11 - Adapt 5: B Cell Flashcards
What parts of B & T cell development is similar
- rearrangement of gene segments
- screening processes to avoid self reactivity
- production of subsets with discrete function
B & T cell development differences - location
T cells start in bone marrow then move to thymus for positive & negative selection
B cells start in bone marrow then go through negative selection in bone marrow
Differences between B & T cells - screening processes
T cells - positive & negative
B cells - negative
B and T cell development differences - eventual outcomes of antigen receptor stimulation
T cells require presentation & differentiate into helper or killer subset
Most B cells require T cell help for activation and secrete antibodies
How many Ig receptors does each B cell have
A single type
Once the B cell binds to antigen, what happens
Each cell will create a clone of cells bearing the same Ag receptor as the original
Once the B cell undergoes antigen-dependent proliferation, what happens?
It differentiates into memory cells or plasma cells which have massive ER & Golgi on diagrams
Plasma B cells produce what
Antibodies
2 B cell responses are elicited by distinct Ag types that are
T dependent (TD) responses & T independent (TI) responses
T dependent responses depend on
Help from helper T cells
T dependent responses are usually generated upon recognition of
Protein antigen
T independent responses are generated upon exposure to
Multitalented/polymerized Ag
TI-1 Ag binds to
B cells through PRRs
TI-2 Ag binds through
Cross-link/clustering of large numbers of B cell receptors (BCR)
T dependent B cell responses - B cells bind Ag via BCR which induces
Initial activation & proliferation events & proliferation induces formation of germinal centers in lymph nodes/spleen
T dependent responses - once germinal centers are formed, what happens to the Ag?
Some Ag is internalized and processed, then presented on cell surface by MHC class 2 molecules
TD B cell responses: interaction with helper T cells provides conditions for
Differentiation and memory cell production
How many signals do B cells get (TD response)
3
Signal 1 TD B cell response
Antigen binding onto B cell
Signal 2 - TD B cell responses
Antigen is presented on MHC 2 - T cell receptor sees it and CD4 ligand engages & drives signal back into B cells. T cell then undergoes proliferation & produces cytokines
Signal 3 TD cell response
Cytokines travel back into B cell
What causes clustering of B cell receptors upon Ag binding?
Ag binding causes oligomerization of Ag bound BCR molecules in the plane of the membrane
What is an integrin
Proteins that hold cells together maintained by cytoskeleton
In TD B cell responses, Ag receptor clustering induces
Internalization and Ag presentation
TD B cell responses - once signalling begins, BCR-Ag complexes are internalized and the internalized Ag proteins are proceses in the
Exogenous pathway
TD B cell response - Ag peptide fragments are presented
In MHC 2 molecules on the B cell surface to solicit T cell help
TD B cell response - Ag engagement upregulates
B cell CD40 (transmembrane protein) to facilitate B cell - T cell interaction. This part drives signal 2 & 3
After TD antigen activation, a conventional naive B cell turns into
Antigen activated GC precursor B cell
Antigen activated GC precursor B cells undergo proliferation and SHM which means
SHM = somatic hypermutation
What happens during SHM phase
Point mutations in areas of already recombined genes which increase affinity of B cell for antigen
What zone does proliferation and somatic hypermutation happen in
Dark zone of medullary cortex of spleen
after TD antigen activation, a conventional naive B cell migrates to
A germinal centre, usually medullary cortex of spleen
After proliferation & SHM, what process does the B cell go through
Selection
During the selection process what happens
T cell help - T cell binds to B cell & antigen presented on MHC2
What zone does selection happen in
Light zone
TD B cell response- after T cell binds to B cell, which always happens in light zone, what happens?
Differentiation & CSR
TD B cell response - what is differentiation
The B cell turning into either a plasmablast or memory B cell precursor cell
TD B cell response - what is CSR
Class Switch Recombination which allows for recombining of constant region of heavy chain & converts B cell receptor from IgM to IgA, IgG,,etc
TD B cell response - the type of Ig the receptor changes to depends on the
Constant region being replaced during CSR
TD B cell response - somatic hypermutation (SHM) affinity selection occurs within the
Germinal center
SHM produces individual point mutations in
Ig heavy and light-chain rearrangements
SHM - mutations increase over time and with repeated exposures which is followed by
Affinity selection result in increased affinity for Ag over time
Somatic hypermutation - mutational apparatus targets
Mutational hot spots are sequence motifs far more likely to be targeted. This prevents mutations in non-Ig areas
Ag-induced selection of B cells with higher affinity - this means
B cells that can bind, process, and present more Ag to T cells for cytokine assistance survive - B cells are selected for survival of fittest
Higher affinity B cells may actually steal Ag from
Lower-affinity B cells promoting their own survival signals (the girls are fighting 💅)
Class switch recombination occurs within the germinal center after
Ag contact
During CSR, recombination occurs between
Donor and acceptor switch regions which contain targeting sites for AID (an enzyme that induces CSR)
Signals that cause CSR:
B cells must receive costimulatory signals from CD40 to engage in CSR
What determines which Ig isotope is going to be produced?
Which cytokine signal from T cells is received
Why does the type of Ig produced matter?
Clinical relevance! If you have a mucosal pathway, you need Ig that is on mucosal surface so IgA
Are most newly generated B cells lost at the end of the primary immune response?
Yes
After the Ag is cleared, are most of the effector signals required?
No
What does B cell memory provide
A rapid/strong response to secondary infection & high affinity antibody right away
IgM-bearing memory B cells generated prior to
Somatic hypermutation
IgG-bearing memory cells have undergone SHM producing
Higher affinity antibody
T-independent Ag stimulate antibody production without
Need for T cell help
How do we know T independent B cell responses exist
Nude mice - have a Foxn1 mutation that results in no T cells but they still create antibodies
TI-1 Ag - typically
Bacterial cell wall components, bind to innate immunity PRRs on B cells
T1-2 Ag - typically responds to
Polymeric protein Ag and capsular polysaccharides, crosslink many more IgM BCRs
There are 2 novel subclasses of B cells that mediate responses to T-independent Ag
B-1 B cells & marginal zone B cells
B1-B cells are
CD5+, primarily in pleural cavities and are 5% of B cell popn in humans/mice, primarily produce IgM
B1-B cells produce
“Natural” antibodies independent of T cell help that bind a broad spectrum of Ag with low affinity
(Natural means a more broad scale of antigens on surface)
Marginal zone B cells must receive
Low level signals through BCR for survival
Marginal zone B cells respond to
Blood borne Ag entering the immune system through the spleen
Marginal zone B cells can renew themselves in
The periphery & differentiate from T2 B cells in the spleen
Shutting down BCR signalling may be required to
Stop proliferation when it is no longer needed
What shuts down unnecessary BCR signalling?
Negative signalling through CD22
CD22 bears ITIMs, which do what
Act as a brake to the immune system, usually in B cell negative regulation
Do ITIMs and ITAMs have the same function
No, they are enemies & have opposite functions. ITIMs act as an immune system inhibitor, whereas ITAMs are immune system activators
Negative signalling through FcyRIIb (CD32) receptor inhibits
B cell activation. It possesses ITIMs similar to those in CD22
FcyRIIb process simply explained (sorry i dont get it when its complicated)
Once antibody binds to FcyRIIb, that process tells B cells that there is already soluble antibody floating around, and more does not need to be produced
Phosphatases are recruited to phosphorylated ITIMs, which
Strips phosphates from signalling molecules and inactivates ITIMs
CD5 acts as a negative regulator of
B cell signalling
CD5 is induced on B-2 B cells following
BCR-CD40 (this is signal 2) engagement
Many CD5+ B cells secrete
IL-10
B-10 cells act as negative regulators by
Secreting IL-10 which shuts down inflammatory responses by T cells and APCs. A small popn of splenic B cells responsible for this
What mechanisms increase antibody response efficiency?
Somatic hypermutation, affinity selection, and class switching recombination
Antibody responses can be generated with and without T cell assistance - true or false ?
TRUE !!!! It depends on antigen type and structure
Antibody mediates the clearance and destruction of pathogen in a variety of ways - true or false?
True! Each Ab isotope has distinct properties and traits that enable it to do so. IgG, IgA, IgM, and IgE all mediate different effector functions
Antibody-mediated effector functions
Neutralization, agglutination, opsinization, complement activation, and antibody-dependent cell-mediated cytotoxicity
Neutralization
Protects against viral or bacteria infection or the damaging effects of toxins
Agglutination
Enhances neutralization and more efficient clearance of pathogens from the body
Opsonization
Promotes or enhances the engulfment of antigens by phagocytes
Complement activation
Results in the generation of the membrane attack complex (MAC), creating pores in pathogen membranes and killing the microbe
Antibody dependent Cell-mediated Cytotoxicity
Activates the killing activity of several types of cytotoxic cells eg natural killer cells
What is the first Ab produced in an immune response?
IgM
Effector functions of IgM
-Tends to be lower affinity
- very good at complement fixation leading to MAC complex and target lysis
- also good at forming dense Ab-pathogen complexes that are efficiently engulfed by macrophage
IgM size
Pentavalent - 10 total Ag binding sites so lots of binding sites for Ag, bug clusters
IgG ab functions
All variants bind to Fc receptors, which enhances phagocytosis by macrophages
IgG has several subclasses, which means what
There’s a division of labour between subclasses, as each of them has a speciality when dealing with different types of pathogens (however, we don’t care what the specialities are for this class)
IgA is typically found in
Major isotype found in secretions - mucus in gut, milk, tears, saliva etc
IgA is effective in neutralizing
Toxins and pathogens but does NOT fix complement so does NOT drive inflammation
IgA has a long half-life in secretions due to
Protease-resistant amino acid sequence in Fc region
Effector functions in IgE
Best known for role in allergy and asthma, but may also play a role in protection against parasitic helminths and Protozoa
IgE effects
Degranulation of eosinophils/basophils & release of molecules such as histamine to damage large pathogens
Fc receptors mediate many
Effector functions of Ab
Fc receptor signalling involves
Multiple FcRs need to be cross-linked to initiate a signal & signal may be positive (enhancing effector function) or negative (inhibiting effector function)
Outcome depends on whether receptor is associated with ITAM/ITIM
FcyR
Most diverse group of FcRs, four families total & are main mediators of Ab functions in the body
Most FcyR are activating receptors (3 activating, 1 inhibiting family). What are functions?
Will induce phagocytosis if expressed by macrophages
Will induce degranulation if expressed by cytotoxic cells
FceR expressed by
Granulocytes - mast cells/basophils & eosinophils
FceR triggers a signalling cascade that releases
Histamines, proteases, and other inflammatory mediators - most often associated with allergy symptoms
FceR expressed by
Myeloid cells - monocytes/macrophages, granulocytes, dendritic cells
FcaR contributes to pathogen destruction by
Triggering ADCC and phagocytosis & stimualates myeloid cells to release inflammatory cytokines and generate superoxide free radicals to help kill internalized pathogens
FcuR & Fca/uR recently described. What are they expressed on?
FcuR- B, T, & NK cells - regulates cell activation
Fca/uR - B cells, macrophages, and follicular dendritic cells - binds IgM with high affinity (induces phagocytosis) and IgA with intermediate affinity
PolyIgR - polymeric immunoglobulin receptor that is expressed by epithelial cells and initiates transport of
IgA and IgM from blood to lumen of multiple tracts (GI, Resp, and Repro).
Responsible for carrying Ab into tears and milk and populating gut mucosa with IgA to protect against ingested microbes and toxins
