b cell development Flashcards
What does the bone marrow contain and provide for maturation of B
lymphocytes
a. Unique microenvironment for maturation (HSC)
b. Bone marrow stromal cells for support
c. Cell to cell integration and production of cytokines
d. Provide survival and differentiation signals via these cytokines ( & other
molectules)
The maturation of B cells can be identified by changes in enzyme activity, CD
marker expression & synthesis of BCR. Some of these markers include:
a. CD34, CD10, CD127, CD19**, CD20 **
b. RAG 1 & RAG 2
c. TdT
d. Btk
e. Pre-B Cell Receptor
f. BCR (IgM first then IgD)
CD127 Is?
IL-7 Receptor
What stages of B-Cell development occur in bone marrow?
a. Development from Stem cell to Pro B Cell
Stem cell H- chain/L – chain, Ig Status
Germline & No Ig
What is the order of development of B cells?
Stem cell early PRO-B cell Late PRO- B cell large PRE-B cell small PRE-B cell Immature B-cell
Which chain (heavy or light) is re-arranged first
The heavy chain (H-chain) genes are rearranged before the light chain
genes
At the early pro-B cell level in the bone marrow gene arrangement of the H-
Chain starts how?
Joining of the D and J genes on both chromosomes (maternal and paternal)
If early pro-B cell level rearrangement productive what does the heavy chain
undergo at late pro-B cell stage?
a. V- DJ rearrangement takes place on the first chromosome (either paternal
or maternal).
b. If V- DJ rearrangement on the first chromosome not productive then
attempt on second chromosome
c. If V- DJ rearrangement on the second chromosome not productive then apoptosis signaled (about 50%)
What must accompany any productive heavy chain re-arrangement before
approval to become pre-B cell? (involving IgM)
The C(mu) gene is selected to complete the genes needed for IgM (VDJC(mu)) and further re-arranged with V-DJ
If late pro-B cell level rearrangement productive what does the cell become?
Still late Pro- B cell
What is the final step before a Pro-B cell can become a pre-B cell?
Functionality checking of µ chain
How is this functionality checking done?
a. The late pro-B cell will express a surrogate light chain, to test if the heavy
chain can successfully pair with a potential light chain.
b. Expression of both the recombined heavy chain with the surrogate light
chain forms the pre-BCR, which is the first checkpoint in the
developing B cell.
c. If this pairing is appropriate, the pre-BCR will be expressed on the
surface of the cell
d. This expression will initiate signaling that indicates the cell is ready to
progress to the large pre-B-cell stage.
e. If the heavy chain is in some way defective or unable** to pair with the
surrogate light chain, the B cell will not receive the appropriate maturation
signals and will subsequently die by apoptosis**
What is the significance of Bruton’s tyrosine Kinase (Btk) in this process?
a. Operates as apart of the signal to tell the cell that it has successfully
rearranged its Ig H chain and has made functional u chain to shut down
further H chain rearrangement, stop synthesis of surrogate chain, start L
chain gene rearrangement.
***b. Mutation in Bkt gene results in lack of differentiation to immature B cell level. Common in boys – X-linked agammaglobulinemia.
What occurs at pre-B cell stage?
Light chain gene re-arrangement
What becomes activated and starts gene re-arrangement of light chain in SMALL
pre-B cell?
RAG Enzyme
Where does L-Chain rearrangement start in PRE-B Cell?
a. Recombination begins at the kappa ()-chain loci;
b. If not productive on both chromosomes the cell will then attempt recombination at the lambda ()-chain loci.
c. If none of these four opportunities work productive then signal for
apoptosis
If re-arrangement productive what happens?
Once productive it will pair with the previously made heavy chain and
be expressed on the cell surface as BCR complex & be called an Immature B- Cell.
Immature B-cell express?
IgM
What has to occur before B-cell can leave bone marrow?
Check for self reactivity (third checkpoint)
Self reactivity/ Self tolerance involves?
a. Immature B cell exposed to self antigen in bone marrow
b. If it recognizes self antigens in the bone marrow – allowed to change the light chain (kappa or lamda) but not the u chain
c. If not, leaves bone marrow and enter secondary lymphoid where expresses
both IgD & Ig M
How do B-cell express both IgD & IgM with the same VDJ (heavy chain)?
Alternative mRNA splicing
Maturation and survival of B-cells requires?
Access to lymphoid tissue. If a B cell meets an antigen it proliferate to form the germinal center and became plasma cells – move to the medulla-
produce Ig. Those that do not meet the antigen of their specificity they
move to the medulla enter efferent vessel.