T cell activation and generation of effector T cells

Question 1

Key concepts to know for this lecture from before (PART 1)

Answer 1

• T cell undergoes early developmental process in the thymus to generate T cells expressing clonal TCR
• This TCR recognises antigen in the context of MHC
• CD4 TCR recognises MHC I|/peptide complex
• CD8 TCR recognises MHC I/peptide complex
• Self tolerance established

Question 2

Key concepts to know tor this lecture from betore (PART 2)

Answer 2

• Self MHC restriction established
• Mature but naïve T cell exported from the thymus in resting state but can recognise MHC
• MHC class I presents endogenous antigen
• MHC class 2 presents exogenous antigen

Question 3

Describe the circulation and re-circulation of naive and activated T cells?

Answer 3

1. Naive T cells travel through blood vessel and enter lymph nodes (LNs) via high endothelial venules
   a. Travel to different LNs via efferent lymphatic vessel until it reaches an LN with an antigen
   b. T cell activates via binding to antigen
   c. Activated T cell moves back to circulation via thoracic duct in heart -> sites of infection
2. Activated T cells at site of infection attack microbe
   a. Move into LN via afferent lymphatic vessel using dendritic cell

Question 4

State the sites of activation of Naive T cells and where effector T cell occurs? Also what the outcome is?

Answer 4

• Secondary lymphoid organs : spleen and lymph nodes
• Activation of naive T cells -> development of effector cells
• Activation of effector T cells at site of infection -> eradication of microbe

Question 5

What type of antigen presenting cells activate T cells?

Answer 5

• Activated professional APCs
• Dendritic cell (main one)
• Macrophage
• B-lymphocyte

Question 6

What do they express in regards to T cell activation?

Answer 6

• Only these express high levels of MHC class lI
• These also express co-stimulatory molecules
• Change the expression of molecules essential for T cell activation

Question 7

State the 3 different signals (in order) required for Naive T cell activation into effector or memory T cells?

Answer 7

• Antigen recognition
• Co-stimulation
• Cytokines

Question 8

Signal 1 - MHC-TCR
What is antigen recognition?

Answer 8

• AR is the signal that initiates the immune response, so that the immune response is antigen- specific
• TCR in T cell recognises the antigen in the context of MHC:
• CD4 TCR recognises MHC I|/peptide complex
• CD8 TCR recognises MHC I/peptide complex

Question 9

Signal 2 - Co-stimulation
Which molecules produce the co-stimulatory signals for naive T cells?

Answer 9

• Molecules which produce Co-stimulation molecules
• Professional APCs -> dendritic (common), macrophages, B cells -> Activated via T cells -> Produce co-stimulation

Question 10

Describe the process behind co-stimulatory molecules and their effect in T cell activation? (PART 1)

Answer 10

• Co-stimulatory molecules are only provided when APC is activated via T cell
• T cells activate APCs via CD40 - CD40L interaction, enhancing T cell responses.
• Upon activation, T cells upregulate CD40L, which binds to CD40 on DCs and stimulates the production of co-stimulatory molecules and cytokines by the DCs, thus enhancing T cell proliferation and differentiation.

Question 11

Describe the process behind co-stimulatory molecules and their effect in T cell activation? (PART 2)

Answer 11

• Co-stimulation process:
• T cell activation (via antigen) -> Increased CD40L -> Binds to CD40 on DCs -> Increased co-stimulatory molecules + cvtokines via DCs -> Increased T cell proliferation + differentiation

Question 12

Describe the effect of negative co-stimulatory molecules?

Answer 12

• They inhibit the downstream effector processes initiated by TCR MHC/peptide interaction
• Reduce inflammation after the infection has cleared •
• Not expressed by naïve T cells, there are induced upon activation
• Effect of negative co-stimulatory molecules
• Inhibit the downstream effector process -> Decreases inflammation after infection is cleared -> upon activation of T cells to control response

Question 13

Give examples of + ve and -ve co-stimulatory molecules expressed by APC? VD

Answer 13

• Positive: B7 - CD28! B7-1 (CD80), B7-2 (CD86)
• Negative: CTLA-4 PD-1 (programmed cell death protein 1) -> expressed in T cells in peripheral tissues

Question 14

How are Positive and negative co-stimulatory molecules expressed by APC?

Answer 14

• Positive -> B7 -> CD28 (expressed on T cell -> co-stimulation of naive T cells) -> B7-1 (CD80) OR B7-2 (CD86)
• Negative -> CTLA-4, PD-1, LAG3
• PD-1 (programmed cell death protein 1) -> Expressed in T cells in peripheral tissue

Question 15

What is CTLA-4?

Answer 15

• CTLA-4 = Cytotoxic T-lymphocyte antigen 4
• CTLA-4 is expressed approx 2-3 days post stimulation

Question 16

Describe key features of CTLA-4

Answer 16

• It has high affinity/avidity for CD80/B7-1 but opposing effects to CD28: Binds laterallya
• It is mostly expressed in T cells in secondary lymphoid organs.
• Peak levels of expression lower than CD28 but avidity of interaction is much higher
• Therefore, competes favourably with CD28 for ligation to CD80/86

Question 17

Signal 3 - cytokines
What is the role of cytokines in activation of naive T cells?

Answer 17

• Various forms of cytokines induce the differentiation of naive CD4 T cells down distinct effector pathways.
• Each effector T cells expresses a master controller transcription factor
• This transcription factor controls the expression of effector cytokines

Question 18

Describe role of IL-2 in T cell?

Answer 18

IL-2 is a growth, survival and differentiation factor for T cells and Tregs. Main role is for induction of T cell proliferation
- Secreted to Eff T cells and leads to proliferation and differentiation
- Also to Treg cells and these inhibit secretion of IL-2 to effector T cells for regulation

Question 19

State key surface markers expressed during T cell activation?

Answer 19

• CD69 (maybe)
• VD

Question 20

State the 4 key steps following succesful signalling via TCR, that a naive T cell will occur?

Answer 20

1. Modify the expression of surface molecules
2. Upregulate cytokine production
3. Undergo active rounds of proliferation
   a. Upregulate expression of pro-survival genes
   b. Upregulate expression of IL-2 and IL-2R-a
4. Differentiate into effector or memory cells

Question 21

Elements of polarization
What induces T cell polarisation into the different subsets?

Answer 21

• The polarising cytokines - These are generated by the stimulating APC
• Which cytokines they produce depends on:
• The cellular origin of the APC
• The maturation and activation status of the APC
• Which pathogens or inflammatory mediators were encountered by the APC
• In which tissue environment the encounter takes place

Question 22

Name the main CD4+ T cell functional subsets with polarising cytokines, master TF, effector cytokine + effector function for main 2?

Answer 22

VD
Seems important

Question 23

What does TH1 polarization occur in response to?

Answer 23

TH1 polarisation occurs in response to the presence of intracellular pathogens such as viruses and bacteria that are ingested by and destroyed by phagocytes.

Question 24

State the functions (3) and master TF of Th1 cells?

Answer 24

• Master TF -> T-Bet
• Functions:
  1. They produce IFN-gamma
  2. Help to activate macrophages to ingest and destroy microbes
  3. Induce antibody class switching to IgG (opsonization).
  All helpful response in eliminating an intracellular pathogen

Question 25

Describe the development of TH2 cells?

Answer 25

• TH2 polarization occurs in response to phagocyte - independent immune responses.
• TH2 polarizing cytokine is IL-4
• Dendritic cells do not make IL-4
• Eosinophils, basophils and mast cells produce IL-4. ILCs also produce IL-4
• Transcription factors: IL-4 activates STAT6 which promotes expression of GATA 3
• GATA 3 is a transcriptional activator of IL-4 and IL-13 genes

Question 26

State the functions of TH2 cells (4)?

Answer 26

• TH2 cells produce IL-4, IL-5 and IL-13, effector cytokines that help eliminate extracellular parasitic infections such as worms
• Promote class switching to IgE, which causes inflammatory cytokines to be released by eosinophils and mast cells.
• They also increase intestinal movement and mucus production.
• IgE also mediates allergy