Lecture 20 - T cells - Development & Activation Flashcards by Eleanor Nicklason

List some general features of T cell development

(Similar to B cells)
• Rearrangement of T cell receptor
• Testing for successful rearrangement
• Assembly of heterodimeric receptor

• Produced in BM but develop in thymus

Differences:
 • Two sets of TCR: αβ & γδ
 • More diverse subsets:
- helpers
- killers

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What is important in terms of discovery about the thymus?

The last organ in the body to be ascribed a function

Function described in the 1950’s by Jacques Miller

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What is DiGeorge Syndrome?

No thymus present - thymic aplasia

* Lack of T cells

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Describe the time-frame of production of T cells

Highest levels occurring before puberty
Levels drop of markedly
Some low level of production occurring throughout life

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What happens to the thymus at the onset of puberty?

Involutes

Becomes replaced by fatty tissue

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Describe the structure of the thymus, including cell types in the various areas

• Capsule

Cortex: (superiorally)
• Thymocytes
• Cortical epithelial cells

Medulla: (inferiorally)
• Macrophages
• DCs
• HEV

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Which types of cells from the BM enter the thymus?

How do they enter?

Thymocytes come from the BM

* Enter through HEVs

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What are the fates of thymocytes?

α/β T cells
γ/δ T cells
Invariant NKT cells

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Where are γ/δ T cells found?

Mucosal surfaces
• Skin

Look like Langerhans cells

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Which thymocyte derivatives have invariant usage of the TCR genes?

γ/δ T cells & Invariant NKT cells

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Describe the progression of development of T cells

Describe what is happening at each of these stages

DN1
• TCR genes in germ line configuration

Thymocytes interact with cortical epithelial cells

DN2
• Dβ-Jβ rearrangement
• Cells become responsive to IL-2 through up regulation of CD25
DN3
• Vβ-DβJβ rearrangement
• Pre-TCR testing w/ pTα and CD3 once β is rearranged
DN4
• Proliferation of thymocytes with successful β chain rearrangement
Double positive
• Vα-Jα rearrangement
• Positive selection
Single positive
• Either CD4+8- or CD4-8+

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What is the role of IL-2 in T cell development?

When do T cells become responsive to it?

How do they becomes responsive to it?

Stimulates proliferation (growth factor)

Become responsive to IL-2 at the DN2 stage

Through expression of CD25 (α chain, high affinity IL-2R)

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What is being tested at the Pre-TCR stage?

What happens if there is success?

Viability of the β chain rearrangement

If success:
• Signals to the nucleus to turn on CD4 and CD8 transcription
• Becomes double positive thymocyte

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Describe the expression of CD4 and CD8 over time

Thymocytes: don’t express either
DN: don’t express either
DP: express both
SP: express one or the other, depending on what sort of stimulation was received

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Which chain of the TCR is rearranged first?

β chain is rearranged first (it is like the heavy chain)

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How does a T cell become responsive to IL-2?

CD25: α chain, rendering IL-2R high affinity

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What is the hallmark of DN2?

Upregulation of CD25

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When does a big burst of proliferation occur in B & T cell development?

Why does this occur?

B cell: once heavy chain has been rearranged

T cell: once β chain has been rearranged

This produces many clones, all with the same β chain (or heavy chain)
These clones all rearrange their light/α chain independently
The individual β chain can thus be used in many different TCRs

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When does massive culling of T cells occur?

After rearrangement of β chain

Those cells that don’t produce successful chains are culled

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Describe the different locations of the developing T cells

Medulla:
1. Thymocytes enter through HEV in medulla

Cortex:

DN1 migrate up to cortex to interact w/ cortical epithelial cells
DN2 → DN3
DN3 → DN4 aka Double negative migrate down a bit

Medulla:
5. Mature CD4+ and CD8+ T cells in medulla

What is a double positive thymocyte?

Compare this with double negative thymocytes

Double positive: Express both CD4 and CD8

Double negative: express neither CD4 or CD8

Which non-immune cells in the cortex and the medulla of the thymus are really important for T cell development?

Cortex:
• Cortical epithelial cells

Medulla:
• Dendritic cells
• Medulla epithelial cells

What is the structure of the pre-TCR?

β chain + pTα

What is pTα?

Surrogate α chain

Describe the process of TCR rearrangement

1. Simultaneous rearrangement of γ, δ and β chains 2. If a successful pre-TCR is formed, signals halt the rearrangement of γ & δ chains (allelic exclusion) 3. Rearrangement of α chain locus NB If β chain rearrangement is unsuccessful, it goes on to form γ:δ TCR

What is the importance of the developing T cells going into the cortex of the thymus?

Positive selection * Here they interact with cortical thymic epithelial cells * It interacts with the MHC on the thymic stromal cells * If this works, then it gets a survival signal * All these thymocytes in the cortex are tested for their ability to interact with MHC * The antigen in the MHC is irrelevant at this stage (It is not just the antigen that binds the TCR, but also the MHC molecule that is important)

What are the various outcomes of the different affinities of TCR to MHC

Strong binding • Likely to respond to self antigen • Negatively selected Intermediate binding: positively selected No binding: 'death by neglect' • Do not receive the correct signals

Compare which cells undergo the following: • death by neglect • positive • negative selection

Positive selection: intermediate affinity Negative selection: too high affinity Death by neglect: no affinity (affinity: TCR to MHC)

What is the importance of DC's in the medulla of the thymus?

Perform negative selection | If TCR binds any antigen presented (because there will only be self antigen at this stage)

Compare the different outcomes when there is recognition of specific peptide + MHC by the TCR in the thymus and in the periphery

Thymus: deletion Periphery: activation

Describe how the periphery is represented in the thymus

• Transcription factor: AIRE (Autoimmune regulator) in medullary epithelial cells (MEC) • AIRE stimulates the transcription of all sorts of proteins that typically only occur in organs in the body - pancreas - eye etc. • These antigens are presented in the context of MHC by MECs and DCs in the medulla

What is APECED? Describe the clinical features as well as the pathogenesis

Autoimmune polyendocrine syndrome Defect in AIRE gene ``` Experience weird diseases: • Problem w/ thyroid: very small thyroid & gonads • Lack pigments in skin • Lack hair • Struggle to absorb food • Anaemia • Hepatitis ``` This is all associated with T cell infiltrate * AIRE hasn't exposed developing T cells to all the antigens around the body * T cells have not been negatively selected * Auto-reactive T cells destroy organs

Describe the mechanism of lineage commitment (CD4/CD8) of α/β T cells

1. Pre-TCR success signals → CD4 and CD8 expression CD4(high)CD8(high) 2. Positive selection 3. Down regulation of CD4 & CD8 expression CD4(low)CD8(low) 4. CD4 re-expressed at low levels CD4(high)CD8(low) 5a. If the TCR, at this point, interacts with MHC II (ThPOK) → CD4 lineage 5b. If MHC I binds the TCR (no ThPOK), CD4 is repressed and CD8 is upregulated → CD8 lineage

What do 'DN' and 'DP' stand for?

DN: double negative DP: double positive

What is the 'default lineage' of T cells?

CD4+ T cell If binding of MHC II-TCR doesn't happen, then CD4 expression is down regulated, and CD8 expression is upregulated

Where (physically) does TCR rearrangement occur?

In the cortex of the thymus

Why is CD25 important? | At what stage of T cell development is it seen?

Receptor for IL-2 Needed for proliferation and stimulation of growth Present at the DN2 and DN3 stages

What do CECs do?

Cortical epithelial cells Positive selection • of those T cells that have intermediate affinity for MHC expressed on the CECs

Where does positive selection occur? | What about negative selection?

Positive: cortex Negative: medulla

Compare the role of the following cells in the thymic medulla: • DC • Medullary epithelial cell

MEC: • AIRE expressed • AIRE turns on transcription of many different genes • Antigen from all over the body expressed • Presentation of the antigen on MHC to DP thymocytes DCs: • Presentation of self antigens to DP thymocytes (Negative selection)

At which stage is α chain rearrangement happening?

Double positive thymocyte i.e. after pre-TCR testing at the DN3 stage

At which stage is the pre-TCR checkpoint?

DN3

Expression of which gene leads to the CD4 T cell lineage?

ThPOK

When is α:β lineage committed?

At the DN3 stage i.e. once successful rearrangement of the β chain occurs

Which cell expresses AIRE?

Medullary epithelial cells in thymus

Which cell performs negative selection on developing T cells?

DCs in medulla of thymus

Which thymocytes are found in the medulla?

Only mature SP thymocytes

What is happening at the DN4 stage?

T cells which successfully rearranged the beta chain under much proliferation Each of these T cells independently rearranges the alpha chain. This leads to maximising the probability that a T cell with a functional TCR will be produced

Interaction with which cell leads to positive selection of DP thymocytes?

Cortical epithelial cells