CD4+ & CD8+ T cells (cell mediated immunity II)

Question 1

What is the general role of effector CD4+ T cells? How is this achieved?

Answer 1

Effector CD4+ T cells enhance the function of other cells of the immune system to eradicate pathogens (rather than directly destroying the pathogens themselves).

Effector CD4+ T cells do this by SECRETING SPECIFIC CYTOKINES that enhance/activate the function of other immune cells.

Question 2

There are different subsets of effector CD4+ T cells.

T helper 1 cells target _____ pathogens.

T helper 2 cells target ______ _______.

T helper 17 cells target __________ _______.

T follicular helper cells target _____ types.

T regulatory helper cells regulate ___ _____ _______.

Answer 2

intracellular

helminth parasites

extracellular bacteria

all types (activate B cells)

T cell responses

Question 3

What are the three main features that characterize CD4+ T cell subsets?

Answer 3

1) polarizing (“fate-specifying”) cytokines
2) master transcriptional regulator
3) effector cytokines

Question 4

1) What are polarizing or fate-specifying cytokines?

2) How do polarizing cytokines carry out their function?

3) Are the polarizing cytokines the same for all?

Answer 4

1) Polarizing cytokines are cytokines produced by APC (one of the three signals required for T cell activation by APC) and delivered to T cells that communicate what subset the T cell should be.

2) Polarizing cytokines induce the expression of the MASTER TRANSCRIPTIONAL REGULATOR that, in turn, regulates the expression of a signature set of EFFECTOR cytokines.

These cytokines work with signals from the TCR and costimulatory molecules during priming to determine the FATE of the CD4+ T cells.

3) No, they are not the same. The polarizing or fate-specifying cytokines produced by the APC DEPEND on the type of pathogen (PAMPs) recognized by PRRs on the APCs. Different cytokines are secreted from APCs, generating different subsets of T cells.

Question 5

What are the fate-specifying cytokines of:

1) T(H)1 cells

2) T(H)2 cells

3) T(H)17 cells

Answer 5

T(H)1 cells: IFN-y, IL-12

T(H)2 cells: IL-4

T(H)17 cells: TGF-β, IL-6, IL-23

Question 6

What are the master transcriptional regulators of:

1) T(H)1 cells

2) T(H)2 cells

3) T(H)17 cells

Answer 6

T(H)1 cells: T-bet

T(H)2 cells: GATA-3

T(H)17 cells: RORyT

Question 7

What are the effector cytokines of:

1) T(H)1 cells

2) T(H)2 cells

3) T(H)17 cells

Answer 7

T(H)1 cells: IFN-y

T(H)2 cells: IL-4, IL-5, IL-13

T(H)17 cells: IL-17, IL-22

Question 8

For some subtypes of T cells, the effector cytokines generated are the same as the polarizing cytokines. What does this lead to?

Answer 8

Amplification/creating more of those T cells.

*by creating more of the polarizing cytokines, more cells are transformed into that specific subtype.

Question 9

What are the general events that take place during the differentiation of naive CD4+ T cells into effector CD4+ T cells?

Answer 9

Dendritic cell displaying foreign antigen migrates to peripheral lymphoid organ from site of infection while naive T cell are also entering the lymph node via the bloodstream.

Naive CD4+ T cells recognize antigens presented by dendritic cells in peripheral lymphoid organs and are stimulated to PROLIFERATE and DIFFERENTIATE into EFFECTOR cells, and then enter the circulation.

Effector CD4+ T cells migrate through blood vessels in tissues by BINDING to ENDOTHELIAL cells that have been ACTIVATED by CYTOKINES produced in response to infection in the target tissues.

Effector T cells recognize the antigen in the tissues and respond by secreting cytokines that recruit more leukocytes and activate phagocytes to eradicate the infection.

Question 10

Fill in the blanks regarding the differentiation of naive CD4+ T cell to the T(H)1 subset.

1) In response to ______ microbes, ____ is produced by dendritic cells and macrophages, while _____ is produced by NK cells.

2) The two polarizing cytokines activate the master transcriptional regulator ______ which promotes the expression of the effector cytokine, _____.

3) The effector cytokine contributes to ________ activation which enables them to destroy __________ microorganisms more efficiently.

Answer 10

1) intracellular; IL-12; IFN-y

2) T-bet; IFN-y

3) macrophage; intracellular

*intracellular pathogens: pathogens that can survive/replicate within macrophages (intracellular bacteria, viruses, protozoans).

Question 11

(T/F) Intracellular pathogens can evade macrophage defences by several mechanisms. IFN-y secreted by the T(H)1 cells serves to enhance the microbicidal activity of macrophages so that it can destroy the pathogen and control its growth + spread.

Answer 11

True1

Question 12

(T/F) T(H)1 cells only have one function and it is to activate macrophages.

Answer 12

False!

T(H)1 cells do more than just activate macrophages.

The goal is to coordinate immune responses to target intracellular pathogens which involves other activities too such as inducing monocyte differentiation in the bone marrow, accumulation of macrophages at site of infection, etc.

Question 13

Fill in the blanks regarding the differentiation of naive CD4+ T cell to the T(H)2 subset.

1) The T(H)2 fate-specfying cytokine _____ is produced by activated __ cells or by ___ cells and ______, especially in response to ________.

2) This cytokine activates the master transcriptional regulator _______, which stimulates the naive cell to the subset.

3) T(H)2 cells produce ____, ____, and _____, each of which serves a critical role in the T(H)2 response.

Answer 13

1) IL-4; T; mast; eosinophils; helminths

2) GATA-3

3) IL-4, IL-5, and IL-13

Question 14

1) What are helminth parasites?

2) Briefly describe the roles of effector cytokine secreted by T(H)2 cells.

Answer 14

1) Helminth parasites are multicellular organisms that are too large to be efficiently enfolded by macrophages.

2) T(H)2 cells produce cytokines that recruit and activate EOSINOPHILS (IL-5) and MAST CELLS (IL-4) and promote enhanced barrier immunity at mucosal surface to eradicate helminths (IL-13).

TH2 cells function to EXPEL INTESTINAL HELMINTHS and to REPAIR TISSUE INJURY; TH2 cells work in conjunction with IL-4-secreting T(FH) cells to produce high levels of IgE which ACTIVATE mast cells & eosinophils.

Question 15

What are the general events that take place during the differentiation of naive CD8+ T cells into effector CD8+ T cells?

Answer 15

Dendritic cell displaying foreign antigen migrates to peripheral lymphoid organ from the site of infection. Naive CD8 + T cell enters the lymph node via the bloodstream.

Naive CD8 + T cells recognize antigens presented by dendritic cells in peripheral lymphoid organs, causing them to proliferate & differentiate into effector (and memory) cells, which then enter the circulation.

Effector CD8 + T cells migrate to TISSUES at the SITE of INFECTION (or tumour growth, graft rejection). These cells recognize the antigen (via MHC I presentation) and subsequently respond by killing the cell where the antigen is produced.

Question 16

State the sentences as true or false.

1) CTLs are specialized to kill cells infected with intracellular pathogens. They kill target cells that display peptide fragments of cytosolic pathogens, most notably fungi, bound to MHC class I molecules.

2) CD4+ T cells target cancer cells.

3) Since the effector function of CTLs is so destructive (i.e. they kill their target cells), naive CD8+ T cells require additional co-stimulatory activity to drive them to become activated effector cells.

Answer 16

1) false: CTLs are specialized to kill cells infected with intracellular pathogens. They kill target cells that display peptide fragments of cytosolic pathogens, most notably VIRUSES, bound to MHC class I molecules.

2) false: CD8+ T cells target cancer cells.

3) true! these cells require an extra layer of regulation to become activated!

Question 17

Which cells provide the additional co-stimulatory activities needed to drive naive CD8+ T cells to become activated effector CD8+ T cells?

Answer 17

Most CD8+ T cell responses require help from CD4+ T cells!

CD4+ T cells that recognize related antigens presented by the APC can amplify the activation of naive CD8+ T cells by further activating the APC.

Question 18

The APC first activates the CD4+ T cells to express _____ and _____ ligand.

This ligand binds _____ on the APC which delivers a signal that increases the expression of ____ and ______ on the APC.

4-1BBL then binds to ____ on the CD8+ T cell, which in turn provides additional co-stimulation to the naive CD8+ T cell

The IL-2 produced by activated CD4+ T cells also acts to promote effector CD8+ T-cell ___________.

Answer 18

IL-2; CD40

CD40; B7; 4-1BBL

4-1BB

Differentiation

Question 19

Which subset of CD4+ T cells provide help for CD8+ T cells?

Answer 19

T(H)1

*help = additional layer of regulation to become activated since the effector functions of CTLs are so destructive.

Question 20

Which one of the statements is true?

1) Specific recognition of peptide:MHC I complexes on a target cell by a cytotoxic T cell leads to the death of the target cell by apoptosis.

2) Cytotoxic T cells can not be recycled to kill multiple targets.

Answer 20

1!

Cytotoxic T cells CAN be recycled to kill multiple targets.

Question 21

1) What kind of pathways do the CTLs use to induce apoptosis?

2) What is the difference between the two pathways?

3) What are the similarities between the pathways?

Answer 21

1) Cytotoxic T cells can induce apoptosis in target cells via the intrinsic or extrinsic pathways of apoptosis (can initiate both).

2)
CTLs use perforin GRANZYME-mediated cell killing to initiate the mitochondrial (intrinsic) pathway.

CTLs use FAS/FasL-mediated cell killing to initiate the death receptor (extrinsic) pathway.

3) Both pathways rely on cysteine proteases called “caspases and so they converge on the executioner caspase 3.

Question 22

Briefly describe the mechanisms of the intrinsic and extrinsic pathways used by CTLs to induce apoptosis.

Answer 22

1) Perforin/granzyme-mediated cell killing: when a CTL binds a target cell, CTLs release granule contents into the immune synapse. Perforin induces the uptake of granzymes into target cell endosomes and release into cytosol, activating caspases.

2) Fas/Fas-L mediated cell killing: FasL on CTL interacts with Fas on the target cell, causing apoptosis of the target cell.

Question 23

Match the stages of CTL-mediated killing of target cells by perforin/granzyme to the right order:

1) Stage 1
2) Stage 2
3) Stage 3
4) Stage 4
5) Stage 5

A) CTL Granule Exocytosis - granzyme and perforin released

B) Dissociation of CTL from target cell

C) Conjugate Formation - very close contact of the CTL with the target cell, preventing granule content from spreading to nearby non-infected cells.

D) Apoptosis of target cell

E) CTL Cytoplasmic Rearrangement - allows for secretion and transport of the granule directly to the ZONE OF CONTACT.

Answer 23

Stage 1: Conjugate Formation - very close contact of the CTL with the target cell, preventing granule content from spreading to nearby non-infected cells.

Stage 2: CTL Cytoplasmic Rearrangement - allows for secretion and transport of the granule directly to the ZONE OF CONTACT.

Stage 3: CTL Granule Exocytosis - granzyme and perforin released

Stage 4: Dissociation of CTL from target cell

Stage 5: Apoptosis of target cell

Question 24

(T/F) Multiple CTLs can work on one target cell.

Answer 24

True!

Question 25

The point of membrane contact between CTL and a target cell is mediated by interaction between ____ and ____.

Answer 25

LFA-1 & ICAM-1

Question 26

1) What kind of binding occurs initially with a CD8 cell and the target cell? What does this allow for?

2) What happens when the target cell does not present the specific antigen of the CD8 cell?

3) What happens when the CD8 cell finds its specific antigen?

Answer 26

1) The initial interaction of CD8 cell with target is made by NONSPECIFIC adhesion molecules. This nonspecific binding allows the CTL to remain in contact with target cell and SCAN THE TARGET CELL SURFACE for specific peptide:MHC complexes.

2) If the target cell does not present its specific antigen, the CTL DISSOCIATES and SCANS other cells UNTIL IT FINDS its specific antigen.

3) When there is antigen-specific recognition, the signalling though the TCR INCREASES STRENGTH of the adhesive interactions. This prolongs the contact between the two cells and stimulates the CTL to deliver its effector molecules.

Question 27

What is CTL polarization?

Answer 27

Polarization of CTLs occurs when there is binding of the TCR to the peptide:MHC complexes on the target cell.

It is when there is a REORGANIZATION of ACTIN cytoskeleton at the site of contact, which aligns the MICROTUBUBLE ORGANIZING CENTRE (MTOC). This aligns the secretory apparatus towards the target cell.

Question 28

(T/F) Collision and initial binding of CTL to target cell has an effect on the location of granules.

Answer 28

FALSE!

Collision and initial binding of CTL to target cell DOES NOT HAVE AN EFFECT on the location of granules.

Question 29

Why is CTL polarization important?

Answer 29

After CTL polarization, proteins stored in CTL granules are specifically directed onto the target cell at the site of contact.

This polarization is important to ensure that the CTL or uninfected neighbouring cells are not killed.

Question 30

What is the difference between the granules in unbound cytotoxic T cells vs a cytotoxic T cell bound to a target cell?

Answer 30

Unbound cytotoxic T cell: granules DISPERSED THROUGHOUT the cell.

Cytotoxic T cell bound to target cell: granules cluster at site of CELL-CELL contact.

Question 31

Effector molecules are released from T-cell granules in a HIGHLY POLAR FASHION.

What happens to the granules:

1) initially where there is contact with a target cell

2) after 1 minute of contact

3) after 4 minutes

4) after 40 minutes

Answer 31

1) initially, the granules are DISTANT from the point of contact

2) the granules begin to move toward the target cell after 1 minute

3) granules complete their migration after 4 minutes

4) granule contents are released into the space between the CTL and target cell. the target cell begins to undergo apoptosis!

Question 32

What are the three proteins in granules of cytotoxic T cells and their actions on target cells?

Answer 32

1) Perforin: polymerizes in the target cell membrane to form pores, facilitating delivery of granzyme into the target cell

2) Granzymes: are serine proteases which activate apoptosis in the cytoplasm of target cell by cleaving various proteins (BID & pro-caspase 3)

3) Granulysin: has antimicrobial actions and can induce apoptosis

Question 33

Fill in the blanks regarding the mechanism of perforin and granzyme:

1) Engagement of TCR by peptide:MHC complex causes directed release of _______ and ______ complexed with _________.

2) Granzyme __ is delivered into the cytosol of the infected cell via pores formed by performin and targets ___ & _______

3) Truncated BID (tBID) disrupts ________ ______ membrane and activated caspase 3 cleaves ______, releasing ______ ______ _______.

4) Release of _______ __ into cytosol activates apoptosis, and _____ induces DNA fragmentation.

Answer 33

1) perforin; granzymes; serglycin (a scaffold protein)

2) B; BID; pro-caspase 3

3) Mitochondrial outer; ICAD; caspase-activated DNase (CAD)

4) cytochrome c; CAD