APCs & T cells Flashcards
Where does T cell activation occur? Why?
Secondary lymphoid tissue only
Greater chances of APC presenting to T cell (greater chance of contact)
What is required for naive T cell activation?
Recognition of cognate antigen on MHC
and
B7:CD28 costimulation
Why can naive T cells only be activated by APCs?
APCs are the only cells that express B7
All naive T cells leave the thymus and go to ___ ____ ___. They travel through __ __ __ to get into this area.
secondary lymphoid tissues
high endothelial venules
What happens to naive T cells if they are not activated after sampling MHC:peptides in secondary lymphoid tissue?
Leave that secondary lymphoid tissue and travel to another one. “This bitch is clean. Let’s bail.”
Once a naive T cell meets its MHC:peptide and co-stimulation, what happens to the T cell? In general….
T cell proliferates.
Differentiates into effector T cells, TH1 or TH2, or they become memory cells.
Selectins, such as L-selectin and P-selectin, bind to what? Where are these found?
Provide examples of these targets.
Bind to mucin-like vascular addressins (have carb moities)
HEV
CD34, PECAM-1 (CD31), GlyCAM-1,
What is CD31? What does it facilitate?
PECAM-1 (mucin-like vascular addressin)
Binds to selectins.
Facilitates movement of lymphocytes through endothelial cells into secondary lymphoid tissue
What is PECAM-1?
CD31 (mucin-like vascular addressin)
Binds to selectins.
Facilitates movement of lymphocytes through endothelial cells into secondary lymphoid tissue
What is GlyCAM-1?
mucin-like vascular addressin
also known as: sulfated sialyl-Lewis X
Binds to selectins.
Facilitates movement of lymphocytes through endothelial cells into secondary lymphoid tissue
What is CD34?
mucin-like vascular addressin that faciliates movement of lymphocytes via selectins (L-selectin) through HEV into secondary lymphoid tissue
What are some examples of addressins that facilitate movement of lymphocytes through HEV into secondary lymphoid tissue? What do they bind to on the lymphocyte?
CD34, PECAM-1 (CD31), GlyCAM-1
L-selectin or P-selectin
What is another name for sulfated sialyl-Lewis X?
GlyCAM-1
Integrins are expressed on?
Provide 2 examples.
What do they bind to?
lymphocytes
LFA-1 and VLA-4
immunoglobulin superfamily members: ICAMs and VCAM-1, respectively
LFA-1:ICAM is on naive T cells
VLA-4:VCAM-1 occurs on activated T cells
What are LFA-1 and VLA-4?
What do each bind to?
What is the difference between LFA-1 and VLA-4?
integrins on lymphocytes that helps tighten the attachment of lymphocytes to HEV in order to diapedese.
LFA-1 binds to ICAMs
VLA-4 binds to VCAM-1
LFA-1:ICAM interaction is between a naive T cell and HEV
VLA-4: VCAM-1 interaction is between activated T cell and activated endothelium.
Describe in detail the process of a naive T cell extravasating into secondary lymphoid tissue.
Naive T cells express chemokine receptors, and L-selectin.
Naive T cells are slowed down when its L-selectins roll on GlyCAM-1 (sialyl Lewis X) or CD34 that are present on high endothelial venules.
With this supplementing attachment, the T cell becomes closely associated with the surface of the HEV, where chemokines that are present between endothelial cells bind to chemokine receptors on the T cell.
This results in a signaling to the T cell’s nucleus to activate LFA-1 (integrin) on the surface. LFA-1 now binds ICAM-1 on the surface of HEVs WITH HIGH AFFINITY.
T cell diapedeses through HEV into secondary lymphoid tissue.
What would a genetic deficiency or an inhibitor of LFA-1 produce?
inability of T cells to closely adhere to HEV and consequently not be able to reach secondary lymphoid tissue.
When a T cell is active (Effector T cell), what “tool” does it use to extravasate into infected tissue? What receptor does this “tool” bind to in order to diapedese into infected tissue?
Effector T cells express the integrin VLA-4 that binds to VCAM-1 on infected activated endothelial cells (received signal to upregulate VCAM-1 from cytokines).
APCs are the only cells that express B7, but the expression is conditional. What does this mean?
Dendritic cells only express B7 after it phagocytoses antigen and after it reaches secondary lymphoid tissue.
Macrophages and B-cells induce B7 expression when their PRRs recognize PAMPS.
Dendritic cells are important APCs for what kind of determinants?
Macrophages are important APCs for what kind of determinants?
B cells are important APCs for what kind of determinants?
viral determinants. DCs are easily infected by viruses.
bacterial/fungal determinants due to the plethora of PRRs on surface that recognize bacterial determinants.
Anything, but remember their Ig are specific.
Despite these rules, all APCs can present determinants from any source.
Langerhans cells are also known as what?
immature dendritic cells
Describe dendritic cell recognition of pathogen and process of presentation.
Where are DCs found?
Where does it activate T cells?
Do they need any help activating T cells?
DC reside all over the body, particularly underneath epithelial tissue. They recognize PRRs and take up viruses effectively. When PRR is engaged, DC migrates to nearest secondary lymphoid tissue. Here, they are activated fully functionally APCs. They upregulate expression of MHC I and II. They express B7 now and sample naive T cells.
Due to upregulation of MHC I & II and B7, they can efficiently sample T cells.
They do not need assistance in activating T cells because DC express a lot of B7.
What is DC-CK?
Chemotactic factor secreted by dendritic cells that have been activated by PRRs to recruit naive T cells to sample their MHC:peptides
What is the name of the chemotactic factor secreted by dendritic cells to recruit T cells for sampling?
DC-CK
How do B-cells recognize and present antigens?
B-cells recognize pathogens via their numerous antibodies on the surface. Pathogens are endocytosed, including Ig.
Degraded
Peptides are placed on MHC usually Class II molecules.
PRRs on surface also recognize pathogen simultaneously and upregulate B7.
Naive T cells sample MHC:peptide with B7 co-stimulation.
What if an effector TH2 cell recognizes its cognate determinant on a B cell’s MHC? What happens? Does this require B7 co-stimulation?
TH2 activates B cell to become a plasma cell.
This does not require any co-stimulation.
What is the purpose of LFA-1 on naive T cells? What do they bind?
They bring the naive T cell and the APC closer together so that MHC:peptide can come in closer contact with TCR.
They bind ICAM-1.
What would happen if there was a deficiency of LFA-1 on naive T cells?
The T cell would be unable to efficiently sample peptide bound to MHC molecules
What are the initial adhesions of a naive T cell to an antigen presenting cell?
LFA-1 (tcell) to ICAM-1 (APC)
When a naive T cell’s TCR finally binds MHC:peptide after initial LFA-1:ICAM-1 interactions, what happens to these interactions? What happens to the CD4 or CD8 interaction?
When TCR binds MHC:peptide, the LFA-1:ICAM-1 binding becomes 2 to 10 times stronger. Since there are multiple LFA-1:ICAM-1 interactions occurring, this collective increase in binding is called increased avidity. The connection between the T cell and APC is now very strong.
The LFA-1 receives a signal from the TCR to induce conformational change, making the binding of LFA-1:ICAM-1 much stronger.
- CD4 and CD8 binding from T cell to MHC is likely to maintain affinity similar to an adhesion molecule.
How does a naive T cell become activated?
2 activation signals
1) Recognition of cognate determinant on MHC by TCR
* There must be multiple TCR:MHC interactions occurring for cumulative threshold to be met.
2) Co-stimulation of multiple B7’s to T cell’s CD28
* Again, a certain threshold must be met for activation.
What happens when a naive T cell is activated?
When thresholds are met and the T cell is activated, it upregulates production of IL-2 receptors, especially the alpha chain (beta & gamma chains already exist on cell surface)
- and produces IL-2 (aka T cell growth factor).
- Secretes IL-2
- IL-2 binds IL-2 receptors on own cell (autocrine)
- Stimulates cell to proliferate and EVENTUALLY differentiate.
What is IL-2? What does it do?
Autocrine produced by newly activated T-cell.
Stimulates T cell to proliferate
What “factor” induces proliferation and differentiation of T cells?
IL-2
IL-2 receptors on T cells are upregulated when?
Explain how T cells “up-regulate” IL-2 receptors. What makes this up-regulation productive?
T cells are activated
beta & gamma IL-2 receptors already exist on cell surface and bind weakly to IL-2. Up-regulation of IL receptors increases production of the alpha part of the IL-2 receptor. This increases the affinity of the receptor many-fold.
Also, increase in IL-2 production increases autocrine growth and proliferation of T cell.
A genetic deficiency of IL-2 causes what?
Cannot produce an effector T cell response to infection.
SCID
A genetic deficiency of one of the chains of the IL-2 receptor would cause ?
lack of effector T cell response because cannot receive signal to proliferate
SCID
What is peripheral tolerance?
Eliminating self-reactive T cells from body.
- Naive T cells that receive 1st activation signal of TCR to MHC:cognate antigen must also receive co-stimulation from CD28:B7 fairly quickly.
- If T cell does not obtain B7 signal quickly, it becomes anergic and dies.
- This backup mechanism prevents most self-reactive T cells from ever becoming activated, and actually result in removal of many of them from the repertoire!
What up-regulates B7 expression APCs?
PRRs recognizing PAMPs
If a T cell receptor binds its cognate antigen on a MHC but does not receive co-stimulation, what happens to the T cell?
What is this called?
Is this common?
- Becomes anergic and dies (like a bitch)
- peripheral tolerance
- Yes, This is a mechanism that prevents most self-reactive T cells from every becoming activated.
During an infection, PRRs on macrophages recognize PAMPs from the bacteria. This up-regulates what?
Bacteria are engulfed, processed, and peptide determinants are presented on MHC II molecules to the surface.
Non-bacterial proteins are engulfed, processed and also presented on MHC II molecules to the surface.
What is the likely outcome of this scenario?
B7
- autoimmune reaction –> autoimmune disease
CD8 T-cells are more difficult to activate than CD4 T-cells. WHY? What is the reason for their heightened activation requirements?
CD8 T-cells are cytotoxic when activated, so want to limit the destruction. Unlike CD4 which secrete a lot of cytokines when activated.
What are 3 ways for CD8 T cells to be activated?
1) Dendritic cells express HIGH levels of B7 when enter secondary lymphoid tissue — This high level can overcome threshold to activate CD8 T cells
- activated CD8 T cells make IL-2, driving its own proliferation and differentiation
2) Macrophages do not express enough B7 to activate CD8 T cells, so need help
- Effector CD4 T HELPER cell can activate Macrophages or B-cells to express even more B7. This activates naive CD8 T cells
3) If both CD4 and CD8 T cells are binding MHC cognate antigen on APC in close proximity.
- CD4 T cell is bound to cognate antigen(MHC) with B7 co-stimulation
- This activates the CD4 T cell to produce IL-2 (Just like all activated Tcells do)
- This IL-2 binds to the neighboring CD8 T-cell that is not meeting its co-stimulation (but bound to MHC&cognate antigen)
- This binding of IL-2 stimulates the CD8 T-cell to now become active.
What attracts neutrophils to sites of inflammation?
C5a, IL-8, IL-17 (from TH17), and LTB4
What molecule on naive T-cell INITIATES entry into secondary lymphoid tissue?
L-Selectin
Do activated effector CD8 T-cells express L-selectin?
- No. It would be counter productive for an activated CD8 T cell to find APCs in secondary lymphoid tissue and kill it.
- Want APCs there to present to naive T cells
Why do you think there are 3 different cell types that serve as APC? WHY? How are they complementary?
- Dendritic cells underlie epithelial cells all over the body; macrophages are found everywhere; B cells are concentrated in secondary lymphoid tissue
- DC very good at presenting viral antigens
- Mac very good at presenting bacterial and fungal antigens
- B cells can take up soluble determinants and particulate in secondary lymphoid tissue
- Swelling from inflammed tissue will drive particulate and antigens to secondary lymph tissue to increase the odds of a high affinity B cell to meet its cognate antigen in order to better present to T-cells
- Trade-off is that B-cells have high specificity and may not find cognate antigen efficiently
- There is no issue with B-cells only residing in secondary lymphoid tissue. Antigens are always driven to secondary lymphoid tissue by swelling, edema, inflammation
- Redundancy (complement each other)
What is the most important UNIQUE feature of APC types?
- They express B7
- B7.1 = CD80
- B7.2 = CD86
What is CD80?
B7.1
What is CD86?
B7.2
What are the other names for B7?
- B7.1 = CD80
- B7.2 = CD86
Where do Tregs reside in the body and how do they perform their effector functions?
Secondary Lymphoid tissue
If recognize self-reactive “antigen”, will secrete anti-inflammatory cytokines: TGF-beta and IL-10.
What do TGF-beta and IL-10 do? What secretes them?
anti-inflammatory cytokines that downregulate any activation or stimulation of immune response nearby.
Tregs
Do Effector T cells require co-stimulation?
No. Dammit.
They are adults. They can do what they fucking want.
What are the MAIN 3 effector T cell types? Briefly describe their roles.
CTL - most responsible for limiting virus infections by recognizing and killing infected cells
TH1 - promote immune responses that are most effective for clearance of intracellular infections (cell-mediated responses)
TH2 - promote immune responses that are most effective for clearance of extracellular infections (antibody responses)
T helper 1 cells influence B cells and Macrophages. How so?
provide co-stimulation of B-cells so that they can become activated.
activate macrophages to become more productive and efficient – Better at killing pathogens they have taken up
T helper 2 cells influence B cells and macrophages. how so?
provide co-stimulation of B-cells so that they can become activated.
Down-regulate macrophages. Tell Mac’s to die in order to release pathogen components to healthier macrophages.
What 2 primary cytokines to T helper 1 cells secrete?
What do these cytokines do?
Interferon-gamma and IL-2
IFN-gamma: Activates macrophages to become more productive
IL-2: Provides co-stimulation for the activation of CD8 T cells
What 2 primary cytokines to T helper 2 cells secrete?
What do these cytokines do?
IL-4 and IL-5
IL-4: Promotes proliferation of B-cells into plasma cells and induces class switching to IgE
IL-5: Stimulates B cell growth and Ig production and secretion
TH1 cytokines stimulate what response?
TH2 cytokines stimulate what response?
TH1: Increase opsoninization to increase uptake by macrophages or killing by NK cells. Appropriate isotype switching to opsonizing Ig such as IgG1 for intracellular pathogen killing.
TH2: Increase neutralization by Ig. Increase production and appropriate neutralizing Ig isotypes in order to kill extracellular pathogens, such as IgA and IgE.
What drives the differentiation of TH0 cells into TH1 or TH2?
The cytokines that surround TH0 at activation
and
The concentration of antigens being processed and presented on MHC.
What drives differentiation of TH0 to become TH1?
Interferon-gamma and IL-12
and
High concentration of antigen presentation on MHC Class 1 due to intracellular pathogen (more foreign protein available)
What drives differentiation of TH0 to become TH2?
IL-4, IL-5, and IL-6 (IL-4 is most important)
and
Low concentration of antigen presentation on MHC Class 2 due to extracellular pathogen (less foreign protein available)
How does a TH0 receive cytokine signaling to become a TH1 cell? Where do these cytokines come from?
Dendritic cells and macrophages infected with viruses or intracellular pathogens secrete IL-12.
IL-12 induces the production of interferon gamma in nearby NK cells.
IFN-gamma and IL-12 cause TH0 cells to become TH1
How does a TH0 receive cytokine signaling to become a TH2 cell? Where do these cytokines come from?
NK cells that recognize extracellular pathogens secrete IL-4.
- IL-4 induces TH0 to become TH2
What are Treg cells? Where do they originate from?
What cytokines do they release when activated?
What transcription factor drives the production of these cytokines in Treg cells?
Regulatory CD4 T cells (NOT CD8) that sample MHC:peptides for self-reactive determinants
- Differentiate from TH0 during thymic development in the medulla during negative selection
- IL-10 and TGF-beta
- FoxP3
How do T cells differentiate into Tregs?
In the thymus, T cells that bind to MHC:peptides with high affinity during negative selection either become anergic and die OR… they follow differentiation of T regs
- TSLP (thymic stromal lymphopoietin) is secreted by Hassall’s corpuscle.
- TSLP stimulates medullary dendritic cells to up-regulate B7 on their surfaces.
- TGF-beta is also secreted by medullary dendritic cells.
- T Cells that bind MHC:peptide and B7 with high affinity on dendritic cells mature and leave thymus as Tregs
What is TSLP?
thymic stromal lymphopoietin that is secreted by Hassall’s corpuscle that signals medullary DCs to produce B7 and TGF-beta to differentiate T cells into Tregs
What do Tregs do in the periphery?
Tregs sample MHC peptide complexes in the secondary lymphoid tissues, and if they recognize their cognate determinant (which is most likely self- derived) they produce TGF-β and IL-10. These cytokines down-regulate activation of other T cells that are sampling peptides on that same APC.
Name the cytokines produced and the transcription factor for Tregs, TH1 and TH2.
(VERY HIGH YIELD)
Tregs - TGF-beta and IL10; FoxP3
TH1 - IFN-gamma and IL-2; T-bet
TH2 - IL-4 and IL-5; GATA-3
What are TH17 cells? What do they secrete?
Another T helper cell
Secrete IL-17 and IL-22
What is IL-17? What secretes it?
Chemoattractant for neutrophils
TH17 cells
What is IL-22? What secretes it?
a cytokine that stimulates antimicrobial peptide production by epithelial cells
TH17 cells
