Humoral Immunity: B Cell Activation, Affinity... Flashcards
What are the 2 phases of the B cell life cycle?
- Antigen-independent Phase
2. Antigen-dependent phase
How does a B cell life cycle start?
B cell starts life as a stem cell and differentiates into a pro-B cell
Describe the first recombination that occurs in the B cell life cycle
Undergoes D→ J recombination to join D and J segments (selected randomly) from the gene segment repertoire
What is the second recombination a B cell undergoes?
V segment is recombined with DJ segment
What is the role of V(D)J recombination?
The Recombined V->DJ segment Hard codes in the heavy chain variable region
Expressed with the μ constant region to form a heavy chain
What makes a B cell a Pre-B cell?
Pro-B cell becomes a Pre-B cell when it expresses a heavy chain + a light chain placeholder
How is the light chain of a B cell encoded?
Pre-B cell undergoes another round of recombination to join V and J segments of either kappa/lambda chain
This hard codes in the variable and constant light chain regions
What is an immature B cell?
B cell becomes immature B cell once it expresses both heavy and light chain IgM
What mechanisms do B cells undergo to produce variability?
Other mechanisms present to introduce variability during V(D)J recombination e.g:
Junctional diversity:
- Junctional flexibility
- P and N nucleotide addition
What is the purpose of the variability mechanisms B cells undergo?
Generates more diversity even between B cells with the same combination of gene segments
Describe how a mature B cell is formed
Mature B cell is formed when the B cell is capable of expressing both IgM and IgD through alternative splicing
Where do mature B cells reside?
Mature B cells recirculate between the bloodstream, spleen and lymph nodes
When does the antigen-dependent phase (2nd phase) of B cell life cycle occur?
Occurs after pathogen (virus / bacteria) invades the body
Describe how stem cells differentiate into lymphoid cells
As stem cell develops, it undergoes lymphoid progenitor stem cell differentiation to give rise to B-cells and T cells (which migrate to thymus)
How are T cell receptors formed?
T-cells have their own TCR generated through a similar VDJ recombination process
What are T helper cells?
T helper cells are a subset of T-cells involved in B cell activation during infection
Where do activated B cells migrate during antigen-dependent phase?
Activated B cells migrate to the GC (germinal centre) where it undergoes affinity maturation
What is the role of affinity maturation process?
Affinity maturation improves the affinity of B-cells for the attacking pathogen antigens
What 3 steps are involved in affinity maturation?
- Clonal expansion
- Somatic hypermutation (in the dark zone)
- Selection (in the light zone)
How often does affinity maturation occur?
Affinity maturation process is repeated several times until highest possible affinity available
How does class switching occur?
Antibody receives signals that indicate which pathogens they’re fighting
Ab then undergoes class switching to gain the appropriate effector function
What is the purpose of class switching?
Class switching enables the presence of appropriate effector functions
What different types do B cells differentiate into after affinity maturation and class switching?
B-cells Differentiate into plasma cells, secreting antibodies whilst maintaining some BCRs on their surface.
A few become memory B cells.
Where do the differentiated B cells of the antigen-dependent phase reside?
Both memory and plasma B cells circulate in the bloodstream
How are B cells activated?
When the body encounters a pathogen, a subset of naive B cells that recognise the pathogen are activated.
(1 billion+ unique naive B-cells patrol the bloodstream. A small portion are activated by the pathogen, the rest keep patrolling)
How many B cells are in the body?
~1 billion+ naive B cells patrol the bloodstream.
A small portion are activated by the pathogen, the rest keep patrolling
How many B cells are activated when encountered with a pathogen?
Only a small portion of the naive cells are activated by the pathogen, while the rest remain patrolling
What are the 2 stages of B cell activation?
- T-cell independent B cell activation
- T-cell dependent B cell activation
When the pathogen invades, the B-cells are partially activated when they bind to and process the antigens. Then, B-cells make clones of itself through clonal expansion.
Some clones become the first defence army, secreting IgM to hold off the invading pathogen.
The other clones migrate to the lymph node to wait for T-cell activation.
T-cell-dependent B-cell activation requires a triple verification process to ensure B-cells not activated by mistake.
What is the first action of B cells once activated?
(T-cell independent)
Once activated B cells form clones of themselves (clonal expansion)
What are the roles of the B cell clones formed from clonal expansion?
Some clones will become first defence, secreting IgM against the pathogen.
Other clones migrate to the lymph node for T cell activation
How is B cell activation regulated?
T-cell-dependent B-cell activation Requires triple verification process to ensure the B cells aren’t activated by mistake
Outline T-cell-dependent B cell activation
- B-cell encounter (BCR) + internalise pathogen’s antigen.
- Antigen is presented on B cell surface via MHC II
- Must be activated by T cell(which has also been activated by same pathogen)
- Pathogen is engulfed by DCs and presented on DC surface
- Thelper cells detect antigen on DC surface and are activated
- B cell is activated
- CD40-CD40L interaction confirms T helper cell => produces cytokines
- The Fully activated B cells undergo affinity maturation, class switching and differentiate into plasma cells(which secrete antibodies)
What are the 3 signals required for B cell activation?
- Antigen binding to BCRs
- Co-stimulation by activated Th cell specific to the same antigen
- Th cell-derived cytokines (signalling)
Summarise B cell activation
- Differentiation and clonal expansion of activated B cells
- 3-signal verification
- Signal transduction pathway
What are signal transduction pathways?
These are complex signal pathways for cell proliferation, differentiation and survival
What activates the B cell signal transduction pathways?
Antigen-BCR binding > Activation of tyrosine kinase
Outline the signal transduction pathway of B cells
- Antigen binds to BCR
- Activates protein tyrosine kinases (e.g. SYK)
- SYK phosphorylates downstream proteins (for cell proliferation, differentiation, survival)
After VDJ/VJ recombination, the body produces ~1bn ……………………
resting circulating B-cells (naive B-cells).
What is a naive B cell?
Naive B cell: Not exposed to antigen yet
Which B cells are selected for during affinity maturation?
Each B-cell has a unique BCR on its surface - BCR that binds the pathogen best is activated
How is the best affinity BCR chosen for?
The chosen B cell(that best binds the pathogen) clones itself (clonal expansion) and those clones undergo affinity maturation
Describe the initial Ab-antigen affinity
When the B-cell antibody initially generated (by VDJ recombination) binds to the antigen, it binds at a low-affinity
Describe the low affinity antibody-antigen complex
- Antigen takes longer to bind antibody
- Antigen binds loosely to antibody
- Antigen quickly detaches after binding
How does affinity maturation process mould shape of antibody to better fit the antigen?
When B cells activated, affinity maturation process generates mutations in variable region genes to select for genes with highest affinity.
Slowly moulds shape of variable region to fit better with the antigen.
Where does affinity maturation occur?
Affinity maturation occurs in the germinal centre of the lymph node
What are germinal centres?
GCs = circular cell clusters on the periphery of the lymph node
Which 2 types of cell aid with affinity maturation process?
Tfh - T Follicular Helper Cells: The Only T-cell that can enter GC
FDCs - Follicular Dendritic Cells: not normal DCs, presents antigens in the GC
Where do other T and Th cells reside in lymph node?
Other T cells remain in the T cell zone of the lymph node
What are the components of the Germinal centre?
The germinal centre (GC) is composed of:
- Light zone
- Dark zone
Which 2 processes occur in the Germinal Centre?
2 processes occur in the GC:
- Affinity maturation
- Class switching
When do B cells enter germinal centre?
When the B-cell is activated by Thelper cells, it migrates into the GC dark zone for clonal expansion
What are the stages of affinity maturation?
Once cloned, B cells undergo affinity maturation(consisting of):
- Somatic hypermutation (dark zone)
- Selection (light zone)
Which B cells undergo class switching?
The B cell with the highest affinity for the antigen will undergo class switching
What do Mature B cells differentiate into?
- plasma cells OR
- memory B cells
What is AID?
+ Its role?
AID = activation-induced deaminase
Generates point mutations within variable region gene of B cells at random points = somatic hypermutation
Describe the outcome of somatic hypermutation
All B cells that were previously clones now differ from one another
When do B cells enter light zone of the germinal centre?
Hypermutated B cells enter GC light zone for selection
What is the role of FDCs in affinity maturation?
FDC (follicular dendritic cells) present the antigen on its surface during selection
How do hypermutated B cells interact with FDCs and Tfh cells?
B cells compete for the limited amount of antigens on the FDC surface and present the antigen to Tfh cells
What is the role of Tfh cells in affinity maturation?
Tfh cells provide ‘survival’ signal to the B-cell during selection
What happens to low affinity B cells during affinity maturation?
B cells with low affinity (unable to bind to FDC) undergo apoptosis due to lack of survival signal from Tfh cells
What is the result of affinity maturation on high affinity B cells?
B cells that survive the process migrate back to dark zone to repeat process until affinity is high enough.
How does affinity change with each affinity maturation cycle?
Ab affinity improves with each cycle of affinity maturation
The process is random
Without the survival signals from the T follicular helper cells, B-cells would die off.
Those that survive migrate back to the dark zone.
The process begins again until Ab affinity is high enough.
