#3 Anibody Diversity 01.09 Flashcards
name 3 things that contribute to the diverse B cell repertoire
- somatic recombination
- class switching
- affinity maturation aka somatic hypermutation
what is the first step in development of a naive B cell?
rearrange Ig genes, i.e., light and heavy chains
what Ig is ALWAYS produced first?
pentameric IgM
each Ig molecule produced by any one B cell is ___
identical
what is the antibody (Ab) repertoire?
the complete collection of Ab/BCR specificities generated by somatic recombination, circulating through the lymphoid tissue
where are B cells made?
bone marrow
where does negative selection of B cells occur?
bone marrow
where does the B cell precursor rearrange its Ig genes?
bone marrow
what is one of the indicators that a B cell is mature?
expression of IgD with IgM
what activates a B cell?
binding the proper antigen
what do activated B cells give rise to?
plasma cells
memory cells
(bone marrow and lymph tissue)
when is a B cell specificity determined?
before infection
somatic recombination is antigen ____
independent
what 2 enzymes mediate somatic recombination?
RAG-1 and RAG-2 (recombination activation genes)
what happens if you are lacking RAG-1 and/or RAG-2?
no functional lymphocytes aka B or T cells
what region of the Ig is somatic recombination generating?
the variable region of the light and heavy chains
what combines to make the light chain variable region?
V+J
what are the two types of light chains?
kappa and lambda
can a B cell have both kappa and lambda light chains?
no! only one
what is the extra gene segment that heavy chains have as compared to light chains?
D
gene segmentation difference between kappa and lambda light chains
kappa: all the Js, then a single C
lambda: J and C alternating
what is allelic exclusion?
when B cell chain rearrangement occurs, it only happens on one chromosome at a time, otherwise a B cell would express 2 Ab at once
what occurs first, heavy or light chain rearrangement?
heavy
what region is changed during class switching?
the constant region of the heavy chain
what region is changed during somatic recombinaton?
variable region of heavy (VDJ) and light (VJ)
what combines to make the heavy chain variable region?
V+D+J
3 things that create diversity of BCR molecules in somatic recombination:
- V+J or V+D+J giving many different variable regions of light and heavy chains, respectively
- the pairing of light and heavy chains
- the imprecise joining process of the gene segments (NT that randomly hang on edge)
what is the order of events in somatic recombination of light and heavy chains?
somatic recombination (of germline DNA) –> Tx (of the rearranged DNA) –> spicing out of introns (from 1* RNA transcript) –> TL (of RNA) –> polypeptide chain = Ig
order of somatic recombination for the variable region of a heavy chain
- D+J
2. V+DJ
how many Ab/BCRs are coded for by Ig genes?
~10^11
order of Ig heavy chain constant region gene segments ***
Cμ = IgM Cδ = IgD Cγ3 = IgG3 Cγ1= IgG1 Cα1 = IgA1 Cγ2 = IgG2 Cγ4 = IgG4 Cε = IgE Cα2 = IgA2
what is the one Ig gene segment that does not have a preceding switch region?
IgD
where does recombination occur during class switching?
at the switch region
what stage of development is the B cell in when it is undergoing class switching? where is its location?
B cell is already activated
proliferating in germinal center Rxn
what is a terminally class-switched B cell?
when a cell and all of its daughter cells will express new Ig/Ab bc all other heavy chain options have been deleted
what is the one Ab type that you cannot class switch to?
IgD
which Ab has no significant secretion?
IgD
which Ab do you always make first?
IgM
when a B cell is mature, it expresses…
IgM
IgD
what is somatic hypermutation/affinity maturation?
mutation that occurs at high frequency in the rearranged variable-region DNA of Ig genes in activated B cells
what does somatic hypermutation/affinity maturation result in?
production of variant Ab
some have higher affinity for antigen
in somatic hypermutation, which B cells are positively selected for?
those that express higher affinity Ab
in somatic hypermutation/affinity maturation, B cells that express ____ affinity for Ab are ____ selected for
higher
positively
quick-hand version of the constant region of heavy chain
M D G3 G1 A1 G2 G4 E A2
three mechanisms for generation of Ab diversity
- cassette system of gene segments via somatic recombination
- class switching
- somatic hypermutation/affinty maturation
somatic recombination is Ab ____
independent
class switching is Ab ____
dependent
somatic hypermutation is Ab ____
dependent
what changes and what doesn’t change in class switching?
changes: the isotype the B cell produces
no change: the antigen specificity
what is a critical growth factor that is necessary to mature B cells?
IL-7
what produces IL-7?
stromal cells in bone marrow
what stage do B cells begin responding to IL-7?
late pro-B cell
name the stages of B cell development**
- stem cell
- early pro-B cell
- late pro-B cell
- large pre-B cell
- small pre-B cell
- immature B cell
- mature B cell
B cell development: stem cell
germline for both heavy and light chains
B cell development: early pro B
D-J
begin heavy chain rearranging of variable
region
:: no surface Ig
B cell development: late pro B
V-DJ
fully rearranged heavy chain variable coding region
:: no surface Ig
B cell development: large pre-B cell
VDJ
translation of heavy chain
B cell development: small pre B
V-J
begin light chain rearrangement of variable region
B cell development: immature B cell
VJ
light chain fully rearranged
begin expressing IgM
B cell development:
IgD and IgM expressed
what can IgD do? what can it not do?
can do: bind antigens as well (same affinity) as IgM
can’t: be secreted in significant amounts
what is Bruton’s thymidine Kinase (BtK)?**
-involved in signal transduction from cell-surface receptors during B cell development
what does a lack of Bruton’s thymidine Kinase (BtK) mean?**
lack of B cell repertoire
what do RAG-1 and RAG-2 (recombination activation genes) do?**
- make double stranded breaks in DNA during somatic recombination
- critical for rearrangement of B and T cell rearrangement of heavy+light/beta+alpha
a lack of RAG-1 and RAG-2 (recombination activation genes) means what?**
wont’ have B or T cells / no lymphocytes
what is the role of terminal deoxynucleotidyl transferase (TdT)?**
- catalyzes addition of N nucleotides at the junctions between rearranging gene segments of light and heavy chains
- backfills gaps
what is activation-induced cytidine deaminase (AID)?**
- catalyzes switch recombination (class switching)
- required for somatic hypermutation/affinity maturation
what would a lack of activation-induced cytidine deaminase (AID) mean?**
- all Ig would be IgM
- very little ability to increase affinity of Ab for antigens in germinal center Rxn
where is the immature B cell self-selected for?
in the bone marrow
if an immature B cell comes into contact with something, what is it most likely?
self/host-cell
selection of self-tolerant B cells: contact with a multivalent self molecule
death/apop in bone marrow
selection of self-tolerant B cells: soluble self molecule
migrates to periphery
anergic B cell
describe an anergic B cell
IgD»_space; IgM
:: cannot be activated or respond to stimuli
short life span
selection of self-tolerant B cells: low affinity noncrosslinking self molecule
migrates to periphery
mature B cell :: IgD and IgM ratio nml
becomes part of repertoire
unclear of self-reactive or not
if a B cell is not positively selected for, what happens?
it fails to enter the lymphoid follicles
if a B cell is positively selected for, what happens?
enters lymphoid follicles
stimulation by antigen –> memory B cells, express IgG, IgA, IgE
what % of B cells express Ab that have both kappa and lambda light chains?
trick question!
0%
B cell class switching to IgA can be further signaled to class switching to IgG3. T/F
F
