Genetics of Ig and TCR diversity - Hudig Flashcards
What is responsible for the diversity of Ig and TCR’s?
Genes for Ig and TCR contain cassettes for the variable region of the receptor. Each locus has 2 or 3 cassettes of minigenes that are combined in variable ways.
What is a cassette?
Regions of the genome in which there are many genes that get selected.
Name the cassettes and state what they encode.
There is a V, D and J cassette. All antigen receptor gene loci contain V and J cassettes but only Ig heavy chain loci and TCR beta chain and delta chain loci have the D cassette.
How do the cassettes work?
When an Ig or TCR is generated different genes from the V and J or the V, D and J cassettes come together to form different variable regions for the receptors. Also, somatic mutation occurs in hotspots in the Ig variable regions - but not in the TCR genes.
What 7 chromosomes have cassettes?
- Ig heavy chain (VH)
- Ig lambda light chain (V lambda)
- Ig kappa light chain (V kappa)
- TCR alpha chain (Valpha)
- TCR beta chain (Vbeta)
- TCR delta chain (Vdelta)
- TCR gamma chain (Vgamma)
The V denotes that these genes encode the entire variable region of the chains.
What enzymes are essential for the rearrangements of these genes in the cassettes?
RAG1 and RAG2 - these are recombinase enzymes that are responsible for the cutting and splicing of the genes. Loss of both of these would cause SCID.
Describe how the Ig or TCR is generated.
In a naive lymphocyte there are genes for the variable region - V, D (in some cases) , J and constant regions of a receptor. The DNA is germline DNA inherited from mom and dad. To produce a receptor random genes from the V cassette, D cassette and J are cut out and the genes that are left are spliced together by RAG1 and RAG2. TdT or terminal deoxynucleotide transferase randomly fills in the sites between the minicassette selections.That B or T cell now has only that DNA that was kept and will make receptors from that DNA. For Ig’s there is also somatic mutation in the CDR region after the V region minigenes are rearranged. In this way B cells with the highest affinity for a certain antigen will be the ones “chosen” for proliferation.
What happens when B cells switch the isotype of the antibody they produce?
With T cell help, the constant region is changed while the VDJ or variable region remains the same. The B cell cannot switch back because DNA is spliced out. The first antibody produced is IgM. IL-4 induces a switch to IgG1 or IgE (at high concentrations). gIFN induces a switch to IgG and inhibits the production of IgE, and TGF-B induces a switch to IgG or IgA.
What surface Ig’s do memory cells have?
IgA or IgE or IgG
At which stage can B cells switch isotype production?
Specific, proliferating B cells can switch.
What are polyclonal antibodies?
Antibodies made in vivo.
What are monoclonal antibodies (Mab)?
Antibodies derived in lab.
How are monoclonal antibodies generated?
A specific plasma cell (the antigen specificity came from and immunized animal) is fused to a tumor cell to form a hybridoma. In this way the source of antibody is immortal and the amounts of Mab that can be obtained are unlimited. At first the antibody is a mouse Mab to a human epitope. Next the mouse constant regions are replaced, next intervening mouse VH are replaced with human and you have a fully humanized Mab for a heavy chain - do the same for the light chain and you have a complete fully humanized Mab.
