Hudig: Genetics of Ig and TCR Diversity

Question 1

There is not enough DNA in human cells to encode all the different antibodies and T cell receptors. So what accounts for the diversity among T and B cells (each with a different receptor, etc)?

Answer 1

1. VDJ recombination: variable minigenes recombine to form heavy and light chains of Igs and T cell receptors
2. Ig classes switch from IgM/IgD to IgE, IgA, IgG
3. somatic mutations of Ig/ag-binding sites

Question 2

The receptors for T and B cells have variable regions and constant regions. Which part is variable? Which part is constant?

Answer 2

variable regions are on the end where the antigen binding site is located; constant region is the remainder of the structure

Question 3

Each locus for Igs and TCRs contains (blank) for the variable regions

Answer 3

cassettes or minigenes (V, D, and J)

Question 4

Which loci contain D minigene cassettes?

Answer 4

Ig heavy chain
TCR beta chain
TCR delta chain

**note that the Ig LIGHT chains are not formed from a D minigene (only a J and a V)

Question 5

Do somatic mutations occur in TCRs?

Answer 5

no; only in Ig light and heavy chain variable regions

Question 6

B cells have 3 chromosomes with minigene V cassettes. What are they?

Answer 6

Ig heavy chain
Ig light lambda
Ig light kappa

**these are all on different chromosomes

Question 7

T cells have 4 chromosomes with minigene V cassettes. What are they?

Answer 7

TCR alpha, beta, delta, and gamma

Question 8

For all of the minigene rearrangements, which proteins are essential?

Answer 8

RAG1, RAG2 and TdT

Question 9

What would happen if you had complete loss of either RAG1 or RAG2?

Answer 9

complete loss of both B and T cells (SCID)

**can’t undergo VDJ recombination to generate Igs and TCRs

Question 10

DNA inherited in the embryo and kept unchanged in the gonadal germline cells.

Answer 10

germline DNA

Question 11

DNA found in differentiated cells. It’s the same as germline DNA for most cells.

Answer 11

somatic DNA

Question 12

In T and B cells, why is the somatic DNA for antigen receptors different from the germline DNA?

Answer 12

DNA segments are selected from the germline DNA and the DNA is rearranged (with some DNA removed) to form somatic genes that will encode the light and heavy chain proteins.

Question 13

Germline rearrangements occur in B and T cells when some (blank) are used and some are discarded. The DNA is rearranged without (blank). The various mature B cells will have different (blank) for Ig, different receptors, and will make different (blank)

Answer 13

minigenes; mutation; DNAs; antibodies

Question 14

What are 3 important enzymes used in Ig V heavy chain germline rearrangement?

Answer 14

1. RAG1 - used for DNA breaking and rejoining inside the minicassettes
2. RAG2
3. TdT - fills rejoining site with “wildcard” nucleotide

Question 15

At the end of germline rearrangement for the Ig HEAVY chain, you will have 1 (blank) minigene, 1 (blank) minigene, 1 (blank) minigine, and a space filled in by (blank)

Answer 15

V (heavy); D (heavy); J (heavy); TdT

Question 16

Why is germline rearrangement important?

Answer 16

It allows for over 4 million different antibodies to be generated!

Question 17

After the variable region minigenes are rearranged, what occurs in the coding regions each time an antigen is re-encountered? Why?

Answer 17

somatic mutation of the antibody; this occurs because antigens select the B cells with the highest affinity mutations and those are the B cells that will proliferate

**think about it like this: when you have an infection, you will begin producing Ab. At some point in time, you have made enough Ab that the amount of antigen decreases. So, the remaining B cells must compete to bind the antigen - those with the highest affinity, will “win” and will be selected

Question 18

What will happen after antigen exposure to a virgin B cell that originally expresses either IgD or IgM?

Answer 18

After antigen response, the antibody isotype will switch

Question 19

This happens in individual B cells to change the isotypes that they AND their progeny secrete

Answer 19

antibody switch

Question 20

What is unique about the antibody specificity after antibody switch has occurred?

Answer 20

the Ig specificity is retained

Question 21

What controls the Ig switch?

Answer 21

cytokines

Question 22

When TH2s secrete IL4, the Ig will switch to which isoform?

Answer 22

IgE

Question 23

When TH1s secrete IFNgamma, the Ig will switch to which isoform?

Answer 23

IgG3

Question 24

Which type of receptors for memory B cells have?

Answer 24

IgA
IgG
IgE

Question 25

What is the difference between polyclonal and monoclonal antibodies?

Answer 25

polyclonal antibodies are made in VIVO, they are antibodies that are secreted by DIFFERENT B cell lineages within the body (whereas monoclonal antibodies come from a SINGLE cell lineage). They are a collection of immunoglobulin molecules that react against a specific antigen, each identifying a DIFFERENT epitope.; in VITRO, you could make a monoclonal antibody which is a preparation of ALL identical antibodies derived from one immune B cell

Question 26

Why would you fuse a tumor with an immune B cell that is producing a monoclonal Ab to generate a hybridoma?

Answer 26

this will allow for a source of identical antibodies that is immortal; it will be an antigen-specific antibody

Question 27

A mouse Mab to a human epitope, with the mouse constants replaced

Answer 27

human monoclonal Ab

Question 28

Why in the heck would you want to infuse portions of a mouse monoclonal antibody into a human?

Answer 28

Well, it will be fully “humanized” and will be accepted as well AND can confer some beneficial function to humans that has been found in a mouse model

Question 29

What is an example of a benefit that a human monoclonal antibody to a flu strain can provide to a ferret?

Answer 29

Human MaB CR6261 can protect ferrets against influenza H5N1