Lecture 11 - B lymphocytes: I

Natural antibodies can be considered part of innate immunity. Which cells produce these?

B1 B-cells

What are ‘Natural antibodies’?

Antibodies produced by B1 B-cells

• IgM antibodies

Which type of B-cell does not require the help of T-cells?

• T-cell independent (TI)

B1 B-cells

Which type of B-cell does require the help of T-cells?

• T-cell dependent (TD)

B2 B-cells

Where is most of the variability found on BCRs?

Found in the antigen binding sites

• Specifically within hypervariable regions of the binding sites

Which antibody isotype forms pentamers?

IgM

Which antibody isotype has low affinity for antigens but high avidity?

IgM

What is avidity?

The accumulated strength of multiple affinities

• E.g. IgM has low affinity for antigens but high avidity due to forming pentamers (10 binding sites)

Which two antibodies can activate complement?

IgM and IgG

Which two antibody isotypes are co-expressed as BCRs during early immune responses?

IgM and IgD

Which antibody isotype has a flexible hinge region

IgD

What is IgD’s flexible hinge region thought to allow?

Allow for conformational changes as a BCR

• Perhaps inhibits signals from IgM BCRs

Which antibody isotype is the most abundant in the body?

IgG

How many subclasses of IgG are there?

Four subclasses

• IgG1 to IgG4

Which Fc family recognises IgG?

FcRy family

• Gamma family

Which antibody isotype is associated with allergy?

IgE

IgE binds what to produce allergy symptoms?

FcRe on mast cells. When antigen binds IgE, mast cell degranulates

• Products of this degranulation then cause allergy

IgA is dominant where?

Dominant in secretions

• E.g. Milk, mucus, etc

How is IgA able to have two different binding sites?

IgA can separate the two heavy chains

• The heavy chains can then recombine, allowing for two different binding sites in one IgA molecule

Which antibody isotype forms dimers?

IgA

What is the role of Fc receptors?

Bind antibody isotypes in order to activate immune cells.

• Depending on the isotype of the antibody, the response to a pathogen will be different

How is a unique BCR heavy chain binding site encoded?

• Heavy chain breaks and D and J regions join

• Heavy chain breaks again and V and DJ regions join

• Any DNA between these breaks is lost

• As there are many D, J and V regions, the random breaking will result in unique BCR generation

How is a unique BCR heavy chain transcribed?

• Following DNA breaking, the VDJ region is transcribed 

• pre-mRNA is spliced into mRNA

  • Alternate splicing allows for different isotypes of the same binding region (IgM or IgD)

• A similar process then occurs on the light chain

• This results in a surface BCR being produced 

What are Ig alpha and Ig beta?

Produce a separate structure to the BCR which helps it move to the plasma membrane

• Also becomes an essential component until plasma cell differentiation

What BCR do Pro- B-cells express on their surface?

No BCR on the surface.

Express an internal re-arranged heavy chain which is held in the ER

What BCR to Pre- B-cells express on their surface?

Express pre-BCR on their surface.

• Here the antigen binding site is checked

• pre-BCR uses a surrogate light chain which doesn’t vary between cells

• Once the heavy chain is accepted, heavy chain rearrangement stops and light chain rearrangement begins

What BCR do immature B-cells express on their surface?

Express mature BCR on their surface.

• Light chain is rearranged and associates with the heavy chain

• Ig alpha and Ig beta remain with the BCR until B-cell differentiation 

What happens if the BCR expressed by immature B-cells binds antigen present in the bone marrow?

The B-cell is seen as auto-reactive and either

• The cell is killed

• Or the cell ‘edits’ their BCR by rearranging and expressing a new light chain

What happens if the BCR expressed by immature B-cells binds soluble antigen present in the bone marrow?

The B-cells exit the bone marrow as anergic B-cells (less responsive)

What is unique about anergic B-cells?

Less BCR on their surface and reduced Ig alpha stability

• This causes impaired BCR signalling

• This is to prevent auto-immunity

Immature B-cells which encounter no antigens in the bone marrow become what?

Transitional B-cells (T1 cells and T2 cells)

T2 B-cells which develop into mature B-cells in the germinal centre become what?

Follicular (FO) B-cells

T2 B-cells which develop into mature B-cells in the marginal zone become what?

Marginal zone (MZ) B-cells

• Function more like B1 B-cells

B1 B-cells don’t develop in the bone marrow like B2 B-cells. Where do they develop?

Generated in the liver

• Then move to the peritoneal and pleural cavities to respond to antigens

What cytokine is essential for B-cell survival in the periphery where they compete with each other for antigens?

BAFF

• Binds BAFFR on B-cells

Once a B-cell binds antigen on a follicular DC, what happens?

B-cell moves to the B-cell/T-cell border. 

• Here B-cells in the follicle can interact with T-cells in the T-cell zone

What are T-cells which enter the B-cell follicle called?

T-follicular helper (Tfh) cells

• These help B-cells become activated 

Once a B-cell is activated by a CD4 T-cell, what happens?

The B-cell will begin developing a germinal centre for rapid proliferation and hypermutation

• However, some B-cells will leave the follicle and secrete their BCR rather than developing a germinal centre 

What is the function of the Ig alpha/beta dimer?

Essential for

• Surface expression of mIgM

• Signalling function of mIgM

Fab fragments of antibodies binding to the BCR causes what?

No signal production

F(ab)2 fragments of antibodies binding to the BCR causes what?

A weak signal generation

Anti-F(ab)2 antibodies binding F(ab)2 fragments which are binding BCRs causes what?

Strong signal because of extensive BCR cross-linking

How does an antigen binding a BCR result in BCR signalling?

BCRs are clustered together and are constantly recruiting phosphatases 

• Constant removal of phosphates on the BCRs which inhibits them

Antigen binding turns off these phosphatases which turns off BCR inhibition

Increased kinase activity and BCR signalling results 

What happens if a B2 B-cell is given a very strong signal?

The B-cell will be killed.

• A strong signal caused by abundant antigens suggests a self antigen and so an auto-reactive B-cell 

B-cell need a strong signal to be activated but strong signals will kill the B-cell. How is this avoided?

• Weak BCR signals are given by antigen binding

• T-cells then help the B-cell via TCR-MHC class II binding

• This produces a strong signal without the cell being killed 

What is linked recognition?

B-cell BCR binds an antigen

B-cell then processes a different part of the antigen and presents it on MHC class II molecules to T-cells 

T-cell can then recognise and activate the B-cell

• This avoids the TCR not recognising an non-immunogenic protein and preventing BCR activation 

Why can B-cells be considered APCs?

Just like DCs, B-cells can also provide all 3 signals to T-cells in order to activate them.

• Can present antigen to T-cell (Signal 1)

• Can provide co-stimulation (Signal 2)

• Can secrete cytokines to T-cell (Signal 3)