Flashcards in Generation Of Receptor Diversity In The Immune System Deck (36):
What do BCR bind to?
What do TCR bind to?
1. Pathogenic components like peptides, proteins, lipids, carbs, glycolipids etc
2. Peptides that are embedded in self-HLA groove (MHC1 or 2)
When a B cell recognizes and antigen via the cell surface BCR, what happens?
The B cell becomes activated and releases antibodies (Ig)
How do BCR and TCR differ in structure and where they are locateD?
BCR have two sets of identical heavy and light chains. BCR can release antibody into circulation.
TCR has a heterodimer (one alpha, one beta) and must remain a transmembrane protein
What is HLA? What type of lymphocyte receptor relies on HLA?
Human leukocyte antigen
TCR need to recognize foreign antigens presented in self-HLA to be effective
Each receptor specificity on a single cell is called a ______________.
How many different receptors exist for both B and T cells?
10^10 to 10^16
How many genes are in the genome B and T cells?
How are we able to get 10^11 receptors with only 24,000 genes?
VDJ DNA recombination provides countless combinations of DNA elements to generate a large repertoire of receptors
Where do recombination events occur for B and T cells?
Primary lymphoid organs
B- bone marrow
Are abTCR and dgTCR formed by genes and subgenes on the same chromosome?
no they form from different chromosomes
How many receptors are expressed on the surface of a single B or T cell?
50000 to 100,000 but all receptors on a single cell have the same specificity
Amino acids from what region of the B cell bind the antigen?
What must have caused these regions?
Hypervariable regions that have different AAs for particular positions (HV1-3 or CDR1-3)
Because there is variation in AA sequence, there must have been variation in genetic code and nucleotide sequence for these particular regions.
How many constant domains are there on a BCR?
4 or 5 (one on the light chain, 3 on IgG,A,D and one on the light chain and 4 on IgM, IgE)
What is meant by 100% variability when referring to the hypervariable regions?
all 20 AA can be used at that position
The nucleotide sequence that encodes the AA of the variable region of BCR is unique to ____________________.
the B cell from which it was derived
What are the three nucleotide stretches that make up the variable Ig domain of the BCR heavy chain?
How do they allow for increased variability in the BCR HV regions?
these DNA segments can recombine in different combinations
When they sequenced the nucleotides for the chromosomal region for the BCR heavy chain, what did they discover?
The locus contained over 100 different Vs, 7Ds and 6Js followed by constant regions
What is a pre-B cell?
A cell destined to be a B cell that does not yet have a BCR. This is where VDJ recombination occurs
What are the names of the enzymes that make double stranded cuts in DNA to recombine VDJ segments?
Recombination Activating Genes (RAGs)
Which two nucleotide segments get combined first when there is recombination of VDJ?
D and J are joined first.
Then the V gene element joins the DJ group.
What are the two major factors of VDJ recombination that contribute to the variability of the variable domain?
1. The combination of one V, one D and one J is random and the DNA between those segments is looped out and never used again
2. The joining event is not precise so there is some additional nucleotides added or removed at the junction of the VDJ segments
How are the RAG enzymes able to recognize where to bind on DNA in the variable region coding regions?
They recognize Recombination Signal Sequences (RSS) that have stretches of nonomer/heptamers
Describe the molecular mechanism of VDJ recombination.
1. The chromosome to be rearranged is opened
2. RAG1/2 recognize RSS nonomers/heptamers
3. RAG1/2 makes a dsDNA cut, loops it off and cleaves it
4. The cut pieces are joined together by additional TdT (terminal deoxynucleotide transferase) that fill in gaps in nucleotides around the RSS
Where does RAG 2 bind?
trimethylated lysine4 histone 3
Where does RAG1 bind?
heptamer/nonomer of RSS
The DNA that is looped out and lost when you are doing VDJ recombination is called what?
Why are they used in newborn screening?
TRECs- T cell receptor excision circles
If TRECs are missing, that means the DNA is not recombining which means you will have less variability in your lymphocyte receptors --> T cell deficiency
What is Omenn disease?
RAG gene mutation so there is not recombination of B and T cells. The child will have high eosinophil count and low lymphocyte count (the innate immune response tries to "cover" for the bad adaptive immune system)
What is meant by oligoclonal? What disorder is this seen in?
SCID or Omenns and it means that the T cells that are being made have the same specificity. The RAG mutation means there is not a diverse number of TCRs or BCRs
How is the recombination process different for the heavy and light chains of the BCR?
The process is the same, but the elements used for the variable region are distinct.
1. The heavy chain, light chain and TCRab use different chromosomes
2. Light chain only uses VJ (no D)
3. b chain of TCR uses VDJ
4. a chain of TCR uses VJ
What receptor segments uses VDJ recombination? What segments only recombine VJ?
VDJ- BCR heavy chain and TCRb
VJ- BCR light chain and TCRa
Why must allelic exclusion occur?
B and T cells are diploid meaning they have 2 chromosome sites for BCR heavy chain, light chain, and TCR a and b chains.
If there was not allelic exclusion, the chromosomes would recombine differently and you would have 2 different heavy chains (when they are supposed to be homogenous)
What occurs during allelic exclusion?
Rearrangement at one allele encoding the BCR heavy chain occurs. If it produces protein product, the second allele encoding BCR heavy chain is shut off. If no protein product is made, the second BCR allele will recombine.
Once the BCR heavy chains are made, what structure is formed and what does it signal the B cell to do?
The pre-BCR is formed which signals the B cell to start light chain recombination on the other chromosome.(kappa-2 tries, lamba 2 tries where recombination occurs until there is a protein product then the other alleles are shut off)
Which chain is produced first for the TCR?
B chain. Once it is make it is the pre-TCR which signals to the T cell to start recombination of the alpha chain
What 4 things does the pre-B cell receptor do?
1. survival and proliferation of pre-b cells
2. inhibits H chain recombination (allelic exclusion)
3. stimulates kappa light chain
4. shuts off surrogate light chain transcription