Module 6 T-cell activation

Question 1

Where is adaptive immune response initiated?

Answer 1

Secondary lymphoid tissues;
Spleen for blood infections; secondary lymphoid tissues, for all other kinds of mucosal infections

Question 2

What’s the difference between immature dendritic cells, and mature dendritic cells?

Answer 2

Immature dendritic cells cells are in skin and other preferred tissues, they are active in the capture, uptake and processing the antigens, but these properties are lost, moving to secondary lymphoid tissues
Mature dendritic cells that are typically in the lymph nodes, they specialize in interacting with T cells

Question 3

Where does the dendritic cell and T cell interactions happen?

Answer 3

The interaction happens in the outer most part of the cortex of the lymph nodes. The place where T cells congregate.

Question 4

Who can activate naïve CD8 T cells?

Answer 4

Dendritic cells are the only cells capable of activating naïve CD8 T cells.

Question 5

Who can activate naïve CD4 T cells?

Answer 5

Naïve CD4 T cells are mainly activated by dendritic cells as well, but it can also be activated by macrophages

Question 6

What is cross presentation pathway and why is it necessary?

Answer 6

MHC I pathway is normally used to present endogenous antigens that infected a cell. MHC I is expressed by all cells because all cells are vulnerable to viral infections.
However, most viral infections are specific to certain tissue types. And the T cell activations can only happen in spleen and a secondary lymphoid tissues.
Cross-presentation is of particular importance, because it permits the presentation of exogenous antigens, which are normally presented by MHC II on the surface of dendritic cells, to also be presented through the MHC I pathway. The MHC I pathway is normally used to present endogenous antigens that have infected a particular cell. However, cross presenting cells are able to utilize the MHC I pathway in order to remain uninfected, while still triggering an adaptive immune response of activated cytotoxic CD8+ T cells against infected peripheral tissue cells.

Question 7

What cell is capable of cross presentation?

Answer 7

Mainly dendritic cells

Question 8

How is dendritic cell activated?

Answer 8

Dendritic cell is activated it through TLRs . All the toll like receptors are expressed by dendritic cells, making them highly sensitive to the presence of all manner of pathogens.

Question 9

How does T cells reach the lymph nodes?

Answer 9

There are two routes circulating naïve T cells can enter a lymph node.
The first route is for T cells to enter the lymph node in the arterial blood and then transfer into the lymphoid tissue at a high endothelial venule (HEV).
The second route is for T cells to enter the lymph node in the afferent lymph that comes from an upstream lymph node.

Question 10

What is homing?

Answer 10

The process by which naïve T cells leave the bloodstream and enter the T cell zone of a lymph node is called homing.

Question 11

How is homing achieved and regulated?

Answer 11

This process is determined and controlled by a series of chemokines and cell adhesion molecules

Question 12

How do naïve and activated T cells leave the lymph nodes?

Answer 12

This process is controlled by a receptor called the S1P receptor. The S1P receptor expressed on a T cell surface is capable of recognizing S1P, a lipid made in all cells. And the S1P gradient helps T cell leave the lymph node in the efferent lymph.

Question 13

How many signals are required for T cell activation?

Answer 13

1. TCR + Antigen peptide: MHC complex ligation + C3
2. T cell co-receptor (CD4 or CD8) + MHC molecule ligation
3. CD28 (expressed by T cells) + B7 (expressed by dendritic cells) ligation

Question 14

What is B7 and its properties

Answer 14

B7 is a molecule is expressed by the dendritic cells and it is a co-stimulator. Co-stimulators are exclusively expressed by professional antigen presenting cells. Its expression is not constitutive but depends on the presence of infection. This ensures that naïve T cells, do not respond to specific antigen in the absence of infection, and also prevents the activation of your T cells expressing receptors that recognize self antigens.

Question 15

What is a co-stimulators receptor?

Answer 15

CD28 and CTLA4 are both co- stimulator receptors. CD 28 is the only B7 receptor in all naïve T cells. CTLA4 is structurally similar to CD28 with much higher binding affinity.
The interaction of B7 and CTLA4 acts as a break that inhibits both of the activation and proliferation of T cells.

Question 16

The intracellular transduction pathway to activate T cells

Answer 16

Clustering of T-cell receptors and co-receptors initiates signaling within the T cell (as oppose to cross-linking and clustering of antigen receptors that are required for B cell intracellular transduction)

Lck (Kinase): normally attached to the cytoplasmic tail of CD4; after clustering of CD4 and antigen receptors, it can phosphorylate CD3 ITAMs

ITAMs (immunoreceptor tyrosine based activation motifs)

ZAP70: a tyrosine kinase and activated by Lck. An essential kinase in T cell signal transduction.

ZAP70 leads to transcription factor activation

Question 17

Autocrine vs Paracrine

Answer 17

T-cell proliferation is stimulated by the IL-2 released by the same cell is call Autocrine.

Question 18

IL-2

Answer 18

IL2 is at the center of T cell proliferation and differentiation. Naïve T cell expresses low affinity IL2 receptor. Once the T cell is activated, it starts to make IL-2 and a high affinity IL2 receptor.

Question 19

What is T-cell anergy / permanent state of tolerance?

Answer 19

In the absence of B7 and co-stimulation, ligation of TCR and co-receptor + antigen:MHC complex gives rise to a signal that causes the T cell to enter a state of anergy (ie no longer respond to any external signal)
It is an irreversible mechanism of self tolerance. The defining characteristic of anergic T cells is their failure to make IL-2 so they could not proliferate and differentiate.

Question 20

What is a adjuvant in protein vaccine?

Answer 20

An adjuvant is needed with a pure protein vaccine due to lack of co-stimulators signal.

Question 21

What are the five types of effector CD4 T cells risen from naive CD4 T cell activation?

Answer 21

CD4 T cells: Th1, Th2, Th17, Tfh and Treg

Question 22

Why does naïve CD8 T cells requires stronger activation than that for naïve CD4 T cells?

Answer 22

Because CD8 T cells, even though are functionally homogenous, they need to interact with a wide range of target cells. These comprise all cells that are susceptible to viral or microbial infection, which is in essence all the cells of human body.

Question 23

What are the key differences between effector T cells and naïve T cells?

Answer 23

1. They are distinguished by the adhesion molecules present on their cell surfaces.
   Naïve T cells: L-selectin: enables entering secondary lymphoid tissues
   Effector T cells:
   no L-selectin, therefore prevents re-entering secondary lymphoid tissues;
   VLA-4: an intergrin that binds VCAM-1 and enables them to leave the blood and enter tissues that are infected
   Increased expression of LFA-1 and CD-2
2. They’re also distinguished by their activation. Co- stimulation mediated by CD 28 binding to B7 is not needed to activate the effector T cell response.

Question 24

How do effector T cell functions?

Answer 24

Effector T cell functions are mediated by cytokines and cytotoxins
T cell synapse is formed in which a T cell receptor and Co-receptor engage the peptide antigen bonded to MHC on the target cell. These effector proteins, such as cytotoxins and cytokines, are delivered to the target cell via the synapse.

Question 25

What are cytokines functions?

Answer 25

Cytokines changes patterns of gene expression in the cells targeted by effector T cells, such as the JAK-STAT signaling pathway

Question 26

Describe JAK-STAT signaling pathway

Answer 26

JAK: Janus kinases, a form of tyrosine kinase
STATs: signal, transducers, and activators of transcription

Question 27

How is JAK-STAT naturally inhibited?

Answer 27

It can be inhibited by phosphatases and suppressors of cytokines signaling (SOCs)

Question 28

How does the cytotoxic T cells kill its target cells?

Answer 28

Cytotoxic T cells kills their target cells by inducing a apoptosis

Question 29

What is Th1 cell’s main function?

Answer 29

macrophage activation; the principal function of Th1 CD4 cells is to make a macrophages more proficient in the uptake and killing on pathogens

Question 30

How does Th1 cell activate macrophages?

Answer 30

Macrophage requires two signals for an activation:
1. INF-gamma; which binds the INF-gamma receptor on macrophages
2. CD40 ligand; binds to CD40 on the macrophages

Question 31

The synapse formed by macrophages and Th1 cell enables?

Answer 31

It ensures that a macrophage activation is an antigen specific process

Question 32

What is Tfh’s main function

Answer 32

Main function is to activate B cells; such a cooperation between T cells and B cells is at the core of the adaptive immune response, and was first form of the T cell help to be described by immunologists;
This activation also happens in secondary lymphoid tissues, TFH cells move from the T cell area of the secondary lymphoid tissues to the nearby B cell area

Question 33

What is Treg’s main function?

Answer 33

It releases immunosuppressive cytokines: IL-4, IL-10 and TGF-beta

Question 34

Describe the path of a T cell activation and function

Answer 34

1. All forms of adaptive immune response are started by the activation of pathogen specific naïve T cells
2. T cell activation takes place in secondary lymphoid tissues where dendritic cells carrying antigens from the site of infection meet antigen specific naïve T cells coming from the blood or lymph.
3. Antigen specific naïve T cells are activated by T cell receptor and CD3: antigen MHC complex & CD4 or CD8 co-receptor and co-stimulatory signal (CD28: B7)
4. The naïve T cell activation, induces many changes in the T cells pattern of gene expression and protein synthesis. Will be nursed by dendritic cells until differentiation is completed. IL2 plays a major role in differentiation.
5. Different cell adhesion molecules expression to enable the activated T cells leave the secondary lymphoid tissue and enter the bloodstream to eventually go to the infected peripheral tissue.
6. Effector T cell does not need co-stimulatory signals from B7 and only need to be activated by specific antigen
7. Once activated, they form synapses with the target cells and release cytokines and cytotoxins

Question 35

T cell priming?

Answer 35

T cell priming refers to the process by which naive T cells are activated and differentiate into effector T cells in response to an antigen. This process occurs in secondary lymphoid organs such as lymph nodes and spleen.

Question 36

Linked recognition?

Answer 36

Cooperation between Tfh and naïve B cells, bearing specificity for different epitopes of the same antigen and where is the B cells serves as an antigen presenting cells

Question 37

What signals are required for B cell activation?

Answer 37

1. B cell activation requires cross-linking of the B cell receptors by antigens.
2. B cell activation also requires signals from the B cell co-receptors (CD19, CR2/CD21)
3. The activation of most naïve B cells (B2 cells) also requires conjugation with a CD4 Tfh cell that recognizes pathogen derived peptides presented by MHC class II on the B cell

Question 38

The steps required for B cell activation?

Answer 38

1. Follicular dendritic cells in the B cell area store intact antigens and display them to B cells.
2. Antigen activated B cells, move close to the T cell area to find a Tfh cell
3. Once activated, they move to medullary cords for clonal expansion
4. Somatic hypermutation and isotype switching occur in the specialized microenvironment of the primary follicle of the cortex
5. Antigen mediated selection of the centrocytes drives affinity maturation of the B cell response in the germinal center of the cortex
6. Cytokines made by Tfh cells guide B-cell isotope switching

Question 39

What is cross-linking of the B cell receptors by antigen?

Answer 39

Cross-linking of the B cell receptors is required to initiate the primary B cell response

Cross-linking —> ITAMs of Ig-alpha and Ig-beta are phosphorylated —> Syk (tyrosine kinase) binding —> change gene expression

Question 40

Is B cell co-receptor required for B cell activation?

Answer 40

B cell co-receptor is required for B cell activation

The B cell co-receptor comprises three proteins:

1. CR2 / CD21 (Complement receptor 2) — (binding) C3d (C3b derivatives of C3b fragments deposited on a pathogen)
2. CD19 (signaling chain of the co-receptor)
3. CD81 (binds CD19 and brings it to the B cell surface)

Question 41

How do B cell receptors and co-receptor cooperate in B cell activation?

Answer 41

The cytoplasmic tale of CD19 is phosphorylated by tyrosine kinase associate with the B cell receptor

Question 43

First step of maturation for naive B cells?

Answer 43

Antigens presented by FDC will be processed and presented by B cells.

FDC: Follicular dendritic cells; they are different from the myeloid dendritic cells that present antigens to naïve T cells, and different from myeloid dendritic cells that secrete type I interferons

If antigen is recognized by the B cell receptor, CD69 is expressed and prevents surface expression of the S1P receptor by B cell which keeps B cell in the lymph node for continuous differentiation.

Question 44

CD4 Tfh and B cell synapse

Answer 44

Effective B cell mediated immunity depends on help from CD4 Tfh cells

Once the cognate pair between CD4 Tfh and B cell is formed, Tfh cell will express CD40 ligand — binding CD40 on B cells

Once CD40L—CD40 —> Activation of NFkB —> CDM—> release cytokines for B cell differentiation

The cognate pair moves from cortex —> Medullary cords —> both cells divide and proliferate —> B lymphoblasts / plasmablasts (stage between immature and mature) —> secreting IgM —>

Part goes to in the infection site and the other part moves back to primary follicle in the cortex for Somatic hypermutation and Isotype switching

B1 cells don’t need CD4 Tfh help

Question 45

Where does somatic hypermutation and isotype switching happen?

Answer 45

It happens in the primary follicles of the cell area —> the cognate pair of B and T cells cause both B and T cells to divide and proliferate —> this interaction induces the B cells to produce AID

AID (activation induced cytidine deaminase): A DNA modifying enzyme that is essential for somatic hypermutation and isotype switching —> Centre lasts —> Germline center formation in the secondary follicle —> affinity maturation

Question 46

How does affinity maturation work?

Answer 46

After hypermutation, a centrocyte’s B cell receptor can have a affinity for specific antigen that is higher lower or equal to that of the unmutated immunoglobulin.

Newly formed centrocyte, moved to the light zone and compete for access to the limited amount of antigen displayed on the dendrites of the FDCs

If a centrocytes’ BCR binds antigen with sufficient strength, FDC and centrocyte form a synapse —> moving to a region where Tfh concentrates —> engagement of Tfh — B cell MHC II antigen complex —> expression of Bcl-Xl, a protein that protects cells from apoptosis

Question 47

what other functions does Tfh cell serve?

Answer 47

1. Cytokines made by Tfh cells guide BCL isotype switching
2. Tfh also determines the differentiation of antigen activated B cells into plasma cells, or memory cells.