Selection of T cell receptor repertoire and CD4/CD8 lineage commitment

Question 1

Where do T cells come from?

Answer 1

1. Multipotent Lymphoid Progenitors Migrate from the Bone Marrow to the Thymus (thymus small organ on top of the heart).

→ Cells in the thymus produce chemokines (e.g. thymosin, thymotaxin, thymopoietin, thymic factors) which are secreted and transported to the bone marrow by blood.

→ T cells can recognise these chemokines and follow the ‘trail’ to the thymus.

2. In the Thymus, the Lymphoid Progenitors Differentiate to pre-T Cells and are Educated to Differentiate Self from Non-self
3. Positively Selected T Cells Emigrate from the Thymus to Mediate and Effect the Cognate Immune Response

Question 2

What happens in Hassall’s corpuscle?

Answer 2

A place where aggregation of mature lymphocytes, particularly T lymphocytes

Question 3

Talk about the pathway of thymocytes at different developmental stages found in distinct parts of the thymus

Answer 3

→ Subcapsular region - immature CD3-4-8, you find double negative thymocytes here

→ Cortex going towards cortico-medullary junction - immature CD3+4+8+, you find double positive thymocytes here

→ Cortico-medullary junction into medulla etc: MATURE CD4+8- and immature CD8+4- thymocytes

Question 4

What 3 molecules make up the T cell receptor complex?

Answer 4

TCR and CD4 and CD8 all present on the surface of a T cell as the T cell receptor complex- can both bind to MHC MHC- major histocompatibility complex CD- cluster of differentiation

Question 5

What can double negatives (DN’s) be subdivided into?

Answer 5

DN’s can be further subdivided into DN1 through DN4

Question 6

In terms of cells being double negative/double positive, what do we tend to see from before they mature and after?

Answer 6

In the thymus, the majority of cells are double negative, but through process of maturation, cells become double positive.
This is not the same as seen in other organs.

Question 7

Which thymocytes are favoured during early fetal development?

Answer 7

Gamma delta T cells are favoured during early fetal development but then numbers go down

Question 8

How is antigen recognition by gamma-delta T cells different to alpha beta T cells?

Answer 8

→ Gamma-delta T cells bearing specific receptors end up in skin (Vg5), gut (Vg2), uterus (Vg6), etc. gamma-delta T cells are not MHC restricted!
→ Antigen is recognized directly, more like an antibody In some cases ligands for the gamma-delta TCR are self proteins upregulated under stress conditions In humans, circulating gamma-delta cells recognize a phospholipid antigen from → Mycobacterium tuberculosis Play a role in cancer surveillance
→ However, majority are alpha-beta T cells (90%), whereas there’s about 10% gamma-delta T cells

Question 9

What does a double positive thymocyte need to do to progress to the single positive stage?

Answer 9

Functional TCR
Alpha chain rearrangement CD4 and MHC II (To be a CD4+ cell) CD8, MHC I and TAP (To be a CD8+ cell) ERK signalling Calcineurin signalling

Question 10

Most cells fail to complete thymocyte maturation- what do these cells undergo?

Answer 10

Apoptosis these cells are removed from tissues by macrophages

Question 11

What does positive selection ensure?

Answer 11

→ Positive selection ensures that only T cells that are useful and can engage in recognition are selected DP CD4/CD8 cells bind to MHC-I or MHC-II on thymic epithelial cells – it is a random event which one binds Following adequate binding of CD4:MHC-II, CD8 is downregulated and vice versa

→ From here on, the SP CD4 or CD8 T cells are ready for negative selection Unselected cells die by apoptosis

Question 12

What does negative selection ensure?

Answer 12

→ Negative selection ensures that self-reactive cells are removed, as they would cause autoimmunity
→ This is determined based on affinity of TCR for presented self-peptide: high – kill him, low – keep him
→ This ensures that remaining T cells are only reactive to foreign peptides Self-reactive cells are not removed immediately but go through further TCR rearrangements (second chance) – before they are eventually removed if still self-reactive A bit of a problem for T cells: thymus does not represent all self-antigens … but it has a transcription activator gene which can induce expression of other tissue specific proteins (kidney, heart etc)
→ This gene is called AIRE (Autoimmune Regulator): this allows negative selection against most bodily self-proteins

Question 13

How are T cells negatively selected against self-antigens not present in thymus?

Answer 13

The Transcription Factor Autoimmune Regulator (AIRE) Mediates Ectopic Gene Expression in the Thymic Medullary Stroma – other tissue specific genes…kidney, heart, liver, lungs, gut, … apart from brain and testes

→ This is known as promiscuous gene expression – about 10% of all genes in thymus are expressed this way
→ This eliminates many self-reactive T cells

Question 13

Summarise postive and negative selection of T cells

Answer 13

picture

Question 14

Summarise T cell development and migration

Answer 14

picture

Question 15

Regulatory T cells may develop in the Thymus
(High expression of CD25 and Foxp3)- what do we know about these regulatory t cells?

Answer 15

→ Do not proliferate in response to MHC self-peptide complexes
→ T Regs accumulate in Hassall corpuscles and later migrate to different tissues

Main role: dampen T cell response

Question 16

What do T cells that pass both positive and negative selection become?

Answer 16

→ T cells that pass both positive and negative selection become
→ Conventional T cells
→ They migrate to secondary lymphoid organs looking for their target antigen
→ ‘Immunological synapse’
→ If they encounter specific antigen, they get activated, proliferate and become effector T cells
→ Some become memory T cells
→ If they don’t find the target they, eventually die by apoptosis
→ After period of circulation

Question 17

Summary of lecture

Answer 17

→ TCR chains undergo V(D)J recombination to generate diversity; they also exhibit allelic exclusion
→ TCRb chain is selected with an invariant pTa chain at the DN3 stage
→ TCRa chain is selected with pre-existing TCRb chain at the DP stage
→ DP Thymocytes Undergo Positive and Negative → Selection to Generate a Population of Mature T Lymphocytes that can Recognize Self MHC with Intermediate Affinity
→ Recent Evidence Indicates that Positive and → Negative Selection Are Mediated by Distinct Pathways
→ APC in the thymus may present self antigen that effect negative selection