Germinal centres, B cell differentiation, and regulation Flashcards
Where does a B cell contact its Ag in the LN and where does it move to?
- contact Ag in follicle
- moves to paracortex (T cell zone) after contact
Where do B cells receive their second signal (co stimualtion)?
Receive costimulation from activated Th cells
Describe how B cells present Ag to T cells to get help
- B cells edocytose Ag bound to BCR (RME)
- peptides from internalized Ag.s are very efficiently present in MHC-ll
- cell-cell complex formed when MHC-II+Ag is recognized by activated Th cells
- allows B cells to receive costimulatory signals
What are the conditions that have to be met for a B cell presenting Ag to a Th cell?
- B cells receive help from T cells that recognize the same Ag
- epitope recognized by BCR does not have to be the same as that recognized by the TCR (linked recognition)
- B cells can also act as APCs and present Ag to naive T cells
What is the importance of linked recognition?
prevent auto-Ab production
What type of T cells do B cells present Ag to? What signals do they receive?
- B cells present to Th2 and TFH cells
- cotimulatory signal = CD40/CD40L
- cytokines = IL-4 and 21
When do T cells express CD40L?
Upregulated upon activation
What are the roles of Th2 cells in the context of B cell activation? (is it the primary activator subtype, how this subtype created, what does it do for B cells)
- not the primary B cell activator subtype
- naive T cells differentiate to Th2 in the presence of IL-4
- Th2 help limited to some B cell activation and induce class switch to IgE
What are the roles of TFH cells in the context of B cell activation? (is it the primary activator subtype, how this subtype created, what does it do for B cells)
- primary B cell activator subtype
- differentiate into TFH in the presence of IL-6 and 21
- TFH express Bcl-6 (master transcriptional regulator)
- migrate to B cell zone and are required to form germinal centres
- high CXCR5 and low CCR7 expression (attracts and retains TFH to B cell follicle)
- produce IL-21 to support differentiation of B cells
What are the effects of CD40L and cytokines (IL-2, 4, 5, 6 and 21) on B cells?
CD40L:
- transmembr protein expressed on the surface of activated Th2 and TFH cells
- signal required for affinity maturation, isotype switching, and memory
cytokines:
- secreted by Th cells to promote B cell proliferation and differentiation
- influences class switching
What is X-linked hyper-IgM syndrome? (what causes it, what are the effects)
- due to LOF mutations in CD40L
- effects
- impaired humoral immune response to TD Ag.s
- also have some defects in cell-mediated immunity
- don’t form germinal centres –> lack B cell memory, fail to make high affinity Ab.s, no class switching –> elevated serum levels of IgM
Why in X-linked hyper-IgM syndrome is cell-mediated immunity affected?
naive CD8+ T cells require CD40L stimulation
What are the three fates of B cells receiving T cell help?
- entry into germinal center
- IgM-bearing memory cells from primary response
- primary focus
What determines if a B cell will go into a germinal centre or primary focus?
High IRF-4 and BLIMP-1 expression –> inhibit Pax-5 and Bcl-6 (germinal center fate factors) –> plasma cell fate –> Ab secretion
What happens to B cells that go into primary focus?
- requires high IRF-4 and BLIMP-1
- formed at the T/B cell border in LN
- rapid production of IgM Ab.s
- can class switch to IgG
- low Ag affinity
- most die by apoptosis
What is blimp-1?
TS repressor critical for plasma cell differentiation, down regulates genes involved in:
- BCR signaling
- Ag presentation
- proliferation
What happened when Blimp-1 was knocked out in B cells?
- impaired Ig signaling
- lack of plasma cells
How can we discern plasma cells from other B cells?
plasma cells have low levels of B220 and high levels of syndecan-1
What cells are found in germinal centres?
- germinal centre B cells (specific)
- FDCs
- non-specific IgD+ B cells
- macrophages
- TFH cells
What are the three important functions of germinal centres?
- site of affinity maturation and SMH
- isotype switching/CSR
- memory B cell formation
note: all these features distinguish TD from TI Ag.s
Describe the germinal centre reaction
T cell zone:
1. naive B cell is activated by TD Ag
2. migrates to germinal centre
Germinal centre (Dark zone)
1. activated B cell enters dark zone
2. B cell begins to proliferate and go under SHM
3. B cell moves to light zone
Germinal centre (light zone)
1. selection of high affinity, mutated, receptor-bearing B cells
2. those that are selected against go under apoptosis
3. those that are selected for can differentiate into plasma cells or memory cells OR can undergo CSR
4. B cells that have class switched can go under light zone dark zone re-circulation
What is affinity maturation?
production of Abs with high affinity for Ag
What mutations occur in SHM and where do they occur?
- type = point muts, insertions, dels
- occur = VDJ exons and VJ exons
What enzyme does SHM require, what does it do?
AID (activation induced cytidine deaminase):
- deaminates cytosine into uridine
How can a U/G mismatch be repaired?
- different NTs subsitituted for modified base
- bases around modified base can also be modified
- modifies sequence of V regions of heavy and light chains
Where do FDCs present Ag to centrocytes (mutated BCR)?
in the light zone
Describe the competition for Ag that occurs in the light zone
- centrocytes with highest affinity for Ab can compete for limited Ag and are selected to survive (receive survival signals from TFHs and FDCs)
- high affinity BCRs internalize Ag and present it to TFH cells and receive additional signals to promote cell survival and further differentiation - centrocytes with no or little affinity undergo apoptosis
Describe what switch regions are, where they are located in genes, and what they facilitate
- switch regions are upstream of each isotype constant region except for delta
- switch regions are introns
- isotype switching involves DNA recombination between switch regions (switch recombination)
Describe switch recombination
- switch regions contain targeting sites for AID
- AID initiates DSBs in s(mu) and s(gamma) regions/whatever constant region it will switch to
- excision circle (interfering DNA) is created and lost
What proteins help in ending the B cell response?
- CD22 = B cell co-inhibitory R
- -ve regulation by Fc(gamma)Rllb
Describe the structure of CD22, where is it expressed?
- expressed on the surface of B cells, associated with BCR
- intraceullar tail is phosphorylated by Lyn when BCR is activated
- intracellular tail has 3 ITIMS
- phosphorylated ITIMs recruit SHP-1
What is SHP-1 and what is it’s role in BCR signaling? What happens if SHP-1 is not expressed?
- SHP-1 is a hematopoietic cell specific tyr phosphotase that has two SH2 domains
- both SH2 domains are required to interact with phosphorylated ITIMs in order to get stable binding of SHP-1 to CD22
- SHP-1 limits the magnitude and duration of BCR signaling by de-phosphorylating tyr-phosphorylated proteins
- mic deficient in SHP-1 or CD22 develop autoimmune disease
Describe negative regulation of B cells by Fc(gamma)RIIb
- circulating Ag-IgG complexes bind to Fc(gamma)RIIb on B cell surface
- cross linking of bound Ag between BCR and Fc(gamma)RIIb brings Fc(gamma)RIIb close to activated Src kinases (e.g. Lyn)
- Lyn phosphorylates ITIMs of Fc(gamma)RIIb
- attracts phosphotases that reverse activating signals
