Antigen Receptors & B Cell Development Flashcards
What are the three groups of Ig genes?
light chain Kappa and Gamma, and the H (heavy) chain
What do the H and L chains consist of?
V, D, and J gene segments
What can the 3 gene families be divided into?
V-region genes and C -region genes
are the kappa, lambda, and H genes located on the same or different chromosomes?
different!
Which end of the Ig locus are the V region exons on?
the 5’ end, downstream of which are the J (joining) segments and (in the H chain), the D (diversity) region
What do the J and D regions code for?
carboxy-terminal ends of the variable region, including the third CDR region
What is the variable region coded by in a light chain?
V and J exons
What is the variable region coded by in a heavy chain?
V, J, and D exons
Are Ig genes only in B cells?
No! they are in all cells, but are only expressed in B cells
describe gene rearrangement in the heavy chain
- joining of a D and a J gene segment with the deletion of the intervening DNA
- Joining of V gene segment to the combined DJ gene segment, with the deletion of any D regions 5’ of the one that was joined
- the VDJ is spliced to the C region
describe the gene rearrangement in the light chain
One V segment joins with one J segment, then the VJ is spliced to C
do gene rearrangements occur in all cells of the body?
No! This type of gene rearrangement only occurs in lymphocytes - in B and T cell receptor genes
for the light chain, how many exons are required to make up the variable region?
2! You need a V exon and a J exon
do B cells make Ig chains on one or both alleles?
only on one - allelic exclusion
for the heavy chain, how many exons are required to make up the variable region?
3! You need to combine a V, J, and D exon
What is significant about pro-B cells in regard to their Ig rearrangement?
Pro B cells undergo DJ and VDJ rearrangement. There is no light chain rearrangement
Define a pre-B lymphocyte
earliest cell type that produces a detectable Ig gene product, contains cytoplasmic mu-heavy chains composed of V and C domains
where are pre-B cells found?
only in hematopoietic tissues, such as bone marrow and the fetal liver
what is the Pre-B cell receptor comprised of?
surrogate light chain, mu-heavy chain, IgAlpha and IgBeta
How can you define a immature B-lymphocyte?
Kappa and Lambda chains begin to be produced
These associate with Mu chains and the assembled IgM antibody is expressed on the cell surface where they serve as receptors against specific Ag
These immature B cells will go to the periphery
What are IgM bearing B cells recently derived from the bone marrow called, and why?
Immature B cells, as they do not proliferate or differentiate in response to Ag
What is allelic exclusion in the context of B cells?
Only one IgH and one IgL allele are productively arranged
What genes do a mature B cell express?
A mature B cell expresses both IgM and IgD
they co-express original lambda and kappa light chains with alternatively spliced gamma and delta heavy chains, and thus IgM and IgD are both expressed on the surface
What is the specificity of the membrane Ig on a mature B cell?
both IgD and IgM have the same V region and thus the same specificity
Can a B cell produce Ig with different types of V regions?
NO!!! once a cell produces a complete heavy or light chain (immature and mature B cells), it cannot produce another heavy or light chain with a different V region
Where are mature B cells found?
Once B cells acquire Ig and therefore Ag specificity, they migrate out of the bone marrow and are found in the circulation and in lymphoid tissue
What is the recombination of V, D, and J regions mediated by?
specific DNA recombination sequences (RSS) found on the 3’ end of the V exon, the 5’ end of the J exon, and flanking the D exon
What enzymes are involved in the recombination event, and what do they do?
RAG1 and RAG2 recognize RSS, align them, and cleave the intervening DNA sequence in between the two exons.
What are BRECs and why are they important?
B-Cell Recombination Excision Circle are the circularized portions of DNA that were excised from between two joined exons. They can be used to detect if a transplant has engrafted.
How do you obtain secreted vs membrane bound Ig?
Alternative splicing
how can a B cell simultaneously produce IgD and IgM with the same Vh and Vl?
Mu and Delta chains with the same Vh domain arise from differential splicing of the primary transcript (nuclear DNA)
What are three ways of generating antibody diversity?
Combinatorial joining of V, D, and J gene segments
Junctional Diversity: N-region addition
Combination of H and L chains
Describe how combinatorial joining of VDJ segments can generate antibody diversity
max number of combinations is the product of the number of different V, D(if present), and J regions
What is N-region addition?
addition of nucleotides (up to 20), not present in the germline, can be added at the junctions of rearranged VDJ genes during rearrangement
occurs in the CDR3 region - on either side of the D region
What is the enzyme that mediates N-region addition?
TdT - terminal deoxyribonucleotide transferase
What happens to B lineage cells in the bone marrow that have non-functional V(D)J gene rearrangements or that express self-reactive antibody?
B cells with non-functional V(D)J genes are deleted
B cells that are self-reactive can be deleted, become anergic, or be rescued via receptor editing
what is the difference in response to multivalent vs soluble self molecule?
multivalent –> apoptose
soluble –> migrate to periphery and become anergic
What kind of TCR do the majority of T cells have? What is the alternative?
The majority of T cells have alpha-beta TCR. ~5% have gamma-delta TCR
Describe the structure of the alpha-beta TCR
The alpha-beta TCR is a disulfide-linked heterodimer of alpha and beta chains
Each chain has 2 Ig like domains - the N terminus is a variable domain, and the carboxy terminus is a constant domain
How do TCR genes rearrange?
They have the same conserved VDJ rearrangement system
The alpha chain has VJ rearrangement and the beta chain has VDJ rearrangement
The V domain is coded by a V and a J gene segment
Is the bone marrow an antigen dependent or independent environment?
Antigen independent –> B cells will mature without antigenic stimulation
What will the alternative splicing machinery choose between when determining if an IgM will be membrane bound or secreted?
It will either include the MC (membrane component) in the transcript, or will splice upstream of the MC at the SC (secreted component), removing the transmembrane portion from the transcript
What is the ballpark number of Ag specificities a B cell can produce?
~10 MILLION
In which chain does N nucleotide addition occur?
The heavy, not the light
Which chain does receptor editing occur on?
The light chain
How does gene editing happen?
RAG will recombine an upstream V with a downstream J to produce a new VJ combination
Where does receptor editing occur?
Only in the bone marrow
What is occurring in a pro-B cell?
heavy chain D-J (early Pro-B) and V-DJ (late Pro-B) rearrangement
what is occurring in a pre-B cell?
Light chain VJ rearrangement
What is a pre-B cell receptor, and what is its purpose?
A Mu heavy chain with a surrogate light chain that is expressed on the cell surface of a large pre-B cell
Receives signal to stop rearrangement of the heavy chain (no more RAG activity)
Will a B cell make both kappa and lambda light chains?
No - one B cell will only make either kappa OR lambda (in humans about 60/40) –> kappa is made first, if it doesn’t work then lambda will be made
What does AID do?
Mediates isotype switching
How does AID recognize where to act?
there are switch regions in front of all the Ch regions, except for delta that bind AID
How many nucleotides are in the human genome?
~3 billion
Are tumors monoclonal or polyclonal?
monoclonal
