B cell Activation and Antibodies Flashcards
Where does B cell development start and where is it completed?
-It begins in the bone marrow and continues in the spleen.
Why is there no positive selection for BCRs?
Because BCRs don’t bind to MHC, they bind free-flowing antigens, hence they don’t need to undergo positive selection to test whether they bind to MHC or not
So how do they bind to TFH then??
What is the purpose of the second round of negative selection in the spleen?
-To present more self-antigens to B-cells
What happens to self-reactive B-cells?
-Receptor editing in light chains (toleranceqq)
or
-Cell death
IN CLASS QUIZ1: Which enzyme is not involved in the light chain recombination?
Tdt gets turned off in recombination of the BCR light chain
-Others involved: Artemis, RAG1/2, Exonucleases
What does the development of BCRs in the spleen look like?
T1 subset:
-Since the spleen filters the blood, it can expose blood-borne antigens to B-cells
-Recombination enzymes like RAG1/2 are turned off at this stage, so no light chain editing is possible
T2 subset:
-creates IgM and IgD antibodies and upregulates receptors for BAFF (B-cell activating factor) that upon binding create a survival signal -> if it doesn’t the cell will undergo APOPTOSIS
At which part of the antibodies does recombination occur?
VDJ recombination including P and N nucleotides at the variable region on the top of the heavy chain:
-The lower part reflects the isotypes: IgM, IgD, IgG, IgA, IgE
Which Ab isotypes are expressed by mature B-cells?
-IgM and IgD
-This is achieved by splicing different products from one single mRNA + removing unused J regions
-Note that IgD and IgM still bind the same antigen bc the VDJ region at the binding site is the same, only the c region is changed -> in case of isotype switching to give the antibody different abilities
How is the membrane and secreted form of antibodies generated?
-Splicing that determines if the membrane-bound (BCR) or secreted form (Antibody) is produced by removing the transmembrane domain
-Once the B-cell finds an antigen, it will start creating the secreted version which is the antibody
How does the Ig isotype switch occur?
-Depending on the pathogen and cytokines secreted by the T helper cell (stimulated by DC)
-genetic DNA recombination:
VDJ M D -> cutting off M or D segment for A,G or E
What are Marginal zone B-cells and B-1 B cells?
The normal B cell is a B-2 B-cell !!
-Marginal zone B-cells derives from T2 subset B-cell during development in the spleen -> the ones that are too sensitive to self-antigens are converted into marginal zone B-cells, which are specialized to recognize carbohydrate Ag
-> they have IgM Ab and cant switch their Isotype, innate type of B-cells
-B1 B cells derived from the stem cells from the gut
limited receptor repertoire Tdt is minimally expressed in the precursor of B1 cells
-they also bin carbohydrates
-they need interaction with self-carbohydrates first, and never get support from T cells
What are T-Independent B Cell Responses?
-Response of B-cell without the help of T cells
-Therefore it needs a strong signal to get activated, one simple antigen is not enough (we do not want a response to that without T cell help anyway bc it could be self)
TI-1 antigens: bacterial cell wall components and LPS -> bind through PRRs and BCRs
TI-2 antigens: polymeric proteins or polysaccharides -> bind BCRs with the help of complement
What is the T-dependent B Cell Response?
- B-Cell binds Anitigen
2.CD40 (B-cell) interact with CD40L - Cytokines -> helps turn B-cell on and start maturation process
this is something T independent dont get, they can secrete out IgM and IgD but nothing else
How do B cells get exposed to antigens?
B-cells don’t have Antigen-presenting cells, instead they are waiting in the secondary lymph tissues for antigens (from dead pathogens) to come along through the lymph system
How do antigens reach the B-cells in the secondary lymph tissues?
There is a barrier bc we don’t want every Ag to be in the lymph node and cause an infection
-Small Ag though can pass directly through the gaps and reach the B-cells
-Larger Ags are captured by subcapsular sinus macrophages (SCSMs) and handed off to B cells in the follicles
-Follicular dendritic cells collect Ags and present them to B-cells
Once B-cells have bind to antigens, what does the B-cell do with it?
-It will present the Ag through exogenous pathway through MHC-II
-B-cells position themselves from follicles into T cell-rich zone to find a T-cell that help them to develop
-Once they find one they will create the 3 signals:
on the B-cell side: 1. Antigen binding -> 2. CD40-CD40L and CD80/86, 3. Cytokines
on T-cell side: MHC-II with peptide to TCR 2. CD28 to CD 80 on B -cell and 3. cytokines
Which type of T cell is involved in B cell development in the germinal zone?
-Development of T-cells and B-cells goes sidewise
-T-cells need antigen-presenting cells to expose them to antigens for the development
-When B-cells find antigens there are no T1 or T2 cells at that point, only naive T cells-> so if B-cells find antigens they present it to TFH that direct the B-cell differentiation process
Which development steps occur in the germinal center?
-Some of the naive B cells will start making antibodies (IgM, IgD), some will turn into plasma cells, some will turn into memory cells
-Other will undergo further development through somatic Hypermutation and Class-switching recombination
What is Somatic Hypermutation?
individual point mutations in Ig heavy- and light-chain rearrangements at the top of the antibody on the variable regions
Follicle DC presents Ag: B-cells that don’t bind to Ag well enough after mutation die
-The purpose is the selection of B-cells with higher affinity to a particular Ag (these ones will receive the survival signal)
Explain Class switch Recombination:
IgM and IgD are the first isotypes created, they are broad
IgA (mucosal), G (viral), and E (helminth) are more targeted to a specific type of antigen
-The cytokine signal (PAMP, PRR, cytokine, Th) determines which isotype is to be produced
-The enzyme used is the activation-induced deaminase (AID), also used in SHM
How does the Class switch Recombination work?
-It happens at the lower end of the antibody -> constant region
AID induces double-strand breaks by a mutation in switch regions, gets rid of the M/D segment, and they are looped together with the new class (IgG, E, A) -> the variable binding site on top (somatic hypermutated) stays the same!
What are the plasma cells?
-They are activated and class-switched and ready to produce antibodies (secreting version)
-They lose their external BCR
-found within 5-6 days of immune responses in lymph nodes
-long-lived in bone marrow (but not able to class-switch)
How are Memory cells created?
-Some B-cells that have been activated by T helper cells (3 signals) don’t convert into plasma cells but they are kept as memory cells
-bc plasma cells don’t have BCR anymore, memory cells do have BCRs and can interact with antigens that are coming back and turn into plasma cells or create new memory cells
Two types:
-IgG bearing: have undergone SHM producing higher-affinity antibody -> EARLY MEMORY RESPONSE
-IgM bearing: generated prior SHM (survive longer): not differentiated but can undergo differentiation in case the pathogen has changed and form new plasma and memory cells -> LATE MEMORY RESPONSE
What advantages do IgM memory cells have?
-They might have lower affinity but:
-capable of class-switching
-Further SHM
-in case that pathogen has changed, IgM memory cells can create new and better plasma cells based on that new antigen, and memory cells
