L6 - Lymphocyte activation: B cells Flashcards
How do B cells act differently to T cells in response to antigens?
B cells don’t deal with antigens themselves
They release antibodies into the blood which recognise the pathogens
What are memory B cells?
When some B cells keep the BCR on their surface – memory B cells
What are antibodies used for?
Neutralisation
Opsonisation
Complement activation
B cell antibody production
B cells also need several signals to become activated
Naïve B cells express membrane (m) Ig/BCR (IgM or IgD) & encounter non-self Ag in secondary lymphoid tissue
Binding of Ag to BCR provides signal 1 to the B cell
Basis of Ag/BCR signal 1
BCR-associated polypeptides involved in signalling – Ig-alpha & Ig-beta allow BCR to go to the cell surface
Crosslinking BCR activates intracellular kinases
Has the same signalling motifs in its signalling molecules – ITAMs (yellow bits)
ITAM (immunoreceptor tyrosine-based activation motif) – series of AA that have 2 tyrosine residues that get phosphorylated
How can signal 1 be increased/enhanced?
- If antigen has activated complement cascade
- Lots of C3b
- Complement receptor 2 (CR2) on B cell surface (CD21)
- CR2/CD19/CD81 form the BCR co-receptor complex
- Augments the signal
What happens if an Ag that binds BCR is coated with complement?
it can also bind CR2 on B cells to give an increased signal 1
Other molecules in the complex: CD19 & CD81
Antigen binds to both the BCR & CR2 to induce a stronger signal
How do B cells receive signal 2?
Bind signal 2 differently depending on the type of Ag they bind
What are the types of Ag that B cells can bind to?
T (thymus)-independent Ag (TI)
T (thymus)-dependent Ag (TD)
T-independent Ag (TI)
SIGNAL 2
Signal 2 is provided by either the:
1) Antigen itself
2) Extensive cross-linking of BCR
DOESN’T INVOLVE T CELLS
Induce antibodies in absence of T cells
Antibodies made this way cannot class switch so isn’t a great response
T-dependent Ag (TD)
SIGNAL 2
Signal 2 is provided by CD4+ T cells
INVOLVES T CELLS – most antigens
• Drives a very good B cell response
B cells don’t go to the thymus – if you take out the thymus you get no T cells – no T cells means B cells don’t respond properly
MHC II presents peptide derived from antigen on the B cell
Signal 2 comes from the T cell in TD not the antigen itself like in TI
What are the 2 types of TI antigens?
TI-1 Ag
TI-2 Ag
TI-1 Ag
As well as binding to BCR, TI-1 Ag also bind to other receptors on all B cells providing signal 2
Eg. lipopolysaccharide from gram -ive bacteria binds to TLR4 expressed by B cells
In high concentrations can act as polyclonal activators (mitogens) for B cells
The 2 signals (1 from BCR, 2 from TLR) lead to B cell activation, proliferation & antibody secretion
TI-2 Ag
Contain repeated epitopes
• Often polysaccharides – same sugar repeated lots
• Important in some bacterial infections – coated
Will therefore cross-link many BCR on the same B cell
Take longer to induce B cell activation
Develops after age 5 – need conjugative vaccines
TD Ag response compared to TI Ag
Antibody responses seen to TD Ag are much better than those to TI Ag – the classical acquired response
Require T cells
Process of TD Ag antibody response
1) B cell recognises coat protein of the virus, takes it up, processes it & presents it to the T cell
2) Doesn’t necessarily present bits of the coat protein
3) T recognises the internal parts of the virus – peptide shown by the MHC
4) When the T cell activates the B cell, B cell makes BCR to bind coat proteins
5) T cell epitope is inside the virus whereas B cell is outside the virus
6) Both parts end up being presented
B cells act as APC for TD Ag
Why do you make conjugate vaccines?
Converting a TI Ag to a TD Ag is a way of improving the efficiency of a vaccine
How do you make conjugate vaccines?
A sugar cannot be processed & presented –
1) Can take the sugar, add it to a protein (chemically or genetically)
2) Now have a conjugate protein – when it gets taken up by the B cell it creates peptide fragments
3) Can then present these fragments to the T cell
4) T helper cell recognises it & delivers signal 2 to the B cell
5) B cell now makes antibodies against the sugar that are T-dependent (TD)
6) Makes a better response – can now be classed switched
Couldn’t naturally do this as a response to a sugar
Example of a conjugate vaccine
Haemophilus influenzae type b
Protective response requires antibodies to capsular polysaccharide (which is a TI Ag)
Coupling this to a protein such as tetanus toxoid converts it to a TD Ag
This allows young children to be immunized and protected (TI antibodies not produced until about 5 years old)
What are B/CD4+ T cell interactions important for?
Good antibody responses
B cells enter lymph node from blood to find antigens & T cell to help
If a B cell comes into contact with its specific Ag it can then be activated
If Ag is TD, B cell presents peptide from Ag to CD4+ TH cells at the boundary of the T/B areas within the lymph node forming B/T cell conjugates
Activated B cells
Conjugates of B lymphoblasts & T cells move to primary follicles (B cell areas)
Form germinal centres in follicle – B cells proliferating in the lymph node as a result of getting help from T cells
B cells divide rapidly to become centroblasts & undergo:
• Somatic hypermutation of Ig genes
• Isotype switching
Differentiate into non-dividing centrocytes (smaller)
B/CD4+ T cell interactions
B cell binds Ag via BCR & presents peptide on MHC II from Ag to activated CD4+ TH cell
• As a result T cell then expresses CD40 ligand (CD40L)
• Secretes cytokines
B cell receives signal 2 from T cell via CD40/CD40L binding & via cytokine from T cells binding receptors –> B cell proliferation
CD40 signal also induces activation induced deaminase (AID) which is required for class switching & somatic hypermutatiom
What is AID?
Activation induced deaminase (AID)
Required for class switching & somatic hypermutatiom
Follicular T helper cells (TFH)
Found predominantly in the B cell follicles of the lymph node
Specialised to help B cells
Secrete either TH1 or TH2 type cytokines
Can be identified with specific markers that differ from other sub-sets of CD4 TH cells
What can germinal centre (GC) B cells do?
Can either:
- Differentiate into plasma cells
• Secrete various isotypes
• High affinity antibody, somatically mutated - Form long lived memory cells – recirculate
- Die within lymphoid tissue
• If BCR no longer binds antigen (as a result of unsuccessful V region created or somatic mutation)
Somatic hypermutation
Introduces point mutations into V regions of the Ig
Approx. 1 mutation/V region/cell division – 106 x normal DNA mutation rate
Enzymes primarily involved include:
• AID & DNA repair genes
Where are follicular dendritic cells (FDC) found?
In germinal centres
What do follicular dendritic cells (FDC) do?
Not bone marrow derived dendritic APC
Only live in GC – not the same as DC
Cells in primary follicle that capture intact Ag for B cells to bind BCR
FDC capture Ag via FcR & CR as immune complexes
Oversee B cell affinity maturation
How do FDC oversee B cell affinity maturation?
Centrocytes undergoing somatic hypermutation express mutated BCR on surface
Centrocytes thus compete with each other for Ag on FDC & for signals from T-FH cell
If mutated BCR binds Ag on FDC better than un-mutated BCR, will present more efficiently & receive CD40 signal from TFH cell
Failure = apoptosis
Those centrocytes that have generated higher affinity BCR survive to differentiate into plasma cells
Role of CD40 expressed by B cells
Protects centrocytes from apoptosis
Allows isotype switching
CD40L deficiency/mutations = hyper-IgM syndrome – cannot switch isotypes
Isotype (class) switching
IgM produced initially – co-expressed with IgD by differential mRNA splicing
IgG, IgA & IgE produced later – AID recombines DNA to change constant region of antibody without affecting existing VDJ region
Deficiency of AID also leads to hyper-IgM syndrome
B cell activation summary
- B cell comes out of the bone marrow and hopefully isn’t auto-reactive
- While expanding its mutating its BCR to get a better BCR
- Affinity maturation process in the germinal centers
- If the BCR is really good & gets lots of T cell help it becomes a plasma cell
- May make IgM or told to switch depending on the cytokine
- DC tells T cell to what T cell to become
- T cell then tells B cell what antibody to make
- Some B cells wont release BCR and will keep them on the surface as memory B cells
