B Cell Development Flashcards
What is required for the generation of antibodies?
Ig gene rearrangements
What antibodies are made in the B Cells?
IgM
What is the gene rearrangement process for the light chain?
The germ line DNA has the V gene rearrange with the J gene into a VJ gene. Then the genes between them are deleted. This forms the LVJ necessary for the expression of the light chain.
What is the gene rearrangement process for the heavy chain?
The germ line DNA has the D and J rearrangement as the first to occur. Then a V and DJ rearrangement occurs and leads to the VDJ formation which is necessary for the heavy chain.
Is Ab synthesis antigen driven?
No. The process occurs entirely independently and does so at random.
For the heavy chain rearrangement, are there residual V genes?
Yes, upstream
For the heavy chain rearrangement, are there residual D genes?
No, both upstream and downstream D genes will all be deleted
For the heavy chain rearrangement, are there residual J genes?
Yes, downstream
What are the steps in B cell development?
ProB -> PreB -> Immature B -> Mature B cells
What is allelic exclusion?
Only one Vh and Vl is rearranged per B cell
What process is occurring during the Pro B cell stage?
D-J and V-DJ rearrangement of the heavy chain
What occurs on the surface in the Pre B cell stage?
Heavy chain binds to a surrogate light chain (as the true light chain rearrangement has not yet occurred) which takes it to the surface and gets a signal to turn off heavy chain rearrangement
What else is occurring intracellularly in the Pre B cell stage?
V-J rearrangement of the light chain
What occurs in an immature B cell?
VJ rearrangement of the light chain is complete and so complete IgM can be expressed on the cell surface
What occurs in mature B cells?
They have gained antigen specificity and as a result, they can express IgM and IgD on the cell surface via alternative splicing
What heavy chain is made by B cells?
Mu heavy chain
What mediates the V, D and J recombination?
The RSS (recombination signal sequence) are highly conserved stretches of seven or nine nucleotides separated by non-conserved 12 or 23 nucleotide spacers.
What recognizes the RSS and cleaves it?
RAG1 and RAG2
What are BRECs and why are they important?
B cell Recombination Excision Circles. They are important because they can be used in marrow transplant patients in order to determine whether or not transplants have been accepted.
How are secreted and membrane IgM generated with the same VDJ region?
Alternative mRNA splicing
How are IgM and IgD generated from the same B cell?
Alternative mRNA splicing
Where does the difference in the secreted and membrane IgM lie?
Difference in secreted and membrane antibodies lies in the C region of the gene
What mediates the V/D/J rearrangements?
RSS and the RAG1 and RAG2 enzymes
What mediates the isotope switch rearrangements?
Mediated by switch regions and
AID (activation induced cytidine deaminase)
What are the factors that contribute to the diversity of the antibodies?
- Combinatorial V(D)J gene joining
- Combination of the Vh and Vl chains
- N nucleotide addition by TdT
- Somatic hypermutation
What is somatic hypermutation?
It occurs after the cells leave the bone marrow and the cells divide, there is a massive increase in the rate of mutation in order to adapt to any possible antigen.
What is N nucleotide addition?
TdT enzyme steps in after excision and randomly
adds nucleotides to broken V and J ends after the BREC leaves and before they are ligated. The number and what is added is entirely random and this allows for the generation of entirely new DNA as a result.
What enzyme is responsible to N nucleotide addition?
Terminal deoxynucleotidyl Transferase (TdT)
What happens when the V(D)J rearrangements are non-functional?
The cells will die
What is the most extreme outcome if an Ab is found to be self reactive?
They will get an apoptotic signal in the bone marrow and be destroyed
Besides apoptosis, what else may happen to self reactive antibodies?
Self reactive Ab that reach the periphery will become
anergic and they do not react to antigens which
are self
What is a process that can fix self reactive antibodies?
Receptor editing
How does receptor editing work?
Anti-self B cells can undergo receptor editing which occurs with the up regulation of the protein RAG, leading to more rearrangements and the generation of a new B cell. If the new B cell is no longer self reactive, it can migrate to the periphery and become a mature B cell.
Can receptor editing occur in the light chain?
Yes, there are V genes upstream and J genes downstream available for further rearrangement
Can receptor editing occur in the heavy chain?
No. There are no further D genes available for rearrangement.
What is the TCR and what is it similar to?
T cell receptor. It is similar to the Fab of the Ab on the B cells
Where are T cells made?
In the thymus
What rearrangement occurs on the alpha chain of TCR?
VJ
What rearrangement occurs on the beta chain of the TCR?
VDJ
What is the mediator of the TCR rearrangement?
RSS and RAG is the enzyme
What are the excised loops of DNA from TCR rearrangement called?
T cell Rearrangement Excision Circles (TRECs)
Do T cells undergo somatic hypermutation?
No
How are the G, E and A Ab generated?
Isotype class switch rearrangement
What is the switch region?
Where DNA switching occurs
Where does the isotope class switch mutation occur?
Outside of the bone marrow
