B cell activation

Question 1

Where about in lymph node can the B cells be found?

Answer 1

(Blood to lymphoid tissue, to B cell area=) Primary lymphoid follicle (B cells drained via the lymphatics back to the blood.)

Question 2

What are the key chemokines involved in recruiting and maintaining B cells within the lymph node.

Answer 2

CCL21 and CXCL13

Question 3

What is CCL21?

Answer 3

This chemokine induces B cell entry into lymph node from blood vessel

Question 4

What are Follicular Dendritic cells (FDC)?

Answer 4

Different from Dendritic cells, FDC they are like stromal cells and they produce CXCL13 chemokine.

Question 5

What does CXCL13 do?

Answer 5

induces B cell entry into lymphoid follicles.

This requires the CXCR5 receptor in order to recruit cells into follicles

Question 6

Describe the steps needed to mature a B cell in the lymph node

Answer 6

1) CCL21 attracts immature B cells to HEV
2) CCL21 and CCL19 attract B cells to lymph node
3) CXCL13 attacts B cell to primary follicle/B cell area
4) Interacts with follicular DC and cytokines drives maturation of B cell
5) Mature B cells recirculate

Question 7

What is the role of FDC in terms of positive selection?

Answer 7

They regulate the positive selection of B cells through FDC secretion of BAFF

Question 8

What is the function of BAFF in terms of positive selection of mature B cells

Answer 8

Cytokine B cell activating factor from the tumour necrosis factor family.
BAFF activates a particular signalling pathway in B cell involving protein kinase-beta.
Mature B cells must receive positive signal in order to survive, be metabolically fit (lots of ATP, and glucose stored) and ready for proliferation.

Without BAFF, B cells die and there is immunodeficiency

Question 9

What is the purpose of lymphotoxin-alpha?

Answer 9

B cells maintain FDC by producing lymphotoxin alpha which regulates survival of FDC cells.

Question 10

What are the three major types of mature B cell?

Answer 10

1) Conventional B cells (B2 cells, Follicular B cells)
2) Marginal zone B cells
3) B1B cells

Question 11

Describe conventional B cells?

Answer 11

They circulate between lymph node follicles and blood, determined by BAFF for survial and readiness.

Question 12

Describe Marginal B cells?

Answer 12

They are resident in marginal zone of the spleen where they respond to antigens in to blood, dependent on BAFF for survival. They are responsive esp. to polysaccharide. pathogens

Question 13

Describe B1 B cells?

Answer 13

They are mostly in the peritoneal/pleural cavities and produce IgM

These are less dependent on BAFF

Question 14

Describe conventional B2 cells?

Answer 14

They circulate between lymph node follicles and blood, determined by BAFF for survial and readiness.

Their primary location is secondary lymphoid tissue.

Question 15

Describe B1 B cells?

Answer 15

They are mostly in the peritoneal/pleural cavities and produce IgM.

These are produced and secreted without antigenic stimulation.

These are less dependent on BAFF

Question 16

Describe some differences between B1 and B2 cells

Answer 16

B1 cells produced in the fetus before B2 cells after birth

B2 is the major set of cells in humans with a more diverse variable region repertoire.

B1 is self renewing, but B2 is replaced from the bone marrow.

Question 17

Describe some differences between B1 and B2 cells

Answer 17

B1 cells produced in the fetal liver before B2 cells after birth

B2 is the major set of cells in humans with a more diverse variable region repertoire.

B1 is self renewing, but B2 is replaced from the bone marrow.

Question 18

Where do B cells come into contact with antigen and become activated?

Answer 18

Within secondary lymphoid tissues (such as spleen, lymph nodes, mucosal issue)

Question 19

What and where in lymph node are Oposonized antigens trapped

Answer 19

1) subcapsular/medullary sinus macrophages
2) FDC in follicle

3 DC in T cell zone.

Question 20

Describe how B cells are activated.

Answer 20

microbial debris and antigens from pahgocytosis released and travel via afferent lymphatic vessels into lymph node. They migrate through lymph node and drain into efferent lymphatic vessels.

While in lymph nodes, cells with receptors for complement (C3), Fc receptors for antibodies bind to recognised antigens; FDCs retain antigen for long time due to lack of phagocytosis/processing ability.

Question 21

What benefit is there from FDC lack of antigen processing?

Answer 21

Maximises time immune system can traffic B cells through lymph node until the specific B cell can match antigen on FDC.

Question 22

What happens when a B cell responds to an antigen?

Answer 22

1) Proliferation of the specific B cell; clonally expand to increase response.
2) Differentiation into plasma cells
3) antigen specific Ab

Question 23

What happens when a B cell responds to an antigen in terms of migration?

Answer 23

• once activated B cells move to medullary cords to become IgM plasma cells
• others move to primary follicles at which point changes to become secondary follicle with a GERMINAL CENTER.

Question 24

What are Germinal centers?

Answer 24

The transient structure of intense B cell proliferation.

Here, genes are altered to perform class switching

Question 25

What are Germinal centers? What happens within germinal center?

Answer 25

The transient structure of intense B cell proliferation.

Here, genes are altered to perform class switching.

Within Germinal center, activated B cells proliferate into centroblasts.

Question 26

What are Germinal centers? What happens within germinal center?

Answer 26

The transient structure of intense B cell proliferation.

Here, genes are altered to perform class switching.

Within Germinal center, activated B cells into large proliferating centroblasts.
Centroblasts undergo isotype switching to become smaller centrocytes.

Centrocytes differentiate, proliferate into plasma cells.

Question 27

What happens when a B cell responds to an antigen?

Answer 27

)IgM is always the Ig of BCR; BCR form clusters of receptors on surface which when activated by antigen initiate strong signals into cells. In addition, co-receptors enhance signal.

) Proliferation of the specific B cell; clonally expand to increase response.

) Differentiation into plasma cells

3) antigen specific Ab

Question 28

What happens when a B cell responds to an antigen in terms of migration?

Answer 28

• before antigen, BCR evenly distributed, in presence of antigen IgM BCR at one end; lots of receptors in a small surface area.
• once activated B cells move to medullary cords to become IgM plasma cells
• others move to primary follicles at which point changes to become secondary follicle with a GERMINAL CENTER.

Question 29

What happens when a B cell responds to an antigen?

Answer 29

1) Activation: IgM is always the Ig of BCR; BCR form clusters of receptors on surface which when activated by antigen initiate strong signals into cells. In addition, co-receptors enhance signal. AND, T cell help or alternatively microbial signals are needed to activate
2) Proliferation of the specific B cell; clonally expand to increase response.

3) Differentiation into plasma cells
3) antigen specific Ab

Question 30

Describe co-receptors and the signalling pathway.

Answer 30

Complement receptor CR2 on B cell binds to C3d attached to bacterial cell.

The alpha chain of BCR associated SRC family of kinases (Fyn, Lyn, Blk) then phosphorylate cytoplasmic tails of CD19

This starts signalling.

Question 31

What is a function of phosphorylation of alpha and beta BCR subunits

Answer 31

Phosphorylation sites are docking sites for other adaptor molecules for certain signalling pathways

Question 32

Which types of B cells are T cell independant?

Answer 32

MZ B cells and B1 cells

Question 33

Describe B cell activation T cell antigens

Answer 33

BCR and antigen bind BUT this is a weak signal.

For strong signal, polysacc. of bacterium close by this can engage TLR4 ; innate PRR (pathogen recognition receptors) like Toll (TLR4) act as second activation signal.

A multivalent antigen molecules engages multiple BCR gives a stron signal and there is no need for co-stimulation.

There is no germinal centre response, therefore short lived plasma response which produce low affinity IgM only antibodies; as there is no somatic mutation and there is no class switching

Question 34

Which B cells are activated by T cell dependant antigens?

Answer 34

MZ and follicular B cells.

Question 35

Describe B cell activation by T cell dependant antigens.

Answer 35

• B cells bind to antigen captured by FDC via adhesion (ICAM-1) and costimulaory (CD40) molecules
• BCR-antigen-attached APC/FDC internalized into B cell via endocytosis.
• Degraded within B cell
• Peptides presented on MHC.
• B cell then migrates towards T cell zone and interacts with effector T cells (Th1/Th2) with the same TCR of same specificity as the peptide it presents.
• B cell and T cell stimulate eachother via cytokines for co-stimulation (signal 2)
• stimulation of helper T cell means CD40 ligand is expressed for co-stimulation (signal 2)

Question 36

What happens after B cell makes contact with T cell and has been stimulated by signal 1 and 2.

Answer 36

• B cell goes back to B cell area: lymphoid follicle

- it differentiates in to a Germinal Center B cell which establishes germinal center reaction of proliferation

Question 37

What provides the continual signalling needed for Germinal centre reaction?

Answer 37

• BCR constantly engaged with antigen
• T follicular helper cell (Tfh cells) migrates to germinal center from T cell zone and gives co stimulatory cytokines (CD40L)

Question 38

List what occurs in the germinal center reaction (in B cell zone of lymph nodes)

Answer 38

1 )B cell survival increases

2) B cell proliferation increases
3) Ig isotype switching with same specificity but different heavy chain (IgM into other) via cell cytokines
4) Somatic Hypermutation
5) Differentiation of plasma cell.

Question 39

What enzyme is important in proliferating B cells during isotype switching?

Answer 39

AID: Activation-induced cytidine deaminase which targets swich regions causing nicks in both DNA strands

Question 40

What is the purpose of somatic Hypermutation?

Answer 40

To enhance production of antibodies that strongly bind to antigen; low affinity BCR die by neglect.

Question 41

What is Somatic Hypermutation?

Answer 41

The process where high affinity antibodies are generated and selected by the process of affinity maturation in the Germline center.

AID enzyme introduces random single nucleotide mutations in heavy, light chain hypervariable regions

Subsequent mutations that increase affinity are selected; over time, we are re-exposed to antigen, there are more point mutations so antigen binding site will change.

Question 42

In what way does allelic exclusion help clonal selection?

Answer 42

It means clonal selection is more efficient due to all daughter cells expressing same Ig specificity.

Exclusion/suppression helps stop other specificities emerging after clonal selection; ability to make antibody for one antigen is greatly diminished and increases infections.

Question 43

What problem is caused by faulty allelic exclusion in terms of repertoire

Answer 43

If two different antigen-specific antibodies made by one B cell, and one is self reactive, negative selection will kill B cell as it made self reactive Ab. Cell is lost.

However if this B cell is required to make a specific antigen to a pathogen; the non reactive Ab made, there is no defense as cell was deleted. This creates holes in our repertoire.