Lecture 10 - T-cell mediated responses II

Question 1

Tregs: what do they do, what are the types, and what is their master transcription factor?

Answer 1

Suppress harmful T-cell responses - prevent self-recognising CD4+ T-cells from triggering an immune response

• Foxp3+ - CD4+/CD25+, the ‘typical’ Treg with the Foxp3 master TF
• Tr1 - CD4+/Foxp3-, produced by IL-10 stimulation, secrete IL-10 and TGFβ
• Th3 - CD4+/Foxp3-, produce large amounts of TGFβ

Question 2

Foxp3+ Tregs: what do they present on their surface, what are the types, and how are they developed?

Answer 2

CD4+ and CD25+

• natural/thymic Tregs (nTregs/tTregs)
• Induced/peripheral Tregs (iTregs/pTregs)

In the thymus:
* Two-step model
* TGFβ dependent

In the periphery:
* TGF-β dependent - TGF-β, IL-2, retinoic acid, vitamin D (the latter three require TGF-β presence)

Question 3

Thymus Treg development: what are the mechanisms of the Two-step model?

Answer 3

• Strong TCR engagement leads to upregulation of CD25
• CD25 is a high affinity IL2R and will react to IL-2 which promotes Foxp3 expression

Question 4

Thymus Treg development: what are the mechanisms of the TGFβ-dependent model?

Answer 4

• TCR stimulus in the thymus along with TGFβ signalling results in Foxp3 expression

Question 5

Periphery Treg development: what are the mechanisms?

Answer 5

TGF-β dependent, the molecules promoting Treg development require TGF-β presence:
* TGF-β
* IL-2
* Retinoic acid
* Vitamin D

Question 6

Tr1 cells: what are their surface markers, what are they produced by and what do they secrete?

Answer 6

CD4+ (Foxp3-)

Produced by IL-10 stimulation

Secrete IL-10 and TGFβ

Question 7

Th3 cells: what are their surface markers and what do they secrete?

Answer 7

CD4+ (Foxp3-)

Secrete large amounts of TGFβ

Question 8

IPEX: what is it and what happens if you have it?

Answer 8

Immunodysregulation polyendocrinopathy enteropathy X-linked syndrome

Malfunctions in Tregs - often result in autoimmune diseases

Question 9

Foxp3+ Tregs: how do they suppress T cells?

Answer 9

• Cytokine dependent - produce TGF-β and IL-10, suppressing activated T-cells or preventing T-cells from becoming activated
• Induction of effector T-cell death - Granzymes/perforin induce cell death
• Disruption of effector T-cell metabolism - Taking up IL-2 and preventing T-cells from getting the required survival signal and converting ATP to adenosine, removing a T-cell energy source
• Targeting of dendritic cells - binding B7 on DCs and preventing TC-DC association

Question 10

Cytokine dependent T-cell suppression

Answer 10

Produce TGF-β and IL-10 (anti-proliferative cytokines), suppressing activated T-cells or preventing T-cells from becoming activated

Question 11

Induction of effector T-cell death

Answer 11

Perforin and granzyme secretion causes apoptosis of T-cells

Question 12

Disruption of effector T-cell metabolism

Answer 12

• Take up IL-2 at a greater rate than CD4+ T-cells through CD25+ so they can’t become activated
• Can also produce adenosine from ATP which inhibits energy-requiring T-cells

Question 13

Targeting of dendritic cells to suppress T-cells

Answer 13

CTLA4 binds to CD80 and CD86 (signal 2 of T-cell activation) with higher affinity, making it unavailable for T-cells by both downregulating it and transendocytosis

Question 14

CTLA-4

Answer 14

Cytotoxic T-lymphocyte-associated protein

Question 15

Treg plasticity: are Tregs able to present multiple T-cell TFs, why, and what is a TF that is expressed in all Tregs?

Answer 15

Foxp3+ should be unable to become another type of T-cell - however hybrid Tregs can occur (ie Foxp3 and T-bet)

Hybrid Tregs become hyper-specialised to shut down the type it is (T-bet hybrid can be localised better to Th1 areas and can then shut them down more efficiently)

GATA3 is expressed in all Tregs

Question 16

CD8+ cells: what is required for its activation, what are the pathways for its activation, and why is the second pathway used most of the time?

Answer 16

DC MHC I presentation through self-infection presentation or cross-presentation (prevents unwanted destruction)

• Direct DC activation, - MHC I, strong costimulation, then IL-2 (etc) production
• CD4+ induced activation - once CD4+ t-cells are activated, they bind to DCs and promote expression of costimulatory molecules

Needs much higher costimulation than CD4+ T-cells so often needs help from CD4+ T-cells for activation

Question 17

CD8+ direct DC activation: how does it work, how frequently does it happen, and what happens?

Answer 17

MHC I binds followed by strong costimulation resulting in activation

Not often - such high levels of costimulation needed

• IL-2 production results in proliferation of antigen-specific CTLs, results in the death of infected cells

Question 18

CD8+ indirect activation: how does it work?

Answer 18

• CD4+ T-cells are activated by DCs
• CD4+ T-cell costimulatory molecules induce higher expression of costimulatory molecules in the dendritic cell
• Enhanced costimulation activates CD8+ T-cells
• Activated CD4+ T-cells produce IL-2 which helps drive CD8+ T-cell proliferation

Question 19

CD8+ T-cell sensitivity to costimulation

Answer 19

• Require much more costimulation to be activated than a CD4+ T-cell
• Much more sensitive to PD1 costimulatory inhibition

Question 20

CTLs: what are they, what do they do, what cytokines are they affected by, how is collateral damage prevented, and what is the process of their killing?

Answer 20

Cytotoxic T-lymphocytes - CD8+ T-cells

• Recognise cell(s) and programs cell death
• Produce cytokines (ie IFNγ, IL-2, TNFα, LTα)

IL-2 promotes proliferation, IFNγ promotes differentiation

Secrete cytotoxic granules directly at the target cell by remodelling its cytoskeleton and pointing its filaments towards the target cell

Cytotoxic granules released:
* Perforin - forms pores in the membrane
* Granzyme - uses holes formed by perforin to enter the target cell and can then induce cell death

Question 21

Granzyme-induced cell death pathways: what are they and how do they work?

Answer 21

BID pathway:
* BID cleaved by granzymes
* Truncated BID disrupts mitochondrial outer membrane
* Cytochrome C released
* Caspase 9 activated
* DNA cleavage - cell death induced

Pro-caspase-3 pathway:
* Cleaves pro-caspase-3 into caspase 3
* Caspase 3 cleaves ICAD forming CAD
* CAD cleaves DNA - cell death induced

Question 22

CTLs: what cytokines may that release and what do they do?

Answer 22

• IFNγ to activate macrophages, induce MHC I expression, inhibit viral replication, etc
• TNF

Question 23

iCAD: what is it, what pathway is it in, and what does it produce?

Answer 23

Inhibitor of caspase-activated DNase

Pro-caspase-3

(CAD)

Question 24

CAD: what is it, what pathway is it in, and what does it produce?

Answer 24

Caspase-activated DNase

Pro-caspase-3

Cleaves DNA, causing DNA damage and inducing cell death

Question 25

TNFα and LTα: what are they, what are they produced by, and what do they do?

Answer 25

Tumour necrosis factor α and lymphotoxin α

CTLs

Help IFNγ to activate macrophages