Antigen Receptors and B-cell development

Question 1

Describe the order of gene rearrangement in the light chains of B cells

Answer 1

Germline DNA –> somatic recombination –> VJ rearrangement –> transcription –> mRNA splicing –> VLC -> translation –> light chain

Question 2

Describe the order of gene rearrangement in the heavy chains of B-cells

Answer 2

DJ joining, V joins DJ

Question 3

How do B cells violate the golden rule of antibody synthesis (that each B cell may only synthesize one specific antibody)?

Answer 3

They can synthesize two or more heavy chain isotypes simultaneously for the cell surface (I.e. IgM and IgD)

Question 4

Describe the identifying features of proB cells

Answer 4

ProB cells are the precursors of PreB cells. They have IgH DJ gene rearrangements and no light chain gene rearrangements.

Question 5

Describe the identifying features of preB cells

Answer 5

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

Question 6

Where are preB-lymphocytes found?

Answer 6

Hematopoietic tissues (bone marrow and fetal liver)

Question 7

Describe the identifying features of immature B cells

Answer 7

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.

Question 8

What are the identifiable features of mature B cells

Answer 8

IgD and IgM are expressed on the cell surface

Mature B cells are responsive to antigens.

Question 9

Which comes first, light chain or heavy chain rearrangement?

Answer 9

heavy chain rearrangement, followed by light chain rearrangement

Question 10

What is allelic exclusion?

Answer 10

Only one IgH and one IgL allele are productively rearranged.

Only 1 of 2 alleles will be expressed

Question 11

What sequences are recognized by the RAG1 and RAG2 enzymes? Where are they located?

Answer 11

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

Question 12

What are the three major steps in the VDJ rearrangement? What happens to the excised DNA?

Answer 12

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)

Question 13

What can PCR of BREC tell us?

Answer 13

It can be used to demonstrate that B cells are developing in the bone marrow

Question 14

How do we obtain IgM and IgD with the same VDJ rearrangement?

Answer 14

Alternative splicing

Question 15

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?

Answer 15

1) Combinatorial Joining of V, D, and J Gene Segments
2) Junctional diversity: N-region adding
3) Combinations of H and L Chain Proteins

Question 16

What is junctional diversity?

Answer 16

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

Question 17

Where does the junctional diversity addition occur (heavy chain, light chain, or both)?

Answer 17

ONly on the heavy chain

Question 18

What happens to B cells where the VDJ rearrangements are not functional?

Answer 18

The cells die via apoptosis

Question 19

What happens to B cells that express self-reactive antibodies?

Answer 19

They can become anergic, can be deleted (apoptosis), or they can be rescued by receptor editing.

Question 20

Which enzymes recognize and align two RSSs and cleave the DNA between the exon and the RSS?

Answer 20

RAG1 and RAG2

Question 21

How can the b cell produce both membrane and secreted (soluble) IgM?

Answer 21

Alternative RNA splicing

Question 22

Where does receptor editing occur?

Answer 22

In the bone marrow

Question 23

What happens to mature b-cells in the bone marrow?

Answer 23

They migrate to the periphery

Question 24

Describe the structure of the T-cell receptors

Answer 24

They are a heterodimer of either alpha/beta or delta/gamma chains, linked by disulfide bonds

Question 25

How do T-cells develop a TCRs for a wide range of antigens?

Answer 25

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

Question 26

Where does TCR rearrangement occur?

Answer 26

THe thymus

Question 27

Where does somatic hypermutation occur for T cell receptors?

Answer 27

Nowhere. TCRs do not undergo somatic hypermutation..

Question 28

Which is the most abundant Ab isotype in secretions such as saliva, tears and breast milk?

Answer 28

IgA