Lecture 15 Flashcards
In a humoral immune response, explain the steps that occur with B cells, beginning with precursor B cells and ending with a long lived plasma cell.
Precursor cells undergo VDJ recombination to become Immature B cells (can now express IgM)
Immature B cells go to the spleen where they become mature B cells (express IgM still)
Mature B cells react to an antigen and become a lymphoblast
Lymphoblasts either become short lived plasma cells (secrete IgM) or go the GCs
Lymphoblasts in GCs undergo hypermutation/antigen selection to become either a memory B cell or a long lived plasma cell
After a B cell has been activated by an Ag add undergoes class switching in a GC, what are the 4 Ig’s that it may now express?
IgA
IgG
IgE
IgD
“AGED mnemonic”
What has to occur to a B cell so that it may form the secreted form of IgM? explain this process.
B cell activation
Alternative processing of a primary RNA transcript must occur to splice out the transmembrane domain of the RNA transcript
(it can now form membrane bound or secreted IgM)
Cu1, Cu2, Cu3, and Cu4 are the four exons of the Cu gene. Which of these will create a membrane bound Ig and which will create a secreted Ig?
Cu1 creates a membrane bound Ig
(resting B cell)
Cu4 creates a secreted Ig
(B cell differentiation)
Compare mIg to sIg and explain what type of cell makes them.
mIg = membrane bound Ab (BCR)
formed by activated B cells that have recognized an antigen
sIG = secreted Ab
formed by Plasma cells (B cells) that have bound to an Ag and begun effector functions
Describe the process of affinity maturation in terms of what types of Ags it occurs with, where it occurs, and how it occurs.
occurs ONLY in TD (thymus dependent) Ags (protein Ags)
occurs in the GC
Somatic hypermutation of the “Ig V genes” occurs and Abs with high affinity are selected (apoptosis if they do not eventually increase affinity)
What 2 interactions in the GC are required for somatic hypermutation to occur in the process of affinity maturation? what presents the Ag so that the Ab mutations can be tested for their affinity to the Ag?
Tfh cells and CD40:CD40L interactions
Follicular DC’s (FDCs) present the Ag in the form of ICs (Immune Complexes)
Explain where clonal expansion fits into the process of Affinity maturation in the GC. Also explain what is going on in the dark and light zones of a GC during affinity maturation.
The Dark zone is full of proliferating B cells (clonal expansion) that are being mutated in hopes to increase their affinity
(mutations cannot occur without a cell division so that is why clonal expansion occurs here)
The light zone is where FDCs are testing the newly mutated B cells and selecting those with the highest affinity for the Ag
(FDCs give survival signal to B cells that bind to the Ag and high affinity B cells are more likely to bind)
Describe AID in terms of what cells produce it, when, and it’s function. (be specific)
Also include a description of the other 2 enzymes that are involved in this process that eventually allows mutations to occur.
AID is expressed by B cells in GCs to initiates somatic hypermutation of the Ag binding site
it usually induces a cytosine to uracil deamination point mutation that is resolved by base excision and mismatch repair (error prone to make mutations on purpose)
UNG (uracil N-glycosylase) removes the U residues to generate an “abasic site”
APE1 then nicks this abasic site to create double DNA stand breaks that allow mutations to occur
True or False:
During the hypermutation that occurs during B cell affinity maturation, both the heavy and the light chain Ig genes can be mutated. explain.
True
both the heavy and light chains account for a portion of the Ag binding portion of the Ab, so they can will both get mutated
The affinity maturation process is cyclical in nature. What happens approximately once each “round” of this process? about how many AA substitutions can occur in a single clone of a B cell?
1 mutation of the V region of the Ig gene accumulates per round
up to 10 AA substitutions
True or False:
There are equal amounts of mutations in B cells that mutate to become IgG, compared to B cells that remain IgM. explain.
False
There are FAR MORE mutations in B cells that mutate to become IgG and these mutations correlate with the increasing affinity of IgG compared to IgM
(IgM does not need a high affinity because it has so many binding sites. IgG does not have than many binding sites)
In the GC, during the Affinity maturation process, describe the 2 different survival signals that B cells must receive in order to avoid apoptosis (these are the signals that do not originate from within the B cell)
- The B cell must bind with the Ag displayed on the FDC for the FDC to give it a survival signal
- Tfh cells will give a survival signal via it’s CD40L to B cells with high affinity for the Ag
When a B cell recognizes an Ag, describe the 2 survival mechanisms that B cells with high affinity express from within when undergoing the affinity maturation process.
- expression of Bcl-2 anti-apoptotic proteins
2. expression of endogenous inhibitors of Fas (if this does not occur, the Fas will induce apoptosis)
State the 2 pathologies associated with the hypermutation and isotype switching that occurs to B cells in the GC. (include a description for the mechanism that causes them)
B cell Lymphomas caused by chromosomal translocation of an oncogene into an Ig gene loci
Autoimmunity disorders caused by the creation of a self-reactive B cell clone
Which part of which chain determines the isotype that the Ig will become after it undergoes isotype switching? What region determines the Abs specificity to Ags? what are the 2 possible Ag types that the B cell has BEFORE isotype switching occurs?
The constant region of the heavy chain
The variable regions (on both the heavy and light chain)
IgM or IgD (usually a mix of both)
Describe where isotype switching primarily occurs and where it occasionally occurs. Include which cells are responsible in for the process of isotype switching the B cell’s Ag in these respective areas.
It primarily occurs in the GC, driven by Tfh cells
SOME isotype switching occurs in the extrafollicular foci, driven by extrafollicular Th cells
Each class of Abs mediates distinct _____ functions. After a helper T cell signals the B cell via it’s CD40L, what other signal tells the B cell what type of Ig class it will become?
effector
Cytokines secreted by the Tfh cells that were activated by the Ag (IFN-gamma, IL-4, TGF-beta/APRIL/BAFF)
State the type of Ig class that will develop from a B cell that has been activated by a helper T cell and is then exposed to the following cytokine.
None:
IFN-gamma (Th1):
IL-4 (Th2):
TGF-beta/APRIL/BAFF:
None: IgM
IFN-gamma (Th1): IgG subclasses (IgG1, IgG3)
IL-4 (Th2): IgE, IgG4
TGF-beta/APRIL/BAFF: IgA
True or False:
Anatomical location plays a big factor in determining what Ig isotype an activated B cell will become. explain.
TRUE
local cytokines present in different anatomical locations are what signal activated B cells to become a certain isotype that is needed (ex. IgA in mucosal tissues)
Resident tissue macrophages play a HUGE role in secreting these local cytokines (alongside DCs and Treg cells that also secrete them)
Isotype switching cannot occur without a functional CD40/CD40L interaction between a B cell and T cell. Why is this so? Describe the arrangement of C and S genes in mature B cells and how they behave during CSR (class-switch recombination).
CD40 engagement induces AID in the B cell
AID (activation-induced deaminase) is what carries out the mutations that cause isotype switching and affinity maturation
every C gene (this determines the Ag isotype) is preceded by an S gene (“switching”) that is the site of “looping out” deletions that occur during isotype switching via alternative splicing
What is a germline transcript and how is it involved in the process of CSR? State the 2 Ig types the B cell has before CSR occurs and the 3 possible Ig types it may have after CSR occurs.
germline transcripts sequences near the S region that serve to open the chromatin structure so that the S regions may be looped out and deleted
B cells initially express IgM/IgD and after CSR can either express IgE, IgA, or IgG
Compare short lived plasma cells with long lived plasma cells in terms of the location they are generated, if they are TD or TD responses, and where they go to secrete Abs after they are formed.
Short lived plasma cells: are TI responses
Generated in the extrafollicular compartments of the LN
Move to secondary lymphoid organs or peripheral non-lymphoid tissues to secrete Abs
Long lived Plasma cells: are TD responses
Generated in the GC of the LN
Enter circulation and home to the Bone marrow to secrete Abs
Compare the signals that induce the formation of short lived and long lived plasma cells
short lived: B cell activation, BCR, CD40, TLRs and cytokines
long lived: B cell activation, BCR, and IL-21
These are maintained by signals from BAFF (this is why they live so long)
What is the difference between a plasmablast and a long lived plasma cell? which of these 2 cell types secretes almost 50% of all Abs in the blood?
plasmablasts are generated in the GC and enter circulation, headed for the bone marrow
Once they reach the bone marrow, they become long lived plasma cells
(these are basically the same cell, just at different points in it’s development)
Long-lived plasma cells secrete up to 50% of the Abs in the blood (makes sense bc they spend a lot of time as a long lived plasma cell, compared to the time spent being a plasmablast)
True or False.
Plasma cells may continue to secrete Abs for months or years, even after the Ag is no longer present. explain.
True
stay ready ain’t gotta get ready lol
What happens if a long lived plasma cell does not get exposed to BAFF?
it will die via apoptosis (no inflammation during apoptosis)
Describe the developmental pathway beginning with a Pre-B cell in the bone marrow and ending with a long lived plasma cell/memory cell.
Pre-B cells leave the bone marrow as Naive B cells (IgM)
Naive B cells enter either the spleen or the LN
In the spleen, Naive B cells become marginal B cells and eventually become short lived plasma cells
In the LN, Naive B cells react with an antigen and become a follicular B cell (Different Ig is chosen here) (can become a short lived plasma cell from this point)
Follicular B cells can then become either long lived plasma cells or memory cells from this point on
Long lived plasma cells will re-circulate to the bone marrow where they continue to secrete Abs
Memory cells are generated in the GC for ___(TD or TI) Ags? Explain how Memory B cells manage to survive for so long without Ag stimulation.
TD (protein Ags)(If it were a TI Ag, it would become a normal short lived effector cell, not a memory cell)
Memory B cells express high levels of Bcl-2 (anti-apoptotic protein)
State the 2 types of Ags that are expressed by Memory B cells. Why is Ab production so greatly accelerated in memory B cells upon a secondary activation with its specific Ag?
High affinity BCRs
Ig molecules of switched isotypes (so anything other than IgM/IgD)
The Abs production (reaction to the Ag) is so accelerated bc the memory cells have already been activated by this antigen in the GC (basically, they are not naive).
State the 2 places that memory cells can be found
- some remain in the lymphoid organ that they were generated in
- others exit GCs and recirculate between the blood an the lymphoid organs
Describe the Ab responses to TI Ags in terms of the affinity and type of Isotypes that can be formed. Describe what type of Ag will elicit a TI response.
These Abs (produced in the absence of Th cells) have a low affinity
IgM is the main isotype, however IgG subtypes can also be achieved
Describe what it means to say that most TI Ags are Multivalent. Explain how this can activate a B cell without the help of a T cell
Multivalent: composed of repeated identical Ag epitopes
Multivalent Ags cause “cross-linking” of the BCR complex, which induces enough stimulation to activate the B cell without a T cell helping.
Which subset of B cells will usually respond to TI Ags in the peritoneal cavity, and mucosal sites? Which subset of B cells will respond to polysaccharides in the spleen and eventually differentiate into short lived plasma cells that produce IgM?
B-1 B cells
MZ B cells
Explain the interaction that occurs in the spleen between the MZ Macrophages and B-2 Cells. What is a main difference between this presentation and the presentation of other Ags via MHC molecules?
MZ macrophages present TI Ags on their surface for long periods of time
TI Ags cannot be processed and presented like TD Ags (so they are simply stuck to the surface in order to be presented)
What are the 2 types of costimulation that can occur in order to potentiate B cell activation? What is another type of activation that non-microbial Ags are able to induce?
CR2/CD21 and PAMPS recognized by TLRs
non-microbial Ags can activate the complement system
_____ produced by non-T cells may stimulate isotype switching in TI responses. What is the dominant Ab class induced by pneumococcal capsular polysaccharide (TI)?
cytokines
IgG2
In a TI response what will the following cytokines stimulate the B cells to do?
TGF-beta:
BAFF:
TGF-beta: causes a switch to IgA
BAFF: induces the synthesis of AID (also a survival signal)
_____immunity is the major defense against encapsulated bacteria. which individuals are especially susceptible to pneumococcal, meningococcus, and haemophilus infections?
Humoral
Individuals with immunodeficiencies of humoral immunity
Describe Natural Abs in terms of the type of Ags they respond to, where in the body they are present, and what type of exposure produces them.
TI Ags (they have a low affinity for non-carbohydrate Ags
They are present in the circulation
They are made WITHOUT overt exposure to pathogens
Natural Abs are thought to be produced by bacteria that is already living in the body (non infectious pathogens). In the GI tract what cells make these and in the spleen what cells make these?
peritoneal B1 cells
MZ B cells (B2 cells)
Describe Conjugate Vaccines in terms of what pathogen they are for and their structure. While conjugate vaccines and traditional vaccines will allow a person to mount a large secondary response to a pathogen, what is the main process that does not occur in conjugate vaccine secondary responses?
Conjugate vaccine: vaccines for capsular polysaccharide pathogens that are basically a polysaccharide linked to a foreign protein to form a hapten-carrier conjugate
There is no isotype switching/affinity maturation in a conjugate vaccine secondary response
FcgammaRIIB serves as a physiologic control mechanism on B cells. Explain the mechanism by which this occurs and what type of diseases this prevents.
If an Ag-Ab complex SIMULTANEOUSLY binds to the BCR (activates if this is the only thing bound) and the FC portion of FCgammaRIIB interacts with the FC receptor, then the B cell activation is inhibited
This prevents uncontrolled Ab production, which prevents diseases such as lupus
