B Lymphocyte Development Flashcards
what is the goal of B cell development
- generate a diverse antigen receptor
- alter or eliminate self-reactive B cells/BCRs
- promote foreign reactive B cells to become mature B cells in SLO’s
basic pathway of B cells in the body as they develop and mature
- generation of BCR in the bone marrow
- negative selection in the bone marrow
- migration of B cells through the circulatory system to SLO’s
- B cell activation in SLO’s
- antibody secretion and memory cells made - present in bone marrow and lymphoid tissue
which part of the bone marrow stroma is responsible for secreting growth factors
mesenchymal stem cells
what is the order of early stages in B cell development up to an immature B cell
primed multipotent progenitor - common lymphoid progenitor - early pro-B cell - late pro-B cell - pre-B cell - immature B cell
which proteins commit the common lymphoid progenitor cell to becoming a B-cell precursor
E2A
FOX01
EBF
PAX5
what are the 3 check points in B cell development
- Large pre-B cell with pre-B receptor
- negative selection 1: central tolerance
- negative selection 2: peripheral tolerance
which stages of B cell development happen in the bone marrow vs in the SLO’s
bone marrow: stem cell to immature B cell
SLO: transitional B cell to mature B cell
which stages of B cell development are antigen-independent vs dependent
antigen-independent: stem cell to small pre-B cell
antigen-dependent: immature B cell to mature B cell
why does negative selection 1 create central tolerance?
the immature B cell is still in the bone marrow so it is presented with self-antigens
what are the surrogate light chain components in B cell development and when are they present
y5 and Vpre-B
present from stem cell to small pre-B cell
what proteins are involved in B-cell development and what are their functions
RAG1/2: lymphoid specific recombinase
y5/VpreB: surrogate light-chain components
Kit/IL-7R: growth factor receptor
when does VDJ recombination happen in B cell development
D-J rearranging = early pro-B cell
V-DJ rearranging = Late pro-B cell
rearranged = large pre-B cell
when does VJ recombination happen in B cell development
V-J rearranging = small pre-B cell
VJ rearranged = immature B cell
appearance of surface Ig in B cell development
large pre-B cell: u chain transiently at surface as part of pre-B cell receptor
small pre-B cell: intracellular u chain
Immature B cell: IgM expressed on cell surface
mature B cell: IgD and IgM alternatively spliced H-chain transcripts
what happens if VDJ rearrangement on first and second chromosome of B cell passes vs fails
pass = signalled to survive and become pre-B cells
fail = die by apoptosis (about 50%)
Checkpoint #1 of B cell development: pre-BCR testing
- pre-B cell pairs with surrogate light chain to form pre-BCR
- results in proliferation and progression to L chain rearrangement
- tests VDJ recombination of only 1 H chain per chromosome (allelic exclusion)
is pre-BCR signalling antigen dependent or independent
independent
what does pre-BCR testing allow?
testing of H chain functionality
characteristics of the pre-B cell receptor
- inhibition of H chain recombination (allelic exclusion)
- proliferation of pre-B cells
- stimulation of k light chain recombination
- shut off surrogate light chain transcription
in gene rearrangement of small pre-B cells what is the order of light chain rearranging
- rearrange k on 1st chromosome
- rearrange k of 2nd chromosome
- rearrange y on 1st chromosome
- rearrange y on 2nd chromosome
what is the first choice of the light chain gene in rearrangement
k light chain gene
(y is second choice)
checkpoint #2 in B cell development: Negative selection 1
- tests whether VDJ recombination of L chain pairs with H chain to produce a BCR that does not recognize self-antigens
- L chain exhibits allelic AND isotypic exclusion
- immature B cell stage
- happens in central lymphoid organs
- creates central tolerance
what is the difference between L and H chain exclusions
L chain = allelic and isotypic exclusion
H chain = only allelic exclusion
what is isotypic exclusion
Ensures each B cell expresses only one type of light chain (either kappa or lambda), creating uniform antibody structures and further enhancing antibody specificity
what is allelic exclusion
Ensures that each B cell uses only one allele for the immunoglobulin heavy and light chains, so it produces antibodies with a single specificity
binding of immature B cells to self-molecules in bone marrow can lead to…
receptor editing or apoptosis
what is receptor editing
additional Ig L chain rearrangements give immature B cells in bone marrow additional chances to replace autoreactive BCR with a non-reactive BCR
what happens in negative selection 1 if B cells passes vs fails
pass = B cell migrates to periphery
fail = receptor editing or apoptosis
how does replacement of L chains by receptor editing rescue self reactive B cells
it changes its antigen specificity
do both B and T cells have receptor editing?
NO! only B cells have this
brief process of receptor editing
- strong ligation of IgM by self antigen
- arrest of B-cell development and continues L chain rearrangement leads to low cell-surface IgM
- new receptor specificity is expressed
Checkpoint #3 of B cell development: Negative selection 2
- tests whether BCR with central tolerance fails to recognize any new antigens expressed in the periphery
- antigen dependent
- at transitional B cell stage
- happens in secondary lymphoid organs
- creates peripheral tolerance
what are the differences between negative selection of B cells in bone marrow vs in SLO’s
- in SLO’s, BCR’s that recognize self-molecules cannot undergo receptor editing but ones in BM can
- only possible outcome in SLO’s is apoptosis of the B cell
what are Marginal zone B cells
- weakly self-reactive B cells
- outside of B-cell follicle
- high levels of CR CD21
- migrate to marginal zones of white pulp where they make rapid responses to blood-borne pathogens
what happens when transitional B cells enter the follicle
- the B cells receive maturation and survival signals which causes them to differentiate into follicular or marginal zone B cells
what is BAFF
a receptor expressed on follicular dendritic cells which stimulates BAFF-R on T1Bs for survival
what is the difference between the 2 types of transitional B cells
T1B: high IgM, no IgD, BAFF-R, no CD21
T2B: IgM and IgD, BAFF-R and CD21
T1 B cells so not have CD21 or IgD and are excluded from the follicle, what will happen to them
they will fail to receive maturation and die within 2-3 days of leaving the bone marrow
alternative splicing of primary transcripts generates IgM to IgD, at what B cell stage will this happen
immature B cell - only IgM
mature B cell - IgM + IgD
