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39 Cards in this Set

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Where does antigen independent B cell development take place?
The bone marrow
Where does antigen dependent B cell development take place?
Lymph nodes and the spleen
What is involved with antigen independent B-cell development?
The generation of B cells in the bone marrow
What is involved with antigen dependent B cell development?
-Ig class switch, somatic hypermutation
-Germinal center reaction
What three things happen in antigen-independent B-cell development?
1. DNA rearrangements to establish the primary repertoire, creating diversity

2. Allelic exclusion, ensuring that each clone expresses a single antibody on the surface, establishing specificity

3. Deletion of self-reactive clones to establish immunological tolerance
What is allelic exclusion?
Allelic exclusion means that you have two alleles, but a B cell cannot make 2 antibodies, so the expression of the other allele has to be excluded. This is the same for the heavy and light chains.
Which chain is made first?
The heavy chain
Explain heavy chain rearrangement
The heavy chain is rearranged first with the rearrangement of the D to the J element. These elements are juxtaposed and from there no functional antibody can be expressed. Next the V segment is joined to the DJ segment. At this stage it is possible that a functional heavy chain protein can be produced. If this is the case then the heavy chain protein associates with surrogate light chains (VpreB and lambda 5). These resemble a light chain in their structure. They are required to express the heavy chain on the surface so that the cell knows that the heavy chain is functional. If this is the case then heavy chain rearrangements stop and light chain rearrangements progress.
What is the pre-B cell receptor composed of?
Membrane heavy chain
Lambda5, VpreB (surrogate LC)
Igalpha, Igbeta signaling components
What defines a "productive rearrangement"?
When a protein can be expressed
What happens when a productive rearrangement occurs in the HC?
-HC rearrangement stops
-There is proliferation
-LC rearrangement begins
What happens if there is a non-productive rearrangement in the HC?
The other allele attempts to rearrange. If this is successful the productive rearrangment path occurs. if this is unsuccessful the cell dies.
Explain light chain rearrangement
With the light chain we have more possibilities to get a functional light chain because there is no D segment. That means that if you have a VJ recombination that leads to a frame shift, the locus has an opportunity to generate a second rearrangement of the light chain and then a third until a light chain is successfully made. This improved the efficiency of the process.
There are two light chain loci. The kappa locus rearranges first. If the kappa gene doesn’t work, then the lambda chain is rearranged with the same method.
Where are the two checkpoints in BCR development? What do these checkpoints confer?
The first check point is the formation of the pre-BCR and the other is the BCR. These checkpoints confer allelic exclusion.
How does allelic exclusion occur?
There is signaling either through pre-BCR or BCR.

There is reduced expression of RAG genes, those gene products which are necessary to rearrange the immunoglobulin segments. There is evidence that these proteins are actively removed from the system to prevent further rearrangements. Then the locus is shut down by an unknown mechanism.
What is the BCR composed of?
Membrane HC
Kappa or lambda LC
Igalpha, Igbeta signaling molecules
What is central tolerance? What is the main mechanism?
-Tolerance to self antigens that is established in lymphocytes developing in central lymphoid organs.
-Main mechanism: clonal deletion
What is peripheral tolerance? What is the main mechanism?
-Tolerance to self antigens that is establised in lymphocytes in the peripheral tissues
-Clonal deletion, anergy, clonal ignorance
What is clonal ignorance?
B cells can be clonally ignorant if the binding of the antigen is not strong enough to crosslink the antibody. The antigen Is there and it binds, but it is not high so the cell just ignores the antigen.
When are B-cells signaled to become anergic?
When they are self-reactive to soluble self antigens.
Where does the B cell interact with antigen?
T-cell zone
Where do B-cells go after they recognize antigen?
The "dark zone" of the germinal center and eventually they go to the light zone where B cells that react very well to the antigen are selected and eventually these cells go to the subepithelium. This is a zone where mature B cells with mature antibody go to.
Where do B cells with high affinity antigen receptors generated?
The germinal center (GC) reaction
What is involved with antigen-dependent B-cell development?
1. T-cell dependent activation of antigen-specific naive B cells, the precursor of the GC reaction

2. Somatic hypermutation and Ig Class switch during the GC-reaction generates high-affinity antigen-specific B cells with specialized effector functions

3. Differentiation of antigen-selected GC B cells into memory B cells and plasma cells, the carriers of antibody-dependent (humoral) immunity
What occurs in the T-cell zone?
Antigen recognition induces expression of effector molecules by the T cell, which activates the B cell
What occurs in the GC dark zone?
B cell proliferation and somatic hypermutation
What occurs in the GC light zone?
Differentiation to resting memory cells and antibody-secreting plasma cells
Describe somatic hypermutation
-Random mutagenesis, mostly single base changes
-Limited to V(D)J (does not extend to the C regions)
-Hypermutation is 10^6 more than normal mutation rate (10^-3/bp/generation compared to 10^(-9)/bp/generation)
-Occurs only in mature, antigen-activated B-cells
-Combined with selection, results in clones making antibodies with increased affinity for antigen (Affinity maturation)
What type of mutation occurs in SHM?
Single base pair substitutions
What DNA cuts occur with SHM?
This process is associated with DNA strand breaks. In the process, sometimes both strands of DNA are cut by enzymes.
What is the distribution of SHM mutations?
Distribution with in ~2kb of the promotor
Describe affinity maturation
Increase in the affinity for the specific antigen of the antibodies produced during a humoral immune response
-Particularly prominent in secondary (memory) immunizations
Describe Class Switch Recombination
Class switch recombination is a DNA rearrangement that allows the same VDJ to be expressed with different heavy chain constant regions.

You begin with IgM and IgD that are expressed together on the cell surface. Shown here there is an enzyme called activation induced deaminase. The process has similarities to somatic hypermutation and the VDJ recombination in the bone marrow. Eventually the DNA is cut and the VDJ exon is brought together to a Cgamma1 exon to make an antibody with the same specificity for the antigen, but a different constant region with different effector functions.
How do secreted antibodies function in various ways to eliminate foreign invaders?
Neutralization
Opsonization
Component activation
What occurs with self reactive B cells formed by SHM?
During the somatic mutation there is the potential for self reactive B cells to form. The germinal center reaction is controlled by the T cells, which are tolerant against self. If a new B cell receptor is made that recognizes self, it is counter selected by the T cells.
Explain how SHM selects for better binding?
This B cell then undergoes somatic hypermutation in the dark zone and in the light zone is then selected for better antigen binding. This B cell has a lower affinity for the antigen and then can no longer see the antigen then can no longer present the antigen to the T cell. This cell is counterselected and just dies. If you have a better affinity to the antigen it then binds to the antigen, BCRs crosslink, and then eventually presents the antigen to the T cell, gets a signal from the T cell, then has the potential to differentiate into a memory B cell or a plasma cell.
When do plasma cells start to develop?
Day 7
Are B cells long or short lived?
Long
Do memory B cells secrete antibody?
No. The memory B cells have only surface immunoglobulin and they just wait for the antigen to enter the body again.