Antigen Receptors and B-cell development Flashcards
Describe the order of gene rearrangement in the light chains of B cells
Germline DNA –> somatic recombination –> VJ rearrangement –> transcription –> mRNA splicing –> VLC -> translation –> light chain
Describe the order of gene rearrangement in the heavy chains of B-cells
DJ joining, V joins DJ
How do B cells violate the golden rule of antibody synthesis (that each B cell may only synthesize one specific antibody)?
They can synthesize two or more heavy chain isotypes simultaneously for the cell surface (I.e. IgM and IgD)
Describe the identifying features of proB cells
ProB cells are the precursors of PreB cells. They have IgH DJ gene rearrangements and no light chain gene rearrangements.
Describe the identifying features of preB cells
PreB cells are the earliest cell type that synthesizes a detectable Ig gene product containing cytoplasmic mu-heavy chains composed of V and C regions.
There are no light chains in preB cells
Where are preB-lymphocytes found?
Hematopoietic tissues (bone marrow and fetal liver)
Describe the identifying features of immature B cells
IgM is expressed on the cell surface.
Kappa or lambda light chains are produced. They assemble with the heavy chain and are expressed on the cell surface.
They are “immature” because they do not proliferate and differentiate in response to antigens.
What are the identifiable features of mature B cells
IgD and IgM are expressed on the cell surface
Mature B cells are responsive to antigens.
Which comes first, light chain or heavy chain rearrangement?
heavy chain rearrangement, followed by light chain rearrangement
What is allelic exclusion?
Only one IgH and one IgL allele are productively rearranged.
Only 1 of 2 alleles will be expressed
What sequences are recognized by the RAG1 and RAG2 enzymes? Where are they located?
RSS: recombination signal sequence- conserved sequences
RSS is located 3’ of V and 5’ of J. They direct the RAG proteins to the inside ends of the various V, D, and J segments
What are the three major steps in the VDJ rearrangement? What happens to the excised DNA?
1) Loop out
2) Excision of DNA in between
3) Rejoining (DNA ligase)
The DNA that was excised forms a BREC (B-cell recombination excision circle)
What can PCR of BREC tell us?
It can be used to demonstrate that B cells are developing in the bone marrow
How do we obtain IgM and IgD with the same VDJ rearrangement?
Alternative splicing
What are the three main ways in which we generate the antibody diversity needed to capture all of the pathogens our bodies are exposed to?
1) Combinatorial Joining of V, D, and J Gene Segments
2) Junctional diversity: N-region adding
3) Combinations of H and L Chain Proteins
What is junctional diversity?
Nucleotides, called N sequences, which are not present in the germline, can be added to the junctions of rearranged VDJ genes during rearrangement. This addition of new nucleotides is a random process mediated by an enzyme called terminal deoxyribonucleotide transferase (TdT)
– random addition of up to 20 nucleotides
Where does the junctional diversity addition occur (heavy chain, light chain, or both)?
ONly on the heavy chain
What happens to B cells where the VDJ rearrangements are not functional?
The cells die via apoptosis
What happens to B cells that express self-reactive antibodies?
They can become anergic, can be deleted (apoptosis), or they can be rescued by receptor editing.
Which enzymes recognize and align two RSSs and cleave the DNA between the exon and the RSS?
RAG1 and RAG2
How can the b cell produce both membrane and secreted (soluble) IgM?
Alternative RNA splicing
Where does receptor editing occur?
In the bone marrow
What happens to mature b-cells in the bone marrow?
They migrate to the periphery
Describe the structure of the T-cell receptors
They are a heterodimer of either alpha/beta or delta/gamma chains, linked by disulfide bonds
How do T-cells develop a TCRs for a wide range of antigens?
The same way as B-cells- gene rearrangements.
Similar to the light chain, the alpha and gamma genes only have a V and J section
Similar to the heavy chain, the beta and delta genes rearrange a V, D and J segment
Where does TCR rearrangement occur?
THe thymus
Where does somatic hypermutation occur for T cell receptors?
Nowhere. TCRs do not undergo somatic hypermutation..
Which is the most abundant Ab isotype in secretions such as saliva, tears and breast milk?
IgA
