T cell Development - Receptor repertoire selection 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

Question 8

Which thymocytes are favoured during early fetal development?

Answer 8

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

Question 9

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

Answer 9

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 10

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

Answer 10

Functional TCRalpha 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 11

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

Answer 11

apoptosis these cells are removed from tissues by macrophages

Question 12

What does positive selection ensure?

Answer 12

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 13

What does negative selection ensure?

Answer 13

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 14

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

Answer 14

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 15

Summarise postive and negative selection of T cells

Answer 15

Question 16

Summarise T cell development and migration

Answer 16

Question 17

Regulatory T cells may develop in the Thymus

(High expression of CD25 and Foxp3)- what do we know about these regulatory t cells?

Answer 17

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 18

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

Answer 18

•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 19

Summary of lecture

Answer 19

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