Thymus T cell development

Question 1

Where are thymocytes made?

Answer 1

Post-natal, made in the bone marrow.
Foetal - made in the liver - the primary organ of haemopoiesis in embryos.

Question 2

How is the thymus linked to T cells?

Answer 2

The thymus is in the anterior mediastinum of the chest.
Thymocytes are made in the bone marrow, circulate in the blood then enter the thymus.
After around 2 weeks these immature precursor cells are matured into T cells and are exported out of the thymus.

Question 3

What is a thymectomy?

Answer 3

Or Congenital absence of thymus in Di George syndrome, causes failure to thrive.
Fewer total lymphocytes
Reduced graft rejection.
Reduced antibody responses.

Question 4

How does the thymus age?

Answer 4

The thymic gland involutes (shrinks) with age.
Old people have very little thymus function, the thymus becomes mainly fat, and very little cortex and medulla.

Question 5

How does the number of T cells change with age?

Answer 5

The number of T cells produced per day decreases significantly with age - thymic atrophy.
But T-cells are long lived so there are still a steady supply to help with the immune response.
However, in disease, if cancer causes bone marrow to be removed, the individual loses their T cells.

Question 6

What are the main cellular elements of the thymus?

Answer 6

Haemopoietic bone marrow/foetal liver derived precursors, which develop into T cells.
Non-haemopoietic stromal (tissue) cells, that drive the development of T-cell precursors.
Both elements are required for normal T-cell production.

Question 7

What is the stromal microenvironment in the thymus?

Answer 7

The thymus is an epithelial tissue, with epithelial cells specialised for T cell development.
It contains cortical, medullary and subcapsular epithelium - which is just below the capsule.
It also has haemopoietic stromal /accessory cells.

Question 8

What are the accessory cells in the thymus?

Answer 8

Dendritic cells - antigen presenting, found at the cortico-medullary junction.
Macrophages - phagocytosis, remove dead or apoptotic material.
Blood vessels, found at the cortico-medullary junction. Precursors enter, mature T cells exit.

Question 9

What are the stages of T cell development?

Answer 9

Least mature: T cell - lack expression of CD4 and CD8 (double negative).
Dual expression of CD4 and CD8 (double positive).
Most mature: CD4 expression only (CD4 single positive)
CD8 expression only (CD8 single positive).

Question 10

Where are the thymocytes found in the thymus?

Answer 10

They sit in an organised structured way:
Double negative cells sit at the cortico-medullary junction of the cortex, then migrate out through the cortex to the subcapsular epithelium.
They then receive signals to become double positive thymocytes, and migrate to the medulla.
In the medullary epithelium they mature into single positive thymocytes.

Question 11

What are the checkpoints in T-cell development?

Answer 11

Checkpoint 1 - maturation from the CD4-8- (double negative) to CD4+8+ (double positive) stage.
Checkpoint 2 - maturation from the CD4+8+ to the CD4+ or CD8+ stage.

Question 12

What are the stages of the first checkpoint in T cell maturation?

Answer 12

First stage - CD4-CD8- to CD4+CD8+.
Commitment of progenitors to the T-cell lineage.
Cellular expansion (proliferation, as only a few 100 enter the thymus each day max).
Rearrangement of genes encoding the T-cell receptor.

Question 13

What is commitment to the T-cell lineage?

Answer 13

Thymocytes interact with thymic epithelial cells - these express a notch ligand - either delta or jagged.
Notch is a receptor expressed on thymocytes.
If there is a notch-notch ligand interaction then the thymocyte commits to the alpha-beta (aB) T cell lineage.
If no notch signal, the cell defaults to other types of cell - dendritic, natural killer, or gamma delta T cells.

Question 14

What is cellular expansion of early T-cell precursors in the thymus?

Answer 14

It is controlled by soluble growth factors - cytokines, particularly interleukins.
Double negative thymocytes express IL-2, IL-7, IL-15 receptors and c-Kit (stem cell factor receptor).
Thymic epithelial cell produces IL-7, IL-15 and Stem cell factor.
So thymocytes can respond to many types of cytokines and proliferate.

Question 15

What are the key drivers of early cellular expansion?

Answer 15

Stem cell factor and interleukin 7.

Question 16

What are the regions of the T-cell receptor B chain gene?

Answer 16

Variable regions
Diversity regions
Joining regions
Constant regions

Question 17

What is rearrangement of T-cell receptor genes?

Answer 17

Randomly chop up gene and stick it back together.
There are hundreds of each of the regions, so produces great variation.
Each TCR B sequence becomes unique to the TCR.

Question 18

What is the Pre-T cell receptor complex?

Answer 18

Expressed only on CD4-8- thymocytes.
Consists of TCR-B protein, CD3 complex (signalling complex) and pre-T-a, it does not yet have TCR a, so is not the abTCR.
Signalling through pre-TCR is essential for the generation of CD4+8+ thymocytes from double negative cells.

Question 19

What is selection of TCRB chain genes?

Answer 19

Only cells that have successful B chain rearrangement are selected to survive.
Other chains do it incorrectly, because rearrangement is random.
Cells that have rearranged correctly can express pre-T cell receptor complex at the cell surface, which then triggers signalling.

Question 20

What is pre-TCR complex signalling?

Answer 20

Takes place when all the components are present at the cell surface - TCRB, CD3 and pre-Ta:
Stops TCR B rearrangement.
Induces CD4 and CD8 expression.
TCRa rearrangement - to make aBTCR.
Cellular expansion.

Question 21

Why is TCRB rearrangement stopped?

Answer 21

It is stopped by the pre-TCR complex signalling.
This is because there are two copies of all genes (alleles), if the rearrangement of the first allele produces a productive rearrangement then can stop - allelic exclusion.

Question 22

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

Answer 22

T cells can only recognise processed antigens presented by MHC molecules.
But, because TCR rearrangement is random, there needs to be selection on the basis of MHC recognition.
Harmful TCR specificities need removing by negative selection, useful specificities need retaining by positive selection.

Question 23

What are the levels of MHC recognition?

Answer 23

Some TCR don’t recognise the peptide/MHC at all - useless.
Some TCR recognise the peptide/MHC of low affinity or avidity - useful.
Some TCR recognise peptide/MHC at high affinity or avidity - harmful.

Question 24

What are the models of positive and negative selection?

Answer 24

Affinity model
Avidity model
Negative selection occurs via apoptosis in both models.

Question 25

What is the affinity model?

Answer 25

Affinity is the fit between a receptor and its ligand. Low affinity TCR-peptide/MHC interactions give positive selection. High affinity TCR-peptide/MHC interactions give negative selection.

Question 26

What is the avidity model?

Answer 26

Avidity refers to the number of receptors engaged by the ligand. Low avidity TCR-peptide/MHC interactions give positive selection. High avidity TCR-peptide/MHC interactions give negative selection.

Question 27

What is involved in positive selection of thymocytes?

Answer 27

These are double positive thymocytes that express TCR. The cortical epithelium expresses both MHC I and II. CD4 cells are MHC II restricted. CD8 cells are MHC I restricted. The TCR could recognise either MHC I or II.

Question 28

What is positive selection of CD4+ thymocytes?

Answer 28

Forms a trimolecular complex - TCR and CD4 binds to MHC II on the thymic epithelium. This allows loss of expression of CD8, as it is not bound to anything. This produces CD4+ cells, which are MHC II restricted.

Question 29

What is positive selection of CD8+ thymocytes?

Answer 29

Forms a trimolecular complex - TCR and CD8 binds to MHC I on the thymic epithelium. This allows loss of expression of CD4, as it is not bound to anything. This produces CD8+ cells, which are MHC I restricted.

Question 30

What is negative selection of thymocytes?

Answer 30

In the medullary epithelial cells, thymocytes are screened further. High affinity/avidity thymocytes die by negative selection. Low affinity/avidity thymocytes survive and go on to form part of the immune system.

Question 31

How are cells made in the thymus tolerised against antigens expressed in other places?

Answer 31

i.e. to prevent autoimmune disease like diabetes: the thymus covers all the body tissues via promiscuous gene expression, so all tissues are represented. It is regulated by one gene - AIRE.

Question 32

What is AIRE?

Answer 32

Autoimmune regulator gene. Transcription factor expressed by medullary thymic epithelial cells. Regulates promiscuous gene expression so that tolerance to proteins expressed in peripheral tissues can occur in the thymus. Thymocytes which are self-reactive undergo negative selection.

Question 33

How does AIRE work for insulin?

Answer 33

Beta cells of the pancreas make insulin. AIRE directs insulin expression in the thymus, so that T cells developing that recognised insulin undergo negative selection. This protects against diabetes.

Question 34

What is APS-1?

Answer 34

Auto-immune polyendocrine syndrome type 1 - autoimmunity of endocrine glands. Multi-organ autoimmunity. This is due to mutations in the AIRE gene.