T-Cell Mediated Immunity

Question 1

What are the two main stages of T-cell Mediated Immunity?

Answer 1

Two stages:

1. T-cell activation (T-cell priming)
2. Differentiation into effector T-cells

Question 2

*Understand the role of dendritic cells in T-cell activation

Answer 2

when a sharp puncture in the skin occurs, it introduces a pathogen.
Myeloid Dendritic cells- capture the antigen at site of infection and bring it to secondary lymphoid tissue, and present antigen to Naive T- Cells.

Question 3

*Know the two routes by which T-cells can enter the lymph nodes.

Answer 3

T-cells can enter draining lymph node via:

1. Blood
2. Afferent lymph node (from upstream lymph node)

Question 4

*Understand the roles of CCR7, CCL21, CCL19, L-, CD34, GlyCAM—1,
LFA-1, ICAM-1, ICAM-2, ICAM3, and CDSIGN, CD2, LFA-3, CDa28, B7, and
CD4/CD8 in T-cell activation

Answer 4

a

Question 5

*Know the role of Sphingosine 1-phosphate and CD69 in T-cell exit from the
lymph node

Answer 5

a

Question 6

*Know the T-cell activation cascade – the phosphorylation that occur and the
transcription factors induced. What are ITAMs? What is the function of Lck?

Answer 6

TCR/MHC engagement induces Phosphorylation of CD3 cytoplasmic tails and Zeta chain
ITAMs- Immunoreceptor tyrosine based activation motifs
Lck- tyrosine kinase that binds to CD4 or CD8, and phosphorylates CD3 ITAMs; Lck also phosphorylates ZAP-70 which binds phosphorylated residues of the zeta chain
Zap70- initiates the activation of various transcription factors that lead to changes in gene transcription and induces T-cell proliferation, differentiation and effector functions.
transcription factors induced:
1. NFAT
2. NF-Kb
3. AP-1 Fos and Jun (expressed when CD28 is engaged)

Question 7

*Know the role of IL-2

Answer 7

Growth cytokine critical for T-cell activation

IL-2 induces the proliferation, differentiation of activated CD8 T cells into effector t cells or cytoxic t cells.

Question 8

*Know how T-cells become Anergic

Answer 8

when specific antigen is alone and there is NO co-stimulator (B7) - T cell becomes ANERGIC.
Absence of CD28 will result in anergy and T cell will not respond to external stimuli- NO production of IL-2 (irreversible).
antigen and B7 present, but not CD28- T cell stays anergic

Question 9

*Know the Master Regulators for the T-cell subsets

Answer 9

TH1 cells- transcription factor T-bet
TH17 cells- transcription factor ROR-gamma T
TH2 cells- transcription factor GATA3
TFH cells- transcription factor Bc16
T regulatory cells (T reg)- transcription factor FoxP3

Question 10

*Know how cytotoxic T-cells target particular cells for apoptosis

Answer 10

a

Question 11

compare and contrast mature vs immature dendritic cells? What are immature dendritic cells in skin also known as?

Answer 11

immature dendritic cells- immature cells in the SKIN during infection that capture and uptake antigen
mature dendritic cells- cells in secondary lymphoid that specialize in naive T cell activation.
Immature dendritic cells in skin aka Langerhans

Question 12

Describe the changes that occur with dendritic cells at they mature and migrate to lymph node.

Answer 12

In peripheral tissue, MHCII is located in endocytic vesicles.
As dendritic cells enter lymphatic circulation, MHCII LEAVES the endocytic vesicles (goes to cell surface)
As mature dendritic cells enter the T-cell area of lymph node, the MHCII is located on cell surface (coats the dendrites.

Question 13

What happens to dendritic cell morphology as they migrate and mature?

Answer 13

cell morphology changes as the dendritic cells become activated, migrate and mature.
Dendrites (processes) , extend and increase surface area for T-cell interaction.

Question 14

How do macrophages compare to dendrites in lymph node? Compare and contrast the role of macrophages and dendrites.

Answer 14

Macrophages play a COMPLIMENTARY role in the lymph node to dendritic cells.
Dendritic cell- activation of NAIVE T-cells
Macrophages- Remove Pathogen and breakdown products from afferent lymph arriving from site of infection (prevents infection from entering bloodstream)
They act as filters (macrophages)

Question 15

What are the only cells able to activate Naive CD8 T cell? Which cells can activate CD4 T cells?

Answer 15

DENDRITIC Cells- are the ONLY cells capable of activating naive CD8 T cell
Macrophages and dendritic cells- can also activate CD4 T-cells.

Question 16

Describe the process of Antigen presentation by dendritic cells in CD4 T cells.

Answer 16

For CD4 T cells: dendritic cells present pathogen derived peptides on MHC II to naive CD4 T cells.
receptor-mediated endocytosis captures bacteria from extracellular fluid and targets them to be processed into lysosomes. Then internalization of SMALL volumes of extracellular fluid- Mircopinocytosis and Macropinocytosis (non-specific. or receptor mediated)- which internalizes Large volumes of extracellular fluid that captures pathogens not recognized by endocytic vesicles.

Question 17

Describe Antigen presentation by dendritic cells for CD8 T cells.

Answer 17

Virus-infected dendritic cell-
1.viral proteins processed in cytosol and delivered to ER where they bind to MHC I
2. peptide-loaded MHC I are taken to cell surface where they are surveyed by antigen receptors of CD8 T cells
3.Virus particles taken up by (micropinocytosis or phagocytosis) broken down and delivered to the cytosol for further degradation to peptides and
subsequent CROSS-PRESENTATION by MHC class I to the antigen.
4. Transfer or viral antigens from infected dendritic cell to resident dendritic cells.

Question 18

What kind of receptors do Dendritic cells express? What occurs when these receptors are activated?

Answer 18

Dendritic cells express TOLL- LIKE receptors.
TLR engagement leads to:
1. activation of dendritic cells (increasing efficiency by which antigens are taken up, processed, and presented by MHCII)
2. Expression of CCR7 (receptor for CCL21- chemokine in secondary lymph tissue)- dendritic cell leaves the lymph and enters tissue
3. Maturation: (no longer processes antigen, job is to present antigen to naive T cells- MHC I and II expression increases.

Question 19

what is the role of naive T cells? How do T cells enter T-cell area/T-cell zone? What occurs when T-cell encounters dendritic cell?

Answer 19

Naive T cells- monitor antigens presented by dendritic cells
T cells bind endothelial cells of the high endothelial venules (HEV)- squeeze through and enter the cortex of the lymph node- T cell area/T cell zone
T- cells encounter dendritic cells- when binding occurs due to antigen recognition, and the T- cell is retained, and selected for activation.

Question 20

What are three places that infection can occur and where you may see T- Cells?

Answer 20

1. Skin= draining lymph node (skin infection)
2. Blood- in the spleen (during blood infections)
   but infections in mucosal tissues = are made in associated mucosal secondary lymphoid tissue

Question 21

what happens to T-cell if antigen is not encountered? How often do naïve T- cells recirculate?

Answer 21

If antigen is NOT encountered, T-cell leaves via EFFERENT lymphatics
Each naive T- cell recirculates from blood through a lymph node and back to blood every 12 to 24 hours.

Question 22

What are Quiescent, unactivated T-cells?

Answer 22

Quiescent, unactivated, T cells- small, nondividing cells with condensed chromatin, little cytoplasm, and little RNA or protein synthesis.

Question 23

What is Homing? which factors are involved in this process? What is CCR7?

Answer 23

Homing- process by which T-cells leave the bloodstream and enter the T-cell zone of the lymph node
CCL21 and CCL19- chemokines that guide T- cell homing. These proteins are secreted by stromal cells and dendritic cells in the T-cell area and are bound to surface of endothelial cells of endothelial cells- HEV’s
CCR7- receptor expressed on naive T cells that binds to CCL21 and CCL19

Question 24

Describe the step by step process of Homing or moving t-cells out of blood and into lymph node.

Answer 24

Homing process:

1. Circulating T-cell enters the high endothelial venule in the lymph node.
2. Binding of L-selectin to GlyCAM-1 and CD34 allows rolling interaction of T- cell
3. LFA-1 is activated by chemokine bound to extracellular matrix
4. Activated LFA-1 binds tightly to ICAM-1
5. Diapedesis- lymphocyte will squeeze itself and leave the blood and enters the lymph node.

Question 25

Explain how L-selectin, LFA-1, CD34, GlyCAM-1, ICAM-1 and ICAM2, play a role in the homing process.

Answer 25

L-selectin- on T-cell binds CD34 and GlyCAM-1 on HEV’s for attachment
LFA-1 on T- cell and ICAM-1 and ICAM-2 on vascular endothelium STRENGTHENS the attachment to T cell (chemokine receptor engagement increases strength of LFA-1/ICAM binding)
This will allow T-cell to migrate between endothelial cells and enter the lymph-node cortex to source of chemokines.

Question 26

What are the interactions that occur between T-cells and dendritic cells initially?

Answer 26

T-cell and dendritic cells bind by transient interactions at first
1. ICAM-1 and ICAM-2 on dendritic cells bind LFA-1 on the T cell
ICAM-3 on T cell binds LFA-1 on dendritic cell
ICAM-3 on T cell binds CD-SIGN on dendritic cell2.
for increased adhesion- CD2 on T cells binds LFA-3 on dendritic cells
This allows T cells to explore the MHC complexes

Question 27

Describe how T cell and dendritic cell interaction develop and become tighter?

Answer 27

initially, T-cell binds to dendritic cells through LOW-affinity LFA-1:ICAM-1 interactions
-after subsequent binding of T-cell receptors sends signal to LFA-1 which causes a conformational change in LFA-1 that increases affinity and prolongs cell-to cell contact.

Question 28

Compare and contrast the environment in where you would expect to find environment rich in CCL21 and CCL19 vs S1P.

Answer 28

paracortex of lymph node or T-cell zone: environment rich in CCL21 and CCL19
Efferent lymph- environment rich in S1P

Question 29

What occurs if an is NOT encountered? What happens to activated T- cells?

Answer 29

Antigen NOT encountered:
-T cells pass from cortex to the medulla and leave the lymph node (guided by S1P gradient)
-Sphingosine 1-phosphate (S1P)- expression on stromal cells is low in the T- cell area and INCREASES towards the MEDULLA and EFFERENT lymphatic vessels
- Activated T cells- express CD69 which sequesters S1P receptor in the cell.
Once activated, T-cells MATURE, then decrease CD69 expression- now they can LEAVE lymph node.

Question 30

What occurs if an antigen is encountered?

Answer 30

If Antigen encountered:

• if a naive TCR recognizes Peptide:MHC complex, a signal is sent for the cell to be activated.
• LFA-1/ICAM interactions are strengthened by Conformational change
• Conjugate or conjugate pair (T-cell/dendritic cell coupling) . The dendritic cell will present antigen for T-cell to recognize
• T-cells then proliferate and CLONAL EXPANSION occurs
• Dendritic cells maintain contact with all clones- “nursery” for propagation of T- cells.

Question 31

What factors are required to activate a naive T cell?

Answer 31

The combination of antigen-specific signal and co-stimulatory signal are required to activate Naive T cell

Question 32

During encounter with antigen, explain what other structures are necessary for T-cells to be activated. What proteins are on the T cell, vs dendritic cell that aid in the activation?

Answer 32

T-cell receptor and co-receptor engagement is NOT sufficient for T-cell activation- you need CO-STIMULATORY Signal
CD28- located on T-cell
B7- Dendritic cell (only expressed on DC presenting antigen)
-requires Simultaneous signaling from TCR (t-cell receptor), co-receptor and costimulatory receptor (CD28/B7) for T-cell activation

Question 33

During the T- cell activation, what is the synapse? What are the main parts of the synapse and what do they compose of?

Answer 33

Synapse- place where dendritic cells and T-cells meet
Major parts of synapse (helps stabilize reaction)
1. c-SMAC- central supramolecular activation complex inner structure
composed of TCR, CD2, CD4, CD8, CD28 and PKC-theta
2. P-SMAC- peripheral supramolecular activation complex- outer structure
composed of LFA1, ICAM-1, and Talin

Question 34

Describe the step by step process of T-cell activation signaling cascade

Answer 34

1. In resting T cell, the ITAMs of the CD3 complex are NOT phosphorylated
2. Binding of MHC to the T-cell receptors leads to ITAM and zeta chain phosphorylation by Lck
3. Zap-70 binds phosphorylated Zeta chain ITAMs and is phosphorylated by Lck
4. ZAP70 initiates the activation of various transcription cells that leads to changes in gene transcription and induces T-cell proliferation, differentiation, and effector functions.

Question 35

What are the transcription factors that ZAP70 initiates the activation of during T-cell activation signaling? When is Fos and Jun expressed?

Answer 35

transcription factors:

1. NFAT
2. NF-kB
3. AP-1 Fos and Jun (expressed when CD28 is engaged)

Question 36

Explain the signal cascade that occurs when Naïve T- cell recognizes specific antigen presented by dendritic cells.

Answer 36

Process:
1. Naive T cell interacts with dendritic cell presenting its specific antigen
2. Activation of the kinase ZAP-70 initiates a series of reactions that leads to activation of phospholipase-gamma (PLC-gamma)
3. PLC - gamma cleaves PIP2 to DAG and IP3
4. DAG activates protein kinase C- which activates a transcription factor NFkB.
DAG may also activate RASGRP, which activates MAP kinase cascade; this RAs-induced kinase cascade induces and activates Fos (component of AP-1 transcription factor)
5. IP3 increases intracellular CALCIUM concentration, activating phosphatase calcineurin, which activates transcription factor NFAT (nuclear factor of activated T cells)
6. NFkB, NFAT and AP-1 are made which change the pattern of gene expression.
This results in cell division, proliferation and differentiation to effector T cells.

Question 37

Which structures activate NFAT vs NFkB, and AP-1

Answer 37

Protein Kinase C (through DAG) activates NFkB
RAS-induced kinase cascade (RASGRP, MAP) activates Fos which is part of AP-1
Calcinuerin (activated through IP3) activates NFAT
calcinuerin is a phosphatase

Question 38

What is the role of IL-2 for T cells? differentiate its role in naive T cells vs Activated T cells?
What drugs suppress IL-2?

Answer 38

IL-2-growth cytokine CRITICAL for T-cell activation
Naive T cell express low affinity IL-2 receptor (IL-2 receptor, Beta and gamma chains only)
* Activated T- cells express HIGH- AFFINITY IL-2 receptor and also EXPRESS IL-2 (Autocrine signaling)
(secrete IL-2 have IL-2R, Beta, and gamma chains)
Immunosuppressive drugs target IL-12
-Cyclosporin A, Tacrolimus (FK506) and rapamycin (suppress IL-2 expression, so you won’t have CD8 expression)

Question 39

Explain the different responses of T cells that occurs during recognition of self-antigen.

Answer 39

If specific antigen and co-stimulator (B7, CD28) are present, the T cell will be ACTIVATED and PROLIFERATES
when specific antigen is alone and there is NO co-stimulator (B7) - T cell becomes ANERGIC.
Absence of CD28 will result in anergy and T cell will not respond to external stimuli- NO production of IL-2 (irreversible).
antigen and B7 present, but not CD28- T cell stays anergic

Question 40

Describe the defining transcription factors for each effector CD4 T cell. What are their functions based on, and how are they distinguished?

Answer 40

TH1 cells- transcription factor T-bet
TH17 cells- transcription factor ROR-gamma T
TH2 cells- transcription factor GATA3
TFH cells- transcription factor Bc16
T regulatory cells (T reg)- transcription factor FoxP3
These cells have distinct functions based on tissue of origin, nature of pathogen, innate immune response, induced-heterogenous population (interact with few cell types; lymphocytes, phagocytes, granulocytes
They are also distinguished by cytokines that induce differentiation, transcription factors that determine differentiation, cytokines they make.
properties change depending on local environment

Question 41

Differentiate between the processes that occur with CD4 vs CD8 T cells when Dendritic cells present antigens.

Answer 41

When dendritic cells present antigens:

1. CD4 T cell will bind to MHCII and undergo receptor mediated endocytosis of bacteria, MACROPINOCYTOSIS of bacteria or viruses
2. CD8 T cell will bind to MHC I and undergo Viral infection, cross presentation of exogenous viral antigen and transfer of viral antigen from infected dendritic cell to resident dendritic cell.

Question 42

Differentiate between what occurs when dendritic cells presents virus-derived peptides to naive CD8 T cell vs CD4 effector cell.

Answer 42

When dendritic cell present virus-derived peptides:
1. NAIVE CD8 T cell: activated CD8 T cell makes IL-2 that drives its division and development to form a clone of CYTOTOXIC T cells
2. CD4 Effector cell: IL-2 from the CD4 T cell drives the PROLIFERATION and DIFFERENTIATION of the Virus-specific CD8 T cell to form a clone of CYTOTOXIC T cells.
hence if dendritic cells present antigen to either CD8 or CD4 t cells, main goal is to make cytotoxic cells, as CD4 assists.

Question 43

Elaborate more on Cytotoxic CD8 T cells and how they are formed and activated. What is their function?
Which cells leave secondary lymph tissue and which stay in the secondary lymph tissue?

Answer 43

Cytotoxic CD8 T cells- functionally HOMOGENOUS- interact with many target cell types- viral or microbial infection
They cause DEFENSE against intracellular infections
Antigens are presented on MHCI
some infections require effector CD4 T- cell help- production of IL-2
CD8 effector T cells and TH1, TH2, TH17 and regulatory T-cells leave secondary lymph tissue and enter circulation
TFH- stay in secondary lymph tissue and move to B cell area.

Question 44

What is the use of VLA-4 and VCAM cells? What do activated T cells not have anymore compared to naïve T cells.

Answer 44

VLA-4- ONLY expressed by ACTIVATING T-cells. it allows cells to LEAVE blood and enter TISSUES by binding to VCAM1 that is expressed on endothelial cell of infected tissue.
Activated T cells NO longer express L-Selectin (this prevents them from entering secondary lymph tissue.

Question 45

Differentiate between the differences in integrins expressed by naive T cells vs effector T cells.

Answer 45

All T cells (naive)- have integrin alpha and Beta molecule, LFA-1 and ICAM-1 (adhesion molecule on target cell)
Effector T cells have Integrin alpha, and Beta subunits, VLA-4 and VCAM-1 on activated endothelial cell (allowing cells to leave blood and enter tissue.

Question 46

Describe the process of a CD8 T cell becoming an effector T cell.

Answer 46

Process:

1. Recognition of antigen- Naive CD8 T cell will be activated by recognizing a Naive CD8 T cell.
2. Proliferation and Differentiation- IL-2 will be secreted and will bind to IL-2 receptor which induces proliferation and differentiation of activated CD8 T cells (effector)
3. Effector Function- The effector CD8 T cells will recognize and kill virus-infected epithelial target cells.

Question 47

Describe the major cells of CD8 T cells and CD4 T cells and their functions

Answer 47

CD8 T cells: CYTOTOXIC T cells that kill virus-infected cells
CD4 T cells:
TH1 cells- help macrophages to suppress intracellular infections
TH2 cells- help basophils, mast cells, eosinophils, and B cells respond to parasite infections
TFH cells- help B cells become activated, switch isotype, and increase antibody affinity
TH17 cells- enhance the neutrophil response to fungal and extracellular bacterial infections
T regulatory cells (T-reggs)- suppress the activation of other effector T- cell populations

Question 48

Explain the granules that are present in CD8 T cells and also the function of IFN-gamma.

Answer 48

CD8 T cells contain cytotoxins that are packaged into granules- which are released at SYNAPSE only
IFN-gamma- produced by CD8 T cells, and they INHIBIT viral replication in infected cell and increase processing of viral antigens, presentation, and activates local MACROPHAGES.

Question 49

How are granules in CD8 T cells released?

Answer 49

Process:

1. Encounter and nonspecific adhesion- Cytotoxic T cells and target cell encounter one another (non-specifically)
2. SPECIFIC recognition redistributes cytoskeleton and cytoplasmic components (LG, MTOC, GA) of T cell
3. Release of lytic granules at the site of cell contact (synapse)

Question 50

How can individual cytotoxic T cells target numerous infected cells?

Answer 50

1.First the cytotoxic CD8 T cell will recognize virus-infected cell
2. Then CD8 T cell will program first target cell to die
3. CD8 T cell will then move to second target cell
4. The First target cell will die, second is dying and the third is being attacked.
hence Cyotoxic CD8 can target numerous cells at once.

Question 51

What happens to cells that are targeted by cytotoxic T cells? What are the components involved?

Answer 51

cells killed by cytotoxic T cells die by APOPTOSIS

• Perforin, granulysin, and serglycin make a pore on target cell and deliver granzymes
• GRANZYMES (serine proteases) will Initiate CASCADE of proteolytic cleavages and activation of Nucleases (that degrade DNA, eventual loss of membrane integrity)

Question 52

How do TH1 T cells induce macrophage activation?

Where are cytokines delivered?

Answer 52

TH1 T cells- induce macrophage activation by IFN-gamma (CD8 also expresses IFN-gamma) and CD40 ligand production:
1. phagosomes fuse more efficiently with lysosomes
2. Increase in NO (Nitric Oxide) and proteases- destroy pathogens
cytokines are delivered via EXOCYTOSIS to the SYNAPSE (specific activation)

Question 53

Explain how TFH cells activate B- cells. What kind of recognition occurs between these two cells?

Answer 53

1. After activation, TFH cells move to B cell area of lymph node to interact with circulating naive B cells
2. Naive B-cell then internalizes pathogen- presentation via MHCII. TFH cell recognizes pathogen and binds and makes CD40 Ligand
   LINKED RECOGNITION- B cell and TFH cell are SPECIFIC for epitopes of the SAME antigen
   recognizes a peptide derived from B cell’s antigen. Then Naive B cell and TFH cell exchange signals that begin process of B- cell activation

Question 54

How do Tregs suppress an immune response? what kind of molecule do they express high levels of?

Answer 54

Tregs express HIGH LEVELS of CD25 (alpha chain IL-2 receptor)
They make anti-inflammatory and immunosuppressive cytokines (IL-4, IL-10, and TGF- Beta)
They also interact with dendritic cells in secondary lymph tissue (and prevent interaction with naive T cells_
Have direct contact with effector T cells- cytokines.

Question 55

Which molecules express IFN-gamma?

Answer 55

CD8 T cells and TH1 cells

Question 56

What must occur for suppression of autoreactive T cells by T reg cells?

Answer 56

Suppression of autoreactive T cells by T reg cells needs BOTH T cells (T reg and CD4 T cells ?) to engage the same-antigen presenting cell