T cell dependent B cell responses

Question 1

When do the B cells and T cells in lymph nodes meet each other?

Answer 1

• The B cells and T cells never meet each other outside their zone except when a protein-antigen comes by and allows a specific B cell and specific T cell to leave their zone and interact with each other somewhere in between (germinal center/GC)

Question 2

How do B cells and T cells meet?

Answer 2

• The B cell encounters its protein antigen, takes it up and displays it on MHC class II.
  o Leads to increased expression of chemokine receptor CCR7 (normally found on T cells), which allows it to migrate towards the T cell zone
• The T cell encounters its peptide-displaying dendritic cell
  —-Leads to increased expression of chemokine receptor CXCR5 (normally found on B cells)
• The B cell and T cell then migrate towards each other in the lymph node due to induction of chemokine receptors.
  o For T cells, this is because of increased expression of CXCR5
  o For B cells, this is because of increased expression of CCR7
• Both cells meet and the T cell activates the B cell.

Question 3

How does the B cell take up display antigen on MHC molecules?

Answer 3

o Full process: B cell specific for a protein will bind protein by surface immunoglobulin (or antigen receptor)-> endocytose that protein to break it up into peptides->express peptide from that protein on the surface of the B cell bound by an MHC molecule

Question 4

How does a T cell encounter its peptide-displaying dendritic cell?

Answer 4

o Protein antigen gets seen in tissue by dendritic cell-> antigen gets taken up by dendritic cell and is displayed on surface as a peptide-> dendritic cell goes to lymph node and activate T cell specific for its peptide leading to T cell proliferation and increased expression of chemokine receptor called CXCR5 (normally found on B cells)-> T cell migrates towards B cell zone due to the increased expression of CXCR5

Question 5

Identify the signals that T cells provide to B cells that lead to B cell activation and expansion.

Answer 5

• Both cells meet and the T cell activates the B cell.
  o T cell binds to MHC molecule/peptide presented by B cell
  o B cell has CD40 on its surface which interacts with the CD40 ligand (CD40L) present on activated T cells-> this induces signalling and causes B cell to proliferate

Question 6

What happens once B cells are activated by T cells?

Answer 6

• Once B cells are activated through the T cells, they start to proliferate and some differentiate into short-lived plasma cells and start producing antibody
  o These short-lived plasma cells concentrate/produce a small amount of antibody
  o Antibody produced is mostly IgM which bind with low affinity-> these are useful to control infection, but can’t yet be used to eradicate infection

Question 7

What is the germinal center made of?

Answer 7

• Germinal center is made of B cells, T follicular helper cells and follicular dendritic cells (FDCs)

Question 8

What is the purpose of follicular dendritic cells?

Answer 8

o FDCs usually show antigen to B cells and are only found in the follicle

Question 9

What is the aim of the germinal center?

Answer 9

• Germinal center accomplishes production of B cells that have antibodies that have very high affinity for antigen and leads to different isotype production

Question 10

What does the germinal center reaction require?

Answer 10

o It requires T cell-B cell collaboration and only occurs in response to protein antigens

Question 11

Where does the germinal center reaction occur?

Answer 11

o Takes place in secondary lymphoid organs such as lymph nodes and spleen

Question 12

What does the germinal center reaction lead to?

Answer 12

o It leads to generation of high affinity antibodies and isotype switched antibodies (such as IgG, IgA, IgE)
o It leads to the generation of long lived plasma cells and memory B cells

Question 13

What happens to the T cells and B cells during the germinal center reaction

Answer 13

 Signals generated by T cell recognition, in addition to upregulating expression of CD40, causes upregulation of CXCR5 on the T cell and the interacting B cell
 The T cell changes into a T follicular helper cell
 CXCR5 expressed on both the B cell and T cell allows them to migrate into the follicle-> in the follicle, B cell undergoes proliferation (clonal expansion of B cells) and T follicular helper

Question 14

What do long-lived plasma cells do?

Answer 14

 Long lived plasma cell makes the high affinity antibodies of isotypes other than IgM

Question 15

What do long lived memory B cells do?

Answer 15

 Long lived memory B cells that don’t actively produce antibody, but that will do so very quickly upon re-exposure to the antigen (such as a subsequent infection)
* Upon subsequent infection, the memory B cells will become long lived plasma cells that make lots of specific antibody

Question 16

What happens due to the germinal center reaction?

Answer 16

The GC reaction occurs in response to protein antigen and has several outcomes:
* Antibody affinity for antigen increases due to mutations in the immunoglobin heavy and light chain variable regions
* B cells differentiate into long-lived plasma cells that secrete high affinity antibodies
* B cells differentiate into memory B cells that can generate more plasma cells
* There is isotype switching at the immunoglobulin genes to create IgA, IgG, or IgE antibodies with the same antigen specificity as the original IgM

Question 17

How does antibody affinity get fine-tuned/increased in the germinal center reaction? Describe the two main steps involved

Answer 17

• B cell expresses Ig membrane molecule through genes for the heavy chain and light chain
  o B cells in the germinal centre have identical antigen binding sites
• Mutations are induced in the variable region of the IgGs in the germinal center by T follicular helper cell signals-> this leads to change in antigen binding site of the antibody which might affect affinity
  o TFH cells signal to B cells and induce high rates of mutation in the variable regions of the immunoglobulin (Ig) genes. This ‘somatic hypermutation’ occurs in both the heavy and light chain genes.
• Follicular dendritic cells have protein antigen bound to surface-> show the protein antigen to different subclones of the B cells->depending on how tightly thee antibody-antigen binds, the B cell will get a signal to survive: if it doesn’t bind well, they die by apoptosis
  o B cells that have high affinity antibodies to the antigen presented by the follicular dendritic cells will survive and proliferate
   Selection for antibodies that have been changed by that process of mutation so that they have these high affinity antibodies

Question 18

How does isotype/class switching in the germinal center reaction occur? Give an example of how it would go from IgM to IgG

Answer 18

• Isotype/class switching involves the cutting and rearranging of DNA and occurs in the constant region of Ig heavy chain genes
  o For IgM production, the VDJ gene component (which encodes for binding) is right next to the u (mu) gene which encodes for the IgM isotype
   The mRNA therefore produces both VDJ and mu, leading to IgM
  o Signals given to the B cell by the T follicular helper cell leads to the gamma heavy chain being produced instead with an intact VDJ
   Switch regions are upstream each exon encoding a type of heavy chain such as the gamma heavy chain for IgG
   T follicular helper cell signals initiate cutting of DNA at the switch regions-> the DNA is re-ligated so that the original VDJ is connected to a new heavy chain gene (such as a gamma heavy chain segment)

Question 19

When do B cells undergo isotype switching>

Answer 19

• B cells undergo isotype switching in response to T cell signals in the GC reaction

Question 20

What does isotype switching do in the GC reaction vs what does it not do?

Answer 20

• Isotype switching alters the antibody heavy chain, which can change the ability of the antibody to perform functions such as activation of complement and mast cells, but does NOT affect antibody affinity for antigen

Question 21

What does generation of different antibody isotypes contribute to?

Answer 21

• Generation of different antibody isotypes contributes to effective immunity

Question 22

What are the best immunoglobulin isotypes for complement fixation?

Answer 22

o Best done by IgG1, IgG3 and IgM

Question 23

What are the best immunoglobulin isotypes for mast cell activation?

Answer 23

IgE

Question 24

What are the best immunoglobulin isotypes for neutralization?

Answer 24

o In blood tissues-> IgG
o In gut lumen-> IgA

Question 25

What are the best immunoglobulin isotypes for complement fixation for opsonization?

Answer 25

IgG1 and IgG3

Question 26

What are the best immunoglobulin isotypes for antibody-dependent cellular cytotoxicity?

Answer 26

IgG1 and IgG3

Question 27

What are the best immunoglobulin isotypes for passive immunity to the fetus?

Answer 27

o Dependent on IgG made by the mother-> this is specific to IgG