The Humoral Immune Response Flashcards
What stage of antigen independent B-cell development does positive selection for a good BCR occur in?
Pre-B cell stage
What stage of antigen independent B-cell development is characterized by B-cell tolerance?
What are the three features of B-cell tolerance?
Immature B-cell stage.
- BCR editing
- Deletion
- Anergy
What antigen dependent events occur in the paracortex to help the B-cell differentiate?
- APC presents the antigen to a T-cell
- T-cell proliferates and differentiates
- T-cell interacts with B cell (CD40/40L) to provide “signal 2” to the B cell
- The B-cell receives cytokines and either:
a. ) forms a Primary focus to release Ab
b. ) migrate to the cortex of the LN or spleen to enter a follicle for further differentiation
What events occur in the dark zone of the germinal center?
What events occur in the light zone?
B-cells enter the germinal center from the paracortex and in the dark zone:
- clonally expand
- somatic mutation (AID dependent)
In the light zone:
- FDC helps select the most fit B-cell
- B-cells go through class switching
What are the four major differences between the 2nd and 1st response to an antigen?
- 2nd is quicker
- more antibody is made
- IgG>>IgM
- The antibody has higher affinity for the antigen
The primary IgM response arises from the activation of ___________________.
During the lag phase, what three major events occur?
virgin (naive) B-cells
Lag Phase events:
- B-cell recognizes antigen
- Clonal expansion of B-cell
- Differentiation in the primary focus
Because in the primary response, the B-cells differentiate in the primary focus rather than the germinal center, what will be missing from the IgG cells?
Affinity maturation (no somatic rearrangement or FDC “survival of the fittest”)
What is the time line of IgM titer after first antigen exposure?
7 days- IgM appears
10 days- IgM peaks
14 days- IgM returns to baseline
You take a serum patient sample and the IgM titer is higher than the IgG titer. What information does this give you about the infection?
It is a primary infection (first exposure to this antigen) and it is a very RECENT INFECTION (IgM develops before IgG)
What B cells are critical for fueling a secondary immune response?
Where are they located?
Memory B-cells (class-switched and mutated)
They are located in the marginal sinus or subcapsular sinus
Which antibody increases quicker upon secondary exposure to an antigen? Why?
IgG predominates and arises faster because memory B cells have higher affinity IgG BCR.
What two factors cause the secondary immune response to be quicker and stronger?
- Memory B-cells are in the marginal sinus so they will encounter the antigen faster
- Memory-B cells have reduced activation threshold (don’t need two signals like naive B-cells)
What type of dendritic cell is found in the paracortex of the LN?
What type of DC is found in the cortex?
Interdigitating DCs are in the T-cell zone to present antigen to the T-cells
FDC are in the B-cell follicles in the cortex of the LN
What are the three products of a successful immune response?
- Effector T- Cells
- Memory T and B cells
- Plasma cells and antibodies
What determines whether a cell will be a B-cell or a plasma cell?
RNA splicing events because there are (s) and (M) components that stand for secreted or membrane bound.
If the (s) exon is spliced next to the VDJ and Fc it will be secreted
What homing receptors on activated B and T cells will direct them to the inflamed tissue?
VLA4 to VCAM1 on the inflammed tissue
LFA1 to ICAM1 on the inflammed tissue
The presence of what cell allows the Memory B-cell to sample invading antigens?
Macrophages are in the subcapsular and marginal sinus so they can engulf and present antigens to the memory B-cells
Where would one find short-lived plasma cells?
Where do they secrete antibodies?
medullary cords of the LN or red pulp of the spleen.
They secrete antibodies into the efferent lymphatics
When plasma cells, Ig and effector lymphocytes flow out the efferent lymphatics, where do the plasma cells go?
What type of cell do they become and where do these cells secrete their antibodies?
They go to the bone marrow where they become long-lived plasma cells. They secrete Ig into the blood, contributing to serum levels of Ab.
In order for BCRs to generate a signal, what needs to happen?
Two or more adjacent IgM and/or IgD need to be crosslinked by the antigen
What kind of antigen is best for crosslinking on adjacent BCRs?
- Aggregated antigens
- Antigens folded in native conformation
What are the steps involved in BCR activation? (8)
- Adjacent BCRs use crosslinking to bind the antigen
- ITAM motifs of the CD79 (Iga and Igb) get phosphorylated
- The phosphorylated ITAM recruits Syk
- Syk phosphorylates several molecules via BTK intermediate
- BTK (via BLNK) phosphorylates and activates PLCgamma2
- PLC generates DAG and IP3
- IP3 and DAG activate calcium, calcineurin, NFAT, PKC, NFKb, MAP kinases
- These activate TF that regulate cell division, differentiation, co-stim, cytokines
What is the role of signaling via the B-cell coreceptor?
To amplify the BCR-mediated signal
Antigen coated with _______________ has the potential to engage a key B-cell coreceptor, ___________________ also known as ______________________.
- complement C3d
- CD21
- complement receptor 2 (CR2)
When C3d binds the CD21 on the B-cell, what other component of the B-cell coreceptor is phosphorylated and activated?
CD19 which recruits PI3K, VAV, Fyn and Lyn
What does CD19 do after it is phosphorylated?
recruits signaling molecules: VAV, Fyn, Lyn and PI3K
What recruits PI3K, VAV, Fyn and Lyn to the B-cell?
What do these signaling factors do?
They are recruited by CD19 and augment phosphorylation of ITAMs
Co-ligation of the BCR and CD21 complex has the net effect of:
amplifying signal 1.
(signal 2 is NOT involved. signal 2 for the B cell is the interaction between CD40/40L with the T cell)
What are two mechanisms by which the activated B-cell can be shut off or ramped down?
- The removal of antigen removes ligand for the BCR so this will ramp down B-cell activation
- CD32B (FcRgammaIIb) and CD22 are molecules on the surface of B cells that can transmit a negative signal by activating phosphatases in the cell to dephosphorylate key signaling molecules
Signal delivered by ________ to ___________ constitutes signal 2 for B-cell activation. Absence of the ligand, _____ results in ____________________syndrome because:
- CD40L on the T cell
- CD40 on the Bcell
- CD40L
- Hyper IgM Syndrome - because T cell/Bcell linked recognition is required for mature B cells to be activated and enter the germinal centers (where somatic mutation and class switching occur)
What two molecules help amplify signal 1 of B-cell activation?
CD21 reacts to antigen bathed in complement (C3d) and phosphorylates CD19 which activates Fyn and Lyn, PI3k and VAV to help phosphorylate ITAMs.
What molecules dampen the BCRs signal by binding CD32B (FCgammaRIIb)?
Ab+Antigen immune complex
What molecule on the B-cell delivers signal one to the T-cell?
MHC/peptide complex
What structure on the B-cell delivers signal 2 to the T-cell?
CD80/CD86 (B7-1 and B7-2) on the B-cell engage with the CD28 of T-cells
What are the three major characteristics of T-dependent B-cell maturation?
- memory
- class-switching
- affinity maturation (somatic mutation, FDC survival of the fittest)
What are the two types of antigens that can do T-indenpendent activation of B-cells?
- TI-1 ligands like LPS that can trigger B-cells polyclonally as a mitogen (not antigen specific)
- TI-2 ligands which are polymeric antigens that are able to cross-link specific BCRs to a high degree of crosslinking
TI-1 ligands like ____ trigger all B-cells ___________.
LPS; polyclonally
TI-2 lignads are _______________________ that cross-link specific BCRs to a high degree of cross-linking.
Examples include: (4)
polymeric antigens
- flagellin
- dextran
- Ficoll
- *** bacterial polysaccharides
Can TI-1 and TI-2 antigens make germinal centers?
No because to make a germinal center there must be linked recognition by the T-cell.
They are able to make primary centers though so they can make IgM and IgG but there is no class-switching or somatic mutation
Where are TI-2 B-cells mostly induced?
- marginal zone
- In B1 cells
What type of receptor is engaged on the B-cell by a TI-1 antigen?
What type of cell does it turn the B-cell into?
TLR which results in the differentiation of the B-cell into an IgM-secreting cell
What does the TI-1 antigen contain that allows it to activate the B-cell without a T-cell?
What does the TI-2 antigen have that allows it to activate the B-cell?
- a mitogen
- it is polyvalent so it can crosslink BCR to a high degree
What do TI-2 antigens still require in order to initiate optimal antibody response?
Cytokine release from T-cells
What surface molecule is expressed by B1 cells?
Where are B1 cells usually located?
CD5
They are expressed in the peritoneal and pleural cavities (primary B-cell found in newborns)
What are the benefits of the T-cell independent B-cell activations?
- FAST production of IgM
- agglutination of antigen
- Binding of complement
What are the major targets for drug development in B-cell activation?
Syk, BTK, PI3K, NFK-b, MAP kinases
What two drugs target calcineurin?
Cyclosporin and FK506
What is Bruton’s disease also called?
What gene is mutated in this disease?
What cell levels are normal?
What cell levels are depleted?
X-linked agammaglobulinemia
Mutated BTK gene
Normal Pre-B cell count
Low activated B-cells and serum Ig
What are three bacteria that operate as TI-2 antigens?
Why are they T-independent?
Who is most susceptible to infection by TI-2 antigens?
- Strep pneumonia
- Hemophilis influenza
- meningococcal polysaccharides
They are unable to be presented by MHC molecules so they can’t depend on T-cells
Children up to 2 years old and people who have had splenectomy are most susceptible because they lack marginal zone B cells
How should patients who have had a splenectomy (lack marginal zone) be immunized against TI-2 antigens?
Conjugate the pathogen to a protein that can bind non- marginal zone cells be internalized and proteolyzed and presented on the MHC
