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B Cell repertoire - role of B cell antigen receptors (BCR)
description
Transcript of B Cell repertoire - role of B cell antigen receptors (BCR)
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B Cell repertoire - role of B cell antigen receptors (BCR)
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To generate enough antibodies for a large number of pathogens, adaptive immunity could have evolvedby producing one gene for each antibody. Disadvantages/problems:
- how does the body know of all the pathogens that they would encounter?
But, our immune system generates a random set of antibodies by shuffling a relatively small number of genes.
- waste of resources
- would need a large genome
- do not provide the immune system with the flexibility of fighting new infections in a specific way
- generation of antibodies against self-antigens
Disadvantages/problems:
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B cell development
Ig heavychain
VpreBplus 5
Ig lightchain
proB preB I ImmatureBHeavy chain
generearrangement
preB IILight chain
generearrangement
matureB
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Generation of a diverse repertoire of immunoglobulins (Ig) (1) - heavy chain gene
VH segments JH segmentsD segments
Recombination ofD and JH segments
Recombination ofV and DJH segments
Constantregion
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V segments J segments
Recombination ofV and J segments
Generation of a diverse repertoire of immunoglobulins (Ig) (2) - light chain gene
Constant region
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Generation of a diverse repertoire of antibodies
(1) Random rearrangement of variable gene segments
(2) Different combinations of heavy (IgH) and light chains (Ig or Ig) (3) Addition or deletion of nucleotides at the junction where the variable gene segments are joined
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Proteins:
VpreB + 5 = surrogate light chain - part of pre-BCR
RAG-1 + RAG-2 = recombinase
nucleases = nick and digest hairpin loops of intermediate
TdT (terminal deoxynucleotide transferase) = for N-addition
IgH, Ig, Ig = containing variable regions for recognition of antigens
Ig, Ig = (1) for cell surface expression of immunoglobulin; (2) for signaling
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proB preB IImmature
BpreB IImature
B
positive selection of B cells -BCR signals promote proliferation and differentiation
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5 and VpreB KO mice- Delayed B cell development, but not abolished - normal numbersof peripheral B cells 6 months later- No apparent defect in allelic exclusion
Role of preB receptor:- suppress RAG genes expression, such that allelic exclusion of IgH isachieved- promote proliferation of preB cells to finish rearrangements of IgL
Function of 5 and VpreB - may select for heavy chain variable regions - shaping the primary B cell repertoire
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Antigen-dependent maturation of B cells:
- Fix IgH chain expression in transgenic mice- These mice utilize endogenous light chains- Isolated immature and mature B cells separately -> measure theusage of light chain variable gene segments
Hypothesis:If maturation of immature B cells (to mature B cells) is random, antibody repertoire remains unchanged
Result:Repertoire significantly changed
Conclusion:Maturation of immature B cells is not random, suggesting antigendependent
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V-A V-B V-C V-A V-B V-C
Stochasticevents
Antigen-dependentselection
Testing dependence on antigen selection during maturation of B cells
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Activation of B cell antigen receptor (BCR) signaling
YY
BCR
Syk
-p-p
-p-p
p
p
p
p
SLP65Bruton tyrosine kinasePLC-
YY
YY
antigen
Lyn/Fyn/Blk
Proliferation
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Ligand-independent signaling through (pre-) B cell BCR
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-p-p
p
p
YY
proB preB IImmature& mature
B
preB II
Normal mouse
MT mouse
CD22- CD43+
CD22+ CD43-
MHC II+CD23+
CD22- CD43+
X X
CD22+ CD43-
MHC II+CD23+
CD22- CD43+
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-p-p
p
p
YY
-p-p
p
p
YY
-p-p
p
p
YY
-p-p
p
p
YY
Strong signal Weak maintenance signal
Antigen-dependent and independent (pre-) BCR signals
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proB preB IImmature
BpreB IImature
B
5MTSyk
BLNKBtkVavCD45Lyn
Regulation of positive selection by signaling pathways
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proB preB IImmature
BpreB IImature
B
negative selection of B cells -BCR signals induces death or anergy
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Activated B
deletion
B cell tolerance
Self-reactiveImmature B
deletion
Rescued
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Receptor editing by secondary rearrangement
V segments J segments
Recombination ofV and J segments
Constant region
SecondaryRecombination
(1)
(2) Rearrangement of the germline allele
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Receptor editing:
(1) Abnormally high or low levels of receptor signals stimulate receptor editing
(2) Interleukin-4, in combination with LPS or anti-CD40, stimulate expression of RAG-1 and -2 genes
(3) Interleukin-7, in combination with anti-CD40, stimulate RAG-1 and -2 genes expression
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Receptor editing (continued):
(4) c-myb and Pax-5 proteins activate RAG-2 gene expression
(5) E2A activate RAG-1 and RAG-2 genes expression
(6) OcaB, a transcription factor, promotes Ig gene recombination through the activation of germline gene transcription
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-p-p
p
p
YY
-p-p
p
p
YY
-p-p
p
p
YY
Strong signalleads to apoptosis
Weak signal leads to anergy
Deletion and anergy as mechanisms of immune tolerance -IgHEL/HEL mouse model
HEL
-p-p
p
p
YY
IgHEL
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Self-reactivity Shapes the B cell repertoire
AnergyMature B cell
Failure to enterthe periphery
B cell fate
Deletion
Strength of BCR signal
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IgM
preB IImmature
BpreB IImature
B
Antigen-inducedactivation
proB
IgG
Expression of Ig isotypes in normal B cells
MT
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Discussion of Seagal et al, 2003, J. Exp. Med.
Previous observation:
Hypothesis/Proposal:
Conclusion:
Results:
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Staining for cell surface maker
B CELL
PE
FITC
PE
FITC
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7