Thymus and T cell development Flashcards
Which cells express Foxn1?
Thymic epithelial cells (TECs) and also skin cells, however, the function of Foxn1 in TECs is different than in skin cells.
What is CCL25?
It is a chemokine secreted by thymic epithelial cells. It binds to CCR9 receptor. It attracts lymphocyte precursors into the thymus.
What is DLL4?
Expressed on cortical thymic epithelial cells. It binds to the receptor on the lymphocyte progenitors called Notch1 and this is imposing the T cell lineage on the precursor cell.
What is Notch1?
It is a receptor on lymphocyte progenitor cells which binds to DLL4 on cortical thymic epithelial cells which imposes the T cell lineage on the precursor cell.
What is KIT-Ligand?
It is a cytokine, produced by thymic epithelial cells, which drives T cell expansion and proliferation in the thymus.
Where in the thymus does negative control take place?
In the medulla.
Where in the thymus does positive control take place?
In the cortex.
What is the role of the thymic cortex?
T cell specification, positive selection.
What is the role of the thymic medulla?
Negative selection, Treg development.
What is Foxn1?
It is a transcription factor, expressed by thymic epithelial cells. It controls thymic epithelial cell function, it controls chemokines, T cell specification, T cell expansion etc.
What drives Foxn1 expression?
Wnt molecules, Bmp4, Bmp7, Shh (sonic hedgehog).
What is AIRE?
It is a transcriptional regulator (autoimmune regulator). It allows medullary thymic epithelial cells to express modules of genes normally associated with other organs and tissues in the periphery.
What cells express AIRE?
Medullary thymic epithelial cells.
What happens when a patient has a mutation in AIRE?
They lose expression of those peripheral tissue antigens which leads to generation of autoreactive T cells which leads to severe autoimmunity.
What is Fezf2?
It is another transcriptional regulator like AIRE which allows medullary thymic epithelial cells to express modules of genes normally associated with other organs and tissues in the periphery.
What transcriptional regulators are found in medullary thymic epithelial cells?
AIRE and Fezf2.
Does TCR recognise only one epitope or antigen?
No, it recognises a range of peptides presented by MHC but with different levels of affinity.
What does a very strong affinity of TCR to self-antigen lead to during negative selection?
T cell deletion.
What does a very weak affinity of TCR to peptide presented on MHC lead to during positive selection in the thymus?
T cell anergy and cell death as no survival signals are given to the T cell.
What is FGF7?
FGF7 (Fibroblast growth factor 7), also known as KGF (Keratinocyte Growth Factor), binds to the FGFR2iiib receptor on thymic epithelial cells and drives their proliferation.
What can lead to the loss of thymic tissue (thymic atrophy)?
Inherited primary genetic defects (mutations in Foxn1), acquired acute loss (pregnancy, infection, radiation) or progressive loss (ageing, sex steroids).
How can you potentially restore thymus function?
Stimulate residual thymic tissue (if there is still some thymic tissue left), transplant thymus cells and tissues (if there is no thymic tissue left), Interleukin 22 treatment.
What can be challenging with restoring or manipulating the thymus?
We need to ensure a proper balance between cortex and medulla. If we restore the cortex but not the medulla it will lead to immune protection, however, it could also lead to autoimmunity. So we need to ensure the control and balance between the cortex and the medulla.
What is a Di George Syndrome?
It is a genetic deficiency which is associated with the micro-deletion in chromosome 22. It is characterised by developmental defects like palate formation and parathyroid. Patients with this disease have either a small (hypoplastic) thymus or an absent (aplastic) thymus. Because of the defects of the thymus, these individuals are immunosuppressed.
What’s a potential treatment for patients with a Di George Syndrome?
Doing an in vitro culture of the thymic tissue from the pediatric donor who underwent cardiac surgery and transplanting the culture into a patient with Di George syndrome. There is a certain recovery in T cell number but also there is a certain level of autoimmunity as MHC is not perfectly genetically matched between the donor and the patient. However, the gain outweighs the risk.
What is Lymphotoxin-beta receptor?
It is a receptor on stromal cells in the thymic niche. It controls endothelial cells in the thymus, entry and exit points and other stromal populations which control the development of T cells. There is a study which showed that injecting an antibody stimulating lymphotoxin-beta receptor alongside bone marrow transplant supported the t cell lineage recovery.
How does bone marrow transplant affect T cell lineage?
As haematopoietic stem cells (HSC) are firstly irradiated and new HSC are transplanted from the donor, when the thymus is large, it still takes time for new T cells to develop which leads to the window of infectious susceptibility, however, when the thymus is small for example in an elderly patient, there is less clustering space for T cells to develop, there is a window for infectious susceptibility and there is a reduced diversity of newly generated T cells.
How can you enhance T cell lineage recovery?
Treatment with IL-22 or ablation of sex steroids, possible use of FGF7, however, it has previously shown positive effects on mice but not on humans.
How do the double-negative T cells become double-positive?
Double negative T cells become double positive after the rearrangement of the beta chain of the T cell receptor. This takes place in the cortex of the thymus.
How do double-positive T cells become single-positive?
After the rearrangement of the alpha chain of T cell receptor, this takes place in the cortex of the thymus.
Do we have a single epithelial cell progenitor that gives rise to both the medulla and cortex, or do we have a separate progenitor for the cortex and a separate one for the medulla?
There is a single progenitor cell, which gives rise to both the medulla and cortex. It was shown in two experiments. One where a single thymic epithelial cell bound to a fluorescent die was injected back into the developing thymus, which was then implanted into the renal capsule and developed both cortical and medullary thymic epithelial cells and then the second experiment used FoxN1 KO mice which did not have the thymus and a single progenitor cell with FoxN1 expression led to the development of fully functional thymus.
What happens to mice deficient in Rag2?
No TCR and BCR rearrangement, no T cells and B cells.
Are TECs controlling T cells development?
Yes, but there is some sort of symbiotic relationship as it seems like T cells are controlling TECs as well.
Do we have specific markers for bipotent thymic epithelial precursors?
Not yet
What can be found in the thymic niche?
Thymic epithelial cells, fibroblasts, macrophages, dendritic cells, progenitor cells, chemokines
What is a potential stem cell-based treatment strategy for thymic atrophy?
Reprogramming of fibroblasts to the thymic epithelial lineage by switching on FoxN1 expression which has been reported to support development of the thymus as well as support T cell development in vivo.
How does IL-23 support thymus regeneration?
The decrease in double-positive thymocytes alerts dendritic cells, which secrete IL-23. IL-23 binds to IL-23 receptor on ILC3 (Innate lymphoid cell type 3) cells, which begin secreting IL-22. IL-22 binds to IL-22 receptor on thymic epithelial cells enhancing thymus recovery
What does Foxn1 mutation lead to?
It leads to the absence of the thymus and no development of T cells. Severe immunosuppression.
Where does the thymus used for research come from?
From paediatric patients undergoing cardiac surgery. The thymus is located just in front of the heart, covering it, surgeons need to remove part of the thymus to perform cardiac surgery. With parents consent, the thymic tissue is then directed to research.
How do progenitor cells enter the thymus?
Lymphoid precursors leave the bone marrow into the bloodstream, chemokine CCL25 recruits progenitors from the blood into the thymus by binding to CCR9 receptor.
Is the thymus the same size throughout life?
No, as people age, more medulla is replaced with fat tissue, and fewer T cells are generated. However, T cells are very long-lived, so that does not affect immunity.
What are the two main cellular elements of the thymus?
- Haematopoietic (precursor cells)
- Non-haematopoietic (stromal cells)
What are the stages of T cell development?
- Double negative precursors (CD4-CD8-)
- Double positive precursors (CD4+CD8+) after the rearrangement of the beta chain of TCR
- Single positive (CD4+ or CD8+) after the rearrangement of the alpha chain of TCR
- positive selection (T cell receptors which recognise MHC class I or II with strong enough affinity get selected and receive survival signals (cortex))
- negative selection (T cells with a strong affinity to self-peptides presented are deleted from the repertoire (medulla))
- cells that passed negative control are becoming mature naive T cells and are released to the periphery
What growth factors mediate the cellular expansion of double-negative thymocytes in the thymus?
IL-7, IL-15, and SCF (Stem cell factor) are secreted by thymic epithelial cells. This was confirmed by studies on IL-7 receptor and c-Kit KO mice whose thymus contained less than 1 million thymocytes, compared to WT mice with around 120 million thymocytes.
How many genes encode T cell receptors?
Only 2 genes (beta chain and alpha chain). However, genes consist of variable, diversity, joining and constant regions which can randomly rearrange, giving rise to many different combinations of T cell receptors.
What is a Pre-T cell receptor complex?
It is expressed only on double negative thymocytes, it consists of TCR-beta protein, CD3 complex and pre-T-alpha. Only cells which rearranged their beta chain successfully can progress to rearranging the alpha chain. If the rearrangement of the beta chain was successful it will:
- stop TCR-beta rearrangement,
- lead to CD4 and CD8 (double positive) expression,
- TCR-alpha rearrangement
- cellular expansion
What are the possible outcomes of TCR rearrangement?
- Does not recognise peptide/MHC at all - cell death
- Recognise peptide/MHC of low affinity and avidity - progress to maturation
- Recognise peptide/MHC of high affinity and avidity - cell death
What does the rearrangement of alpha chain of TCR lead to?
It leads to the generation of mature TCRS, which either recognises MHC II, which leads to the loss of CD8 expression or recognises MHC I, which leads to the loss of CD4 expression. This leads to the creation of single-positive thymocytes.
What disease is caused by a mutation in Aire?
APECED
What happens with Foxn1 KO mice?
They don’t have thymus and no T cells produced, severe immunodeficiency.
Do we have a dual epithelial progenitor pool or a single epithelial progenitor pool?
We have a single epithelial cell progenitor, which gives rise to both cortex and medulla thymic epithelial cells.
Nude mice
Lacks Foxn1 and lacks thymus
Possible stem-cell-based strategies for thymic regeneration?
- Isolate bipotent thymic epithelial precursor and grow them in vitro (no markers identified yet)
- Use embryonic stem cells
- Use induced pluripotent stem cells (iPSC)
