Germinal Centre

Question 1

Where do germinal centres arise?

Answer 1

secondary lymphoid organs including the lymph and spleen

Question 2

What does the GC cortex contain?

Answer 2

B cell follicles

Question 3

What does the paracortex contain?

Answer 3

The T cell zone

Question 4

What result would we want from a productive GC reaction?

Answer 4

1) Plasma cells = short lived, produce large amount of high affinity antibodies
2) Memory b cells = can differentiate into plasma cells upon antigen re challenge

Question 5

What are some examples of B cells in the GC?

Answer 5

1) centroblasts: rapidly proliferate, undergo somatic hypermutation of their Ig to create diverse antibodies
2) memory b cells = launch a faster response to antigens
3) B cells that differentiate to become plasma cells = more antibodies

Question 6

What is the role of t follicular helper cells?

Answer 6

help B cells produce high affinity, class switched antibodies

Question 7

What do follicular dendritic cells do?

Answer 7

present antigens to B cells -> display antigens in the form of immune complexes so B cells can test their new receptors for affinity

Question 8

What do dendritic cells do?

Answer 8

present antigens to T cells in complex with MHC class II = activate naive T cells

Question 9

What transcription factor do Tfh cells express and why is this important

Answer 9

Bcl-6 = bcl-6 controls the expression of CXCR5 which guides tfh cells to migrate into b cell follicles

Question 10

What are the steps in the GC?

Answer 10

1) dendritic cell with antigen + naive T cell = tfh cell
2) activation of b cell by tfh
3) b cell proliferates and mutation occurs, GC expands,
4) b cells go from dark zone -> light zone and undergo selection
5) mutated b cells come in contact with FDC
6) memory cells and activation

Question 11

What is the primary function of the immunological synapse?

Answer 11

to ensure effective and sustained T cell activation

Question 12

What is the spreading phase in an immunological synapse?

Answer 12

1) after contact with a T cell and and APC, the T cell undergoes spreading to maximise area of contact with APC
2) involves actin polymerisation = allows T cell to flatten out over the surface of the APC.
3)TCR microclusters form between the two cells -> microclusters contain CD28 and initiates early signalling cascade

Question 13

What is the contraction phase of an immunological synapse?

Answer 13

1) reorganisation of the cytoskeleton and pulling back of the contact area, concentrating the interactions between T cell and APC
2) TCR microclusters move towards centre of the interface -> into the central supramolecular activation cluster
3)forms a more stable and mature immunological synapse

Question 14

What happens within the cSMAC?

Answer 14

• TCR-MHC interactions are maintained and CD28 bind to their ligands on the APC

Question 15

Why is ICOS important in Tfh cell differentiation?

Answer 15

• binding of ICOS to its ligand ICOS-L provides a critical signal for Tfh cell differentiation
  -upregulates Bcl-6 (required for Tfh differentiation)
  -induces CXCR5- without ICOS Tfh cannot migrate to follicles where germinal centres form

Question 16

Why is IL-21 important in Tfh cell differentiation?

Answer 16

1)helps stabilise the expression of BCL-6
2)promotes expansion and survival of Tfh cells
3) supports B cell proliferation, somatic hypermutation -> leads to production of high affinity antibodies

Question 17

How does GC seeding occur?

Answer 17

1) activated b cells present antigens via MHC II molecules on their surface
2) they then move to the T-B border where they encounter Tfh cells
3) after contact between b cells and tfh cells -> co stimulatory signals are delivered
4) after receiving these signals the b cells undergo proliferation and either become plasma cell/ migrate into the follicle
5) b cells proliferate and create GC structure

Question 18

What occurs in the dark zone?

Answer 18

proliferating B cells (centroblasts) undergo somatic hypermutation in their Ig genes -> point mutations in the variable regions of the BCR generates B cells with varying affinities for the antigen

Question 19

What occurs in the light zone?

Answer 19

mutated b cells (centrocytes) migrate to the light zone where they compete for survival signals based on the affinity of their newly mutated BCRs

Question 20

What is selection and affinity maturation in regards to the light zone?

Answer 20

1) B cells that bind antigen with high affinity are selected to undergo further proliferation
2) low affinity B cells that fail to compete for antigen and Tfh undergo apoptosis

Question 21

What happens to successful B cells in the light zone?

Answer 21

they re-enter the dark zone to undergo additional rounds of proliferation and somatic hypermutation -> increases affinity more

Question 22

What role does CXCR4 play in migration to the light zone?

Answer 22

1) in the dark zone CXCR4 interacts with CXCL12 = keeps b cells in dark zone
2) CXCR4 is down regulated which makes B cells less responsive to CXCL12
3) CXCL13 is the ligand for CXCR5 -> guides B cells to the light zone for selection

Question 23

Which cells secrete CXCL12?

Answer 23

cortical reticular cells

Question 24

Which cells secrete CXCL13?

Answer 24

Follicular dendritic cells

Question 25

What happens to B cells with little to no affinity?

Answer 25

apoptosis - removal of inefficient BCR clones

Question 26

What is the difference between the expression of centroblasts and centrocytes?

Answer 26

centroblasts express CXCR4 and CXCR5 whereas centrocytes only express CXCR5

Question 27

What forms when b cells (centrocytes) process and present captured antigens with MHC II to Tfh cells?

Answer 27

Immunological synapse

Question 28

What must b cells receive from Tfh cells to prevent the induction of apoptosis?

Answer 28

CD40L and CD40

Question 29

Why is IL21 important in Tfh - centrocyte interactions?

Answer 29

• promotes b cell survival and proliferation
• up regulates the expression of BCL- 6 in Tfh which promotes their activity and also promotes B cell proliferation

Question 30

What does a lack of CD40 lead to?

Answer 30

hyper IgM syndrome

Question 31

Where do memory cells reside?

Answer 31

lymphoid tissues or target organs
spleen is a major reservoir of memory cells

Question 32

Which interleukin up regulates plasma cell production?

Answer 32

IL-21

Question 33

Which interleukin inhibits differentiation into a plasma cell?

Answer 33

IL-24 prevents b cells from going down the plasma cell path and drives them to become memory b cells

Question 34

How would you image the immunological synapse?

Answer 34

total internal reflection microscopy

Question 35

What sort of competition is there in the GC?

Answer 35

• Naive B cells that also express CXCR5 are also drawn into the light zone where they can capture antigen from FDCs and join the GC response

Question 36

What protein do memory cells express?

Answer 36

CD27 - easy to detect

Question 37

What things can a B cell do to give itself a better chance of success in the GC?

Answer 37

• capture more antigen from FDCs
• more mutations = higher affinity
• more proliferation
• inhibit other B cells