Handout 12 part 1: Differentiation and Functions of CD8+ T cells Flashcards
How is the activation of B cells initiated?
by specific recognition of Ags by the surface BCRs
How does a B cell proliferate–what stimulation helps it proliferate and differentiate?
Ags and other stimuli like T helper cells stimulate the proliferation and differentiation of the specific B cell clone
After a B cell proliferates, what types of cells can it become?
Can differentiate into a plasma cell that produces IgM (secretes Ab)
cell that expresses IgG or other isotype (isotype switching)
High affinity Ig expressing B cell (affinity maturation)
Memory cells
Sequence of events in humoral immune respose
Recognition of Ag by Naive IgM+ or IgD+ B cell–>Activated B cell–>Proliferation which can become one of the following:
Plasma cell–>Ab secretion
IgG expressing B cell–>Isotype switching
High affinity Ig expressing B cell–>Affinity maturation
High affinity Ig expressing B cell–>Memory B cell
Where do mature Ag-responsive B cells develop? Do they need an Ag?
Develop from bone marrow; no they develop from bone marrow in the absence of Ag
How are humoral immune responses initiated?
by the recognition of Ags by specific BCR on B lymphocytes
How are B cells activated?
Ag binds to membrane IgM and IgD on mature, naive B cells and activates these cells
How do B cells proliferate into plasma cells and memory cells?
Activation leads to their proliferation which leads to clonal expansion, followed by differentiation which generates Ab-secreting plasma cells and memory B cells
How many Ab-secreting cells does one single B cell give rise to?
5000 Ab-secreting cells per week
How many Abs molecules are produced every day when humoral immune response is at its peak?
10^12 Ab molecules
What is heavy chain isotype (class) switching?
When activated B cells produce Abs other than IgM and IgD
What is affinity maturation?
when an activated B cell produces Abs that bind to Ags with increasing affinity which progressively dominate the developing humoral immune response
What two things determines whether a response is primary or secondary in humoral immunity?
The type and amount of Abs produced
Humoral immune response can be T dependent or T independent. Ab responses to what kind of Ags are T dependent?
Ab response to protein Ags require help of CD4+ helper T lymphocytes so proteins are T-dependent Ags
What kind of helper T cell facilitates the formation of germinal centers?
specialized type of helper T cell, called follicular helper T cell
Where are germinal centers generated? What occurs in this area?
generated in secondary lymphoid organs where many steps of T-dependent humoral immune responses occur
What must activated B cells differentiate into in T-dependent responses?
activated B cells must differentiate into Ab-secreting plasma cells
From where and to where do plasma cells migrate? What do they secrete?
Plasma cells migrate from germinal centers in the peripheral lymphoid organs to the bone marrow where they live for many years. They continuously secrete Abs that provide immediate protection
What kind of Ags are T-independent Ags?
Multivalent Ags are T independent and do not require Ag specific helper T lymphocyte. They are non-protein and have repeating epitopes like:
polysaccharides,
some lipids and
nucleic acids
T-independent responses are elicited by engagement of ____ and may be potentiated by signals from ___ ____ on the B cell
BCR, other receptors
What kind of B cells differentiate into memory cells?
B cells activated by protein Ags
Are memory T (B??) cells always active?
No, they survive in a resting state without secreting Abs for many years but mount rapid responses on subsequent encounters with the Ags
What kind of Ags initiate isotype switching and affinity maturation?
typically seen in humoral immune responses to protein Ags
What drives isotype switching and affinity maturation?
T-cell dependent signals
Primary and secondary Ab responses to protein Ags differ both qualitatively and quantitatively. What are some major differences between primary and secondary immune responses?
Primary response: Naive B cells stimulated by Ag becomes activated and differentiate into Ab-secreting cells that produce Abs specific for the eliciting Ag
Secondary response: elicicited when the SAME Ag stimulates memory B cells leading to production of greater quantities of specific Abs than in primary response
What is different in terms of the quantity and quality of response between primary and secondary response?
secondary response develops more rapidly and larger amounts of Abs are produced than in primary response
Does primary or secondary response have increased heavy chain isotype switching and affinity maturation to protein Ags?
Secondary response
Primary vs. secondary response graph
Primary: First infection (slower response): Naive B cell–>Activated B cell–>short lived plasma cells in bone marrow–>low-level Ab production
Secondary: Repeat infection (rapid response, high levels of Ab produced): Memory B cell–>Plasma cell–>Long-lived plasma cells in bone marrow–> Memory B cell
Primary vs. Secondary response:
Peak response: Smaller/larger
Ab affinity: high/low affinity
Induced by: what kind of Ag?
Primary response:
Peak response smaller
Ab affinity: lower average affinity, more variable
Induced by all immunogens
Secondary response:
Peak response larger
Ab affinity: higher average affinity (affinity maturation)
Induced by: mainly protein Ags
What kind of B cells respond to protein Ags and are therefore T-dependent Ab responses? Which kind of B cells are T-independent B cells?
Follicular B cells respond to protein Ags (T-dependent)
Marginal zone B cells in spleen and B-1 cells in mucosal sites mediate responses to multivalent Ags and are T-independent
Where are follicular B cells found?
In the spleen and in other lymphoid organs
Sequence of events for T-dependent B cell response
Follicular B cells in spleen/other lymphoid organ–>protein Ag and helper T cell–> T dependent isotype-switched, high affinity Abs; long lived plasma cells
Sequence of events for T-independent B cell response
Marginal zone B cells in spleen/lymphoid organ OR B-1 cell in mucosal tissues/peritoneal cavity–>polysaccharides, lipids, etc–>T independent, mainly IgM; short lived plasma cells
To initiate Ab responses, what must happen to the Ag?
It must be captured and transported to the B cell areas of lymphoid organs
Most mature naive B lymphocytes are what kinds of cells? What is another name for these cells?
They are follicular B cells which are also called reciruculating B cells
Where do Follicular B cells circulate? Where do they migrate from?
Follicular B cells constantly recirculate in the blood and migrate from one secondary lymphoid organ (spleen, lymphnodes, mucosal lymphoid tissues)
In secondary lymphoid tissues, follicular B cells migrate into the ___ ___ ___ of these tissues called ___
B cell zones
follicles
What chemokine guides the movement of follicular B cells into lymphoid follicles (B cell zone)?
chemokine CDCL13 secreted by follicular DCs which is the major stromal cell type in the follicle
CXCL13 attracts ___ _ ___ into follicles
naive B cells
Ag may be delivered to naive B cells in lymphoid organs in different forms and via multiple routes. How are most Ags from tissue transported into lymphoid organs?
Ags transported to lymph nodes via afferent lymphatic vessels that drain into subcapsular sinus of the LNs
Soluble Ags, Microbes and Ag-Ab complexes and large Ags are delivered to B cell zone of follicle in different ways. What are these differences?
Soluble Ags (smaller than 70 kD) reaches B cell zone of follicle and interact directly with specific B cells
Microbes and Ag-Ab complexes are captured by subcapsular sinus macrophages which deliver Ags to follicles
Large Ags are captured by resident DCs and transported into follicles where they can activate B cells
What receptor do follicular DCs express that plays a role in Ag presentation?
Follicular DCs express CR2 complement receptor so they can present Ags to follicular B cells
What do the complement receptor CR2 bind to?
In the spleen, Ags in immune complexes bind to the CR2 complement receptor
What transfers the immune complex-containing Ags to follicular B cells?
Marginal zone B cells
How are blood-borne pathogens delivered to marginal zone B cells?
They are captured by plasmacytoid dendritic cells (pDCs) in the blood and transported to the spleen where they may be delivered to marginal zone B cells
How are polysaccharide Ags captured?
They are captured by macrophages in the marginal zone of splenic lymphoid follicles and displayed or transferred to B cells in this area
Is the Ag presented to B cells processed by the APC?
No, the Ag presented to B cells is generally in its intact, native conformation and is not processed by APC
- Small Ags are delivered to B cells in the follicles through ___ ___ and via ____
- Larger Ags are delivered by ___ ___ ___ or by __ in the ___
- afferent lymphatics, conduits
2. subcapsular sinus macrophages, DCs in medulla
Follicular B cell survival depends on signals from the BCR as well as on inputs received from cytokine called ___
BAFF (B cell-activating factor of the TNF family)
What cells produce BAFF? Where?
BAFF is mainly produced by myeloid cells in lymphoid follicles and in bone marrow
What is the function of the BAFF?
It provides maturation and survival signals through the BAFF receptor
How is the Ag-specific B lymphocyte activation initiated?
by the binding of Ag to membrane Ig molecules which together with the associated Iga and IgB proteins make up the Ag receptor complex of mature B cells
What initiates the process of B cell activation? What else occurs at the same time?
BCR
BCR internalizes the bound Ag into endosomal vesicles
If the Ag is a protein, how it it processed and presented?
It is processed and presented within class II MHC on the B cell surface for recognition by T helper cells
B cell activation is facilitated by the ___/___ coreceptor on B cells
CR2/CD21
Simultaneous ____ recognition and ___ signaling may contribute to B cell activation
PAMP, TLRs
How is the activation of B cells through BCRs enhanced?
by complement-coated Ags that can ligate both the BCR and complement receptor 2 (CR2)
How are non-microbial polysaccaride Ags able to induce Ab response without T cell help?
by activating complement
What are the cellular responses induced by Ag-induced cross-linking of BCRs?
- production of proteins that promote survival and proliferation, expression of costimulators and cytokine receptors that promote interactions with and responsiveness to helper T cells
- Migration of the cells toward T cells due to expression of CCR7
Ag binding to and cross-linking of memrane Ig–>Changes in activated B cells–>Functional consequences:
Naive B lymphocyte recognizes Ag–>Expression of proteins that promote survival and cell cycling–>Result?
Increased B cell survival, proliferation
Ag binding to and cross-linking of memrane Ig–>Changes in activated B cells–>Functional consequences:
Naive B lymphocyte recognizes Ag–>Ag presentation, increased B7 expression–>Result?
Interaction with helper T cells
Ag binding to and cross-linking of memrane Ig–>Changes in activated B cells–>Functional consequences:
Naive B lymphocyte recoginizes Ag–>increased expression of cytokine receptors–>Result?
Responsiveness to cytokines
Ag binding to and cross-linking of memrane Ig–>Changes in activated B cells–>Functional consequences:
Naive B lymphocyte recognizes Ag–>increased expression of CCR7–>Result?
Micration from follicle to T cell areas
In T cell-dependent responses, immune responses are initiated by the recognition of Ags by B cells and _____
Then, the activated B cells and T helper cells migrate towards each other and interact causing what?
CD4+ T cells
proliferation and differentiation
B-T cell interaction in extrafollicular sites leads to ___ ___ and __ __ ___ __ ___
isotype switching
short-lived plasma cell generation
Activation of T cells by B cells results in the induction of ______ ___ ___
follicular helper T-cells
The late events in T cell dependent responses occur in germinal centers. What happens in the germinal center?
somatic mutation and affinity maturation, isotype switching, generation of memory B cell and long-lived plasma cells
How are B cell Ags presented to T cells?
Protein Ags recognized by membrane Ig are endocytosed and processed, and peptide fragments are presented within class II MHC molecules. Then, helper T cells recognize MHC-peptide complexes on B cells and stimulate B cell response
What happens after the hapten (B cell eptiope) is recongized by a specific Bcell?
The hapten carrier conjugate is endocytosed, carrier protein is processed in B cell, peptides from carrier (T cell epitopes) are presented to helper T cell
Sequence of events for B cell Ag presentation to T cells
Receptor (conformational epitope-specific Bcell receptor) mediated endocytosis of Ag–>Ag processing and presentation within class II MHC peptide complex–> T cell recognition of Ag
What ligand do helper T cells that are activated by Ags presented by B cells express?
CD40L
What does CD40L bind to?
CD40L (expressed by helper T cells) binds to CD40 on B cells and stimulates B cell proliferation and differentiation
Aside from the CD40-CD40L binding, cytokines produced by what kind of cell contributes to B cell responses?
Cytokines produced by helper T cell
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
slide 26
The germinal center is within the ___ and includes a __ and __ zone
follicle
dark, light
The dark zone contains what kinds of cells? The light zone contains what kind of cells?
Dark zones contains proliferating B cells (can stain with anti-Ki67 Ab thats red to detect cycling cells)
Light zone contains follicular DCs which can be stained with an anti-CD23 Ab
How is the dark zone formed?
Activated B cells migrate into the follicle and proliferate, forming the dark zone
Which B cells undergo extensive isotype switching and somatic hypermutation of Ig V genes?
Activated and proliferated B cells in the DARK ZONE
What happens after the B cells migrate from the dark zones into the light zones?
They encouter follicular DCs displaying Ag and T follicular helper cells
How is it determined which B cells survive and differentiate?
B cells with the highest affinity Ig receptors are selected to survive and they differentiate into Ab secreting cells and memory B cells
Where do the Ab-secreting cells then go after being in the germinal center?
They leave and reside in the bone marrow as long-lived plasma cells, and the memory B cells enter the recirculating lymphocyte pool
Germinal center sequence of events:
Activation of B cells by T helper cell and migration into germinal center–>B cell proliferation__>somatic mutation and affinity maturation; isotype switching–>Exit of high-affinity Ab-secreting cells and memory B cells
When and how do T cells differentiate into Follicular helper T cells?
Within 4-7 days of Ag exposure, activated Ag-specific B cells induce some previously activated T cells to differentiate into Follicular T helper cells
What cytokine plays a critical role in drawing Tfh cells into lymphoid follicles and in germinal center formation and function?
CXCL13
What costimulators, cytokines and transcription factors do follicular helper T cells express?
ICOS (inducible costimulator)
PD-1 (programmed death-1)
cytokine IL-21
transcription factor Bcl-6
Follicular Helper T cells are distinct from which other T cells?
Distinct from other T cells like Th1, Th2, Treg, and Th17 subsets of effector T cells
Tfh cells play important roles in the ____ and ___ of B cells in the germinal center. What cytokines and receptors/ligands does it use?
activation, differentiation
uses: ICOSL, CD40L, IL-21R on B cells
The generation of follicular helper T cells requires the sequential activation of T cell–first by ___, and then by ___ ___. The Tfh cells migrate to ___ where they activate B cells
DC, activated B cells
Germinal centers
What cytokines do Tfh cells secrete? What do each of these cytokines do?
secretes IL-21 neededfor GC development and generation of plasma cells
IFN-gamma or IL-4 which controls isotype switching
