Lecture 1 Flashcards
T-cell recognize only ONE specific non-self peptide. However, there is a large TCR repertoire made up by many T-cells generated in the body so that we can recognize ANY antigens. When does this process occur?
This occurs during normal thymus development. T-cells are eliminated if they react with self-antigens or kept to survive if they do not.
CD4+ T helper cells will recognize antigens to presented on MHC Class II molecules that are expressed on what cells?
- Dendritic cells 2. Macrophages 3. B-cells
What are the two phenotypes of CD4+ T- helper cells?
Th1 Th2
CD8+ cytotoxic T cells recognize peptides on MHC Class I molecules. What is their action?
They kill host cells that are infected with pathogens and then activate macrophages.
What part of the MHC Class II and MHC Class I molecule for T cells bind to?
The non-polymorphic part.
Where do naive T-cells circulate through?
The lymph node.
When do naive T cells activate?
When they encounter a antigen.
What transports antigens to the lymph nodes?
mature (activated) dendritic cells
Naive T cells will interact how with antigens?
They will interact with many different DC’s until they find the antigen for their TCR.
Once naive T-cells are activated, what do they do?
They become EFFECTOR CELLS and then: 1. Stay in the lymphoid organ and help B-cells. 2. Go to sites of infection and help activate macrophages.
What are the phases of T-cells responses?
- Naive T-cells circulate throughout the LN.
- Dendritic cells bring antigens to the LN.
- Naive T-cells search through the DC until they find an antigen that they match with.
- Naive T cell is activated by [antigen+co-stimulatory signal]. What happens next depends on the type of T-cell.
- A. CD4+ Helper T-cell–> IL2–> Clonal expansion (proliferation) and differentiation–>
- CD4+ effector T helper cells will activate macrophages, B cells and other cells
- Memory CD4+ T cells
- B. CD8+ T-cells–> proliferate and differentiate –>
- CTL cells–> kill infected host and activate macrophages
- CTL Memory cells
When a T cell recognizes the antigen+costimulatory signal, what responses do the T-cells make?
- Proliferation 2. Differentiation into effector and memory cells 3. Cytokines are secreted
After the antigen is eliminated, does the number of effector T-cells increase, decrease or stay the same?
They will decrease. As a result, we will only be left with memory-T cells
In order for T-cells to proliferate and differentiate into an effector signals, we need three signals. What are they?
First signal- Antigen Second signal- Costimulation Third signal- Cytokine ACC
In order to activate a NAIVE T cell, it must recognize an antigen that is presented by what kind of cells?
ONLY a dendritic cell.
In order to activate a EFFECTOR T-cell, it must recognize antigens that are presented by what kind of cells?
Tissue macrophages and B-cells
When a TCR recognizes an antigen, there are inhibitory and activating signals. What is the inhibitory signal?
CTLA4: CD80/86 (B7-1, B7-2)
When a TCR recognizes an antigen, there are inhibitory and activating signals. What is the activating signal?
CD28: CD80/86
What is CD28?
Where it is located and what does it bind to?
CD28 is a co-stimulator involved in signal transduction.
Located on T-cells and binds to CD80/86 (B7-1/B7-2) that is located on APCs.
How do T-cells adhere with APCs?
LFA-1: ICAM-1
ICAM-1 is located on APCs and endothelium.
What do CD4 or CD8 co-receptors on the T-cells do?
CD4 co-receptor binds to class II MHC on APCs and helps with signal tranduction.
CD8 co-receptor is binds to class I MHC on all nucleated cells and helps with signal transduction.
What is CD3?
T-cell signaling complex.
What are ITAMs?
ITAMs (immunoreceptor tyrosine-based activation motifs) are parts of the signaling complex
- whose tyrosine residues get phosphorylated
- where [tyrosine kinases] attach.
What are ITIMs?
ITIMs (Immunoreceptor tyrosine based inhibitory motifs) are part of the signaling complex where [tyrosine phosphotases] bind.
They are located on the cystolic tails of PD-1, which have an inhibitory function.
What are co-stimulatory molecules?
Co-stimulatory molecules are located on APCs after they encounter pathogens. They bind to the co-stimulatory receptor that is on the naive T-cell and help to promote the response of the pathogen.
What are adhesion molecules?
They help T-cells bind stronger with APCs.
What are superantigens (SAgs)?
(SAgs) made by some bacteria are the most powerful T-cell mitogens. They are not peptides. Only 0.1pg/ml of the SAg needs to bind to the T-cell in an uncontrolled manner and it will result in fever, shock and death due to toxic shock syndrome.
Where do SAgs bind?
- SAgs glue both the [MHC class II molecules] and SOME [V-region of the B subunit] of the TCR at the same time. This activates T-cells. Because SAgs are not peptides, they do not bind to the peptide binding groove.
- When the SAg bind, this causes the T-cells to make a massive amount of pro-inflammatory cytokines (TNF, IL-1 and IL-2)
- Can cause shock
Example of bacterial SAg?
Staphylococcal enterotoxins (SE)- causes food postening
Toxic Shock Syndrome Toxin (TSST)
Signal 2 is costimulation. Explain this process.
- Immature DC’s have ____ levels of co-stimulatory molecules.
- What happens when an antigen is recognized, but there is no co-stimatulation?
- What causes DC’s to express co-stimulator molecules?
- Immature DC’s have a low levesl of co-stimulatory molecules. Pathogens or cytokines in the innate immune system will trigger DC’s to make co-stimulatory molecules
- When an antigen is recognized without co-stimulation, it may make the T-cell unresponsive or anergic (tolerant)
Activated APC’s have increased expression of co-stimulators and secrete cytokines.What is an example of the cytokine secreted? (signal 3)
IL-12- causes naive T-cells–> Th1 type of effector T-helper cells.
Example of co-stimulators molexules
CD80/CD86
T-cell co-stimulation by CD-28
The best characterized costimulatory pathway in T-cell acitvation is when T-cell surface receptor CD28 binds to the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) that are on activated APCs.
This makes sure that T-cells only respond when they need to.
Many receptors like CD28 and ligands like B7 have been identified. How do they regulate T-cell responses?
They regulate them positively and negatively. As a result, T-cell activation is influenced by a balance between activating and inhibitory receptors of the CD28 family.
What co-stimulator LIGANDS do only [DC’s, macrophages and B-cells] express and receptors on the T-cell do they bind to?
- B7-1 (CD80) bind to CD28 or CTLA-4
- B7-2 (CD-86) bind to CD28 or CTLA-4
CD28?
Expression:
Major function:
Expression: T-cell; constituitive
Major function: co-stimulates naive T cells and generates regulatory T cells
CTLA-4
Expression:
Major function:
A co-stimulatory receptor
Expression: T-cells; inducible
Major function: negatively regulates immune responses and self-tolerance
What ligands to [DCs, macrophages B-cell and OTHER cells] express and what receptor do they bind to?
ICOS-L (CD275).
Inducible T-cell Costimulator
Binds to ICOS.
ICOS
Expression:
Major function:
ICOS-L (CD275)
Expression: T cells (induced)
Major function:
- A costimulator for effector and regulatory T-cells
- Generates follicular helper T-cell
What ligands are expressed on [macrophages, B-cells, endothelial, epithelial and tumor cells] and what receptor do they bind to?
- PD-L1 (program death-L1)
- PD-L2 (not expressed on tumor cells)
They both bind to the PD-1 receptor.
PD-1
Expression:
Major function:
Co-stimatory receptor
Expression: Induced receptor on T-cells, B-cells, myeloid cells
Major function: negatively regulates T-cells
What are the inhibitory co-receptors on T-cells?
- CTLA4
- PD1
They are structurally similar to CD-28.
CTLA4 and PD1 are both inhibitor co-stimulator receptors on T-cells that serve as _____________
checkpoints that regulate T-cell responses.
CTLA-4 is an inhibitory co-stimulator that regulates the T-cell immune response. Tell me about it.
Naive and memory T-cells have high levels of CD28 located on the surface. But, they do not express CTLA-4; it is stored in intracellular vesicles.
When naive T-cells respond to an antigen, the CTLA-4 receptor is transported to the surface. The stronger the stimulation, the more CTLA-4 that goes to the surface.
Thus, it regulates the amount of T-cells activated by damping the sigal.
PD1 is an inhibitory co-stimulator that regulates the T-cell immune response. Tell me about it.
PD-1 regulates inflammatory responses in tissues by effector T-cells recognizing Ag in peripheral tissues.
- When a T-cell is activated, it increases the amount of PD1 receptors and expresses it IN TISSUES.
- Inflammatory signals in the tissue will induce the expression of PD1L due to IFN-y released from Th1 cells.
- When PD1-L binds to PD1, it downregulates the activity of T-cells and limits tissue damage.
If too many PD1 receptors are made, it can induce an exhausted or anergic state in T-cells.
______ is a rheostat for immune responses.
PD-1
Cytokines that are released from the T-cell will determine the outcome of antigen recognition with regards to effector T-cell differentiation.
What does IL-12 cause?
IL12–> STAT4 --> T-bet–> Th1 cells
Cytokines that are released from the T-cell will determine the outcome of antigen recognition with regards to effector T-cell differentiation.
What does IL-4 cause?
IL4–> STAT-6–> GATA-3–> Th2 cells
Cytokines that are released from the T-cell will determine the outcome of antigen recognition with regards to effector T-cell differentiation.
What does IL-6 cause?
IL-6–> STAT3–> RORyt (retinoic acid receptor-related orphan receptor-yt)–> Th17 molecules
Cytokines that are released from the T-cell will determine the outcome of antigen recognition with regards to effector T-cell differentiation.
What does TGF-BR cause?
TGF-BR–> SMAD2-SMAD4–> FOXP3–> Regulatory T-cells
CD8+ T cells recognize antigens that are presented on class 1 MHC-cells. What happens when an antigen is presented to it?
- Naive T-cells recognize antigens and are co-stimulated by CD28-CD80 on DCs in the lymph nodes.
- CD8+ T-cells proliferate and differentiate into [CTLs and memory cells].
- CD8+ CTLs are released into circulation and go to the site of the antigen.
- CTLs recognize antigens in the tissue and release lysosome granules, which have perforin and granzymes. This kills the target cells that are making the antigen.
- CTLs will release IFN-y, which activates macrophages.
What is the role of T-bet in that CD8+ T-cells?
T-bet is a transcription factor for perforin, granzymes and IFN-y involved in CTL differentiation.
Describe the signaling cascade in the activation of T-cells
- TCR recognizes the antigen
- The LCK that is associated with CD4/CD8 moves near the CD3 Complex.
- LCK will doubly phosphorylate ITAMS on their __ chains.
- ZAP 70 binds to the SH2 domain of ITAM
- LCH will phosphorylate and activate ZAP70.
- ZAP-70 will then phosphorylate LAT and SLP-76, which function as scaffolds to recruitother signaling molecules
- Phosphorylated Lat will then recruit: GADS and GRB2.
- GRB2 will then phosphorylate and activate PLC1.
- PLC1 will then make [IP3 and DAG].
- IP3 will then increase Ca2+ in the cytosol and activate NFAT.
- DAG will activate PKC, which activates NF-kB and RAS, which activates MAPK and AP-1.
After the CD4+ T cell is activated, what is the first change in protein we will see?
An increase in c-Fos (a transcription factor) 0-3 hours after activation.
What are the 5 biological actions of IL-2 in the adaptive immune response?
- IL-2 is an autocrine growth factor for T-cells.
- It increases cytotoxicity of NK cells and CD8+ T-cells.
- Co-stimulates T-cells to make–> [IL4, IL5 and IFN-y].
- Promotes the development of T-regulatory cells.
- Induces a autocrine activation-induced death in T-cells.
IL-2 will stimulate the survival, proliferation and differentiation of antigen-activated T-cells. How does it do so?
- Induce anti-apoptotic protein, Bcl-2
- Helps the cell cycle progress by degrading p27, which inhibits the cell cycle.
What is required for the survival and function of Tregg cells?
IL-2.
IL2 is the ONLY cytokine that can maintain Treg cells.
What causes proliferation and differentiation of NK cells and B cells in vitro?
IL-2
When T-cells are activated, CD69 expression is altered. How and what happens?
S1PR1 (sphingosine 1- phosphate receptor 1) on T cells and S1P concentrations between [lymph organs] and the circulatory system play a big role T-cells leaving the LN.
- CD69 binds to CD69L, which reduces the amount of S1PR1 that is expressed on the surface.
- As a result, activated T-cells remain in the LN long enough to cause them to proliferate and differentiate into [effector and memory cells].
- Once the cell divides, CD69 decrease, the activated T-cell re-expressed high levels of S1PR1; allowing effector and memory cells to exit lymphoid organs.
Explain the expression of CD25 (IL-2Ralpha) in activated T-cells
- A resting naive cell expresses IL-2RByc complex, which has low affinity for IL-2.
- When IL-2 is released, this causes the IL-2Ralpha (CD25) to come from within the cell and combine with the IR-2RByc complex and form–> IL-2RalphaByc complex (which has high affinity for IL-2).
Now the complete IL-2 receptor is able to bind IL-2 strongly. Thus, IL-2 binds to the same T-cell that it was made from and acts as an autocrine cytokine.
Explain the expression of CD40L in activated T-cells
- CD40L (CD154) expression on the T-cell increases 24-48 hours after it recognizes the antigen.
- CD40L will bind to CD40 on the APC–> Increase in B7 molecules & secretion of cytokines that activate T-cells.
-
Describe clonal expansion of T-cells.
1-2 days after the T-cell is activated by the [antigen + costimulatory molecule], we see an increase in antigen-specific clones (clonal expansion). S/O to IL-2.
A majority of the clones are specific for a few, less than 5 immunodominant peptides. Effector cells can use these to fight infections.
The expansion of CD4+ T cells appears to be 100-fold –> 1000-fold less than that of CD8+ cells. This may be because of the difference between the two cells:
CTL kill infected cells via direct contact. Thus, may are needed to kill are large number of infected cell. CD4+, on the other hand, release cytokines and activate other effector cells, so we will need a small number of them.
How do we get T-cells to decline their response?
When the antigen is eliminated, the T-cells responses decline, allowing us to maintain homeostasis.
- As the level of co-stimulation and IL-2 decrease, the levels of anti-apoptotic proteins drop.
- IL-2 starvation causes the intrinsic pathway of the mitochondria to begin apoptosis.
- Inhibitory receptors: CTLA4 and PD
- Apoptosis due to death receptors TNFRI and Fas
- Treg cell products are inhibited.
What makes up the most abundant lymphocyte population in the body during the lifetime?
Memory T cells
Where do a majority of memory T-cells live?
Tissue sites
What influences the fate between the development of effector and memory cells?
Transcription factors
What causes effector cells to differentiate into CD4 T-cells?
T-bet
What TF promotes the generation of memory cells?
Blimp-1
What determines the function and status among the three types of T cells: naive, effector and memory?
DNA methylation.
Effector and T memory cells have similar methylation patterns. Naive cells, on the other hand, have more methyl group.
Thus, this proves that memory T-cells are made from effector T-cells via epigenetic changes.
Properities of Memory cells.
- What may help them survive for a while?
- In terms of initating a response, how to memory cells compare to naive T-cells?
- Do we have more memory T-cells specific for an antigen or naive T cells for the same antigen?
When an antigen is not present, memory cells are dormant.
- They have increased levels of anti-apoptic proteins and they undergo slow self-renewing, which may cause them to live longer.
- They initate a larger and quicker response: responding in 1-3 days compared to the 5-7 days naive cells take.
- We have a 10-100 fold more memory cells than naive.
What is the maintenance of memory cells dependent on?
Cytokines, but it does not require antigens
Memory T-cells express antipoptic proteins, helping them live for a while. What cytokines cause these to be expressed?
IL-7
IL-15
What are the three phases that memory T-cells go through?
- Memory generation
- Memory homeostasis
- Immunosenescence
Memory T-cell generation
- Memory T-cells are mostly made after the exposure to an antigen during [0-20 years].
- At ages 30-65, their levels plateau and are maitained via homeostasis.
- After 65, they show senescent changes (remain the same)
How are CD4+ and CD8+ memory T cells subsubdivided into subsets?
Based on where they live and their function
Central memory T-cells (Tcm cells)
Live?
Function?
- Live in LN, spleen and in the blood.
- They proliferate (due to high IL-2) and make many effector cells on antigen
Effector memory T-cells (TEM cells)
- Located in blood.
- Do not proliferate. Instead, they
1. Make IFN-y and TNF
2. Become cytotoxic
Resident tissue memory T cells (TRM Cells)
- Located in epithelial barrier.
- Make IFN-y and TNF; they are specific for pathogens that have been encountered previously through that barrier epithelium.
Naive T lymphocytes home to lymph nodes as a result of L-selectin, integrin, and chemokine receptor CCR7 binding to their ligands on high endothelial ve- nules (HEVs). Chemokines expressed in lymph nodes bind to CCR7 on naive T cells, enhancing integrin-dependent adhesion and migration through the HEV. The phospholipid, sphingosine 1-phosphate (S1P), plays a role in the exit of T cells from lymph nodes, by binding to the receptor, called S1PR1 (type 1 sphingosine 1-phosphate receptor). Activated T lymphocytes, including the majority of effector cells, home to sites of infection in peripheral tissues, and this migration is mediated by E-selectin and P-selectin, integrins, and chemokines secreted at inflammatory sites. Follicular helper T (Tfh) cells (not shown) are effector cells that remain in lymphoid organs, because they express a chemokine receptor (CXCR5) that draws them into lymphoid follicles, where they can interact with resident B lymphocytes. B, This table summarizes the functions of the principal T cell homing receptors and chemokine receptors and their ligands. ICAM-1, Intercellular adhesion molecule 1; LFA-1, leukocyte function– associated antigen 1; VCAM-1, vascular cell adhesion molecule 1; VLA-4, very late antigen 4.
Naive T-cells live in LN, as a result of [L-selectin, integrin and chemokine receptor CCR7] binding to ligands on HEV.
- Chemokines in the LN bind to CCR7 on naive T-cells and move through the HEV.
- SLP binds to S1PR1 causes T-cells to leave the LN.
- E-selectine, P-selectin integrins and chemokines at the site of infection will cause the activated T-lymphocyte to go to the site of infection.
What are the 3 T-cell homing receptors located on naive- T cells?
- L-selectin
- LFA-1
- CCR7
L-selectin is a naive T-cell receptor. What does it bind to and what does it do?
Binds [L-selectin ligans].
Function: Causes naive T-cells to weakly adhere to HEV in lymph node.
LFA-1 is a naive T-cell receptor. What does it bind to and what does it do?
Binds to ICAM-1.
Causes the naive-Tcell to firmly adhere on HEV.
CCR7 is a naive T-cell receptor. What does it bind to and what does it do?
- binds to CCL19 or CCL21.
- activates integrins and chemotaxis.
What are the receptors/ligands on activated (effector and memory) T cells?
- Ligands for E and P selectin.
- LFA-1 or VLA-4
- CXCR3
Ligands for E and P-selectin
Activated T-cells express ligands for E and P selectin.
This allows them to weakly adhere effector and memory T cells to the activated endothelium
LFA-1 and VLA-4
What does it bind to? What is the function?
Receptors on the T-cell.
Bind to ICAM-1 or VCAM-1.
Cause the T-cell to firmly adhere to the endothelium
CXCR3 is a activated T-cell receptor. What does it bind to and what does it do?
Binds to the CXCL10 ligand.
Activates integrins and chemotaxis.
