Co-stimulatory molecules & T cell signalling

Question 1

What 3 signals does primary T cell activation require?

Answer 1

Signal 1 = TCR recognition of correct MHC peptide combination
Signal 2 = Co-stimulatory molecules engagement
Signal 3 = cytokine instruction

Question 2

What 2 things do the TCR recognise?

Answer 2

MHC protein itself
Antigenic peptide presented by MHC protein

Question 3

How does the TCR recognise MHC complexes?

Answer 3

TCR recognises both MHC protein and peptide antigen being presented by it
Binds with diagonal footprint that cuts across both alpha helices with the peptide in between

Question 4

What is CD8 required for?

Answer 4

Acts as a co-receptor for MHC I
Required for T cell to make an effective response

Question 5

How can TCR and CD8 bind to MHC I at the same time?

Answer 5

Distant binding sites
TCR binds to alpha-1 alpha-2 domains
CD8 binds to the support domains (alpha-3 and beta-2m)

Question 6

What happens after TCR engagement?

Answer 6

Triggers signalling responses inside T cell
Up regulates expression of many genes
Main function is upregulation of IL-2 and IL-2 receptor which sets up an amplificatory loop that drives T cell clonal expansion

Question 7

What does the TCR assemble with to form a signalling complex?

Answer 7

CD3 subunits and zeta chain/ZAP70 in ER
Allows the receptor to transmit signals into the cell following activation

Question 8

What does TCR engagement trigger activation of?

Answer 8

Nuclear Factor for Activated T cells (NF-AT)

Question 9

What does ZAP70 do?

Answer 9

Protein that when activated, signals the cell to release calcium from intracellular stores
Calcium will bind to phosphatase
Phosphatase will take off phosphates from NF-AT, activating it

Question 10

Where is NF-AT usually found?

Answer 10

In the cytoplasm

Question 11

What happens when NF-AT is dephosphorylated?

Answer 11

Goes into nucleus and can bind to its target promoters by sequence specific recognition of DNA
Binds to IL-2 promoter and turns it on

Question 12

What happens when NF-AT is dephosphorylated?

Answer 12

Goes into nucleus and can bind to its target promoters by sequence specific recognition of DNA
Binds to IL-2 promoter and turns it on

Question 13

What is central tolerance?

Answer 13

T cells with high affinity for self are eliminated in the thymic medulla

Question 14

What T cells are allowed to leave the thymus?

Answer 14

Those with moderate affinity for self
These T cells are still naive (antigen inexperienced)

Question 15

What are anergic T cells?

Answer 15

If antigen challenge not accompanied by signals of infection then T cells not activated and enter a period of extended inactivity (anergic)
Also called peripheral tolerance
NF-AT does not bind IL-2 promoter

Question 16

What does peripheral tolerance help to ensure?

Answer 16

Lack of self reactivity

Question 17

What is the second signal?

Answer 17

Survival signal
Upregulation of costimulatory proteins on DCs
B7.1and B7.2 (CD80 and CD86) recognised by CD28 on T cell surface

Question 18

How can T cells be activated without co-stimulation?

Answer 18

A humanised monoclonal anti-CD28 antibody called TGN1412 is capable of binding to the CD28 receptor and activating signalling

Question 19

How can co-stimulation be enhanced?

Answer 19

CD40L on T cells binds CD40 on DCs to increase production of co-stimulatory proteins

Question 20

How can co-stimulation be decreased?

Answer 20

CTLA-4 on T cells binds co-stimulatory proteins on DCs with higher avidity than CD28 so acts as a decoy

Question 21

Describe CTLA-4 and its significance

Answer 21

It is a negative regulator of T cell activation
It is constitutively expressed in Treg cells but only upregulated in conventional T cells after activation
It acts as an off switch when bound to CD80 or cd86 on the surface of APCs

Question 22

What is the 3rd signal required for T cell activation?

Answer 22

Differentiation signal
Release of cytokines by APC to stimulate differentiation of the naive T cell
e.g. IL-6, IL-12, TGF-B, IL-4

Question 23

What are the cytokines required by CD8+ cells to develop effector functions?

Answer 23

IL-12
Type 1 IFNs

Question 24

What cytokine induces development of Treg cells?

Answer 24

TGF-B

Question 25

What cytokine induces development of Tfh cells?

Answer 25

IL-6

Question 26

What cytokine induces development of Th17 cells?

Answer 26

TGF-B
IL-6

Question 27

What cytokine induces development of Th1 cells?

Answer 27

IL-12
IFN-gamma

Question 28

What cytokine induces development of Th2 cells?

Answer 28

IL-4

Question 29

What is the type of cytokine released dependent on?

Answer 29

The type of pathogen e.g DCs that are virally infected produce IL-12 and IFN-gamma which promote Th1 differentiation