Generation Of Receptor Diversity In The Immune System

Question 1

What do BCR bind to?

What do TCR bind to?

Answer 1

1. Pathogenic components like peptides, proteins, lipids, carbs, glycolipids etc
2. Peptides that are embedded in self-HLA groove (MHC1 or 2)

Question 2

When a B cell recognizes and antigen via the cell surface BCR, what happens?

Answer 2

The B cell becomes activated and releases antibodies (Ig)

Question 3

How do BCR and TCR differ in structure and where they are locateD?

Answer 3

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

Question 4

What is HLA? What type of lymphocyte receptor relies on HLA?

Answer 4

Human leukocyte antigen

TCR need to recognize foreign antigens presented in self-HLA to be effective

Question 5

Each receptor specificity on a single cell is called a ______________.

Answer 5

clonotype

Question 6

How many different receptors exist for both B and T cells?

Answer 6

10^10 to 10^16

Question 7

How many genes are in the genome B and T cells?

Answer 7

24,000

Question 8

How are we able to get 10^11 receptors with only 24,000 genes?

Answer 8

VDJ DNA recombination provides countless combinations of DNA elements to generate a large repertoire of receptors

Question 9

Where do recombination events occur for B and T cells?

Answer 9

Primary lymphoid organs
B- bone marrow
T- thymus

Question 10

Are abTCR and dgTCR formed by genes and subgenes on the same chromosome?

Answer 10

no they form from different chromosomes

Question 11

How many receptors are expressed on the surface of a single B or T cell?

Answer 11

50000 to 100,000 but all receptors on a single cell have the same specificity

Question 12

Amino acids from what region of the B cell bind the antigen?

What must have caused these regions?

Answer 12

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.

Question 13

How many constant domains are there on a BCR?

Answer 13

4 or 5 (one on the light chain, 3 on IgG,A,D and one on the light chain and 4 on IgM, IgE)

Question 14

What is meant by 100% variability when referring to the hypervariable regions?

Answer 14

all 20 AA can be used at that position

Question 15

The nucleotide sequence that encodes the AA of the variable region of BCR is unique to ____________________.

Answer 15

the B cell from which it was derived

Question 16

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?

Answer 16

Variable (V)
Diversity (D)
Joining (J)

these DNA segments can recombine in different combinations

Question 17

When they sequenced the nucleotides for the chromosomal region for the BCR heavy chain, what did they discover?

Answer 17

The locus contained over 100 different Vs, 7Ds and 6Js followed by constant regions

Question 18

What is a pre-B cell?

Answer 18

A cell destined to be a B cell that does not yet have a BCR. This is where VDJ recombination occurs

Question 19

What are the names of the enzymes that make double stranded cuts in DNA to recombine VDJ segments?

Answer 19

Recombination Activating Genes (RAGs)

Question 20

Which two nucleotide segments get combined first when there is recombination of VDJ?

Answer 20

D and J are joined first.

Then the V gene element joins the DJ group.

Question 21

What are the two major factors of VDJ recombination that contribute to the variability of the variable domain?

Answer 21

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

Question 22

How are the RAG enzymes able to recognize where to bind on DNA in the variable region coding regions?

Answer 22

They recognize Recombination Signal Sequences (RSS) that have stretches of nonomer/heptamers

Question 23

Describe the molecular mechanism of VDJ recombination.

Answer 23

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

Question 24

Where does RAG 2 bind?

Answer 24

trimethylated lysine4 histone 3

Question 25

Where does RAG1 bind?

Answer 25

heptamer/nonomer of RSS

Question 26

The DNA that is looped out and lost when you are doing VDJ recombination is called what?
Why are they used in newborn screening?

Answer 26

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

Question 27

What is Omenn disease?

Answer 27

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)

Question 28

What is meant by oligoclonal? What disorder is this seen in?

Answer 28

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

Question 29

How is the recombination process different for the heavy and light chains of the BCR?

Answer 29

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

Question 30

What receptor segments uses VDJ recombination? What segments only recombine VJ?

Answer 30

VDJ- BCR heavy chain and TCRb

VJ- BCR light chain and TCRa

Question 31

Why must allelic exclusion occur?

Answer 31

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)

Question 32

What occurs during allelic exclusion?

Answer 32

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.

Question 33

Once the BCR heavy chains are made, what structure is formed and what does it signal the B cell to do?

Answer 33

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)

Question 34

Which chain is produced first for the TCR?

Answer 34

B chain. Once it is make it is the pre-TCR which signals to the T cell to start recombination of the alpha chain

Question 35

What 4 things does the pre-B cell receptor do?

Answer 35

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

Question 36

What are the 5 roles of the pre-T cell receptor?

Answer 36

1. survival and proliferation of pre-t cells
2. Inhibition of B chain gene recombination
3. stimulation of a chain recombination
4. express CD4 and CD8
5. shut off pTa transcription