Lecture 4 : T-Cell Development

Question 1

What is the Thymus?

Answer 1

Primary site of T cell production and development

Question 2

What is thymic atrophy?

Answer 2

When the thymus shrinks in size with age. This means less T cells will be produced over time, but thats fine because T cells are long lived, eg. Memory T-cells can survive for life.

Question 3

What two cellular elements are required for normal T-cell production?

Answer 3

• Haematopoietic precursors → the immature cells that will become T cells
• Stromal cells → support/teaching cells in the thymus that provide signals for development

One becomes the T cell; the other tells it how to develop.

Question 4

Stromal def

Answer 4

Supporting cells. In the Thymus, they are non-T cells (mainly epithelial) that provide signals, structure and guidance to allow T cells to fully develop.

Question 5

3 types of stromal microenvironments in Thymus :

Answer 5

1. Subcapsular epithelium
   - Where T cells arrive
   - Early development, so immature cells (CD4⁻ CD8⁻)
2. Cortical Epithelium (cortex)
   - Middle stage of development (CD4⁺ CD8⁺)
   - T cells are checked to see if they work properly
3. Medullary Epithelium (medulla)
   - Final stage of development
   - Checking if they cause harm, if not then they’re good to go

Also present: macrophages, dendritic cells, blood vessels
Each supports different stages of T-cell development

Question 6

What is MHC?

Answer 6

Major Histocompatibility Complex

They are proteins on any cell surface that collect parts from inside the cell and display them on the outside surface, which T cells receptors interact with, letting the T cell know if the cell is infected or not.

Ie. The antigen is the small protein pieces collected from inside the cell, MHC is the DISPLAY PLATFORM. ONLY IN CONTEXT OF T CELLS.

Question 7

What is CD4 and CD8?

Answer 7

They are co-receptors on T cells which INTERACT WITH THE MHC molecules.

CD4 helps T cells recognise MHC II → helper T cells

CD8 helps T cells recognise MHC I → cytotoxic T cells

Question 8

Timeline of T cell life for easy understanding

Answer 8

Bone marrow precursor → thymus (DN → DP → SP, selection) → mature T cell → blood/lymph

Question 9

Piecing a lot of it together

Answer 9

T-cell precursors originate in the bone marrow and migrate to the thymus, where they undergo staged development within distinct thymic microenvironments.

In the subcapsular epithelium, thymocytes are double negative (CD4⁻ CD8⁻). At this stage, they commit to the T-cell lineage, undergo extensive proliferation, and produce the TCR β chain, (a part of the final T cell receptor, what happens here is the genes encoding the T-cell receptor (TCR β chain) are rearranged). Successful β-chain formation is tested via signalling through the pre-T-cell receptor (ie. to see if it works); cells that fail undergo apoptosis.

Thymocytes then migrate to the cortex, where they become double positive (CD4⁺ CD8⁺) and express a complete αβ T-cell receptor (final receptor). Here, positive selection occurs: thymocytes must recognise self-MHC molecules with low affinity to survive. Recognition of MHC II leads to commitment to the CD4⁺ lineage, while recognition of MHC I leads to commitment to the CD8⁺ lineage.

Finally, thymocytes migrate to the medulla as single positive (CD4⁺ or CD8⁺) cells, where negative selection removes cells that bind self-peptide/MHC with high affinity. (This ensures self-tolerance before mature, ie. it wont attack our own cells), naïve (mature but unactivated) T cells exit the thymus and enter the circulation.

Question 10

What are the 2 checkpoints in T cell development

Answer 10

DN → DP (CD4⁻CD8⁻ to CD4⁺CD8⁺)

DP → SP (CD4⁺CD8⁺ to CD4⁺ or CD8⁺)

Question 11

What happens during DN → DP

Answer 11

• Commitment to the T-cell lineage
• Extensive cellular expansion (proliferation)
• Rearrangement of genes encoding the T-cell receptor (TCR β chain)

Question 12

What happens during DP → SP

Answer 12

Question 13

Which signals drive early thymocyte proliferation?

Answer 13

• IL-7
• IL-15
• Stem Cell Factor (c-Kit ligand)

Question 14

What happens if IL-7R or c-Kit is missing?

Answer 14

Severe reduction in thymocytes
→ These signals are essential

Question 15

What does the TCR β chain look like when rearranged?

Answer 15

Question 16

what is pre-TCR?

Answer 16

Question 17

What is the need for positive and negative selection in Thymus?

Answer 17

Positive selection: retain useful TCRs (ones with a good enough affinity to function)

Negative selection: eliminate autoreactive TCRs (too high affinity, can cause harm)

A self-peptide is a small fragment of your own body’s proteins that is presented on MHC molecules.

Question 18

How does affinity determine thymocyte fate?

Answer 18

Low affinity TCR–peptide/MHC interaction → positive selection

High affinity interaction → negative selection

Question 19

How does avidity determine thymocyte fate?

Answer 19

refers to number of receptors engaged by ligand.

Low avidity TCR-peptide/MHC interactions give positive selection
High avidity TCR-peptide/MHC interactions give negative selection

Question 20

Site of positive selection

Answer 20

In the cortex, mediated by cortical thymic epithelial cells.

Question 21

How is CD4 or CD8 fate determined during positive selection?

Answer 21

Recognition of MHC II → CD4⁺ T cells

Recognition of MHC I → CD8⁺ T cells

Question 22

What is negative selection and where does it occur?

Answer 22

Deletion of thymocytes with high-affinity self-reactive TCRs

Occurs mainly in the medulla

Question 23

Why is tolerance to tissue-specific antigens challenging?

Answer 23

During negative selection, T cells are checked to ensure they do not attack self-antigens. But a lot of self antigens are in specific tissues around the body, which aren’t present in the Thymus, so in theory a T cell which could attack an antigen undetected in the Thymus, but present in another tissue, and cause an autoimmune disorder in that part of the body.

Question 24

What is the solution to the above? (What is AIRE?)

Answer 24