Lecture 13 - B lymphocyte Development

What does B-Cell development result in?

1. Generate a diverse antigen receptor (BCR)

2. Alter or eliminate self-reactive B cells/BCRs

3. Promote foreign reactive B cells to become mature B cells in the 2ndary lymphoid tissues

What is the fist antibody expressed by the BCR?

IgM

What happens to self-reactive BCRs?

They don’t receive a survival signal and die by apoptosis

Where do BCRs start to sample self antigens initially?

In the bone marrow

Where does B cell development take place? Where does maturation take place?

Bone marrow and then in the secondary lymphoid organs (spleen)

What is the first step in B cell development?

Must receive a survival signal for precursor cells that comes from the epithelial thymus or the bone marrow, this is IL-7

What is IL-7 Important for?

For both B cells and T cells as it is a survival signal for precursor cells

What is the signal that T cells receive for lineage commitment?

Notch for T cells and E2A transcription factor for B cells (there are more for B cells)

What are all the B cell lineage commitment factors?

FOXO1, EBF, PAX5 and E2A, E2A for initial commitment and then the other are expressed at different stages in development

When is FOXO1 and EBF expressed?

common lymphoid progenitor

When is E2A first expressed?

Lymphoid-myeloid primed multipotent progenitor

When is PAX5 expressed?

Early pro-B cell

What are the stages in B cell development?

1. Common lymphoid progenitor (Stem cell)

2. Early pro-B cell

3. Late pro-B cell

4. Large Pre-B cell

5. Small Pre-B cell

6. Immature B cell

7. Transitional B cell

8. Mature B cell (still innate, not activated)

Which stages in B cell development are antigen-independent?

Stem cell, early pro-B cell, late pro-B cell, large pro-B cell and small pre-B cell

Which of these stages are Antigen dependent?

Immature B cell, Transitional B cell and Mature B cell

Where is RAG1 and RAG 2 expressed? Why?

In early pro-B cell (DJ recombination heavy chain), late pro-B cell (VDJ recombination heavy chain) and then again in small pre-B cell (VJ light chain recombination)

Where is IL-7R and Kit active?

Kit is early pro B cell and IL-7R is early pro B cell-Large pre-B cell, these are growth factor receptors

What are the surrogate light chain components? When are they active?

They are lambda5 and VpreB, they are active from early pre-B cell until beginning/mid small pre-B cell

What happens in the early pro-B cell

Heavy chain begins to rearrange first and this is where RAG 1/2 turn on, this happens on both chromosomes at the same time

What happens in late pro-B cell?

V segment of the heavy chain begins rearrangement and joins DJ, it will start with the first chromosome, and if that fails it will have a second chance at the second chromosome, if this fails then there will be no survival signal and the cell dies, after this RAG 1 and 2 turn off

What does the survival signal in pro-B cells trigger?

Movement into large pre-B cells

What happens in large pre-B cells?

Here the pre-BCR is formed to check the productivity of Heavy chain rearragment, this is the first checkpoint, the heavy chain will pair with the surrogate light chain (VpreB or lambda5) to form pre-BCR, if the test is positive it will lead to proliferation

What is the pre-BCR made up of?

The heavy chain and the surrogate light chain, it will be made in the cell and expressed on the surface for testing

What is the surrogate light chain made of?

it is made of lambda5 or VpreB

How is pre-BCR testing an example of allelic exclusion?

It will test the first Heavy chain chromosome and if it works then it will not test the second

What happens in the small pre-B cell?

Light chain begins to rearrange, RAG 1 and 2 will turn back on and the surrogate light chain will stop being expressed, it will start be trying on K of first chromosome then K on second, then lambda on first and then lamda on second, if it fails for both kapa and lambda on both chromosomes the cell will dies by apoptosis

What is the first checkpoint?

Pre-BCR signaling, positive test results in proliferation and light chain rearrangement

Where does Immature B cell stage occur?

In the bone marrow

Where is checkpoint 2

In the immature B cell in the bone marrow

What is checkpoint 2 and where does it occur?

Test to see if VDJ of light chain pairs with the heavy chain and does not recognize self-antigens expressed in the bone marrow, cells that do die by apoptosis or do receptor editing

Where is the first stage of negative selection?

In the bone marrow testing for self reactivity to self antigens in the bone marrow, this is checkpoint 2

What does the light chain exhibit?

Allelic and isotypic exclusion (one chromosome and one type kapa/lambda)

What does the first negative selection and checkpoint 2 accomplish?

Establishment of central tolerance

What happens if the BCR recognizes self-antigens in the bone marrow?

It will undergo receptor editing of the light chain until it runs out of machinery and then it will die by apoptosis, or it could just die from apoptosis

What is isotypic exclusion?

only kapa or lambda expressed, if kapa is successfully rearranged, then lambda can’t rearrange

What happens in Transitional B cells?

They leave the bone marrow and go to the secondary lymphoid organs (spleen in this case), and undergo the third checkpoint, this is another round of negative selection where it can recognize foreign antigens or peripheral self-antigens, this establishes peripheral tolerance, if it does not recognize self-antigens then it receives survival signal BAFF (spleen) (BAFFR binds BAFF), those that do bind self-antigens will not receive a signal and they will die by apoptosis

What cells express BAFF?

BAFFR is the receptor for BAFF and is the survival signal in the spleen, this is expressed by follicular dendritic cells.

What BCR do immature B cells express?

IgM

What is different between negative selection 1 and 2

Negative selection 2 can’t undergo receptor editing

Where does peripheral tolerance occur?

Secondary lymphoid organs (spleen)

Where do B cells complete maturation?

B-Cell follicles in the spleen

If immature B cells receive a BAFF survival signal, what do they express?

IgD

What are marginal zone B cells?

they are weakly self-reactive and express high CR CD21, they migrate to the marginal zones of splenic white pulp and they make rapid responses to blood-borne antigens or pathogens

What cells make quick responses to blood-borne antigens/pathogens?

Marginal zone B-cells

What is the difference between T1B (transitional 1 B cells) and T2B (transitional 2 B cells)?

T1B have IgM, no IgD and express BAFFR, T2B have IgM, IgD, BAFFR and CD21

What do transitional 1 B cells become?

TB1 become Tb2 then either follicular B cells or marginal B cells

Where does the survival signal come from for transitional B cells?

BAFF comes from follicular dendritic cells and binds BAFFR on T1B

What Igs do mature B cells have? How is IgD formed?

They have IgM and IgD, IgD is formed by alternative splicing of genes