs, where B cells differentiate after encountering an antigen.
16
Q
What is shown in the maturation phase of the B cell development diagram?
A
transition from stem cell to mature naïve B cell, which is antigen-independent and occurs in the bone marrow.
17
Q
What is the differentiation phase in B cell development?
A
antigen-dependent and occurs in the secondary lymphoid organs, where activated mature B cells become either antibody-secreting plasma cells or memory B cells.
18
Q
Why is diversity in antigen receptors crucial for B cells?
A
BCR must recognize a vast array of potential antigens, so genetic rearrangements create a diverse pool of BCRs, each with a different specificity.
19
Q
What is clonal deletion in the context of B cell development?
A
Clonal deletion is the process by which potentially self-reactive pre-B cells are removed to prevent the immune system from attacking the body’s own tissues.
20
Q
What is clonal deletion in the context of B cell development?
A
process by which potentially self-reactive pre-B cells are removed to prevent the immune system from attacking the body’s own tissues.
21
Q
When do specific immature B cells proliferate and differentiate?
A
Upon encountering their specific antigen and receiving necessary co-stimulatory signals, activated specific immature B cells proliferate and differentiate into effector cells.
22
Q
What is the role of memory B cells?
A
persist after an antigen has been eliminated, providing a quicker and more robust response upon re-exposure to the same antigen.
23
Q
What is the purpose of genetic rearrangement in B cells?
A
ensures that each B cell has a unique receptor, contributing to the immune system’s ability to recognize an immense variety of antigens.
24
Q
How is the specificity of B cell receptors (BCRs) determined?
A
specificity of BCRs is defined by genetic rearrangements that occur during B cell development in the bone marrow.
25
What are the two main processes in B cell development?
maturation phase where B cells achieve immunocompetence, and differentiation phase where they become specialized effector cells.
26
What triggers the activation of naïve B cells?
when their receptors bind to a specific antigen and they receive co-stimulatory signals, leading to their proliferation and differentiation
27
SLO
secondary lymphoid organ
28
what is the maturation phase characterized by
CD45R
29
What happens to naive B cells upon encountering an antigen?
enter the antigen-dependent phase (activation and differentiation), where they interact with T helper cells in peripheral lymphoid organs.
30
What are the possible outcomes for an activated B cell?
undergo class switching, affinity maturation, become plasma cells secreting antibodies, or develop into memory B cells.
31
What processes enhance B cell effectiveness?
Class switching changes the antibody isotype produced, and affinity maturation increases the antibody's ability to bind to the antigen.
32
What percentage of activated B cells survive to become effector cells?
10% survive, with the majority (~90%) undergoing cell death if they do not successfully undergo class switching or affinity maturation.
33
What is the role of memory B cells after the elimination of an antigen?
provide long-term immunity by responding more quickly and effectively upon re-exposure to the same antigen.
34
What are the stages of B cell development in the bone marrow?
Stem cell, early pro-B cell, late pro-B cell, large pre-B cell, small pre-B cell, immature B cell.
35
What marks the successful development of B cells in the bone marrow?
The successful generation of functional B cell receptor (BCR) expression, specifically IgM.
36
The successful generation of functional B cell receptor (BCR) expression, specifically IgM.
Heavy chain (H-chain) and Light chain (L-chain) bonded together by disulfide bridges.
37
What are the two types of regions found in H-chains and L-chains?
Constant region and variable region
38
Constant region on H and L-chains
determines the antibody subclass
39
Variable region on H and L-chains
determines antigen-binding affinity
40
Describe the heavy chain loci
V, D, and J gene segments
41
Describe the light chain loci
κ locus includes V and J segments, and the λ locus has multiple V and J segments.
42
What is the V(D)J rearrangement process in B cells?
process of somatic recombination that creates the variable region of the immunoglobulin heavy and light chains, generating the unique antigen-binding site of each B cell's receptor.
43
What happens during the D-J rearranging stage of B cell development?
DJ, gene segments of the heavy chain (H-chain) undergo recombination. This is the first step in the variable region gene rearrangement.
44
What is the V-DJ rearranging step in B cell development?
Following D-J recombination, a V gene segment is joined to the D-J complex in the late pro-B cell stage, completing the rearrangement for the heavy chain locus.
45
What indicates the completion of heavy chain rearrangement in B cell development?
successful VDJ recombination of the heavy chain genes is confirmed in the large pre-B cell stage, allowing for the expression of the μ chain as part of the pre-B cell receptor on the cell surface
46
After heavy chain rearrangement, what is the next step for the light chain genes?
light chain genes begin their rearrangement with the V to J joining in the small pre-B cell stage, preparing the light chains to pair with the heavy chain.
47
What completes the immunoglobulin light chain gene rearrangement?
completion of the VJ rearrangement allows for the full assembly of the light chain, which can then pair with the heavy chain to form a functional B cell receptor.
48
When is immunoglobulin first expressed on the B cell surface?
Once both heavy and light chain rearrangements are complete, the B cell receptor, primarily IgM, is expressed on the surface of the immature B cell, indicating readiness for antigen encounter.
49
Main IgG subclasses
IgA, IgG, IgM, IgE and IgD
50
IgD function
antigen receptor on B cells that have not been exposed to antigens.
51
IgE Function
Binds to allergens and triggers histamine release from mast cells and basophils, and is associated with allergy and defense against parasitic infections.
52
IgM
First antibody to be produced in response to an antigen, primary activator of the complement system.
53
IgA subclasses and functions
IgA1: (most abundant) found in various secretions such as saliva and tears.
IgA2: More resistant to proteases and predominant in mucosal areas like the gut.
54
IgG subclasses (1-2)
IgG1: Most abundant IgG subclass in serum, effective in opsonization and complement activation.
IgG2: Responds mainly to polysaccharide antigens.
55
IgG subclasses (3-4)
IgG3: Has a high affinity for antigens and can effectively activate complement.
IgG4: Does not activate complement and is involved in response to repeated exposure to antigens (e.g., allergies).
56
The μ (mu) chain (heavy chain)
pairs with a surrogate light chain (made up of VpreB and λ5 proteins) and the signaling components Igα and Igβ to form the pre-BCR.
57
Heavy chain functions
Large protein chains - contribute to antibody size and shape
58
Light chain function
Smaller protein chains
59
Constant regions in antibodies
decide antibody class and whether they are soluble or membrane-bound
60
Variable antibody region
antibody's unique antigen-binding specificity
61
What processes contribute to the vast diversity of antibodies
Combinatorial diversity (VDJ),
Junctional diversity,
Pairing of different heavy and light chains,
Somatic hypermutation
62
Junctional diversity
Additional diversity is introduced at the junctions during the recombination process.
63
Somatic hypermutation
After antigen stimulation, mutations in the V regions can increase the affinity of the antibody for its antigen.
64
In what order do gene rearrangements occur for antibody production?
The heavy chain VDJ is rearranged first, located on chromosome 14.
Light chain rearrangement follows, with the kappa chain on chromosome 2 or the lambda chain on chromosome 22.
The first functional antibody produced is always IgM, expressed on the surface of immature B cells as part of the BCR.
65
Kappa and Lambda on what chromosomes
kappa chain on chromosome 2, lambda chain on chromosome 22.
66
What are pseudogenes in the context of immunoglobulin loci?
Pseudogenes= non-functional gene segments within immunoglobulin loci that cannot produce a protein. They reduce evolutionary pressure by providing a reservoir of genetic material that can potentially evolve new functions.
67
What role do Recombination Signal Sequences (RSS) play in antibody gene rearrangement?
Conserved non-coding DNA sequences that guide the recombination of gene segments in antibody genes. They consist of a conserved heptamer, a non-conserved spacer, and a conserved nonamer, and are crucial for the correct joining of gene segments.
68
69
What happens during gene rearrangement if gene segments are in the same or opposite orientations?
same=looping out and deletion of the intervening DNA occur. opposite orientations= the coiled region is retained in the DNA in an inverted orientation.
70
Enzymatic steps involved in the rearrangement of immunoglobulin genes
RAG1/RAG2 complex recognizes and binds to RSS.
Ku70:Ku80 forms a ring around the DNA, associating with the DNA-PKcs to form DNA-PK.
Artemis with nuclease activity cleaves DNA.
DNA ligase IV:XRCC4 joins the DNA ends.
71
What is the 12/23 rule in immunoglobulin gene rearrangement?
Requirement that a 12-nucleotide spacer RSS can only be joined to a 23-nucleotide spacer RSS, ensuring diverse yet accurate recombination of immunoglobulin gene segments.
72
Functions of Junctions in antibody gene rearrangement ?
variable (V), diversity (D, for heavy chains only), and joining (J) gene segments come together.
73
V(D)J recombination is facilitated by enzymes
RAG1 and RAG2
74
Non-conserved spacers
A variable length sequence of either 12 or 23 nucleotides (hence the 12/23 rule) that provides the physical space between the heptamer and nonamer.
75
Conserved Nonamer
9 nucleotides, providing a binding site for the RAG complex.
76
Conserved Heptamer
7 nucleotides, recognized by recombination activating genes (RAG) during V(D)J recombination
77
Where are N-nucleotides more common and what is their role?
contribute to junctional diversity in the antigen-binding sit- more common in heavy-chain V-D and D-J junctions than in light-chain junctions.
78
What are N-nucleotides added by
terminal deoxynucleotidyl transferase (TdT)
79
How many times can L-chain genes rearrange and why is this significant?
L-chain genes can rearrange multiple times.
This flexibility allows B cells to try different light-chain combinations to express a functional BCR if the first rearrangement is nonproductive.
80
How is antibody diversity generated during B cell development?
Back:
81
How is antibody diversity generated during B cell development?
Combinatorial diversity arises from different V(D)J gene segment combinations.
Junctional diversity results from the addition of P and N nucleotides at gene segment junctions.
82
P Nucleotides
created when the hairpin structures at the coding ends of gene segments are opened during V(D)J recombination.
83
At which stages can developing B cells be lost during rearrangement?
cells can fail to produce a functional receptor at both the heavy-chain and light-chain gene rearrangement stages.
Cells that fail to rearrange successfully are typically eliminated, ensuring only functional B cells proceed to maturation.
84
How do chromosomal rearrangements occur during B cell development?
Rearrangements happen one chromosome at a time to prevent simultaneous productive rearrangements on both chromosomes, which would reduce diversity.
This sequential process ensures that if a productive rearrangement occurs, the other chromosome's rearrangement is halted.
85
V(D)J recombination does not produce a viable coding sequence for BCR, rearrangement fails at L(less as is rearranged multiple times) or H-chain, B cells that produce a receptor that binds too strongly to "self" antigens.
86
Stringent Selection
process during B cell development where only B cells with correctly formed and non-self-reactive BCRs are allowed to mature and survive.
87
Negative selection (stringent)
Eliminates B cells that have failed to produce a functional BCR or that react with self-antigens, preventing autoimmunity.
88
Positive Selection (Stringent)
Ensures that B cells with functional, non-self-reactive BCRs continue their development into mature B cells.
89
What is the function of the pre-B cell receptor (pre-BCR)?
stops further heavy-chain gene rearrangement and triggers the B cell to enter the cell cycle.
Ensuring allelic exclusion and progression to light-chain gene rearrangement.
90
What is allelic exclusion and why is it important in B cell development?
Allelic exclusion ensures that each B cell expresses only one specificity of antibody, preventing the expression of different receptor types on the same cell, which would complicate antigen recognition.
91
What are the key characteristics of immature B cells?
Completed all V(D)J rearrangements and express surface IgM. They have a short lifespan and must check for autoreactivity before maturing and leaving the bone marrow.
92
What is the significance of surface IgM+ B cells?
Surface IgM+ B cells are an indication that the B cell has successfully produced a functional BCR. However, these cells are also tested for self-reactivity to prevent autoimmunity.
93
What happens during the autoreactivity check for B cells?
B cells are tested for binding to self-antigens. Those that bind strongly to self-antigens undergo apoptosis, a process known as negative selection, to prevent autoimmune reactions.
94
How do B cells determine if they are binding too strongly to self-antigens?
BCR binds with high affinity to a self-antigen present in the bone marrow, this signals that the B cell may be autoreactive = negative selection (apoptosis)
95
What is the central tolerance in B-cell development
allowing self-reactive B cells to change their specificity by rearranging their light-chain genes, starting with the kappa (κ) locus and potentially moving to the lambda (λ) locus.
96
What are the main stages in B-cell development for SLO?
maturation in the bone marrow, where B cells express IgM as a signal to leave, and IgD to transition to secondary lymphoid organs (SLO). The V region of the B-cell receptor remains unchanged through these stages
97
How do B cells produce different immunoglobulin types?
B cells can produce IgM or IgD through alternative splicing of the primary transcript of the B-cell receptor gene. This process allows B cells to switch the type of antibody they express without changing the specificity for the antigen.
98
Where do newly produced B cells go after maturation?
Newly produced B cells leave the bone marrow and enter the circulation to survey for antigens in secondary lymphoid organs (SLOs).
99
How do B cells initially respond to antigens?
B cells can become low-affinity plasma cells producing IgM antibodies in a T cell-independent manner within a few days of antigen recognition
100
What is isotype switching in B cells?
Isotype switching is a process that occurs after interaction with helper T cells, leading to the production of different classes of antibodies beyond IgM, typically within 1-2 weeks.
101
What is isotype switching in B cells?
Isotype switching is a process that occurs 1-2 weeks after antigen recognition where B cells change the class of antibody they produce, such as from IgM to other isotypes like IgG, IgA, or IgE.
102
What happens during the germinal center formation?
B cells undergo somatic mutation and affinity maturation over several weeks, leading to the production of high-affinity plasma cells and memory B cells.
103
What is the result of somatic mutation and affinity maturation in B cells?
These processes result in the selection of B cells that produce high-affinity antibodies, which are crucial for effectively neutralizing antigens and providing long-term immunity.
104
What are high-affinity plasma cells and memory B cells?
High-affinity plasma cells secrete large amounts of specific antibodies, while memory B cells persist in the body, providing a rapid and effective response upon re-exposure to their specific antigen
105
What triggers somatic hypermutation in B cells?
Somatic hypermutation is a high rate mutation process that occurs only after antigen activation and requires T cell signaling. It typically takes place in germinal centers.
106
What T cell signals initiate somatic hypermutation in B cells?
T helper cells when CD40 ligand (CD40L) on the T cell binds to CD40 on the B cell.
107
cytokines secreted by T cells to their receptors on B cells, influencing stomatic hypermutation
IL4 and IL5, IFN y, TGF-B- they affect AID to induce mutations
108
What are the possible outcomes of somatic hypermutation?
This process can lead to the production of B cells with either improved antigen affinity, referred to as affinity maturation, or non-functional receptors due to random mutations.
109
Activation-Induced Cytidine Deaminase (AID)
expressed upon B cell activation and targets actively transcribed genes. It modifies cytidine bases to uridine in single-stranded DNA, which can then be processed by DNA repair mechanisms.
110
What is class switching in B cells?
Class switching involves genetic rearrangement of the C-region of the H-chain, allowing B cells to produce different classes of antibodies (like IgG, IgA, etc.) instead of IgM.
111
Why is T cell signaling necessary for B cell maturation?
aids B cells to undergo somatic hypermutation and class switching, essential for the maturation process and the generation of high-affinity antibodies.
112
Stomatic hypermutation - TGF-β
Involved in class switching to IgA and IgG2b in mice, and has a regulatory role in the immune system.
113
What are CDRs in antibody structure?
Specific regions within the variable regions of Ig heavy and light chains that directly contact antigens. These regions show high variability, determining the unique specificity for antigens.
114
CDRs
Complementarity Determining Regions
115
What role do framework regions play in antibody structure?
provide the structural support for the CDRs, helping to maintain the overall structure of the antibody's variable regions.
116
What are isotypes and allotypes in the context of immunoglobulins?
refer to different classes of heavy or light chains within the same host (e.g., IgM, IgG), while allotypes are genetic variants within the constant regions of antibodies, differing between individuals of the same species.
117
What is the role of mismatch repair in somatic hypermutation?
Mismatch repair enzymes identify and repair the mismatches created by AID, which can result in additional mutations further diversifying the antibody genes.
118
What is allelic exclusion in B cells?
ensuring that each B cell expresses an antibody with a unique specificity. Through gene rearrangement, only one allele of the heavy or light chain gene is productively rearranged and expressed, preventing the expression of the other allele.
119
Affinity Maturation
The process by which B cells in the germinal centre of lymph nodes increase the affinity of their antibody for the antigen.
120
What are isotypes
different classes of immunoglobulins (e.g., IgM, IgG) within the same species
121
What are allotypes
genetic variants within the constant regions of antibodies among different individuals
122
Idiotypes
unique antigen-binding sites determined by the variable regions of the antibodies.
123
What are the functions of UNG in somatic hypermutation?
In somatic hypermutation, AID converts cytosine to uracil in DNA. UNG removes the uracil, leaving an abasic site.
124
What are the functions of APE1 in somatic hypermutation
cuts the DNA at this site, creating a single-strand break.
125
UNG and APE1 function
initiates the error-prone repair process that introduces mutations into the variable regions of immunoglobulin genes, increasing antibody diversity.
126
What is the role of mismatch repair enzymes in antibody diversification?
After AID activity, mismatch repair enzymes recognize and repair the uracils paired incorrectly with other DNA bases, often introducing errors. These mutations can result in a variety of outcomes, including transitions or transversions. The process enhances the diversity of antibodies, enabling a more effective immune response.
127
transitions
purine replaced by another purine or a pyrimidine by another pyrimidine
128
transversion
purine replaced by a pyrimidine or vice versa
129
What initiates class switching in B cells?
Class switching is initiated by the enzyme AID in response to signals from T helper cells, including CD40 ligand interaction and cytokines such as IFN-γ for IgG class switching, or IL-4 for IgE class switching.
130
What is the significance of switch regions in class switching?
repetitive DNA sequences located upstream of the constant regions of the heavy chain immunoglobulin genes. They are the sites where class switch recombination occurs, changing the antibody class that a B cell produces without altering the specificity for the antigen.
131
What are R-loops and their role in class switching?
RNA-DNA hybrid structures formed during transcription of the immunoglobulin gene. They serve as substrates for AID, leading to mutations and recombination events necessary for class switching.
132
133
What is allelic exclusion in B cell development?
Allele, thereby producing antibodies with a single specificity. This process prevents the expression of immunoglobulin genes from both alleles of a chromosome, maintaining the monospecificity of antibodies.
134
How does transcription through the switch region initiate class switching
Transcription through the switch region forms R-loop structures that expose single-stranded DNA, which serves as a substrate for AID. This leads to double-strand breaks in DNA, initiating the recombination process that results in class switching.
135
What is the role of DNA repair enzymes in class switching
Enzymes like DNA-PK and others involved in DSBR are essential for re-joining the DNA ends after class switching, ensuring the integrity of the genome and the successful change of the antibody class.
136
DSBR
double-strand break repair
137
In the context of B cell class switching, what are the substrates for AID
RNA-DNA hybrid structures known as R-loops.
