B-cell Development and Antibody Production - Borghesi

Question 1

Where are fetal B-cells derived and how are they different from adult B-cells.

Answer 1

Fetal B-cells are derived from hematopoetic stem cells in the fetal liver and mature into B1 cells. They have a limited repertoire (all IgM).

Question 2

Where are adult B-cells derived and how are they different from fetal B-cells.

Answer 2

Adult B-cells are derived from the bone marrow and mature into B2 cells. They have a massively diverse repertoire and are the source of memory unlike fetal B-cells.

Question 3

What are some of the hallmarks of adaptive immunity?

Answer 3

1. Clonality - Each B-cell produces Ig’s of a unique specificity.
2. Specificity - Each receptor binds to a unique antigen.
3. Diversity - Millions of different antigen specificities exist at any given time.

Question 4

What are the functions of antibodies?

Answer 4

1. Prevents dissemination of pathogen
2. Inhibit pathogen replication
3. Activate complement
4. Bind to Fc receptors on macrophages, DCs, NK cells

Question 5

Are antigen-antibody interactions covalent or not? Why is this important?

Answer 5

Antigen-antibody interactions are non-covalent. This is important because the longer the antigen sits in the binding pocket, the more persistent the signaling.

Question 6

What are some shared structural features between Igs and TCRs?

Answer 6

Igs have a light and heavy chain composing the antigen binding sites, similar to how TCRs have alpha and beta chains composing their antigen binding sites.

Question 7

What is a key difference between antibodies (Igs) and TCRs?

Answer 7

Antibodies can be both membrane-bound and secreted whereas TCRs are only membrane-bound.

Question 8

VDJ Recombination: Step 1

Answer 8

Recombinational Signal Sequence (RSS) in V and J genes are recognized by Rag1/2.

Question 9

VDJ Recombination: Step 2

Answer 9

Rag complex cuts a single strand of DNA, exposing a free 3’ hydroxyl.

Question 10

VDJ Recombination: Step 3

Answer 10

Free 3’ OH introduced by Rag complex attacks the other strand resulting in a complete double stranded break and hairpin formation.

Question 11

VDJ Recombination: Step 4

Answer 11

Artemis enzyme removes the hairpinned DNA.

Question 12

VDJ Recombination: Step 5

Answer 12

TdT (terminal deoxynucleotidyl transferase) adds or deletes non-coded nucleotides, permanently changing the DNA sequence.

Question 13

VDJ Recombination: Step 6

Answer 13

DNA repair enzymes join V segments with J segments. DNA that used to be between these segments form a signal joint and are lost.

Question 14

Example Question: What would happen in an infant with a null mutation in the Rag gene?

Answer 14

No B or T-cell development.

Question 15

Example Question: What would happen in an infant with partial loss of function in Rag?

Answer 15

Autoimmunity due to lack of regulatory B and T cells, therefore no immune dampening.

Question 16

Example Question: What would happen in an infant with a null mutation in artemis?

Answer 16

No B or T-cell development because the DNA during recombination remains hairpinned.

Question 17

B-cell Development: Step 1

Answer 17

VDJ recombination of heavy chain and joins surrogate light chain on B-cell surface. A basal signal is transduced back to the cell to indicate successful formation and trafficking of heavy chain.

Question 18

B-cell Development: Step 2

Answer 18

After successful heavy chain selection, the light chain undergoes VDJ recombination. The light chain is trafficked to the surface and combined with the heavy chain. A tonic signal is transduced indicating successful joining of heavy and light chain.

Question 19

B-cell Development: Step 3

Answer 19

Test for autoreactivity:
1. Strong self-reactive signaling B-cells undergo clonal deletion by apoptosis
2. Weak reactivity to soluble self molecules leads to anergy –> Non-reactive
3. Weak reactivity to membrane-bound molecules leads to receptor editing of the light chain.

Question 20

Allelic Exclusion in B-cell development

Answer 20

You have alleles from each parent. If successful recombination occurs with one allele, the expression of the others shuts down to inhibit further rearrangement.

Question 21

Discover of Bregs

Answer 21

Clinical trial which used drugs to block either T-cells or B-cells in the context of tissue transplants. Blocking B-cells resulted in high rejection rates indicating that B-cells have an immune dampening role as well.

Question 22

What is the phenotype of Tregs?

Answer 22

Foxp3+ (transcription factor), and CD4/CD25 surface markers.

Question 23

What is the phenotype of Bregs?

Answer 23

No consensus definition or transcription factors – Many phenotypes

Question 24

What do Bregs do?

Answer 24

Rapidly secrete IL-10 to dampen immune responses.