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?
Hard codes in heavy chain variable region
Expressed with a new 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 with 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
Codes in variable and constant light chain regions
What is an immature B cell?
B cell becomes immature B cell once expressing 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 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 formed when the B cell is capable of expressing both IgM and IgD through alternative splicing
Where do mature B cells reside?
Recirculate between bloodstream, spleen and lymph nodes
When does the antigen-dependent 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 undergoes lymphoid progenitor stem cell differentiation to give rise to B and T cells (which migrate to thymus)
How are T cell receptors formed?
T cell have their own TCR generated through similar VDJ recombination processes
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 attacking pathogens
What 3 steps are involved in affinity maturation?
- Clonal expansion
- Somatic hypermutation
- Selection
How often does affinity maturation occur?
Process is repeated several times until highest possible affinity available
How does class switching occur?
Antibodies receive signals to inform them about the pathogens they’re fighting to undergo class switching
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?
Differentiates 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
How many B cells are in the body?
More than ~1 billion naive B cells patrol the bloodstream
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
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 (IgM)
Others migrate to lymph node for T cell activation
How is B cell activation regulated?
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) and internalise pathogen
- Antigen presented on B cell surface via MHC II receptor
- Must be activated by T cell (also activated by same pathogen)
- Pathogen engulfed by DC and presented on surface
- Th cells detect antigen on DC surface and are activated
- B cell activated
- CD40-CD40L interaction confirms T helper cell => produces cytokines
- Fully activated B cells undergo affinity maturation, class switching and differentiate into plasma cells to secrete antibodies
What are the 3 signals required for B cell activation?
- Antigen binding to BCRs
- Co-stimulation by activated Th cell specific to same antigen
- Th cell-derived cytokines
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?
BCR binding > Activation of tyrosine kinase
Outline the signal transduction pathway of B cells
- Antigen binds to BCR
- Tyrosine protein kinases (e.g. SYK)
- SYK phosphorylates downstream proteins
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 continues cloning itself (clonal expansion) and those clones undergo affinity maturation
Describe the initial Ab-antigen affinity
When antibody initially generated binds to the antigen it binds at a low-affinity
Describe the low affinity antibody-antigen complex
- Antigen takes longer to bind antibody
- Binds loosely to antibody
- 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?
These are 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: only T cell that can enter GC
FDCs - Follicular dendritic cells: not normal DCs
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 a light and 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 B cells are activated by Th cells, they migrate 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) and selection in the 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?
AID (activation-induced cytidine deaminase)
What is the role of AID?
Generates point mutations within variable region gene of B cells at random points
(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 FDCs in affinity maturation?
FDC (follicular dendritic cells) present the antigen on its surface during selection
What is the role of Tfh cells in affinity maturation?
Tfh cells provide ‘survival’ signal 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 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 cycles of affinity maturation
The process is random
